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diff --git a/old/54210-8.txt b/old/54210-8.txt deleted file mode 100644 index 462342d..0000000 --- a/old/54210-8.txt +++ /dev/null @@ -1,33979 +0,0 @@ -The Project Gutenberg EBook of The Principles of Chemistry. Volume II (of -2), by D. Mendeléeff - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Principles of Chemistry. Volume II (of 2) - -Author: D. Mendeléeff - -Editor: T. A. Lawson - -Translator: George Kamensky - -Release Date: February 19, 2017 [EBook #54210] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK PRINCIPLES OF CHEMISTRY, VOL II *** - - - - -Produced by Chris Curnow, Jens Nordmann and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - THE - PRINCIPLES OF CHEMISTRY - - By D. MENDELÉEFF - - TRANSLATED FROM THE RUSSIAN (SIXTH EDITION) BY - - GEORGE KAMENSKY, A.R.S.M. - - OF THE IMPERIAL MINT, ST PETERSBURG: MEMBER OF THE RUSSIAN - PHYSICO-CHEMICAL SOCIETY - - EDITED BY - - T. A. LAWSON, B.Sc. PH.D. - - EXAMINER IN COAL-TAR PRODUCTS TO THE CITY AND GUILDS OF LONDON - INSTITUTE FELLOW OF THE INSTITUTE OF CHEMISTRY - - IN TWO VOLUMES - - VOLUME II. - - - LONGMANS, GREEN, AND CO - 39 PATERNOSTER ROW, LONDON - NEW YORK AND BOMBAY - 1897 - - All rights reserved - - * * * * * - - - - - TABLE III. - - _The periodic dependence of the composition of the simplest compounds - and properties of the simple bodies upon the atomic weights of - the elements._ - - +-------------------------+--------------------------------+ - | | | - |Molecular composition of | | - |the higher hydrogen and | Atomic weights of the elements | - |metallo-organic compounds| | - |-------------------------+--------------------------------+ - | | | - | | | - |E=CH_{3}, C_{2}H_{5}, &c.| | - | | | - | | | - |[1] [2] [3] [4] | [5] [6] | - | | | - | HH| H 1,005 (mean) | - | | Li 7·02 (Stas) | - | | Be 9·1 (Nilson Pettersson)| - | BE_{3} -- --| B 11·0 (Ramsay Ashton) | - | CH_{4} C_{2}H_{6} | | - | C_{2}H_{4} C_{2}H_{2} | C 12·00 (Roscoe) | - | NH_{3} N_{2}H_{4} --| N 14·04 (Stas) | - | OH_{2} --| O 16 (conventional) | - | FH| F 19·0 (Christiansen) | - | | | - | NaE| Na 23·04 (Stas) | - | MgE_{2} --| Mg 24·3 (Burton) | - | AlE_{3} -- --| Al 27·1 (Mallet) | - |SiH_{4} Si_{2}E_{6} -- --| Si 28·4 (Thorpe Young) | - | PH_{3} P_{2}H_{4} --| P 31·0 (v. d. Plaats) | - | SH_{2} --| S 32·06 (Stas) | - | ClH| Cl 35·45 (Stas) | - | | | - | | K 39·15 (Stas) | - | | Ca 40·0 (Dumas) | - | | Sc 44·0 (Nilson) | - | | Ti 48·1 (Thorpe) | - | | V 51·2 (Roscoe) | - | | Cr 52·1 (Rawson) | - | | Mn 55·1 (Marignac) | - | | Fe 56·0 (Dumas) | - | | Co 58·9 (Zimmermann) | - | | Ni 59·4 (Winkler) | - | | Cu 63.6 (Richards) | - | ZnE_{2} --| Zn 65·3 (Marignac) | - | GaE_{3} -- --| Ga 69·9 (Boisbaudran) | - | GeE_{4} -- -- --| Ge 72·3 (Winkler) | - | AsH_{3} -- --| As 75·0 (Dumas) | - | SeH_{2} --| Se 79·0[A] (Pettersson) | - | BrH| Br 79·95 (Stas) | - | | | - | | Rb 85·5 (Godeffroy) | - | | Sr 87·6 (Dumas) | - | | Y 89 (Clève) | - | | Zr 90·6 (Bailey) | - | | Nb 94 (Marignac) | - | | Mo 96·1 (Maas) | - | | Unknown metal | - | | | - | | Ru 101·7 (Joly) | - | | Rh 102·7 (Seubert) | - | | Pd 106·4 (Keller Smith) | - | | Ag 107·92 (Stas) | - | CdE_{2} --| Cd 112·1 (Lorimer Smith) | - | InE_{3} -- --| In 113·6 (Winkler) | - | SnE_{4} -- -- --| Sn 119·1 (Classen) | - | SbH_{3} -- --| Sb 120·4 (Schneider) | - | TeH_{2} --| Te 125·1 (Brauner) | - | | | - | | Cs 132·7 (Godeffroy) | - | | Ba 137·4 (Richards) | - | | La 138·2 (Brauner) | - | | Ce 140·2 (Brauner) | - | | | - | | Ta 182·7 (Marignac) | - | | W 184·0 (Waddel) | - | | Unknown element. | - | | | - | | Ir 193·3 (Joly) | - | | Pt 196·0 (Dittmar McArthur) | - | | Au 197·5 (Mallet) | - | HgE_{2} --| Hg 200·5 (Erdmann Mar.) | - | TlE_{3} -- --| Tl 204·1 (Crookes) | - | PbE_{4} -- -- --| Pb 206·90 (Stas) | - | BiE_{3} -- --| Bi 208·9 (Classen) | - | | Five unknown elements. | - | | Th 232·4 (Krüss Nilson) | - | | Unknown element. | - | | U 239·3 (Zimmermann) | - +-------------------------+--------------------------------+ - - +----------------------------------------------------------------------+ - | | - | | - | Composition of the saline compounds, X = Cl | - | | - +----------------------------------------------------------------------+ - | Br, (NO_{3}), 1/2 O, 1/2 (SO_{4}), OH, (OM) = Z, where M = K, | - | 1/2 Ca, 1/3 Al, &c. | - |Form RX RX_{2} RX_{3} RX_{4} RX_{5} RX_{6} RX_{7} RX_{8}| - |Oxi- R_{2}O RO R_{2}O_{3} RO_{2} R_{2}O_{5} RO_{3} R_{2}O_{7} RO_{4}| - |des | - | [7] [8] [9] [10] [11] [12] [13] [14] | - | | - | X or H_{2}O | - | iX | - | -- BeX_{2} | - | -- -- BX_{3} | - | | - | -- CO -- COZ_{2} | - | N_{2}O NO NOZ NO_2 NO_{2}Z | - | -- OX_{2} | - | FZ | - | | - | NaX | - | -- MgX_{2} | - | -- -- AlX_{3} | - | -- -- -- SiOZ_{2} | - | -- -- PX_{3} -- POZ_{3} | - | -- SX_{2} -- SOZ_{2} -- SO_{2}Z_{2} | - | ClZ -- ClOZ -- ClO_{2}Z -- ClO_{3}Z | - | | - | KX | - | -- CaX_{2} | - | -- -- ScX_{3} | - | -- TiX_{2} TiX_{3} TiX_{4} | - | -- VO VOX -- VOZ_{3} | - | -- CrX_{2} CrX_{3} CrO_{2} -- CrO_{2}Z_{2} | - | -- MnX_{2} MnX_{3} MnO_{2} -- MnO_{2}Z_{2} MnO_{3}Z | - | -- FeX_{2} FeX_{3} -- -- FeO_{2}Z_{2} | - | -- CoX_{2} CoX_{3} CoO_{2} | - | -- NiX_{2} NiX_{3} | - | CuX CuX_{2} | - | -- ZnX_{2} | - | -- -- GaX_{3} | - | -- GeX_{2} -- GeX_{4} | - | -- AsS AsX_{3} AsS_{2} AsO_{2}Z | - | -- -- -- SeOZ_{2} -- SeO_{2}Z_{2} | - | BrZ -- BrOZ -- BrO_{2}Z -- BrO_{3}Z | - | | - | RbX | - | -- SrX_{2} | - | -- -- YX_{3} | - | -- -- -- ZrX_{4} | - | -- -- NbX_{3} -- NbO_{2}Z | - | -- -- MoX_{3} MoX_{4} -- MoO_{2}Z_{2} | - |(eka-manganese, Em = 99). EmO_{3}Z | - | RuO_{4}| - | -- RuX_{2} RuX_{3} RuX_{4} -- RuO_{2}Z_{2} RuO_{3}Z | - | -- RhX_{2} RhX_{3} RhX_{4} -- RhO_{2}Z_{2} | - | PdX PdX_{2} -- PdX_{4} | - | AgX | - | -- CdX_{2} | - | -- InX_{2} InX_{3} | - | -- SnX_{2} -- SnX_{4} | - | -- -- SbX_{3} -- SbO_{2}Z | - | -- -- -- TeOZ_{2} -- TeO_{2}Z_{2} | - | IZ -- IZ_{3} -- IO_{2}Z -- IO_{3}Z | - | | - | CsX | - | -- BaX_{2} | - | -- -- LaX_{3} | - | -- -- CeX_{3} CeX_{4} | - | Little known Di = 142.1 and Yb = 173.2, and over 15 unknown elements.| - | -- -- -- -- TaO_{2}Z | - | -- -- -- WX_{4} -- WO_{2}Z_{2} | - | | - | OsO_{4}| - | -- -- OsX_{3} OsX_{4} -- OsO_{2}Z_{2} -- | - | -- -- IrX_{3} IrX_{4} -- IrO_{2}Z_{2} | - | -- PtX_{2} -- PtX_{4} | - | AuX -- AuX_{3} | - | HgX HgX_{2} | - | TlX -- TlX_{3} | - | -- PbX_{2} -- PbOZ_{2} | - | -- -- BiX_{3} -- BiO_{2}Z | - | | - | -- -- -- ThX_{4} | - | | - | -- -- -- UO_{2} -- UO_{2}X_{2} UO_{4}| - +----------------------------------------------------------------------+ - - +-------------------------+------------+---------+---------------------+ - | | | Lower | Simple bodies | - |Molecular composition of | |hydrogen +-----+-------+-------| - |the higher hydrogen and | Peroxides | com- | Sp. | Sp. |Melting| - |metallo-organic compounds| | pounds | gr | vol. | point | - |-------------------------+------------+---------+-----+-------+-------| - | | | | | | | - | | | | | | | - |E=CH_{3}, C_{2}H_{5}, &c.| | | | | | - | | | | | | | - | | | | | | | - |[1] [2] [3] [4] | [15] | [16] |[17] | [18] | [19] | - | | | | | | | - | HH|H_{2}O_{2} | -- |*0·05| 20 | -250°?| - | | -- | -- | 0·59| 11·9 | 180° | - | | -- | BeH | 1·64| 5·5 | 900°?| - | BE_{3} -- --| -- | -- | 2·5 | 4·4 |1,300°?| - | CH_{4} C_{2}H_{6} | | | | | | - | C_{2}H_{4} C_{2}H_{2} |C_{2}O_{5}* | -- |*1·9 | 6·3 |2,600°?| - | NH_{3} N_{2}H_{4} --|N_{2}O_{6}* | N_{3}H |*0·6 | 23 | -203° | - | OH_{2} --|O_{3} | -- |*0·9 | 18 | -230°?| - | FH| -- | -- |?1·0 | 19 | ? | - | | | | | | | - | NaE|NaO | Na_{2}H | 0·98| 23·5 | 96° | - | MgE_{2} --| -- | MgH | 1·74| 14 | 500° | - | AlE_{3} -- --| -- | -- | 2·6 | 11 | 600° | - |SiH_{4} Si_{2}E_{6} -- --| -- | -- | 2·3 | 12 |1,300°?| - | PH_{3} P_{2}H_{4} --| -- | P_2H | 2·2 | 14 | 44° | - | SH_{2} --|S_{2}O_{7} | -- | 2·07| 15 | 114° | - | ClH| -- | -- |*1·3 | 27 | -75° | - | | | | | | | - | |KO_{2} | K_{2}H | 0·87| 45 | 58° | - | |CaO_{2} | CaH | 1·56| 26 | 800° | - | | -- | -- |?2·5 | ?18 |1,200°?| - | |TiO_{3} | -- | 3·6 | 13 |2,500°?| - | | -- | -- | 5·5 | 9 |3,000°?| - | |Cr_{2}O_{7} | -- | 6·7 | 7·7 |2,000°?| - | | -- | -- | 7·5 | 7·3 |1,500° | - | | -- |Fe_{n}H* | 7·8 | 7·2 |1,450° | - | | -- | -- | 8·6 | 6·8 |1,400° | - | | -- | Ni_{n}H | 8·7 | 6·8 |1,350° | - | |Cu_{2}O_{5}*| CuH | 8·8 | 7·2 |1,054° | - | ZnE_{2} --|ZnO_{2} | -- | 7·1 | 9·2 | 418° | - | GaE_{3} -- --| -- | -- | 5·96| 11·7 | 30° | - | GeE_{4} -- -- --| -- | -- | 5·47| 13·2 | 900° | - | AsH_{3} -- --| -- |As_{4}H* | 5·65| 13·3 | 500° | - | SeH_{2} --| -- | -- | 4·8 | 16 | 217° | - | BrH| -- | -- | 3·1 | 26 | -7° | - | | | | | | | - | |RbO |Rb_{2}H* | 1·5 | 57 | 39° | - | |SrO_{2} | SrH | 2·5 | 35 | 600°?| - | | -- | -- |*3·4 | *26 |1,000°?| - | | -- |Zr_{4n}H*| 4·1 | 22 |1,500°?| - | | -- |Nb_{n}H* | 7·1 | 13 |1,800°?| - | |Mo_{2}O_{7} | -- | 8·6 | 11 |2,200°?| - | | -- | -- | -- | -- | -- | - | | | | | | | - | | -- |Ru_{n}H* |12·2 | 8·4 |2,000°?| - | | -- |Rh_{n}H* |12·1 | 8·6 |1,900°?| - | | -- | Pd_{2}H |11·4 | 8·3 |1,500° | - | |AgO | -- |10·5 | 10·3 | 950° | - | CdE_{2} --|CdO_{2} | -- | 8·6 | 13 | 320° | - | InE_{3} -- --| -- | -- | 7·4 | 14 | 176° | - | SnE_{4} -- -- --|SnO_{3} | -- | 7·2 | 16 | 232° | - | SbH_{3} -- --| -- | -- | 6·7 | 18 | 432° | - | TeH_{2} --| -- | -- | 6·4 | 20 | 455° | - | IH| -- | -- | 4·9 | 26 | 114° | - | | | | | | | - | | -- |Cs_{2}H* | 2·37| 56 | 27° | - | |BaO_{2} | BaH | 3·75| 36 | ? | - | | -- | -- | 6·1 | 23 | ? | - | | -- | -- | 6·6 | 21 | 700°?| - | | | | | | | - | | -- |Ta_{n}H* |10·4 | 18 | ? | - | |W_{2}O_{7} | -- |19·1 | 9·6 |2,600° | - | | | | | | | - | | | | | | | - | | -- | -- |22·5 | 8·5 |2,700°?| - | | -- | Ir_nH* |22·4 | 8·6 |2,000° | - | | -- |Pt_{n}H* |21·4 | 9·2 |1,775° | - | | -- | -- |19·3 | 10 |1,045° | - | HgE_{2} --| -- | -- |13·6 | 15 | -39° | - | TlE_{3} -- --| -- | -- |11·8 | 17 | 294° | - | PbE_{4} -- -- --| -- | -- |11·3 | 18 | 328° | - | BiE_{3} -- --| -- | -- | 9·8 | 21 | 269° | - | | | | | | | - | | -- | -- |11·1 | 21 | ? | - | | | | | | | - | | -- | -- |18·7 | 13 |2,400°?| - +-------------------------+------------+---------+-----+-------+-------+ - [A] From analogy there is reason for thinking that the atomic weight - of selenium is really slightly less than 79·0. - -Columns 1, 2, 3, and 4 give the molecular composition of the hydrogen -and metallo-organic compounds, exhibiting the most characteristic forms -assumed by the elements. The first column contains only those which -correspond to the form RX_{4}, the second column those of the form -RX_{3}, the third of the form RX_{2}, and the fourth of the form RX, so -that the periodicity stands out clearly (see Column 16). - -Column 5 contains the symbols of all the more or less well-known -elements, placed according to the order of the magnitude of their atomic -weights. - -Column 6 contains the atomic weights of the elements according to the -most trustworthy determinations. The names of the investigators are given -in parenthesis. The atomic weight of oxygen, taken as 16, forms the basis -upon which these atomic weights were calculated. Some of these have been -recalculated by me on the basis of Stas's most trustworthy data (_see_ -Chapter XXIV. and the numbers given by Stas in the table, where they are -taken according to van der Plaats and Thomsen's calculations). - -Columns 7-14 contain the composition of the saline compounds of the -elements, placed according to their forms, RX, RX_{2} to RX_{8} (in the -14^{th} column). If the element R has a metallic character like H, Li, -Be, &c., then X represents Cl, NO_{3}, 1/2 SO_{4}, &c., haloid radicles, -or (OH) if a perfect hydrate is formed (alkali, aqueous base), or 1/2 O, -1/2 S, &c. when an anhydrous oxide, sulphide, &c. is formed. For -instance, NaCl, Mg(NO_{3})_{2}, Al_{2}(SO_{4})_{3}, correspond to NaX, -MgX_{2}, and AlX_{3}; so also Na(OH), Mg(OH)_{2}, Al(OH)_{3}, Na_{2}O, -MgO, Al_{2}O_{3}, &c. But if the element, like C or N, be of a metalloid -or acid character, X must be regarded as (OH) in the formation of -hydrates; (OM) in the formation of salts, where M is the equivalent of a -metal, 1/2 O in the formation of an anhydride, and Cl in the formation of -a chloranhydride; and in this case (_i.e._ in the acid compounds) Z is -put in the place of X; for example, the formulæ COZ_{2}, NO_{2}Z, -MNO_{2}Z, FeO_{2}Z_{2}, and IZ_{3} correspond to CO(NaO)_{2} = -Na_{2}CO_{3}, COCl_{2}, CO_{2}, NO_{2}(NaO) = NaNO_{3}, NO_{2}Cl, -NO_{2}(OH) = HNO_{3}; MnO_{3}(OK) = KMnO_{4}, ICl, &c. - -The 15th column gives the compositions of the peroxides of the -elements, _taking them as anhydrous_. An asterisk (*) is attached to -those of which the composition has not been well established, and a dash -(--) shows that for a given element no peroxides have yet been obtained. -The peroxides contain more oxygen than the higher saline oxides of the -same elements, are powerfully oxidising, and easily give peroxide of -hydrogen. This latter circumstance necessitates their being referred to -the type of peroxide of hydrogen, if bases and acids are referred to the -type of water (see Chapter XV., Note 7 and 11 bis). - -The 16th column gives the composition of the lower hydrogen -compounds like N_{3}H and Na_{2}H. They may often be regarded as alloys -of hydrogen, which is frequently disengaged by them at a comparatively -moderate temperature. They differ greatly in their nature from the -hydrogen compounds given in columns 1-4 (_see_ Note 12). - -Column 17 gives the specific gravity of the elements in a solid and -a liquid state. An asterisk (*) is placed by those which can either only -be assumed from analogy (for example, the sp. gr. of fluorine and -hydrogen, which have not been obtained in a liquid state), or which vary -very rapidly with a variation of temperature and pressure (like oxygen -and nitrogen), or physical state (for instance, carbon in passing from -the state of charcoal to graphite and diamond). But as the sp. gr. in -general varies with the temperature, mechanical condition, &c., the -figures given, although chosen from the most trustworthy sources, can -only be regarded as approximate, and not as absolutely true. They clearly -show a certain periodicity; for instance, the sp. gr. diminishes from Al -on both sides (Al, Mg, Na, with decreasing atomic weight; and Al, Si, P, -S, Cl, with increasing atomic weight, it also diminishes on both sides -from Cu, Ru, and Os.) - -The same remarks refer to the figures in the 18th column, which -gives the so-called atomic volumes of the simple bodies, or the quotient -of their atomic weight and specific gravity. For Na, K, Rb, and Cs the -atomic volume is greatest among the neighbouring elements. For Ni, Pd, -and Os it is least, and this indicates the periodicity of this property -of the simple bodies. - -The last (19th) column gives the melting points of the simple -bodies. Here also a periodicity is seen, i.e. a maximum and minimum value -between which there are intermediate values, as we see, for instance, in -the series Cl, K, Ca, Sc, and Ti, or in the series Cr, Mn, Fe, Co, Ni, -Cu, Zn, Ga, and Ge. - - * * * * * - - - - - CHAPTER XV - - THE GROUPING OF THE ELEMENTS AND THE PERIODIC LAW - - -It is seen from the examples given in the preceding chapters that the sum -of the data concerning the chemical transformations proper to the -elements (for instance, with respect to the formation of acids, salts, -and other compounds having definite properties) is insufficient for -accurately determining the relationship of the elements, inasmuch as this -may be many-sided. Thus, lithium and barium are in some respects -analogous to sodium and potassium, and in others to magnesium and -calcium. It is evident, therefore, that for a complete judgment it is -necessary to have, not only qualitative, but also quantitative, exact and -measurable, data. When a property can be measured it ceases to be vague, -and becomes quantitative instead of merely qualitative. - -Among these measurable properties of the elements, or of their -corresponding compounds, are: (_a_) isomorphism, or the analogy of -crystalline forms; and, connected with it, the power to form crystalline -mixtures which are isomorphous; (_b_) the relation of the volumes of -analogous compounds of the elements; (_c_) the composition of their -saline compounds; and (_d_) the relation of the atomic weights of the -elements. In this chapter we shall briefly consider these four aspects of -the matter, which are exceedingly important for a natural and fruitful -grouping of the elements, facilitating, not only a general acquaintance -with them, but also their detailed study. - -Historically the first, and an important and convincing, method for -finding a relationship between the compounds of two different elements is -by _isomorphism_. This conception was introduced into chemistry by -Mitscherlich (in 1820), who demonstrated that the corresponding salts of -arsenic acid, H_{3}AsO_{4}, and phosphoric acid, H_{3}PO_{4}, crystallise -with an equal quantity of water, show an exceedingly close resemblance in -crystalline form (as regards the angles of their faces and axes), and are -able to crystallise together from solutions, forming crystals containing -a mixture of the isomorphous compounds. Isomorphous substances are those -which, with an equal number of atoms in their molecules, present an -analogy in their chemical reactions, a close resemblance in their -properties, and a similar or very nearly similar crystalline form: they -often contain certain elements in common, from which it is to be -concluded that the remaining elements (as in the preceding example of As -and P) are analogous to each other. And inasmuch as crystalline forms are -capable of exact measurement, the external form, or the relation of the -molecules which causes their grouping into a crystalline form, is -evidently as great a help in judging of the internal forces acting -between the atoms as a comparison of reactions, vapour densities, and -other like relations. We have already seen examples of this in the -preceding pages.[1] It will be sufficient to call to mind that the -compounds of the alkali metals with the halogens RX, in a crystalline -form, all belong to the cubic system and crystallise in octahedra or -cubes--for example, sodium chloride, potassium chloride, potassium -iodide, rubidium chloride, &c. The nitrates of rubidium and cæsium appear -in anhydrous crystals of the same form as potassium nitrate. The -carbonates of the metals of the alkaline earths are isomorphous with -calcium carbonate--that is, they either appear in forms like calc spar or -in the rhombic system in crystals analogous to aragonite.[1 bis] -Furthermore, sodium nitrate crystallises in rhombohedra, closely -resembling the rhombohedra of calc spar (calcium carbonate), CaCO_{3}, -whilst potassium nitrate appears in the same form as aragonite, CaCO_{3}, -and the number of atoms in both kinds of salts is the same: they all -contain one atom of a metal (K, Na, Ca), one atom of a non-metal (C, N), -and three atoms of oxygen. The analogy of form evidently coincides with -an analogy of atomic composition. But, as we have learnt from the -previous description of these salts, there is not any close resemblance -in their properties. It is evident that calcium carbonate approaches more -nearly to magnesium carbonate than to sodium nitrate, although their -crystalline forms are all equally alike. Isomorphous substances which are -perfectly analogous to each other are not only characterised by a close -resemblance of form (homeomorphism), but also by the faculty of entering -into analogous reactions, which is not the case with RNO_{3} and RCO_{3}. -The most important and direct method of recognising perfect -isomorphism--that is, the absolute analogy of two compounds--is given by -that property of analogous compounds of separating from solutions _in -homogeneous crystals, containing the most varied proportions_ of the -analogous substances which enter into their composition. These quantities -do not seem to be in dependence on the molecular or atomic weights, and -if they are governed by any laws they must be analogous to those which -apply to indefinite chemical compounds.[2] This will be clear from the -following examples. Potassium chloride and potassium nitrate are not -isomorphous with each other, and are in an atomic sense composed in a -different manner. If these salts be mixed in a solution and the solution -be evaporated, independent crystals of the two salts will separate, each -in that crystalline form which is proper to it. The crystals will not -contain a mixture of the two salts. But if we mix the solutions of two -isomorphous salts together, then, under certain circumstances, crystals -will be obtained which contain both these substances. However, this -cannot be taken as an absolute rule, for if we take a solution saturated -at a high temperature with a mixture of potassium and sodium chlorides, -then on evaporation sodium chloride only will separate, and on cooling -only potassium chloride. The first will contain very little potassium -chloride, and the latter very little sodium chloride.[3] But if we take, -for example, a mixture of solutions of magnesium sulphate and zinc -sulphate, they cannot be separated from each other by evaporating the -mixture, notwithstanding the rather considerable difference in the -solubility of these salts. Again, the isomorphous salts, magnesium -carbonate, and calcium carbonate are found together--that is, in one -crystal--in nature. The angle of the rhombohedron of these magnesia-lime -spars is intermediate between the angles proper to the two spars -individually (for calcium carbonate, the angle of the rhombohedron is -105° 8´; magnesium carbonate, 107° 30´; CaMg(CO_{3})_{2}, 106° 10´). -Certain of these _isomorphous mixtures_ of calc and magnesia spars appear -in well-formed crystals, and in this case there not unfrequently exists a -simple molecular proportion of strictly definite chemical combination -between the component salts--for instance, CaCO_{3},MgCO_{3}--whilst in -other cases, especially in the absence of distinct crystallisation (in -dolomites), no such simple molecular proportion is observable: this is -also the case in many artificially prepared isomorphous mixtures. The -microscopical and crystallo-optical researches of Professor Inostrantzoff -and others show that in many cases there is really a mechanical, although -microscopically minute, juxtaposition in one whole of the heterogeneous -crystals of calcium carbonate (double refracting) and of the compound -CaMgC_{2}O_{6}. If we suppose the adjacent parts to be microscopically -small (on the basis of the researches of Mallard, Weruboff, and others), -we obtain an idea of isomorphous mixtures. A formula of the following -kind is given to isomorphous mixtures: for instance, for spars, RCO_{3}, -where R = Mg, Ca, and where it may be Fe,Mn ..., &c. This means that the -Ca is partially replaced by Mg or another metal. Alums form a common -example of the separation of isomorphous mixtures from solutions. They -are double sulphates (or seleniates) of alumina (or oxides isomorphous -with it) and the alkalis, which crystallise in well-formed crystals. If -aluminium sulphate be mixed with potassium sulphate, an alum separates, -having the composition KAlS_{2}O_{8},12H_{2}O. If sodium sulphate or -ammonium sulphate, or rubidium (or thallium) sulphate be used, we obtain -alums having the composition RAlS_{2}O_{8},12H_{2}O. Not only do they all -crystallise in the cubic system, but they also contain an equal atomic -quantity of water of crystallisation (12H_{2}O). Besides which, if we mix -solutions of the potassium and ammonium (NH_{4}AlS_{2}O_{8},12H_{2}O) -alums together, then the crystals which separate will contain various -proportions of the alkalis taken, and separate crystals of the alums of -one or the other kind will not be obtained, but each separate crystal -will contain both potassium and ammonium. Nor is this all; if we take a -crystal of a potassium alum and immerse it in a solution capable of -yielding ammonia alum, the crystal of the potash alum will continue to -grow and increase in size in this solution--that is, a layer of the -ammonia or other alum will deposit itself upon the planes bounding the -crystal of the potash alum. This is very distinctly seen if a colourless -crystal of a common alum be immersed in a saturated violet solution of -chrome alum, KCrS_{2}O_{8},12H_{2}O, which then deposits itself in a -violet layer over the colourless crystal of the alumina alum, as was -observed even before Mitscherlich noticed it. If this crystal be then -immersed in a solution of an alumina alum, a layer of this salt will form -over the layer of chrome alum, so that one alum is able to incite the -growth of the other. If the deposition proceed simultaneously, the -resultant intermixture may be minute and inseparable, but its nature is -understood from the preceding experiments; the attractive force of -crystallisation of isomorphous substances is so nearly equal that the -attractive power of an isomorphous substance induces a crystalline -superstructure exactly the same as would be produced by the attractive -force of like crystalline particles. From this it is evident that one -isomorphous substance may _induce the crystallisation_[4] of another. -Such a phenomenon explains, on the one hand, the aggregation of different -isomorphous substances in one crystal, whilst, on the other hand, it -serves as a most exact indication of the nearness both of the molecular -composition of isomorphous substances and of those forces which are -proper to the elements which distinguish the isomorphous substances. -Thus, for example, ferrous sulphate or green vitriol crystallises in the -monoclinic system and contains seven molecules of water, -FeSO_{4},7H_{2}O, whilst copper vitriol crystallises with five molecules -of water in the triclinic system, CuSO_{4},5H_{2}O; nevertheless, it may -be easily proved that both salts are perfectly isomorphous; that they are -able to appear in identically the same forms and with an equal molecular -amount of water. For instance, Marignac, by evaporating a mixture of -sulphuric acid and ferrous sulphate under the receiver of an air-pump, -first obtained crystals of the hepta-hydrated salt, and then of the -penta-hydrated salt FeSO_{4},5H_{2}O, which were perfectly similar to the -crystals of copper sulphate. Furthermore, Lecoq de Boisbaudran, by -immersing crystals of FeSO_{4},7H_{2}O in a supersaturated solution of -copper sulphate, caused the latter to deposit in the same form as ferrous -sulphate, in crystals of the monoclinic system, CuSO_{4},7H_{2}O. - - [1] For instance the analogy of the sulphates of K, Rb, and Cs (Chapter - XIII., Note 1). - - [1 bis] The crystalline forms of aragonite, strontianite, and witherite - belong to the rhombic system; the angle of the prism of CaCO_{3} is - 116° 10´, of SrCO_{3} 117° 19´, and of BaCO_{3} 118° 30´. On the - other hand the crystalline forms of calc spar, magnesite, and - calamine, which resemble each other quite as closely, belong to the - rhombohedral system, with the angle of the rhombohedra for CaCO_{3} - 105° 8´, MgCO_{3} 107° 10´, and ZnCO_{3} 107° 40´. From this - comparison it is at once evident that zinc is more closely allied - to magnesium than magnesium to calcium. - - [2] Solutions furnish the commonest examples of indefinite chemical - compounds. But the isomorphous mixtures which are so common among - the crystalline compounds of silica forming the crust of the earth, - as well as alloys, which are so important in the application of - metals to the arts, are also instances of indefinite compounds. And - if in Chapter I., and in many other portions of this work, it has - been necessary to admit the presence of definite compounds (in a - state of dissociation) in solutions, the same applies with even - greater force to isomorphous mixtures and alloys. For this reason - in many places in this work I refer to facts which compel us to - recognise the existence of definite chemical compounds in all - isomorphous mixtures and alloys. This view of mine (which dates - from the sixties) upon isomorphous mixtures finds a particularly - clear confirmation in B. Roozeboom's researches (1892) upon the - solubility and crystallising capacity of mixtures of the chlorates - of potassium and thallium, KClO_{3} and TlClO_{3}. He showed that - when a solution contains different amounts of these salts, it - deposits crystals containing either an excess of the first salt, - from 98 p.c. to 100 p.c., or an excess of the second salt, from - 63·7 to 100 p.c.; that is, in the crystalline form, either the - first salt saturates the second or the second the first, just as in - the solution of ether in water (Chapter I.); moreover, the - solubility of the mixtures containing 36·3 and 98 p.c. KClO_{3} is - similar, just as the vapour tension of a saturated solution of - water in ether is equal to that of a saturated solution of ether in - water (Chapter I., Note 47). But just as there are solutions - miscible in all proportions, so also certain isomorphous bodies can - be present in crystals in all possible proportions of their - component parts. Van 't Hoff calls such systems 'solid solutions.' - These views were subsequently elaborated by Nernst (1892), and Witt - (1891) applied them in explaining the phenomena observed in the - coloration of tissues. - - [3] The cause of the difference which is observed in different - compounds of the same type, with respect to their property of - forming isomorphous mixtures, must not be looked for in the - difference of their volumetric composition, as many investigators, - including Kopp, affirm. The molecular volumes (found by dividing - the molecular weight by the density) of those isomorphous - substances which do give intermixtures are not nearer to each other - than the volumes of those which do not give mixtures; for example, - for magnesium carbonate the combining weight is 84, density 3·06, - and volume therefore 27; for calcium carbonate in the form of calc - spar the volume is 37, and in the form of aragonite 33; for - strontium carbonate 41, for barium carbonate 46; that is, the - volume of these closely allied isomorphous substances increases - with the combining weight. The same is observed if we compare - sodium chloride (molecular volume = 27) with potassium chloride - (volume = 37), or sodium sulphate (volume = 55) with potassium - sulphate (volume = 66), or sodium nitrate 39 with potassium nitrate - 48, although the latter are less capable of giving isomorphous - mixtures than the former. It is evident that the cause of - isomorphism cannot be explained by an approximation in molecular - volumes. It is more likely that, given a similarity in form and - composition, the faculty to give isomorphous mixtures is connected - with the laws and degree of solubility. - - [4] A phenomenon of a similar kind is shown for magnesium sulphate in - Note 27 of the last chapter. In the same example we see what a - complication the phenomena of dimorphism may introduce when the - forms of analogous compounds are compared. - -Hence it is evident that isomorphism--that is, the analogy of forms and -the property of inducing crystallisation--may serve as a means for the -discovery of analogies in molecular composition. We will take an example -in order to render this clear. If, instead of aluminium sulphate, we add -magnesium sulphate to potassium sulphate, then, on evaporating the -solution, the double salt K_{2}MgS_{2}O_{8},6H_{2}O (Chapter XIV., Note -28) separates instead of an alum, and the ratio of the component parts -(in alums one atom of potassium per 2SO_{4}, and here two atoms) and the -amount of water of crystallisation (in alums 12, and here 6 equivalents -per 2SO_{4}) are quite different; nor is this double salt in any way -isomorphous with the alums, nor capable of forming an isomorphous -crystalline mixture with them, nor does the one salt provoke the -crystallisation of the other. From this we must conclude that although -alumina and magnesia, or aluminium and magnesium, resemble each other, -they are not isomorphous, and that although they give partially similar -double salts, these salts are not analogous to each other. And this is -expressed in their chemical formulæ by the fact that the number of atoms -in alumina or aluminium oxide, Al_{2}O_{3}, is different from the number -in magnesia, MgO. Aluminium is trivalent and magnesium bivalent. Thus, -having obtained a double salt from a given metal, it is possible to judge -of the analogy of the given metal with aluminium or with magnesium, or of -the absence of such an analogy, from the composition and form of this -salt. Thus zinc, for example, does not form alums, but forms a double -salt with potassium sulphate, which has a composition exactly like that -of the corresponding salt of magnesium. It is often possible to -distinguish the bivalent metals analogous to magnesium or calcium from -the trivalent metals, like aluminium, by such a method. Furthermore, the -specific heat and vapour density serve as guides. There are also indirect -proofs. Thus iron gives ferrous compounds, FeX_{2}, which are isomorphous -with the compounds of magnesium, and ferric compounds, FeX_{3}, which are -isomorphous with the compounds of aluminium; in this instance the -relative composition is directly determined by analysis, because, for a -given amount of iron, FeCl_{2} only contains two-thirds of the amount of -chlorine which occurs in FeCl_{3}, and the composition of the -corresponding oxygen compounds, _i.e._ of ferrous oxide, FeO, and ferric -oxide, Fe_{2}O_{3}, clearly indicates the analogy of the ferrous oxide -with MgO and of the ferric oxide with Al_{2}O_{3}. - -Thus in the building up of similar molecules in crystalline forms we -see one of the numerous means for judging of the internal world of -molecules and atoms, and one of the weapons for conquests in the -invisible world of molecular mechanics which forms the main object of -physico-chemical knowledge. This method[5] has more than once been -employed for discovering the analogy of elements and of their compounds; -and as crystals are measurable, and the capacity to form crystalline -mixtures can be experimentally verified, this method is a numerical and -measurable one, and in no sense arbitrary. - - [5] The property of solids of occurring in regular crystalline - forms--the occurrence of many substances in the earth's crust in - these forms--and those geometrical and simple laws which govern the - formation of crystals long ago attracted the attention of the - naturalist to crystals. The crystalline form is, without doubt, the - expression of the relation in which the atoms occur in the - molecules, and in which the molecules occur in the mass, of a - substance. Crystallisation is determined by the distribution of the - molecules along the direction of greatest cohesion, and therefore - those forces must take part in the crystalline distribution of - matter which act between the molecules; and, as they depend on the - forces binding the atoms together in the molecules, a very close - connection must exist between the atomic composition and the - distribution of the atoms in the molecule on the one hand, and the - crystalline form of a substance on the other hand; and hence an - insight into the composition may be arrived at from the crystalline - form. Such is the elementary and _a priori_ idea which lies at the - base of all researches into _the connection between composition and - crystalline form_. Haüy in 1811 established the following - fundamental law, which has been worked out by later investigators: - That the fundamental crystalline form for a given chemical compound - is constant (only the combinations vary), and that with a change of - composition the crystalline form also changes, naturally with the - exception of such limiting forms as the cube, regular octahedron, - &c., which may belong to various substances of the regular system. - The fundamental form is determined by the angles of certain - fundamental geometric forms (prisms, pyramids, rhombohedra), or the - ratio of the crystalline axes, and is connected with the optical - and many other properties of crystals. Since the establishment of - this law the description of definite compounds in a solid state is - accompanied by a description (measurement) of its crystals, which - forms an invariable, definite, and measurable character. The most - important epochs in the further history of this question were made - by the following discoveries:--Klaproth, Vauquelin, and others - showed that aragonite has the same composition as calc spar, whilst - the former belongs to the rhombic and the latter to the hexagonal - system. Haüy at first considered that the composition, and after - that the arrangement, of the atoms in the molecules was different. - This is dimorphism (_see_ Chapter XIV., Note 46). Beudant, - Frankenheim, Laurent, and others found that the forms of the two - nitres, KNO_{3} and NaNO_{3}, exactly correspond with the forms of - aragonite and calc spar; that they are able, moreover, to pass from - one form into another; and that the difference of the forms is - accompanied by a small alteration of the angles, for the angle of - the prisms of potassium nitrate and aragonite is 119°, and of - sodium nitrate and calc spar, 120°; and therefore dimorphism, or - the crystallisation of one substance in different forms, does not - necessarily imply a great difference in the distribution of the - molecules, although some difference clearly exists. The researches - of Mitscherlich (1822) on the dimorphism of sulphur confirmed this - conclusion, although it cannot yet be affirmed that in dimorphism - the arrangement of the atoms remains unaltered, and that only the - molecules are distributed differently. Leblanc, Berthier, - Wollaston, and others already knew that many substances of - different composition appear in the same forms, and crystallise - together in one crystal. Gay-Lussac (1816) showed that crystals of - potash alum continue to grow in a solution of ammonia alum. Beudant - (1817) explained this phenomenon as the _assimilation_ of a foreign - substance by a substance having a great force of crystallisation, - which he illustrated by many natural and artificial examples. But - Mitscherlich, and afterwards Berzelius and Henry Rose and others, - showed that such an assimilation only exists with a similarity or - approximate similarity of the forms of the individual substances - and with a certain degree of chemical analogy. Thus was established - the idea of _isomorphism_ as an analogy of forms by reason of a - resemblance of atomic composition, and by it was explained the - variability of the composition of a number of minerals as - isomorphous mixtures. Thus all the garnets are expressed by the - general formula: (RO)_{3}M_{2}O_{3}(SiO_{2})_{3}, where R = Ca, Mg, - Fe, Mn, and M = Fe, Al, and where we may have either R and M - separately, or their equivalent compounds, or their mixtures in all - possible proportions. - - But other facts, which render the correlation of form and - composition still more complex, have accumulated side by side with - a mass of data which may be accounted for by admitting the - conceptions of isomorphism and dimorphism. Foremost among the - former stand the phenomena of _homeomorphism_--that is, a nearness - of forms with a difference of composition--and then the cases of - polymorphism and hemimorphism--that is, a nearness of the - fundamental forms or only of certain angles for substances which - are near or analogous in their composition. Instances of - homeomorphism are very numerous. Many of these, however, may be - reduced to a resemblance of atomic composition, although they do - not correspond to an isomorphism of the component elements; for - example, CdS (greenockite) and AgI, CaCO_{3} (aragonite) and - KNO_{3}, CaCO_{3} (calc spar) and NaNO_{3}, BaSO_{4} (heavy spar), - KMnO_{4} (potassium permanganate), and KClO_{4} (potassium - perchlorate), Al_{2}O_{3} (corundum) and FeTiO_{3} (titanic iron - ore), FeS_{2} (marcasite, rhombic system) and FeSAs (arsenical - pyrites), NiS and NiAs, &c. But besides these instances there are - homeomorphous substances with an absolute dissimilarity of - composition. Many such instances were pointed out by Dana. - Cinnabar, HgS, and susannite, PbSO_{4}3PbCO_{3} appear in very - analogous crystalline forms; the acid potassium sulphate - crystallises in the monoclinic system in crystals analogous to - felspar, KAlSi_{3}O_{8}; glauberite, Na_{2}Ca(SO_{4})_{2}, augite, - RSiO_{3} (R = Ca, Mg), sodium carbonate, Na_{2}CO_{3},10H_{2}O, - Glauber's salt, Na_{2}SO_{4},10H_{2}O, and borax, - Na_{2}BrO_{7},10H_{2}O, not only belong to the same system - (monoclinic), but exhibit an analogy of combinations and a nearness - of corresponding angles. These and many other similar cases might - appear to be perfectly arbitrary (especially as a _nearness_ of - angles and fundamental forms is a relative idea) were there not - other cases where a resemblance of properties and a distinct - relation in the variation of composition is connected with a - resemblance of form. Thus, for example, alumina, Al_{2}O_{3}, and - water, H_{2}O, are frequently found in many pyroxenes and - amphiboles which only contain silica and magnesia (MgO, CaO, FeO, - MnO). Scheerer and Hermann, and many others, endeavoured to explain - such instances by _polymetric isomorphism_, stating that MgO may be - replaced by 3H_{2}O (for example, olivine and serpentine), SiO_{2} - by Al_{2}O_{3} (in the amphiboles, talcs), and so on. A certain - number of the instances of this order are subject to doubt, because - many of the natural minerals which served as the basis for the - establishment of polymeric isomorphism in all probability no longer - present their original composition, but one which has been altered - under the influence of solutions which have come into contact with - them; they therefore belong to the class of _pseudomorphs_, or - false crystals. There is, however, no doubt of the existence of a - whole series of natural and artificial homeomorphs, which differ - from each other by atomic amounts of water, silica, and some other - component parts. Thus, Thomsen (1874) showed a very striking - instance. The metallic chlorides, RCl_{2}, often crystallise with - water, and they do not then contain less than one molecule of water - per atom of chlorine. The most familiar representative of the order - RCl_{2},2H_{2}O is BaCl_{2},2H_{2}O, which crystallises in the - rhombic system. Barium bromide, BaBr_{2},2H_{2}O, and copper - chloride, CuCl_{2},2H_{2}O, have nearly the same forms: potassium - iodate, KIO_{4}; potassium chlorate, KClO_{4}; potassium - permanganate, KMnO_{4}; barium sulphate, BaSO_{4}; calcium - sulphate, CaSO_{4}; sodium sulphate, Na_{2}SO_{4}; barium formate, - BaC_{2}H_{2}O_{4}, and others have almost the same crystalline form - (of the rhombic system). Parallel with this series is that of the - metallic chlorides containing RCl_{2},4H_{2}O, of the sulphates of - the composition RSO_{4},2H_{2}O, and the formates - RC_{2}H_{2}O_{4},2H_{2}O. These compounds belong to the monoclinic - system, have a close resemblance of form, and differ from the first - series by containing two more molecules of water. The addition of - two more molecules of water in all the above series also gives - forms of the monoclinic system closely resembling each other; for - example, NiCl_{2},6H_{2}O and MnSO_{4},4H_{2}O. Hence we see that - not only is RCl_{2},2H_{2}O analogous in form to RSO_{4} and - RC_{2}H_{2}O_{4}, but that their compounds with 2H_{2}O and with - 4H_{2}O also exhibit closely analogous forms. From these examples - it is evident that the conditions which determine a given form may - be repeated not only in the presence of an isomorphous - exchange--that is, with an equal number of atoms in the - molecule--but also in the presence of an unequal number when there - are peculiar and as yet ungeneralised relations in composition. - Thus ZnO and Al_{2}O_{3} exhibit a close analogy of form. Both - oxides belong to the rhombohedral system, and the angle between the - pyramid and the terminal plane of the first is 118° 7´, and of the - second 118° 49´. Alumina, Al_{2}O_{3}, is also analogous in form to - SiO_{2}, and we shall see that these analogies of form are - conjoined with a certain analogy in properties. It is not - surprising, therefore, that in the complex molecule of a siliceous - compound it is sometimes possible to replace SiO_{2} by means of - Al_{2}O_{3}, as Scheerer admits. The oxides Cu_{2}O, MgO, NiO, - Fe_{3}O_{4}, CeO_{2}, crystallise in the regular system, although - they are of very different atomic structure. Marignac demonstrated - the perfect analogy of the forms of K_{2}ZrF_{6} and CaCO_{3}, and - the former is even dimorphous, like the calcium carbonate. The same - salt is isomorphous with R_{2}NbOF_{5} and R_{2}WO_{2}F_{4}, where - R is an alkali metal. There is an equivalency between CaCO_{3} and - K_{2}ZrF_{6}, because K_{2} is equivalent to Ca, C to Zr, and F_{6} - to O_{3}, and with the isomorphism of the other two salts we find - besides an equal contents of the alkali metal--an equal number of - atoms on the one hand and an analogy to the properties of - K_{2}ZrF_{6} on the other. The long-known isomorphism of the - corresponding compounds of potassium and ammonium, KX and NH_{4}X, - may be taken as the simplest example of the fact that an analogy of - form shows itself with an analogy of chemical reaction even without - an equality in atomic composition. Therefore the ultimate progress - of the entire doctrine of the correlation of composition and - crystalline forms will only be arrived at with the accumulation of - a sufficient number of facts collected on a plan corresponding with - the problems which here present themselves. The first steps have - already been made. The researches of the Geneva _savant_, Marignac, - on the crystalline form and composition of many of the double - fluorides, and the work of Wyruboff on the ferricyanides and other - compounds, are particularly important in this respect. It is - already evident that, with a definite change of composition, - certain angles remain constant, notwithstanding that others are - subject to alteration. Such an instance of the relation of forms - was observed by Laurent, and named by him _hemimorphism_ (an - anomalous term) when the analogy is limited to certain angles, and - _paramorphism_ when the forms in general approach each other, but - belong to different systems. So, for example, the angle of the - planes of a rhombohedron may be greater or less than 90°, and - therefore such acute and obtuse rhombohedra may closely approximate - to the cube. Hausmannite, Mn_{3}O_{4}, belongs to the tetragonal - system, and the planes of its pyramid are inclined at an angle of - about 118°, whilst magnetic iron ore, Fe_{3}O_{4}, which resembles - hausmannite in many respects, appears in regular octahedra--that - is, the pyramidal planes are inclined at an angle of 109° 28´. This - is an example of paramorphism; the systems are different, the - compositions are analogous, and there is a certain resemblance in - form. Hemimorphism has been found in many instances of saline and - other substitutions. Thus, Laurent demonstrated, and Hintze - confirmed (1873), that naphthalene derivatives of analogous - composition are hemimorphous. Nicklès (1849) showed that in - ethylene sulphate the angle of the prism is 125° 26´, and in the - nitrate of the same radicle 126° 95´. The angle of the prism of - methylamine oxalate is 131° 20´, and of fluoride, which is very - different in composition from the former, the angle is 132°. Groth - (1870) endeavoured to indicate in general what kinds of change of - form proceed with the substitution of hydrogen by various other - elements and groups, and he observed a regularity which he termed - _morphotropy_. The following examples show that morphotropy recalls - the hemimorphism of Laurent. Benzene, C_{6}H_{6}, rhombic system, - ratio of the axes 0·891 : 1 : 0·799. Phenol, C_{6}H_{5}(OH), and - resorcinol, C_{6}H_{4}(OH)_{2}, also rhombic system, but the ratio - of one axis is changed--thus, in resorcinol, 0·910 : 1 : 0·540; - that is, a portion of the crystalline structure in one direction is - the same, but in the other direction it is changed, whilst in the - rhombic system dinitrophenol, C_{6}H_{3}(NO_{2})_{2}(OH) = - O·833 : 1 : 0·753; trinitrophenol (picric acid), - C_{6}H_{2}(NO)_{3}(OH) = 0·937 : 1 : 0·974; and the potassium salt - = 0·942 : 1 : 1·354. Here the ratio of the first axis is - preserved--that is, certain angles remain constant, and the - chemical proximity of the composition of these bodies is undoubted. - Laurent compares hemimorphism with architectural style. Thus, - Gothic cathedrals differ in many respects, but there is an analogy - expressed both in the sum total of their common relations and in - certain details--for example, in the windows. It is evident that we - may expect many fruitful results for molecular mechanics (which - forms a problem common to many provinces of natural science) from - the further elaboration of the data concerning those variations - which take place in crystalline form when the composition of a - substance is subjected to a known change, and therefore I consider - it useful to point out to the student of science seeking for matter - for independent scientific research this vast field for work which - is presented by the correlation of form and composition. The - geometrical regularity and varied beauty of crystalline forms offer - no small attraction to research of this kind. - -The regularity and simplicity expressed by the exact laws of crystalline -form repeat themselves in the aggregation of the atoms to form molecules. -Here, as there, there are but few forms which are essentially different, -and their apparent diversity reduces itself to a few fundamental -differences of type. There the molecules aggregate themselves into -crystalline forms; here, the atoms aggregate themselves into molecular -forms or into _the types of compounds_. In both cases the fundamental -crystalline or molecular forms are liable to variations, conjunctions, -and combinations. If we know that potassium gives compounds of the -fundamental type KX, where X is a univalent element (which combines with -one atom of hydrogen, and is, according to the law of substitution, able -to replace it), then we know the composition of its compounds: K_{2}O, -KHO, KCl, NH_{2}K, KNO_{3}, K_{2}SO_{4}, KHSO_{4}, -K_{2}Mg(SO_{4})_{2},6H_{2}O, &c. All the possible derivative crystalline -forms are not known. So also all the atomic combinations are not known -for every element. Thus in the case of potassium, KCH_{3}, K_{3}P, -K_{2}Pt, and other like compounds which exist for hydrogen or chlorine, -are unknown. - -Only a few fundamental types exist for the building up of atoms into -molecules, and the majority of them are already known to us. If X stand -for a univalent element, and R for an element combined with it, then -eight atomic types may be observed:-- - - RX, RX_{2}, RX_{3}, RX_{4}, RX_{5}, RX_{6}, RX_{7}, RX_{8}. - -Let X be chlorine or hydrogen. Then as examples of the first type we -have: H_{2}, Cl_{2}, HCl, KCl, NaCl, &c. The compounds of oxygen or -calcium may serve as examples of the type RX_{2}: OH_{2}, OCl_{2}, OHCl, -CaO, Ca(OH)_{2}, CaCl_{2}, &c. For the third type RX_{3} we know the -representative NH_{3} and the corresponding compounds N_{2}O_{3}, NO(OH), -NO(OK), PCl_{3}, P_{2}O_{3}, PH_{3}, SbH_{3}, Sb_{2}O_{3}, B_{2}O_{3}, -BCl_{3}, Al_{2}O_{3}, &c. The type RX_{4} is known among the hydrogen -compounds. Marsh gas, CH_{4}, and its corresponding saturated -hydrocarbons, C_{_n_}H_{2_n_ + 2}, are the best representatives. Also -CH_{3}Cl, CCl_{4}, SiCl_{4}, SnCl_{4}, SnO_{2}, CO_{2}, SiO_{2}, and a -whole series of other compounds come under this class. The type RX_{5} is -also already familiar to us, but there are no purely hydrogen compounds -among its representatives. Sal-ammoniac, NH_{4}Cl, and the corresponding -NH_{4}(OH), NO_{2}(OH), ClO_{2}(OK), as well as PCl_{5}, POCl_{3}, &c., -are representatives of this type. In the higher types also there are no -hydrogen compounds, but in the type RX_{6} there is the chlorine compound -WCl_{6}. However, there are many oxygen compounds, and among them SO_{3} -is the best known representative. To this class also belong -SO_{2}(OH)_{2}, SO_{2}Cl_{2}, SO_{2}(OH)Cl, CrO_{3}, &c., all of an acid -character. Of the higher types there are in general only oxygen and acid -representatives. The type RX_{7} we know in perchloric acid, ClO_{3}(OH), -and potassium permanganate, MnO_{3}(OK), is also a member. The type -RX_{8} in a free state is very rare; osmic anhydride, OsO_{4}, is the -best known representative of it.[6] - - [6] The still more complex combinations--which are so clearly expressed - in the crystallo-hydrates, double salts, and similar - compounds--although they may be regarded as independent, are, - however, most easily understood with our present knowledge as - aggregations of whole molecules to which there are no corresponding - double compounds, containing one atom of an element R and many - atoms of other elements RX_{_n_}. The above types embrace all cases - of direct combinations of atoms, and the formula MgSO_{4},7H_{2}O - cannot, without violating known facts, be directly deduced from the - types MgX_{_n_} or SX_{_n_}, whilst the formula MgSO_{4} - corresponds both with the type of the magnesium compounds MgX_{2} - and with the type of the sulphur compounds SO_{2}X_{2}, or in - general SX_{6}, where X_{2} is replaced by (OH)_{2}, with the - substitution in this case of H_{2} by the atom Mg, which always - replaces H_{2}. However, it must be remarked that the sodium - crystallo-hydrates often contain 10H_{2}O, the magnesium - crystallo-hydrates 6 and 7H_{2}O, and that the type PtM_{2}X_{6} is - proper to the double salts of platinum, &c. With the further - development of our knowledge concerning crystallo-hydrates, double - salts, alloys, solutions, &c., in the _chemical sense_ of feeble - compounds (that is, such as are easily destroyed by feeble chemical - influences) it will probably be possible to arrive at a perfect - generalisation for them. For a long time these subjects were only - studied by the way or by chance; our knowledge of them is - accidental and destitute of system, and therefore it is impossible - to expect as yet any generalisation as to their nature. The days of - Gerhardt are not long past when only three types were recognised: - RX, RX_{2}, and RX_{3}; the type RX_{4} was afterwards added (by - Cooper, Kekulé, Butleroff, and others), mainly for the purpose of - generalising the data respecting the carbon compounds. And indeed - many are still satisfied with these types, and derive the higher - types from them; for instance, RX_{5} from RX_{3}--as, for example, - POCl_{3} from PCl_{3}, considering the oxygen to be bound both to - the chlorine (as in HClO) and to the phosphorus. But the time has - now arrived when it is clearly seen that the forms RX, RX_{2}, - RX_{3}, and RX_{4} do not exhaust the whole variety of phenomena. - The revolution became evident when Würtz showed that PCl_{5} is not - a compound of PCl_{3} + Cl_{2} (although it may decompose into - them), but a whole molecule capable of passing into vapour, PCl_{5} - like PF_{5} and SiF_{4}. The time for the recognition of types even - higher than RX_{8} is in my opinion in the future; that it will - come, we can already see in the fact that oxalic acid, - C_{2}H_{2}O_{4}, gives a crystallo-hydrate with 2H_{2}O; but it may - be referred to the type CH_{4}, or rather to the type of ethane, - C_{2}H_{6}, in which all the atoms of hydrogen are replaced by - hydroxyl, C_{2}H_{2}O_{4}2H_{2}O = C_{2}(OH)_{6} (_see_ Chapter - XXII., Note 35). - -The four lower types RX, RX_{2}, RX_{3}, and RX_{4} are met with in -compounds of the elements R with chlorine and oxygen, and also in their -compounds with hydrogen, whilst the four higher types only appear for -such acid compounds as are formed by chlorine, oxygen, and similar -elements. - -Among the oxygen compounds the _saline oxides_ which are capable of -forming salts either through the function of a base or through the -function of an acid anhydride attract the greatest interest in every -respect. Certain elements, like calcium and magnesium, only give one -saline oxide--for example, MgO, corresponding with the type MgX_{2}. But -the majority of the elements appear in several such forms. Thus copper -gives CuX and CuX_{2}, or Cu_{2}O and CuO. If an element R gives a higher -type RX_{_n_}, then there often also exist, as if by symmetry, lower -types, RX_{_n_-2}, RX_{_n_-4}, and in general such types as differ from -RX_{_n_} by an even number of X. Thus in the case of sulphur the types -SX_{2}, SX_{4}, and SX_{6} are known--for example SH_{2}, SO_{2}, and -SO_{3}. The last type is the highest, SX_{6}. The types SX_{5} and SX_{3} -do not exist. But even and uneven types sometimes appear for one and the -same element. Thus the types RX and RX_{2} are known for copper and -mercury. - -Among the _saline_ oxides only the _eight types_ enumerated below are -known to exist. They determine the possible formulæ of the compounds of -the elements, if it be taken into consideration that an element which -gives a certain type of combination may also give lower types. For this -reason the rare type of the _suboxides_ or quaternary oxides R_{4}O (for -instance, Ag_{4}O, Ag_{2}Cl) is not characteristic; it is always -accompanied by one of the higher grades of oxidation, and the compounds -of this type are distinguished by their great chemical instability, and -split up into an element and the higher compound (for instance, Ag_{4}O = -2Ag + Ag_{2}O). Many elements, moreover, form transition oxides whose -composition is intermediate, which are able, like N_{2}O_{4}, to split up -into the lower and higher oxides. Thus iron gives magnetic oxide, -Fe_{3}O_{4}, which is in all respects (by its reactions) a compound of -the suboxide FeO with the oxide Fe_{2}O_{3}. The independent and more or -less stable saline compounds correspond with the following eight -types:-- - - R_{2}O; salts RX, hydroxides ROH. Generally basic like K_{2}O, Na_{2}O, - Hg_{2}O, Ag_{2}O, Cu_{2}O; if there are acid oxides of this - composition they are very rare, are only formed by distinctly acid - elements, and even then have only feeble acid properties; for - example, Cl_{2}O and N_{2}O. - - R_{2}O_{2} or RO; salts RX_{2}, hydroxides R(OH)_{2}. The most simple - basic salts R_{2}OX_{2} or R(OH)X; for instance, the chloride - Zn_{2}OCl_{2}; also an almost exclusively basic type; but the basic - properties are more feebly developed than in the preceding type. - For example, CaO, MgO, BaO, PbO, FeO, MnO, &c. - - R_{2}O_{3}; salts RX_{3}, hydroxides R(OH)_{3}, RO(OH), the most simple - basic salts ROX, R(OH)X_{3}. The bases are feeble, like - Al_{2}O_{3}, Fe_{2}O_{3}, Tl_{2}O_{3}, Sb_{2}O_{3}. The acid - properties are also feebly developed; for instance, in B_{2}O_{3}; - but with the non-metals the properties of acids are already clear; - for instance, P_{2}O_{3}, P(OH)_{3}. - - R_{2}O_{4} or RO_{2}; salts RX_{4} or ROX_{2}, hydroxides R(OH)_{4}, - RO(OH)_{2}. Rarely bases (feeble), like ZrO_{2}, PtO_{2}; more - often acid oxides; but the acid properties are in general feeble, - as in CO_{2}, SO_{2}, SnO_{2}. Many intermediate oxides appear in - this and the preceding and following types. - - R_{2}O_{5}; salts principally of the types ROX_{3}, RO_{2}X, - RO(OH)_{3}, RO_{2}(OH), rarely RX_{5}. The basic character (X, a - halogen, simple or complex; for instance, NO_{3}, Cl, &c.) is - feeble; the acid character predominates, as is seen in N_{2}O_{5}, - P_{2}O_{5}, Cl_{2}O_{5}; then X = OH, OK, &c., for example - NO_{2}(OK). - - R_{2}O_{6} or RO_{3}; salts and hydroxides generally of the type - RO_{2}X_{2}, RO_{2}(OH)_{2}. Oxides of an acid character, as - SO_{3}, CrO_{3}, MnO_{3}. Basic properties rare and feebly - developed as in UO_{3}. - - R_{2}O_{7}; salts of the form RO_{3}X, RO_{3}(OH), acid oxides; for - instance, Cl_{2}O_{7}, Mn_{2}O_{7}. Basic properties as feebly - developed as the acid properties in the oxides R_{2}O. - - R_{2}O_{8} or RO_{4}. A very rare type, and only known in OsO_{4} and - RuO_{4}. - -It is evident from the circumstance that in all the higher types the -_acid hydroxides_ (for example, HClO_{4}, H_{2}SO_{4}, H_{3}PO_{4}) and -salts with a single atom of one element contain, like the higher saline -type RO_{4}, _not more than four atoms of oxygen_; that the formation of -the saline oxides is governed by a certain common principle which is best -looked for in the fundamental properties of oxygen, and in general of the -most simple compounds. The hydrate of the oxide RO_{2} is of the higher -type RO_{2}2H_{2}O = RH_{4}O_{4} = R(HO)_{4}. Such, for example, is the -hydrate of silica and the salts (orthosilicates) corresponding with it, -Si(MO)_{4}. The oxide R_{2}O_{5}, corresponds with the hydrate -R_{2}O_{5}3H_{2}O = 2RH_{3}O_{4} = 2RO(OH)_{3}. Such is orthophosphoric -acid, PH_{3}O_{3}. The hydrate of the oxide RO_{3} is RO_{3}H_{2}O = -RH_{2}O_{4} = RO_{2}(OH)_{2}--for instance, sulphuric acid. The hydrate -corresponding to R_{2}O_{7} is evidently RHO = RO_{3}(OH)--for example, -perchloric acid. Here, besides containing O_{4}, it must further be -remarked that _the amount of hydrogen in the hydrate is equal to the -amount of hydrogen in the hydrogen compound_. Thus silicon gives SiH_{4} -and SiH_{4}O_{4}, phosphorus PH_{3} and PH_{3}O_{4}, sulphur SH_{2} and -SH_{2}O_{4}, chlorine ClH and ClHO_{4}. This, if it does not explain, at -least connects in a harmonious and general system the fact that _the -elements are capable of combining with a greater amount of oxygen, the -less the amount of hydrogen which they are able to retain_. In this the -key to the comprehension of all further deductions must be looked for, -and we will therefore formulate this rule in general terms. An element R -gives a hydrogen compound RH_{_n_}, the hydrate of its higher oxide will -be RH_{_n_}O_{4}, and therefore the higher oxide will contain -2RH_{_n_}O_{4} - _n_H_{2}O = R_{2}O_{8 - _n_}. For example, chlorine -gives ClH, hydrate ClHO_{4}, and the higher oxide Cl_{2}O_{7}. Carbon -gives CH_{4} and CO_{2}. So also, SiO_{2} and SiH_{4} are the higher -compounds of silicon with hydrogen and oxygen, like CO_{2} and CH_{4}. -Here the amounts of oxygen and hydrogen are equivalent. Nitrogen combines -with a large amount of oxygen, forming N_{2}O_{5}, but, on the other -hand, with a small quantity of hydrogen in NH_{3}. _The sum of the -equivalents of hydrogen and oxygen_, occurring in combination with an -atom of nitrogen, is, as always in the higher types, equal to _eight_. It -is the same with the other elements which combine with hydrogen and -oxygen. Thus sulphur gives SO_{3}; consequently, six equivalents of -oxygen fall to an atom of sulphur, and in SH_{2} two equivalents of -hydrogen. The sum is again equal to eight. The relation between -Cl_{2}O_{7} and ClH is the same. This shows that the property of elements -of combining with such different elements as oxygen and hydrogen is -subject to one common law, which is also formulated in the system of the -elements presently to be described.[7] - - [7] The hydrogen compounds, R_{2}H, in equivalency correspond with the - type of the suboxides, R_{4}O. Palladium, sodium, and potassium - give such hydrogen compounds, and it is worthy of remark that - according to the periodic system these elements stand near to each - other, and that in those groups where the hydrogen compounds R_{2}H - appear, the quaternary oxides R_{4}O are also present. - - Not wishing to complicate the explanation, I here only touch on the - general features of the relation between the hydrates and oxides - and of the oxides among themselves. Thus, for instance, the - conception of the ortho-acids and of the normal acids will be - considered in speaking of phosphoric and phosphorous acids. - - As in the further explanation of the periodic law only those oxides - which give salts will be considered, I think it will not be - superfluous to mention here the following facts relative to the - peroxides. Of the _peroxides_ corresponding with hydrogen peroxide, - the following are at present known: H_{2}O_{2}, Na_{2}O_{2}, - S_{2}O_{7} (as HSO_{4}?), K_{2}O_{4}, K_{2}O_{2}, CaO_{2}, TiO_{3}, - Cr_{2}O_{7}, CuO_{2}(?), ZnO_{2}, Rb_{2}O_{2}, SrO_{2}, - Ag_{2}O_{2}, CdO_{2}, CsO_{2}, Cs_{2}O_{2}, BaO_{2}, Mo_{2}O_{7}, - SnO_{3}, W_{2}O_{7}, UO_{4}. It is probable that the number of - peroxides will increase with further investigation. A periodicity - is seen in those now known, for the elements (excepting Li) of the - first group, which give R_{2}O, form peroxides, and then the - elements of the sixth group seem also to be particularly inclined - to form peroxides, R_{2}O_{7}; but at present it is too early, in - my opinion, to enter upon a generalisation of this subject, not - only because it is a new and but little studied matter (not - investigated for all the elements), but also, and more especially, - because in many instances only the hydrates are known--for - instance, Mo_{2}H_{2}O_{8}--and they perhaps are only compounds of - peroxide of hydrogen--for example, Mo_{2}H_{2}O_{8} = 2MoO_{3} + - H_{2}O_{2}--since Prof. Schöne has shown that H_{2}O_{2} and - BaO_{2} possess the property of combining together and with other - oxides. Nevertheless, I have, in the general table expressing the - periodic properties of the elements, endeavoured to sum up the data - respecting all the known peroxide compounds whose characteristic - property is seen in their capability to form peroxide of hydrogen - under many circumstances. - -In the preceding we see not only the regularity and simplicity which -govern the formation and properties of the oxides and of all the -compounds of the elements, but also a fresh and exact means for -recognising the analogy of elements. Analogous elements give compounds of -analogous types, both higher and lower. If CO_{2} and SO_{2} are two -gases which closely resemble each other both in their physical and -chemical properties, the reason of this must be looked for not in an -analogy of sulphur and carbon, but in that identity of the type of -combination, RX_{4}, which both oxides assume, and in that influence -which a large mass of oxygen always exerts on the properties of its -compounds. In fact, there is little resemblance between carbon and -sulphur, as is seen not only from the fact that CO_{2} is the _higher -form_ of oxidation, whilst SO_{2} is able to further oxidise into SO_{3}, -but also from the fact that all the other compounds--for example, SH_{2} -and CH_{4}, SCl_{2} and CCl_{4}, &c.--are entirely unlike both in type -and in chemical properties. This absence of analogy in carbon and sulphur -is especially clearly seen in the fact that the highest saline oxides are -of different composition, CO_{2} for carbon, and SO_{3} for sulphur. In -Chapter VIII. we considered the limit to which carbon tends in its -compounds, and in a similar manner there is for every element in its -compounds a tendency to attain a certain highest limit RX_{_n_}. This -view was particularly developed in the middle of the present century by -Frankland in studying the metallo-organic compounds, _i.e._ those in -which X is wholly or partially a hydrocarbon radicle; for instance, X = -CH_{3} or C_{2}H_{5} &c. Thus, for example, antimony, Sb (Chapter XIX.) -gives, with chlorine, compounds SbCl_{3} and SbCl_{5} and corresponding -oxygen compounds Sb_{2}O_{3} and Sb_{2}O_{5}, whilst under the action of -CH_{3}I, C_{2}H_{5}I, or in general EI (where E is a hydrocarbon radicle -of the paraffin series), upon antimony or its alloy with sodium there are -formed SbE_{3} (for example, Sb(CH_{3})_{3}, boiling at about 81°), -which, corresponding to the lower form of combination SbX_{3}, are able -to combine further with EI, or Cl_{2}, or O, and to form compounds of the -limiting type SbX_{5}; for example, SbE_{4}Cl corresponding to NH_{4}Cl -with the substitution of nitrogen by antimony, and of hydrogen by the -hydrocarbon radicle. The elements which are most chemically analogous are -characterised by the fact of their giving compounds of similar form -RX_{_n_}. The halogens which are analogous give both higher and lower -compounds. So also do the metals of the alkalis and of the alkaline -earths. And we saw that this analogy extends to the composition and -properties of the nitrogen and hydrogen compounds of these metals, which -is best seen in the salts. Many such groups of analogous elements have -long been known. Thus there are analogues of oxygen, nitrogen, and -carbon, and we shall meet with many such groups. But an acquaintance with -them inevitably leads to the questions, what is the cause of analogy and -what is the relation of one group to another? If these questions remain -unanswered, it is easy to fall into error in the formation of the groups, -because the notions of the degree of analogy will always be relative, and -will not present any accuracy or distinctness Thus lithium is analogous -in some respects to potassium and in others to magnesium; beryllium is -analogous to both aluminium and magnesium. Thallium, as we shall -afterwards see and as was observed on its discovery, has much kinship -with lead and mercury, but some of its properties appertain to lithium -and potassium. Naturally, where it is impossible to make measurements one -is reluctantly obliged to limit oneself to approximate comparisons, -founded on apparent signs which are not distinct and are wanting in -exactitude. But in the elements there is one accurately measurable -property, which is subject to no doubt--namely, that property which is -expressed in their atomic weights. Its magnitude indicates the relative -mass of the atom, or, if we avoid the conception of the atom, its -magnitude shows the relation between the masses forming the chemical and -independent individuals or elements. And according to the teaching of all -exact data about the phenomena of nature, the mass of a substance is that -property on which all its remaining properties must be dependent, because -they are all determined by similar conditions or by those forces which -act in the weight of a substance, and this is directly proportional to -its mass. Therefore it is most natural to seek for a dependence between -the properties and analogies of the elements on the one hand and their -atomic weights on the other. - -This is the fundamental idea which leads _to arranging all the elements -according to their atomic weights_. A periodic repetition of properties -is then immediately observed in the elements. We are already familiar -with examples of this:-- - - F = 19, Cl = 35·5, Br = 80, I = 127, - Na = 23, K = 39, Rb = 85, Cs = 133, - Mg = 24, Ca = 40, Sr = 87, Ba = 137. - -The essence of the matter is seen in these groups. The halogens have -smaller atomic weights than the alkali metals, and the latter than the -metals of the alkaline earths. Therefore, _if all the elements be -arranged in the order of their atomic weights, a periodic repetition of -properties is obtained_. This is expressed by the _law of periodicity_, -_the properties of the elements, as well as the forms and properties of -their compounds, are in periodic dependence or (expressing ourselves -algebraically) form a periodic function of the atomic weights of the -elements_.[8] Table I. of _the periodic system of the elements_, which is -placed at the very beginning of this book, is designed to illustrate this -law. It is arranged in conformity with the eight types of oxides -described in the preceding pages, and those elements which give the -oxides, R_{2}O and consequently salts RX, form the 1st group; the -elements giving R_{2}O_{2} or RO as their highest grade of oxidation -belong to the 2nd group; those giving R_{2}O_{3} as their highest oxides -form the 3rd group, and so on; whilst the elements of all the groups -which are nearest in their atomic weights are arranged in series from 1 -to 12. The even and uneven series of the same groups present the same -forms and limits, but differ in their properties, and therefore two -contiguous series, one even and the other uneven--for instance, the 4th -and 5th--form a period. Hence the elements of the 4th, 6th, 8th, 10th, -and 12th, or of the 3rd, 5th, 7th, 9th, and 11th, series form analogues, -like the halogens, the alkali metals, &c. The conjunction of two series, -one even and one contiguous uneven series, thus forms one large _period_. -These periods, beginning with the alkali metals, end with the halogens. -The elements of the first two series have the lowest atomic weights, and -in consequence of this very circumstance, although they bear the general -properties of a group, they still show many peculiar and independent -properties.[9] Thus fluorine, as we know, differs in many points from the -other halogens, and lithium from the other alkali metals, and so on. -These lightest elements may be termed _typical elements_. They include-- - - H. - Li, Be, B, C, N, O, F. - Na, Mg.... - -In the annexed table all the remaining elements are arranged, not in -groups and series, but _according to periods_. In order to understand the -essence of the matter, it must be remembered that here the atomic weight -gradually increases along a given line; for instance, in the line -commencing with K = 39 and ending with Br = 80, the intermediate elements -have intermediate atomic weights, as is clearly seen in Table III., where -the elements stand in the order of their atomic weights. - - I. II. III. IV. V. VI. VII. I. II. III. IV. V. VI. VII. - { Even Series. } Mg Al Si P S Cl - K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br - Rb Sr Y Zr Nb Mo -- Ru Rh Pd Ag Cd In Sn Sb Te I - Cs Ba La Ce Di? -- -- -- -- -- -- -- -- -- -- -- -- - -- -- Yb -- Ta W -- Os Ir Pt Au Hg Tl Pb Bi -- -- - -- -- -- Th -- U { Uneven Series } - -The same degree of analogy that we know to exist between potassium, -rubidium, and cæsium; or chlorine, bromine, and iodine; or calcium, -strontium, and barium, also exists between the elements of the other -vertical columns. Thus, for example, zinc, cadmium, and mercury, which -are described in the following chapter, present a very close analogy with -magnesium. For a true comprehension of the matter[10] it is very -important to see that all the aspects of the distribution of the elements -according to their atomic weights essentially express one and the same -fundamental _dependence_--_periodic properties_.[11] The following points -then must be remarked in it. - - [8] The periodic law and the periodic system of the elements appeared - in the same form as here given in the first edition of this work, - begun in 1868 and finished in 1871. In laying out the accumulated - information respecting the elements, I had occasion to reflect on - their mutual relations. At the beginning of 1869 I distributed - among many chemists a pamphlet entitled 'An Attempted System of the - Elements, based on their Atomic Weights and Chemical Analogies,' - and at the March meeting of the Russian Chemical Society, 1869, I - communicated a paper 'On the Correlation of the Properties and - Atomic Weights of the Elements.' The substance of this paper is - embraced in the following conclusions: (1) The elements, if - arranged according to their atomic weights, exhibit an evident - _periodicity_ of properties. (2) Elements which are similar as - regards their chemical properties have atomic weights which are - either of nearly the same value (platinum, iridium, osmium) or - which increase regularly (_e.g._ potassium, rubidium, cæsium). (3) - The arrangement of the elements or of groups of elements in the - order of their atomic weights corresponds with their so-called - _valencies_. (4) The elements, which are the most widely - distributed in nature, have _small_ atomic weights, and all the - elements of small atomic weight are characterised by sharply - defined properties. They are therefore typical elements. (5) The - _magnitude_ of the atomic weight determines the character of an - element. (6) The discovery of many yet unknown elements may be - expected. For instance, elements analogous to aluminium and - silicon, whose atomic weights would be between 65 and 75. (7) The - atomic weight of an element may sometimes be corrected by aid of a - knowledge of those of the adjacent elements. Thus the combining - weight of tellurium must lie between 123 and 126, and cannot be - 128. (8) Certain characteristic properties of the elements can be - foretold from their atomic weights. - - The entire periodic law is included in these lines. In the series - of subsequent papers (1870-72, for example, in the _Transactions_ - of the Russian Chemical Society, of the Moscow Meeting of - Naturalists, of the St. Petersburg Academy, and Liebig's _Annalen_) - on the same subject we only find applications of the same - principles, which were afterwards confirmed by the labours of - Roscoe, Carnelley, Thorpe, and others in England; of Rammelsberg - (cerium and uranium), L. Meyer (the specific volumes of the - elements), Zimmermann (uranium), and more especially of C. Winkler - (who discovered germanium, and showed its identity with - ekasilicon), and others in Germany; of Lecoq de Boisbaudran in - France (the discoverer of gallium = ekaaluminium); of Clève (the - atomic weights of the cerium metals), Nilson (discoverer of - scandium = ekaboron), and Nilson and Pettersson (determination of - the vapour density of beryllium chloride) in Sweden; and of Brauner - (who investigated cerium, and determined the combining weight of - tellurium = 125) in Austria, and Piccini in Italy. - - I consider it necessary to state that, in arranging the periodic - system of the elements, I made use of the previous researches of - Dumas, Gladstone, Pettenkofer, Kremers, and Lenssen on the atomic - weights of related elements, but I was not acquainted with the - works preceding mine of De Chancourtois (_vis tellurique_, or the - spiral of the elements according to their properties and - equivalents) in France, and of J. Newlands (Law of Octaves--for - instance, H, F, Cl, Co, Br, Pd, I, Pt form the first octave, and O, - S, Fe, Se, Rh, Te, Au, Th the last) in England, although certain - germs of the periodic law are to be seen in these works. With - regard to the work of Prof. Lothar Meyer respecting the periodic - law (Notes 12 and 13), it is evident, judging from the method of - investigation, and from his statement (Liebig's _Annalen, Supt. - Band 7_, 1870, 354), at the very commencement of which he cites my - paper of 1869 above mentioned, that he accepted the periodic law in - the form which I proposed. - - In concluding this historical statement I consider it well to - observe that no law of nature, however general, has been - established all at once; its recognition is always preceded by many - hints; the establishment of a law, however, does not take place - when its significance is recognised, but only when it has been - confirmed by experiment, which the man of science must consider as - the only proof of the correctness of his conjectures and opinions. - I therefore, for my part, look upon Roscoe, De Boisbaudran, Nilson, - Winkler, Brauner, Carnelley, Thorpe, and others who verified the - adaptability of the periodic law to chemical facts, as the true - founders of the periodic law, the further development of which - still awaits fresh workers. - - [9] This resembles the fact, well known to those having an acquaintance - with organic chemistry, that in a series of homologues (Chapter - VIII.) the first members, in which there is the least carbon, - although showing the general properties of the homologous series, - still present certain distinct peculiarities. - - [10] Besides arranging the elements (_a_) in a successive order - according to their atomic weights, with indication of their - analogies by showing some of the properties--for instance, their - power of giving one or another form of combination--both of the - _elements_ and of their compounds (as is done in Table III. and in - the table on p. 36), (_b_) according to periods (as in Table I. at - the commencement of volume I. after the preface), and (_c_) - according to groups and series or small periods (as in the same - tables), I am acquainted with the following methods of expressing - the periodic relations of the elements: (1) By a curve drawn - through points obtained in the following manner: The elements are - arranged along the horizontal axis as abscissæ at distances from - zero proportional to their atomic weights, whilst the values for - all the elements of some property--for example, the specific - volumes or the melting points, are expressed by the ordinates. - This method, although graphic, has the theoretical disadvantage - that it does not in any way indicate the existence of a limited - and definite number of elements in each period. There is nothing, - for instance, in this method of expressing the law of periodicity - to show that between magnesium and aluminium there can be no other - element with an atomic weight of, say, 25, atomic volume 13, and - in general having properties intermediate between those of these - two elements. The actual periodic law does not correspond with a - continuous change of properties, with a continuous variation of - atomic weight--in a word, it does not express an uninterrupted - function--and as the law is purely chemical, starting from the - conception of atoms and molecules which combine in multiple - proportions, with intervals (not continuously), it _above all_ - depends on there being but few types of compounds, which are - arithmetically simple, _repeat themselves_, and offer no - uninterrupted transitions, so that each period can only contain a - definite number of members. For this reason there can be no other - elements between magnesium, which gives the chloride MgCl_{2}, and - aluminium, which forms AlX_{3}; there is a break in the - continuity, according to the law of multiple proportions. The - periodic law ought not, therefore, to be expressed by geometrical - figures in which continuity is always understood. Owing to these - considerations I never have and never will express the periodic - relations of the elements by any geometrical figures. (2) _By a - plane spiral._ Radii are traced from a centre, proportional to the - atomic weights; analogous elements lie along one radius, and the - points of intersection are arranged in a spiral. This method, - adopted by De Chancourtois, Baumgauer, E. Huth, and others, has - many of the imperfections of the preceding, although it removes - the indefiniteness as to the number of elements in a period. It is - merely an attempt to reduce the complex relations to a simple - graphic representation, since the equation to the spiral and the - number of radii are not dependent upon anything. (3) _By the lines - of atomicity_, either parallel, as in Reynolds's and the Rev. S. - Haughton's method, or as in Crookes's method, arranged to the - right and left of an axis, along which the magnitudes of the - atomic weights are counted, and the position of the elements - marked off, on the one side the members of the even series - (paramagnetic, like oxygen, potassium, iron), and on the other - side the members of the uneven series (diamagnetic, like sulphur, - chlorine, zinc, and mercury). On joining up these points a - periodic curve is obtained, compared by Crookes to the - oscillations of a pendulum, and, according to Haughton, - representing a cubical curve. This method would be very graphic - did it not require, for instance, that sulphur should be - considered as bivalent and manganese as univalent, although - neither of these elements gives stable derivatives of these - natures, and although the one is taken on the basis of the lowest - possible compound SX_{2}, and the other of the highest, because - manganese can be referred to the univalent elements only by the - analogy of KMnO_{4} to KClO_{4}. Furthermore, Reynolds and Crookes - place hydrogen, iron, nickel, cobalt, and others outside the axis - of atomicity, and consider uranium as bivalent without the least - foundation. (4) Rantsheff endeavoured to classify the elements in - their periodic relations by a system dependent on solid geometry. - He communicated this mode of expression to the Russian Chemical - Society, but his communication, which is apparently not void of - interest, has not yet appeared in print. (5) _By algebraic - formulæ_: for example, E. J. Mills (1886) endeavours to express - all the atomic weights by the logarithmic function A = 15(_n_ - - 0·9375_t_), in which the variables _n_ and _t_ are whole numbers. - For instance, for oxygen _n_ = 2, _t_ = 1; hence A = 15·94; for - antimony _n_ = 9, _t_ = 0; whence A = 120, and so on. _n_ varies - from 1 to 16 and _t_ from 0 to 59. The analogues are hardly - distinguishable by this method: thus for chlorine the magnitudes - of _n_ and _t_ are 3 and 7; for bromine 6 and 6; for iodine 9 and - 9; for potassium 3 and 14; for rubidium 6 and 18; for cæsium 9 and - 20; but a certain regularity seems to be shown. (6) A more natural - method of expressing the dependence of the properties of elements - on their atomic weights is obtained by _trigonometrical - functions_, because this dependence is periodic like the functions - of trigonometrical lines, and therefore Ridberg in Sweden (Lund, - 1885) and F. Flavitzky in Russia (Kazan, 1887) have adopted a - similar method of expression, which must be considered as worthy - of being worked out, although it does not express the absence of - intermediate elements--for instance, between magnesium and - aluminium, which is essentially the most important part of the - matter. (7) The investigations of B. N. Tchitchérin (1888, - _Journal of the Russian Physical and Chemical Society_) form the - first effort in the latter direction. He carefully studied the - alkali metals, and discovered the following simple relation - between their atomic volumes: they can all be expressed by A(2 - - 0·0428A_n_), where A is the atomic weight and _n_ = 1 for lithium - and sodium, 4/8 for potassium, 3/8 for rubidium, and 2/8 for - cæsium. If _n_ always = 1, then the volume of the atom would - become zero at A = 46-2/3, and would reach its maximum when A = - 23-1/3, and the density increases with the growth of A. In order - to explain the variation of _n_, and the relation of the atomic - weights of the alkali metals to those of the other elements, as - also the atomicity itself, Tchitchérin supposes all atoms to be - built up of a primary matter; he considers the relation of the - central to the peripheric mass, and, guided by mechanical - principles, deduces many of the properties of the atoms from the - reaction of the internal and peripheric parts of each atom. This - endeavour offers many interesting points, but it admits the - hypothesis of the building up of all the elements from one primary - matter, and at the present time such an hypothesis has not the - least support either in theory or in fact. Besides which the - starting-point of the theory is the specific gravity of the metals - at a definite temperature (it is not known how the above relation - would appear at other temperatures), and the specific gravity - varies even under mechanical influences. L. Hugo (1884) - endeavoured to represent the atomic weights of Li, Na, K, Rb, and - Cs by geometrical figures--for instance, Li = 7 represents a - central atom = 1 and six atoms on the six terminals of an - octahedron; Na, is obtained by applying two such atoms on each - edge of an octahedron, and so on. It is evident that such methods - can add nothing new to our data respecting the atomic weights of - analogous elements. - - [11] Many natural phenomena exhibit a dependence of a periodic - character. Thus the phenomena of day and night and of the seasons - of the year, and vibrations of all kinds, exhibit variations of a - periodic character in dependence on time and space. But in - ordinary periodic functions one variable varies continuously, - whilst the other increases to a limit, then a period of decrease - begins, and having in turn reached its limit a period of increase - again begins. It is otherwise in the periodic function of the - elements. Here the mass of the elements does not increase - continuously, but abruptly, by steps, as from magnesium to - aluminium. So also the valency or atomicity leaps directly from 1 - to 2 to 3, &c., without intermediate quantities, and in my opinion - it is these properties which are the most important, and it is - their periodicity which forms the substance of the periodic law. - It expresses _the properties of the real elements_, and not of - what may be termed their manifestations visually known to us. The - external properties of elements and compounds are in periodic - dependence on the atomic weight of the elements only because these - external properties are themselves the result of the properties of - the real elements which unite to form the 'free' elements and the - compounds. To explain and express the periodic law is to explain - and express the cause of the law of multiple proportions, of the - difference of the elements, and the variation of their atomicity, - and at the same time to understand what mass and gravitation are. - In my opinion this is still premature. But just as without knowing - the cause of gravitation it is possible to make use of the law of - gravity, so for the aims of chemistry it is possible to take - advantage of the laws discovered by chemistry without being able - to explain their causes. The above-mentioned peculiarity of the - laws of chemistry respecting definite compounds and the atomic - weights leads one to think that the time has not yet come for - their full explanation, and I do not think that it will come - before the explanation of such a primary law of nature as the law - of gravity. - - It will not be out of place here to turn our attention to the - many-sided correlation existing between the undecomposable - _elements and the compound carbon radicles_, which has long been - remarked (Pettenkofer, Dumas, and others), and reconsidered in - recent times by Carnelley (1886), and most originally in - Pelopidas's work (1883) on the principles of the periodic system. - Pelopidas compares the series containing eight hydrocarbon - radicles, C_{_n_}H_{2_n_ + 1}, C_{_n_}H_{2_n_} &c., for instance, - C_{6}H_{13}, C_{6}H_{12}, C_{6}H_{11}, C_{6}H_{10}, C_{6}H_{9}, - C_{6}H_{8}, C_{6}H_{7}, and C_{6}H_{6}--with the series of the - elements arranged in eight groups. The analogy is particularly - clear owing to the property of C_{_n_}H_{2_n_+1} to combine with - X, thus reaching saturation, and of the following members with - X_{2}, X_{3} ... X_{8}, and especially because these are followed - by an aromatic radicle--for example, C_{6}H_{5}--in which, as is - well known, many of the properties of the saturated radicle - C_{6}H_{13} are repeated, and in particular the power of forming a - univalent radicle again appears. Pelopidas shows a confirmation of - the parallel in the property of the above radicles of giving - oxygen compounds corresponding with the groups in the periodic - system. Thus the hydrocarbon radicles of the first group--for - instance, C_{6}H_{13} or C_{6}H_{5}--give oxides of the form - R_{2}O and hydroxides RHO, like the metals of the alkalis; and in - the third group they form oxides R_{2}O_{3} and hydrates RO_{2}H. - For example, in the series CH_{3} the corresponding compounds of - the third group will be the oxide (CH)_{2}O_{3} or - C_{2}H_{2}O_{3}--that is, formic anhydride and hydrate, CHO_{2}H, - or formic acid. In the sixth group, with a composition of C_{2}, - the oxide RO_{3} will be C_{2}O_{3}, and hydrate - C_{2}H_{2}O_{4}--that is, also a bibasic acid (oxalic) resembling - sulphuric, among the inorganic acids. After applying his views to - a number of organic compounds, Pelopidas dwells more particularly - on the radicles corresponding with ammonium. - - With respect to this remarkable parallelism, it must above all be - observed that in the elements the atomic weight increases in - passing to contiguous members of a higher valency, whilst here it - decreases, which should indicate that the periodic variability of - elements and compounds is subject to some higher law whose nature, - and still more whose cause, cannot at present be determined. It is - probably based on the fundamental principles of the internal - mechanics of the atoms and molecules, and as the periodic law has - only been generally recognised for a few years it is not - surprising that any further progress towards its explanation can - only be looked for in the development of facts touching on this - subject. - -1. The composition of the higher oxygen compounds is determined by the -groups: the first group gives R_{2}O, the second R_{2}O_{2} or RO, the -third R_{2}O_{3}, &c. There are eight types of oxides and therefore eight -groups. Two groups give a period, and the same type of oxide is met with -twice in a period. For example, in the period beginning with potassium, -oxides of the composition RO are formed by calcium and zinc, and of the -composition RO_{3} by molybdenum and tellurium. The oxides of the even -series, of the same type, have stronger basic properties than the oxides -of the uneven series, and the latter as a rule are endowed with an acid -character. Therefore the elements which exclusively give bases, like the -alkali metals, will be found at the commencement of the period, whilst -such purely acid elements as the halogens will be at the end of the -period. The interval will be occupied by intermediate elements, whose -character and properties we shall afterwards describe. It must be -observed that the acid character is chiefly proper to the elements with -small atomic weights in the uneven series, whilst the basic character is -exhibited by the heavier elements in the even series. Hence elements -which give acids chiefly predominate among the lightest (typical) -elements, especially in the last groups; whilst the heaviest elements, -even in the last groups (for instance, thallium, uranium) have a basic -character. Thus the basic and acid characters of the higher oxides are -determined (_a_) by the type of oxide, (_b_) by the even or uneven -series, and (_c_) by the atomic weight.[11 bis] The groups are indicated -by Roman numerals from I. to VIII. - -2. _The hydrogen compounds_ being volatile or gaseous substances which -are prone to reaction--such as HCl, H_{2}O, H_{3}N, and H_{4}C[12]--are -only formed by the elements of the uneven series and higher groups giving -oxides of the forms R_{2}O_{_n_}, RO_{3}, R_{2}O_{5}, and RO_{2}. - -3. If an element gives a hydrogen compound, RX_{_m_}, it forms an -_organo-metallic compound_ of the same composition, where X = -C_{_n_}H_{2_n_ + 1}; that is, X is the radicle of a saturated -hydrocarbon. The elements of the uneven series, which are incapable of -giving hydrogen compounds, and give oxides of the forms RX, RX_{2}, -R_{X}3, also give organo-metallic compounds of this form proper to the -higher oxides. Thus zinc forms the oxide ZnO, salts ZnX_{2} and zinc -ethyl Zn(C_{2}H_{5})_{2}. The elements of the even series do not seem to -form organo-metallic compounds at all; at least all efforts for their -preparation have as yet been fruitless--for instance, in the case of -titanium, zirconium, or iron. - -4. The atomic weights of elements belonging to contiguous periods differ -approximately by 45; for example, K<Rb, Cr<Mo, Br<I. But the elements of -the typical series show much smaller differences. Thus the difference -between the atomic weights of Li, Na, and K, between Ca, Mg, and Be, -between Si and C, between S and O, and between Cl and F, is 16. As a -rule, there is a greater difference between the atomic weights of two -elements of one group and belonging to two neighbouring series (Ti-Si = -V-P = Cr-S = Mn-Cl = Nb-As, &c. = 20); and this difference attains a -maximum with the heaviest elements (for example, Th-Pb = 26, Bi-Ta = 26, -Ba-Cd = 25, &c.). Furthermore, the difference between the atomic weights -of the elements of even and uneven series also increases. In fact, the -differences between Na and K, Mg and Ca, Si and Ti, are less abrupt than -those between Pb and Th, Ta and Bi, Cd and Ba, &c. Thus even in the -magnitude of the differences of the atomic weights of analogous elements -there is observable a certain connection with the gradation of their -properties.[12 bis] - -5. According to the periodic system every element occupies a certain -position, determined by the group (indicated in Roman numerals) and -series (Arabic numerals) in which it occurs. These indicate the atomic -weight, the analogues, properties, and type of the higher oxide, and of -the hydrogen and other compounds--in a word, all the chief quantitative -and qualitative features of an element, although there yet remain a whole -series of further details and peculiarities whose cause should perhaps be -looked for in small differences of the atomic weights. If in a certain -group there occur elements, R_{1}, R_{2}, R_{3}, and if in that series -which contains one of these elements, for instance R_{2}, an element -Q_{2} precedes it and an element T_{2} succeeds it, then the properties -of R_{2} are determined by the properties of R_{1}, R_{3}, Q_{2}, and -T_{2}. Thus, for instance, the atomic weight of R_{2} = 1/4(R_{1} + -R_{3} + Q_{2} + T_{2}). For example, selenium occurs in the same group as -sulphur, S = 32, and tellurium, Te = 125, and, in the 7th series As = 75 -stands before it and Br = 80 after it. Hence the atomic weight of -selenium should be 1/4(32 + 125 + 75 + 80) = 78, which is near to the -truth. Other properties of selenium may also be determined in this -manner. For example, arsenic forms H_{3}As, bromine gives HBr, and it is -evident that selenium, which stands between them, should form H_{2}Se, -with properties intermediate between those of H_{3}As and HBr. Even the -physical properties of selenium and its compounds, not to speak of their -composition, being determined by the group in which it occurs, may be -foreseen with a close approach to reality from the properties of sulphur, -tellurium, arsenic, and bromine. _In this manner it is possible to -foretell the properties of still unknown elements._ For instance in the -position IV, 5--that is, in the IVth group and 5th series--an element is -still wanting. These unknown elements may be named after the preceding -known element of the same group by adding to the first syllable the -prefix _eka_-, which means _one_ in Sanskrit. The element IV, 5, follows -after IV, 3, and this latter position being occupied by silicon, we call -the unknown element ekasilicon and its symbol Es. The following are the -properties which this element should have on the basis of the known -properties of silicon, tin, zinc, and arsenic. Its atomic weight is -nearly 72, higher oxide EsO_{2}, lower oxide EsO, compounds of the -general form EsX_{4}, and chemically unstable lower compounds of the form -EsX_{2}. Es gives volatile organo-metallic compounds--for instance, -Es(CH_{3})_{4}, Es(CH_{3})_{3}Cl, and Es(C_{2}H_{5})_{4}, which boil at -about 160°, &c.; also a volatile and liquid chloride, EsCl_{4}, boiling -at about 90° and of specific gravity about 1·9. EsO_{2} will be the -anhydride of a feeble colloidal acid, metallic Es will be rather easily -obtainable from the oxides and from K_{2}EsF_{6} by reduction, EsS_{2} -will resemble SnS_{2} and SiS_{2}, and will probably be soluble in -ammonium sulphide; the specific gravity of Es will be about 5·5, EsO_{2} -will have a density of about 4·7, &c. Such a prediction of the properties -of ekasilicon was made by me in 1871, on the basis of the properties of -the elements analogous to it: IV, 3, = Si, IV, 7 = Sn, and also II, 5 = -Zn and V, 5 = As. And now that this element has been discovered by C. -Winkler, of Freiberg, it has been found that its actual properties -entirely correspond with those which were foretold.[13] In this we see a -most important confirmation of the truth of the periodic law. This -element is now called germanium, Ge (_see_ Chapter XVIII.). It is not the -only one that has been predicted by the periodic law.[14] We shall see in -describing the elements of the third group that properties were foretold -of an element ekaaluminium, III, 5, El = 68, and were afterwards verified -when the metal termed 'gallium' was discovered by De Boisbaudran. So also -the properties of scandium corresponded with those predicted for -ekaboron, according to Nilson.[15] - - [11 bis] True peroxides (_see_ Note 7), like H_{2}O_{2}, BaO_{2}, - S_{2}O_{7} (Chapter XX.), must not be confused with true saline - oxides even if the latter contain much oxygen (for instance, - N_{2}O_{5}, CrO_{3}, &c.) although one and the other easily - oxidise. The difference between them is seen in their fundamental - properties: the saline oxides correspond to water, while the - peroxides correspond in their reactions and origin to peroxide of - hydrogen. This is clearly seen in the difference between Na_{2}O - and Na_{2}O_{2} (Chapter XII.). Therefore the peroxides should - also have their periodicity. An element R, giving a highest degree - of oxidation, R_{2}O_{_n_}, may give both a lower degree of - oxidation, R_{2}O_{_n_ - _m_} (where _m_ is evidently less than - _n_), and peroxides, R_{2}O_{_n_ + 1}, R_{2}O_{_n_ + 2}, or even - more oxygen. This class of oxides, to which attention has only - recently been turned (Berthelot, Piccini, &c.), may perhaps on - further study give the possibility of generalising the capability - of the elements to give unstable complex higher forms of - combination, such as double salts, and in my opinion should in the - near future be the field of new and important discoveries. And in - contemporary chemistry, salts, saline oxides, hydrogen compounds, - and other combinations of the elements corresponding to them - constitute an important and very complex problem for - generalisation, which is satisfied by the periodic law in its - present form, to which it has risen from its first state, in which - it gave the means of foreseeing (_see_ later on) the existence of - unknown elements (Ga, Sc, and Ge), their properties, and many - details respecting their compounds. Until those improvements in - the periodic system which have been proposed by Prof. Flavitzky - (of Kazan) and Prof. Harperath (of Cordoba, in the Argentine - Republic), Ugo Alvisi (Italy), and others give similar practical - results, I think it unnecessary to discuss them further. - - [12] The hydrides generalised by the periodic law are those to which - metallo-organic compounds correspond, and they are themselves - either volatile or gaseous. The hydrogen compounds like Na_{2}H, - BaH, &c. are distinguished by other signs. They resemble alloys. - They show (_see_ end of last chapter) a systematic harmony, but - they evidently should not be confused with true hydrides, any more - than peroxides with saline oxides. Moreover, such hydrides have, - like the peroxides, only recently been subjected to research, and - have been but little studied. The best known of these compounds - are given in the 16th column of Table III., and it will be seen - that they already exhibit a periodicity of properties and - composition. - - [12 bis] The relation between the atomic weights, and especially the - difference = 16, was observed in the sixth and seventh decades of - this century by Dumas, Pettenkofer, L. Meyer, and others. Thus - Lothar Meyer in 1864, following Dumas and others, grouped together - the tetravalent elements carbon and silicon; the trivalent - elements nitrogen, phosphorus, arsenic, antimony, and bismuth; the - bivalent oxygen, sulphur, selenium, and tellurium; the univalent - fluorine, chlorine, bromine, and iodine; the univalent metals - lithium, sodium, potassium, rubidium, cæsium, and thallium, and - the bivalent metals beryllium, magnesium, strontium and - barium--observing that in the first the difference is, in general - = 16, in the second about = 46, and the last about = 87-90. The - first germs of the periodic law are visible in such observations - as these. Since its establishment this subject has been most fully - worked out by Ridberg (Note 10), who observed a periodicity in the - variation of the differences between the atomic weights of two - contiguous elements, and its relation to their atomicity. A. - Bazaroff (1887) investigated the same subject, taking, not the - arithmetical differences of contiguous and analogous elements, but - the ratio of their atomic weights; and he also observed that this - ratio alternately rises and falls with the rise of the atomic - weights. I will here remark that the relation of the eighth group - to the others will be considered at the end of this work in - Chapter XXII. - - [13] The laws of nature admit of no exceptions, and in this they - clearly differ from such rules and maxims as are found in grammar, - and other inventions, methods, and relations of man's creation. - The confirmation of a law is only possible by deducing - consequences from it, such as could not possibly be foreseen - without it, and by verifying those consequences by experiment and - further proofs. Therefore, when I conceived the periodic law, I - (1869-1871, Note 9) deduced such logical consequences from it as - could serve to show whether it were true or not. Among them was - the prediction of the properties of undiscovered elements and the - correction of the atomic weights of many, and at that time little - known, elements. Thus uranium was considered as trivalent, U = - 120; but as such it did not correspond with the periodic law. I - therefore proposed to double its atomic weight--U = 240, and the - researches of Roscoe, Zimmermann, and others justified this - alteration (Chapter XXI.). It was the same with cerium (Chapter - XVIII.) whose atomic weight it was necessary to change according - to the periodic law. I therefore determined its specific heat, and - the result I obtained was verified by the new determinations of - Hillebrand. I then corrected certain formulæ of the cerium - compounds, and the researches of Rammelsberg, Brauner, Clève, and - others verified the proposed alteration. It was necessary to do - one or the other--either to consider the periodic law as - completely true, and as forming a new instrument in chemical - research, or to refute it. Acknowledging the method of experiment - to be the only true one, I myself verified what I could, and gave - every one the possibility of proving or confirming the law, and - did not think, like L. Meyer (Liebig's _Annalen, Supt. Band 7_, - 1870, 364), when writing about the periodic law that 'it would be - rash to change the accepted atomic weights on the basis of so - uncertain a starting-point.' ('Es würde voreilig sein, auf so - unsichere Anhaltspunkte hin eine Aenderung der bisher angenommenen - Atomgewichte vorzunehmen.') In my opinion, the basis offered by - the periodic law had to be verified or refuted, and experiment in - every case verified it. The starting-point then became general. No - law of nature can be established without such a method of testing - it. Neither De Chancourtois, to whom the French ascribe the - discovery of the periodic law, nor Newlands, who is put forward by - the English, nor L. Meyer, who is now cited by many as its - founder, ventured to foretell the _properties_ of undiscovered - elements, or to alter the 'accepted atomic weights,' or, in - general, to regard the periodic law as a new, strictly established - law of nature, as I did from the very beginning (1869). - - [14] When in 1871 I wrote a paper on the application of the periodic - law to the determination of the properties of hitherto - undiscovered elements, I did not think I should live to see the - verification of this consequence of the law, but such was to be - the case. Three elements were described--ekaboron, ekaaluminium, - and ekasilicon--and now, after the lapse of twenty years, I have - had the great pleasure of seeing them discovered and named - Gallium, Scandium, and Germanium, after those three countries - where the rare minerals containing them are found, and where they - were discovered. For my part I regard L. de Boisbaudran, Nilson, - and Winkler, who discovered these elements, as the true - corroborators of the periodic law. Without them it would not have - been accepted to the extent it now is. - - [15] Taking indium, which occurs together with zinc, as our example, we - will show the principle of the method employed. The equivalent of - indium to hydrogen in its oxide is 37·7--that is, if we suppose - its composition to be like that of water; then In = 37·7, and the - oxide of indium is In_{2}O. The atomic weight of indium was taken - as double the equivalent--that is, indium was considered to be a - bivalent element--and In = 2 × 37·7 = 75·4. If indium only formed - an oxide, RO, it should be placed in group II. But in this case it - appears that there would be no place for indium in the system of - the elements, because the positions II., 5 = Zn = 65 and II., 6 = - Sr = 87 were already occupied by known elements, and according to - the periodic law an element with an atomic weight 75 could not be - bivalent. As neither the vapour density nor the specific heat, nor - even the isomorphism (the salts of indium crystallise with great - difficulty) of the compounds of indium were known, there was no - reason for considering it to be a bivalent metal, and therefore it - might be regarded as trivalent, quadrivalent, &c. If it be - trivalent, then In = 3 × 37·7 = 113, and the composition of the - oxide is In_{2}O_{3}, and of its salts InX_{3}. In this case it at - once falls into its place in the system, namely, in group III. and - 7th series, between Cd = 112 and Sn = 118, as an analogue of - aluminium or dvialuminium (dvi = 2 in Sanskrit). All the - properties observed in indium correspond with this position; for - example, the density, cadmium = 8·6, indium = 7·4, tin = 7·2; the - basic properties of the oxides CdO, In_{2}O_{3}, SnO_{2}, - successively vary, so that the properties of In_{2}O_{3} are - intermediate between those of CdO and SnO_{2} or Cd_{2}O_{2} and - Sn_{2}O_{4}. That indium belongs to group III. has been confirmed - by the determination of its specific heat, (0·057 according to - Bunsen, and 0·055 according to me) and also by the fact that - indium forms alums like aluminium, and therefore belongs to the - same group. - - The same kind of considerations necessitated taking the atomic - weight of titanium as nearly 48, and not as 52, the figure derived - from many analyses. And both these corrections, made on the basis - of the law, have now been confirmed, for Thorpe found, by a series - of careful experiments, the atomic weight of titanium to be that - foreseen by the periodic law. Notwithstanding that previous - analyses gave Os = 199·7, Ir = 198, and Pt = 187, the periodic law - shows, as I remarked in 1871, that the atomic weights should rise - from osmium to platinum and gold, and not fall. Many recent - researches, and especially those of Seubert, have fully verified - this statement, based on the law. Thus a true law of nature - anticipates facts, foretells magnitudes, gives a hold on nature, - and leads to improvements in the methods of research, &c. - -6. As a true law of nature is one to which there are no exceptions, the -periodic dependence of the properties on the atomic weights of the -elements gives a _new means for determining by the equivalent the atomic -weight_ or atomicity of imperfectly investigated but known elements, for -which no other means could as yet be applied for determining the true -atomic weight. At the time (1869) when the periodic law was first -proposed there were several such elements. It thus became possible to -learn their true atomic weights, and these were verified by later -researches. Among the elements thus concerned were indium, uranium, -cerium, yttrium, and others. - -7. The periodic variability of the properties of the elements in -dependence on their masses presents a distinction from other kinds of -periodic dependence (as, for example, the sines of angles vary -periodically and successively with the growth of the angles, or the -temperature of the atmosphere with the course of time), in that the -weights of the atoms do not increase gradually, but by leaps; that is, -according to Dalton's law of multiple proportions, there not only are -not, but there cannot be, any transitive or intermediate elements between -two neighbouring ones (for example, between K = 39 and Ca = 40, or Al = -27 and Si = 28, or C = 12 and N = 14, &c.) As in a molecule of a hydrogen -compound there may be either one, as in HF, or two, as in H_{2}O, or -three, as in NH_{3}, &c., atoms of hydrogen; but as there cannot be -molecules containing 2-1/2 atoms of hydrogen to one atom of another -element, so there cannot be any element intermediate between N and O, -with an atomic weight greater than 14 or less than 16, or between K and -Ca. Hence the periodic dependence of the elements cannot be expressed by -any algebraical continuous function in the same way that it is possible, -for instance, to express the variation of the temperature during the -course of a day or year. - -8. The essence of the notions giving rise to the periodic law consists -in a general physico-mechanical principle which recognises the -correlation, transmutability, and equivalence of the forces of nature. -Gravitation, attraction at small distances, and many other phenomena are -in direct dependence on the mass of matter. It might therefore have been -expected that chemical forces would also depend on mass. A dependence is -in fact shown, the properties of elements and compounds being determined -by the masses of the atoms of which they are formed. The weight of a -molecule, or its mass, determines, as we have seen, (Chapter VII. and -elsewhere) many of its properties independently of its composition. Thus -carbonic oxide, CO, and nitrogen, N_{2}, are two gases having the same -molecular weight, and many of their properties (density, liquefaction, -specific heat, &c.) are similar or nearly similar. The differences -dependent on the nature of a substance play another part, and form -magnitudes of another order. But the properties of atoms are mainly -determined by their mass or weight, and are in dependence upon it. Only -in this case there is a peculiarity in the dependence of the properties -on the mass, for this _dependence is determined by a periodic law_. As -the mass increases the properties vary, at first successively and -regularly, and then return to their original magnitude and recommence a -fresh period of variation like the first. Nevertheless here as in other -cases a small variation of the mass of the atom generally leads to a -small variation of properties, and determines differences of a second -order. The atomic weights of cobalt and nickel, of rhodium, ruthenium, -and palladium, and of osmium, iridium, and platinum, are very close to -each other, and their properties are also very much alike--the -differences are not very perceptible. And if the properties of atoms are -a function of their weight, many ideas which have more or less rooted -themselves in chemistry must suffer change and be developed and worked -out in the sense of this deduction. Although at first sight it appears -that the chemical elements are perfectly independent and individual, -instead of this idea of the nature of the elements, the notion of the -dependence of their properties upon _their mass_ must now be established; -that is to say, the subjection of the individuality of the elements to a -common higher principle which evinces itself in gravity and in all -physico-chemical phenomena. Many chemical deductions then acquire a new -sense and significance, and a regularity is observed where it would -otherwise escape attention. This is more particularly apparent in the -physical properties, to the consideration of which we shall afterwards -turn, and we will now point out that Gustavson first (Chapter X., Note -28) and subsequently Potilitzin (Chapter XI., Note 66) demonstrated the -direct dependence of the reactive power on the atomic weight and that -fundamental property which is expressed in the forms of their compounds, -whilst in a number of other cases the purely chemical relations of -elements proved to be in connection with their periodic properties. As a -case in point, it may be mentioned that Carnelley remarked a dependence -of the decomposability of the hydrates on the position of the elements in -the periodic system; whilst L. Meyer, Willgerodt, and others established -a connection between the atomic weight or the position of the elements in -the periodic system and their property of serving as media in the -transference of the halogens to the hydrocarbons.[16] Bailey pointed out -a periodicity in the stability (under the action of heat) of the oxides, -namely: (_a_) in the even series (for instance, CrO_{3}, MoO_{3}, WO_{3}, -and UO_{3}) the higher oxides of a given group decompose with greater -ease the smaller the atomic weight, while in the uneven series (for -example, CO_{2}, GeO_{2}, SnO_{2}, and PbO_{2}) the contrary is the case; -and (_b_) the stability of the higher saline oxides in the even series -(as in the fourth series from K_{2}O to Mn_{2}O_{7}) decreases in passing -from the lower to the higher groups, while in the uneven series it -increases from the Ist to the IVth group, and then falls from the IVth to -the VIIth; for instance, in the series Ag_{2}O, CdO, In_{2}O_{3}, -SnO_{2}, and then SnO_{2}, Sb_{2}O_{5}, TeO_{3}, I_{2}O_{7}. K. Winkler -looked for and actually found (1890) a dependence between the -reducibility of the metals by magnesium and their position in the -periodic system of the elements. The greater the attention paid to this -field the more often is a distinct connection found between the variation -of purely chemical properties of analogous substances and the variation -of the atomic weights of the constituent elements and their position in -the periodic system. Besides, since the periodic system has become more -firmly established, many facts have been gathered, showing that there are -many similarities between Sn and Pb, B and Al, Cd and Hg, &c., which had -not been previously observed, although foreseen in some cases, and a -consequence of the periodic law. Keeping our attention in the same -direction, we see that the most widely distributed elements in nature are -those with small atomic weights, whilst in organisms the lightest -elements exclusively predominate (hydrogen, carbon, nitrogen, oxygen), -whose small mass facilitates those transformations which are proper to -organisms. Poluta (of Kharkoff), C. C. Botkin, Blake, Brenton, and others -even discovered a correlation between the physiological action of salts -and other reagents on organisms and the positions occupied in the -periodic system by the metals contained in them.[17] - - [16] Meyer, Willgerodt, and others, guided by the fact that Gustavson - and Friedel had remarked that metalepsis rapidly proceeds in the - presence of aluminium, investigated the action of nearly all the - elements in this respect. For example, they took benzene, added - the metals to be experimented on to it, and passed chlorine - through the liquid in diffused light. When, for instance, sodium, - potassium, barium, &c. are taken, there is no action on the - benzene; that is, hydrochloric acid is not disengaged; but if - aluminium, gold, or, in general, any metal having this power of - aiding chlorination (Halogen-überträger) is employed, then the - action is clearly seen from the volumes of hydrochloric acid - evolved (especially if the metallic chloride formed is soluble in - benzene). Thus, in group I., and in general among the even and - light elements, there are none capable of serving as agents of - metalepsis; but aluminium, gallium, indium, antimony, tellurium, - and iodine, which are contiguous members in the periodic system, - are excellent transmitters (carriers) of the halogens. - - [17] The periodic relations enumerated above appertain to the real - elements, and not to the elements in the free state as we know - them; and it is very important to note this, because the periodic - law refers to the real elements, inasmuch as the atomic weight is - proper to the real element, and not to the 'free' element, to - which, as to a compound, a molecular weight is proper. Physical - properties are chiefly determined by the properties of molecules, - and only indirectly depend on the properties of the atoms forming - the molecules. For this reason the periods, which are clearly and - quite distinctly expressed--for instance, in the forms of - combination--become to some extent involved (complicated) in the - physical properties of their members. Thus, for instance, besides - the _maxima_ and _minima_ corresponding with the periods and - groups, new molecules appear; thus, as regards the melting-point - of germanium, a local maximum appears, which was, however, - foreseen by the periodic law when the properties of germanium - (ekasilicon) were forecast. - -As, from the necessity of the case, the physical properties must be in -dependence on the composition of a substance, _i.e._ on the quality and -quantity of the elements forming it, so for them also a dependence on the -atomic weight of the component elements must be expected, and -consequently also on their periodic distribution. We shall meet with -repeated proofs of this in the further exposition of our treatise, and -for the present will content ourselves with citing the discovery by -Carnelley in 1879 of the dependence of the magnetic properties of the -elements on the position occupied by them in the periodic system. -Carnelley showed that all the elements of the _even series_ (beginning -with lithium, potassium, rubidium, cæsium) belong to the number of -magnetic (paramagnetic) substances; for example, according to Faraday and -others,[17 bis] C, N, O, K, Ti, Cr, Mn, Fe, Co, Ni, Ce, are magnetic; and -the elements of the _uneven series are diamagnetic_, H, Na, Si, P, S, Cl, -Cu, Zn, As, Se, Br, Ag, Cd, Sn, Sb, I, Au, Hg, Tl, Pb, Bi. - - [17 bis] The relation of certain elements (for instance, the analogues - of Pt) among diamagnetic and paramagnetic bodies is sometimes - doubtful (probably partly owing to the imperfect purity of the - reagents under investigation). This subject has been studied in - some detail by Bachmetieff in 1889. - -Carnelley also showed that the _melting-point_ of elements varies -periodically, as is seen by the figures in Table III. (nineteenth -column),[18] where all the most trustworthy data are collected, and -predominance is given to those having maximum and minimum values.[19] - - [18] It is evident that many of the figures, especially those exceeding - 1000°, have been determined with but little exactitude, and some, - placed in Table III. with the sign (?), I have only given on the - basis of rough and comparative determinations, calculated from the - melting-points of silver and platinum, now established by many - observers. In Table III., besides the large periods whose maxima - correspond with carbon, silicon, titanium, ruthenium (?), and - osmium (?), there are also small periods in the melting-points, - and their maxima correspond with sulphur, arsenic, antimony. The - minima correspond with the halogens and metals of the alkalis. A - distinct periodicity is also seen in taking the coefficients of - linear expansion (chiefly according to Fizeau); for instance, in - the vertical series (according to the magnitude of the atomic - weight), Fe, Co, Ni, Cu, the linear expansion in millionths of an - inch = 12, 13, 17, and 29; for Rh, Pd, Ag, Cd, In, Sn, and Sb the - coefficients are 8, 12, 19, 31, 46, 26, and 12, so that a maximum - is reached at In. In the series Ir (7), Pt (5), Au (14), Hg (60), - Tl (31), Pb (29), and Bi (14), the maximum is at Hg and the - minimum at Pt. Raoul Pictet expressed this connection by the fact - that he found the product [alpha](_t_ + 273)[3root](A/_d_) to be - nearly constant for all elements in the free state, and nearly - equal to 0·045, and being the coefficient of linear expansion, _t_ - + 273, the melting-point calculated from the absolute zero - (-273°), and [3root](A/_d_), the mean distance between the atoms, - if A is the atomic weight and _d_ the sp. gr. of an element. - Although the above product is not strictly constant, nevertheless - Pictet's rule gives an idea of the bond between magnitudes which - ought to have a certain connection with each other. De Heen, - Nadeschdin, and others also studied this dependence, but their - deductions do not give a general and exact law. - - [19] Carnelley found a similar dependence in comparing the - melting-points of the metallic chlorides, many of which he - redetermined for this purpose. The melting-points (and - boiling-points, in brackets) of the following chlorides are known, - and a certain regularity is seen to exist in them, although the - number (and degree of accuracy) of the data is insufficient for a - generalisation:-- - - LiCl 598° BeCl_{2} 600° BCl_{3} -20° - NaCl 772° MgCl_{2} 708° AlCl_{3} 187° - KCl 734° CaCl_{2} 719° ScCl_{3} ? - {CuCl 434° ZnCl_{2} 262° GaCl_{3} 76° - {(993°) (680°) (217°) - AgCl 451° CdCl_{2} 541° InCl_{3} ? - {TlCl 427° PbCl_{2} 498° BiCl_{3} 227° - {(713°) (908°) - - We will also enumerate the following data given by Carnelley, - which are interesting for comparison: HCl -112° (-102°); RbCl - 710°, SrCl_{2} 825°, CsCl 631°, BaCl_{2} 860°, SbCl_{3} 73° - (223°), TeCl_{2} 209° (327°), ICl 27°, HgCl_{2} 276° (303°), - FeCl_{3} 306°, NbCl_{5} 194° (240°), TaCl_{3} 211° (242°), WCl_{6} - 190°. The melting-points of the bromides and iodides are higher or - lower than those of the corresponding chlorides, according to the - atomic weight of the element and number of atoms of the halogen, - as is seen from the following examples:--1. KCl 734°, KBr 699°, KI - 634°; 2. AgCl 454°, AgBr 427° AgI 527°; 3. PbCl_{2} 498° (900°), - PbBr_{2} 499° (861°), PbI_{2} 383° (906°); 4. SnCl_{4} below -20° - (114°), SnBr_{2} 30° (201°), SnI_{4} 146° (295°) (_see_ Chapter - II. Note 27, and Chapter XI. Note 47^{bis}, &c.) - - Laurie (1882) also observed a periodicity in the _quantity of - heat_ developed in the formation of the chlorides, bromides, and - iodides (fig. 79), as is seen from the following figures, where - the heat developed is expressed in thousands of calories, and - referred to a molecule of chlorine, Cl_{2}, so that the heat of - formation of KCl is doubled, and that of SnCl_{4} halved, &c.: Na - 195 (Ag 59, Au 12), Mg 151 (Zn 97, Cd 93, Hg 63), Al 117, Si 79 - (Sn 64), K 211 (Li 187), Ca 170 (Sr 185, Ba 194), whence it is - seen that the greatest amount of heat is evolved by the metals of - the alkalis, and that in each period it falls from them to the - halogens, which evolve very little heat in combining together. - Richardson, by comparing the heats of formation of the fluorides - also came to the conclusion that they are in periodic dependence - upon the atomic weights of the combined elements. - - [Illustration: FIG. 79.--Laurie's diagram for expressing the - periodic variation of the heat of formation of the chlorides. The - abscissæ give the atomic weights from 0 to 210, and the ordinates - the amounts of heat from 0 to 220 thousand calories evolved in the - combination with Cl_{2}, (_i.e._ with 71 parts of chlorine). The - apices of the curve correspond to Li, Na, K, Rb, Cs, and the lower - extremities to F, Cl, Br, and I.] - - In this respect it may not be superfluous to remark (1) that - Thomsen, whose results I have employed above, observed a - correlation in the calorific equivalents of analogous elements, - although he did not remark their periodic variation; (2) that the - uniformity of many thermochemical deductions must gain - considerably by the application of the periodic law, which - evidently repeats itself in calorimetric data; and if these data - frequently lead to true forecasts, this is due to the periodicity - of the thermal as well as of many other properties, as Laurie - remarked; and (3) that the heat of formation of the oxides is also - subject to a periodic dependence which differs from that of the - heat of formation of the chlorides, in that the greatest quantity - corresponds with the bivalent metals of the alkaline earths - (magnesium, calcium, strontium, barium), and not with the - univalent metals of the alkalis, as is the case with chlorine, - bromine, and iodine. This circumstance is probably connected with - the fact that chlorine, bromine, and iodine are univalent - elements, and oxygen bivalent (compare, for instance, Chapter XI., - Note 13, Chapter XXII., Note 40, Chapter XXIV., Note 28^{bis}, - &c.) - - Keyser (1892), in investigating the spectra of the alkali metals - and metals of the alkaline earths, came to the conclusion that in - this respect also there is a regularity of a periodic character in - dependence upon the atomic weights. Probably a closer and - systematic study of many of the properties of the elements and of - complex and simple bodies formed by them will more and more - frequently lead to similar conclusions, and to extending the range - of application of the periodic law. - -There is no doubt that many other physical properties will, when -further studied, also prove to be in periodic dependence on the atomic -weights,[19 bis] but at present only a few are known with any -completeness, and we will only refer to the one which is the most easily -and frequently determined--namely, the _specific gravity_ in a solid and -liquid state, the more especially as its connection with the chemical -properties and relations of substances is shown at every step. Thus, for -instance, of all the metals those of the alkalis, and of all the -non-metals the halogens, are the most energetic in their reactions, and -they have the lowest specific gravity among the adjacent elements, as is -seen in Table III., column 17. Such are sodium, potassium, rubidium, -cæsium among the metals, and chlorine, bromine, and iodine among the -non-metals; and as such less energetic metals as iridium, platinum, and -gold (and even charcoal or the diamond) have the highest specific gravity -among the elements near to them in atomic weight; therefore the degree of -the condensation of matter evidently influences the course of the -transformations proper to a substance, and furthermore this dependence on -the atomic weight, although very complex, is of a clearly periodic -character. In order to account for this to some extent, it may be -imagined that the lightest elements are porous, and, like a sponge, are -easily penetrated by other substances, whilst the heavier elements are -more compressed, and give way with difficulty to the insertion of other -elements. These relations are best understood when, instead of the -specific gravities referring to a unit of volume,[20] the _atomic volumes -of the elements_--that is, the quotient _A_/_d_ of the atomic weight _A_ -by the specific gravity _d_--are taken for comparison. As, according to -the entire sense of the atomic theory, the actual matter of a substance -does not fill up its whole cubical contents, but is surrounded by a -medium (ethereal, as is generally imagined), like the stars and planets -which travel in the space of the heavens and fill it, with greater or -less intervals, so the quotient _A_/_d_ only expresses the _mean_ volume -corresponding to the sphere of the atoms, and therefore [3root]_A_/_d_ -_is the mean distance between the centres of the atoms_. For compounds -whose molecules weigh _M_, the mean magnitude of the atomic volume is -obtained by dividing the mean molecular volume _M_/_d_ by the number of -atoms _n_ in the molecule.[21] The above relations may easily be -expressed from this point of view by comparing the atomic volumes. Those -comparatively light elements which easily and frequently enter into -reaction have the greatest atomic volumes: sodium 23, potassium 45, -rubidium 57, cæsium 71, and the halogens about 27; whilst with those -elements which enter into reaction with difficulty, the mean atomic -volume is small; for carbon in the form of a diamond it is less than 4, -as charcoal about 6, for nickel and cobalt less than 7, for iridium and -platinum about 9. The remaining elements having atomic weights and -properties intermediate between those elements mentioned above have also -intermediate atomic volumes. Therefore _the specific gravities and -specific volumes of solids and liquids stand in periodic dependence on -the atomic weights_, as is seen in Table III., where both _A_ (the atomic -weight) and _d_ (the specific gravity), and _A_/_d_ (specific volumes of -the atoms) are given (column 18). - - [19 bis] Probably, besides thermo-chemical data (Note 19), the - refractive index, cohesion, ductility, and similar properties of - corresponding compounds or of the elements themselves will be - found to exhibit a dependence of the magnitude of the atomic - weight upon the periodic law. - - [20] Having occupied myself since the fifties (my dissertation for the - degree of M.A. concerned the specific volumes, and is printed in - part in the _Russian Mining Journal_ for 1856) with the problems - concerning the relations between the specific gravities and - volumes, and the chemical compositions of substances, I am - inclined to think that the direct investigation of specific - gravities gives essentially the same results as the investigation - of specific volumes, only that the latter are more graphic. Table - III. of the periodic properties of the elements clearly - illustrates this. Thus, for those members whose volume is the - greatest among the contiguous elements, the specific gravity is - least--that is, the periodic variation of both properties is - equally evident. In passing, for instance, from silver to iodine - we have a successive decrease of specific gravity and successive - increase of specific volume. The periodic alternation of the rise - and fall of the specific gravity and specific volume of the free - elements was communicated by me in August 1869 to the Moscow - Meeting of Russian Naturalists. In the following year (1870) L. - Meyer's paper appeared, which also dealt with the specific volume - of the elements. - - [21] In my opinion the mean volume of the atoms of compounds deserves - more attention than has yet been paid to it. I may point out, for - instance, that for feebly energetic oxides the mean volume of the - atom is generally nearly 7; for example, the oxides SiO_{2}, - Sc_{2}O_{3}, TiO_{2}, V_{2}O_{5}, as well as ZnO, Ga_{2}O_{3}, - GeO_{2}, ZrO_{2}, In_{2}O_{3}, SnO_{2}, Sb_{2}O_{5}, &c., whilst - the mean volume of the atom of the alkali and acid oxides is - greater than 7. Thus we find in the magnitudes of the mean volumes - of the atom in oxides and salts both a periodic variation and a - connection with their energy of essentially the same character as - occurs in the case of the free elements. - -Thus we find that in the large periods beginning with lithium, sodium, -potassium, rubidium, cæsium, and ending with fluorine, chlorine, bromine, -iodine, the extreme members (energetic elements) have a small density and -large volume, whilst the intermediate substances gradually increase in -density and decrease in volume--that is, as the atomic weight increases -the density rises and falls, again rises and falls, and so on. -Furthermore, the energy decreases as the density rises, and the greatest -density is proper to the atomically heaviest and least energetic -elements; for example, Os, Ir, Pt, Au, U. - -In order to explain the relation between the volumes of the elements and -of their compounds, the densities (column S) and volumes (column M/_s_) -of some of the higher saline oxides arranged in the same order as in the -case of the elements are given on p. 36. For convenience of comparison -the volumes of the oxides are all calculated per two atoms of an element -combined with oxygen. For example, the density of Al_{2}O_{3} = 4·0, -weight Al_{2}O_{3} = 102, volume Al_{2}O_{3} = 25·5. Whence, knowing the -volume of aluminium to be 11, it is at once seen that in the formation of -aluminium oxide, 22 volumes of it give 25·5 volumes of oxide. A distinct -periodicity may also be observed with respect to the specific gravities -and volumes of the higher saline oxides. Thus in each period, beginning -with the alkali metals, the specific gravity of the oxides first rises, -reaches a maximum, and then falls on passing to the acid oxides, and -again becomes a minimum about the halogens. But it is especially -important to call attention to the fact that the volume of the alkali -oxides is less than that of the metal contained in them, which is also -expressed in the last column, giving this difference for each atom of -oxygen.[22] Thus 2 atoms of sodium, or 46 volumes, give 24 volumes of -Na_{2}O, and about 37 volumes of 2NaHO--that is, the oxygen and hydrogen -in distributing themselves in the medium of sodium have not only not -increased the distance between its atoms, but have brought them nearer -together, have drawn them together by the force of their great affinity, -by reason, it may be presumed, of the small mutual attraction of the -atoms of sodium. Such metals as aluminium and zinc, in combining with -oxygen and forming oxides of feeble salt-forming capacity, hardly vary in -volume, but the common metals and non-metals, and especially those -forming acid oxides, always give an increased volume when oxidised--that -is, the atoms are set further apart in order to make room for the oxygen. -The oxygen in them does not compress the molecule as in the alkalis; it -is therefore comparatively easily disengaged. - - [22] The volume of oxygen (judging by the table on p. 36) is evidently - a variable quantity, forming a distinctly periodic function of the - atomic weight and type of the oxide, and therefore the efforts - which were formerly made to find the volume of the atom of oxygen - in the volumes of its compounds may be considered to be futile. - But since a distinct contraction takes place in the formation of - oxides, and the volume of an oxide is frequently less than the - volume in the free state of the element contained in it, it might - be surmised that the volume of oxygen in a free state is about 15, - and therefore the specific gravity of solid oxygen in a free state - would be about O·9. - - S M/_s_ Volume of Oxygen - - H_{2}O 1·0 18 ?- 22 - Li_{2}O 2·0 15 - 9 - Be_{2}O_{2} 3·06 16 + 2·6 - B_{2}O_{3} 1·8 39 + 10·0 - C_{2}O_{4} 1·6 55 + 10·6 - N_{2}O_{5} 1·64 66 ?+ 4 - - Na_{2}O 2·6 24 - 22 - Mg_{2}O_{2} 3·5 23 - 4·5 - Al_{2}O_{3} 4·0 26 + 1·3 - Si_{2}O_{4} 2·65 45 + 5·2 - P_{2}O_{5} 2·39 59 + 6·2 - S_{2}O_{6} 1·96 82 + 8·7 - Cl_{2}O_{7} ?1·92 95 + 6 - - K_{2}O 2·7 35 - 35 - Ca_{2}O_{2} 3·25 34 - 8 - Sc_{2}O_{3} 3·86 35 ? 0 - Ti_{2}O_{4} 4·2 38 + 3 - V_{2}O_{5} 3·49 52 + 6·7 - Cr_{2}O_{6} 2·74 73 + 9·5 - Cu_{2}O 5·9 24 + 9·6 - Zn_{2}O_{2} 5·7 23 + 4·8 - Ga_{2}O_{3} ?5·1 36 + 4 - Ge_{2}O_{4} 4·7 44 + 4·5 - As_{2}O_{5} 4·1 56 + 6·0 - - Sr_{2}O_{2} 4·7 44 - 13 - Y_{2}O_{3} 5·0 45 ?- 2 - Zr_{2}O_{4} 5·5 44 0 - Nb_{2}O_{5} 4·7 57 + 6 - MoO_{6} 4·4 65 + 6·8 - Ag_{2}O 7·5 31 + 11 - Cd_{2}O_{3} 8·0 32 + 3 - In_{2}O_{3} 7·18 38 + 2·7 - Sn_{2}O_{4} 7·O 43 + 2·7 - Sb_{2}O_{5} 6·5 49 + 2·6 - TeO_{6} 5·1 68 + 4·7 - - Ba_{2}O_{2} 5·7 52 - 10 - La_{2}O_{3} 6·5 50 + 1 - Ce_{2}O_{4} 6·74 50 + 2 - Ta_{2}O_{5} 7·5 59 + 4·6 - W_{2}O_{6} 6·8 68 + 8·2 - Hg_{2}O_{2} 11·1 39 + 4·5 - Pb_{2}O_{4} 8·9 53 + 4·2 - Th_{2}O_{4} 9·86 54 + 2 - -As the volumes of the chlorides, organo-metallic and all other -corresponding compounds, also vary in a like periodic succession with a -change of elements, it is evidently possible to indicate the properties -of substances yet uninvestigated by experimental means, and even those of -yet undiscovered elements. It was possible by following this method to -foretell, on the basis of the periodic law, many of the properties of -scandium, gallium, and germanium, which were verified with great accuracy -after these metals had been discovered.[23] The periodic law, therefore, -has not only embraced the mutual relations of the elements and expressed -their analogy, but has also to a certain extent subjected to law the -doctrine of the types of the compounds formed by the elements: it has -enabled us to see a regularity in the variation of all chemical and -physical properties of elements and compounds, and has rendered it -possible to foretell the properties of elements and compounds yet -uninvestigated by experimental means; thus it has prepared the ground for -the building up of atomic and molecular mechanics.[24] - - [23] As an example we will take indium oxide, In_{2}O_{3}. Its sp. gr. - and sp. vol. should be the mean of those of cadmium oxide, - Cd_{2}O_{2}, and stannic oxide, Sn_{2}O_{4}, as indium stands - between cadmium and tin. Thus in the seventies it was already - evident that the volume of indium oxide should be about 38, and - its sp. gr. about 7·2, which was confirmed by the determinations - of Nilson and Pettersson (7·179) made in 1880. - - [24] As the distance between, and the volumes of, the molecules and - atoms of solids and liquids certainly enter into the data for the - solution of the problems of molecular mechanics, which as yet have - only been worked out to any extent for the gaseous state, the - study of the specific gravity of solids, and especially of - liquids, has long had an extensive literature. With respect to - solids, however, a great difficulty is met with, owing to the - specific gravity varying not only with a change of isomeric state - (for example, for silica in the form of quartz = 2·65, and in - tridymite = 2·2) but also directly under mechanical pressure (for - example, in a crystalline, cast, and forged metal), and even with - the extent to which they are powdered, &c., which influences are - imperceptible in liquids. Compare Chapter XIV., Note 55^{bis}. - - Without going into further details, we may add to what has been - said above that the conception of specific volumes and atomic - distances has formed the subject of a large number of researches, - but as yet it is only possible to lay down a few generalisations - given by Dumas, Kopp, and others, which are mentioned and - amplified by me in my work cited in Note 20, and in my memoirs on - this subject. - - 1. Analogous compounds and their isomorphs have frequently - approximately the same molecular volumes. - - 2. Other compounds, analogous in their properties, exhibit - molecular volumes which increase with the molecular weight. - - 3. When a contraction takes place in combination in a gaseous - state, then contraction is in the majority of instances also to be - observed in the solid or liquid state--that is, the sum of the - volumes of the reacting substances is greater than the volume of - the resultant substance or substances. - - 4. In decomposition the reverse takes place to that which occurs - in combination. - - 5. In substitution (when the volumes in a state of vapour do not - vary) a very small change of volume generally takes place--that - is, the sum of the volumes of the reacting substances is almost - equal to the sum of the resultant substances. - - 6. Hence it is impossible to judge the volume of the component - substances from the volume of a compound, although it is possible - to do so from the product of substitution. - - 7. The replacement of H_{2} by sodium, Na_{2}, and by barium, Ba, - as well as the replacement of SO_{4} by Cl_{2}, scarcely changes - the volume, but the volume increases with the replacement of Na by - K, and decreases with the replacement of H_{2}, by Li_{2} Cu, and - Mg. - - 8. There is no need for comparing volumes in a solid and liquid - state at the so-called corresponding temperatures--that is at - temperatures at which vapour tension is equal in each case. The - comparison of volumes at the ordinary temperature is sufficient - for finding a regularity in the relations of volumes (this - deduction was developed with particular detail by me in 1856). - - 9. Many investigators (Perseau, Schröder, Löwig, Playfair and - Joule, Baudrimont, Einhardt) have sought in vain for a multiple - proportion in the specific volumes of solids and liquids. - - 10. The truth of the above is seen very clearly in comparing the - volumes of polymeric substances. The volumes of their molecules - are equal in a state of vapour, but are very different in a solid - and liquid state, as is seen from the close resemblance of the - specific gravities of polymeric substances. But as a rule the more - complex polymerides are denser than the simpler. - - 11. We know that the hydroxides of light metals have generally a - smaller volume than the metals, whilst that of magnesium hydroxide - is considerably greater, which is explained by the stability of - the former and instability of the latter. In proof of this we may - cite, besides the volumes of the true alkali metals, the volume of - barium (36) which is greater than that of its stable hydroxide - (sp. gr. 4·5, sp. vol. 30). The volumes of the salts of magnesium - and calcium are greater than the volume of the metal, with the - single exception of the fluoride of calcium. With the heavy metals - the volume of the compound is always greater than the volume of - the metal, and, moreover, for such compounds as silver iodide, AgI - (_d_ = 5·7), and mercuric iodide, HgI_{2} (_d_ = 6·2, and the - volumes of the compounds 41 and 73), the volume of the compound is - greater than the sum of the volumes of the component elements. - Thus the sum of the volumes Ag + I = 36, and the volume of AgI = - 41. This stands out with particular clearness on comparing the - volumes K + I = 71 with the volume of KI, which is equal to 54, - because its density = 3·06. - - 12. In such combinations, between solids and liquids, as - solutions, alloys, isomorphous mixtures, and similar feeble - chemical compounds, the sum of the reacting substances is always - very nearly that of the resulting substance, but here the volume - is either slightly larger or smaller than the original; speaking - generally, the amount of contraction depends on the force of - affinity acting between the combining substances. I may here - observe that the present data respecting the specific volumes of - solid and liquid bodies deserve a fresh and full elaboration to - explain many contradictory statements which have accumulated on - this subject. - - - - - CHAPTER XVI - - ZINC, CADMIUM, AND MERCURY - - -These three metals give, like magnesium, oxides RO, which form feebly -energetic bases, and like magnesium they are volatile. The volatility -increases with the atomic weight. Magnesium can be distilled at a white -heat, zinc at a temperature of about 930°, cadmium about 770°, and -mercury about 351°. Their oxides, RO, are more easily reducible than -magnesia, and mercuric oxide is the most easily reducible. The properties -of their salts RX_{2} are very similar to the properties of MgX_{2}. -Their solubility, power of forming double and basic salts, and many other -qualities are in many respects identical with those of MgX_{2}. The -greater or less ease with which they are oxidised, the instability of -their compounds, the density of the metals and their compounds, their -scarcity in nature, and many other properties gradually change with the -increase of atomic weight, as might be expected from the periodicity of -the elements. Their principal characteristics, as contrasted with -magnesium, find a general expression in the fact that zinc, cadmium, and -mercury are heavy metals. - -_Zinc_ stands nearest to magnesium in atomic weight and in properties. -Thus zinc sulphate, or white vitriol, easily crystallises with seven -molecules of water, ZnSO_{4},7H_{2}O. It is isomorphous with Epsom salts, -and parts with difficulty with the last molecule of water; it forms -double salts--for instance, ZnK_{2}(SO_{4})_{2},6H_{2}O--exactly as -magnesium sulphate does.[1] _Zinc oxide_, ZnO, is a white powder, almost -insoluble in water,[2] like magnesia, from which, however, it is -distinguished by its solubility in solutions of sodium and potassium -hydroxides.[3] Zinc chloride[4] is decomposed by water, combines with -ammonium chloride, potassium chloride, &c., just like magnesium chloride, -forms an oxychloride, and also combines with zinc oxide.[4 bis] - - [1] Zinc sulphate is often obtained as a by-product--for instance, in - the action of galvanic batteries containing zinc and sulphuric - acid. When the anhydrous salt is heated it forms zinc oxide, - sulphurous anhydride, and oxygen. The solubility in 100 parts of - water at O° = 43, 20° = 53, 40° = 63-1/2, 60° = 74, 80° = 84-1/2, - 100° = 95 parts of anhydrous zinc sulphate--that is to say, it is - closely expressed by the formula 43 + 0·52_t_. - - An admixture of iron is often found in ordinary sulphate of zinc in - the form of ferrous sulphate, FeSO_{4}, isomorphous with the zinc - sulphate. In order to separate it, chlorine is passed through the - solution of the impure salt (when the ferrous salt is converted - into ferric), the solution is then boiled, and zinc oxide is - afterwards added, which, after some time has elapsed, precipitates - all the ferric oxide. Ferric oxide of the form R_{2}O_{3} is - displaced by zinc oxide of the form RO. - - [2] Zinc oxide is obtained both by the combustion and oxidation of - zinc, and by the ignition of some of its salts--for instance, those - of carbonic and nitric acids; it is likewise precipitated by - alkalis from a solution of ZnX_{2} in the form of a gelatinous - hydroxide. The oxide produced by roasting zinc blende (by burning - in the air, when the sulphur is converted into sulphurous - anhydride) contains various impurities. For purification, the oxide - is mixed with water, and the sulphurous anhydride formed by - roasting the blende is passed through it. Zinc bisulphite, - ZnSO_{3},H_{2}SO_{3}, then passes into solution. If a solution of - this salt be evaporated, and the residue ignited, zinc oxide, free - from many of its impurities, will remain. Zinc oxide is a light - white powder, used as a paint instead of _white lead_; the basic - salt, corresponding with magnesia alba, is used for the same - purpose. V. Kouriloff (1890) by boiling the hydrate of the oxide - with a 3 p.c. solution of peroxide of hydrogen obtained - Zn_{2}H_{2}O_{4} or the hydrate of the peroxide - (= ZnO_{2}ZnH_{2}O_{2} or a compound of 2ZnO with H_{2}O_{2}), - which did not part with its oxygen at 100°, but only above 120°. - Cadmium gives a similar compound of a yellow colour. Magnesium, - although it does form such a compound, does so with great - difficulty. - - [3] For the solution of one part of the oxide 55,400 parts of water are - required. Nevertheless, even in such a weak solution, zinc oxide - (hydroxide, ZnH_{2}O_{2}) changes the colour of red litmus paper. - Zinc oxide is obtained in the wet way by adding an alkali hydroxide - to a solution of a zinc salt--for instance: ZnSO_{4} + 2HKO = - K_{2}SO_{4} + ZnH_{2}O_{2}. The gelatinous precipitate of zinc - hydroxide is _soluble_ in an excess of alkali, which clearly - distinguishes it from magnesia. This solubility of zinc hydroxide - in alkalis is due to the power of zinc oxide to form a compound, - although an unstable one, with alkalis--that is to say, points to - the fact that zinc oxide already partly belongs to the intermediate - oxides. The oxides of the metals above mentioned (except BeO) do - not show this property. The property which metallic zinc itself has - of dissolving in caustic alkali with the disengagement of hydrogen - (the solution is facilitated by contact with platinum or iron) - depends on the formation of such a compound of the oxides of zinc - and the alkali metals. The solution of zinc hydroxide, - ZnH_{2}O_{2}, in potash (in a strong solution), proceeds when these - hydrates are taken in proportion to ZnH_{2}O_{2} + KHO. If such a - solution be evaporated to dryness, water extracts only caustic - potash from the fused residue. When a solution of zinc hydroxide in - strong alkali is mixed with a large mass of water, nearly all the - oxide of zinc is precipitated; and, therefore, in weak solutions, a - large quantity of the alkali is required to effect solution, which - points to the decomposition of the zinc-alkali compounds by water. - If strong alcohol be added to a solution of zinc oxide in sodium - hydroxide, the crystallo-hydrate, 2Zn(OH)(ONa),7H_{2}O, separates. - - [4] _Zinc chloride_, ZnCl_{2}, is generally employed in the arts in the - form of a solution obtained by dissolving zinc in hydrochloric - acid. This solution is used for soldering metals, impregnating - wood, &c. The reason why it is thus employed may be understood from - its properties. When evaporated it first parts with its water of - crystallisation; on being further heated, however, it loses all - traces of water, and forms an oily mass of anhydrous salt which - solidifies on cooling. This substance melts at 250°, commences to - volatilise at about 400°, and boils at 730°. The soldering of - metals--that is, the introduction of an easily fusible metal - between two contiguous metallic objects--is hindered by any film of - oxide upon them; and, as heated metals easily oxidise, they are - naturally difficult to solder. Zinc chloride is used to prevent the - oxidation. It fuses on being heated, and, covering the metal with - an oily coating, prevents contact with the air; but even if any - oxide has formed, the free hydrochloric acid generally existing in - the zinc chloride solution dissolves it, and in this way the - metallic surface of the metals to be soldered is preserved fit for - the adhesion of the liquid solder, which, on cooling, binds the - objects together. Much zinc chloride is used also for steeping wood - (telegraph-posts and railway-sleepers) in order to preserve it from - decaying quickly; this preservative action is in all probability - mainly due to the poisonous character of zinc salts (corrosive - sublimate is still more poisonous, and a still better agent to - preserve wood from decay), since decay is due to the action of - lower organisms. - - The specific gravity of solutions containing _p_ per cent. of zinc - chloride, ZnCl_{2}, is as follows: - - _p_ = 10 20 30 40 50 - 15°/4° = 1·093 1·184 1·293 1·411 1·554 - _ds/dt_ = -3 -5 -7 -8 -9 - - The last line shows the change of specific gravity for 1° in - ten-thousandth parts for temperatures near 15°. More accurate - determinations of Cheltzoff, personally communicated by him, led - him to conclude that solutions of zinc chloride follow the same - laws as the solutions of sulphuric acid, which will be considered - in Chapter XX.: (1) from H_{2}O to ZnCl_{2},120H_{2}O _s_ = S_{0} + - 92·85_p_ + 0·1748_p_^2; (2) from thence to ZnCl_{2},40H_{2}O _s_ = - S_{0} + 93·96_p_ - 0·0126_p_^2; (3) thence to ZnCl_{2},25H_{2}O _s_ - = 11481·5 + 96·45(_p_ - 15·89) + 0·4567(_p_ - 15·89)^2; (4) thence - to ZnCl_{2},10H_{2}O _s_ = 12212·1 + 104·82(_p_ - 23·21) + - 0·7992(_p_ - 23·21)^2; (5) thence to _p_ = 65 p.c. _s_ = 14606·3 + - 140·96(_p_ - 43·05) + 1·4905(_p_ - 43·05)^2, where _s_ is the - specific gravity of the solution at 15°, containing _p_ p.c. of - ZnCl_{2} by weight, taking water at 4° = 10000, and where S_{0} = - 9991·6 (specific gravity of water at 15°). The compound of zinc - chloride with hydrochloric acid has been mentioned in Vol. I. - Chapter X. - - Zinc chloride has a great affinity for water; it is not only - soluble in it, but in alcohol, and on being dissolved in water - becomes considerably heated, like magnesium and calcium chlorides. - Zinc chloride is capable of taking up water, not only in a free - state, but also in chemical combination with many substances. Thus, - for instance, it is used in organic researches for removing the - elements of water from many of the organic compounds. - - [4 bis] When mixed with zinc oxide it forms, with remarkable ease, a - very hard mass of zinc oxychloride, which is applied in the arts; - for instance, in painting, to resist the action of water, or for - cementing such objects as are destined to remain in water. Zinc - oxychloride, ZnCl_{2},3ZnO,2H_{2}O (= Zn_{2}OCl_{2},2ZnH_{2}O_{2}), - is also formed from a solution of zinc chloride by the action of a - small quantity of ammonia on it after heating the precipitate - obtained with the liquid for a considerable time; the admixture of - ammonium salts with a mixture of a strong solution of zinc chloride - with its oxide makes a similar mass, which does not solidify so - rapidly, and is therefore more useful for some purposes. Moisture - and cold do not change the hardened mass of oxychloride, and it - also resists the action of many acids, and a temperature of 300°, - which makes it a useful cement for many purposes. A solution of - magnesium chloride with magnesium oxide forms a similar - oxychloride. The mass solidifies best when there are equal - quantities by weight of zinc in the chloride and oxide, and - therefore when it has the composition Zn_{2}OCl_{2} In preparing - such a cement, naturally zinc oxide alone may be taken, and the - requisite quantity of hydrochloric acid added to it. The capacity - of ZnCl_{2} to combine with water, ZnO, and HCl (and also with - other metallic chlorides) indicates its property to combine with - molecules of other substances, and therefore its compounds with - NH_{3}, and especially a compound, ZnCl_{2}2NH_{3}, similar to - sal-ammoniac, might be expected (_i.e._ 2NH_{4}Cl, in which H_{2} - is replaced by Zn). And indeed it has long been known that ZnCl_{2} - absorbs ammonia and gives solid substances capable of dissociating - with the disengagement of NH_{3}. Among these compounds Isambert - and V. Kouriloff (1894) obtained ZnCl_{2}6NH_{3}, ZnCl_{2}4NH_{3}, - ZnCl_{2}2NH_{3}, and ZnCl_{2}NH_{3}. The dissociation tension of - the two last-mentioned compounds at 218° is equal to 43·6 mm. and - 6·7 mm. CdCl_{2} also forms similar compounds with NH_{3} - (Kouriloff, 1894). - -Zinc, like many heavy metals, is often _found in nature in combination -with sulphur_, forming the so-called _zinc blende_,[5] ZnS. It sometimes -occurs in large masses, often crystallised in cubes; it is frequently -translucent, and has a metallic lustre, although this is not so clearly -developed as in many other metallic sulphides with which we shall -hereafter become acquainted. The ores of zinc also comprise the -carbonate, calamine, and silicate, _siliceous calamine_. - - [5] This mineral has been given the name of 'mock-ore,' on account of - its having the appearance (considerable density, 4·06, &c.) of - ordinary metallic ores; it deceived the first miners, because it - did not, like other ores, give metal when simply roasted in air and - fused with charcoal. The white zinc oxide, formed by burning the - vapours of zinc, was also called 'nihil album,' or 'white nothing,' - on account of its lightness. - -[Illustration: FIG. 80.--Distillation of zinc in a crucible placed in a -furnace. _o c_, tube along which the vapour passes and condenses.] - -Metallic zinc (spelter) is most frequently obtained from the ores -containing the carbonate[6]--that is, from calamine, which is sometimes -found in thick veins: for instance, in Poland, Galicia, in some places on -the banks of the Rhine, and in considerable masses in Belgium and -England. In Russia beds of zinc ore are met with in Poland and the -Caucasus, but the output is small. In Sweden, as early as the fifteenth -century, calamine was worked up into an alloy of zinc and copper (brass), -and Paracelsus produced zinc from calamine; but the technical production -of the metal itself, long ago practised in China, only commenced in -Europe in 1807--in Belgium, when the Abbé Donnet discovered that zinc was -volatile. From that time the production increased until it is now about -150 million kilograms in Germany alone. - - [6] It may be here mentioned that by the word _ore_ is meant a hard, - heavy substance dug out of the earth, which is used in - metallurgical works for obtaining the usual heavy metals long known - and used. The natural compounds of sodium, or magnesium, are not - called ores, because magnesium and sodium have not been long - obtainable in quantity. The heavy metals, those which are easily - reduced and do not easily oxidise, are exclusively those which are - directly applied in manufactures. Ores either contain the metals - themselves (for instance, ores of silver or bismuth), and the - metals are then said to be in a native state, or else their sulphur - compounds (blende, mock-ore, pyrites--as, for example, galena, PbS; - zinc blende, ZnS; copper pyrites, CuFeS) or oxides (as the ores of - iron), or salts (calamine, for instance). Zinc is incomparably - rarer than magnesium, and is only well known because it is - transformed from its ores into a metal which finds direct use in - many branches of industry. - -The reduction of metallic zinc from its ores is based on the fact that -zinc oxide[7] is easily reduced by charcoal at a red heat: ZnO + C = Zn + -CO. The zinc thus obtained is in a finely divided state and impure, being -mixed with other metals reduced with it, but the greater portion is -_converted into vapour_, from which it easily passes into a liquid or -solid state. The reduction and distillation are carried on in earthenware -retorts, filled with a mixture of the divided ore and charcoal. The -vapours of zinc and gases formed during the reaction escape by means of a -pipe leading downwards, and are led to a chamber where the vapours are -cooled. By this means they do not come into contact with the air, because -the neck of the retort is filled with gaseous carbonic oxide, and -therefore the zinc does not oxidise; otherwise its vapour would burn in -the air.[7 bis] The vapours of zinc, entering into the cooling chamber, -condense into white zinc powder or zinc dust. When the neck of the retort -is heated the zinc is obtained in a liquid state, and is cast into -plates, in which form it is generally sold. - - [7] Ores, when extracted from the earth by the miners, are often - enriched by sorting, washing, and other mechanical operations. The - sulphurous ores (and likewise others) are then generally roasted. - Roasting an ore means heating it to redness in air. The sulphur - then burns, and passes off in the form of sulphurous anhydride, - SO_{2}, and the metal oxidises. The roasting is carried on in order - to obtain an oxide instead of a sulphur compound, the oxide being - reducible by charcoal. These methods, introduced ages ago, are met - with in nearly all metallurgical works for practically all ores. - For this reason the preparatory treatment of zinc blende furnishes - zinc oxide: this is already contained in calamine. - - [7 bis] with very impure ores, especially such as contain lead (PbS - often accompanies zinc), the vapour of the reduced zinc is allowed - to pass directly into the air. It burns and gives ZnO, which is - used as a pigment. - -Commercial zinc is generally impure, containing a mixture of lead, -particles of carbon, iron, and other metals carried over with the -vapours, although they are not volatile at a temperature approaching -1000°. If it be required to obtain pure zinc from the commercial article, -it is subjected to a further distillation in a crucible with a pipe -passing through the bottom, the vapours formed by the heated zinc only -having exit through the pipe cemented into the bottom of the crucible. -Passing through this pipe, the vapours condense to a liquid, which is -collected in a receiver. Zinc thus purified is generally re-melted and -cast into rods, and in this form is often used for physical and chemical -researches where a pure article is required.[8] - - [8] This zinc, although homogeneous, still contains certain impurities, - to remove which it is necessary to prepare some salt of zinc in a - pure state and transform it into carbonate, which latter is then - distilled with charcoal; and, as thin sheets of zinc can only be - obtained from very pure metal, they are frequently made use of in - cases where pure zinc is required. In order to remove the arsenic - from zinc, it was proposed to melt it and mix it with anhydrous - magnesium chloride, by which means vapours of zinc chloride and - arsenic chloride are formed. Perfectly pure zinc is made (V. Meyer - and others) by decomposing, by means of the galvanic current, a - solution of zinc sulphate to which an excess of ammonia has been - added. The zinc used for Marsh's arsenic test (Chapter XIX.) is - purified from As by fusing it with KNO_{3} and then with ZnCl_{2}. - -Metallic zinc has a bluish-white colour; its lustre, compared with many -other metals, is insignificant. When cast it exhibits a crystalline -structure. Its specific gravity is about 7--that is, varies from 6·8 to -7·2, according to the degree of compression (by forging, rolling, &c.) to -which it has been subjected. It is very ductile, considering its -hardness. For this reason it chokes up files when being worked. Its -malleability is considerable when pure, but in the ordinary impure -condition in which it is sold, it is impossible to roll it at the -ordinary temperature, as it easily breaks. At a temperature of 100°, -however, it easily undergoes such operations, and can then be drawn into -wire or rolled into sheets. If heated further it again becomes brittle, -and at 200° may be even crushed into powder, so completely does it lose -its molecular cohesion. It melts at 418°, and distils at 930°. - -Zinc does not undergo any change in the atmosphere. Even in very damp air -it only becomes slowly coated with a very thin white coating of oxide. -For this reason it is available for all objects which are only in contact -with air. Therefore sheet zinc may be used for roofing and many other -purposes.[9] This great unchangeability of zinc in the air shows its -slight energy with regard to oxygen compared with the metals already -mentioned, which are capable of reducing zinc from solutions. But zinc -plays this part with regard to the remaining metals--for example, it -reduces salts of lead, copper, mercury, &c. Although zinc is an almost -unoxidisable metal at the ordinary temperature, it burns in the air on -being heated, particularly when in the form of shavings or in the -condition of vapour. At the ordinary temperature zinc does not decompose -water--at any rate, if the metal be in a dense mass. But even at a -temperature of 100° zinc begins little by little to decompose water; it -easily displaces the hydrogen of acids at the ordinary temperature, and -of alkalis on being heated. - - [9] Cornices and other architectural ornaments, remarkable for their - lightness and beauty, are stamped out of sheet zinc. Zinc-roofing - does not require painting, but it melts during a conflagration, and - even burns at a strong heat. Many iron vessels, &c., are covered - with zinc ('galvanised') in order to prevent them from rusting. - -In this respect the action of zinc varies a great deal with the degree -of its purity. Weak sulphuric acid (corresponding with the composition -H_{2}SO_{4},8H_{2}O) at the ordinary temperature does not act at all on -chemically pure zinc, and even a stronger solution acts very slowly. If -the temperature be raised, and particularly if the zinc be previously -slightly heated, so as to cover the surface with a film of oxide, -chemically pure zinc acts on sulphuric acid. Thus, for example, one cubic -centimetre of zinc in sulphuric acid having a composition -H_{2}SO_{4},6H_{2}O at the ordinary temperature in two hours only -dissolves to the extent of 0·018 gram, and at a temperature of 100° about -3·5 grams. If we compare this slow action with that rapid evolution of -hydrogen which occurs in the case of commercial zinc, we see that the -influence of those impurities in the zinc is very great. Every particle -of charcoal or iron introduced into the mass of the zinc, and likewise -the connection of the zinc with a piece of another electro-negative -metal, assists such a dissolution. The slowness of the action of -sulphuric acid on pure zinc (and likewise on amalgamated zinc) may also -be explained by the fact that a layer of hydrogen[10] collects on the -surface of the metal, preventing contact between the acid and the -metal.[10 bis] - - [10] Veeren (1891) proved this by simple experiments, finding that in - vacuo the solution proceeds far more rapidly for both pure and - commercial zinc, and still more rapidly in the presence of - oxidising agents (which absorb the hydrogen) like CrO_{3} and - H_{2}O_{2}. - - [10 bis] The addition of cupric sulphate, or, better still, a few drops - of platinic chloride (the metals become reduced), to the sulphuric - acid greatly accelerates the evolution of the hydrogen, because in - this case, as with commercial zinc, galvanic couples are formed - locally by the copper or platinum and the zinc, under the - influence of which the zinc rapidly dissolves. The action of acids - on metallic zinc of various degrees of purity has been the subject - of many investigations, particularly important with reference to - the application of zinc in galvanic batteries, whilst some - investigations have direct significance for chemical mechanics, - although from many points of view the matter is not clear. I - consider it useful to mention certain of these investigations. - - Calvert and Johnson made the following series of observations on - the action of sulphuric acid of various degrees of concentration - on 2 grams of pure zinc during two hours. In the cold the - concentrated acid, H_{2}SO_{4}, does not act, H_{2}SO_{4},2H_{2}O - dissolves about 0·002 gram, but principally forms hydrogen - sulphide, which is obtained also when the dilution reaches - H_{2}SO_{4},7H_{2}O, when 0·035 gram of zinc is dissolved. When - largely diluted with water, pure hydrogen begins to be disengaged. - H_{2}SO_{4},2H_{2}O at 130° gives a mixture of hydrogen sulphide - and sulphurous anhydride dissolving 0·156 gram of zinc. - - Bouchardat showed that if in a vessel made of glass or sulphur - dilute sulphuric acid acting on a piece of zinc liberates one part - of hydrogen, then the same acid with the same piece of zinc in the - same time will liberate 4 parts of hydrogen if the vessel be made - of tin--that is, zinc forms a galvanic couple with tin; in a - leaden vessel 9 parts of hydrogen are set free, with a vessel of - antimony or bismuth 13 parts, silver or platinum 38 parts, copper - 50 parts, iron 43 parts. If a salt of platinum be added to the - dilute sulphuric acid (1 part of acid and 12 parts of water), - Millon determined that the rapidity of the action on the zinc is - increased 149 times, and by the addition of copper sulphate is - rendered 45 times greater than the action of pure sulphuric acid. - The salts which are added are reduced to metals by the zinc, their - contact serving to promote the reaction because they form local - galvanic currents. - - According to the observations of Cailletet, if, at the ordinary - pressure, sulphuric acid with zinc liberates 100 parts of - hydrogen, then with a pressure of 60 atmospheres 47 parts will be - liberated and 1 part at a pressure of 120 atmospheres. With a - reduced pressure under the receiver of an air-pump 168 parts are - liberated. Helmholtz showed that a reduced pressure also exercises - its influence on galvanic elements. - - Debray, Löwel, and others showed that zinc liberates hydrogen and - forms basic salts and zinc oxide with solutions of many salts--for - instance, MCl_{_n_}, aluminium sulphate, and alum, Sodium and - potassium carbonates scarcely act, because they form carbonates. - The salts of ammonia act more strongly than the salts of potassium - and sodium; the zinc remains bright. It is evident that this - action is founded on the formation of double salts and basic - salts. - - The variation with concentration in the rate of the action of - sulphuric acid on zinc (containing impurities) under otherwise - uniform conditions is in evident connection with the electrical - conductivity of the solution and its viscosity, although, when - largely diluted, the action is almost proportional to the amount - of acid in a known volume of the solution. Forging, casting the - molten metal, and similar mechanical influences change the density - and hardness of zinc, and also strongly influence its power of - liberating hydrogen from acids. Kayander showed (1881) that when - magnesium is submitted to the action of acids: (_a_) the action - depends, not on the nature of the acid, but on its basicity; (_b_) - the increase of the action is more rapid than the growth of the - concentration; and (_c_) there is a decrease of action with the - increase of the coefficient of internal friction and electrical - conductivity. - - Spring and Aubel (1887) measured the volume of hydrogen disengaged - by an alloy of zinc and a small quantity of lead (0·6 p.c.), - because the action of acids is then uniform. In order to deal with - a known surface, spheres were taken (9·5 millimetres diameter) and - cylinders (17 mm. dia.), the sides of which were covered with wax - in order to limit the action to the end surfaces. During the - commencement of the action of a definite quantity of acid the - rapidity increases, attains a maximum, and then declines as the - acid becomes exhausted. The results for 5, 10, and 15 per cent. of - hydrochloric acid are given below. H denotes the number of cubic - centimetres of hydrogen, D the time in seconds elapsing after the - zinc spheres have been plunged into the acid. At 15° were - obtained: - - H = 50 100 200 400 600 800 1000 - 5 p.c. D = 714 1152 1755 2731 3908 6234 15462 - 10 p.c. D = 301 455 649 995 1573 2746 6748 - 15 p.c. D = 106 151 233 440 826 1604 4289 - - - At 35°: - - 5 p.c. D = 462 705 1058 1700 2525 4132 8499 - 10 p.c. D = 96 148 239 460 835 1594 3735 - 15 p.c. D = 44 64 112 255 505 1011 2457 - - - At 55°: - - 5 p.c. D = 178 276 408 699 1164 2105 5093 - 10 p.c. D = 34 60 113 258 491 970 2457 - 15 p.c. D = 24 35 58 136 239 610 1593 - - In consequence of the complex character of the phenomenon, the - authors themselves do not consider their determinations as being - conclusive, and only give them a relative significance; and in - this connection it is remarkable that hydrobromic acid under - similar conditions (with an equivalent strength) gives a greater - (from 2 to 5 times) rapidity of action than hydrochloric acid, but - sulphuric acid a far smaller velocity (nearly 25 times smaller). - It is also remarkable that during the reaction the metal becomes - much more heated than the acid. - - It may be mentioned that zinc dust and zinc itself, when heated - with hydrated lime and similar hydrates, disengages hydrogen: this - method has even been proposed for obtaining hydrogen for filling - war balloons. - -The action of zinc on acids, and the consequent formation of zinc -salts, interferes with its application in many cases, particularly for -the preservation of liquids either containing or capable of developing -acid. For this reason zinc vessels ought not to be used for the -preparation or preservation of food, as this often contains acids which -form poisonous salts with the zinc. Even ordinary water, containing -carbonic acid, slowly attacks zinc. - -Finely divided zinc, or _zinc dust_, obtained in the distillation of the -metal when the receiver is not heated up to the melting point, on account -of its presenting a large surface of contact and containing foreign -matter (particularly zinc oxide), has in the highest degree the property -of decomposing acids, and even water, which it easily decomposes, -particularly if slightly heated. On this account zinc dust is often used -in laboratories and factories as a reducing agent. A similar influence of -the finely divided state is also noticed in other metals--for instance, -copper and silver--which again shows the close connection between -chemical and physico-mechanical phenomena. We must first of all turn to -this close connection for an explanation of the widely spread application -of zinc in galvanic batteries, where the chemical (latent, potential) -energy of the acting substances is transformed into (evident, kinetic) -galvanic energy, and through this latter into heat, light, or mechanical -work. - -Hermann and Stromeyer, in 1819, showed that _cadmium_ is almost always -found with zinc, and in many respects resembles it. When distilled the -cadmium volatilises sooner, because it has a lower boiling point. -Sometimes the zinc dust obtained by the first distillation of zinc -contains as much as 5 per cent. of cadmium. When zinc blende, containing -cadmium, is roasted, the zinc passes into the state of oxide, and the -cadmium sulphide in the ore oxidises into cadmium sulphate, CdSO_{4}, -which resists tolerably well the action of heat; therefore if roasted -zinc blende be washed with water, a solution of cadmium sulphate will be -obtained, from which it is very easy to prepare metallic cadmium. -Hydrogen sulphide may be used for separating cadmium from its solutions; -it gives _a yellow precipitate of cadmium sulphide_, CdS (according to -the equation CdSO_{4} + H_{2}S = H_{2}SO_{4} + CdS),[11] which, on -account of its characteristic colour, is used as a pigment.[11 bis] -Cadmium sulphide, when strongly heated in air, leaves cadmium oxide, from -which the metal may be obtained in precisely the same way as in the case -of zinc. - - [11] It may be here remarked that sulphate of zinc (especially in the - presence of mineral acids) does not give a precipitate of sulphide - of zinc, or is only slightly precipitated by sulphuretted - hydrogen. - - [11 bis] Sulphide of cadmium appears in two varieties of a similar - chemical but different physical character: one is of a lemon - colour, and the other bright red. Kloboukoff (1890) studied the - physical properties of these varieties more closely. The sp. gr. - of the former is 3·906, and of the latter 4·513. They belong to - different crystallographic systems. The first variety may be - converted into the second by friction or pressure, but the second - cannot be converted into the first variety by these means. - -Cadmium is a white metal, and when freshly cut is almost as white and -lustrous as tin. It is so soft that it may be easily cut with a knife, -and so malleable that it can be easily drawn into wire, rolled into -sheets, &c. Its specific gravity is 8·67, melting point 320°, boiling -point 770°; its vapours burn, forming a brown powder of the oxide.[12] -Next to mercury it is the most volatile metal; hence Deville determined -the density of its vapours compared with hydrogen, and found it to be -equal to 57·1; therefore the molecule contains _one atom_ whose weight = -112. V. Meyer found the like for zinc; the molecule of mercury also -contains one atom. - - [12] Amongst the compounds of cadmium very closely allied to the - compounds of zinc, we must mention _cadmium iodide_, CdI_{2}, - which is used in medicine and photography. This salt crystallises - very well: it is prepared by the direct action of iodine, mixed - with water, on metallic cadmium. One part of cadmium iodide at 20° - requires for its solution 1·08 part of water. It may be remarked - that cadmium chloride at the same temperature requires 0·71 part - of water to dissolve it, so that the iodine compound of this metal - is less soluble than the chloride, whilst the reverse relation - holds in the case of the corresponding compounds of the alkali or - alkaline earthy metals. Cadmium sulphate crystallises well, and - has the composition 3CdSO_{4},8H_{2}O, thus differing from zinc - sulphate. - - Cadmium oxide is soluble, although sparingly, in alkalis, but in - the presence of tartaric and certain other acids the alkaline - solution of cadmium oxide does not change when boiled, whilst a - _diluted_ solution in that case deposits cadmium oxide: this may - also serve for separating zinc compounds from those of cadmium. - Cadmium is precipitated from its salts by zinc, which fact may - also be taken advantage of for separating cadmium; for this - reason, in an alloy of zinc and cadmium, acids first of all - extract the zinc. Cadmium is in all respects less energetic than - zinc. Thus, for instance, it decomposes water with difficulty, and - this only when strongly heated. It even acts but slowly on acids, - but then displaces hydrogen from them. It is necessary here to - call attention to the fact that for alkali and alkaline earthy - metals (of the even series) the highest atomic weight determines - the greatest energy; but cadmium (of the uneven series), whilst - having a larger atomic weight than zinc, is less energetic. The - salts of cadmium are colourless, like those of zinc. De Schulten - obtained a crystalline oxychloride, Cd(OH)Cl by heating marble - with a solution of cadmium chloride in a sealed tube at 200°. - - _Mercury_ resembles zinc and cadmium in many respects, but - presents that distinction from them which is always noticed in all - the heaviest metals (with regard to atomic weight and density) - compared with the lighter ones--namely, that it oxidises with more - difficulty, and its compounds are more easily decomposed.[12 bis] - Besides compounds of the usual type RX_{2}, it also gives those of - the lower type, RX, which are unknown for zinc and cadmium.[13] - Mercury therefore gives salts of the composition HgX (mercurous - salts) and HgX_{2} (mercuric salts), the oxides having the formulæ - Hg_{2}O and HgO respectively. - - [12 bis] According to its atomic weight, mercury follows gold in the - periodic system, just as cadmium follows silver and zinc follows - copper:-- - - Ni = 59 Cu = 63 Zn = 65 - Pd = 106 Ag = 108 Cd = 112 - Pt = 196 Au = 198 Hg = 200 - - Eventually we shall see the near relation of platinum, palladium, - and nickel, and also of gold, silver, and copper, but we will now - point out the parallelism between these three groups. The relation - between the physical and also chemical properties is here - strikingly similar. Nickel, palladium, and platinum are very - difficult to fuse (far more so than iron, ruthenium, and osmium, - which stand before them). Copper, silver, and gold melt far more - easily in a strong heat than the three preceding metals, and zinc, - cadmium, and mercury melt still more easily. Nickel, palladium, - and platinum are very slightly volatile; copper, silver, and gold - are more volatile; and zinc, cadmium, and mercury are among the - most volatile metals. Zinc oxidises more easily than copper, and - is reduced with more difficulty, and the same is true for mercury - as compared with gold. These properties for cadmium and silver are - intermediate in the respective groups. Relations of this kind - clearly show the nature of the periodic law. - - [13] Thus thallium, lead, and bismuth, following mercury according to - their atomic weights, form, besides compounds of the highest - types, TlX_{3}, PbX_{4}, and BiX_{5}, also the lower ones TlX, - PbX_{2}, and BiX_{3}. - -Mercury is found _in nature_ almost exclusively in combination with -sulphur (like zinc and cadmium, but is still rarer than them) in the form -known as cinnabar, HgS (Chapter XX., Note 29). It is far more rarely met -with in the native or metallic condition, and this in all probability has -been derived from cinnabar. Mercury ore is found only in a few -places--namely, in Spain (in Almaden), in Idria, Japan, Peru, and -California. About the year 1880 Minenkoff discovered a rich bed of -cinnabar in the Bahmout district (near the station of Nikitovka), in the -Government of Ekaterinoslav, so that now Russia even exports mercury to -other countries. Cinnabar is now being worked in Daghestan in the -Caucasus. Mercury ores are easily reduced to metallic mercury, because -the combination between the metal and the sulphur is one of but little -stability. Oxygen, iron, lime, and many other substances, when heated, -easily destroy the combination. If iron is heated with cinnabar, iron -sulphide is formed; if cinnabar is heated with lime, mercury and calcium -sulphide and sulphate are formed, 4HgS + 4CaO = 4Hg + 3CaS + CaSO_{4}. On -being heated in the air, or roasted, the sulphur burns, oxidises, forming -sulphurous anhydride, and vapours of metallic mercury are formed. Mercury -is more easily distilled than all other metals, its boiling point being -about 351°, and therefore its separation from natural admixtures, -decomposed by one of the above-mentioned methods, is effected at the -expense of a comparatively small amount of heat. The mixture of mercury -vapour, air, and products of combustion obtained is cooled in tubes (by -water or air), and the mercury condenses as liquid metal.[14] - - [14] During the condensation of the vapours of mercury in works, a part - forms a black mass of finely-divided particles, which gives - metallic mercury when worked up in centrifugal machines, or on - pressure, or on re-distillation. In mercury we observe a tendency - to easily split up into the finest drops, which are difficult to - unite into a dense mass. It is sufficient to shake up mercury with - nitric and sulphuric acids in order to produce such a mercury - _powder_. The mercury separated (for instance, reduced by - substances like sulphurous anhydride) from solutions, forms such a - powder. According to the experiments of Nernst, this disintegrated - mercury when entering into reactions develops more heat than the - dense liquid metal--that is to say, the work of disintegration - reappears in the form of heat. This example is instructive in - considering thermochemical deductions. - -Mercury, as everybody knows, is a liquid metal at the ordinary -temperature. In its lustre and whiteness it resembles silver.[15] At -39° -mercury is transformed into a malleable crystalline metal; at 0° its -specific gravity is 13·596, and in the solid state at -40° it is -14·39.[16] Mercury does not change in the air--that is to say, it does -not oxidise at the ordinary temperature--but at a temperature approaching -the boiling-point, as was stated in the Introduction, it oxidises, -forming mercuric oxide. Both metallic mercury and its compounds in -general produce salivation, trembling of the hands, and other unhealthy -symptoms which are found in the workmen exposed to the influence of -mercurial vapours or the dust of its compounds. - - [15] Mercury may sometimes be obtained in a perfectly pure state from - works (in iron bottles holding about 35 kilos), but after being - used in laboratories (for baths, calibration, &c.) it contains - impurities. It may be purified mechanically in the following way: - a paper filter with a fine hole (pricked with a needle) is placed - in a glass funnel and mercury is poured into it, which slowly - trickles through the hole, leaving the impurities upon the filter. - Sometimes it is squeezed through chamois leather or through a - block of wood (as in the well-known experiment with the air-pump). - It may be purified from many metals by contact with dilute nitric - acid, if small drops of mercury are allowed to pass through a long - column of it (from the fine end of a funnel); or by shaking it up - with sulphuric acid in air. Mercury may be purified by the action - of an electric current, if it be covered with a solution of - HgNO_{3}. But the complete purification of mercury for barometers - and thermometers can only be attained by distillation, best in a - vacuum (the vapour-tension of mercury is given in Chapter II., - Note 27). For this purpose Weinhold's apparatus is most often - used. The principle of this apparatus is very ingenious, the - distillation being effected in a Torricellian vacuum continuously - supplied with fresh mercury, whilst the condensed mercury is - continuously removed. This process of distillation requires very - little attention, and gives about one kilo of pure mercury per - hour. - - [16] If the volume of _liquid_ mercury at 0° be taken as 1000000, then, - according to the determinations of Regnault (recalculated by me in - 1875), at _t_ it will be 1000000 + 180·1_t_ + 0·02_t_^2. - -As many of the compounds of mercury decompose on being heated--for -instance, the oxide or carbonate[17]--and as zinc, cadmium, copper, iron, -and other metals separate mercury from its salts,[18] it is evident that -mercury has less chemical energy than the metals already described, even -than zinc and cadmium. Nitric acid, when acting on _an excess_ of mercury -at the ordinary temperature, gives mercurous nitrate, HgNO_{3}.[19] The -same acid, under the influence of heat and when in excess (nitric oxide -being liberated), forms mercuric nitrate, Hg(NO_{3})_{2}. This,[20] both -in its composition and properties, resembles the salts of zinc and -cadmium. Dilute sulphuric acid does not act on mercury, but strong -sulphuric acid dissolves it, with evolution of _sulphurous anhydride_ -(not hydrogen), and on being slightly heated with an excess of mercury it -forms the sparingly soluble mercurous sulphate, Hg_{2}SO_{4}; but if -mercury be strongly heated with an excess of the acid, the mercuric salt, -HgSO_{4},[21] is formed. Alkalis do not act on mercury, but the -non-metals chlorine, bromine, sulphur, and phosphorus easily combine with -it. They form, like the acids, two series of compounds, HgX and HgX_{2}. -The oxygen compound of the first series is the suboxide of mercury, or -mercurous oxide, Hg_{2}O, and of the second order the oxide HgO, mercuric -oxide. The chlorine compound corresponding with the suboxide is HgCl -(calomel), and with the oxide HgCl_{2} (corrosive sublimate or mercuric -chloride). In the compounds HgX, mercury resembles the metals of the -first group, and more especially silver. In the mercuric compounds there -is an evident resemblance to those of magnesium, cadmium, &c. Here the -atom of mercury is bivalent, as in the type RX_{2}.[22] Every soluble -mercurous compound (corresponding with the type of the suboxide of -mercury), HgX, forms a white precipitate of calomel, HgCl, with -hydrochloric acid or a metallic chloride, because HgCl is very slightly -soluble in water, HgX + MCl = HgCl + MX. In soluble mercuric compounds, -HgX_{2}, hydrochloric acid and metallic chlorides do not form a -precipitate, because corrosive sublimate, HgCl_{2}, is soluble in water. -Alkali hydroxides precipitate the yellow mercuric oxide from a solution -of HgX_{2}, and the black mercurous oxide from HgX. Potassium iodide -forms a dirty greenish precipitate, HgI, with mercurous salts, HgX, and a -red precipitate, HgI_{2}, with the mercuric salts, HgX_{2}. These -reactions distinguish the mercuric from the mercurous salts, which latter -represent the transition from the mercuric salts to mercury itself, 2HgX -= Hg + HgX_{2}. The salts, HgX, as well as HgX_{2}, are reduced by -nascent hydrogen (_e.g._ from Zn + H_{2}SO_{4}), by such metals as zinc -and copper, and also by many reducing agents--for example, -hypophosphorous acid, the lowest grade of oxidation of phosphorus, by -sulphurous anhydride, stannous chloride, &c. Under the action of these -reagents the mercuric salts are first transformed into the mercurous -salts, and the latter are then reduced to metallic mercury. This reaction -is so delicate that it serves to detect the smallest quantity of mercury; -for instance, in cases of poisoning, the mercury is detected by immersing -a copper plate in the solution to be tested, the mercury being then -deposited upon it (more readily on passing a galvanic current). The -copper plate, on being rubbed, shows a silvery white colour; on being -heated, it yields vapours of mercury, and then again assumes its original -red colour (if it does not oxidise). The mercurous compounds, HgX, under -the action of oxidising agents, even air, pass into mercuric compounds, -especially in the presence of acids (otherwise a basic salt is produced), -2HgX + 2HX + O = 2HgX_{2} + H_{2}O; but the mercuric compounds, when in -contact with mercury, change more or less readily, and turn into -mercurous compounds, HgX_{2} + Hg = 2HgX. For this reason, in order to -preserve solutions of mercurous salts, a little mercury is generally -added to them. - - [17] All salts of mercury, when mixed with sodium carbonate and heated, - give mercurous or mercuric carbonates; these decompose on being - heated, forming carbonic anhydride, oxygen, and vapours of - mercury. - - [18] Spring (1888) showed that, solid dry HgCl is gradually decomposed - in contact with metallic copper. According to the determinations - of Thomsen, the formation of a gram of mercurial compounds from - their elements develops the following amounts of heat (in - thousands of units): Hg_{2} + O, 42; Hg + O, 31; Hg + S, 17; Hg + - Cl, 41; Hg + Br, 34; Hg + I, 24; Hg + Cl_{2}, 63; Hg + Br_{2}, 51; - Hg + I_{2}, 34; Hg + C_{2}N_{2}, 19. These numbers are less than - the corresponding ones for potassium, sodium, calcium, barium, and - for zinc and cadmium--for instance, Zn + O, 85; Zn + Cl_{2}, 97; - Zn + Br_{2}, 76; Zn + I_{2}, 49; Cd + Cl_{2}, 93; Cd + Br_{2}, 75; - Cd + I_{2}, 49. - - [19] This salt easily forms the crystallo-hydrate HgNO_{3},H_{2}O, - corresponding with ortho-nitric acid, H_{3}NO_{4} (the terms - ortho-, pyro-, and meta-acids are explained in the chapter on - Phosphorus), with the substitution of Hg for H. In an aqueous - solution this salt can only be preserved in the presence of free - mercury, otherwise it forms basic salts, which will be mentioned - hereafter (Chapter VI., Note 59). - - [20] Mercuric nitrate, Hg(NO_{3})_{2},8H_{2}O, crystallises from a - concentrated solution of mercury in an excess of boiling nitric - acid. Water decomposes this salt; at the ordinary temperature - crystals of a basic salt of the composition - Hg(NO_{3})_{2},HgO,2H_{2}O are formed, and with an excess of water - the insoluble yellow basic salt Hg(NO_{3})_{2},H_{2}O,2HgO. These - three salts correspond with the type of ortho-nitric acid, - (H_{3}NO_{4})_{2}, in which mercury is substituted for 1, 2 and 3 - times H_{2}. As all these salts still contain water, it is - possible that they correspond with the tetrahydrate = N_{2}O_{5} + - 4H_{2}O = N_{2}O(OH)_{8} if ortho-nitric acid = N_{2}O_{5} + - 3H_{2}O = 2NO(OH)_{3}. - - [21] To obtain the mercuric salt a large excess of strong sulphuric - acid must be taken and strongly heated. With a small quantity of - water colourless crystals of HgSO_{4},H_{2}O may be obtained. An - excess of water, especially when heated, forms the basic salt (as - in Note 20), HgSO_{4},2HgO, which corresponds with trihydrated - sulphuric acid, SO_{3} + 3H_{2}O = S(OH)_{6}, with the - substitution of H_{6} by 3Hg, which in mercuric salts is - equivalent to H_{6}. Le Chatelier (1888) gives the following ratio - between the amounts of equivalents per litre: - - HgSO_{4} 0·318 0·890 1·80 2·02 - SO_{3} 0·752 1·42 2·10 2·40 - - --that is, the relative amount of free acid decreases as the - strength of the solution increases. - - [22] The question of the molecular weight of calomel--that is, whether - the mercury in the salts of the suboxide is monatomic or - diatomic--long occupied the minds of chemists, although it is not - of very great importance. It is only recently (1894) that this - question can be considered as answered, thanks to the researches - of V. Meyer and Harris, in favour of diatomicity--that is, that - calomel is analogous to peroxide of hydrogen and contains - Hg_{2}Cl_{2} (like O_{2}H_{2}) in its molecule if corrosive - sublimate contains HgCl_{2} (like water OH_{2}). As a matter of - fact, direct experiment gives the vapour density of calomel as - about 118--that is, indicates that its molecule contains HgCl, - whilst the molecule of the sublimate, judging also by the vapour - density (nearly 136), contains HgCl_{2}; it might therefore be - concluded that the mercury in the suboxide is not only monovalent - (corresponding to H) but also monatomic, whilst in the oxide it is - divalent and diatomic. Instances of a variable atomicity, as shown - by the vapour density, are known in N_{2}O, NO, and NH_{3}, CO and - CO_{2}, PCl_{3} and PCl_{5}, and it might therefore be supposed - that the present was a similar instance. But there are also - instances of a variable equivalency which do not correspond to a - variation of atomicity--for example, OH_{2} (water) and OH - (peroxide of hydrogen), CH_{4} (methane), C_{2}H_{5} (ethyl), and - CH_{2} (ethylene), &c. Here, according to the law of substitution, - the residues of OH_{2} and CH_{4} combine together and give - molecules; OHOH = O_{2}H_{2} (peroxide of hydrogen) and - CH_{3}CH_{3} = C_{2}H_{6} (ethane), &c. The same may be assumed - also to be the relation of calomel to sublimate; the residue HgCl, - which is combined with Cl in sublimate, corresponds to HgCl_{2}, - and in calomel it may be supposed that this residue is combined - with itself, forming the molecule Hg_{2}Cl_{2}. On this view of - the composition of the molecule of calomel it would follow that in - the state of vapour it breaks up into two molecules, HgCl_{2} and - Hg, when the vapour density would be about 118 (because that of - sublimate is about 136 and that of mercury about 100), and that in - cooling this mixture (like a mixture of HCl and NH_{3}) again - gives Hg_{2}Cl_{2}. It was therefore necessary to prove that - calomel is decomposed in the state of vapour. This was not - effected for a long time, although Odling, as far back as the - thirties, showed that gold becomes amalgamated (_i.e._ absorbs - metallic mercury) in the vapour of calomel, but not in the vapour - of sublimate. Recently, however, V. Meyer and Harris (1894) have - shown that a greater amount of the vapour of mercury than of - calomel passes (at about 465°) through a porous clay cell, - containing calomel. This proves that the vapour of calomel - contains a mixture of the vapours of Hg and HgCl_{2}, as would - follow from the second hypothesis. Moreover, on introducing a - heated piece of KHO into the vapour of calomel, Meyer observed the - formation, not of suboxide (black), but of oxide of mercury - (yellow). Therefore the molecular formula of calomel must be taken - as Hg_{2}Cl_{2} (and not HgCl). - -The lowest oxygen compound of mercury--that is, _mercurous oxide_, -Hg_{2}O--does not seem to exist, for the substance precipitated in the -form of a black mass by the action of alkalis on a solution of mercurous -salts gradually separates on keeping into the yellow mercuric oxide and -metallic mercury, as does also a simple mechanical mixture of oxide, HgO, -with mercury (Guibourt, Barfoed). The other compound of mercury with -oxygen is already known to us as _mercuric oxide_, HgO, obtained in the -form of a red crystalline substance by the oxidation of mercury in the -air, and precipitated as a yellow powder by the action of sodium -hydroxide on solutions of salts of the type HgX_{2}. In this case it is -amorphous and more amenable to the action of various reagents (Chap. XI., -Note 32) than when it is in the crystalline state. Indeed, on -trituration, the red oxide is changed into a powder of a yellow colour. -It is very sparingly soluble in water, and forms an alkaline solution -which precipitates magnesia from the solution of its salts. - -Mercury combines directly with chlorine, and the first product of -combination is _calomel_ or _mercurous chloride_, Hg_{2}Cl_{2}. This is -obtained, as above stated, in the form of a white precipitate by mixing -solutions of mercurous salts with hydrochloric acid or with metallic -chlorides. A precipitate of calomel is also obtained by reducing a -boiling aqueous solution of corrosive sublimate, HgCl_{2}, with -sulphurous anhydride. It is likewise produced by heating corrosive -sublimate with mercury.[22 bis] Calomel may be distilled (although in so -doing it decomposes and recombines on cooling from a state of vapour); -its vapour density equals 118 compared with hydrogen (= 1) (_see_ Note -23). In the solid state its specific gravity is 7·0; it crystallises in -rhombic prisms, is colourless, but has a yellowish tint, turns brown from -the action of light, and when boiled with hydrochloric acid decomposes -into mercury and corrosive sublimate. It is used as a strong purgative. -_Corrosive sublimate or mercuric chloride_, HgCl_{2}, can be obtained -from or converted into calomel by many methods.[23] An excess of chlorine -(for instance, _aqua regia_) converts calomel and also mercury into -corrosive sublimate. It owes its name corrosive sublimate to its -volatility, and, in medicine up to the present day, it is termed -_Mercurius sublimatus seu corrosivus_. The vapour density, compared with -hydrogen (= 1) is 135; therefore its molecule contains HgCl_{2}. It forms -colourless prismatic crystals of the rhombic system, boils at 307°, and -is soluble in alcohol. It is usually prepared by subliming a mixture of -mercuric sulphate with common salt, HgSO_{4} + 2NaCl = Na_{2}SO_{4} + -HgCl_{2}. Corrosive sublimate combines with mercuric oxide, forming an -oxychloride or basic salt,[23 bis] of the composition HgCl_{2},2HgO -(magnesium and zinc form similar compounds). This compound is obtained by -mixing a solution of corrosive sublimate with mercuric oxide or with a -solution of sodium bicarbonate. In general, with both mercurous and -mercuric salts, there is a marked tendency to form basic salts.[24] - - [22 bis] Calomel (in Japanese 'Keyfun') has been prepared in Japan (and - China) for many centuries, by heating mercury in clay crucibles - with sea salt, which contains MgCl_{2} and gives HCl. The vapour - of the mercury reacts with this HCl and the oxygen of the air and - forms calomel: 2Hg + 2HCl + O = Hg_{2}Cl_{2} + H_{2}O. The calomel - collects on the lid of the crucible in the form of a sublimate - (Divers, 1894). - - [23] HgCl_{2} is partially converted into calomel even in the act of - dissolving in ordinary water, especially under the action of light. - - [23 bis] As feebly energetic bases (for instance, the oxides MgO, ZnO, - PbO, CuO, Al_{2}O_{3}, Bi_{2}O_{3}, &c.), mercuric oxide (_see_ - Notes 20, 21) and mercurous oxide easily give basic salts, which - are usually directly formed by the action of water on the normal - salt, according to the general equation (for mercuric compounds, - RX_{2}): - - _n_RX_{2} + _m_H_{2}O = 2_m_HX + (_n_-_m_)RX_{2}_m_RO - neutral salt water acid basic salt - - or else are produced directly from the normal salt and the oxide - or its hydroxide. Thus mercurous nitrate, when treated with water, - forms basic salts of the composition 6(HgNO_{3}),Hg_{2}O,H_{2}O, - 2(HgNO_{3}),Hg_{2}O,H_{2}O, and 3(HgNO_{3}),Hg_{2}O,H_{2}O, the - first two of which crystallise well. Naturally it is possible - either to refer similar salts to the type of hydrates--for - instance, the second salt to the hydrate N_{2}O_{5},4H_{2}O--or to - view it as a compound, HgNO_{3},HgHO, but our present knowledge of - basic salts is not sufficiently complete to admit of - generalisations. However, it is already possible to view the - subject in the following aspects: (1) basic salts are principally - formed from feeble bases; (2) certain metals (mentioned above) - form them with particular ease, so that one of the causes of the - formation of many basic salts must depend on the property of the - metal itself; (3) those bases which readily form basic salts as a - rule also readily form double salts; (4) in the formation of basic - salts, as also everywhere in chemistry, where sufficient facts - have accumulated, we clearly see the conditions of equally - balanced heterogeneous systems, such as we saw, for instance, in - the formation of double salts, crystallo-hydrates, &c. - - The mercuric salts often form double salts (confirming the third - thesis), and mercuric chloride easily combines with ammonia, - forming Hg(NH_{4})_{2}Cl_{4}, or in general HgCl_{2}_n_MCl. If a - mixture of mercurous and potassium sulphates be dissolved in - dilute sulphuric acid, the solution easily yields large colourless - crystals of a double salt of the composition - K_{2}SO_{4},3HgSO_{4},2H_{2}O. Boullay obtained crystalline - compounds of mercuric chloride with hydrochloric acid, and - mercuric iodide with hydriodic acid; and Thomsen describes the - compound HgBr_{2},HBr,4H_{2}O as a well-crystallised salt, melting - at 13°, and having, in a molten state, a specific gravity 3·17 and - a high index of refraction. Moreover, the capacity of salts for - forming basic compounds has been considerably cleared up since the - investigation (by Würtz, Lorenz, and others) of glycol, - C_{2}H_{4}(OH)_{2} (and of polyatomic alcohols resembling it), - because the ethers C_{2}H_{4}X_{2}, corresponding with it, are - capable of forming compounds containing - C_{2}H_{4}X_{2}_n_C_{2}H_{4}O. - - On the other hand, there is reason to think that the property of - forming basic salts is connected with the polymerisation of bases, - especially colloidal ones (_see_ the chapter on Silica, Lead - Salts, and Tungstic Acid). - - [24] Mercuric iodide, HgI_{2}, is obtained first as a yellow, and then - as a red, precipitate on mixing solutions of mercuric salts and - potassium iodide, and is soluble in an excess of the latter (in - consequence of the formation of the double salt, HgKI_{3}); of - ammonium chloride (for a similar reason), &c. It crystallises at - the ordinary temperature in square prisms of a red colour. On - being heated, these change into yellow rhombic crystals, - isomorphous with mercuric chloride. This yellow form of mercuric - iodide is very unstable, and when cooled and triturated easily - again assumes the more stable red form. When fused, a yellow - liquid is obtained. _Mercuric cyanide_, Hg(CN)_{2}, forms one of - the most stable metallic cyanides. It is obtained by dissolving - mercuric oxide in prussic acid, and by boiling Prussian blue with - water and mercuric oxide, ferric oxide being then obtained in the - precipitate. Mercuric cyanide is a colourless crystalline - substance, soluble in water, and distinguished by its great - stability; sulphuric acid does not liberate prussic acid from it, - and even caustic potash does not remove the cyanogen (a complex - salt is probably produced), but the halogen acids disengage HCN. - Like the chloride, it combines with mercuric oxide, forming the - oxycyanide, Hg_{2}O(CN)_{2}, and it shows a very marked tendency - to form double compounds--for example, K_{2}Hg(CN)_{4}. The alkali - chlorides and iodides form similar compounds--for instance, the - salt HgKI(CN)_{2} crystallises very well, and is produced by - directly mixing solutions of potassium iodide and mercuric - cyanide. - - Wells (1889) and Vare obtained and investigated many such double - salts, and showed the possibility of the formation, not only of - HgCl_{2}MCl and HgCl_{2}2MCl where M is a metal of the - alkalis--for example, Cs--but also of HgCl_{2}3MCl,2(HgCl_{2})MCl, - and in general _n_HgX_{2}_m_MX, where X stands for various - haloids. - -Mercury has a remarkable power of forming very unstable compounds with -ammonia, in which the mercury replaces the hydrogen, and, if a mercuric -compound be taken, its atom occupies the place of two atoms of the -hydrogen in the ammonia. Thus Plantamour and Hirtzel showed that -precipitated mercuric oxide dried at a gentle heat, when continuously -heated (up to 100°-150°) in a stream of dry ammonia, leaves a brown -powder of _mercuric nitride_, N_{2}Hg_{3}, according to the equation 3HgO -+ 2NH_{3} = N_{2}Hg_{3} + 3H_{2}O.[24 bis] This substance, which is -attacked by water, acids, and alkalis (giving a white powder), is very -explosive when struck or rubbed, evolving nitrogen, proving that the bond -between the mercury and the nitrogen is very feeble.[25] By the action of -liquefied ammonia on yellow mercuric oxide Weitz also obtained an -explosive compound, dimercurammonium hydroxide, N_{2}Hg_{4}O, which -corresponds with an ammonium oxide, (NH_{4})_{2}O, in which the whole of -the hydrogen is replaced by mercury. A solution of ammonia reacts with -mercuric oxide, forming the hydroxide, NHg_{2}.OH, to which a whole -series of salts, NHg_{2}X, correspond; these are generally insoluble in -water and capable of decomposing with an explosion. But salts of the same -type, but with one atom of mercury, NH_{2}HgX, are more frequently and -more easily formed; they were principally studied by Kane, although known -much earlier. Thus, if ammonia be added to a solution of corrosive -sublimate (or, still better, in reverse order), a precipitate is obtained -known as white precipitate (_Mercurius præcipitatus albus_) or -_mercurammonium chloride_, NH_{2}HgCl, which may also be regarded not -only as sal-ammoniac with the substitution of H_{2} by mercury, but also -as HgX_{2}, where one X represents Cl and the other X represents the -ammonia radicle, HgCl_{2} + 2NH_{3} = NH_{2}.HgCl + NH_{4}Cl. When -heated, mercurammonium chloride decomposes, yielding mercurous chloride; -when heated with dry hydrochloric acid it forms ammonium chloride and -mercuric chloride. Other simple and double salts of mercurammonium, -NH_{2}HgX, are also known. Pici (1890) showed that all the compounds -HgH_{2}NX may be regarded as compounds of the above-named Hg_{2}NX with -NH_{4}X because their sum equals 2HgH_{2}X.[25 bis] - - [24 bis] _See_ Chapter XIX., Note 6 bis: Hg_{3}P_{2}. In studying the - metallic nitrides it is necessary to keep the corresponding - phosphides in mind. - - [25] Hg_{3}N_{2} is similar in composition to Mg_{3}N_{2}, &c. (Chapter - XIV.) The readiness with which mercuric nitride explodes shows - that the connection between the nitrogen and the mercury is very - unstable, and explains the circumstance that the so-called - _mercury fulminate_, or _fulminating mercury_, is an exceedingly - explosive substance. This substance is prepared in large - quantities for explosive mixtures; it enters into the composition - of percussion caps, which explode when struck, and ignite - gunpowder. Mercury fulminate was discovered by Howard, and from - that time has been prepared in the following way: one part of - mercury is dissolved in twelve parts of nitric acid, of sp. gr. - 1·36, and when the whole of the mercury is dissolved, 5·5 parts of - 90 p.c. alcohol are added, and the mass is shaken. A reaction then - commences, accompanied by a rise in temperature due to the - oxidation of the alcohol. As a matter of fact, many oxidation - products are produced during the action of the nitric acid on the - alcohol (glycolic acid, ethers, &c.) When the reaction becomes - tolerably vigorous, the same quantity of alcohol is added as at - the commencement, when a grey precipitate of the fulminate - separates. This salt has the composition C_{2}Hg(NO_{2})N. It - explodes when struck or heated. The mercury in it may be replaced - by other metals--for instance, copper or zinc, and also silver. - The silver salt, C_{2}Ag_{2}(NO_{2})N, is obtained in a precisely - analogous manner, and is even more explosive. Under the action of - alkali chlorides, only half the silver is replaced by the alkali - metal, but if the whole of the silver be replaced by an alkali - metal, then the salt decomposes. This is evidently because - combinations of this kind proceed in virtue of the formation of - substances in which mercury, and metals akin to it, are connected - in an unstable way with nitrogen. Potassium and other light metals - are incapable of entering into such connection and therefore, the - substitution of potassium for mercury entails the splitting-up of - the combination. Investigations of the fulminates were carried on - by Gay-Lussac and Liebig, but only the investigations of L. N. - Shishkoff fully cleared up the composition and relation of these - substances to the other carbon compounds. Shishkoff showed that - fulminates correspond with the nitro-acid, C_{2}H_{2}(NO_{2})N. - The explosiveness of the group depends partly on its containing at - the same time NO_{2} and carbon; we already know that all such - nitrogen compounds are explosive. If we imagine that the NO_{2} is - replaced by hydrogen, we shall have a substance of the composition - C_{2}H_{3}N. This is acetonitrile--that is, acetic acid + NH_{3} - - 2H_{2}O, or ethenyl nitrile, as shown in Chapter VI. The formation - of an acetic compound by the action of nitric acid on alcohol is - easily understood, because acetic acid is produced by the - oxidation of alcohol, and the production of the elements of - ammonia, indispensable for the formation of a nitrile, is - accounted for by the fact that nitric acid under the action of - reducing substances in many cases forms ammonia. Moreover a - certain analogy has been found between fulminating acid and - hydroxylamine, but details upon this subject must be looked for in - works on organic chemistry. The explosiveness of fulminating - mercury, the rapidity of its decomposition (gunpowder, and even - guncotton, burn more slowly and explode less violently), and the - force of its explosion, are such that a small quantity (loosely - covered) will shatter massive objects. - - The investigations of Abel on the communication of explosion from - one substance to another are remarkable. If guncotton be ignited - in an open space, it burns quietly; but if fulminating mercury be - exploded by the side of it, the decomposition of the guncotton is - effected instantaneously, and it then shatters the objects upon - which it lies, so rapid is the decomposition. Abel explains this - by supposing that the explosion of the fulminating salt brings the - molecules of guncotton into a uniform or as it were harmonious - state of vibration, which causes the rapid decomposition of the - whole mass. This rapid decomposition of explosive substances - defines the distinction between explosion and combustion. Besides - this, Berthelot showed that from that form of powerful molecular - concussion which takes place during the explosion of fulminating - mercury, the state of strain and stability of equilibrium of - substances which are endothermal, or capable of decomposing with - the disengagement of heat--for instance, cyanogen, nitro - compounds, nitrous oxide, &c.--is generally destroyed. Thorpe - showed that carbon bisulphide, CS_{2}, also an endothermal - substance, decomposes into sulphur and charcoal, when fulminating - mercury is exploded in contact with it. - - [25 bis] The capacity for replacing hydrogen in chloride of ammonium by - metals also belongs to Zn and Cd. Kvasnik (1892), by the action of - ammonia upon alcoholic solutions of CdCl_{2} and ZnCl_{2}, - obtained substances of the general formula M(NH_{3}Cl)_{2}, formed - as it were from two molecules of sal-ammoniac by the substitution - of two atoms of hydrogen by a diatomic metal. These substances - appear as white, finely crystalline powders. Under the action of - heat half the ammonia passes off, and a compound of the - composition MClNH_{3}Cl is formed. The compounds of cadmium and - zinc are distinguished from each other by the former being more - volatile than the latter. - - We may further remark that in the series Mg, Zn, Cd, and Hg the - capacity to form double salts of diverse composition increases - with the atomic weight. Thus, according to Wells and Walden's - observations (1893), the ratio _n_ : _m_ for the type - _n_MCl_m_RCl_{2} (M = K, Li, Na ... R = Mg, Zn ...) is for Mg 1 : - 1, for Zn 3 : 1, 2 : 1, and 1 : 1; for Cd, besides this, salts are - known with the ratio 4 : 1, and for Hg 3 : 1, 2 : 1, 1 : 1, 2 : 3, - 1 : 2, and 1 : 5. - -Mercury as a liquid metal is capable of dissolving other metals and -forming metallic solutions. These are generally called 'amalgams.' The -formation of these solutions is often accompanied by the development of a -large amount of heat--for instance, when potassium and sodium are -dissolved (Chapter XII., Note 39); but sometimes heat is absorbed, as, -for instance, when lead is dissolved. It is evident that phenomena of -this kind are exceedingly similar to the phenomena accompanying the -dissolution of salts and other substances in water, but here it is easy -to demonstrate that which is far more difficult to observe in the case of -salts: the solution of metals in mercury is accompanied by the formation -of definite chemical compounds of the mercury with the metals dissolved. -This is shown by the fact that when pressed (best of all in chamois -leather) such solutions leave solid, definite compounds of mercury with -metals. It is, however, very difficult to obtain them in a pure state, on -account of the difficulty of separating the last traces of mercury, which -is mechanically distributed between the crystals of the compounds. -Nevertheless, in many cases such compounds have undoubtedly been -obtained, and their existence is clearly shown by the evident crystalline -structure and characteristic appearance of many amalgams. Thus, for -instance, if about 2-1/2 p.c. of sodium be dissolved in mercury, a hard, -crystalline amalgam is obtained, very friable and little changeable in -air. It contains the compound NaHg_{5} (Chapter XII., Note 39). Water -decomposes it, with the evolution of hydrogen, but more slowly than other -sodium amalgams, and this action of water only shows that the bond -between the sodium and the mercury is weak, just like the connection -between mercury and many other elements--for instance, nitrogen. Mercury -directly and easily dissolves potassium, sodium, zinc, cadmium, tin, -gold, bismuth, lead, &c., and from such solutions or alloys it is in most -cases easy to extract definite compounds--thus, for instance, the -compounds of mercury and silver have the compositions HgAg and -Ag_{2}Hg_{3}. Objects made of copper when rubbed with mercury become -covered with a white coating of that metal, which slowly forms an -amalgam; silver acts in the same way, but more slowly, and platinum -combines with mercury with still greater difficulty. This metal only -readily forms an amalgam when in the form of a fine powder. If salts of -platinum in solution are poured on to an amalgam of sodium, the latter -element reduces the platinum, and the platinum separated is dissolved by -the mercury. Almost all metals readily form amalgams if their solutions -are decomposed by a galvanic current, where mercury forms the negative -pole. In this way an amalgam may even be made with iron, although iron in -a mass does not dissolve in mercury. Some amalgams are found in -nature--for instance, silver amalgams. Amalgams are used in considerable -quantities in the arts. Thus the solubility of silver in mercury is taken -advantage of for extracting that metal from the ore by means of -amalgamation, and for silvering by fire. The same is the case with gold. -Tin amalgam, which is incapable of crystallising and is obtained by -dissolving tin in mercury, composes the brilliant coating of ordinary -looking-glasses, which is made to adhere to the surface of the polished -glass by simply pressing by mechanical means sheets of tin foil bathed in -mercury on to the cleansed surface of the glass.[26] (_See_ 'The Nature -of Amalgams,' by W. L. Dudley; Toronto, 1889.) - - [26] I consider it appropriate here to call attention to the want of an - element (ekacadmium) between cadmium and mercury in the periodic - system (Chapter XV.) But as in the ninth series there is not a - single known element, it may be that this series is entirely - composed of elements incapable of existing under present - conditions. However, until this is proved in one way or another, - it may be concluded that the properties of ekacadmium will be - between those of cadmium and mercury. It ought to have an atomic - weight of about 155, to form an oxide EcO, a slightly stable oxide - Ec_{2}O. Both ought to be feeble bases, easily forming double and - basic salts. The volume of the oxide will be nearly 17·5, because - the volume of cadmium oxide is about 16, and that of mercuric - oxide 19. Therefore the density of the oxide will approach 171 ÷ - 17·5 = 9·7. The metal ought to be easily fusible, oxidising when - heated, of a grey colour, with a specific volume, about 14 - (cadmium = 13, mercury = 15), and, therefore, its specific gravity - (155 ÷ 14) will nearly = 11. Such a metal is unknown. But in 1879 - Dahl, in Norway, discovered in the island of Oterö, not far from - Kragerö, in a vein of Iceland spar in a nickel mine, traces of a - new metal which he called norwegium, and which presented a certain - resemblance to ekacadmium. Perfect purity of the metal was not - attained, and therefore the properties ascribed to norwegium must - be regarded as approximate, and likely to undergo considerable - alteration on further study. A solution of the roasted mineral in - acid was twice precipitated by sulphuretted hydrogen, and again - ignited; the oxide obtained was easily reduced. When the metal was - dissolved in hydrochloric acid largely diluted with water, and the - solution boiled, the basic salt was precipitated, and thus freed - from the copper which remained in the solution. The reduced metal - had a density 9·44, and easily oxidised. If the composition NgO be - assigned to the oxide, then Ng = 145·9. It fused at 254°; the - hydroxide was soluble in alkalis and potassium carbonate. In any - case, if norwegium is not a mixture of other metals, it belongs to - the uneven series, because the heavy metals of the even series are - not easily reducible. Brauner thinks that norwegium oxide is - Ng_{2}O_{3}, the atom Ng = 219, and places it in Group VI., series - 11, but then the feebly acid higher oxide, NgO_{3}, ought to be - formed. - - Amongst the metals accompanying zinc which have been named, but - not authentically separated, must be included the _actinium_ of - Phipson (1881). He remarked that certain sorts of zinc give a - white precipitate of zinc sulphide which blackens on exposure to - light and then becomes white in the dark again. Its oxide, closely - resembling in many ways cadmium oxide, is insoluble in alkalis, - and it forms a white metallic sulphide, blackening on exposure to - light. As no further mention has been made of it since 1882, its - existence must be regarded as doubtful. - - - - - CHAPTER XVII - - BORON, ALUMINIUM, AND THE ANALOGOUS METALS OF THE THIRD GROUP - - -If the elements of small atomic weight which we have hitherto discussed -be placed in order, it will be clearly seen that, judging by the formulæ -of their higher compounds, one element is wanting between beryllium and -carbon. For lithium gives LiX, beryllium forms BeX_{2}, and then comes -carbon giving CX_{4}. Evidently to complete the series we must look for -an element forming RX_{3}, and having an atomic weight greater than 9 and -less than 12. And _boron_ is such a one; its atomic weight is 11, and its -compounds are expressed by BX_{3}. Lithium and beryllium are metals; -carbon has no metallic properties; boron appears in a free state in -several forms which are intermediate between the metals and non-metals. -Lithium gives an energetic caustic oxide, beryllium forms a very feeble -base; hence one would expect to find that the oxide of boron, B_{2}O_{3}, -has still more feeble basic properties and some acid properties, all the -more as CO_{2} and N_{2}O_{5}, which follow after B_{2}O_{3} in their -composition and in the periodic system, are acid oxides. And, indeed, the -only known _oxide of boron_ exhibits a feeble basic character, together -with the properties of a feeble acid oxide. This is even seen from the -fact that a solution of boron oxide reddens blue litmus and acts on -turmeric paper as an alkali, and these reactions may be used for -determining the presence of B_{2}O_{3} in solutions. By themselves the -alkali borates have an alkaline reaction, which clearly indicates the -feeble acid character of boric acid. If they are mixed in solution with -hydrochloric acid, boric acid is liberated, and if a piece of turmeric -paper be immersed in this solution and then dried, the excess of -hydrochloric acid volatilises, while the boric acid remains on the paper -and communicates a _brown coloration_ to it, just like alkalis. - -Boron trioxide or boric anhydride enters into the composition of many -minerals, in the majority of cases in small quantities as an isomorphous -admixture, not replacing acids but bases, and most frequently alumina -(Al_{2}O_{3}), for as a rule the amount of alumina decreases as that of -the boric anhydride increases in them. This substitution is explained by -the similarity between the atomic composition of the oxides of aluminium -(alumina) and boron. The subdivision of oxides into basic and acid can in -no way be sharply defined, and here we meet with the most conclusive -proof of the fact, for the oxides of boron and aluminium belong to the -number of intermediate oxides, closely approaching the limit separating -the basic from the acid oxides. Their type (Chapter XV.) R_{2}O_{3} is -intermediate between those of the basic oxides R_{2}O and RO and those of -the acid oxides R_{2}O_{5} and RO_{3}. If we turn our attention to the -chlorides, we remark that lithium chloride is soluble in water, is not -volatile, and is not decomposed by water; the chlorides of beryllium and -magnesium are more volatile, and although not entirely, still are -decomposed by water; whilst the chlorides of boron and aluminium are -still more volatile and are decomposed by water. Thus the position of -boron and aluminium in the series of the other elements is clearly -defined by their atomic weights, and shows us that we must not expect any -new and distinct functions in these elements. - -Boron was originally known in the form of sodium borate, -Na_{2}B_{4}O_{7},10H_{2}O, or _borax_, or _tincal_, which was exported -from Asia, where it is met with in solution in certain lakes of Thibet; -it has also been discovered in California and Nevada, U.S.A.[1] Boric -acid was afterwards found in sea-water and in certain mineral springs.[2] -Its presence may be discovered by means of the green coloration which it -communicates to the flame of alcohol, which is capable of dissolving free -boric acid.[3] Many of the boron compounds employed in the arts are -obtained from the impure boric acid which is extracted in Tuscany from -the so-called _suffioni_. In these localities, which present the remains -of volcanic action, steam mixed with nitrogen, hydrogen sulphide, small -quantities of boric acid, ammonia, and other substances, issue from the -earth.[3 bis] The boric acid partially volatilises with the steam, for if -a solution of boric acid be boiled, the distillate will always contain a -certain amount of this substance.[4] - - [1] Borax is either directly obtained from lakes (the American lakes - give about 2,000 tons and the lakes of Thibet about 1,000 tons per - annum), or by heating native calcium borate (_see_ Note 2) with - sodium carbonate (about 4,000 tons per annum), or it is obtained - (up to 2,000 tons) from the Tuscan impure boric acid and sodium - carbonate (carbonic anhydride is evolved). Borax gives - supersaturated solutions with comparative ease (Gernez), from which - it crystallises, both at the ordinary and higher temperatures, in - octahedra, containing Na_{2}B_{4}O_{7},5H_{2}O. Its sp. gr. is - 1·81. But if the crystallisation proceeds in open vessels, then at - temperatures below 56°, the ordinary prismatic crystallo-hydrate - B_{4}Na_{2}O_{7},10H_{2}O is obtained. Its sp. gr. is 1·71, it - effloresces in dry air at the ordinary temperature, and at 0° 100 - parts of water dissolve about 8 parts of this crystallo-hydrate, at - 50° 27 parts, and at 100° 201 parts. Borax fuses when heated, loses - its water and gives an anhydrous salt which at a red heat fuses - into a mobile liquid and solidifies into a transparent amorphous - _glass_ (sp. gr. 2·37), which before hardening acquires the pasty - condition peculiar to common molten glass. Molten borax dissolves - many oxides and on solidifying acquires characteristic tints with - the different oxides; thus oxide of cobalt gives a dark blue glass, - nickel a yellow, chromium a green, manganese an amethyst, uranium a - bright yellow, &c. Owing to its fusibility and property of - dissolving oxides, borax is employed in soldering and brazing - metals. Borax frequently enters into the composition of strass and - fusible glasses. - - [2] We may mention the following among the minerals which contain - boron: calcium borate, (CaO)_{3}(B_{2}O_{3})(H_{2}O)_{6}, found and - extracted in Asia Minor, near Brusa; _boracite_ (stassfurtite), - (MgO)_{6}(B_{2}O_{3})_{8},MgCl_{2}, at Stassfurt, in the regular - system, large crystals and amorphous masses (specific gravity - 2·95), used in the arts; _ereméeffite_ (Damour), AlBO_{3} or - Al_{2}O_{3}B_{2}O_{3}, found in the Adulchalonsk mountains in - colourless, transparent prisms (specific gravity 3·28) resembling - apatite; _datholite_, (CaO)_{2}(SiO_{2})_{2}B_{2}O_{3},H_{2}O; and - ulksite, or the boron-sodium carbonate from which a large quantity - of borax is now extracted in America (Note 1). As much as 10 p.c. - of boric anhydride sometimes enters into the composition of - tourmalin and axinite. - - [3] This green coloration is best seen by taking an alcoholic solution - of volatile ethyl borate, which is easily obtained by the action of - boron chloride on alcohol. - - [3 bis] P. Chigeffsky showed in 1884 (at Geneva) that in the - evaporation of saline solutions many salts are carried off by the - vapour--for instance, if a solution of potash containing about - 17-20 grams of K_{2}CO_{3} per litre be boiled, about 5 milligrams - of salt are carried off for every litre of water evaporated. With - Li_{2}CO_{3} the amount of salt carried over is infinitesimal, and - with Na_{2}CO_{3} it is half that given by K_{2}CO_{3}. The - volatilisation of B_{2}O_{3} under these circumstances is - incomparably greater--for instance, when a solution containing 14 - grams of B_{2}O_{3} per litre is boiled, every litre of water - evaporated carries over about 350 milligrams of B_{2}O_{3}. When - Chigeffsky passed steam through a tube containing B_{2}O_{3} at - 400°, it carried over so much of this substance that the flame of a - Bunsen's burner into which the steam was led gave a distinct green - coloration; but when, instead of steam, air was passed through the - tube there was no coloration whatever. By placing a tube with a - cold surface in steam containing B_{2}O_{3}, Chigeffsky obtained a - crystalline deposit of the hydrate B(OH)_{3} on the surface of the - tube. Besides this, he found that the amount of B_{2}O_{3} carried - over by steam increases with the temperature, and that crystals of - B(OH)_{3} placed in an atmosphere of steam (although perfectly - still) volatilise, which shows that this is not a matter of - mechanical transfer, but is based on the capacity of B_{2}O_{3} and - B(OH)_{3} to pass into a state of vapour in an atmosphere of steam. - - [4] How it is that these vapours containing boric acid are formed in - the interior of the earth is at present unknown. Dumas supposes - that it depends on the presence of _boron sulphide_, B_{2}S_{3} - (others think boron nitride), at a certain depth in the earth. This - substance may be artificially prepared by heating a mixture of - boric acid and charcoal in a stream of carbon bisulphide vapour, - and by the direct combination of boron and the vapour of sulphur at - a white heat. The almost non-crystalline compound B_{2}S_{3}, sp. - gr. 1·55, thus obtained is somewhat volatile, has an unpleasant - smell, and is very easily decomposed by water, forming boric acid - and hydrogen sulphide, B_{2}S_{3} + 3H_{2}O = B_{2}O_{3} + 3H_{2}S. - It is supposed that a bed of boron sulphide lying at a certain - depth below the surface of the earth comes into contact with sea - water which has percolated through the upper strata, becomes very - hot, and gives steam, hydrogen sulphide, and boric acid. This also - explains the presence of ammonia in the vapours, because the sea - water certainly passes through crevices containing a certain amount - of animal matter, which is decomposed by the action of heat and - evolves ammonia. There are several other hypotheses for explaining - the presence of the vapours of boric acid, but owing to the want of - other known localities the comparison of these hypotheses is at - present hardly possible. The amount of boric anhydride in the - vapours which escape from the Tuscan fumerolles and suffioni is - very inconsiderable, less than one-tenth per cent., and therefore - the direct extraction of the acid would be very uneconomical, hence - the heat contained in the discharged vapours is made use of for - evaporating the water. This is done in the following manner. - Reservoirs are constructed over the crevices evolving the vapours, - and the water of some neighbouring spring is passed into them. The - vapours are caused to pass through these reservoirs, and in so - doing they give up all their boric acid to the water and heat it, - so that after about twenty-four hours it even boils; still this - water only forms a very weak solution of boric acid. This solution - is then passed into lower basins and again saturated by the vapours - discharged from the earth, by which means a certain amount of the - water is evaporated and a fresh quantity of boric acid absorbed; - the same process is repeated in another reservoir, and so on until - the water has collected a somewhat considerable amount of boric - acid. The solution is drawn from the last reservoir A into settling - vessels B D, and then into a series of vessels _a_, _b_, _c_. In - these vessels, which are made of lead, the solution is also - evaporated by the vapours escaping from the earth, and attains a - density of 10° to 11° Baumé. It is allowed to settle in the vessel - C, in which it cools and crystallises, yielding (not quite pure) - crystalline boric acid. At temperatures above 100°, for instance, - with superheated steam, boric acid volatilises with steam very - easily. - - [Illustration: FIG. 81.--Extraction of boric acid in Tuscany.] - -If boric acid be introduced into an excess of a strong hot solution of -sodium hydroxide, then, on slowly cooling, the salt NaBO_{2},4H_{2}O -crystallises out. This salt contains an equivalent of Na_{2}O to one -equivalent B_{2}O_{3}. It might be termed a neutral salt did it not -possess strongly alkaline reactions and easily split up into the alkali -and the more stable borax or biborate of sodium mentioned above, which -contains 2B_{2}O_{3} to Na_{2}O.[5] This salt is prepared by the action -of boric acid on a solution of sodium carbonate. Borax may be perfectly -purified by crystallisation. If a saturated and hot solution of borax be -mixed with strong hydrochloric acid, common salt and a normal crystalline -hydrate of _boric acid_ are formed. The composition of this hydrate is -B(HO)_{3}, according to the form BX_{3}--that is, of the composition -B_{2}O_{3},3H_{2}O. This is the easiest method of obtaining pure boric -acid. The water is easily expelled from this hydrate; it loses half at -100° and the remainder on further heating, and the remaining B_{2}O_{3} -or boric anhydride fuses at 580° (according to Carnelley), forming at -first a ductile (easily drawn out into threads), tenacious mass and then -a colourless liquid solidifying to a transparent glass, which absorbs -moisture from the atmosphere and then becomes cloudy.[6] Only the -alkaline salts of boric acid are soluble in water, but all borates are -soluble in acids, owing to their easy decomposability and the solubility -of boric acid itself. Although boric anhydride, B_{2}O_{3}, absorbs -3H_{2}O from damp air, still in the presence of water it always[7] -combines with a less quantity of bases (borax only contains 1/6). -However, fused boric anhydride forms a crystalline compound with -magnesium of the same type as the hydrate (MgO)_{3}B_{2}O_{3} (Ebelmann), -and even with sodium it forms (Na_{2}O)_{3}B_{2}O_{3} or Na_{3}BO_{3} -(Benedict). As a rule, the salts of boric acid contain less base, -although they are all able to form saline compounds with bases when -fused. Generally, vitreous fluxes are formed by this means,[8] which when -fused recall ordinary aqueous solutions in many respects. Some of them -crystallise on solidifying, and then they have, like salts, a definite -composition. The property of boric anhydride of forming higher grades of -combination with basic oxides when fused explains the power of fused -borax to dissolve metallic oxides, and the experiments of Ebelmann on the -preparation of artificial crystals of the precious stones by means of -boric anhydride. Boric anhydride is, although with difficulty, volatile -at a high temperature, and therefore if it dissolves an oxide, it may be -partially driven off from such a solution by prolonged and powerful -ignition; in which case the oxides previously in solution separate out in -a crystalline form, and frequently in the same forms as those in which -they occur in nature--for example, crystals of alumina, which by itself -fuses with difficulty, have been obtained in this manner. It dissolves in -molten boric anhydride, and separates out in natural rhombohedric -crystals. In this way Ebelmann also obtained _spinel_--that is, a -compound of magnesium and aluminium oxides which occurs in nature.[9] - - [5] Metals, like Na, K, Li, give salts of the type of borax, MBO_{2} or - MH_{2}BO_{3}. A solution of borax, Na_{2}B_{4}O_{7}, has an - alkaline reaction, decomposes ammonia salts with the liberation of - ammonia (Bolley), absorbs carbonic anhydride like an alkali, - dissolves iodine like an alkali (Georgiewics), and seems to be - decomposed by water. Thus Rose showed that strong solutions of - borax give a precipitate of silver borate with silver nitrate, - whilst dilute solutions precipitate silver oxide, like an alkali. - Georgiewics even supposes (1888) boric anhydride to be entirely - void of acid properties; for all acids, on acting on a mixture of - solutions of potassium iodide and iodate, evolve iodine, but boric - acid does not do this. With dilute solutions of sodium hydroxide - Berthelot obtained a development of heat equal to 11-1/2 thousand - calories per equivalent of alkali (40 grams sodium hydroxide) when - the ratio Na_{2}O : 2B_{2}O_{3} (as in borax) was taken, and only 4 - thousand calories when the ratio was Na_{2}O : B_{2}O_{3}, whence - he concludes that water powerfully decomposes those sodium borates - in which there is more alkali than in borax. Laurent (1849) - obtained a sodium compound, Na_{2}O,4B_{2}O_{3},10H_{2}O, - containing twice as much boric anhydride as borax, by boiling a - mixture of borax with an equivalent quantity of sal-ammoniac until - the evolution of ammonia entirely ceased. - - Hence it is evident that feeble acids are as prone to, and as - easily, form acid salts (that is, salts containing much acid oxide) - as feeble bases are to give basic salts. These relations become - still clearer on an acquaintance with such feeble acids as silicic, - molybdic, &c. This variety of the proportions in which bases are - able to form salts recalls exactly the variety of the proportions - in which water combines with crystallo-hydrates. But the want of - sufficient data in the study of these relations does not yet permit - of their being generalised under any common laws. - - With respect to the feeble acid energy of boric anhydride I think - it useful to add the following remarks. Carbonic anhydride is - absorbed by a solution of borax, and displaces boric anhydride; but - it is also displaced by it, not only on fusion, but also on - solution, as the preparation of borax itself shows. Sulphuric - anhydride is absorbed by boric acid, forming a compound - B(HSO_{4})_{3}, where HSO_{4} is the radicle of sulphuric acid - (D'Ally). With phosphoric acid, boric acid forms a stable compound, - BPO_{4}, or B_{2}O_{3}P_{2}O_{5}, undecomposable by water, as - Gustavson and others have shown. With respect to tartaric acid, - boric anhydride is able to play the same part as antimonious oxide. - Mannitol, glycerol, and similar polyhydric alcohols also seem able - to form particularly characteristic compounds with boric anhydride. - All these aspects of the subject require still further explanation - by a method of fresh and detailed research. - - [6] Ditte determined the sp. gr.:-- - - 0° 12° 80° - B_{2}O_{3} 1·8766 1·8470 1·6988 - B(OH)_{3} 1·5463 1·5172 1·3828 - Solubility 1·95 2·92 16·82 - - The last line gives the solubility, in grams, of boric acid, - B(OH)_{3}, per 100 c.c. of water, also according to the - determinations of Ditte. - - [7] It is evident that, in the presence of basic oxides, water competes - with them, which fact in all probability determines both the amount - of water in the salts of boric acid as well as their decomposition - by an excess of water. In confirmation of the above-mentioned - competing action between water and bases, I think it useful to - point out that the crystallo-hydrate of borax containing 5H_{2}O - may be represented as B(HO)_{3}, or rather as B_{2}(OH)_{6}, with - the substitution of one atom of hydrogen by sodium, since - Na_{2}B_{4}O_{7},5H_{2}O = 2B_{2}(OH)_{5}(ONa). The composition of - the acid boric salts is very varied, as is seen from the fact that - Reychler (1893) obtained (Cs_{2}O)3B_{2}O_{3}, (Rb_{2}O)2B_{2}O_{3} - (corresponding to borax) and (Li_{2}O)B_{2}O_{3}, and that Le - Chatelier and Ditte obtained, for CaO, MgO, &c., (RO)B_{2}O_{3}, - (RO)_{2}3B_{2}O_{3}, (RO)2B_{2}O_{3}, (RO)_{2}B_{2}O_{3}, and even - (RO)_{3}B_{2}O_{3}. - - [8] A glass can only be formed by those slightly volatile oxides which - correspond with feeble acids, like silica, phosphoric and boric - anhydrides, &c., which themselves give glassy masses, like quartz, - glacial phosphoric acid, and boric anhydride. They are able, like - aqueous solutions and like metallic alloys, to solidify either in - an amorphous form or to yield (or even be wholly converted into) - definite crystalline compounds. This view illustrates the position - of solutions amongst the other chemical compounds, and allows all - alloys to be regarded from the aspect of the common laws of - chemical reactions. I have therefore frequently recurred to it in - this work, and have since the year 1850 introduced it into various - provinces of chemistry. - - [9] If boric acid in its aqueous solutions proves to be exceedingly - feeble, unenergetic, and easily displaced from its salts by other - acids, yet in an anhydrous state, as anhydride, it exhibits the - properties of an energetic acid oxide, and it _displaces_ the - anhydrides of other acids. This of course does not mean that the - acid then acquires new chemical properties, but only depends on the - fact that the anhydrides of the majority of acids are much more - volatile than boric anhydride, and therefore the salts of many - acids--even of sulphuric acid--are decomposed when fused with boric - anhydride. - - By itself boric acid is used in the arts in small quantity, chiefly - for the preservation of meat and fish (which must be afterwards - well washed in water) and of milk, and for soaking the wicks of - stearin candles; the latter application is based on the fact that - the wicks, which are made of cotton twist, contain an ash which is - infusible by itself but which fuses when mixed with boric acid. - -Free _boron_ was obtained (1809) by Davy, Gay-Lussac, and Thénard when -they obtained the metals of the alkalis, for boric anhydride when fused -with sodium gives up its oxygen to the sodium, and free boron is -liberated as an _amorphous_ powder like charcoal.[10] It is of a brown -colour, specific gravity 2·45 (Moissan), and when dry does not alter in -the air at the ordinary temperature; but it burns when ignited to 700°, -and in so doing combines not only with the oxygen of the air, but also -with the nitrogen. However, the combustion is never complete, because the -boric anhydride formed on the surface covers the remaining mass of the -boron, and so preserves it from the action of the oxygen. Acids, even -sulphuric (forming SO_{2}) and phosphoric (forming phosphorus), easily -oxidise amorphous boron, especially when heated, converting it into boric -acid. Alkalis have the same action on it, only in this case hydrogen is -evolved. Boron decomposes steam at a red heat, also with evolution of -hydrogen. - - [10] _Amorphous boron_ is prepared by mixing 100 parts of powdered - boric anhydride with 50 parts of sodium in small lumps; this - mixture is thrown into a powerfully heated cast-iron crucible, - covered with a layer of ignited salt, and the crucible covered. - Reaction proceeds rapidly; the mass is stirred with an iron rod, - and poured directly into water containing hydrochloric acid. The - action is naturally accompanied by the formation of sodium borate, - which is dissolved, together with the salt, by the water, whilst - the boron settles at the bottom of the vessel as an insoluble - powder. It is washed in water, and dried at the ordinary - temperature. Magnesium, and even charcoal and phosphorus, are also - able to reduce boron from its oxide. Boron, in the form of an - amorphous powder, very easily passes through filter-paper, remains - suspended in water, and colours it brown, so that it appears to be - soluble in water. Sulphur precipitated from solutions shows the - same (colloidal) property. When borax is fused with magnesium - powder, it gives a brown powder of a compound of boron and - magnesium, Mg_{2}B (Winkler, 1890), but when a mixture of 1 part - of magnesium and 3 parts of B_{2}O_{3} is heated to redness - (Moissan, 1892), it forms amorphous boron in the form of a - chestnut-coloured powder, which, after being washed with water, - hydrochloric and hydrofluoric acids, is fused again with - B_{2}O_{3} in an atmosphere of hydrogen in order to prevent the - access of the nitrogen of the air, which is easily absorbed by - incandescent amorphous boron. - - Sabatier (1891) considers that a certain amount of gaseous hydride - of boron is evolved in the action of hydrochloric acid upon the - alloys of magnesium and boron, because the gas disengaged burns - with a green flame. Still, the existence of hydride of boron - cannot be regarded as certain. - - Under the action of the heat of the electric furnace boron forms - with carbon a _carbide_, BC, as Mühlhäuser and Moissan showed in - 1893. - -Amorphous boron, like charcoal, dissolves in certain molten metals. The -property of fused _aluminium of dissolving boron_ in considerable -quantity is very striking; on cooling such a solution, the boron -partially combined with the aluminium separates out in a crystalline -form, and its properties are then exceedingly remarkable. The crystalline -boron may be obtained by heating (to 1,300°) the pulverulent boron with -aluminium in a well-closed crucible, the access of air being prevented as -far as possible. After cooling, crystals are observed on the surface of -the aluminium, and may easily be separated by dissolving the latter in -hydrochloric acid, which does not act on the crystals. The specific -gravity of the crystals is 2·68; they are partially transparent, but are -for the most part coloured dark brown; they contain about 4 p.c. of -carbon and up to 7 p.c. of aluminium, so that they cannot be considered -as pure boron. Nevertheless, the properties of this _crystalline_ -substance, which was obtained by Wöhler and Deville, are very remarkable. -It most closely resembles _the diamond in its properties_--in fact, these -crystals have the lustre and high refracting power proper to the diamond -only, whilst their hardness competes with that of the diamond. Their -powder polishes even the diamond, and like the diamond scratches the -sapphire and corundum. Crystalline boron is much more stable with respect -to chemical reagents than the amorphous variety, and as it resembles the -diamond, so amorphous boron, on the other hand, distinctly recalls -certain of the properties of charcoal; thus a certain resemblance exists -between boron and carbon in a free state, which is further justified by -the proximity of their positions in the periodic system. - -Among the other compounds of boron, those with nitrogen and the halogens -are the most remarkable. As already mentioned above, amorphous boron -combines directly with _nitrogen_ at a red heat. If it be heated in a -glass tube in a stream of nitric oxide, perfect combustion takes place, -5B + 3NO = B_{2}O_{3} + 3BN. If the residue be treated with nitric acid, -the boric anhydride dissolves, whilst the _boron nitride_ remains[11] as -an extremely light white powder, which is sometimes partially crystalline -and greasy to the touch, like talc. It is infusible and unchanged, even -at the melting-point of nickel. In general, it is remarkable for its -great stability with respect to chemical reagents. Nitric and -hydrochloric acids, as well as alkaline solutions, and hydrogen and -chlorine at a red heat, have no action on it. When fused with potash, it -evolves ammonia, and when ignited in steam it also yields ammonia: 2BN + -3H_{2}O = B_{2}O_{3} + 2NH_{3}.[12] - - [11] At first boron nitride was obtained by heating boric acid with - potassium cyanide or other cyanogen compounds. It may be more - simply prepared by heating anhydrous borax with potassium - ferrocyanide, or by heating borax with ammonium chloride. For this - purpose one part of borax is intimately mixed with two parts of - dry ammonium chloride, and the mixture heated in a platinum - crucible. A porous mass is formed, which after crushing and - treating with water and hydrochloric acid, leaves boron nitride. - _Boron fluoride_, BF, is known, corresponding to BN; this body was - obtained by Besson and Moissan (1891). The action of phosphorus - upon iodide of boron, BI_{3}, forms PBI_{2}, and when heated to - 500° in hydrogen it forms BP, which gives PH_{3} with fused KHO. - - [12] When fused with potassium carbonate it forms potassium cyanate, - BN + K_{2}CO_{3} = KBO_{2} + KCNO. All this shows that boron - nitride is a nitrile of boric acid, BO(OH) + NH_{3} - 2H_{2}O = - BN. The same is expressed by saying that boron nitride is a - compound of the type of the boron compounds BX_{3}, with the - substitution of X_{3} by nitrogen, as the trivalent radicle of - ammonia, NH_{3}. - -No less remarkable is the compound of boron with fluorine--_boron -fluoride_, BF_{3}. It is produced in many instances when compounds of -boron and of fluorine are brought together.[13] The most convenient -method of preparing it is by heating a mixture of calcium fluoride with -boric anhydride and sulphuric acid, 3CaF_{2} + B_{2}O_{3} + 3H_{2}SO_{4} -= 3CaSO_{4} + 3H_{2}O + 2BF_{3}.[14] It is a colourless liquefiable _gas_ -(the liquid boils at -100°), which on coming into contact with damp air -forms white fumes, owing to its combining with water. One volume of water -dissolves as much as 1,050 volumes of this gas (Bazaroff), forming a -liquid which disengages boron fluoride when heated, and distils over -unaltered. Boron fluoride chars organic matter, owing to its taking up -the water from it, and in this respect it acts like sulphuric acid. The -behaviour of boron fluoride with water must be understood as a reversible -reaction, since with water it yields hydrofluoric and boric acids, whilst -they, acting on one another, re-form boron fluoride and water. A state of -equilibrium is set up between these four substances (and between two -reversible reactions) which is distinctly dependent on the mass of the -water.[14 bis] When boron fluoride is in great excess, the equilibrated -system, which is capable of distilling over (sp. gr. of the liquid 1·77), -has a composition BF_{3},2H_{2}O (or B_{2}O_{3},H_{2}O,6HF). It has also -its corresponding salts.[15] It is a caustic liquid, having the -properties of a powerful acid; but it does not act on glass, which shows -that there is no free hydrofluoric acid present. Under the action of -water this system changes, with the formation of boric acid and -hydroborofluoric acid (HBF_{4}) according to the equation -4BF_{3}H_{4}O_{2} = 3HBF_{4} + BH_{3}O_{3} + 5H_{2}O.[16] This -hydroborofluoric acid has its corresponding salts--for instance, KBF_{4}. -On evaporating the aqueous solution this free acid decomposes, with the -evolution of hydrofluoric acid, and a stable system is again obtained: -2HBF_{4} + 5H_{2}O = B_{2}F_{6}H_{10}O_{5} + 2HF. The resultant solution -(containing 2BF_{3},5H_{2}O, sp. gr. 1·58), which is identical with that -formed by the evaporation of a solution of boric acid with hydrofluoric -acid, again only contains a compound of boron fluoride with water. -Probably there are various other possible and more or less stable states -of equilibrium and definite compounds of boron fluoride, hydrofluoric -acid, and water. - - [13] Boron fluoride is frequently evolved on heating certain compounds - occurring in nature containing both boron and fluorine. If calcium - fluoride is heated with boric anhydride, calcium borate and boron - fluoride are formed, and the latter, as a gas, is volatilised: - 2B_{2}O_{3} + 3CaF_{2} = 2BF_{3} + Ca_{3}B_{2}O_{6}. The calcium - borate, however, retains a certain amount of calcium fluoride. - - [14] In order to avoid the formation of silicon fluoride the - decomposition should not be carried on in glass vessels, which - contain silica, but in lead or platinum vessels. Boron fluoride by - itself does not corrode glass, but the hydrofluoric acid liberated - in the reaction may bring a part of the silica into reaction. - Boron fluoride should be collected over mercury, as water acts on - it, as we shall see afterwards. - - [14 bis] It appears to me that from this point of view it is possible - to understand the apparently contradictory results of different - investigators, especially those of Gay-Lussac (and Thénard), Davy, - Berzelius, and Bazaroff. In the form in which the reaction of - BF_{3} on water is given here, it is evident that the act of - solution in water is accompanied by complex but direct chemical - transformations, and I think that this example should prove the - justness of those observations upon the nature of solutions which - are given in Chapter I. - - [15] They are called fluoborates. They may be prepared directly from - fluorides and borates. Such compounds of halogens with oxygen - salts are known in nature (for instance, apatite and boracite), - and may be artificially prepared. The composition of the - fluoborates--for example, K_{4}BF_{3}O_{2}--may be expressed as - that of a double salt, BO(OK),3KF. If an excess of water - decomposes them (Bazaroff), this does not prove that they do not - exist as such, for many double salts are decomposed by water. - - [16] Fluoboric acid contains boron fluoride and water, hydrofluoboric - acid, boron fluoride, and hydrofluoric acid. It is evident that on - the one side the competition between water and hydrofluoric acid, - and, on the other hand, their power to combine, are among the - forces which act here. From the fact that hydroborofluoric acid, - HBF_{4}, can only exist in an aqueous solution, it must be assumed - that it forms a somewhat stable system only in the presence of - 3H_{2}O. - -Nothing of this kind occurs with boron chloride, because hydrochloric -acid does not act on boric acid. However, amorphous boron at 400° burns -in chlorine, and at 410° forms _boron chloride_, BCl_{3}. The boron burns -in the chlorine, forming a gas which, in a freezing mixture, condenses -into a liquid boiling at 17°, and gives up its excess of chlorine, if -there be any, to mercury. The specific gravity of this liquid is 1·42 at -6°. Boron chloride may also be directly obtained from boric anhydride by -the simultaneous action of charcoal and chlorine at a high temperature: -B_{2}O_{3} + 3C + 3Cl_{2} = 2BCl_{3} + 3CO. It is also obtained by the -action of phosphoric chloride on boric anhydride in a closed tube at 200° -It is completely decomposed by water, like the chloranhydride of an acid, -boric acid being formed; hence it fumes in the air: 2BCl_{3} + 6H_{2}O = -2BH_{3}O_{3} + 6HCl. Boron forms with bromine a similar compound, -BBr_{3}, specific gravity at 6° = 2·64, boiling at 90°. The vapour -densities of the fluoride, chloride, and bromide of boron show that they -contain three atoms of the halogen in the molecule--that is, that boron -is a trivalent element forming BX_{3}.[16 bis] - - [16 bis] Iodide of boron, BI_{3}, was obtained by Moissan (1891), by - heating a mixture of the vapours of HI and BCl_{3} in a tube, or - by the action of iodine vapour (at 750°) or HI upon amorphous - boron. BI_{3} is a solid substance which dissolves in benzol and - CS_{2}, reacts with water, melts at 43°, boils at 210°, has a - density 3·3 at 50°, and partially decomposes in the light. Besson - (1891) obtained BIBr_{2} (boiling at 125°), and BI_{2}Br (boiling - at 180°) by heating (300-400°) a mixture of the vapours of HI and - BBr_{3}, and showed that NH_{3} combines with BBr_{3} and BI_{3} - in various proportions. - -As in the first group lithium is followed by sodium, giving a more basic -oxide, so in the second group beryllium is followed by magnesium, and so -also in the third group there is, besides the lightest element, boron, -whose basic character is scarcely defined, _aluminium_, Al = 27, whose -oxide, alumina, has somewhat distinct basic properties, which, although -not so powerful as in magnesium oxide, are more distinct than in boric -anhydride. Among the elements of the third group, aluminium is the most -widely distributed in nature; it will be sufficient to mention that it -enters into the composition of clay to demonstrate the universal -distribution of aluminium in the earth's crust. - -Alumina is so named from its being the metal of alums (_alumen_). - -_Clay_, which is so widely distributed and familiar to everybody, is the -insoluble residue obtained after the action of water containing carbonic -acid on many rocks, and especially on the felspars contained in some of -them. Felspar is a compound containing potash or soda, alumina, and -silica. The primary rocks, like granite, contain many similar compounds -(_see_ Chapter XVIII.: Felspars). Felspar is acted on by water containing -carbonic acid, all the alkalis (potash and soda), and a portion of the -silica passing into the water as substances which are soluble and carried -away by it, whilst the alumina and silica left from the felspar remain on -the spot where the solution has taken place. This is the original method -of the formation of clay in its primary deposits among rocks along whose -crevices the atmospheric water has permeated. Such primary deposits often -contain a white pure clay, termed _kaolin_ or _porcelain clay_. But such -clay is a rarity, because the conditions for its formation are rarely met -with. The water, whilst acting chemically on rocks, at the same time -destroys them _mechanically_, and carries off the finely divided residues -of disintegration with it. Clay is most easily subjected to this -mechanical action of water, because it is composed of grains of -exceedingly small size and void of any visible crystalline structure, -which easily remain suspended in water. The cloudy water of running -mountain streams generally contains particles of clay in suspension, -owing to the above-described chemical and mechanical action of the water -on the minerals contained in the mountain rocks. Together with these -minute particles of clay the water carries away the coarser components on -which it is not able to act--for example, splinters of rock, grains of -mica, quartz, &c. They were originally held together by those minerals -which form clay. When the water acts on these binding minerals, a sandy -mass is formed which water bears away. The cloudy water in which the -particles of clay and sand are held in suspension carries them to, and -deposits them at, the estuaries of rivers, lakes, seas, and oceans. The -coarser particles are first deposited and form sand and similar -disintegrated rocky matter, whilst the clay, owing to its finely divided -state, is carried on further, and is only deposited in the still parts of -the rivers, lakes, &c. Such disintegrations of rocks and separations of -clay from sand have been gradually going on during the millions of years -of the earth's existence, and are now proceeding, and have been the cause -of the formation of the immense deposits of sandstone and clay now -forming a part of the earth's strata. Such beds of clay may have been -transferred by currents and streams from one locality to another, so that -we must distinguish between primary and secondary deposits of clay. In -places these beds of clay have, owing to long exposure under water, and -perhaps partially owing to the action of heat, undergone compression, and -have formed the rocky masses known as clay slates and schists, which -sometimes form entire mountains. Roofing slates belong to this class of -rocks. - -From what has been said above it will be evident that these deposits can -never consist of a chemically pure and homogeneous substance, but will -contain all kinds of extraneous insoluble finely divided matter, and -especially sand--that is, fragments of rock, chiefly quartz (SiO_{2}). It -is, however, possible to considerably purify clay from these impurities, -owing to the fact that they are the result of mechanical disintegration, -whilst the clay has been formed as a residue of the chemical alteration -of rocky matter, and therefore its particles are incomparably more minute -than the particles of sand and other rock fragments mixed with it. This -difference in the size of the grains causes the clay to remain longer in -suspension when shaken up in water than the coarser grains of sand. If -clay be shaken up in water, and especially if it be previously boiled in -it, and if after the first portion has settled the cloudy water be -decanted, it will give a deposit of a very much purer clay than the -original. This method is employed for purifying kaolin designed for the -manufacture of the best kinds of china, earthenware, &c. A similar method -is also employed in the investigation of earths for determining the -_composition of soils_ chiefly composed of a mixture of sand, clay, -limestone, and mould. The limestone is soluble in dilute acids, but -neither the clay nor sand passes into solution by this means, and -therefore the limestone is easily separated in the investigation of -soils. The clay is separated from the sand by a mechanical method similar -to that described above, and termed _levigation_.[17] - - [17] The process of _levigation_ is based on the difference in the - diameters of the particles of clay and sand. In density these - particles differ but little from each other, and therefore a - stream of water of a certain velocity can only carry away the - particles of a certain diameter, whilst the particles of a larger - diameter cannot be borne away by it. This is due to the resistance - to falling offered by the water. This resistance to substances - moving in it increases with the velocity, and therefore a - substance falling into water will only move with an increasing - velocity until its weight equals the resistance offered by the - water, and then the velocity will be uniform. And as the weight of - the minute particles of clay is small, the maximum velocity - attained by them in falling is also small. A detailed account of - the theory of falling bodies in liquid, and of the experiments - bearing on this subject, may be found in my work, _Concerning the - Resistance of Liquids and Aeronautics_, 1880. The minute particles - of clay remain suspended longer in water, and take longer to fall - to the bottom. Heavy particles, although of small dimensions, fall - more quickly, and are borne away by water with greater difficulty - than the lighter. In this way gold and other heavy ores are washed - free from sand and clay, and the coarser portions and heavier - particles are left behind. A current of water of a certain - velocity cannot carry away with it particles of more than a - definite diameter and density, but by increasing the velocity of - the current a point may be arrived at when it will bear away - larger particles. A description of apparatus for the observation - of phenomena of this kind is given by Schöne in his memoir in the - Transactions of the Moscow Society of Natural Sciences for 1867. - In order to be able accurately to vary the velocity of the current - of water, a cylinder is employed in which the earth to be - experimented on is placed, and water is introduced through the - conical bottom of the cylinder. The rate at which the water rises - in the cylinder will vary according to the quantity of water - flowing per unit of time into the vessel, and consequently - particles of various sizes will be carried away by the water - flowing over the upper edges of the vessel. Schöne showed by - direct experiment that a current of water having a velocity of 0·1 - mm. per second will carry away particles having a diameter of not - more than 0·0075 mm., that is, only the most minute; with a - velocity _v_ = 0·2 mm. per second, particles having a diameter _d_ - = 0·011 mm. are carried away; with _v_ = 0·3 mm., _d_ = 0·0146 - mm.; with _v_ = 0·4 mm., _d_ = 0·017 mm.; with _v_ = 0·5 mm., _d_ - = 0·02 mm.; with _v_ = 1 mm., _d_ = 0·03 mm.; with _v_ = 4 mm., - _d_ = 0·07 mm.; with _v_ = 10 mm., _d_ = 0·137 mm.; with _v_ = 12 - mm., _d_ = 0·15 mm.; and therefore if the current does not exceed - one of these velocities, it will only carry away or wash away - particles having a diameter less than that indicated. The sand and - other particles mixed with the clay will then remain in the - vessel. The very minute particles obtained after levigation are - all considered as clay, although not only clay but other rock - residue may also exist in it as very fine particles. However, this - is very seldom the case, and the fine mud separated from all clays - has practically the same composition as the purest kinds of - kaolin. - - The relation between the amounts of clay and sand in soils used - for the cultivation of plants is very important, because a soil - rich in clay is denser, heavier, shrinks up under the action of - heat, and does not readily yield to the plough in dry or wet - weather, whilst a soil rich in sand is friable, crumbling, easily - parts with its moisture and dries rapidly, but is comparatively - easily worked. Neither crumbling sand nor pure clay can be - regarded as a good _cultivating soil_. The difference in the - amounts of clay and sand in a soil has also a purely chemical - signification. Sand is easily permeated by the air, because its - particles are not closely packed together. Hence the chemical - change of manures proceeds very easily in sandy soils. But on the - other hand such soils do not retain the nutritious principles - contained in the manure, nor the water necessary for the - nourishment of plants by means of their roots. Solutions of - nutritious substances, containing salts of potassium, phosphoric - acid, &c., when passed through sand only leave a portion - moistening the surface of its particles. The sand has only to be - washed with pure water and all the adhering films of solution are - washed away. It is not so with clay. If the above solutions be - passed through a layer of clay the retention of the nutritive - substances of these solutions will be very marked; this is partly - because of the very large surface which the minute particles of - clay expose. The nutritive elements dissolved in water are - retained by the particles of clay in a peculiar manner--that is, - the absorptive power of clay is very great compared to that of - sand--and this has a great significance in the economy of nature - (Chapter XIII., p. 547). It is evident that for cultivation the - most convenient soils in every respect will be those containing a - definite mixture of clay and sand, and indeed the most fertile - soils have this composition. The study of fertile soils, which is - so important for a knowledge of the natural conditions for the - application of fertilisers, belongs, strictly speaking, to the - province of agriculture. In Russia the first foundation of a - scientific fertilisation has been laid by Dokuchaeff. As an - example only, we will give the composition of four soils; (1) The - black earth of the Simbirsk Government; (2) a clay soil from the - Smolensk Government; (3) a more sandy soil from the Moscow - Government; and (4) a peaty soil from near St. Petersburg. These - analyses were made in the laboratory of the St. Petersburg - University about 1860, in connection with experiments on - fertilisation (conducted by me) by the Imperial Free Economical - Society. 10,000 grams of air-dried soil contain the following - quantities (in grams) of substances capable of dissolving in - acids, and of serving for the nourishment of plants. - - (1) (2) (3) (4) - - Na_{2}O 11 5 4 4 - K_{2}O 58 10 7 5 - MgO 92 33 19 7 - CaO 134 17 14 11 - P_{2}O_{5} 7 1 7 3 - N 44 11 13 16 - S 13 7 7 6 - Fe_{2}O_{3} 341 155 111 46 - - By chemical and mechanical analysis, the chief component parts per - 100 parts of air-dried soil are - - Clay 46 29 12 10 - Sand 40 67 86 84 - Organic matter 3·7 1·7 0·6 4·1 - Hygroscopic water 6·3 1·3 0·8 1·9 - Weight of a litre in grams 1150 1270 1350 960 - - The black earth excels the other soils in many respects, but - naturally its stores are also exhausted by cultivation if nothing - be returned to it in the form of fertilisers; and the improvement - of a soil (for instance, by the addition of marl or peat, and by - drainage and watering), and its fertilisation, if carried on in - conformity with its composition and with the properties of the - plants to be cultivated, are capable of rendering not only every - soil fit for cultivation, but also of improving its value, so that - in the course of time whole countries (like Holland) may clearly - improve their agricultural position, whilst under the ordinary - _régime_ of continued exhaustion of the soil, entire regions (as, - for instance, many parts of Central Asia) may be rendered unfit - for any agriculture. - -By treating clay with strong sulphuric acid, which dissolves the alumina -in it, and then (by means of an alkaline carbonate) dissolving the silica -which was combined with the alumina in the clay (but not that occurring -in the form of sand, &c., which is hardly dissolved by carbonate of soda -solution at all even on boiling), we may form an idea of the proportion -between the component parts of a clay; and by igniting it at a high -temperature, we may determine the amount of water held in it. In the -purer sorts of clay dried at 100° (sp. gr. of pure kaolin is about 2·5) -this proportion is about 2SiO_{2} : 2H_{2}O : Al_{2}O_{3}. In this case -the conversion of felspar into kaolin is expressed by the equation:-- - - K_{2}O,Al_{2}O_{3},6SiO_{2} = Al_{2}O_{3},2SiO_{2} + K_{2}O,4SiO_{2}; - Felspar Kaolin - -the compound K_{2}O,4SiO_{2} passes into solution. - -But as a rule clays contain from 45 to 60 p.c. of silica, from 20 to 30 -p.c. of alumina, and about 12 p.c. of water; and it cannot be supposed -that clays are always homogeneous, because they are an aggregation of -residues (of silico-aluminous compounds) which are unacted on by water. -Nevertheless, clays always contain a hydrous compound of alumina and -silica, which is able to give up the alumina contained by it as a base to -strong sulphuric acid, forming aluminium sulphate, which is soluble in -water. After this treatment the silica remains, and is soluble in a -solution of an alkaline carbonate.[18] - - [18] Everyone knows that a mixture of clay and water is endowed with - the property of taking a given form when subjected to a moderate - pressure. This plasticity of clay renders it an invaluable - material for practical purposes. From clay are moulded and - manufactured a variety of objects, beginning with the common brick - and ending with the most delicate china works of art. This - _plasticity of clay_ increases with its purity. When articles made - of clay are dried, the well-known hard mass is obtained; but water - washes it away, and furthermore, the cohesion of its particles is - not sufficiently great for it to resist the impression of blows, - shocks, &c. If such an article be subjected to the action of heat, - its volume first decreases, then it begins to lose water, and it - shrinks still further (in the case of a compact mass approximately - by 1/5 of its linear measurement). On the other hand, a great - coherence of particles is obtained, and thus burnt clay has the - hardness of stone. Pure clay, however, shrinks so considerably - when burnt that the form given to it is destroyed and cracks - easily form; such vessels are also porous, so that they will not - hold water. The addition of sand--that is, silica in fine - particles--or of _chamotte_--that is, already burnt and crushed - clay--renders the mass much more dense and incapable of cracking - in the furnace. Nevertheless, such clay articles (bricks, - earthenware vessels, &c.) are still porous to liquids after being - burnt, because the clay in the furnace is only baked and does not - fuse. In order to obtain articles impervious to water the clay - must either be mixed with substances which form a glassy mass in - the furnace, permeating the clay and filling up its pores, or else - only the surface of the article is covered with such a glassy - fusible substance. In the first case the purest kinds of clay give - what is known as china, in the second case porcelain or 'faïence.' - So, for instance, by covering the surface of clay articles with a - layer of the oxides of lead and tin, the well-known white glaze is - obtained, because the oxides of these metals give a white gloss - when fused with silica and clay. In the preparation of china, - fluor spar and finely ground silica is mixed up into the clay; - these ingredients give a mass which is infusible but softens in - the furnace, so that all the particles of the clay cohere in this - softened mass, which hardens on cooling. A glaze composed of - glassy substances, which only fuse at a high temperature, is also - applied to the surface of china articles. - -Clay is the source from which alumina, Al_{2}O_{3}, and the majority of -the compounds of aluminium are prepared. Among these compounds the most -important are the alums--that is, the double sulphates of potassium (and -allied metals) and aluminium, AlK(SO_{4})_{2},12H_{2}O. When clay is -treated with sulphuric acid diluted with a certain amount of water, -aluminium sulphate, Al_{2}(SO_{4})_{3}, is formed; and if potassium -carbonate or sulphate be added to this solution, a double salt or alum is -obtained in solution. The alums crystallise easily, and are prepared on a -very large manufacturing scale owing to their being employed in the -process of dyeing. Alums are soluble in water, and, on the addition of -ammonia to their solutions, they give _hydrous alumina_, or _aluminium -hydroxide_, as a white gelatinous precipitate, which is insoluble in -water but easily soluble in acids, even when dilute, and in aqueous soda -or potash. The solubility of alumina in acids indicates the basic -character of the oxide, and its solubility in alkalis and its power of -forming compounds with them shows the weakness of this basic character. -However, the feeblest acids, even carbonic acid, take up the alkali from -such a solution, and the alumina then separates out in a precipitate as -the hydroxide. It must also be remembered as characteristic of the -salt-forming properties of alumina that it does not combine with such -feeble acids as carbonic, sulphurous, or hypochlorous, &c.--that is, its -compounds with these acids are decomposed by water. It is also important -to observe that the hydroxide is not soluble in aqueous ammonia. - -_Alumina_, Al_{2}O_{3}--that is, the anhydrous aluminium oxide--is -met with in nature, sometimes in a somewhat pure state, having -crystallised in transparent crystals, which are often coloured by -impurities (chromic, cobaltic, and ferric compounds). Such are the ruby -and sapphire, the former red and the latter blue. They have a specific -gravity 4·0, are distinguished by their very great hardness, which is -second only to that of the diamond, and they represent the purest form of -alumina. They are found in Ceylon and other islands of the Indian -Archipelago, embedded in a rock matrix.[18 bis] _Corundum_ is the same -crystallised anhydrous alumina coloured brown by a trace of oxide of -iron. A very much larger portion of this impurity occurs in _emery_, -which is found in crystalline masses in Asia Minor and in Massachusetts, -and owing to its extreme hardness is employed for polishing stones and -metals. In this anhydrous and crystalline state the aluminium oxide is a -substance which very powerfully resists the action of reagents, and is -insoluble both in solutions of the alkalis and in strong acids. It is -only capable of passing into solution after being fused with alkalis.[19] -Alumina may be obtained in this form by artificial means if the hydroxide -be ignited and then fused in the oxyhydrogen flame.[20] Alumina also -occurs in nature in combination with water--as, for instance, in the -rather rare minerals _hydrargillite_ (sp. gr. 2·3), Al_{2}O_{3},3H_{2}O = -2Al(HO)_{3}, and _diaspore_, Al_{2}O_3,H_{2}O = 2AlO(HO) (sp. gr. 3·4). A -less pure hydrate, mixed with ferric oxide, sometimes occurs in masses -(at Baux in the south of France) and is termed _bauxite_; it contains -Al_{2}O_{3},2H_{2}O = Al_{2}O(HO)_{4} (sp. gr. 2·6). When bauxite is -ignited with sodium carbonate, carbonic anhydride is liberated and the -alumina then combines with the sodium oxide, forming a saline aluminate -of the oxides of aluminium and sodium. This is taken advantage of in -practice for the preparation of pure alumina compounds on a large scale, -for bauxite is found in large masses (in the South of France, in Austria, -and in Carolina in South America), and the resultant compound of alumina -and sodium is soluble in water and does not contain ferric oxide. This -solution when subjected to the action of carbonic anhydride gives a -precipitate of aluminium hydroxide,[21] which with acids forms aluminium -salts. If aqueous ammonia be added to a solution of aluminium sulphate a -gelatinous precipitate is formed, which at first remains suspended in the -liquid and then on settling forms a gelatinous mass, which itself -indicates the _colloidal property of aluminium hydroxide_. The following -points are characteristic of this colloidal state: (1) in an anhydrous -state such a colloidal substance is insoluble in water, as alumina is; -(2) in the hydrated state, it is gelatinous and insoluble in water; and -(3) it is also capable of existing in solutions, from which it separates -out in a non-crystalline state, forming a substance resembling glue. -These different states of colloids were distinguished by Graham, who gave -them the following very characteristic names. He called the gelatinous -form of the hydrate _hydrogel_, _i.e._ a gelatinous hydrate, and the -soluble form of the aqueous compound, _hydrosol_, from the Latin for a -soluble hydrate. Alumina readily and frequently assumes these states. The -gelatinous hydrate of alumina is its hydrogel. It is, as has been already -mentioned, insoluble in water, and, like all similar hydrogels, shows not -the faintest sign of crystallisation; it is apt to vary in many of its -properties with the amount of water it contains, and loses its water on -ignition, leaving a white powder of the anhydrous oxide. The hydrogel of -alumina is soluble both in acids and alkalis. It may also be obtained by -the evaporation of its solutions in such feebly energetic acids as -volatile acetic acid. These properties are very frequently made use of in -the arts, and especially in _the processes of dyeing_, because the -hydrogel of alumina in precipitating attracts a number of colouring -matters from their solutions, the precipitate being thus coloured by the -dyes attracted.[22] The preparation of fixed dyes and the employment of -aluminous compounds (mordants) in the processes of dyeing are founded on -this fact.[23] When precipitated upon the fibres of tissues (calicoes, -linens, &c.) the aluminium hydroxide renders them impermeable to water; -this may be taken advantage of for the preparation of waterproof tissues. - - [18 bis] Frémy (1890) obtained transparent rubies, which crystallised - in rhombohedra, and resembled natural rubies in their hardness, - colour, size, and other properties. He heated together a mixture - of anhydrous alumina containing more or less caustic potash, with - barium fluoride and bichromate of potassium. The latter is added - to give the ruby its colour, and is taken in small quantity (not - more than 4 parts by weight to 100 parts of alumina). The mixture - is put into a clay crucible, and heated (for from 100 hours to 8 - days) in a reverberatory furnace at a temperature approaching - 1,500°. At the end of the experiment the crucible was found to - contain a crystalline mass, and the walls were covered with - crystals of the ruby of a beautiful rose colour. It was found that - the access of moist air was indispensable for the reaction. - According to Frémy, the formation of the ruby may be here - explained by the formation of fluoride of aluminium which under - the action of the moist air at the high temperature of the furnace - gives the ruby and hydrofluoric acid gas. - - [19] The effects of purely mechanical subdivision on the solubility of - alumina are evident from the fact that native anhydrous alumina, - when converted into an exceedingly fine powder by means of - levigation, dissolves in a mixture of strong sulphuric acid and a - small quantity of water, especially when heated in a closed tube - at 200°, or when fused with acid sulphate of potassium (_see_ - Chapter XIII., Note 9). - - [20] The preparation of crystallised alumina is given on p. 65, and in - Note 18 bis. When alumina, moistened with a solution of cobalt - salt, is ignited, it forms a blue mass called Thénard's salt. This - coloration is taken advantage of not only in the arts, but also - for distinguishing alumina from other earthy substances resembling - it. - - [21] The treatment of bauxite is carried on on a large scale, chiefly - in order to obtain alumina from alkaline solutions, free from - ferric oxide, because in dyeing it is necessary to have salts of - aluminium which do not contain iron. But this end, it would seem, - may also be obtained by igniting alumina containing ferric oxide - in a stream of chlorine mixed with hydrocarbon vapours, as ferric - chloride then volatilises. K. Bayer observed that in the treatment - of bauxite with soda, about 4 molecules of sodium hydroxide pass - into solution to 1 molecule of alumina, and that on agitating this - solution (especially in the presence of some already precipitated - aluminium hydroxide), about two-thirds of the alumina is - precipitated, so that only 1 molecule of alumina to 12 molecules - of sodium hydroxide remains in solution. This solution is - evaporated directly, and used again. He therefore treats bauxite - directly with a solution of NaHO at 170° in a closed boiler, and - on cooling adds hydrated alumina to the resultant solution. The - greater part of the dissolved alumina then precipitates on this - hydrated alumina, and the solution is used over again. The - hydroxide which separates from the alkaline solution contains - Al(OH)_{3}. All these properties bear a great resemblance to those - of boric acid. It may be taken for granted that the relation - between sodium hydroxide and alumina in solution varies with the - mass of water. - - If lime be added to a solution of alumina in alkali (sodium - aluminate) calcium aluminate is precipitated, from which acids - first extract the lime, leaving aluminium hydroxide, which is - easily soluble in acids (Loewig). When sodium aluminate is mixed - with a solution of sodium bicarbonate, a double carbonate of the - alkali and aluminium is precipitated, which is easily soluble in - acids. - - [22] These coloured precipitates of alumina are termed _lakes_, and are - employed in dyeing tissues and in the formation of various - pigments--such as pastels, oil colours, &c. Thus, if organic - colouring matters, such as logwood, madder, &c., are added to a - solution of any aluminium salt, and then an alkali is added, so - that alumina may be precipitated, these pigments, which are by - themselves soluble in water, will come down with the precipitate. - This shows that alumina is able to combine with the colouring - matter, and that this compound is not decomposed by water. The - dyes then become insoluble in water. If a dye be mixed with starch - paste and aluminium acetate, and then, by means of engraved blocks - having a design in relief, we transfer this mixture to a fabric - which is then heated, the aluminium acetate will leave the - hydrogel of alumina which binds the colouring matter, and water - will no longer be able to wash the pigment from the material--that - is, a so-called 'fixed' dye is obtained. In the case of dyeing a - fabric a uniform tint, it is first soaked in a solution of - aluminium acetate and then dried, by which means the acetic acid - is driven off, while the hydrogel of alumina adheres to the fibres - of the material. If the latter be then passed through a solution - of a dye in water, the former will be attracted to the portions - covered with alumina, and closely adhere to them. If certain parts - of the material be protected by the application of an acid, such - as tartaric, C_{4}H_{6}O_{6}, oxalic, citric, &c. (these acids - being non-volatile), the alumina will be dissolved in those parts, - and the pigment will not adhere, so that after washing, a white - design will be obtained on those parts which have been so - protected. - - In dye-works the aluminium acetate is generally obtained in - solution by taking a solution of alum, and mixing it with a - solution of lead acetate. In this case lead sulphate is - precipitated and aluminium acetate remains in solution, together - with either acetate or sulphate of potassium, according to the - amount of acetate of lead first taken. The complete decomposition - will be as follows: KAl(SO_{4})_{2} + 2Pb(C_{2}H_{3}O_{2})_{2} = - KC_{2}H_{3}O_{2} + Al(C_{2}H_{3}O_{2})_{3} + 2PbSO_{4}, or the - less complete decomposition, 2KAl(SO_{4})_{2} + - 3Pb(C_{2}H_{3}O_{2})_{2} = 2Al(C_{2}H_{3}O_{2})_{3} + K_{2}SO_{4} - + 3PbSO_{4}. If the resultant solution of aluminium acetate be - evaporated or further boiled, the acetic acid passes off and the - hydrogel of alumina remains. - - As the salt of potassium obtained in the solution passes away with - the water used for washing, and the salt of lead precipitated has - no practical use, this method for the preparation of aluminium - acetate cannot be considered economical; it is retained in the - process of dyeing mainly because both the salts employed, alum and - sugar of lead, easily crystallise, and it is easy to judge of - their degree of purity in this form. Indeed, it is very important - to employ pure reagents in dyeing, because if impurity is - present--such as a small quantity of an iron compound--the tint of - the dye changes; thus madders give a red colour with alumina, but - if oxide of iron be present the red changes into a violet tint. - The aluminium hydroxide is soluble in alkalis, whilst ferric oxide - is not. Therefore sodium aluminate--that is, the dissolved - compound of alumina and caustic soda--obtained, as already - described, from bauxite, is sometimes employed in dyeing. Every - aluminium salt gives a solution containing sodium aluminate free - from iron, when it is mixed with excess of caustic soda. This - solution, when mixed with a solution of ammonium chloride, gives a - precipitate of the hydrogel of alumina: Al(OH)_{3} + 3NaHO + - 3NH_{4}Cl = Al(OH)_{3} + 3NaCl + 3NH_{4}OH. There was originally - free soda, and on the addition of sal-ammoniac there is free - ammonia, and this does not dissolve alumina, therefore the - hydrogel of the latter is precipitated. - - [23] Another direct method for the preparation of pure aluminium - compounds consists in the treatment of _cryolite_ containing - aluminium fluoride together with sodium fluoride, AlNa_{3}F_{6}. - This mineral is exported from Greenland, and is also found in the - Urals. It is crushed and heated in reverberatory furnaces with - lime, and the resultant mass is treated with water; sodium - aluminate is then obtained in solution, and calcium fluoride in - the precipitate AlNa_{3}F_{6} + 3CaO = 3CaF_{2} + AlNa_{3}O_{3}. - -_The hydrosol_ of alumina--_i.e._ the soluble aluminium hydroxide--is -more difficult to obtain.[24] In order to obtain this soluble variety of -alumina, Graham took a solution of its hydrogel in hydrochloric -acid--that is, a solution of aluminium chloride, which is able to -dissolve a still further quantity of the hydrogel of alumina, forming a -basic salt having probably one of the compositions Al(HO)Cl_{2} or -Al(HO)_{2}Cl. When such a solution, considerably diluted with water, is -subjected to dialysis--that is, to diffusion through a membrane[25]--the -hydrochloric acid diffuses through the membrane and leaves the alumina in -the form of hydrosol. The resultant solution, even when only containing -two or three per cent. of alumina, passes into the hydrogel state with -such facility that it is sufficient to transfer it from one vessel to -another which has not been previously washed with water, for the entire -mass to solidify into a jelly. But a solution containing not more than -one-half per cent. of alumina may even be boiled without coagulating; -however, after the lapse of several days this solution will of its own -accord yield the hydrogel of alumina.[25 bis] - - [24] Crum first prepared a solution of basic acetate of alumina--that - is, a salt containing as large as possible an excess of aluminium - hydroxide with as small as possible a quantity of acetic acid. The - solution must be dilute--that is, not contain more than one part - of alumina per 200 of water--and if this solution be heated in a - closed vessel (so that the acetic acid cannot evaporate) to the - boiling point of water, for one and a half to two days, then the - solution, which apparently remains unaltered, loses its original - astringent taste, proper to solutions of all the salts of alumina, - and has instead the purely acid taste of vinegar. The solution - then no longer contains the salt, but acetic acid and the hydrosol - of alumina in an uncombined state; they may be isolated from each - other by evaporating the acetic acid in shallow vessels at the - ordinary temperature, and with a thin layer of liquid the alumina - does not separate as a precipitate. When the acid vapours cease to - come off there remains a solution of the hydrosol of alumina, - which is tasteless and has no action on litmus paper. When - concentrated, this solution acquires a more and more gluey - consistency, and when completely evaporated over a water-bath it - leaves a non-crystalline glue-like hydrate, whose composition is - Al_{2}H_{4}O_{5} = Al_{2}O_{3},2H_{2}O. The smallest quantity of - alkalis, and of many acids and salts, will convert the hydrosol - into the hydrogel of alumina--that is, convert the aluminium - hydroxide from a soluble into an insoluble form, or, as it is - said, cause the hydrate to coagulate or gelatinise. The smallest - amount of sulphuric acid and its salts will cause the alumina to - gelatinise--that is, cause the hydrogel to separate. Many such - colloidal solutions are known (Vol. I. p. 98, Note 57). - - [25] In a dialyser, Vol. I. p. 63, Note 18. - - [25 bis] The different states in which the hydrates of alumina occur - and are prepared resemble similar varieties of the hydrates of the - oxides of iron and chromium, of molybdic and tungstic acids, as - well as of phosphoric and silicic acids, of many sulphides, - proteid substances, &c. We shall therefore have occasion to recur - to this subject in the further course of this work. - - The most remarkable peculiarity of Graham's solution is that it - solidifies on litmus paper, and leaves a blue ring on it, which - shows the alkaline--that is, basic--character of the alumina in - such a solution. If in the dialysis the basic hydrochloric acid - salt be replaced by a similar acetic acid salt, a hydrosol of - alumina is obtained which does not act upon litmus. - -With respect to alumina as a base, it is very important to observe that -it is not only capable of combining with other bases[26] but that it does -not give salts with feeble volatile acids (like carbonic and -hypochlorous); it forms salts which are easily decomposed by water, -especially when heated,[27] as well as double and basic salts,[28] _so -that it forms a clear example of a feeble base_.[29] To these -characteristics of alumina we must add that it not only gives compounds -of the type AlX_{3}, but also the polymeric type Al_{2}X_{6}, even when X -is a simple univalent haloid like chlorine. Deville and Troost showed -(1857) that the vapour density of aluminium chloride (at about 400°) is -9·37 with respect to air--that is, nearly 135 with respect to hydrogen, -and therefore the formula of its molecule is expressed by Al_{2}Cl_{6}, -and not AlCl_{3},[30] although in the case of boron, arsenic, and -antimony, which give oxides R_{2}O_{3} of the same composition as -Al_{2}O_{3}, the chlorine compounds form non-polymeric molecules, -BCl_{3}, AsCl_{3}, SbCl_{3}.[31] This duplication (polymerisation) of the -form AlX_{3} is connected with the facility with which the salts of -aluminium combine with other salts to form double salts and with -aluminium hydroxide itself to form basic salts. - - [26] Compounds of alumina with bases (aluminates, _see_ Note 21) are - sometimes met with in nature. Such are spinel (_see_ p. 65), - MgO,Al_{2}O_{3} = MgAl_{2}O_{4}, chrysoberyl, BeAl_{2}O_{4}, and - others. Magnetic oxide of iron, FeO,Fe_{2}O_{3} = Fe_{3}O_{4}, and - compounds like it, belong to the same class. Here we evidently - have a case of combination 'by analogy,' as in solutions and - alloys, accompanied by the formation of strictly definite saline - compounds, and such instances form a clear transition from - so-called solutions and certain mixtures to the type of true - salts. - - [27] Not only aluminium acetate (Note 24), but also every other - aluminium salt with a volatile acid, parts with its acid on - heating an aqueous solution--that is, is decomposed by water, and - forms either basic salts or a hydrate of alumina. By dissolving - aluminium hydroxide in nitric acid we may easily obtain a - well-crystallising _aluminium nitrate_, Al(NO_{3})_{3},9H_{2}O, - which fuses at 73° without decomposing (Ordway), gives a basic - salt, 2Al_{2}O_{3},6HNO_{3}, at 100°, and at 140° leaves the - aluminium hydroxide perfectly free from the elements of nitric - acid. But the solutions of this salt, like those of the acetate, - are also able to yield aluminium hydroxide. From all this it is - evident that we must suppose that the solutions of this and - similar salts contain an equilibrated dissociated system, - containing the salt, the acid, and the base, and their compounds - with water, as well as partly the molecules of water itself. Such - examples much more clearly confirm those conceptions of solutions - which are given in the first chapter than a general preliminary - acquaintance with the subject can do. - - [28] As an example of native basic salts we may cite _alunite_, or - alum-stone (sp. gr. 2·6), which sometimes occurs in crystals, but - more frequently in fibrous masses. It has been found in masses in - the Caucasus (at Zaglik, forty versts distance from Elizabetpol), - and at Tolfa, near Rome. Its composition is - K_{2}O,3Al_{2}O_{3},4SO_{3},6H_{2}O (alunite contains 9H_{2}O). It - is soluble in water but not decomposed by it, but after being - slightly ignited it gives up alum to it. It may be artificially - prepared by heating a mixture of alum with aluminium sulphate in a - closed tube at 230°. - - [29] As the colloidal properties are particularly sharply developed in - those oxides (Al_{2}O_{3}, SiO_{2}, MoO_{3}, SnO_{2}, &c.) which - show (like water also) the properties of feeble bases and feeble - acids, there is probably some causal reason for this coincidence, - all the more so since among organic substances--gelatins, - albumins, &c.--the representatives of the colloids also have the - property of feebly combining with bases and acids. - - [30] Since Deville's experiments the question of the density of - aluminium chloride has been frequently re-investigated. The - subject has more especially occupied the attention of Nilson, - Pettersson, Friedel and Crafts, and V. Meyer and his - collaborators. In general, it has been found that at low - temperatures (up to 440°) the density is constant, and indicates a - molecule Al_{2}Cl_{6}; whilst depolymerisation probably (although - it is not yet certain) takes place at higher temperatures, and the - molecule AlCl_{3} is obtained. Along with this there has been, and - still is, a difference of opinion as to the vapour density of - aluminium ethyl and methyl--whether for instance, Al(CH_{3})_{3} - or Al_{2}(CH_{3})_{6} expresses the molecule of the latter. The - interest of these researches is intimately connected with the - question of the valency of aluminium, if we hold to the opinion - that elements in their various compounds have a constant and - strictly definite valency. In this case the formula AlCl_{3} or - Al(CH_{3})_{3} would show that Al is trivalent, and that - consequently the compounds of aluminium are Al(OH)_{3}, AlO_{3}Al, - and, in general, AlX_{3}. But if the molecule be Al_{2}Cl_{6}, it - is--for the followers of the doctrine of the invariable valency of - the elements--incompatible with the idea of the trivalency of - aluminium, and they assume it to be quadrivalent like carbon, - likening Al_{2}Cl_{6} to ethane C_{2}H_{6} = CH_{3}CH_{3}, - although this does not explain why Al does not form AlCl_{4}, or, - in general, AlX_{4}. In this work another supposition is - introduced; according to this, although aluminium, as an element - of group III., gives compounds of the type AlX_{5}, this does not - exclude the possibility of these molecules combining with others, - and consequently with _each other_--that is, forming Al_{2}X_{6}; - just as the molecules of univalent elements exist either as H_{2}, - Cl_{2}, &c., or as Na, and the molecules of bivalent elements - either as Zn, or as S_{2}, or even S_{6}. In the first place it - must be recognised that the limiting form does not exhaust all - power of combination, it only exhausts the capacity of the element - for combining with X's, but the saturated substance may afterwards - combine with _whole molecules_, which fact is best proved by the - capacity of substances to form crystalline compounds with water, - ammonia, &c. But in some substances this faculty for further - combinations is less developed (for instance, in carbon - tetrachloride, CCl_{4}), whilst in others it is more so. AlX_{3} - combines with many other molecules. Now if a limiting form, which - does not combine with new X's, nevertheless combines with other - whole molecules, it will naturally in some instances combine with - itself, will polymerise. In this manner the mind clearly grasps - the idea that the same forces which cause S_{2} to unite itself to - Cl_{2}, or C_{2}H_{4} to Cl_{2}, &c., also unite molecules of a - similar kind together; thus _polymerisation_ ceases to be an - isolated fragmentary phenomenon, and chemical combinations 'by - analogy' acquire a particular and important interest. In - conformity with these views the following proposition may be made - concerning the compounds of aluminium. They are of the type - AlX_{3} in the limit, like BX_{3}, but those limiting forms are - still able to combine to form AlX_{3},RZ, and the aluminium - chloride is a compound of this kind--_i.e._ (AlX_{3})_{2}. In - boron, for example, in BCl_{3}, this tendency to form further - compounds is less developed. Hence boron chloride appears as - BCl_{3}, and not (BCl_{3})_{2}. Polymerisation is not only - possible when a substance has not attained the limit (although it - is more probable then), but also when the limiting form has been - reached, if only the latter has the faculty of combining with - other whole molecules. We may therefore conclude that aluminium, - like boron, is trivalent in the same sense that lithium and sodium - are univalent, magnesium bivalent, and carbon tetravalent. In a - word, there is no reason to consider that aluminium is capable of - forming compounds AlX_{4}, and in that way to explain the - existence of the molecule Al_{2}Cl_{6}. Furthermore, there are - many reasons for thinking that AlF_{3}, Al_{2}O_{3}, and other - empirical formulæ do not express the molecular weights of these - compounds, but that they are much higher: Al_{_n_}F_{3_n_}, - Al_{2_n_}O_{3_n_}. In recent years convincing proofs of the truth - of the above statements have been obtained, and of the independent - existence of AlX_{3} in a state of vapour; for Comb has determined - the vapour density of the volatile acetyl of aluminium acetate - Al(C_{3}H_{7}O_{2})_{3} (which melts at 193°, boils at 315°, and - distils without a trace of decomposition), and has found that it - exactly corresponds to the above molecular composition. On the - other hand, Louise and Roux (1889) by employing the method of - 'freezing point depression' of solutions (Chapter I., Note 49) - found that the molecules Al_{2}(C_{2}H_{5})_{6} and - Al_{2}(C_{5}H_{11})_{6}, &c., correspond to the type Al_{2}X_{6}. - Thus it may now be accepted that the molecular composition of the - compounds of aluminium in their simplest form is AlX_{3}, but that - they may polymerise and give Al_{2}X_{6} or, in general, - Al_{2}X_{3_n_}. - - [31] In the case of gallium, as a close analogue of aluminium, Lecoq de - Boisbaudran (1880) showed that probably the molecule gallium - chloride contains Ga_{2}Cl_{6} at low temperatures and high - pressures, and that it dissociates into GaCl_{3} at high - temperatures and low pressures. The molecule of indium chloride - seems to exist only in the simplest form, InCl_{3}. - -_Aluminium sulphate_, Al_{2}(SO_{4})_{3}, which is obtained by treating -clay or the hydrates of alumina with sulphuric acid, crystallises in the -cold with 27H_{2}O, or at the ordinary temperature in pearly crystals, -which are greasy to the touch and contain 16H_{2}O.[32] Its solutions act -like sulphuric acid--for instance, they evolve hydrogen with zinc, -forming basic salts, which are sometimes met with in nature (_aluminite_, -Al_{2}O_{3},SO_{3},9H_{2}O, _alumiane_, Al_{2}O_{3},2SO_{3}, and others), -and may be obtained by the decomposition of normal salts and by the -direct solution of the hydroxide in normal salts: these exhibit a varying -composition, (Al_{2}O_{3})_{_n_}(SO_{3})_{_m_}(H_{2}O)_{_q_}, where _m/n_ -is less than 3. Aluminium sulphate is now prepared (from the pure hydrate -obtained from bauxite, Note 21) in large quantities for dyeing purposes -(instead of alums) as a mordant. With solutions of the alkali sulphates -(potassium, sodium, ammonium, rubidium, and cæsium sulphates), the normal -salt easily forms double salts, termed _alums_--for example, the ordinary -crystalline alum contains KAl(SO_{4})_{2},12H_{2}O, or -K_{2}SO_{4},Al_{2}(SO_{4})_{3},24H_{2}O. In the ammonium alums (which -leave a residue of alumina when ignited) the potassium is replaced by -ammonium (NH_{4}). Alums are used in large quantities, because there is -scarcely any other salt which crystallises so easily. In this respect the -alums formed by potassium and ammonium are equally convenient to purify, -because they present a considerable difference in their solubility at the -ordinary and higher temperatures. If the crystallisation be conducted -rapidly, the salt separates in minute crystals, but if it be slowly -deposited, especially in large masses, as in factories, then crystals -several centimetres long are sometimes obtained. At a higher temperature -alums are very much more soluble, and crystallise with greater -difficulty, and are therefore less easily freed from impurities; at 0° -100 parts of water dissolve 3 parts, at 30° 22 parts, at 70° 90 parts, -and at 100° 357 parts of potassium alum.[33] The solubility of ammonium -alum is slightly less. The specific gravity of potassium alum is 1·74, of -ammonium alum 1·63, and of sodium alum 1·60. Alums easily part with their -water of crystallisation; thus potash alum partially effloresces when -exposed to the air, and loses 9 mol. H_{2}O under the receiver of an -air-pump. At 100°, dry air passed over alums takes up nearly all their -water. As we have already mentioned (Chapter XV.), the law of isomorphous -substitutions exhibits itself more clearly in the alums than in any other -salts, and all alums not only contain the same amount of water of -crystallisation, MR(SO_{4})_{2},12H_{2}O (where M = K, NH_{4}, Na; R = -Al, Fe, Cr), and appear in crystals whose planes are inclined at equal -angles, but they also give every possible kind of isomorphous mixture. -The aluminium in them is easily replaced by iron, chromium, indium and -sometimes by other metals, whilst the potassium may be substituted by -sodium, rubidium, ammonium, and thallium, and the sulphuric acid may be -replaced by selenic and chromic acids. - - [32] The pure salt (16H_{2}O) is not hygroscopic. In the presence of - impurities the amount of water increases to 18H_{2}O, and the salt - becomes hygroscopic. - - [33] The common form of crystals of alums is octahedral, but if this - solution contains a certain small excess of alumina above the - ratio 2Al(OH)_{3} to K_{2}SO_{4}, and not more sulphuric acid than - 3H_{2}SO_{4} to 2Al(OH)_{3}, then it easily forms combinations of - the cube and octahedron, and these alums are called 'cubic' alums. - They are valued by the dyer because they can contain no iron in - solution, for oxide of iron is precipitated before alumina, and if - the latter be in excess there can be no oxide of iron present. - These alums were long exported from Italy, where they were - prepared from alunite (Note 28). - -_Aluminium chloride_, Al_{2}Cl_{6}, is obtained, like other similar -chlorides, (for instance MgCl_{2}) either directly from chlorine and the -metal, or by heating to redness an intimate mixture of the amorphous -anhydrous oxide and charcoal in a stream of dry chlorine.[33 bis] The -resultant sublimate is very volatile,[34] and forms a crystalline, easily -fusible mass, which deliquesces in the air and easily dissolves in water, -with the evolution of a large amount of heat.[34 bis] On evaporating this -solution, hydrochloric acid and aluminium hydroxide are liberated. But if -the solution be heated in a closed tube, with an excess of hydrochloric -acid, then, on cooling, crystals of AlCl_{3},6H_{2}O are obtained--that -is, aluminium chloride both combines with water and is decomposed by it. -And the faculty of the type AlX_{3} for combining with other molecules is -seen in the compounds of AlCl_{3} with many other chlorine compounds. -Thus, for example, a mixture of aluminium chloride with sulphur -tetrachloride gives Al_{2}Cl_{6},SCl_{4}, under the action of chlorine, -whilst with phosphorus pentachloride it forms AlCl_{3},PCl_{5}; it also -combines with NOCl. Thus, the compounds AlCl_{3},NOCl, AlCl_{3},POCl_{3}, -AlCl_{3},3NH_{3}, AlCl_{3},KCl, AlCl_{3},NaCl are known.[35] The compound -of aluminium and sodium chlorides, AlNaCl_{4}, is very fusible and much -more stable in the air than aluminium chloride itself. It seems to be of -the same type as the alums. This compound, AlNaCl_{4}, is employed in the -extraction of metallic aluminium, as we shall presently proceed to -describe. Aluminium bromide, which is obtained by the direct combination -of metallic aluminium with bromine, closely resembles the chloride; it -melts at 90°, volatilises at 270°, and its vapour density indicates the -formula Al_{2}Br_{6}. Aluminium iodide is obtained by heating iodine with -finely divided aluminium in a closed tube; it is so easily decomposed by -oxygen that its vapour even explodes when mixed with it.[36] - - [33 bis] It is also formed by the action of hydrochloric acid upon - metallic aluminium (Nilson and Pettersson), by heating alumina in - a mixture of the vapours of naphthaline and HCl (Faure, 1889), and - by the action of dry HCl upon an alloy of 14 p.c., or more of Al - and copper (Mobery). - - [34] Aluminium chloride fuses at 178°, boils at 183° (pressure 755 mm., - at 168° under a pressure of 250 mm., and at 213° under 2,278 mm.), - according to Friedel and Crafts, so that it boils immediately - after fusion. According to Seubert and Pallard (1892), - Al_{2}Cl_{6} fuses at 193°. Aluminium bromide fuses at about 92°, - and the iodide at 185° according to Weber, at 125° according to - Deville and Troost. - - All these halogen compounds of aluminium are soluble in water. - _Aluminium fluoride_, AlF_{3} (Al_{_n_}F_{3_n_}), is insoluble in - water. It is obtained by dissolving alumina in hydrofluoric acid; - a solution is then formed, but it contains an excess of - hydrofluoric acid. When this solution is evaporated, crystals - containing Al_{2}F_{6},HF,H_{2}O are obtained. They are also - insoluble in water. By saturating the above solution with a large - quantity of alumina, and then evaporating, we obtain crystals - having the composition Al_{2}F_{6},7H_{2}O. All these compounds, - when ignited, leave insoluble anhydrous aluminium fluoride. It - forms colourless rhombohedra, which are non-volatile, of sp. gr. - 3·1, and are decomposed by steam into alumina and hydrofluoric - acid. The acid solution apparently contains a compound which has - its corresponding salts; by the addition of a solution of - potassium fluoride, a gelatinous precipitate of AlK_{3}F_{6} is - obtained. A similar compound occurs in nature--namely, - AlNa_{3}F_{6}, or _cryolite_, sp. gr. 3·0. - - [34 bis] In this respect aluminium chloride resembles the chloranhydrides - of the acids, and probably in the aqueous solution the elements of - the hydrochloric acid are already separated, at least partially, - from the aluminium hydroxide. The solution may also be obtained by - the action of aluminium hydroxide on hydrochloric acid. - - [35] Here we see an instance in confirmation of what has been said in - Note 30--_i.e._ the action of the molecule AlCl_{3}. We will cite - still another instance confirming the power of alumina to enter - into complex combinations. Alumina, moistened with a solution of - calcium chloride, gives, when ignited, an anhydrous crystalline - substance (tetrahedral), which is soluble in acids, and contains - (Al_{2}O_{3})_{6}(CaO)_{10}CaCl_{2}. Even clay forms a similar - stony substance, which might be of practical use. - - Among the most complex compounds of aluminium, _ultramarine_, or - _lapis lazuli_, must be mentioned. It occurs in nature near Lake - Baikal, in crystals, some colourless and others of various - tints--green, blue, and violet. When heated it becomes dull and - acquires a very brilliant blue colour. In this form it is used for - ornaments (like malachite), and as a brilliant blue pigment. At - the present time ultramarine is prepared artificially in large - quantities, and this process is one of the most important - conquests of science; for the blue tint of ultramarine has been - the object of many scientific researches, which have culminated in - the manufacture of this native substance. The most characteristic - property of ultramarine is that when placed in sulphuric acid it - evolves hydrogen sulphide and becomes colourless. This shows that - the blue colour of ultramarine is due to the presence of - sulphides. If clay be heated in a furnace with sodium sulphate and - charcoal (forming sodium sulphide) without access of air, a white - mass is obtained, which becomes green when heated in the air, and - when treated with water leaves a colourless substance known as - 'white ultramarine.' When ignited in the air it absorbs oxygen and - turns blue. The coloration is ascribed to the presence of metallic - sulphides or polysulphides, but it is most probable that silicon - sulphide, or its oxysulphide, SiOS, is present. At all events the - sulphides play an important part, but the problem is not yet quite - settled. The formula Na_{8}Al_{6}Si_{6}O_{24}S is ascribed to - white ultramarine. The green probably contains more sulphur, and - the blue a still larger quantity. The last is supposed to contain - Na_{8}Al_{6}Si_{6}O_{24}S_{3}. It is more probable (according to - Guckelberger, 1882) that the composition of the blue varies - between Si_{18}Al_{18}Na_{20}S_{6}O_{71} and - Si_{18}Al_{12}Na_{20}S_{6}O_{69}. The latter may be expressed as - (Al_{2}O_{3})_{6}(SiO_{2})_{18}(Na_{2}O)_{10}S_{6}O_{5}, which - would indicate the presence of insufficiently-oxidised sulphur in - ultramarine. - - [36] At the ordinary temperature aluminium does not decompose water, - but if a small quantity of iodine, or of hydriodic acid and - iodine, or of aluminium iodide and iodine, is added to the water, - then hydrogen is abundantly evolved. It is evident that here the - reaction proceeds at the expense of the formation of Al_{2}I_{6}, - and that this substance, with water, gives aluminium hydroxide and - hydriodic acid, which, with aluminium, evolves hydrogen. Aluminium - probably belongs to those metals having a greater affinity for - oxygen than for the halogens (Note 36 tri). - -_Metallic Aluminium_ was first prepared by Wöhler in 1822 as a grey -powder by the action of potassium on aluminium chloride. He afterwards -(in 1845) obtained it as a white compact metal, unoxidisable in the air, -and only slowly attacked by acids. Owing to the vast and wide occurrence -of clay, many efforts have been made in investigating in detail the -methods for the extraction of this metal. These efforts were brought to a -successful issue (1854) by Sainte-Claire Deville, who is also renowned -for his doctrine of dissociation. Experiments on a large scale have -proved that metallic aluminium, although possessed of great lightness, -strength, and durability, is not so generally suitable for technical -purposes as was at first thought. Nitric and many other acids, indeed, do -not act on it, but the alkalis, alkaline substances, and even salts--for -instance, moist table salt--humidity, &c.,[36 bis] tarnish it, and hence -objects made of aluminium suffer at the surfaces, alter, and cannot, as -was hoped, replace the precious metals, from which it differs in its -extreme lightness. But the alloys made with aluminium (especially with -copper, for example aluminium bronze) are very valuable in their -properties and applications. - - [36 bis] As an example we may mention that if mercury comes in contact - with metallic aluminium and especially if it be rubbed upon the - surface of aluminium moistened with a dilute acid, the Al becomes - rapidly oxidised (Al_{2}O_{3} being formed). The oxidation is - accompanied by a very curious appearance, as it were of wool (or - fur) formed by threads of oxide of aluminium growing upon the - metal. This was first pointed out by Cass in 1870, and - subsequently by A. Sokoleff in 1892. This interesting and curious - phenomenon has not to my knowledge been further studied. - - I think it necessary, however, to add that according to Lubbert - and Rascher's researches (1891), wine, coffee, milk, oil, urine, - earth, &c., have no more action upon aluminium vessels than upon - copper, tin, and other similar articles. In the course of four - months ordinary vinegar dissolved 0·35 grm. of Al per sq. - centimetre, whilst a 5 per cent. solution of common salt dissolved - about 0·05 grm. of aluminium. Ditte (1890) showed that Al is acted - upon by nitric and sulphuric acids, although only slowly (owing to - the formation of a layer of gas, as in Chapter XVI., Note 10) and - that the reaction proceeds much more rapidly in vacuo or in the - presence of oxidising agents. Al is even oxidised by water on the - surface, but the thin coating of alumina formed prevents further - action. In the course of twelve hours nitric acid sp. gr. 1·383 - dissolved at 17° about 20 grms. of aluminium (containing only a - small amount of Si, 1-1/4 p.c.) from a sq. metre of surface (Le - Rouart, 1891). - - The Deville method for the preparation of metallic aluminium is - based on the decomposition of the above-mentioned compound of - sodium and aluminium chlorides by metallic sodium. The compound is - obtained by passing the vapour of aluminium chloride (evolved from - a mixture of alumina, extracted from bauxite or cryolite, with - charcoal ignited in a stream of chlorine) over red-hot salt, when - the compound AlNaCl_{4}, is itself volatilised, and may in this - manner be obtained pure. A mixture of this compound with salt and - fluor spar, or with cryolite, is heated with a certain excess of - sodium, cut into small lumps. On a large scale this operation is - carried on in special furnaces with a small access of air and at a - high temperature. The decomposition takes place chiefly according - to the equation NaAlCl_{4} + 3Na = 4NaCl + Al. Neither charcoal - nor zinc will reduce the oxygen compounds of aluminium; even - sodium and potassium do not act on alumina. Moreover, metallic - aluminium, like magnesium, is able to reduce even the metals of - the alkalis from their oxygen compounds. This is connected with - the fact that the atom of oxygen evolves more heat in combining - with Al (and Mg) than it does in combining with other metals; - whilst on the other hand, chlorine (and the other halogens) evolve - more heat in combining with the metals of the alkalis.[36 tri] - - [36 tri] In addition to the data given in Chapters XI., XIII., and - in Chapter XV., Note 19, the following are the amounts of heat in - thousands of units, evolved in the formation of the oxides and - chlorides from the metals taken in gram-atomic quantities: - - Na_{2}O 100; MgO 140*; 1/3Al_{2}O_{3} 120*; - 1/3Fe_{2}O_{3} 63*; - Na_{2}Cl_{2} 195; MgCl_{2} 151; 1/3Al_{2}Cl_{6} 107; - 1/3Fe_{2}Cl_{6} 64. - - The asterisks following the oxides of Mg, Al and Fe call attention - to the fact that the existing data refer to the formation of the - hydrates of these metals, from which the heat of formation of the - anhydrous oxides may easily be assumed, because the heat of - hydration (for example, MgO + H_{2}O) has not yet been determined. - -Since the close of the eighties the metallurgy of aluminium has taken a -new direction, based upon the action of an electric current upon cryolite -at a high temperature,[37] and the solution of oxide of aluminium -(obtained from bauxite or in the form of corundum) in it; under these -conditions metallic aluminium is reduced at the negative pole (cathode) -in a sufficiently pure state, and if the cathode be copper, forms alloys -with it. Such are Hall's and Cowle's (both in the United States) and the -Neuhausen process (where the current is obtained from a dynamo worked by -the Falls of the Rhine at Schaffhausen). As an example, we will describe -(in the words of Prof. D. P. Konovaloff, who became acquainted with this -process at the Chicago Exhibition), Hall's process as applied near -Pittsburg, where it gives about 1,500 kilos of Al a day. An iron box -(about 1 metre long and 1/2 metre wide), provided with a well rammed down -charcoal lining, is charged with a mixture of cryolite and Al_{2}O_{3} -(from bauxite), over which salt is strewn, and a current of 5,000 ampères -at 20 volts is passed through the mixture. The anode is composed of a -carbon cylinder (about 9 cm. in diameter), while the charcoal lining -forms the cathode. When the temperature inside the box is raised to a red -heat by the current, the mixture fuses and the Al_{2}O_{3} begins to -decompose. The Al liberated collects at the bottom of the box, whilst the -oxygen evolved burns the charcoal anode. When the decomposition is at an -end, and the resistance of the mass increases, a fresh quantity of -Al_{2}O_{3} is added, and this is continued until the amount of -impurities accumulated in the furnace and passing into the metal becomes -too great.[37 bis] - - [37] Cryolite under the action of the current at about 1,000° gives off - the vapour of Na which reduces the Al, but it recombines with the - liberated fluorine and again passes into the fused mass. It is - important to obtain aluminium at as low a temperature as possible, - but the action proceeds far more easily with the solution (alloy) - of oxide of aluminium in cryolite. - - [37 bis] The cost of working this process can be brought as low as 20 - cents per lb. or about 2-1/2 fcs. per kilo. In England, Castner, - prior to the introduction of the electric method, obtained Al by - taking a mixture of 1,200 parts of the double salt NaAlCl_{4}, 600 - parts of cryolite, and 350 parts of Na, and obtained about 120 - parts of Al, so that the cost of this process is about 1-1/2 time - that of the electric method. - - Buchner found that sulphide of aluminium, Al_{2}S_{3}, is more - suitable for the preparation of Al by the electrolytic method than - Al_{2}O_{3}, but since the formation of Al_{2}S_{3} by heating a - mixture of Al_{2}O_{3}, and charcoal in sulphur vapour proceeds - with difficulty, Gray (1894) proposed to prepare Al_{2}S_{3} by - heating a mixture of charcoal, sulphate of aluminium, and sodium - fluoride. The resultant molten mixture of NaF and Al_{2}S_{3} - gives aluminium directly under the action of an electric current. - -Aluminium has a white colour resembling that of tin--that is, it is -greyer than silver and has the feebly dull lustre of tin, but compared to -tin and pure silver, aluminium is very hard. Its density is 2·67--that -is, it is nearly four times lighter than silver and three times lighter -than copper. It melts at an incipient red heat (600°), and in so doing is -but slightly oxidised. At the ordinary temperature it does not alter in -the air, and in a compact mass it burns with great difficulty at a white -heat, but in thin sheets, into which it may be rolled, or as a very fine -wire, it burns with a brilliant white light, since it forms an infusible -and non-volatile oxide. Aluminium itself is non-volatile at a furnace -heat. These properties render Al a very good reducing agent, and N. N. -Beketoff showed that it reduces the oxides of the alkali metals (Chapter -XIII., Note 42 bis). Dilute sulphuric acid has scarcely any action on it, -but the strong acid dissolves it, especially with the aid of heat. Nitric -acid, dilute or strong, has no action whatever on it. On the other hand, -hydrochloric acid dissolves aluminium with great ease, as do also -solutions of caustic soda and potash. In the latter cases hydrogen is -evolved.[38] - - [38] Aluminium, when heated to the high temperature of the electric - furnace, dissolves carbon and forms an alloy which, according to - Moissan, when rapidly treated with _cold_ hydrochloric acid leaves - a compound C_{3}Al_{4} in the form of a yellow crystalline - transparent powder, sp. gr. 2·36 (_see_ Chapter VIII. Note 12 - bis). This _carbide of aluminium_ C_{3}Al_{4} corresponds to - methane CH_{4}, for Al replaces H_{3} and carbon O_{2} or H_{4}, - that is, it is equal to three molecules of CH_{4} with the - substitution of twelve atoms of H in it by four of Al, or, what is - the same thing, it is the duplicated molecule of Al_{2}O_{3} with - the substitution of O_{6} by C_{3}. And indeed C_{3}Al_{4} under - the action of water forms marsh gas and hydrate of alumina: - C_{3}Al_{4} + 12H_{2}O = 3CH_{4} + 4Al(OH)_{3}. This decomposition - gives a new aspect of the synthesis of hydrocarbons, and quite - agrees with what should follow from the action of water upon the - metallic carbides as applied by me for explaining the origin of - naphtha (Chapter VIII., Notes 57, 58, and 59). Frank (1894) by - heating Al with carbon obtained a similar although not quite pure - compound, which (like CaC_{2}) evolves acetylene with hydrochloric - acid _i.e._ probably has the composition AlC_{3}. - -Aluminium forms alloys with different metals with great ease. Among them -the copper alloy is of practical use. It is called _aluminium bronze_. -This alloy is prepared by dissolving 11 p.c. by weight of metallic -aluminium in molten copper at a white heat. The formation of the alloy is -accompanied by the development of a considerable quantity of heat, so -that it glows to a bright white heat. This alloy, which corresponds with -the formula AlCu_{3}, presents an exceedingly homogeneous mass, -especially if perfectly pure copper be taken. It is distinguished for its -capacity to fill up the most minute impressions of the mould into which -it may be cast, and by its extraordinary elasticity and toughness, so -that objects cast from it may be hammered, drawn, &c., and at the same -time it is fine-grained and exceedingly hard, takes an excellent polish, -and, what is most important, its surface then remains almost unchangeable -in the air, and has a colour and lustre which may be compared to that of -gold alloys. Hence aluminium bronze is much used in the arts for making -spoons, watches, vessels, forks, knives, and for ornaments, &c. No less -important is the fact that the admixture of one-thousandth part of -aluminium with steel renders its castings homogeneous (free from -cavities) to an extent that could not be arrived at by other means, nor -does the quality of the steel in any respect deteriorate by this -admixture, but rather is it improved. In a pure state, aluminium is only -employed for such objects as require the hardness of metals with -comparative lightness, such as telescopes and various physical apparatus -and small articles. - -According to the periodic system of the elements, the analogues of -magnesium are zinc, cadmium, and mercury in the second group. So also in -the third group, to which aluminium belongs, we find its corresponding -analogues _gallium_, _indium_, and _thallium_. They are all three so -rarely and sparingly met with in nature that they could only be -discovered by means of the spectroscope. This fact shows that they are -partially volatile, as should be the case according to the property of -their nearest neighbours, the very volatile zinc, cadmium and mercury. As -with them, in gallium, indium, and thallium the density of the metal, -decomposability of compounds, &c., rises with the atomic weight. But here -we find a peculiarity which does not exist in the second group. In the -latter, the fusibility increases with the atomic weight of magnesium, -zinc, cadmium, and mercury; indeed, the heaviest metal--mercury--is a -liquid. In the third group it is not so. In order to understand this it -is sufficient to turn our attention to the elements of the further groups -of the uneven series--for instance, to group V., containing phosphorus, -arsenic, and antimony, or to group VI., with sulphur, selenium, and -tellurium, and also to group VII., where chlorine, bromine and iodine are -situated. In all these instances the fusibility decreases with a rise of -atomic weight; the members of the higher series, the elements of a high -atomic weight, fuse with greater difficulty than the lighter elements. -The representatives of the uneven series of group III., aluminium, -gallium, indium, thallium, forming, as they do, a transition, all show an -intermediate behaviour. Here the most fusible of all is the medium metal -gallium,[38 bis] which fuses at the heat of the hand; whilst indium, -thallium, and aluminium fuse at much higher temperatures. - - [38 bis] The same is the case in group IV. of the uneven series, where - tin is the most fusible. Thus the temperature of fusion rises on - both sides of tin (silicon is very infusible; germanium, 900°; - tin, 230°; lead, 326°); as it also does in group III., starting - from gallium, for indium fuses at 176°, less easily than gallium - but more easily than thallium (294°). Aluminium also fuses with - greater difficulty than gallium. - -Zinc (group II.), which has an atomic weight 65, should be followed in -group III. by an element with an atomic weight of about 69. It will be in -the same group as Al and should consequently give R_{2}O_{3}, RCl_{3}, -R_{2}(SO_{4})_{3}, alums and similar compounds analogous to those of -aluminium. Its oxide should be more easily reducible to metal than -alumina, just as zinc oxide is more easily reduced than magnesia. The -oxide R_{2}O_{3} should, like alumina, have feeble but clearly expressed -basic properties. The metal reduced from its compounds should have a -greater atomic volume than zinc, because in the fifth series, proceeding -from zinc to bromine, the volume increases. And as the volume of zinc = -9·2, and of arsenic = 18, that of our metal should be near to 12. This is -also evident from the fact that the volume of aluminium = 11, and of -indium = 14, and our metal is situated in group III., between aluminium -and indium. If its volume = 11·5 and its atomic weight be about 69, then -its density will be nearly 5·9. The fact that zinc is more volatile than -magnesium gives reason for thinking that the metal in question will be -more volatile than aluminium, and therefore for expecting its discovery -by the aid of the spectroscope, &c. - -These properties were indicated by me for the analogue of aluminium in -1871, and I named it (_see_ Chapter XV.) eka-aluminium. In 1875, Lecoq de -Boisbaudran, who had done much work in spectrum analysis, discovered a -new metal in a zinc blende from the Pyrenees (Pierrefitte). He recognised -its individuality and difference from zinc, cadmium, indium, and the -other companions of zinc by means of the spectroscope; but he only -obtained some fractions of a centigram of it in a free state. -Consequently only a few of its reactions were determined, as, for -instance, that barium carbonate precipitates the new oxide from its salts -(alumina, as is known, is also precipitated). Lecoq de Boisbaudran named -the newly discovered metal _gallium_. As one would expect the same -properties for eka-aluminium as were observed in gallium, I pointed out -this fact at the time in the Memoirs of the Paris Academy of Sciences. -All the subsequent observations of Lecoq de Boisbaudran confirmed the -identity between the properties of gallium and those indicated for -eka-aluminium. Immediately after this the ammonium alum of gallium was -obtained, but the most convincing proof of all was found in the fact that -the density of gallium although first apparently different (4·7) from -that indicated above, afterwards, when the metal was carefully purified -from sodium (which was first used as a reducing agent), proved to be just -that (5·9) which would have been looked for in the analogue of aluminium; -and, what was very important, the equivalent (23·3) and atomic weight -(69·8) determined by the specific heat (0·08) were shown by experiment to -be such as would be expected. These facts confirmed the universality and -applicability of the periodic system of the elements. It must be remarked -that previous to it there was no means of either foretelling the -properties or even the existence of undiscovered elements.[39] - - [39] The spectrum of gallium is characterised by a brilliant violet - line of wave-length = 417 millionths of a millimetre. The metal - can be separated from the solution, containing a mixture of the - many metals occurring in the zinc blende, by making use of the - following reactions: it is precipitated by sodium carbonate in the - first portions; it gives a sulphate which, on boiling, easily - decomposes into a basic salt, very slightly soluble in water; and - it is deposited in a metallic state from its solutions by the - action of a galvanic current. It fuses at +30°, and, when once - fused, remains liquid for some time. It oxidises with difficulty, - evolves hydrogen from hydrochloric acid and from potassium - hydroxide, and, like all feeble bases (for instance, alumina and - indium oxide), it easily forms basic salts. The hydroxide is - soluble in a solution of caustic potash, and slightly so in - caustic ammonia. Gallium forms volatile GaCl_{3} and GaCl_{2} - (Nilson and Pettersson). - -Much more light has been thrown on that element of the aluminium group -which follows after cadmium (its position in the periodic system is III., -7, that is, it is in group III. in the 7th series). This is _indium_, In, -which also occurs in small quantities in certain zinc ores. It was -discovered (1863) by Reich and Richter (and more fully investigated by -Winkler) in the Freiberg zinc ores, and was named indium from the fact -that it gives to the flame of a gas-burner a blue coloration, owing to -the indigo blue spectral lines proper to it. The equivalent (_see_ -Chapter XV., Note 15), specific heat, and other properties of the metal -confirm the atomic weight In = 113.[40] - - [40] The vapour density of indium chloride, InCl_{3} (Note 31), - determined by Nilson and Pettersson, confirms this atomic weight. - Indium is separated from zinc and cadmium, with which it occurs, - by taking advantage of the fact that its hydroxide is insoluble in - ammonia, that the solutions of its salts give indium when treated - with zinc (hence indium is dissolved after zinc by acids) and that - they give a precipitate with hydrogen sulphide even in acid - solutions. Metallic indium is grey, has a sp. gr. of 7·42, fuses - at 176°, and does not oxidise in the air; when ignited, it first - gives a black suboxide, In_{4}O_{3}, then volatilises and gives a - brown oxide, In_{2}O_{3}, whose salts, InX_{3}, are also formed by - the direct action of acids on the metal, hydrogen being evolved. - Caustic alkalis do not act on indium, from which it is evident - that it is less capable of forming alkaline compounds than - aluminium is; however, with potassium and sodium hydroxides, - solutions of indium salts give a colourless precipitate of the - hydroxide, which is soluble in an excess of the alkali, like the - hydroxides of aluminium and zinc. Its salts do not crystallise. - Nilson and Pettersson (1889), by the action of HCl upon In, - obtained volatile crystalline, InCl_{2}, and by treating this - compound with In, InCl also. - -Inasmuch as we found among the analogues of magnesium in group II. a -metal, mercury, heavier and more easily reduced than the rest, and giving -two grades of oxidation, so we should expect to find a metal among the -analogues of aluminium in group III. which would be heavy, easily -reduced, and give two grades of oxidation, and would have an atomic -weight greater than 200. Such is _thallium_. It forms compounds of a -lower type, TlX, besides the higher unstable type TlX_{3}, just as -mercury gives HgX_{2} and HgX. In the form of the thallic oxide, -Tl_{2}O_{3}, the base is but feebly energetic, as would be expected by -analogy with the oxides Al_{2}O_{3}, Ga_{2}O_{3}, and In_{2}O_{3}, whilst -in thallous oxide, Tl_{2}O, the basic properties are sharply defined, as -might be expected according to the properties of the type R_{2}O (Chapter -XV.). _Thallium_ was discovered in 1861 by Crookes and by Lamy in certain -pyrites. When pyrites are employed in the manufacture of sulphuric acid, -they are burned, and give besides sulphurous anhydride the vapours of -various substances which accompany the sulphur, and are volatile. Among -these substances arsenic and selenium are found, and together with them, -thallium. These substances accumulate in a more or less considerable -quantity in the tubes through which the vapours formed in the combustion -of the pyrites have to pass. When the methods of spectrum analysis were -discovered (1860), a great number of substances were subjected to -spectroscopic research, and it was observed that those sublimations which -are obtained in the combustion of certain pyrites contained an element -having a very sharply-defined and characteristic spectrum--namely, in the -green portion of the spectra it gave a well-defined band (wave-length 535 -millionth millimetres) which did not correspond with any then known -element.[41] - - [41] Thallium was afterwards found in certain micas and in the rare - mineral crookesite, containing lead, silver, thallium, and - selenium. Its isolation depends on the fact that in the presence - of acids thallium forms thallous compounds, TlX. Among these - compounds the chloride and sulphate are only slightly soluble, and - give with hydrogen sulphide a black precipitate of the sulphide - Tl_{2}S, which is soluble in an excess of acid, but insoluble in - ammonium sulphide. - -Under the action of a galvanic current solutions of thallium salts -deposit the metal in the form of a heavy powder. It is of a grey colour -like tin, is soft like sodium, and has a metallic lustre. Its specific -gravity is 11·8, it melts at 290°, and volatilises at a high temperature. -When heated slightly above its melting point it forms an insoluble (in -water) higher oxide, Tl_{2}O_{3}, as a dark-coloured powder, generally -however accompanied by the lower oxide Tl_{2}O, which is also black but -soluble in water and alcohol. This solution has a distinctly alkaline -reaction. This _thallous oxide_, melts at 300°, and is easily obtained -from the hydroxide TlHO by igniting it without access of air (in the -presence of air the incandescent thallous oxide partly passes into -thallic oxide). _Thallous hydroxide_, TlOH, crystallises with one -molecule H_{2}O in yellow prisms which are very easily soluble in water. -Metallic thallium may be used for its preparation, as the metal in the -presence of water attracts oxygen from the air and forms the hydroxide. -But metallic thallium does not decompose water, although it gives a -hydroxide which is soluble in water.[41 bis] All the other data for the -chemical and physical properties of thallium, of its two grades of -oxidation and of their corresponding salts, are expressed by the position -occupied by this metal in virtue of its atomic weight Tl = 204, between -mercury Hg = 200, and lead Pb = 206. - - [41 bis] The best method of preparing thallous hydroxide, TlOH, is by - the decomposition of the requisite quantity of baryta by thallous - sulphate, which is slightly soluble in water; barium sulphate is - then obtained in the precipitate and thallous hydroxide in - solution. This solubility of the hydroxide is exceedingly - characteristic, and forms one of the most important properties of - thallium. These lower (thallous) compounds are of the type TlX, - and recall the salts of the alkalis. The salts TlX are colourless, - do not give a precipitate with the alkalis or ammonia, but are - precipitated by ammonium carbonate, because thallous carbonate, - Tl_{2}CO_{3}, is sparingly soluble in water. Platinic chloride - gives the same kind of precipitate as it does with the salts of - potassium--that is, thallous platinochloride, PtTl_{2}Cl_{6}. All - these facts, together with the isomorphism of the salts TlX with - those of potassium, again point out what an important significance - the types of compounds have in the determination of the character - of a given series of substances. Although thallium has a greater - atomic weight and greater density than potassium, and although it - has a less atomic volume, nevertheless thallous oxide is analogous - to potassium oxide in many respects, for they both give compounds - of the same type, RX. We may further remark that thallous - fluoride, TlF, is easily soluble in water as well as thallous - silicofluoride, SiTl_{2}F_{6}, but that thallous cyanide, TlCN, is - sparingly soluble in water. This, together with the slight - solubility of thallous chloride, TlCl, and sulphate, Tl_{2}SO_{4}, - indicates an analogy between TlX and the salts of silver, AgX. - - As regards the higher oxide or the _thallic oxide_, Tl_{2}O_{3}, - the thallium is trivalent in it--that is, it forms compounds of - the type TlX_{3}. The hydroxide, TlO(OH), is formed by the action - of hydrogen peroxide on thallous oxide, or by the action of - ammonia on a solution of thallic chloride, TlCl_{3}. It is - obtained as a brown precipitate, insoluble in water but easily - soluble in acids, with which it gives thallic salts, TlX_{3}. - Thallic chloride, which is obtained by cautiously heating the - metal in a stream of chlorine, forms an easily fusible white mass, - which is soluble in water and able to part with two-thirds of its - chlorine when heated. An aqueous solution of this salt yields - colourless crystals containing one equivalent of water. It is - evident from the above that all the thallic salts can easily be - reduced to thallous salts by reducing agents such as sulphurous - anhydride, zinc, &c. Besides these salts, thallic sulphate, - Tl_{2}(SO_{4})_{3},7H_{2}O, thallic nitrate - Tl(NO_{3})_{3},4H_{2}O, &c., are known. These salts are decomposed - by water, like the salts of many feeble basic metals--for example, - aluminium. - -Gallium, indium, and thallium belong to the uneven series, and there -should be elements of the even series in group III. corresponding with -calcium, strontium, and barium in group II. These elements should in -their oxides R_{2}O_{3} present basic characters of a more energetic kind -than those shown by alumina, just as calcium, strontium, and barium give -more energetic bases than magnesium, zinc, and cadmium. Such are -_yttrium_ and _ytterbium_, which occur in a rare Swedish mineral called -_gadolinite_, and are therefore termed the gadolinite metals. To these -belong also the metal _lanthanum_, which accompanies the two other metals -_cerium_ and _didymium_ in the mineral _cerite_, and it therefore belongs -to the cerite metals. All these metals and certain others accompanying -them, give basic oxides R_{2}O_{3}. At first their formula was supposed -to be RO, but the application of the periodic system required their being -counted as elements of groups III. and IV., which was also confirmed by -the determination of the specific heats of these metals,[42] and better -still by the fact that Nilson and Clève, in their researches on the -gadolinite metals (1879), discovered that they contain a peculiar and -very rare element, _scandium_, which by the magnitude of its atomic -weight, Sc = 44, and in all its properties, exactly corresponds with the -metal (previously foretold on the basis of the periodic system) -_ekaboron_, whose properties were determined by taking the cerite and -gadolinite metals as forming oxides R_{2}O_{3}.[43] - - [42] The specific heat of cerium determined (1870) by me, and - afterwards confirmed by Hillebrand, corresponds with that atomic - weight of cerium according to which the composition of two oxides - should be Ce_{2}O_{3} and CeO_{2}. Hillebrand also obtained - metallic lanthanum and didymium by decomposing their salts by a - galvanic current, and he found their specific heats to be near - that of cerium and about 0·04, and it is therefore justifiable to - give them an atomic weight near that of cerium, as was done on the - basis of the periodic law. Up to 1870 yttrium oxide was also given - the formula RO. Having re-determined the equivalent of yttrium - oxide (with respect to water), and found it to be 74·6, I - considered it necessary to also ascribe to it the composition - Y_{2}O_{3}, because then it falls into its proper place in the - periodic system. If the equivalent of the oxide to water be 74·6, - it contains 58·6 of metal per 16 of oxygen, and consequently one - part by weight of hydrogen replaces 29·3 of yttrium, and if it be - regarded as bivalent (oxide RO), it would not, by its atomic - weight 58·6, find a place in the second group. But if it be taken - as trivalent--that is, if the formula of its oxide be R_{2}O_{3} - and salts RX_{3}--then Y = 88, and a position is open for it in - the third group in the sixth series after rubidium and strontium. - These alterations in the atomic weights of the cerite and - gadolinite metals were afterwards accepted by Clève and other - investigators, who now ascribe a formula R_{2}O_{3} to all the - newly discovered oxides of these metals. But still the position in - the periodic system of certain elements--for example of holmium, - thulium, samarium, and others--has not yet been determined for - want of a sufficient knowledge of their properties in a state of - purity. - - [43] So, for example, in 1871, in the _Journal of the Russian - Physico-Chemical Society_ (p. 45) and in Liebig's _Annalen_, Supt. - Band viii. 198, I deduced, on the basis of the periodic law, an - atomic weight 44 for ekaboron, and Nilson in 1888 found that of - scandium, which is ekaboron, to be Sc = 44·03, The periodic law - showed that the specific gravity of the ekaboron oxide would be - about 8·5, that it would have decided but feeble basic properties - and that it would give colourless salts. And this proved to be the - case with scandium oxide. In describing scandium, Clève and Nilson - acknowledge that the particular interest attached to this element - is due to its complete identity with the expected element - ekaboron. And this accurate foretelling of properties could only - be arrived at by admitting that alteration of the atomic weights - of the cerite and gadolinite metals which was one of the first - results of the application of the periodic system of the elements - to the interpretation of chemical facts. In my first memoirs, - namely, in the _Bulletin of the St. Petersburg Academy of - Sciences_, vol. viii. (1870), and in Liebig's _Annalen_ (_l. c._ - p. 168) and others, I particularly insisted on the necessity of - altering the then accepted atomic weights of cerium, lanthanum, - and didymium. Clève, Höglund, Hillebrand and Norton, and more - especially Brauner, and others accepted the proposed alteration, - and gave fresh proofs in favour of the proposed alterations of - these atomic weights. The study of the fluorides was particularly - important. Placing cerium in the fourth group, the composition of - its highest oxide would then be CeO_{2}, and its compounds CeX_{4} - and the lower oxide, Ce_{2}O_{3} or CeX_{3}. Brauner obtained the - fluoride CeF_{4},H_{2}O corresponding with the first, and a double - crystalline salt, 3KF,2CeF_{4},2H_{2}O, without any admixture of - compound of the lower grade CeX_{3}, which generally occur - together with the majority of salts corresponding with CeX_{4}. It - will be seen from these formulæ and from the tables of the - elements, that cerium and didymium do not belong to the third - group, which is now being described, but we mention them here for - convenience, as all the cerite and gadolinite metals have much in - common. These metals, which are rare in nature, resemble each - other in many respects, always accompany each other, are with - difficulty isolated from each other, and stand together in the - periodic system of the elements; they have acquired a peculiar - interest owing to their having been in 1870 the objects of the - study of Marignac, Delafontaine, Soret, Lecoq de Boisbaudran, - Brauner, Clève, Nilson, the professors of Upsala, and others. - - The cerite and gadolinite metals occur in rare siliceous minerals - from Sweden, America, the Urals, and Baikal, such as cerite (in - Sweden), gadolinite, and orthite; and in still rarer minerals - formed by titanic, niobic, and tantalic acids, such as euxenite in - Norway and America, and samarskite in Norway, the Urals and - America, and in a few rare fluorides and phosphates. Among the - latter, monazite is found in somewhat considerable quantities in - Brazil and North Carolina; this contains the phosphate of cerium, - CePO_{4} (= Ce_{2}O_{3}P_{2}O_{3}), together with didymium, - thorium and lanthanum (according to W. Edron and Shapleigh's - analyses), and is now used for preparing that mixture of the - oxides of the rare metals (especially ThO_{2}, Ce_{2}O_{3}, - La_{2}O_{3}, &c.), which is employed for incandescent burners - (Auer von Welsbach), as it has been found by experiment that these - oxides when raised to incandescence in a non-luminous gas flame, - give a far more brilliant flame with a smaller consumption of gas, - besides being suitable for such non-luminous gases as water gas. - The insufficiency of material to work upon, and the difficulty of - separating the oxides from each other, are the chief reasons why - the composition of the compounds of these rare metals is so - imperfectly known. Cerite is the most accessible of these - minerals. Besides silica it contains more than 50 p.c. of the - oxides of cerium, lanthanum (from 4 p.c.), and didymium. The - decomposition of its powder by sulphuric acid gives sulphates, all - of which are soluble in water. The other minerals mentioned above - are also decomposed in the same manner. The solution of sulphates - is precipitated with free oxalic acid, which forms salts insoluble - in water and dilute acids with all the cerite and gadolinite - oxides. The oxides themselves are obtained by igniting the - oxalates. When ignited in the air the cerium passes from its - ordinary oxide Ce_{2}O_{3} into the higher oxide CeO_{2}, which is - so feeble a base that its salts are decomposed by water, and it is - insoluble in dilute nitric acid. Therefore it is always possible - to remove all the cerium oxide by repeated ignitions and solutions - in sulphuric acid. The further separation of the metals is mainly - based on four methods employed by many investigators. - - (_a_) A solution of the mixed salts is treated with an excess of - solid potassium sulphate. Double salts, such as - Ce_{2}(SO_{4})_{3},3K_{2}SO_{4}, are thus formed. The gadolinite - metals, namely yttrium, ytterbium, and erbium, then remain in - solution--that is, their double salts are soluble in a solution of - potassium sulphate, whilst the cerite metals--namely, cerium, - lanthanum, and didymium--are precipitated, that is, their double - salts are insoluble in a saturated solution of potassium sulphate. - This ordinary method of separation, however, appears from the - researches of Marignac to be so untrustworthy that a considerable - amount of didymium and the other metals remain in the soluble - portion, owing to the fact that, although individually insoluble, - they are dissolved when mixed together. Thus erbium and terbium - occur both in the solution and precipitate. Nevertheless, - beryllium, yttrium, erbium, and ytterbium belong to the soluble, - and scandium, cerium, lanthanum, didymium, and thorium to the - insoluble portion. The insoluble salt of scandium, for example - (_i.e._ insoluble in a solution of potassium sulphate), has a - composition Sc_{2}(SO_{4})_{3},3K_{2}SO_{4}. - - (_b_) The oxides obtained by the ignition of the oxalates are - dissolved in nitric acid (the nitrates of the cerite metals easily - form double salts with those of the alkali metals, and as - some--for example, the ammonio-lanthanum salt--crystallise very - well, they should be studied and applied to the analytical - separation of these metals), the solution is then evaporated to - dryness, and the residue fused. All nitrates are destroyed by - heat; those of aluminium and iron, &c., very easily, those of the - cerite and gadolinite metals also easily (although not so easily - as the above) but in different degrees and sequence; so that by - carrying on the decomposition carefully from the beginning it is - possible to destroy the nitrate of only one metal without touching - the others, or leaving them as insoluble basic salts. This method, - like the preceding and the two following, must be repeated as many - as seventy times to attain a really constant product of fixed - properties, that is, one in which the decomposed and undecomposed - portions contain one and the same oxide. This method, due to - Berlin and worked out by Bunsen, has given in the hands of - Marignac and Nilson the best results, especially for the - separation of the gadolinite metals, ytterbium and scandium. - - (_c_) A solution of the salts is partially precipitated by - ammonia; that is, the solution is mixed with a small quantity of - ammonia insufficient for the precipitation of the entire quantity - of the bases (fractional precipitation). Thus, the didymium - hydroxide is first precipitated from a mixture of the salts of - didymium and lanthanum. A partial separation may be effected by - repeating the solution of the precipitate and fractional - precipitation, but a perfectly pure product is scarcely - attainable. - - (_d_) The formates having different degrees of solubility - (lanthanum formate 420 parts of water per one of salt, didymium - formate 221, cerium formate 360, yttrium and erbium formates - easily soluble) give a possible means of separating certain of the - gadolinite metals from each other by a method of fractional - solution and precipitation, as Bunsen, Bahr, Clève, and others - have pointed out. - - (_e_) Crookes (1893) took advantage of the fractional - precipitation of alcoholic solutions of the chlorides by amylene, - and by this means separated, for example, erbium, terbium, and - others. - - (_f_) Lastly, oxide of thorium ThO_{2} (Chapter VIII., Note 59) is - separated by means of its solubility in a solution of sodium - carbonate. - - A good method of separating these metals is not known, for they - are so like each other. There are also only a few _methods of - distinguishing_ them from each other, and we can only add the - following four to the above. - - ^a The faculty of oxidising into a higher oxide. This is very - characteristic for cerium, which gives the oxides Ce_{2}O_{3} and - CeO_{2} or Ce_{2}O_{4}. Didymium also gives one colourless oxide, - Di_{2}O_{3}, which is capable of forming salts (of a lilac - colour), and another, according to Brauner, Di_{2}O_{5} which is - dark brown and does not form salts, so far as is known, and (like - ceric oxide) acts as an oxidising agent, like the higher oxides of - tellurium, manganese, lead, and others. Lanthanum, yttrium, and - many others are not capable of such oxidation. The presence of the - higher oxides may be recognised by ignition in a stream of - hydrogen, by which means the higher oxides are reduced to the - lower, which then remain unaltered. - - ^b The majority of the salts of the gadolinite and cerite metals - are colourless, but those of didymium and erbium are - rose-coloured, the salts of the higher oxide of cerium, CeX_{4}, - yellow, of the higher oxide of terbium, yellow, &c. Thus, the - first metals obtained from gadolinite were yttrium, giving - colourless, and erbium, giving rose-coloured, salts. Afterwards it - was found that the salts of erbium of former investigators - contained numerous colourless salts of scandium, ytterbium, &c., - so that a coloration sometimes indicates the presence of a small - impurity, as was long known to be the case in minerals, and - therefore this point of distinction cannot be considered - trustworthy. - - ^c In a solid state and in solutions, the salts of didymium, - samarium, holmium, &c., give characteristic absorption spectra, as - we pointed out in Chapter XIII., and this naturally is connected - with the colour of these salts. The most important point is, that - those metals which do not give an absorption spectrum--for - example, lanthanum, yttrium, scandium, and ytterbium--may be - obtained free from didymium, samarium, and the other metals giving - absorption spectra, because the presence of the latter may be - easily recognised by means of the spectroscope, whilst the - presence of the former in the latter cannot be distinguished, and - therefore the purification of the former can be carried further - than that of the latter. We may further remark that the - sensitiveness of the spectrum reaction for didymium is so great - that it is possible with a layer of solution half a metre thick to - recognise the presence of 1 part of didymium oxide (as salt) in - 40,000 parts of water. Cossa determined the presence of didymium - (together with cerium and lanthanum) in apatites, limestones, - bones, and the ashes of plants by this method. The main group of - dark lines of didymium correspond with wave-lengths of from 580 to - 570 millionths mm.; and the secondary to about 520, 730, 480, &c. - The chief absorption bands of samarium are 472-486, 417, 500, and - 559. Besides which, Crookes applied the investigation of the - spectra of the phosphorescent light which is emitted by certain - earths in an almost perfect vacuum, when an electric discharge is - passed through it, to the discovery and characterisation of these - rare metals. But it would seem that the smallest admixture of - other oxides (for example, bismuth, uranium) so powerfully - influences these spectra that the fundamental distinctions of the - oxides cannot be determined by this method. Besides which, the - spectra obtained by the passage of sparks through solutions or - powders of the salts are determined and applied to distinguishing - the elements, but as spectra vary with the temperature and - elasticity (concentration) this method cannot be considered as - trustworthy. - - ^d The most important point of distinction of individual metallic - oxides is given by the direct _determination of their equivalent - with respect to water_--that is, the amount of the oxide by weight - which combines (like water) with 80 parts by weight of sulphuric - anhydride, SO_{3}, for the formation of a normal salt. For this - purpose the oxide is weighed and dissolved in nitric acid, - sulphuric acid is then added, and the whole is evaporated to - dryness over a water-bath and then heated over a naked flame - sufficiently strongly to drive off the excess of sulphuric acid, - but so as not to decompose the salt (the product would in that - case not be perfectly soluble in water); then, knowing the weight - of the oxide and of the anhydrous sulphate, we can find the - equivalent of the oxide. The following are the most trustworthy - figures in this connection: scandium oxide 45·35 (Nilson), yttrium - oxide 75·7 (Clève; according to my determination, 1871--74·6), - cerous oxide--that is, the lower form of oxidation of cerium, - according to various investigators (Bunsen, Brauner, and others) - from 108 to 111, the higher oxide of cerium from 85 to 87, - lanthanum oxide, according to Brauner, 108, didymium oxide (in - salts of the ordinary lower form of oxidation) about 112 - (Marignac, Brauner, Clève), samarium oxide about 116 (Clève), - ytterbium oxide 131·3 (Nilson). It may not be superfluous here to - draw attention to the fact that the equivalent of the oxides of - all the gadolinite and cerite metals for water distribute - themselves into four groups with a somewhat constant difference of - nearly 30. In the first group is scandium oxide with equivalent - 45, in the second, yttrium oxide 76, in the third, lanthanum, - cerium, didymium, and samarium oxides with equivalent about 110, - and, in the fourth, erbium, ytterbium, and thorium oxides with - equivalent about 131. The common difference of period is nearly - 45. And if we ascribe the type R_{2}O_{3} to all the oxides--that - is, if we triple the weight of the equivalent of the oxide--we - shall obtain a difference of the groups nearly equal to 90, which, - for two atoms of the metal, forms the ordinary periodic difference - of 45. If one and the same type of oxide R_{2}O_{3} be ascribed to - all these elements (as now generally accepted, in many cases there - being insufficiently trustworthy data), then the atomic weights - should be Sc = 44, Y = 89, La = 138, Ce = 140, Di = 144, - (neodymium 140, praseodymium 144), Sm = 150, Yb = 173, also - terbium 147, holmium 162, alphayttrium 157, erbium 166, thulium - 170, decipium 171. It should be observed that there may be - instances of basic salts. If, for example, an element with an - atomic weight 90 gave an oxide RO_{2}, but salts ROX_{2}, then by - counting its oxide as R_{2}O_{3} its atomic weight would be 159. - - All the points distinguishing many gadolinite and cerite elements - have not been sufficiently well established in certain cases (for - example, with decipium, thulium, holmium, and others). At present - the most certain are yttrium, scandium, cerium, and lanthanum. In - the case of didymium, for example, there is still much that is - doubtful. Didymium, discovered in 1842 by Mosander after - lanthanum, differs from the latter in its absorption spectrum and - the lilac-rose colour of its salts. Delafontaine (1878) separated - samarium from it. Welsbach showed that it contains two particular - elements, neodymium (salts bluish-red) and praseodymium (salts - apple-green), and Becquerel (1887) by investigating the spectra of - crystals, recognised the presence of six individual elements. - Probably, therefore, many of the now recognised elements contain a - mixture of various others, and as yet there is not enough - confirmation of their individuality. As regards yttrium, scandium, - cerium, and lanthanum, which have been established without doubt, - I think that, owing to their great rarity in nature and chemical - art, it would be superfluous to describe them further in so - elementary a work as the present. We may add that Winkler (1891) - obtained a hydrogen compound of lanthanum, whose composition - (according to Brauner) is La_{2}H_{3}, as would be expected from - the composition of Na_{2}H, Mg_{2}H_{2}, &c. C. Winkler (1891), on - reducing CeO_{2} with magnesium, also remarked a rapid absorption - of hydrogen, and showed that a _hydride of cerium_, CeH_{2}, - corresponding to CaH, and the other similar hydrides of metals of - the alkaline earths, is formed (Chapter XIV., Note 63). - -The brevity of this work and the great rarity of the above-mentioned -elements will give me the right to exclude their description, all the -more as the principles of the periodic system enable many of their -properties to be foreseen, and as their practical uses (cerium oxalate is -used in medicine, and didymium oxide in the manufacture of glass, a -mixture of the oxides of lanthanum and similar metals is employed for -giving a bright light, as this mixture emits a brilliant white light when -brought to incandescence) are very limited, by reason of their great -rarity in nature, and the difficulty of separating them from one another. - - - - - CHAPTER XVIII - - SILICON AND THE OTHER ELEMENTS OF THE FOURTH GROUP - - -Carbon, which gives the compounds CH_{4}, and CO_{2}, belongs to the -fourth group of elements. The nearest element to carbon is silicon, which -forms the compounds SiH_{4} and SiO_{2}; its relation to carbon is like -that of aluminium to boron or phosphorus to nitrogen. As carbon composes -the principal and most essential part of animal and vegetable substances, -so is silicon almost an invariable component part of the rocky formations -of the earth's crust. Silicon hydride, SiH_{4}, like CH_{4}, has no acid -properties, but silica, SiO_{2}, shows feeble acid properties like -carbonic anhydride. In a free state silicon is also a non-volatile, -slightly energetic non-metal, like carbon. Therefore the form and nature -of the compounds of carbon and silicon are very similar. In addition to -this resemblance, silicon presents one exceedingly important distinction -from carbon: namely, the nature of the higher degree of oxidation. That -is, silica, silicon dioxide, or silicic anhydride, SiO_{2} is a solid, -non-volatile, and exceedingly infusible substance, very unlike carbonic -anhydride, CO_{2}, which is a gas. This expresses the essential -peculiarity of silicon. The cause of this distinction may be most -probably sought for in the polymeric composition of silica compared with -carbonic anhydride. The molecule of carbonic anhydride contains CO_{2}, -as seen by the density of this gas. The molecular weight and vapour -density of silica, were it volatile, would probably correspond with the -formula SiO_{2}, but it might be imagined that it would correspond to a -far higher atomic weight of Si_{_n_}O_{2_n_}, principally from the fact -that SiH_{4} is a gas like CH_{4}, and SiCl_{4} is a liquid and volatile, -boiling at 57°--that is, even lower than CCl_{4}, which boils at 76°. In -general, analogous compounds of silicon and carbon have nearly the same -boiling points if they are liquid and volatile.[1] From this it might be -expected that silicic anhydride, SiO_{2}, would be a gas like carbonic -anhydride, whilst in reality silica is a hard non-volatile substance,[1 -bis] and therefore it may with great certainty be considered that in this -condition it is polymeric with SiO_{2}, as on polymerisation--for -instance, when cyanogen passes into paracyanogen, or hydrocyanic acid -into cyanuric acid (Chapter IX.)--very frequently gaseous or volatile -substances change into solid, non-volatile, and physically denser and -more complex substances.[2] We will first make acquaintance with free -silicon and its volatile compounds, as substances in which the analogy of -silicon with carbon is shown, not only in a chemical but also in a -physical sense.[3] - - [1] Chloroform, CHCl_{3}, boils at 60°, and silicon chloroform, - SiHCl_{3}, at 34°; silicon ethyl, Si(C_{2}H_{5})_{4}, boils at - about 150°, and its corresponding carbon compound, - C(C_{2}H_{5})_{4}, at about 120°; ethyl orthosilicate, - Si(OC_{2}H_{5})_{4}, boils at 160°, and ethyl orthocarbonate, - C(OC_{2}H_{5})_{4}, at 158°. The specific volumes in a liquid - state--that is, those of the silicon compounds--generally are - slightly greater than those of the carbon compounds; for example, - the volumes of CCl_{4} = 94, SiCl_{4} = 112, CHCl_{3} = 81, - SiHCl_{3} = 82, of C(OC_{2}H_{5})_{4} = 186, and - Si(OC_{2}H_{5})_{4} = 201. The corresponding salts have also nearly - equal specific volumes; for example, CaCO_{3} = 37, CaSiO_{3} = 41. - It is impossible to compare SiO_{2} and CO_{2}, because their - physical states are so widely different. - - [1 bis] But silica fuses and volatilises (Moissan) in the heat of the - electric furnace, about 3000°, SiO_{2} is also partially volatile - at the temperature attained in the flame of detonating gas (Cremer, - 1892). - - [2] A property of intercombination is observable in the atoms of - carbon, and a faculty for intercombination, or polymerisation, is - also seen in the unsaturated hydrocarbons and carbon compounds in - general. In silicon a property of the same nature is found to be - particularly developed in silica, SiO_{2}, which is not the case - with carbonic anhydride. The faculty of the molecules of silica for - combining both with other molecules and among themselves is - exhibited in the formation of most varied compounds with bases, in - the formation of hydrates with a gradually decreasing proportion of - water down to anhydrous silica, in the colloid nature of the - hydrate (the molecules of colloids are always complex), in the - formation of polymeric ethereal salts, and in many other properties - which will be considered in the sequel. Having come to this - conclusion as to the polymeric state of silica since the years - 1850-1860, I have found it to be confirmed by all subsequent - researches on the compounds of silica, and, if I mistake not, this - view has now been very generally accepted. - - [3] It was only after Gerhardt, and in general subsequently to the - establishment of the true atomic weights of the elements (Chapter - VII.), that a true idea of the atomic weight of silicon and of the - composition of silica was arrived at from the fact that the - molecules of SiCl_{4}, SiF_{4}, Si(OC_{2}H_{5})_{4}, &c., never - contain less than 28 parts of silicon. - - The question _of the composition of silica_ was long the subject of - the most contradictory statements in the history of science. In the - last century Pott, Bergmann, and Scheele distinguished silica from - alumina and lime. In the beginning of the present century Smithson - for the first time expressed the opinion that silica was an acid, - and the minerals of rocks salts of this acid. Berzelius determined - the presence of oxygen in silica--namely, that 8 parts of oxygen - were united with 7 of silicon. The composition of silica was first - expressed as SiO (and for the sake of shortness S only was - sometimes written instead). An investigation in the amount of - silica present in crystalline minerals showed that the amount of - oxygen in the bases bears a very varied proportion to the amount of - oxygen in the silica, and that this ratio varies from 2 : 1 to 1 : - 3. The ratio 1 : 1 is also met with, but the majority of these - minerals are rare. Other more common minerals contain a larger - proportion of silica, the ratio between the oxygen of the bases and - the oxygen of the silica being equal to 1 : 2, or thereabouts; such - are the augites, labradorites, oligoclase, talc, &c. The higher - ratio 1 : 3 is known for a widely distributed series of natural - silicates--for example, the felspars. Those silicates in which the - amount of oxygen in the bases is equal to that in the silica are - termed _monosilicates_; their general formula will be - (RO)_{2}SiO_{2} or (R_{2}O_{3})_{2}(SiO_{2})_{3}. Those in which - the ratio of the oxygen is equal to 1 : 2 are termed _bisilicates_, - and their general formula will be ROSiO_{2} or - R_{2}O_{3}(SiO_{2})_{3}. Those in which the ratio is 1 : 3 will be - _trisilicates_, and their general formula (RO)_{2}(SiO_{2})_{3} or - (R_{2}O_{3})_{2}(SiO_{2})_{9}. - - In these formulæ the now established composition of SiO_{2}--that - is, that in which the atom of Si = 28--is employed. Berzelius, who - made an accurate analysis of the composition of felspar, and - recognised it as a trisilicate formed by the union of potassium - oxide and alumina with silica, in just the same manner as the alums - are formed by sulphuric acid, gave silica the same formula as - sulphuric anhydride--that is, SiO_{3}. In this case the formula of - felspar would be exactly similar to that of the alums--that is, - KAl(SiO_{4})_{2}, like the alums, KAl(SO_{4})_{2}. If the - composition of silica be represented as SiO_{3}, the atom of - silicon must be recognised as equal to 42 (if O = 16; or if O = 8, - as it was before taken to be, Si = 21). - - The former formulæ of silica, SiO (Si = 14) and SiO_{3} (Si = 42), - were first changed into the present one, SiO_{2} (Si = 28), on the - basis of the following arguments:--An excess of silica occurs in - nature, and in siliceous rocks free silica is generally found side - by side with the silicates, and one is therefore led to the - conclusion that it has formed acid salts. It would therefore be - incorrect to consider the trisilicates as normal salts of silica, - for they contain the largest proportion of silica; it is much - better to admit another formula with a smaller proportion of oxygen - for silica, and it then appears that the majority of minerals are - normal or slightly basic salts, whilst some of the minerals - predominating in nature contain an excess of silica--that is, - belong to the order of acid salts. - - At the present time, when there is a general method (Chapter VII.) - for the determination of atomic weights, the volumes of the - volatile compounds of silica show that its atomic weight Si = 28, - and therefore silica is SiO_{2}. Thus, for example, the vapour - density of silicon chloride with respect to air is, as Dumas showed - (1862), 5·94, and hence with respect to hydrogen it is 85·5, and - consequently its molecular weight will be 171 (instead of 170 as - indicated by theory). This weight contains 28 parts of silicon and - 142 parts of chlorine, and as an atom of the latter is equal to - 35·5, the molecule of silicon chloride contains SiCl_{4}. As two - atoms of chlorine are equivalent to one of oxygen, the composition - of silica will be SiO_{2}--that is, the same as stannic oxide, - SnO_{2}, or titanic oxide, TiO_{2}, and the like, and also as - carbonic and sulphurous anhydrides, CO_{2} and SO_{2}. But silica - bears but little physical resemblance to the latter compounds, - whilst stannic and titanic oxides resemble silica both physically - and chemically. They are non-volatile, crystalline insoluble, are - colloids, also form feeble acids like silica, &c., and they might - therefore be expected to form analogous compounds, and be - isomorphous with silica, as Marignac (1859) found actually to be - the case. He obtained stannofluorides, for example an easily - soluble strontium salt, SrSnF_{6},2H_{2}O, corresponding with the - already long known silicofluorides, such as SrSiF_{6},2H_{2}O. - These two salts are almost identical in crystalline form - (monoclinic; angle of the prism, 83° for the former and 84° for the - latter; inclination of the axes, 103° 46´ for the latter and 103° - 30´ for the former), that is, they are isomorphous. We may here add - that the specific volume of silica in a solid form is 22·6, and of - stannic oxide 21·5. - -Free silicon can be obtained in an amorphous or crystalline state. -Amorphous silicon is produced, like aluminium, by decomposing the double -fluoride of sodium and silicon (sodium silicofluoride) by means of -sodium: Na_{2}SiF_{6} + 4Na = 6NaF + Si. By treating the mass thus -obtained with water the sodium fluoride may be extracted and the residue -will consist of brown, powdery silicon. In order to free it from any -silica which might be formed, it is treated with hydrofluoric acid. This -silicon powder is not lustrous; when heated it easily ignites, but does -not completely burn. It fuses when very strongly heated, and has then the -appearance of carbon.[4] Crystalline silicon is obtained in a similar -way, but by substituting an excess of aluminium for the sodium: -3Na_{2}SiF_{6} + 4Al = 6NaF + 4AlF_{3} + 3Si. The part of the aluminium -remaining in the metallic state dissolves the silicon, and the latter -separates from the solution on cooling in a crystalline form. The excess -of aluminium after the fusion is removed by means of hydrochloric and -hydrofluoric acid. The best silicon crystals are obtained from molten -zinc; 15 parts of sodium silicofluoride are mixed with 20 parts of zinc -and 4 parts of sodium, and the mixture is thrown into a strongly heated -crucible, a layer of common salt being used to cover it; when the mass -fuses it is stirred, cooled, treated with hydrochloric acid, and then -washed with nitric acid. Silicon, especially when crystalline, like -graphite and charcoal, does not in any way act on the above-mentioned -acids. It forms black, very brilliant, regular octahedra having a -specific gravity of 2·49; it is a bad conductor of electricity, and does -not burn even in pure oxygen (but it burns in gaseous fluorine). The only -acid which acts on it is a mixture of hydrofluoric and nitric acids; but -caustic alkalis dissolve in it like aluminium, with evolution of -hydrogen, thus showing its acid character. In general silicon strongly -resists the action of reagents, as do also boron and carbon. Crystalline -silicon was obtained in 1855 by Deville, and amorphous silicon in 1826 by -Berzelius.[4 bis] - - [4] A similar form of silicon is obtained by fusing SiO_{2} with - magnesium, when an alloy of Si and Mg is also formed (Gattermann). - Warren (1888) by heating magnesium in a stream of SiF_{4} obtained - silicon and its alloy with magnesium. Winkler (1890) found that - Mg_{5}Si_{3} and Mg_{2}Si are formed when SiO_{2} and Mg are heated - together at lower temperatures, whilst at a high temperature Si - only is formed. - - [4 bis] It is very remarkable that silicon decomposes carbonic - anhydride at a white heat, forming a white mass which, after being - treated with potassium hydroxide and hydrofluoric acid, leaves a - very stable yellow substance of the formula SiCO, which is formed - according to the equation, 3Si + 2CO_{2} = SiO_{2} + 2SiCO. It is - also slowly formed when silicon is heated with carbonic oxide. It - is not oxidised when heated in oxygen. A mixture of silicon and - carbon when heated in nitrogen gives the compound Si_{2}C_{2}N, - which is also very stable. On this basis Schützenberger recognises - a group, C_{2}Si_{2}, as capable of combining with O_{2} and N, - like C. - - We may add that Troost and Hautefeuille, by heating amorphous - silicon in the vapour of SiCl_{4}, obtained crystalline silicon, - and probably at the same time lower compounds of Si and Cl were - temporarily formed. In the vapour of TiCl_{4} under the same - conditions crystalline titanium is formed (Levy, 1892). - -Silicon hydride, SiH_{4}, analogous to marsh gas was obtained first of -all in an impure state, mixed with hydrogen, by two methods: by the -action of an alloy of silicon and magnesium on hydrochloric acid,[5] and -by the action of the galvanic current on dilute sulphuric acid, using -electrodes of aluminium, containing silicon. In these cases silicon -hydride is set free, together with hydrogen, and the presence of the -hydride is shown by the fact that the hydrogen separated ignites -spontaneously on coming into contact with the air, forming water and -silica. The formation of silicon hydride by the action of hydrochloric -acid on magnesium silicide is perfectly akin to the formation of -phosphuretted hydrogen by the action of hydrochloric acid on calcium -phosphide, to the formation of hydrogen sulphide by the action of acids -on many metallic sulphides, and to the formation of hydrocarbons by the -action of hydrochloric acid on white cast iron. On heating silicon -hydride--that is, on passing it through an incandescent tube, it is -decomposed into silicon and hydrogen, just like the hydrocarbons, but the -caustic alkalis, although without action on the latter, react with -silicon hydride according to the equation: SiH_{4} + 2KHO + H_{2}O = -SiK_{2}O_{3} + 4H_{2}. - - [5] This alloy, as Beketoff and Cherikoff showed, is easily obtained by - directly heating finely divided silica (the experiment may be - conducted in a test tube) with magnesium powder (Chapter XIV., - Notes 17, 18). The substance formed, when thrown into a solution of - hydrochloric acid, evolves spontaneously inflammable and impure - silicon hydride, so that the self-inflammability of the gas is - easily demonstrated by this means. - - In 1850-60 Wöhler and Buff obtained an alloy of silicon and - magnesium by the action of sodium on a molten mixture of magnesium - chloride, sodium silicofluoride, and sodium chloride. The sodium - then simultaneously reduces the silicon and magnesium. - - Friedel and Ladenburg subsequently prepared silicon hydride in a - pure state, and showed that it is not spontaneously inflammable in - air, at the ordinary pressure, but that, like PH_{3}, and like the - mixture prepared by the above methods, it easily takes fire in air - under a lower pressure or when mixed with hydrogen. They prepared - the pure compound in the following manner: Wöhler showed that when - dry hydrochloric acid gas is passed through a slightly heated tube - containing silicon it forms a very volatile colourless liquid, - which fumes strongly in air; this is a mixture of silicon chloride, - SiCl_{4}, and _silicon chloroform_, SiHCl_{3}, which corresponds - with ordinary chloroform, CHCl_{3}. This mixture is easily - separated by distillation, because silicon chloride boils at 57°, - and silicon chloroform at 36°. The formation of the latter will be - understood from the equation Si + 3HCl = H_{2} + SiHCl_{3}. It is - an anhydrous inflammable liquid of specific gravity 1·6. It forms a - transition product between SiH_{4} and SiCl_{4}, and may be - obtained from silicon hydride by the action of chlorine and - SbCl_{5}, and is itself also transformed into silicon chloride by - the action of chlorine. Gattermann obtained SiHCl_{3} by heating - the mass obtained after the action (Note 4) of Mg upon SiO_{2}, in - a stream of chlorine (with HCl) at about 470°. Friedel and - Ladenburg, by acting on anhydrous alcohol with silicon chloroform, - obtained an ethereal compound having the composition - SiH(OC_{2}H_{5})_{3}. This ether boils at 136°, and when acted on - by sodium disengages silicon hydride, and is converted into ethyl - orthosilicate, Si(OC_{2}H_{5})_{4}, according to the equation - 4SiH(OC_{2}H_{5})_{3} = SiH_{4} + 3Si(OC_{2}H_{5})_{4} (the sodium - seems to be unchanged), which is exactly similar to the - decomposition of the lower oxides of phosphorus, with the evolution - of phosphuretted hydrogen. If we designate the group C_{2}H_{5}, - contained in the silicon ethers by Et, the parallel is found to be - exact: - - 4PHO(OH)_{2} = PH_{3} + 3PO(OH)_{3}; - 4SiH(OEt)_{3} = SiH_{4} + 3Si(OEt)_{4}. - -_Silicon chloride_, SiCl_{4}, is obtained from amorphous anhydrous -silica (made by igniting the hydrate) mixed with charcoal,[6] heated to a -white heat in a stream of dry chlorine--that is, by that general method -by which many other chloranhydrides having acid properties are obtained. -Silicon chloride is purified from free chlorine by distillation over -metallic mercury. Free silicon forms the same substance when treated with -dry chlorine. It is a volatile colourless liquid, which boils at 59° and -has a specific gravity of 1·52. It fumes strongly in air, has a pungent -smell, and in general has the characteristic properties of the acid -chloranhydrides. It is completely decomposed by water, forming -hydrochloric acid and silicic acid, according to the equation: SiCl_{4} + -4H_{2}O = Si(OH)_{4} + 4HCl.[7] - - [6] The amorphous silica is mixed with starch, dried, and then charred - by heating the mixture in a closed crucible. A very intimate - mixture of silica and charcoal is thus formed. In Chapter XI., Note - 13, we saw that elements like silicon disengage more heat with - oxygen than with chlorine, and therefore their oxygen compounds - cannot be directly decomposed by chlorine, but that this can be - effected when the affinity of carbon for oxygen is utilised to aid - the action. When the mass obtained by the action of Mg upon SiO_{2} - is heated to 300° in a current of chlorine, it easily forms - SiCl_{4} (Gattermann): besides which two other compounds, - corresponding to SiCl_{4}, are formed, namely: Si_{2}Cl_{6}, which - boils at 145° and solidifies at -1°, and Si_{3}Cl_{8}, which boils - at about 212°. These substances, which answer to corresponding - carbon compounds (C_{2}H_{6} and C_{3}H_{8}), act upon water and - form corresponding oxygen compounds; for instance, Si_{2}Cl_{6} + - 4H_{2}O = (SiO_{2}H)_{2} + 6HCl gives the analogue of oxalic acid - (CO_{2}H)_{2}. This substance is insoluble in water, decomposes - under the action of friction and heat with an explosion, and should - be called _silico-oxalic acid_, Si_{2}H_{2}O_{4} (_see_ later, Note - 11 ^{bis}). - - [7] Silicon chloride shows a similar behaviour with alcohol. This is - accompanied by a very characteristic phenomenon; on pouring silicon - chloride into anhydrous alcohol a momentary evolution of heat is - observed, owing to a reaction of double decomposition, but this is - immediately followed by a powerful cooling effect, due to the - disengagement of a large amount of hydrochloric acid--that is, - there is an absorption of heat from the formation of gaseous - hydrochloric acid. This is a very instructive example in this - respect; here two processes occurring simultaneously--one chemical - and the other physical--are divided from each other by time, the - latter process showing itself by a distinct fall in temperature. In - the majority of cases the two processes proceed simultaneously, and - we only observe the difference between the heat developed and - absorbed. In acting on alcohol, silicon chloride forms ethyl - orthosilicate, SiCl_{4} + 4HOC_{2}H_{5} = 4HCl + - Si(OC_{2}H_{5})_{4}. This substance boils at 160°, and has a - specific gravity 0·94. Another salt, ethyl metasilicate, - SiO(OC_{2}H_{5})_{2}, is also formed by the action of silicon - chloride on anhydrous alcohol; it volatilises above 300°, having a - sp. gr. 1·08. It is exceedingly interesting that these two ethereal - salts are both volatile, and both correspond with silica, SiO_{2}: - the first ether corresponds to the hydrate Si(OH)_{4}, orthosilic - acid, and the second to the hydrate SiO(OH)_{2}, metasilicic acid. - As the nature of hydrates may be judged from the composition of - salts, so also, with equal right, can ethereal salts serve the same - purpose. The composition of an ethereal salt corresponds with that - of an acid in which the hydrogen is replaced by a hydrocarbon - radicle--for instance, by C_{2}H_{5}. And, therefore, it may be - truly said that there exist at least the two silicic acids above - mentioned. We shall afterwards see that there are really several - such hydrates; that these ethereal salts actually correspond with - hydrates of silica is clearly shown from the fact that they are - decomposed by water, and that in moist air they give alcohol and - the corresponding hydrate, although the hydrate which is obtained - in the residue always corresponds with the second ethereal salt - only--that is, it has the composition SiO(OH)_{2}; this form - corresponds also to carbonic acid in its ordinary salts. This - hydrate is formed as a vitreous mass when the ethyl silicates are - exposed to air, owing to the action of the atmospheric moisture on - them. Its specific gravity is 1·77. - - _Silicon bromide_, SiBr_{4}, as well as silicon bromoform, - SiHBr_{3}, are substances closely resembling the chlorine compounds - in their reactions, and they are obtained in the same manner. - Silicon iodoform, SiHI_{3}, boils at about 220°, has a specific - gravity of 3·4, reacts in the same manner as silicon chloroform, - and is formed, together with silicon iodide, SiI_{4}, by the action - of a mixture of hydrogen and hydriodic acid on heated silicon. - Silicon iodide is a solid at the ordinary temperature, fusing at - about 120°; it may be distilled in a stream of carbonic anhydride, - but easily takes fire in air, and behaves with water and other - reagents just like silicon chloride. It may be obtained by the - direct action of the vapour of iodine on heated silicon. Besson - (1891) also obtained SiCl_{3}I (boils at 113°), SiCl_{2}I_{2} - (172°), and SiClI_{3} (220°), and the corresponding bromine - compounds. All the halogen compounds of Si are capable of absorbing - 6NH_{3} and more. Besides which Besson obtained SiSCl_{2} by - heating Si in the vapour of chloride of sulphur; this compound - melts at 74°, boils at 185°, and gives with water the hydrate of - SiO_{2}, HCl, and H_{2}S. - -The most remarkable of the haloid compounds of silicon is _silicon -fluoride_, SiF_{4}. It is a gaseous substance only liquefied by intense -cold, -100°, and is obtained (Chapter XI.) directly by the action of -hydrofluoric acid on silica and its compounds (SiO_{2} + 4HF = 2H_{2}O + -SiF_{4}), and also by heating fluorspar with silica (2CaF_{2} + 3SiO_{2} -= 2CaSiO_{3} + SiF_{4}).[8] In order to prepare silicon fluoride, sand or -broken glass is mixed with an equal quantity by weight of fluorspar and 6 -parts by weight of strong sulphuric acid, and the mixture is gently -heated. It fumes strongly in air, reacting with the aqueous vapours, -although it is produced from silica and hydrofluoric acid with the -separation of water. It is evident that a reverse reaction occurs here; -that is to say, the water reacts with the silicon fluoride, but the -reaction is not complete. This phenomenon is similar to that which occurs -when water decomposes aluminium chloride, but at the same time -hydrochloric acid dissolves aluminium hydroxide and forms the same -aluminium chloride. The relative amount of water present (together with -the temperature) determines the limit and direction of the reaction. The -faculty which silicon fluoride has of reacting with water is so great -that it takes up the elements of water from many substances--for -instance, like sulphuric acid, it chars paper. Water dissolves about 300 -volumes of this gas, but in this case it is not a common dissolution -which takes place, but a reaction. During the first absorption of silicon -fluoride by water, silicic acid is separated in the form of a jelly, but -a certain quantity of the silicon fluoride also remains in the liquid, -because the hydrofluoric acid formed dissolves the other part of the -silica[9] and forms the so-called _hydrofluosilicic acid_: H_{2}SiF_{6} = -SiF_{4} + 2HF = SiH_{2}O_{3} + 6HF - 3H_{2}O. That is to say, a -metasilicic acid, SiH_{2}O_{3}, in which O_{3} is replaced by F_{6}. This -view of the composition of hydrofluosilicic acid may be admitted, because -it forms a whole series of crystallisable and well defined salts. In -general, the whole reaction of water on silicon fluoride may be expressed -by the equation: 3SiF_{4} + 3H_{2}O = SiO(OH)_{2} + 2SiH_{2}F_{6}. -Hydrofluosilicic acid and silicic acid resemble each other as much, and -differ as much, in their chemical character as water and hydrofluoric -acid. For this reason silicic acid is a feebler acid than -hydrofluosilicic acid, and in addition to this the former is insoluble, -and the latter soluble, in water.[10] Hydrofluosilicic acid is also -formed if silicic acid be dissolved in a solution of hydrofluoric acid. -It is incapable of volatilising without decomposition, and on heating the -concentrated acid silicon fluoride is evolved, leaving an aqueous -solution of hydrofluoric acid. This is the reason why solutions of -hydrofluosilicic acid corrode glass. This decomposition may be further -accelerated by the addition of sulphuric acid, or even of other acids. -Hydrofluosilicic acid, when acting on potassium and barium salts, gives -precipitates, because the salts of these metals are but sparingly soluble -in water: thus 2KX + H_{2}SiF_{6} = 2HX + K_{2}SiF_{6}. The potassium -salt is obtained in the form of very fine octahedra, but the precipitate -does not form quickly, and at first appears as a jelly. Nevertheless, the -decomposition is complete, and it is taken advantage of for obtaining -their corresponding acids from salts of potassium.[10 bis] - - [8] This property of calcium fluoride of converting silica into a gas - and a vitreous fusible slag of calcium silicate is frequently taken - advantage of in the laboratory and in practice in order to remove - silica. The same reaction is employed for preparing silicon - fluoride on a large scale in the manufacture of hydrofluosilicic - acid (see sequel). - - [9] The amount of heat developed by the solution of silicic acid, - SiO_{2}_n_H_{2}O, in aqueous hydrofluoric acid, _x_HF_n_H_{2}O, - increases with the magnitude of _x_ and normally equals _x_5,600 - heat units, where _x_ varies between 1 and 8. However, when _x_ = - 10 the maximum amount of heat is developed (= 49,500 units), and - beyond that the amount decreases (Thomsen). - - [10] In reality, however, it would seem that the reaction is still more - complex, because the aqueous solution of silicon fluoride does not - yield a hydrate of silica, but a fluo-hydrate (Schiff), - Si_{2}O_{3}(OH)F, corresponding to the (pyro) hydrate - Si_{2}O_{3}(OH)_{2}, equal to SiO(OH)_{2}SiO_{2}, so that the - reaction of silicon fluoride on water is expressed by the - equation: 5SiF_{4} + 4H_{2}O = 3SiH_{2}F_{6} + Si_{2}O_{3}(OH)F + - HF. However, Berzelius states that the hydrate, when well washed - with water, contains no fluorine, which is probably due to the - fact that an excess of water decomposes Si_{2}O_{3}(OH)F, forming - hydrofluoric acid and the compound Si_{2}O_{3}(OH)_{2}. Water - saturated with silicon fluoride disengages silicon fluoride and - hydrofluoric acid when treated with hydrochloric acid, the - gelatinous precipitate being simultaneously dissolved. It may be - further remarked that hydrofluosilicic acid has been frequently - regarded as SiO_{2},6HF, because it is formed by the solution of - silica in hydrofluoric acid, but only two of these six hydrogens - are replaced by metals. On concentration, solutions of the acid - begin to decompose when they reach a strength of 6H_{2}O per - H_{2}SiF_{6}, and therefore the acid may be regarded as - Si(OH)_{4},2H_{2}O,6HF, but the corresponding salts contain less - water, and there are even anhydrous salts, R_{2}SiF_{6}, so that - the acid itself is most simply represented as H_{2}SiF_{6}. - - If gaseous silicon fluoride be passed directly into water, the - gas-conducting tube becomes clogged with the precipitated silicic - acid. This is best prevented by immersing the end of the tube - under mercury, and then pouring water over the mercury; the - silicon fluoride then passes through the mercury, and only comes - into contact with the water at its surface, and consequently the - gas-conducting tube remains unobstructed. The silicic acid thus - obtained soon settles, and a colourless solution with a pleasant - but distinctly acid taste is procured. - - Mackintosh, by taking 9 p.c. of hydrofluoric acid, observed that - in the course of an hour its action on opal attained 77 p.c. of - the possible, and did not exceed 1-1/2 p.c. of its possible action - on quartz during the same time. This shows the difference of the - structure of these two modifications of silica, which will be more - fully described in the sequel. - - [10 bis] The sodium salt is far more soluble in water, and crystallises - in the hexagonal system. The magnesium salt, MgSiF_{6}, and - calcium salt are soluble in water. The salts of hydrofluosilicic - acid may be obtained not only by the action of the acid on bases - or by double decompositions, but also by the action of - hydrofluoric acid on metallic silicates. Sulphuric acid decomposes - them, with evolution of hydrofluoric acid and silicon fluoride, - and the salts when heated evolve silicon fluoride, leaving a - residue of metallic fluoride, R_{2}F_{2}. - -Silicon, having so much in common with carbon, is also able to combine -with it in the proportion given by the law of substitution, that is, it -forms a carbide of silicon CSi, called _carborundum_ and obtained by -Mühlhäuser and Acheson in the United States, and by Moissan in France -(1891), and others, by reducing silica with carbon in the electrical -furnace at a temperature of about 2500°[11], _i.e._ by the action of an -electrical current upon a mixture of carbon and SiO_{2} with NaCl. After -treating the resultant mass with acids and washing with water, -carborundum is obtained in transparent, lustrous grains of a greenish -color, possessing great hardness (greater than corundum) and therefore -used for polishing the hardest kinds of steel and stones. The specific -gravity is about 3·1. Carborundum does not alter at a red heat, does not -burn, and apparently approaches the diamond in its properties. (Moissan -obtained, 1894, a similar very hard compound for boron, B_{6}C, sp. gr. -2·5.) - - [11] _See_ Note 4 bis. Probably Schützenberger had already obtained CSi - in his researches together with other silicon compounds. An - amorphous, less hard compound of the same alloy is also obtained - together with the hard crystalline CSi. - -According to the principle of substitution, if silicon forms SiH_{4}, a -series of hydrates, or hydroxyl derivatives, ought to exist corresponding -to it. The first hydrate of an alcoholic character ought to have the -composition SiH_{3}(OH); the second hydrate SiH_{2}(OH)_{2}; the third, -SiH(OH)_{3};[11 bis] and the last, Si(OH)_{4}. The last is a hydrate of -silica, because it is equal to SiO_{2} + 2H_{2}O); and it is formed by -the action of water on silicon chloride, when all four atoms of chlorine -are replaced by four hydroxyl groups. It does not, however, remain in -this state, but easily loses part of its water. - - [11 bis] The following consideration is very important in explaining - the nature of the lower hydrates which are known for silicon. If - we suppose water to be taken up from the first hydrates (just as - formic acid is CH(OH)_{3}, _minus_ water), we shall obtain the - various lower hydrates corresponding with silicon hydride. When - ignited they should, like phosphorous and hypophosphorous acids, - disengage silicon hydride, and leave a residue of silica - behind--_i.e._ of the oxide corresponding to the highest - hydrate--just as organic hydrates (for example, formic acid with - an alkali) form carbonic anhydride as the highest oxygen compound. - Such imperfect hydrates of silicon, or, more correctly speaking, - of silicon hydride, were first obtained by Wöhler (1863) and - studied by Geuther (1865), and were named after their - characteristic colours. (_See_ Note 6). - - _Leucone_ is a white hydrate of the composition SiH(OH)_{3}. It is - obtained by slowly passing the vapour of silicon chloroform into - cold water: SiHCl_{3} + 3H_{2}O = SiH(OH)_{3} + 3HCl. But this - hydrate, like the corresponding hydrate of phosphorus or carbon, - does not remain in this state of hydration, but loses a portion of - its water. The carbon hydrate of this nature, CH(OH)_{3}, loses - water and forms formic acid, CHO(OH); but the silicon hydrate - loses a still greater proportion of water, 2SiH(OH)_{3}, parting - with 3H_{2}O, and consequently leaving Si_{2}H_{2}O_{3}. This - substance must be an anhydride; all the hydrogen previously in the - form of hydroxyl has been disengaged, two remaining hydrogens - being left from SiH_{4}. The other similar hydrate is also white, - and has the composition Si_{3}H_{2}O (nearly). It may be regarded - as the above white hydrate + SiO_{2}. A yellow hydrate, known as - _chryseone_ (silicone), is obtained by the action of hydrochloric - acid on an alloy of silicon and calcium; its composition is about - Si_{6}H_{4}O_{3}. Most probably, however, chryseone has a more - complex composition, and stands in the same relation to the - hydrate SiH_{2}(OH)_{3} as leucone does to the hydrate - SiH(OH)_{3}, because this very simply expresses the transition of - the first compound into the second with the loss of water, - SiH_{2}(OH)_{3} - H_{2} + H_{2}O = SiH(OH)_{3}. When these lower - hydrates are ignited without access of air, they are decomposed - into hydrogen, silicon, and silica--that is, it may be supposed - that they form silicon hydride (which decomposes into silicon and - hydrogen) and silica (just as phosphorous and hypophosphorous - acids give phosphoric acid and phosphuretted hydrogen). When - ignited in air, they burn, forming silica. They are none of them - acted on by acids, but when treated with alkalis they evolve - hydrogen and give silicates; for example, leucone: SiH_{2}O_{3} + - 4KHO = 2SiK_{2}O_{3} + H_{2}O + 2H_{2}. They have no acid - properties. - -Silica or silicic anhydride, both in the free state and in combination -with other oxides, enters into the composition of most of the rocky -formations of the earth's crust. These silicious compounds are substances -varying so much in their properties, crystalline forms, and relations to -one another that they are comprised in a special branch of natural -science (like the carbon compounds), and are treated of in works on -mineralogy; so that, in dealing with them further, we shall only give a -short description of these various compounds. It is first of all -necessary to turn to the description of silica itself, especially as it -is not unfrequently met with in nature in a separate state, and often -forms whole masses of rocky formations, called 'quartz.' In an anhydrous -condition silica appears in the greatest variety of natural -forms--sometimes in well-formed crystals, hexagonal prisms, terminated by -hexagonal pyramids. If the crystals are colourless and transparent, they -are called _rock crystal_. This is the purest form of silica. Prismatic -crystals of rock crystal sometimes attain considerable size, and as they -are remarkable for their unchangeability, great hardness, and high index -of refraction, they are used for ornaments, for seals, making necklaces, -&c.[12] Rock crystal coloured with organic matter in contact with which -it has been produced has a brown or greyish colour, and then bears the -name of _cairngorm_ or _smoky quartz_. In this form it has the same uses -as rock crystal, especially as it is often found in large masses. The -same mineral, frequently occurs, coloured red or pink by manganese or -iron oxides, especially in aqueous formations, and is then known as -_amethyst_. When finely coloured the amethyst is used as a precious -stone, but amethysts most frequently occur as small crystals in the -cavities formed in other rocky formations, and especially in those formed -in silica itself. A similar anhydrous silica is often found in -transparent non-crystalline masses, having the same specific gravity as -rock crystal itself (2·66). In this case it is called _quartz_. Sometimes -it forms complete rocky formations, but more often penetrates or is -interspersed through other rocky formations, together with other -siliceous compounds. Thus, in granite, quartz is mixed with felspar and -similar substances. Sometimes the colouring of quartz is so considerable -that it is hardly transparent in thin sheets, but it is often found in -transparent masses slightly coloured with various tints. The existence in -nature of enormous masses of quartz proves that it resists the action of -water. When water destroys rocky formations, the siliceous minerals which -they contain are partly dissolved and partly transformed into clay, &c. -But the quartz remains untouched, in the form of grains in which it -existed in the rocky formation; sometimes, when crushed, it is carried -away by the water and deposited. This is the nature of _sand_. Naturally, -sometimes other rocky substances which are not changed by water, or only -slightly acted on by it, are found in sand; but as these latter are more -or less changed by the continuous action of water, it is not unusual to -find sand which consists almost entirely of pure quartz. Common sand is -generally coloured yellow or reddish-brown by foreign mineral matter, -consisting principally of ferruginous minerals and clays. The purest or -so-called quartz sand is, however, rarely found, and is recognised by the -absence of colour, and also by the test that when shaken in water it does -not form any turbidity: this shows the absence of clay; when fused with -bases it forms a colourless glass, and on this account is a valuable -material for the manufacture of glass. Sands were formed at all periods -of the earth's existence; the ancient ones, compressed by strata of more -recent formation and permeated with various substances (deposited from -the infiltrating water), are sometimes solidified into rock, called -_sandstone_, composing, in some places, whole mountain chains, and -serviceable as a most excellent building material, on account of the -slight change it undergoes under the influence of atmospheric agencies, -and on account of the facility with which it may be wrought from rocky -formations into immense regularly-shaped flags--the latter property is -due to the primary laminar structure of the sand formations deposited, as -above-mentioned, by water. Many grindstones and whetstones are made from -such rocks. - - [12] Two modifications of rock crystal are known. They are very easily - distinguished from each other by their relation to polarised - light; one rotates the plane of polarisation to the right and the - other to the left--in the one the hemihedral faces are right and - in the other they are left; this opposite rotatory power is taken - advantage of in the construction of polarisers. But, with this - physical difference--which is naturally dependent on a certain - difference in the distribution of the molecules--there is not only - no observable difference in the chemical properties, but not even - in the density of the mass. Perfectly pure rock crystal is a - substance which is most invariable with respect to its specific - gravity. The numerous and accurate determinations made by - Steinheil on the specific gravity of rock crystal show that (if - the crystal be free from flaws) it is very constant and is equal - to 2·66. - -Perfectly pure anhydrous silica is not only known in the condition of -rock crystal and quartz having a specific gravity of 2·6, but also in -another special form, having other chemical and physical properties. This -variety of silica has a specific gravity of 2·2, and is formed by fusing -rock crystal or heating silicic acid.[12 bis] Silicic acid, when heated -to a dull red heat, parts entirely with the water it contains, and leaves -an exceedingly fine amorphous mass of silica (easily levigated, but -difficult to moisten); it is characterised by such excessive friability -that, when lightly blown on, a large mass of it rises into the air like a -cloud of dust. A mass of anhydrous silica maybe poured in this way from -one vessel to another like a liquid, and like the latter it takes a -horizontal position in the vessel containing it.[13] Anhydrous silica, -like quartz, does not fuse in the heat of a furnace, but it fuses in the -oxyhydrogen flame to a colourless glassy mass exactly similar to that -formed in the same way from rock crystal. In this condition silica has a -specific gravity of 2·2.[13 bis] Both forms of silica are insoluble in -ordinary acids, and even when they are in the state of powder, alkalis in -solution act very slowly and feebly on them; rock crystal offers much -greater resistance to the action of alkalis than the powder obtained by -heating the hydrate. The latter is quite soluble, although but slowly, in -hot alkaline solutions. This last property appertains in a greater degree -to anhydrous silica having a specific gravity of 2·2 than to that which -has a specific gravity of 2·6. Hydrofluoric acid more easily transforms -the former into silicon fluoride than it does the latter. Both varieties -of silica, when taken in the form of powder, easily combine with bases, -forming, on being fused with an alkali, a vitreous slag, which is a salt -corresponding with silica. Glass is such a salt, formed of alkalis and -alkaline earthy bases; if the glass does not contain any of the -latter--that is, if only alkaline glass be taken--a mass soluble in water -is obtained. In order to obtain such _soluble glass_, potassium or sodium -carbonates, or better a mixture of the two (fusion mixture), is fused -with fine sand. A still better and further saturation of the alkalis with -silica is effected by the action of alkaline solutions on the silicon -hydrate met with in nature; for instance, an alkaline solution is often -made use of to act on the so-called _tripoli_, or collection of siliceous -skeletons of the lowest microscopical infusoria, which is sometimes found -in considerable layers in the form of a sandy mass. Tripoli is used for -polishing, not only on account of the considerable hardness of the -silica, but also because the microscopic bodies of the infusoria have a -pointed shape, which, however, is not angular, so that they do not -scratch metals like sand.[14] The alkaline solutions of silica obtained -by boiling tripoli with caustic soda under pressure contain various -proportions of silica and alkali.[14 bis] In order that it may contain -the greatest amount of silica, silicic acid should be added to the heated -solution. Silicic acid is formed by taking any solution containing silica -and alkali, and adding to it, by degrees, some acid--for instance, -sulphuric or hydrochloric; if the experiment be carried on carefully and -the solution be concentrated, the whole mass thickens to a jelly, due to -the gelatinous form of the _silicic acid_ separated from the salt by the -action of the acid. The decomposition may be expressed by the following -equation: Si(ONa)_{4} + 4HCl = 4NaCl + Si(OH)_{4}. The hydrate separated, -Si(OH)_{4}, easily loses part of the water and forms a jelly, the whole -mass gelatinising if the solution be strong enough.[15] - - [12 bis] Several other modifications are known as minute crystals. For - example, there is a particular mineral first found in Styria and - known as _tridymite_. Its specific gravity 2·3 and form of - crystals clearly distinguish it from rock crystal; its hardness is - the same as that of quartz--that is, slightly below that of the - ruby and diamond. - - [13] There is a distinct rise of temperature (about 4°) when amorphous - silica is moistened with water. Benzene and amyl alcohol also give - an observable rise of temperature. Charcoal and sand give the same - result, although to a less extent. - - [13 bis] Silica also occurs in nature in two modifications. The opal - and tripoli (infusorial earth) have a specific gravity of about - 2·2, and are comparatively easily soluble in alkalis and - hydrofluoric acid. Chalcedony and flint (tinted quartzose - concretions of aqueous origin), agate and similar forms of silica - of undoubted aqueous origin, although still containing a certain - amount of water, have a specific gravity of 2·6, and correspond - with quartz in the difficulty with which they dissolve. This form - of silica sometimes permeates the cellulose of wood, forming one - of the ordinary kinds of petrified wood. The silica may be - extracted from it by the action of hydrofluoric acid, and the - cellulose remains behind, which clearly shows that silica in a - soluble form (see sequel) has permeated into the cells, where it - has deposited the hydrate, which has lost water, and given a - silica of sp. gr. 2·6. The quartzose stalactites found in certain - caves are also evidently of a similar aqueous origin; their sp. - gr. is also 2·6. As crystals of amethyst are frequently found - among chalcedonies, and as Friedau and Sarrau (1879) obtained - crystals of rock crystal by heating soluble glass with an excess - of hydrate of silica in a closed vessel, there is no doubt but - that rock crystal itself is formed in the wet way from the - gelatinous hydrate. Chroustchoff obtained it directly from soluble - silica. Thus this hydrate is able to form not only the variety - having the specific gravity 2·2 but also the more stable variety - of sp. gr. 2·6; and both exist with a small proportion of water - and in a perfectly anhydrous state in an amorphous and crystalline - form. All these facts are expressed by recognising silica as - dimorphous, and their cause must be looked for in a difference in - the degree of polymerisation. - - [14] Deposits of perfectly white tripoli have been discovered near - Batoum, and might prove of some commercial importance. - - [14 bis] Alkaline solutions, saturated with silica and known as _soluble - glass_, are prepared on a large scale for technical purposes by - the action of potassium (or sodium) hydroxide in a steam boiler on - tripoli or infusorial earth, which contains a large proportion of - amorphous silica. All solutions of the alkaline silicates have an - alkaline reaction, and are even decomposed by carbonic acid. They - are chiefly used by the dyer, for the same purposes as sodium - aluminate, and also for giving a hardness and polish to stucco and - other cements, and in general to substances which contain lime. A - lump of chalk when immersed in soluble glass, or better still when - moistened with a solution and afterwards washed in water (or - better in hydrofluosilicic acid, in order to bind together the - free alkali and make it insoluble), becomes exceedingly hard, - loses its friability, is rendered cohesive, and cannot be - levigated in water. This transformation is due to the fact that - the hydrate of silica present in the solution acts upon the lime, - forming a stony mass of calcium silicate, whilst the carbonic acid - previously in combination with the lime enters into combination - with the alkali and is washed away by the water. - - [15] The equation given above does not express the actual reaction, for - in the first place silica has the faculty of forming compounds - with bases, and therefore the formula SiNa_{4}O_{4} is not rightly - deduced, if one may so express oneself. And, in the second place, - silica gives several hydrates. In consequence of this, the hydrate - precipitated does not actually contain so high a proportion of - water as Si(OH)_{4}, but always less. The insoluble gelatinous - hydrate which separates out is able (before, but not after, having - been dried) to dissolve in a solution of sodium carbonate. When - dried in air its composition corresponds with the ordinary salts - of carbonic acid--that is, SiH_{2}O_{3}, or SiO(OH)_{2}. If - gradually heated it loses water by degrees, and, in so doing, - gives various degrees of combination with it. The existence of - these degrees of hydration, having the composition - SiH_{2}O_{3}_n_SiO_{2}, or, in general, _n_SiO_{2}_m_H_{2}O, where - _m_ < _n_, must be recognised, because most varied degrees of - combination of silica with bases are known. The hydrate of silica, - when not dried above 30°, has a composition of nearly - H_{4}Si_{3}O_{8} = (H_{2}SiO_{3})_{2}SiO_{2}, but at 60° contains - a greater proportion of silica--that is, it loses still more - water; and at 100° a hydrate of the composition - SiH_{2}O_{3}2SiO_{2}, and at 250° a hydrate having approximately a - composition SiH_{2}O_{3}7SiO_{2} is obtained. - - These data show the complexity of the molecules of anhydrous - silica. The hydrates of silica easily lose water and give the - hydrates (SiO_{2})_{_n_}(H_{2}O)_{_m_}, where _m_ becomes smaller - and smaller than _n_. In the natural hydrates, this decrement of - water proceeds quite consecutively, and, so to say, imperceptibly, - until _n_ becomes incomparably greater than _m_, and when the - ratio becomes very large, anhydrous silica of the two - modifications 2·6 and 2·2 is obtained. The composition - (SiO_{2})_{10},H_{2}O still corresponds with 2·9 p.c. of water, - and natural hydrates often contain still less water than this. - Thus some opals are known which contain only 1 p.c. of water, - whilst others contain 7 and even 10 p.c. As the artificially - prepared gelatinous hydrate of silica when dried has many of the - properties of native opals, and as this hydrate always loses water - easily and continually, there can be no doubt that the transition - of (SiO_{2})_{_n_}(H_{2}O)_{_m_} into anhydrous silica, both - amorphous and crystalline (in nature, chalcedony), is accomplished - gradually. This can only be the case if the magnitude of _n_ be - considerable, and therefore the molecule of silica in the hydrate - is undoubtedly complex, and hence the anhydrous silica of sp. gr. - 2·2 and 2·6 does not contain SiO_{2}, but a complex molecule, - Si_{_n_}O_{2_n_}--that is, the structure of silica is polymeric - and complex, and not simple as represented above by the formula - SiO_{2}. - -Neither of the two varieties of anhydrous silica, nor the various -natural gelatinous hydrates, are directly soluble in water. There is, -however, a condition of silica known which is soluble in water, _soluble -silica_, and silica is found in this state in nature. Small quantities of -soluble silica are met with in all waters. Certain mineral springs, and -especially hot springs--of which the best known are the Geysers of -Iceland and those in the North American National Park (Yellowstone -Valley)--contain a considerable amount of silica in solution. Such water, -permeating the objects it meets with--for instance, wood--penetrates into -them and deposits silica inside them, that is, transforms them into a -petrified condition. Siliceous stalactites, and also many (if not all) -forms of silica are formed by such water. The absorption of silica by -plants by means of their roots, and also by the lower organisms having -siliceous bodies, is due also to their nourishing themselves with the -solutions containing silica continually formed in nature. Thus, in -plants, in the straws of the grasses, in hard shave-grass, and especially -in the knots of bamboo and other straw-like plants, a considerable -quantity of silica is deposited, which must previously have been absorbed -by the plants. - -Silicic acid is a colloid. The gelatinous silicon hydrate is its -hydrogel, the soluble hydrate is the hydrosol (Chapter XII.) Both -varieties may be easily obtained from the alkaline silicates and from -water-glass. The very same substances--that is, aqueous solutions of -soluble glass and acid--taken in the same proportion, may produce either -the gelatinous or the soluble silica, according to the way these -solutions are mixed together. If the acid be added little by little to -the _alkaline silicate_, with continuous stirring, a moment arrives when -the whole mass thickens to a jelly, hydrogel; in this case the silicic -acid is formed in the midst of the alkaline solution and becomes -insoluble. But if the mixing be done in the reverse order--that is, if -the soluble glass be added to the acid, or if a quantity of acid be -rapidly poured into the solution of the salt--then the separation of the -silica takes place in the midst of the acid liquid, and it is obtained in -the form of the soluble hydrate, the hydrosol.[16] - - [16] The presence of an excess of acid aids the retention of the silica - in the solution, because the gelatinous silica obtained in the - above manner, but not heated to 60°--that is, containing more - water than the hydrate H_{2}SiO_{3}--is more soluble in water - containing acid than in pure water. This would seem to indicate a - feeble tendency of silica to combine with acids, and it might even - have been imagined that in such a solution the hydrate of silica - is held in combination by an excess of acid, had Graham not - obtained soluble silica perfectly free from acid, and if there - were not solutions of silica free from any acid in nature. At all - events a tolerably strong solution of free silica or silicic acid - may be obtained from soluble glass diluted with water. The - solution, besides silica, will contain sodium chloride and an - excess of the acid taken. If this solution remains for some time - exposed to the air, or in a closed vessel, and under various other - conditions, it is found that, after a time, insoluble gelatinous - silica separates out--that is, the soluble form of silica is - unstable, like the soluble form of alumina. The analogous forms of - molybdic or tungstic acids may be heated, evaporated, and kept for - a long period of time without the soluble form being converted - into the insoluble. - -The hydrosol of silica prepared by mixing an excess of hydrochloric acid -with a solution of sodium silicate, may be freed from the admixtures both -of hydrochloric acid and salt, sodium chloride, _by means of -dialysis_,[17] as Graham showed (in 1861) in enquiring into the nature of -colloids (Chapter I.), and making many other important chemical -investigations. The solution, containing the acid, salt, and silica, all -dissolved in water, is poured into a dialyser--that is, a vessel with a -porous diaphragm surrounded by water. Certain substances pass more easily -through the diaphragm than others. This may be represented thus: the -passage through the diaphragm proceeds in both directions, and if the -solutions on each side of the diaphragm be equally strong, there will be -equal numbers of molecules of the soluble substance passing into either -side in a given time, some passing quickly and others slowly. The -metallic chlorides and hydrochloric acid belong to the series of -crystalloids which easily pass through a diaphragm, and therefore the -hydrochloric acid and sodium chloride contained in the above-mentioned -dialyser pass from the solution through the diaphragm into the water of -the external vessel with considerable rapidity. The aqueous solution of -colloidal silica also penetrates through the diaphragm, but very much -more slowly. But if the amount of the substance dissolved is not equal on -either side of the diaphragm, the whole system strives to attain a state -of equilibrium; that is, the given substance penetrates through the -diaphragm from the side where it is in excess to the part where there is -a smaller quantity of it. All substances which are soluble in water have -the faculty of penetrating through a membrane swollen in water, but the -velocity of penetration is not equal, and in this respect the dialyser -separates substances like a sieve. The silica passes less rapidly through -the diaphragm than the sodium chloride and hydrochloric acid, so that by -repeatedly changing the external water it is easy to effect the -extraction of the chlorine compounds from the dialyser, which will -finally only contain a solution of silica. This extraction (of HCl and -NaCl) may be so complete that the liquid taken from the dialyser will not -give any precipitate with a solution of silver nitrate. Graham obtained -in this way soluble silica having a distinctly acid reaction, which, -however, disappeared on the addition of a very minute quantity of alkali; -for ten parts of silica in the solution it was sufficient to take one -part of alkali in order to give the liquid an alkaline reaction, so -slightly energetic are the acid properties of silicic acid. The solution -of silica obtained by this method becomes gelatinous on standing, on -being heated, or on evaporation under the receiver of an air-pump, &c. -The hydrosol is transformed into the hydrogel, the soluble hydrate into -the gelatinous. - - [17] _See_ Chapter I., Note 18. A solution of water-glass mixed with an - excess of hydrochloric acid is poured into the dialyser, and the - outer vessel is filled with water, which is continually renewed. - The water carries off the sodium chloride and hydrochloric acid, - and the hydrosol remains in the dialyser. - -Thus in addition to the gelatinous form of the silicic acid, there -exists also a variety of this substance, soluble in water, as is the case -with alumina. Such variation in properties and exactly the same relations -with regard to water characterise an immense series of other substances -having a great significance in nature. The number of such substances is -especially great among organic compounds, and particularly in those -classes of them which compose the principal material of the bodies of -animals and plants. It is sufficient to mention, for instance, the -gelatin which is familiar to all as carpenter's and other glues, and in -the form of size and jelly. The same substance is also known in the -solution which is used to join objects together. In a peculiar insoluble -condition it enters into the composition of hides and bones. These -various forms of gelatin differ in the same way as the different -varieties of silica. The property of forming a jelly is exactly the same -as in silica, and the adhesiveness of the solutions of both substances is -identical; soluble silica adheres like a solution of gelatin. The same -properties are again shown by starch, rosin, and albumin, and by a series -of similar substances. The diaphragms used in dialysis are also -insoluble, gelatinous, forms of colloids. The bodies of animals and -plants consist largely of similar matter, insoluble in water, -corresponding with the gelatinous or insoluble silicon hydrate, or with -glue. The albumin which coagulates when eggs are boiled is a typical form -of the gelatinous condition of such substances in the body. These slight -indications are sufficient in order to show how great is the significance -of those transformations which are so well marked in silica. The facts -discovered by _Graham_ in 1861-1864 comprise the most essential -acquisitions in the general association of these phenomena of nature in -the history of organic forms. The facility of transit from hydrogel to -hydrosol is the first condition of the possibility of the development of -organisms. The blood contains hydrosols, and the hydrogels of the same -substances are contained in the muscles and tissues, and especially on -the surface, of the body. All tissues are formed from the blood, and in -that case the hydrosols are converted into hydrogels.[18] The absence of -crystallisation, the property, apparently under the influence of feeble -agencies, of passing from the soluble condition to the insoluble, to the -gelatinous condition of the hydrogel, constitute the fundamental -properties of all colloids.[19] - - [18] A similar process occurs in plants--for example, when they secrete - a store of material for the following year in their bulbs, roots, - &c. (for instance, the potato in its tubers), the solutions from - the leaves and stems penetrate into the roots and other parts in - the form of hydrosols, where they are converted into - hydrogels--that is, into an insoluble form, which is acted on with - difficulty and is easily kept unaltered until the period of - growth--for example, until the following spring--when they are - reconverted into hydrosols, and the insoluble substance re-enters - into the sap, and serves as a source of the hydrogels in the - leaves and other portions of plants. - - [19] As regards their chemical composition the colloids are very - complex--that is, they have a high molecular weight and a large - molecular volume--in consequence of which they do not penetrate - through membranes, and are easily subject to variation in their - physical and chemical properties (owing to their complex structure - and polymerism?) They have but little chemical energy, and are - generally feeble acids, if belonging to the order of oxides or - hydrates, such as the hydrates of molybdic and tungstic acids - (Chapter XXI.). But now the number of substances capable, like - colloids, of passing into aqueous solutions and of easily - separating out from them, as well as of appearing in an insoluble - form, must be supplemented by various other substances, among - which soluble gold and silver (Chapter XXIV.) are of particular - interest. So that now it may be said that the capacity of forming - colloid solutions is not limited to a definite class of compounds, - but is, if not a general, at all events, an exceedingly widely - distributed phenomenon. - -Silica, as regards its _salt forming properties_, stands in the series of -oxides on the boundary line on the side of the acids in just such a place -as alumina occupies on the side of the bases--that is, aluminium -hydroxide is the representative of the feeblest bases and silicic acid is -the least energetic of acids (at least in the presence of water--that is, -in aqueous solutions); in alumina, however, the basic properties are -distinctly expressed, while in silica the acid properties preponderate. -Like all feeble acid oxides it is capable of forming, with other acids, -saline compounds which are but slightly stable and are very easily -decomposed in the presence of water. The chief peculiarity of the -silicates consists in the number of their types. The salts formed with -nitric or sulphuric acid exist in one, two, and three tolerably stable -forms, but for acids like silicic acid the number of forms is very great, -almost unlimited. The natural silicates in particular furnish proof of -this fact; they contain various bases in combination with silica, and for -one and the same base there often exist various degrees of combination. -As feeble bases are capable of forming basic salts in addition to normal -salts--that is, a compound of a normal salt with a feeble base (either -the hydroxide or the oxide)--so the feeble acid oxides (although not all) -form, in addition to normal salts, highly acid salts--that is, normal -salts _plus_ acid (hydrate or anhydride). Such acids are boric, -phosphoric, molybdic, chromic, and especially silicic, acid. - -In order to explain these relations it is necessary first to recollect -the existence of the various hydrates of silica, or silicic acids,[20] -and then to turn our attention to the similarity between silicon -compounds and metallic alloys. Silica is an oxide having the appearance -of, and in many respects the same properties as, those oxides which -combine with it, and if two metals are capable of forming homogeneous -alloys in which there exist definite or indefinite compounds, it is -permissible to assume a similar power of forming alloys in the case of -analogous oxides. Such alloys are found in indefinite, amorphous masses -in the form of glass, lava, slags, and a number of similar siliceous -compounds which do not contain any definite types of combination, but -nevertheless are homogeneous throughout their mass. By slow cooling, or -under other circumstances, definite crystalline compounds may--and -sometimes do--separate from this homogeneous mass, as also sometimes -definite crystalline alloys separate from metallic alloys. - - [20] This is in accordance with the generally-accepted representation - of the relations between salts and the hydrates of acids, but it - is of little help in the study of siliceous compounds. Generally - speaking, it becomes necessary to explain the property of - (SiO_{2})_{_n_} to combine with (RO)_{_m_}, where _n_ may be - greater than _m_, and where R may be H_{2}, Ca, &c. Here we are - aided by those facts which have been attained by the investigation - of carbon compounds, especially with respect to glycol. Glycol is - a compound having the composition C_{2}H_{6}O_{2}, only differing - from alcohol, C_{2}H_{6}O, by an extra atom of oxygen. This - hydrate contains two hydroxyl groups, which may be successively - replaced by chlorine, &c. Hence the composition of glycol should - be represented as C_{2}H_{4}(OH)_{2}. It has been found that - glycol forms so-called polyglycols. Their origin will be - understood from the fact that glycol as a hydrate has a - corresponding anhydride of the composition C_{2}H_{4}O, known as - ethylene oxide. This substance is ethane, C_{2}H_{6}, in which two - hydrogens are replaced by one atom of oxygen. Ethylene oxide is - not the only anhydride of glycol, although it is the simplest one, - because C_{2}H_{4}O = C_{2}H_{4}(OH)_{2} - H_{2}O. Various other - anhydrides of glycol are possible, and have actually been - obtained, of the composition _n_C_{2}H_{4}(OH)_{2} - (_n_ - - 1)H_{2}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. These imperfect - anhydrides of glycol, or _polyglycols_, still contain hydroxyls - like glycol itself, and therefore are of an alcoholic character in - the same sense as glycol itself. They are obtained by various - methods, and, amongst others, by the direct combination of - ethylene oxide with glycol, because C_{2}H_{4}(OH)_{2} + (_n_ - - 1)C_{2}H_{4}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. The most - important circumstance, from a theoretical point of view, is that - these polyglycols may be distilled without undergoing - decomposition, and that the general formula given above expresses - their actual molecular composition. Hence we have here a direct - combination of the anhydride with the hydrate, and, moreover, a - repeated one. The formula A_{_n_}H_{2}O may be used to express the - composition of glycol and polyglycols with respect to ethylene - oxide in the most simple manner, if A stand for ethylene oxide. - When _n_ = 1 we have glycol, when _n_ is greater than 1 a - polyglycol. Such also is the relationship of the salts of hydrate - of silica, if A stand for silica, and if we imagine that H_{2}O - may also be taken _m_ times. Such a representation of the - _polysilicic acids_ corresponds with the representation of the - polymerism of silica. Laurent supposed the existence of several - polymeric forms, Si_{2}O_{4}, Si_{3}O_{6}, &c., besides silica, - SiO_{2}. - -The formation of crystalline rocks in nature is partly of such a -nature. By aqueous or igneous agency, but in any case in a liquid -condition, those oxides which form the earth's crust and her crystalline -minerals came into mutual contact. First of all they formed a shapeless -mass, of which lava, glass, slags and solutions are examples, but little -by little, or else suddenly, some definite compounds of certain oxides -existing in this alloy or in the shapeless mass were formed. This is -entirely similar to two metals forming a homogeneous alloy,[21] and under -known circumstances (for instance, on cooling the alloy, or in the case -of aqueous solution when the two metals are simultaneously liberated from -the solution), definite crystalline compounds are separated. In any case -there is no doubt that there is less distinction between silica and -bases, than between bases and such anhydrides as, for instance, sulphuric -or nitric, or even carbonic, as is seen on comparing the physical and -chemical properties of silica and various kinds of oxides. Alumina, -especially, is exceedingly near akin to silica; not only in the hydrated -state, but also in the anhydrous condition, there exists a certain -similarity between the crystalline forms of alumina and silica, in the -uncombined state. Both are very hard, transparent, inactive, -non-volatile, infusible, and crystallise in the hexagonal system--in a -word, they are remarkably similar, and for this reason they are capable, -like two kindred metals, of entering into many different degrees of -combination. Isomorphous mixtures--that is, differing by the substitution -of oxides akin both in their physical and chemical characters--are very -frequently met with among minerals, and the study of the latter gave the -principal impetus to the study of isomorphism. Thus, in a whole series of -minerals, lime and magnesia are found in variable and interchangeable -proportions. Exactly the same may be said of potassium and sodium, of -alumina and ferric oxide, of manganous, ferrous, magnesium oxides, &c. -Such isomorphism does not, however, extend without change of form and -properties beyond certain rather narrow limits.[22] What I mean by this -is that lime is not always replaced totally, but often only in small -quantities, by magnesia, or by the manganous and ferrous oxides, without -changing the crystalline form. The same may be observed with regard to -potassium and lithium, which may be in part, but not completely, replaced -by sodium. On the total substitution of one metal for another, often -(although not invariably) the entire nature of the substance is changed; -for instance, _enstatite_ (or bronzite) is a magnesium bisilicate with a -small isomorphous substitution of calcium for magnesium; its composition -is expressed by the formula MgSiO_{3}, it belongs to the rhombic system. -On the entire substitution of calcium, _wollastonite_, CaSiO_{3}, of the -monoclinic system, is obtained; when manganese is substituted, -_rhodonite_, of the triclinic system, is produced; but in all of them the -angles of the prism are 86° to 88°.[23] - - [21] For us the latter have not a saline character, only because they - are not regarded from this point of view, but an alloy of sodium - and zinc is, in a broad sense, a salt in many of its reactions, - for it is subject to the same double decompositions as sodium - phosphide or sulphide, which clearly have saline properties. The - latter (sodium phosphide), when heated with ethyl iodide, forms - ethyl phosphide, and the former--_i.e._ the alloy of zinc and - sodium--gives zinc ethyl; that is, the element (P, S, Zn) which - was united with the sodium passes into combination with the ethyl: - RNa + EtI = REt + NaI. By combining sodium successively with - chlorine, sulphur, phosphorus, arsenic, antimony, tin, and zinc, - we obtain substances having less and less the ordinary appearance - of salts, but if the alloy of sodium and zinc cannot be termed a - salt, then perhaps this name cannot be given to sodium sulphide, - and the compounds of sodium with phosphorus. The following - circumstance may also be observed: with chlorine, sodium gives - only one compound (with oxygen, at the most three), with sulphur - five, with phosphorus probably still more, with antimony naturally - still more, and the more analogous an element is to sodium, the - more varied are the proportions in which it is able to combine - with it, the less are the alterations in the properties which take - place by this combination, and the nearer does the compound formed - approach to the class of compounds known as indefinite chemical - compounds. In this sense a siliceous alloy, containing silica and - other acids, is a salt. The oxide to a certain extent plays the - same part as the sodium, whilst the silica plays the part of the - acid element which was taken up successively by zinc, phosphorus, - sulphur, &c., in the above examples. Such a comparison of the - silica compounds with alloys presents the great advantage of - including under one category the definite and indefinite silica - compounds which are so analogous in composition--that is, brings - under one head such crystalline substances as certain minerals, - and such amorphous substances as are frequently met with in - nature, and are artificially prepared, as glass, slags, enamels, - &c. - - If the compounds of silica are substances like the metallic - alloys, then (1) the chemical union between the oxides of which - they are composed must be a feeble one, as it is in all compounds - formed between analogous substances. In reality such feeble - agencies as water and carbonic acid are able, although slowly, to - act on and destroy the majority of the complex silica compounds in - rocks, as we saw in the preceding chapter; (2) their formation, - like that of alloys, should not be accompanied by a considerable - alteration of volume; and this is actually the case. For example, - felspar has a specific gravity of about 2·6, and therefore, taking - its composition to be K_{2}O,Al_{2}O_{3},6SiO_{2}, we find its - volume, corresponding with this formula, to be 556·8. 2·6 = 214, - the volume of K_{2}O = 35, of Al_{2}O_{3} = 26, and of SiO_{2} = - 22·6. Hence the sum of the volumes of the component oxides, 35 + - 26 + 6 × 22·6 = 196, which is very nearly equal to that of the - felspar; that is, its formation is attended by a slight expansion, - and not by contraction, as is the case in the majority of other - cases when combinations determined by strong affinities are - accomplished. In the case in question the same phenomenon is - observed as in solutions and alloys--that is, as in cases of - feeble affinities. So also the specific gravity of glass is - directly dependent on the amount of those oxides which enter into - its composition. If in the preceding example we take the sp. gr. - of silica to be, not 2·65, but 2·2, its volume = 27·3, and the sum - of the volumes will be = 224--that is, greater than that of - orthoclase. - - [22] It is, however, easy to imagine, and experience confirms the - supposition, that in a complex siliceous compound containing for - instance sodium and calcium, the whole of the sodium may be - replaced by potassium, and _at the same time_ the whole of the - calcium by magnesium, because then the substitution of potassium - for the sodium will produce a change in the nature of the - substance contrary to that which will occur from the calcium being - replaced by magnesium. That increase in weight, decrease in - density, increase of chemical energy, which accompanies the - exchange of sodium for potassium will, so to speak, be compensated - by the exchange of calcium for magnesium, because both in weight - and in properties the sum of Na + Ca is very near to the sum of K - + Mg. _Pyroxene_ or _augite_ can be taken as an example; its - composition may be expressed by the formula CaMgSi_{2}O_{6}; that - is, it corresponds with the acid H_{2}SiO_{3}; it is a bisilicate. - In many respects it closely resembles another mineral called - '_spodumene_' (they are both monoclinic). This latter has the - composition Li_{6}Al_{8}Si_{15}O_{45}. On reducing both formulæ to - an equal contents of silica the following distinction will be - observed between them: spodumene - (Li_{2}O)_{6}(Al_{2}O_{3})_{8}30SiO_{2}; augite - (CaO)_{15}(MgO)_{15}30SiO_{2}. That is, the difference between - them consists in the sum of the magnesia and lime (MgO)_{15} + - (CaO)_{15} replacing the sum of the lithium oxide and alumina - (Li_{2}O)_{6} + (Al_{2}O_{3})_{8}; and in the chemical relation - these sums are near to one another, because magnesium and calcium, - both in forms of oxidation and in energy (as bases), in all - respects occupy a position intermediate between lithium and - aluminium, and therefore the sum of the first may be replaced by - the sum of the second. - - If we take the composition of spodumene, as it is often - represented to be, Li_{2}O,Al_{2}O_{3},4SiO_{2}, the corresponding - formula of augite will be (CaO)_{2},(MgO)_{2},4SiO_{2}, and also - the amount of oxygen in the sum of Li_{2}OAl_{2}O_{3} will be the - same as in (CaO)_{2}(MgO)_{2}. I may remark, for the sake of - clearness, that lithium belongs to the first, aluminium to the - third group, and calcium and magnesium to the intermediate second - group; lithium, like calcium, belongs to the even series, and - magnesium and aluminium to the uneven. - - The representation of the substitutions of analogous compounds - here introduced was first deduced by me in 1856. It finds much - confirmation in facts which have been subsequently discovered--for - example, with respect to tourmalin. Wülfing (1888), on the basis - of a number of analyses (especially of those by Röggs), states - that all varieties contain an isomorphous mixture of alkali and - magnesia tourmalin; into the composition of the former there - enters 12SiO_{2},3B_{2}O_{3},8Al_{2}O_{3},2Na_{2}O,4H_{2}O, and of - the latter 12SiO_{2},3B_{2}O_{3},5Al_{2}O_{3},12MgO,3H_{2}O. Hence - it is seen that the former contains in addition the sum of - 3Al_{2}O_{3},2Na_{2}O,H_{2}O, whilst in the latter this sum of - oxides is replaced by 12MgO, in which there is as much oxygen as - in the sum of the more clearly-defined base 2Na_{2}O and less - basic 3Al_{2}O_{3}H_{2}O--that is, the relation is just the same - here as between augite and spodumene. - - [23] With respect to the silica compounds of the various oxides, it - must be observed that only the _alkali salts_ are known in a - soluble form; all the others only exist in an insoluble form, so - that a solution of the alkali compounds of silica, or soluble - glass, gives a precipitate with a solution of the salts of the - majority of other metals, and this precipitate will contain the - silica compounds of the other bases. The maximum amount of the - gelatinous hydrate of silica, which dissolves in caustic potash, - corresponds with the formation of a compound, 2K_{2}O,9SiO_{2}. - But this compound is partially decomposed, with the precipitation - of hydrate of silica, on cooling the solution. Solutions - containing a smaller amount of silica may be kept for an - indefinite time without decomposing, and silica does not separate - out from the solution; but such compounds crystallise from the - solutions with difficulty. However, a crystalline bisilicate (with - water) has been obtained for sodium having the composition - Na_{2}O,SiO_{2}--_i.e._ corresponding to sodium carbonate. The - whole of the carbonic acid is evolved, and a similar soluble - sodium metasilicate is obtained on fusing 3·5 parts of sodium - carbonate with 2 parts of silica. If less silica be taken a - portion of the sodium carbonate remains undecomposed; however, a - substance may then be obtained of the composition Si(ONa)_{4}, - corresponding with orthosilicic acid. It contains the maximum - amount of sodium oxide capable of combining with silica under - fusion. It is a sodium orthosilicate, (Na_{2}O)_{2},SiO_{2}. - - Calcium carbonate, and the carbonates of the alkaline earths in - general, also evolve all their carbonic acid when heated with - silica, and in some instances even form somewhat fusible - compounds. Lime forms a fusible slag of _calcium silicate_, of the - composition CaO,SiO_{2} and 2CaO,3SiO_{2}. With a larger - proportion of silica the slags are infusible in a furnace. The - magnesium _slags_ are less fusible than those with lime, and are - often formed in smelting metals. Many compounds of the metals of - the alkaline earths with silica are also met with in nature. For - instance, among the magnesium compounds there is _olivine_, - (MgO)_{2},SiO_{2}, sp. gr. 3·4, which occurs in meteorites, and - sometimes forms a precious stone (peridote), and occurs in slags - and basalts. It is decomposed by acids, is infusible before the - blow-pipe, and crystallises in the rhombic system. _Serpentine_ - has the composition 3MgO,2SiO_{2},2H_{2}O; it sometimes forms - whole mountains, and is distinguished for its great cohesiveness, - and is therefore used in the arts. It is generally tinted green; - its specific gravity is 2·5; it is exceedingly infusible, even - before the blowpipe. It is acted on by acids. Among the magnesium - compounds of silica, _talc_ is very widely used. It is frequently - met with in rocks which are widely distributed in nature, and - sometimes in compact masses; it can be used for writing like a - slate pencil or chalk, and being greasy to the touch, is also - known as _steatite_. It crystallises in the rhombic system, and - resembles mica in many respects; like it, it is divisible into - laminæ, greasy to the touch, and having a sp. gr. 2·7. These - laminæ are very soft, lustrous, and transparent, and are infusible - and insoluble in acids. The composition of talc approaches nearly - to 6MgO,5SiO_{2},2H_{2}O. - - Among the crystalline silicates the following minerals are - known:--_Wollastonite_ (tabular-spar), crystallises in the - monoclinic system; sp. gr. 2·8; it is semi-transparent, - difficultly fusible, decomposed by acids, and has the composition - of a metasilicate, CaOSiO_{2}. But isomorphous mixtures of calcium - and magnesium silicates occur with particular frequency in nature. - The _augites_ (sp. gr. 3·3), diallages, hypersthenes, hornblendes - (sp. gr. 3·1), amphiboles, common asbestos, and many similar - minerals, sometimes forming the essential parts of entire rock - formations, contain various relative proportions of the - bisilicates of calcium and magnesium partially mixed with other - metallic silicates, and generally anhydrous, or only containing a - small amount of water. In the pyroxenes, as a rule, lime - predominates, and in the amphiboles (also of the monoclinic - system) magnesia predominates. Details upon this subject must be - looked for in works upon mineralogy. - -The most remarkable complex siliceous compounds are the _felspars_, which -enter into nearly all the primary rocks like porphyry, granite, gneiss, -&c. These felspars always contain, in addition to silica and alumina, -oxides presenting more marked basic properties, such as potash, soda, and -lime. Thus the _orthoclase_ (adularia), or ordinary felspar (monoclinic) -of the granites, contains K_{2}O,Al_{2}O_{3},6SiO_{2}; _albite_ contains -the same substances, only with Na_{2}O instead of K_{2}O (it already -appertains to the triclinic system); _anorthite_ contains lime, and its -composition is CaO,Al_{2}O_{3},2SiO_{2}. On expressing the two last as -containing equal quantities of oxygen, we have:-- - - Albite Na_{2} Al_{2} Si_{6} O_{16} - Anorthite Ca_{2} Al_{4} Si_{4} O_{16} - -It is then evident that on the conversion of albite into anorthite, -Na_{2}Si_{2} is replaced by Ca_{2}Al_{2}, and this sum, both in chemical -energy and in the form of oxide, may be considered as corresponding with -the first, because sodium and silicon are extreme elements in chemical -character (from groups I. and IV.), and calcium and aluminium are means -between them (from groups II. and III.), and actually both these felspar -minerals are not only of one (triclinic) system, but form (Tchermak, -Schuster) all possible kinds of definite compounds (isomorphous mixtures) -between themselves, as indicated by their composition and all their -properties. Thus oligoclase, andesine, labradorite, &c. (plagioclases), -are nothing more than mutual combinations of albite and anorthite. -Labradorite consists of albite, in combination with 1 to 2 molecules of -anorthite. The class of _zeolites_ corresponds to the felspars; they are -hydrated compounds of a similar composition to the felspars. Thus -_natrolite_ contains Na_{2}O,Al_{2}O_{3},3SiO_{2},2H_{2}O, and _analcime_ -presents the same composition, but contains 4SiO_{2} instead of 3SiO_{2}. -In general, the felspars and zeolites contain RO,Al_{2}O_{3},_n_SiO_{2}, -where _n_ varies considerably.[24] - - [24] The majority of the siliceous minerals have now been obtained - artificially under various conditions. Thus N. N. Sokoloff showed - that slags very frequently contain peridote. Hautefeuille, - Chroustchoff, Friedel, and Sarasin obtained felspar identical in - all respects with the natural minerals. The details of the methods - here employed must be looked for in special works on mineralogy; - but, as an example, we will describe the method of the preparation - of felspar employed by Friedel and Sarasin (1881). From the fact - that felspar gives up potassium silicate to water even at the - ordinary temperature (Debray's experiments), they concluded that - the felspar in granites had an aqueous origin (and this may be - supposed to be the case from geological data); then, in the first - place, its formation could not be accomplished unless in the - presence of an excess of a solution of potassium silicate. In - order to render this argument clear I may mention, as an example, - that carnallite is decomposed by water into easily soluble - magnesium chloride and potassium chloride, and therefore if it is - of aqueous origin it could not be formed otherwise than from a - solution containing an excess of magnesium chloride, and, in the - second place, from a strongly-heated solution; again, felspar - itself and its fellow-components in granites are anhydrous. On - these facts were based experiments of heating hydrates of silica - with alumina and a solution of potassium silicate in a closed - vessel. The mixture was placed in a sealed platinum tube, which - was enclosed in a steel tube and heated to dull redness. When the - mixture contained an excess of silica the residue contained many - crystals of rock crystal and tridymite, together with a powder of - felspar, which formed the main product of the reaction when the - proportion of hydrate of silica was decreased, and a mixture of a - solution of potassium silicate with alumina precipitated together - with the silica by mixing soluble glass with aluminium chloride - was employed. The composition, properties, and forms of the - resultant felspar proved it to be identical with that found in - nature. The experiments approach very nearly to the natural - conditions, all the more as felspar and quartz are obtained - together in one mixture, as they so often occur in nature. - -Such complex silicates are generally insoluble in water,[25] and if -they undergo change in it, it is but very slow, and more often only in -the presence of carbonic acid. Some of the silicates which are insoluble -in water are easily and directly decomposed by acids; for instance, the -zeolites and those fused silicates which contain a large quantity of -energetic bases--such as lime. Many of the silicates, like glass,[26] are -hardly changed by acids, particularly if they contain much silica, whilst -fusion with alkalis leads to the formation of compounds rich in bases, -after which acids decompose the alloys formed.[27] - - [25] The application of _cements_ is based on this principle; they are - those sorts of 'hydraulic' lime which generally form a stony mass, - which hardens even under water, when mixed with sand and water. - - The hydraulic properties of cements are due to their containing - calcareous and silico-aluminous compounds which are able to - combine with water and form hydrates, which are then unacted on by - water. This is best proved, in the first place, by the fact that - certain slags containing lime and silica, and obtained by fusion - (for example, in blast-furnaces), solidify like cements when - finely ground and mixed with water; and, in the second place, by - the method now employed for the manufacture of artificial cements - (formerly only peculiar and comparatively rare natural products - were used). For this purpose a mixture of lime and clay is taken, - containing about 25 p.c. of the latter; this mixture is then - heated, not to fusion, but until both the carbonic anhydride and - water contained in the clay are expelled. This mass when finely - ground forms Portland cement, which hardens under water. The - process of hardening is based on the formation of chemical - compounds between the lime, silica, alumina, and water. These - substances are also found combined together in various natural - minerals--for example, in the zeolites, as we saw above. In all - cases cement which has set contains a considerable amount of - water, and its hardening is naturally due to hydration--that is, - to the formation of compounds with water. Well-prepared and very - finely-ground cement hardens comparatively quickly (in several - days, especially after being rammed down), with 3 parts (and even - more) of coarse sand and with water, into a stony mass which is as - hard and durable as many stones, and more so than bricks and - limestone. Hence not only all maritime constructions (docks, - ports, bridges, &c.), but also ordinary buildings, are made of - Portland cement, and are distinguished for their great durability. - A combination of ironwork (ties, girders) and cement is - particularly suitable for the construction of aqueducts, arches, - reservoirs, &c. Arches and walls made of such cements may be much - less thick than those built up of ordinary stone. Hence the - production and use of cement rapidly increases from year to year. - The origin of accurate data respecting cements is chiefly due to - Vicat. In Russia Professor Schuliachenko has greatly aided the - extension of accurate data concerning Portland cement. Many works - for the manufacture of cement have already been established in - various parts of Russia, and this industry promises a great future - in the arts of construction. - - [26] _Glass_ presents a similar complex composition, like that of many - minerals. The ordinary sorts of white glass contain about 75 p.c. - of silica, 13 p.c. of sodium oxide, and 12 p.c of lime; but the - inferior sorts of glass sometimes contain up to 10 p.c. of - alumina. The mixtures which are used for the manufacture of glass - are also most varied. For example, about 300 parts of pure sand, - about 100 parts of sodium carbonate, and 50 of limestone are - taken, and sometimes double the proportion of the latter. Ordinary - _soda-glass_ contains sodium oxide, lime, and silica as the chief - component parts. It is generally prepared from sodium sulphate - mixed with charcoal, silica, and lime (Chapter XII.), in which - case the following reaction takes place at a high temperature: - Na_{2}SO_{4} + C + SiO_{2} = Na_{2}SiO_{3} + SO_{2} + CO. - Sometimes potassium carbonate is taken for the preparation of the - better qualities of glass. In this case a glass, _potash-glass_, - is obtained containing potassium oxide instead of sodium oxide. - The best-known of these glasses is the so-called Bohemian glass or - crystal, which is prepared by the fusion of 50 parts of potassium - carbonate, 15 parts of lime, and 100 parts of quartz. The - preceding kinds of glass contain lime, whilst crystal glass - contains lead oxide instead. Flint glass--that is, the lead glass - used for optical instruments--is prepared in this manner, - naturally from the purest possible materials. - _Crystal-glass_--_i.e._ glass containing lead oxide--is softer - than ordinary glass, more fusible and has a higher index of - refraction. However, although the materials for the preparation of - glass be most carefully sorted, a certain amount of iron oxides - falls into the glass and renders it greenish. This coloration may - be destroyed by adding a number of substances to the vitreous - mass, which are able to convert the ferrous oxide into ferric - oxide; for example, manganese peroxide (because the peroxide is - deoxidised to manganous oxide, which only gives a pale violet tint - to the glass) and arsenious anhydride, which is deoxidised to - arsenic, and this is volatilised. The manufacture of glass is - carried on in furnaces giving a very high temperature (often in - regenerative furnaces, Chapter IX.). Large clay crucibles are - placed in these furnaces, and the mixture destined for the - preparation of the glass, having been first roasted, is charged - into the crucibles. The temperature of the furnace is then - gradually raised. The process takes place in three separate - stages. At first the mass intermixes and begins to react; then it - fuses, evolves carbonic acid gas, and forms a molten mass; and, - lastly, at the highest temperature, it becomes homogeneous and - quite liquid, which is necessary for the ultimate elimination of - the carbonic anhydride and solid impurities, which latter collect - at the bottom of the crucible. The temperature is then somewhat - lowered, and the glass is taken out on tubes and blown into - objects of various shapes. In the manufacture of window-glass it - is blown into large cylinders, which are then cut at the ends and - across, and afterwards bent back in a furnace into the ordinary - sheets. After being worked up, all glass objects have to be - subjected to a slow cooling (_annealing_) in special furnaces, - otherwise they are very brittle, as is seen in the so-called - 'Rupert's drops,' formed by dropping molten glass into water; - although these drops preserve their form, they are so brittle that - they break up into a fine powder if a small piece be knocked off - them. Glass objects have frequently to be polished and chased. In - the manufacture of mirrors and many massive objects the glass is - cast and then ground and polished. Coloured glasses are either - made by directly introducing into the glass itself various oxides, - which give their characteristic tints, or else a thin layer of a - coloured glass is laid on the surface of ordinary glass. Green - glasses are formed by the oxides of chromium and copper, blue by - cobalt oxide, violet by manganese oxide, and red glass by cuprous - oxide and by the so-called purple of Cassius--_i.e._ a compound of - gold and tin--which will be described later. A yellow coloration - is obtained by means of the oxides of iron, silver, or antimony, - and also by means of carbon, especially for the brown tints for - certain kinds of bottle-glass. - - From what has been said about glass it will be understood that it - is impossible to give a definite formula for it, because it is a - non-crystalline or amorphous alloy of silicates; but such an alloy - can only be formed within certain limits in the proportions - between the component oxides. With a large proportion of silica - the glass very easily becomes clouded when heated; with a - considerable proportion of alkalis it is easily acted on by - moisture, and becomes cloudy in time on exposure to the air; with - a large proportion of lime it becomes infusible and opaque, owing - to the formation of crystalline compounds in it; in a word, a - certain proportion is practically attained among the component - oxides in order that the glass formed may have suitable - properties. Nevertheless, it may be well to remark that the - composition of common glass approaches to the formula - Na_{2}O,CaO,4SiO_{2}. - - The coefficient of cubical expansion of glass is nearly equal to - that of platinum and iron, being approximately 0·000027. The - specific heat of glass is nearly 0·18, and the specific gravity of - common soda glass is nearly 2·5, of Bohemian glass 2·4, and of - bottle glass 2·7. Flint glass is much heavier than common glass, - because it contains the heavier oxide of lead, its specific - gravity being 2·9 to 3·2. - - [27] It must be recollected that although acids seem to act only feebly - on the majority of silicates, nevertheless a finely-levigated - powder of siliceous compounds is acted on by strong acids, - especially with the aid of heat, the basic oxides being taken up - and gelatinous silica left behind. In this respect sulphuric acid - heated to 200° with finely-divided siliceous compounds in a closed - tube acts very energetically. - -According to the periodic law, the nearest analogues of silicon ought to -be elements of the uneven series, because silicon, like sodium, -magnesium, and aluminium, belongs to the uneven series.[28] Immediately -after silicon follows ekasilicon or _germanium_, Ge = 72, whose -properties were predicted (1871) before Winkler (1886) in Freiberg, -Saxony (Chapter XV. § 5), discovered this element in a peculiar silver -ore called _argyrodite_, Ag_{6}GeS_{5}.[29] Easily reduced from the oxide -by heating with hydrogen and charcoal, and separated from its solutions -by zinc, metallic germanium proved to be greyish white, easily -crystallisable (in octahedra), brittle, fusible (under a coating of fused -borax) at about 900°, and easily oxidisable; the specific gravity = -5·469, the atomic weight = 72·3, and the specific heat = 0·076,[30] as -might be expected for this element according to the periodic law. The -corresponding _germanium dioxide_, GeO_{2}, is a white powder having a -specific gravity of 4·703; water, especially when boiling, dissolves this -dioxide (1 part of GeO_{2} requires for solution 247 parts of water at -20°, 95 parts at 100°). It forms soluble salts with alkalis and is but -sparingly soluble in acids.[31] In a stream of chlorine the metal forms -_germanium chloride_, GeCl_{4}, which boils at 86°, and has a specific -gravity of 1·887 at 18°; water decomposes it, forming the oxide. All -these properties[32] of germanium, showing its analogy to silicon and -tin, form a most beautiful demonstration of the truth of the periodic -law.[33] - - [28] Such elements as silicon, tin, and lead were only brought together - under one common group by means of the periodic law, although the - quadrivalency of tin and lead was known much earlier. Generally - silicon was placed among the non-metals, and tin and lead among - the metals. - - [29] At first (February 1886) the want of material to work on, the - absence of a spectrum in the Bunsen's flame, and the solubility of - many of the compounds of germanium, presented difficulties in the - researches of Professor Winkler, who, on analysing argyrodite by - the usual method, obtained a constant loss of 7 p.c., and was thus - led to search for a new element. The presence of arsenic and - antimony in the accompanying minerals also impeded the separation - of the new metal. After fusion with sulphur and sodium carbonate, - argyrodite gives a solution of a sulphide which is precipitated by - an _excess_ of hydrochloric acid; germanium sulphide is soluble in - ammonia and then precipitated by hydrochloric acid, as a _white_ - precipitate, which is dissolved (or decomposed) by water. After - being oxidised by nitric acid, dried and ignited germanium - sulphide leaves the oxide GeO_{2}, which is reduced to the metal - when ignited in a stream of hydrogen. - - [30] G. Kobb determined the spectrum of germanium, when the metal was - taken as one of the electrodes of a powerful Ruhmkorff's coil. The - wave-lengths of the most distinct lines are 602, 583, 518, 513, - 481, 474, millionths of a millimetre. - - [31] If germanium or germanium sulphide be heated in a stream of - hydrochloric acid, it forms a volatile liquid, boiling at 72°, - which Winkler regarded as germanium chloride, GeCl_{2}, or - germanium chloroform, GeHCl_{3}. It is decomposed by water, - forming a white substance, which may perhaps be the hydrate of - germanious oxide, GeO, and acts as a powerfully reducing agent in - a hydrochloric acid solution. - - [32] Under certain circumstances germanium gives a blue coloration like - that of ultramarine, as Winkler showed, which might have been - expected from the analogy of germanium with silicon. - - [33] Winkler expressed this in the following words (_Jour. f. pract. - Chemie_, 1886 [2], 34, 182-183): '... es kann keinem Zweifel mehr - unterliegen, dass das neue Element nichts Anderes, als das vor - fünfzehn Jahren von _Mendeléeff_ prognosticirte _Ekasilicium_ - ist.' - - 'Denn einen schlagenderen Beweis für die Richtigkeit der Lehre von - der Periodicität der Elemente, als den, welchen die Verkörperung - des bisher hypothetischen "Ekasilicium" in sich schliesst, kann es - kaum geben, und er bildet in Wahrheit mehr, als die blosse - Bestätigung einer kühn aufgestellten Theorie, er bedeutet eine - eminente Erweiterung des chemischen Gesichtfeldes, einen mächtigen - Schritt in's Reich der Erkenntniss.' - -The increase of atomic weight from silicon 28 to germanium 72 is 44--that -is, about the same difference as there is in the atomic weights of -chlorine and bromine; between germanium and its next analogue, _tin_ (Sn -= 118), the difference is 46--that is, almost as much as the amount by -which the atomic weight of iodine exceeds that of bromine. - -Metallic tin is rarely met with in _nature_; it occurs in the veins of -ancient formations, almost exclusively in the form of oxide, SnO_{2}, -called _tin-stone_. The best known tin deposits are in Cornwall and in -Malacca. In Russia, tin ores have been found in small quantities on the -shores of Lake Ladoga, in Pitkarand. The crushed ore may easily be -separated from the earthy matter accompanying it by washing on inclined -tables, as the tin-stone has a specific gravity of 6·9, whilst the -impurities are much lighter. _Tin oxide is very easily reduced_ to -metallic tin by heating with charcoal. For this reason tin was known in -ancient times, and the Ph[oe]nicians brought it from England. Metallic -tin is cast into ingots of considerable weight or into thin sticks or -rods. Tin has a white colour, rather duller than that of silver. It fuses -easily at 232°, and crystallises on cooling. Its specific gravity is -7·29. The crystalline structure of ordinary tin is noticed in bending tin -rods, when a peculiar sound is heard, produced by the fracture of the -particles of tin along the surfaces of crystalline structure. - -When pure tin is cooled to a low temperature it splits up into separate -crystals, the bond between the particles is lost, the tin assumes a grey -colour, becomes less brilliant--in a word, its properties become changed, -as Fritzsche showed. This depends on the peculiar structure which the tin -then acquires, and is particularly remarkable because it is effected by -cold in a solid.[33 bis] If such tin be fused, or even simply heated, it -becomes like ordinary tin, but is again changed when cooled. When in this -condition tin has a specific gravity of 7·19. Similarly, tin is obtained -by the action of the galvanic current on a solution of tin chloride; it -then appears in crystals of the cubic system, and has a specific gravity -of 7·18--that is, the same as when cooled.[34] - - [33 bis] Emilianoff (1890) states that in the cold of the Russian - winter 30 out of 200 tin moulds for candles were spoilt through - becoming quite brittle. - - [34] The tin deposited by an electric current from a neutral solution - of SnCl_{2} easily oxidises and becomes coated with SnO (Vignon, - 1889). - -Tin is softer than silver and gold, and is only surpassed by lead in -this respect. In addition to this it is very ductile, but its tenacity is -very slight, so that wire made from it will bear but little strain. In -consequence of its ductility it is easily worked, by forging and rolling -into very thin sheets (tin foil), which are used for wrapping many -articles to preserve them from moisture, &c. In this case, however, and -in many others, lead is mixed with the tin, which, within certain limits, -does not alter the ductility. Whilst so soft at the ordinary temperatures -tin becomes brittle at 200°, before fusing. Tin powder may be easily -obtained if the metal be fused and then stirred whilst cooling. At a -white heat tin may be distilled, but with more difficulty than zinc. If -molten tin comes into contact with oxygen, it oxidises, forming stannic -oxide, SnO_{2}, _and its vapour burns_ with a white flame. _At ordinary -temperatures tin does not oxidise_, and this very important property of -tin allows it to be applied in many cases for covering other metals to -prevent their oxidising. This is termed _tinning_. Iron and copper are -frequently tinned. Iron and steel sheets, coated with tin, bear the name -of tin plate (for the most part made in England), and are used for -numerous purposes. Tin plate is prepared by immersing iron sheets, -previously thoroughly cleansed by acid and mechanical means, into molten -tin.[34 bis] - - [34 bis] If after this the coating of tin be rapidly cooled--for - instance, by dashing water over it--it crystallises into diverse - star-shaped figures, which become visible when the sheets are - first immersed in dilute aqua regia and then in a solution of - caustic soda. - - The coating of iron by tin, guards it against the direct access of - air, but it only preserves the iron from oxidation so long as it - forms a perfectly continuous coating. If the iron is left bare in - certain places, it will be powerfully oxidised at these spots, - because the tin is electro-negative with respect to the iron, and - thus the oxidation is confined entirely to the iron in the - presence of tin. Hence a coating of tin over iron objects only - partially preserves them from rusting. In this respect a coating - of zinc is more effectual. However, a dense and invariable alloy - is formed over the surface of contact of the iron and tin, which - binds the coating of tin to the remaining mass of the iron. Tin - may be fused with cast iron, and gives a greyish-white alloy, - which is very easily cast, and is used for casting many objects - for which iron by itself would be unsuitable owing to its ready - oxidisability and porosity. The coating of copper objects by tin - is generally done to preserve the copper from the action of acid - liquids, which would attack the copper in the presence of air and - convert it into soluble salts. Tin is not acted on in this manner, - and therefore copper vessels for the preparation of food should be - tinned. - -Tin with copper forms _bronze_, an alloy which is most extensively used -in the arts. Bronze has various colours and a variety of physical -properties, according to the relative amount of copper and tin which it -contains. With an excess of copper the alloy has a yellow colour; the -admixture of tin imparts considerable hardness and elasticity to the -copper. An alloy containing 78 parts of copper and about 22 per cent. of -tin is so elastic that it is used for casting bells, which naturally -require a very elastic and hard alloy.[35] For casting statues and -various large or small ornamental articles alloys containing 2 to 5 p.c. -of tin, 10 to 30 p.c. of zinc, and 65 to 85 p.c. of copper are used.[36] -Tin is also often used alloyed with lead, for making various objects--for -instance, drinking vessels. - - [35] The ancient Chinese alloys, containing about 20 p.c. of tin - (specific gravity of alloys about 8·9), which have been rapidly - cooled, are distinguished for their resonance and elasticity. - These alloys were formerly manufactured in large quantities in - China for the musical instruments known as _tom-toms_. Owing to - their hardness, alloys of this nature are also employed for - casting guns, bearings, &c., and an alloy containing about 11 p.c. - of tin (corresponding with the ratio Cu_{15}Sn) is known as - gun-metal. The addition of a small quantity of phosphorus, up to 2 - p.c., renders bronze still harder and more elastic, and the alloy - so formed is now used under the name of phosphor-bronze. - - The alloy SnCu_{3} is brittle, of a bluish colour, and has nothing - in common with either copper or tin in its appearance or - properties. It remains perfectly homogeneous on cooling, and - acquires a crystalline structure (Riche). All these signs clearly - indicate that the alloy SnCu_{3} is a product of chemical - combination, which is also seen to be the case from its density, - 8·91. Had there been no contraction, the density of the alloy - would be 8·21. It is the heaviest of all the alloys of tin and - copper, because the density of tin is 7·29 and of copper 8·8. The - alloy SnCu_{4}, specific gravity 8·77, has similar properties. All - the alloys except SnCu_{3} and SnCu_{4} split up on cooling; a - portion richer in copper solidifies first (this phenomenon is - termed the _liquation_ of an alloy), but the above two alloys do - not split up on cooling. In these and many similar facts we can - clearly distinguish a _chemical union between the metals_ forming - an alloy. The alloys of tin and copper were known in very remote - ages, before iron was used. The alloys of zinc and tin are less - used, but alloys composed of zinc, tin, and copper frequently - replace the more costly bronze. Concerning the alloys of lead - _see_ Note 46. - - [36] An excellent proof of the fact that alloys and solutions are - subject to law is given, amongst others, by the application of - Raoult's method (Chapter I., Note 49) to solutions of different - metals in tin. Thus Heycock and Neville (1889) showed that the - temperature of solidification of molten tin (226°·4) is lowered by - the presence of a small quantity of other metals in proportion to - the concentration of the solution. The following were the - reductions of the temperature of solidification of tin obtained by - dissolving in it atomic proportions of different metals (for - example, 65 parts of zinc in 11,800 parts of tin); Zn 2°·53, Cu - 2°·47, Ag 2°·67, Cd 2°·16, Pb 2°·22, Hg 2°·3, Sb 2° [rise], Al - 1°·34. As Raoult's method (Chapter VII.) enables the molecular - weight to be determined, the almost perfect identity of the - resultant figures (except for aluminium) shows that the molecules - of copper, silver, lead, and antimony contain _one atom in the - molecule_, like zinc, mercury, and cadmium. They obtained the same - result (1890) for Mg, Na, Ni, Au, Pd, Bi and In. It should here be - mentioned that Ramsay (1889) for the same purpose (the - determination of the molecular weight of metals on the basis of - their mutual solution) took advantage of the variation of the - vapour tension of mercury (_see_ Vol. I., p. 134), containing - various metals in solution, and he also found that the - above-mentioned metals contain but one atom in the molecule. - -Tin decomposes the vapour of water when heated with it, liberating the -hydrogen and forming stannic oxide. Sulphuric acid, diluted with a -considerable quantity of water, does not act, or at all events acts very -slightly, on tin, but tin reduces hot strong sulphuric acid, when not -only sulphurous anhydride but also sulphuretted hydrogen is evolved. -Hydrochloric acid acts very easily on tin, with evolution of hydrogen and -formation of stannous chloride, SnCl_{2}, in solution, which, with an -excess of hydrochloric acid and access of air, is converted into stannic -chloride: SnCl_{2} + 2HCl + O = SnCl_{4} + H_{2}O.[36 bis] Nitric acid -diluted with a considerable quantity of water dissolves tin at the -ordinary temperature, whilst the nitric acid itself is reduced, forming, -amongst other products, ammonia and hydroxylamine. Here the tin passes -into solution in the form of stannous nitrate. Stronger nitric acid (also -more dilute, when heated) transforms the tin into its highest grade of -oxidation, SnO_{2}, but the latter then appears as the so-called -metastannic acid, which does not dissolve in nitric acid, and therefore -the tin does not pass into solution. Feeble acids--for instance, carbonic -and organic acids--do not act on tin even in the presence of oxygen, -because tin does not form any powerful bases. - - [36 bis] The action of a mixture of hydrochloric acid and tin forms an - excellent means of reducing, wherein both the hydrogen liberated - by the mixture (at the moment of separation) and the stannous - chloride act as powerful reducing and deoxidising agents. Thus, - for instance, by this mixture nitro-compounds are transformed into - amido-compounds--that is, the elements of the group NO_{2} are - reduced to NH_{2}. - -It is important to remark as a characteristic of tin that it is reduced -from its solutions by many metals which are more easily oxidised, as, for -instance, by zinc. - -_In combination_, _tin_ appears in the two types, SnX_{4} and -SnX_{2},[37] compounds of the intermediate type, Sn_{2}X_{6}, being also -known, but these latter pass with remarkable facility in most cases into -compounds of the higher and lower types, and therefore the form SnX_{3} -cannot be considered as independent. - - [37] Many volatile compounds of tin are known, whose molecular weights - can therefore be established from their vapour densities. Among - these may be mentioned stannic chloride, SnCl_{4}, and stannic - ethide, Sn(C_{2}H_{5})_{4} (the latter boils at about 150°). But - V. Meyer found the vapour density of stannous chloride, SnCl_{2}, - to be variable between its boiling point (606°) and 1100°, owing, - it would seem, to the fact that the molecule then varies from - Sn_{2}Cl_{4} to SnCl_{2}, but the vapour density proved to be less - than that indicated by the first and greater than that shown by - the second formula, although it approaches to the latter as the - temperature rises--that is, it presents a similar phenomenon to - that observed in the passage of N_{2}O_{4} into NO_{2}. - -_Stannous oxide_, SnO, in an anhydrous condition is obtained by boiling -solutions of stannous salts with alkalis, the first action of the alkali -being to precipitate a white hydrate of stannous oxide, Sn(OH)_{2}SnO. -The latter when heated parts with water as easily as the hydrate of -copper oxide. In this form stannous oxide is a black crystalline powder -(specific gravity 6·7) capable of further oxidation when heated. The -hydrate is freely soluble in acids, and also in potassium and sodium -hydroxides, but not in aqueous ammonia.[38] This property indicates the -feeble basic properties of this lower oxide, which acts in many cases as -a reducing agent.[39] Among the compounds corresponding with stannous -oxide the most remarkable and the one most frequently used is stannous -chloride or _chloride of tin_, SnCl_{2}, also called proto-chloride of -tin (because it is the lowest chloride, containing half as much Cl as -SnCl_{4}). It is a transparent, colourless, crystalline substance, -melting at 250° and boiling at 606°. Water dissolves it, without visible -change (in reality partial decomposition occurs, as we shall see -presently). It is also soluble in alcohol. It is obtained by heating tin -in dry hydrochloric acid gas, the hydrogen being then liberated, or by -dissolving metallic tin in hot strong hydrochloric acid and then -evaporating quickly. On cooling, crystals of the monoclinic system are -obtained having the composition SnCl_{2},2H_{2}O. An aqueous solution of -this substance absorbs oxygen from the atmosphere, and gives a -precipitate containing stannic oxide. From this it follows that a -solution of stannous chloride will act as a reducing agent, a fact -frequently made use of in chemical investigations--for example, for -reducing metals from their solutions--since even mercury may be reduced -to a metallic state from its salts by means of stannous chloride. This -reducing property is also employed in the arts, especially in the dyeing -industry, where this substance in the form of a crystalline salt finds an -extensive application, and is known as _tin salt_ or tin crystals. - - [38] When rapidly boiled, an alkaline solution of stannous oxide - deposits tin and forms stannic oxide, 2SnO = Sn + SnO_{4}, which - remains in the alkaline solution. - - [39] Weber (1882) by precipitating a solution of stannous chloride with - sodium sulphite (this salt as a reducing agent prevents the - oxidation of the stannous compound) and dissolving the washed - precipitate in nitric acid, obtained crystals of _stannous - nitrate_, Sn(NO_{3})_{2},20H_{2}O, on refrigerating the solution. - This crystallo-hydrate easily melts, and is deliquescent. Besides - this, a more stable anhydrous basic salt, Sn(NO_{3})_{2},SnO, is - easily formed. In general, stannous oxide as a feeble base easily - forms basic salts, just as cupric and lead oxides do. For the same - reason SnX_{2} easily forms double salts. Thus a potassium salt, - SnK_{2}Cl_{4},H_{2}O, and especially an ammonium salt, - Sn(NH_{4})_{2}Cl_{4},H_{2}O, called _pink salt_, are known. Some - of these salts are used in the arts, owing to their being more - stable than tin salts alone. Stannous bromide and iodide, SnBr_{2} - and SnI_{2}, resemble the chloride in many respects. - - Among other stannous salts a sulphate, SnSO_{4}, is known. It is - formed as a crystalline powder when a solution of stannous oxide - in sulphuric acid is evaporated under the receiver of an air-pump. - The feeble basic character of the stannous oxide is clearly seen - in this salt. It decomposes with extreme facility, when heated, - into stannic oxide and sulphurous anhydride, but it easily forms - double salts with the salts of the alkali metals. - - In gaseous hydrochloric acid, stannous chloride, SnCl_{2},2H_{2}O, - forms a liquid having the composition SnCl_{2},HCl,3H_{2}O (sp gr. - 2·2, freezes at -27°), and a solid salt, SnCl_{2},H_{2}O (Engel). - -_Stannic oxide_, SnO_{2}, occurring in nature as _tinstone_, or -_cassiterite_, is formed during the oxidation or combustion of heated tin -in air as a white or yellowish powder which fuses with difficulty. It is -prepared in large quantities, being used as a white vitreous mixture for -coating ordinary tiles and similar earthenware objects with a layer of -easily fusible glass or enamel. Acid solutions of stannic oxide treated -with alkalis, and alkaline solutions treated with acids, give a -precipitate of stannic hydroxide, Sn(OH)_{4}, also known as stannic acid, -which, when heated, gives up water and leaves the anhydride, SnO_{2}, -which is insoluble in acids, clearly showing the feebleness of its basic -character. When fused with alkali hydroxides (not with their carbonates -or acid sulphates), an alkaline compound is obtained which is soluble in -water. Stannic hydroxide, like the hydrates of silica, is a colloidal -substance, and presents several different modifications, depending on the -method of preparation, but having an identical composition; the various -hydroxides have also a different appearance, and act differently with -reagents. For instance, a distinction is made between ordinary stannic -acid and metastannic acid. _Stannic acid_ is produced by precipitation by -soda or ammonia from a freshly-prepared solution of stannic chloride, -SnCl_{4}, in water; on drying the precipitate thus obtained, a -non-crystalline mass is formed, which is freely soluble in strong -hydrochloric or nitric acids, and also in potassium and sodium -hydroxides. This ordinary stannic acid may be still better obtained from -sodium stannate by the action of acids. _Metastannic acid_ is insoluble -in sulphuric and nitric acids. It is obtained in the form of a heavy -white powder by treating tin with nitric acid; hydrochloric acid does not -dissolve it immediately, but changes it to such an extent that, after -pouring off the acid, water extracts the stannic chloride, SnCl_{4}, -already formed. Dilute alkalis not only dissolve metastannic acid, but -also transform it into salts, which, slowly, yet completely, dissolve in -_pure water_, but are insoluble even in dilute alkali hydroxides. Dilute -hydrochloric acid, especially when boiling, changes the ordinary hydrate -into metastannic acid. On this depends, by the way, the formation of a -white precipitate, stannic hydroxide, from solutions of stannous and -stannic chlorides diluted with water. The stannic oxide first dissolved -changes under the influence of hydrochloric acid into metastannic acid, -which is insoluble in water in the presence of hydrochloric acid. -Solutions of metastannic acid differ from solutions of ordinary stannic -acid, and in the presence of alkali they change into solutions of -ordinary acid, so that metastannic acid corresponds principally with the -acid compounds of stannic oxide, and ordinary stannic acid with the -alkaline compounds.[40] Graham obtained a soluble colloidal hydroxide; it -is subject to the same transformations that are in general peculiar to -colloids. - - [40] Frémy supposes the cause of the difference to consist in a - difference of polymerisation, and considers that the ordinary acid - corresponds with the oxide SnO_{2}, and the meta-acid with the - oxide Sn_{5}O_{10}, but it is more probable that both are - polymeric but in a different degree. Stannic acid with sodium - carbonate gives a salt of the composition Na_{2}SnO_{3}. The same - salt is also obtained by fusing metastannic acid with sodium - hydroxide, whilst metastannic acid gives a salt, - Na_{2}SnO_{3},4SnO_{2} (Frémy), when treated with a dilute - solution of alkali; moreover, stannic acid is also soluble in the - ordinary stannate, Na_{2}SnO_{3} (Weber), so that both stannic - acids (like both forms of silica) are capable of polymerisation, - and probably only differ in its degree. In general, there is here - a great resemblance to silica, and Graham obtained a solution of - stannic acid by the direct dialysis of its alkaline solution. The - main difference between these acids is that the meta-acid is - soluble in hydrochloric acid, and gives a precipitate with - sulphuric acid and stannous chloride, which do not precipitate the - ordinary acid. Vignon (1889) found that more heat is evolved in - dissolving stannic acid in KHO than metastannic. - -Stannic oxide shows the properties of a slightly energetic and -intermediate oxide (like water, silica, &c.); that is to say, it forms -saline compounds both with bases and with acids, but both are easily -decomposed, and are but slightly stable. But still the acid character is -more clearly developed than the basic, as in silica, germanic oxide, and -lead dioxide. This determines the character of the compounds SnX_{4}, -corresponding to stannic chloride, SnCl_{4} (also called tetrachloride of -tin). It is obtained in an anhydrous condition by the direct action of -chlorine on tin, and is then easily purified, because it is a liquid -boiling at 114°, and therefore can be easily distilled. Its specific -gravity is 2·28 (at 0°), and it fumes in the open air (spiritus fumans -libavii), reacting on the moisture of the air, thus showing the -properties of a chloranhydride. Water however does not at first decompose -it, but dissolves it, and on evaporation gives the crystallo-hydrate -SnCl_{4},5H_{2}O. If but little water be taken, crystals containing -SnCl_{4},3H_{2}O are formed, which part with one-third of the water when -placed under the receiver of the air-pump. A large quantity of water -however, especially on heating, causes a precipitate of metastannic -acid[41] and formation of HCl. - - [41] The formation of the compound SnCl_{4},3H_{2}O is accompanied by - so great a contraction that these crystals, although they contain - water, are heavier than the anhydrous chloride SnCl_{4}. The - penta-hydrated crystallo-hydrate absorbs dry hydrochloric acid, - and gives a liquid of specific gravity 1·971, which at 0° yields - crystals of the compound SnCl_{4},2HCl,6H_{2}O (it corresponds - with the similar platinum compound), which melt at 20° into a - liquid of specific gravity 1·925 (Engel). - - Stannic chloride combines with ammonia (SnCl_{4},4NH_{3}), - hydrocyanic acid, phosphoretted hydrogen, phosphorus pentachloride - (SnCl_{4},PCl_{5}), nitrous anhydride and its chloranhydride - (SnCl_{4},N_{2}O_{3} and SnCl_{4},2NOCl), and with metallic - chlorides (for example, K_{2}SnCl_{6}, (NH_{4})_{2}SnCl_{6}, &c.) - In general, a highly-developed faculty for combination is observed - in it. - - Tin does not combine directly with iodine, but if its filings be - heated in a closed tube with a solution of iodine in carbon - bisulphide, it forms stannic iodide, SnI_{4}, in the form of red - octahedra which fuse at 142° and volatilise at 295°. The fluorine - compounds of tin have a special interest in the history of - chemistry, because they give a series of double salts which are - isomorphous with the salts of hydrofluosilicic acid, SiR_{2}F_{6}, - and this fact served to confirm the formula SiO_{2} for silica, as - the formula SnO_{2} was indubitable. Although _stannic fluoride_, - SnF_{4}, is almost unknown in the free state, its corresponding - double salts are very easily formed by the action of hydrofluoric - acid on alkaline solutions of stannic oxide; thus, for example, a - crystalline salt of the composition SnK_{2}F_{6},H_{2}O is - obtained by dissolving stannic oxide in potassium hydroxide and - then adding hydrofluoric acid to the solution. The barium salt, - SnBaF_{6},3H_{2}O, is sparingly soluble like its corresponding - silicofluoride. The more soluble salt of strontium, - SnSrF_{6},2H_{2}O, crystallises very well, and is therefore more - important for the purposes of research; it is isomorphous with the - corresponding salt of silicon (and titanium); the magnesium salt - contains 6H_{2}O. - - Stannic sulphide, SnS_{2}, is formed, as a yellow precipitate, by - the action of sulphuretted hydrogen on acid solutions of stannic - salts; it is easily soluble in ammonium and potassium sulphides, - because it has an acid character, and then forms thiostannates - (see Chapter XX.). In an anhydrous state it has the form of - brilliant golden yellow plates, which may be obtained by heating a - mixture of finely-divided tin, sulphur, and sal-ammoniac for a - considerable time. It is sometimes used in this form under the - name of mosaic gold, as a cheap substitute for gold-leaf in - gilding wood articles. On ignition it parts with a portion of its - sulphur, and is converted into stannous sulphide SnS. It is - soluble in caustic alkalis. Hydrochloric acid does not dissolve - the anhydrous crystalline compound, but the precipitated powdery - sulphide is soluble in boiling strong hydrochloric acid, with the - evolution of hydrogen sulphide. - -_The alkali compounds of stannic oxide_--that is, the compounds in which -it plays the part of an acid, corresponding in this respect to the -compounds of silica and other anhydrides of the composition RO_{2}--are -very easily formed and are used in the arts. Their composition in most -cases corresponds with the formula SnM_{2}O_{3}--that is, SnO(MO)_{2}, -similar to CO(MO)_{2}, where M = K, Na. Acids, even feeble acids like -carbonic, decompose the salts, like the corresponding compounds of -alumina or silica. In order to obtain _potassium stannate_, which -crystallises in rhombohedra, and has the composition -SnK_{2}O_{3},3H_{2}O, potassium hydroxide (8 parts) is fused, and -metastannic acid (3 parts) gradually added. _Sodium stannate_ is prepared -in practice in large quantities by heating a solution of caustic soda -with lead oxide and metallic tin. In this last case an alkaline solution -of lead oxide is formed, and the tin acts on the solution in such a way -as to reduce the lead to the metallic state, and itself passes into -solution. It is very remarkable that lead displaces tin when in -combination with acids, whilst tin, on the contrary, displaces lead from -its alkali compounds. By dissolving the mass obtained in water, and -adding alcohol, sodium stannate is precipitated, which may then be -dissolved in water and purified by re-crystallisation. In this case it -has the composition SnNa_{2}O_{3},3H_{2}O if separated from strong -solutions, and SnNa_{2}O_{3},10H_{2}O when crystallised at a low -temperature from dilute solutions. In the arts this salt is used as a -mordant in dyeing operations. With a cold solution of sodium hydroxide -metastannic acid forms a salt of the composition -(NaHO)_{2},5SnO_{2},3H_{2}O, from which Frémy drew his conclusions -concerning the polymerism of metastannic acid. Tin, like other metals and -many metalloids, gives a peroxide form of combination or _perstannic -oxide_. This substance was obtained by Spring (1889) in the form of a -hydrate, H_{2}Sn_{2}O_{7} = 2(SnO_{3})H_{2}O, by mixing a solution of -SnCl_{2}, containing an excess of HCl, with freshly prepared peroxide of -barium. A cloudy liquid is then obtained, and this after being subjected -to dialysis leaves a gelatinous mass which on drying is found to have the -composition Sn_{2}H_{2}O_{7}. Above 100° this substance gives off oxygen -and leaves SnO_{2}. It is evident that SnO_{3} bears the same relation to -SnO_{2} as H_{2}O_{2} to H_{2}O or ZnO_{2} to ZnO, &c. - -Tin occupies the same position amongst the analogues of silicon as -cadmium and indium amongst the analogues of magnesium and aluminium -respectively, and as in each of these cases the heavier analogues with a -high atomic weight and a special combination of properties--namely, -mercury and thallium--are known, so also for silicon we have _lead_ as -the heaviest analogue (Pb = 206), with a series of both kindred and -special properties. The higher type, PbX_{4}--for instance, PbO_{2}--is -in a chemical sense far less stable than the lower type, PbX. The -ordinary compounds of lead correspond with the latter, and in addition to -this, PbO, although not particularly energetic, is still a decided base -easily forming basic salts, PbX_{2}(PbO)_{n}. Although the compounds -PbX_{4}, are unstable they offer many points of analogy with the -corresponding compounds of tin SnO_{2}; this is seen, for instance, in -the fact that PbO_{2} is a feeble acid, giving the salt PbK_{2}O_{3}, -that PbCl_{4} is a liquid like SnCl_{4} which is not affected by -sulphuric acid, and that PbF_{4} gives double salts, like SnF_{4} or -SiF_{4} (Brauner 1894. See Chapter II., Note 49 bis); Pb(C_{2}H_{5})_{4} -also resembles Sn(C_{2}H_{5})_{4} &c. All this shows that lead is a true -analogue of tin, as Hg is of cadmium.[41 bis] - - [41 bis] Although this has long been generally recognised from the - resemblance between the two metals, still from a chemical point of - view it has only been demonstrated by means of the periodic law. - -_Lead_ is found in nature in considerable masses, in the form of galena, -_lead sulphide_, PbS.[42] The specific gravity of galena is 7·58, colour -grey; it crystallises in the regular system, and has a fine metallic -lustre. Both the native and artificial sulphides are insoluble in acids -(hydrogen sulphide gives a black precipitate with the salts PbX_{2}).[42 -bis] When heated, lead melts, and in the open air is either totally or -partially transformed into white lead sulphate, PbSO_{4}, as it also is -by many oxidising agents (hydrogen peroxide, potassium nitrate). Lead -sulphate is also insoluble in water,[43] and lead is but rarely met with -in this form in nature. The chromates, vanadates, phosphates, and similar -salts of lead are also somewhat rare. The carbonate, PbCO_{2}, is -sometimes found in large masses, especially in the Altai region. Lead -sulphide is often worked for extracting the silver which it contains; and -as the lead itself also finds manifold industrial applications, this work -is carried out on an exceedingly large scale. Many methods are employed. -Sometimes the lead sulphide is decomposed by heating it with cast iron. -The iron takes up the sulphur from the lead and forms easily-fusible iron -sulphide, which does not mix with the heavier reduced lead. But another -process is more frequently used: the lead ore (it must be clean; that is, -free from earthy matter, which may be easily removed by washing) is -heated in a reverberatory furnace to a moderate temperature with a free -access of air. During this operation part of the lead sulphide oxidises -and forms lead sulphate, PbSO_{4}, and lead oxide. When the oxidation of -part of the lead has been attained, it is necessary to shut off the air -supply and increase the temperature, then the oxidised compounds of the -lead enter into reaction with the remaining lead sulphide, with formation -of sulphurous anhydride and metallic lead. At first from PbS + O_{3}, PbO -+ SO_{2} are formed, and also from PbS + O_{4} lead sulphate PbSO_{4}, -and then PbO and PbSO_{4} react with the remaining PbS, according to the -equations 2PbO + PbS = 3Pb + SO_{2} and also PbSO_{4} + PbS = 2Pb + -2SO_{2}.[44] - - [42] Mixed ores of copper compounds together with PbS and ZnS are - frequently found in the most ancient primary rocks. As the - separation of the metals themselves is difficult, the ores are - separated by a method of selection or mechanical sorting. Such - mixed ores occur in Russia, in many parts of the Caucasus, and in - the Donetz district (at Nagolchik). - - [42 bis] Lead sulphide in the presence of zinc and hydrochloric acid is - completely reduced to metallic lead, all the sulphur being given - off as hydrogen sulphide. - - [43] Lead sulphate, PbSO_{4}, occurs in nature (_anglesite_) in - transparent brilliant crystals which are isomorphous with barium - sulphate, and have a specific gravity of 6·3. The same salt is - formed on mixing sulphuric acid or its soluble salts with - solutions of lead salts, as a heavy white precipitate, which is - insoluble in water and acids, but dissolves in a solution of - ammonium tartrate in the presence of an excess of ammonia. This - test serves to distinguish this salt from the similar salts of - strontium and barium. - - [44] According to J. B. Hannay (1894) the last named decomposition - (PbS + PbSO_{4} = 2Pb + 2SO_{2}) is really much more complicated, - and in fact a portion of the PbS is dissolved in the Pb, forming a - slag containing PbO, PbS and PbSO_{4}, whilst a portion of the - lead volatilises with the SO_{2} in the form of a compound - PbS_{2}O_{2}, which is also formed in other cases, but has not yet - been thoroughly studied. - - Besides these methods for extracting lead from PBS in its ores, - roasting (the removal of the S in the form of SO_{2}) and smelting - with charcoal with a blast in the same manner as in the - manufacture of pig iron (Chapter XXII.) are also employed. - - We may add that PbS in contact with Zn and hydrochloric acid - (which has no action upon PbS alone) entirely decomposes, forming - H_{2}S and metallic lead: PbS + Zn + 2HCl = Pb + ZnCl_{2} + - H_{2}S. - - As lead is easily reduced from its ores, and the ore itself has a - metallic appearance, it is not surprising that it was known to the - ancients, and that its properties were familiar to the alchemists, - who called it 'Saturn.' Hence metallic lead, reduced from its - salts in solution by zinc, having the appearance of a tree-like - mass of crystals, is called 'arbor saturni,' &c. - -The appearance of lead is well known; its specific gravity is 11·3; the -bluish colour and well-marked metallic lustre of freshly-cut lead quickly -disappear when exposed to the air, because it becomes coated with a -layer--although a very thin layer--of oxide and salts formed by the -moisture and acids in the atmosphere. It melts at 320°, and crystallises -in octahedra on cooling. Its softness is apparent from the flexibility of -lead pipes and sheets, and also from the fact that it may be cut with a -knife, and also that it leaves a grey streak when rubbed on paper. On -account of its being so soft, lead naturally cannot be applied in many -cases where most metals may be used; but on the other hand it is a metal -which is not easily changed by chemical reagents, and as it is capable of -being soldered and drawn into sheets, &c., lead is most valuable for many -technical uses. Lead pipes are used for conveying water[45] and many -other liquids, and sheet lead is used for lining all kinds of vessels -containing liquids--(acids, for instance) which act on other metals. This -particularly refers to sulphuric and hydrochloric acids, because at a low -temperature they do not act on lead, and if they form lead sulphate, -PbSO_{4}, and chloride, PbCl_{2}, these salts being insoluble in water -and in acids, cover the lead and protect it from further corrosion.[46] -All soluble preparations of lead are poisonous. At a white heat lead may -be partially distilled; the vapours oxidise and burn. Lead may also be -easily oxidised at low temperatures. Lead only decomposes water at a -white heat, and does not liberate hydrogen from acids, with the exception -only of very strong hydrochloric acid and then only when boiling. -Sulphuric acid diluted with water does not act on it, or only acts very -feebly at the surface; but strong sulphuric acid, when heated, is -decomposed by it, with the evolution of sulphurous anhydride. The best -solvent for lead is nitric acid, which transforms it into a soluble salt, -Pb(NO_{3})_{2}. - - [45] Freshly laid new lead pipes contaminate the water with a certain - amount of lead salts, arising from the presence of oxygen, - carbonic acid, &c., in the water. But the lead pipes under the - action of running water soon become coated with a film of - salts--lead sulphate, carbonate, chloride, &c.--which are - insoluble in water, and the water pipes then become harmless. - - [46] Lead is used in the arts, and owing to its considerable density, - it is cast, mixed with small quantities of other metals, into - shot. A considerable amount is employed (together with mercury) in - extracting gold and silver from poor ores, and in the manufacture - of chemical reagents, and especially of lead chromate. _Lead - chromate_, PbCrO_{4}, is distinguished for its brilliant yellow - colour, owing to which it is employed in considerable quantities - as a dye, mainly for dyeing cotton tissues yellow. It is formed on - the tissue itself, by causing a soluble salt of lead to react on - potassium chromate. Lead chromate is met with in nature as 'red - lead ore.' It is insoluble in water and acetic acid, hut it - dissolves in aqueous potash. The so-called pewter vessels often - consist of an alloy of 5 parts of tin and 1 part of lead, and - solder is composed of 1 to 2 parts of tin with 1/2 part of lead. - Amongst the alloys of lead and tin, Rudberg states that the alloy - PbSn_{3} stands out from the rest, since, according to his - observations, the temperature of solidification of the alloy is - 187°. - -Although acids thus have directly but little effect on lead, and this is -one of its most important practical properties, _yet when air has free -access, lead (like copper) very easily reacts with many acids_, even with -those which are comparatively feeble. The action of acetic acid on lead -is particularly striking and often applied in practice. If lead be -plunged into acetic acid it does not change at all and does not pass into -solution, but if part of the lead be immersed in the acid, and the other -part remain in contact with the air, or if lead be merely covered with a -thin layer of acetic acid in such a way that the air is practically in -contact with the metal, then it unites with the oxygen of the air to form -oxide, which combines with the acetic acid and forms lead acetate, -soluble in water. The formation of lead oxide is especially marked from -the fact that with a sufficient quantity of air not only is the normal -lead acetate formed but also the basic salts.[47] - - [47] The normal lead acetate, known in trade as _sugar of lead_, owing - to its having a sweetish taste, has the formula - Pb(C_{2}H_{3}O_{2})_{2},3H_{2}O. This salt only crystallises from - acid solutions. It is capable of dissolving a further quantity of - lead oxide or of metallic lead in the presence of air. A basic - salt of the composition Pb(C_{2}H_{3}O_{2})_{2},PbH_{2}O_{2} is - then formed which is soluble in water and alcohol. As in this salt - the number of atoms is even and the same as in the hydrate of - acetic acid, C_{2}H_{4}O_{2},H_{2}O = C_{2}H_{3}(OH)_{3}, it may - be represented as this hydrate in which two of hydrogen are - replaced by lead--that is, as C_{2}H_{3}(OH)(O_{2}Pb). This basic - salt is used in medicine as a remedy for inflammation, for - bandaging wounds, &c., and also in the manufacture of white lead. - Other basic acetates of lead, containing a still greater amount of - lead oxide, are known. According to the above representation of - the composition of the preceding lead acetate, a basic salt of the - composition (C_{2}H_{3})_{2}(O_{2}Pb)_{3} would be also possible, - but what appear to be still more basic salts are known. As the - character of a salt also depends on the property of the base from - which it is formed, it would seem that lead forms a hydroxide of - the composition HOPbOH, containing two water residues, one or both - of which may be replaced by the acid residues. If both water - residues are replaced, a normal salt, XPbX, is obtained, whilst if - only one is replaced a basic salt, XPbOH, is formed. But lead does - not only give this normal hydroxide, but also polyhydroxides, - Pb(OH),_n_PbO, and if we may imagine that in these polyhydroxides - there is a substitution of both the water residues by acid - residues, then the power of lead for forming basic salts is - explained by the properties of the base which enters into their - composition. - -When oxidising in the presence of air,[48] when heated or in the -presence of an acid at the ordinary temperature, lead forms compounds of -the type PbX_{2}. _Lead oxide_, PbO, known in industry as _litharge_, -silberglätte (this name is due to the fact that silver is extracted from -the lead ores of this kind) and massicot. If the lead is oxidised in air -at a high temperature, the oxide which is formed fuses, and on cooling is -easily obtained in fused masses which split up into scales of a yellowish -colour, having a specific gravity of 9·3; in this form it bears the name -of litharge. Litharge is principally used for making lead salts, for the -extraction of metallic lead, and also for the preparation of drying -oils--for instance, from linseed oil.[49] When oxidised carefully and -slightly heated, lead forms a powdery (not fused) oxide known under the -name of _massicot_. It is best prepared in the laboratory by heating lead -nitrate, or lead hydroxide. It has a yellow colour, and differs from -litharge in the greater difficulty with which it forms lead salts with -acids. Thus, for instance, when massicot is moistened with water it does -not attract the carbonic acid of the air so easily as litharge does. It -may, however, be imagined that the cause of the difference depends only -on the formation of dioxide on the surface of the lead oxide, on which -the acids do not act. In any case lead oxide is comparatively easily -soluble in nitric and acetic acids. It is but slightly soluble in water, -but communicates an alkaline reaction to it, since it forms the -hydroxide. This hydroxide is obtained in the shape of a white precipitate -by the action of a small quantity of an alkali hydroxide on a solution of -a lead salt. An excess of alkali dissolves the hydroxide separated, which -fact demonstrates the comparatively indistinct basic properties of lead -oxide. The normal lead hydroxide, which should have the composition -Pb(OH)_{2}, is unknown in a separate state, but it is known in -combination with lead oxide as Pb(OH)_{2},2PbO or Pb_{3}O_{2}(OH)_{2}. -The latter is obtained in the form of brilliant, white, octahedral -crystals when basic lead acetate is mixed with ammonia and gently heated. -The basic qualities of this hydroxide are shown distinctly by its -absorbing the carbonic anhydride of the air. When an alkaline solution of -the hydroxide is boiled, it deposits lead oxide in the form of a -crystalline powder. - - [48] Few compounds are known of the lower type PbX, and still fewer of - the intermediate type PbX_{3}. To the first type belongs the - so-called lead suboxide, Pb_{2}O, obtained by the ignition of lead - oxalate, C_{2}PbO_{4}, without access of air. It is a black - powder, which easily breaks up under the action of acids, and even - by the simple action of heat, into metallic lead and lead oxide. - This is the character of all suboxides. They cannot be regarded as - independent salt-forming oxides, neither can those forms of - oxidation of lead which contain more oxygen than the oxide of - lead, PbO, and less than the dioxide, PbO_{2}. As we shall see, at - least two such compounds are formed. Thus, for example, an oxide - having the composition Pb_{2}O_{3} is known, but it is decomposed - by the action of acids into lead oxide, which passes into - solution, and lead dioxide, which remains behind. Such is red - lead. (See further on.) - - [49] In the boiling of drying oils, the lead oxide partially passes - into solution, forming a saponified compound capable of attracting - oxygen and solidifying into a tar-like mass, which forms the oil - paint. Perhaps, however, glycerine partially acts in the process. - - Ossovetsky by saturating drying oil with the salts of certain - metals obtained oil colours of great durability. - - A mixture of very finely-divided litharge with glycerine (50 parts - of litharge to 5 c.c. of anhydrous glycerine) forms a very quick - (two minutes) setting cement, which is insoluble in water and - oils, and is very useful in setting up chemical apparatus. The - hardening is based on the reaction of the lead oxide with - glycerine (Moraffsky). - -Lead oxide forms but few soluble salts--for instance, the nitrate and the -acetate. The majority of its salts (sulphate, PbSO_{4}; carbonate, -PbCO_{3}; iodide, PbI_{2}, &c.) are insoluble in water. These salts are -colourless or light yellow if the acid be colourless. In lead oxide _the -faculty of forming basic salts_, PbX_{2}_n_PbO or PbX_{2}_n_PbH_{2}O_{2}, -is strongly developed. A similar property was observed in magnesium and -also in the salts of mercury, but lead oxide forms basic salts with still -greater facility, although double salts are in this case more rarely -formed.[50] - - [50] It is very instructive to observe that lead not only easily forms - basic salts, but also salts containing several acid groups. Thus, - for example, lead carbonate occurs in nature and forms compounds - with lead chloride and sulphate. The first compound, known as - _corneous lead_, _phosgenite_, has the composition - PbCO_{3},PbCl_{2}; it occurs in nature in bright cubical crystals, - and is prepared artificially by simply boiling lead chloride with - lead carbonate. A similar compound of normal salts, - PbSO_{4},PbCO_{3}, occurs in nature as _lanarkite_ in monoclinic - crystals. _Leadhillite_ contains PbSO_{4},3PbCO_{3}, and also - occurs in yellowish, monoclinic, tabular crystals. We will turn - our attention to these salts of lead, because it is very probable - that their formation is allied to the formation of the basic - salts, and the following considerations may lead to the - explanation of the existence of both. In describing silica we - carefully developed the conception of polymerisation, which it is - _also indispensable to recognise in the composition of many other - oxides_. Thus it may be supposed that PbO_{2} is a similar - polymerised compound to SiO_{2}--_i.e._ that the composition of - lead peroxide will be Pb_{_n_}O_{2_n_}, because lead methyl, - PbMe_{4}, and lead ethyl, PbEt_{4}, are volatile compounds, whilst - PbO_{2} is non-volatile, and is very like silica in this respect, - and not in the least like carbonic anhydride. Still more should a - polymeric structure, Pb_{_n_}O_{_n_}, be ascribed to lead oxide, - since it differs as little from lead dioxide in its physical - properties as carbonic oxide does from carbonic anhydride, and - being an unsaturated compound is more likely to be capable of - intercombination (polymerisation) than lead dioxide. These - considerations respecting the complexity of lead oxide could have - no real significance, and could not be accepted, were it not for - the existence of the above-mentioned basic and mixed salts. The - oxide apparently corresponds with the composition - Pb_{_n_}X_{2_n_}, and since, according to this representation, the - number of X's in the salts of lead is considerable, it is obvious - that they may be diverse. When a part of these X's is replaced by - the water residue (OH) or by oxygen, X_{2} = O, and the other - parts by an _acid residue_, X, then basic salts are obtained, but - if a part of the X's is replaced by acid residues of one kind, and - the other part by acid residues of another kind, then those mixed - salts about which we are now speaking are formed. Thus, for - example, we may suppose, for a comparison of the composition of - the majority of the salts of lead, that _n_ = 12, and then the - above-mentioned compounds will present themselves in the following - form:--Lead oxide, Pb_{12}O_{12}, its crystalline hydrate, - Pb_{12}O_{8}(OH)_{8}, lead chloride, Pb_{12}Cl_{24}, lead - oxychloride, Pb_{12}Cl_{12}O_{6}, the other oxychloride, - Pb_{12}(OH)_{8}Cl_{6}O_{6}, mendipite (_see_ Note 51), - Pb_{12}Cl_{8}O_{8}, normal lead carbonate, Pb_{12}(CO_{3})_{12}, - crystalline basic salt, Pb_{12}(OH)_{6}(CO_{3})_{6}, white lead, - Pb_{12}(CO_{3})_{8}(HO)_{8}, corneous lead, - Pb_{12}Cl_{12}(CO_{3})_{6}, lanarkite, - Pb_{12}(CO_{3})_{6}(SO_{4})_{6}, leadhillite, - Pb_{12}(CO_{3})_{9}(SO_{4})_{3}, &c. The number 12 is only taken - to avoid fractional quantities. Possibly the polymerisation is - much higher than this. The theory of the polymerisation of oxides - introduced by me in the first edition of this work (1869) is now - beginning to be generally accepted. - -Amongst the soluble lead salts, that best known and most often applied in -practical chemistry is _lead nitrate_, obtained directly by dissolving -lead or its oxide in nitric acid. The normal salt, Pb(NO_{3})_{2}, -crystallises in octahedra, dissolves in water, and has a specific gravity -of 4·5. When a solution of this salt acts on white lead or is boiled with -litharge, the basic salt, having a composition Pb(OH)(NO_{3}), is formed -in crystalline needles, sparingly soluble in cold water but easily -dissolved in hot water, and therefore in many respects resembling lead -chloride. When the nitrate is heated, either lead oxide is obtained or -else the oxide in combination with peroxide. - -_Lead chloride_, PbCl_{2}, is precipitated from the soluble salts of -lead when a strong solution is treated with hydrochloric acid or a -metallic chloride. It is soluble in considerable quantities in hot water, -and therefore if the solutions be dilute or hot, the precipitation of -lead chloride does not occur, and if a hot solution be cooled, the salt -separates in brilliant prismatic crystals. It fuses when heated (like -silver chloride), but is insoluble in ammonia. This salt is sometimes met -with in nature, and when heated in air is capable of exchanging half its -chlorine for oxygen, forming the basic salt or lead oxychloride, -PbCl_{2}PbO, which may also be obtained by fusing PbCl_{2} and PbO -together. The reaction of lead chloride with water vapour leads to the -same conclusion, showing the feeble basic character of lead 2PbCl_{2} + -H_{2}O = PbCl_{2},PbO + 2HCl. When ammonia is added to an aqueous -solution of lead chloride a white precipitate is formed, which parts with -water on being heated, and has the composition Pb(OH)Cl,PbO. This -compound is also formed by the action of metallic chlorides on other -soluble basic salts of lead.[51] - - [51] A similar basic salt having a white colour, and therefore used as - a substitute for white lead, is also obtained by mixing a solution - of basic lead acetate with a solution of lead chloride. Its - formation is expressed by the equation: 2PbX(OH),PbO + PbCl_{2} = - 2Pb(OH)Cl,PbO + PbX_{2}. Similar basic compounds of lead are met - with in nature--for instance, _mendipite_, PbCl,2PbO, which - appears in brilliant yellowish-white masses. The ignition of red - lead with sal-ammoniac results in similar polybasic compounds of - lead chloride, forming the _Cassel's_, or _mineral yellow_ of the - composition PbCl_{2}_n_PbO. _Lead iodide_, PbI_{2}, is still less - soluble than the chloride, and is therefore obtained by mixing - potassium iodide with a solution of a lead salt. It separates as a - yellow powder, which may be dissolved in boiling water, and on - cooling separates in very brilliant crystalline scales of a golden - yellow colour. The salts PbBr_{2}, PbF_{2}, Pb(CN)_{2}, - Pb_{2}Fe(CN)_{6} are also insoluble in water, and form white - precipitates. - -Lead carbonate, or _white lead_, is the most extensively used basic lead -salt. It has the valuable property of 'covering,' which only to a certain -extent appertains to lead sulphate and other white powdery substances -used as pigments. This faculty of 'covering' consists in the fact that a -small quantity of white lead mixed with oil spreads uniformly, and if -such a mixture be spread over a surface (for instance, of wood or metal) -the surface is quickly covered--that is, light does not penetrate through -even a very thin layer of superposed white lead; thus, for example, the -grain of the wood remains invisible.[52] White lead, or _basic lead -carbonate_, after being dried at 120°, has a composition -Pb(OH)_{2},2PbCO_{3}.[53] It may be obtained by adding a solution of -sodium carbonate to a solution of one of the basic salts of lead--for -instance, the basic acetate--and likewise by treating this latter with -carbonic acid. For this purpose the solution of basic acetate is poured -into the vessel _f_; it is prepared in the vat A, containing litharge, -into which the pump P delivers the solution of the acetate, which remains -after the action of carbonic anhydride on the basic salt. In A a basic -salt is formed having a composition approaching to -Pb_{4}(OH)_{6}(C_{2}H_{3}O_{2})_{2}; carbonic anhydride, 2CO_{2}, is -passed through this solution and precipitates white lead, -Pb_{2}(OH)_{2}(CO_{3})_{2}, and normal lead acetate, -Pb(C_{2}H_{3}O_{2})_{2}, remains in the solution, and is pumped back into -the vat A containing lead oxide, where the normal salt is again (on being -agitated) converted into the basic salt. This is run into the vessel E, -and thence into _f_. Into the latter carbonic anhydride is delivered from -the generator D, and forms a precipitate of white lead.[53 bis] - - [52] It is remarkable that a peculiar kind of attraction exists between - boiled linseed oil and white lead, as is seen from the following - experiments. White lead is triturated in water. Although it is - heavier than water, it remains in suspension in it for some time - and is thoroughly moistened by it, so that the trituration may be - made perfect; boiled linseed oil is then added, and shaken up with - it. A mixture of the oil and white lead is then found to settle at - the bottom of the vessel. Although the oil is much lighter than - the water it does not float on the top, but is retained by the - white lead and sinks under the water together with it. There is - not, however, any more perfect combination nor even any solution. - If the resultant mass be then treated with ether or any other - liquid capable of dissolving the oil, the latter passes into - solution and leaves the white lead unaltered. - - [53] It may be regarded as a salt corresponding with the normal hydrate - of carbonic acid, C(OH)_{4}, in which three-quarters of the - hydrogen is replaced by lead. A salt is also known in which all - the hydrogen of this hydrate of carbonic acid is replaced by - lead--namely, the salt containing CO_{4}Pb_{2}. This salt is - obtained as a white crystalline substance by the action of water - and carbonic acid on lead. The normal salt, PbCO_{3}, occurs in - nature under the name of white lead ore (sp. gr. 6·47), in - crystals, isomorphous with aragonite, and is formed by the double - decomposition of lead nitrate with sodium carbonate, as a heavy - white precipitate. Thus both these salts resemble white lead, but - the first-named salt is exclusively used in practice, owing to its - being very conveniently prepared, and being characterised by its - great covering capacity, or 'body,' due to its fine state of - division. - - [53 bis] One of the many methods by which white lead is prepared - consists in mixing massicot with acetic acid or sugar of lead, and - leaving the mixture exposed to air (and re-mixing from time to - time), containing carbonic acid, which is absorbed from the - surface by the basic salt formed. After repeated mixings (with the - addition of water), the entire mass is converted into white lead, - which is thus obtained very finely divided. - -[Illustration: FIG. 82.--Manufacture of white lead.] - -In order to mark the transition from lead oxide, PbO, into lead dioxide -PbO_{2} (plumbic anhydride), it is necessary to direct our attention to -the intermediate oxide, or _red lead_, Pb_{3}O_{4}.[54] In the arts it is -used in considerable quantities, because it forms a very durable -yellowish-red paint used for colouring the resins (shellac, colophony, -&c.) composing sealing wax. It also forms a very good cheap oil paint, -used especially for painting metals, more particularly because drying -oils--for instance, hemp seed, linseed oils--very quickly dry with red -lead and with lead salts. Red lead is prepared by slightly heating -massicot, for which purpose two-storied stoves are used. In the lower -story the lead is turned into massicot, and in the higher one, having the -lower temperature (about 300°), the massicot is transformed into red -lead. Frémy and others showed the instability of red lead prepared by -various methods, and its decomposition by acids, with formation of lead -dioxide, which is insoluble in acids, and a solution of the salts of lead -oxide. The artificial production (synthesis) of red lead by double -decomposition was most important. For this purpose Frémy mixed an -alkaline solution of potassium plumbate, K_{2}PbO_{3} (prepared by -dissolving the dioxide in fused potash),[54 bis] with an alkaline -solution of lead oxide. In this way a yellow precipitate of minium -hydrate is formed, which, when slightly heated, loses water and turns -into bright red anhydrous minium Pb_{3}O_{4}. - - [54] If lead hydroxide be dissolved in potash and sodium hypochlorite - be added to the solution, the oxygen of the latter acts on the - dissolved lead oxide, and partially converts it into dioxide, so - that the so-called lead sesquioxide is obtained; its empirical - formula is Pb_{2}O_{3}. Probably it is nothing but a lead - salt--_i.e._ is referable to the type of dioxide of lead, or its - hydroxide, PbO(OH)_{2}, in which two atoms of hydrogen are - replaced by lead, PbO(O_{2}Pb). The brown compound precipitated by - the action of dilute acids--for example, nitric--splits up, even - at the ordinary temperature, into insoluble lead dioxide and a - solution of a lead salt. This compound evolves oxygen when it is - heated. It dissolves in hydrochloric acid, forming a yellow - liquid, which probably contains compounds of the composition - PbCl_{2} and PbCl_{4}, but even at the ordinary temperature the - latter soon loses the excess of chlorine, and then only lead - chloride, PbCl_{2}, remains. In order to see the relation between - red lead and lead sesquioxide, it must be observed that they only - differ by an extra quantity of lead oxide--that is, red lead is a - basic salt of the preceding compound, and if the compound - Pb_{2}O_{3} may be regarded as PbO_{3}Pb, then red lead should be - looked on as PbO_{3}Pb,PbO--that is, as basic lead plumbate. - - [54 bis] Frémy obtained potassium plumbate in the following manner. - Pure lead dioxide is placed in a silver crucible, and a strong - solution of pure caustic potash is poured over it. The mixture is - heated and small quantities are removed from time to time for - testing, which consists in dissolving in a small quantity of water - and decomposing the resultant solution with nitric acid. There is - a certain moment during the heating when a considerable amount of - insoluble lead dioxide is precipitated on the addition of the - nitric acid; the solution then contains the salt in question, and - the heating must be stopped, and a small amount of water added to - dissolve the potassium plumbate formed. On cooling the salt - separates in somewhat large crystals, which have the same - composition as the stannate--that is, PbO(KO)_{2},3H_{2}O. - -Minium is the first and most ordinary means of producing _lead -dioxide_, or plumbic anhydride, PbO_{2},[55] because when red lead is -treated with dilute nitric acid it gives up lead oxide, and PbO_{2} -remains, on which dilute nitric acid does not act. The composition of -minium is Pb_{3}O_{4}, and therefore the action of nitric acid on it is -expressed by the equation: Pb_{3}O_{4} + 4HNO_{3} = PbO_{2} + -2Pb(NO_{3})_{2} + 2H_{2}O. The dioxide may also be obtained by treating -lead hydroxide suspended in water with a stream of chlorine. Under these -conditions the chlorine takes up the hydrogen from the water, and the -oxygen passes over to the lead oxide.[56] When a strong solution of lead -nitrate is decomposed by the electric current, the appearance of -crystalline lead dioxide is also observed upon the positive pole; it is -also found in nature in the form of a black crystalline substance having -a specific gravity of 9·4. When artificially produced it is a fine dark -powder, resisting the action of acids, but nevertheless when treated with -strong sulphuric acid it evolves oxygen and forms lead sulphate, and with -hydrochloric acid it evolves chlorine. The oxidising property of lead -dioxide depends of course on the facility of its transition into the more -stable lead oxide, which is easily understood from the whole history of -lead compounds. In the presence of alkalis it transforms chromium oxide -into chromic acid, whilst lead chromate, PbCrO_{4}, is formed, remaining, -however, in solution, on account of its being soluble in caustic alkalis. -The oxidising action of lead dioxide on sulphurous anhydride is most -striking, as it immediately absorbs it, with formation of lead sulphate. -This is accompanied by a change of colour and development of heat, -PbO_{2} + SO_{2} = PbSO_{4}. When triturated with sulphur the mixture -explodes, the sulphur burning at the expense of the oxygen of the lead -dioxide. _Tetrachloride of lead_, PbCl_{4}, belongs to the same class of -lead compounds as PbO_{2}. This chloride is formed by the action of -strong hydrochloric acid upon PbO_{2}, or, in the cold, by passing a -stream of chlorine through water containing PbCl_{2} in suspension. The -resultant yellow solution gives off chlorine when heated. With a solution -of sal ammoniac (Nicolukin, 1885) it gives a precipitate of a double -salt, (NH_{4})_{2}PbCl_{6} (very slightly soluble in a solution of sal -ammoniac), which when treated with strong sulphuric acid (Friedrich, -1890) gives PbCl_{4} as a yellow liquid sp. gr. 3·18, which solidifies at --18°, and when heated gives PbCl_{2} + Cl_{2}. It is not acted upon by -H_{2}SO_{4} like SnCl_{4}. Tetrafluoride of lead (Brauner) belongs to the -same class of compounds, it easily forms double salts and decomposes with -the evolution of fluorine (Chapter II., Note 49 bis).[56 bis] - - [55] Lead dioxide is often called lead peroxide, but this name leads to - error, because PbO_{2} does not show the properties of true - peroxides, like hydrogen or barium peroxides, but is endowed with - acid properties--that is, it is able to form true salts with - bases, which is not the case with true peroxides. Lead dioxide is - a normal salt-forming compound of lead, as Bi_{2}O_{5} is for - bismuth, CeO_{2} for cerium, and TeO_{3} for tellurium, &c. They - all evolve chlorine when treated with hydrochloric acid, whilst - true peroxides form hydrogen peroxide. The true lead peroxide, if - it were obtained, would probably have the composition Pb_{2}O_{5}, - or, in combination with peroxide of hydrogen, H_{2}Pb_{2}O_{7} = - H_{2}O_{2} + Pb_{2}O_{5}, judging from the peroxides corresponding - with sulphuric, chromic, and other acids, which we shall - afterwards consider. - - As a proof of the fact, that the form PbO_{2}, or PbX_{4}, is the - highest normal form of any combination of lead, it is most - important to remark that it might be expected that the action of - lead chloride, PbCl_{2}, on zinc-ethyl, ZnEt_{2}, would result in - the formation of zinc chloride, ZnCl_{2}, and lead-ethyl, - PbEt_{2}, but that in reality the reaction proceeds otherwise. - Half of the lead is set free, and lead tetrethyl, PbEt_{4}, is - formed as a colourless liquid, boiling at about 200° (Butleroff, - Frankland, Buckton, Cahours, and others). The type PbX_{4} is not - only expressed in PbEt_{4} and PbO_{2}, but also in PbF_{4}, - obtained by Brauner. - - [56] According to Carnelley and Walker, the hydrate - (PbO_{2})_{3},H_{2}O is then formed; it loses water at 230°. The - anhydrous dioxide remains unchanged up to 280°, and is then - converted into the sesquioxide, Pb_{2}O_{3}, which again loses - oxygen at about 400°, and forms red lead, Pb_{3}O_{4}. Red lead - also loses oxygen at about 550°, forming lead oxide, PbO, which - fuses without change at about 600°, and remains constant as far as - the limit of the observations made (about 800°). - - The best method for preparing pure lead dioxide consists in mixing - a hot solution of lead chloride with a solution of bleaching - powder (Fehrman). - - [56 bis] The plumbates of Ca and other similar metals, mentioned in - Chapter III., Note 7, also belong to the form PbX_{4}. - -Amongst the elements of the second and third groups it was observed that -the elements were more basic in the even than in the uneven series. It is -sufficient to remember calcium, strontium, and barium in the even, and -magnesium, zinc, and cadmium in the uneven series. In addition to this, -in the even series, as the atomic weight increases, in the same type of -oxidation the basic properties increase (the acid properties decrease); -for example, in the second group, calcium, strontium, barium. The same -also appears in the fourth and all the following groups. In the even -series of the fourth group titanium, zirconium, cerium, and thorium are -found. All their highest oxides, RO_{2}, even the lightest, titanic -oxide, TiO_{2}, have more highly developed basic properties than silica, -SiO_{2}, and in addition to this the basic properties are more distinctly -seen in zirconium dioxide, ZrO_{2}, than in titanic oxide, TiO_{2}, -although the acid property of combining with bases still remains. In the -heaviest oxides, cerium dioxide, CeO_{2}, and thorium dioxide, ThO_{2}, -no acid properties are observed, these being both purely basic oxides. In -Chapter XVII. (Note 43) we already pointed out this higher oxide of -cerium. As the above-mentioned elements are rather rare in nature, have -but little practical application, and do not present any new forms of -combination, it is unadvisable to dwell on them in this treatise. - -_Titanium_ is found in nature in the form of its anhydride or oxide, -TiO_{2}, mixed with silicon in many minerals, but the oxide is also found -separately in the form of semi-metallic _rutile_ (sp. gr. 4·2). Another -titanic mineral is found as a mixture in other ores, known as _titanic -iron ore_ (in the Thuensky mountains of the southern Ural; it is known as -_thuenite_), FeTiO_{3}. This is a salt of ferrous oxide and titanic -anhydride. It crystallises in the rhombohedric system, has a metallic -lustre, grey colour, sp. gr. 4·5. The third mineral in which titanium is -found in considerable quantities in nature is _sphene_ or _titanite_, -CaTiSiO_{5} = CaO,SiO_{2},TiO_{2}, sp. gr. 3·5, colour yellow, green, or -the like, crystallises in tablets. The fourth, but rare, titanic mineral -is _peroffskite_, calcium titanate, CaTiO_{3}; it forms blackish-grey or -brown cubic crystals, sp. gr. 4·02, and occurs in the Ural and other -localities. It may be prepared artificially by fusing sphene in an -atmosphere of water vapour and carbonic anhydride. At the end of the last -century Klaproth showed the distinction between titanic compounds and all -others then known.[57] - - [57] The compounds of titanium are generally obtained from rutile; the - finely-ground ore is fused with a considerable amount of acid - potassium sulphate, until the titanic anhydride, as a feeble base, - passes into solution. After cooling, the resultant mass is ground - up, dissolved in cold water, and treated with ammonium - hydrosulphide; a black precipitate then separates out from the - solution. This precipitate contains TiO_{2} (as hydrate) and - various metallic sulphides--for example, iron sulphide. It is - first washed with water and then with a solution of sulphurous - anhydride until it becomes colourless. This is due to the iron - sulphide contained in the precipitate, and rendering it black, - being converted into dithionate by the action of the sulphurous - acid. The titanic acid left behind is nearly pure. The - considerable volatility of titanium chloride may also be taken - advantage of in preparing the compounds of titanium from rutile. - It is formed by heating a mixture of rutile and charcoal in dry - chlorine; the distillate then contains _titanium chloride_, - TiCl_{4}. It may be easily purified, owing to its having a - constant boiling point of 136°. Its specific gravity is 1·76; it - is a colourless liquid, which fumes in the air, and is perfectly - soluble in water if it be not heated. When hot water acts on - titanic chloride, a large proportion of titanic acid separates out - from the solution and passes into metatitanic acid. A similar - decomposition of acid solutions of titanic acid is accomplished - whenever they are heated, and especially in the presence of - sulphuric acid, just as with metastannic acid, which titanic acid - resembles in many respects. On igniting the titanic acid a - colourless powder of the anhydride, TiO_{2}, is obtained. In this - form it is no longer soluble in acids or alkalis, and only fuses - in the oxyhydrogen flame; but, like silica, it dissolves when - fused with alkalis and their carbonates; as already mentioned, it - dissolves when fused with a considerable excess of acid potassium - sulphate--that is, it then reacts as a feeble base. This shows the - basic character of titanic anhydride; it has at once, although - feebly developed, both basic and acid properties. The fused mass, - obtained from titanic anhydride and alkali when treated with - water, parts with its alkali, and a residue is obtained of a - sparingly-soluble poly-titanate, K_{2}TiO_{3}_n_TiO_{2}. The - hydrate, which is precipitated by ammonia from the solutions - obtained by the fusion of TiO_{2} with acid potassium sulphate, - when dried forms an amorphous mass of the composition Ti(OH)_{4}. - But it loses water over sulphuric acid, gradually passing into a - hydrate of the composition TiO(OH)_{2}, and when heated it parts - with a still larger proportion of water; at 100° the hydrate - Ti_{2}O_{3}(OH)_{2} is obtained, and at 300° the anhydride itself. - The higher hydrate, Ti(OH)_{4}, is soluble in dilute acid, and the - solution may be diluted with water; but on boiling the sulphuric - acid solution (though not the solution in hydrochloric acid), all - the titanic acid separates in a modified form, which is, however, - not only insoluble in dilute acids, but even in strong sulphuric - acid. This hydrate has the composition Ti_{2}O_{3}(OH)_{2}, but - shows different properties from those of the hydrate of the same - composition described above, and therefore this modified hydrate - is called _metatitanic acid_. It is most important to note the - property of the ordinary gelatinous hydrate (that precipitated - from acid solutions by ammonia) of dissolving in acids, the more - so since silica does not show this property. In this property a - transition apparently appears between the cases of common solution - (based on a capacity for unstable combination) and the case of the - formation of a hydrosol (the solubility of germanium oxide, - GeO_{2}, perhaps presents another such instance). If titanium - chloride be added drop by drop to a dilute solution of alcohol and - hydrogen peroxide, and then ammonia be added to the resultant - solution, a yellow precipitate of _titanium trioxide_, - TiO_{3}H_{2}O, separates out, as Piccini, Weller, and Classen - showed. This substance apparently belongs to the category of true - peroxides. - - Titanium chloride absorbs ammonia and forms a compound, - TiCl_{4},4NH_{3}, as a red-brown powder which attracts moisture - from the air and when ignited forms _titanium nitride_, - Ti_{3}N_{4}. Phosphuretted hydrogen, hydrocyanic acid, and many - similar compounds are also absorbed by titanium chloride, with the - evolution of a considerable amount of heat. Thus, for example, a - yellow crystalline powder of the composition TiCl_{4},2HCN is - obtained by passing dry hydrocyanic acid vapour into cold titanium - chloride. Titanium chloride combines in a similar manner with - cyanogen chloride, phosphorus pentachloride, and phosphorus - oxychloride, forming molecular compounds, for example - TiCl_{4},POCl_{3}. This faculty for further combination probably - stands in connection, on the one hand, with the capacity of - titanium oxide to give polytitanates, TiO(MO)_{2},_n_TiO_{2}; on - the other hand, it corresponds with the kindred faculty of stannic - chloride for the formation of poly-compounds (Note 41), and lastly - it is probably related to the remarkable behaviour of titanium - towards nitrogen. Metallic titanium, obtained as a grey powder by - reducing potassium titanofluoride, K_{2}TiF_{6}, (sp. gr. 3·55 K. - Hofman 1893), with iron in a charcoal crucible, combines directly - with nitrogen at a red heat. If titanic anhydride be ignited in a - stream of ammonia, all the oxygen of the titanic oxide is - disengaged, and the compound TiN_{2} is formed as a dark violet - substance having a copper-red lustre. A compound Ti_{5}N_{6} is - also known; it is obtained by igniting the compound Ti_{3}N_{4} in - a stream of hydrogen, and is of a golden-yellow colour with a - metallic lustre. To this order of compounds also belongs the - well-known and chemically historical compound known as _titanium - nitrocyanide_; its composition is Ti_{5}CN_{4}. This substance - appears as infusible, sometimes well-formed, cubical crystals of - sp. gr, 4·3, and having a red copper colour and metallic lustre; - it is found in blast furnace slag. It is insoluble in acids but is - acted on by chlorine at a red heat, forming titanium chloride. It - was at first regarded as metallic titanium; it is formed in the - blast furnace at the expense of those cyanogen compounds - (potassium cyanide and others) which are always present, and at - the expense of the titanium compounds which accompany the ores of - iron. Wöhler, who investigated this compound, obtained it - artificially by heating a mixture of titanic oxide with a small - quantity of charcoal, in a stream of nitrogen, and thus proved the - direct power for combination between nitrogen and titanium. When - fused with caustic potash, all the nitrogen compounds of titanium - evolve ammonia and form potassium titanate. Like metals they are - able to reduce many oxides--for example, oxides of copper--at a - red heat. Among the alloys of titanium, the crystalline compound - Al_{4}Ti is remarkable. It is obtained by directly dissolving - titanium in fused aluminium; its specific gravity is 3·11. The - crystals are very stable, and are only soluble in aqua regia and - alkalis. - -The comparatively rare element _zirconium_, Zr = 90, is very similar to -titanium, but has a more basic character. It is rarer in nature than -titanium, and is found principally in a mineral called _zircon_, -ZrSiO_{4} = ZrO_{2}.SiO_{2}, crystallising in square prisms, sp. gr. 4·5. -It has considerable hardness and a characteristic brownish-yellow colour, -and is occasionally found in the form of transparent crystals, as a -precious stone called hyacinth.[58] Metallic zirconium was obtained, by -Berzelius and Troost, by the action of aluminium on potassium -zirconofluoride in the same way that silicon is prepared; it forms a -crystalline powder, similar in appearance to graphite and antimony, but -having a very considerable hardness, not much lustre, sp. gr. 4·15. In -many respects it resembles silicon; it does not fuse when heated, and -even oxidises with difficulty, but liberates hydrogen when fused with -potash. When fused with silica it liberates silicon. With carbon in the -electrical furnace it forms ZrC_{2}, with hydrogen it gives ZrH_{2} (like -CaH_{2}, Winkler, Vol. I., p. 621); hydrochloric and nitric acids act -feebly on it, but aqua regia easily dissolves it. It is distinguished -from silicon by the fact that hydrofluoric acid acts on it with great -facility, even in the cold and when diluted, whilst this acid does not -act on silicon at all. - - [58] The formula ZrO was first given to the oxide of zirconium as a - base, in this case Zr = 45 whilst the present atomic weight is Zr - = 90--that is, the formula of the oxide is now recognised as being - ZrO_{2}. The reasons for ascribing this formula to the compounds - of zirconium are as follows. In the first place, the investigation - of the crystalline forms of the zirconofluorides--for example, - K_{2}ZrF_{6}, MgZrF_{6},5H_{2}O--which proved to be analogous in - composition and crystalline form with the corresponding compounds - of titanium, tin, and silicon. In the second place, the specific - heat of Zr is 0·067, which corresponds with the combining weight - 90. The third and most important reason for doubling the combining - weight of zirconium was given by Deville's determination of the - vapour density of _zirconium chloride_, ZrCl_{4}. This substance - is obtained by igniting zirconium oxide mixed with charcoal in a - stream of dry chlorine, and is a colourless, saline substance - which is easily volatile at 440°. Its density referred to air was - found to be 8·15, that is 117 in relation to hydrogen, as it - should be according to the molecular formula of this substance - above-cited. It exhibits, however, in many respects, a saline - character and that of an acid chloranhydride, for zirconium oxide - itself presents very feebly developed acid properties but clearly - marked basic properties. Thus zirconium chloride dissolves in - water, and on evaporation the solution only partially disengages - hydrochloric acid--resembling magnesium chloride, for example. - Zirconium was discovered and characterised as an individual - element by Klaproth. - - Pure compounds of zirconium are generally prepared from zircon, - which is finely ground, but as it is very hard it is first heated - and thrown into cold water, by which means it is disintegrated. - Zircon is decomposed or dissolved when fused with acid potassium - sulphate, or still more easily when fused with acid potassium - fluoride (a double soluble salt, K_{2}ZrF_{6}, is then formed); - however, zirconium compounds are generally prepared from powdered - zircon by fusing it with sodium carbonate and then boiling in - water. An insoluble white residue is obtained consisting of a - compound of the oxides of sodium and zirconium, which is then - treated with hydrochloric acid and the solution evaporated to - dryness. The silica is thus converted into an insoluble form, and - zirconium chloride obtained in solution. Ammonia precipitates - _zirconium hydroxide_ from this solution, as a white gelatinous - precipitate, ZrO(OH)_{2}. When ignited this hydroxide loses water - and in so doing undergoes a spontaneous recalescence and leaves a - white infusible and exceedingly hard mass of _zirconium oxide_, - ZrO_{2}, having a specific gravity of 5·4 (in the electrical - furnace ZrO_{2} fuses and volatilises like SiO_{2}, Moissan). - Owing to its infusibility, zirconium oxide is used as a substitute - for lime and magnesia in the Drummond light. This oxide, in - contradistinction to titanium oxide, is soluble, even after - prolonged ignition, in hot strong sulphuric acid. The hydroxide is - easily soluble in acids. The composition of the salts is ZrX_{4}, - or ZrOX_{2}, or ZrOX_{2},ZrO_{2}, just as with those of its - analogues. But although zirconium oxide forms salts in the same - way with acids, it also gives salts with bases. Thus it liberates - carbonic anhydride when fused with sodium carbonate, forming the - salts Zr(NaO)_{4}, ZrO(NaO)_{2}, &c. Water, however, destroys - these salts and extracts the soda. - -The very similar element _thorium_ (Th = 232) was distinguished by -Berzelius from zirconium. It is very rarely met with, in _thorite_ and -_orangeite_, ThSiO_{4},2H_{2}O. The latter is isomorphous with zircon -(sp. gr. 4·8).[59] - - [59] Thorium has also been found in the form of oxide in certain - pyrochlores, euxenites, monazites, and other rare minerals - containing salts of niobium and phosphates. The compounds of - thorium are prepared by decomposing thorite or orangeite with - strong sulphuric acid at its boiling point; this renders the - silica insoluble, and the thorium oxide passes into solution when - the residue is treated with cold water, after having been - previously boiled with water (boiling water does not dissolve the - oxide of thorium). Lead and other impurities are separated by - passing sulphuretted hydrogen through the solution, and the - thorium hydroxide is then precipitated by ammonia. If this - hydroxide be dissolved in the smallest possible amount of - hydrochloric acid, and oxalic acid be then added, thorium oxalate - is obtained as a white precipitate, which is insoluble in an - excess of oxalic acid; this reaction is taken advantage of for - separating this metal from many others. It, however, resembles the - cerite metals (Chapter XVII., Note 43) in this and many other - respects. The thorium hydroxide is gelatinous; on ignition it - leaves an infusible oxide, ThO_{2}, which, when fused with borax, - gives crystals of the same form as stannic oxide or titanic - anhydride; sp. gr. 9·86. But the basic properties are much more - developed in thorium oxide than in the preceding oxides, and it - does not even disengage carbonic acid when fused with sodium - carbonate--that is, it is a much more energetic base than - zirconium oxide. The hydrate, ThO_{2}, however, is soluble in a - solution of Na_{2}CO_{3} (Chapter XVII., Note 43). Thorium - chloride, ThCl_{4} is obtained as a distinctly crystalline - sublimate when thorium oxide, mixed with charcoal, is ignited in a - stream of dry chlorine. When heated with potassium, thorium - chloride gives a metallic powder of thorium having a sp. gr. 11·1. - It burns in air, and is but slightly soluble in dilute acids. The - atomic weight of thorium was established by Chydenius and - Delafontaine on the basis of the ismorphism of the double - fluorides. - - - - - CHAPTER XIX - - PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP - - -Nitrogen is the lightest and most widely distributed representative of -the elements of the fifth group, which form a higher saline oxide of the -form R_{2}O_{5}, and a hydrogen compound of the form RH_{3}. Phosphorus, -arsenic, bismuth, and antimony belong to the uneven series of this group. -_Phosphorus_ is the most widely distributed of these elements. There is -hardly any mineral substance composing the mass of the earth's crust -which does not contain some--it may be a small--amount of phosphorus -compounds in the form of the salts of phosphoric acid. The soil and -earthy substances in general usually contain from one to ten parts of -phosphoric acid in 10,000 parts. This amount, which appears so small, -has, however, a very important significance in nature. No plant can -attain its natural growth if it be planted in an artificial soil -completely free from phosphoric acid. Plants equally require the presence -of potash, magnesia, lime, and ferric oxide, among basic, and of -carbonic, sulphuric, nitric, and phosphoric anhydrides, among acid -oxides. In order to increase the fertility of a more or less poor soil, -the above-named nutritive elements are introduced into it by means of -fertilisers. Direct experiment has proved that these substances are -undoubtedly necessary to plants, but that they must be all present -simultaneously and in small quantities, and that an excess, like an -insufficiency, of one of these elements is necessarily followed by a bad -harvest, or an imperfect growth, even if all the other conditions (light, -heat, water, air) are normal. The phosphoric compounds of the soil -accumulated by plants pass into the organism of animals, in which these -substances are assimilated in many instances in large quantities. Thus -the chief component part of bones is calcium phosphate, Ca_{3}P_{2}O_{8}, -and it is on this that their hardness depends.[1] - - [1] Dry bones contain about one-third of gelatinous matter and about - two-thirds of ash, chiefly calcium phosphate. The salts of - phosphoric acid are also found in the mass of the earth as separate - minerals; for example, the _apatites_ contain this salt in a - crystalline form, combined with calcium chloride or fluoride, - CaR_{2},3Ca_{3}(PO_{4})_{2}, where R = F or Cl, sometimes in a - state of isomorphous mixture. This mineral often crystallises in - fine hexagonal prisms; sp. gr. 3·17 to 3·22. Vivianite is a - hydrated ferrous phosphate, Fe_{3}(PO_{4})_{2},8H_{2}O. Phosphates - of copper are frequently found in copper mines; for example, - _tagilite_, Cu_{3}(PO_{4})_{2},Cu(OH)_{2},2H_{2}O. Lead and - aluminium form similar salts. They are nearly all insoluble in - water. The turquoise, for instance, is hydrated phosphate of - alumina, (Al_{2}O_{3})_{2},P_{2}O_{5}5H_{2}O, coloured with a salt - of copper. Sea and other waters always contain a small amount of - phosphates. The ash of sea-plants, as well as of land-plants, - always contains phosphates. Deposits of calcium phosphate are often - met with; they are termed _phosphorites_ and _osteolites_, and are - composed of the fossil remains of the bones of animals; they are - used for manure. Of the same nature are the so-called guano - deposits from Baker's Island, and entire strata in Spain, France, - and in the Governments of Orloff and Kursk in Russia. It is evident - that if a soil destined for cultivation contain very little - phosphoric acid, the fertilisation by means of these minerals will - be beneficial, but, naturally, only if the other elements necessary - to plants be present in the soil. - -Phosphorus was first extracted by Brand in 1669, by the ignition of -evaporated urine. After the lapse of a century Scheele, who knew of the -existence of a more abundant source of phosphorus in bones, pointed out -the method which is now employed for the extraction of this element. -Calcium phosphate in bones permeates a nitrogenous organic substance, -which is called ossein, and forms a gelatin. When bones are treated -exclusively for the extraction of phosphorus, neglecting the gelatin, -they are burnt, in which case all the ossein is burnt away. When, -however, it is desired to preserve the gelatin, the bones are immersed in -cold dilute hydrochloric acid, which dissolves the calcium phosphate and -leaves the gelatin untouched; calcium chloride and acid calcium -phosphate, CaH_{4}(PO_{4})_{2}, are then obtained in the solution. When -the bones are directly burnt in an open fire their mineral components -only are left as an ash, containing about 90 per cent. of calcium -phosphate, Ca_{3}(PO_{4})_{2}, mixed with a small amount of calcium -carbonate and other salts. This mass is treated with sulphuric acid, and -then the same substance is obtained in the solution as was obtained from -the unburnt bones immersed in hydrochloric acid--_i.e._ the acid calcium -phosphate soluble in water, in which reaction naturally the chief part of -the sulphuric acid is converted into calcium sulphate: - - Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = 2CaSO_{4} + CaH_{4}(PO_{4})_{2}. - Ca_{3}(PO_{4})_{2} + 4HCl = 2CaCl_{2} + CaH_{4}(PO_{4})_{2}. - -On evaporating the solution, crystallisable acid calcium phosphate is -obtained. The extraction of the phosphorus from this salt consists in -_heating it with charcoal to a white heat_. When heated, the acid -phosphate, CaH_{4}(PO_{4})_{2}, first parts with water, and forms the -metaphosphate, Ca(PO_{3})_{2}, which for the sake of simplicity may be -regarded, like the acid salt, as composed of pyrophosphate and phosphoric -anhydride, 2Ca(PO_{3})_{2} = Ca_{2}P_{2}O_{7} + P_{2}O_{5}. The latter, -with charcoal, gives phosphorus and carbonic oxide, P_{2}O_{5} + 5C = -P_{2} + 5CO. So that in reality a somewhat complicated process takes -place here, yielding ultimately products according to the following -equation: - - 2CaH_{4}(PO_{4})_{2} + 5C = 4H_{2}O + Ca_{2}P_{2}O_{7} + P_{2} + 5CO. - -After the steam has come over, phosphorus and carbonic oxide distil over -from the retort and calcium pyrophosphate remains behind.[1 bis] - - [1 bis] By subjecting the pyrophosphate to the action of sulphuric or - hydrochloric acid it is possible to obtain a fresh quantity of the - acid salt from the residue, and in this manner to extract all the - phosphorus. It is usual to take burnt bones, but mineral - phosphorites, osteolites, and apatites may also be employed as - materials for the extraction of phosphorus. Its extraction for the - manufacture of matches is everywhere extending, and in Russia, in - the Urals, in the Government of Perm, it has attained such - proportions that the district is able to supply other countries - with phosphorus. A great many methods have been proposed for - facilitating the extraction of phosphorus, but none of them differ - essentially from the usual one, because the problem is dependent on - the liberation of phosphoric acid by the action of acids, and on - its ultimate reduction by charcoal. Thus the calcium phosphate may - be mixed directly with charcoal and sand, and phosphorus will be - liberated on heating the mixture, because the silica displaces the - phosphoric anhydride, which gives carbonic oxide and phosphorus - with the charcoal. It has also been proposed to pass hydrochloric - acid over an incandescent mixture of calcium phosphate and - charcoal; the acid then acts just as the silica does, liberating - phosphoric anhydride, which is reduced by the charcoal. It is - necessary to prevent the access of air in the condensation of the - vapours of phosphorus, because they take fire very easily; hence - they are condensed under water by causing the gaseous products to - pass through a vessel full of water. For this purpose the condenser - shown in fig. 83 is usually employed. - -[Illustration: FIG. 83.--Preparation of phosphorus. The mixture is -calcined in the retort _c_. The vapours of phosphorus pass through _a_ -into water without coming into contact with air. The phosphorus condenses -in the water, and the gases accompanying it escape through _i_.] - -As phosphorus melts at about 40°, it condenses at the bottom of the -receiver in a molten liquid mass, which is cast under water in tubes, and -is sold in the form of sticks. This is common or _yellow phosphorus_. It -is a transparent, yellowish, waxy substance, which is not brittle, almost -insoluble in water, and easily undergoes change in its external -appearance and properties under the action of light, heat, and of various -substances. It crystallises (by sublimation or from its solution in -carbon bisulphide) in the regular system, and[2] (in contradistinction to -the other varieties) is easily soluble in carbon bisulphide, and also -partially in other oily liquids. In this it recalls common sulphur. Its -specific gravity is 1·84. It fuses at 44°, and passes into vapour at -290°; it is easily inflammable, and must therefore be handled with great -caution; careless rubbing is enough to cause phosphorus to ignite. Its -application in the manufacture of matches is based on this.[2 bis] It -emits light in the air owing to its slow[3] oxidation, and is therefore -kept under water (such water is phosphorescent in the dark, like -phosphorus itself). It is also very easily oxidised by various oxidising -agents and takes up the oxygen from many substances.[3 bis] Phosphorus -enters into direct combination with many metals and with sulphur, -chlorine, &c., with development of a considerable amount of heat. It is -very poisonous although not soluble in water. - - [2] Vernon (1891) observed that ordinary (yellow) phosphorus is - dimorphous. If it be melted and by careful cooling be brought in a - liquid form to as low a temperature as possible, it gives a variety - which melts at 45°·3 (the ordinary variety fuses at 44°·3), sp. gr. - 1·827 (that of the ordinary variety is 1·818) at 13°, crystallises - in rhombic prisms (instead of in forms belonging to the cubical - system). This is similar to the relation between octahedral and - prismatic sulphur (Chapter XX.). - - [2 bis] According to Herr Irinyi (an Hungarian student), the first - phosphorus matches were made in Austria at Roemer's works in 1835. - - [3] The absorption of the oxygen of the atmosphere at a constant - ordinary temperature by a large surface of phosphorus proceeds so - uniformly, regularly, and rapidly, that it may serve, as Ikeda - (Tokio, 1893) has shown, for demonstrating the law of the velocity - (rate) of reaction, which is considered in theoretical chemistry, - and shows that the rate of reaction is proportional to the active - mass of a substance--_i.e._ _dx_/_dt_ = _k_(A - _x_) where _t_ is - the time, A the initial mass of the reacting substance--in this - case oxygen--_x_ the amount of it which has entered into reaction, - and _k_ the coefficient of proportionality. Ikeda took a test-tube - (diameter about 10 mm.), and covered its outer surface with a - coating of phosphorus (by melting it in a test-tube of large - diameter, inserting the smaller test-tube, and, when the phosphorus - had solidified, breaking away the outer test-tube), and introduced - it into a definite volume of air, contained in a Woulfe's bottle - (immersed in a water bath to maintain a constant temperature), one - of whose orifices was connected with a mercury manometer showing - the fall of pressure, _x_. Knowing that the initial pressure of the - oxygen (in air nearly 750 × ·0209) was about 155 mm. = A, the - coefficient of the rate of reaction _k_ is given, by the law of the - variation of the rate of reaction with the mass of the reacting - substance, by the equation: _k_ = (1/_t_)log(A/(A - _x_)), where - _t_ is the time, counting from the commencement, of the experiment - in minutes. When the surface of the phosphorus was about 11 sq. - cm., the following results were actually obtained. - - _t_ = 10 20 30 40 50 60 minutes - _x_ = 10·5 21·5 31·1 40·7 49·1 57·3 mm - 10,000 _k_ = 32 32 32 33 33 33 - - The constancy of _k_ is well shown in this case. The determination - takes a comparatively short time, so that it may serve as a lecture - experiment, and demonstrates one of the most important laws of - chemical mechanics. - - [3 bis] Not only do oxidising agents like nitric, chromic, and similar - acids act upon phosphorus, but even the alkalis are attacked--that - is, phosphorus acts as a reducing agent. In fact it reduces many - substances, for instance, copper from its salts. When phosphorus is - heated with sodium carbonate, the latter is partially reduced to - carbon. If phosphorus be placed under water slightly warmed, and a - stream of oxygen be passed over it, it will burn under the water. - -Besides this, there is a red variety of phosphorus, which differs -considerably from the above. _Red phosphorus_ (sometimes wrongly called -_amorphous phosphorus_) is partially formed when ordinary phosphorus -remains exposed to the action of light for a long time. It is also formed -in many reactions; for example, when ordinary phosphorus combines with -chlorine, bromine, iodine, or oxygen, a portion of it is converted into -red phosphorus. Schrötter, in Vienna, investigated this variety of -phosphorus, and pointed out by what methods it may be produced in -considerable quantities. Red phosphorus is a powdery red-brown opaque -substance of specific gravity 2·14. It does not combine so energetically -with oxygen and other substances as yellow phosphorus, and evolves less -heat in combining with them.[4] Common phosphorus easily oxidises in the -air; red phosphorus does not oxidise at all at the ordinary temperature; -hence it does not phosphoresce in the air, and may be very conveniently -kept in the form of powder. It does not, like yellow phosphorus, fuse at -44°. After being converted into vapour at 290° or 300°, it again passes -into the ordinary variety when slowly cooled. Red phosphorus is not -soluble in carbon bisulphide and other oily liquids, which permits of its -being freed from any admixture of the ordinary phosphorus. It is not -poisonous, and is used in many cases for which the ordinary phosphorus is -unsuitable or dangerous; for example, in the manufacture of matches, -which are then not poisonous or inflammable by accidental friction, and -therefore the red variety has now replaced the ordinary -phosphorus.[4 bis] - - [4] The thermochemical determinations for phosphorus and its compounds - date from the last century, when Lavoisier and Laplace burnt - phosphorus in oxygen in an ice calorimeter. Andrews, Despretz, - Favre, and others have studied the same subject. The most accurate - and complete data are due to Thomsen. To determine the heat of - combustion of yellow phosphorus, Thomsen oxidised it in a - calorimeter with iodic acid in the presence of water, and a mixture - of phosphorous and phosphoric acids was thus formed (was not any - hypophosphoric acid formed?--Salzer), and the iodic acid converted - into hydriodic acid. It was first necessary to introduce two - corrections into the calorimetric result obtained, one for the - oxidation of the phosphorous into phosphoric acid, knowing their - relative amounts by analysis, and the other for the deoxidation of - the iodic acid. The result then obtained expresses the conversion - of phosphorous into hydrated phosphoric acid. This must be - corrected for the heat of solution of the hydrate in water, and for - the heat of combination of the anhydride with water, before we can - obtain the heat evolved in the reaction of P_{2} with O_{5} in the - proportion for the formation of P_{2}O_{5}. It is natural that with - so complex a method there is a possibility of many small errors, - and the resultant figures will only present a certain degree of - accuracy after repeated corrections by various methods. Of such a - kind are the following figures determined by Thomsen, which we - express in thousands of calories:--P_{2} + O_{5} = 370; P_{2} + - O_{3} + 3H_{2}O = 400; P_{2} + O_{5} + a mass of water = 405. Hence - we see that P_{2}O_{5} + 3H_{2}O = 30; 2PH_{3}O_{4} + an excess of - water = 5. Experiment further showed that crystallised PH_{3}O_{4}, - in dissolving in water, evolves 2·7 thousand calories, and that - fused (39°) PH_{3}O_{4} evolves 5·2 thousand calories, whence the - heat of fusion of H_{3}PO_{4} = 2·5 thousand calories. For - phosphorous acid, H_{3}PO_{3}, Thomsen obtained P_{2} + O_{3} + - 3H_{2}O = 250, and the solution of crystallised H_{3}PO_{3} in - water = -0·13, and of fused H_{3}PO_{3} = +2·9. For hypophosphorous - acid, H_{3}PO_{2}, the heats of solution are nearly the same (-0·17 - and +2·1), and the heat of formation P_{2} + O + 3H_{2}O = 75; - hence its conversion into 2H_{3}PO_{3} evolves 175 thousand - calories, and the conversion of 2H_{3}PO_{3} into 2H_{3}PO_{4} = - 150 thousand calories. For the sake of comparison we will take the - combination of chlorine with phosphorus, also according to Thomsen, - per 2 atoms of phosphorus, P_{2} + 3Cl_{2} = 151, P_{2} + 5Cl_{2} = - 210 thousand calories. In their reaction on an excess of water - (with the formation of a solution), 2PCl_{3} = 130, 2PCl_{5} = 247, - and 2POCl_{3} = 142 thousand calories. - - Besides which we will cite the following data given by various - observers: heat of fusion for P (that is, for 31 parts of - phosphorus by weight) -0·15 thousand calories; the conversion of - yellow into red phosphorus for P, from +19 to +27 thousand - calories; P + H_{3} = 4·3, HI + PH_{3} = 24, PH_{3} + HBr = 22 - thousand calories. - - At the ordinary temperature (20° C.) phosphorus is not oxidised by - pure oxygen; oxidation only takes place with a slight rise of - temperature, or the dilution of the oxygen with other gases - (especially nitrogen or hydrogen), or a decrease of pressure. - - [4 bis] Ordinary phosphorus takes fire at a temperature (60°) at which - no other known substance will burn. Its application to the - manufacture of matches is based on this property. In order to - illustrate the easy inflammability of common (yellow) phosphorus, - its solution in carbon bisulphide may be poured over paper; this - solvent quickly evaporates, and the free phosphorus spread over a - large surface takes fire spontaneously, notwithstanding the cooling - effect produced by the evaporation of the bisulphide. The majority - of _phosphorus matches_ are composed of common phosphorus mixed - with some oxidising substance which easily gives up oxygen, such as - lead dioxide, potassium chlorate, nitre, &c. For this purpose - common phosphorus is carefully triturated under warm water - containing a little gum; lead dioxide and potassium nitrate are - then added to the resultant emulsion, and the match ends, - previously coated with sulphur or paraffin, are dipped into this - preparation. After this the matches are dipped into a solution of - gum and shellac, in order to preserve the phosphorus from the - action of the air. When such a match containing particles of yellow - phosphorus is rubbed over a rough surface, it becomes (especially - at the point of rupture of the brittle gummy coating) slightly - heated, and this is sufficient to cause the phosphorus to take fire - and burn at the expense of the oxygen of the other ingredients. - -The heads of the 'safety' matches do not contain any phosphorus, but -only substances capable of burning and of supporting combustion. Red -phosphorus is spread over a surface on the box, and it is the friction -against this phosphorus which ignites the matches. There is no danger of -the matches taking fire accidentally, nor are they poisonous.[5] This red -phosphorus is prepared by heating the ordinary phosphorus at 230° to -270°; it is evident that this must be done in an atmosphere incapable of -supporting combustion--for example, in nitrogen, carbonic anhydride, -steam, &c. On a large scale, ordinary phosphorus is placed in closed iron -vessels,[5 bis] and immersed in a bath of different proportions of tin -and lead, by which means the temperature of 250° necessary for the -conversion is easily attained. It is kept at this temperature for some -time. The temperature is at first cautiously raised, and the air is thus -partially expelled by the heat, and also by the evolution of steam (the -phosphorus is damp when put in), whilst the remaining oxygen is also -partially absorbed by the phosphorus, so that an atmosphere of nitrogen -is produced in the iron vessel. Red phosphorus enters into all the -reactions proper to yellow phosphorus, only with greater difficulty and -more slowly;[6] and, as its vapour tension (volatility) is less than that -of the yellow variety, it may be supposed that a polymerisation takes -place in the passage of the yellow into the red modification, just as in -the passage of cyanogen into paracyanogen, or of cyanic acid into -cyanuric acid (Chapter IX. Notes 39 bis and 48). - - [5] In the so-called 'safety' or Swedish matches (which are not - poisonous, and do not take fire from accidental friction) a mixture - of red phosphorus and glass forms the surface on which the matches - are struck, and the matches themselves do not contain any - phosphorus at all, but a mixture of antimonious sulphide, - Sb_{2}S_{3} (or similar combustible substances) and potassium - chlorate (or other oxidising agents). The combustion, when once - started by contact with the red phosphorus, proceeds by itself at - the expense of the inflammatory and combustible elements contained - in the tip of the match. The mixture applied on the match itself - must not be liable to take fire from a blow or friction. The - mixture forming the heads of the 'safety' matches has the following - approximate composition: 55-60 parts of chlorate of potassium, 5-10 - parts of peroxide of manganese (or of K_{2}Cr_{2}O_{7}), about 1 - part of sulphur or charcoal, about 1 part of pentasulphide of - antimony, Sb_{2}S_{5}, and 30-40 parts of rouge and powdered glass. - This mixture is stirred up in gum or glue, and the matches are - dipped into it. The paper on which the matches are struck is coated - with a mixture of red phosphorus and trisulphide of antimony, - Sb_{2}S_{3}, stirred up in dextrine. - - [5 bis] Phosphorus only acts on iron at a red heat. The boiler is - provided with a safety valve and gas-conducting tube, which is - immersed in mercury or other liquid to prevent the admission of air - into the boiler. - - [6] The specific heat of the yellow variety is 0·189--that is, greater - than that of the red variety, which is 0·170. The sp. gr. of the - yellow is 1·84, and of the red prepared at 260° 2·15, and of that - prepared at 580° and above (_i.e._ 'metallic' phosphorus, _see_ - below) = 2·34. At 230° the pressure of the vapour of ordinary - phosphorus = 514 millimetres of mercury, and of the red = 0--that - is to say, the red phosphorus does not form any vapour at this - temperature; at 447° the vapour tension of ordinary phosphorus is - at first = 5500 mm., but it gradually diminishes, whilst that of - red phosphorus is equal to 1636 mm. - - Hittorf, by heating the lower portion of a closed tube containing - red phosphorus to 530° and the upper portion to 447°, obtained - crystals of the so-called 'metallic' phosphorus at the upper - extremity. As the vapour tensions (according to Hittorf, at 530° - the vapour tension of yellow phosphorus = 8040 mm., of red = 6139 - mm., and of metallic = 4130 mm.) and reactions are different, - _metallic phosphorus_ may be regarded as a distinct variety. It is - still less energetic in its chemical reaction than red phosphorus, - and it is denser than the two preceding varieties: sp. gr. = 2·34. - It does not oxidise in the air; is crystalline, and has a metallic - lustre. It is obtained when ordinary phosphorus is heated with lead - for several hours at 400° in a closed vessel, from which the air - has been exhausted. The resultant mass is then treated with dilute - nitric acid, which first dissolves the lead (phosphorus is - electro-negative to lead, and does not, therefore, act on the - nitric acid at first) and leaves brilliant rhombohedral crystals of - phosphorus of a dark violet colour with a slight metallic lustre, - which conduct an electric current incomparably better than the - yellow variety; this also is characteristic of the metallic state - of phosphorus. - - The researches of Lemoine partially explain the passage of yellow - (ordinary) phosphorus into its other varieties. He heated a closed - glass globe containing either ordinary or red phosphorus, in the - vapour of sulphur (440°), and then determined the amount of the red - and yellow varieties after various periods of time, by treating the - mixture with carbon bisulphide. It appeared that after the lapse of - a certain time a mixture of definite and equal composition is - obtained from both--that is, between the red and yellow varieties a - state of equilibrium sets in like that of dissociation, or that - observed in double decompositions. But at the same time, the - progress of the transformation appeared to be dependent on the - relative quantity of phosphorus taken per volume of the globe - (_i.e._ upon the pressure). Neglecting the latter, we will cite as - an example the amounts of the red phosphorus transformed into the - ordinary, and of the ordinary not converted into red, per 30 grams - of red or yellow taken per litre capacity of the globe, heated to - 440°. When red phosphorus was taken, 4·75 grams of yellow - phosphorus were formed after two hours, four grams after eight - hours, three grams after twenty-four hours, and the last limit - remained constant on further heating. When thirty grams of yellow - phosphorus were taken, five grams remained unaltered after two - hours, four grams after eight hours, and after twenty-four hours - and more three grams as before. Troost and Hautefeuille showed that - liquid phosphorus in general changes more easily into the red than - does phosphorus vapour, which, however, is able, although slowly, - to deposit red phosphorus. - - The question presents itself as to whether phosphorus in a state of - vapour is the ordinary or some other variety? Hittorf (1865) - collected many data for the solution of this problem, which leave - no doubt that (as experimental figures show) the density of the - vapour of phosphorus is always the same, although the vapour - tension of the different varieties and their mixtures is very - variable. This shows that the different varieties of phosphorus - only occur in a liquid and solid state, as indeed is implied in the - idea of polymerisation. Strictly speaking, the vapour of phosphorus - is a particular state of this substance, and the molecular formula - P_{4} refers only to it, and not to any other definite state of - phosphorus. But Raoult's solution method showed that in a benzene - solution the fall of the freezing point indicates for ordinary - phosphorus a molecule P_{4}, judging by the determinations of - Paterno and Nasini (1888), Hirtz (1890), and Beckmann (1891), who - obtained for sulphur by the same method a molecular weight = S_{6}, - in conformity with the vapour density. Further research in this - direction will perhaps show the possibility of finding the - molecular weight of red phosphorus, if a means be discovered for - dissolving it without converting it into the yellow variety. - - I think it will not be out of place here to draw the reader's - attention to the fact that red phosphorus, which we must recognise - as polymeric with the yellow, stands nearer to nitrogen, whose - molecule is N_{2}, in its small inclination towards chemical - reactions, although judging by its small vapour tension it must be - more complex than ordinary (yellow and white) phosphorus. - -The vapour of phosphorus is colourless; its density remains constant -between 300° and 1000° (Dumas, 1833; Mitscherlich, Deville, and Troost, -1859, and others). The density with respect to air has been determined as -from 4·3 to 4·5. Hence, referred to hydrogen, it is 4·4 × 14·4 = 63, -corresponding with a molecular weight 124, _i.e._ the molecule of -phosphorus in a state of vapour contains P_{4}. The reader will remember -that the molecule of nitrogen contains N_{2}, of sulphur S_{6} or S_{2}, -and of oxygen O_{2} or O_{3}. - -The chemical energy of phosphorus in a free state more nearly approaches -that of sulphur than nitrogen. Phosphorus is combustible and inflames at -60°; but having in the act of combination parted with a portion of its -energy in the form of heat it becomes analogous to nitrogen, so long as -there is no question of its reduction back again into phosphorus. Nitric -acid is easily reduced to nitrogen, whilst phosphoric acid is reduced -with very much greater difficulty. All the compounds of phosphorus are -less volatile than those of nitrogen. Nitric acid, HNO_{3}, is easily -distilled; metaphosphoric acid, HPO_{3}, is generally said to be -non-volatile; triethylamine, N(C_{2}H_{5})_{3}, boils at 90°, and -triethylphosphine, P(C_{2}H_{5})_{3}, at 127°. - -Phosphorus not only combines easily and directly with oxygen, but also -with chlorine, bromine, iodine, sulphur, and with certain metals, and red -phosphorus when heated combines with hydrogen also.[6 bis] So, for -instance, when fused with sodium under naphtha, phosphorus gives the -compound Na_{3}P_{2}. Zinc, absorbing the vapour of phosphorus, gives the -phosphide Zn_{3}P_{2} (sp. gr. 4·76); tin, SnP; copper, Cu_{2}P; even -platinum combines with phosphorus (PtP_{2}, sp. gr. 8·77).[6 tri] Iron, -when combined even with a small quantity of phosphorus, becomes -brittle.[7] Some of these compounds of phosphorus are obtained by the -action of phosphorus on the solutions of metallic salts, and by the -ignition of metallic oxides in the vapour of phosphorus, or by heating -mixtures of phosphates with charcoal and metals. Phosphides do not -exhibit the external properties of salts, which are so clearly seen in -the chlorides and still distinctly observable in the sulphides. _The -phosphides of the metals_ of the alkalis and of the alkaline earths are -even immediately and very easily decomposed by water, whereas this is -found to be the case with only a very few sulphides, and still more -rarely and indistinctly with the chlorides. We may take calcium phosphide -as an example.[7 bis] Phosphorus is laid in a deep crucible, and covered -with a clay plug, over which lime is strewn. At a red heat the vapours of -phosphorus combine with the oxygen of the lime and form phosphoric -anhydride, which forms a salt with another portion of the lime, whilst -the liberated calcium combines with the phosphorus and forms calcium -phosphide. Its composition is not quite certain; it may be CaP -(corresponding with liquid phosphuretted hydrogen). This substance is -remarkable for the following reaction: if we take water--or, better -still, a dilute solution of hydrochloric acid--and throw calcium -phosphide into it, bubbles of gas are evolved, which take fire -spontaneously in the air and form white rings. This is owing to the fact -that the liquid hydrogen phosphide, PH_{2}, is first formed, thus, CaP + -2HCl = CaCl_{2} + PH_{2}, which, owing to its instability, very easily -splits up into the solid phosphide, P_{2}H, and gaseous phosphide, -PH_{3}; 5PH_{2} = P_{2}H + 3PH_{3}; the latter corresponds with ammonia. -The mixture of the gaseous and liquid phosphides takes fire spontaneously -in the air, forming phosphoric acid. The same hydrogen phosphides are -formed when water acts on sodium phosphide (P_{2}Na_{3}). A similar -mixture of gaseous liquid and solid phosphuretted hydrogen (Retgers 1894) -is formed by heating (in a glass tube) red phosphorus in a stream of dry -hydrogen. Hence we see that there are _three compounds of phosphorus with -hydrogen_. (1) The first or solid yellow phosphide, P_{2}H (more probably -P_{4}H_{2}), is obtained by the action of strong hydrochloric acid on -sodium phosphide; it takes fire when struck or at 175°. (2) The liquid, -PH_{2}, or more correctly expressed as the molecule, P_{2}H_{4}, is a -colourless liquid which takes fire spontaneously in the air, boils at -30°, is very unstable, and is easily decomposed (by light or hydrochloric -acid) into the two other phosphides of hydrogen. It is prepared by -passing the gases evolved by the action of water on calcium phosphide -through a freezing mixture.[8] And, lastly, (3), gaseous hydrogen -phosphide, _phosphine_, PH_{3}, which is distinguished as being the most -stable. It is a colourless gas, which does not take fire in the air. It -has an odour of garlic, and is very poisonous. It resembles ammonia in -many of its properties.[8 bis] It is easily decomposed by heat, like -ammonia, forming phosphorus and hydrogen; but it is very slightly soluble -in water, and does not saturate acids, although it forms compounds with -some of them which resemble ammonium salts in their form and properties. -Among them the _compound with hydriodic acid_, PH_{4}I, analogous to -ammonium iodide, is remarkable. This compound crystallises on sublimation -in well-formed cubes, like sal-ammoniac, which it resembles in many -respects. However, this compound does not enter into those reactions of -double decomposition which are proper to sal-ammoniac, because its saline -properties are very feebly developed. Phosphuretted hydrogen also -combines, like ammonia, with certain chloranhydrides; but they are -decomposed by water, with the evolution of phosphine. Ogier (1880) showed -that hydrochloric acid also combines with phosphine under a pressure of -20 atmospheres at +18°, and under the ordinary pressure at -35°, forming -the crystalline phosphonium chloride PH_{4}Cl, corresponding to -sal-ammoniac. Hydrobromic acid does the same with greater ease, and -hydriodic acid with still greater facility, forming phosphonium iodide, -PH_{4}I.[9] - - [6 bis] Retgers (see further on) showed this in 1894, and observed that - As when heated also combines with hydrogen. - - [6 tri] The capacity of mercury (Chapter XVI., Note 25 bis) to give - unstable compounds with nitrogen gives rise to the supposition that - similar compounds exist with phosphorus also. Such a compound was - obtained by Granger (1892) by heating mercury with iodide of - phosphorus in a closed tube at 275°-300°. After removing the iodide - of mercury formed, there remain fine rhombic crystals having a - metallic lustre, and composition Hg_{3}P_{2}. This compound is - stable, does not alter at the ordinary temperature and only - decomposes at a red heat; when heated in air it burns with a flame. - Nitric and hydrochloric acids do not act upon it, but it is easily - decomposed by aqua regia. A phosphide of copper, Cu_{2}P_{2}, was - obtained by Granger (1893) by heating a mixture of water, finely - divided copper and red phosphorus in a sealed tube to 130°. The - excess of copper was afterwards washed away by a solution of NH_{3} - in the presence of air. - - [7] The metallic compounds of phosphorus possess a great chemical - interest, because they show a transition from metallic alloys (for - instance, of Sb, As) to the sulphides, halogen salts, and oxides, - and on the other hand to the nitrides. Although there are already - many fragmentary data on the subject, the interesting province of - the metallic phosphides cannot yet be regarded as in any way - generalised. The varied applications (phosphor-iron, - phosphor-bronze, &c.), which the phosphides have recently acquired - should give a strong incentive to the complete and detailed study - of this subject, which would, in my opinion, help to the - explanation of chemical relations beginning with alloys (solutions) - and ending with salts and the compounds of hydrogen (hydrides), - because the phosphor-metals, as is proved by direct experiment, - stand in the same relation to phosphuretted hydrogen as the - sulphides do towards sulphuretted hydrogen, or as the metallic - chlorides to hydrochloric acid. - - [7 bis] Many other compounds of phosphorus are also capable of forming - phosphuretted hydrogen. Thus BP also gives PH_{3} (_see_ Chapter - XVII., Note 12). According to Lüpke (1890) phosphuretted hydrogen - is formed by phosphide of tin. The latter is prepared by treating - molten tin covered with a layer of carbonate of ammonium, with red - phosphorus; 200-300 c.c. of water are then poured into a flask, 3-5 - grams of this phosphide of tin dropped in, and after driving out - the air by a stream of carbonic acid, hydrochloric acid (sp. gr. - 1·104) is poured in. The disengagement of phosphuretted hydrogen - takes place on heating the flask in a water bath. The following is - another easy method for preparing PH_{3}. A mixture of 1 part of - zinc dust (fume) and 2 parts of red phosphorus are heated in an - atmosphere of hydrogen (the mixture burns in air). Combination - takes place accompanied by a flash, and a grey mass of Zn_{3}P_{2} - is formed which gives PH_{3} when treated with dilute H_{2}SO_{4}. - - [8] The spontaneous inflammability of the hydride PH_{2} in air is very - remarkable, and it is particularly interesting that its analogues - in composition, P(C_{2}H_{5})_{2} (the formula must be doubled) and - Zn(C_{2}H_{5})_{2}, also take fire spontaneously in air. - - [8 bis] The analogy between PH_{3} and NH_{3} is particularly clear in - the hydrocarbon derivatives. Just as NH_{2}R, NHR_{2}, and NR_{3}, - where R is CH_{3}, and other hydrocarbon radicles, correspond to - NH_{3}, so there are actually similar compounds corresponding to - PH_{3}. These compounds form a branch of organic chemistry. - - [9] The periodic law and direct experiment (the molecular weight) show - that PH_{3} is the normal compound of P and H and that it is more - simple than PH_{2} or P_{2}H_{4}, just as methane, CH_{4}, is more - simple than ethane, C_{2}H_{6}, whose empirical composition is - CH_{3}. The formation of liquid phosphuretted hydrogen may be - understood from the law of substitution. The univalent radicle of - PH_{3} is PH_{2}, and if it is combined with H in PH_{3} it - replaces H in liquid phosphuretted hydrogen, which thus gives - P_{2}H_{4}. This substance corresponds with free amidogen - (hydrazine), N_{2}H_{4} (Chapter VI.) Probably P_{2}H_{4} is able - to combine with HI, and perhaps also with 2HI, or other - molecules--that is, to give a substance corresponding to - phosphonium iodide. - - _Phosphonium iodide_, PH_{4}I, may be prepared, according to - Baeyer, in large quantities in the following manner:--100 parts of - phosphorus are dissolved in dry carbon bisulphide in a tubulated - retort: when the mixture has cooled, 175 parts of iodide are added - little by little, and the carbon bisulphide is then distilled off, - this being done towards the end of the operation in a current of - dry carbonic anhydride at a moderate temperature. The neck of the - retort is then connected with a wide glass tube, and the tubulure - with a funnel furnished with a stopcock, and containing 50 parts of - water. This water is added drop by drop to the phosphorous iodide, - and a violent reaction takes place, with the evolution of hydriodic - acid and phosphonium iodide. The latter collects as crystals in the - glass tube and the retort itself. It is purified by further - distillations; more than 100 parts may be obtained. Baeyer - expresses the reaction by the equation P_{2}I + 2H_{2}O = PH_{4}I + - PO_{2}; and the compound PO_{2} may be represented as phosphorous - phosphoric anhydride: P_{2}O_{5} + P_{2}O_{3} = 4PO_{2}. As a - better proportion we may take 400 grams of phosphorus, 680 grams of - iodine, and 240 grams of water, and express the formation thus: 13P - + 9I + 21H_{2}O = 3H_{4}P_{2}O_{7} + 7PH_{4}I + 2HI (Chapter XI., - Note 77). - - Phosphonium iodide and even phosphine act as reducing agents in - solutions of many metallic salts. Cavazzi showed that with a - solution of sulphurous anhydride phosphine gives sulphur and - phosphoric acid. - -_Phosphuretted hydrogen, or phosphine_, PH_{3}, is generally prepared by -the action of caustic potash on phosphorus.[10] Small pieces of -phosphorus are dropped into a flask containing a strong solution of -caustic potash and heated. Potassium hypophosphite, H_{2}KPO_{2}, is then -obtained in solution; gaseous phosphuretted hydrogen is evolved: - - P_{4} + 3KHO + 3H_{2}O = 3(KH_{2}PO_{2}) + PH_{3}. - -Liquid phosphuretted hydrogen (and free hydrogen) is also formed, -together with the phosphine, so that the gaseous product, on escaping -from the water into the air, takes fire spontaneously, forming beautiful -white rings of phosphoric acid. In this experiment, as in that with -calcium phosphide, it is the liquid, P_{2}H_{4}, that takes fire; but the -phosphine set light to by it also burns, PH_{3} + O_{4} = PH_{3}O_{4}. -The same phosphuretted hydrogen, PH_{3}, may be obtained pure, and not -spontaneously combustible, by igniting the hydrates of phosphorous acid -(4PH_{3}O_{3} = PH_{3} + 3PH_{3}O_{4}) and hypophosphorous acid -(2PH_{3}O_{2} = PH_{3} + PH_{3}O_{4}); or, more simply, by the -decomposition of calcium phosphide by hydrochloric acid, because then all -the liquid phosphide, P_{2}H_{4}, is decomposed into non-volatile P_{2}H -and gaseous PH_{3}. Pure phosphine liquefies when cooled to -90°, boils -at -85°, and solidifies at -135° (Olszewski). When phosphorus burns in an -excess[10 bis] of _dry_ oxygen, then only _phosphoric anhydride_, -P_{2}O_{5} is formed. It is prepared by dropping pieces of phosphorus -through a wide tube, fixed into the upper neck of a large glass globe, on -to a cup suspended in the centre of the globe. These lumps are set alight -by touching them with a hot wire, and the phosphorus burns into -P_{2}O_{5}. The dry air necessary for its combustion is forced into the -globe through a lateral neck, and the white flakes of phosphoric -anhydride formed are carried by the current of air through a second -lateral neck into a series of Woulfe's bottles, where they settle as -friable white flakes. Phosphoric anhydride may also be formed by passing -dry air through a solution of phosphorus in carbon bisulphide. All the -materials for the preparation of this substance must be carefully dried, -because it _combines_ with great eagerness _with water_, at the same time -developing a large amount of heat and forming metaphosphoric acid, -HPO_{3}, from which the water cannot be separated by heat. Phosphoric -anhydride is a colourless snow-like substance, which attracts moisture -from the air with the utmost avidity. It fuses at a red heat, and then -_volatilises_. Its affinity for water is so great that it takes it up -from many substances. Thus it converts sulphuric acid into sulphuric -anhydride, and carbohydrates (wood, paper) are carbonised, and give up -the elements of water when brought into contact with it. - - [10] The air must first be expelled from the flask by hydrogen, or some - other gas which will not support combustion, as otherwise an - explosion might take place owing to the spontaneous inflammability - of the phosphuretted hydrogen. - - The combustion of phosphuretted hydrogen in oxygen also takes - place under water when the bubbles of both gases meet, and it is - very brilliant. The phosphuretted hydrogen obtained by the action - of phosphorus on caustic potash always contains free hydrogen, and - often even the greater part of the gas evolved consists of - hydrogen. - - _Pure phosphuretted hydrogen_ (not containing hydrogen or liquid - or solid phosphides) is obtained by the action of a solution of - potash on phosphonium iodide: PH_{4}I + KHO = PH_{3} + KI + H_{2}O - (in just the same way as ammonia is liberated from ammonium - chloride). The reaction proceeds easily, and the purity of the gas - is seen from the fact that it is entirely absorbed by bleaching - powder and is not spontaneously inflammable. Its mixture with - oxygen explodes when the pressure is diminished (Chapter XVIII., - Note 8). The vapours of bromine, nitric acid, &c., cause it to - again acquire the property of inflaming in the air; that is, they - partially decompose it, forming the liquid hydride, P_{2}H_{4}. - Oppenheim showed that when red phosphorus is heated at 200° with - hydrochloric acid in a closed tube it forms the compound - PCl_{3}(H_{3}PO_{3}), together with phosphine. - - [10 bis] If there be a deficiency of oxygen, _phosphorous anhydride_ - P_{2}O_{3} is formed. It was obtained by Thorpe and Tutton (1890) - and is easily volatilised, melts at 22°·5, boils without change - (in an atmosphere of N_{2} or CO_{2}) at 173°, and is therefore - easily separated from P_{2}O_{3}, which volatilises with - difficulty. The vapour density shows that the molecular weight is - double, _i.e._ P_{4}O_{6} (like As_{2}O_{3}). Although colourless, - phosphorous anhydride (its density in a state of fusion at 24° = - 1·936) turns yellow and reddens in sun-light (possibly red - phosphorus separates out ?), and decomposes at 400° forming - hypophosphorous anhydride P_{2}O_{4} (Note 11) and phosphorus. It - passes into P_{2}O_{5} in air and oxygen, and when slightly heated - in oxygen becomes luminous, and ultimately takes fire. Cold water - slowly transforms P_{2}O_{3} into phosphoric acid, but hot water - gives an explosion and leads to the formation of PH_{3}, - (P_{4}O_{6} + 6H_{2}O = PH_{3} + 3PH_{3}O_{4}). Alkalis act in the - same manner. It takes fire in chlorine and forms POCl_{3} and - PO_{2}Cl, and combines with sulphur at 160°, forming - P_{2}S_{2}O_{3} (the molecular formula is double this) a substance - which volatilises in vacuo and is decomposed by water into H_{2}S - and phosphoric acid, _i.e._ it may be regarded as P_{2}O_{5}, in - which O_{2} has been replaced by two atoms of sulphur. Judging - from the above, the mixture of P_{2}O_{3} and P_{2}O_{5} formed in - the combustion of phosphorus in air is transformed into P_{2}O_{5} - in an excess of oxygen. - -When moist phosphorus slowly oxidises in the air, it not only forms -phosphorous and phosphoric acids, but also _hypophosphoric acid_, -H_{4}P_{2}O_{6}, which when in a dry state easily splits up at 60° into -phosphorous and metaphosphoric acids (H_{4}P_{2}O_{6} = H_{3}PO_{3} + -HPO_{3}), but differs from a mixture of these acids in that it forms -well-characterised salts, of which the sodium salt, -H_{2}Na_{2}P_{2}O_{6}, is but slightly soluble in water (the sodium salts -of phosphoric and phosphorous acids are easily soluble), and that it does -not act as a reducing agent, like mixtures containing phosphorous -acid.[11] - - [11] Salzer proved the existence of hypophosphoric acid (it is also - called subphosphoric acid), in which many chemists did not - believe. Drawe (1888) and Rammelsberg (1892) investigated its - salts. It may be obtained in a free state by the following method. - The solution of acid produced by the slow oxidation of moist - phosphorus is mixed with a solution (25 p.c.) of sodium acetate. A - salt, Na_{2}H_{2}P_{2}O_{6},6H_{2}O, crystallises out on cooling; - it is soluble in 45 parts of water, and gives a precipitate of - Pb_{2}P_{2}O_{6} with lead salts (Ag_{4}P_{2}O_{6} with salts of - silver). The lead salt is decomposed by a current of hydrogen - sulphide, when lead sulphide is precipitated, while the solution, - evaporated under the receiver of an air-pump, gives crystals of - H_{4}P_{2}O_{6},2H_{2}O, which easily lose water and give - H_{4}P_{2}O_{6}. The salts in which the H_{4} is replaced by - Ni_{2}, or NiNa_{2}, or CdNa_{2}, &c., are insoluble in water. - - In order to see the relation between phosphoric acid and - hypophosphoric acid which does not contain the elements of - phosphorous acid (because it does not reduce either gold or - mercury from their solutions), but which nevertheless is capable - of being oxidised (for example, by potassium permanganate) into - phosphoric acid, it is simplest to apply the law of substitution. - This clearly indicates the relation between oxalic acid, - (COOH)_{2}, and carbonic acid, OH(COOH). The relation between the - above acids is exactly the same if we express phosphoric acid as - OH(POO_{2}H_{2}), because in this case P_{2}H_{4}O_{6}, or - (POO_{2}H_{2})_{3}, will correspond with it just as oxalic does - with carbonic acid. A similar relationship exists between - hyposulphuric or dithionic acid, (SO_{2}OH)_{2}, and sulphuric - acid, OH(SO_{2}OH), as we shall find in the following chapter. - Dithionic acid corresponds with the anhydride S_{2}O_{5}, - intermediate between SO_{2} and SO_{3}; oxalic acid with - C_{2}O_{3}, intermediate between CO and CO_{2}; hypophosphoric - acid corresponds with the anhydride P_{2}O_{4}, intermediate - between P_{2}O_{3} and P_{2}O_{5}, and the analogue of N_{2}O_{4}. - -Judging by the general law of the formation of acids (Chapter XV.), the -series of phosphorus compounds should include the following _ortho-acids_ -and their corresponding anhydrides, answering to phosphuretted hydrogen, -H_{3}P:-- - - H_{3}PO_{4}, phosphoric acid, and P_{2}O_{5}, anhydride, - H_{3}PO_{3}, phosphorous acid, and P_{2}O_{3}, anhydride, - H_{3}PO_{2}, hypophosphorous acid, and P_{2}O, anhydride.[12] - -The last of these (the analogue of N_{2}O) is almost unknown. Phosphoric -anhydride (P_{2}O_{5}) with a small quantity of water does not at first -give orthophosphoric acid, PH_{3}O_{4}, but a compound P_{2}O_{5},H_{2}O, -or PHO_{3}, whose composition corresponds with that of nitric acid; this -is _metaphosphoric acid_. Even with an excess of water, combining with -phosphoric anhydride, this metaphosphoric acid, and not the ortho-, -passes at first into solution. Metaphosphoric acid in solution only -passes into orthophosphoric acid when the solution is heated or after a -lapse of time. - - [12] Besides the hydrates enumerated, a compound, PH_{3}O, should - correspond with PH_{3}. This hydrate, which is analogous to - hydroxylamine, is not known in a free state, but it is known as - triethylphosphine oxide, P(C_{2}H_{5})_{3}O, which is obtained by - the oxidation of triethylphosphine, P(C_{2}H_{5})_{3}. It must be - observed that there may also be lower oxides of phosphorus - corresponding with PH_{3}, like N_{2}O and NO, and there are even - indications of the formation of such compounds, but the data - concerning them cannot be considered as firmly established. - -_Orthophosphoric acid_[13] is obtained by oxidising phosphorus with -nitric acid until the phosphorus entirely passes into solution and the -lower oxides of nitrogen cease to be evolved. The reaction takes place -best with dilute nitric acid, and when aided by heat. The resultant -solution is evaporated to a syrup. If a weighed quantity of phosphorus -(dried in a current of dry carbonic anhydride) be taken, a crystalline -mass of the acid can be obtained by evaporating the solution until it -consists only of the quantity[14] of phosphoric acid corresponding with -the amount of phosphorus taken (from 31 parts of P, 98 parts of -solution). The acid fuses at +39°; specific gravity of the liquid 1·88. -Phosphorus pentachloride, PCl_{5}, and oxychloride, POCl_{3} (see further -on), give orthophosphoric acid and hydrochloric acid with water. The two -other varieties of phosphoric acid, with which we shall presently become -acquainted, give the same ortho-acid when under the influence of acids, -with particular ease when boiled and more slowly in the cold. By itself -orthophosphoric acid (either in solution or when dry) does not pass into -the other varieties; it does not oxidise, and therefore presents the -limiting and stable form. When heated to 300°, it loses water and passes -into pyrophosphoric acid, 2H_{3}PO_{4} = H_{2}O + H_{4}P_{2}O_{7}, whilst -at a red heat it loses twice as much water and is converted into -metaphosphoric acid, H_{3}PO_{4} = H_{2}O + HPO_{3}. In aqueous solution -orthophosphoric acid differs clearly from pyro- or metaphosphoric acids, -because the solutions of these latter acids give different reactions: -thus orthophosphoric acid does not precipitate albumin, does not give a -precipitate with barium chloride, and forms a yellow precipitate of -silver orthophosphate, Ag_{3}PO_{4}, with silver nitrate (in the presence -of alkalis, but not otherwise); whilst a solution of pyrophosphoric acid, -H_{4}P_{2}O_{7}, although it does not precipitate albumin or barium -chloride, gives a white precipitate of silver pyrophosphate, -Ag_{4}P_{2}O_{7}, with silver nitrate; and a solution of metaphosphoric -acid, HPO_{3}, precipitates both albumin and barium chloride, and gives a -white precipitate of silver metaphosphate, AgPO_{3}, with silver nitrate. -These points of distinction were studied by Graham, and are exceedingly -instructive. They show that the solution of a substance does not -determine the maximum of chemical combination with water, that solutions -may contain various degrees of combination with water, and that there is -a clear difference between the water serving for solution and that -entering into chemical combination. Graham's experiments also showed that -the water whose removal or combination determines the conversion of -ortho- into meta- and pyrophosphoric acids differs distinctly from water -of crystallisation, for he obtained the salts of ortho-, meta-, and -pyrophosphoric acids with water of crystallisation, and they differed in -their reactions, like the acids themselves. This water of crystallisation -was expelled with greater ease than the water of constitution of the -hydrates in question.[14 bis] - - [13] Phosphoric acid, being a soluble and almost non-volatile - substance, cannot be prepared like hydrochloric and nitric acids - by the action of sulphuric acid on the alkali phosphates, although - it is partially liberated in the process. For this purpose the - salts of barium or lead may be taken, because they give insoluble - salts, thus Ba_{3}(PO_{4})_{2} + 3H_{2}SO_{4} = 3BaSO_{4} + - 2H_{3}PO_{4}. Bone ash contains, besides calcium phosphate, sodium - and magnesium phosphates, and fluorides and other salts, so that - it cannot give directly a pure phosphoric acid. - - [14] If this is not done the orthophosphoric acid, PH_{3}O_{4}, loses a - portion of its water, and then, as with an excess of water, it - does not crystallise. - - [14 bis] The difference between the reactions of ortho-, meta- and - pyrophosphoric acids, established by Graham (_see_ p. 163), is of - such importance for the theory of hydrates and for explaining the - nature of solutions, that in my opinion its influence upon - chemical thought has been far from exhausted. At the present time - many such instances are known both in organic (for instance, the - difference between the reactions of the solutions of certain - anhydrides and hydrates of acids), and inorganic chemistry (for - example, the difference between the rose and purple cobalt - compounds, Chapter XXII. &c.) They essentially recall the long - known and generalised difference between C_{2}H_{4} (ethylene), - C_{2}H_{6}O (ethyl alcohol = ethylene + water), and C_{4}H_{10}O - (ethyl ether = 2 ethylene + water = 2 alcohol - water); but to the - present day the numerous analogous phenomena existing among - inorganic substances are only considered as a simple difference in - degrees of affinity, distinguishing the water of constitution - (hydration), crystallisation, and solution without penetrating - into the difference of the structure or distribution of the - elements, which exists here and gives rise to a distinct isomerism - of solutions. In my opinion the progress of chemistry, especially - with regard to solutions, should make rapid strides when the cause - of the isomerism of solutions, for instance, of ortho- and - pyrophosphoric acids, has become as clear to us as the cause of - many well-studied instances of the isomerism, polymerism, and - metamerism of organic compounds. Here it forms one of those many - important problems which remain for the chemistry of the future in - a state of only indistinct presentiments and in the form of facts - empirically known but insufficiently comprehended. - -Orthophosphoric acid has a pleasant acid taste and a distinctly acid -reaction; it is used as a medicine, and is not poisonous (phosphorous -acid is poisonous). Alkalis, like sodium, potassium, and ammonium -hydroxides, saturate the acid properties of phosphoric acid when taken in -the ratio 2NaHO : H_{3}PO_{4}--that is, when salts of the composition -HNa_{2}PO_{4} are formed. When taken in the ratio NaHO : H_{3}PO_{4}, a -solution having an acid reaction is obtained, and when 3NaHO : -H_{3}PO_{4}--that is, when the salt Na_{3}PO_{4} is formed--an alkaline -reaction is obtained. Hence many chemists (Berzelius) even regarded the -salts of composition R_{2}HPO_{4} as normal, and considered phosphoric -acid to be bibasic. But the salt Na_{2}HPO_{4} also shows a feeble -alkaline reaction, so that it is impossible to judge the characteristic -peculiarities of acids by the reactions on litmus paper, as we already -know from many examples. Orthophosphoric acid is tribasic, because it -contains three equivalents of hydrogen replaceable by metals, forming -salts, such as NaH_{2}PO_{4}, Na_{2}HPO_{4}, and Na_{3}PO_{4}. It is also -tribasic, because with silver nitrate its soluble salts always give -Ag_{3}PO_{4},[15] a salt with three equivalents of silver, and because by -double decomposition with barium chloride it forms a salt of the -composition Ba_{3}(PO_{4})_{2}, and silver and barium hardly ever give -basic salts. With the metals of the alkalis, phosphoric acid forms -soluble salts, but the normal salts of the metals of the alkaline earths, -R_{3}(PO_{4})_{2} and even R_{2}H_{2}(PO_{4}), are insoluble in water, -but dissolve in feeble acids, such as phosphoric and acetic, because they -then form soluble acid salts, especially RH_{4}(PO_{4})_{2}.[16] - - [15] Silver orthophosphate, Ag_{3}PO_{4}, is yellow, sp. gr. 7·32, and - insoluble in water. When heated it fuses like silver chloride, and - if kept fused for some length of time it gives a white - pyrophosphate (the decomposition which causes this is not known). - It is soluble in aqueous solutions of phosphoric, nitric, and even - acetic acids, of ammonia, and many of its salts. If silver nitrate - acts on a dimetallic orthophosphate--for instance, - Na_{2}HPO_{4}--it still gives Ag_{3}PO_{4}, nitric acid being - disengaged: Na_{2}HPO_{4} + 3AgNO_{3} = Ag_{3}PO_{4} + 2NaNO_{3} + - HNO_{3}. When alcohol is added to silver orthophosphate, - Ag_{3}PO_{4}, dissolved in syrupy phosphoric acid, it precipitates - a white salt (the alcohol takes up the free phosphoric acid) - having the composition Ag_{2}HPO_{4}, which is immediately - decomposed by water into the normal salt and phosphoric acid. - - [16] The researches of Thomsen showed that in very dilute aqueous - solutions the majority of monobasic acids--nitric, acetic, - hydrochloric, &c. (but hydrofluoric acid more and hydrocyanic - less)--HX evolve the following amounts of heat (in thousands of - calories) with caustic soda: NaHO + 2HX = 14; NaHO + HX = 14; - 2NaHO + HX = 14; that is, if _n_ be a whole number _n_NaHO + HX = - 14 and NaHO + _n_HX = 14. Hence reaction here only takes place - between one molecule of NaHO and one molecule of acid, and the - remaining quantity of acid or alkali does not enter into the - reaction. In the case of bibasic acids, H_{2}R´´ (sulphuric, - dithionic, oxalic, sulphuretted hydrogen, &c.), NaHO + 2H_{2}R´´ = - 14; NaHO + H_{2}R´´ = 14; 2NaHO + H_{2}R´´ = 28; _n_NaHO + - H_{2}R´´ = 28; that is, with an excess of acid (NaHO + 2H´_{2}R´´) - 14 thousand units of heat are developed, and with an excess of - alkali 28. When phosphoric acid is taken (but not all tribasic - acids--for instance, not citric) the general character of the - phenomenon is similar to the preceding, namely, NaHO + - 2H_{3}PO_{4} = 14·7; NaHO + H_{3}PO_{4} = 14·8; 2NaHO + - H_{3}PO_{4} = 27·1; 3NaHO + H_{3}PO_{4} = 34·0; 6NaHO + - H_{3}PO_{4} = 35·3; or, in general terms, NaHO + _n_H_{3}PO_{4} = - 14 (approximately) and _n_NaHO + H_{3}PO_{4} = 35 and not 42, - which shows a peculiarity of phosphoric acid. In the case of - energetic acids, when one equivalent (23 grams) of sodium (in the - form of hydroxide) replaces one equivalent (1 gram) of hydrogen - (with the formation of water and in dilute solutions), 14,000 heat - units are evolved; and this is true for phosphoric acid when in - H_{3}PO_{4}, Na or Na_{2} replaces H or H_{2}, but when Na_{3} - replaces H_{3} less heat is developed. This will be seen from the - following scheme based on the preceding figures: H_{3}PO_{4} + - NaHO = 14·8; NaH_{2}PO_{4} + NaHO = 12·3; Na_{2}HPO_{4} + NaHO = - 5·9; with Na_{3}PO_{4} + NaHO, a very small amount of heat is - evolved, as may be judged from the fact that Na_{3}PO_{4} + 3NaHO - = 1·3, but still heat is evolved. It must be supposed that in - acting on phosphoric acid in the presence of a large quantity of - water, a certain portion of the sodium hydroxide remains as alkali - uncombined with the acid. Thus, on increasing the mass of the - alkali, heat is still evolved, and a fresh interchange between Na - and H takes place. Hence water shows a decomposing action on the - alkali phosphates. The same decomposing action of water is seen, - but to a less extent, with Na_{2}HPO_{4}, as may be judged both - from the reactions of this salt and from the amount of heat - developed by NaH_{2}PO_{4} with NaHO. Such an explanation is in - accordance with many facts concerning the decomposition of salts - by water already known to us. Recent researches made by Berthelot - and Louguinine have confirmed the above deductions made by me in - the first edition (1871) of this work. At the present time views - of this nature are somewhat generally accepted, although they are - not sufficiently strictly applied in other cases. As regards - PH_{3}O_{4} it may be said that: on the substitution of the first - hydrogen this acid acts as a powerful acid (like HCl, HNO_{3}, - H_{2}SO_{4}); on the substitution of the second hydrogen as a - weaker acid (like an organic acid); and on the substitution of the - third, as an alcohol, for instance phenol, having the properties - of a feeble acid. - -Phosphoric anhydride, or any of its hydrates, when ignited with an excess -of sodium hydroxide, carbonate, &c., forms normal or _trisodium -orthophosphate_, Na_{3}PO_{4}, but when a solution of sodium carbonate is -decomposed by orthophosphoric acid, only the salt Na_{2}HPO_{4} is -formed; and when an excess of sodium chloride is ignited with -orthophosphoric acid, hydrochloric acid is evolved, and the acid salt -H_{2}NaPO_{4} alone is formed. These facts clearly indicate the small -energy of phosphoric acid with respect to the formation of the -tri-metallic salt, which is seen further from the fact that the salt -Na_{3}PO_{4} has an alkaline reaction, decomposes in the presence of -water and carbonic acid, forming Na_{2}HPO_{4}, corrodes glass vessels in -which it is boiled or evaporated, just like solutions of the alkalis, -disengages, like them, ammonia from ammonium chloride, and crystallises -from solutions, as Na_{3}PO_{4},12H_{2}O, only in the presence of an -excess of alkali. At 15° the crystals of this salt require five parts of -water for solution; they fuse at 77°. - -_Disodium orthophosphate_, or common sodium phosphate, Na_{2}HPO_{4}, is -more stable both in solution and in the solid state. As it is used in -medicine and in dyeing, it is prepared in considerable quantities, most -frequently from the impure phosphoric acid obtained by the action of -sulphuric acid on bone ash. The solution thus formed--which contains, -besides phosphoric and sulphuric acids, salts of sodium, calcium, and -magnesium--is heated, and sodium carbonate added so long as carbonic -anhydride is disengaged. A precipitate is formed containing the insoluble -salts of magnesium and calcium, whilst the solution contains sodium -phosphate, Na_{2}HPO_{4}, with a small quantity of other salts, from -which it may be easily purified by crystallisation. At the ordinary -temperature its solutions, especially in the presence of a small amount -of sodium carbonate, give finely-formed inclined prismatic crystals, -Na_{2}HPO_{4},12H_{2}O; when the crystallisation takes place above 30° -they only contain 7H_{2}O. The former crystals even lose a portion of -their water of crystallisation at the ordinary temperature (the salt -effloresces), and form the second salt with 7H_{2}O; whilst under the -receiver of an air-pump and over sulphuric acid they also part with this -water.[17] When ignited they lose the last molecule of water of -constitution, and give sodium pyrophosphate, Na_{4}P_{2}O_{7}. - - [17] Na_{2}HPO_{4},12H_{2}O has a sp. gr. 1·53. Poggiale determined the - solubility in 100 parts of water (1) of the anhydrous ortho-salt - Na_{2}HPO_{4}, and (2) of the corresponding pyro-salt - Na_{4}P_{2}O_{7}:-- - - 0° 20° 40° 80° 100° - I. 1·5 11·1 30·9 81 108 - II. 3·2 6·2 13·5 30 40 - - At temperatures of 20° to 100° the ortho-salt is so very much less - soluble that this difference alone already indicates the - deeply-seated alteration in constitution which takes place in the - passage from the ortho- to the pyro-salts. - -_Monosodium orthophosphate_, NaH_{2}PO_{4}, crystallises with one -equivalent of water; its solution has an acid reaction. At 100° the salt -only loses this water of crystallisation, and at about 200° it parts with -all its water, forming the metaphosphate NaPO_{3}. It is prepared from -ordinary sodium phosphate by adding phosphoric acid until the solution -does not give a precipitate with barium chloride, and then evaporating -and crystallising the solution. The solution of this salt does not absorb -carbonic anhydride, and does not give a precipitate with salts of -calcium, barium, &c.[18] - - [18] The _ammonium orthophosphates_ resemble the sodium salts in many - respects, but the instability of the di- and tri-metallic salts is - seen in them still more clearly than in the sodium salts; thus - (NH_{4})_{3}PO_{4}, and even (NH_{4})_{2}HPO_{4}, lose ammonia in - the air (especially when heated, even in solutions); - NH_{4}H_{2}PO_{4} alone does not disengage ammonia and has an acid - reaction. The crystals of the first salt contain 3H_{2}O, and are - only formed in the presence of an excess of ammonia; both the - others are anhydrous, and may be obtained like the sodium salts. - When ignited these salts leave metaphosphoric acid behind; for - example, (NH_{4})_{2}HPO_{4} = 2NH_{3 + H_{2}O + HPO_{3}. Ammonia - also enters into the composition of many double phosphates. - Ammonium sodium orthophosphate, or simply phosphate, - NH_{4}NaHPO_{4},4H_{2}O, crystallises in large transparent - crystals from a mixture of the solutions of disodium phosphate and - ammonium chloride (in which case sodium chloride is obtained in - the mother liquid), or, better still, from a solution of - monosodium phosphate saturated with ammonia. It is also formed - from the phosphates in urine when it ferments. This salt is - frequently used in testing metallic compounds by the blow-pipe, - because when ignited it leaves a vitreous metaphosphate, NaPO_{3}, - which, like borax, dissolves metallic oxides, forming - characteristic tinted glasses. - - When a solution of trisodium phosphate is added to a solution of a - magnesium salt it gives a white precipitate of the normal - orthophosphate Mg_{2}(PO_{4})_{2},7H_{2}O. If the trisodium salt - be replaced by the ordinary salt, Na_{2}HPO_{4}, a precipitate is - also formed, and MgHPO_{4},7H_{2}O is obtained. It might be - thought that the normal salt Mg_{3}(PO_{4})_{2} would be - precipitated if disodium phosphate was added to ammonia and a salt - of magnesium, but in reality _ammonium magnesium orthophosphate_, - MgNH_{4}PO_{4},6H_{2}O, is precipitated as a crystalline powder, - which loses ammonia and water when ignited, and gives a - pyrophosphate, Mg_{2}P_{2}O_{7}. This salt occurs in nature as the - mineral struvite, and in various products of the changes of animal - matter. If we consider that the above salt parts with ammonia with - difficulty, and that the corresponding salt of sodium is not - formed under the same conditions (MgNaPO_{4},9H_{2}O is obtained - by the action of magnesia on disodium phosphate), if we turn our - attention to the fact that the salts of calcium and barium do not - form double salts as easily as magnesium, and remember that the - salts of magnesium in general easily form double ammonium salts, - we are led to think that this salt is not really a normal, but an - acid salt, corresponding with Na_{2}HPO_{4}, in which Na_{2} is - replaced by the equivalent group NH_{3}Mg. - - The common normal _calcium phosphate_, Ca_{3}(PO_{4})_{2}, occurs - in minerals, in animals, especially in bones, and also probably in - plants, although the ash of many portions of plants, as a rule, - contains less lime than the formation of the normal salt requires. - Thus 100 parts of the ash (from 5,000 parts of grain) of rye grain - contain 47·5 of phosphoric anhydride and only 2·7 of lime, and - even the ash of the whole of the rye (including the straw) - contains twice as much phosphoric anhydride as lime, and the - normal salt contains almost equal weights of these substances. - Only the ash of grasses, and especially of clover, and of trees, - contains in the majority of cases more lime than is required for - the formation of Ca_{3}P_{2}O_{8}. This salt, which is insoluble - in water, dissolves even in such feeble acids as acetic and - sulphurous, and even in water containing carbonic acid. The latter - fact is of immense importance in nature, since by reason of it - rain water is able to transfer the calcium phosphates in the soil - into solutions which are absorbed by plants. The solubility of the - normal salt in acids takes place by virtue of the formation of an - acid salt, which is evident from the quantity of acid required for - its solution, and more especially from the fact that the acid - solutions when evaporated give crystalline scales of the acid - calcium phosphate, CaH_{4}(PO_{4})_{2}, soluble in water. This - solubility of the acid salt forms the basis of the treatment by - acids of bones, phosphorites, guano, and other natural products - containing the normal salt and employed for fertilising the soil. - The perfect decomposition requires at least 2H_{2}SO_{4} to - Ca_{3}(PO_{4})_{2}, but in reality less is taken, so that only a - portion of the normal salt is converted into the acid salt. - Hydrochloric acid is sometimes used. (In practice such mixtures - are known as _superphosphates_). Certain experiments, however, - show that a thorough grinding, the presence of organic, and - especially of nitrogenous, substances, and the porous structure of - some calcium phosphates (for example, in burnt bones), render the - treatment of phosphoric manures by acids superfluous--that is, the - crop is not improved by it. - -As a hydrate, orthophosphoric acid should be expressed, after the fashion -of other hydrates, as containing three water residues (hydroxyl groups), -_i.e._ as PO(OH)_{3}. This method of expression indicates that the type -PX_{5}, seen in PH_{4}I, is here preserved, with the substitution of -X_{2} by oxygen and X_{3} by three hydroxyl groups. The same type appears -in POCl_{3}, PCl_{5}, PF_{5}, &c. And if we recognise phosphoric acid as -PO(OH)_{3}, we should expect to find three anhydrides corresponding with -it: (1) [PO(OH)_{2}]_{2}O, in which two of the three hydroxyls are -preserved; this is pyrophosphoric acid, H_{4}P_{2}O_{7}. (2) PO(OH)O, -where only one hydroxyl is preserved. This is metaphosphoric acid. (3) -(PO)_{2}O_{3} or P_{2}O_{5}, that is, perfect phosphoric anhydride. -Therefore, _pyro- and metaphosphoric acids are imperfect anhydrides_ (or -anhydro-acids) _of orthophosphoric acid_.[19] - - [19] In this sense the ortho-acid itself might be regarded as an - anhydro-acid, counting P(HO)_{5} as the perfect hydrate, if PH_{5} - existed; but as in general the normal hydrates correspond with the - existing hydrogen compounds with the addition of up to 4 atoms of - oxygen, therefore PH_{3}O_{4} is the normal acid, just as - SH_{2}O_{4} and ClHO_{4}; while NHO_{3}, CH_{2}O_{3} are - meta-acids, or higher normal acids (NH_{3}O_{4} and CH_{4}O_{4}) - with the loss of a molecule of water. - - In order to see the relation between the ortho-, pyro-, and - metaphosphoric acids, the first thing to remark in them is that - the anhydride P_{2}O_{5} is combined with 3, 2, and 1 molecules of - water. In the absence of data for the molecular weight of ortho- - and pyrophosphoric acids it is necessary to mention that all - existing data for metaphosphoric acid indicate (Note 21) that its - molecule is much more complex and contains at least - H_{3}P_{3}O_{9} or H_{6}P_{6}O_{18}. The explanation of the - problems which here present themselves can, it seems to me, be - only looked for after a detailed study of the phenomena of the - polymerisations of mineral substances, and of those complex acids, - such as phosphomolybdic, which we shall hereafter describe - (Chapter XXI.) A similar instance is exhibited in the solubility - of hydrate of silica (produced by the action of silicon fluoride - on water) in fused metaphosphoric acid, with the formation, on - cooling, of an octahedral compound (sp. gr., 3·1) containing - SiO_{2},P_{2}O_{5}. A certain indication (but no proof) that - ordinary orthophosphoric acid is polymerised is given by - Staudenmaier (1893), who obtained a salt, K_{5}H_{4}P_{3}O_{12}, - by the action of a solution of KH_{2}PO_{4} upon K_{2}CO_{3}; and - a compound, KH_{3}P_{2}O_{8}, corresponding to the doubled - molecule of H_{3}PO_{4}, by the action of KH_{2}PO_{4} upon - H_{3}PO_{4} itself. - -_Pyrophosphoric acid_, H_{4}P_{2}O_{7}, is formed by heating -orthophosphoric acid to 250° when it loses water.[19 bis] Its normal -salts are formed by igniting the dimetallic salts of orthophosphoric acid -of the types HM_{2}PO_{4}. Thus from the disodium salt we obtain sodium -pyrophosphate, Na_{4}P_{2}O_{7} (it crystallises from water with -10H_{2}O, is very stable, fuses when heated, has an alkaline reaction, -and does not form ortho-salts when its solution is boiled): and from the -monosodium salt NaH_{2}PO_{4} the acid salt Na_{2}H_{2}P_{2}O_{7} (easily -soluble in water) is formed; this has an acid reaction, and when ignited -further gives the meta-salt.[20] - - [19 bis] According to Watson (1893) the ortho-acid is partially - transformed into the pyro-acid at 230°, whilst at 260° the latter - begins to volatilise. At 300° the meta-acid only is formed. - - [20] The method of preparation of the acid itself consists in - converting the sodium salt, Na_{4}P_{2}O_{7}, by double - decomposition with water and a salt of lead, into insoluble lead - pyrophosphate, Pb_{2}P_{2}O_{7}, which is then suspended in water - and decomposed by sulphuretted hydrogen; lead sulphide is thus - precipitated, and pyrophosphoric acid remains in solution. This - solution cannot be heated, or the pyro-acid will pass into the - ortho-, but must be evaporated under the receiver of an air-pump. - It concentrates to a syrup and crystallises, and when ignited in - this form loses water, and forms metaphosphoric acid. It resembles - orthophosphoric acid in many respects; its salts with the alkalis - are also soluble, and the others insoluble in water but soluble in - acids. When heated in solution with acid it gives orthophosphoric - acid, as well as when fused with an excess of alkali. - - Witt heated ammonium chloride with phosphoric acid (hydrochloric - acid was evolved), ignited the residue to drive off ammonia, and - obtained pyrophosphoric acid in the residue. - -_Metaphosphoric acid_, HPO_{3} (the analogue of nitric acid), is formed -by the ignition of the pyro- and ortho-acids (or, better, of their -ammonium salts), as a vitreous, hygroscopic, fused mass (glacial -phosphoric acid, _acidum phosphoricum glaciale_), soluble in water and -volatilising without decomposition. It is also formed in the first slow -action of cold water on the anhydride, but metaphosphoric acid gradually -changes into the ortho-acid when its solution is boiled, or when it is -kept for any length of time, especially in the presence of acids.[21] - - [21] As when using phenolphthalein as an indicator in neutralising by - an alkali metaphosphoric acid is monobasic, and orthophosphoric - acid is bibasic, it is possible by means of this difference to - follow the transition of meta- into orthophosphoric acid. Sabatier - (1888) carried on an investigation of this nature, and found that - the rate of transformation is dependent on the temperature, and is - subject to the general laws of the rate of chemical - transformations which belongs to physical chemistry. - - Metaphosphoric acid has a particular interest in respect to the - variations to which its salts are subject. The metaphosphates are - formed by the ignition of the acid orthophosphates, MH_{2}PO_{4}, - or MNH_{4}HPO_{4}, or of the acid pyrophosphates, - M_{2}H_{2}P_{2}O_{7}, or M_{2}(NH_{4})_{2}P_{2}O_{7}, water and - ammonia being given off in the process. The properties of the - metaphosphates, which have a similar composition to nitrates--for - instance, NaPO_{3}, or Ba(PO_{3})_{2}--vary according to the - duration of the ignition to which the ortho-, or pyrophosphates - from which they are prepared have been subjected. When the salts - NaH_{2}PO_{4} or NH_{4}NaHPO_{4} are strongly ignited, a salt - NaPO_{3} is formed, which deliquesces in the air, and gives a - gelatinous precipitate with salts of the alkaline earths. But, as - Graham (in 1830-40), and many others, especially Fleitmann and - Henneberg (in 1840-50), and Tamman (in the nineties), observed, - under other conditions the salts of the same composition acquire - other properties. The above chemists recognise five polymeric - forms of metaphosphates, (HPO_{3})_{_n_}. We will follow the - nomenclature and researches of Fleitmann. - - _Monometaphosphoric acid._ The salts are distinguished for their - insolubility in water; even the salts NaPO_{3}, KPO_{3}, are - insoluble. They are obtained by igniting the monometallic - orthophosphates--for example, RH_{2}PO_{4}--up to the temperature - at which all water is evolved (316°), but not to fusion. No double - salts are known. - - _Dimetaphosphoric acid_, on the contrary, easily forms double - salts--for example, KNaP_{2}O_{6}, and also the copper potassium - salt, &c. The copper salt is obtained by evaporating a solution of - copper oxide in orthophosphoric acid. A blue ortho-salt, - CuRHO_{4}, first separates from the solution, then a light-blue - pyro-salt, Cu_{2}P_{2}O_{7}; and above 350°, when metaphosphoric - acid itself begins to volatilise, the dimetaphosphate, - CuP_{2}O_{6}, is formed. The residue is washed with water, and - decomposed with a hot solution of sodium sulphide, when the sodium - salt, Na_{2}P_{2}O_{6}, is obtained in solution. This salt, when - evaporated with alcohol, gives crystals containing 2 mol. H_{2}O, - which, however, retain their solubility (in 7 parts of water) - after the water is driven off at 100°. When fused, these crystals - give a deliquescent salt (hexa-metaphosphate). The solution of the - salt has a neutral reaction, which only after prolonged boiling - becomes acid, owing to the formation of orthophosphate, - NaH_{2}PO_{4}. The soluble salts of dimetaphosphoric acid give the - insoluble silver salt, Ag_{2}P_{2}O_{6}, with silver nitrate, and - a precipitate of BaP_{2}O_{6}2H_{2}O with barium chloride. - - _Trimetaphosphoric acid_ is obtained as the sodium salt - Na_{3}P_{3}O_{9} when any other metaphosphate of sodium is fused - and _slowly_ cooled, then dissolved in a slight excess of warm - water, and the resultant solution evaporated. The crystals contain - 6 mol. H_{2}O, and dissolve in four parts of water. An acid - reaction is only obtained, as with the preceding salt, after - prolonged boiling with water. The acid is a true analogue of - nitric acid, because _all its metallic salts are soluble_. - - _Hexametaphosphoric acid._ Fleitmann so named the ordinary - metaphosphoric acid (glacial) which attracts moisture. The - deliquescent sodium salt is obtained, like the trimetaphosphate, - only by _rapid_ cooling. It is also formed by fusing silver oxide - with an excess of phosphoric acid. The sodium salt is soluble in - water, and gives viscous, elastic precipitates with salts of Ba, - Ca, and Mg. Lubert (1893) obtained salts of Ag, Pb, &c. - - Jawein and Thillot (1889), who investigated the sodium salts of - metaphosphoric acid by Raoult's method, came to the conclusion - that the salts of di- and tri-metaphosphoric acid behave in such a - manner that their molecule must be represented as non-polymerised - NaPO_{3}, whilst those of hexametaphosphoric acid behave as - (NaPO_{3})_{4}. At all events, the series of salts which Fleitmann - and Henneberg regard as monometaphosphates--_i.e._ as - non-polymerised--are most probably the most polymerised, because - they are insoluble. - - According to Tamman's researches, vitreous metaphosphoric acid - contains a mixture consisting chiefly of two varieties, differing - in the solubility and degree of stability of their salts. The - least stable corresponds to Fleitmann's hexa-acid, and gives three - isomeric salts. Tamman came to the conclusion that there exist - polymers also in the form of penta-, ortho-, and - deca-metaphosphoric acids. Without going into details upon this - subject, I do not think it superfluous to point out that the - undoubted capability of metaphosphoric acid to polymerise should - be connected with its faculty of combining with water, whilst the - degree of polymerisation and the number of polymeric forms cannot - yet be considered as sufficiently explained. - -In order to see the relation between phosphoric acid and the lower acids -of phosphorus, it is simplest to imagine the substitution of hydroxyl in -H_{3}PO_{4} or PO(OH)_{3} by hydrogen. Then from orthophosphoric acid, -PO(OH)_{3}, we shall obtain phosphorous acid, POH(OH)_{2}, and -hypophosphorous acid, POH(OH); and, furthermore, phosphorous acid should -be bibasic if orthophosphoric acid was tribasic, and hypophosphorous acid -should be monobasic. This conclusion[21 bis] is, in fact, true, and hence -all the acids of phosphorus may be referred to one common type, PX_{5}, -whose representatives are PH_{4}I and PCl_{5}, POCl_{3}, PCl_{2}F_{3}, -&c. - - [21 bis] The bibasity of H_{3}PO_{3}, established by Würtz, has been - proved by many direct experiments (see, for instance, Note 22), - among which we may mention that Amat (1892) took a mixture of the - aqueous solutions of Na_{2}HPO_{3} and NaHO and added absolute - alcohol to it. Two layers were formed; the upper, alcoholic, - contained all the excess of NaHO, whilst the lower only contained - the salt Na_{2}HPO_{3}, which was therefore unable to react with - the excess of NaHO. Amat also obtained NaH_{2}PO_{3} by saturating - H_{3}PO_{3} with soda until he obtained a neutral reaction with - methyl-orange. The replacement of one atom of H by sodium here, as - in phosphoric acid (Note 16), gives more heat than the replacement - of the second atom. For the third atom there is no formation of a - salt, and therefore no evolution of heat. The monometallic - salts--for example, NaH_{2}PO_{3}--or the ammonia salts, when - heated to 160°, give, as Amat had previously shown, a salt of - bibasic pyrophosphorous acid, Na_{2}H_{2}P_{2}O_{5}. - -_Phosphorous acid_, PH_{3}O_{3}, is generally obtained from phosphorus -trichloride, PCl_{3}, by the action of water: PCl_{3} + 3H_{2}O = 3HCl + -PH_{3}O_{3}. Both acids formed are soluble in water, but are easily -separated, because hydrochloric acid is volatile whilst phosphorous acid -volatilises with difficulty, and if a small amount of water be originally -taken the hydrochloric acid nearly all passes off directly. Concentrated -solutions of phosphorous acid give crystals of H_{3}PO_{3}, which fuse at -70°, attract moisture from the air, and deliquesce when ignited, giving -phosphine and phosphoric acid,[22] and are oxidised into orthophosphoric -acid by many oxidising agents. In its salts only two hydrogen atoms are -replaced by metals (Würtz); the salts of the alkaline metals are soluble, -and give precipitates with salts of the majority of other metals. - - [22] Phosphorous acid, when subjected to the action of nascent hydrogen - (zinc and sulphuric acid), evolves phosphine, and when boiled with - an excess of alkali it evolves hydrogen (PH_{3}O_{3} + 3KHO = - PK_{3}O_{4} + 2H_{2}O + H_{2}); owing to its liability to - oxidation, it is a reducing agent--for instance, it reduces cupric - chloride to cuprous chloride, and precipitates silver from the - nitrate and mercury from its salts. - - These reactions are perhaps connected with the fact that in this - acid one atom of hydrogen should be considered as in the same - condition as in phosphuretted hydrogen, which is expressed by the - formula PHO(OH)_{2}, if we represent it as PH_{4}X, with the - substitution of two of the hydrogen atoms by oxygen and of HX by - two of hydroxyl. The direct passage of phosphorous chloride into - phosphorous acid would, however, indicate that all the three atoms - of hydrogen in it occur in the form of hydroxyl, because no - difference is known between the three atoms of chlorine in - PCl_{3}--they all react alike, as a rule. However, Menschutkin, by - acting on alcohol, C_{2}H_{5}OH, with phosphorous chloride, - obtained hydrochloric acid and a substance P(C_{2}H_{5}O)Cl_{2}, - and from it by the action of bromine he obtained ethyl bromide, - C_{2}H_{5}Br, and a compound PBrOCl_{2}, which proves, to a - certain extent, the existence of a difference between the three - atoms of chlorine in phosphorous chloride. If we turn our - attention to the formation of phosphine by the ignition of - phosphorous acid, we see that 4PH_{3}O_{3} only evolve 3H in the - form of PH_{3}, and therefore the residue--that is, - 3PH_{3}O_{4}--will still contain one hydrogen of the same nature - as in phosphine, because in 4PH_{3}O_{3} we should recognise four - such hydrogens as in phosphine. We arrive at the same conclusion - by examining the decomposition of hypophosphorous acid, - 2PH_{3}O_{2} = PH_{3} + PH_{3}O_{4}. In the two molecules of the - monobasic hypophosphorous acid taken, there are only two atoms of - hydrogen replaceable by metals, whilst in the molecule of the - resultant phosphoric acid there are three. Perhaps relations of - this nature determine the relative stability of the dimetallic - salts of orthophosphoric acid. - -The monobasic _hypophosphorous acid_, PH_{3}O_{2}, gives salts -PH_{2}O_{2}Na, (PH_{2}O_{2})_{2}Ba, &c.; the two remaining atoms of -hydrogen (which exist in the same form as in phosphine, PH_{3}) are not -replaceable by metals, and this determines the property of these salts of -evolving phosphuretted hydrogen when heated (especially with alkalis). In -acting on substances liable to reduction it is this hydrogen which acts, -and, for example, _reduces_ gold and mercury from the solutions of their -salts, or converts cupric into cuprous salts. In all these instances the -hypophosphorous acid is converted into phosphoric acid. Under the action -of zinc and sulphuric acid it gives phosphine, PH_{3}. Nevertheless, -neither hypophosphorous acid nor its dry salts absorb oxygen from the -air. The salts of hypophosphorous acid are more soluble than those of the -preceding acids of phosphorus. Thus the sodium salt PNaH_{2}O_{2} does -not give a precipitate with barium chloride, and the salts of calcium, -barium, and many other metals are soluble.[23] The hypophosphites are -prepared by boiling an alkali with phosphorus so long as phosphuretted -hydrogen is evolved. The acid itself is obtained from barium -hypophosphite (prepared in the same manner by boiling phosphorus in -baryta water), by decomposing its solution with sulphuric acid. By -concentration of the solution of hypophosphorous acid (it must not be -heated above 130°, at which temperature it decomposes) a syrup is formed -which is able to crystallise. In the solid state hypophosphorous acid -fuses at +17°, and has the properties of a clearly defined acid. - - [23] Calcium hypophosphite is used in medicine. According to Cavazzi, a - mixture of sodium hypophosphite, NaH_{2}PO_{2}, and sodium nitrate - explodes violently. - -The types PX_{3} and PX_{5}, which are evident for the hydrogen and -oxygen compounds of phosphorus, are most clearly seen in its halogen -compounds,[24] to the consideration of which we will proceed, fixing our -attention more especially on the chlorine compounds, as being the most -important from the historical, theoretical, and practical point of view. - - [24] Fluorine and bromine give PX_{3} and PX_{5}, like chlorine. With - respect to iodine PI_{5} is, in a chemical sense, a very unstable - substance, and generally _phosphorus tri-iodide_ only is formed - (from yellow or red phosphorus and iodine in the requisite - proportions). It is a red crystalline substance, fuses at 55°, is - easily decomposed by water, forming phosphorous and hydriodic - acids, and when heated it evolves iodine vapours and forms - phosphorus di-iodide, PI_{2}. This substance may be obtained in - the same manner as the preceding by taking a smaller proportion of - iodine (8 parts of iodine to 1 part of phosphorus, whilst the - tri-iodide requires 12·3); it also forms red crystals, which melt - at 110°. When decomposed by water it not only gives phosphorous - and hydriodic acids, but also phosphine and a yellow substance (a - lower oxide of phosphorus). In its composition di-iodide of - phosphorus corresponds with liquid phosphuretted hydrogen, PH_{2}, - and probably its molecular weight is much higher: P_{2}I_{4} or - P_{3}I_{6}, &c. As the iodine compounds of phosphorus give - hydriodic and phosphorous acids with water, and as both these - substances are reducing agents in the presence of water (and - hydrates), iodide of phosphorus also acts as a reducing agent. - -Phosphorus burns in chlorine, forming phosphorous chloride, PCl_{3}, and -with an excess of chlorine, phosphoric chloride, PCl_{5}. The -oxychloride, POCl_{3}, as the simplest chloranhydride according to the -type PX_{5}, and also phosphoric chloride, correspond with -orthophosphoric acid, PO(OH)_{3}, while phosphorous chloride, PCl_{3}, -corresponds with phosphorous acid and the type PX_{3}. Phosphoric -oxychloride, POCl_{3}, is a colourless liquid, boiling at 110°. -Phosphorus trichloride is also a colourless liquid, boiling at 76°,[25] -whilst phosphoric chloride is a solid yellowish substance, which -volatilises without melting at about 168°. They are all heavier than -water, and form types of the _chloranhydrides_ or chlorine compounds of -the non-metallic elements whose hydrates are acids, just as NaCl or -BaCl_{2} are types of halogen metallic salts. - - [25] In a liquid state the density of phosphorous chloride at 10° = - 1·597, and therefore its molecular volume = 137·5/1·597 = 86·0, - and that of phosphorus oxychloride is equal to 153·5/1·693 = 90·7; - hence the addition of oxygen has produced considerable increase in - volume, just as in the conversion of sulphur dichloride, SCl_{2}, - into sulphuryl chloride, SOCl_{2}, the volume changes from 64 to - 71. It is the same with the boiling-points; phosphorus trichloride - boils at 70°, the oxychloride at 100°, sulphur dichloride at 64°, - and sulphuryl chloride at 78°--that is, the addition of oxygen - raises the boiling points. - - _The vapour density_ of phosphorus trichloride and oxychloride - corresponds with their formulæ (Cahours, Würtz)--namely, is equal - to half the molecular weight referred to hydrogen. But it is not - so with phosphorus pentachloride. Cahours showed that the vapour - density of phosphorus pentachloride referred to air = 3·65, to - hydrogen = 52·6, whilst according to the formula PCl_{5} it should - be = 104·2. Hence this formula corresponds with four, and not with - two, molecules. This shows that the vapour of phosphoric chloride - contains two and not one molecule, that in a state of vapour it - splits up, like sal-ammoniac, sulphuric acid, &c. The products of - disruption must here be phosphorous chloride, PCl_{3}, and - chlorine, Cl_{2}, bodies which easily re-form phosphoric chloride, - PCl_{5}, at a lower temperature. This decomposition of phosphoric - chloride in its conversion into vapour is confirmed by the fact - that the vapour of this almost colourless substance shows the - greenish-yellow colour proper to chlorine. This dissociation of - phosphoric chloride has been considered by some chemists as a sign - that phosphorus, like nitrogen, does not give volatile compounds - of the type PX_{5}, and that such substances are only obtained as - unstable molecular compounds which break up when distilled; for - example, PH_{3},HI, PCl_{3},Cl_{2}, NH_{3},HCl, &c. To prove that - the molecule PCl_{5} actually exists, Würtz in 1870 observed that - when mixed with the vapour of phosphorous chloride the vapour of - phosphoric chloride distils over (from 160° to 190°) perfectly - colourless, and has a density which is really near to the - formula--namely, to 104--and the same density was determined for - the pentachloride in an atmosphere of chlorine. Hence at low - temperatures and in admixture with one of the products of - dissociation, there is no longer that decomposition which occurs - at higher temperatures--that is, we have here a case of - dissociation proceeding at moderate temperatures. - - An important proof in favour of the type PX_{5} is exhibited by - phosphorus pentafluoride PF_{5}, obtained by Thorpe as a - colourless gas which only corrodes glass after the lapse of time; - it may be kept over mercury, and has a normal density. It is - formed when liquid arsenic trifluoride, AsF_{3}, is added to - phosphoric chloride surrounded by a freezing mixture: 3PCl_{5} + - 5AsF_{3} = 3PF_{5} + 5AsCl_{3}. - - In general, fluorine and phosphorus give stable compounds: PF_{3}, - POF_{3}, and PF_{5}, as would be expected from the fact that in - passing from Cl to I (_i.e._ as the atomic weight of the halogen - increases) the stability of the compounds with P and the tendency - to give PX_{5} (Note 24) decreases. _Phosphorus trifluoride_ is - obtained by heating a mixture of ZnF_{2} and PBr_{3}, by the - action of AsF_{3} upon PCl_{3}, by heating phosphide of copper - with PbF_{2}, &c. It is a strong-smelling gas, which liquefies at - -10° under a pressure of 40 atmospheres, giving a colourless - liquid. It dissolves easily in (is absorbed by, reacts with) - water, and acts upon glass; when mixed with Cl_{2} it combines - with it (Poulenc, 1891), forming PCl_{2}F_{3}, a colourless gas of - normal density, which is transformed into a liquid at 8°, - decomposes into PF_{3} + Cl_{2} at 250°, and, with a small amount - of water, gives _oxy-fluoride_ of phosphorus, POF_{3} (with a - large amount of water it gives PH_{3}O_{4}), which Moissan (1891) - obtained by the action of dry HF upon P_{2}O_{5}, and Thorpe and - Tutton (1890) by heating a mixture of cryolite and P_{2}O_{5}. It - is a gas of normal density, like PF_{3}, and was obtained by - Moissan by the action of fluorine upon PF_{3} (PSF_{3}, _see_ - Chapter XX., Note 20). Thus the forms PX_{3} and PX_{5} not only - exist in many solid and non-volatile substances, but also as - vapours. - -If a piece of phosphorus be dropped into a flask containing chlorine, it -burns when touched with a red-hot wire, and combines with the chlorine. -If the phosphorus be in excess, liquid _phosphorus trichloride_, PCl_{3}, -is always formed, but if the chlorine be in excess the solid -pentachloride is obtained. The trichloride is generally prepared in the -following manner. Dry chlorine (passed through a series of Woulfe's -bottles containing sulphuric acid) is led into a retort containing sand -and phosphorus. The retort is heated, the phosphorus melts, spreads -through the sand, and gradually forms the trichloride, which distils over -into a receiver, where it condenses. _Phosphoric chloride_ or _phosphorus -pentachloride_, PCl_{5}, is prepared by passing dry chlorine into a -vessel containing phosphorus trichloride (purified by distillation). -Phosphorous chloride combines directly with oxygen, but more rapidly with -ozone or with the oxygen of potassium chlorate (3PCl_{3} + KClO_{3} = -3POCl_{3} + KCl), forming _phosphorus oxychloride_, POCl_{3} (Brodie). -This compound is also formed by the first action of water on phosphoric -chloride; for example, if two vessels, one containing phosphoric chloride -and the other water, are placed under a bell jar, after a certain time -the crystals of the chloride disappear and hydrochloric acid passes into -the water. The aqueous vapour acts on the pentachloride, and the -following reaction occurs: PCl_{5} + H_{2}O = POCl_{3} + 2HCl, the result -being that liquid phosphorus oxychloride is found in one vessel, and a -solution of hydrochloric acid in the other. However, an excess of water -directly transforms phosphoric chloride into orthophosphoric acid, -PCl_{5} + 4H_{2}O = PH_{3}O_{4} + 5HCl,[26] since POCl_{3} reacts with -water (3H_{2}O), forming 3HCl and phosphoric acid PO(OH)_{3}. - - [26] Phosphorus oxychloride is obtained by the action of phosphoric - chloride on hydrates of acids (because alkalis decompose - phosphorus oxychloride), according to the equation PCl_{5} + RHO = - POCl_{3} + RCl + HCl, where RHO is an acid. The reaction only - proceeds according to this equation with monobasic acids, but then - RCl is volatile, and therefore a mixture is obtained of two - volatile substances, the acid chloride and phosphorus oxychloride, - which are sometimes difficult to separate; whilst if the hydrate - be polybasic the reaction frequently proceeds so that an anhydride - is formed: RH_{2}O_{2} + PCl_{5} = RO + POCl_{3} + 2HCl. If the - anhydride be non-volatile (like boric), or easily decomposed (like - oxalic), it is easy to obtain pure oxychloride. Thus phosphorus - oxychloride is often prepared by acting on boric or oxalic acid - with phosphoric chloride. It is also formed when the vapour of - phosphoric chloride is passed over phosphoric anhydride, - P_{2}O_{5} + 3PCl_{5} = 5POCl_{3}. This forms an excellent example - in proof of the fact that the formation of one substance from two - does not necessarily show that the resultant compound contains the - molecules of these substances in its molecule. But other - oxychlorides of phosphorus are also formed by the interaction of - phosphoric anhydride and chloride; thus at 200° the - chloranhydride, PO_{2}Cl, or chloranhydride of metaphosphoric - acid, is formed (Gustavson). The chloranhydride of pyrophosphoric - acid, P_{2}O_{3}Cl_{4}, was obtained (Hayter and Michaelis), - together with NOCl, &c., by the action of NO upon cold PCl_{3}, as - a fuming liquid boiling at 210°. - -The above chlorine compounds serve not only as a type of the -chloranhydrides, but also as a means for the preparation of other _acid -chloranhydrides_. Thus the conversion of acids XHO into chloranhydrides, -XCl, is generally accomplished by means of _phosphorus pentachloride_. -This fact was discovered by Chancel, and adopted by Gerhardt as an -important method for studying organic acids. By this means organic acids, -containing, as we know, RCOOH (where R is a hydrocarbon group, and where -carboxyl may repeat itself several times by replacing the hydrogen of -hydrocarbon compounds), are converted into their chloranhydrides, RCOCl. -With water they again form the acid, and resemble the chloranhydrides of -mineral acids in their general properties. - -Since carbonic acid, CO(OH)_{2}, contains two hydroxyl groups, its -perfect chloranhydride, COCl_{2}, _carbonic oxychloride_, _carbonyl -chloride_ or _phosgene gas_, contains two atoms of chlorine, and differs -from the chloranhydrides of organic acids in that in them one atom of -chlorine is replaced by the hydrocarbon radicle RCOCl, if R be a -monatomic radicle giving a hydrocarbon RH. It is evident, on the one -hand, that in RCOCl the hydrogen is replaced by the radicle COCl, which -is also able to replace several atoms of hydrogen (for example, -C_{2}H_{4}(COCl)_{2} corresponds with the bibasic succinic acid); and, on -the other hand, that the reactions of the chloranhydrides of organic -acids will answer to the reactions of carbonyl chloride, as the reactions -of the acids themselves answer to those of carbonic acid. Carbonyl -chloride is obtained directly from dry carbon monoxide and chlorine[27] -exposed to the action of light, and forms a colourless gas, which easily -condenses into a liquid, boiling at +8°, specific gravity 1·43, and -having the suffocating odour belonging to all chloranhydrides. Like all -chloranhydrides, it is immediately decomposed by water, forming carbonic -anhydride, according to the equation COCl_{2} + H_{2}O = CO_{2} + 2HCl, -and thus expresses the type proper to all chloranhydrides of both mineral -and organic acids.[28] - - [27] The direct action of the sun's rays, or of magnesium light, is - necessary to start the reaction between carbonic oxide and - chlorine, but when once started it will proceed rapidly in - diffused light. An excess of chlorine (which gives its coloration - to the colourless phosgene) aids the completion of the reaction, - and may afterwards be removed by metallic antimony. Porous - substances, like charcoal, aid the reaction. Phosgene may be - prepared by passing a mixture of carbonic anhydride and chlorine - over incandescent charcoal. Lead or silver chloride, when heated - in a current of carbonic oxide, also partially form phosgene gas. - Carbon tetrachloride, CCl_{4}, also forms it when heated with - carbonic anhydride (at 400°), with phosphoric anhydride (200°), - and most easily of all with sulphuric anhydride (2SO_{3} + CCl_{4} - = COCl_{2} + S_{2}O_{5}Cl_{2}, this is pyrosulphuryl chloride). - Chloroform, CHCl_{3}, is converted into carbonyl chloride when - heated with SO_{2}(OH)Cl (the first chloranhydride of sulphuric - acid); CHCl_{3} + SO_{3}HCl = COCl_{2} + SO_{2} + 2HCl (Dewar), - and when oxidised by chromic acid. - - Among the reactions of phosgene we may mention the formation of - urea with ammonia, and of carbonic oxide when heated with metals. - - [28] We are already acquainted with some of the chloranhydrides of the - inorganic acids--for instance, BCl_{3}, and SiCl_{4}--and here we - shall describe those which correspond with sulphuric acid in the - following chapter. It may be mentioned here that when hydrochloric - acts on nitric acid (aqua regia, Vol. I. p. 467) there is formed, - besides chlorine, the oxychlorides NOCl and NO_{2}Cl, which may be - regarded as chloranhydrides of nitric and nitrous acids (nitrogen - chloride, Vol. I. p. 476). The former boils at -5°, the latter at - +5°, the specific gravity of the first at -12° = 1·416, and at - -18° = 1·433 (Geuther), and of the second = 1·3; the first is - obtained from nitric oxide and chlorine, the second from nitric - peroxide and chlorine, and also by the action of phosphoric - chloride on nitric acid. If the gases evolved by aqua regia be - passed into cold and strong sulphuric acid, they form crystals of - the composition NHSO_{3} (like chamber crystals), which melt at - 86°, and with sodium chloride form acid sodium sulphate and the - oxychloride NOCl. This chloranhydride of nitric acid is termed - _nitrosyl chloride_. - - _Cyanogen chloride_, CNCl, is the gaseous chloranhydride of cyanic - acid; it is formed by the action of chlorine on aqueous mercury - cyanide, Hg(CN)_{2} + 2Cl_{2} = HgCl_{2} + 2CNCl. When chlorine - acts on cyanic acid, it forms not only this cyanogen chloride, but - also polymerides of it--a liquid, boiling at 18°, and a solid, - boiling at 190°. The latter corresponds with cyanuric acid, and - consequently contains C_{3}N_{3}Cl_{3}. Details concerning these - substances must be looked for in works on organic chemistry. - -In order to show the general method for the preparation of acid -chloranhydrides, we will take that of acetic acid, CH_{3}·COOH, as an -example. Phosphorus pentachloride is placed in a glass retort, and acetic -acid poured over it; hydrochloric acid is then evolved, and the substance -distilling over directly after is a very volatile liquid, boiling at 50°, -and having all the properties of the chloranhydrides. With water it forms -hydrochloric and acetic acids. The reaction here taking place may be -explained thus: the substitution of the oxygen taken from the acetic acid -(from its carboxyl) by two atoms of chlorine from the PCl_{5} should be -as follows: CH_{3}·COOH + PCl_{5} = CH_{3}·COHCl_{2} + POCl_{3}. But the -compound CH_{3}·COHCl_{2} does not exist in a free state (because it -would indicate the possibility of the formation of compounds of the type -CX_{6}, and carbon only gives those of the type CX_{4}); it therefore -splits up into HCl and the chloranhydride CH_{3}·COCl. The general scheme -for the reaction of phosphorus pentachloride with hydrates ROH is exactly -the same as with water; namely, ROH with PCl_{5}, gives POCl_{3} + HCl + -RCl--that is a chloranhydride.[28 bis] - - [28 bis] This reaction indeed proceeds very easily and completely with - a number of hydroxides, if they do not react on hydrochloric acid - and phosphorus oxychloride, which is the case when they have - alkaline properties. When the hydroxide is bibasic and is present - in excess, it not unfrequently happens that the elements of water - are taken up: R(OH)_{2} + PCl_{5} = RO + 2HCl + POCl_{3}. The - anhydride RO may then be converted into chloranhydride, RO + - PCl_{5} = RCl_{2} + POCl_{3}--that is, phosphorus pentachloride - brings about the substitution of O by Cl_{2}. Thus carbonyl - chloride, COCl_{2}, boron chloride, 2BCl_{3}, and succinic - chloride, C_{4}H_{4}O_{2}Cl_{2}, &c., are respectively obtained by - the action of phosphoric chloride on carbonic, boric, and succinic - anhydrides. Phosphorus pentachloride reacts in a similar manner on - the aldehydes, RCHO, forming RCHCl_{2}, and on the chloranhydrides - themselves--for example, with acetic chloride, CH_{3}.COCl (when - heated in a closed tube), it forms a substance having the - composition CH_{3}CCl_{3}. - - Phosphorus trichloride and oxychloride act in a similar manner to - phosphoric chloride. When phosphorus trichloride acts on an acid, - 3RHO + PCl_{3} = 3RCl + P(HO)_{3}. If a salt is taken, then by the - action of phosphorus oxychloride a corresponding chloranhydride - and salt of orthophosphoric acid are easily formed: 3R(KO) + - POCl_{3} = 3RCl + PO(KO)_{3}. The chloranhydride RCl is always - more volatile than its corresponding acid, and distils over before - the hydrate RHO. Thus acetic acid boils at 117°, and its - chloranhydride at 50°. Phosphoric and phosphorous acids are very - slightly volatile, whilst their chloranhydrides are comparatively - easily converted into vapour. The faculty of the chloranhydrides - to react at the expense of their own chlorine determines their - great importance in chemistry. For instance, suppose we require to - know the molecular formula of some hydrate which does not pass - into a state of vapour and does not give a chloranhydride with - hydrochloric acid--that is, which has not any basic or alkaline - properties; we must then endeavour to obtain this chloranhydride - by means of phosphoric chloride, and it frequently happens that - the corresponding chloranhydride is volatile. The resultant - chloranhydride is then converted into vapour, and its composition - is determined; and if we know its composition we are able to - decide that of its corresponding hydrate. Thus, for example, from - the formula of silicon chloride, SiCl_{4}, or of boron chloride, - BCl_{3}, we can judge the composition of their corresponding - hydrates, Si(HO)_{4}, B(HO)_{3}. Having obtained the - chloranhydride RCl or RCl_{_n_}, it is possible by its means to - obtain many other compounds of the same radicle R according to the - equation MX + RCl = MCl + RX. M may be = H, K, Ag, or other metal. - The reaction proceeds thus if M forms a stable compound with - chlorine--for example, silver chloride, hydrochloric acid, and R, - an unstable substance. Hence, a chloranhydride is frequently - employed for the formation of other compounds of a given radicle; - for instance, with ammonia they form amides RNH_{2}, and with - salts ROK, with anhydrides R_{2}O, &c. - -Containing, as they do, chlorine, which easily reacts with hydrogen, -phosphorus pentachloride, trichloride, and oxychloride enter into -reaction with ammonia, and give a series of amide and nitrile compounds -of phosphorus. Thus, for example, when ammonia acts on the oxychloride we -obtain sal-ammoniac (which is afterwards removed by water) and an -orthophosphoric triamide, PO(NH_{2})_{3}, as a white insoluble powder on -which dilute acids and alkalis do not act, but which, when fused with -potassium hydroxide, gives potassium phosphate and ammonia like other -amides. When ignited, the triamide liberates ammonia and forms the -nitrile PON, just as urea, CO(NH_{2})_{2}, gives off ammonia and forms -the nitrile CONH. This nitrile, called _monophosphamide_, PON, naturally -corresponds with metaphosphoric acid, namely, with its ammonium salt. -NH_{4}PO_{3} - H_{2}O = PO_{2}·NH_{2}, an as yet unknown amide, and -PO_{2}·NH_{2} - H_{2}O gives the nitrile PON. This relation is confirmed -by the fact that PON, moistened with water, gives metaphosphoric acid -when ignited. It is the analogue of nitrous oxide, NON. It is a very -stable compound, more so than the preceding.[29] - - [29] The reaction of ammonia on phosphorus pentachloride is more - complex than the preceding. This is readily understood: to the - oxychloride, POCl_{3}, tere corresponds a hydrate PO(OH)_{3}, and - a salt PO(NH_{4}O)_{3}, and consequently also an amide - PO(NH_{2})_{3}, whilst the pentachloride, PCl_{5}, has no - corresponding hydrate P(OH)_{5}, and therefore there is no amide - P(NH_{2})_{5}. The reaction with ammonia will be of two kinds: - either instead of 5 mol. NH_{3}, only 3 mol. NH_{3} or still less - will act; _i.e._ PCl_{2}(NH_{2})_{3}, PCl_{3}(NH_{2})_{2}, &c. are - formed; or else the pentachloride will act like a mixture of - chlorine with the trichloride, and then as the result there will - be obtained the products of the action of chlorine on those amides - which are formed from phosphorus trichloride and ammonia. It would - appear that both kinds of reaction proceed simultaneously, but - both kinds of products are unstable, at all events complex, and in - the result there is obtained a mixture containing sal-ammoniac, - &c. The products of the first kind should react with water, and we - should obtain, for example, PCl_{3}(NH_{2})_{2} + 2H_{2}O = 3HCl - and PO(HO)(NH_{2})_{2}. This substance has not actually been - obtained, but the compound PONH(NH_{2}) derived from it by - elimination of the elements of water is known, and is termed - _diphosphamide_; it is, however, more probable that it is a - nitrile than an amide, because only amides contain the group - NH_{2}. It is a colourless, stable, insoluble powder, which - possibly corresponds with pyrophosphoric acid, more especially - since when heated it evolves ammonia and gives and leaves - phosphoryl nitride, PON--that is, the nitrile of metaphosphoric - acid. The amide corresponding with the pyrophosphate - P_{2}O_{3}(NH_{4}O)_{4} should be P_{2}O_{3}(NH_{2})_{4}, and the - nitriles corresponding to the latter would be - P_{2}O_{2}N(NH_{2})_{3}, P_{2}ON_{2}(NH_{2})_{2}, and - P_{2}N_{3}(NH_{2}). The composition of the first is the same as - that of the above diphosphamide. The third pyrophosphoric nitrile - has a formula P_{2}N_{4}H_{2}, and this is the composition of the - body known as _phospham_, PHN_{2} (in a certain sense this is the - analogue of N_{3}H polymerised, Chapter VI.) Indeed, phospham has - been obtained by heating the products of the action of ammonia on - phosphoric chloride, as an insoluble and alkaline powder, which - gives ammonia and phosphoric acid when subjected to the action of - water. The same substance is obtained by the action of ammonium - chloride on phosphoric chloride (PNCl_{2} is first formed, and - reacts further with ammonia, forming phospham), and by igniting - the mass which is formed by the action of ammonia on phosphorus - trichloride. Formerly the composition of phospham was supposed to - be PHN_{2}, now there is reason to think that its molecular weight - is P_{3}H_{3}N_{6}. - - The above compounds correspond with normal salts, but nitriles and - amides corresponding to acid salts are also possible, and they - will be acids. For example, the amide PO(HO)_{2}(NH_{2}), and its - nitrile, will be either PN(HO)_{2} or PO(HO)(NH), but at all - events of the composition PNH_{2}O_{2}, and having acid - properties. The ammonium salt of this _phosphonitrilic acid_ (it - is called phosphamic acid), PNH(NH_{4})O_{2}, is obtained by the - action of ammonia on phosphoric anhydride, P_{2}O_{5} + 4NH_3 = - H_{2}O + 2PNH(NH_{4})O_{2}. A non-crystalline soluble mass is thus - formed, which is dissolved in a dilute solution of ammonia and - precipitated with barium chloride, and the resultant barium salt - is then decomposed with sulphuric acid, and thus a solution of the - acid of the above composition is obtained. - - It is evident from the theory of the formation of amides and - nitriles (Chapter IX.) that very many compounds of this kind can - correspond with the acids of phosphorus; but as yet only a few are - known. The easy transitions of the ortho-, meta-, and - pyrophosphoric acids, by means of the hydrogen of ammonia, into - the lower acids, and conversely, tend to complicate the study of - this very large class of compounds, and it is rarely that the - nature of a product thus obtained can be judged from its - composition; and this all the more that instances of isomerism and - polymerism, of mixture between water of crystallisation and of - constitution, &c., are here possible. Many data are yet needed to - enable us to form a true judgment as to the composition and - structure of such compounds. As the best proof of this we will - describe the very interesting and most fully investigated compound - of this class, PNCl_{2}, called _chlorophosphamide_, or nitrogen - chlorophosphorite. It is formed in small quantities when the - vapour of phosphoric chloride is passed over ignited sal-ammoniac. - Besson (1892) heated the compound PCl_{5}8NH_{3} (which is easily - and directly formed from PCl_{5} and NH_{3}) under a pressure of - about 50 mm. (of mercury) to 200°, and obtained brilliant crystals - of PNCl_{2}, which melted at 106° (in the residue after the - distillation of sal-ammoniacal phospham). The chlorine in it is - very stable--quite different from that in phosphoric chloride. - Indeed, the resultant substance is not only insoluble in water - (though soluble in alcohol and ether), but it is not even - moistened by it, and distils over, together with steam, without - being decomposed. In a free state it easily crystallises in - colourless prisms, fuses at 114°, boils at 250° (Gladstone, - Wichelhaus), and when fused with potash gives potassium chloride - and the amidonitrile of phosphoric acid. Judging from its formula - and the simplicity of its composition and reactions, it might be - thought that the molecular weight of this substance would be - expressed by the formula PCl_{2}N, that it corresponds with PON - and with PCl_{5} (like POCl_3), with the substitution of Cl_3 by - N, just as in POCl_3 two atoms of chlorine are replaced by oxygen; - but all these surmises are incorrect, because its vapour density - (referred to hydrogen--Gladstone, Wichelhaus) = 182--that is, the - molecular formula must be three times greater, P_{3}N_{3}Cl_{6}. - The polymerisation (tripling) is here of exactly the same kind as - with the nitriles. - -The most important analogue of phosphorus is _arsenic_, the metallic -aspect of which and the general character of its compounds of the types -AsX_{3} and AsX_{5} at once recall the metals. The hydrate of its highest -oxide, arsenic acid (ortho-arsenic acid), H_{3}AsO_{4}, is an oxidising -agent, and gives up a portion of its oxygen to many other substances; -but, nevertheless, it is very like phosphoric acid. Mitscherlich -established the conception of isomorphism by comparing the salts of these -acids.[30] - - [30] It is necessary to remark that, although arsenic is so closely - analogous to phosphorus (especially in the higher forms of - combination, RX_{3} and RX_{5}), at the same time it exhibits a - certain resemblance and even isomorphism with the corresponding - compounds of sulphur (especially the metallic compounds of the - type MAs, corresponding with MS). Thus compounds containing - metals, arsenic, and sulphur are very frequently met with in - nature. Sometimes the relative amounts of arsenic and sulphur - vary, so that an isomorphous substitution between the arsenides - and sulphides must be recognised. Besides FeS_{2} (ordinary - pyrites), and FeAs_{2}, iron forms an arsenical pyrites containing - both sulphur and arsenic, which from its composition, FeAsS or - FeS_{2}FeAs_{2}, resembles the two preceding. - -Arsenic occurs _in nature_, not only combined with metals, but also, -although rarely, native and also in combination with sulphur in two -minerals--one red, _realgar_, As_{2}S_{2}, and the other yellow, -_orpiment_, As_{2}S_{3} (Chapter XX., Note 29). Arsenic occurs, but more -rarely, in the form of salts of arsenic acid--for instance, the so-called -cobalt and nickel blooms, two minerals which are found accompanying other -cobalt ores, are the arsenates of these metals. Arsenic is also found in -certain clays (ochres) and has been discovered in small quantities in -some mineral springs, but it is in general of rarer occurrence in nature -than phosphorus. Arsenic is most frequently extracted from arsenical -pyrites, FeSAs, which, when roasted without access of air, evolves the -vapour of arsenic, ferrous sulphide being left behind. It is also -obtained by heating arsenious anhydride with charcoal, in which case -carbonic oxide is evolved. In general, the oxides and other compounds are -very easily reduced. Solid _arsenic_ is a steel-grey brittle _metal_, -having a bright lustre and scaly structure. Its specific gravity is 5·7. -It is opaque and infusible, but volatilises as a yellow vapour which on -cooling deposits rhombohedral crystals.[30 bis] The vapour density of -arsenic is 150 times greater than that of hydrogen--that is, its -molecule, like that of phosphorus, contains 4 atoms, As_{4}. When heated -in the air, arsenic easily oxidises into white arsenious anhydride, -As_{2}O_{3}, but even at the ordinary temperature it loses its lustre -(becomes dull), owing to the formation of a coating of a lower oxide. The -latter appears to be as volatile as arsenious anhydride, and it is -probable that it is owing to the presence of this compound that the -vapours of arsenious compounds, when heated with charcoal (for example, -in the reducing flame of a blow-pipe), have the characteristic smell of -garlic, because the vapour of arsenic itself has not this odour. - - [30 bis] According to Retgers (1893) the arsenic mirror (see further - on) is an unstable variety of metallic arsenic, whilst the brown - product which is formed together with it in Marsh's apparatus is a - lower hydride AsH. Schuller and McLeod (1894), however, recognise - a peculiar yellow variety of arsenic. - -Arsenic easily combines with bromine and chlorine;[31] nitric acid and -aqua regia also oxidise it into the higher oxide, or rather its hydrate, -arsenic acid.[32] As far as is known, it does not decompose steam, and it -acts exceedingly slowly on those acids, like hydrochloric, which are not -capable of oxidising. - - [31] Hydrochloric acid dissolves arsenious anhydride in considerable - quantities, and this is probably owing to the formation of - unstable compounds in which the arsenious anhydride plays the part - of a base. A compound called _arsenious oxychloride_, having the - composition AsOCl, is even known. It is formed when arsenious - anhydride is added little by little to boiling arsenic - trichloride, As_{2}O_{3} + AsCl_{3} = 3AsOCl. It is a transparent - substance, which fumes in air, and combines with water to form a - crystalline mass having the composition As_{2}(OH)_{4}Cl_{2}. When - heated it decomposes into arsenious chloride and a fresh - oxychloride of a more complex composition, As_{6}O_{8}Cl_{2}· - Arsenic trichloride, when treated with a small quantity of water, - forms the crystalline compound, As_{2}(HO)_{4}Cl_{2}, mentioned - above. These compounds resemble the basic salts of bismuth and - aluminium. The existence of these compounds shows that arsenic is - of a more metallic or basic character than phosphorus. - Nevertheless _arsenic trichloride_, AsCl_{3}, resembles phosphorus - trichloride in many respects. It is obtained by the direct action - of chlorine on arsenic, or by distilling a mixture of common salt, - sulphuric acid, and arsenious anhydride. The latter mode of - preparation already indicates the basic properties of the oxide. - Arsenious chloride is a colourless oily liquid, boiling at 130°, - and having a sp. gr. of 2·20. It fumes in air like other - chloranhydrides, but it is much more slowly and imperfectly - decomposed by water than phosphorus trichloride. A considerable - quantity of water is required for its complete decomposition into - hydrochloric acid and arsenious anhydride. It forms an excellent - example of the transition from true metallic chlorides to true - chloranhydrides of the acids. It hardly combines with chlorine, - _i.e._ if AsCl_{5} is formed it is very unstable. _Arsenic - tribromide_, AsBr_{3}, is formed as a crystalline substance, - fusing at 20° and boiling at 220°, by the direct action of - metallic arsenic on a solution of bromine in carbon bisulphide, - the latter being then evaporated. The specific gravity of arsenic - tribromide is 3·36. Crystalline arsenic tri-iodide, AsI_{3}, - having a sp. gr. 4·39, may be obtained in a like manner; it may be - dissolved in water, and on evaporation separates out from the - solution in an anhydrous state--that is, it is not decomposed--and - consequently behaves like metallic salts. _Arsenic trifluoride_, - AsF_{3}, is obtained by heating fluor spar and arsenious anhydride - with sulphuric acid. It is a fuming, colourless, and very - poisonous liquid, which boils at 63° and has a sp. gr. of 2·73. It - is decomposed by water. It is very remarkable that fluorine forms - a pentafluoride of arsenic also, although this compound has not - yet been obtained in a separate state, but only in combination - with potassium fluoride. This compound, K_{3}AsF_{8}, is formed as - prismatic crystals when potassium arsenate, K_{3}AsO_{4}, is - dissolved in hydrofluoric acid. - - [32] _Arsenic acid_, H_{3}AsO_{4}, corresponding with orthophosphoric - acid, is formed by oxidising arsenious anhydride with nitric acid, - and evaporating the resultant solution until it attains a sp. gr. - of 2·2; on cooling it separates in crystals having the above - composition. This hydrate corresponds with the normal salts of - arsenic acid; but on dissolving in water (without heating), and on - cooling a strong solution, crystals containing a greater amount of - water, namely, (AsH_{3}O_{4})_{2},H_{2}O, separate. This water, - like water of crystallisation, is very easily expelled at 100°. At - 120° crystals having a composition identical with that of - pyrophosphoric acid, As_{2}H_{4}O_{7}, separate, but water, on - dissolving this hydrate with the development of heat, forms a - solution in no way differing from a solution of ordinary arsenic - acid, so that it is not an independent pyroarsenic acid that is - formed. Neither is there any true analogue of metaphosphoric acid, - although the compound AsHO_{3} is formed at 200°, and on - solidifying forms a mass having a pearly lustre and sparingly - soluble in cold water; but on coming into contact with warm water - it becomes very hot, and gives ordinary orthoarsenic acid in - solution. Arsenic acid forms three series of salts, which are - perfectly analogous to the three series of orthophosphates. Thus - the normal salt, K_{3}AsO_{4}, is formed by fusing the other - potassium arsenates with potassium carbonate; it is soluble in - water and crystallises in needles which do not contain water. - Di-potassium arsenate, K_{2}HAsO_{4}, is formed in solution by - mixing potassium carbonate and arsenic acid until carbonic - anhydride ceases to be evolved; it does not crystallise, and has - an alkaline reaction; hence it corresponds perfectly with the - sodium phosphate. As was mentioned above, arsenic acid itself acts - as an oxidising agent; for example, it is used in the manufacture - of aniline dyes for oxidising the aniline, and it is prepared in - large quantities for this purpose. When sulphuretted hydrogen is - passed through its solution, sulphuric acid and arsenious - anhydride are obtained in solution. Arsenic acid is very easily - soluble in water, and its solution has an exceedingly acid - reaction, and when boiled with hydrochloric acid evolves chlorine, - like selenic, chromic, manganic, and certain other higher metallic - acids. - - _Arsenic anhydride_, As_{2}O_{5}, is produced when arsenic acid is - heated to redness. It must be carefully heated, as at a bright red - heat it decomposes into oxygen and arsenious anhydride. Arsenic - anhydride is an amorphous substance almost entirely insoluble in - water, but it attracts moisture from the air, deliquesces, and - passes into the acid. Hot water produces this transformation with - great ease. - -_Arseniuretted hydrogen_, _arsine_, AsH_{3}, resembles phosphuretted -hydrogen in many respects. This colourless gas, which liquefies into a -mobile liquid at -40°, has a disagreeable garlic-like odour, is only -slightly soluble in water, and is exceedingly poisonous. Even in a small -quantity it causes great suffering, and if present to any considerable -amount in air it even causes death. The other compounds of arsenic are -also poisonous, with the exception of the insoluble sulphur compound and -some compounds of arsenic acid. Arseniuretted hydrogen, AsH_{3}, is -obtained by the action of water on the alloy of arsenic and sodium, -sodium hydroxide and arseniuretted hydrogen being formed. It is also -formed by the action of sulphuric acid on the alloy of arsenic and zinc: -Zn_{3}As_{2} + 3H_{2}SO_{4} = 2AsH_{3} + 3ZnSO_{4}.[33] The oxygen -compounds of arsenic are very easily reduced by the action of hydrogen at -the moment of its evolution from acids, and the reduced arsenic then -combines with the hydrogen; hence, if a certain amount of an oxygen -compound of arsenic be put into an apparatus containing zinc and -sulphuric acid (and thus serving for the evolution of hydrogen), the -hydrogen evolved will contain arseniuretted hydrogen. In this case it is -diluted with a considerable amount of hydrogen. But its presence in the -most minute quantities may be easily recognised from the fact that it is -_easily decomposed_ by heat (200° according to Brunn) into metallic -arsenic and hydrogen, and therefore if such impure hydrogen he passed -through a moderately-heated tube metallic arsenic will be deposited as a -bright layer on the part of the tube which was heated (_see_ Note 30 -bis). This reaction is so sensitive that it enables the most minute -traces of arsenic to be discovered; hence it is employed in medical -jurisprudence, as a test in poisoning cases. It is easy to discover the -presence of arsenic in common zinc, copper, sulphuric and hydrochloric -acids, _&c._ by this method. It is obvious that in testing for poison by -Marsh's apparatus it is necessary to take zinc and sulphuric acid quite -free from arsenic. The arsenic deposited in the tube may be driven as a -volatile metal from one place to another in the current of hydrogen -evolved, owing to its volatility. This forms a distinction between -arseniuretted and antimoniuretted hydrogen, which is decomposed by heat -in just the same way as arseniuretted hydrogen, but the mirror given by -Sb is not so volatile as that formed by As. - -[Illustration: FIG. 84.--Formation and decomposition of arseniuretted -hydrogen. Hydrogen is evolved in the Woulfe's bottle, and when the gas -comes off, a solution containing arsenic is poured through the funnel. -The presence of AsH_{3} is recognised from the deposition of a mirror of -arsenic when the gas-conducting tube is heated. If the escaping hydrogen -be lighted, and a porcelain dish be held in the flame, a film of arsenic -is deposited on it. The gas is dried by passing through the tube -containing calcium chloride. This apparatus is used for the detection of -arsenic by Marsh's test.] - - [33] The formation of arseniuretted hydrogen is accompanied by the - absorption of 37,000 heat units, while phosphine evolves 18,000 - (Ogier), and ammonia 27,000. Sodium (0·6 p.c.) amalgam, with a - strong solution of As_{2}O_{3}, gives a gas containing 86 vols. of - arsenic and 14 vols. of hydrogen (Cavazzi). - -If hydrogen contains arseniuretted hydrogen, it also gives metallic -arsenic when it burns, because in the reducing flame of hydrogen the -oxygen attracted combines entirely with the hydrogen and not with the -arsenic, so that if a cold object, such as a piece of china, be held in -the hydrogen flame the arsenic will be deposited upon it as a metallic -spot.[34] - - [34] This spot, or the metallic ring which is deposited on the heated - tube, may easily be tested as to whether it is really due to - arsenic or proceeds from some other substance reduced in the - hydrogen flame--for instance, carbon or antimony. The necessity - for distinguishing arsenic from antimony is all the more - frequently encountered in medical jurisprudence, from the fact - that preparations of antimony are very frequently used as - medicine, and antimony behaves in the hydrogen apparatus just like - arsenic, and therefore in making an investigation for poisoning by - arsenic it is easy to mistake it for antimony. The best method to - distinguish between the metallic spots of arsenic and antimony is - to test them with a solution of sodium hypochlorite, free from - chlorine, because this will dissolve arsenic and not antimony. - Such a solution is easily obtained by the double decomposition of - solutions of sodium carbonate and bleaching powder. A solution of - potassium chlorate acts in the same manner, only more slowly. - Further particulars must be looked for in analytical works. - - Arseniuretted hydrogen, like phosphuretted hydrogen, is only - slightly soluble in water, has no alkaline properties--that is, it - does not combine with acids--and acts as a reducing agent. When - passed into a solution of silver nitrate it gives a blackish brown - precipitate of metallic silver, the arsenic being oxidised. If - acting on copper sulphate and similar salts, arseniuretted - hydrogen sometimes forms arsenides--_i.e._ it reduces the metallic - salt with its hydrogen, and is itself reduced to arsenic. - Sulphuric, and even hydrochloric, acid reduces arseniuretted - hydrogen to arsenic, and it is still more easily decomposed by - arsenious chloride, and with phosphorous chloride it gives the - compound PAs. Arseniuretted hydrogen gives metallic arsenic with - an acid solution of arsenious anhydride (Tivoli). - -The most common compound of arsenic is the solid and volatile _arsenious -anhydride_, As_{2}O_{3}, which corresponds with phosphorous and nitrous -anhydrides. This very poisonous, colourless, and sweet-tasting substance -is generally known under the name of arsenic, or _white arsenic_. The -corresponding hydrate is as yet unknown; its solutions, when evaporated, -yield crystals of arsenious anhydride. It is chiefly prepared for the -dyer, and is also used as a vermin killer, and sometimes in medicine; it -is a product from which all other compounds of arsenic can be prepared. -It is obtained as a by-product in roasting cobalt and other ores -containing arsenic. Arsenical pyrites are sometimes purposely roasted for -the extraction of arsenious anhydride. When arsenical ores are burnt in -the air, the sulphur and arsenic are converted into the oxides -As_{2}O_{3} and SO_{2}. The former is a solid at the ordinary -temperature, and the latter gaseous, and therefore the arsenious -anhydride is deposited as a sublimate in the cooler portion of the flues -through which the vapours escape from the furnace. It collects in -condensing chambers especially constructed in the flues. The deposit is -collected, and after being distilled gives arsenious anhydride in the -form of a vitreous non-crystalline mass. This is one of the varieties of -arsenious anhydride, which is also known in two crystalline forms. When -sublimed--_i.e._ when it rapidly passes from the state of vapour to the -solid state--it appears in the regular system in the form of -octahedra.[35] It is obtained in the same form when it is crystallised -from acid solutions. The specific gravity of the crystals is 3·7. The -other crystalline form (in prisms) belongs to the rhombohedral system, -and is also formed by sublimation when the crystals are deposited on a -heated surface, or when it is crystallised from alkaline solutions.[36] - - [35] According to Mitscherlich's determination, the vapour density of - arsenious anhydride is 199 (H = 1)--that is, it answers to the - molecular formula As_{4}O_{6}. Probably this is connected with the - fact that the molecule of free arsenic contains As_{4}. V. Meyer - and Biltz, however, showed (1889) that at a temperature of about - 1,700° the vapour density of arsenic corresponds with the molecule - As_{2}, and not As_{4}, as at lower temperatures. - - [36] Arsenious anhydride is obtained in an amorphous form after - prolonged heating at a temperature near to that at which it - volatilises, or, better still, by heating it in a closed vessel. - It then fuses to a colourless liquid, which on cooling forms a - transparent vitreous mass, whose specific gravity is only slightly - less than that of the crystalline anhydride. On cooling, this - vitreous mass undergoes an internal change, in which it - crystallises and becomes opaque, and acquires the appearance of - porcelain. The following difference between the vitreous and - opaque varieties is very remarkable: when the vitreous variety is - dissolved in strong and hot hydrochloric acid it gives crystals of - the anhydride on cooling, and this crystallisation _is accompanied - by the emission of light_ (which is visible in the dark), and the - entire liquid glows as the crystals begin to separate. The opaque - variety does not emit light when the crystals separate from its - hydrochloric acid solution. It is also remarkable that the - vitreous variety passes into the opaque form when it is - pounded--that is, under the action of a series of blows. Thus, - several varieties of arsenious anhydride are known, but as yet - they are not characterised by any special chemical distinctions, - and even differ but little in their specific gravities, so that it - cannot be said that the above differences are due to any isomeric - transformation--that is, to an arrangement of the atoms in the - molecule--but probably only depend on a difference in the - distribution of the molecules, or, in other terms, are physical - and not chemical variations. One part of the vitreous anhydride - requires twelve parts of boiling water for its solution, or - twenty-five parts at the ordinary temperature. The opaque variety - is less soluble, and at the ordinary temperature requires about - seventy parts of water for its solution. - -Solutions of arsenious anhydride have a sweet metallic taste, and give -_a feeble acid reaction_. Its solubility increases with the admixture of -acids and alkalis. This shows the property of arsenious anhydride of -forming salts with acids and alkalis. And in fact compounds of it with -hydrochloric acid (Note 31), sulphuric anhydride (_see_ further on), and -with the alkali oxides are known.[37] If silver nitrate be added to a -solution of arsenious anhydride, it does not give any precipitate unless -a certain amount of the arsenious anhydride is saturated with an -alkali--for instance, ammonia. It then gives a precipitate of silver -arsenite, Ag_{3}AsO_{3}. This is yellow, soluble in an excess of ammonia, -and anhydrous; it distinctly shows that arsenious acid is tribasic, and -that it differs in this respect from phosphorous acid, in which only two -atoms of hydrogen can be replaced by metals.[38] The feeble acid -character of arsenious anhydride is confirmed by the formation of saline -compounds with acids. In this respect the most remarkable example is the -anhydrous compound with sulphuric acid, having the composition -As_{2}O_{3},SO_{3}. It is formed in the roasting of arsenical pyrites in -those spaces where the arsenious anhydride condenses, a portion of the -sulphurous anhydride being converted into sulphuric anhydride, SO_{3}, at -the expense of the oxygen of the air. The compound in question forms -colourless tabular crystals, which are decomposed by water with formation -of sulphuric acid and arsenious anhydride.[39] - - [37] Arsenious anhydride does not oxidise in air, either in a dry state - or in solution, but in the presence of alkalis it absorbs oxygen - from the air, and acts as an excellent reducing agent. This - probably is connected with the fact that arsenic acid is much more - energetic than arsenious acid, and that it is arsenic acid which - is formed by the oxidation of the latter in the presence of - alkalis. Arsenious anhydride is easily reduced to arsenic by many - metals, even by copper. - - [38] The feebleness of the acid properties of arsenious anhydride is - seen in the fact that if it be dissolved in ammonia water, and - then a still stronger solution of ammonia be added, prismatic - crystals separate having the composition of ammonium metarsenite, - NH_{4}AsO_{3}. This ammonium salt deliquesces in air, and loses - all its ammonia. The magnesium salt is tri-metallic, - Mg_{3}(AsO_{3})_{2}; it is insoluble in water, and is formed by - mixing an ammoniacal solution of arsenious anhydride with an - ammoniacal solution of a magnesium salt. It is insoluble even in - ammonia, although it dissolves in an excess of acids. Magnesium - hydroxide gives the same salt with arsenious solutions, and hence - magnesia is one of the best antidotes for arsenic poisoning. _The - arsenites of copper_ are much used in the manufacture of colours, - more especially of pigments. They are distinguished by their - insolubility in water and by their remarkably vivid green colour, - but at the same time by their poisonous character. Not only do - such pigments applied to wall papers or other materials easily - dust off from them, but they give exhalations containing AsH_{3}. - The cupric salts, CuX_{2}, when mixed with an alkaline solution of - arsenious acid, give a green precipitate of a copper salt called - _Scheele's green_. Its composition is probably CuHAsO_{3}. Ammonia - dissolves it, and gives a colourless solution, containing cuprous - arsenate--that is, the cupric compound is reduced and the arsenic - subjected to a further oxidation. The so-called _Schweinfurt - green_ was still more used, especially in former times; it is an - insoluble green cupric salt, which resembles the preceding in many - respects, but has a different tint. It is prepared by mixing - boiling solutions of arsenious acid and cupric acetate. Arsenious - acid forms an insoluble compound with ferric hydroxide, resembling - the phosphate; and this is the reason why freshly precipitated - oxide of iron is employed as an _antidote for arsenic_. The - freshly precipitated oxide of iron, taken immediately after - poisoning by arsenic, converts the arsenious acid into an - insoluble state, by forming a compound on which the acids of the - stomach have no action, so that the poisoning cannot proceed. It - is remarkable that the inhabitants of certain mountainous - countries accustom themselves to taking arsenic, as a means which, - according to their experience, helps to overcome the fatigue of - mountain ascents. Arsenious anhydride and certain of its salts are - also used in medicine, naturally only in small quantities. When - taken internally arsenic passes into the blood, and is mainly - excreted by the urine. - - [39] Adie (1889) obtained compounds of As_{2}O_{3} with 1, 2, 4, and 8 - SO_{3} by the direct action of ordinary and Nordhausen sulphuric - acid upon As_{2}O_{3}. Weber had previously obtained - As_{2}O_{3}SO_{3} (which disengages SO_{3} at 225°), and also - other As_{2}O_{3}_n_SO_{3} (where _n_ = 3, 6, and 8), by the - action of the vapours of SO_{3} upon As_{2}O_{3} at a definite - temperature. The compound As_{2}O_{3},8SO_{3} loses SO_{3} at - 100°. Oxide of antimony, Sb_{2}O_{3}, gives similar compounds. - Adie (1891) also obtained (by the action of SO_{3} upon - H_{3}PO_{4}) a compound H_{3}PO_{4}3SO_{3} in the form of a - viscous liquid decomposed by water. - -_Antimony_ (stibium), Sb = 120, is another analogue of phosphorus. In -its external appearance and the properties of its compounds it resembles -the metals still more closely than arsenic. In fact, antimony has the -appearance, lustre, and many of the characteristic properties of the -metals. Its oxide, Sb_{2}O_{3}, exhibits the earthy appearance of rust or -of lime, and has distinctly basic properties, although it corresponds -with nitrous and phosphorous anhydride, and is able, like them, to give -saline compounds with bases. At the same time antimony presents, in the -majority of its compounds, an entire analogy with phosphorus and arsenic. -Its compounds belong to the type SbX_{3} and SbX_{5}. It is found in -nature chiefly in the form of sulphide, Sb_{2}S_{3}. This substance -sometimes occurs in large masses in mineral veins and is known in -mineralogy under the name of antimony glance or _stibnite_, and -commercially as _antimony_ (Chapter XX., Note 29). The most abundant -deposits of antimony ore occur in Portugal (near Oporto on the Douro). -Besides which antimony partially or totally replaces arsenic in some -minerals; thus, for example, a compound of antimony sulphide and arsenic -sulphide with silver sulphide is found in red silver ore. But in every -case antimony is a rather rare metal found in few localities. In Russia -it is known to occur in Daghestan in the Caucasus. It is extracted -chiefly for the preparation of alloys with lead and tin, which are used -for casting printing type.[40] Some of its compounds are also used in -medicine, the most important in this respect being antimony -pentasulphide, Sb_{2}S_{5} (_sulfur auratum antimonii_), and tartar -emetic, which is a double salt derived from tartaric acid and has the -composition C_{4}H_{4}K(SbO)O_{6}. Even the native antimony sulphide is -used in large quantities as a purgative for horses and dogs. Metallic -antimony is extracted from the glance, Sb_{2}S_{2}, by roasting, when the -sulphur burns away and the antimony oxidises, forming the oxide -Sb_{2}O_{3}, which is then heated with charcoal, and thus reduced to a -_metallic state_. The reduction may be carried on in the laboratory on a -small scale by fusing the sulphide with iron which takes up the -sulphur.[40 bis] - - [40] Printers' type consists of an alloy known as 'type-metal,' - containing usually about 15 parts of antimony to 85 parts of lead; - sometimes (for example, for stereotypes) from 10 to 15 per cent. - Bi or 8 per cent. Sn and even Cu is added. The hardness of the - alloy, which is essential for printing, evidently depends upon the - presence of antimony, but an excess must be avoided, since this - renders the alloy brittle, and the type after a time loses its - sharpness. - - [40 bis] Antimony is prepared in a state of greater purity by heating - with charcoal the oxide obtained by the action of nitric acid on - the impure commercial metallic antimony. This is based on the fact - that by the action of the acid, antimony forms the oxide - Sb_{2}O_{3}, which is but slightly soluble in water. The arsenic, - which is nearly always present, forms soluble arsenious and - arsenic acids, and remains in solution. The purest antimony is - easily obtained from tartar emetic, by heating it with a small - quantity of nitre. Metallic antimony also occurs, although rarely, - native; and as it is very easily obtained, it was known to the - alchemists of the fifteenth century. Very pure metallic antimony - may be deposited by the electric current from a solution of - antimonious sulphide in sodium sulphide after the addition of - sodium chloride to the solution. - -Metallic antimony has a white colour and a brilliant lustre; it remains -untarnished in the air, for the metal does not oxidise at the ordinary -temperature. It crystallises in rhombohedra, and always shows a -distinctly crystalline structure which gives it quite a different aspect -from the majority of the metals yet known. It is most like tellurium in -this respect. Antimony is brittle, so that it is very easily powdered; -its specific gravity is 6·7, it melts at about 432°, but only volatilises -at a bright red heat. When heated in the air--for instance, before the -blow-pipe--it burns and gives white odourless fumes, consisting of the -oxide. This oxide is termed antimonious oxide, although it might as well -be termed antimonious anhydride. It is given the first name because in -the majority of cases its compounds with acids are used, but it forms -compounds with the alkalis just as easily. - -Antimonious oxide, like arsenious anhydride, crystallises either in -regular octahedra or in rhombic prisms; its specific gravity is 5·56; -when heated it becomes yellow and then fuses, and when further heated in -air it oxidises, forming an oxide of the composition Sb_{2}O_{4}. -Antimonious oxide is insoluble in water and in nitric acid, but it easily -dissolves in strong hydrochloric acid and in alkalis, as well as in -tartaric acid or solutions of its acid salts. When dissolved in the -latter it forms tartar emetic. It is precipitated from its solutions in -alkalis and acids (by the action of acids on the former and alkalis on -the latter). It occurs native but rarely. As a base it gives salts of the -type SbOX (as if the basic salts = SbX_{3}, Sb_{2}O_{3}) and hardly ever -forms salts, SbX_{3}. In the antimonyl salts, SbOX, the group SbO is -univalent, like potassium or silver. The oxide itself is (SbO)_{2}O, the -hydroxide, SbO(OH), &c.; tartar emetic is a salt in which one hydrogen of -tartaric acid is replaced by potassium and the other by antimonyl, SbO. -Antimonious oxide is very easily separated from its salts by any base, -but it must be observed that this separation does not take place in the -presence of tartaric acid, owing to the property of tartaric acid of -forming a soluble double salt--_i.e._ tartar emetic.[41] - - [41] As antimonious oxide answers to the type SbX_{3}, it is evident - that compounds may exist in which antimony will replace three - atoms of hydrogen; such compounds have been to some extent - obtained, but they are easily converted by water into substances - corresponding with the ordinary formulæ of the compounds of - antimony. Thus tartar emetic, C_{4}H_{4}(SbO)KO_{6}, loses water - when heated, and forms C_{4}H_{2}SbKO_{6}--that is, tartaric acid, - C_{2}H_{6}O_{6}, in which one atom of hydrogen is replaced by - potassium and three by antimony. But this substance is reconverted - into tartar emetic by the action of water. - - A similar compound is seen in that _intermediate oxide of - antimony_ which is formed when antimonious oxide is heated in air: - its composition is SbO_{2} or Sb_{2}O_{4}. This oxide may be - regarded as orthantimonic acid, SbO(HO)_{3}, in which three atoms - of hydrogen are replaced by antimony in that state in which it - occurs in oxide of antimony--_i.e._ SbO(SbO_{3}) = Sb_{2}O_{4}. - Oxide of antimony is also formed when antimonic acid is ignited; - it then loses water and oxygen, and gives this intermediate oxide - as a white infusible powder, of sp. gr. 6·7. It is somewhat - soluble in water, and gives a solution which turns litmus paper - red. - -If metallic antimony, or antimonious oxide, be oxidised by an excess of -nitric acid and the resultant mass be carefully evaporated to dryness, -_metantimonic acid_, SbHO_{3}, is formed. Its corresponding potassium -salt, 2SbKO_{3},5H_{2}O, is prepared by fusing metallic antimony with -one-fourth its weight of nitre and washing the resultant mass with cold -water. This potassium salt is only slightly soluble in water (in 50 -parts) and the sodium salt is still less so. An ortho-acid, SbH_{3}O_{4}, -also appears to exist;[41 bis] it is obtained by the action of water on -antimony pentachloride, but it is very unstable, like the pentachloride, -SbCl_{5}, itself, which easily gives up Cl_{2}, leaving antimony -trichloride, SbCl_{3}, and this is decomposed by water, forming an -oxychloride--SbOCl, only slightly soluble in water. When antimonic acid -is heated to an incipient red heat, it parts with water and forms the -anhydride, Sb_{2}O_{5}, of a yellow colour and specific gravity 6·5.[42] - - [41 bis] Beilstein and Blaese (1889), after preparing many salts of - antimonic acid, came to the conclusion that it is monobasic, but - all the salts still contain water, so that their general type is - mostly: MSbO_{3}3H_{2}O, for example, M = Li, Hg (salts of the - suboxide), 1/2 Pb, &c. The type of the ortho-salts, M_{2}SbO_{4}, - is quite unknown, although it is reproduced in the thio-compounds, - for instance, Schlippe's salt, Na_{2}SbS_{4}, but this salt also - contains water of crystallisation, 9H_{2}O (Chapter XX., Note 29). - - [42] Among the other compounds of antimony, _antimoniuretted hydrogen_, - SbH_{3}, resembles arseniuretted hydrogen in its mode of formation - and properties (it splits up at 150°, Brunn 1890; when liquified, - it boils at -65° and solidifies at -92°), whilst the halogen - compounds differ in many respects from those of arsenic. When - chlorine is passed over an excess of antimony powder, it forms - _antimony trichloride_, SbCl_{3}, but if the chlorine be in excess - it forms the _pentachloride_, SbCl_{5}. The trichloride is a - crystalline substance which melts at 72° and distils at 230°, - whilst the pentachloride is a yellow liquid, which splits up into - chlorine and the trichloride when heated; at 140° it begins to - give off chlorine abundantly, carrying away the vapour of the - trichloride with it; and at 200° the decomposition is complete, - and pure antimonious chloride only passes over. This property of - antimony pentachloride has caused it to be applied in many cases - for the transference of chlorine; all the more that when it has - given up its chlorine, it leaves the trichloride, which is able to - absorb a fresh amount of chlorine; and therefore many substances - which are unable to react directly with gaseous chlorine do so - with antimony pentachloride, and in the presence of a small - quantity of it chlorine will act on them, just as oxygen is able, - in the presence of nitrogen oxides, to oxidise substances which - could not be oxidised by means of free oxygen. Thus carbon - bisulphide is not acted on by chlorine at low temperatures--this - reaction requires a high temperature--but in the presence of - antimony pentachloride its conversion into carbon tetrachloride - takes place at low temperatures. Antimony tri- and pentachloride, - having the character of chloranhydrides, fume in air, attract - moisture, and are decomposed by water, forming antimonious and - antimonic acids. But in the first action of water the trichloride - does not evolve all its chlorine as hydrochloric acid, which is - intelligible in view of the fact that antimonious anhydride is - also a base, and is therefore able to react with acids; indeed - antimony sulphide dissolved in an excess of hydrochloric acid - (hydrogen sulphide is evolved) gives an aqueous solution of - antimony trichloride, which, when carefully distilled, even gives - the anhydrous compound. Antimony trichloride is only decomposed by - an excess of water, and then not completely, for with a large - quantity of water it forms _powder of algaroth_--_i.e._ antimony - oxychloride. The first action of water consists in the formation - of _oxychloride_, SbOCl--that is, a salt corresponding to oxide of - antimony as a base. If antimony oxide or antimony chloride be - dissolved in an excess of hydrochloric acid, and the solution - diluted with a considerable amount of water, then this same powder - of algaroth is precipitated. The composition varies with the - relative amount of water; namely, between the limits SbOCl and - Sb_{4}O_{5}Cl_{2}. The latter compound is, as it were, a basic - salt of the former, because its composition = 2(SbOCl)Sb_{2}O_{3}. - - With bromine and iodine, antimony forms compounds similar to those - with chlorine. Antimonious bromide, SbBr_{3}, crystallises in - colourless prisms, melts at 94°, and boils at 270°; antimonious - iodide, SbI_{3}, forms red crystals of sp. gr. 5·0; antimony - trifluoride, SbF_{3} separates from a solution of antimonious - oxide in hydrofluoric acid, and SbF_{5} is formed by a similar - treatment of antimonic acid. The latter gives easily-soluble - double salts with the fluorides of the metals of the alkalis. - - De Haën (1887) obtained very stable double soluble salts, - SbF_{3},KCl (100 parts of water dissolve 57 parts of salt), - SbF_{3},K_{2}SO_{4}, &c., which he proposed to make use of in the - arts as very easily crystallisable and soluble salts of antimony. - - Engel, by passing hydrochloric acid gas into a saturated solution - of antimonious chloride at 0°, obtained a compound - HCl,2SbCl_{3},2H_{2}O, and with the pentachloride a compound - SbCl_{5},5HCl,10H_{2}O. Bismuth trichloride, BiCl_{3}, gives a - similar compound. - - Saunders (1892) obtained 5RbCl,3SbCl_{3} and RbCl,SbCl_{3}. Ditte - and Metzner (1892) showed that Sb and Bi dissolve in hydrochloric - acid only owing to the participation of the oxygen of the air or - of that dissolved in the acid. - -The heaviest analogue of nitrogen and phosphorus is _bismuth_, Bi = 208. -Here, as in the other groups, the basic, metallic, properties increase -with the atomic weight. Bismuth does not give any hydrogen compound and -the highest oxide, Bi_{2}O_{5}, is a very feeble acid oxide. Bismuthous -oxide, Bi_{2}O_{3}, is a base, and bismuth itself a perfect metal. To -explain the other properties of bismuth it must further be remarked that -in the eleventh series it follows mercury, thallium and lead, whose -atomic weights are near to that of bismuth, and that therefore it -resembles them and more especially its nearest neighbour, lead. Although -PbO and PbO_{2}, represent types different from Bi_{2}O_{3} and -Bi_{2}O_{5}, they resemble them in many respects, even in their external -appearance, moreover the lower oxides both of Pb and Bi are basic and the -higher acid, which easily evolve oxygen. But judging by the formula, -Bi_{2}O_{3} is a more feeble base than PbO. They both easily give basic -salts. - -Bismuth forms compounds of two types, BiX_{3} and BiX_{5},[43] which -entirely recall the two types we have already established for the -compounds of lead. Just as in the case of lead, the type PbX_{2}, is -basic, stable, easily formed, and passes with difficulty into the higher -and lower types, which are unstable, so also in the case of bismuth the -type of combination BiX_{3} is the usual basic form. The higher type of -combination, BiX_{5},[44] in fact behaves toward this stable type, -BiX_{3}, in exactly the same manner as lead dioxide does to the monoxide; -and bismuthic acid is obtained by the action of chlorine on bismuth oxide -suspended in water, in exactly the same way as lead dioxide is obtained -from lead oxide. It is an oxidising agent like lead dioxide, and even the -acid character in bismuthic acid is only slightly more developed than in -lead dioxide. Here, as in the case of lead (minium), intermediate -compounds are easily formed in which the bismuth of the lower oxide plays -the part of a base combined with the acid which is formed by the higher -form of the oxidation of bismuth. - - [43] Metallic bismuth is very easily obtained when the compounds of the - oxide are reduced by powerful reducing agents, but when less - powerful reducing agents--for example, stannous oxide--are taken, - bismuth suboxide is formed as a black crystalline powder. It is a - compound of the type BiX_{2}, its composition being BiO; it is - decomposed by acids into the metal and oxide, which passes into - solution. - - [44] The type BiX_{5} is represented by the pentoxide, Bi_{2}O_{5}, its - metahydrate, Bi_{2}O_{5},H_{2}O, or BiHO_{3}, known as bismuthic - acid, and the pyrohydrate, Bi_{2}H_{4}O_{7}. _Bismuth pentoxide_ - is obtained by the prolonged passage of chlorine through a boiling - solution of potassium hydroxide (sp. gr. 1·38), containing bismuth - oxide in suspension; the precipitate is washed with water, with - boiling nitric acid (but not for long, as otherwise the bismuthic - acid is decomposed), then again with water, and finally the - resultant bright red powder of the hydrate BiHO_{3} is dried at - 125°. The prolonged action of nitric acid on bismuthic anhydride, - Bi_{2}O_{5}, results in the formation of the compound - Bi_{2}O_{4},H_{2}O, which decomposes in moist air, forming - Bi_{2}O_{3}. The density of bismuthic anhydride is 5·10, of the - tetroxide, Bi_{2}O_{4}, 3·60, and of bismuthic acid, BiHO_{3}, - 5·75. _Pyrobismuthic acid_, Bi_{2}H_{4}O_{7}, forms a brown - powder, which loses a portion of its water at 150°, and decomposes - on further heating, with the evolution of oxygen and water. It is - obtained by the action of potassium cyanide on a solution of - bismuth nitrate. The meta-salts of bismuthic acid are known, for - example KBiO_{3}. They generally occur, however, in combinations - with metabismuthic acid itself. Thus André (1891) took a solution - of the double salt of BiBr_{3} and KBr, treated it with bromine - after adding ammonia, and obtained a red-brown precipitate, which - after being washed (for several weeks) had the composition - KBiO_{3},HBiO_{3} When washed with dilute nitric acid this salt - gave bismuthic acid. - -[Illustration: FIG. 85.--Furnace used for the extraction of bismuth from -its ores.] - -In nature, bismuth occurs in only a few localities and in small -quantities, most frequently in a native state, and more rarely as oxide -and as a compound of bismuth sulphide with the sulphides of other metals, -and sometimes in gold ores. It is extracted from its native ores by -simple fusion in the furnace shown in fig. 85. This furnace contains an -inclined iron retort, into the upper extremity of which the ore is -charged, and the molten _metal_ flows from the lower extremity. It is -refined by re-melting, and the pure metal may be obtained by dissolving -in nitric acid, decomposing the resultant salt with water, and reducing -the precipitate by heating it with charcoal. Bismuth is a metal which -crystallises very well from a molten state. Its specific gravity is 9·8; -it melts at 269°, and if it be melted in a crucible, allowed to cool -slowly, and the crust broken and the remaining molten liquid poured out, -perfect rhombohedral crystals of bismuth are obtained on the sides of the -crucible.[44 bis] It is brittle, has a grey-coloured fracture with a -reddish lustre, is not hard, and is but very slightly ductile and -malleable; it volatilises at a white heat and easily oxidises. It recalls -antimony and lead in many of its properties. When oxidised in air, or -when the nitrate is ignited, bismuth forms the _oxide_, Bi_{2}O_{3}, as a -white powder which fuses when heated and resembles massicot. The addition -of an excess of caustic potash to a solution of a bismuthous salt gives a -white precipitate of the hydroxide, BiO(OH), which loses its water and -gives the anhydrous oxide when boiled with a solution of caustic potash. -Both the hydroxide and oxide easily dissolve in acids and form bismuthous -salts. - - [44 bis] Hérard (1889) obtained a peculiar variety of bismuth by - heating pure crystalline bismuth to a bright red heat in a stream - of nitrogen. A greenish vapour was deposited in the cooler - portions of the apparatus in the form of a grey powder, which - under the microscope had the appearance of minute globules. An - atmosphere of nitrogen is necessary for this transformation, other - gases such as hydrogen and carbonic oxide do not favour the - transition. The resultant amorphous bismuth fuses at 410° (the - crystalline variety at 269°), sp. gr. 9·483. (Does it not contain - a nitride?) - -_Bismuthous oxide_, Bi_{2}O_{3}, is a feeble and unenergetic base. The -normal hydroxide of the oxide Bi_{2}O_{3} is Bi(OH)_{3}; it parts with -water and forms a metahydroxide (bismuthyl hydroxide), BiO(OH). Both of -these hydroxides have their corresponding saline compounds of the -composition BiX_{3} and BiOX. And the form BiOX is nothing else but the -type of the basic salt, because 3ROX = RX + R_{2}O_{3}. It is evident -that in the type BiX_{3} the bismuth replaces three atoms of hydrogen. -And indeed with phosphoric acid solutions of the bismuthous salts give a -precipitate of the composition BiPO_{4}. On the other hand, in the form -of compounds BiOX or Bi(OH)_{2}X, the univalent group (BiO) or -(BiH_{2}O_{2}) is combined with X. Many bismuth salts are formed -according to the type BiOX. For instance the carbonate, (BiO)_{2}CO_{3}, -which corresponds with the other carbonates M_{2}CO_{3}. It is obtained -as a white precipitate when a solution of sodium carbonate is added to a -solution of a bismuth salt.[45] The compound radicle BiO is not a special -natural grouping, as it was formerly represented to be; it is simply a -mode of expression for showing the relation between the compound in -question and the compounds of other oxides. - - [45] Basic bismuth carbonate is employed for whitening the skin - (veloutine, &c.) - -Three _salts of nitric acid_ are known containing bismuthous oxide. If -metallic bismuth or its oxide be dissolved in nitric acid, it forms a -colourless transparent solution containing a salt which separates in -large transparent crystals containing Bi(NO_{3})_{3},5H_{2}O. When heated -at 80° these crystals melt in their water of crystallisation, and in so -doing lose a portion of their nitric acid together with water, forming a -salt whose empirical formula is Bi_{2}N_{2}H_{2}O_{9}. If the preceding -salt belongs to the type BiX_{3}, this one should belong to the form -BiOX, because it = BiO(NO_{3}) + Bi(H_{2}O_{2})(NO_{3}). This salt may be -heated to 150° without change. When the first colourless crystalline salt -dissolves in water _it is decomposed_. There is no decomposition if an -excess of acid be added to the water--that is to say, the salt is able to -exist in an acid solution without decomposing, without separation of the -so-called basic salt--but by itself it cannot be kept in solution; water -decomposes this salt, acting on it like an alkali. In other words the -basic properties of bismuthic oxide are so feeble that even water acts by -taking up a portion of the acid from it. Here we see one of the most -striking facts, long since observed, confirming that action of water on -salts about which we have spoken in Chapter X. and elsewhere. This action -on water may be expressed thus:--BiX_{3} + 2H_{2}O = Bi(OH)_{2}X + 2XH. A -salt of the type Bi(OH)_{2}X is obtained in the precipitate. But if the -quantity of acid, HX, be increased, the salt BiX_{3} is again formed and -passes into solution. The quantity of the salt BiOX which passes into -solution on the addition of a given quantity of acid depends indisputably -on the amount (mass) of water (Muir). The solution, which is perfectly -transparent with a small amount of water, becomes cloudy and deposits the -salt of the type BiOX, when diluted. The white flaky precipitate of -Bi(OH)_{2}NO_{3} formed from the normal salt Bi(NO_{3})_{3} by mixing it -with five parts of water, and in general with a small amount of water, is -used in medicine under the name of magistery of bismuth.[46] - - [46] With an excess of water a further quantity of acid is separated - and a still more basic salt formed. The ultimate product, on which - an excess of water has apparently no action whatever, is a - substance having the composition BiO(NO_{3}).BiO(OH). In the - latter salt we see the limit of change, and this limit appears to - show that the type of the saline compounds of bismuthic oxide is - of the form Bi_{2}X_{6}, and not BiX_{3}; but it is very probable, - on the basis of the examples which we considered in the case of - lead, that this type should be still further polymerised in order - to give a correct idea of the type of the bismuthous compounds. If - we refer all the bismuthous compounds to this type, Bi_{2}X_{6}, - we shall obtain the following expression for the composition of - the nitrates: normal salt, Bi_{2}(NO_{3})_{6}, first basic salt, - Bi_{2}O(OH)_{2}(NO_{3})_{2}, magistery of bismuth, - Bi_{2}(OH)_{4}(NO_{3})_{2}, and the limiting form - Bi_{2}O_{2}(OH)(NO_{3}). - - The general character of bismuthous oxide in its compounds is well - exemplified in the nitrate; bismuthous chloride, BiCl_{3}, which - is obtained by heating bismuth in chlorine, or by dissolving it in - aqua regia, and then distilling without access of air, is also - decomposed by water in exactly the same manner, and forms basic - salts--for instance, first, BiOCl, like the above salt of nitric - acid. Bismuth chloride boils at 447° and probably its formula is - BiCl_{3}. Polymerisation may take place in some compounds and not - in others. A volatile compound of the composition - Bi(C_{2}H_{5})_{3} is also known as a liquid which is insoluble in - water and decomposes with explosion when heated at 130°. Double - salts containing chloride of bismuth are: 2(KCl)BiCl_{3}2H_{2}O - (from a solution of Bi_{2}O_{3} and KCl in hydrochloric acid) and - KClBiCl_{3}H_{2}O. Bigham (1892) also obtained KBr(SO_{4})_{2} in - tabular crystals by treating the above-named double salt with - strong sulphuric acid. The composition of this salt recalls that - of alum. - -Metallic bismuth is used in the preparation of fusible alloys. The -addition of bismuth to many metals renders them very hard, and at the -same time generally lowers their melting point to a considerable extent. -Thus Wood's metal, which contains one part of cadmium, one part of tin, -two parts of lead, and four parts of bismuth, fuses at about 60°, and in -general many alloys composed of bismuth, tin, lead, and antimony melt -below or about the boiling point of water.[47] - - [47] As the metals contained in alloys like the above (bismuth, lead, - tin, cadmium) are difficultly volatile and their alloys are - fusible, they may be employed in the place of mercury in many - physical experiments conducted at or above 70°, and they offer the - advantage that they do not give any vapour having an appreciable - tension (mercury at 100°, 0·75 mm.) Bismuth expands in passing - into a molten state, but it has a temperature of maximum density. - According to Luedeking the mean coefficient of expansion of liquid - bismuth is 0·0000442 (between 270° and 303°), and of solid bismuth - 0·0000411. - -Just as in group II., side by side with the elements zinc, cadmium, and -mercury in the uneven series, we found calcium, strontium, and barium in -the even series; and as in group IV., parallel to silicon, germanium, -tin, and lead, we noticed thallium, zirconium, cerium, and thorium; so -also in group V. we find, beside those elements of the uneven series just -considered by us, a series of analogues in the even series, which, with a -certain degree of similarity (mainly quantitative, or relative to the -atomic weights), also present a series of particular (qualitative) -independent points of distinction. In the even series are known -_vanadium_, which stands between titanium and chromium, _niobium_, -between zirconium and molybdenum, and _tantalum_, situated near tungsten -(an element of group VI. like chromium and molybdenum). Just as bismuth -is similar in many respects to its neighbour lead, so also do these -neighbouring elements resemble each other, even in their external -appearance, not to mention the quality of their compounds, naturally -taking into account the differences of type corresponding with the -different groups. The occurrence in group V. determines the type of the -oxides, R_{2}O_{3} and R_{2}O_{5}, and the development of an acid -character in the higher oxides. The occurrence in the even series -determines the absence of volatile compounds, RH_{3}, for these metals, -and a more basic character of the oxides of a given composition than in -the uneven series, &c.[48] Vanadium, niobium, and tantalum belong to the -category of rare metals, and are exceedingly difficult to obtain pure, -more especially owing to their similarity to, and occurrence with, -chromium, tungsten and other metals, and also in combination among -themselves; therefore it is natural that they have been far from -completely studied, although since 1860 chemists have devoted not a -little time to their investigation. The researches carried out by -Marignac, at Geneva, on niobium, and by Sir Henry Roscoe, at Manchester, -on vanadium deserve special attention. The undoubted external resemblance -of the compounds of chromium and vanadium, as well as the want of -completeness in the knowledge of the compounds of vanadium, long caused -its oxides to be considered analogous in atomic composition to those -formed by chromium. The higher oxide of vanadium was therefore supposed -to have the formula VO_{3}. But the fact of the matter is, that the -chemical analogy of the elements does not hold in one direction only; -vanadium is at one and the same time the analogue of chromium, and -consequently of the elements like sulphur of group VI, and also the -analogue of phosphorus, arsenic, and antimony; just as bismuth stands in -respect to lead and antimony. Investigation has shown that the compounds -of vanadium are always accompanied by those of phosphorus as well as of -iron, and that it is even more difficult to separate it from the -compounds of phosphorus than from those of iron and tungsten. We should -have to extend our description considerably if we wished to give the -complete history, even of vanadium alone, not to mention niobium and -tantalum, all the more as questions would not unfrequently arise -concerning the compounds of these elements which have not yet been fully -elucidated. We shall therefore limit ourselves to pointing out the most -important features in the history of these elements, the more so since -the minerals themselves in which they occur are exceedingly rare and only -accessible to a few investigators. - - [48] Although, guided by Brauner, who showed that didymium gives a - higher oxide, Di_{2}O_{5}, I place this element in the fifth - group, still I am not certain as to its position, because I - consider that the questions relating to this metal are still far - from being definitely answered. - -An important point in the history of the members of this group is the -circumstance that they form volatile compounds with chlorine, similar to -the compounds of the elements of the phosphorus group, namely, of the -type RX_{5}. The vapour densities of the compounds of this kind were -determined, and served as the most important basis for the explanation of -the atomic composition of these molecules. In this we see the power of -general and fundamental laws, like the law of Avogadro-Gerhardt. An -oxychloride, VOCl_{3}, is known for vanadium, which is the perfect -analogue of phosphorus oxychloride. It was formerly considered to be -vanadium chloride, for just as in the case of uranium (Chapter XXI.), its -lower oxide, VO, was considered to be the metal, because it is -exceedingly difficultly reduced--even potassium does not remove all the -oxygen, besides which it has a metallic appearance, and decomposes acids -like a metal; in a word, it simulates a metal in every respect. _Vanadium -oxychloride_ is obtained by heating the trioxide, V_{2}O_{3}, mixed with -charcoal, in a current of hydrogen; the lower oxide of vanadium is then -formed, and this, when heated in a current of dry chlorine, gives the -oxychloride VOCl_{3} as a reddish liquid which does not act on sodium and -may be purified by distillation over this metal. It fumes in the air, -giving reddish vapours; it reacts on water, forming hydrochloric and -vanadic acids; hence, on the one hand it is very similar to phosphorus -oxychloride, and on the other hand to chromium oxychloride, CrO_{2}Cl_{2} -(Chapter XXI.). It is of a yellow colour, its specific gravity is 1·83, -it boils at 120°, and its vapour density is 86 with respect to hydrogen; -therefore the above formula expresses its molecular weight.[49] - - [49] When the vapours of vanadium oxychloride are heated with zinc in a - closed tube at 400°, they lose a portion of their chlorine and - form a green crystalline mass of sp. gr. 2·88, which is - deliquescent in air and has the composition VOCl_{2}. Only its - vapour density is unknown, and it would be extremely important to - determine whether its molecular composition is that given above, - or whether it corresponds with the formula V_{2}O_{2}Cl_{4}. - Another less volatile oxychloride, VOCl, is formed with it as a - brown insoluble substance, which is, however, soluble in nitric - acid like the preceding. Roscoe obtained a still less chlorinated - substance, namely, (VO)_{2}Cl; but it may only consist of a - mixture of VO and VOCl. At all events, we here find a graduated - series such as is met with in the compounds of very few other - elements. - -_Vanadic anhydride_, V_{2}O_{5}, is obtained either in small quantities -from certain clays where it accompanies the oxides of iron (hence some -sorts of iron contain vanadium) and phosphoric acid, or from the rare -minerals: _volborthite_, CuHVO_{4}, or basic vanadate of copper; -_vanadinite_, PbCl_{2}3Pb_{3}(VO_{4})_{2}; lead vanadate, -Pb_{3}(VO_{4})_{2}, &c. The latter salts are carefully ignited for some -time with one-third of their weight of nitre; the fused mass thus formed -is powdered and boiled in water: the yellow solution obtained contains -potassium vanadate. The solution is neutralised with acid, and barium -chloride added; a meta-salt, Ba(VO_{3})_{2}, is then precipitated as an -almost insoluble white powder, which gives a solution of vanadic acid -when boiled with sulphuric acid. (The precipitate is at first yellow, as -long as it remains amorphous, but it afterwards becomes crystalline and -white.) The solution thus obtained is neutralised with ammonia, which -thus forms ammonium (meta) vanadate, NH_{4}VO_{3}, which, when -evaporated, gives colourless crystals, insoluble in water containing -sal-ammoniac; hence this salt is precipitated by adding solid -sal-ammoniac to the solution. Ammonium vanadate, when ignited, leaves -vanadic acid behind. In this it differs from the corresponding chromium -salt, which is deoxidised into chromium oxide when ignited. In general, -vanadic acid has but a small oxidising action. It is reduced with -difficulty, like phosphoric or sulphuric acid, and in this differs from -arsenic and chromic acids. Vanadic acid, like chromic acid, separates -from its solution as the anhydride V_{2}O_{5}, and not in a hydrous -state. Vanadic anhydride, V_{2}O_{5}, forms a reddish-brown mass, which -easily fuses and re-solidifies into transparent crystals having a violet -lustre (another point of resemblance to chromic acid); it dissolves in -water, forming a yellow solution with a slightly acid reaction.[50] - - [50] Strong acids and alkalis dissolve vanadic anhydride in - considerable quantities, forming yellow solutions. When it is - ignited, especially in a current of hydrogen, it evolves oxygen - and forms the lower oxides; V_{2}O_{4} (acid solutions of a green - colour, like the salts of chromic oxide), V_{2}O_{3}, and the - lowest oxide, VO. The latter is the metallic powder which is - obtained when the vanadium oxychloride is heated in an excess of - hydrogen, and was formerly mistaken for metallic vanadium. When a - solution of vanadic acid is treated with metallic zinc it forms a - blue solution, which seems to contain this oxide. It acts as a - reducing agent (and forms a close analogue to chromous oxide, - CrO). Metallic _vanadium_ can only be obtained from vanadium - chloride which is quite free from oxygen. Moissan (1893) obtained - it by reducing the oxide with carbon in the electric furnace, and - considered it to be most infusible of the metals in the series Pt, - Cr, Mo, U, W, and V (he also obtained a compound of vanadium and - carbon). The specific gravity of this metal is 5·5. It is of a - grey-white colour, is not decomposed by water, and is not oxidised - in air, but burns when strongly heated, and can be fused in a - current of hydrogen (forming perhaps a compound with hydrogen). It - is insoluble in hydrochloric acid, but easily dissolves in nitric - acid, and when fused with caustic soda it forms sodium vanadate. - - As regards the salts of vanadic acid, three different classes are - known; the first correspond with metavanadic acid, VMO_{3} = - M_{2}OV_{2}O_{5}, the second correspond with the dichromates--that - is, have the composition V_{4}M_{2}O_{11}, which is equal to - M_{2}O + 2V_{2}O_{5}--and the third correspond with orthovanadic - acid, VM_{3}O_{4} or 3M_{2}O + V_{2}O_{5}. The latter are formed - when vanadic anhydride is fused with an excess of an alkaline - carbonate. - - Vanadic acid gives the so-called 'complex' acids (which are - considered more fully in Chapter XXI. in speaking of Mo and - W)--_i.e._ acids formed of two acids assimilated into one. Thus - Friedheim (1890) obtained phosphor-vanadic acid, and - Schmitz-Dumont (1890) a similar arseno-vanadic acid. The former is - obtained by heating V_{2}O_{5} with sirupy phosphoric acid. The - resultant golden-yellow tabular crystals have the composition - H_{2}OV_{2}O_{5}P_{2}O_{5}9H_{2}O, and there are corresponding - salts--for example, (NH_{4})_{2}V_{2}O_{5}P_{2}O_{5} with 3 and - 7H_{2}O, &c. These salts cannot be separated by crystallisation, - so that there are 'complexes' of these acids in a whole series of - salts (and also in nature). It may be supposed (Friedheim) that - V_{2}O_{5} here, as it were, plays the part of a base, or that - those acids may be looked upon as double salts. Among the true - double salts of vanadium (Nb and Ta) very many are known among the - fluorides, such as VF_{3}2NH_{4}F, VOF_{2}2NH_{4}F, - VO_{2}F,3NH_{4}F, &c. (Pettersson, Piccini, and Georgi, 1890-92). - - Vanadium was discovered at the beginning of this century by - Del-Rio, and afterwards investigated by Sefström, but it was only - in 1868 that Roscoe established the above formulæ of the vanadic - compounds. - -_Niobium and tantalum_[51] occur as acids in rare minerals, and are -mainly extracted from _tantalite_ and _columbite_, which are found in -Bavaria, Finland, North America, and in the Urals. These minerals are -composed of the ferrous salts of niobic and tantalic acids; they contain -about 15 per cent. of ferrous oxide in isomorphous mixture with manganous -oxide, in combination with various proportions of tantalic and niobic -anhydrides. These minerals are first fused with a considerable amount of -potassium bisulphate, and the fused mass is boiled in water, which -dissolves the ferrous and potassium salts and leaves an insoluble residue -of impure niobic and tantalic acids. This raw product is then treated -with ammonium sulphide, in order to extract the tin and tungsten, which -pass into solution. The residue containing the acids (according to -Marignac) is then treated with hydrofluoric acid, in which it entirely -dissolves, and potassium fluoride is added to the resultant hot solution; -on cooling, a sparingly soluble double fluoride of potassium and tantalum -separates out in fine crystals, while the much more soluble niobium salt -remains in solution. The difference in the solubility of these double -salts in water acidified with hydrofluoric acid (in pure water the -solution becomes cloudy after a certain time) is so great that the -tantalum compound requires 150 parts of water for its solution, and the -niobium compound only 13 parts. The Greenland columbite (specific gravity -5·36) only contains niobic acid, and that from Bodenmais, Bavaria -(specific gravity 6·06) almost equal quantities of tantalic and niobic -acids. Having isolated tantalic and niobic salts, Marignac found that the -relation between the potassium and fluorine in them is very -variable--that is, that there exist various double salts of fluoride of -potassium, and of the fluorides of the metals of this group, but that -with an excess of hydrofluoric acid both the tantalum and niobium -compounds contain seven atoms of fluorine to two of potassium, whence it -must be concluded that the simplest formula for these double salts will -be K_{2}RF_{7} = RF_{5},2KF; that is, that the type of the higher -compounds of niobium and tantalum is RX_{5}, and hence is similar to -phosphoric acid. A chloride, TaCl_{5}, may be obtained from pure tantalic -acid by heating it with charcoal in a current of chlorine. This is a -yellow crystalline substance, which melts at 211°, and boils at 241°; its -vapour density with respect to hydrogen is 180, as would follow from the -formula TaCl_{5}. It is completely decomposed by water into tantalic and -hydrochloric acids. _Niobium pentachloride_ may be prepared in the same -manner; it fuses at 194°, and boils at 240°. When treated with water this -substance gives a solution containing niobic acid, which only separates -out on boiling the solution. Delafontaine and Deville found its vapour -density to be 9·3 (air = 1), as is shown by its formula NbCl_{5}.[52] - - [51] The researches made by Roscoe were preceded by those of Marignac - in 1865, on the _compounds_ of _niobium_ and _tantalum_, to which - were also ascribed different formulæ from those now recognised. - Tantalum was discovered simultaneously with vanadium by Hatchett - and Ekeberg, and was afterwards studied by Rose, who in 1844 - discovered niobium in it. Notwithstanding the numerous researches - of Hermann (in Moscow), Kobell, Rose, and Marignac, still there is - not yet any certainty as to the purity of, and the properties - ascribed to, the compounds of these elements. They are difficult - to separate from each other, and especially from the cerite metals - and titanium, &c., which accompany them. Before the investigations - of Rose the highest oxide of tantalum was supposed to belong to - the type TaX_{6}--that is, its composition was taken as TaO_{3}, - and to the lower oxide was ascribed a formula TaO_{2}. Rose gave - the formula TaO_{2} to the higher oxide, and discovered a new - element called niobium in the substance previously supposed to be - the lower oxide. He even admitted the existence of a third element - occurring together with tantalum and niobium, which he named - pelopium, but he afterwards found that pelopic acid was only - another oxide of niobium, and he considered it probable that the - higher oxide of this element is NbO_{2}, and the lower - Nb_{2}O_{3}. Hermann found that niobic acid which was considered - pure contained a considerable quantity of tantalic acid, and - besides this he admitted the existence of another special metallic - acid, which he called ilmenic acid, after the locality (the Ilmen - mountains of the Urals) of the mineral from which he obtained it. - V. Kobell recognised still another acid, which he called dianic - acid, and these diverse statements were only brought into - agreement in the sixties by Marignac. He first of all indicated an - accurate method for the separation of tantalic and niobic - compounds, which are always obtained in admixture. - - [52] If niobic acid be mixed with a small quantity of charcoal and - ignited in a stream of chlorine, a difficultly-fusible and - difficultly-volatile oxychloride, NbOCl_{3} separates. The vapour - density of this compound with respect to air is 7·5, and this - vapour density perfectly confirms the accuracy of the formulæ - given by Marignac, and indicates the quantitative analogy between - the compounds of niobium and tantalum, and those of phosphorus and - arsenic, and consequently also of vanadium. In their qualitative - relations (as is evident also from the correspondence of the - atomic weights), the compounds of tantalum and niobium exhibit a - great analogy with the compounds of molybdenum and tungsten. Thus - zinc, when acting on acid solutions of tantalic and niobic - compounds, gives a blue coloration, exactly as it does with those - of tungsten and molybdenum (also titanium). These acids form the - same large number of salts as those of tungsten and molybdenum. - The anhydrides of the acids are also insoluble in water, but as - colloids are sometimes held in solution, just like those of - titanic and molybdic acids. Furthermore, niobium is in every - respect the nearest analogue of molybdenum, and tantalum of - tungsten. _Niobium_ is obtained by reducing the double fluoride of - niobium and sodium, with sodium. It is difficult to obtain in a - pure state. It is a metal on which hydrochloric acid acts with - some energy, as also does hydrofluoric acid mixed with nitric - acid, and also a boiling solution of caustic potash. _Tantalum_, - which is obtained in exactly the same way, is a much heavier - metal. It is infusible, and is only acted on by a mixture of - hydrofluoric and nitric acids. Rose in 1868 showed that in the - reduction of the double fluoride, NbF_{5},2KF, by sodium, a - greyish powder is obtained after treating with water. The specific - gravity of this powder is 6·8, and he considers it to be niobium - hydride, NbH. Neither did he obtain metallic niobium when he - reduced with magnesium and aluminium, but an alloy, Al_{3}Nb, - having a sp. gr. of 4·5. - - Niobium, so far as is known, unites in three proportions with - oxygen. NbO, which is formed when NbOF_{3},2KF is reduced by - sodium; NbO_{2}, which is formed by igniting niobic acid in a - stream of hydrogen, and niobic anhydride, Nb_{2}O_{5}, a white - infusible substance, which is insoluble in acids, and has a - specific gravity of 4·5. Tantalic anhydride closely resembles - niobic anhydride, and has a specific gravity of 7·2. _The - tantalates and niobates_ present the type of ortho-salts--for - example, Na_{2}HNbO_{4},6H_{2}O, and also of pyro-salts, such as - K_{3}HNb_{2}O_{7},6H_{2}O, and of meta-salts--for example, - KNbO_{3},2H_{2}O. And, besides these, they give salts of a more - complex type, containing a larger amount of the elements of the - anhydride; thus, for instance, when niobic anhydride is fused with - caustic potash it forms a salt which is soluble in water, and - crystallises in monoclinic prisms, having the composition - K_{8}Nb_{6}O_{19},16H_{2}O. There is a perfectly similar - isomorphous salt of tantalic acid. Tantalite is a salt of the type - of metatantalic acid, Fe(TaO_{3})_{2}. The composition of - Yttrotantalite appears to correspond with orthotantalic acid. - - - - - CHAPTER XX - - SULPHUR, SELENIUM, AND TELLURIUM - - -The acid character of the higher oxides RO_{3} of the elements of group -VI. is still more clearly defined than that of the higher oxides of the -preceding groups, whilst feeble basic properties only appear in the -oxides RO_{3} of the elements of the even series, and then only for those -elements having a high atomic weight--that is, under those two conditions -in which, as a rule, the basic characters increase. Even the lower types -RO_{2} and R_{2}O_{3}, &c., formed by the elements of group VI., are acid -anhydrides in the uneven series, and only those of the elements of the -even series have the properties of peroxides or even of bases. - -_Sulphur_ is the typical representative of group VI., both on account of -the fact that the acid properties of the group are clearly defined in it, -and also because it is more widely distributed in nature than any of the -other elements belonging to this group. As an element of the uneven -series of group VI., sulphur gives H_{2}S, sulphuretted hydrogen, SO_{3}, -sulphuric anhydride, and SO_{2}, sulphurous anhydride. And in all of them -we find acid properties--SO_{3} and SO_{2} are anhydrides of acids, and -H_{2}S is an acid, although a feeble one. As an element sulphur has all -the properties of a true non-metal; it has not a metallic lustre, does -not conduct electricity, is a bad conductor of heat, is transparent, and -combines directly with metals--in short it has all the properties of the -non-metals, like oxygen and chlorine. Furthermore, sulphur exhibits a -great qualitative and quantitative _resemblance to oxygen_, especially in -the fact that, like oxygen, it combines _with two atoms of hydrogen_, and -forms compounds resembling oxides with metals and non-metals. From this -point of view sulphur is bivalent, if the halogens are univalent.[1] The -chemical character of sulphur is expressed by the fact that it forms a -very slightly stable and feebly energetic acid with hydrogen. The salts -corresponding with this acid are the sulphides, just as the oxides -correspond to water and the chlorides to hydrochloric acid. However, as -we shall afterwards see more fully, the sulphides are more analogous to -the former than to the latter. But although combining with metals, like -oxygen, sulphur also forms chemically stable compounds with oxygen, and -this fact impresses a peculiar character on all the relations of this -element.[2] - - [1] The character of sulphur is very clearly defined in the - organo-metallic compounds. Not to dwell on this vast subject, which - belongs to the province of organic chemistry, I think it will be - sufficient for our purpose to compare the physical properties of - the ethyl compounds of mercury, zinc, sulphur and oxygen. The - composition of all of them is expressed by the general formula - (C_{2}H_{5})_{2}R, where R = Hg, Zn, S, or O. They are all - volatile: mercury ethyl, Hg(C_{2}H_{5})_{2}, boils at 159°, its sp. - gr. is 2·444, molecular volume = 106; zinc ethyl boils at 118°, sp. - gr. 1·882, volume 101; ethyl sulphide, S(C_{2}H_{5})_{2}, boils at - 90°, sp. gr. 0·825, volume 107; common ether, or ethyl oxide, - O(C_{2}H_{5})_{2}, boils at 35°, sp. gr. 0·736, volume 101, in - addition to which diethyl itself, (C_{2}H_{5})_{2} = C_{4}H_{10}, - boils about 0°, sp. gr. about 0·62, volume about 94. Thus the - substitution of Hg, S, and O scarcely changes the volume, - notwithstanding the difference of the weights; the physical - influence, if one may so express oneself, of these elements, which - are so very different in their atomic weights, is almost alike. - - [2] Therefore in former times sulphur was known as an amphid element. - Although the analogy between the compounds of sulphur and oxygen - has been recognised from the very birth of modern chemistry (owing, - amongst other things, to the fact that the oxides and sulphides are - the most widely spread metallic ores in nature), still it has only - been clearly expressed by the periodic system, which places both - these elements in group VI. Here, moreover, stands out that - parallelism which exists between SO_{2} and ozone OO_{2}, between - K_{2}SO_{3} and peroxide of potassium K_{2}O_{4} (Volkovitch in - 1893 again drew attention to this parallelism). - -Sulphur belongs to the number of those elements which _are very widely -distributed in nature_, and occurs both free and combined in various -forms. The atmosphere, however, is almost entirely free from compounds of -sulphur, although a certain amount of them should be present, if only -from the fact that sulphurous anhydride is emitted from the earth in -volcanic eruptions, and in the air of cities, where much coal is burnt, -since this always contains FeS_{2}. Sea and river water generally contain -more or less sulphur in the form of sulphates. The beds of gypsum, sodium -sulphate, magnesium sulphate, and the like are formations of undoubtedly -aqueous origin. The sulphates contained in the soil are the source of the -sulphur found in plants, and are indispensable to their growth. Among -vegetable substances, the proteïds always contain from one to two per -cent. of sulphur. From plants the albuminous substances, together with -their sulphur, pass into the animal organism, and therefore the -decomposition of animal matter is accompanied by the odour of -sulphuretted hydrogen, as the product into which the sulphur passes in -the decomposition of the albuminous substances. Thus a rotten egg emits -sulphuretted hydrogen. Sulphur occurs largely in nature, as the various -insoluble sulphides of the metals. Iron, copper, zinc, lead, antimony, -arsenic, &c., occur in nature combined with sulphur. These _sulphides_ -frequently have a metallic lustre, and in the majority of cases occur -crystallised, and also very often several sulphides occur combined or -mixed together in these crystalline compounds. If they are yellow and -have a metallic lustre they are called pyrites. Such are, for example, -copper pyrites, CuFeS_{2}, and iron pyrites, FeS_{2}, which is the -commonest of all. They are all also known as glances or blendes if they -are greyish and have a metallic lustre--for example, zinc blende, lead -glance, PbS, antimony glance, Sb_{2}S_{3}, &c. And, lastly, sulphur -occurs _native_. It occurs in this form in the most recent geological -formations in admixture with limestone and gypsum, and most frequently in -the vicinity of active or extinct volcanoes. As the gases of volcanoes -contain sulphur compounds--namely, sulphuretted hydrogen and sulphurous -anhydride, which by reacting on one another may produce sulphur, which -also frequently appears in the craters of volcanoes as a sublimate--it -might be imagined that the sulphur was of volcanic origin. But on a -nearer acquaintance with its mode of occurrence, and more especially -considering its relation to gypsum, CaSO_{4}, and limestone, the present -general opinion leads to the conclusion that the 'native' sulphur has -been formed by the reduction of the gypsum by organic matter and that its -occurrence is only indirectly connected with volcanic agencies. Near -Tetush, on the Volga, there are beds containing gypsum, sulphur, and -asphalt (mineral tar). In Europe the most important deposits of sulphur -are in the south of Sicily from Catania to Girgenti.[3] There are very -rich deposits of sulphur in Daghestan near Cherkai and Cherkat in Khyut, -near Mount Kanabour-bam, near Petrovsk, and in the Kira Koumski steppes -in the Trans-Caspian provinces, which are able to supply the whole of -Russia with this mineral. Abundant deposits of sulphur have also been -found in Kamtchatka in the neighbourhood of the volcanoes. The method of -separation of the sulphur from its earthy impurities is based on the fact -that sulphur melts when it is heated. The fusion is carried on at the -expense of a portion of the sulphur, which is burnt, so that the -remainder may melt and run from the mass of the earth. This is carried on -in special furnaces called calcaroni, built up of unhewn stone in the -neighbourhood of the mines.[4] - - [3] When in Sicily, I found, near Caltanisetta, a specimen of sulphur - with mineral tar. In the same neighbourhood there are naphtha - springs and mud volcanoes. It may be that these substances have - reduced the sulphur from gypsum. - - The chief proof in favour of the origin of sulphur from gypsum is - that in treating the deposits for the extraction of the sulphur it - is found that the proportion of sulphur to calcium carbonate never - exceeds that which it would be had they both been derived from - calcium sulphate. - - [4] Naturally only those ores of sulphur which contain a considerable - amount of sulphur can be treated by this method. With poor ores it - is necessary to have recourse to distillation or mechanical - treatment in order to separate the sulphur, but its price is so low - that this method in most cases is not profitable. - - The sulphur obtained by the above-described method still contains - some impurities, but it is frequently made use of in this form for - many purposes, and especially in considerable quantities for the - manufacture of sulphuric acid, and for strewing over grapes. For - other purposes, and especially in the preparation of gunpowder, a - purer sulphur is required. Sulphur may be purified by distillation. - The crude sulphur is called _rough_, and the distilled sulphur - _refined_. The arrangement given in fig. 86 is employed for - refining sulphur. The rough sulphur is melted in the boiler _d_, - and as it melts it is run through the tube F into an iron retort B - heated by the naked flame of the furnace. Here the sulphur is - converted into vapour, which passes through a wide tube into the - chamber G, surrounded by stone walls and furnished with a - safety-valve S. - -[Illustration: FIG. 86.--Refining sulphur by sublimation.] - -Sulphur is purified by distillation in special retorts (see fig. 86) by -passing the vapour into a chamber G built of stone. The first portions of -the vapour entering into the condensing chamber are condensed straightway -from the vapour into a solid state, and form a fine powder known as -_flowers of sulphur_.[5] But when the temperature of the receiver attains -the melting point of sulphur, it passes into a liquid state and is cast -into moulds (like sealing wax), and is then known under the name of _roll -sulphur_.[6] - - [5] Flowers of sulphur always contain a certain amount of the oxides of - sulphur. - - [6] Sulphur may be extracted by various other means. It may be - extracted from iron pyrites, FeS_{2}, which is very widely - distributed in nature. From 100 parts of iron pyrites about half - the sulphur contained, namely, about 25 parts, may be extracted by - heating without the access of air, a lower sulphide of iron, which - is more stable under the action of heat, being left behind. Alkali - waste (Chapter XII.), containing calcium sulphide and gypsum, - CaSO_{4}, may be used for the same purpose, but native sulphur is - so cheap that recourse can only be had to these sources when the - calcium sulphide appears as a worthless by-product. The most simple - process for the extraction of sulphur from alkali waste, in a - chemical sense, consists in evolving sulphuretted hydrogen from the - calcium sulphide by the action of hydrochloric acid. The - sulphuretted hydrogen when burnt gives water and sulphurous - anhydride, which reacts on fresh sulphuretted hydrogen with the - separation of sulphur. The combustion of the sulphuretted hydrogen - may be so conducted that a mixture of 2H_{2}S and SO_{2} is - straightway formed, and this mixture will deposit sulphur (Chapter - XII., Note 14). Gossage and Chance treat alkali waste with carbonic - anhydride, and subject the sulphuretted hydrogen evolved to - incomplete combustion (this is best done by passing a mixture of - sulphuretted hydrogen and air, taken in the requisite proportions, - over red-hot ferric oxide), by which means water and the vapour of - sulphur are formed: H_{2}S + O = H_{2}O + S. - -In an uncombined state sulphur exists in _several modifications_, and -forms a good example of the facility with which an alteration of -properties can take place without a change of composition--that is, as -regards the material of a substance. Common sulphur has the well-known -yellow colour. This colour fades as the temperature falls, and at -50° -sulphur is almost colourless. It is very brittle, so that it may be -easily converted into a powder, and it presents a crystalline structure, -which, by the way, shows itself in the unequal expansion of lumps of -sulphur by heat. Hence when a piece of sulphur is heated by the warmth of -the hand, it emits sounds and sometimes cracks, which probably also -depends on the bad heat-conducting power of this substance. It is easily -obtained in a crystalline form by artificial means, because although -insoluble in water it dissolves in carbon bisulphide, and in certain -oils.[7] Solutions of sulphur in carbon bisulphide when evaporated at the -ordinary temperature yield well-formed transparent crystals of sulphur in -the form of rhombic octahedra, in which form it occurs native. The -specific gravity of these crystals is 2·045. Fused sulphur, cast into -moulds and cooled, has, after being kept a long time, a specific gravity -2·066; almost the same as that of the crystalline sulphur of the above -form, which shows that common sulphur is the same as that which -crystallises in octahedra. The specific heat of octahedral sulphur is -0·17; it melts at 114°, and forms a bright yellow mobile liquid. On -further heating, the fused sulphur undergoes an alteration, which we -shall presently describe, first observing that the above octahedral state -of sulphur is its most stable form. Sulphur may be kept at the ordinary -temperature in this form for an indefinite length of time, and many other -modifications of sulphur pass into this form after being left for a -certain time at ordinary temperature. - - [7] One hundred parts of liquid carbon bisulphide, CS_{2}, dissolve - 16·5 parts of sulphur at -11°, 24 parts at 0°, 37 parts at 15°, 46 - parts at 22°, and 181 parts at 55°. The saturated solution boils at - 55°, whilst pure carbon bisulphide boils at 47°. The solution of - sulphur in carbon bisulphide reduces the temperature, just as in - the solution of salts in water. Thus the solution of 20 parts of - sulphur in 50 parts of carbon bisulphide at 22° lowers the - temperature by 5°; 100 parts of benzene, C_{6}H_{6}, dissolves - 0·965 part of sulphur at 26°, and 4·377 parts at 71°; chloroform, - CHCl_{3}, dissolves 1·2 part of sulphur at 22°, and 16·35 parts at - 174°. - -If sulphur be melted and then slightly cooled, so that it forms a crust -on the surface and over the sides of the crucible, while the internal -mass remains liquid, then the sulphur takes another crystalline form as -it solidifies. This may be seen by breaking the crust, and pouring out -the remaining molten sulphur.[8] It is then found that the sides of the -crucible are covered with _prismatic crystals_ of the monoclinic system; -they have a totally different appearance from the above-described -crystals of rhombic sulphur. The prismatic crystals are brown, -transparent, and less dense than the crystals of rhombic sulphur, their -specific gravity being only 1·93, and their melting point higher--about -120°. These crystals of sulphur cannot be kept at the ordinary -temperature, which is indeed evident from the fact that in time they turn -yellow; the specific gravity also changes, and they pass completely into -the ordinary modification. This is accompanied by a considerable -development of heat, so that the temperature of the mass may rise 12°. -Thus sulphur is _dimorphous_--that is, it exists in two crystalline -forms, and in both forms it has independent physical properties. However, -no chemical reactions are known which distinguish the two modifications -of sulphur, just as there are none distinguishing aragonite from -calcspar.[9] - - [8] If the experiment be made in a vessel with a narrow capillary tube, - the sulphur fuses at a lower temperature (occurs, as it were, in a - supersaturated state), and solidifying at 90°, appears in a rhombic - form (Schützenberger). - - [9] If sulphur be cautiously melted in a U tube immersed in a salt - bath, and then gradually cooled, it is possible for all the sulphur - to remain liquid at 100°. It will now be in a state of superfusion; - thus also by careful refrigeration water may be obtained in a - liquid state at -10°, and a lump of ice then causes such water to - form ice, and the temperature rises to 0°. If a prismatic crystal - of sulphur be thrown into one branch of the U tube containing the - liquid sulphur at 100°, and an octahedral crystal be thrown into - the other branch, then, as Gernez showed, the sulphur in each - branch will crystallise in the corresponding form, and both forms - are obtained at the same temperature; therefore it is not the - influence of temperature only which causes the molecules of sulphur - to distribute themselves in one or another form, but also the - influence of the crystalline parts already formed. This phenomenon - is essentially analogous to the phenomena of supersaturated - solutions. - -If molten sulphur be heated to 158° it loses its mobility and becomes -thick and very dark-coloured, so that the crucible in which it is heated -may be inverted without the sulphur running out. When heated above this -temperature the sulphur again becomes liquid, and at 250° it is very -mobile, although it does not acquire its original colour, and at 440° it -boils. These modifications in the properties of sulphur depend not only -on the variations of temperature, but also on a change of structure. If -sulphur, heated to about 350°, be poured in a thin stream into cold -water, it does not solidify into a solid mass, but retains its brown -colour and _remains soft_, may be stretched out into threads, and is -elastic, like guttapercha. But in this soft and ductile state, also, it -does not remain for a long time. After the lapse of a certain period this -soft transparent sulphur hardens, becomes opaque, passes into the -ordinary yellow modification of sulphur, and in so doing develops heat, -just as in the conversion of the prismatic into the octahedral variety. -The soft sulphur is characterised by the fact that a certain portion of -it is insoluble in carbon bisulphide. When soft sulphur is immersed in -this liquid, only a portion of common sulphur passes into solution, -whilst a certain portion is quite insoluble and remains so for a long -time. The maximum proportion of insoluble sulphur is obtained by heating -slightly above 170°. It melts at 114°. An exactly similar _insoluble -amorphous sulphur_ is obtained in certain reactions in the wet way, when -sulphur separates out from solutions. Thus sodium thiosulphate, -Na_{2}S_{2}O_{3}, when treated with acids, gives a precipitate of -sulphur, which is insoluble in carbon bisulphide. The action of water on -sulphur chloride also gives a similar modification of sulphur. Certain -sulphides, when treated with nitric acid, also yield sulphur in this -form.[10] - - [10] A certain amount of insoluble sulphur remains for a long time in - the mass of soft sulphur, changing into the ordinary variety. - Freshly-cooled soft sulphur contains about one-third of insoluble - sulphur, and after the lapse of two years it still contains about - 15 p.c. Flowers of sulphur, obtained by the rapid condensation of - sulphur from a state of vapour, also contains a certain amount of - insoluble sulphur. _Rapidly distilled and condensed sulphur_ also - contains some insoluble sulphur. Hence a certain amount of - insoluble sulphur is frequently found in roll sulphur. The action - of light on a solution of sulphur converts a certain portion into - the insoluble modification. Insoluble sulphur is of a lighter - colour than the ordinary variety. It is best prepared by - vaporising sulphur in a stream of carbonic anhydride, hydrochloric - acid, &c., and collecting the vapour in cold water. When condensed - in this manner it is nearly all insoluble in carbon bisulphide. It - then has the form of hollow spheroids, and is therefore lighter - than the common variety: sp. gr. 1·82. An idea of the - modifications taking place in sulphur between 110° and 250° may be - formed from the fact that at 150° liquid sulphur has a coefficient - of expansion of about 0·0005, whilst between 150° and 250° it is - less than 0·0003. - - Engel (1891), by decomposing a saturated solution of hyposulphite - of sodium (Note 42) with HCl in the cold (the sulphur is not - precipitated directly in this case), obtained, after shaking up - with chloroform and evaporation, crystals of sulphur (sp. gr. - 2·135), which, after several hours, passed into the insoluble (in - CS_{2}) state, and in so doing became opaque, and increased in - volume. But if a mixture of solution of Na_{2}S_{2}O_{3} and HCl - be allowed to stand, it deposits sulphur, which, after sufficient - washing, is able to dissolve in water (like the colloid varieties - of the metallic sulphides, alumina, boron, and silver), but this - colloid _solution of sulphur_ soon deposits sulphur insoluble in - CS_{2}. - - When a solution of sulphuretted hydrogen in water is decomposed by - an electric current the sulphur is deposited on the positive pole, - and has therefore an electro-negative character, and this sulphur - is soluble in carbon bisulphide. When a solution of sulphurous - acid is decomposed in the same manner, the sulphur is deposited on - the negative pole, and is therefore electro-positive, and the - sulphur so deposited is insoluble in carbon bisulphide. The - sulphur which is combined with metals must have the properties of - the sulphur contained in sulphuretted hydrogen, whilst the sulphur - combined with chlorine is like that which is combined with oxygen - in sulphurous anhydride. Hence Berthelot recognises the presence - of soluble sulphur in metallic sulphides, and of the insoluble - modification of amorphous sulphur in sulphur chloride. Cloez - showed that the sulphur precipitated from solutions is either - soluble or insoluble, according to whether it separates from an - alkaline or acid solution. If sulphur be melted with a small - quantity of iodine or bromine, then on pouring out the molten mass - it forms amorphous sulphur, which keeps so for a very long time, - and is insoluble, or nearly so, in carbon bisulphide. This is - taken advantage of in casting certain articles in sulphur, which - by this means retain their tenacity for a long time; for example, - the discs of electrical machines. - -At temperatures of 440° to 700° the vapour density of sulphur is 6·6 -referred to air--_i.e._ about 96 referred to hydrogen. Hence, at these -temperatures _the molecule of sulphur contains six atoms_, it has the -composition S_{6}. The agreement between the observations of Dumas, -Mitscherlich, Bineau, and Deville confirms the accuracy of this result. -But in this respect the properties of sulphur were found to be variable. -When heated to higher temperatures, that is to say, _above_ 800°, the -vapour density of sulphur is found to be one-third of this quantity, -_i.e._ about 32 referred to hydrogen. At this temperature _the molecule -of sulphur_, like that of hydrogen, oxygen, nitrogen, and chlorine, -_contains two atoms_; hence the molecular formula is then S_{2}. This -variation in the vapour density of sulphur evidently corresponds with a -polymeric modification, and may be likened to the transformation of -ozone, O_{3}, into oxygen, O_{2}, or better still, of benzene, -C_{6}H_{6}, into acetylene, C_{2}H_{2}.[11] - - [11] Here, however, it is very important to remark that both benzene - and acetylene can exist at the ordinary temperature, whilst the - sulphur molecule S_{2} only exists at high temperatures; and if - this sulphur be allowed to cool, it passes first into S_{6} and - then into a liquid state. Were it possible to have sulphur at the - ordinary temperature in both the above modifications, then in all - probability the sulphur in the state S_{2} would present totally - different properties from those which it has in the form S_{6}, - just as the properties of gaseous acetylene are far from being - similar to those of liquid benzene. Sulphur, in the form of S_{2}, - is probably a substance which boils at a much lower temperature - than the variety with which we are now dealing. Paterno and Nasini - (1888), following the method of depression or fall of the - freezing-point in a benzene solution, found that the molecule of - sulphur in solution contains S_{6}. - - One must here call attention to the fact that sulphur, with all - its analogy to oxygen (which also shows itself in its faculty to - give the modification S_{2}), is also able to give a series of - compounds containing more atoms of sulphur than the analogous - oxygen compounds do of oxygen. Thus, for instance, compounds of 5 - atoms of sulphur with 1 atom of barium, BaS_{5}, are known, - whereas with oxygen only BaO_{2} is known. On every side one - cannot but see in sulphur a faculty for the union of a greater - number of atoms than with oxygen. With oxygen the form of ozone, - O_{3}, is very unstable, the stable form is O_{2}; whilst with - sulphur S_{6} is the stable form, and S_{2} is exceedingly - unstable. Furthermore, it is remarkable that sulphur gives a - higher degree of oxidation, H_{2}SO_{4}, corresponding, as it - were, with its complex composition, if we suppose that in S_{6} - four atoms of sulphur are replaced by oxygen and one by two atoms - of hydrogen. The formulæ of its compounds, K_{2}SO_{4}, - K_{2}S_{2}O_{3}, K_{2}S_{5}, BaS_{5}, and many others, have no - analogues among the compounds of oxygen. They all correspond with - the form S_{6} (one portion of the sulphur being replaced by - oxygen and another by metals), which is not attained by oxygen. In - this faculty of sulphur to hold many atoms of other substances the - same forces appear which cause many atoms of sulphur to form one - complex molecule. - -_In its faculty for combination_, sulphur most closely resembles oxygen -and chlorine; like them, it combines with nearly all elements, with the -development of heat and light, forming sulphur compounds, but as a rule -this only takes place at a high temperature. At the ordinary temperature -it does not enter into reactions, owing, amongst other things, to the -fact that it is a solid. In a molten state it acts on most metals and on -the halogens. It burns in air at about 300°, and with carbon at a red -heat, but it does not combine with nitrogen. - -Fine wires, or the powders of the greater number of metals, burn in the -vapour of sulphur. The direct combination of hydrogen with sulphur is -restricted by a limit--that is, at a given temperature and under other -given conditions it does not proceed unrestrictedly; there is no -explosion or recalescence. Sulphuretted hydrogen, H_{2}S, decomposes at -its temperature of combination--that is, it is easily dissociated.[12] -The same phenomenon is repeated here as with water, except that the -temperatures at which the attraction of hydrogen for sulphur begins and -ceases are much lower than in the case of oxygen and hydrogen. The -temperature at which combination takes place is here, as in many other -instances, nearly the same as that at which dissociation begins. Hence -_sulphuretted hydrogen_ is formed in a small quantity by the direct -ignition of a mixture of the vapour of sulphur and hydrogen. However, the -temperature must not be high, because otherwise the whole of the -sulphuretted hydrogen is decomposed; but at lower temperatures a small -amount of sulphuretted hydrogen is formed by direct combination.[13] -Sulphuretted hydrogen however, like all other hydrogen compounds, may be -easily obtained by the double decomposition of its corresponding metallic -compounds, the replacement of the metal by hydrogen being effected by the -action of acids on the sulphides. The metallic sulphides are, as a rule, -easily formed. A sulphide, when mixed with a non-volatile acid, may give, -by double decomposition, a salt of the acid taken and sulphuretted -hydrogen, M_{2}S + H_{2}SO_{4} = H_{2}S + M_{2}SO_{4}. However, it is not -all sulphides nor solutions of all acids that will evolve sulphuretted -hydrogen, which fact is exceedingly characteristic, because, for example, -all carbonates evolve carbonic anhydride when treated with any acid. -Sulphuric acid will only evolve sulphuretted hydrogen from those -sulphides which contain a metal capable of decomposing the acid with the -evolution of hydrogen. Thus zinc, iron, calcium, magnesium, manganese, -potassium, sodium, &c., form sulphides which evolve sulphuretted hydrogen -when treated with sulphuric acid, and the metals themselves evolve -hydrogen with acids.[14] The sulphides of those metals which do not -liberate hydrogen from acids do not generally act on acids--that is, do -not form sulphuretted hydrogen with them; such are, for example, the -sulphides of lead, silver, copper, mercury, tin, &c. Therefore, the -_modus operandi_ of the formation of sulphuretted hydrogen by the action -of acids on metallic sulphides may be looked on as a phenomenon of the -combination of hydrogen, at the moment of its evolution, with the -sulphur, which is combined with the metal. Such a representation is all -the more simple as all the circumstances under which sulphuretted -hydrogen is formed are exactly similar to the conditions of the formation -of hydrogen itself. Thus the usual mode of preparing sulphuretted -hydrogen is by the action of _sulphuric acid on ferrous sulphide_, in -which the same apparatus and method are employed as in the preparation of -hydrogen, only replacing the metallic iron or zinc by ferrous sulphide or -zinc sulphide. The reaction between sulphide of iron and sulphuric acid -takes place at the ordinary temperature, and is accompanied by just as -small a development of heat as in the liberation of hydrogen itself, FeS -+ H_{2}SO_{4} = FeSO_{4} + H_{2}S.[15] - - [12] In the formation of potassium sulphide, K_{2}S (that is, in the - combination of 32 parts of sulphur with 78 parts of potassium), - about 100 thousand heat units are developed. Nearly as much heat - is developed in the combination of an equivalent quantity of - sodium; about 90 thousand heat units in the formation of calcium - or strontium sulphide; about 40 thousand for zinc or cadmium - sulphide, and about 20 thousand for iron, cobalt, or nickel - sulphide. Less heat is evolved in the combination of sulphur with - copper, lead, and silver. According to Thomsen, sulphur develops - heat with hydrogen in solutions. The reaction I_{2},Aq,H_{2}S = - 21,830 calories. But, as the reaction I_{2} + H_{2} + Aq develops - 26,842 calories, it follows that the reaction H_{2} + S develops - 4,512 calories. - - [13] If sulphur be melted in a flask and heated nearly to its boiling - point, as Lidoff showed, the addition, drop by drop (from a funnel - with a stopcock) of heavy (0·9) naphtha oil (of lubricating - oleonaphtha), &c., is followed by a regular evolution of - sulphuretted hydrogen. This is analogous to the action of bromine - or iodine on paraffin and other oils, because hydrobromic or - hydriodic acid is then formed (Chapter XI.) A certain amount of - hydrogen sulphide is even formed when sulphur is boiled with - water. - - [14] However, the matter is really much more complicated. Thus zinc - sulphide evolves sulphuretted hydrogen with sulphuric or - hydrochloric acids, but does not react with acetic acid and is - oxidised by nitric acid. Ferrous sulphide evolves sulphuretted - hydrogen with acids, whilst the bisulphide, FeS_{2}, does not - react with acids of ordinary strength. This absence of action - depends, among other things, on the form in which the native iron - pyrites occurs; it is a crystalline, compact, and very dense - substance; and acids in general react with great difficulty on - such metallic sulphides. This is seen very clearly in the case of - zinc sulphide; if this substance is obtained by double - decomposition, it separates as a white precipitate, which evolves - sulphuretted hydrogen with great ease when treated with acids. - Zinc sulphide is obtained in the same form when zinc is fused with - sulphur, but native zinc sulphide--which occurs in compact masses - of zinc blende, and has a metallic lustre--is not decomposed or - scarcely decomposed by sulphuric acid. - - Another source of complication in the behaviour of the metallic - sulphides towards acids depends on the action of water, and is - shown in the fact that the action varies with different degrees of - dilution or proportion of water present. The best known example of - this is antimonious sulphide, Sb_{2}S_{3}, for strong hydrochloric - acid, containing not more water than corresponds with HCl,6H_{2}O, - even decomposes native antimony glance, with evolution of - sulphuretted hydrogen, whilst dilute acid has no action, and in - the presence of an excess of water the reaction 2SbCl_{3} + - 3H_{2}S = Sb_{2}S_{3} + 6HCl occurs, whilst in the presence of a - small amount of water the reaction proceeds in exactly the - opposite direction. Here the participation of water in the - reaction and its affinity are evident. - - The facts that lead sulphide is insoluble in acids, that zinc - sulphide is soluble in hydrochloric acid but insoluble in acetic - acid, that calcium sulphide is even decomposed by carbonic acid, - &c.--all these peculiarities of the sulphides are in correlation - with the amount of heat evolved in the reaction of the oxides with - hydrogen sulphide and with acids, as is seen from the observations - of Favre and Silberman, and from the comparisons made by Berthelot - in the Proceedings of the Paris Academy of Sciences, 1870, to - which we refer the reader for further details. - - [15] _Ferrous sulphide_ is formed by heating a piece of iron to an - incipient white heat, and then removing it from the furnace and - bringing it into contact with a piece of sulphur. Combination then - proceeds, accompanied by the development of heat, and the ferrous - sulphide formed fuses. The sulphide of iron thus formed is a - black, easily-fusible substance, insoluble in water. When damp it - attracts oxygen from the air, and is converted into green vitriol, - FeSO_{4}. If all the iron does not combine with the sulphur in the - method described above, the action of sulphuric acid will evolve - hydrogen as well as hydrogen sulphide. - - We will not describe the details of the preparation of - sulphuretted hydrogen employed as a reagent in the laboratory, - because, in the first place, the methods are essentially the same - as in the preparation of hydrogen, and, in the second place, - because the apparatus and methods employed are always described in - text-books of analytical chemistry. Ferrous sulphide may be - advantageously replaced by calcium sulphide or a mixture of - calcium and magnesium sulphides. A solution of magnesium - hydrosulphide, MgS,H_{2}S, is very convenient, as at 60° it - evolves a stream of pure hydrogen sulphide. A paste, consisting of - CuS with crystals of MgCl_{2} and water, may also be employed, - since it only evolves H_{2}S when heated (Habermann). - -_In nature_ sulphuretted hydrogen is formed in many ways. The most usual -mode of its formation is by the decomposition of albuminous substances -containing sulphur, as mentioned above. Another method is by the reducing -action of organic matter on sulphates, and by the action of water and -carbonic acid on the sulphides formed by this reduction. Volcanic -eruptions are a third source of sulphuretted hydrogen in nature. Although -sulphuretted hydrogen is formed in small quantities everywhere, it -nevertheless soon disappears from the atmosphere, owing to its being -easily decomposed by oxidising agencies. Many mineral waters contain -sulphuretted hydrogen, and smell of it; they are called 'sulphur waters.' - -Sulphuretted hydrogen, at the ordinary temperature, is a colourless gas, -having a very unpleasant odour. It has, as its composition H_{2}S shows, -a specific gravity seventeen times greater than hydrogen, and therefore -it is somewhat heavier than air. Sulphuretted hydrogen _liquefies_ at -about -74°, and at the ordinary temperature when subjected to a pressure -of 10 to 15 atmospheres; at -85° it is converted into a solid crystalline -mass.[15 bis] The easy liquefaction of sulphuretted hydrogen is evidently -allied to its solubility. One volume of water at 0° dissolves 4·37 -volumes of sulphuretted hydrogen, at 10° 3·58 volumes, and at 20° 2·9 -volumes.[16] The solutions impart a very feeble red coloration to litmus -paper. This gas is poisonous. One part in fifteen hundred parts of air -will kill birds. Mammalia die in an atmosphere containing 1/200 of this -gas. - - [15 bis] Liquid sulphuretted hydrogen is most easily obtained by the - decomposition of hydrogen polysulphide, which we shall presently - describe, by the action of heat, and in the presence of a small - amount of water. If poured into a bent tube, like that described - for the liquefaction of ammonia (Chapter VI.), the hydrogen - polysulphide is decomposed by heat, in the presence of water, into - sulphur and sulphuretted hydrogen, which condenses in the cold end - of the tube into a colourless liquid. - - [16] Sulphuretted hydrogen is still more soluble in alcohol than in - water; one volume at the ordinary temperature dissolves as much as - eight volumes of the gas. The solutions in water and alcohol - undergo change, especially in open vessels, owing to the fact that - the water and alcohol dissolve oxygen from the atmosphere, which, - acting on the sulphuretted hydrogen, forms water and sulphur. The - solution may be so altered in this manner that every trace of - sulphuretted hydrogen disappears. Solutions of sulphuretted - hydrogen in glycerine change much more slowly, and may therefore - be kept for a long time as reagents. De Forcrand obtained a - hydrate, H_{2}S,16H_{2}O, resembling the hydrates given by many - gases. - -Sulphuretted hydrogen is very easily _decomposed_ into its component -parts by the action of heat or a series of electric sparks. Hence it is -not surprising that sulphuretted hydrogen undergoes change under the -action of many substances having a considerable affinity for hydrogen and -oxygen. Very many metals[17] evolve hydrogen with sulphuretted hydrogen, -so that in this respect it presents the property of an acid; for -instance, 2H_{2}S + Sn = 2H_{2} + SnS_{2}. This may be taken advantage of -for determining the composition of sulphuretted hydrogen, because a given -volume then leaves the same volume of hydrogen. On the other hand, -oxygen,[18] chlorine,[19] and even iodine decompose sulphuretted -hydrogen, removing the hydrogen from it and leaving free sulphur, so that -in this reaction the sulphur is replaced by the above-named elements; for -example, H_{2}S + Br_{2} = 2HBr + S. In no other hydrogen compound is it -so easy to show the _substitution_, both of hydrogen and of the element -combined with it, as in hydrogen sulphide. This clearly proves the feeble -union between the elements forming this gas. Compounds containing a -considerable amount of oxygen, with which they easily part, can -accomplish the separation of the sulphur very easily. Such are, for -instance, nitrous acid, chromic acid, and even ferric oxide and the -higher oxides like it. Thus, if sulphuretted hydrogen be passed into a -solution of chromic acid or an acid solution of ferric oxide, water is -formed, _and the sulphur is separated in a free state_. Thus, -sulphuretted hydrogen acts as a _reducing agent_, in virtue of the -hydrogen it contains. Salts of iodic, chlorous, chloric, and other acids -are reduced by sulphuretted hydrogen, their oxygen acting mainly on its -hydrogen; but in the presence of an excess of a powerful oxidising agent -a portion of the sulphur may also be oxidised to sulphurous anhydride. -The reducing action of sulphuretted hydrogen is frequently applied in -chemical manipulations for the preparation of lower oxides, and for the -conversion of certain oxygen compounds into hydrogen compounds: thus, the -higher oxides of nitrogen are converted into ammonia by it, and in the -presence of alkalis the nitro-compounds are converted into ammonia -derivatives. The reaction of sulphuretted hydrogen on sulphurous -anhydride belongs to this class of phenomena, the chief products of which -are sulphur and water, 2H_{2}S + SO_{2} = 2H_{2}O + S_{3}. - - [17] Some metals evolve hydrogen from sulphuretted hydrogen at the - ordinary temperature. For example, the light metals, and copper - and silver (especially with the access of air?) among the heavy - metals. Hence articles made of silver turn black in the presence - of vapours containing sulphuretted hydrogen, because silver - sulphide is black. Zinc and cadmium act at a red heat, but not - completely. - - [18] If sulphuretted hydrogen escapes from a fine orifice into the air, - it will burn when lighted, and be transformed into sulphurous - anhydride and water. But if it burns in a limited supply of - air--for instance, when a cylinder is filled with it and - lighted--then only the hydrogen burns, which has, judging from the - amount of heat developed in its combustion and from all its - properties, a greater affinity for oxygen than sulphur. In this - respect the combustion of sulphuretted hydrogen resembles that of - hydrocarbons. - - [19] Hence bleaching powder and chlorine destroy the disagreeable smell - of sulphuretted hydrogen. (For the reaction of hydrogen sulphide - and iodine, _see_ Chapter XI. p. 504.) - -The acid character of sulphuretted hydrogen is clearly seen in its -action on alkalis and salts.[19 bis] Thus lead oxide and its salts in the -presence of sulphuretted hydrogen form water or an acid, and sulphide of -lead: PbX_{2} + H_{2}S = PbS + 2HX. This reaction takes place even in the -presence of powerful acids, because lead sulphide is one of those -sulphides which are unacted on by acids, and in solutions the reaction is -a complete one. This reaction is taken advantage of for the preparation -of many acids, by first converting into a lead salt, and then submitting -this salt to the action of sulphuretted hydrogen. For example, lead -formate with sulphuretted hydrogen gives formic acid. Sulphuretted -hydrogen in acting on a number of metallic acid substances in solution or -in an anhydrous state also forms corresponding sulphates: (1) if it does -not reduce the acid; (2) if the sulphur compound corresponding with the -anhydride of the acid be insoluble in water, the reaction proceeds in -solutions; (3) if the sulphuretted hydrogen and the acid taken do not -come in contact with an alkali, on which they would be able to act first; -and (4) if the sulphur compound be not decomposed by water. Thus -solutions of arsenious acid give a precipitate of arsenious sulphide, -As_{2}S_{3}, with sulphuretted hydrogen. This reaction proceeds not only -in the presence of water, but also of acids, because the latter do not -decompose the resultant sulphur compounds. The type of the decomposition -is the same as with bases--that is, the sulphur and oxygen change places: -RO_{_n_} + _n_H_{2}S = RS_{_n_} + _n_H_{2}O. Some sulphides corresponding -with acid anhydrides are decomposed by water, and therefore are not -formed in the presence of water. Such, for example, are the sulphides of -phosphorus.[20] - - [19 bis] Perfectly dry H_{2}S (Hughes 1892) has no action upon - perfectly dry salts, just as dry HCl does not react with dry - NH_{3} or metals (Chapter IX., Note 29). - - [20] The sulphide P_{4}S is obtained by cautiously fusing the requisite - proportions of common phosphorus and sulphur under water; it is a - liquid which solidifies at 0°, and may be distilled without - undergoing change, but it fumes in air and easily takes fire. The - higher sulphide, P_{2}S, has similar properties. But little heat - is evolved in the formation of these compounds, and it may be - supposed that they are formed by the direct conjunction of whole - molecules of phosphorus and sulphur; but if the proportion of - sulphur be increased, the reaction is accompanied by so - considerable a rise of temperature that an explosion takes place, - and for the sake of safety red phosphorus must be used, mixed as - intimately as possible with powdered sulphur and heated in an - atmosphere of carbonic anhydride. The higher compounds are - decomposed by water. By increasing the proportion of sulphur, the - following compounds have been obtained: P_{4}S_{3} as prisms - (fuses at 165°, Rebs), soluble in carbon bisulphide, and unaltered - by air and water; _phosphorus trisulphide_, P_{2}S_{3}, is the - analogue of P_{2}O_{3}; it is a light yellow crystalline compound - only slightly soluble in carbon bisulphide, fusible and volatile, - decomposed into hydrogen sulphide and phosphorous acid by water, - and, like the highest compound of sulphur and phosphorus, - P_{2}S_{5}, it forms thio-salts with potassium sulphide, &c. This - _phosphorus pentasulphide_ corresponds with phosphoric anhydride; - like the trisulphide it gives hydrogen sulphide and phosphoric - acid with an excess of water. It reacts in many respects like - phosphoric chloride. The sulphide PS_{2} is also known; the vapour - density of this compound seems to indicate a molecule P_{3}S_{6}. - - _Phosphorus sulphochloride_, PSCl_{3}, corresponds with phosphorus - oxychloride. It is a colourless, pleasant-smelling liquid, boiling - at 124°, and of sp. gr. 1·63; it fumes in air and is decomposed by - water: PSCl_{3} + 4H_{2}O = PH_{3}O_{4} + H_{2}S + 3HCl. It is - obtained when phosphoric chloride is treated with hydrogen - sulphide, hydrochloric acid being also formed; it is also produced - by the action of phosphoric chloride on certain sulphides--for - example, on antimonious sulphide, also by the (cautious) action of - phosphorus on sulphur chloride: 2P + 3S_{2}Cl_{2} = 2PSCl_{3} + - 4S, by the action of PCl_{5} upon certain sulphides, for example, - Sb_{2}S_{3}, by the reaction: 3MCl + P_{2}S_{5} = PSCl_{3} + - M_{3}PS_{4} (Glatzel, 1893), and in the reaction 3PCl_{3} + - SOCl_{2} = PCl_{5} + POCl_{3} + PSCl_{3}, showing the reducing - action of phosphorus trichloride, which is especially clear in the - reaction SO_{3} + PCl_{3} = SO_{2} + POCl_{3}. Thorpe and Rodger - (1889), by heating 3PbF_{2} or BiF_{3} with phosphorus - pentasulphide (and also by heating AsF_{3} and PSCl_{3} to 150°), - obtained thiophosphoryl fluoride as a colourless, spontaneously - inflammable gas (see further on, Note 74 bis, and Chapter XIX., - Note 25). The action of PSCl_{3} upon NaHO gives a salt of - monothiophosphoric acid (Würtz, Kubierschky), H_{3}PSO_{3}, which - gives soluble salts of the alkalis. - -The metallic sulphides corresponding with the metallic oxides have -either a feeble alkaline or a feeble acid character, according to the -character of the corresponding oxide, and therefore by combining together -they are able to form saline substances--that is, salts in which the -oxygen is replaced by sulphur. Thus sulphuretted hydrogen having the -properties of a feeble acid[21] has, at the same time, the properties of -water, and forms the type of the sulphur derivatives, which may also be -formed by means of sulphuretted hydrogen, just as the oxides may be -formed by the aid of water. But as sulphuretted hydrogen has acid -properties, it combines more easily with the basic metallic sulphides. -Hence, for instance, there exists a compound of sulphuretted hydrogen -with potassium sulphide, potassium hydrosulphide, 2KHS = K_{2}S + H_{2}S, -just as there are potassium hydroxides; but there are scarcely any -compounds of sulphuretted hydrogen with the sulphides corresponding with -acids. Thus the sulphides of the metals may be regarded either as salts -of sulphuretted hydrogen or as oxides of the metals in which the oxygen -is replaced by sulphur. In general terms the sulphides exhibit the same -degrees of difference with respect to their solubility in water as do the -oxides. Thus the oxides of the alkali metals, and of some of the metals -of the alkaline earths, are soluble in water, whilst those of nearly all -the other metals are insoluble. The same may be said as to the sulphides; -the sulphides of the metals of the alkalis and certain of the alkaline -earths are soluble in water, whilst those of the other metals are -insoluble. Those metals, like aluminium, whose oxides--for example, -Al_{2}O_{3}--have intermediate properties and do not form compounds with -feeble acids, at least in a wet way, also do not form sulphides by this -method, although these may be obtained indirectly. And in general the -sulphides of the metals are easily formed in a wet way, and with -particular ease if they are insoluble in water. In this case their salts -enter into double decomposition with sulphuretted hydrogen, or with -soluble sulphides, and give an insoluble sulphide--for instance, a salt -of lead gives lead sulphide with sulphuretted hydrogen. By the action of -sulphuretted hydrogen on a salt of a metal, a free acid must be formed -besides the metallic sulphide. Thus if a metal M be in a state of -combination MX_{2}, then by the action of sulphuretted hydrogen there -will be formed, besides MS,[22] an acid 2HX. It is evident that -sulphuretted hydrogen will not precipitate an insoluble sulphide from the -salts of those metals whose sulphides react with free acid, such as zinc, -iron, manganese, &c. The reaction FeCl_{2} + H_{2}S = FeS + 2HCl, and the -like, do not take place because the acid acts on the ferrous sulphide. -Antimonious sulphide is not acted on by dilute hydrochloric acid, but it -is decomposed by strong acid, and therefore in presence of an excess of -hydrochloric acid antimonious chloride does not entirely react with -hydrogen sulphide, whilst the reaction 2SbCl_{3} + 3H_{2}S = Sb_{2}S_{3} -+ 6HCl is a complete one in a dilute solution and with a small quantity -of acid. Those metallic sulphides which are decomposed by acids may be -obtained in a wet way by the double decomposition of the salts of the -metals, not with hydrogen sulphide, but with soluble metallic sulphides, -such as sulphide of ammonium or of potassium, because then no free acid -is formed, but a salt of the metal (potassium or ammonium) which was -taken as a soluble sulphide. So, for example, FeCl_{2} + K_{2}S = FeS + -2KCl.[23] - - [21] Sulphuretted hydrogen does not saturate the alkaline properties of - alkali hydroxides, so that a solution of potassium hydroxide will - not under any circumstances give a neutral liquid with - sulphuretted hydrogen. In this case the sulphuretted hydrogen - forms in solution only an acid salt with the potassium: KHO + - H_{2}S = KHS + H_{2}O. It must be supposed that the normal salt is - not formed in the solution--that is, that the reaction 2KHO + - H_{2}S = K_{2}S + 2H_{2}O does not take place. This is seen from - the fact that a development of heat, depending on the formation of - potassium hydrosulphide, KHS, is remarked when as much hydrogen - sulphide is passed into a solution of potassium hydroxide as it - will absorb. But if a further quantity of potassium hydroxide be - added to the resultant solution, heat is not developed, whilst if - alkali be added to potassium acid sulphate or sodium acid - carbonate, heat is developed. It must not be concluded from this - that H_{2}S is a monobasic acid, for here there is a question of - the decomposing action of water upon K_{2}S; K_{2}S and H_{2}O in - reacting on each other should absorb heat if the reaction of KHS - upon KHO evolves heat. Furthermore, it must be taken into account - that potassium oxide, K_{2}O, and the anhydrous oxides like it, - also do not exist in solutions, for whenever they are formed they - immediately react with the water, forming caustic potash, KHO, &c. - In the same way, directly potassium sulphide, K_{2}S, is formed in - water it is decomposed into potassium hydroxide and hydrosulphide: - K_{2}S + H_{2}O = KHO + KHS. Potassium sulphide, K_{2}S, in a - solid state corresponds with K_{2}O, although neither can exist in - solution. - - [22] During recent years (beginning with Schulze, 1882) it has been - found that many metallic sulphides which were considered totally - insoluble do, under certain circumstances, form very unstable - solutions in water, as already mentioned in Chapter I., Note 57. - Arsenic sulphide is very easily obtained in the form of a solution - (hydrosol). Solutions of copper and cadmium sulphides may also be - easily obtained by precipitating their salts CuX_{2}, or CdX_{2}, - with ammonium sulphide, and washing the precipitate; but they are - re-precipitated by the addition of foreign salts. - - [23] In reality the preceding reaction should be expressed thus: - FeCl_{2} + 2KHS = FeS + 2KCl + H_{2}S (Note 21), because in the - presence of water not K_{2}S but KHS reacts. But as the - sulphuretted hydrogen takes no part in the reaction, it is usual - to express the formation of such sulphides without taking the - hydrogen sulphide proceeding from the potassium or ammonium - hydrosulphides into account. It is not usual to employ potassium - sulphide but ammonium sulphide--or, to speak more accurately, - ammonium hydrosulphide--in order to avoid the formation of a - non-volatile salt of potassium and to have, together with the - formation of the sulphide, a salt of ammonium which can always be - driven off by evaporating the solution and igniting the - residue--for instance: FeCl_{2} + (NH_{4})_{2}S = FeS + 2NH_{4}Cl. - Thus the metallic sulphides may be divided into three chief - classes: (1) _those soluble in water_, (2) _those insoluble in - water but reacting with acids_, and (3) _those insoluble both in - water and acids_. The third class may be easily subdivided into - two groups; to the first group belong those sulphides which - correspond with bases or basic oxides, and are therefore unable to - play the part of an acid with the sulphides of the alkalis, and - are insoluble in NH_{4}HS, whilst the sulphides of the second - group are of an acid character, and give soluble thio-salts with - the sulphides of the alkaline metals, in which they play the part - of an acid. To this group belong those metals whose corresponding - oxides have acid properties. It must be observed, however, that - not all metallic acids have corresponding sulphides, partly owing - to the fact that certain acids are reducible by sulphuretted - hydrogen, especially when their lower degrees of oxidation are of - a basic character. Such are, for instance, the acids of chromium, - manganese, &c. Sulphuretted hydrogen converts them into lower - oxides, having the properties of bases. Those bases which do not - combine with feeble acids, such as carbonic acid and hydrogen - sulphide, give a precipitate of hydroxide with ammonium - sulphide--for example, aluminium salts react in this manner. This - difference of the metals in their behaviour towards sulphuretted - hydrogen gives a very valuable means of separating them from each - other, and _is taken advantage of in analytical chemistry_. If, - for instance, the metals of the first and third groups occur - together, it is only necessary to convert them into soluble salts, - and to act on the solution of the salts with sulphuretted - hydrogen; this will precipitate the metals of the third group in - the form of sulphides, whilst the metals of the first group will - not be in the least acted on. Such a method of separating the - metals is considered more fully in analytical chemistry, and we - will therefore limit ourselves here to pointing out to which - groups the most common metals belong, and the colour which is - proper to the sulphide precipitated. - - _Metals which are precipitated by sulphuretted hydrogen_, as - sulphides from a solution of their salts, even in the presence of - free acid: - - The precipitate is soluble in ammonium sulphide: - - _Platinum_ (dark brown) | _Antimony_ (orange) - _Gold_ (dark brown) | _Arsenic_ (yellow) - _Tin_ (yellow and brown) | - - The precipitate is insoluble in ammonium sulphide: - - _Copper_ (black) | _Mercury_ (black) - _Silver_ (black) | _Lead_ (black) - _Cadmium_ (yellow) | - - _Metals which are precipitated by ammonium sulphide_ from neutral - solutions, but not precipitated from acid solutions by - sulphuretted hydrogen: - - The sulphide precipitated is soluble in hydrochloric acid: - - _Zinc_ (white) | _Manganese_ (rose colour) | _Iron_ (black) - - The sulphide precipitated is not soluble in dilute hydrochloric - acid: - - _Nickel_ (black) | _Cobalt_ (black) - - - A hydroxide, and not a sulphide, is precipitated: - - _Chromium_ (green) | _Aluminium_ (white) - - The metals of the alkalis and of the alkaline earths are not - precipitated either by sulphuretted hydrogen or ammonium sulphide. - The metals of the alkaline earths when in acid solutions in the - form of phosphates and many other salts are precipitated by - ammonium sulphide, because the latter neutralises the free acid, - with formation of an ammonium salt of the acid and evolution of - sulphuretted hydrogen. - -Metallic sulphides may be obtained by many other means besides the -action of sulphuretted hydrogen on salts and oxides, or by the simple -combination of metals with sulphur when heated or fused. Thus they may -also be formed by the reduction of sulphates by heating them with -charcoal or other means. Charcoal takes up the oxygen from many -sulphates, leaving corresponding sulphides. Thus sodium sulphate, -Na_{2}SO_{4}, when heated with charcoal, forms sodium sulphide, Na_{2}S. -Besides which metallic sulphides are also obtained by heating metals or -their oxides in the vapours of many sulphur compounds--for example, in -the vapour of carbon bisulphide, CS_{2}, when the carbon takes up the -oxygen and the sulphur combines with the metal. The sulphides formed in -this manner are often crystalline, and often appear with those properties -and in that crystalline form in which they occur in nature. Besides which -we must mention that many of the sulphides of the metals are oxidised in -air at the ordinary, and especially at a higher, temperature, forming -either SO_{2} and the oxide of the metal or sulphates. This oxidation -proceeds with particular ease, even at the ordinary temperature, when a -metallic sulphide is precipitated from its solutions, as a fine powder -containing water. The sulphides of iron and manganese, &c., are very -easily oxidised in this manner. But if these hydrates be ignited, they -lose their water (the ignition must be carried on in a stream of hydrogen -to prevent their oxidation during the process), become denser, and are no -longer oxidised at the ordinary temperature. Those sulphides whose -corresponding sulphates are decomposed by heat part with their sulphur in -the form of sulphurous anhydride when they are ignited in air, and the -metal, as a rule, remains behind as oxide. This is taken advantage of in -the treatment of sulphurous ores. The process is called _roasting_. - -Hydrogen not only forms sulphuretted hydrogen with sulphur, but it also -combines with it in several other proportions, just as it combines with -oxygen, forming not only water but also hydrogen peroxide. Moreover these -_polysulphides of hydrogen_ are also unstable, like hydrogen peroxide, -and are also obtained from the corresponding polysulphides of the metals -of the alkaline earths, just as hydrogen peroxide is obtained from barium -peroxide. Thus calcium forms not only calcium sulphide, CaS, but also as -bi-, tri-, and pentasulphide, CaS_{5}, and all these compounds are -soluble in water. Sodium also combines with sulphur in the same -proportions, forming sulphides from Na_{2}S to Na_{2}S_{5}. If an acid be -added to a solution of a polysulphide, it gives sulphur, sulphuretted -hydrogen, and a salt of the metal. For instance, MS_{5}, + 2HCl = MCl_{2} -+ H_{2}S + 4S. If we reverse the operation, and pour a solution of a -polysulphide into an acid, sulphur is not precipitated, but an oily -liquid is formed which is heavier than water and insoluble in it. This is -the polysulphide of hydrogen: MS_{5} + 2HCl = MCl_{2} + H_{2}S_{5}. As -Rebs showed (1888), whatever polysulphide be taken--of sodium, for -instance--it always gives one and the same _hydrogen pentasulphide_,[24] -of specific gravity 1·71 (15°). It can only be preserved in the absence -of water and at low temperatures, and then not for long: for, especially -in the presence of alkalis and when slightly warmed, it splits up very -easily into sulphuretted hydrogen and sulphur.[25] - - [24] Rebs took di-, tri-, tetra-, and pentasulphides of sodium, - potassium, and barium, which he prepared by dissolving sulphur in - solutions of the normal sulphides; on adding hydrochloric acid he - always obtained hydrogen pentasulphide, whence it is evident that - 4H_{2}S_{n} = (_n_ - 1)H_{2}S_{5} + (5 - _n_)H_{2}S. For example, - if H_{2}S_{2} were formed, it would decompose according to the - equation 4H_{2}S_{2} = H_{2}S_{5} + 3H_{2}S. The hydrogen - pentasulphide formed breaks up into hydrogen sulphide and sulphur - when brought into contact with water. Previous to Rebs' researches - many chemists stated that all polysulphides gave the bisulphide - H_{2}S_{2}, and Hofmann recognised only hydrogen trisulphide, - H_{2}S_{3}. - - [25] The formation of the polysulphides of hydrogen, H_{2}S_{n} is - easily understood from the law of substitution, like that of the - saturated hydrocarbons, C_{n}H_{2n + 2}, knowing that sulphur - gives H_{2}S, because the molecule of sulphuretted hydrogen may be - divided into H and HS. This radicle, HS, is equivalent to H. By - substituting this radicle for hydrogen in H_{2}S we obtain (HS)HS - = H_{2}S_{2}, (HS)(HS)S = H_{2}S_{3}, &c., in general H_{2}S_{n}. - The homologues of CH_{4}, C_{n}H_{2n + 2} are formed in this - manner from CH_{4}, and consequently the polysulphides H_{2}S_{n} - are the homologues of H_{2}S. The question arises why in - H_{2}S_{n} the apparent limit of _n_ is 5--that is, why does the - substitution end with the formation of H_{2}S_{5}? The answer - appears to me to be clearly because in the molecule of sulphur, - S_{6}, there are six atoms of sulphur (Note 11). The forces in one - and the other case are the same. In the one case they hold S_{6} - together, in the other S_{5} and H_{2}; and, judging from H_{2}S, - the two atoms of hydrogen are equal in power and significance to - the atom of sulphur. Just as hydrogen peroxide, H_{2}O_{2}, - expresses the composition of ozone, O_{3}, in which O is replaced - by H_{2}, so also H_{2}S_{5} corresponds with S_{6}. - -The soluble sulphides and polysulphides of the metals of the alkalis -and alkaline earths--for example, of ammonium,[26] potassium,[27] and -calcium,[28]--have the appearance and properties of salts, just as the -hydrated oxides have, whilst the sulphides of the metals of the higher -groups resemble their oxides and have not at all the appearance -of salts, and this is more especially the case with regard to the -crystalline forms in which they frequently occur in nature.[29] - - [26] _Ammonium sulphide_, (NH_{4})_{2}S, may be prepared by passing - sulphuretted hydrogen into a vessel full of dry ammonia, or by - passing both dry gases together into a very cold receiver. In the - latter case it is necessary to prevent the access of air, and to - have an excess of ammonia. Under these circumstances, two volumes - of ammonia combine with one volume of sulphuretted hydrogen, and - form a colourless, very volatile, crystalline substance, having a - very unpleasant odour, which is very poisonous and exceedingly - unstable. When exposed to the air it absorbs oxygen and acquires a - yellow colour, and then contains oxygen and polysulphide compounds - (because a portion of the hydrogen sulphide gives water and - sulphur). It is soluble in water and forms a colourless solution, - which, however, in all probability contains free ammonia and the - acid salt--that is, ammonium hydrosulphide, NH_{4}HS, or - (NH_{4})_{2},S,H_{2}S. This salt is formed when dry ammonia is - mixed with an excess of dry sulphuretted hydrogen. The compound - contains equal volumes of the components NH_{3} + H_{2}S = - (NH_{4})HS. It crystallises in an anhydrous state in colourless - plates, and may be easily volatilised (dissociating like ammonium - chloride), even at the ordinary temperature; it has an alkaline - reaction, absorbs oxygen from the air, is soluble in water, and - its solution is usually prepared by saturating an aqueous solution - of ammonia with sulphuretted hydrogen. According to the ordinary - rule, these salts, like other ammonium salts, split up into - ammonia and sulphuretted hydrogen when they are distilled. - - A solution of ammonium sulphide is able to dissolve sulphur, and - it then contains compounds of hydrogen polysulphide and ammonia. - Some of these compounds may be obtained in a crystalline form. - Thus Fritzsche obtained a compound of ammonia with hydrogen - pentasulphide, or ammonium pentasulphide, (NH_{4})_{2}S_{5}, in - the following manner: He saturated an aqueous solution of ammonia - with sulphuretted hydrogen, added powdered sulphur to it, and - passed ammonia gas into the solution, which then absorbed a fresh - amount. After this he again passed sulphuretted hydrogen into the - solution, and then added sulphur, and then again ammonia. After - repeating this several times, orange-yellow crystals of - (NH_{4})_{2}S_{5} separated out from the liquid. These crystals - melted at 40° to 50°, and were very unstable. - - When a solution of ammonium hydrosulphide, prepared by saturating - a solution of ammonia with sulphuretted hydrogen, is exposed to - the air, it turns yellow, owing to the presence of an ammonium - polysulphide, whose formation is due to the sulphuretted hydrogen - being oxidised by the air and converted into water and sulphur, - which is dissolved by the ammonium sulphide. In certain analytical - reactions it is usual to employ a solution of ammonium sulphide - which has been kept for some time and acquired a yellow colour. - This yellow sulphide of ammonium deposits sulphur when saturated - with acids, whilst a freshly-prepared solution only evolves - sulphuretted hydrogen. The yellow solution furthermore contains - ammonium thiosulphate, which is derived not only from the - oxidation of the ammonium sulphide, but also from the action of - the liberated sulphur on the ammonia, just as an alkaline salt of - thiosulphuric acid and a sulphide are formed by the action of - sulphur on a solution of a caustic alkali. - - [27] _Potassium sulphide_, K_{2}S, is obtained by heating a mixture of - potassium sulphate and charcoal to a bright-red heat. It may be - prepared in solution by taking a solution of potassium hydroxide, - dividing it into two equal parts, and saturating one portion with - sulphuretted hydrogen so long as it is absorbed. This portion will - then contain the acid salt KHS (Note 21). The two portions are - then mixed together, and potassium sulphide will then be obtained - in the solution. This solution has a strongly alkaline reaction, - and is colourless when freshly prepared, but it very easily - undergoes change when exposed to the air, forming potassium - thiosulphate and polysulphides. When the solution is evaporated at - low temperatures under the receiver of an air-pump, it yields - crystals containing K_{2}S,5H_{2}O (heated at 150°, they part with - 3 mol. H_{2}O, and at higher temperatures they lose nearly all - their water without evolving sulphuretted hydrogen). When they are - ignited in glass vessels they corrode the glass. When a solution - of caustic potash, completely saturated with sulphuretted - hydrogen, is evaporated under the receiver of an air-pump it forms - colourless rhombohedra of _potassium hydrosulphide_, - 2(KHS),H_{2}O,K_{2}S,H_{2}S,H_{2}O. These crystals are - deliquescent in the air, but do not change in a vacuum when heated - up to 170°, and at higher temperatures they lose water but do not - evolve sulphuretted hydrogen. The anhydrous compound, KHS, fuses - at a dark-red heat into a very mobile yellow liquid, which - gradually becomes darker in colour and solidifies to a red mass. - It is remarkable that when a solution of the compound KHS is - boiled it somewhat easily evolves half its sulphuretted hydrogen, - leaving potassium sulphide, K_{2}S, in solution; and a solution of - the latter in water is also able to evolve sulphuretted hydrogen - on prolonged boiling, but the evolution cannot be rendered - complete, and, therefore, at a certain temperature, a solution of - potassium sulphide will not be capable of absorbing sulphuretted - hydrogen at all. From this we must conclude that potassium - hydroxide, water, and sulphuretted hydrogen form a system whose - complex equilibrium is subject to the laws of dissociation, - depends on the relative mass of each substance, on the - temperature, and the dissociation pressure of the component - elements. Potassium sulphide is not only soluble in water, but - also in alcohol. - - Berzelius showed that in addition to potassium sulphide there also - exist potassium bisulphide, K_{2}S_{2}; trisulphide, K_{2}S_{3}; - tetrasulphide, K_{2}S_{4}; and pentasulphide, K_{2}S_{5}. - According to the researches of Schöne, the last three are the most - stable. These different compounds of potassium and sulphur may be - prepared by fusing potassium hydroxide or carbonate with an excess - of sulphur in a porcelain crucible in a stream of carbonic - anhydride. At about 600° potassium pentasulphide is formed; this - is the highest sulphur compound of potassium. When heated to 800° - it loses one-fifth of its sulphur and gives the tetrasulphide, - which at this temperature is stable. At a bright-red heat--namely, - at about 900°--the trisulphide is formed. This compound may be - also formed by igniting potassium carbonate in a stream of carbon - bisulphide, in which case a compound, K_{2}CS_{3}, is first formed - corresponding to potassium carbonate, and carbonic anhydride is - evolved. On further ignition this compound splits up into carbon - and potassium trisulphide, K_{2}S_{3}. The tetrasulphide may also - be obtained in solution if a solution of potassium sulphide be - boiled with the requisite amount of sulphur without access of air. - This solution yields red crystals of the composition - K_{2}S_{4},2H_{2}O when it is evaporated in a vacuum. These - crystals are very hygroscopic, easily soluble in water, but very - sparingly in alcohol; when ignited they give off water, - sulphuretted hydrogen, and sulphur. If a solution of potassium - sulphide be boiled with an excess of sulphur it forms the - pentasulphide, which, however, is decomposed on prolonged boiling - into sulphuretted hydrogen and potassium thiosulphate: K_{2}S_{5} - + 3H_{2}O = K_{2}S_{2}O_{3} + 3H_{2}S. A substance called _liver - of sulphur_ was formerly frequently used in chemistry and - medicine. Under this name is known the substance which is formed - by boiling a solution of caustic potash with an excess of flowers - of sulphur. This solution contains a mixture of potassium - pentasulphide and thiosulphate, 6KHO + 12S = 2K_{2}S_{5} + - K_{2}S_{2}O_{3} + 3H_{2}O. The substance obtained by fusing - potassium carbonate with an excess of sulphur was also known as - liver of sulphur. If this mixture be heated to an incipient - dark-red heat it will contain potassium thiosulphate, but at - higher temperatures potassium sulphate is formed. In either case a - polysulphide of potassium is also present. The sulphides of - sodium, for example Na_{2}S, NaHS, &c., in many respects closely - resemble the corresponding potassium compounds. - - [28] The metals of the alkaline earths, like those of the alkalis, form - several compounds with sulphur; thus calcium forms compounds with - one and with five atoms of sulphur. There are doubtless also - intermediate sulphides. If sulphuretted hydrogen be passed over - ignited lime it forms water and _calcium sulphide_, which may also - be formed by heating calcium sulphate with charcoal, whilst if - sulphur be heated with lime or with calcium carbonate, then - naturally oxygen compounds (calcium thiosulphate and sulphate) are - formed at the same time as calcium sulphide. The prolonged action - of the vapour of carbon bisulphide, especially when mixed with - carbonic anhydride, on strongly ignited calcium carbonate entirely - converts it into sulphide. Calcium sulphide is generally obtained - as an almost colourless, opaque, brittle mass, which is infusible - at a white heat, and is soluble in water. The act of solution (as - with K_{2}S, Note 21) is partly accompanied by a double - decomposition with the water. When heated, dry calcium sulphide - does not absorb oxygen from the air. An excess of water decomposes - it, like many other metallic sulphides, precipitating lime (as a - product of the decomposition the lime hinders the action of the - water upon the CaS; see soda refuse, Chapter XII., Note 12), and - forming a hydrosulphide, CaH_{2}S_{2}, in solution. This compound - is also formed by passing sulphuretted hydrogen through an aqueous - solution of calcium sulphide or lime. Its solution, like that of - calcium sulphide, has an alkaline reaction. It decomposes when - evaporated, and absorbs oxygen from the air. _Calcium - pentasulphide_, CaS_{5}, is not known in a pure state, but may be - obtained in admixture with calcium thiosulphate by boiling a - solution of lime or calcium sulphide with sulphur: 3CaH_{2}O_{2} + - 12S = 2CaS_{5} + CaS_{2}O_{3} + 3H_{2}O. A similar compound in an - impure form is formed by the action of air on alkali waste, and is - used for the preparation of thiosulphates. - - Many of the sulphides of the metals of the alkaline earths are - phosphorescent--that is, they have the faculty of _emitting - light_, after having been subjected to the action of sunlight, or - of any bright source of light (Canton phosphorus, &c.). The - luminosity lasts some time, but it is not permanent, and gradually - disappears. This phosphorescent property is inherent, in a greater - or less degree, to nearly all substances (Becquerel), but for a - very short time, whilst with calcium sulphide it is comparatively - durable, lasting for several hours, and Dewar (1894) showed that - it is far more intense at very low temperatures (for instance, in - bodies cooled in liquid oxygen to -182°). It is due to the - excitation of the surfaces of substances by the action of light, - and is determined by those rays which exhibit a chemical action. - Hence daylight or the light of burning magnesium, &c., acts more - powerfully than the light of a lamp, &c. Warnerke has shown that a - small quantity of magnesium lighted near the surface of a - phosphorescent substance rapidly excites the greatest possible - intensity of luminosity; this enabled him to found a method of - measuring the intensity of light--_i.e._ to obtain a constant unit - of light--and to apply it to photography. The nature of the change - which is accomplished on the surface of the luminous substance is - at present unknown, but in any case it is a renewable one, because - the experiment may be repeated for an infinite number of times and - takes place in a vacuum. The intensity and tint of the light - emitted depend on the method of preparation of the calcium - sulphide, and on the degree of ignition and purity of the calcium - carbonate taken. According to the observations of Becquerel, the - presence of compounds of manganese, bismuth, &c., sodium sulphide - (but not potassium sulphide), &c., although in minute traces, is - perfectly indispensable. This gives reason for thinking that the - formation (in the dark) and decomposition (in light) of double - salts like MnS,Na_{2}S perhaps form the chemical cause of the - phenomena. Compounds of strontium and barium have this property to - even a greater extent than calcium sulphide. These compounds may - be prepared as in the following example: A mixture of sodium - thiosulphate and strontium chloride is prepared; a double - decomposition takes place between the salts, and, on the addition - of alcohol, strontium thiosulphate, SrS_{2}O_{3}, is precipitated, - which, when ignited, leaves strontium sulphide behind. The - strontium sulphide thus prepared emits (when dry) a greenish - yellow light. It contains a certain amount of sulphur, sodium - sulphide, and strontium sulphate. By ignition at various - temperatures, and by different methods of preparation, it is - possible to obtain mixtures which emit different coloured lights. - - [29] As examples, we will describe the sulphides of arsenic, antimony, - and mercury. Arsenic trisulphide, or _orpiment_, As_{2}S_{3}, - occurs native, and is obtained pure when a solution of arsenious - anhydride in the presence of hydrochloric acid comes into contact - with sulphuretted hydrogen (there is no precipitate in the absence - of free acid). A beautiful yellow precipitate is then obtained: - As_{2}O_{3} + 3H_{2}S = 3H_{2}O + As_{2}S_{3}; it fuses when - heated, and volatilises without decomposition. As_{2}S_{3} is - easily obtained in a colloid form (Chapter I., Note 57). When - fused it forms a semi-transparent, yellow mass, and it is thus - that it enters the market. The specific gravity of native orpiment - is 3·4, and that of the artificially-fused mass is 2·7. It is used - as a yellow pigment, and owing to its insolubility in water and - acids it is less injurious than the other compounds corresponding - to arsenious acid. According to the type AsX_{2}, realgar, AsS, is - known, but it is probable that the true composition of this - compound is As_{4}S_{4}--that is, it presents the same relation to - orpiment as liquid phosphuretted hydrogen does to gaseous. - _Realgar_ (_Sandaraca_) occurs native as brilliant red crystals of - specific gravity 3·59, and may be prepared artificially by fusing - arsenic and sulphur in the proportions indicated by its formulæ. - It is prepared in large quantities by distilling a mixture of - sulphur and arsenical pyrites. Like orpiment it dissolves in - calcium sulphide, and even in caustic potash. It is used for - signal lights and fireworks, because it deflagrates and gives a - large and very brilliant white flame with nitre. - - With antimony, sulphur gives a tri- and a pentasulphide. The - former, Sb_{2}S_{3}, which corresponds with antimonious oxide, - occurs native (Chapter XIX.) in a crystalline form; its sp. gr. is - then 4·9, and it presents brilliant rhombic crystals of a grey - colour, which fuse when heated. A substance of the same - composition is obtained as an amorphous orange powder by passing - sulphuretted hydrogen into an acid solution of antimonious oxide. - In this respect antimonious oxide again reacts like arsenious - acid, and the sulphides of both are soluble in ammonium and - potassium sulphides, and, especially in the case of arsenious - sulphide, are easily obtained in colloidal solutions. By prolonged - boiling with water, antimonious sulphide may be entirely converted - into the oxide, hydrogen sulphide being evolved (Elbers). Native - antimony sulphide, or the orange precipitated trisulphide when - fused with dry, or boiled with dissolved, alkalis, forms a - dark-coloured mass (Kermes mineral) formerly much used in - medicine, which contains a mixture of antimonious sulphide and - oxide. There are also compounds of these substances. A so-called - antimony vermilion is much used as a dye; it is prepared by - boiling sodium thiosulphate (six parts) with antimony trichloride - (five parts) and water (fifty parts). This substance probably - contains an oxysulphide of antimony--that is, a portion of the - oxygen in the oxide of antimony in it is replaced by sulphur. Red - antimony ore, and antimony glass, which is obtained by fusing the - trisulphide with antimonious oxide, have a similar composition, - Sb_{2}OS_{2}. In the arts, the _antimony pentasulphide_, - Sb_{2}S_{5}, is the most frequently used of the sulphur compounds - of antimony. It is formed by the action of acids on the so-called - Schlippe's salt, which is a _sodium thiorthantimonate_, - SbS(NaS)_{3}, corresponding with (Chapter XIX., Note 41 bis) - orthantimonic acid, SbO(OH)_{3}, with the replacement of oxygen by - sulphur. It is obtained by boiling finely-powdered native antimony - trisulphide with twice its weight of sodium carbonate, and half - its weight of sulphur and lime, in the presence of a considerable - quantity of water. The processes taking place are as follows:--The - sodium carbonate is converted into hydroxide by the lime, and then - forms sodium sulphide with the sulphur; the sodium sulphide then - dissolves the antimony sulphide, which in this form already - combines with the greatest amount of sulphur, so that a compound - is formed corresponding with antimony pentasulphide dissolved in - sodium sulphide. The solution is filtered and crystallised, care - being taken to prevent access of air, which oxidises the sodium - sulphide. This salt crystallises in large, yellowish crystals, - which are easily soluble in water and have the composition - Na_{3}SbS_{4},9H_{2}O. When heated they lose their water of - crystallisation and then fuse without alteration; but when in - solution, and even in crystalline form, this salt turns brown in - air, owing to the oxidation of the sulphur and the breaking up of - the compound. As it is used in medicine, especially in the - preparation of antimony pentasulphide, it is kept under a layer of - alcohol, in which it is insoluble. Acids precipitate antimony - pentasulphide from a solution of this salt, as an orange powder, - insoluble in acids and very frequently used in medicine (_sulfur - auratum antimonii_). This substance when heated evolves vapours of - sulphur, and leaves antimony trisulphide behind. - - Mercury forms compounds with sulphur of the same types as it does - with oxygen. Mercurous sulphide, Hg_{2}S, easily splits up into - mercury and mercuric sulphide. It is obtained by the action of - potassium sulphide on mercurous chloride, and also by the action - of sulphuretted hydrogen on solutions of salts of the type HgX. - Mercuric sulphide, HgS, corresponding with the oxide, is cinnabar; - it is obtained as a black precipitate by the action of an excess - of sulphuretted hydrogen on solutions of mercuric salts. It is - insoluble in acids, and is therefore precipitated in their - presence. If a certain amount of water containing sulphuretted - hydrogen be added to a solution of mercuric chloride, it first - gives a white precipitate of the composition - Hg_{3}S_{2}Cl_{2}--that is, a compound HgCl,2HgS, a sulphochloride - of mercury like the oxychloride. But in the presence of an excess - of sulphuretted hydrogen, the black precipitate of mercuric - sulphide is formed. In this state it is not crystalline (the red - variety is formed by the prolonged action of polysulphides of - ammonium upon the black HgS), but if it be heated to its - temperature of volatilisation it forms a red crystalline sublimate - which is identical with native cinnabar. In this form its specific - gravity is 8·0, and it forms a red powder, owing to which it is - used as a red pigment (vermilion) in oil, pastel, and other - paints. It is so little attacked by reagents that even nitric acid - has no action on it, and the gastric juices do not dissolve it, so - that it is not poisonous. When heated in air, the sulphur burns - away and leaves metallic mercury. On a large scale cinnabar is - usually prepared in the following manner: 300 parts of mercury and - 115 parts of sulphur are mixed together as intimately as possible - and poured into a solution of 75 parts of caustic potash in 425 - parts of water, and the mixture is heated at 50° for several - hours. Red mercury sulphide is thus formed, and separates out from - the solution. The reaction which takes place is as follows: A - soluble compound, K_{2}HgS_{2}, is first formed; this compound is - able to separate in colourless silky needles, which are soluble in - the caustic potash, but are decomposed by water, and at 50°; this - solution (perhaps by attracting oxygen from the air) slowly - deposits HgS in a crystalline form. - - Spring conducted an interesting research (at Liège, 1894) upon the - conversion of the black amorphous sulphide of mercury, HgS, into - red crystalline cinnabar. This research formed a sequel to - Spring's classical researches on the influence of high pressures - upon the properties of solids and their capacity for mutual - combination. He showed, among other things, that ordinary solids - and even metals (for instance, Pb), after being considerably - compressed under a pressure of 20,000 atmospheres, return on - removal of the pressure to their original density like gases. But - this is only true when the compressed solid is not liable to an - allotropic variation, and does not give a denser variety. Thus - prismatic sulphur (sp. gr. 1·9) passes under pressure into the - octahedral (sp. gr. 2·05) variety. Black HgS (precipitated from - solution) has a sp. gr. 7·6, while that of the red variety is 8·2, - and therefore it might be expected that the former would pass into - the latter under pressure, but experiments both at the ordinary - and a higher temperature did not give the looked-for result, - because even at a pressure of 20,000 atmospheres the black - sulphide was not compressed to the density of cinnabar (a pressure - of as much as 35,000 atmospheres was necessary, which could not be - attained in the experiment). But Spring prepared a black HgS, - which had a sp. gr. of 8·0, and this, under a pressure of 2,500 - atmospheres, passed into cinnabar. He obtained this peculiar black - variety of HgS (sp. gr. 8·0) by distilling cinnabar in an - atmosphere of CO_{2}, when the greater portion of the HgS is - redeposited in the form of cinnabar. Under the action of a - solution of polysulphide of ammonium, this variety of HgS passes - more slowly into the red variety than the precipitated variety - does, while under pressure the conversion is comparatively easy. - - It is worthy of remark, that Linder and Picton obtained complex - compounds of many of the sulphides of the heavy metals (Ca, Hg, - Sb, Zn, Cd, Ag, Au) with H_{2}S, for example H_{2}S,7CuS (by the - action of H_{2}S upon the hydrate of oxide of copper), H_{2}S,9CuS - (in the presence of acetic acid and with an excess of H_{2}S), &c. - Probably we have here a sort of 'solid' solution of H_{2}S in the - metallic sulphides. - -As the acids derived from chlorine, phosphorus, and carbon are the -oxidised hydrogen compounds of these elements, so also we can form an -idea of the acid hydrates of sulphur, or of _the normal acids of -sulphur_, by representing them as the oxidised products of sulphuretted -hydrogen-- - - HCl H_{2}S H_{3}P H_{4}C - HClO H_{2}SO(?) H_{3}PO(?) H_{4}CO - HClO_{2} H_{2}SO_{2}(?) H_{3}PO_{2} H_{4}CO_{2} - HClO_{3} H_{2}SO_{3} H_{3}PO_{3} H_{4}CO_{3} - HClO_{4} H_{2}SO_{4} H_{3}PO_{4} H_{4}CO_{4}[30] - -In the case of chlorine, if not all the hydrates, at all events salts of -all the normal hydrates are known, whilst in the case of sulphur only the -acids H_{2}S, H_{2}SO_{3} and H_{2}SO_{4} are known. But, on the other -hand, the latter are obtained not only as hydrates but also as stable -anhydrides, SO_{2} and SO_{3}, which are formed with the evolution of -heat from sulphur and oxygen; 32 parts of sulphur in combining with 32 -parts of oxygen--that is, in forming SO_{2}--evolve 71,000 heat -units,[31] and if the oxidation proceeds to the formation of SO_{3}, -103,000 heat units are evolved. These figures may be compared with those -which correspond with the passage of carbon into CO and CO_{2}, when -29,000 and 97,000 units of heat are evolved. This determines the -stability of the higher oxides of sulphur, and also expresses the -peculiarity of sulphur as an element which, although an analogue of -oxygen, forms stable compounds with it, and thus fundamentally differs -from chlorine. The higher and lower oxides of chlorine are powerful -oxidising agents, whilst the higher oxide of sulphur, SO_{3}, has but -feeble oxidising powers, and the lower oxide, SO_{2}, frequently acts as -a reducing agent, and is formed by the direct combustion of sulphur, just -as carbonic anhydride, CO_{2}, proceeds from the combustion of carbon. In -the combustion of sulphur, and also in the oxidation (roasting) of the -sulphides and polysulphides by their ignition in air, _sulphurous oxide_, -or _sulphurous anhydride_, or _sulphur dioxide_, SO_{2},[31 bis] is -exclusively formed. It is prepared on a large scale by burning sulphur or -roasting iron pyrites or other sulphides[32] for the manufacture of -sulphuric acid (Chapter VI.), and for direct application in the -manufacture of wine or for bleaching tissues and other purposes. In the -latter instances its application is based on the fact that sulphurous -anhydride acts on certain vegetable matters, and has the property of a -reducing and feeble acid.[32 bis] - - [30] CH_{4} gives CH_{4}O or CH_{3}(OH), wood spirit; CH_{4}O_{2} or - CH_{2}(OH)_{2}, which decomposes into water and CH_{2}O--that is, - methylene oxide or formaldehyde; CH_{4}O_{3} = CH(OH)_{3} = H_{2}O - + CHO(OH), or formic acid; and CH_{4}O_{4} = C(OH)_{4} = 2H_{2}O + - CO_{2}. There are four typical hydrogen compounds, RH, RH_{2}, - RH_{3}, and RH_{4}, and each of them has its typical oxide. Beyond - H_{4} and O_{4} combination does not proceed. - - [31] Rhombic sulphur, 71,080 heat units; monoclinic sulphur, 71,720 - units, according to Thomsen. - - [31 bis] However, when sulphur or metallic sulphides burn in an excess - of air, there is always formed a certain, although small, amount - of SO_{3}, which gives sulphuric acid with the moisture of the - air. - - [32] The enormous amount of sulphuric acid now manufactured is chiefly - prepared by roasting native pyrites, but a considerable amount of - the SO_{2} for this purpose is obtained by roasting zinc blende - (ZnS) and copper and lead sulphides. A certain amount is also - procured from soda refuse (Note 6) and the residues obtained from - the purification of coal gas. - - [32 bis] Sulphurous anhydride is also obtained by the decomposition of - many sulphates, especially of the heavy metals, by the action of - heat; but this requires a very powerful heat. This formation of - sulphurous anhydride from sulphates is based on the decomposition - proper to sulphuric acid itself. When sulphuric acid is strongly - heated (for instance, by dropping it upon an incandescent surface) - it is decomposed into water, oxygen, and sulphurous - anhydride--that is, into those compounds from which it is formed. - A similar decomposition proceeds during the ignition of many - sulphates. Even so stable a sulphate as gypsum does not resist the - action of very high temperatures, but is decomposed in the same - manner, lime being left behind. The decomposition of sulphates by - heat is accomplished with still greater facility in the presence - of sulphur, because in this case the liberated oxygen combines - with the sulphur and the metal is able to form a sulphide. Thus - when ferrous sulphate (green vitriol) is ignited with sulphur, it - gives ferrous sulphide and sulphurous anhydride: FeSO_{4} + 2S = - FeS + 2SO_{2}, and this reaction may even be used for the - preparation of this gas. At 400° sulphuric acid and sulphur give - an extremely uniform stream of pure sulphurous anhydride, so that - it is best prepared on a manufacturing scale by this method. Iron - pyrites, FeS_{2}, when heated to 150° with sulphuric acid (sp. gr. - 1·75) in cast-iron vessels also gives an abundant and uniform - supply of sulphurous anhydride. - -In the laboratory--that is, on a small scale--sulphurous anhydride is -best prepared by deoxidising sulphuric acid by heating it with charcoal, -or copper, sulphur, mercury, &c. Charcoal produces this decomposition of -sulphuric acid at but moderately high temperatures; it is itself -converted into carbonic anhydride,[32 tri] and therefore when sulphuric -acid is heated with charcoal it evolves a mixture of sulphurous and -carbonic anhydrides: C + 2H_{2}SO_{4} = CO_{2} + 2SO_{2} + 2H_{2}O. The -metals which are unable to decompose water, and which do not, therefore, -expel hydrogen from sulphuric acid, are frequently capable of decomposing -sulphuric acid, with the evolution of sulphurous anhydride, just as they -decompose nitric acid, forming the lower oxides of nitrogen. These metals -are silver, mercury, copper, lead, and others. Thus, for example, the -action of copper on sulphuric acid may be expressed by the following -equation: Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O. In the -laboratory this reaction is carried on in a flask with a gas-conducting -tube, and does not take place unless aided by heat.[33] - - [32 tri] Mellitic acid is formed at the same time (Verneuille). - - [33] The thermochemical data connected with this reaction are as - follows: A molecule of hydrogen H_{2}, in combining with oxygen (O - = 16) develops about 69,000 heat units, whilst the molecule of - SO_{2}, in combining with oxygen only develops about 32,000 heat - units--that is, about half as much--and therefore those metals - which cannot decompose water may still be able to deoxidise - sulphuric into sulphurous acid. Those metals which decompose water - and sulphuric acid with the evolution of hydrogen, evolve in - combining with sixteen parts by weight of oxygen more heat than - hydrogen does--for example, K_{2}, Na_{2}, Ca develop about or - more than 100,000 heat units; Fe, Zn, Mn about 70,000 to 80,000 - heat units; whilst those metals which neither decompose water nor - evolve hydrogen from sulphuric acid, but are still capable of - evolving sulphurous anhydride from it, develop less heat with - oxygen than hydrogen, but nearly the same amount, if not more - than, sulphurous anhydride develops--for example, Cu and Hg - develop about 40,000 and Pb about 50,000 heat units. - -In its physical and chemical properties sulphurous anhydride presents a -great _resemblance to carbonic anhydride_. It is a heavy gas, somewhat -considerably soluble in water, very easily condensed into a liquid; it -forms normal and acid salts, does not evolve oxygen under the direct -action of heat,[34] although such metals as sodium and magnesium burn in -it, just as in carbonic anhydride. It has a suffocating odour, which is -well known owing to its being evolved when sulphur or sulphur matches are -burnt. In characterising the properties of sulphurous anhydride, it is -very important to remember (Chapter II.) also that it is more easily -liquefied (at -10°, or at 0° under two atmospheres pressure) than -carbonic anhydride (thirty-six atmospheres at 0°),[35] that it is more -soluble than carbonic anhydride (Vol. I. p. 79); at 0°, 100 vols. of -water dissolve 180 vols. of carbonic anhydride and 688 vols. of sulphuric -anhydride), that the molecular weight of SO_{2} = 64 and of CO_{2} = 44, -and that the density of liquid sulphurous anhydride at 0° = 1·43 -(molecular volume = 45) and of carbonic anhydride = 0·95 (molecular -volume = 49). Although sulphur dioxide is the anhydride of an acid, -nevertheless, like carbonic anhydride, it does not form any stable -compounds with water, but gives a solution from which it may be entirely -expelled by the action of heat.[36] The acid character of sulphurous -anhydride is clearly expressed by the fact that it is entirely absorbed -by alkalis, with which it forms acid and normal salts easily soluble in -water. With salts of barium, calcium, and the heavy metals, the normal -salts of the alkalis, M_{2}SO_{3}, give precipitates exactly like those -formed by the carbonates. In general, the salts of sulphurous acid are -closely analogous to the corresponding carbonates. - - [34] That is, it only dissociates and re-forms the original product on - cooling. - - [35] At a given temperature the pressure of this gas evolved from any - salt will be less than that of carbonic anhydride, if we compare - the separation of a gas from its salts with the phenomenon of - evaporation, as was done in discussing the decomposition of - calcium carbonate. - - Liquid sulphurous anhydride is used on a large scale (Pictet) for - the production of cold. - - [36] De la Rive, Pierre, and more especially Roozeboom, have - investigated the crystallo-hydrate which is formed by sulphurous - anhydride and water at temperatures below 7° under the ordinary - pressure, and in closed vessels (at temperatures below 12°). Its - composition is SO_{2},7H_{2}O, and density 1·2. This hydrate - corresponds with the similar hydrate CO_{2},8H_{2}O obtained by - Wroblewsky. - -_Acid sodium sulphite_, NaHSO_{3}, may be obtained by passing sulphurous -anhydride into a solution of sodium hydroxide. It is also formed by -saturating a solution of sodium carbonate with the gas (carbonic -anhydride is then given off), and as the solubility of the acid sulphite -is much greater than that of the carbonate, a further quantity of the -latter may be dissolved after the passage of the sulphurous anhydride, so -that ultimately a very strong solution of the sulphite may be formed in -this manner, from which it may be obtained in a crystalline form, either -by cooling and evaporating (without heating, for then the salt would give -off sulphurous anhydride) or by adding alcohol to the solution. When -exposed to the air this salt loses sulphurous anhydride and attracts -oxygen, which converts it into sodium sulphate. The acid sulphites of the -alkali metals are able to combine not only with oxygen, but also with -many other substances--for example, a solution of the sodium salt -dissolves sulphur, forming sodium thiosulphate, gives crystalline -compounds with the aldehydes and ketones, and dissolves many bases, -converting them into double sulphites. Having the faculty of attracting -or absorbing oxygen, acid sodium sulphite is also able to absorb -chlorine, and is therefore employed, like sodium thiosulphate, for the -removal of chloride (as an antichlor), especially in the bleaching of -fabrics, when it is necessary to remove the last traces of the chlorine -held in the tissues, which might otherwise have an injurious effect on -them. If a solution of an alkali hydroxide be divided into two parts, and -one half is saturated with sulphurous anhydride, and then the other half -added to it, a normal salt will be obtained in the solution, having an -alkaline reaction, like a solution of sodium carbonate. The acid salt has -a neutral reaction.[36 bis] Like sodium carbonate, _normal sodium -sulphite_ has the composition Na_{2}SO_{3},10H_{2}O, and its maximum -solubility is at 33°--in a word, it very closely resembles sodium -carbonate. Although this salt does not give off sulphurous anhydride from -its solution, it is able, like the acid salt, to absorb oxygen from the -air, and is then converted into sodium sulphate.[37] - - [36 bis] Schwicker (1889) by saturating NaHSO_{3} with potash, or - KHSO_{3} with soda, obtained NaKSO_{3}, in the first instance with - H_{2}O, and in the second instance with 2H_{2}O, probably owing to - the different media in which the crystals are formed. In general - sulphurous acid easily forms double salts. - - [37] The normal salts of calcium and magnesium are slightly, and the - acid salts easily, soluble in water. These acid sulphites are much - used in practice; thus calcium bisulphite is employed in the - manufacture of cellulose from sawdust, for mixing with fibrous - matter in the manufacture of paper. - -Besides the acid character we must also point out the reducing character -of sulphurous anhydride. The reducing action of sulphurous acid, its -anhydride and salts, is due to their faculty of passing into sulphuric -acid and sulphates. The reducing action of the sulphites is particularly -energetic, so that they even convert nitric oxide into nitrous oxide: -K_{2}SO_{3} + 2NO = K_{2}SO_{4} + N_{2}O. The salts of many of the higher -oxides are converted into those of the lower--for example, FeX_{3} into -FeX_{2}, CuX_{2} into CuX, HgX_{2} into HgX; thus 2FeX_{3} + SO_{2} + -2H_{2}O = 2FeX_{2} + H_{2}SO_{4} + 2HX. In the presence of water, -sulphurous anhydride is oxidised by chlorine (SO_{2} + 2H_{2}O + Cl_{2} = -H_{2}SO_{4} + 2HCl), iodine, nitrous acid, hydrogen peroxide, -hypochlorous acid, chloric acid, and other oxygen compounds of the -halogens, chromic, manganic, and many other metallic acids and higher -oxides, as well as all peroxides. Free oxygen in the presence of spongy -platinum is able to oxidise sulphurous anhydride even in the absence of -water, in which case sulphuric anhydride SO_{3} is formed, so that the -latter may be prepared by passing a mixture of sulphurous anhydride and -oxygen over incandescent spongy platinum, or, as it is now prepared on a -large scale in chemical works, by passing this mixture over asbestos or -pumice stone moistened with a solution of platinum salt and ignited. -Sulphurous anhydride is completely absorbed by certain higher oxides--for -instance, by barium peroxide and lead dioxide (PbO_{2} + SO_{2} = -PbSO_{4}).[38] - - [38] This reaction is taken advantage of in removing sulphurous - anhydride from a mixture of gases. Lead dioxide, PbO_{2}, is - brown, and when combined with sulphurous anhydride it forms lead - sulphate, PbSO_{4}, which is white, so that the reaction is - evident both from the change in colour and development of heat. - Sulphurous anhydride is slowly decomposed by the action of light, - with the separation of sulphur and formation of sulphuric - anhydride. This explains the fact that sulphurous anhydride - prepared in the dark gives a white precipitate of silver sulphite, - Ag_{2}SO_{3}, with silver chlorate, AgClO_{4}, but when prepared - in the light, even in diffused light, it gives a dark precipitate. - This naturally depends on the fact that the sulphur liberated then - forms silver sulphide, which is black. - -There are, however, cases where sulphurous anhydride acts as an oxidising -agent--that is, it is _deoxidised_ in the presence of substances which -are capable of absorbing oxygen with still greater energy than the -sulphurous anhydride itself. This oxidising action proceeds with the -formation of sulphuretted hydrogen or of sulphides, while the reducing -agent is oxidised at the expense of the oxygen of the sulphurous -anhydride. In this respect, the action of stannous salts is particularly -remarkable. Stannous chloride, SnCl_{2}, in an aqueous solution gives a -precipitate of stannic sulphide, SnS_{2}, with sulphurous anhydride--that -is, the latter is deoxidised to sulphuretted hydrogen, while SnX_{2} is -oxidised into SnX_{4}. A solution of sulphurous anhydride has also an -oxidising action on zinc. The zinc passes into solution, but no hydrogen -is evolved,[39] because a salt of _hydrosulphurous acid_, ZnS_{2}O_{4}, -is formed. The free acid is still less stable than the salt. - - [39] Schönebein observed that the liquid turns yellow, and acquires the - faculty of decolorising litmus and indigo. Schützenberger showed - that this depends on the formation of a zinc salt of a peculiar - and very powerfully-reducing acid, for with cupric salts the - yellow solution gives a red precipitate of cuprous hydrate or - metallic copper, and it reduces salts of silver and mercury - entirely. An exactly similar solution is obtained by the action of - zinc on sodium bisulphite without access of air and in the cold. - The yellow liquid absorbs oxygen from the air with great avidity, - and forms a sulphate. If the solution be mixed with alcohol, it - deposits a double sulphite of zinc and sodium, - ZnNa_{2}(SO_{3})_{2}, which does not decolorise litmus or indigo. - The remaining alcoholic solution deposits colourless crystals in - the cold, which absorb oxygen with great energy in the presence of - water, but are tolerably stable when dried under the receiver of - an air-pump. The solution of these crystals has the - above-mentioned decolorising and reducing properties. These - crystals contain a sodium salt of a lower acid; their composition - was at first supposed to be HNaSO_{2}, but it was afterwards - proved that they do not contain hydrogen, and present the - composition Na_{2}S_{2}O_{4} (Bernthsen). The same salt is formed - by the action of a galvanic current on a solution of sodium - bisulphite, owing to the action of the hydrogen at the moment of - its liberation. If SO_{2} resembles CO_{2} in its composition, - then hyposulphurous acid H_{2}S_{2}O_{4} resembles oxalic acid - H_{2}C_{2}O_{4}. Perhaps an analogue of formic acid SH_{2}O_{2} - will be discovered. - -The faculty of sulphurous anhydride of combining with various substances -is evident from the above-cited reactions, where it combines with -hydrogen and with oxygen, and this faculty also appears in the fact that, -like carbonic oxide, it combines with chlorine, forming a chloranhydride -of sulphuric acid, SO_{2}Cl_{2}, to which we shall afterwards return. The -same faculty for combination also appears in the salts of sulphurous -acid, in their liability to oxidation and in the exceedingly -characteristic formation of a peculiar series of salts obtained by -Pelouze and Frémy. At a temperature of -10° or below, nitric oxide NO is -absorbed by alkaline solutions of the alkali sulphites, forming a -peculiar series of _nitrosulphates_. At a higher temperature these salts -are not formed but the nitric oxide is reduced to nitrous oxide. But in -the cold the liquid saturated with nitric oxide after a certain time -gives prismatic crystals resembling those of nitre. The composition of -the potassium salt is K_{2}SN_{2}O_{3}--that is, the salt contains the -elements of potassium sulphite and of nitric oxide.[40] - - [40] The instability of this salt is very great, and may be compared to - that of the compound of ferrous sulphate with nitric oxide, for - when heated under the contact influence of spongy platinum, - charcoal, &c., it splits up into potassium sulphate and nitrous - oxide. At 130° the dry salt gives off nitric oxide, and re-forms - potassium sulphite. The free acid has not yet been obtained. These - salts resemble the series of _sulphonitrites_ discovered by Frémy - in 1845. They are obtained by passing sulphurous anhydride through - a concentrated and strongly alkaline aqueous solution of potassium - nitrite. They are soluble in water, but are precipitated by an - excess of alkali. The first product of the action has the - composition K_{3}NS_{3}HO_{9}. It is then converted by the further - action of sulphurous anhydride, cold water, and other reagents - into a series of similar complex salts, many of which give - well-formed crystals. One must suppose that the chief cause of the - formation of these very complex compounds is that they contain - unsaturated compounds, NO, KNO_{2}, and KHSO_{3}, all of which are - subject to oxidation and further combination, and therefore easily - combine among each other. The decomposition of these compounds, - with the evolution of ammonia, when their solutions are heated is - due to the fact that the molecule contains the deoxidant, - sulphurous anhydride, which reduces the nitrous acid, NO(OH), to - ammonia. In my opinion the composition of the sulphonitrites may - be very simply referred to the composition of ammonia, in which - the hydrogen is partly replaced by the radicle of the sulphates. - If we represent the composition of potassium sulphate as - KO.KSO_{3}, the group KSO_{3} will be equivalent (according to the - law of substitution) to HO and to hydrogen. It combines with - hydrogen, forming the potassium acid sulphite, KHSO_{3}. Hence the - group KSO_{3} may also replace the hydrogen in ammonia. Judging by - my analysis (1870) the extreme limit of this substitution, - N(HSO_{3})_{3}, agrees with that of the sulphonitrite, which is - easily formed, simultaneously with alkali, by the action of - potassium sulphite on potassium nitrite, according to the equation - 3K(KSO_{3}) + KNO_{2} + 2H_{2}O = N(KSO_{3})_{3} + 4HKO. The - researches of Berglund, and especially of Raschig (1887), fully - verified my conclusions, and showed that we must distinguish the - following types of salts, corresponding with ammonia, where X - stands for the sulphonic group, HSO_{3}, in which the hydrogen is - replaced by potassium; hence X = KSO_{3}: (1) NH_{2}X, (2) - NHX_{2}, (3) NH_{3}, (4) N(OH)XH, (5) N(OH)X_{2}, (6) N(OH)_{2}X, - just as NH_{2}(OH) is hydroxylamine, NH(OH)_{2}, is the hydrate of - nitrous oxide, and N(OH)_{3} is orthonitrous acid, as follows from - the law of substitution. This class of compounds is in most - intimate relation with the series of sulphonitrous compounds, - corresponding with 'chamber crystals' and their acids, which we - shall consider later. - -There are also several other substances, formed by the oxides of -nitrogen and sulphur, which belong to this class of complex and, under -some circumstances, unstable compounds. In the manufacture of sulphuric -acid, both these classes of oxides come into contact with each other in -the lead chambers, and if there be insufficient water for the formation -of sulphuric acid they give crystalline compounds, termed _chamber -crystals_. As a rule, the composition of the crystals is expressed by the -formula NHSO_{3}. This is a compound of the radicles NO_{2} of nitric -acid, and HSO_{3} of sulphuric acid, or nitro-sulphuric acid, -NO_{2}.SHO_{3}, if sulphuric acid be expressed as OH.SHO_{3} and nitric -by NO_{2}.OH. The tabular crystals of this substance fuse at about 70°, -are formed both by the direct action of nitrous anhydride or nitric -peroxide (but not NO, which is not absorbed by sulphuric acid) on -sulphuric acid (Weltzien and others), and especially on sulphuric acid -containing an anhydride and the lower oxides of sulphur and nitric -acid.[41] - - [41] In the sulphuric acid chambers the lower oxides of nitrogen and - sulphur take part in the reaction. They are oxidised by the oxygen - of the air, and form nitro-sulphuric acid--for example, 2SO_{2} + - N_{2}O_{3} + O_{2} + H_{2}O = 2NHSO_{5}. This compound dissolves - in strong sulphuric acid without changing, and when this solution - is diluted (when the sp. gr. falls to 1·5), it splits up into - sulphuric acid and nitrous anhydride, and by the action of - sulphurous anhydride is converted into nitric oxide, which by - itself (in the absence of nitric acid or oxygen) is insoluble in - sulphuric acid. These reactions are taken advantage of in - retaining the oxides of nitrogen in the Gay-Lussac coke-towers, - and for extracting the absorbed oxides of nitrogen from the - resultant solution in the Glover tower. Although nitric oxide is - not absorbed by sulphuric acid, it reacts (Rose, Brüning) on its - anhydride, and forms sulphurous anhydride and a crystalline - substance, N_{2}S_{2}O_{9} = 2NO + 3SO_{3} - SO_{2} = - N_{2}O_{3}2SO_{3}. This may be regarded as the anhydride of - nitro-sulphuric acid, because N_{2}S_{2}O_{9} = 2NHSO_{5} - - H_{2}O; like nitro-sulphuric acid, it is decomposed by water into - nitro-sulphuric acid and nitrous anhydride. Since boric and - arsenious anhydrides, alumina and other oxides of the form - R_{2}O_{3} are able to combine with sulphuric anhydride to form - similar compounds decomposable by water, the above compound does - not present any exceptional phenomenon. The substance NOClSO_{3} - obtained by Weber by the action of nitrosyl chloride upon - sulphuric anhydride belongs to this class of compounds. - -_Thiosulphuric acid_, H_{2}S_{2}O_{3}--that is, a compound of sulphurous -acid and sulphur--also belongs to the products of combination of -sulphurous acid. In the same way that sulphurous acid, H_{2}SO_{3}, gives -H_{2}SO_{4} with oxygen, so it gives H_{2}S_{2}O_{3} with sulphur. In a -free state it is very unstable, and it is only known in the form of its -salts proceeding from the direct action of sulphur on the normal -sulphites; if endeavours be made to separate it in a free state, it -immediately splits up into those elements from which it might be -formed--that is, into sulphur and sulphurous acid. The most important of -its salts is the _sodium thiosulphate_ (known as hyposulphite), -Na_{2}S_{2}O_{3},5H_{2}O, which occurs in colourless crystals, and is -unacted on by atmospheric oxygen either when in a dry state or in -solution. Many other salts of this acid are easily formed by means of -this salt,[41 bis] although this cannot be done with all bases, for such -bases as alumina, ferric oxide, chromium oxide, and others do not give -compounds with thiosulphuric acid, just as they do not form stable -compounds with carbonic acid. Whenever these salts might be formed, they -(like the acid) split up into sulphurous acid and sulphur, and -furthermore the elements of thiosulphuric acid in many cases act in a -reducing manner, forming sulphuric acid and taking up the oxygen from -reducible oxides. Thus when treated with a thiosulphate the soluble -ferric salts give a precipitate of sulphur and form ferrous salts. The -thiosulphates of the metals of the alkalis are obtained directly by -boiling a solution of their sulphites with sulphur: Na_{2}SO_{3} + S = -Na_{2}S_{2}O_{3}. The same salts are formed by the action of sulphurous -anhydride on solutions of the sulphides; thus sodium sulphide dissolved -in water gives sulphur and sodium thiosulphate when a stream of -sulphurous anhydride is passed through it: 2Na_{2}S + 3SO_{2} = -2Na_{2}S_{2}O_{3} + S. The polysulphides of the alkali metals when left -exposed to the air attract oxygen and also form thiosulphates.[42] - - [41 bis] Many double salts of thiosulphuric acid are known, for - instance, PbS_{2}O_{3},3Na_{2}S_{2}O_{3},12H_{2}O; - CaS_{2}O_{3},3K_{2}S_{2}O_{3},5H_{2}O, &c. (Fortman, Schwicker, - Fock, and others). - - [42] Thus when alkali waste, which contains calcium sulphide, undergoes - oxidation in the air it first forms a calcium polysulphide, and - then calcium thiosulphate, CaS_{2}O_{3}. When iron or zinc acts on - a solution of sulphurous acid, besides the hyposulphurous acid - first formed, a mixture of sulphite and thiosulphate is obtained - (Note 39), 3SO_{2} + Zn_{2} = ZnSO_{3} + ZnS_{2}O_{3}. In this - case, as in the formation of hyposulphurous acid, there is no - hydrogen liberated. One of the most common methods for preparing - thiosulphates consists in the _action of sulphur on the alkalis_. - The reaction is accomplished by the formation of sulphides and - thiosulphates, just as the reaction of chlorine on alkalis is - accompanied by the formation of hypochlorites and chlorides; hence - in this respect the thiosulphates hold the same position in the - order of the compounds of sulphur as the hypochlorites do among - the chlorine compounds. The reaction of caustic soda on an excess - of sulphur may be expressed thus: 6NaHO + 12S = 2Na_{2}S_{5} + - Na_{2}S_{2}O_{3} + 3H_{2}O. Thus sulphur is soluble in alkalis. On - a large scale sodium thiosulphate, Na_{2}S_{2}O_{3}, is prepared - by first heating sodium sulphate with charcoal, to form sodium - sulphide, which is then dissolved in water and treated with - sulphurous anhydride. The reaction is complete when the solution - has become slightly acid. A certain amount of caustic alkali is - added to the slightly acid solution; a portion of the sulphur is - thus precipitated, and the solution is then boiled and evaporated - when the salt crystallises out. The saturation of the solution of - sodium sulphide by sulphurous anhydride is carried on in different - ways--for example, by means of coke-towers, by causing the - solution of sulphide to trickle over the coke, and the sulphurous - anhydride, obtained by burning sulphur, to pass up the coke-tower - from below. An excess of sulphurous anhydride must be avoided, as - otherwise sodium trithionate is formed. Sodium thiophosphate is - also prepared by the double decomposition of the soluble calcium - thiosulphate with sodium sulphate or carbonate, in which case - calcium sulphate or carbonate is precipitated. The calcium - thiosulphate is prepared by the action of sulphurous anhydride on - either calcium sulphide or alkali waste. A dilute solution of - calcium thiosulphate may be obtained by treating alkali waste - which has been exposed to the action of air with water. On - evaporation, this solution gives crystals of the salt containing - CaS_{2}O_{3},5H_{2}O. A solution of calcium thiosulphate must be - evaporated with great care, because otherwise the salt breaks up - into sulphur and calcium sulphide. Even the crystallised salt - sometimes undergoes this change. - - The crystals of sodium thiosulphate are stable, do not effloresce - and at 0° dissolve in one part of water, and at 20° in 0·6 part. - The solution of this salt does not undergo any change when boiled - for a short time, but after prolonged boiling it deposits sulphur. - The crystals fuse at 56°, and lose all their water at 100°. When - the dry salt is ignited it gives sodium sulphide and sulphate. - With acids, a solution of the thiosulphate soon becomes cloudy and - deposits an exceedingly fine powder of sulphur (Note 10). If the - amount of acid added be considerable, it also evolves sulphurous - anhydride: H_{2}S_{2}O_{3} = H_{2}O + S + SO_{2}. Sodium - thiosulphate has many practical uses; it is used in photography - for dissolving silver chloride and bromide. Its solvent action on - silver chloride may be taken advantage of in extracting this metal - as chloride from its ores. In dissolving, it forms a double salt - of silver and sodium: AgCl + Na_{2}S_{2}O_{3} = NaCl + - AgNaS_{2}O_{3}. Sodium thiosulphate is an _antichlor_--that is, a - substance which hinders the destructive action of free chlorine - owing to its being very easily oxidised by chlorine into sulphuric - acid and sodium chloride. The reaction with iodine is different, - and is remarkable for the accuracy with which it proceeds. The - iodine takes up half the sodium from the salt and converts it into - a tetrathionate; 2Na_{2}S_{2}O_{3} + I_{2} = 2NaI + - Na_{2}S_{4}O_{6}, and hence this reaction is employed for the - determination of free iodine. As iodine is expelled from potassium - iodide by chlorine, it is possible also to determine the amount of - chlorine by this method if potassium iodide be added to a solution - containing chlorine. And as many of the higher oxides are able to - evolve iodine from potassium iodide, or chlorine from hydrochloric - acid (for example, the higher oxides of manganese, chromium, &c.), - it is also possible to determine the amounts of these higher - oxides by means of sodium thiosulphate and liberated iodine. This - forms the basis of the iodometric method of volumetric analysis. - The details of these methods will be found in works on analytical - chemistry. - - On adding a solution of a _lead salt_ gradually to a solution of - sodium thiosulphate a white precipitate of lead thiosulphate, - PbS_{2}O_{3}, is formed (a soluble double salt is first formed, - and if the action be rapid, lead sulphide). When this substance is - heated at 200°, it undergoes a change and takes fire. Sodium - thiosulphate in solution rapidly reduces cupric salts to cuprous - salts by means of the sulphurous acid contained in the - thiosulphate, but the resultant cuprous oxide is not precipitated, - because it passes into the state of a thiosulphate and forms a - double salt. These double cuprous salts are excellent reducing - agents. The solution when heated gives a black precipitate of - copper sulphide. - - The following formulæ sufficiently explain the position held by - thiosulphuric acid among the other acids of sulphur: - - Sulphurous acid SO_{2}H(OH) - Sulphuric acid SO_{2}OH(OH) - Thiosulphuric acid SO_{2}SH(OH) - Hyposulphurous acid SO_{2}H(SO_{2}H) - Dithionic acid SO_{2}OH(SO_{2}OH) - - At one time it was thought that all the salts of thiosulphuric - acid only existed in combination with water, and it was then - supposed that their composition was H_{4}S_{2}O_{4}, or - H_{2}SO_{2}, but Popp obtained the anhydrous salts. - -Although sulphur, oxidising at a high temperature, only forms a small -quantity of sulphuric anhydride, SO_{3}, and nearly all passes into -sulphurous anhydride, still the latter may be converted into the higher -oxide, or _sulphuric anhydride_, SO_{3}, by many methods. Sulphuric -anhydride is a solid crystalline substance at the ordinary temperature; -it is easily fusible (15°), and volatile (46°), and rapidly attracts -moisture. Although it is formed by the combination of sulphurous -anhydride with oxygen, it is capable of further combination. Thus it -combines with water, hydrochloric acid, ammonia, with many hydrocarbons, -and even with sulphuric acid, boric and nitrous anhydrides, &c., and also -with bases which burn directly in its vapour, forming sulphates in the -presence of traces of moisture (_see_ Chapter IX., Note 29). The -oxidation of sulphurous anhydride, SO_{2}, into sulphuric anhydride, -SO_{3}, is effected by passing a mixture of the former and dry oxygen or -air over incandescent spongy platinum. An increase of pressure -accelerates the reaction (Hanisch). If the product be passed into a cold -vessel, crystalline sulphuric anhydride is deposited upon the sides of -the vessel, but as it is difficult to avoid all traces of moisture it -always contains compounds of its hydrates: H_{2}S_{2}O_{7} and -H_{2}S_{4}O_{13}, whose presence so modifies the properties of the -anhydride (Weber) that formerly two modifications of the anhydride were -recognised. The same sulphuric anhydride may be obtained from certain -anhydrous sulphates, or those which are almost so, which are decomposed -by heat, whilst an impure but perfectly anhydrous anhydride is formed by -distillation over phosphoric anhydride. For instance, acid sodium -sulphate, NaHSO_{4}, and the pyro- or di-sulphate, Na_{2}S_{2}O_{7} -(Chapter XII.) formed from it, when ignited evolve sulphuric anhydride. -Green vitriol--that is, ferrous sulphate, FeSO_{4}--belongs to the number -of those sulphates which easily give off sulphuric anhydride under the -action of heat. It contains water of crystallisation and parts with it -when it is heated, but the last equivalent of water is driven off with -difficulty, just as is the case with magnesium sulphate, MgSO_{4}7H_{2}O; -however, when thoroughly heated, this evolution of sulphuric anhydride -does take place, although not completely, because at a high temperature a -portion of it is decomposed by the ferrous oxide (SO_{3} + 2FeO), which -is converted into ferric oxide, Fe_{2}O_{3}, and in consequence part of -the sulphuric anhydride is converted into sulphurous anhydride. Thus the -products of the decomposition of ferrous sulphate will be: ferric oxide, -Fe_{2}O_{3}, sulphurous anhydride, SO_{2}, and sulphuric anhydride, -SO_{3}, according to the equation: 2FeSO_{4} = Fe_{2}O_{3} + SO_{2} + -SO_{3}. As water still remains with the ferrous sulphate when it is -heated, the result will partially consist of the hydrate H_{2}SO_{4}, -with anhydride, SO_{3}, dissolved in it. Sulphuric acid was for a long -time prepared in this manner; the process was formerly carried on on a -large scale in the neighbourhood of Nordhausen, and hence the sulphuric -acid prepared from ferrous sulphate is called _fuming Nordhausen acid_. -At the present time the fuming acid is prepared by passing the volatile -products of the decomposition of ferrous sulphate through strong -sulphuric acid prepared by the ordinary method. The sulphurous anhydride -is insoluble in it, but it absorbs the sulphuric anhydride. Sulphuric -anhydride may be prepared not only by igniting FeSO_{4} or sodium -pyrosulphate, Na_{2}S_{2}O_{7} (the decomposition proceeds at 600°), but -also by heating a mixture of the latter and MgSO_{4} (Walters); in the -former case a stable double salt MgNa_{2}(SO_{4})_{2} finally remains. It -is also obtained by the direct combination of SO_{2} and O under the -action of spongy platinum or asbestos coated with platinum black (C. -Winkler's process). Nordhausen sulphuric acid fumes in air, owing to its -containing and easily giving off sulphuric anhydride, and it is therefore -also called _fuming sulphuric acid_; these fumes are nothing but the -vapour of sulphuric anhydride combining with the moisture in the air and -forming non-volatile sulphuric acid (hydrate).[43] - - [43] Nordhausen sulphuric acid may serve as a very simple means for the - preparation of sulphuric anhydride. For this purpose the - Nordhausen acid is heated in a glass retort, whose neck is firmly - fixed in the mouth of a well-cooled flask. The access of moisture - is prevented by connecting the receiver with a drying-tube. On - heating the retort the vapours of sulphuric anhydride will pass - over into the receiver, where they condense; the crystals of - anhydride thus prepared will, however, contain traces of sulphuric - acid--that is, of the hydrate. By repeatedly distilling over - phosphoric anhydride, it is possible to obtain the pure anhydride, - SO_{3}, especially if the process be carried on without access of - air in a closed vessel. - - The ordinary sulphuric anhydride, which is imperfectly freed from - the hydrate, is a snow-white, exceedingly volatile substance, - which crystallises (generally by sublimation) in long silky - prisms, and only gives the pure anhydride when carefully distilled - over P_{2}O_{5}. Freshly prepared crystals of almost pure - anhydride fuse at 16° into a colourless liquid having a specific - gravity at 26° = 1·91, and at 47° = 1·81; it volatilises at 46°. - After being kept for some time the anhydride, even containing only - small traces of water, undergoes a change of the following nature: - A small quantity of sulphuric acid combines by degrees with a - large proportion of the anhydride, forming polysulphuric acids, - H_{2}SO_{4},_n_SO_{3}, which fuse with difficulty (even at 100°, - Marignac), but decompose when heated. In the entire absence of - water this rise in the fusing point does not occur (Weber), and - then the anhydride long remains liquid, and solidifies at about - +15°, volatilises at 40°, and has a specific gravity 1·94 at 16°. - We may add that Weber (1881), by treating sulphuric anhydride with - sulphur, obtained a blue lower oxide of sulphur, S_{2}O_{3}. - Selenium and tellurium also give similar products with SO_{3}, - SeSO_{3}, and TeSO_{3}. Water does not act upon them. - -Nordhausen sulphuric acid contains a peculiar compound of SO_{3} and -H_{2}SO_{4}, or _pyrosulphuric acid_; an imperfect anhydride of sulphuric -acid, H_{2}S_{2}O_{7}, analogous in composition with the salts -Na_{2}S_{2}O_{7}, K_{2}Cr_{2}O_{7}, and bearing the same relation to -H_{2}SO_{4} that pyrophosphoric acid does to H_{3}PO_{4}. The bond -holding the sulphuric acid and anhydride together is unstable. This is -obvious from the fact that the anhydride may easily be separated from -this compound, by the action of heat. In order to obtain the definite -compound, the Nordhausen acid is cooled to 5°, or, better still, a -portion of it is distilled until all the anhydride and a certain amount -of sulphuric acid have passed over into the distillate, which will then -solidify at the ordinary temperature, because the compound -H_{2}SO_{4},SO_{3} fuses at 35°. Although this substance reacts on water, -bases, &c., like a mixture of SO_{3} + H_{2}SO_{4}, still since a -definite compound, H_{2}S_{2}O_{7}, exists in a free state and gives -salts and a chloranhydride, S_{2}O_{5}Cl_{2},[44] we must admit the -existence of a definite pyrosulphuric acid, like pyrophosphoric acid, -only that the latter has a far greater stability and is not even -converted into a perfect hydrate by water. Further, the salts -M_{2}S_{2}O_{7} dissolved in water react in the same manner as the acid -salts MHSO_{4}, whilst the imperfect hydrates of phosphoric acid (for -example, PHO_{3}, H_{4}P_{2}O_{7}) have independent reactions even in an -aqueous solution which distinguish them and their salts from the perfect -hydrates. - - [44] Pyrosulphuric chloranhydride, or _pyrosulphuryl chloride_, - S_{2}O_{5}Cl_{2}, corresponds to pyrosulphuric acid, in the same - way that sulphuryl chloride, SO_{2}Cl_{2}, corresponds to - sulphuric acid. The composition S_{2}O_{5}Cl_{2} = SO_{2}Cl_{2} + - SO_{3}. It is obtained by the action of the vapour of sulphuric - anhydride on sulphur chloride: S_{2}Cl_{2} + 5SO_{3} = 5SO_{2} + - S_{2}O_{5}Cl_{2}. It is also formed (and not sulphuryl chloride, - SO_{2}Cl_{2}, Michaelis) by the action of phosphorus pentachloride - in excess on sulphuric acid (or its first chloranhydride, - SHO_{3}Cl). It is an oily liquid, boiling at about 150°, and of - sp. gr. 1·8. According to Konovaloff (Chapter VII.), its vapour - density is normal. It should be noticed that the same substance is - obtained by the action of sulphuric anhydride on sulphur - tetrachloride, and also on carbon tetrachloride, and this - substance is the last product of the metalepsis of CH_{4}, and - therefore the comparison of SCl_{2} and S_{2}Cl_{2} with products - of metalepsis (_see_ later) also finds confirmation in particular - reactions. Rose, who obtained pyrosulphuryl chloride, - S_{2}O_{5}Cl_{2}, regarded it as SCl_{6},5SO_{3}, for at that time - an endeavour was always made to find two component parts of - opposite polarity, and this substance was cited as a proof of the - existence of a hexachloride, SCl_{6}. Pyrosulphuryl chloride is - decomposed by cold water, but more slowly than chlorosulphuric - acid and the other chloranhydrides. - - The relation between pyrosulphuric acid and the normal acid will - be obvious if we express the latter by the formula OH(SO_{3}H), - because the sulphonic group (SO_{3}H) is then evidently equivalent - to OH, and consequently to H, and if we replace both the hydrogens - in water by this radicle we shall obtain (SO_{3}H)_{2}O--that is, - pyrosulphuric acid. - -[Illustration: FIG. 87.--Concentration of sulphuric acid in glass -retorts. The neck of each retort is attached to a bent glass tube, whose -vertical arm is lowered into a glass or earthenware vessel acting as a -receiver for the steam which comes over from the acid, as the former -still contains a certain amount of acid.] - -_Sulphuric acid_, H_{2}SO_{4}, is formed by the combination of its -anhydride, SO_{3}, and water, with the evolution of a large amount of -heat; the reaction SO_{3} + H_{2}O develops 21,300 heat units. The method -of its preparation on a large scale, and most of the methods employed for -its formation, are dependent on the oxidation of sulphurous anhydride, -and the formation of sulphuric anhydride, which forms sulphuric acid -under the action of water. The technical method of its manufacture has -been described in Chapter VI. The acid obtained _from the lead chambers_ -contains a considerable amount of water, and is also impure owing to the -presence of oxides of nitrogen, lead compounds, and certain impurities -from the burnt sulphur which have come over in a gaseous and vaporous -state (for example, arsenic compounds). For practical purposes, hardly -any notice is taken of the majority of these impurities, because they do -not interfere with its general qualities. Most frequently endeavours are -only made to remove, as far as possible, all the water which can be -expelled.[45] That is, the object is to obtain the hydrate, H_{2}SO_{4}, -from the dilute acid (60 per cent.), and this is effected by evaporation -by means of heat. Every given mixture of water and sulphuric acid begins -to part with a certain amount of aqueous vapour when heated to a certain -definite temperature. At a low temperature either there is no evaporation -of water, or there can even be an absorption of moisture from the air. As -the removal of the water proceeds, the vapour tension of the residue -decreases for the same temperature, and therefore the more dilute the -acid the lower the temperature at which it gives up a portion of its -water. In consequence of this, the removal of water from dilute solutions -of sulphuric acid may be easily carried on (up to 75 p.c. H_{2}SO_{4}) in -lead vessels, because at low temperatures dilute sulphuric acid does not -attack lead. But as the acid becomes more concentrated the temperature at -which the water comes over becomes higher and higher, and then the acid -begins to act on lead (with the evolution of sulphuretted hydrogen and -conversion of the lead into sulphate), and therefore lead vessels cannot -be employed for the complete removal of the water. For this purpose the -evaporation is generally carried on in glass or platinum retorts, like -those depicted in figs. 87 and 88. - - [45] The removal of the water, or concentration to almost the real - acid, H_{2}SO_{4}, is effected for two reasons: in the first place - to avoid the expense of transit (it is cheaper to remove the water - than to pay for its transit), and in the second place because many - processes--for instance, the refining of petroleum--require a - strong acid free from an excess of water, the weak acid having no - action. When in the manufacture of chamber acid, both the - Gay-Lussac tower (cold, situated at the end of the chambers) and - the Glover tower (hot, situated at the beginning of the plant, - between the chambers and ovens for the production of SO_{2}) are - employed, a mixture of nitrose (_i.e._ the product of the - Gay-Lussac tower) and chamber acid containing about 60 p.c. - H_{2}SO_{4}, is poured into the Glover tower, where under the - action of the hot furnace gases containing SO_{2}, and the water - held in the chamber acid (1) N_{2}O_{3} is evolved from the - nitrose; (2) water is expelled from the chamber acid; (3) a - portion of the SO_{2} is converted into H_{2}SO_{4}; and (4) the - furnace gases are cooled. Thus, amongst other things, the Glover - tower facilitates the concentration of the chamber acid (removal - of H_{2}O), but the product generally contains many impurities. - -[Illustration: FIG. 88.--Concentration of sulphuric acid in platinum -retorts.] - -_The concentration of sulphuric acid_ in glass retorts is not a -continuous process, and consists of heating the dilute 75 per cent. acid -until it ceases to give off aqueous vapour, and until acid containing -93-98 per cent. H_{2}SO_{4} (66° Baumé) is obtained--and this takes place -when the temperature reaches 320° and the density of the residue reaches -1·847 (66° Baumé).[46] The platinum vessels designed for the continuous -concentration of sulphuric acid consist of a still _B_, furnished with a -still head _E_, a connecting pipe _E F_, and a syphon tube _H R_, which -draws off the sulphuric acid concentrated in the boiler. A stream of -sulphuric acid previously concentrated in lead retorts to a density of -about 60° Baumé--_i.e._ to 75 per cent. or a sp. gr. of 1·7--runs -continuously into the retort through a syphon funnel _E´_. The apparatus -is fed from above, because the acid freshly supplied is lighter than that -which has already lost water, and also because the water is more easily -evaporated from the freshly supplied acid at the surface. The platinum -retort is heated, and the steam coming off[47] is condensed in a worm _F -G_, whilst as fresh dilute acid is supplied to the boiler the acid -already concentrated is drawn off through the syphon tube _H B_, which is -furnished with a regulating cock by means of which the outflow of the -concentrated acid from the bottom of the retort can be so regulated that -it will always present one and the same specific gravity, corresponding -with the strength required. For this purpose the acid flowing from the -syphon is collected in a receiver _R_, in which a hydrometer, indicating -its density, floats; if its density be less than 66° Baumé, the -regulating cock is closed sufficiently to retard the outflow of sulphuric -acid, so as to lengthen the time of its evaporation in the retort.[48] - - [46] The difficulty with which the last portions of water are removed - is seen from the fact that the boiling becomes very irregular, - totally ceasing at one moment, then suddenly starting again, with - the rapid formation of a considerable amount of steam, and at the - same time bumping and even overturning the vessel in which it is - held. Hence it is not a rare occurrence for the glass retorts to - break during the distillation; this causes platinum retorts to be - preferred, as the boiling then proceeds quite uniformly. - - [47] According to Regnault, the vapour tensions (in millimetres of - mercury) of the water given off by the hydrates of sulphuric acid, - H_{2}SO_{4},_n_H_{2}O, are-- - - _t_=5° 15° 30° - - _n_ = 1 0·1 0·1 0·2 - 2 0·4 0·7 1·5 - 3 0·9 1·6 4·1 - 4 1·3 2·8 7·0 - 5 2·1 4·2 10·7 - 7 3·2 6·2 15·6 - 9 4·1 8·0 19·6 - 11 4·4 9·0 22·2 - 17 5·5 10·6 26·1 - - According to Lunge, the vapour tension of the aqueous vapour given - off from solutions of sulphuric acid containing _p_ per cent. - H_{2}SO_{4}, at _t_°, equals the barometric pressure 720 to 730 - mm. - - _p_= 10 20 30 40 50 60 70 80 85 90 95 - - _t_= 102° 105° 108° 114° 124° 141° 170° 207° 233° 262° 295° - - The latter figures give the temperature at which water is easily - expelled from solutions of sulphuric acid of different strengths. - But the evaporation begins sooner, and concentration may be - carried on at lower temperatures if a stream of air be passed - through the acid. Kessler's process is based upon this (Note 48). - - [48] The greatest part of the sulphuric acid is used in the soda - manufacture, in the conversion of the common salt into sulphate. - For this purpose an acid having a density of 60° Baumé is amply - sufficient. Chamber acid has a density up to 1·57 = 50° to 51° - Baumé; it contains about 35 per cent. of water. About 15 per cent. - of this water can be removed in leaden stills, and nearly all the - remainder may be expelled in glass or platinum vessels. Acid of - 66° Baumé, = 1·847, contains about 96 per cent. of the hydrate - H_{2}SO_{4}. The density falls with a greater or less proportion - of water, the maximum density corresponding with 97-1/2 per cent. - of the hydrate H_{2}SO_{4}. The concentration of H_{2}SO_{4} in - platinum retorts has the disadvantage that sulphuric acid, upwards - of 90 per cent. in strength, does corrode platinum, although but - slightly (a few grams per tens of tons of acid). The retorts - therefore require repairing, and the cost of the platinum exceeds - the price obtained for concentrating the acid from 90 per cent. to - 98 per cent. (in factories the acid is not concentrated beyond - this by evaporation in the air). This inconvenience has lately - (1891, by Mathey) been eliminated by coating the inside of the - platinum retorts with a thin (0·1 to 0·02 mm.) layer of gold which - is 40 times less corroded by sulphuric acid than platinum. Négrier - (1890) carries on the distillation in porcelain dishes, Blond by - heating a thin platinum wire immersed in the acid by means of an - electric current, but the most promising method is that of Kessler - (1891), which consists in passing hot air over sulphuric acid - flowing in a thin stream in stone vessels, so that there is no - boiling but only evaporation at moderate temperatures: the - transference of the heat is direct (and not through the sides of - the vessels), which economises the fuel and prevents the - distilling vessels being damaged. - - When, by evaporation of the water, sulphuric acid attains a - density of 66° Baumé (sp. gr. 1·84), it is impossible to - concentrate it further, because it then distils over unchanged. - _The distillation of sulphuric acid_ is not generally carried on - on a large scale, but forms a laboratory process, employed when - particularly pure acid is required. The distillation is effected - either in platinum retorts furnished with corresponding condensers - and receivers, or in glass retorts. In the latter case, great - caution is necessary, because the boiling of sulphuric acid itself - is accompanied by still more violent jerks and greater - irregularity than even the evaporation of the last portions of - water contained in the acid. If the glass retort which holds the - strong sulphuric acid to be distilled be heated directly from - below, it frequently jerks and breaks. For greater safety the - heating is not effected from below, but at the sides of the - retort. The evaporation then does not proceed in the whole mass, - but only from the upper portions of the liquid, and therefore goes - on much more quietly. The acid may be made to boil quietly also by - surrounding the retort with good conductors of heat--for example, - iron filings, or by immersing a bunch of platinum wires in the - acid, as the bubbles of sulphuric acid vapour then form on the - extremities of the wires. - -Strictly speaking, _sulphuric acid is not volatile_, and at its -so-called boiling-point it really decomposes into its anhydride and -water; its boiling-point (338°) being nothing else but its temperature of -decomposition. The products of this decomposition are substances boiling -much below the temperature of the decomposition of sulphuric acid. This -conclusion with regard to the process of the distillation of sulphuric -acid may be deduced from Bineau's observations on the vapour-density of -sulphuric acid. This density referred to hydrogen proved to be half that -which sulphuric acid should have according to its molecular weight, -H_{2}SO_{4}, in which case it should be 49, whilst the observed density -was equal to 24·5. Besides which, Marignac showed that the first portions -of the sulphuric acid distilling over contain less of the elements of -water than the portion which remains behind, or which distils over -towards the end. This is explained by the fact that on distillation the -sulphuric acid is decomposed, but a portion of the water proceeding from -its decomposition is retained by the remaining mass of sulphuric acid, -and therefore at first a mixture of sulphuric acid and sulphuric -anhydride--_i.e._ fuming sulphuric acid--is obtained in the distillate. -It is possible by repeating the distillation several times and only -collecting the first portions of the distillate, to obtain a distinctly -fuming acid. To obtain the definite hydrate H_{2}SO_{4} it is necessary -to refrigerate a highly concentrated acid, of as great a purity as -possible, to which a small quantity of sulphuric anhydride has been -previously added. Sulphuric acid containing a small quantity (a fraction -of a per cent. by weight) of water only freezes at a very low -temperature, while the pure normal acid, H_{2}SO_{4}, solidifies when it -is cooled below 0°, and therefore the normal acid first crystallises out -from the concentrated sulphuric acid. By repeating the refrigeration -several times, and pouring off the unsolidified portion, it is possible -to obtain a pure _normal hydrate_, H_{2}SO_{4}, which melts at 10°·4. -Even at 40° it gives off distinct fumes--that is, it begins to evolve -sulphuric anhydride, which volatilises, and therefore even in a dry -atmosphere the hydrate H_{2}SO_{4} becomes weaker, until it contains -1-1/2 p.c. of water.[49] - - [49] Thus it appears that so common, and apparently so stable, a - compound as sulphuric acid decomposes even at a low temperature - with separation of the anhydride, but this decomposition is - restricted by a limit, corresponding to the presence of about - 1-1/2 p.c. of water, or to a composition of nearly - H_{2}O,12H_{2}SO_{4}. - - Now there is no reason for thinking that this substance is a - definite compound; it is an equilibrated system which does not - decompose under ordinary circumstances below 338°. Dittmar carried - on the distillation under pressures varying between 30 and 2,140 - millimetres (of mercury), and he found that the composition of the - residue hardly varies, and contains from 99·2 to 98·2 per cent. of - the normal hydrate, although at 30 mm. the temperature of - distillation is about 210° and at 2,140 mm. it is 382°. - Furthermore, it is a fact of practical importance that under a - pressure of two atmospheres the distillation of sulphuric acid - proceeds very quietly. - - Sulphuric acid may be _purified_ from the majority of its - impurities by distillation, if the first and last portions of the - distillate be rejected. The first portions will contain the oxides - of nitrogen, hydrochloric acid, &c., and the last portions the - less volatile impurities. The oxides of nitrogen may be removed by - heating the acid with charcoal, which converts them into volatile - gases. Sulphuric acid may be freed from arsenic by heating it with - manganese dioxide and then distilling. This oxidises all the - arsenic into non-volatile arsenic acid. Without a preliminary - oxidation it would partially remain as volatile arsenious acid, - and might pass over into the distillate. The arsenic may also be - driven off by first reducing it to arsenious acid, and then - passing hydrochloric acid gas through the heated acid. It is then - converted into arsenious chloride, which volatilises. - -In a concentrated form sulphuric acid is commercially known as _oil of -vitriol_, because for a long time it was obtained from green vitriol and -because it has an oily appearance and flows from one vessel into another -in a thick and somewhat sluggish stream, like the majority of oily -substances, and in this clearly differs from such liquids as water, -spirit, ether, and the like, which exhibit a far greater mobility. Among -its reactions the first to be remarked is its faculty for the formation -of many compounds. We already know that it combines with its anhydride, -and with the sulphates of the alkali metals; that it is soluble in water, -with which it forms more or less stable compounds. Sulphuric acid, when -mixed with water, develops a very considerable amount of heat.[50] - - [50] The amount of heat developed by the mixture of sulphuric acid with - water is expressed in the diagram on p. 77, Volume I., by the - middle curve, whose abscissæ are the percentage amounts of acid - (H_{2}SO_{4}) in the resultant solution, and ordinates the number - of units of heat corresponding with the formation of 100 cubic - centimetres of the solution (at 18°). The calculations on which - the curve is designed are based on Thomsen's determinations, which - show that 98 grams or a molecular amount of sulphuric acid, in - combining with _m_ molecules of water (that is, with _m_=18 grams - of water), develop the following number of units of heat, R:-- - - _m_ = 1 2 3 5 9 - R = 6379 9418 11137 13108 14952 - _c_ = 0·432 0·470 0·500 0·576 0·701 - T = 127° 149° 146° 121° 82° - - _m_ = 19 49 100 200 - R = 16256 16684 16859 17066 - _c_ = 0·821 0·914 0·954 0·975 - T = 145° 19° 9° 5° - - _c_ stands for the specific heat of H_{2}SO_{4}_m_H_{2}O - (according to Marignac and Pfaundler), and T for the rise in - temperature which proceeds from the mixture of H_{2}SO_{4} with - _m_H_{2}O. The diagram shows that contraction and rise of - temperature proceed almost parallel with each other. - - Besides the normal hydrate H_{2}SO_{4}, _another definite - hydrate_, H_{2}SO_{4},H_{2}O (84·48 per cent. of the normal - hydrate, and 15·52 per cent. of water) is known; it - crystallises[50 bis] extremely easily in large six-sided prisms, - which form above 0°--namely, at about +8°·5; when heated to 210° - it loses water.[51] If the hydrates H_{2}SO_{4} and - H_{2}SO_{4},H_{2}O exist at low temperatures as definite - crystalline compounds, and if pyrosulphuric acid, - H_{2}SO_{4}SO_{3}, has the same property, and if they all - decompose with more or less ease on a rise of temperature, with - the disengagement of either SO_{3} or H_{2}O, and in their - ordinary form present all the properties of simple solutions, it - follows that between sulphuric anhydride, SO_{3}, and water, - H_{2}O, there exists a consecutive series of homogeneous liquids - or solutions, among which we must distinguish _definite - compounds_, and therefore it is quite justifiable to look for - other definite compounds between SO_{3} and H_{2}O, beyond the - conditions for a change of state. In this respect we may be guided - by the variation of properties of any kind, proceeding - concurrently with a variation in the composition of a solution. - - [50 bis] Pickering (1890) showed (_a_) that dilute solutions of - sulphuric acid containing up to H_{2}SO_{4} + 10H_{2}O deposit ice - (at -0°·12 when there is 2,000H_{2}O per H_{2}SO_{4}, at -0°·23 - when there is 1,000H_{2}O, at -1°·04 when there is 200H_{2}O, at - -2°·12 when there is 100H_{2}O, at -4°·5 when there is 50H_{2}O, - at -15°·7 when there is 20H_{2}O, and at -61° when the composition - of the solution is H_{2}SO_{4} + 10H_{2}O); (_b_) that for higher - concentrations crystals separate out at a considerable degree of - cold, having the composition H_{2}SO_{4}4H_{2}O, which melt at - -24°·5, and if either water or H_{2}SO_{4} be added to this - compound the temperature of crystallisation falls, so that a - solution of the composition 12H_{2}SO_{4} + 100H_{2}O gives - crystals of the above hydrate at -70°, 15H_{2}SO_{4} + 100H_{2}O - at -47°, 30H_{2}SO_{4} + 100H_{2}O at -32°, 40H_{2}SO_{4} + - 100H_{2}O at -52°; (_c_) that if the amount of H_{2}SO_{4} be - still greater, then a hydrate H_{2}SO_{4}H_{2}O separates out and - melts at +8°·5, while the addition of water or sulphuric acid to - it lowers the temperature of crystallisation so that the - crystallisation of H_{2}SO_{4}H_{2}O from a solution of the - composition H_{2}SO_{4} + 1·73H_{2}O takes place at -22°, - H_{2}SO_{4} + 1·5H_{2}O at -6°·5, H_{2}SO_{4} + 1·2H_{2}O at - +3°·7, H_{2}SO_{4} + 0·75H_{2}O at +2°·8, H_{2}SO_{4} + 0·5H_{2}O - at -16°; (_d_) that when there is less than 40H_{2}O per - 100H_{2}SO_{4}, refrigeration separates out the normal hydrate - H_{2}SO_{4}, which melts at +10°·35, and that a solution of the - composition H_{2}SO_{4} + 0·35H_{2}O deposits crystals of this - hydrate at -34°, H_{2}SO_{4} + 0·1H_{2}O at -4°·1, H_{2}SO_{4} + - 0·05H_{2}O at +4°·9, while fuming acid of the composition - H_{2}SO_{4} + 0·05SO_{3} deposits H_{2}SO_{4} at about +7°. Thus - the temperature of the separation of crystals clearly - distinguishes the above four regions of solutions, and in the - space between H_{2}SO_{4} + H_{2}O and +25H_{2}O a particular - hydrate H_{2}SO_{4}4H_{2}O separates out, discovered by Pickering, - the isolation of which deserves full attention and further - research. I may add here that the existence of a hydrate - H_{2}SO_{4}4H_{2}O was pointed out in my work, _The Investigation - of Aqueous Solutions_, p. 120 (1887), upon the basis that it has - at all temperatures a smaller value for the coefficient of - expansion _k_ in the formula S_{_t_} = S_{0}/(1 - _kt_) than the - adjacent (in composition) solutions of sulphuric acid. And for - solutions approximating to H_{2}SO_{4}10H_{2}O in their - composition, _k_ is constant at all temperatures (for more dilute - solutions the value of _k_ increases with _t_ and for more - concentrated solutions it decreases). This solution (with - 10H_{2}O) forms the point of transition between more dilute - solutions which deposit ice (water) when refrigerated and those - which give crystals of H_{2}SO_{4}4H_{2}O. According to R. Pictet - (1894) the solution H_{2}SO_{4}10H_{2}O freezes at -88° (but no - reference is made as to what separates out), _i.e._ at a lower - temperature than all the other solutions of sulphuric acid. - However, in respect to these last researches of R. Pictet (for - 88·88 p.c. H_{2}SO_{4} -55°, for H_{2}SO_{4}H_{2}O +3·5°, for - H_{2}SO_{4}2H_{2}O -70°, for H_{2}SO_{4}4H_{2}O -40°, &c.) it - should be remarked that they offer some quite improbable data; for - example, for H_{2}SO_{4}75H_{2}O they give the freezing point as - 0°, for H_{2}SO_{4}300H_{2}O +4°·5, and even for - H_{2}SO_{4}1000H_{2}O +0°·5, although it is well known that a - small amount of sulphuric acid lowers the temperature of the - formation of ice. I have found by direct experiment that a frozen - solidified solution of H_{2}SO_{4} + 300H_{2}O melted completely - at 0°. - - [51] With an excess of snow, the hydrate H_{2}SO_{4},H_{2}O, like the - normal hydrate, gives a freezing mixture, owing to the absorption - of a large amount of heat (the latent heat of fusion). In melting, - the molecule H_{2}SO_{4} absorbs 960 heat units, and the molecule - H_{2}SO_{4}H_{2}O 3,680 heat units. If therefore we mix one gram - molecule of this hydrate with seventeen gram molecules of snow, - there is an absorption of 18,080 heat units, because 17H_{2}O - absorbs 17 × 1,430 heat units, and the combination of the - monohydrate with water evolves 9,800 heat units. As the specific - heat of the resultant compound H_{2}SO_{4},18H_{2}O = 0·813, the - fall of temperature will be -52°·6. And, in fact, a very low - temperature may be obtained by means of sulphuric acid. - -But only a few properties have been determined with sufficient accuracy. -In those properties which have been determined for many solutions of -sulphuric acid, it is actually seen that the above-mentioned definite -compounds are distinguished by distinctive marks of change. As an example -we may cite the variation of the specific gravity with a variation of -temperature (namely K = _ds/dt_, if _s_ be the sp. gr. and _t_ the -temperature). For the normal hydrate, H_{2}SO_{4}, this factor is easily -determined from the fact that-- - - _s_ = 18528 - 10·65_t_ + 0·013_t_^2, - -where _s_ is the specific gravity at _t_ (degrees Celsius) if the sp. -gr. of water at 4° = 10,000. Therefore K = 10·65 - 0·026_t_. This means -that at 0° the sp. gr. of the acid H_{2}SO_{4} decreases by 10·65 for -every rise of a degree of temperature, at 10° by 10·39, at 20° by 10·13, -at 30° by 9·87.[52] And for solutions containing slightly more anhydride -than the acid H_{2}SO_{4} (_i.e._ for fuming sulphuric acid), as well as -for solutions containing more water, K is greater than for the acid -H_{2}SO_{4}. Thus for the solution SO_{3},2H_{2}SO_{4}, at 10° K = 11·0. -On diluting the acid H_{2}SO_{4} K again increases until the formation of -the solution H_{2}SO_{4},H_{2}O (K = 11·1 at 10°), and then, on further -dilution with water, it again decreases. Consequently both hydrates -H_{2}SO_{4} and H_{2}SO_{4},H_{2}O are here expressed by an alteration of -the magnitude of K. - - [52] For example, if it be taken that at 19° the sp. gr. of pure - sulphuric acid is 1·8330, then at 20° it is 1·8330 - (20 - - 19)10·13 = 1·8320. - -[Illustration: FIG. 89.--Diagram showing the variation of the factor -(_ds/dp_) of the specific gravity of solutions of sulphuric acid. The -percentage quantities of the acid, H_{2}SO_{4}, are laid out on the axes -of abscissæ. The ordinates are the factors or rises in sp. gr. (water -at 4 = 10,000) with the increase in the quantity of H_{2}SO_{4}.] - -This shows that in liquid solutions it is possible by studying the -variation of their properties (without a change of physical state) to -recognise the presence or formation of definite hydrate compounds, and -therefore an exact investigation of the properties of solutions, of their -specific gravity for instance, should give direct indications of such -compounds.[53] The mean result of the most trustworthy determinations of -this nature is given in the following tables. The first of these tables -gives the specific gravities (in vacuo, taking the sp. gr. of water at 4° -= 1), at 0° (column 3), 15° (column 4), and 30° (column 5),[53 bis] for -solutions having the composition H_{2}SO_{4} + _n_H_{2}O (the value of -_n_ is given in the first column), and containing _p_ (column 2) per -cent. (by weight in vacuo) of H_{2}SO_{4}.[53 tri] - - _n_ _p_ 0° 15° 30° - 100 5·16 1·0374 1·0341 1·0292 - 50 9·82 1·0717 1·0666 1·0603 - 25 17·88 1·1337 1·1257 1·1173 - 15 26·63 1·2040 1·1939 1·1837 - 10 35·25 1·2758 1·2649 1·2540 - 8 40·50 1·3223 1·3110 1·2998 - 6 47·57 1·3865 1·3748 1·3622 - 5 52·13 1·4301 1·4180 1·4062 - 4 57·65 1·4881 1·4755 1·4631 - 3 64·47 1·5635 1·5501 1·5370 - 2 73·13 1·6648 1·6500 1·6359 - 1 84·48 1·7940 1·7772 1·7608 - 0·5 91·59 1·8445 1·8284 1·8128 - H_{2}SO_{4} 100 1·8529 1·8372 1·8221 - - [53] Unfortunately, notwithstanding the great number of fragmentary and - systematic researches which have been made (by Parks, Ure, Bineau, - Kolbe, Lunge, Marignac, Kremers, Thomsen, Perkin, and others) for - determining the relation between the sp. gr. and composition of - solutions of sulphuric acid, they contain discrepancies which - amount to, and even exceed, 0·002 in the sp. gr. For instance, at - 15°·4 the solution of composition H_{2}SO_{4}3H_{2}O has a sp. gr. - 1·5493 according to Perkin (1886), 1·5501 according to Pickering - (1890), and 1·5525 according to Lunge (1890). The cause of these - discrepancies must be looked for in the methods employed for - determining the composition of the solutions--_i.e._ in the - inaccuracy with which the percentage amount of H_{2}SO_{4} is - determined, for a difference of 1 p.c. corresponds to a difference - of from 0·0070 (for very weak solutions) to 0·0118 (for a solution - containing about 73 p.c.) in the specific gravity (that is the - factor _ds/dp_) at 15°. As it is possible to determine the - specific gravity with an accuracy even exceeding 0·0002, the - specific gravities given in the adjoining tables are only averages - and most probable data in which the error, especially for the - 30-80 p.c. solutions cannot be less than 0·0010 (taking water at - 4° as 1). - - [53 bis] Judging from the best existing determinations (of Marignac, - Kremers, and Pickering) for solutions of sulphuric acid - (especially those containing more than 5 p.c. H_{2}SO_{4}) within - the limits of 0° and 30° (and even to 40°), the variation of the - sp. gr. with the temperature _t_ may (within the accuracy of the - existing determinations) be perfectly expressed by the equation - S_{_t_} = S_{_0_} + A_t_ + B_t_^2. It must be added that (1) three - specific gravities fully determine the variation of the density - with _t_; (2) _ds/dt_ = A + 2B_t_--_i.e._ the factor of the - temperature is expressed by a straight line; (3) the value of A - (if _p_ be greater than 5 p.c.) is negative, and numerically much - greater than B; (4) the value of B for dilute solutions containing - less than 25 p.c. is negative; for solutions approximating to - H_{2}SO_{4}3H_{2}O in their composition it is equal to 0, and for - solutions of greater concentration B is positive; (5) the factor - _ds/dp_ for all temperatures attains a maximum value about - H_{2}SO_{4}H_{2}O; (6) on dividing _ds/dt_ by S_{_0_}, and so - obtaining the coefficient of expansion _k_ (_see_ Note 53), a - minimum is obtained near H_{2}SO_{4} and H_{2}SO_{4}4H_{2}O, and a - maximum at H_{2}SO_{4}H_{2}O for all temperatures. - - [53 tri] These data (as well as those in the following table) have been - recalculated by me chiefly upon the basis of Kremer's, - Pickering's, Perkin's, and my own determinations; all the - requisite corrections have been introduced, and I have reason for - thinking that in each of them the probable error (or difference - from the true figures, now unknown) of the specific gravity does - not exceed ±0·0007 (if water at 4° = 1) for the 25-80 p.c. - solutions, and ±0·0002 for the more dilute or concentrated - solutions. - -In the second table the first column gives the percentage amount _p_ (by -weight) of H_{2}SO_{4}, the second column the weight in grams (S_{15}) of -a litre of the solution at 15° (at 4° the weight of a litre of water = -1,000 grams), the third column, the variation (_d_S/_dt_) of this weight -for a rise of 1°, the fourth column, the variation _d_S/_dp_ of this -weight (at 15°) for a rise of 1 per cent. of H_{2}SO_{4}, the fifth -column, the difference between the weight of a litre at 0° and 15° (S_{0} -- S_{15}), and the sixth column, the difference between the weight of a -litre at 15° and 30° (S_{15} - S_{30}). - - _p_ _S__{15} _dS__{15}/_dt_ _dS__{15}/_dp_ _S__{0}- _S__{15}- - _S__{15} _S__{30} - 0 999·15 0·148 7·0 0·7 3·4 - 5 1033·0 0·27 6·8 3·1 5·0 - 10 1067·7 0·38 7·1 5·2 6·4 - 20 1141·9 0·58 7·7 8·6 8·9 - 30 1221·3 0·69 8·2 10·4 10·4 - 40 1306·6 0·75 8·8 11·3 11·2 - 50 1397·9 0·79 9·9 11·9 11·8 - 60 1501·2 0·86 10·8 13·0 12·7 - 70 1613·1 0·93 11·6 14·1 13·8 - 80 1731·4 1·04 11·0 15·8 15·4 - 90 1819·9 1·08 5·4 16·4 16·0 - 95 1837·6 1·03 +1·7 15·8 15·1 - 100 1837·2 1·03 -1·9[54] 15·7 15·1 - -The figures in these tables give the means of finding the amount of -H_{2}SO_{4} contained in a solution from its specific gravity,[55] and -also show that 'special points' in the lines of variation of the specific -gravity with the temperature and percentage composition correspond to -certain definite compounds of H_{2}SO_{4} with OH_{2}. This is best seen -in the variation of the factors (_d_S/_dt_ and _d_S/_dp_) with the -temperature and composition (columns 3, 4, second table). We have already -mentioned how the factor of temperature points to the existence of -hydrates, H_{2}SO_{4} and H_{2}SO_{4},H_{2}O. As regards the factor -_d_S/_dp_ (giving the increase of sp. gr. with an increase of 1 per cent. -H_{2}SO_{4}) the following are the three most salient points: (1) In -passing from 98 per cent. to 100 per cent. the factor is negative, and at -100 per cent. about -0·0019 (_i.e._ at 99 per cent. the sp. gr. is about -1·8391, and at 100 per cent. about 1·8372, at 15°, the amount of -H_{2}SO_{4} has increased whilst the sp. gr. has decreased), but as soon -as a certain amount of SO_{3} is added to the definite compound -H_{2}SO_{4} (and 'fuming' acid formed) the specific gravity rises (for -example, for H_{2}SO_{4} 0·136 SO_{3} the sp. gr. at 15° = 1·866), that -is the factor becomes positive (and, in fact, greater by +0·01), so that -the formation of the definite hydrate H_{2}SO_{4} is accompanied by a -distinct and considerable break in the continuity of the factor[55 bis]; -(2) The factor (_d_S/_dp_) in increasing in its passage from dilute to -concentrated solutions, attains a maximum value (at 15° about 0·012) -about H_{2}SO_{4}2H_{2}O, _i.e._ at about the hydrate corresponding to -the form SX_{6}; proper to the compounds of sulphur, for S(OH)_{6} = -H_{2}SO_{4}2H_{2}O; the same hydrate corresponds to the composition of -gypsum CaSO_{4}2H_{2}O, and to it also corresponds the greatest -contraction and rise of temperature in mixing H_{2}SO_{4} with H_{2}O -(_see_ Chapter I., Note 28); (3) The variation of the factor (_d_S/_dp_) -under certain variations in the composition proceeds so uniformly and -regularly, and is so different from the variation given under other -proportions of H_{2}SO_{4} and H_{2}O, that the sum of the variations of -_d_S/_dp_ is expressed by a series of straight lines, if the values of -_p_ be laid along the axis of abscissæ and those of _d_S/_dp_ along the -ordinates.[56] Thus, for instance, for 15°, at 10 per cent. _d_S/_dp_ = -0·0071, at 20 per cent. = 0·0077, at 30 per cent. = 0·0082, at 40 per -cent. = 0·0088, that is, for each 10 per cent. the factor increases by -about 0·0006 for the whole of the above range, but beyond this it becomes -larger, and then, after passing H_{2}SO_{4}2H_{2}O, it begins to fall -rapidly. Such changes in the variation of the factor take place -apparently about definite hydrates,[56 bis] and especially about -H_{2}SO_{4}4H_{2}O, H_{2}SO_{4}2H_{2}O and H_{2}SO_{4}H_{2}O. All this -indicating as it does the special chemical affinity of sulphuric acid for -water, although of no small significance for comprehending the nature of -solutions (_see_ Chapter I. and Chapter VII.), contains many special -points which require detailed investigation, the chief difficulty being -that it requires great accuracy in a large number of experimental data. - - [54] The factor _d_S/_dp_ passes through 0, that is, the specific - gravity attains a maximum value at about 98 p.c. This was - discovered by Kohlrausch, and confirmed by Chertel, Pickering, and - others. - - [55] Naturally under the condition that there is no other ingredient - besides water, which is sufficiently true. For commercial acid, - whose specific gravity is usually expressed in degrees of Baumé's - hydrometer, we may add that at 15° - - Specific gravity 1 1·1 1·2 1·3 1·4 1·5 1·6 1·7 1·8 - Degree Baumé 0 13 24 33·3 41·2 48·1 54·1 59·5 64·2 - - 66° Baumé (the strongest commercial acid or oil of vitriol) - corresponds to a sp. gr. 1·84. - - By employing the second table (by the method of interpolation) the - specific gravity, at a given temperature (from 0° to 30°) can be - found for any percentage amount of H_{2}SO_{4}, and therefore - conversely the percentage of H_{2}SO_{4} can be found from the - specific gravity. - - [55 bis] Whether similar (even small) breaks in the continuity of the - factor _dS_/_dp_ exist or not, for other hydrates (for instance, - for H_{2}SO_{4}H_{2}O and H_{2}SO_{4}4H_{2}O) cannot as yet be - affirmed owing to the want of accurate data (Note 53). In my - investigation of this subject (1887) I admit their possibility, - but only conditionally; and now, without insisting upon a similar - opinion, I only hold to the existence of a distinct break in the - factor at H_{2}SO_{4}, being guided by C. Winkler's observations - ond the specific gravities of fuming sulphuric acid. - - [56] In 1887, on considering all the existent observations for a - temperature 0°, I gave the accompanying scheme (p. 243) of the - variation of the factor _ds_/_dp_ at 0°. - - I did not then (1887) give this scheme an absolute value, and now - after the appearance of two series of new determinations (Lunge - and Pickering in 1890), which disagree in many points, I think it - well to state quite clearly: (1) that Lunge's and Pickering's new - determinations have not added to the accuracy of our data - respecting the variation of the specific gravity of solutions of - sulphuric acid; (2) that the sum total of existing data does not - negative (within the limit of experimental accuracy) the - possibility of a rectilinear and broken form for the factors - _ds_/_dp_; (3) that the supposition of 'special points' in - _ds_/_dp_, indicating definite hydrates, finds confirmation in all - the latest determinations; (4) that the supposition respecting the - existence of hydrates determining a break of the factor _ds_/_dp_ - is in in way altered if, instead of a series of broken straight - lines, there be a continuous series of curves, nearly approaching - straight lines; and (5) that this subject deserves (as I mentioned - in 1887) new and careful elaboration, because it concerns that - foremost problem in our science--solutions--and introduces a - special method into it--that is, the study of differential - variations in a property which is so easily observed as the - specific gravity of a liquid. - - [56 bis] These hydrates are: (_a_) H_{2}SO_{4} = SO_{3}H_{2}O (melts - at + 10°·4); (_b_) H_{2}SO_{4}H_{2}O = SO_{3}2H_{2}O - (crystallo-hydrate, melts at +8°·5); (_c_) H_{2}SO_{4}2H_{2}O (is - apparently not crystallisable); (_d_) one of the hydrates between - H_{2}SO_{4}6H_{2}O and H_{2}SO_{4}3H_{2}O, most probably - H_{2}SO_{4}4H_{2}O = SO_{3}5H_{2}O, for it crystallises at -24°·5 - (Note 50 bis); and (_e_) a certain hydrate with a large proportion - of water, about H_{2}SO_{4}150H_{2}O. The existence of the last is - inferred from the fact that the factor _ds_/_dp_ first falls, - starting from water, and then rises, and this change takes place - when _p_ is less than 5 p.c. Certainly a change in the variation - of _ds_/_dp_ or _ds_/_dt_ does take place in the neighbourhood of - these five hydrates (Pickering, 1890, recognised a far greater - number of hydrates). I think it well to add that if the - composition of the solutions be expressed by the percentage amount - of molecules--_r__{1}SO_{3} + (100 - _r__{1})H_{2}O we find that - for H_{2}SO_{4}, _r__{1} = 50, for H_{2}SO_{4}2H_{2}O _r__{1} = 25 - = 50/2, for H_{2}SO_{4}H_{2}O, _r__{1} = 33·333 = 50 · 2/3, while - for H_{2}SO_{4}4H_{2}O, _r__{1} = 16·666 = 50 · 1/3--_i.e._ that - the chief hydrates are distributed symmetrically between H_{2}O - and H_{2}SO_{4}. Besides which I may mention that my researches - (1887) upon the abrupt changes in the factor for solutions of - sulphuric acid, and upon the correspondence of the breaks of - _ds_/_dp_ with definite hydrates, received an indirect - confirmation not only in the solutions of HNO_{3}, HCl, - C_{2}H_{6}O, C_{3}H_{8}O, &c., which I investigated (in my work - cited in Chapter I., Note 19), but also in the careful - observations made by Professor Cheltzoff on the solutions of - FeCl_{3} and ZnCl_{2} (Chapter XVI., Note 4) which showed the - existence in these solutions of an almost similar change in - _ds_/_dp_ as is found in sulphuric acid. The detailed researches - (1893) made by Tourbaba on the solutions of many organic - substances are of a similar nature. Besides which, H. Crompton - (1888), in his researches on the electrical conductivity of - solutions of sulphuric acid, and Tammann, in his observations on - their vapour tension, found a correlation with the hydrates - indicated as above by the investigation of their specific - gravities. The influence of mixtures of a definite composition - upon the chemical relations of solutions is even exhibited in such - a complex process as electrolysis. V. Kouriloff (1891) showed that - mixtures containing about 3 p.c., 47 p.c. and 73 p.c. of sulphuric - acid--_i.e._ whose composition approaches that of the hydrates - H_{2}SO_{4}150H_{2}O, H_{2}SO_{4}6H_{2}O and - H_{2}SO_{4}2H_{2}O--exhibit certain peculiarities in respect to - the amount of peroxide of hydrogen formed during electrolysis. - Thus a 3 p.c. solution gives a maximum amount of peroxide of - hydrogen at the negative pole, as compared with that given by - other neighbouring concentrations. Starting from 3 p.c., the - formation of peroxide of hydrogen ceases until a concentration of - 47 p.c. is reached. - -The great affinity of sulphuric acid for water is also seen from the -fact that when the strong acid acts on the majority of _organic -substances_ containing hydrogen and oxygen (especially on heating) it -very frequently _takes up these elements in the form of water_. Thus -strong sulphuric acid acting on alcohol, C_{2}H_{6}O, removes the -elements of water from it, and converts it into olefiant gas, C_{2}H_{4}. -It acts in a similar manner on wood and other vegetable tissues, which it -chars. If a piece of wood be immersed in strong sulphuric acid it turns -black. This is owing to the fact that the wood contains carbohydrates -which give up hydrogen and oxygen as water to the sulphuric acid, leaving -charcoal, or a black mass very rich in it. For example, cellulose, -C_{6}H_{10}O_{5}, acts in this manner.[57] - - [57] Cellulose, for instance unsized paper or calico, is dissolved by - strong sulphuric acid. Acid diluted with about half its volume of - water converts it (if the action be of short duration) into - vegetable parchment (Chapter I., Note 18). The action of dilute - solutions of sulphuric acid converts it into hydro-cellulose, and - the fibre loses its coherent quality and becomes brittle. The - prolonged action of strong sulphuric acid chars the cellulose - while dilute acid converts it into glucose. If sulphuric acid be - kept in an open vessel, the organic matter of the dust held in the - atmosphere falls into it and blackens the acid. The same thing - happens if sulphuric acid be kept in a bottle closed by a cork; - the cork becomes charred, and the acid turns black. However, the - chemical properties of the acid undergo only a very slight change - when it turns black. Sulphuric acid which is considerably diluted - with water does not produce the above effects, which clearly shows - their dependence on the affinity of the sulphuric acid for water. - It is evident from the preceding that strong sulphuric acid will - act as a powerful poison; whilst, on the other hand, when very - dilute it is employed in certain medicines and as a fertiliser for - plants. - -We have already had frequent occasion to notice the very _energetic acid -properties_ of sulphuric acid, and therefore we will now only consider a -few of their aspects. First of all we must remember that, with calcium, -strontium, and especially with barium and lead, sulphuric acid forms very -slightly soluble salts, whilst with the majority of other metals it gives -more easily soluble salts, which in the majority of cases are able, like -sulphuric acid itself, to combine with water to form crystallo-hydrates. -Normal sulphuric acid, containing two atoms of hydrogen in its molecule, -is able for this reason alone to form two classes of salts, _normal_ and -_acid_, which it does with great facility _with the alkali metals_. The -metals of the alkaline earths and the majority of other metals, if they -do form acid sulphates, do so under exceptional conditions (with an -excess of strong sulphuric acid), and these salts when formed are -decomposable by water--that is, although having a certain degree of -physical stability they have no chemical stability. Besides the acid -salts RHSO_{4}, sulphuric acid also gives other forms of acid salts. An -entire series of salts having the composition RHSO_{4},H_{2}SO_{4}, or -for bivalent metals RSO_{4},3H_{2}SO_{4},[58] has been prepared. Such -salts have been obtained for potassium, sodium, nickel, calcium, silver, -magnesium, manganese. They are prepared by dissolving the sulphates in an -excess of sulphuric acid and heating the solution until the excess of -sulphuric acid is driven off; on cooling, the mass solidifies to a -crystalline salt. Besides which, Rose obtained a salt having the -composition Na_{2}SO_{4},NaHSO_{4}, and if HNaSO_{4} be heated it easily -forms a salt Na_{2}S_{2}O_{7} = Na_{2}SO_{4},SO_{3}; hence it is clear -that sulphuric anhydride combines with various proportions of bases, just -as it combines with various proportions of water. - - [58] Weber (1884) obtained a series of salts R_{2}O,8SO_{3}_n_H_{2}O - for K, Rb, Cs, and Tl. - -We have already learned that sulphuric acid displaces the acid from the -salts of nitric, carbonic, and many other volatile acids. Berthollet's -laws (Chapter X.) explain this by the small volatility of sulphuric acid; -and, indeed, in an aqueous solution sulphuric acid displaces the much -less soluble boric acid from its compounds--for instance, from borax, and -it also displaces silica from its compounds with bases; but both boric -anhydride and silica, when fused with sulphates, decompose them, -displacing sulphuric anhydride, SO_{3}, because they are less volatile -than sulphuric anhydride. It is also well known that with metals, -sulphuric acid forms salts giving off hydrogen (Fe, Zn, &c.), or sulphur -dioxide (Cu, Hg, &c.).[58 bis] - - [58 bis] Ditte (1890) divides all the metals into two groups with - respect to sulphuric acid; the first group includes silver, - mercury, copper, lead, and bismuth, which are only acted upon by - hot concentrated acid. In this case sulphurous anhydride is - evolved without any by-reactions. The second group contains - manganese, nickel, cobalt, iron, zinc, cadmium, aluminium, tin, - thallium, and the alkali metals. They react with sulphuric acid of - any concentration at any temperature. At a low temperature - hydrogen is disengaged, and at higher temperatures (and with very - concentrated acid) hydrogen and sulphurous anhydride are - simultaneously evolved. - -The reactions of sulphuric acid _with respect to organic substances_ are -generally determined by its acid character, when the direct extraction of -water, or oxidation at the expense of the oxygen of the sulphuric -acid,[59] or disintegration does not take place. Thus the majority of the -saturated hydrocarbons, C_{_n_}H_{2_m_}, form with sulphuric acid a -special class of _sulphonic acids_, C_{_n_}H_{2_m_-1}(HSO_{3}); for -example, benzene, C_{6}H_{6}, forms benzenesulphonic acid, -C_{6}H_{5}.SO_{3}H, water being separated, for the formation of which -oxygen is taken up from the sulphuric acid, for the product contains less -oxygen than the sulphuric acid. It is evident from the existence of these -acids that the hydrogen in organic compounds is replaceable by the group -SO_{3}H, just as it may be replaced by the radicles Cl, NO_{2}, CO_{2}H -and others. As the radicle of sulphuric acid or _sulphoxyl_, SO_{2}OH or -SHO_{3}, contains, like carboxyl (Vol. I., p. 395), one hydrogen -(hydroxyl) of sulphuric acid, the resultant substances are acids whose -basicity is equal to the number of hydrogens replaced by sulphoxyl. Since -also sulphoxyl takes the place of hydrogen, and itself contains hydrogen, -the sulpho-acids are equal to a hydrocarbon + SO_{3}, just as every -organic (carboxylic) acid is equal to a hydrocarbon + CO_{2}. Moreover, -here this relation corresponds with actual fact, because many sulphonic -acids are obtained by the direct combination of sulphuric anhydride: -C_{6}H_{5},(SO_{3}H) = C_{6}H_{6} + SO_{3}. The sulphonic acids give -soluble barium salts, and are therefore easily distinguished from -sulphuric acid. They are soluble in water, are not volatile, and when -distilled give sulphurous anhydride (whilst the hydroxyl previously in -combination with the sulphurous anhydride remains in the hydrocarbon -group; thus phenol, C_{6}H_{5}.OH, is obtained from benzenesulphonic -acid), and they are very energetic, because the hydrogen acting in them -is of the same nature as in sulphuric acid itself.[60] - - [59] For example, the action of hot sulphuric acid on nitrogenous - compounds, as applied in Kjeldahl's method for the estimation of - nitrogen (Volume I. p. 249). It is obvious that when sulphuric - acid acts as an oxidising agent it forms sulphurous anhydride. - - The action of sulphuric acid on the alcohols is exactly similar to - its action on alkalis, because the alcohols, like alkalis, react - on acids; a molecule of alcohol with a molecule of sulphuric acid - separates water and forms an _acid_ ethereal salt--that is there - is produced an ethereal compound corresponding with acid salts. - Thus, for example, the action of sulphuric acid, H_{2}SO_{4}, on - ordinary alcohol, C_{2}H_{5}OH, gives water and sulphovinic acid, - C_{2}H_{5}HSO_{4}--that is, sulphuric acid in which one atom of - hydrogen is replaced by the radicle C_{2}H_{5} of ethyl alcohol, - SO_{2}(OH)(OC_{2}H_{5}), or, what is the same thing, the hydrogen - in alcohol is replaced by the radicle (sulphoxyl) of sulphuric - acid, C_{2}H_{5}O.SO_{2}(OH). - - [60] We will mention the following difference between the sulphonic - acids and the ethereal acid sulphates (Note 59): the former - re-form sulphuric acid with difficulty and the latter easily. Thus - sulphovinic acid when heated with an excess of water is - reconverted into alcohol and sulphuric acid. This is explained in - the following manner. Both these classes of acids are produced by - the substitution of hydrogen by SO_{3}H, or the univalent radicle - of sulphuric acid, but in the formation of ethereal acid sulphates - the SO_{3}H replaces the hydrogen of the hydroxyl in the alcohol, - whilst in the formation of the sulphonic acids the SO_{3}H - replaces the hydrogen of a hydrocarbon. This difference is clearly - evidenced in the existence of two acids of the composition - SO_{4}C_{2}H_{6}. The one, mentioned above, is sulphovinic acid or - alcohol, C_{2}H_{5}.OH, in which the hydrogen of the hydroxyl is - replaced by sulphoxyl = C_{2}H_{5}.OSO_{3}H, whilst the other is - alcohol, in which one atom of the hydrogen in ethyl, C_{2}H_{5}, - is replaced by the sulphonic group--that is = - (C_{2}H_{4})SO_{3}H·OH. The latter is called isethionic acid. It - is more stable than sulphovinic acid. The details as to these - interesting compounds must be looked for in works on organic - chemistry, but I think it necessary to note one of the general - methods of formation of these acids. The sulphites of the - alkalis--for example, K_{2}SO_{3}--when heated with the halogen - products of metalepsis, give a halogen salt and a salt of a - sulphonic acid. Thus methyl iodide, CH_{3}I, derived from marsh - gas, CH_{4}, when heated to 100° with a solution of potassium - sulphite, K_{2}SO_{3}, gives potassium iodide, KI, and potassium - methylsulphonate, CH_{3}SO_{3}K--that is a salt of the sulphonic - acid. This shows that the sulphonic acid may be referred to - sulphurous acid, and that there is a resemblance between sulphuric - and sulphurous acid, which clearly reveals itself here in the - formation of one product from them both. - -Sulphuric acid, as containing a large proportion of oxygen, is a -substance which frequently acts as an oxidising agent: in which case it -is _deoxidised, forming sulphurous anhydride_ and water (or even, -although more rarely, sulphuretted hydrogen and sulphur). Sulphuric acid -acts in this manner on charcoal, copper, mercury, silver, organic and -other substances, which are unable to evolve hydrogen from it directly, -as we saw in describing sulphurous anhydride. - -Although the hydrate of a higher saline form of oxidation (Chapter XV.), -sulphuric anhydride is capable of further oxidation, and forms a kind of -peroxide, just as hydrogen gives hydrogen peroxide in addition to water, -or as sodium and potassium, besides the oxides Na_{2}O and K_{2}O, give -their peroxides, compounds which are in a chemical sense unstable, -powerfully oxidising, and not directly able to enter into saline -combinations. If the oxides of potassium, barium, &c., be compared to -water, then their peroxides must in like manner correspond to hydrogen -peroxide,[61] not only because the oxygen contained in them is very -mobile and easily liberated, and because their reactions are similar, but -also because they can be mutually transformed into each other, and are -able to form compounds with each other, with bases and with water, and -indeed form a kind of peroxide salts.[62] This is also the character of -_persulphuric acid_, discovered in 1878 by Berthelot, and its -corresponding anhydride or peroxide of sulphur S_{2}O_{7}. It is formed -from 2SO_{3} + O with the absorption of heat (-27 thousand heat units), -like ozone from O_{2} + O (-29 thousand units of heat), or hydrogen -peroxide from H_{2}O + O (-21 thousand heat units). - - [61] The reaction BaO + O develops 12,000 heat units, whilst the - reaction H_{2}O + O absorbs 21,000 heat units. - - [62] Schöne obtained a compound of peroxide of barium with peroxide of - hydrogen. If barium peroxide be dissolved in hydrochloric (or - acetic) acid, or if a solution of hydrogen peroxide be diluted - with a solution of barium hydroxide, a pure hydrate is - precipitated having the composition BaO_{2},8H_{2}O (sometimes the - composition is taken as BaO_{2},6H_{2}O). This fact was already - known to Thénard. Schöne showed that if hydrogen peroxide be in - excess, a crystalline compound of the two peroxides, - BaO_{2}H_{2}O_{2}, is precipitated. Schöne also obtained small - well-formed crystals of the same composition by adding a solution - of ammonia to an acid solution of barium peroxide (containing a - barium salt and hydrogen peroxide or a compound of BaO_{2} with - the acid). Thus barium peroxide combines with both water and - hydrogen peroxide. This is a very important fact for the - comprehension of the composition of other peroxides. Moreover, if - the peroxides are able to give hydrates they can also form - corresponding salts, _i.e._ they can combine with bases and acids, - as was afterwards found to be the case on further research into - this subject. - -Peroxide of sulphur is produced by the action of a silent discharge -upon a mixture of oxygen and sulphurous anhydride.[63] With water -S_{2}O_{7} gives persulphuric acid, H_{2}S_{2}O_{8}. The latter is -obtained more simply by mixing strong sulphuric acid (not weaker than -H_{2}SO_{4},2H_{2}O) directly with hydrogen peroxide, or by the action of -a galvanic current on sulphuric acid mixed with a certain amount of -water, and cooled, the electrodes being platinum wires, when persulphuric -acid naturally appears at the positive pole.[64] When an acid of the -strength H_{2}SO_{4},6H_{2}O is taken, at first the hydrate of the -sulphuric peroxide, S_{2}O_{7},H_{2}O only is formed; but when the -concentration about the positive pole reaches H_{2}SO_{4},3H_{2}O, a -mixture of hydrogen peroxide and the hydrate of sulphuric peroxide begins -to be formed. Dilute solutions of sulphuric peroxide can be kept better -than more concentrated solutions, but the latter may be obtained -containing as much as 123 grams of the peroxide to a litre. It is a very -instructive fact that hydrogen peroxide is always formed when strong -solutions of persulphuric acid break up on keeping. So that the bond -between the two peroxides is established both by analysis and synthesis: -hydrogen peroxide is able to produce S_{2}H_{2}O_{8}, and the latter to -produce hydrogen peroxide. A mixture of sulphuric peroxide with sulphuric -acid or water is immediately decomposed, with the evolution of oxygen, -either when heated or under the action of spongy platinum. The same thing -takes place with a solution of baryta, although at first no precipitate -is formed and the decomposition of the barium salt, BaS_{2}O_{8}, with -the formation of BaSO_{4}, only proceeds slowly, so that the solution may -be filtered (the barium salt of persulphuric acid is soluble in water). -Mercury, ferrous oxide, and the stannous salts, are oxidised by -S_{2}H_{2}O_{8}. These are all distinct signs of true peroxides. The same -common properties (capacity for oxidising, property of forming peroxide -of hydrogen, &c.) are possessed by the alkali salts of persulphuric acid, -which are obtained by the action of an electric current upon certain -sulphates, for instance ammonium or potassium sulphate. The ammonium salt -of persulphuric acid, (NH_{4})_{2}S_{2}O_{8}, is especially easily formed -by this means, and is now prepared on a large scale and used (like -Na_{2}O_{2} and H_{2}O_{2}) for bleaching tissues and fibres.[65] - - [63] Anhydrous _sulphuric peroxide_, S_{2}O_{7}, is obtained by the - prolonged (8 to 10 hours) action of a silent discharge of - considerable intensity on a mixture of oxygen and sulphurous - anhydride; the vapour of sulphuric peroxide, S_{2}O_{7}, condenses - as liquid drops, or after being cooled to 0° in the form of long - prismatic crystals, resembling those of sulphuric anhydride. The - anhydrous compound S_{2}O_{7} (and also the hydrated compound) - cannot be preserved long, as it splits up into oxygen and - sulphuric anhydride. Direct experiment shows that a mixture of - equal volumes of sulphurous anhydride and oxygen leaves a residue - of a quarter of the oxygen taken, or half of the whole volume, - which indicates the formula S_{2}O_{7}. This substance is soluble - in water, and it then gives a hydrate, probably having the - composition S_{2}O_{7},H_{2}O = 2SHO_{4}. This solution oxidises - the salts SnX_{2}, potassium iodide, and others, which renders it - possible to prove that the solution actually contains one atom of - oxygen capable of effecting oxidation to two molecules of - sulphuric anhydride. - - In order to fully demonstrate the reality of a peroxide form for - acids, it should be mentioned that some years ago Brodie obtained - the so-called _acetic peroxide_, (C_{2}H_{2}O)_{2}O_{2}, by the - action of barium peroxide on acetic anhydride, (C_{2}H_{3}O)_{2}O. - Its corresponding hydrate is also known. This shows that true - peroxides and their hydrates, with reactions similar to those of - hydrogen peroxide, are possible for acids. A similar higher oxide - has long been known for chromium, and Berthelot obtained a like - compound for nitric acid (Chapter VI., Note 26). - - [64] When an acid of the strength H_{2}SO_{4}6H_{2}O is taken, at first - only the hydrate of the sulphuric peroxide, S_{2}O_{7}H_{2}O, is - formed, but when the concentration at the positive pole reaches - H_{2}SO_{4}3H_{2}O, a mixture of hydrogen peroxide and the hydrate - of sulphuric peroxide begins to be formed. A state of equilibrium - is ultimately arrived at when the amounts of these substances - correspond to the proportion S_{2}O_{7} : 2H_{2}O_{2}, which, as - it were, answers to a new hydrate, S_{2}O_{9}2H_{2}O. But its - existence cannot be admitted because the sulphuric peroxide can be - easily distinguished from the hydrogen peroxide in the solution - owing to the fact that the former does not act on an acid solution - of potassium permanganate, whilst the hydrogen peroxide disengages - both its own oxygen and that of the permanganic acid, converting - it into manganous oxide. Their common property of liberating - iodine from an acid solution of the potassium iodide enables the - sum of the active oxygen in them both to be determined. - - [65] If a solution of sulphuric acid which has been first subjected to - electrolysis be neutralised with potash or baryta, the salt which - is formed begins to decompose rapidly with the evolution of oxygen - (Berthelot, 1890). On saturating with caustic baryta, the solution - of the salt formed may be separated from the sulphate of barium, - and then the composition of the resultant compound, BaS_{2}O_{8}, - may be determined from the amount of oxygen disengaged. Marshall - (1891) studied the formation of this class of compounds more - fully; he subjected a saturated solution of bisulphate of - potassium to electrolysis with a current of 3-3-1/2 ampères; - before electrolysis dilute sulphuric acid is added to the liquid - surrounding the negative pole, and during electrolysis the - solution at the anode is cooled. The electrolysis is continued - without interruption for two days, and a white crystalline deposit - separates at the anode. To avoid decomposition, the latter is not - filtered through paper, but through a perforated platinum plate, - and dried on a porous tile. The mother liquor, with the addition - of a fresh solution of bisulphate of potassium, is again subjected - to electrolysis and the crystals formed at the anode are again - collected, &c. The salt so obtained may be recrystallised by - dissolving it in hot water and rapidly cooling the solution after - filtration; a small proportion of the salt is decomposed by this - treatment. Rapid cooling is followed by the formation of small - columnar crystals; slow cooling gives large prismatic crystals. - The composition of the salt is determined either by igniting it, - when it forms sulphate of potassium, or else by titrating the - active oxygen with permanganate: its composition was found to - correspond to the salt of persulphuric acid, K_{2}S_{2}O_{8}. The - solution of the salt has a neutral reaction, and does not give a - precipitate with salts of other metals. K_{2}S_{2}O_{8} is the - most insoluble of the salts of persulphuric acid. With nitrate of - silver it forms persulphate of silver, which gives peroxide of - silver under the action of water according to the equation - Ag_{2}S_{2}O_{8} + 2H_{2}O = Ag_{2}O_{2} + 2H_{2}SO_{4}. With an - alkaline solution of a cupric salt (Fehling's solution) it forms a - red precipitate of peroxide of copper. Manganese and cobalt salts - give precipitates of MnO_{2} and Co_{2}O_{3}. Ferrous salts are - rapidly oxidised, potassium iodide slowly disengages iodine at the - ordinary temperature. All these reactions indicate the powerful - oxidising properties of K_{2}S_{2}O_{8}. In oxidising in the - presence of water it gives a residue of KHSO_{4}. The - decomposition of the dry salt begins at 100° but is not complete - even at 250°. The freshly prepared salt is inodorous, but after - being kept in a closed vessel it evolves a peculiar smell - different from that of ozone. The ammonium salt of persulphuric - acid, (NH_{4})_{2}S_{2}O_{8}, is obtained in a similar manner. It - is soluble to the extent of 58 parts per 100 parts by weight of - water. The decomposition of the ammonium salt by the hydrated - oxide of barium gives the barium salt, BaS_{2}O_{8}4H_{2}O, which - is soluble to the extent of 52·2 parts in 100 parts of water at - 0°. The crystals do not deliquesce in the air and decompose in the - course of several days; they decompose most rapidly in perfectly - dry air. Solutions of the pure salt decompose slowly at the - ordinary temperature; on boiling barium sulphate is gradually - precipitated, oxygen being liberated simultaneously. To completely - decompose this salt it is necessary to boil its solution for a - long time. Alcohol dissolves the solid salt; the anhydrous salt - does not separate from the alcoholic solution, but a hydrate - containing one molecule of water, BaS_{2}O_{8}H_{2}O, which is - soluble in water but insoluble in absolute alcohol. Solid barium - persulphate decomposes even when slightly heated. The free acid, - which may serve for the preparation of other salts, is obtained by - treating the barium salt with sulphuric acid. The lead salt, - PbS_{2}O_{8}, has been obtained from the free acid; it - crystallises with two or three molecules of water. It is soluble - in water, deliquesces in the air, and with alkalis gives a - precipitate of the hydrated oxide which rapidly oxidises into the - binoxide. - - Traube, before Marshall's researches, thought that the - electrolysis of solutions of sulphuric acid did not give - persulphuric acid but a persulphuric oxide having the composition - SO_{4}. On repeating his former researches (1892) Traube obtained - a persulphuric oxide by the electrolysis of a 70 per cent. - solution of sulphuric acid, and he separated it from the solution - by means of barium phosphate. Analysis showed that this substance - corresponded to the above composition SO_{4}, and therefore Traube - considers it very likely that the salts obtained by Marshall - corresponded to an acid H_{2}SO_{4} + SO_{4}, _i.e._ that the - indifferent oxide, SO_{4}, can combine with sulphuric acid and - form peculiar saline compounds. - -In order to understand the relation of sulphuric peroxide to sulphuric -acid we must first remark that hydrogen peroxide is to be considered, in -accordance with the law of substitution, as water, H(OH), in which H is -replaced by (OH). Now the relation of H_{2}S_{2}O_{8} to H_{2}SO_{4} is -exactly similar. The radicle of sulphuric acid, equivalent to hydrogen, -is HSO_{4};[65 bis] it corresponds with the (OH) of water, and therefore -sulphuric acid, H(SHO_{4}), gives (SHO_{4})_{2} or S_{2}H_{2}O_{8}, in -exactly the same manner as water gives (HO)_{2}--_i.e._ H_{2}O_{2}.[66] - - [65 bis] Or one of those supposed ions which appear at the positive - pole in the decomposition of sulphuric acid by the action of a - galvanic current. - - [66] If this be true one would expect the following peroxide hydrates: - for phosphoric acid, (H_{2}PO_{4})_{2} = H_{4}P_{2}O_{8} = 2H_{2}O - + 2PO_{3}; for carbonic acid, (HCO_{3})_{2} = H_{2}C_{2}O_{6} = - H_{2}O + C_{2}O_{5}; and for lead the true peroxide will be also - Pb_{2}O_{5}, &c. Judging from the example of barium peroxide (Note - 62), these peroxide forms will probably combine together. It seems - to me that the compounds obtained by Fairley for uranium are very - instructive as elucidating the peroxides. In the action of - hydrogen peroxide in an acid solution on uranium oxide, UO_{3}, - there is formed a uranium peroxide, UO_{4},4H_{2}O (U = 240), but - hydrogen peroxide acts on uranium oxide in the presence of caustic - soda; on the addition of alcohol a crystalline compound containing - Na_{4}UO_{8},4H_{2}O is precipitated, which is doubtless a - compound of the peroxides of sodium, Na_{2}O_{2}, and uranium, - UO_{4}. It is very possible that the first peroxide, - UO_{4},4H_{2}O, contains the elements of hydrogen peroxide and - uranium peroxide, U_{2}O_{7}, or even U(OH)_{6},H_{2}O_{2}, just - as the peroxide form lately discovered by Spring for tin perhaps - contains Sn_{2}O_{3},H_{2}O_{2}. - -The largest part _of the sulphuric acid made_ is used for reacting on -sodium chloride in the manufacture of sodium carbonate; for the -manufacture of the volatile acids, like nitric, hydrochloric, &c., from -their corresponding salts; for the preparation of ammonium sulphate, -alums, vitriols (copper and iron), artificial manures, superphosphate -(Chapter XIX., Note 18) and other salts of sulphuric acid; in the -treatment of bone ash for the preparation of phosphorus, and for the -solution of metals--for example, of silver in its separation from -gold--for cleaning metals from rust, &c. A large amount of oil of vitriol -is also used in treatment of organic substances; it is used for the -extraction of stearin, or stearic acid, from tallow, for refining -petroleum and various vegetable oils, in the preparation of -nitro-glycerine (Chapter VI., Notes 37 and 37 bis), for dissolving indigo -and other colouring matters, for the conversion of paper into vegetable -parchment, for the preparation of ether from alcohol, for the preparation -of various artificial scents from fusel oil, for the preparation of -vegetable acids, such as oxalic, tartaric, citric, for the conversion of -non-fermentable starchy substances into fermentable glucose, and in a -number of other processes. It would be difficult to find another -artificially-prepared substance which is so frequently applied in the -arts as sulphuric acid. Where there are not works for its manufacture, -the economical production of many other substances of great technical -importance is impossible. In those localities which have arrived at a -high technical activity the amount of sulphuric acid consumed is -proportionally large; sulphuric acid, sodium carbonate, and lime are the -most important of the artificially-prepared agents employed in factories. - -Besides the normal acids of sulphur, H_{2}SO_{3}, H_{2}SO_{3}S, and -H_{2}SO_{4}, corresponding with sulphuretted hydrogen, H_{2}S, in the -same way that the oxy-acids of chlorine correspond with hydrochloric -acid, HCl, there exists a peculiar series of acids which are termed -_thionic acids_. Their general composition is S_{_n_}H_{2}O_{6}, where -_n_ varies from 2 to 5. If _n_ = 2, the acid is called dithionic acid. -The others are distinguished as trithionic, tetrathionic, and -pentathionic acids. Their composition, existence, and reactions are very -easily understood if they be referred to the class of the sulphonic -acids--that is, if their relation to sulphuric acid be expressed in just -the same manner as the relation of the organic acids to carbonic acid. -The organic acids, as we saw (Chapter IX.), proceed from the hydrocarbons -by the substitution of their hydrogen by carboxyl--that is, by the -radicle of carbonic acid, CH_{2}O_{3} - HO = CHO_{2}. The formation of -the acids of sulphur by means of sulphoxyl may be represented in the same -manner, HSO_{3} = H_{2}SO_{4} - HO. Therefore to hydrogen H_{2}, there -should correspond the acids H.SHO_{3}, sulphurous, and SHO_{3}.SHO_{3} = -S_{2}H_{2}O_{6}, or dithionic; to SH_{2} there should correspond the -acids SH(SHO_{3}) = H_{2}S_{2}O_{3} (thiosulphuric), and S(SHO_{3})_{2} = -H_{2}S_{3}O_{6} (trithionic); to S_{2}H_{2} the acids S_{2}H(SHO_{3}) = -H_{2}S_{3}O_{2} (unknown), and S_{2}(SHO_{3})_{2} = H_{2}S_{4}O_{6} -(tetrathionic); to S_{3}H_{2} the acids S_{3}H(SHO_{3}) and -S_{3}(SHO_{3})_{2} = H_{2}S_{5}O_{6} (pentathionic). We know that iodine -reacts directly with the hydrogen of sulphuretted hydrogen and combines -with it, and if thiosulphuric acid contains the radicle of sulphuretted -hydrogen (or hydrogen united with sulphur) of the same nature as in -sulphuretted hydrogen, it is not surprising that iodine reacts with -sodium thiosulphate and forms sodium tetrathionate. Thus, thiosulphuric -acid, HS(SHO_{3}), when deprived of H, gives a radicle which immediately -combines with another similar radicle, forming the tetrathionate -S_{2}(SO_{2}HO)_{2}. On this view[67] of the structure of the thionic -acids and salts, it is also clear how all the thionic acids, like -thiosulphuric acid, easily give sulphur and sulphides, with the exception -only of dithionic acid, H_{2}S_{2}O_{6}, which, judging from the above, -stands apart from the series of the other thionic acids. Dithionic acid -stands in the same relation to sulphuric acid as oxalic acid does to -carbonic acid. Oxalic acid is dicarboxyl, (CHO_{2})_{2} = -C_{2}H_{2}O_{4}, and so also dithionic acid is disulphoxyl, (SHO_{3})_{2} -= S_{2}H_{2}O_{6}. Oxalic acid when ignited decomposes into carbonic -anhydride and carbonic oxide, CO, and dithionic acid when heated -decomposes into sulphuric anhydride and sulphurous anhydride, SO_{2}, and -SO_{2} stands in the same relation to SO_{3} as CO to CO_{2}. This also -explains the peculiarity of the calcium, barium, and lead, &c. salts of -the thionic acids being easily soluble (although the corresponding salts -of H_{2}SO_{3}, H_{2}SO_{4}, and H_{2}S dissolve with difficulty), -because the former are similar to the salts of the sulphonic acids, which -are also soluble in water. Thus the thionic acids are _disulphonic -acids_, just as many dicarboxylic acids are known--for example, -CH_{2}(CO_{2}H)_{2}, C_{6}H_{4}(CO_{2}H)_{2}.[68] - - [67] This view was communicated by me in 1870 to the Russian Chemical - Society. - - [68] _Dithionic acid_, H_{2}S_{2}O_{6}, is distinguished among the - thionic acids as containing the least proportion of sulphur. It is - also called hyposulphuric acid, because its supposed anhydride, - S_{2}O_{5}, contains more O than sulphurous oxide, SO_{2} or - S_{2}O_{4}, and less than sulphuric anhydride, SO_{3} or - S_{2}O_{6}. Dithionic acid, discovered by Gay-Lussac and Welter, - is known as a hydrate and as salts, but not as anhydride. The - method for preparing dithionic acid usually employed is by the - action of finely-powdered manganese dioxide on a solution of - sulphurous anhydride. On shaking, the smell of the latter - disappears, and the manganese salt of the acid in question passes - into solution; MnO_{2} + 2SO_{2} = MnS_{2}O_{6}. If the - temperature be raised, the dithionate splits up into sulphurous - anhydride and manganese sulphate, MnSO_{4}. Generally owing to - this a mixture of manganese sulphate and dithionate is obtained in - the solution. They may be separated by mixing the solution of the - manganese salts with a solution of barium hydroxide, when a - precipitate of manganese hydroxide and barium sulphate is - obtained. In this manner barium dithionate only is obtained in - solution. It is purified by crystallisation, and separates as - BaS_{2}O_{6},2H_{2}O; this is then dissolved in water, and - decomposed with the requisite amount of sulphuric acid. Dithionic - acid, H_{2}S_{2}O_{6}, then remains in solution. By concentrating - the resultant solution under the receiver of an air-pump it is - possible to obtain a liquid of sp. gr. 1·347, but it still - contains water, and on further evaporation the acid decomposes - into sulphuric acid and sulphurous anhydride: H_{2}S_{2}O_{6} = - H_{2}SO_{4} + SO_{2}. The same decomposition takes place if the - solution be slightly heated. Like all the thionic acids, dithionic - acid is readily attacked by oxidising agents, and passes into - sulphurous acid. No dithionate is able to withstand the action of - heat, even when very slight, without giving off sulphurous - anhydride: K_{2}S_{2}O_{6} = K_{2}SO_{4} + SO_{2}. The alkali - dithionates have a neutral reaction (which indicates the energetic - nature of the acid) are soluble in water, and in this respect - present a certain resemblance to the salts of nitric acid (their - anhydrides are: N_{2}O_{5} and S_{2}O_{5}). Klüss (1888) described - many of the salts of dithionic acid. - - Langlois, about 1840, obtained a peculiar thionic acid by heating - a strong solution of acid potassium sulphite with flowers of - sulphur to about 60°, until the disappearance of the yellow - coloration first produced by the solution of the sulphur. On - cooling, a portion of the sulphur was precipitated, and crystals - of a salt of _trithionic acid_, K_{2}S_{3}O_{6} (partly mixed with - potassium sulphate), separated out. Plessy afterwards showed that - the action of sulphurous acid on a thiosulphate also gives sulphur - and trithionic acid: 2K_{2}S_{2}O_{3} + 3SO_{2} = 2K_{2}S_{3}O_{6} - + S. A mixture of potassium acid sulphite and thiosulphate also - gives a trithionate. It is very possible that a reaction of the - same kind occurs in the formation of trithionic acid by Langloid's - method, because potassium sulphite and sulphur yield potassium - thiosulphate. The potassium thiosulphate may also be replaced by - potassium sulphide, and on passing sulphurous anhydride through - the solution thiosulphate is first formed and then trithionate: - 4KHSO_{3} + K_{2}S + 4SO_{2} = 3K_{2}S_{3}O_{6} + 2H_{2}O. The - sodium salt is not formed under the same circumstances as the - corresponding potassium salt. The sodium salt does not crystallise - and is very unstable: the barium salt is, however, more stable. - The barium and potassium salts are anhydrous, they give neutral - solutions and decompose when ignited, with the evolution of - sulphur and sulphurous anhydride, a sulphate being left behind, - K_{2}S_{3}O_{6} = K_{2}SO_{4} + SO_{2} + S. If a solution of the - potassium salt be decomposed by means of hydrofluosilicic or - chloric acid, the insoluble salts of these acids are precipitated - and trithionic acid is obtained in solution, which however very - easily breaks up on concentration. The addition of salts of - copper, mercury, silver, &c., to a solution of a trithionate is - followed, either immediately or after a certain time, by the - formation of a black precipitate of the sulphides whose formation - is due to the decomposition of the trithionic acid with the - transference of its sulphur to the metal. - - _Tetrathionic acid_, H_{2}S_{4}O_{6}, in contradistinction to the - preceding acids, is much more stable in the free state than in the - form of salts. In the latter form it is easily converted into - trithionate, with liberation of sulphur. Sodium tetrathionate was - obtained by Fordos and Gélis, by the action of iodine on a - solution of sodium thiosulphate. The reaction essentially consists - in the iodine taking up half the sodium of the thiosulphate, - inasmuch as the latter contains Na_{2}S_{2}O_{3}, whilst the - tetrathionate contains NaS_{2}O_{3} or Na_{2}S_{4}O_{6}, so that - the reaction is as follows: 2Na_{2}S_{2}O_{3} + I_{2} = 2NaI + - Na_{2}S_{4}O_{6}. It is evident that tetrathionic acid stands to - thiosulphuric acid in exactly the same relation as dithionic acid - does to sulphurous acid; for the same amount of the other elements - in dithionate, KSO_{3}, and tetrathionate, KS_{2}O_{3}, there is - half as much metal as in sulphite, K_{2}SO_{3}, and thiosulphate, - K_{2}S_{2}O_{3}. If in the above reaction the sodium thiosulphate - be replaced by the lead salt PbS_{2}O_{3}, the sparingly-soluble - lead iodide PbI_{2} and the soluble salt PbS_{4}O_{6} are - obtained. Moreover the lead salt easily gives tetrathionic acid - itself (PbSO_{4} is precipitated). The solution of tetrathionic - acid may be evaporated over a water-bath, and afterwards in a - vacuum, when it gives a colourless liquid, which has no smell and - a very acid reaction. When dilute it may be heated to its - boiling-point, but in a concentrated form it decomposes into - sulphuric acid, sulphurous anhydride, and sulphur: H_{2}S_{4}O_{6} - = H_{2}SO_{4} + SO_{2} + S_{2}. - - _Pentathionic acid_, H_{2}S_{5}O_{6}, also belongs to this series - of acids. But little is known concerning it, either as hydrate or - in salts. It is formed, together with tetrathionic acid, by the - direct action of sulphurous acid on sulphuretted hydrogen in an - aqueous solution; a large proportion of sulphur being precipitated - at the same time: 5SO_{2} + 5H_{2}S = H_{2}S_{5}O_{6} + 5S + - 4H_{2}O. - - If, as was shown above, the thionic acids are disulphonic acids, - they may be obtained, like other sulphonic acids, by means of - potassium sulphite and sulphur chloride. Thus Spring demonstrated - the formation of potassium trithionate by the action of sulphur - dichloride on a strong solution of potassium sulphite: 2KSO_{3}K + - SCl_{2} = S(SO_{3}K)_{2} + 2KCl. If sulphur chloride be taken, - sulphur also is precipitated. The same trithionate is formed by - heating a solution of double thiosulphates; for example, of - AgKS_{2}O_{3}. Two molecules of the salts then form silver - sulphide and potassium trithionate. If the thiosulphate be the - potassium silver salt SO_{3}K(AgS), then the structure of the - trithionate must necessarily be (SO_{3}K)_{2}S. Previous to - Spring's researches, the action of iodine on sodium thiosulphate - was an isolated accidentally discovered reaction; he, however, - showed its general significance by testing the action of iodine on - mixtures of different sulphur compounds. Thus with iodine, I_{2}, - the mixture Na_{2}S + Na_{2}SO_{3} forms 2NaI + Na_{2}S_{2}O_{3}, - whilst the mixture Na_{2}S_{2}O_{3} + Na_{2}SO_{3} + I_{2} gives - 2NaI + Na_{2}S_{3}O_{6}--that is, trithionic acid stands in the - same relation to thiosulphuric acid as the latter does to - sulphuretted hydrogen. We adopt the same mode of representation: - by replacing one hydrogen in H_{2}S by sulphuryl we obtain - thiosulphuric acid, HSO_{3}.HS, and by replacing a second hydrogen - in the latter again by sulphuryl we obtain trithionic acid, - (HSO_{3})_{2}S. Furthermore, Spring showed that the action of - sodium amalgam on the thionic acids causes reverse reactions to - those above indicated for iodine. Thus sodium thiosulphate with - Na_{2} gives Na_{2}S + Na_{2}SO_{3}, and Spring showed that the - sodium here is not a simple element taking up sulphur, but itself - enters into double decomposition, replacing sulphur; for on taking - a potassium salt and acting on it with sodium, KSO_{3}(SK) + NaNa - = KSO_{3}Na + (SK)Na. In a similar way sodium dithionate with - sodium gives sodium sulphite: (NaSO_{3})_{2} + Na_{2} = - 2NaSO_{3}Na; sodium trithionate forms NaSO_{3}Na and NaSO_{3}.SNa, - and tetrathionate forms sodium thiosulphate, - (NaSO_{3})S_{2}(NaSO_{3}) + Na_{2} = 2(NaSO_{3})(NaS). - - In all the oxidised compounds of sulphur we may note the presence - of the elements of sulphurous anhydride, SO_{2}, the only product - of the combustion of sulphur, and in this sense the compounds of - sulphur containing one SO_{2} are-- - - H HO C_{6}H_{5} HS - SO_{2} SO_{2} SO_{2} SO_{2} - HO HO HO HO - - Sulphurous Sulphuric Benzene sulphonic Thiosulphuric - acid acid acid acid - - while, according to this mode of representation, the thionic acids - are-- - - HO HO HO HO - SO_{2} SO_{2} SO_{2} SO_{2} - S S_{2} S_{3} - SO_{2} SO_{2} SO_{2} SO_{2} - HO HO HO HO - - Dithionic Trithionic Tetrathionic Pentathionic - - Hence it is evident that SO_{2} has (whilst CO_{2} has not) the - faculty for combination, and aims at forming SO_{2}X_{2}. These - X_{2} can = O, and the question naturally suggests itself as to - whether the O_{2} which occurs in SO_{2} is not of the same nature - as this oxygen which adds itself to SO_{2}--that is, whether - SO_{2} does not correspond with the more general type SX_{4}, and - its compounds with the type SX_{6}? To this we may answer 'Yes' - and 'No'--'Yes' in the general sense which proceeds from the - investigation of the majority of compounds, especially metals, - where RO corresponds with RCl_{2}, RX_{2}; 'No' in the sense that - sulphur does not give either SH_{4}, SH_{6}, or SCl_{6}, and - therefore the stages SX_{4} and SX_{6} are only observable in - oxygen compounds. With reference to the type SX_{6} a hydrate, - S(HO)_{6}, might be expected, if not SCl_{6}. And we must - recognise this hydrate from a study of the compounds of sulphuric - acid with water. In addition to what has been already said - respecting the complex acids formed by sulphur, I think it well to - mention that, according to the above view, still more complex - oxygen acids and salts of sulphur may be looked for. For instance, - the salt Na_{2}S_{4}O_{8} obtained by Villiers (1888) is of this - kind. It is formed together with sodium trithionate and sulphur, - when SO_{2} is passed through a cold solution of Na_{2}S_{2}O_{3}, - which is then allowed to stand for several days at the ordinary - temperature: 2Na_{2}S_{2}O_{3} + 4SO_{2} = Na_{2}S_{4}O_{8} + - Na_{2}S_{3}O_{6} + S. It may be assumed here, as in the thionic - acids, that there are two sulphoxyls, bound together not only by - S, but also by SO_{2}, or what is almost the same thing, that the - sulphoxyl is combined with the residue of trithionic acid, _i.e._ - replaces one aqueous residue in trithionic acid. - -Sulphur exhibits an acid character, not only in its compounds with -hydrogen and oxygen, but also in those with other elements. The compound -of sulphur and carbon has been particularly well investigated. It -presents a great analogy to carbonic anhydride, both in its elementary -composition and chemical character. This substance is the so-called -carbon bisulphide, CS_{2}, and corresponds with CO_{2}. - -The first endeavours to obtain a compound of sulphur with carbon were -unsuccessful, for although sulphur does combine directly with carbon, yet -the formation of this compound requires distinctly definite conditions. -If sulphur be mixed with charcoal and heated, it is simply driven off -from the latter, and not the smallest trace of carbon bisulphide is -obtained. The formation of this compound requires that the charcoal -should be first heated to a red heat, but not above, and then either the -vapour of sulphur passed over it or lumps of sulphur thrown on to the -red-hot charcoal, but in small quantities, so as not to lower the -temperature of the latter. If the charcoal be heated to a white heat, the -amount of carbon bisulphide formed is less. This depends, in the first -place, on the carbon bisulphide dissociating at a high temperature.[69] -In the second place, Favre and Silberman showed that in the combustion of -one gram of carbon bisulphide (the products will be CO_{2} + 2SO_{2}) -3,400 heat units are evolved--that is, the combustion of a molecular -quantity of carbon bisulphide evolves 258,400 heat units (according to -Berthelot, 246,000). From a molecule of carbon bisulphide in grams we may -obtain 12 grams of carbon, whose combustion evolves 96,000 heat units, -and 64 grams of sulphur, evolving by combustion (into SO_{2}) 140,800 -heat units. Hence we see that the component elements separately evolve -less heat by their combustion (237,000 heat units) than carbon bisulphide -itself--that is, that heat should be evolved (at the ordinary -temperature) and not absorbed in its decomposition, and therefore that -the formation of carbon bisulphide from charcoal and sulphur is in all -probability accompanied by an absorption of heat.[70] It is therefore not -surprising that, like other compounds produced with an absorption of heat -(ozone, nitrous oxide, hydrogen peroxide, &c.), carbon bisulphide is -unstable and easily converted into the original substances from which it -is obtained. And indeed if the vapour of carbon bisulphide be passed -through a red-hot tube, it is decomposed--that is, it dissociates--into -sulphur and carbon. And this takes place at the temperature at which this -substance is formed, just as water decomposes into hydrogen and oxygen at -the temperature of its formation. In this absorption of heat in the -formation of carbon bisulphide is explained the facility with which it -suffers reactions of decomposition, which we shall see in the sequel, and -its main difference from the closely analogous carbonic anhydride. - -[Illustration: FIG. 90.--Apparatus for the manufacture of carbon -bisulphide. - - [69] Even light decomposes carbon bisulphide, but not to the extent of - separating carbon; under the action of the sun's rays it is - decomposed into sulphur and solid substance which is considered to - be carbon monosulphide; it is of a red colour, and its sp. gr. is - 1·66. (The formation of a red liquid compound C_{3}S_{2} has also - been remarked.) Thorpe (1889) observed a complete decomposition of - carbon bisulphide under the action of a liquid alloy of potassium - and sodium; it is accompanied by an explosion and the deposition - of carbon and sulphur. A similar complete decomposition of carbon - bisulphide is also accomplished by the action of mercury fulminate - (Chapter XVI., Note 26), and is due to the fact that _at the - ordinary temperature_ (at which carbon bisulphide is not produced) - _the decomposition_ of carbon bisulphide takes place with the - development of heat--that is, it presents an exothermal reaction, - like the decomposition of all explosives. It is very possible that - at a higher temperature, when carbon bisulphide is formed, the - _combination_ of carbon with sulphur is also an exothermal - reaction--that is, heat is developed. If this should be the case, - carbon bisulphide would present a most instructive example in - thermochemistry. - - [70] The fact should not be lost sight of that sulphur and charcoal are - solids at the ordinary temperature, whilst carbon bisulphide is a - very volatile liquid, and consequently, in the act of combination, - referred to the ordinary temperature (Note 69), there is, as it - were, a passage into a liquid state, and this requires the - absorption of heat. And furthermore, the molecule of sulphur - contains at least six atoms, and the molecule of carbon in all - probability (Chapter VIII.) a very considerable number of atoms; - thus the reaction of sulphur on charcoal may be expressed in the - following manner: 3C_{_n_} + _n_S_{6} = 3_n_CS_{2}--that is, from - _n_ + 3 molecules there proceed 3_n_ molecules, and as _n_ must be - very considerable, 3_n_ must be greater than 3 + _n_, which - indicates a decomposition in the formation of carbon bisulphide, - although the reaction at first sight appears as one of - combination. This decomposition is seen also from the volumes in - the solid and liquid states. Carbon bisulphide has a sp. gr. of - 1·29; hence its molecular volume is 59. But the volume of carbon, - even in the form of charcoal, is not more than 6, and the volume - of S_{2} is 30; hence 36 volumes after combination give 59 - volumes--an expansion takes place, as in decompositions. - -In the laboratory carbon bisulphide is prepared as follows: A porcelain -tube is luted into a furnace in an inclined position, the upper extremity -of the tube being closed by a cork, and the lower end connected with a -condenser. The tube contains charcoal, which is raised to a red heat, and -then pieces of sulphur are placed in the upper end. The sulphur melts, -and its vapour comes into contact with the red-hot charcoal, when -combination takes place; the vapours condense in the condenser, carbon -bisulphide being a liquid boiling at 48°. On a large scale the apparatus -depicted in fig. 90 is employed. A cast-iron cylinder rests on a stand in -a furnace. Wood charcoal is charged into the cylinder through the upper -tube closed by a clay stopper, whilst the sulphur is introduced through a -tube reaching to the bottom of the cylinder. Pieces of sulphur thrown -into this tube fall on to the bottom of the cylinder, and are converted -into vapour, which passes through the entire layer of charcoal in the -cylinder. The vapour of carbon bisulphide thus formed passes through the -exit tube first into a Woulfe's bottle (where the sulphur which has not -entered into the reaction is condensed), and then into a strongly-cooled -condenser or worm.[71] - - [71] Carbon bisulphide, as prepared on a large scale, is generally very - impure, and contains not only sulphur, but, more especially, other - impurities which give it a very disagreeable odour. The best - method of purifying this malodorous carbon bisulphide is to shake - it up with a certain amount of mercuric chloride, or even simply - with mercury, until the surface of the metal ceases to turn black. - After this the carbon bisulphide must be poured off and distilled - over a water-bath, after mixing with some oil to retain the - impurities. - -Pure carbon bisulphide is a colourless liquid, which refracts light -strongly, and has a pure ethereal smell; at 0° its specific gravity is -1·293, and at 15° 1·271. If kept for a long time it seems to undergo a -change, especially when it is kept under water, in which it is insoluble. -It boils at 48°, and the tension of its vapour is so great that it -evaporates very easily, producing cold,[72] and therefore it has to be -kept in well-stoppered vessels; it is generally kept under a layer of -water, which hinders its evaporation and does not dissolve it.[73] - - [72] If carbon bisulphide be evaporated under the receiver of an - air-pump, or by means of a current of air, it is possible to - obtain a temperature as low as -60°, and the carbon bisulphide - does not solidify at this temperature. However, if a series of - air-bubbles be passed through it by means of bellows, a - crystalline white substance remains which volatilises below 0°: - this a hydrate, H_{2}O,2CS_{2}; it easily decomposes into water - and carbon bisulphide. It is formed in the above experiment by the - moisture held in the air passed through the carbon bisulphide, and - the fall of temperature. - - [73] Strong alcohol is miscible in all proportions with carbon - bisulphide, but dilute alcohol only in a definite amount, owing to - its diminished solubility from the presence of the water in it. - Ether, hydrocarbons, fatty oils, and many other organic substances - are soluble with great ease in carbon bisulphide. This is taken - advantage of in practice for extracting the fatty oils from - vegetable seeds, such as linseed, palm-nuts, or from bones, &c. - The preparation of vegetable oils is usually done by pressing the - seeds under a press, but the residue always contains a certain - amount of oil. These traces of oil can, however, be removed by - treatment with carbon bisulphide. In this manner a solution is - obtained which when heated easily parts with all the carbon - bisulphide, leaving the non-volatile fatty oil behind, so that the - same carbon bisulphide may be condensed and used over again for - the same purpose. It also dissolves iodine, bromine, indiarubber, - sulphur, and tars. - - Carbon bisulphide, especially at high temperatures, very often - acts by its elements in a manner in which carbon and sulphur alone - are not able to react, which will be understood from what has been - said above respecting its endothermal origin. If it be passed over - red-hot metals--even over copper, for instance, not to mention - sodium, &c.--it forms a sulphide of the metal and deposits - charcoal, and if the vapour be passed over incandescent metallic - oxides it forms metallic sulphides and carbonic anhydride (and - sometimes a certain amount of sulphurous anhydride). Lime and - similar oxides give under these circumstances a carbonate and a - sulphide--for example, CS_{2} +3CaO = 2CaS + CaCO_{3}. The - sulphides obtained by this means are often well crystallised, like - those found in nature--for example, lead and antimony sulphides. - -Carbon bisulphide enters into many combinations, which are frequently -closely analogous to the compounds of carbonic anhydride. In this respect -it is a _thio-anhydride_--_i.e._ it has the character of the acid -anhydrides,[73 bis] like carbonic anhydride, with the difference that the -oxygen of the latter is replaced by sulphur. By thio-compounds in general -are understood those compounds of sulphur which differ from the compounds -of oxygen as carbon bisulphide does from carbonic anhydride--that is, -which correspond with the oxygen compounds, but with substitution of -sulphur for oxygen. Thus thiosulphuric acid is monothiosulphuric -acid--that is, sulphuric acid in which one atom of sulphur replaces one -atom of oxygen. With the sulphides of the alkalis and alkaline earths, it -forms saline substances corresponding with the carbonates, and these -compounds may be termed _thiocarbonates_. For example, the composition of -the sodium salt Na_{2}CS_{3} is exactly like that of sodium carbonate. -They are formed by the direct solution of carbon bisulphide in aqueous -solutions of the sulphides; but they are difficult to obtain in a -crystalline form, because they are easily decomposable. When the -solutions of these salts are highly concentrated they begin to decompose, -with the evolution of sulphuretted hydrogen and the formation of a -carbonate, water taking part in the reaction--for example, K_{2}CS_{3} + -3H_{2}O = K_{2}CO_{3} + 3H_{2}S.[74] - - [73 bis] And just as COCl_{2} corresponds to CO_{2}, so also the - chloranhydride, CSCl_{2}, or _thiophosgene_, corresponds to CS_{2}. - - [74] If instead of a sulphide we take an alkali hydroxide, a - thiocarbonate is also formed, together with a carbonate--thus, - 3BaH_{2}O_{2} + 3CS_{2} = 2BaCS_{3} + BaCO_{3} + 3H_{2}O. From the - instability of the thiocarbonates of the alkaline metals we can - clearly see the reason of the difficulty with which the salts of - the heavier metals are formed, whose basic properties are - incomparably weaker than those of the alkali metals. However, - these salts may be obtained by double decomposition. Ammonia in - reacting on carbon bisulphide gives, besides products like those - formed by other alkalis, a whole series of products of as complex - a structure as those substances which are produced by the action - of carbonic anhydride on ammonia. In the ninth chapter we examined - the formation of the ammonium carbonates, and saw the transition - from them into the cyanides. It is not surprising after this that - the action of carbon bisulphide on ammonia not only produces the - above-mentioned salts, but also amidic compounds corresponding - with them, in which the oxygen is wholly or partially replaced by - sulphur. Thus ammonium dithiocarbamate is very easily obtained if - carbon bisulphide be added to an alcoholic solution of ammonia, - and the mixture cooled in a closed vessel. The salt then separates - out in minute yellow crystals, CN_{2}H_{6}S_{2}. - - Carbon bisulphide not only forms compounds with the metallic - sulphides, but also with sulphuretted hydrogen--that is, it forms - _thiocarbonic acid_, H_{2}CS_{3}. This is obtained by carefully - mixing solutions of thiocarbonates with dilute hydrochloric acid. - It then separates in an oily layer, which easily decomposes in the - presence of water into sulphuretted hydrogen and carbon - bisulphide, just as the corresponding carbonic acid (hydrate) - decomposes into water and carbonic anhydride. Carbon bisulphide - combines not only with sodium sulphide, but also with the - bisulphide, Na_{2}S_{2}, not, however, with the trisulphide, - Na_{2}S_{3}. - - The relation of carbon bisulphide to the other carbon compounds - presents many most interesting features which are considered in - organic chemistry. We will here only turn our attention to one of - the compounds of this class. Ethyl sulphide, (C_{2}H_{5})_{2}S, - combines with ethyl iodide, C_{2}H_{5}I, forming a new molecule, - S(C_{2}H_{5})_{3}I. If we designate the hydrocarbon group, for - instance ethyl, C_{2}H_{5}, by Et, the reaction would be expressed - by the following equation : Et_{2}S + EtI = SEt_{3}I. This - compound is of a saline character, corresponds with salts of the - alkalis, and is closely analogous to ammonium chloride. It is - soluble in water; when heated it again splits up into its - components EtI and Et_{2}S, and with silver hydroxide gives a - hydroxide, Et_{3}S·OH, having the property of a distinct and - energetic alkali, resembling caustic ammonia. Thus the compound - group SEt_{3} combines, like potassium or ammonium, with iodine, - hydroxyl, chlorine, &c. The hydroxide SEt_{3}·OH is soluble in - water, precipitates metallic salts, saturates acids, &c. Hence - sulphur here enters into a relation towards other elements similar - to that of nitrogen in ammonia and ammonium salts, with only this - difference, that nitrogen retains, besides iodine, hydroxyl, and - other groups, also H_{4} or Et_{4} (for example, NH_{4}Cl, - NEt_{3}HI, NEt_{4}I), whilst sulphur only retains Et_{3}. - Compounds of the formula SH_{3}X are however unknown, only the - products of substitution SEt_{3}X, &c. are known. The distinctly - alkaline properties of the hydroxide, triethylsulphine hydroxide, - SEt_{3}OH, and also the sharply-defined properties of the - corresponding hydroxide, tetraethylammonium hydroxide, NEt_{4}OH, - depend naturally not only on the properties of the nitrogen and - sulphur entering into their composition, but also on the large - proportion of hydrocarbon groups they contain. Judging from the - existence of the ethylsulphine compounds, it might be imagined - that sulphur forms a compound, SH_{4}, with hydrogen; but no such - compound is known, just as NH_{5} is unknown, although NH_{4}Cl - exists. - -A remarkable example[74 bis] of the thio-compounds is found in -_thiocyanic acid_--_i.e._ cyanic acid in which the oxygen is replaced by -sulphur, HCNS. We know (Chapter IX.) that with oxygen the cyanides of the -alkaline metals RCN give cyanates RCNO; but they also combine with -sulphur, and therefore if yellow prussiate of potash be treated as in the -preparation of potassium cyanide, and sulphur be added to the mass, -potassium thiocyanate, KNCS, is obtained in solution. This salt is much -more stable than potassium cyanate; it dissolves without change in water -and alcohol, forming colourless solutions from which it easily -crystallises on evaporation. It may be kept exposed to air even when in -solution; in dissolving in water it absorbs a considerable amount of -heat, and forms a starting-point for the preparation of all the -thiocyanates, RCNS, and organic compounds in which the metals are -replaced by hydrocarbon groups. Such, for example, is volatile mustard -oil, C_{3}H_{5}CSN (allyl thiocyanate),[75] which gives to mustard its -caustic properties. With ferric salts the thiocyanates give an -exceedingly brilliant red coloration, which serves for detecting the -smallest traces of ferric salts in solution. Thiocyanic acid, HCNS, may -be obtained by a method of double decomposition, by distilling potassium -thiocyanate with dilute sulphuric acid. It is a volatile colourless -liquid, having a smell recalling that of vinegar, is soluble in water, -and may be kept in solution without change.[75 bis] - - [74 bis] Thorpe and Rodger (1889), by heating a mixture of lead - fluoride and phosphorus pentasulphide to 250° in an atmosphere of - dry nitrogen, obtained gaseous _phosphorus fluosulphide_, or - _thiophosphoryl fluoride_, PSF_{3}, corresponding with POCl_{3}. - This colourless gas is converted into a colourless liquid by a - pressure of eleven atmospheres; it does not act on dry mercury, - and takes fire spontaneously in air or oxygen, forming phosphorus - pentafluoride, phosphoric anhydride, and sulphurous anhydride. It - is soluble in ether, but is decomposed by water: PSF_{3} + 4H_{2}O - = H_{2}S + H_{3}PO_{4} + 3HF (Note 20). - - [75] Although mustard oil may be obtained from the thiocyanates, it is - only an isomer of allyl thiocyanate proper, as is explained in - Organic Chemistry. - - [75 bis] Sulphur can only replace half the oxygen in CO_{2}, as is seen - in _carbon oxysulphide_, or monothiocarbonic anhydride COS. This - substance was obtained by Than, and is formed in many reactions. A - certain amount is obtained if a mixture of carbonic oxide and the - vapour of sulphur be passed through a red-hot tube. When carbon - tetrachloride is heated with sulphurous anhydride, this substance - is also formed; but it is best obtained in a pure form by - decomposing potassium thiocyanate with a mixture of equal volumes - of water and sulphuric acid. A gas is then evolved containing a - certain amount of hydrocyanic acid, from which it may be freed by - passing it over wool containing moistened mercuric oxide, which - retains the hydrocyanic acid. The reaction is expressed by the - equation: 2KCNS + 2H_{2}SO_{4} + 2H_{2}O = K_{2}SO_{4} + - (NH_{4})_{2}SO_{4} + 2COS. It is also formed by passing the vapour - of carbon bisulphide over alumina or clay heated to redness - (Gautier; silicon sulphide is then formed). COS is also formed by - passing phosgene over a long layer of asbestos mixed with cadmium - sulphide at 270°; CdS + COCl_{3} = CdCl_{2} + COS (Nuricsán, - 1892). The pure gas has an aromatic odour, is soluble in an equal - volume of water, which, however, acts on it, so that it must be - collected over mercury. When slightly heated, carbon oxysulphide - decomposes into sulphur and carbonic oxide. It burns in air with a - pale blue flame, explodes with oxygen, and yields potassium - sulphide and carbonate with potassium hydroxide: COS + 4KHO = - K_{2}CO_{3} + K_{2}S + 2H_{2}O. - -The sulphur compounds of chlorine Cl_{2}S and Cl_{2}S_{2} may be -regarded on the one hand as products of the metalepsis of the sulphides -of hydrogen, H_{2}S and H_{2}S_{2}; and on the other hand of the oxygen -compounds of chlorine, because chloride of sulphur, Cl_{2}S, resembles -chlorine oxide, Cl_{2}O, whilst Cl_{2}S_{2} corresponds with the higher -oxide of chlorine; or thirdly, we may see in these compounds the type of -the acid chloranhydrides, because they are all decomposed by water, -forming hydrochloric acid, and sulphur tetrachloride, SCl_{4}, is -decomposed with the formation of sulphurous anhydride.[76] - - [76] There is no reason for seeing any contradiction or mutual - incompatibility in these three views, because every analogy is - more or less modified by a change of elements. Thus, for instance, - it cannot be expected that the product of the metalepsis of - hydrogen sulphide would resemble the corresponding products of - water in all respects, because water has not the acid properties - of hydrogen sulphide. In the days of dualism and electrical - polarity it was supposed that the sulphur varied in its nature: in - hydrogen sulphide or potassium sulphide it was considered to be - negative, and in sulphurous anhydride or sulphur dichloride - positive. It then appeared evident that sulphur dichloride would - have no point of analogy with potassium sulphide. But metalepsis, - or its expression in the law of substitution, necessitates such - opinions being laid aside. If we can compare CO_{2}, CH_{4}, - CCl_{4}, CHCl_{3}, CH_{3}(OH) with each other, we cannot recognise - any difference in the sulphur in SH_{2}, SCl_{2}, SK_{2}, or in - general SX_{2}, for otherwise we should have to acknowledge as - many different states of sulphur, carbon, or hydrogen as there are - compounds of sulphur, carbon, or hydrogen. The essential truth of - the matter is that all the elements in a molecule play their part - in the reactions into which it enters. Often this appears to be - contradicted in the result--for example, hydrogen alone may be - replaced; but it is not this hydrogen alone that has determined - the reaction; all the elements present have participated in it. - This may be made clearer by the following rough illustration. - Supposing two regiments of soldiers were fighting against each - other, and that several men were lost by one of the regiments; no - one could say that it was only these men who took part in the - engagement. The other men fired and the bullets flew over the - heads of their opponents. It was not only those who fell who - fought, although they only were removed from the field of battle; - the fighting proceeded among the masses, but only those few were - disabled who went forward and were more conspicuous &c.; not that - the remainder did not take part in the action; they also fought - and were an object of attack, only they remained sound and unhurt. - Hydrogen is lighter than other elements and its atoms more mobile; - it subjects itself more frequently and easily to reactions; but it - is not it alone which reacts, it is even less liable to attack - than other elements. It participates in exceedingly diverse - reactions, not indeed because the hydrogen itself varies, but - because one atom of it puts itself forward, another is hidden, one - is united with carbon, another feebly held by sulphur, one stands - or moves in the neighbourhood of oxygen, another is joined to a - hydrocarbon. All hydrogen atoms are equal, and equally serve as an - object of attack for the atoms of molecules encountering them, but - those only are removed from the sphere of action which are nearer - the surface of a molecule, which are more mobile, or held by a - less sum of forces. So also sulphur is one and the same in sulphur - dichloride, in sulphurous or sulphuric anhydride, in hydrogen - sulphide, in potassium sulphide, but it reacts differently, and - those elements which are with it also vary in their reactions - because they are with it, and it varies its reactions because it - is with them. It is possible to seize on a character common to - substances quantitatively and qualitatively analogous to each - other. It may be admitted that an element in certain forms is not - able to enter into reactions into which in other forms it enters - willingly, if only the requisite conditions are encountered; but - it must not therefore be concluded that an element changes its - essential quality in these different cases. The preceding remarks - touch on questions which are subject to much argument among - chemists, and I mention them here in order to show the treatment - of those most important problems of chemistry which lie at the - basis of this treatise. - -[Illustration: FIG. 91.--Apparatus for the preparation of sulphur -chloride, and similar volatile compounds prepared by combustion in a -stream of chlorine.] - -The compounds of sulphur with chlorine are prepared in the apparatus -depicted in fig. 91. As sulphur chloride is decomposed by water, the -chlorine evolved in the flask C must be dried before coming into contact -with the sulphur. It is therefore first passed through a Woulfe's bottle, -B, containing sulphuric acid, and then through the cylinder D containing -pumice stone moistened with sulphuric acid, and then led into the retort -E, in which the sulphur is heated. The compound which is formed distils -over into the receiver R. A certain amount of sulphur passes over with -the sulphur chloride, but if the resultant distillate be re-saturated -with chlorine and distilled no free sulphur remains, the boiling-point -rises to 144°, and pure sulphur chloride, S_{2}Cl_{2}, is obtained. It -has this formula because its vapour density referred to hydrogen is 68. -It is also obtained by heating certain metallic chlorides (stannous, -mercuric) with sulphur; both the metal and chlorine then combine with the -sulphur. Sulphur chloride is a yellowish-brown liquid, which boils at -144°, and has a specific gravity of 1·70 at 0°. It fumes strongly in the -air, reacting on the moisture contained therein, and has a heavy -chloranhydrous odour. It dissolves sulphur, is miscible with carbon -bisulphide, and falls to the bottom of a vessel containing water, by -which it is decomposed, forming sulphurous anhydride and hydrochloric -acid; but it first forms various lower stages of oxidation of sulphur, -because the addition of silver nitrate to the solution gives a black -precipitate. With hydrogen sulphide it gives sulphur and hydrochloric -acid, and it reacts directly with metals--especially arsenic, antimony, -and tin--forming sulphides and chlorides. In the cold, it absorbs -chlorine and gives _sulphur dichloride_, SCl_{2}. The entire conversion -into this substance requires the prolonged passage of dry chlorine -through sulphur chloride surrounded by a freezing mixture. The -distillation of the dichloride must be conducted in a stream of chlorine, -as otherwise it partially decomposes into sulphur chloride and chlorine. -Pure sulphur dichloride is a reddish-brown liquid, which resembles the -lower chloride in many respects; its specific gravity is 1·62; its odour -is more suffocating than that of sulphur chloride; it volatilises at -64°.[77] - - [77] The observed vapour density of sulphur dichloride referred to - hydrogen is 53·3, and that given by the formula is 51·5. The - smaller molecular weight explains its boiling point being lower - than that of sulphur chloride, S_{2}Cl_{2}. The reactions of both - these compounds are very similar. Sulphur converts the dichloride, - SCl_{2}, into the monochloride, S_{2}Cl_{2}. In one point the - dichloride differs distinctly from the monochloride--that is, in - its capacity for easily giving up chlorine and decomposing. Even - light decomposes it into chlorine and the monochloride. Hence it - acts on many substances in the same manner as chlorine, or - substances which easily part with the latter, such as phosphoric - or antimonic chloride. In distinction to these, however, sulphur - dichloride would appear to distil without any considerable - decomposition, judging by the vapour density. But this is not a - valid conclusion, for if there be a decomposition, then 2SCl_{2} = - S_{2}Cl_{2} + Cl_{2}; now the density of sulphur chloride = 67·5, - and of chlorine = 35·5, and consequently a mixture of equal - volumes of the two = 51·5, just the same as an equal volume of - sulphur dichloride. _Therefore the distillation of sulphur - dichloride is probably nothing but its decomposition._ Hence the - compound SCl_{2}, which is stable at the ordinary temperature, - decomposes at 64°. In the cold it absorbs a further amount of - chlorine, corresponding to SCl_{4}, but even at -10° a portion of - the absorbed chlorine is given off--that is, dissociation takes - place. Thus the tetrachloride is even less stable than the - dichloride. - -_Thionyl chloride_, SOCl_{2}, may be regarded as oxidised sulphur -dichloride; it corresponds with sulphur chloride, S_{2}Cl_{2}, in which -one atom of sulphur is replaced by oxygen. At the same time it is -chlorine oxide (hypochlorous anhydride, Cl_{2}O) combined with sulphur, -and also the chloranhydride of sulphurous acid--that is, SO(HO)_{2}, in -which the two hydroxyl groups are replaced by two atoms of chlorine, or -sulphurous anhydride, SO_{2}, in which one atom of oxygen is replaced by -two atoms of chlorine. All these representations are confirmed by -reactions of formation, or decompositions; they all agree with our -notions of the other compounds of sulphur, oxygen, and chlorine; hence -these definitions are not contradictory to each other. Thus, for -instance, thionyl chloride was first obtained by Schiff, by the action of -dry sulphurous anhydride on phosphorus pentachloride. On distilling the -resultant liquid, thionyl chloride comes over first at 80°, and on -continuing the distillation phosphorus oxychloride distils over at above -100°, PCl_{5} + SO_{2} = POCl_{3} + SOCl_{2}. This mode of preparation is -direct evidence of the oxychloride character of SOCl_{2}. Würtz obtained -the same substance by passing a stream of chlorine oxide through a cold -solution of sulphur in sulphur chloride; the chlorine oxide then combined -directly with the sulphur, S + Cl_{2}O = SOCl_{2}, whilst the sulphur -chloride remained unchanged (sulphur cannot be combined directly with -chlorine oxide, as an explosion takes place). Thionyl chloride is a -colourless liquid, with a suffocating acrid smell; it has a specific -gravity at 0° of 1·675, and boils at 78°. It sinks in water, by which it -is immediately decomposed, like all chloranhydrides--for example, like -carbonyl chloride, which corresponds with it: SOCl_{2} + H_{2}O = SO_{2} -+ 2HCl.[77 bis] - - [77 bis] Hartog and Sims (1893) obtained thionyl bromide, SOBr_{2}, by - treating SOCl_{2} with sodium bromide; it is a red liquid, sp. gr. - 2·62, and decomposes at 150°. - -Normal _sulphuric acid has two corresponding chloranhydrides_; the first, -SO_{2}(OH)Cl, is sulphuric acid, SO_{2}(HO)_{2}, in which one equivalent -of HO is replaced by chlorine; the second has the composition -SO_{2}Cl_{2}--that is, two HO groups are substituted by two of chlorine. -The second chloranhydride, or the compound SO_{2}Cl_{2}, is called -sulphuryl chloride, and the first chloranhydride, SO_{2}HOCl, may be -called chlorosulphonic acid, because it is really an acid; it still -retains one hydroxyl of sulphuric acid, and its corresponding salts are -known. Thus, potassium chloride absorbs the vapour of sulphuric -anhydride, forming a salt, SO_{3}KCl, corresponding with SO_{3}HCl as -acid. In acting on sodium chloride it forms hydrochloric acid and the -salt NaSO_{3}Cl. This first chloranhydride of sulphuric acid, SO_{2}HOCl, -discovered by Williamson, is obtained either by the action of phosphorus -pentachloride on sulphuric acid (PCl_{5} + H_{2}SO_{4} = POCl_{3} + HCl + -HSO_{3}Cl), or directly by the action of dry hydrochloric acid on -sulphuric anhydride, SO_{3} + HCl = HSO_{3}Cl. The most easy and rapid -method of its formation is by direct saturation of cold Nordhausen acid -with dry hydrochloric acid gas (SO_{3} + HCl = HSO_{3}Cl), and -distillation of the resultant solution; the distillate then contains -HSO_{3}Cl. It is a colourless fuming liquid, having an acrid odour; it -boils at 153° (according to my determination, confirmed by Konovaloff), -and its specific gravity at 19° is 1·776. It is immediately decomposed by -water, forming hydrochloric and sulphuric acids, as should be the case -with a true chloranhydride. In the reactions of this chloranhydride we -find the easiest means of introducing the sulphonic group HSO_{3} into -other compounds, because it is here combined with chlorine. The second -chloranhydride of sulphuric acid, or _sulphuryl chloride_, SO_{2}Cl_{2}, -was obtained by Regnault by the direct action of the sun's ray on a -mixture of equal volumes of chlorine and sulphurous oxide. The gases -gradually condense into a liquid, combining together as carbonic oxide -does with chlorine. It is also obtained when a mixture of the two gases -in acetic acid is allowed to stand for some time. The first -chloranhydride, SO_{3}HCl, decomposes when heated at 200° in a closed -tube into sulphuric acid and sulphuryl chloride. It boils at 70°, its -specific gravity is 1·7, it gives hydrochloric and sulphuric acids with -water, fumes in the air, and, judging by its vapour density, does not -decompose when distilled.[78] - - [78] Pyrosulphuryl chloride, S_{2}O_{5}Cl_{2}. See Note 44. Thorpe and - Kirman, by treating SO_{3} with HF, obtained SO_{2}(OH)F, as a - liquid boiling at 163°, but which decomposed with greater facility - and then gave SO_{2}F_{2}. - - The acids of sulphur naturally have their corresponding ammonium - salts, and the latter their amides and nitriles. It will be - readily understood how vast a field for research is presented by - the series of compounds of sulphur and nitrogen, if we only - remember that to carbonic and formic acids there corresponds, as - we saw (Chapter IX.), a vast series of derivatives corresponding - with their ammonium salts. To sulphuric acid there correspond two - ammonium salts, SO_{2}(HO)(NH_{4}O) and SO_{2}(NH_{4}O)_{2}; three - amides: the acid amide SO_{2}(HO)(NH_{2}), or sulphamic acid, the - normal saline compound SO_{2}(NH_{4}O)(NH_{2}), or ammonium - sulphamate, and the normal amide SO_{2}(NH_{2})_{2}, or sulphamide - (the analogue of urea); then the acid nitrile, SON(HO), and two - neutral nitriles, SON(NH_{2}) and SN_{2}. There are similar - compounds corresponding with sulphurous acid, and therefore its - nitriles will be, an acid, SN(HO), its salt, and the normal - compound, SN(NH_{2}). Dithionic and the other acids of sulphur - should also have their corresponding amides and nitriles. Only a - few examples are known, which we will briefly describe. Sulphuric - acid forms salts of very great stability with ammonia, and - ammonium sulphate is one of the commonest ammoniacal compounds. It - is obtained by the direct action of ammonia on sulphuric acid, or - by the action of the latter on ammonium carbonate; it separates - from its solutions in an anhydrous state, like potassium sulphate, - with which it is isomorphous. Hence, the composition of crystals - of ammonium sulphate is (NH_{4})_{2}SO_{4}. This salt fuses at - 140°, and does not undergo any change when heated up to 180°. At - higher temperatures it does not lose water, but parts with half - its ammonia, and is converted into the acid salt, HNH_{4}SO_{4}; - and this acid salt, on further heating, undergoes a further - decomposition, and splits up into nitrogen, water, and acid - ammonium sulphite, HNH_{4}SO_{3}. At the ordinary temperature the - normal salt is soluble in twice its weight of water and at the - boiling-point of water in an equal weight. In its faculty for - combinations this salt exhibits a great resemblance to potassium - sulphate, and, like it, easily forms a number of double salts; the - most remarkable of which are the ammonia alums, - NH_{4}AlS_{2}O_{8},12H_{2}O, and the double salts formed by the - metals of the magnesium group, having, for example, the - composition (NH_{4})_{2}MgS_{2}O_{8},6H_{2}O. Ammonium sulphate - does not give an amide when heated, perhaps owing to the faculty - of sulphuric anhydride to retain the water combined with it with - great force. But the amides of sulphuric acid may be very - conveniently prepared from sulphuric anhydride. Their formation by - this method is very easily understood because an amide is equal to - an ammonium salt less water, and if the anhydride be taken it will - give an amide directly with ammonia. Thus, if dry ammonia be - passed into a vessel surrounded by a freezing mixture and - containing sulphuric anhydride, it forms a white powdery mass - called sulphatammon, having the composition SO_{3},2H_{3}N, and - resembling the similar compound of carbonic acid, CO_{2},2NH_{3}. - This substance is naturally the ammonium salt of sulphamic acid, - SO_{2}(NH_{4}O)NH_{2}. It is slowly acted on by water, and may - therefore be obtained in solution, in which it slowly reacts with - barium chloride, which proves that with water it still forms - ammonium sulphate. If this substance be carefully dissolved in - water and evaporated, it yields well-formed crystals, whose - solution no longer gives a precipitate with barium chloride. This - is not due to the presence of impurities, but to a change in the - nature of the substance, and therefore Rose calls the crystalline - modification _parasulphatammon_. Platinum chloride only - precipitates half the nitrogen as platinochloride from solutions - of sulphat- and parasulphatammon, which shows that they are - ammonium salts, SO_{2}(NH_{4}O)(NH_{2}). It may be that the reason - of the difference in the two modifications is connected with the - fact that two different substances of the composition - N_{2}H_{4}SO_{2} are possible: one is the amide SO_{2}(NH_{2})_{2} - corresponding with the normal salt, and the other is the salt of - the nitrile acid corresponding with acid ammonium sulphate--that - is, SON(ONH_{4}) corresponds with the acid SON(OH) = - SO_{2}(NH_{4}O)OH - 2H_{2}O. Hence there may here be a difference - of the same nature as between urea and ammonium cyanate. Up to the - present, the isomerism indicated above has been but little - investigated, and might be the subject of interesting researches. - - If in the preceding experiment the ammonia, and not the sulphuric - anhydride, be taken in excess, a soluble substance of the - composition 2SO_{2},3NH_{3} is formed. This compound, obtained by - Jacqueline and investigated by Voronin, doubtless also contains a - salt of sulphamic acid--that is, of the amide corresponding with - the acid ammonium sulphate = HNH_{4}SO_{4} - H_{2}O = - (NH_{2})SO_{2}(OH). Probably it is a compound of sulphatammon with - sulphamic acid. Thus it has an acid reaction, and does not give a - precipitate with barium chloride. - - With normal sulphate of ammonium, an amide of the composition - N_{2}H_{4}SO_{2} should correspond, which should bear the same - relation to sulphuric acid as urea bears to carbonic acid. This - amide, known as _sulphamide_, is obtained by the action of dry - ammonia on the sulphuryl chloride, SO_{2}Cl_{2}, just as urea is - obtained by the action of ammonia on carbonyl chloride, - SO_{2}Cl_{2} + 4NH_{3} = N_{2}H_{4}SO_{2} + 2NH_{4}Cl. The - ammonium chloride is separated from the resultant sulphamide with - great difficulty. Cold water, acting on the mixture, dissolves - them both; the cold solution does not gives precipitate with - barium chloride. Alkalis act on it slowly, as they do on urea; but - on boiling, especially in the presence of alkalis or acids, it - easily recombines with water, and gives an ammonium salt. V. - Traube (1892) obtained sulphamide by the reaction of sulphuryl, - dissolved in chloroform, upon ammonia. The resultant precipitate - dissolves when shaken up with water, and the solution (after - boiling with the oxides or lead or silver) is evaporated, when a - syrupy liquid remains. With nitrate of silver the latter gives a - solid compound, which, when decomposed by hydrochloric acid, gives - free sulphamide in large colourless crystals, having the - composition SO_{2}(NH_{2})_{2}. This substance fuses at 81°, - begins to decompose below 100°, and is entirely decomposed above - 250°; it is soluble in water, and the solution has a neutral - reaction and bitter taste. When heated with acids, sulphamide - gradually decomposes, forming sulphuric acid and ammonia. If the - silver compound obtained by the action of sulphamide on nitrate of - silver be heated at 170°-180° until ammonia is no longer evolved, - and the residue be extracted with water acidulated with nitric - acid, a salt separates out from the solution, answering in its - composition to sulphamide, SO_{2}NAg, which = the amide - NH_{3} = - SO_{2}N_{2}H_{4} - NH_{3} = SO_{2}NH. The action of sulphuryl - chloride (and of the other chloranhydrides of sulphur) on ammonium - carbonate always, as Mente showed (1888), results in the formation - of the salt NH(SO_{3}NH_{4})_{2}. - - The nitriles corresponding with sulphuric acid are not as yet - known with any certainty. The most simple nitrile corresponding - with sulphuric acid should have the composition N_{2}H_{8}SO_{4} - - 4H_{2}O = N_{2}S. This would be a kind of cyanogen corresponding - with sulphuric acid. On comparing sulphurous acid with carbonic - acid, we saw that they present a great analogy in many respects, - and therefore it might be expected that nitrile compounds having - the composition NHS and N_{2}S_{2} would be found. The latter of - these compounds is well known, and was obtained by Soubeiron, by - the action of dry ammonia on sulphur chloride. This substance - corresponds with cyanogen (paracyanogen), and is known as - _nitrogen sulphide_, N_{2}S_{2}. It is formed according to the - equation 3SCl_{2} + 8NH_{3} = N_{2}S_{2} + S + 6NH_{4}Cl. The free - sulphur and nitrogen sulphide are dissolved by acting on the - product with carbon bisulphide, the nitrogen sulphide being much - less soluble than the sulphur. It is a yellow substance, which is - excessively irritating to the eyes and nostrils. It explodes when - rubbed with a hard substance, being naturally decomposed with the - evolution of nitrogen; but when heated it fuses without - decomposing, and only decomposes with explosion at 157°. It is - insoluble in water, and only slightly so in alcohol, ether, and - carbon bisulphide; 100 parts of the latter dissolve 1·5 part of - nitrogen sulphide at the boiling point. This solution on cooling - deposits it in minute transparent prisms of a golden yellow - colour. - -In the group of the halogens we saw four closely analogous -elements--fluorine, chlorine, bromine, and iodine--and we meet with the -same number of closely allied analogues in the oxygen group; for besides -sulphur this group also includes _selenium_ and _tellurium_: O, S, Se, -Te. These two groups are very closely allied, both in respect to the -magnitudes of their atomic weights and also in the faculty of the -elements of both groups for combining with metals. The distinct analogy -and definite degree of variance known to us for the halogens, also repeat -themselves in the same degree for the elements of the oxygen group. -Amongst the halogens fluorine has many peculiarities compared to Cl, Br -and I which are more closely analogous, whilst oxygen differs in many -respects from S, Se, Te, which possess greater similarities. The analogy -in a quantitative respect is perfect in both cases. Thus the halogens -combine with H, and the elements of the oxygen group with H_{2}, forming -H_{2}O, H_{2}S, H_{2}Se, H_{2}Te. The hydrogen compounds of selenium and -tellurium are acids like hydrogen sulphide. Selenium, by simple heating -in a stream of hydrogen, partially combines with it directly, but -seleniuretted hydrogen is more readily decomposable by heat than -sulphuretted hydrogen, and this property is still more developed in -telluretted hydrogen. Hydrogen selenide and telluride are gases like -sulphuretted hydrogen, and, like it, are soluble in water, form saline -compounds with alkalis, precipitate metallic salts, are obtained by the -action of acids on their compounds with metals, &c. Selenium and -tellurium, like sulphur, give two normal grades of combination with -oxygen, both of an acid character, of which only the forms corresponding -to sulphurous anhydride--namely, selenious anhydride, SeO_{2}, and -tellurous anhydride, TeO_{2}[79]--are formed directly. These are both -solids, obtained by the combustion of the elements themselves and by the -action of oxidising agents on them. They form feebly energetic acids, -having distinct bibasic properties; however, a characteristic difference -from SO_{2} is observable both in the physical properties of these -compounds and in their stability and capacity for further oxidation, just -as in the series of the halogens already known to us, only in an inverse -order; in the latter we saw that iodine combines more easily than bromine -or chlorine with oxygen, forming more stable oxygen compounds, whereas -here, on the contrary, sulphurous anhydride, as we know, is difficultly -decomposed, parts with its sulphur with difficulty, and is easily -oxidised and especially in its salts, while selenious and tellurous -anhydrides are oxidised with difficulty and easily reduced, even by means -of sulphurous acid. - - [79] _Selenious anhydride_, SeO_{2}, is a volatile solid, which - crystallises in prisms soluble in water. It is best procured by - the action of nitric acid on selenium. The well-known researches - of Nilson (1874) showed that the salts of selenious acid easily - form acid salts, and are so characteristic in many respects that - they may even serve for judging the analogy of types of oxides. - Thus the oxides of the composition RO give normal salts of the - composition RSeO_{3},2H_{2}O, where R = Mn, Co, Ni, Cu, Zn. The - salts of magnesium, barium, and calcium contain a different - quantity of water, as do also the salts of the oxides R_{2}O_{3}. - We here turn attention to the fact that beryllium gives a normal - salt, BeSeO_{3},2H_{2}O, and not a salt analogous to those of - aluminium, scandium, Sc_{2}(SeO_{3})_{3},H_{2}O, yttrium, - Y_{2}(SeO_{3})_{2},12H_{2}O, and other oxides of the form - R_{2}O_{3}, which speaks in favour of the formula BeO. - - _Tellurous anhydride_ is also a colourless solid, which - crystallises in octahedra; it also, when heated, first fuses and - then volatilises. It is insoluble in water, and the decomposition - of its salts gives a hydrate, H_{2}TeO_{3}, which is insoluble. - - It is a very characteristic circumstance that selenious and - tellurous anhydrides are very easily _reduced_ to selenium and - tellurium. This is not only effected by metals like zinc, or by - sulphuretted hydrogen, which are powerful deoxidisers, but even by - sulphurous anhydride, which is able to precipitate selenium and - tellurium from solutions of the selenites and tellurites, and even - of the acids themselves, which is taken advantage of in obtaining - these elements and separating them from sulphur. - - Sulphuric acid, as we know, rarely acts as an oxidising agent. It - is otherwise with selenic and telluric acids, H_{2}SeO_{4} and - H_{2}TeO_{4}, which are powerful oxidising agents--that is, are - easily reduced in many circumstances either into the lower oxide - or even to selenium and tellurium. A powerful oxidising agent is - required in order to convert selenious and tellurous anhydrides - into selenic and telluric anhydrides, and, moreover, it must be - employed in excess. If chlorine be passed through a solution of - potassium selenide, K_{2}Se, telluride, K_{2}Te, selenite, - K_{2}SeO_{3}, or tellurite, K_{2}TeO_{3}, it acts as an oxidiser - in the presence of the water, forming potassium selenate, - K_{2}SeO_{4}, or tellurate, K_{2}TeO_{4}. The same salts are - formed by fusing the lower oxides with nitre. These salts are - isomorphous with the corresponding sulphates, and cannot therefore - be separated from them by crystallisation. The salts of potassium, - sodium, magnesium, copper, cadmium, &c. are soluble like the - sulphates, but those of barium and calcium are insoluble, in - perfect analogy with the sulphates. When copper selenate, - CuSeO_{4}, is treated with sulphuretted hydrogen (CuS is - precipitated), _selenic acid_ remains in solution. On evaporation - and drying in vacuo at 180° it gives a syrupy liquid, which may be - concentrated to almost the pure acid, H_{2}SeO_{4}, having a - specific gravity of 2·6. Cameron and Macallan (1891) showed that - pure H_{2}SeO_{4} only remains liquid in a state of superfusion - whilst the solidified acid melts at +58°, the solid acid - crystallises well, its sp. gr. is then 2·95. The hydrate - H_{2}SeO_{4},H_{2}O melts at +25°. The acid in a superfused state - has a sp. gr. 2·36 and the solid 2·63. Like sulphuric acid strong - selenic acid attracts moisture from the atmosphere; it is not - decomposed by sulphurous acid, but oxidises hydrochloric acid - (like nitric, chromic, and manganic acids), evolving chlorine and - forming selenious acid, H_{2}SeO_{4} + 2HCl = H_{2}SeO_{3} + - H_{2}O + Cl_{2}. _Telluric acid_, H_{2}TeO_{4}, is obtained by - fusing tellurous anhydride with potassium hydroxide and chlorate; - the solution, containing potassium tellurate, is then precipitated - with barium chloride, and the barium tellurate, BaTeO_{4} obtained - in the precipitate is decomposed by sulphuric acid. A solution of - telluric acid is thus obtained, which on evaporation yields - colourless prisms, soluble in water, and containing - TeH_{2}O_{4},2H_{2}O. Two equivalents of water are driven off at - 160°; on further heating the last equivalent of water is expelled, - and then oxygen is given off. It also gives chlorine with - hydrochloric acid, like selenic acid. Its salts also correspond - with those of sulphuric acid. It must, however, be remarked that - telluric and selenic acids are able to give poly-acid salts with - much greater ease than sulphuric acid. Thus, for example, there - are known for telluric acid not only K_{2}TeO_{4},5H_{2}O and - KHTeO_{4},3H_{2}O, but also KHTeO_{4},H_{2}TeO_{4},H_{2}O = - K_{2}TeO_{4},3H_{2}TeO_{4},2H_{2}O. This salt is easily obtained - from acid solutions of the preceding salts and is less soluble in - water. As selenious anhydride is volatile and gives similar - poly-salts, it may be surmised that selenious, tellurous, selenic, - and telluric anhydrides are polymeric as compared with sulphurous - and sulphuric anhydrides, for which reason it would be desirable - to determine the vapour density of selenious anhydride. It would - probably correspond with Se_{2}O_{4} or Se_{3}O_{6}. - - In order to show the very close analogy of selenium to sulphur, I - will quote two examples. Potassium cyanide dissolves selenium, as - it does sulphur, forming potassium selenocyanate, KCNSe, - corresponding with potassium thiocyanate. Acids precipitate - selenium from this solution, because selenocyanic acid, H_{2}CNSe, - when in a free state is immediately decomposed. A boiling solution - of sodium sulphite dissolves selenium, just as it would sulphur, - forming a salt analogous to thiosulphate of sodium, namely, sodium - selenosulphate, Na_{2}SSeO_{3}. Selenium is separated from a - solution of this salt by the action of acid. - -_Selenium_ was obtained in 1817 by Berzelius from the sublimate which -collects in the first chamber in the preparation of sulphuric acid from -Fahlun pyrites. Certain other pyrites also contain small quantities of -selenium. Some native selenides, especially those of lead, mercury, and -copper, have been found in the Hartz Mountains, but only in small -quantities. Pyrites and blendes, in which the sulphur is partially -replaced by selenium, still remain the chief source for its extraction. -When these pyrites are roasted they evolve selenious anhydride, which -condenses in the cooler portions of the apparatus in which the pyrites -are roasted, and is partially or wholly reduced by the sulphurous -anhydride simultaneously formed. The presence of selenium in ores and -sublimates is most simply tested by heating them before the blowpipe, -when they evolve the characteristic odour of garlic. Selenium exhibits -two modifications, like sulphur: one amorphous and insoluble in carbon -bisulphide, the other crystalline and slightly soluble in carbon -bisulphide (in 1,000 parts at 45° and 6,000 at 0°), and separating from -its solutions in monoclinic prisms. If the red precipitate obtained by -the action of sulphurous anhydride on selenious anhydride be dried, it -gives a brown powder, having a specific gravity of 4·26, which when -heated changes colour and fuses to a metallic mass, which gains lustre as -it cools. The selenium acquires different properties according to the -rate at which it is cooled from a fused state; if rapidly cooled, it -remains amorphous and has the same specific gravity (4·28) as the powder, -but if slowly cooled it becomes crystalline and opaque, soluble in carbon -bisulphide, and has a specific gravity of 4·80. In this form it fuses at -214° and remains unchanged, whilst the amorphous form, especially above -80°, gradually passes into the crystalline variety. The transition is -accompanied by the evolution of heat, as in the case of sulphur; thus the -analogy between sulphur and selenium is clearly shown here. In the fused -amorphous form selenium presents a brown mass, slightly translucent, with -a vitreous fracture, whilst in the crystalline form it has the appearance -of a grey metal, with a feeble lustre and a crystalline fracture.[79 bis] -Selenium boils at 700°, forming a vapour whose density is only constant -at a temperature of about 1,400°, when it is equal to 79·4 (referred to -hydrogen)--that is, the molecular formula is then Se_{2}, like sulphur at -an equally high temperature. - - [79 bis] Muthmann, in his researches upon the allotropic forms of - selenium, pointed out (1889) a peculiar modification, which - appears, as it were, as a transition between crystalline and - amorphous selenium. It is obtained together with the crystalline - variety by slowly evaporating a solution of selenium in bisulphide - of carbon, and differs from the crystalline variety in the form of - its crystals; it passes into the latter modification when heated. - Schultz also obtained selenium (like Ag, _see_ Chapter XXIV.) in a - soluble form, but these researches are not so conclusive as those - upon soluble silver, and we shall therefore not consider them more - fully. - -_Tellurium_ is met with still more rarely than selenium (it is known in -Saxony) in combination with gold, silver, lead, and antimony in the -so-called foliated tellurium ore. Bismuth telluride and silver telluride -have been found in Hungary and in the Altai. Tellurium is extracted from -bismuth telluride by mixing the finely-powdered ore with potassium and -charcoal in as intimate a mixture as possible, and then heating in a -covered crucible. Potassium telluride, K_{2}Te, is then formed, because -the charcoal reduces potassium tellurite. As potassium telluride is -soluble in water, forming a red-brown solution which is decomposed by the -oxygen of the atmosphere (K_{2}Te + O + H_{2}O = 2KHO + Te), the mass -formed in the crucible is treated with boiling water and filtered as -rapidly as possible, and the resultant solution exposed to the air, by -which means the tellurium is precipitated.[80] In a free state tellurium -has a perfectly _metallic appearance_; it is of a silver-white colour, -crystallises very easily in long brilliant needles; is very brittle, so -that it can be easily reduced to powder; but it is a bad conductor of -heat and electricity, and in this respect, as in many others, it forms a -transition from the metals to the non-metals. Its specific gravity is -6·18, it melts at an incipient red heat, and takes fire when heated in -air, like selenium and sulphur, burning with a blue flame, evolving white -fumes of tellurous anhydride, TeO_{2}, and emitting an acrid smell if no -selenium be present; but if it be, the odour of the latter preponderates. -Alkalis dissolve tellurium when boiled with it, potassium telluride, -K_{2}Te, and potassium tellurite, K_{2}TeO_{3}, being formed. The -solution is of a red colour, owing to the presence of the telluride, -K_{2}Te; but the colour disappears when the solution is cooled or -diluted, the tellurium being all precipitated: 2K_{2}Te + K_{2}TeO_{3} + -3H_{2}O = 6KHO + 3Te.[81] - - [80] The tellurium thus prepared is impure, and contains a large amount - of selenium. The latter may be removed by converting the mixture - into the salts of potassium, and treating this with nitric acid - and barium nitrate, when barium selenate only is precipitated, - whilst the barium tellurate remains in solution. This method does - not, however, give a pure product, and it appears to be best to - separate the selenium from the tellurium in a metallic form; this - is done by boiling the impure potassium tellurate with - hydrochloric acid, which converts it into potassium tellurite, - from which the tellurium is reduced by sulphurous anhydride. The - metal thus obtained is then fused and distilled in a stream of - hydrogen; the selenium volatilises first, and then the tellurium, - owing to its being much less volatile than the former. - Nevertheless, tellurium is also volatile, and may be separated in - this manner from less volatile metals, such as antimony. Brauner - determined the atomic weight of pure tellurium, and found it to be - 125, but showed (1889) that tellurium purified by the usual - method, even after distillation, contains a large amount of - impurities. - - [81] The decomposition proceeds in the above order in the cold, but in - a hot solution with an excess of potassium hydroxide it proceeds - inversely. A similar phenomenon takes place when tellurium is - fused with alkalis, and it is therefore necessary in order to - obtain potassium telluride to add charcoal. - - Selenium and tellurium form higher compounds with chlorine with - comparative ease. For selenium, SeCl_{2} and SeCl_{4} are known, - and for tellurium TeCl_{2} and TeCl_{4}. The tetrachlorides of - selenium and tellurium are formed by passing chlorine over these - elements. Selenium tetrachloride, SeCl_{4}, is a crystalline, - volatile mass which gives selenious anhydride and hydrochloric - acid with water. Tellurium tetrachloride is much less volatile, - fuses easily, and is also decomposed by water. Both elements form - similar compounds with bromine. Tellurium tetrabromide is red, - fuses to a brown liquid, volatilises, and gives a crystalline - salt, K_{2}TeBr_{6},3H_{2}O, with an aqueous solution of potassium - bromide. - - - - - CHAPTER XXI - - CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE - - -Sulphur, selenium, and tellurium belong to the uneven series of the sixth -group. In the even series of this group there are known _chromium, -molybdenum, tungsten, and uranium_; these give acid oxides of the type -RO_{3}, like SO_{3}. Their acid properties are less sharply defined than -those of sulphur, selenium, and tellurium, as is the case with all -elements of the even series as compared with those of the uneven series -in the same group. But still the oxides CrO_{3}, MoO_{3}, WO_{3}, and -even UO_{3}, have clearly defined acid properties, and form salts of the -composition MO,_n_RO_{3} with bases MO. In the case of the heavy -elements, and especially of uranium, the type of oxide, UO_{3}, is less -acid and more basic, because in the even series of oxides the element -with the highest atomic weight always acquires a more and more pronounced -basic character. Hence UO_{3} shows the properties of a base, and gives -salts UO_{2}X_{2}. The basic properties of chromium, molybdenum, -tungsten, and uranium are most clearly expressed in the lower oxides, -which they all form. Thus chromic oxide, Cr_{2}O_{3}, is as distinct a -base as alumina, Al_{2}O_{3}. - -Of all these elements _chromium_ is the most widely distributed and the -most frequently used. It gives chromic anhydride, CrO_{3}, and chromic -oxide, Cr_{2}O_{3}--two compounds whose relative amounts of oxygen stand -in the ratio 2 : 1. Chromium is, although somewhat rarely, met with in -nature as a compound of one or the other type. The red chromium ore of -the Urals, lead chromate or crocoisite PbCrO_{4}, was the source in which -chromium was discovered by Vauquelin, who gave it this name (from the -Greek word signifying colour) owing to the brilliant colours of its -compounds; the chromates (salts of chromic anhydride) are red and yellow, -and the chromic salts (from Cr_{2}O_{3}) green and violet. The red lead -chromate is, however, a rare chromium ore found only in the Urals and in -a few other localities. Chromic oxide, Cr_{2}O_{3}, is more frequently -met with. In small quantities it forms the colouring matter of many -minerals and rocks--for example, of some serpentines. The commonest ore, -and the chief source of the chromium compounds, is the _chrome iron ore_ -or chromite, which occurs in the Urals[1] and Asia Minor, California, -Australia, and other localities. This is magnetic iron ore, -FeO,Fe_{2}O_{3}, in which the ferric oxide is replaced by chromic oxide, -its composition being FeO,Cr_{2}O_{3}. Chrome iron ore crystallises in -octahedra of sp. gr. 4·4; it has a feeble metallic lustre, is of a -greyish-black colour, and gives a brown powder. It is very feebly acted -on by acids, but when fused with potassium acid sulphate it gives a -soluble mass, which contains a chromic salt, besides potassium sulphate -and ferrous sulphate. In practice the treatment of chrome iron ore is -mainly carried on for the preparation of chromates, and not of chromic -salts, and therefore we will trace the history of the element by -beginning with chromic acid, and especially with the working up of the -chrome iron ore into _potassium dichromate_, K_{2}Cr_{2}O_{7}, as the -most common salt of this acid. It must be remarked that chromic -anhydride, CrO_{3}, is only obtained in an anhydrous state, and is -distinguished for its capacity for easily giving anhydro-salts with the -alkalis, containing one, two, and even three equivalents of the anhydride -to one equivalent of base. Thus among the potassium salts there is known -the normal or yellow chromate, K_{2}CrO_{4}, which corresponds to, and is -perfectly isomorphous with, potassium sulphate, easily forms isomorphous -mixtures with it, and is not therefore suitable for a process in which it -is necessary to separate the salt from a mixture containing sulphates. As -in the presence of a certain excess of acid, the dichromate, -K_{2}Cr_{2}O_{7} = 2K_{2}CrO_{4} + 2HX - 2KX - H_{2}O, is easily formed -from K_{2}CrO_{4}, the object of the manufacturer is to produce such a -dichromate, the more so as it contains a larger proportion of the -elements of chromic acid than the normal salt. Finely-ground chrome iron -ore, when heated with an alkali, absorbs oxygen almost as easily (Chapter -III., Note 7) as a mixture of the oxides of manganese with an alkali. -This absorption is due to the presence of chromic oxide, which is -oxidised into the anhydride, and then combines with the alkali -Cr_{2}O_{3} + O_{3} = 2CrO_{3}. As the oxidation and formation of the -chromate proceeds, the mass turns _yellow_. The iron is also oxidised, -but does not give ferric acid, because the capacity of the chromium for -oxidation is incomparably greater than that of the iron. - - [1] The working of the Ural chrome iron ore into chromium compounds has - been firmly established in Russia, thanks to the endeavours of P. - K. Ushakoff, who constructed large works for this purpose on the - river Kama, near Elabougi, where as much as 2,000 tons of ore are - treated yearly, owing to which the importation of chromium - preparations into Russia has ceased. - -A mixture of lime (sometimes with potash) and chrome iron ore is heated -in a reverberatory furnace, with free access of air and at a red heat for -several hours, until the mass becomes yellow; it then contains normal -calcium chromate, CaCr_O_{4}, which is insoluble in water in the presence -of an excess of lime.[1 bis] The resultant mass is ground up, and treated -with water and sulphuric acid. The excess of lime forms gypsum, and the -soluble calcium dichromate, CaCr_{2}O_{7}, together with a certain amount -of iron, pass into solution. The solution is poured off, and chalk added -to it; this precipitates the ferric oxide (the ferrous oxide is converted -into ferric oxide in the furnace) and forms a fresh quantity of gypsum, -while the chromic acid remains in solution--that is, it does not form the -sparingly-soluble normal salt (1 part soluble in 240 parts of water). The -solution then contains a fairly pure calcium dichromate, which by double -decomposition gives other chromates; for example, with a solution of -potassium sulphate it gives a precipitate of calcium sulphate and a -solution of potassium dichromate, which crystallises when evaporated.[2] - - [1 bis] But the calcium chromate is soluble in water in the presence of - an excess of chromic acid, as may be seen from the fact that a - solution of chromic acid dissolves lime. - - [2] There are many variations in the details of the manufacturing - processes, and these must be looked for in works on technical - chemistry. But we may add that the chromate may also be obtained by - slightly roasting briquettes of a mixture of chrome iron and lime, - and then leaving the resultant mass to the action of moist air - (oxygen is absorbed, and the mass turns yellow). - -_Potassium dichromate_, K_{2}Cr_{2}O_{7}, easily crystallises from acid -solutions in red, well-formed prismatic crystals, which fuse at a red -heat and evolve oxygen at a very high temperature, leaving chromic oxide -and the normal salt, which undergoes no further change: 2K_{2}Cr_{2}O_{7} -= 2K_{2}CrO_{4} + Cr_{2}O_{3} + O_{3}. At the ordinary temperature 100 -parts of water dissolve 10 parts of this salt, and the solubility -increases as the temperature rises. It is most important to note that the -dichromate does not contain water, it is K_{2}CrO_{4} + CrO_{3}; the acid -salt corresponding to potassium acid sulphate, KHSO_{4}, does not exist. -It does not even evolve heat when dissolving in water, but on the -contrary produces cold, _i.e._ it does not form a very stable compound -with water. The solution and the salt itself are poisonous, and act as -powerful oxidising agents, which is the character of chromic acid in -general. When heated with sulphur or organic substances, with sulphurous -anhydride, hydrogen sulphide, &c., this salt is deoxidised, yielding -chromic compounds.[2 bis] Potassium dichromate[3] is used in the arts and -in chemistry as a source for the preparation of all other chromium -compounds. It is converted into yellow pigments by means of double -decomposition with salts of lead, barium, and zinc. When solutions of the -salts of these metals are mixed with potassium dichromate (in dyeing -generally mixed with soda, in order to obtain normal salts), they are -precipitated as insoluble normal salts; for example, 2BaCl_{2} + -K_{2}Cr_{2}O_{7} + H_{2}O = 2BaCrO_{4} + 2KCl + 2HCl. It follows from -this that these salts are insoluble in dilute acids, but the -precipitation is not complete (as it would be with the normal salt). The -barium and zinc salts are of a lemon yellow colour; the lead salt has a -still more intense colour passing into orange. Yellow cotton prints are -dyed with this pigment. The silver salt, Ag_{2}CrO_{4}, is of a bright -red colour. - - [2 bis] The oxidising action of potassium dichromate on organic - substances at the ordinary temperature is especially marked under - the action of light. Thus it acts on gelatin, as Poutven - discovered; this is applied to photography in the processes of - photogravure, photo-lithography, pigment printing, &c. Under the - action of light this gelatin is oxidised, and the chromic anhydride - deoxidised into chromic oxide, which unites with the gelatin and - forms a compound insoluble in warm water, whilst where the light - has not acted, the gelatin remains soluble, its properties being - unaffected by the presence of chromic acid or potassium dichromate. - - [3] Ammonium and sodium dichromates are now also prepared on a large - scale. The sodium salts may be prepared in exactly the same manner - as those of potassium. The normal salt combines with ten - equivalents of water, like Glauber's salt, with which it is - isomorphous. Its solution above 30° deposits the anhydrous salt. - Sodium dichromate crystals contain Na_{2}Cr_{2}O_{7},2H_{2}O. The - _ammonium salts of chromic acid_ are obtained by saturating the - anhydride itself with ammonia. The dichromate is obtained by - saturating one part of the anhydride with ammonia, and then adding - a second part of anhydride and evaporating under the receiver of an - air-pump. On ignition, the normal and acid salts leave chromic - oxide. Potassium ammonium chromate, NH_{4}KCrO_{4}, is obtained in - yellow needles from a solution of potassium dichromate in aqueous - ammonia; it not only loses ammonia and becomes converted into - potassium dichromate when ignited, but also by degrees at the - ordinary temperature. This shows the feeble energy of chromic acid, - and its tendency to form stable dichromates. Magnesium chromate is - soluble in water, as also is the strontium salt. The calcium salt - is also somewhat soluble, but the barium salt is almost insoluble. - The isomorphism with sulphuric acid is shown in the chromates by - the fact that the magnesium and ammonium salts form double salts - containing six equivalents of water, which are perfectly - isomorphous with the corresponding sulphates. The magnesium salt - crystallises in large crystals containing seven equivalents of - water. The beryllium, cerium, and cobalt salts are insoluble in - water. Chromic acid dissolves manganous carbonate, but on - evaporation the solution deposits manganese dioxide, formed at the - expense of the oxygen of the chromic acid. Chromic acid also - oxidises ferrous oxide, and ferric oxide is soluble in chromic - acid. - - One of the chromates most used by the dyer is the insoluble yellow - lead chromate, PbCrO_{4} (Chapter XVIII., Note 46), which is - precipitated on mixing solutions of PbX_{2} with soluble chromates. - It easily forms a basic salt, having the composition PbO,PbCrO_{4}, - as a crystalline powder, obtained by fusing the normal salt with - nitre and then rapidly washing in water. The same substance is - obtained, although impure and in small quantity, by treating lead - chromate with neutral potassium chromate, especially on boiling the - mixture; and this gives the possibility of attaining, by means of - these materials, various tints of lead chromate, from yellow to - red, passing through different orange shades. The decomposition - which takes place (incompletely) in this case is as follows: - 2PbCrO_{4} + K_{2}CrO_{4} = PbCrO_{4},PbO + K_{2}Cr_{2}O_{7}--that - is, potassium dichromate is formed in solution. - -When potassium dichromate is mixed with potassium hydroxide or carbonate -(carbonic anhydride being disengaged in the latter case) it forms the -_normal_ salt, K_{2}CrO_{4}, known as _yellow chromate of potassium_. Its -specific gravity is 2·7, being almost the same as that of the dichromate. -It absorbs heat in dissolving; one part of the salt dissolves in 1·75 -part of water at the ordinary temperature, forming a yellow solution. -When mixed even with such feeble acids as acetic, and more especially -with the ordinary acids, it gives the dichromate, and Graham obtained a -trichromate, K_{2}Cr_{3}O_{10} = K_{2}CrO_{4},2CrO_{3}, by mixing a -solution of the latter salt with an excess of nitric acid. - -_Chromic anhydride_ is obtained by preparing a saturated solution of -potassium dichromate at the ordinary temperature, and pouring it in a -thin stream into an equal volume of pure sulphuric acid.[4] On mixing, -the temperature naturally rises; when slowly cooled, the solution -deposits chromic anhydride in needle-shaped crystals of a red colour -sometimes several centimetres long. The crystals are freed from the -mother liquor by placing them on a porous tile.[4 bis] It is very -important at this point to call attention to the fact that a hydrate of -chromic anhydride is never obtained in the decomposition of chromic -compounds, but always the _anhydride_, CrO_{3}. The corresponding -hydrate, CrO_{4}H_{2}, or any other hydrate, is not even known. -Nevertheless, it must be admitted that chromic acid is bibasic, because -it forms salts isomorphous or perfectly analogous with the salts formed -by sulphuric acid, which is the best example of a bibasic acid. A clear -proof of the bibasicity of CrO_{3} is seen in the fact that the anhydride -and salts give (when heated with sodium chloride and sulphuric acid) a -volatile chloranhydride, CrO_{2}Cl_{2}, containing two atoms of chlorine -as a bibasic acid should.[5] Chromic anhydride is a red crystalline -substance, which is converted into a black mass by heat; it fuses at -190°, and disengages oxygen above 250°, leaving a residue of chromium -dioxide, CrO_{2},[6] and, on still further heating, chromic oxide, -Cr_{2}O_{3}. Chromic anhydride is exceedingly soluble in water, and even -attracts moisture from the air, but, as was mentioned above, it does not -form any definite compound with water. The specific gravity of its -crystals is 2·7, and when fused it has a specific gravity 2·6. The -solution presents perfectly defined acid properties. It liberates -carbonic anhydride from carbonates; gives insoluble precipitates of the -chromates with salts of barium, lead, silver, and mercury. - - [4] The sulphuric acid should not contain any lower oxides of nitrogen, - because they reduce chromic anhydride into chromic oxide. If a - solution of a chromate be heated with an excess of acid--for - instance, sulphuric or hydrochloric acid--oxygen or chlorine is - evolved, and a solution of a chromic salt is formed. Hence, under - these circumstances, chromic acid cannot be obtained from its - salts. One of the first methods employed consisted in converting - its salts into volatile _chromium hexafluoride_, CrF_{6}. This - compound, obtained by Unverdorben, may be prepared by mixing lead - chromate with fluor spar in a dry state, and treating the mixture - with fuming sulphuric acid in a platinum vessel: PbCrO_{4} + - 3CaF_{2} + 4H_{2}SO_{4} = PbSO_{4} + 3CaSO_{4} + 4H_{2}O + CrF_{6}. - Fuming sulphuric acid is taken, and in considerable excess, because - the chromium fluoride which is formed is very easily decomposed by - water. It is volatile, and forms a very caustic, poisonous vapour, - which condenses when cooled in a dry platinum vessel into a red, - exceedingly volatile liquid, which fumes powerfully in air. The - vapours of this substance when introduced into water are decomposed - into hydrofluoric acid and chromic anhydride: CrF_{6} + 3H_{2}O = - CrO_{3} + 6HF. If very little water be taken the hydrofluoric acid - volatilises, and chromic anhydride separates directly in crystals. - The chloranhydride of chromic acid, CrO_{2}Cl_{2} (Note 5), is also - decomposed in the same manner. A solution of chromic acid and a - precipitate of barium sulphate are formed by treating the insoluble - barium chromate with an equivalent quantity of sulphuric acid. If - carefully evaporated, the solution yields crystals of chromic - anhydride. Fritzsche gave a very convenient method of preparing - chromic anhydride, based on the relation of chromic to sulphuric - acid. At the ordinary temperature the strong acid dissolves both - chromic anhydride and potassium chromate, but if a certain amount - of water is added to the solution the chromic anhydride separates, - and if the amount of water be increased the precipitated chromic - anhydride is again dissolved. The chromic anhydride is almost all - separated from the solution when it contains two equivalents of - water to one equivalent of sulphuric acid. Many methods for the - preparation of chromic anhydride are based on this fact. - - [4 bis] They cannot be filtered through paper or washed, because the - chromic anhydride is reduced by the filter-paper, and is dissolved - during the process of washing. - - [5] Berzelius observed, and Rose carefully investigated, this - remarkable reaction, which occurs between chromic acid and sodium - chloride in the presence of sulphuric acid. If 10 parts of common - salt be mixed with 12 parts of potassium dichromate, fused, cooled, - and broken up into lumps, and placed in a retort with 20 parts of - fuming sulphuric acid, it gives rise to a violent reaction, - accompanied by the formation of brown fumes of _chromic - chloranhydride_, or _chromyl chloride_, CrO_{2}Cl_{2}, according to - the reaction: CrO_{3} + 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + H_{2}O - + CrO_{2}Cl_{2}. The addition of an excess of sulphuric acid is - necessary in order to retain the water. The same substance is - always formed when a metallic chloride is heated with chromic acid, - or any of its salts, in the presence of sulphuric acid. The - formation of this volatile substance is easily observed from the - brown colour which is proper to its vapour. On condensing the - vapour in a dry receiver a liquid is obtained having a sp. gr. of - 1·9, boiling at 118°, and giving a vapour whose density, compared - with hydrogen, is 78, which corresponds with the above formula. - Chromyl chloride is decomposed by heat into chromic oxide, oxygen, - and chlorine: 2CrO_{2}Cl_{2} = Cr_{2}O_{3} + 2Cl_{2} + O; so that - it is able to act simultaneously as a powerful oxidising and - chlorinating agent, which is taken advantage of in the - investigation of many, and especially of organic, substances. When - treated with water, this substance first falls to the bottom, and - is then decomposed into hydrochloric and chromic acids, like all - chloranhydrides: CrO_{2}Cl_{2} + H_{2}O = CrO_{3} + 2HCl. When - brought into contact with inflammable substances it sets fire to - them; it acts thus, for instance, on phosphorus, sulphur, oil of - turpentine, ammonia, hydrogen, and other substances. It attracts - moisture from the atmosphere with great energy, and must therefore - be kept in closed vessels. It dissolves iodine and chlorine, and - even forms a solid compound with the latter, which depends upon the - faculty of chromium to form its higher oxide, Cr_{2}O_{7}. The - close analogy in the physical properties of the chloranhydrides, - CrO_{2}Cl_{2} and SO_{2}Cl_{2}, is very remarkable, although - sulphurous anhydride is a gas, and the corresponding oxide, - CrO_{2}, is a non-volatile solid. It may be imagined, therefore, - that chromium dioxide (which will be mentioned in the following - note) presents a polymerised modification of the substance having - the composition CrO_{2}; in fact, this is obvious from the method - of its formation. - - If three parts of potassium dichromate be mixed with four parts of - strong hydrochloric acid and a small quantity of water, and gently - warmed, it all passes into solution, and no chlorine is evolved; on - cooling, the liquid deposits red prismatic crystals, known as - _Peligot's salt_, very stable in air. This has the composition - KCl,CrO_{3}, and is formed according to the equation - K_{2}Cr_{2}O_{7} + 2HCl = 2KCl,CrO_{3} + H_{2}O. It is evident that - this is the first chloranhydride of chromic acid, HCrO_{3}Cl, in - which the hydrogen is replaced by potassium. It is decomposed by - water, and on evaporation the solution yields potassium dichromate - and hydrochloric acid. This is a fresh instance of the reversible - reactions so frequently encountered. With sulphuric acid Peligot's - salt forms chromyl chloride. The latter circumstance, and the fact - that Geuther produced Peligot's salt from potassium chromate and - chromyl chloride, give reason for thinking that it is a compound of - these two substances: 2KCl,CrO_{3} = K_{2}CrO_{4} + CrO_{2}Cl_{2}. - It is also sometimes regarded as potassium dichromate in which one - atom of oxygen is replaced by chlorine--that is, - K_{2}Cr_{2}O_{6}Cl_{2}, corresponding with K_{2}Cr_{2}O_{7}. When - heated it parts with all its chlorine, and on further heating gives - chromic oxide. - - [6] This intermediate degree of oxidation, CrO_{2}, may also be - obtained by mixing solutions of chromic salts with solutions of - chromates. The brown precipitate formed contains a compound, - Cr_{2}O_{3},CrO_{3}, consisting of equivalent amounts of chromic - oxide and anhydride. The brown precipitate of chromium dioxide - contains water. The same substance is formed by the imperfect - deoxidation of chromic anhydride by various reducing agents. - Chromic oxide, when heated, absorbs oxygen, and appears to give the - same substance. Chromic nitrate, when ignited, also gives this - substance. When this substance is heated it first disengages water - and then oxygen, chromic oxide being left. It corresponds with - manganese dioxide, Cr_{2}O_{3},CrO_{3} = 3CrO_{2}. Krüger treated - chromium dioxide with a mixture of sodium chloride and sulphuric - acid, and found that chlorine gas was evolved, but that chromyl - chloride was not formed. Under the action of light, a solution of - chromic acid also deposits the brown dioxide. At the ordinary - temperature chromic anhydride leaves a brown stain upon the skin - and tissues, which probably proceeds from a decomposition of the - same kind. Chromic anhydride is soluble in alcohol containing - water, and this solution is decomposed in a similar manner by - light. Chromium dioxide forms K_{2}CrO_{4} when treated with - H_{2}O_{2} in the presence of KHO. - -The action of hydrogen peroxide on a solution of chromic acid or of -potassium dichromate gives a blue solution, which very quickly becomes -colourless with the disengagement of oxygen. Barreswill showed that this -is due to the formation of a _perchromic anhydride_, Cr_{2}O_{7}, -corresponding with sulphur peroxide. This peroxide is remarkable from the -fact that it very easily dissolves in ether and is much more stable in -this solution, so that, by shaking up hydrogen peroxide mixed with a -small quantity of chromic acid, with ether, it is possible to transfer -all the blue substance formed to the ether.[6 bis] - - [6 bis] Now that persulphuric acid H_{2}S_{2}O_{8} is well known it - might be supposed that perchromic anhydride, Cr_{2}O_{7}, would - correspond to perchromic acid, H_{2}Cr_{2}O_{8}, but as yet it is - not certain whether corresponding salts are formed. Péchard (1891) - on adding an excess of H_{2}O_{2} and baryta water to a dilute - solution of CrO_{2} (8 grm. per litre), observed the formation of a - yellow precipitate, but oxygen was disengaged at the same time and - the precipitate (which easily exploded when dried) was found to - contain, besides an admixture of BaO_{2}, a compound BaCrO_{5}, and - this = BaO_{2} + CrO_{3}, and does not correspond to perchromic - acid. The fact of its decomposing with an explosion, and the mode - of its preparation, proves, however, that this is a similar - derivative of peroxide of hydrogen to persulphuric acid (Chapter - XX.) - -With oxygen acids, chromic acid evolves oxygen; for example, with -sulphuric acid the following reaction takes place: 2CrO_{3} + -3H_{2}SO_{4} = Cr_{2}(SO_{4})_{3} + O_{3} + 3H_{2}O. It will be readily -understood from this that _a mixture of chromic acid_ or _of its salts -with sulphuric acid_ forms an excellent _oxidising agent_, which is -frequently employed in chemical laboratories and even for technical -purposes as a means of oxidation. Thus hydrogen sulphide and sulphurous -anhydride are converted into sulphuric acid by this means. Chromic acid -is able to act as a powerful oxidising agent because it passes into -chromic oxide, and in so doing disengages half of the oxygen contained in -it: 2CrO_{3} = Cr_{2}O_{3} + O_{3}. Thus chromic anhydride itself is a -powerful oxidising agent, and is therefore employed instead of nitric -acid in galvanic batteries (as a depolariser), the hydrogen evolved at -the carbon being then oxidised, and the chromic acid converted into a -non-volatile product of deoxidation, instead of yielding, as nitric acid -does, volatile lower oxides of offensive odour. Organic substances are -more or less perfectly oxidised by means of chromic anhydride, although -this generally requires the aid of heat, and does not proceed in the -presence of alkalis, but generally _in the presence of acids_. In acting -on a solution of potassium iodide, chromic acid, like many oxidising -agents, liberates iodine; the reaction proceeds in proportion to the -amount of CrO_{3} present, and may serve for determining the amount of -CrO_{3}, since the amount of iodine liberated can be accurately -determined by the iodometric method (Chapter XX., Note 42). If chromic -anhydride be ignited in a stream of ammonia, it gives chromic oxide, -water, and nitrogen. In all cases when chromic acid acts as an oxidising -agent in the presence of acids and under the action of heat, the product -of its deoxidation is a chromic salt, CrX_{3}, which is characterised by -the green colour of its solution, so that the _red_ or yellow _solution_ -of a salt of chromic acid is then transformed into a _green solution_ of -a chromic salt, derived from chromic oxide, Cr_{2}O_{3}, which is closely -analogous to Al_{2}O_{3}, Fe_{2}O_{3}, and other bases of the composition -R_{2}O_{3}. This analogy is seen in the insolubility of the anhydrous -oxide, in the gelatinous form of the colloidal hydrate, in the formation -of alums,[7] of a volatile chloride of chromium, &c.[7 bis] - - [7] As a mixture of potassium dichromate and sulphuric acid is usually - employed for oxidation, the resultant solution generally contains a - double sulphate of potassium and chromium--that is, _chrome alum_, - isomorphous with ordinary alum--K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4} + - 20H_{2}O = O_{3} + K_{2}Cr_{2}(SO_{4})_{4},24H_{2}O or - 2(KCr(SO_{4})_{2},12H_{2}O). It is prepared by dissolving potassium - dichromate in dilute sulphuric acid; alcohol is then added and the - solution slightly heated, or sulphurous anhydride is passed through - it. On the addition of alcohol to a cold mixture of potassium - dichromate and sulphuric acid, the gradual disengagement of - pleasant-smelling volatile products of the oxidation of alcohol, - and especially of aldehyde, C_{2}H_{4}O, is remarked. If the - temperature of decomposition does not exceed 35°, a _violet_ - solution of chrome alum is obtained, but if the temperature be - higher, a solution of the same alum is obtained of a _green_ - colour. As chrome alum requires for solution 7 parts of water at - the ordinary temperature, it follows that if a somewhat strong - solution of potassium dichromate be taken (4 parts of water and - 1-1/2 of sulphuric acid to 1 part of dichromate), it will give so - concentrated a solution of chrome alum that on cooling, the salt - will separate without further evaporation. _If the liquid_, - prepared as above or in any instance of the deoxidation of chromic - acid, _be heated_ (the oxidation naturally proceeds more rapidly) - somewhat strongly, for instance, to the boiling-point of water, or - if the violet solution already formed be raised to the same - temperature, it acquires a bright _green colour_, and on - evaporation the same mixture, which at lower temperatures so easily - gives cubical crystals of chrome alum, _does not give any crystals - whatever_. _If the green solution be kept_, however, _for several - weeks_ at the ordinary temperature, it deposits _violet crystals_ - of chrome alum. The green solution, when evaporated, gives a - non-crystalline mass, and the violet crystals lose water at 100° - and turn green. It must be remarked that the transition of the - green modification into the violet is accompanied by a decrease in - volume (Lecoq de Boisbaudran, Favre). If the green mass formed at - the higher temperature be evaporated to dryness and heated at 30° - in a current of air, it does not retain more then 6 equivalents of - water. Hence Löwel, and also Schrötter, concluded that the green - and violet modifications of the alum depend on different degrees of - combination with water, which may be likened to the different - compounds of sodium sulphate with water and to the different - hydrates of ferric oxide. - - However, the question in this case is not so simple, as we shall - afterwards see. Not chrome alum alone, but _all the chromic salts_, - give two, if not three, _varieties_. At least, there is no doubt - about the existence of two--a _green_ and a _violet modification_. - The green chromic salts are obtained by heating solutions of the - violet salts, the violet solutions are produced on keeping - solutions of the green salts for a long time. The conversion of the - violet salts into green by the action of heat is itself an - indication of the possibility of explaining the different - modifications by their containing different proportions (or states) - of water, and, moreover, by the green salts having a less amount of - water than the violet. However, there are other explanations. - Chromic oxide is a base like alumina, and is therefore able to give - both acid and basic salts. It is supposed that the difference - between the green and violet salts is due to this fact. This - opinion of Krüger is based on the fact that alcohol separates out a - salt from the green solution which contains less sulphuric acid - than the normal violet salt. On the other hand, Löwel showed that - all the acid cannot be separated from the green chromic salts by - suitable reagents, as easily as it can be from the same solution of - the violet salts; thus barium salts do not precipitate all the - sulphuric acid from solutions of the green salts. According to - other researches the cause of the varieties of the chromic salts - lies in a difference in the bases they contain--that is, it is - connected with a modification of the properties of the oxide of - chromium itself. This only refers to the hydroxides, but as - hydroxides themselves are only special forms of salts, the - differences observed as yet in this direction between the - hydroxides only confirm the generality of the difference observed - in the chromic compounds (_see_ Note 7 bis). - - The salts of chromic oxide, like those of alumina, are easily - decomposed, give basic and double salts, and have an acid reaction, - as chromic oxide is a feeble base. Potassium and sodium hydroxides - give a _precipitate_ of the hydroxide with chromic salts, CrX_{3}. - The violet and green salts give a _hydroxide soluble in an excess - of the reagent_; but the hydroxide is held in solution by very - feeble affinities, so that it is partially separated by heat and - dilution with water, and completely so on boiling. In an alkaline - solution, chromic hydroxide is easily converted into chromic acid - by the action of lead dioxide, chlorine, and other oxidising - agents. If the chromic oxide occurs together with such oxides as - magnesia, or zinc oxide, then on precipitation it separates out - from its solution in combination with these oxides, forming, for - example, ZnO,Cr_{2}O_{3}. Viard obtained compounds of Cr_{2}O_{3} - with the oxides of Mg, Zn, Cd, &c.) On precipitating the violet - solution of chrome alum with ammonia, a precipitate containing - Cr_{2}O_{3},6H_{2}O is obtained, whilst the precipitate from the - boiling solution with caustic potash was a hydrate containing four - equivalents of water. When fused with borax chromic salts give a - green glass. The same coloration is communicated to ordinary glass - by the presence of traces of chromic oxide. A chrome glass - containing a large amount of chromic oxide may be ground up and - used as a green pigment. Among the hydrates of oxide of chromium - _Guignet's green_ forms one of the widely-used green pigments which - have been substituted for the poisonous arsenical copper pigments, - such as Schweinfurt green, which formerly was much used. Guignet's - green has an extremely bright green colour, and is distinguished - for its great stability, not only under the action of light but - also towards reagents; thus it is not altered by alkaline - solutions, and even nitric acid does not act on it. This pigment - remains unchanged up to a temperature of 250°; it contains - Cr_{2}O_{3},2H_{2}O, and generally a small amount of alkali. It is - prepared by fusing 3 parts of boric acid with 1 part of potassium - dichromate; oxygen is disengaged, and a green glass, containing a - mixture of the borates of chromium and potassium, is obtained. When - cool this glass is ground up and treated with water, which extracts - the boric acid and alkali and leaves the above-named chromic - hydroxide behind. This hydroxide only parts with its water at a red - heat, leaving the anhydrous oxide. - - The chromic hydroxides lose their water by ignition, and in so - doing become spontaneously incandescent, like the ordinary ferric - hydroxide (Chapter XXII.). It is not known, however, whether all - the modifications of chromic oxide show this phenomenon. The - anhydrous _chromic oxide_, Cr_{2}O_{3}, is exceedingly difficultly - soluble in acids, if it has passed through the above recalescence. - But if it has parted with its water, or the greater part of it, and - not yet undergone this self-induced incandescence (has not lost a - portion of its energy), then it is soluble in acids. It is not - reduced by hydrogen. It is easily obtained in various crystalline - forms by many methods. The chromates of mercury and ammonium give a - very convenient method for its preparation, because when ignited - they leave chromic oxide behind. In the first instance oxygen and - mercury are disengaged, and in the second case nitrogen and water: - 2Hg_{2}CrO_{4} = Cr_{2}O_{3} + O_{5} + 4Hg or - (NH_{4})_{2}Cr_{2}O_{7} = Cr_{2}O_{3} + 4H_{2}O + N_{2}. The second - reaction is very energetic, and the mass of salt burns - spontaneously if the temperature be sufficiently high. A mixture of - potassium sulphate and chromic oxide is formed by heating potassium - dichromate with an equal weight of sulphur: K_{2}Cr_{2}O_{7} + S = - K_{2}SO_{4} + Cr_{2}O_{3}. The sulphate is easily extracted by - water, and there remains a bright green residue of the oxide, whose - colour is more brilliant the lower the temperature of the - decomposition. The oxide thus obtained is used as a green pigment - for china and enamel. The anhydrous chromic oxide obtained from - chromyl chloride, CrO_{2}Cl_{2}, has a specific gravity of 5·21, - and forms almost black crystals, which give a green powder. They - are hard enough to scratch glass, and have a metallic lustre. The - crystalline form of chromic oxide is identical with that of the - oxide of iron and alumina, with which it is isomorphous. - - [7 bis] The most important of the compounds corresponding with chromic - oxide is _chromic chloride_, Cr_{2}Cl_{6}, which is known in an - anhydrous and in a hydrated form. It resembles ferric and aluminic - chlorides in many respects. There is a great difference between the - anhydrous and the hydrated chlorides; the former is insoluble in - water, the latter easily dissolves, and on evaporation its solution - forms a hygroscopic mass which is very unstable and easily evolves - hydrochloric acid when heated with water. The anhydrous form is of - a violet colour, and Wöhler gives the following method for its - preparation: an intimate mixture is prepared of the anhydrous - chromic oxide with carbon and organic matter, and charged into a - wide infusible glass or porcelain tube which is heated in a - combustion furnace; one extremity of the tube communicates with an - apparatus generating chlorine which is passed through several - bottles containing sulphuric acid in order to dry it perfectly - before it reaches the tube. On heating the portion of the tube in - which the mixture is placed and passing chlorine through, a - slightly volatile sublimate of chromic chloride, CrCl_{3} or - Cr_{2}Cl_{6}, is formed. This substance forms _violet tabular - crystals_, which may be distilled in dry chlorine without change, - but which, however, require a red heat for their volatilisation. - These crystals are greasy to the touch and insoluble in water, but - if they be powdered and boiled in water for a long time they pass - into _a green solution_. Strong sulphuric acid does not act on the - anhydrous salt, or only acts with exceeding slowness, like water. - Even aqua regia and other acids do not act on the crystals, and - alkalis only show a very feeble action. The specific gravity of the - crystals is 2·99. When fused with sodium carbonate and nitre they - give sodium chloride and potassium chromate, and when ignited in - air they form green chromic oxide and evolve chlorine. On ignition - in a stream of ammonia, chromic chloride forms sal-ammoniac and - chromium nitride, CrN (analogous to the nitrides BN, AlN). Mosberg - and Peligot showed that when chromic chloride is ignited in - hydrogen, it parts with one-third of its chlorine, forming chromous - chloride, CrCl_{2}--that is, there is formed from a compound - corresponding with chromic oxide, Cr_{2}O_{3}, a compound answering - to the _suboxide_, chromous oxide, CrO--just as hydrogen converts - ferric chloride into ferrous chloride with the aid of heat. - _Chromous chloride_, CrCl_{2}, forms colourless crystals easily - soluble in water, which in dissolving evolve a considerable amount - of heat, and form a blue liquid, capable of absorbing oxygen from - the air with great facility, being converted thereby into a chromic - compound. - - The blue solution of chromous chloride may also be obtained by the - action of metallic zinc on the green solution of the hydrated - chromic chloride; the zinc in this case takes up chlorine just as - the hydrogen did. It must be employed in a large excess. Chromic - oxide is also formed in the action of zinc on chromic chloride, and - if the solution remain for a long time in contact with the zinc the - whole of the chromium is converted into chromic oxychloride. Other - chromic salts are also reduced by zinc into _chromous salts_, - CrX_{2}, just as the ferric salts FeX_{3} are converted into - ferrous salts FeX_{2} by it. The chromous salts are exceedingly - unstable and easily oxidise and pass into chromic salts; hence the - reducing power of these salts is very great. From cupric salts they - separate cuprous salts, from stannous salts they precipitate - metallic tin, they reduce mercuric salts into mercurous and ferric - into ferrous salts. Moreover, they absorb oxygen from the air - directly. With potassium chromate they give a brown precipitate of - chromium dioxide or of chromic oxide, according to the relative - amounts of the substances taken: CrO_{3} + CrO = 2CrO_{2} or - CrO_{3} + 3CrO = 2Cr_{2}O_{3}. Aqueous ammonia gives a blue - precipitate, and in the presence of ammoniacal salts a blue liquid - is obtained which turns red in the air from oxidation. This is - accompanied by the formation of compounds analogous to those given - by cobalt (Chapter XXII.). A solution of chromous chloride with a - hot saturated solution of sodium acetate, C_{2}H_{3}NaO_{2}, gives, - on cooling, transparent red crystals of chromous acetate, - C_{4}H_{6}CrO_{4},H_{2}O. This salt is also a powerful reducing - agent, but may be kept for a long time in a vessel full of carbonic - anhydride. - - The insoluble anhydrous _chromic chloride_ CrCl_{3} very easily - _passes into solution_ in the presence of a trace (0·004) of - _chromous chloride_ CrCl_{2}. This remarkable phenomenon was - observed by Peligot and explained by Löwel in the following manner: - chromous chloride, as a lower stage of oxidation, is capable of - absorbing both oxygen and chlorine, combining with various - substances. It is able to decompose many chlorides by taking up - chlorine from them; thus it precipitates mercurous chloride from a - solution of mercuric chloride, and in so doing passes into chromic - chloride: 2CrCl_{2} + 2HgCl_{2} = Cr_{2}Cl_{6} + 2HgCl. Let us - suppose that the same phenomenon takes place when the anhydrous - chromic chloride is mixed with a solution of chromous chloride. The - latter will then take up a portion of the chlorine of the former, - and pass into a soluble hydrate of chromic chloride (hydrochloride - of oxide of chromium), and the original anhydrous chromic chloride - will pass into chromous chloride. The chromous chloride re-formed - in this manner will then act on a fresh quantity of the chromic - chloride, and in this manner transfer it entirely into solution as - hydrate. This view is confirmed by the fact that other chlorides, - capable of absorbing chlorine like chromous chloride, also induce - the solution of the insoluble chromic chloride--for example, - ferrous chloride, FeCl_{2}, and cuprous chloride. The presence of - zinc also aids the solution of chromic chloride, owing to its - converting a portion of it into chromous chloride. The solution of - chromic chloride in water obtained by these methods is perfectly - identical with that which is formed by dissolving chromic hydroxide - in hydrochloric acid. On evaporating the _green solution_ obtained - in this manner, it gives a green mass, containing water. On further - heating it leaves a soluble chromic oxychloride, and when ignited - it first forms an insoluble oxychloride and then chromic oxide; but - no anhydrous chromic chloride, Cr_{2}Cl_{6}, is formed by heating - the aqueous solution of chromic chloride, which forms an important - fact in support of the view that the green solution of chromic - chloride is nothing else but hydrochloride of oxide of chromium. At - 100° the composition of the green hydrate is Cr_{2}Cl_{6},9H_{2}O, - and on evaporation at the ordinary temperature over H_{2}SO_{4} - crystals are obtained with 12 equivalents of water; the red mass - obtained at 120° contains Cr_{2}O_{3},4Cr_{2}Cl_{6},24H_{2}O. The - greater portion of it is soluble in water, like the mass which is - formed at 150°. The latter contains - Cr_{2}O_{3},2Cr_{2}Cl_{6},9H_{2}O = 3(Cr_{2}OCl_{4},3H_{2}O)--that - is, it presents the same composition as chromic chloride in which - one atom of oxygen replaces two of chlorine. And if the hydrate of - chromic chloride be regarded as Cr_{2}O_{3},6HCl, the substance - which is obtained should be regarded as Cr_{2}O_{3},4HCl combined - with water, H_{2}O. The addition of alkalis--for example, - baryta--to a solution of chromic chloride immediately produces a - precipitate, which, however, re-dissolves on shaking, owing to the - formation of one of the oxychlorides just mentioned, which may be - regarded as _basic salts_. Thus we may represent the product of the - change produced on chromic chloride under the influence of water - and heat by the following formulæ: first Cr_{2}O_{3},6HCl or - Cr_{2}Cl_{6},3H_{2}O is formed, then Cr_{2}O_{3},4HCl,H_{2}O or - Cr_{2}OCl_{4},3H_{2}O, and lastly Cr_{2}O_{3},2HCl,2H_{2}O or - Cr_{2}O_{2}Cl_{2},3H_{2}O. In all three cases there are 2 - equivalents of chromium to at least 3 equivalents of water. These - compounds may be regarded as being intermediate between chromic - hydroxide and chloride; chromic chloride is Cr_{2}Cl_{6}, the first - oxychloride Cr_{2}(OH)_{2}Cl_{4}, the second Cr_{2}(OH)_{4}Cl_{2}, - and the hydrate Cr_{2}(OH)_{6}--that is, the chlorine is replaced - by hydroxyl. - - It is very important to remark two circumstances in respect to - this: (1) That the whole of the chlorine in the above compounds is - not precipitated from their solutions by silver nitrate; thus the - normal salt of the composition Cr_{2}Cl_{6},9H_{2}O only gives up - two-thirds of its chlorine; therefore Peligot supposes that the - normal salt contains the oxychloride combined with hydrochloric - acid: Cr_{2}Cl_{6} + 2H_{2}O = Cr_{2}O_{2}Cl_{2},4HCl, and that the - chlorine held as hydrochloric acid reacts with the silver, whilst - that held in the oxychloride does not enter into reaction, just as - we observe a very feebly-developed faculty for reaction in the - anhydrous chromic chloride; and (2) if the green aqueous solution - of CrCl_{3} be left to stand for some time, it ultimately turns - violet; in this form the whole of the chlorine is precipitated by - AgNO_{3}, whilst boiling re-converts it into the green variety. - Löwel obtained the violet solution of hydrochloride of chromic - oxide by decomposing the violet chromic sulphate with barium - chloride. Silver nitrate precipitates all the chlorine from this - violet modification; but if the violet solution be boiled and so - converted into the green modification, silver nitrate then only - precipitates a portion of the chlorine. - - Recoura (1890-1893) obtained a crystallohydrate of violet chromium - sulphate, Cr_{2}(SO_{4})_{3}, with 18 or 15 H_{2}O. By boiling a - solution of this crystallohydrate, he converted it into the green - salt, which, when treated with alkalis, gave a precipitate of - Cr_{2}O_{3},2H_{2}O, soluble in 2H_{2}SO_{4} (and not 3), and only - forming the basic salt, Cr_{2}(OH)_{2}(SO_{4})_{2}. He therefore - concludes that the green salts are basic salts. The cryoscopic - determinations made by A. Speransky (1892) and Marchetti (1892) - give a greater 'depression' for the violet than the green salts, - that is, indicate a greater molecular weight for the green salts. - But as Étard, by heating the violet sulphate to 100°, converted it - into a green salt of the same composition, but with a smaller - amount of H_{2}O, it follows that the formation of a basic salt - alone is insufficient to explain the difference between the green - and violet varieties, and this is also shown by the fact that - BaCl_{2} precipitates the whole of the sulphuric acid of the violet - salt, and only a portion of that of the green salt. A. Speransky - also showed that the molecular electro-conductivity of the green - solutions is less than that of the violet. It is also known that - the passage of the former into the latter is accompanied by an - increase of volume, and, according to Recoura, by an evolution of - heat also. - - Piccini's researches (1894) throw an important light upon the - peculiarities of the green chromium trichloride (or chromic - chloride); he showed (1) that AgF (in contradistinction to the - other salts of silver) precipitates all the chlorine from an - aqueous solution of the green variety; (2) that solutions of green - CrCl_{3},6H_{2}O in ethyl alcohol and acetone precipitate all their - chlorine when mixed with a similar solution of AgNO_{3}; (3) that - the rise of the boiling-point of the ethyl alcohol and acetone - green solutions of CrCl_{3},6H_{2}O (Chapter VII., Note 27 bis) - shows that i in this case (as in the aqueous solutions of MgSO_{4} - and HgCl_{2}) is nearly equal to 1, that is, that they are like - solutions of non-conductors; (4) that a solution of green CrCl_{3} - in methyl alcohol at first precipitates about 7/8 of its chlorine - (an aqueous solution about 2/3) when treated with AgNO_{3}, but - after a time the whole of the chlorine is precipitated; and (5) - that an aqueous solution of the green variety gradually passes into - the violet, while a methyl alcoholic solution preserves its green - colour, both of itself and also after the whole of the chlorine has - been precipitated by AgNO_{3}. If, however, in an aqueous or methyl - alcoholic solution only a portion of the chlorine be precipitated, - the solution gradually turns violet. In my opinion the general - meaning of all these observations requires further elucidation and - explanation, which should be in harmony with the theory of - solutions. Recoura, moreover, obtained compounds of the green salt, - Cr_{2}(SO_{4})_{3}, with 1, 2, and 3 molecules of H_{2}SO_{4}, - K_{2}SO_{4}, and even a compound Cr_{2}(SO_{4})_{3}H_{2}CrO_{4}. By - neutralising the sulphuric acid of the compounds of - Cr_{2}(SO_{4})_{3} and H_{2}SO_{4} with caustic soda, Recoura - obtained an evolution of 33 thousand calories per each 2NaHO, while - free H_{2}SO_{4} only gives 30·8 thousand calories. Recoura is of - opinion that special _chromo sulphuric acids_, for instance - (CrSO_{4})H_{2}SO_{4} = 1/2Cr_{2}(SO_{4})_{3}H_{2}SO_{4}, are - formed. With a still larger excess of sulphuric acid, Recoura - obtained salts containing a still greater number of sulphuric acid - radicles, but even this method does not explain the difference - between the green and violet salts. - - These facts must naturally be taken into consideration in order to - arrive at any complete decision as to the cause of the different - modifications of the chromic salts. We may observe that the green - modification of chromic chloride does not give double salts with - the metallic chlorides, whilst the violet variety forms compounds - Cr_{2}Cl_{6},2RCl (where R = an alkali metal), which are obtained - by heating the chromates with an excess of hydrochloric acid and - evaporating the solution until it acquires a violet colour. As the - result of all the existing researches on the green and violet - chromic salts, it appears to me most probable that their difference - is determined by the feeble basic character of chromic oxide, by - its faculty of giving basic salts, and by the colloidal properties - of its hydroxide (these three properties are mutually connected), - and moreover, it seems to me that the relation between the green - and violet salts of chromic oxide best answers to the relation of - the purpureo to the luteo cobaltic salts (Chapter XXII., Note 35). - This subject cannot yet be considered as exhausted (_see_ Note 7). - - We may here observe that with tin the chromic salts, CrX_{3}, give - at low temperatures CrX_{2} and SnX_{2}, whilst at high - temperatures, on the contrary, CrX_{2} reduces the metal from its - salts SnX_{2}. The reaction, therefore, belongs to the class of - reversible reactions (Beketoff). - - Poulenc obtained anhydrous CrF_{3} (sp. gr. 3·78) and CrF_{2} (sp. - gr. 4·11) by the action of gaseous HF upon CrCl_{2}. A solution of - fluoride of chromium is employed as a mordant in dyeing. Recoura - (1890) obtained green and violet varieties of Cr_{2}Br_{6},6H_{2}O. - The green variety can only be kept in the presence of an excess of - HBr in the solution; if alone its solution easily passes into the - violet variety with evolution of heat. - -_Chromic oxide_, Cr_{2}O_{3}, rarely found, and in small quantities, -in chrome ochre, is formed by the oxidation of chromium and its lower -oxides, by the reduction of chromates (for example, of ammonium or -mercuric chromate) and by the decomposition (splitting up) of the saline -compounds of the oxide itself, CrX_{3} or Cr_{2}X_{6}, like alumina, -which it resembles in forming a feeble base easily giving double and -basic salts, which are either green or violet. - -The reduction of chromic oxide--for instance, in a solution by zinc -and sulphuric acid--leads to the formation of chromous oxide, CrO, and -its salts, CrX_{2}, of a blue colour (_see_ Notes 7 and 7 ^{bis}). The -further reduction[8] of oxide of chromium and its corresponding compounds -gives _metallic chromium_. Deville obtained it (probably containing -carbon) by reducing chromic oxide with carbon, at a temperature near the -melting point of platinum, about 1750°, but the metal itself does not -fuse at this temperature. Chromium has a steel-grey colour and is very -hard (sp. gr. 5·9), takes a good polish, and dissolves in hydrochloric -acid, but cold dilute sulphuric and nitric acids have no action upon it. -Bunsen obtained metallic chromium by decomposing a solution of chromic -chloride, Cr_{2}Cl_{6}, by a galvanic current, as scales of a grey colour -(sp. gr. 7·3). Wöhler obtained crystalline chromium by igniting a mixture -of the anhydrous chromic chloride Cr_{2}Cl_{6} (_see_ Note 7 bis) with -finely-divided zinc, and sodium and potassium chlorides, at the -boiling-point of zinc. When the resultant mass has cooled the zinc may be -dissolved in dilute nitric acid, and grey crystalline chromium (sp. gr. -6·81) is left behind. Frémy also prepared crystalline chromium by the -action of the vapour of sodium on anhydrous chromic chloride in a stream -of hydrogen, using the apparatus shown in the accompanying drawing, and -placing the sodium and the chromic chloride in separate porcelain boats. -The tube containing these boats is only heated when it is quite full of -dry hydrogen. The crystals of metallic chromium obtained in the tube are -grey cubes having a considerable hardness and withstanding the action of -powerful acids, and even of aqua regia. The chromium obtained by Wöhler -by the action of a galvanic current is, on the contrary, acted on under -these conditions. The reason of this difference must be looked for in the -presence of impurities, and in the crystalline structure. But in any -case, among the properties of metallic chromium, the following may be -considered established: it is white in colour, with a specific gravity of -about 6·7, is extremely hard in a crystalline form, is not oxidised by -air at the ordinary temperature, and with carbon it forms alloys like -cast iron and steel. - - [8] The reduction of metallic chromium proceeds with comparative ease - in aqueous solutions. Thus the action of sodium amalgams upon a - strong solution of Cr_{2}Cl_{6} gives (first CrCl_{2}) an amalgam - of chromium from which the mercury may be easily driven off by - heating (in hydrogen to avoid oxidation), and there remains a - spongy mass of easily oxidizable chromium. Plaset (1891), by - passing an electric current through a solution of chrome alum mixed - with a small amount of H_{2}SO_{4} and K_{2}SO_{4}, obtained hard - scales of chromium of a bluish-white colour possessing great - hardness and stability (under the action of water, air, and acids). - Glatzel (1890) reduced a mixture of 2KCl + Cr_{2}Cl_{6} by heating - it to redness with shavings of magnesium. The metallic chromium - thus obtained has the appearance of a fine light-grey powder which - is seen to be crystalline under the microscope; its sp. gr. at 16° - is 6·7284. It fuses (with anhydrous borax) only at the highest - temperatures, and after fusion presents a silver-white fracture. - The strongest magnet has no action upon it. - - Moissan (1893) obtained chromium by reducing the oxide Cr_{2}O_{3} - with carbon in the electrical furnace (Chapter VIII., Note 17) in - 9-10 minutes with a current of 350 ampères and 50 volts. The - mixture of oxide and carbon gives a bright ingot weighing 100-110 - grams. A current of 100 ampères and 50 volts completes the - experiment upon a smaller quantity of material in 15 minutes; a - current of 30 ampères and 50 volts gave an ingot of 10 grams in - 30-40 minutes. The resultant carbon alloy is more or less rich in - chromium (from 87·37-91·7 p.c.). To obtain the metal free from - carbon, the alloy is broken into large lumps, mixed with oxide of - chromium, put into a crucible and covered with a layer of oxide. - This mixture is then heated in the electric furnace and the pure - metal is obtained. This reduction can also be carried on with - chrome iron ore FeOCr_{2}O_{3} which occurs in nature. In this case - a homogeneous alloy of iron and chromium is obtained. If this alloy - be thrown in lumps into molten nitre, it forms insoluble - sesquioxide of iron and a soluble alkaline chromate. This alloy of - iron and chromium dissolved in molten steel (chrome steel) renders - it hard and tough, so that such steel has many valuable - applications. The alloy, containing about 3 p.c. Cr and about 1·3 - p.c. carbon, is even harder than the ordinary kinds of tempered - steel and has a fine granular fracture. The usual mode of preparing - the ferrochromes for adding to steel is by fusing powdered chrome - iron ore under fluxes in a graphite crucible. - -[Illustration: FIG. 92.--Apparatus for the preparation of metallic -chromium by igniting chromic chloride and sodium in a stream of -hydrogen.] - -The two analogues of chromium, _molybdenum_ and _tungsten_ (or wolfram), -are of still rarer occurrence in nature, and form acid oxides, RO_{3}, -which are still less energetic than CrO_{3}. Tungsten occurs in the -somewhat rare minerals, _scheelite_, CaWO_{4}, and _wolfram_; the latter -being an isomorphous mixture of the normal tungstates of iron and -manganese, (MnFe)WO_{4}. Molybdenum is most frequently met with as -_molybdenite_, MoS_{2}, which presents a certain resemblance to graphite -in its physical properties and softness. It also occurs, but much more -rarely, as a yellow lead ore, PbMoO_{4}. In both these forms molybdenum -occurs in the primary rocks, in granites, gneiss, &c., and in iron and -copper ores in Saxony, Sweden, and Finland. Tungsten ores are sometimes -met with in considerable masses in the primary rocks of Bohemia and -Saxony, and also in England, America, and the Urals. The preliminary -treatment of the ore is very simple; for example, the sulphide, MoS_{2}, -is roasted, and thus converted into sulphurous anhydride and molybdic -anhydride, MoO_{3}, which is then dissolved in alkalis, generally in -ammonia. The ammonium molybdate is then treated with acids, when the -sparingly soluble molybdic acid is precipitated. Wolfram is treated in a -different manner. Most frequently the finely-ground ore is repeatedly -boiled with hydrochloric and nitric acids, and the resultant solutions -(of salts of manganese and iron) poured off, until the dark brown mass of -ore disappears, whilst the tungstic acid remains, mixed with silica, as -an insoluble residue; it is treated also with ammonia, and is thus -converted into soluble ammonium tungstate, which passes into solution and -yields tungstic acid when treated with acids. This hydrate is then -ignited, and leaves tungstic anhydride. The general character of molybdic -and tungstic anhydrides is analogous to that of chromic anhydride; they -are anhydrides of a feebly acid character, which easily give polyacid -salts and colloid solutions.[8 bis] - - [8 bis] The atomic composition of the tungsten and molybdenum compounds - is taken as being identical with that of the compounds of sulphur - and chromium, because (1) both these metals give two oxides in - which the amounts of oxygen per given amount of metal stand in the - ratio 2 : 3; (2) the higher oxide is of the latter kind, and, like - chromic and sulphuric anhydrides, it has an acid character; (3) - certain of the molybdates are isomorphous with the sulphates; (4) - the specific heat of tungsten is 0·0334, consequently the product - of the atomic weight and specific heat is 6·15, like that of the - other elements--it is the same with molybdenum, 96·0 × 0·0722 = - 6·9; (5) tungsten forms with chlorine not only compounds WCl_{6}, - WCl_{5}, and WOCl_{4}, but also WO_{2}Cl_{2}, a volatile substance - the analogue of chromyl chloride, CrO_{2}Cl_{2}, and sulphuryl - chloride, SO_{2}Cl_{2}. Molybdenum gives the chlorine compounds, - MOCl_{2}, MOCl_{3}(?), MOCl_{4} (fuses at 194°, boils at 268°; - according to Debray it contains MOCl_{5}), MoOCl_{4}, - MoO_{2}Cl_{2}, and MoO_{2}(OH)Cl. The existence of tungsten - hexachloride, WCl_{6}, is an excellent proof of the fact that the - type SX_{6} appears in the analogues of sulphur as in SO_{3}; (6) - the vapour density accurately determined for the chlorine compounds - MoCl_{4}, WCl_{6}, WCl_{5}, WOCl_{4} (Roscoe) leaves no doubt as to - the molecular composition of the compounds of tungsten and - molybdenum, for the observed and calculated results entirely agree. - - Tungsten is sometimes called scheele in honour of Scheele, who - discovered it in 1781 and molybdenum in 1778. Tungsten is also - known as wolfram; the former name was the name given to it by - Scheele, because he extracted it from the mineral then known as - tungsten and now called scheelite, CaWO_{4}. The researches of - Roscoe, Blomstrand and others have subsequently thrown considerable - light on the whole history of the compounds of molybdenum and - tungsten. - - The ammonium salts of tungsten and molybdic acids when ignited - leave the anhydrides, which resemble each other in many respects. - _Tungsten anhydride_, WO_{3}, is a yellowish substance, which only - fuses at a strong heat, and has a sp. gr. of 6·8. It is insoluble - both in water and acid, but solutions of the alkalis, and even of - the alkali carbonates, dissolve it, especially when heated, forming - alkaline salts. _Molybdic anhydride_, MoO_{3}, is obtained by - igniting the acid (hydrate) or the ammonium salt, and forms a white - mass which fuses at a red heat, and solidifies to a yellow - crystalline mass of sp. gr. 4·4; whilst on further heating in open - vessels or in a stream of air this anhydride _sublimes_ in pearly - scales--this enables it to be obtained in a tolerably pure state. - Water dissolves it in small quantities--namely, 1 part requires 600 - parts of water for its solution. The hydrates of molybdic anhydride - are _soluble also in acids_ (a hydrate, H_{2}MoO_{4}, is obtained - from the nitric acid solution of the ammonium salt), which forms - one of their distinctions from the tungstic acids. But after - ignition molybdic anhydride is insoluble in acids, like tungstic - anhydride; alkalis dissolve this anhydride, easily forming - molybdates. Potassium bitartrate dissolves the anhydride with the - aid of heat. None of the acids yet considered by us form so many - different salts with one and the same base (alkali) as molybdic and - tungstic acids. The composition of these salts, and their - properties also, vary considerably. The most important discovery in - this respect was made by Marguerite and Laurent, who showed that - the salts which contain a large proportion of tungstic acid are - easily soluble in water, and ascribed this property to the fact - that tungstic acid may be obtained _in several states_. The common - tungstates, obtained with an excess of alkali, have an alkaline - reaction, and on the addition of sulphuric or hydrochloric acid - first deposit an acid salt and then a hydrate of tungstic acid, - which is insoluble both in water and acids; but if instead of - sulphuric or hydrochloric acids, we add acetic or phosphoric acid, - or if the tungstate be saturated with a fresh quantity of tungstic - acid, which may be done by boiling the solution of the alkali salt - with the precipitated tungstic acid, a solution is obtained which, - on the addition of sulphuric or a similar acid, does not give a - precipitate of tungstic acid at the ordinary or at higher - temperatures. The solution then contains peculiar salts of tungstic - acid, and if there be an excess of acid it also contains tungstic - acid itself; Laurent, Riche, and others called it _metatungstic - acid_, and it is still known by this name. Those salts which with - acids immediately give the insoluble tungstic acid have the - composition R_{2}WO_{4},RHWO_{4}, whilst those which give the - soluble metatungstic acid contain a far greater proportion of the - acid elements. Scheibler obtained the (soluble) metatungstic acid - itself by treating the soluble barium (meta) tetratungstate, - BaO,4WO_{3}, with sulphuric acid. Subsequent research showed the - existence of a similar phenomenon for molybdic acid. There is no - doubt that this is a case of colloidal modifications. - - Many chemists have worked on the various salts formed by molybdic - and tungstic acids. The tungstates have been investigated by - Marguerite, Laurent, Marignac, Riche, Scheibler, Anthon, and - others. The molybdates were partially studied by the same chemists, - but chiefly by Struvé and Svanberg, Delafontaine, and others. It - appears that for a given amount of base the salts contain one to - eight equivalents of molybdic or tungstic anhydride; _i.e._ if the - base have the composition RO, then the highest proportion of base - will be contained by the salts of the composition ROWO_{3} or - ROMoO_{3}--that is, by those salts which correspond with the normal - acids H_{2}WO_{4} and H_{2}MoO_{4}, of the same nature as sulphuric - acid; but there also exist salts of the composition RO,2WO_{3}, - RO,3WO_{3} ... RO,8WO_{3}. The water contained in the composition - of many of the acid salts is often not taken into account in the - above. The properties of the salts holding different proportions of - acids vary considerably, but one salt may be converted into another - by the addition of acid or base with great facility, and the - greater the proportion of the elements of the acid in a salt, the - more stable, within a certain limit, is its solution and the salt - itself. - - The most common ammonium molybdate has the composition - (NH_{4}HO)_{6},H_{2}O,7MoO_{3} (or, according to Marignac and - others, NH_{4}HMoO_{4}), and is prepared by evaporating an - ammoniacal solution of molybdic acid. It is used in the laboratory - for precipitating phosphoric acid, and is purified for this purpose - by mixing its solution with a small quantity of magnesium nitrate, - in order to precipitate any phosphoric acid present, filtering, and - then adding nitric acid and evaporating to dryness. A pure ammonium - molybdate free from phosphoric acid may then be extracted from the - residue. - - Phosphoric acid forms insoluble compounds with the oxides of - uranium and iron, tin, bismuth, &c., having feeble basic and even - acid properties. This perhaps depends on the fact that the atoms of - hydrogen in phosphoric acid are of a very different character, as - we saw above. Those atoms of hydrogen which are replaced with - difficulty by ammonium, sodium, &c., are probably easily replaced - by feebly energetic acid groups--that is, the formation of - particular complex substances may be expected to take place at the - expense of these atoms of the hydrogen of phosphoric acid and of - certain feeble metallic acids; and these substances will still be - acids, because the hydrogen of the phosphoric acids and metallic - acids, which is easily replaced by metals, is not removed by their - mutual combination, but remains in the resultant compound. Such a - conclusion is verified in the _phosphomolybdic acids_ obtained - (1888) by Debray. If a solution of ammonium molybdate be acidified, - and a small amount of a solution (it may be acid) containing - orthophosphoric acid or its salts be added to it (so that there are - at least 40 parts of molybdic acid present to 1 part of phosphoric - acid), then after a period of twenty-four hours the whole of the - phosphoric acid is separated as a yellow precipitate, containing, - however, not more than 3 to 4 p.c. of phosphoric anhydride, about 3 - p.c. of ammonia, about 90 p.c. of molybdic anhydride, and about 4 - p.c. of water. The formation of this precipitate is so distinct and - so complete that this method is employed for the discovery and - separation of the smallest quantities of phosphoric acid. - Phosphoric acid was found to be present in the majority of rocks by - this means. The precipitate is soluble in ammonia and its salts, in - alkalis and phosphates, but is perfectly insoluble in nitric, - sulphuric, and hydrochloric acids in the presence of ammonium - molybdate. The composition of the precipitate appears to vary under - the conditions of its precipitation, but its nature became clear - when the acid corresponding with it was obtained. If the - above-described yellow precipitate be boiled in aqua regia, the - ammonia is destroyed, and an acid is obtained in solution, which, - when evaporated in the air, crystallises out in yellow oblique - prisms of approximately the composition - P_{2}O_{5},20MoO_{3},26H_{2}O. Such an unusual proportion of - component parts is explained by the above-mentioned considerations. - We saw above that molybdic acid easily gives salts - R_{2}O_n_MoO_{3}_m_H_{2}O, which we may imagine to correspond to a - hydrate MoO_{2}(HO)_{2}_n_Mo_{3}_m_H_{2}O. And suppose that such a - hydrate reacts on orthophosphoric acid, forming water and compounds - of the composition MoO_{2}(HPO_{4})_n_MoO_{3}_m_H_{2}O or - MoO_{3}(H_{2}PO_{4})_{2}_n_MoO_{3}_m_H_{2}O; this is actually the - composition of phosphomolybdic acid. Probably it contains a portion - of the hydrogen replaceable by metals of both the acids H_{3}PO_{4} - and of H_{2}MoO_{4}. The crystalline acid above is probably - H_{3}MoPO_{7},9MoO_{3},12H_{2}O. This acid is really tribasic, - because its aqueous solution precipitates salts of potassium, - ammonium, rubidium (but not lithium and sodium) _from acid - solutions_, and gives a _yellow_ precipitate of the composition - R_{3}MoPO_{7},9MoO_{3},8H_{2}O, where R = NH_{4}. Besides these, - salts of another composition may be obtained, as would be expected - from the preceding. These salts are only stable in acid solutions - (which is naturally due to their containing an excess of acid - oxides), whilst under the action of alkalis they give _colourless_ - phosphomolybdates of the composition R_{3}MoPO_{3},MoO_{2},3H_{2}O. - The corresponding salts of potassium, silver, ammonium, are easily - soluble in water and crystalline. - - Phosphomolybdic acid is an example of the _complex inorganic acids_ - first obtained by Marignac and afterwards generalised and studied - in detail by Gibbs. We shall afterwards meet with several examples - of such acids, and we will now turn attention to the fact that they - are usually formed by weak polybasic acids (boric, silicic, - molybdic, &c.), and in certain respects resemble the cobaltic and - such similar complex compounds, with which we shall become - acquainted in the following chapter. As an example we will here - mention certain complex compounds containing molybdic and tungstic - acids, as they will illustrate the possibility of a considerable - complexity in the composition of salts. The action of ammonium - molybdate upon a dilute solution of purpureocobaltic salts (_see_ - Chapter XXII.) acidulated with acetic acid gives a salt which after - drying at 100° has the composition - Co_{2}O_{3}10NH_{3}7MoO_{3}3H_{2}O. After ignition this salt leaves - a residue having the composition 2CoO_{7}MoO_{3}. An analogous - compound is also obtained for tungstic acid, having the composition - Co_{2}O_{3}10NH_{3}10WO_{3}9H_{2}O. In this case after ignition - there remains a salt of the composition CoO_{5}WO_{3} (Carnot, - 1889). Professor Kournakoff, by treating a solution of potassium - and sodium molybdates, containing a certain amount of suboxide of - cobalt, with bromine obtained salts having the composition: - 3K_{2}OCo_{2}O_{3}12MoO_{3}20H_{2}O (light green) and - 3K_{2}OCo_{2}O_{3}10Mo_{3}10H_{2}O (dark green). Péchard (1893) - obtained salts of the four complex phosphotungstic acids by - evaporating equivalent mixtures of solutions of phosphoric acid and - metatungstic acid (_see_ further on): phosphotrimetatungstic acid - P_{2}O_{5}12WO_{3}48H_{2}O, phosphotetrametatungstic acid - P_{2}O_{5}16WO_{3}69H_{2}O, phosphopentametatungstic acid - P_{2}O_{5}20WO_{3}H_{2}O, and phosphohexametatungstic acid - P_{2}O_{5}24WO_{3}59H_{2}O. Kehrmann and Frankel described still - more complex salts, such as: - 3Ag_{2}O_{4}BaOP_{2}O_{5}22WO_{3}H_{2}O,5BaO_{2} - K_{2}OP_{2}O_{3}22WO_{3}48H_{2}O. - Analogous double salts with 22WO_{3} were also obtained with KSr, - KHg, BaHg, and NH_{4}Pb. Kehrmann (1892) considers the possibility - of obtaining an unlimited number of such salts to be a general - characteristic of such compounds. Mahom and Friedheim (1892) - obtained compounds of similar complexity for molybdic and arsenic - acids. - - For tungstic acid there are known: (1) Normal salts--for example, - K_{2}WO_{4}; (2) the so-called acid salts have a composition like - 3K_{2}O,7WO_{3},6H_{2}O or K_{6}H_{8}(WO_{4})_{7},2H_{2}O; (3) the - tritungstates like Na_{3}O,3WO_{3},3H_{2}O = - Na_{2}H_{4}(WO_{4})_{3},H_{2}O. All these three classes of salts - are soluble in water, but are precipitated by barium chloride, and - with acids in solution give an insoluble hydrate of tungstic acid; - whilst those salts which are enumerated below do not give a - precipitate either with acids or with the salts of the heavy - metals, because they form soluble salts even with barium and lead. - They are generally called metatungstates. They all contain water - and a larger proportion of acid elements than the preceding salts; - (4) the tetratungstates, like Na_{2}O,4WO_{3},10H_{2}O and - BaO,4WO_{3},9H_{2}O for example; (5) the octatungstates--for - example, Na_{2}O,8WO_{3},24H_{2}O. Since the metatungstates lose so - much water at 100° that they leave salts whose composition - corresponds with an acid, 3H_{2}O,4WO_{3}--that is, - H_{6}W_{4}O_{15}--whilst in the meta salts only 2 hydrogens are - replaced by metals, it is assumed, although without much ground, - that these salts contain a particular soluble metatungstic acid of - the composition H_{6}W_{4}O_{15}. - - As an example we will give a short description of the sodium salts. - The normal salt, Na_{2}WO_{4}, is obtained by heating a strong - solution of sodium carbonate with tungstic acid to a temperature of - 80°; if the solution be filtered hot, it crystallises in rhombic - tabular crystals, having the composition Na_{2}WO_{4},2H_{2}O, - which remain unchanged in the air and are easily soluble in water. - When this salt is fused with a fresh quantity of tungstic acid, it - gives a ditungstate, which is soluble in water and separates from - its solution in crystals containing water. The same salt is - obtained by carefully adding hydrochloric acid to the solution of - the normal salt so long as a precipitate does not appear, and the - liquid still has an alkaline reaction. This salt was first supposed - to have the composition Na_{2}W_{2}O_{7},4H_{2}O, but it has since - been found to contain (at 100°) Na_{6}W_{7}O_{24},16H_{2}O--that - is, it corresponds with the similar salt of molybdic acid. - - (If this salt be heated to a red heat in a stream of hydrogen, it - loses a portion of its oxygen, acquires a metallic lustre, and - turns a golden yellow colour, and, after being treated with water, - alkali, and acid, leaves golden yellow leaflets and cubes which are - very like gold. This very remarkable substance, discovered by - Wöhler, has, according to Malaguti's analysis, the composition - Na_{2}W_{3}O_{9}; that is, it, as it were, contains a double - tungstate of tungsten oxide, WO_{2}, and of sodium, - Na_{2}WO_{4},WO_{2}WO_{3}. The decomposition of the fused sodium - salt is best effected by finely-divided tin. This substance has a - sp. gr. 6·6; it conducts electricity like metals, and like them has - a metallic lustre. When brought into contact with zinc and - sulphuric acid it disengages hydrogen, and it becomes covered with - a coating of copper in a solution of copper sulphate in the - presence of zinc--that is, notwithstanding its complex composition - it presents to a certain extent the appearance and reactions of the - metals. It is not acted on by aqua regia or alkaline solutions, but - it is oxidised when ignited in air.) - - The ditungstate mentioned above, deprived of water (having - undergone a modification similar to that of metaphosphoric acid), - after being treated with water, leaves an anhydrous, sparingly - soluble tetratungstate, Na_{2}WO_{4},3WO_{3}, which, when heated at - 120° in a closed tube with water, passes into an easily soluble - metatungstate. It may therefore be said that the metatungstates are - hydrated compounds. On boiling a solution of the above-mentioned - salts of sodium with the yellow hydrate of tungstic acid they give - a solution of metatungstate, which is the hydrated tetratungstate. - Its crystals contain Na_{2}W_{4}O_{13},10H_{2}O. After the hydrate - of tungstic acid (obtained from the ordinary tungstates by - precipitation with an acid) has stood a long time in contact with a - solution (hot or cold) of sodium tungstate, it gives a solution - which is not precipitated by hydrochloric acid; this must be - filtered and evaporated over sulphuric acid in a desiccator (it is - decomposed by boiling). It first forms a very dense solution - (aluminium floats in it) of sp. gr. 3·0, and octahedral crystals of - _sodium metatungstate_, Na_{2}W_{4}O_{13},10H_{2}O, sp. gr. 3·85, - then separate. It effloresces and loses water, and at 100° only two - out of the ten equivalents of water remain, but the properties of - the salt remain unaltered. If the salt be deprived of water by - further heating, it becomes insoluble. At the ordinary temperature - one part of water dissolves ten parts of the metatungstate. The - other metatungstates are easily obtained from this salt. Thus a - strong and hot solution, mixed with a like solution of barium - chloride, gives on cooling crystals of barium metatungstate, - BaW_{4}O_{13},9H_{2}O. These crystals are dissolved without change - in water containing hydrochloric acid, and also in hot water, but - they are partially decomposed by cold water, with the formation of - a solution of metatungstic acid and of the normal barium salt - BaWO_{4}. - - In order to explain the difference in the properties of the salts - of tungstic acid, we may add that a mixture of a solution of - tungstic acid with a solution of silicic acid does not coagulate - when heated, although the silicic acid alone would do so; this is - due to the formation of a silicotungstic acid, discovered by - Marignac, which presents a fresh example of a complex acid. A - solution of a tungstate dissolves gelatinous silica, just as it - does gelatinous tungstic acid, and when evaporated deposits a - crystalline salt of silicotungstic acid. This solution is not - precipitated either by acids (a clear analogy to the - metatungstates) or by sulphuretted hydrogen, and corresponds with a - series of salts. These salts contain one equivalent of silica and 8 - equivalents of hydrogen or metals, in the same form as in salts, to - 12 or 10 equivalents of tungstic anhydride; for example the - crystalline potassium salt has the composition - K_{8}W_{12}SiO_{42},14H_{2}O = 4K_{2}O,12WO_{3},SiO_{2},14H_{2}O. - Acid salts are also known in which half of the metal is replaced by - hydrogen. The complexity of the composition of such complex acids - (for example, of the phosphomolybdic acid) involuntarily leads to - the idea of polymerisation, which we were obliged to recognise for - silica, lead oxide, and other compounds. This polymerisation, it - seems to me, may be understood thus: a hydrate A (for example, - tungstic acid) is capable of combining with a hydrate B (for - example, silica or phosphoric acid, with or without the - disengagement of water), and by reason of this faculty it is - capable of polymerisation--that is, A combines with A--combines - with itself--just as aldehyde, C_{2}H_{4}O, or the cyanogen - compounds are able to combine with hydrogen, oxygen, &c., and are - liable to polymerisation. On this view the molecule of tungstic - acid is probably much more complex than we represent it; this - agrees with the easy volatility of such compounds as the - chloranhydrides, CrO_{2}Cl_{2}, MoO_{2}Cl_{2}, the analogues of the - volatile sulphuryl chloride, SO_{2}Cl_{2}, and with the - non-volatility, or difficult volatility, of chromic and molybdic - anhydrides, the analogues of the volatile sulphuric anhydride. Such - a view also finds a certain confirmation in the researches made by - Graham on the _colloidal_ state of tungstic acid, because colloidal - properties only appertain to compounds of a very complex - composition. The observations made by Graham on the colloidal state - of tungstic and molybdic acids introduced much new matter into the - history of these substances. When sodium tungstate, mixed in a - dilute solution with an equivalent quantity of dilute hydrochloric - acid, is placed in a dialyser, hydrochloric acid and sodium - chloride pass through the membrane, and a solution of tungstic acid - remains in the dialyser. Out of 100 parts of tungstic acid about 80 - parts remain in the dialyser. The solution has a bitter, astringent - taste, and does not yield gelatinous tungstic acid (hydrogel) - either when heated or on the addition of acids or salts. It may - also be evaporated to dryness; it then forms a vitreous mass of the - _hydrosol_ of _tungstic acid_, which adheres strongly to the walls - of the vessel in which it has been evaporated, and is perfectly - soluble in water. It does not even lose its solubility after having - been heated to 200°, and only becomes insoluble when heated to a - red heat, when it loses about 2-1/2 p.c. of water. The dry acid, - dissolved in a small quantity of water, forms a gluey mass, just - like gum arabic, which is one of the representatives of the - hydrosols of colloidal substances. The solution, containing 5 p.c - of the anhydride, has a sp. gr. of 1·047; with 20 p.c., of 1·217; - with 50 p.c., of 1·80; and with 80 p.c., of 3·24. The presence of a - polymerised trioxide in the form of hydrate, H_{2}OW_{3}O_{9} or - H_{2}O_{4}WO_{3}, must then be recognised in the solution: this is - confirmed by Sabaneeff's cryoscopic determinations (1889). A - similar stable solution of molybdic acid is obtained by the - dialysis of a mixture of a strong solution of sodium molybdate with - hydrochloric acid (the precipitate which is formed is - re-dissolved). If MoCl_{4} be precipitated by ammonia and washed - with water, a point is reached at which perfect solution takes - place, and the molybdic acid forms a colloid solution which is - precipitated by the addition of ammonia (Muthmann). The addition of - alkali to the solutions of the hydrosols of tungstic and molybdic - acids immediately results in the re-formation of the ordinary - tungstates and molybdates. There appears to be no doubt but that - the same transformation is accomplished in the passage of the - ordinary tungstates into the metatungstates as takes place in the - passage of tungstic acid itself from an insoluble into a soluble - state; but this may be even actually proved to be the case, because - Scheibler obtained a solution of tungstic acid, before Graham, by - decomposing barium metatungstate (BaO_{4}WO_{3},9H_{2}O) with - sulphuric acid. By treating this salt with sulphuric acid in the - amount required for the precipitation of the baryta, Scheibler - obtained a solution of metatungstic acid which, when containing - 43·75 p.c. of acid, had a sp. gr. of 1·634, and with 27·61 p.c. a - sp. gr. of 1·327--that is, specific gravities corresponding with - those found by Graham. - - Péchard found that as much heat is evolved by neutralising - metatungstic acid as with sulphuric acid. - - Questions connected with the metamorphoses or modifications of - tungstic and molybdic acids, and the polymerisation and colloidal - state of substances, as well as the formation of complex acids, - belong to that class of problems the solution of which will do much - towards attaining a true comprehension of the mechanism of a number - of chemical reactions. I think, moreover, that questions of this - kind stand in intimate connection with the theory of the formation - of solutions and alloys and other so-called indefinite compounds. - -Hydrogen (which does not directly form compounds with Cr, Mo, and W) -reduces molybdic and tungstic anhydride at a red heat; and this forms the -means of obtaining metallic molybdenum and tungsten. _Both metals_ are -infusible, and both under the action of heat form compounds with carbon -and iron (the addition of tungsten to steel renders the latter ductile -and hard).[9] Molybdenum forms a grey powder, which scarcely aggregates -under a most powerful heat, and has a specific gravity of 8·5. It is not -acted on by the air at the ordinary temperature, but when ignited it is -first converted into a brown, and then into a blue oxide, and lastly into -molybdic anhydride. Acids do not act on it--that is, it does not liberate -hydrogen from them, not even from hydrochloric acid--but strong sulphuric -acid disengages sulphurous anhydride, forming a brown mass, containing a -lower oxide of molybdenum. Alkalis in solution do not act on molybdenum, -but when fused with it hydrogen is given off, which shows, as does its -whole character, the acid properties of the metal. The properties of -tungsten are almost identical; it is infusible, has an iron-grey colour, -is exceedingly hard, so that it even scratches glass. Its specific -gravity is 19·1 (according to Roscoe), so that, like uranium, platinum, -&c., it is one of the heaviest metals.[9 bis] Just as sulphur and -chromium have their corresponding persulphuric and perchromic acids, -H_{2}S_{2}O_{8} and H_{2}CrO_{8}, having the properties of peroxides, and -corresponding to peroxide of hydrogen, so also molybdenum and tungsten -are known to give _permolybdic_ and _pertungstic_ acids, H_{2}Mo_{2}O_{8} -and H_{2}W_{2}O_{8}, which have the properties of true peroxides, _i.e._ -easily disengage iodine from KI and chlorine from HCl, easily part with -their oxygen, and are formed by the action of peroxide of hydrogen, into -which they are readily reconverted (hence they may be regarded as -compounds of H_{2}O_{2} with 2MoO_{3} and 2WO_{3}), &c. Their formation -(Boerwald 1884, Kemmerer 1891) is at once seen in the coloration (not -destroyed by boiling), which is obtained on mixing a solution of the -salts with peroxide of hydrogen, and on treating, for instance, molybdic -acid with a solution of peroxide of hydrogen (Péchard 1892). The acid -then forms an orange-coloured solution, which after evaporation in vacuo -leaves Mo_{2}H_{2}O_{8}4H_{2}O as a crystalline powder, and loses 4H_{2}O -at 100°, beyond which it decomposes with the evolution of oxygen.[9 tri] - - [9] Moissan (1893) studied the compounds of Mo and W formed with carbon - in the electrical furnace (they are extremely hard) from a mixture - of the anhydrides and carbon. Poleck and Grützner obtained definite - compounds FeW_{2} and FeW_{2}C_{3} for tungsten. Metallic W and Mo - displace Ag from its solutions but not Pb. There is reason for - believing that the sp. gr. of pure molybdenum is higher than that - (8·5) generally ascribed to it. - - [9 bis] We may conclude our description of tungsten and molybdenum by - stating that their sulphur compounds have an acid character, like - carbon bisulphide or stannic sulphide. If sulphuretted hydrogen be - passed through a solution of a molybdate it does not give a - precipitate unless sulphuric acid be present, when a dark brown - precipitate of _molybdenum trisulphide_, MoS_{3}, is formed. When - this sulphide is ignited without access of air it gives the - bisulphide MoS_{2}; the latter is not able to combine with - potassium sulphide like the trisulphide MoS_{3}, which forms a - salt, K_{2}MoS_{4}, corresponding with K_{2}MoO_{4}. This is - soluble in water, and separates out from its solution in red - crystals, which have a metallic lustre and reflect a green light. - It is easily obtained by heating the native bisulphide, MoS_{2}, - with potash, sulphur, and a small amount of charcoal, which serves - for deoxidising the oxygen compounds. Tungsten gives similar - compounds, R_{2}WS_{4}, where R = NH_{4}, K, Na. They are - decomposed by acids, with the separation of tungsten trisulphide, - WS_{3}, and molybdenum trisulphide, MoS_{3}. Rideal (1892) obtained - W_{2}N_{3} by heating WO_{3} in NH_{3}. This compound exhibited the - general properties of metallic nitrides. - - [9 tri] When peroxide of hydrogen acts upon a solution of potassium - molybdate well-formed yellow crystals belonging to the triclinic - system separate out in the cold. When these crystals are heated in - vacuo they first lose water and then decompose, leaving a residue - composed of the salt originally taken. They are soluble in water - but insoluble in alcohol. Their composition is represented by the - formula K_{2}Mo_{2}O_{8}2H_{2}O. An ammonium salt is obtained by - evaporating peroxide of hydrogen with ammonium molybdate. The - following salts have also been obtained by the action of peroxide - of hydrogen upon the corresponding molybdates: - Na_{2}Mo_{2}O_{6}6H_{2}O--in yellow prismatic crystals; - MgMo_{2}O_{8}10H_{2}O--stellar needles; BaMoO_{8}2H_{2}O--in - microscopic yellow octahedra. A corresponding sodium pertungstate - has been obtained by Péchard by boiling sodium tungstate with a - solution of peroxide of hydrogen for several minutes. The solution - rapidly turns yellow, and no longer gives a precipitate of tungstic - anhydride when treated with nitric acid. When evaporated in vacuo - the solution leaves a thick syrupy liquid from which ray-like - crystals separate out; these crystals are more soluble in water - than the salt originally taken. When heated they also lose water - and oxygen. Their composition answers to the formula - M_{2}W_{2}O_{8}2H_{2}O, where M = Na, NH_{4}, &c. The permolybdates - and pertungstates have similar properties. When treated with oxygen - acids they give peroxide of hydrogen, and disengage chlorine and - iodine from hydrochloric acid and potassium iodide. - - Piccini (1891) showed that peroxide of hydrogen not only combines - with the oxygen compounds of Mo and W, but also with their - fluo-compounds, among which ammonium fluo-molybdate - MoO_{2}F_{2}2NH_{4} and others have long been known. (A few new - salts of similar composition have been obtained by F. Moureu in - 1893.) The action of peroxide of hydrogen upon these compounds - gives salts containing a larger amount of oxygen; for instance, a - solution of MoO_{2}F_{2}2KFH_{2}O with peroxide of hydrogen gives a - yellow solution which after cooling separates out yellow - crystalline flakes of MoO_{3}F_{2}2KFH_{2}O, resembling the salt - originally taken in their external appearance. By employing a - similar method Piccini also obtained: - MoO_{3}F_{2}2RbFH_{2}O--yellow monoclinic crystals; - MoO_{3}F_{2},2CsFH_{2}O,--yellow flakes, and the corresponding - tungstic compounds. All these salts react like peroxide of - hydrogen. - - In speaking of these compounds I for my part think it may be well - to call attention to the fact that, in the first place, the - composition of Piccini's oxy-fluo compounds does not correspond to - that of permolybdic and pertungstic acid. If the latter be - expressed by formulæ with one equivalent of an element, they will - be HMoO_{4} and HWO_{4}, and the oxy-fluo form corresponding to - them should have the composition MoO_{3}F and WO_{3}F while it - contains MO_{3}F_{2} and WO_{3}F_{2}, _i.e._ answers as it were to - a higher degree of oxidation, MoH_{2}O_{3} and W_{3}HO_{3}. But if - permolybdic acid be regarded as 2MoO_{3} + H_{2}O_{2}, _i.e._ as - containing the elements of peroxide of hydrogen, then Piccini's - compound will also be found to contain the original salts + H_{2}O; - for example, from MoO_{2}F_{2}2KFH_{2}O there is obtained a - compound MoO_{2}F_{2}2KFH_{2}O_{2}, _i.e._ instead of H_{2}O they - contain H_{2}O_{2}. In the second place the capacity of the salts - of molybdenum and tungsten to retain a further amount of oxygen or - H_{2}O_{2} probably bears some relation to their property of giving - complex acids and of polymerising which has been considered in Note - 8 bis. There is, however, a great chemical interest in the - accumulation of data respecting these high peroxide compounds - corresponding to molybdic and tungstic acids. With regard to the - peroxide form of uranium, _see_ Chapter XX., Note 66. - -_Uranium_, U = 240, has the highest atomic weight of all the analogues -of chromium, and indeed of all the elements yet known. Its highest -salt-forming oxide, UO_{3}, shows very feeble acid properties. Although -it gives sparingly-soluble yellow compounds with alkalis, which fully -correspond with the dichromates--for example, Na_{2}U_{2}O_{7} = -Na_{2}O,2UO_{3},[10]--yet it more frequently and easily reacts with -acids, HX, forming fluorescent yellowish-green salts of the composition -UO_{2}X_{2}, and in this respect uranic trioxide, UO_{3}, differs from -chromic anhydride, CrO_{3}, although the latter is able to give the -oxychloride, CrO_{2}Cl_{2}. In molybdenum and tungsten, however, we see a -clear transition from chromium to uranium. Thus, for example, chromyl -chloride, CrO_{2}Cl_{2}, is a brown liquid which volatilises without -change, and is completely decomposed by water; molybdenum oxychloride, -MoO_{2}Cl_{2}, is a crystalline substance of a yellow colour, which is -volatile and soluble in water (Blomstrand), like many salts. Tungsten -oxychloride, WO_{2}Cl_{2}, stands still nearer to uranyl chloride in its -properties; it forms yellow scales on which water and alkalis act, as -they do on many salts (zinc chloride, ferric chloride, aluminium -chloride, stannic chloride, &c.), and perfectly corresponds with the -difficultly volatile salt, UO_{2}Cl_{2} (obtained by Peligot by the -action of chlorine on ignited uranium dioxide, UO_{2}), which is also -yellow and gives a yellow solution with water, like all the salts -UO_{2}X_{2}. The property of uranic oxide, UO_{3}, of forming salts -UO_{2}X_{2} is shown in the fact that the hydrated oxide of uranium, -UO_{2}(HO)_{2}, which is obtained from the nitrate, carbonate, and other -salts by the loss of the elements of the acid, is easily soluble in -acids, as well as in the fact that the lower grades of oxidation of -uranium are able, when treated with nitric acid, to form an easily -crystallisable uranyl nitrate, UO_{2}(NO_{3})_{2},6H_{2}O; this is the -most commonly occurring uranium salt.[11] - - [10] Uranium trioxide, or uranic oxide, shows its feeble basic and acid - properties in a great number of its reactions. (1) Solutions of - uranic salts give yellow precipitates with alkalis, but these - precipitates do not contain the hydrate of the oxide, but - compounds of it with bases; for example, 2UO_{2}(NO_{3})_{2} + - 6KHO = 4KNO_{3} + 3H_{2}O + K_{2}U_{2}O_{7}. There are other - _urano-alkali compounds_ of the same constitution; for example, - (NH_{4})_{2}U_{2}O_{7} (known commercially as uranic oxide), - MgU_{2}O_{7}, BaU_{2}O_{7}. They are the analogues of the - dichromates. Sodium uranate is the most generally used under the - name of uranium yellow, Na_{2}U_{2}O_{7}. It is used for imparting - the characteristic yellow-green tint to glass and porcelain. - Neither heat nor water nor acids are able to extract the alkali - from sodium uranate, Na_{2}U_{2}O_{7}, and therefore it is a true - insoluble salt, of a yellow colour, and clearly indicates the acid - character (although feeble) of uranic oxide. (2) The carbonates of - the alkaline earths (for instance, barium carbonate) precipitate - uranic oxide from its salts, as they do all the salts of feeble - bases; for example, R_{2}O_{3}. (3) The _alkaline carbonates_, - when added to solutions of uranic salts, give a _precipitate, - which is soluble in_ _an excess of the reagent_, and particularly - so if the acid carbonates be taken. This is due to the fact that - (4) the uranyl salts _easily form double salts_ with the salts of - the alkali metals, including the salts of ammonium. Uranium, in - the form of these double salts, often gives salts of well-defined - crystalline form, although the simple salts are little prone to - appear in crystals. Such, for example, are the salts obtained by - dissolving potassium uranate, K_{2}U_{2}O_{7}, in acids, with the - addition of potassium salts of the same acids. Thus, with - hydrochloric acid and potassium chloride a well-formed crystalline - salt, K_{2}(UO_{2})Cl_{4},2H_{2}O, belonging to the monoclinic - system, is produced. This salt decomposes in dissolving in pure - water. Among these double salts we may mention the double - carbonate with the alkalis, R_{4}(UO_{2})(CO_{3})_{3} (equal to - 2R_{2}CO_{3} + UO_{2}CO_{3}); the acetates, - R(UO_{2})(C_{2}H_{3}O_{2})_{3}--for instance, the sodium salt, - Na(UO_{2})(C_{2}H_{3}O_{2})_{3}, and the potassium salt, - K(UO_{2})(C_{2}H_{3}O_{2})_{3},H_{2}O; the sulphates, - R_{2}(UO_{2})(SO_{4})_{3},2H_{2}O, &c. In the preceding formula R - = K, Na, NH_{4}, or R_{2} = Mg, Ba, &c. _This property of giving - comparatively stable double salts indicates feebly developed basic - properties_, because double salts are mainly formed by salts of - distinctly basic metals (these form, as it were, the basic element - of a double salt) and salts of feebly energetic bases (these form - the acid element of a double salt), just as the former also give - acid salts; the acid of the acid salts is replaced in the double - salts by the salt of the feebly energetic base, which, like water, - belongs to the class of intermediate bases. For this reason barium - does not give double salts with alkalis as magnesium does, and - this is why double salts are more easily formed by potassium than - by lithium in the series of the alkali metals. (5) The most - remarkable property, proving the feeble energy of uranic oxide as - a base, is seen in the fact that when their composition is - compared with that of other salts those of uranic oxide _always - appear as basic salts_. It is well known that a normal salt, - R_{2}X_{6}, corresponds with the oxide R_{2}O_{3}, where X = Cl, - NO_{3}, &c., or X_{2} = SO_{4}, CO_{3}, &c.; but there also exist - basic salts of the same type where X = HO or X_{2} = O. We saw - salts of all kinds among the salts of aluminium, chromium, and - others. With uranic oxide no salts are known of the types UX_{6} - (UCl_{6}, U(SO_{4})_{3}, alums, &c., are not known), nor even - salts, U(HO)_{2}X_{4} or UOX_{4}, but it always forms salts of the - type U(HO)_{4}X_{2}, or UO_{2}X_{2}. Judging from the fact that - nearly all the salts of uranic oxide retain water in crystallising - from their solutions, and that this water is difficult to separate - from them, it may be thought to be water of hydration. This is - seen in part from the fact that the composition of many of the - salts of uranic oxide may then be expressed without the presence - of water of crystallisation; for instance, U(HO)_{4}K_{2}Cl_{4} - (and the salt of NH_{4}, U(HO)_{4}K_{2}(SO_{4})_{2}, - U(HO)_{4}(C_{2}H_{3}O_{2})_{2}. Sodium uranyl acetate however does - not contain water. - - [11] _Uranyl nitrate_, or uranium nitrate, UO_{2}(NO_{3})_{2},6H_{2}O, - crystallises from its solutions in transparent yellowish-green - prisms (from an acid solution), or in tabular crystals (from a - neutral solution), which effloresce in the air and are easily - soluble in water, alcohol, and ether, have a sp. gr. of 2·8, and - fuse when heated, losing nitric acid and water in the process. If - the salt itself (Berzelius) or its alcoholic solution (Malaguti) - be heated up to the temperature at which oxides of nitrogen are - evolved, there then remains a mass which, after being evaporated - with water, leaves uranyl hydroxide, UO_{2}(HO)_{2} (sp. gr. - 5·93), whilst if the salt be ignited there remains the dioxide, - UO_{2}, as a brick-red powder, which on further heating loses - oxygen and forms the dark olive uranoso-uranic oxide, U_{3}O_{8}. - The solution of the nitrate obtained from the ore is purified in - the following manner: sulphurous anhydride is first passed through - it in order to reduce the arsenic acid present into arsenious - acid; the solution is then heated to 60°, and sulphuretted - hydrogen passed through it; this precipitates the lead, arsenic, - and tin, and certain other metals, as sulphides, insoluble in - water and dilute nitric acid. This liquid is then filtered and - evaporated with nitric acid to crystallisation, and the crystals - are dissolved in ether. Or else the solution is first treated with - chlorine in order to convert the ferrous chloride (produced by the - action of the hydrogen sulphide) into ferric chloride, the oxides - are then precipitated by ammonia, and the resultant precipitate, - containing the oxides Fe_{2}O_{3}, UO_{3}, and compounds of the - latter with potash, lime, ammonia, and other bases present in the - solution (the latter being due to the property of uranic oxide of - combining with bases), is washed and dissolved in a strong, - slightly-heated solution of ammonium carbonate, which dissolves - the uranic oxide but not the ferric oxide. The solution is - filtered, and on cooling deposits a well-crystallising _uranyl - ammonium carbonate_, UO_{2}(NH_{4})_{4}(CO_{3})_{3}, in brilliant - monoclinic crystals which on exposure to air slowly give off - water, carbonic anhydride, and ammonia; the same decomposition is - readily effected at 300°, the residue then consisting of uranic - oxide. This salt is not very soluble in water, but is readily so - in ammonium carbonate; it is obvious that it may readily be - converted into all the other salts of oxides of uranium. Uranium - salts are also purified in the form of _acetate_, which is very - sparingly soluble, and is therefore directly precipitated from a - strong solution of the nitrate by mixing it with acetic acid. - - We may also mention the _uranyl phosphate_, HUPO_{6}, which must - be regarded as an orthophosphate in which two hydrogens are - replaced by the radicle uranyl, UO_{2}, _i.e._ as H(UO_{2})PO_{4}. - This salt is formed as a hydrated gelatinous yellow precipitate, - on mixing a solution of uranyl nitrate with disodium phosphate. - The precipitation occurs in the presence of acetic acid, but not - in the presence of hydrochloric acid. If moreover an excess of an - ammonium salt be present, the ammonia enters into the composition - of the bright yellow gelatinous precipitate formed, in the - proportion UO_{2}NH_{4}PO_{4}. This precipitate is not soluble in - water and acetic acid, and its solution in inorganic acids when - boiled entirely expels all the phosphoric acid. This fact is taken - advantage of for removing phosphoric acids from solutions--for - instance, from those containing salts of calcium and magnesium. - -_Uranium_, which gives an oxide, UO_{3}, and the corresponding salt -UO_{2}X_{2} and dioxide UO_{2}, to which the salts UX_{4} correspond, is -rarely met with in nature. Uranite or the double orthophosphate of uranic -oxide, R(UO_{2})H_{2}P_{2}O_{8},7H_{2}O, where R = Cu or Ca, -uranium-vitriol U(SO_{4})_{2},H_{2}O, samarakite, and æschynite, are very -rarely found, and then only in small quantities. Of more frequent and -abundant occurrence is the non-crystalline, earthy brown uranium ore -known as _pitchblende_ (sp. gr. 7·2), which is mainly composed of the -intermediate oxide, U_{3}O_{8} = UO_{2},2UO_{3}. This ore is found at -Joachimsthal in Bohemia and in Cornwall. It usually contains a number of -different impurities, chiefly sulphides and arsenides of lead and iron, -as well as lime and silica compounds. In order to expel the arsenic and -sulphur it is roasted, ground, washed with dilute hydrochloric acid, -which does not dissolve the uranoso-uranic oxide, U_{3}O_{8}, and the -residue is dissolved in nitric acid, which transforms the uranium oxide -into the nitrate, UO_{2}(NO_{3})_{2}. - -It must be observed that the oxide of uranium, first distinguished by -Klaproth (1789), was for a long time regarded as able to give metallic -uranium under the action of charcoal and other reducing agents (with the -aid of heat). But the substance thus obtained was only the _uranium -dioxide_, UO_{2}. The compound nature of this dioxide,[12] or the -presence of oxygen in it, was demonstrated by Peligot (1841), by igniting -it with charcoal in a stream of chlorine. He thus obtained a volatile -_uranium tetrachloride_, UCl_{4},[13] which, when heated with sodium, -gave _metallic uranium_ as a grey metal, having a specific gravity of -18·7, and liberating hydrogen from acids, with the formation of green -uranous salts, UX_{4}, which act as powerful reducing agents.[14] - - [12] Uranium dioxide, or _uranyl_, UO_{2}, which is contained in the - salts UO_{2}X_{2}, has the appearance and many of the properties - of a metal. Uranic oxide may be regarded as uranyl oxide, - (UO_{2})O, its salts as salts of this uranyl; its hydroxide, - (UO_{2})H_{2}O_{2}, is constituted like CaH_{2}O_{2}. The green - oxide of uranium, uranoso-uranic oxide (easily formed from uranic - salts by the loss of oxygen), U_{3}O_{8} = UO_{2},2UO_{3}, when - ignited with charcoal or hydrogen (dry) gives a brilliant - crystalline substance of sp. gr. about 11·0 (Urlaub), whose - appearance resembles that of metals, and decomposes steam at a red - heat with the evolution of hydrogen; it does not, however, - decompose hydrochloric or sulphuric acid, but is oxidised by - nitric acid. The same substance (i.e. uranium dioxide UO_{2}) is - also obtained by igniting the compound (UO_{2})K_{2}Cl_{4} in a - stream of hydrogen, according to the equation UO_{2}K_{4}Cl_{4} + - H_{2} = UO_{2} + 2HCl + 2KCl. It was at first regarded as the - metal. In 1841 Peligot found that it contained oxygen, because - carbonic oxide and anhydride were evolved when it was ignited with - charcoal in a stream of chlorine, and from 272 parts of the - substance which was considered to be metal he obtained 382 parts - of a volatile product containing 142 parts of chlorine. From this - it was concluded that the substance taken contained an equivalent - amount of oxygen. As 142 parts of chlorine correspond with 32 - parts of oxygen, it followed that 272 - 32 = 240 parts of metal - were combined in the substance with 32 parts of oxygen, and also - in the chlorine compound obtained with 142 parts of chlorine. - These calculations have been made for the now accepted atomic - weight of uranium (U = 240, _see_ Note 14). Peligot took another - atomic weight, but this does not alter the principle of the - argument. - - [13] _Uranium tetrachloride_, uranous chloride, UCl_{4}, corresponds - with uranous oxide as a base. It was obtained by Peligot by - igniting uranic oxide mixed with charcoal in a stream of _dry_ - chlorine: UO_{3} + 3C + 2Cl_{2} = UCl_{4} + 3CO. This green - volatile compound (Note 12) crystallises in regular octahedra, is - very hygroscopic, easily soluble in water, with the development of - a considerable amount of heat, and no longer separates out from - its solution in an anhydrous state, but disengages hydrochloric - acid when evaporated. The solution of uranous chloride in water is - green. It is also formed by the action of zinc and copper (forming - cuprous chloride) on a solution of uranyl chloride, UO_{2}Cl_{2}, - especially in the presence of hydrochloric acid and sal-ammoniac. - Solutions of uranyl salts are converted into uranous salts by the - action of various reducing agents, and among others by organic - substances or by the action of light, whilst the salts UX_{4} are - converted into uranyl salts, UO_{2}X_{2}, by exposure to air or by - oxidising agents. Solutions of the green uranyl salts act as - powerful reducing agents, and give a brown precipitate of the - uranous hydroxide, UH_{4}O_{4}, with potash and other alkalis. - This hydroxide is easily soluble in acids but not in alkalis. On - ignition it does not form the oxide UO_{2}, because it decomposes - water, but when the higher oxides of uranium are ignited in a - stream of hydrogen or with charcoal they yield uranous oxide. Both - it and the chloride UCl_{4}, dissolve in strong sulphuric acid, - forming a green salt, U(SO_{4})_{2},2H_{2}O. The same salt, - together with uranyl sulphate, UO_{2}(SO_{4}), is formed when the - green oxide, U_{3}O_{8}, is dissolved in hot sulphuric acid. The - salts obtained in the latter instance may be separated by adding - alcohol to the solution, which is left exposed to the light; the - alcohol reduces the uranyl salt to uranous salt, an excess of acid - being required. An excess of water decomposes this salt, forming a - basic salt, which is also easily produced under other - circumstances, and contains UO(SO_{4}),2H_{2}O (which corresponds - to the uranic salt). - - [14] The atomic weight of uranium was formerly taken as half the - present one, U = 120, and the oxides U_{2}O_{3}, suboxide UO, and - green oxide U_{3}O_{4}, were of the same types as the oxides of - iron. With a certain resemblance to the elements of the iron - group, uranium presents many points of distinction which do not - permit its being grouped with them. Thus uranium forms a very - stable oxide, U_{2}O_{3}(U = 120), but does not give the - corresponding chloride U_{2}Cl_{6} (Roscoe, however, in 1874 - obtained UCl_{5}, like MoCl_{5} and WCl_{5}), and under those - circumstances (the ignition of oxide of uranium mixed with - charcoal, in a stream of chlorine), when the formation of this - compound might be expected, it gives (U = 120) the chloride - UCl_{2}, which is characterised by its volatility; this is not a - property, to such an extent, of any of the bichlorides, RCl_{2}, - of the iron group. - - The alteration or doubling of the atomic weight of uranium--_i.e._ - the recognition of U = 240--was made for the first time in the - first (Russian) edition of this work (1871), and in my memoir of - the same year in Liebig's _Annalen_, on the ground that with an - atomic weight 120, uranium could not be placed in the periodic - system. I think it will not be superfluous to add the following - remarks on this subject: (1) In the other groups (K--Rb--Cs, - Ca--Sr--Ba, Cl--Br--I) the acid character of the oxides decreases - and their basic character increases with the rise of atomic - weight, and therefore we should expect to find the same in the - group Cr--Mo--W--U, and if CrO_{3}, MoO_{3}, WO_{3} be the - anhydrides of acids then we indeed find a decrease in their acid - character, and therefore uranium trioxide, UO_{3}, should be a - very feeble anhydride, but its basic properties should also be - very feeble. Uranic oxide does indeed show these properties, as - was pointed out above (Note 10). (2) Chromium and its analogues, - besides the oxides RO_{3}, also form lower grades of oxidation - RO_{2}, R_{2}O_{3}, and the same is seen in uranium; it forms - UO_{3}, UO_{2}, U_{2}O_{3} and their compounds. (3) Molybdenum and - tungsten, in being reduced from RO_{3}, easily and frequently give - an intermediate oxide of a blue colour, and uranium shows the same - property; giving the so-called green oxide which, according to - present views, must be regarded as U_{3}O_{8} = UO_{2}2UO_{3}, - analogous to Mo_{3}O_{8}. (4) The higher chlorides, RCl_{6}, - possible for the elements of this group, are either unstable - (WCl_{6}) or do not exist at all (Cr); but there is one single - lower volatile compound, which is decomposed by water, and liable - to further reduction into a non-volatile chlorine product and the - metal. The same is observed in uranium, which forms an easily - volatile chloride, UCl_{4}, decomposed by water. (5) The high sp. - gr. of uranium (18·6) is explained by its analogy to tungsten (sp. - gr. 19·1). (6) For uranium, as for chromium and tungsten, yellow - tints predominate in the form RO_{3}, whilst the lower forms are - green and blue. (7) Zimmermann (1881) determined the vapour - densities of uranous bromide, UBr_{4}, and chloride, UCl_{4} (19·4 - and 13·2), and they were found to correspond to the formulæ given - above--that is, they confirmed the higher atomic weight U = 240. - Roscoe, a great authority on the metals of this group, was the - first to accept the proposed atomic weight of uranium, U = 240, - which since Zimmermann's work has been generally recognised. - -As the salts of uranic oxide are reduced in the absence of organic matter -by the action of light, and as they impart a characteristic coloration to -glass,[15] they find a certain application in photography and glass work. - - [15] Uranium glass, obtained by the addition of the yellow salt - K_{2}U_{2}O_{7} to glass, has a green yellow fluorescence, and is - sometimes employed for ornaments; it absorbs the violet rays, like - the other salts of uranic oxide--that is, it possesses an - absorption spectrum in which the violet rays are absent. The index - of refraction of the absorbed rays is altered, and they are given - out again as greenish-yellow rays; hence, compounds of uranic - acid, when placed in the violet portion of the spectrum, emit a - greenish-yellow light, and this forms one of the best examples - (another is found in a solution of quinine sulphate) of the - phenomenon of fluorescence. The rays of light which pass through - uranic compounds do not contain the rays which excite the - phenomena of fluorescence and of chemical transformation, as the - researches of Stokes prove. - -If we compare together the highly acid elements, sulphur, selenium, and -tellurium, of the uneven series, with chromium, molybdenum, tungsten, and -uranium of the even series, we find that the resemblance of the -properties of the higher form RO_{3} does not extend to the lower forms, -and even entirely disappears in the elements, for there is not the -smallest resemblance between sulphur and chromium and their analogues in -a free state. In other words, this means that the small periods, like Na, -Mg, Al, Si, P, S, Cl, containing seven elements, do not contain any near -analogues of chromium, molybdenum, &c., and therefore their true position -among the other elements must be looked for only in those large periods -which contain two small periods, and whose type is seen in the period -containing: K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, -Br. These large periods contain Ca and Zn, giving RO, Sc, and Ga of the -third group, Ti and Ge giving RO_{2}, V and As forming R_{2}O_{5}, Cr and -Se of the sixth group, Mn and Br of the seventh group, and the remaining -elements, Fe, Co, Ni, form connective members of the intermediate eighth -group, to the description of the representatives of which we shall turn -in the following chapters. We will now proceed to describe _manganese_, -Mn = 55, as an element of the seventh group of the even series, directly -following after Cr = 52, which corresponds with Br = 80 to the same -degree that Cr does with Se = 79. For chromium, selenium, and bromine -very close analogues are known, but for manganese as yet none have been -obtained--that is, it is the only representative of the even series in -the seventh group. In placing manganese with the halogens in one group, -the periodic system of the elements only requires that it should bear an -analogy to the halogens in the higher type of oxidation--_i.e._ in the -salts and acids--whilst it requires that as great a difference should be -expected in the lower types and elements as there exists between chromium -or molybdenum and sulphur or selenium. And this is actually the case. The -elements of the seventh group form a higher salt-forming oxide, -R_{2}O_{7}, and its corresponding hydrate, HRO_{4}, and salts--for -example, KClO_{4}. Manganese in the form of potassium permanganate, -KMnO_{4}, actually presents a great analogy in many respects to potassium -perchlorate, KClO_{4}. The analogy of the crystalline form of both salts -was shown by Mitscherlich. The salts of permanganic acid are also nearly -all soluble in water, like those of perchloric acid, and if the silver -salt of the latter, AgClO_{4}, be sparingly soluble in water, so also is -silver permanganate, AgMnO_{4}. The specific volume of potassium -perchlorate is equal to 55, because its specific gravity = 2·54; the -specific volume of potassium permanganate is equal to 58, because its -specific gravity = 2·71. So that the volumes of equivalent quantities are -in this instance approximately the same whilst the atomic volumes of -chlorine (35·5/1·3 = 27) and manganese (55/7·5) are in the ratio 4 : 1. -In a free state the higher acids HClO_{4} and HMnO_{4} are both soluble -in water and volatile, both are powerful oxidisers--in a word, their -analogy is still closer than that of chromic and sulphuric acids, and -those points of distinction which they present also appear among the -nearest analogues--for example, in sulphuric and telluric acids, in -hydrochloric and hydriodic acids, &c. Besides Mn_{2}O_{7} manganese gives -a lower grade of oxidation, MnO_{3}, analogous to sulphuric and chromic -trioxides, and with it corresponds potassium manganate, K_{2}MnO_{4}, -isomorphous with potassium sulphate.[16] In the still lower grades of -oxidation, Mn_{2}O_{3} and MnO, there is hardly any similarity to -chlorine, whilst every point of resemblance disappears when we come to -the elements themselves--_i.e._ to manganese and chlorine--for manganese -is a metal, like iron, which combines directly with chlorine to form a -saline compound, MnCl_{2}, analogous to magnesium chloride.[17] - - [16] The comparison of potassium permanganate with potassium - perchlorate, or of potassium manganate with potassium sulphate, - shows directly that many of the physical and chemical properties - of substances do not depend on the nature of the elements, but on - the atomic types in which they appear, on the kind of movements, - or on the positions in which the atoms forming the molecule occur. - - [17] If, however, we compare the spectra (Vol. I. p. 565) of chlorine, - bromine, and iodine with that of manganese, a certain resemblance - or analogy is to be found connecting manganese both to iron and to - chlorine, bromine, and iodine. - -Manganese belongs to the number of metals widely distributed in nature, -especially in those localities where iron occurs, whose ores frequently -contain compounds of manganous oxide, MnO, which presents a resemblance -to ferrous oxide, FeO, and to magnesia. In many minerals magnesia and the -oxides allied to it are replaced by manganous oxide; calcspars and -magnesites--_i.e._ R´´CO_{3} in general--are frequently met with -containing manganous carbonate, which also occurs in a separate state, -although but rarely. The soil also and the ash of plants generally -contain a small quantity of manganese. In the analysis of minerals it is -generally found that manganese occurs together with magnesia, because, -like it, manganous oxide remains in solution in the presence of -ammoniacal salts, not being precipitated by reagents. The property of -this manganous oxide, MnO, of passing into the higher grades of oxidation -under the influence of heat, alkalis, and air, gives an easy means not -only of discovering the presence of manganese in admixture with magnesia, -but also of separating these two analogous bases. Magnesia is not able to -give higher grades of oxidation, whilst manganese gives them with great -facility. Thus, for instance, an _alkaline_ solution of sodium -hypochlorite produces a precipitate of manganese dioxide in a solution of -a manganous salt: MnCl_{2} + NaClO + 2NaHO = MnO_{2} + H_{2}O + 3NaCl; -whilst magnesia is not changed under these circumstances, and remains in -the form of MgCl_{2}. If the magnesia be precipitated owing to the -presence of alkali, it may be dissolved in acetic acid, in which -manganese dioxide is insoluble. The presence of small quantities of -manganese may also be recognised by the green coloration which alkalis -acquire when heated with manganese compounds in the air. This green -coloration depends on the property of manganese of giving a green -alkaline manganate: MnCl_{2} + 4KHO + O_{2} = K_{2}MnO_{4} + 2KCl + -2H_{2}O. Thus _the faculty of oxidising in the presence of alkalis_ forms -an essential character of manganese. The higher grades of oxidation -containing Mn_{2}O_{7} and MnO_{3} are quite unknown in nature, and even -MnO_{2} is not so widely spread in nature as the ores composed of -manganous compounds which are met with nearly everywhere. The most -important ore of manganese is its dioxide, or so-called _peroxide_, -MnO_{2}, which is known in mineralogy as _pyrolusite_. Manganese also -occurs as an oxide corresponding with magnetic iron ore, MnO,Mn_{2}O_{3} -= Mn_{3}O_{4}, forming the mineral known as _hausmannite_. The oxide -Mn_{2}O_{3} also occurs in nature as the anhydrous mineral _braunite_, -and in a hydrated form, Mn_{2}O_{3},H_{2}O, called _manganite_. Both of -these often occur as an admixture in pyrolusite. Besides which, manganese -is met with in nature as a rose-coloured mineral, _rhodonite_, or -silicate, MnSiO_{3}. Very fine and rich deposits of manganese ores have -been found in the Caucasus, the Urals, and along the Dnieper. Those at -the Sharapansky district of the Government of Kutais and at Nicopol on -the Dnieper are particularly rich. A large quantity of the ore (as much -as 100,000 tons yearly) is exported from these localities. - -Thus manganese gives oxides of the following forms: MnO, manganous oxide, -and manganous salts, MnX_{2}, corresponding with the base, which -resembles magnesia and ferrous oxide in many respects; Mn_{2}O_{3}, a -very feeble base, giving salts, MnX_{3}, analogous to the aluminium and -ferric salts, easily reduced to MnX_{2}; MnO_{2}, dioxide, generally -called peroxide, an almost indifferent oxide, or feebly acid;[18] -MnO_{3}, manganic anhydride, which forms salts resembling potassium -sulphate;[18 bis] Mn_{2}O_{7}, permanganic anhydride, giving salts -analogous to the perchlorates. - - [18] The name 'peroxide' should only be retained for those _highest_ - oxides (and MnO_{2} stands between MnO and MnO_{3}) which either - by a direct method of double decomposition are able to give - hydrogen peroxide or contain a larger proportion of oxygen than - the base or the acid, just as hydrogen peroxide contains more - oxygen than water. Their type will be H_{2}O_{2}, and they are - exemplified by barium peroxide, BaO_{2}, and sulphur peroxide, - S_{2}O_{7}, &c. Such a dioxide as MnO_{2} is, in all probability, - a salt--that is, a manganous manganate, MnO_{3}MnO, and also, as a - basic salt of a feeble base, capable of combining with alkalis and - acids. Hence the name of manganese peroxide should be abandoned, - and replaced by manganese dioxide. PbO_{2} is better termed lead - dioxide than peroxide. Bisulphide of manganese, MnS_{2}, - corresponding to iron pyrites, FeS_{2}, sometimes occurs in nature - in fine octahedra (and cube combinations), for instance, in - Sicily; it is called Hauerite. - - [18 bis] On comparing the manganates with the permanganates--for - example, K_{2}MnO_{4} with KMnO_{4}--we find that they differ in - composition by the abstraction of one equivalent of the metal. - Such a relation in composition produced by oxidation is of - frequent occurrence--for instance, K_{4}Fe(CN)_{6} in oxidising - gives K_{3}Fe(CN)_{6}; H_{2}SO_{4} in oxidising gives persulphuric - acid, HSO_{4}, or H_{2}S_{7}O_{8}; H_{2}O forms HO or - H_{2}O_{2}, &c. - -_All the oxides of manganese when heated with acids give salts_, MnX_{2}, -corresponding with the lower grade of oxidation, _manganous oxide_, MnO. -Manganic oxide, Mn_{2}O_{3}, is a feebly energetic base; it is true that -it dissolves in hydrochloric acid and gives a dark solution containing -the salt MnCl_{3}, but the latter when heated evolves chlorine and gives -a salt corresponding with manganous oxide MnCl_{2}--_i.e._ at first: -Mn_{2}O_{3} + 6HCl = 3H_{2}O + Mn_{2}Cl_{6}, and then the Mn_{2}Cl_{6} -decomposes into 2MnCl_{2} + Cl_{2}. None of the remaining higher grades -of oxidation have a basic character, but _act as oxidising agents in the -presence of acids_, disengaging oxygen and passing into salts of the -lower grade of oxidation of manganese, MnO. Owing to this circumstance, -_the manganous salts_ are often obtained; they are, for instance, left in -the residue when the dioxide is used for the preparation of oxygen and -chlorine.[19] - - [19] In the preparation of oxygen from the dioxide by means of - H_{2}SO_{4}, MnSO_{4} is formed; in the preparation of chlorine - from HCl and MnO_{2}, MnCl_{2} is obtained. These two manganous - salts may be taken as examples of compounds MnX_{2}. Manganous - sulphate generally contains various impurities, and also a large - amount of iron salt (from the native MnO_{2}), from which it - cannot be freed by crystallisation. Their removal may, however, be - effected by mixing a portion of the liquid with a solution of - sodium carbonate; a precipitate of manganous carbonate is then - formed. This precipitate is collected and washed, and then added - to the remaining mass of the impure solution of manganous - sulphate; on heating the solution with this precipitate, the whole - of the iron is precipitated as oxide. This is due to the fact that - in the solution of the manganese dioxide in sulphuric acid the - whole of the iron is converted into the ferric state (because the - dioxide acts as an oxidising agent), which, as an exceedingly - feeble base precipitated by calcium carbonate and other kindred - salts, is also precipitated by manganous carbonate. After being - treated in this manner, the solution of manganous sulphate is - further purified by crystallisation. If it be a bright red colour, - it is due to the presence of higher grades of oxidation of - manganese; they may be destroyed by boiling the solution, when the - oxygen from the oxides of manganese is evolved and a very faintly - coloured solution of manganous sulphate is obtained. This salt is - remarkable for the facility with which it gives various - combinations with water. By evaporating the almost colourless - solution of _manganous sulphate_ at very low temperatures, and by - cooling the saturated solution at about 0°, crystals are obtained - containing 7 atoms of water of crystallisation, MnSO_{4},7H_{2}O, - which are isomorphous with cobaltous and ferrous sulphates. These - crystals, even at 10°, lose 5 p.c. of water, and completely - effloresce at 15°, losing about 20 p.c. of water. By evaporating a - solution of the salt at the ordinary temperature, but not above - 20°, crystals are obtained containing 5 mol. H_{2}O, which are - isomorphous with copper sulphate; whilst if the crystallisation be - carried on between 20° and 30°, large transparent prismatic - crystals are formed containing 4 mol. H_{2}O (see Nickel). A - boiling solution also deposits these crystals together with - crystals containing 3 mol. H_{2}O, whilst the first salt, when - fused and boiled with alcohol, gives crystals containing 2 mol. - H_{2}O. Graham obtained a monohydrated salt by drying the salt at - about 200°. The last atom of water is eliminated with difficulty, - as is the case with all salts like MnSO_{4}nH_{2}O. The crystals - containing a considerable amount of water are rose-coloured, and - the anhydrous crystals are colourless. The solubility of - MnSO_{4},4H_{2}O (Chapter I., Note 24) per 100 parts of water is: - at 10°, 127 parts; at 37°·5, 149 parts; at 75°, 145 parts; and at - 101°, 92 parts. Whence it is seen that at the boiling-point this - salt is less soluble than at lower temperatures, and therefore a - solution saturated at the ordinary temperature becomes turbid when - boiled. Manganous sulphate, being analogous to magnesium sulphate, - is decomposed, like the latter, when ignited, but it does not then - leave manganous oxide, but the intermediate oxide, Mn_{3}O_{4}. It - gives double salts with the alkali sulphates. With aluminium - sulphate it forms fine radiated crystals, whose composition - resembles that of the alums--namely, - MnAl_{2}(SO_{4})_{4},24H_{2}O. This salt is easily soluble in - water, and occurs in nature. - - _Manganous chloride_, MCl_{2}, crystallises with 4 mol. H_{2}O, - like the ferrous salt, and not with 6 mol. H_{2}O like many - kindred salts--for example, those of cobalt, calcium, and - magnesium; 100 parts of water dissolve 38 parts of the anhydrous - salt at 10° and 55 parts at 62°. Alcohol also dissolves manganous - chloride, and the alcoholic solution burns with a red flame. This - salt, like magnesium chloride, readily forms double salts. A - solution of borax gives a dirty rose-coloured precipitate having - the composition MnH_{4}(BO_{3})_{2}H_{2}O, which is used as a - drier in paint-making. Potassium cyanide produces a yellowish-grey - precipitate, MnC_{2}N_{2}, with manganous salts, soluble in an - excess of the reagent, a double salt, K_{4}MnC_{6}N_{6}, - corresponding with potassium ferrocyanide, being formed. On - evaporation of this solution, a portion of the manganese is - oxidised and precipitated, whilst a salt corresponding to Gmelin's - red salt, K_{3},MnC_{6}N_{6} (_see_ Chapter XXII.), remains in - solution. Sulphuretted hydrogen does not precipitate salts of - manganese, not even the acetate, but ammonium sulphide gives a - flesh-coloured precipitate, MnS; at 320° this sulphide of - manganese passes into a green variety (Antony). Oxalic acid in - strong solutions of manganous salts gives a white precipitate of - the oxalate, MnC_{2}O_{4}. This precipitate is insoluble in water, - and is used for the preparation of manganous oxide itself because - it decomposes like oxalic acid when ignited (in a tube without - access of air), with the formation of carbonic anhydride, carbonic - oxide, and manganous oxide. _Manganous oxide_ thus obtained is a - green powder, which however oxidises with such facility that it - burns in air when brought into contact with an incandescent - substance, and passes into the red intermediate oxide Mn_{3}O_{4}. - In solutions of manganous salts, alkalis produce a precipitate of - the hydroxide MnH_{2}O_{2}, which rapidly absorbs oxygen in the - presence of air and gives the brown intermediate oxide, or, more - correctly speaking, its hydrate. - - Manganous oxide, besides being obtained by the above-described - method from manganous oxalate, may also be obtained by igniting - the higher oxides in a stream of hydrogen, and also from manganese - carbonate. The manganous oxide ignited in the presence of hydrogen - acquires a great density, and is no longer so easily oxidised. It - may also be obtained in a crystalline form, if during the ignition - of the carbonate or higher oxide a trace of dry hydrochloric acid - gas be passed into the current of hydrogen. It is thus obtained in - the form of transparent emerald green crystals of the regular - system, and in this state is easily soluble in acids. - - Manganous oxide in oxidising gives the _red oxide of manganese_, - Mn_{5}O_{4}. This is the most stable of all the oxides of - manganese; it is not only stable at the ordinary but also at a - high temperature--that is, it does not absorb or disengage oxygen - spontaneously. When ignited, all the higher oxides of manganese - pass into it by losing oxygen, and manganous oxide by absorbing - oxygen. This oxide does not give any distinct salts, but it - dissolves in sulphuric acid, forming a dark red solution, which - contains both manganous and manganic (of the _oxide_, Mn_{2}O_{3}) - sulphates. The latter with potassium sulphate gives a manganese - alum, in which the alumina is replaced by its isomorphous oxide of - manganese. But this alum, like the solution of the intermediate - oxide in sulphuric acid, evolves oxygen and leaves a manganous - salt when slightly heated. - - _Manganese dioxide_ is still less basic than the oxide, and - disengages oxygen or a halogen in the presence of acids, forming - manganous salts, like the oxide. However, if it be suspended in - ether, and hydrochloric acid gas passed into the mixture, which is - kept cool, the ether acquires a green colour, owing to the - formation of tetrachloride of manganese, MnCl_{4}, corresponding - with the dioxide which passes into solution. It is however very - unstable, being exceedingly easily decomposed with the evolution - of chlorine. The corresponding fluoride, MnF_{4}, obtained by - Nicklés is much more stable. At all events, manganese dioxide does - not exhibit any well-defined basic character, but has rather an - acid character, which is particularly shown in the compounds - MnF_{4} and MnCl_{4} just mentioned, and in the property of - manganese dioxide of combining with alkalis. If the higher grades - of oxidation of manganese be deoxidised in the presence of - alkalis, they frequently give the dioxide combined with the - alkali--for example, in the presence of potash a compound is - formed which contains K_{2}O,5MnO_{2}, which shows the weak acid - character of this oxide. When ignited in the presence of sodium - compounds manganese dioxide frequently forms Na_{2}O,8MnO_{2} and - Na_{2}O,12MnO_{2}, and lime when heated with MnO_{2} gives from - CaO,3MnO_{2} to (CaO)_{2},MnO_{2} (Rousseau) according to the - temperature. Besides which, perhaps, MnO_{2} is a saline compound, - containing MnOMnO_{3} or (MnO)_{3}Mn_{2}O_{7}, and there are - reactions which support such a view (Spring, Richards, Traube, and - others); for instance it is known that manganous chloride and - potassium permanganate give the dioxide in the presence of - alkalis. - - Manganese dioxide may be obtained from manganous salts by the - action of oxidising agents. If manganous hydroxide or carbonate be - shaken up in water through which chlorine is passed, the - hypochlorite of the metal is not formed, as is the case with - certain other oxides, but manganese dioxide is precipitated: - 2MnO_{2}H_{2} + Cl_{2} = MnCl_{2} + MnO_{2},H_{2}O + H_{2}O. Owing - to this fact, hypochlorites in the presence of alkalis and acetic - acid when added to a solution of manganous salts give hydrated - manganese dioxide, as was mentioned above. Manganous nitrate also - leaves manganese dioxide when heated to 200°. It is also obtained - from manganous and manganic salts of the alkalis, when they are - decomposed in the presence of a small amount of acid; the - practical method of converting the salts MnX_{2} into the higher - grades of oxidation is given in Chapter II., Note 6. - -As the salts of manganous oxide MnX_{2} closely resemble (and are -isomorphous with) the salts of magnesia MgX_{2} in many respects (with -the exception of the fact that MnX_{2} are rose coloured and are easily -oxidised in the presence of alkalis), we will not dwell upon them, but -limit ourselves to illustrating the chemical character of manganese by -describing the metal and its corresponding acids. The fact alone that the -oxides of manganese are not reduced to the metal when ignited in hydrogen -(whilst the oxides of iron give metallic iron under these circumstances), -but only to manganous oxide, MnO, shows that manganese has a considerable -affinity for oxygen--that is, it is difficult to reduce. This may be -effected, however, by means of charcoal or sodium at a very high -temperature. A mixture of one of the oxides of manganese with charcoal or -organic matter gives fused _metallic manganese_ under the powerful heat -developed by coke with an artificial draught. The metal was obtained for -the first time in this manner by Gahn, after Pott, and more especially -Scheele, had in the last century shown the difference between the -compounds of iron and manganese (they were previously regarded as being -the same). Manganese is prepared by mixing one of its oxides in a -finely-divided state with oil and soot; the resultant mass is then first -ignited in order to decompose the organic matter, and afterwards strongly -heated in a charcoal crucible. The manganese thus obtained, however, -contains, as a rule, a considerable amount of silicon and other -impurities. Its specific gravity varies between 7·2 and 8·0. It has a -light grey colour, a feebly metallic lustre, and although it is very hard -it can be scratched by a file. It rapidly oxidises in air, being -converted into a black oxide; water acts on it with the evolution of -hydrogen--this decomposition proceeds very rapidly with boiling water, -and if the metal contain carbon.[20] - - [20] Other chemists have obtained manganese by different methods, and - attributed different properties to it. This difference probably - depends on the presence of carbon in different proportions. - Deville obtained manganese by subjecting the pure dioxide, mixed - with pure charcoal (from burnt sugar), to a strong heat in a lime - crucible until the resultant metal fused. The metal obtained had a - rose tint, like bismuth, and like it was very brittle, although - exceedingly hard. It decomposed water at the ordinary temperature. - Brunner obtained manganese having a specific gravity of about 7·2, - which decomposed water very feebly at the ordinary temperature, - did not oxidise in air, and was capable of taking a bright polish, - like steel; it had the grey colour of cast iron, was very brittle, - and hard enough to scratch steel and glass, like a diamond. - Brunner's method was as follows: He decomposed the manganese - fluoride (obtained as a soluble compound by the action of - hydrofluoric acid on manganese carbonate) with sodium, by mixing - these substances together in a crucible and covering the mixture - with a layer of salt and fluor spar; after which the crucible was - first gradually heated until the reaction began, and then strongly - heated in order to fuse the metal separated. Glatzel (1889) - obtained 25 grms. of manganese, having a grey colour and sp. gr. - 7·39, by heating a mixture of 100 grms. of MnCl_{2} with 200 grms. - KCl and 15 grms. Mg to a bright white heat. Moissan and others, by - heating the oxides of manganese with carbon in the electric - furnace, obtained carbides of manganese--for example, Mn_{3}C--and - remarked that the metal volatilised in the heat of the voltaic - arc. Metallic manganese is, however, not prepared on a large - scale, but only its alloys with carbon (they readily and rapidly - oxidise) and _ferro-manganese_ or a coarsely crystalline alloy of - iron, manganese and carbon, which is smelted in blast-furnaces - like pig-iron (_see_ Chapter XXII.) This ferro-manganese is - employed in the manufacture of steel by Bessemer's and other - processes (see Chapter XXII.) and for the manufacture of manganese - bronze. However, in America, Green and Wahl (1895) obtained almost - pure metallic manganese on a large scale. They first treat the ore - of MnO_{2} with 30 p.c. sulphuric acid (which extracts all the - oxides of iron present in the ore), and then heat it in a reducing - flame to convert it into MnO, which they mix with a powder of Al, - lime and CaF_{2} (as a flux), and heat the mixture in a crucible - lined with magnesia; a reaction immediately takes place at a - certain temperature, and a metal of specific gravity 7·3 is - obtained, which only contains a small trace of iron. - - Manganese gives two compounds with _nitrogen_, Mn_{5}N_{2} and - Mn_{3}N_{2}. They were obtained by Prelinger (1894) from the - amalgam of manganese Mn_{2}Hg_{5} (obtained on a mercury anode by - the action of an electric current upon a solution of MnCl_{2}); - the mercury may be removed from this amalgam by heating it in an - atmosphere of hydrogen, and then metallic manganese is obtained as - a grey porous mass of specific gravity 7·42. If this amalgam be - heated in dry nitrogen it gives Mn_{5}N_{2} (grey powder, sp. gr. - 6·58), but if heated in an atmosphere of NH_{3} it gives (as also - does Mn_{5}N_{2}) Mn_{3}N_{2}, (a dark mass with a metallic - lustre, sp. gr. 6·21), which, when heated in nitrogen is converted - into Mn_{5}N_{2}, and if heated in hydrogen evolves NH_{3} and - disengages hydrogen from a solution of NH_{4}Cl. At all events, - manganese is a metal which decomposes water more easily than iron, - nickel, and cobalt. - -It has been shown above that if manganese dioxide, or any lower oxide of -manganese, be heated with an alkali in the presence of air, the mixture -absorbs oxygen,[21] and forms an alkaline manganate of a green colour: -2KHO + MnO_{2} + O = K_{2}MnO_{4} + H_{2}O. Steam is disengaged during -the ignition of the mixture, and if this does not take place there is no -absorption of oxygen. The oxidation proceeds much more rapidly if, before -igniting in air, potassium chlorate or nitre be added to the mixture, and -this is the method of preparing _potassium manganate_, K_{2}MnO_{4}. The -resultant mass dissolved in a small quantity of water gives a dark green -solution, which, when evaporated under the receiver of an air-pump over -sulphuric acid, deposits green crystals of exactly the same form as -potassium sulphate--namely, six-sided prisms and pyramids. The -composition of the product is not changed by being redissolved, if -perfectly pure water free from air and carbonic acid be taken. But in the -presence of even very feeble acids the solution of this salt changes its -colour and becomes red, and deposits manganese dioxide. The same -decomposition takes place when the salt is heated with water, but when -diluted with a large quantity of unboiled water manganese dioxide does -not separate, although the solution turns red. This change of colour -depends on the fact that potassium manganate, K_{2}MnO_{4}, whose -solution is green, is transformed into potassium permanganate, KMnO_{4}, -whose solution is of a red colour. The reaction proceeding under the -influence of acids and a large quantity of water is expressed in the -following manner: 3K_{2}MnO_{4} + 2H_{2}O = 2KMnO_{4} + MnO_{2} + 4KHO. -If there is a large proportion of acid and the decomposition is aided by -heat, the manganese dioxide and potassium permanganate are also -decomposed, with formation of manganous salt. Exactly the same -decomposition as takes place under the action of acids is also -accomplished by magnesium sulphate, which reacts in many cases like an -acid. When water holding atmospheric oxygen in solution acts on a -solution of potassium manganate, the oxygen combines directly with the -manganate and forms potassium permanganate, without precipitating -manganese dioxide, 2K_{2}MnO_{4} + O + H_{2}O = 2KMnO_{4} + 2KHO. Thus a -solution of potassium manganate undergoes a very characteristic change in -colour and passes from green to red; hence this salt received the name of -_chameleon mineral_.[22] - - [21] Volume I. p. 157, Note 7. - - [22] It was known to the alchemists by this name, but the true - explanation of the change in colour is due to the researches of - Chevillot, Edwards, Mitscherlich, and Forchhammer. The change in - colour of potassium manganate is due to its instability and to its - splitting up into two other manganese compounds, a higher and a - lower: 3MnO_{3} = Mn_{2}O_{7} + MnO_{2}. Manganese trioxide is - really decomposed in this manner by the action of water (see - later): 3MnO_{3} + H_{2}O = 2MnHO_{4} + MnO_{2} (Franke, Thorpe, - and Humbly). The instability of the salt is proved by the fact of - its being deoxidised by organic matter, with the formation of - manganese dioxide and alkali, so that, for instance, a solution of - this salt cannot be filtered through paper. The presence of an - excess of alkali increases the stability of the salt; when heated - it breaks up in the presence of water, with the evolution of - oxygen. - - The method of preparing _potassium permanganate_ will be - understood from the above. There are many recipes for preparing - this substance, as it is now used in considerable quantities both - for technical and laboratory purposes. But in all cases the - essence of the methods is one and the same: a mixture of alkali - with any oxide of manganese (even manganous hydroxide, which may - be obtained from manganous chloride) is first heated in the - presence of air or of an oxidising substance (for the sake of - rapidity, with potassium chlorate); the resultant mass is then - treated with water and heated, when manganese dioxide is - precipitated and potassium permanganate remains in solution. This - solution may be boiled, as the liquid will contain free alkali; - but the solution cannot be evaporated to dryness, because a strong - solution, as well as the solid salt, is decomposed by heat. - - By adding a dilute solution of manganous sulphate to a boiling - mixture of lead dioxide and dilute nitric acid, the whole of the - manganese may be converted into permanganic acid (Crum). - -_Potassium permanganate_, KMnO_{4}, crystallises in well-formed, long -red prisms with a bright green metallic lustre. In the arts the potash is -frequently replaced by soda, and by other alkaline bases, but no salt of -permanganic acid crystallises so well as the potassium salt, and -therefore this salt is exclusively used in chemical laboratories. One -part of the crystalline salt dissolves in 15 parts of water at the -ordinary temperature. The solution is of a very deep _red colour_, which -is so intense that it is still clearly observable after being highly -diluted with water. In a solid state it is decomposed by heat, with -evolution of oxygen, a residue consisting of the lower oxides of -manganese and potassium oxide being left.[22 bis] A mixture of -permanganate of potassium, phosphorous and sulphur takes fire when struck -or rubbed, a mixture of the permanganate with carbon only takes fire when -heated, not when struck. The instability of the salt is also seen in the -fact that its solution is decomposed by peroxide of hydrogen, which at -the same time it decomposes. A number of substances reduce potassium -permanganate to manganese dioxide (in which case the red solution becomes -colourless).[23] Many organic substances (although far from all, even -when boiled in a solution of permanganate) act in this manner, being -oxidised at the expense of a portion of its oxygen. Thus, a solution of -sugar decomposes a cold solution of potassium permanganate. In the -presence of an excess of alkali, with a small quantity of sugar, the -reduction leads to the formation of potassium manganate, because -2KMnO_{4} + 2KHO = O + 2K_{2}MnO_{4} + H_{2}O. With a considerable amount -of sugar and a more prolonged action, the solution turns brown and -precipitates manganese dioxide or even oxide. In the oxidation of many -organic bodies by an alkaline solution of KMnO_{4} generally -three-eighths of the oxygen in the salt are utilised for oxidation: -2KMnO_{4} = K_{2}O + 2MnO_{2} + O_{3}. A portion of the alkali liberated -is retained by the manganese dioxide, and the other portion generally -combines with the substance oxidised, because the latter most frequently -gives an acid with an excess of alkali. A solution of potassium iodide -acts in a similar manner, being converted into potassium iodate at the -expense of the three atoms of oxygen disengaged by two molecules of -potassium permanganate. - - [22 bis] The solution of this salt with an excess of impure commercial - alkali generally acquires a green tint. - - [23] A solution of potassium permanganate gives a beautiful absorption - spectrum (Chapter XIII.) If the light in passing through this - solution loses a portion of its rays in it (if one may so account - for it), this is partially explained by the increased oxidising - power which the solution then acquires. We may here also remark - that a dilute solution of permanganate of potassium forms a - colourless solution with nickel salts, because the green colour of - the solution of nickel salts is complementary to the red. Such a - decolorised solution, containing a large proportion of nickel and - a small proportion of manganese, decomposes after a time, throws - down a precipitate, and re-acquires the green colour proper to the - nickel salts. The addition of a solution of a cobalt salt - (rose-red) to the nickel salt also destroys the colour of both - salts. - -_In the presence of acids, potassium permanganate acts as an oxidising -agent_ with still greater energy than in the presence of alkalis. At any -rate, a greater proportion of oxygen is then available for oxidation, -namely, not 3/8, as in the presence of alkalis, but 5/8, because in the -first instance manganese dioxide is formed, and in the second case -manganous oxide, or rather the salt, MnX_{2}, corresponding with it. -Thus, for instance, in the presence of an excess of sulphuric acid, the -decomposition is accomplished in the following manner: 2KMnO_{4} + -3H_{2}SO_{4} = K_{2}SO_{4} + 2MnSO_{4} + 3H_{2}O + 5O. This -decomposition, however, does not proceed directly on mixing a solution of -the salt with sulphuric acid, and crystals of the salt even dissolve in -oil of vitriol without the evolution of oxygen, and this solution only -decomposes by degrees after a certain time. This is due to the fact that -sulphuric acid liberates free permanganic acid from the permanganate,[24] -which acid is stable in solution. But if, in the presence of acids and a -permanganate, there is a substance capable of absorbing oxygen--for -instance, capable of passing into a higher grade of oxidation--then the -reduction of the permanganic acid into manganous oxides sometimes -proceeds directly at the ordinary temperature. This reduction is very -clearly seen, because the solutions of potassium permanganate are red -whilst the manganous salts are almost colourless. Thus, for instance, -nitrous acid and its salts are converted into nitric acid and decolorise -the acid solution of the permanganate. Sulphurous anhydride and its salts -immediately decolorise potassium permanganate, forming sulphuric acid. -Ferrous salts, and in general salts of lower grades of oxidation capable -of being oxidised in solution, act in exactly the same manner. -Sulphuretted hydrogen is also oxidised to sulphuric acid; even mercury is -oxidised at the expense of permanganic acid, and decolorises its -solution, being converted into mercuric oxide. Moreover, the end point of -these reactions may easily be seen, and therefore, having first -determined the amount of active oxygen in one volume of a solution of -potassium permanganate, and knowing how many volumes are required to -effect a given oxidation, it is easy to determine the amount of an -oxidisable substance in a solution from the amount of permanganate -expended (Marguerite's method). - - [24] If sulphuric acid is allowed to act on potassium permanganate - without any special precautions, a large amount of oxygen is - evolved (it may even explode and inflame), and a violet spray of - the decomposing permanganic acid is given off. But if the pure - salt (_i.e._ free from chlorine) be dissolved in pure well-cooled - sulphuric acid, without any rise in temperature, a green-coloured - liquid settles at the bottom of the vessel. This liquid does not - contain any sulphuric acid, and consists of permanganic anhydride, - Mn_{2}O_{7} (Aschoff, Terreil). It is impossible to prepare any - considerable quantity of the anhydride by this method, as it - decomposes with an explosion as it collects, evolving oxygen and - leaving red oxide of manganese. _Permanganic anhydride_, - Mn_{2}O_{7}, in dissolving in sulphuric acid, gives a green - solution, which (according to Franke, 1887) contains a compound - Mn_{2}SO_{10} = (MnO_{3})_{2}SO_{4}--that is, sulphuric acid in - which both hydrogens are replaced by the group MnO_{3}, which is - combined with OK in permanganate of potassium. This mixture with a - small quantity of water gives Mn_{2}O_{7}, according to the - equation: (MnO_{3})_{2}SO_{4} + H_{2}O = H_{2}SO_{4} + - Mn_{2}O_{7}, and when heated to 30° it gives _manganese trioxide_, - (MnO_{3})_{2}SO_{4} + H_{2}O = 2MnO_{2} + H_{2}SO_{4} + O. Pure - manganese trioxide is obtained if the solution of - (MnO_{3})_{2}SO_{4} be poured in drops on to sodium carbonate. - Then, together with carbonic anhydride, a spray of manganese - trioxide passes over, which may be collected in a well-cooled - receiver, and this shows that the reaction proceeds according to - the equation: (MnO_{3})_{2}SO_{4} + Na_{2}CO_{3} = Na_{2}SO_{4} + - 2MnO_{3} + CO_{2} + O (Thorpe). The trioxide is decomposed by - water, forming manganese dioxide and a solution of _permanganic - acid_: 3MnO_{3} + H_{2}O = MnO_{2} + 2HMnO_{4}. The same acid is - obtained by dissolving permanganic anhydride in water. - - Barium permanganate when treated with sulphuric acid gives the - same acid. This barium salt may be prepared by the action of - barium chloride on the difficultly soluble silver permanganate, - AgMnO_{4}, which is precipitated on mixing a strong solution of - the potassium salt with silver nitrate. The solution of - permanganic acid forms a bright red liquid which reflects a dark - violet tint. A dilute solution has exactly the same colour as that - of the potassium salt. It deposits manganese dioxide when exposed - to the action of light, and also when heated above 60°, and this - proceeds the more rapidly the more dilute the solution. It shows - its oxidising properties in many cases, as already mentioned. Even - hydrogen gas is absorbed by a solution of permanganic acid; and - charcoal and sulphur are also oxidised by it, as they are by - potassium permanganate. This may be taken advantage of in - analysing gunpowder, because when it is treated with a solution of - potassium permanganate, all the sulphur is converted into - sulphuric acid and all the charcoal into carbonic anhydride. - Finely-divided platinum immediately decomposes permanganic acid. - With potassium iodide it liberates iodine (which may afterwards be - oxidised into iodic acid) (Mitscherlich, Fromherz, Aschoff, and - others). Ammonia does not form a corresponding salt with free - permanganic acid, because it is oxidised with evolution of - nitrogen. The oxidising action of permanganic acid in a strong - solution may be accompanied by flame and the formation of violet - fumes of permanganic acid; thus a strong solution of it takes fire - when brought into contact with paper, alcohol, alkaline sulphides, - fats, &c. - - We may add that, according to Franke, 1 part of potassium - permanganate with 13 parts of sulphuric acid at 100° gives brown - crystals of the salt Mn_{2}(SO_{4})_{3},H_{2}SO_{4},4H_{2}O, which - gives a precipitate of hydrated manganese dioxide, H_{2}MnO_{3} = - MnO_{2}H_{2}O, when treated with water. - - Spring, by precipitating potassium permanganate with sodium - sulphite and washing the precipitate by decantation, obtained a - soluble colloidal manganese oxide, whose composition was the mean - between Mn_{2}O_{3} and MnO_{2}--namely, - Mn_{2}O_{3},4(MnO_{2}H_{2}O). - -The oxidising action of KMnO_{4}, like all other chemical reactions, is -not accomplished instantaneously, but only gradually. And, as the course -of the reaction is here easily followed by determining the amount of salt -unchanged in a sample taken at a given moment,[25] the oxidising reaction -of potassium permanganate, in an acid liquid, was employed by Harcourt -and Esson (1865) as one of the first cases for the investigation of the -laws of the _rate of chemical change_[26] as a subject of great -importance in chemical mechanics. In their experiments they took oxalic -acid, C_{2}H_{2}O_{4}, which in oxidising gives carbonic anhydride, -whilst, with an excess of sulphuric acid, the potassium permanganate is -converted into manganous sulphate, MnSO_{4}, so that the ultimate -oxidation will be expressed by the equation: 5C_{2}H_{2}O_{4} + 2MnKO_{4} -+ 3H_{2}SO_{4} = 10CO_{2} + K_{2}SO_{4} + 2MnSO_{4} + 8H_{2}O. The -influence of the relative amount of sulphuric acid is seen from the -annexed table, which gives the measure of reaction _p_ per 100 parts of -potassium permanganate, taken four minutes after mixing, using n -molecules of sulphuric acid, H_{2}SO_{4}, per 2KMnO_{4} + -5C_{2}H_{2}O_{4}: - - _n_ = 2 4 6 8 12 16 22 - _p_ = 22 36 51 63 77 86 92 - -showing that in a given time (4 minutes) the oxidation is the more -perfect the greater the amount of sulphuric acid taken for given amounts -of KMnO_{4} and C_{2}H_{2}O_{4}. It is obvious also that the temperature -and relative amount of every one of the acting and resulting substances -should show its influence on the relative velocity of reaction; thus, for -instance, direct experiment showed the influence of the admixture of -manganous sulphate. When a large proportion of oxalic acid (108 -molecules) was taken to a large mass of water and to 2 molecules of -permanganate 14 molecules of manganous sulphate were added, the quantity -x of the potassium permanganate acted on (in percentages of the potassium -permanganate taken) in t minutes (at 16°) was as follows: - - _t_ = 2 5 8 11 14 44 47 53 61 68 - _x_ = 5·2 12·1 18·7 25·1 31·3 68·4 71·7 75·8 79·8 83·0 - -These figures show that the rate of reaction--that is, the quantity of -permanganate changed in one minute--decreases proportionally to the -decrease in the amount of unchanged potassium permanganate. At the -commencement, about 2·6 per cent. of the salt taken was decomposed in the -course of one minute, whilst after an hour the rate was about 0·5 per -cent. The same phenomena are observed in every case which has been -investigated, and this branch of theoretical or physical chemistry, now -studied by many,[27] promises to explain the course of chemical -transformations from a fresh point of view, which is closely allied to -the doctrine of affinity, because the rate of reaction, without doubt, is -connected with the magnitude of the affinities acting between the -reacting substances. - - [25] For rapid and accurate determinations of this kind, advantage is - taken of those methods of chemical analysis which are known as - 'titrations' (volumetric analysis), and consist in measuring the - volume of solutions of known strength required for the complete - conversion of a given substance. Details respecting the theory and - practice of titration, in which potassium permanganate is very - frequently employed, must be looked for in works on analytical - chemistry. - - [26] The measurements of velocity and acceleration serve for - determining the measure of forces in mechanics, but in that case - the velocities are magnitudes of length or paths passed over in a - unit of time. The velocity of chemical change embodies a - conception of quite another kind. In the first place, the - velocities of reactions are magnitudes of the masses which have - entered into chemical transformations; in the second place, these - velocities can only be relative quantities. Hence the conception - of 'velocity' has quite a different meaning in chemistry from what - it has in mechanics. Their only common factor is time. If _dt_ be - the increment of time and _dx_ the quantity of a substance changed - in this space of time, then the fraction (or quotient) _dx/dt_ - will express the rate of the reaction. The natural conclusion, - come to both by Harcourt and Esson, and previously to them (1850) - by Wilhelmj (who investigated the rate of conversion, or - inversion, of sugar in its passage into glucose), consists in - establishing that this velocity is proportional to the quantity of - substances still unchanged--_i.e._ that _dx/dt_ = C(A - _x_), - where C is a constant coefficient of proportionality, and where A - is the quantity of a substance taken for reaction at the moment - when _t_ = 0 and _x_ = 0--that is, at the beginning of the - experiment, from which the time _t_ and quantity _x_ of substance - changed is counted. On integrating the preceding equation we - obtain log(A/(A - _x_)) = _kt_, where _k_ is a new constant, if we - take ordinary (and not natural) logarithms. Hence, knowing A, _x_, - and _t_, for each reaction, we find _k_, and it proves to be a - constant quantity. Thus from the figures cited in the text for the - reaction 2KMnO_{4} + 108C_{2}H_{2}O_{4} + 14MnSO_{4}, it may be - calculated that _k_ = 0·0114; for example, _t_ = 44, _x_ = 68·4 (A - = 100), whence _kt_ = 0·5004 and _k_ = 0·0114, (_see also_ Chapter - XIV., Note 3, and Chapter XVII., Note 25 bis). - - [27] The researches made by Hood, Van't Hoff, Ostwald, Warder, - Menschutkin, Konovaloff, and others have a particular significance - in this direction. Owing to the comparative novelty of this - subject, and the absence of applicable as well as indubitable - deductions, I consider it impossible to enter into this province - of theoretical chemistry, although I am quite confident that its - development should lead to very important results, especially in - respect to chemical equilibria, for Van't Hoff has already shown - that the limit of reaction in reversible reactions is determined - by the attainment of equal velocities for the opposite reactions. - - - - - CHAPTER XXII - - IRON, COBALT, AND NICKEL - - -Judging from the atomic weights, and the forms of the higher oxides of -the elements already considered, it is easy to form an idea of the seven -groups of the periodic system. Such are, for instance, the typical series -Li, Be, B, C, N, O, F, or the third series, Na, Mg, Al, Si, P, S, Cl. The -seven usual types of oxides from R_{2}O to R_{2}O_{7} correspond with -them (Chapter XV.) The position of the eighth group is quite separate, -and is determined by the fact that, as we have already seen, in each -group of metals having a greater atomic weight than potassium a -distinction ought to be made between the elements of the even and uneven -series. The series of even elements, commencing with a strikingly -alkaline element (potassium, rubidium, cæsium), together with the uneven -series following it, and concluding with a haloid (chlorine, bromine, -iodine), forms a large period, the properties of whose members repeat -themselves in other similar periods. The elements of the eighth group are -situated between the elements of the even series and the elements of the -uneven series following them. And for this reason elements of the eighth -group are found in the middle of each large period. The properties of the -elements belonging to it, in many respects independent and striking, are -shown with typical clearness in the case of iron, the well-known -representative of this group. - -_Iron_ is one of those elements which are not only widely diffused in the -crust of the earth, but also throughout the entire universe. Its oxides -and their various compounds are found in the most diverse portions of the -earth's crust; but here iron is always found combined with some other -element. Iron is not found on the earth's surface in a free state, -because it easily oxidises under the action of air. It is occasionally -found in the native state in meteorites, or aerolites, which fall upon -the earth. - -_Meteoric iron_ is formed outside the earth.[1] Meteorites are -fragments which are carried round the sun in orbits, and fall upon the -earth when coming into proximity with it during their motion in space. -The meteoric dust, on passing through the upper parts of the atmosphere, -and becoming incandescent from friction with the gases, produces that -phenomenon which is familiar under the name of falling stars.[2] Such is -the doctrine concerning meteorites, and therefore the fact of their -containing rocky (siliceous) matter and metallic iron shows that outside -the earth the elements and their aggregation are in some degree the same -as upon the earth itself. - - [1] The composition of meteoric iron is variable. It generally contains - nickel, phosphorus, carbon, &c. The schreibersite of meteoric - stones contains Fe_{4}Ni_{2}P. - - [2] Comets and the rings of Saturn ought now to be considered as - consisting of an accumulation of such meteoric cosmic particles. - Perhaps the part played by these minute bodies scattered throughout - space is much more important in the formation of the largest - celestial bodies than has hitherto been imagined. The investigation - of this branch of astronomy, due to Schiaparelli, has a bearing on - the whole of natural science. - - The question arises as to why the iron in meteorites is in a free - state, whilst on earth it is in a state of combination. Does not - this tend to show that the condition of our globe is very different - from that of the rest? My answer to this question has been already - given in Volume I. p. 377, Note 57. It is my opinion that inside - the earth there is a mass similar in composition to - meteorites--that is, containing rocky matter and metallic iron, - partly carburetted. In conclusion, I consider it will not be out of - place to add the following explanations. According to the theory of - the distribution of pressures (see my treatise, _On Barometrical - Levelling_, 1876, pages 48 _et seq._) in an atmosphere of mixed - gases, it follows that two gases, whose densities are _d_ and - _d__{1}, and whose relative quantities or partial pressures at a - certain distance from the centre of gravity are _h_ and _h__{1}, - will, when at a greater distance from the centre of attraction, - present a different ratio of their masses _x_ : _x__{1}--that is, - of their partial pressures--which may be found by the equation - _d__{1}(log(_h_) - log(_x_)) = _d_(log(_h__{1}) - log(_x__{1})). - If, for instance, _d_ : _d__{1} = 2 : 1, and _h_ = _h__{1} (that is - to say, the masses are equal at the lower height) = 1000, then when - _x_ = 10 the magnitude of _x__{1} will not be 10 (_i.e._ the mass - of a gas at a higher level whose density = 1 will not be equal to - the mass of a gas whose density = 2, as was the case at a lower - level), but much greater--namely, _x__{1} = 100--that is, the - lighter gas will predominate over a heavier one at a higher level. - Therefore, when the whole mass of the earth was in a state of - vapour, the substances having a greater vapour density accumulated - about the centre and those with a lesser vapour density at the - surface. And as the vapour densities depend on the atomic and - molecular weights, those substances which have small atomic and - molecular weights ought to have accumulated at the surface, and - those with high atomic and molecular weights, which are the least - volatile and the easiest to condense, at the centre. Thus it - becomes apparent why such light elements as hydrogen, carbon, - nitrogen, oxygen, sodium, magnesium, aluminium, silicon, - phosphorus, sulphur, chlorine, potassium, calcium, and their - compounds predominate at the surface and largely form the earth's - crust. There is also now much iron in the sun, as spectrum analysis - shows, and therefore it must have entered into the composition of - the earth and other planets, but would have accumulated at the - centre, because the density of its vapour is certainly large and it - easily condenses. There was also oxygen near the centre of the - earth, but not sufficient to combine with the iron. The former, as - a much lighter element, principally accumulated at the surface, - where we at the present time find all oxidised compounds and even a - remnant of free oxygen. This gives the possibility not only of - explaining in accordance with cosmogonic theories the predominance - of oxygen compounds on the surface of the earth, with the - occurrence of unoxidised iron in the interior of the earth and in - meteorites, but also of understanding why the density of the whole - earth (over 5) is far greater than that of the rocks (1 to 3) - composing its crust. And if all the preceding arguments and - theories (for instance the supposition that the sun, earth, and all - the planets were formed of an elementary homogeneous mass, formerly - composed of vapours and gases) be true, it must be admitted that - the interior of the earth and other planets contains metallic - (unoxidised) iron, which, however, is only found on the surface as - aerolites. And then assuming that aerolites are the fragments of - planets which have crumbled to pieces so to say during cooling - (this has been held to be the case by astronomers, judging from the - paths of aerolites), it is readily understood why they should be - composed of metallic iron, and this would explain its occurrence in - the depths of the earth, which we assumed as the basis of our - theory of the formation of naphtha (Chapter VIII., Notes 57-60). - -The most widely diffused terrestrial compound of iron is iron bisulphide, -FeS_{2}, or _iron pyrites_. It occurs in formations of both aqueous and -igneous origin, and sometimes in enormous masses. It is a substance -having a greyish-yellow colour, with a metallic lustre, and a specific -gravity of 5·0; it crystallises in the regular system.[2 bis] - - [2 bis] Immense deposits of iron pyrites are known in various parts of - Russia. On the river Msta, near Borovitsi, thousands of tons are - yearly collected from the detritus of the neighbouring rocks. In - the Governments of Toula, Riazan, and in the Donets district - continuous layers of pyrites occur among the coal seams. Very thick - beds of pyrites are also known in many parts of the Caucasus. But - the deposits of the Urals are particularly vast, and have been - worked for a long time. Amongst these I will only indicate the - deposits on the Soymensky estate near the Kishteimsky works; the - Kaletinsky deposits near the Virhny-Isetsky works (containing 1-2 - p.c. Cu); on the banks of the river Koushaivi near Koushvi (3-5 - p.c. Cu), and the deposits near the Bogoslovsky works (3-5 p.c. - Cu). Iron pyrites (especially that containing copper which is - extracted after roasting) is now chiefly employed for roasting, as - a source of SO_{2}, for the manufacture of chamber sulphuric acid - (Vol. I. p. 291), but the remaining oxide of iron is perfectly - suitable for smelting into pig iron, although it gives a sulphurous - pig iron (the sulphur may be easily removed by subsequent - treatment, especially with the aid of ferro-manganese in Bessemer's - process). The great technical importance of iron pyrites leads to - its sometimes being imported from great distances; for instance, - into England from Spain. Besides which, when heated in closed - retorts FeS_{2} gives sulphur, and if allowed to oxidise in damp - air, green vitriol, FeSO_{4}. - -The oxides are the principal ores used for producing metallic iron. The -majority of the ores contain ferric oxide, Fe_{2}O_{3}, either in a free -state or combined with water, or else in combination with ferrous oxide, -FeO. The species and varieties of iron ores are numerous and diverse. -Ferric oxide in a separate form appears sometimes as crystals of the -rhombohedric system, having a metallic lustre and greyish steel colour; -they are brittle, and form a red powder, specific gravity about 5·25. -Ferric oxide in type of oxidation and properties resembles alumina; it -is, however, although with difficulty, soluble in acids even when -anhydrous. The crystalline oxide bears the name of _specular iron ore_, -but ferric oxide most often occurs in a non-crystalline form, in masses -having a red fracture, and is then known as _red hæmatite_. In this form, -however, it is rather a rare ore, and is principally found in veins. The -hydrates of ferric oxide, ferric hydroxides,[3] are most often found in -aqueous or stratified formations, and are known as _brown hæmatites_; -they generally have a brown colour, form a yellowish-brown powder, and -have no metallic lustre but an earthy appearance. They easily dissolve in -acids and diffuse through other formations, especially clays (for -instance, ochre); they sometimes occur in reniform and similar masses, -evidently of aqueous origin. Such are, for instance, the so-called bog or -lake and peat ores found at the bottom of marshes and lakes, and also -under and in peat beds. This ore is formed from water containing ferrous -carbonate in solution, which, after absorbing oxygen, deposits ferric -hydroxide. In rivers and springs, iron is found in solution as ferrous -carbonate through the agency of carbonic acid: hence the existence of -chalybeate springs containing FeCO_{3}. This ferrous carbonate, or -_siderite_, is either found as a non-crystalline product of evidently -aqueous origin, or as a crystalline spar called _spathic iron ore_. The -reniform deposits of the former are most remarkable; they are called -spherosiderites, and sometimes form whole strata in the jurassic and -carboniferous formations. _Magnetic iron ore_, Fe_{3}O_{4} = -FeO,Fe_{2}O_{3}, in virtue of its purity and practical uses, is a very -important ore; it is a compound of the ferrous and ferric oxides, is -naturally magnetic, has a specific gravity of 5·1, crystallises in -well-formed crystals of the regular system, is with difficulty soluble in -acids, and sometimes forms enormous masses, as, for instance, Mount -Blagodat in the Ural. However, in most cases--for instance, at -Korsak-Mogila (to the north of Berdiansk and Nogaiska, near the Sea of -Azov), or at Krivoi Rog (to the west of Ekaterinoslav)--the magnetic iron -ore is mixed with other iron ores. In the Urals, the Caucasus (without -mentioning Siberia), and in the districts adjoining the basin of the Don, -Russia possesses the richest iron ores in the world. To the south of -Moscow, in the Governments of Toula and Nijninovgorod, in the Olonetz -district, and in the Government of Orloffsky (near Zinovieff in the -district of Kromsky), and in many other places, there are likewise -abundant supplies of iron ores amongst the deposited aqueous formations; -the siderite of Orloffsky, for instance, is distinguished by its great -purity.[4] - - [3] The hydrated ferric oxide is found in nature in a dual form. It is - somewhat rarely met with in the form of a crystalline mineral - called _göthite_, whose specific gravity is 4·4 and composition - Fe_{2}H_{3}O_{4}, or FeHO_{2}--that is, one of oxide of iron to one - of water, Fe_{2}O_{3},H_{2}O; frequently found as brown ironstone, - forming a dense mass of fibrous, reniform deposits containing - 2Fe_{2}O_{3},3H_{2}O--that is, having a composition - Fe_{4}H_{6}O_{9}. In bog ore and other similar ores we most often - find a mixture of this hydrated ferric oxide with clay and other - impurities. The specific gravity of such formations is rarely as - high as 4·0. - - [4] The ores of iron, similarly to all substances extracted from veins - and deposits, are worked according to mining practice by means of - vertical, horizontal, or inclined shafts which reach and penetrate - the veins and strata containing the ore deposits. The mass of ore - excavated is raised to the surface, then sorted either by hand or - else in special sorting apparatus (generally acting with water to - wash the ore), and is subjected to roasting and other treatment. In - every case the ore contains foreign matter. In the extraction of - iron, which is one of the cheapest metals, the dressing of an ore - is in most cases unprofitable, and only ores rich in metal are - worked--namely, those containing at least 20 p.c. It is often - profitable to transport very rich and pure ores (with as much as 70 - p.c. of iron) from long distances. The details concerning the - working and extraction of metals will be found in special treatises - on metallurgy and mining. - -Iron is also found in the form of various other compounds--for instance, -in certain silicates, and also in some phosphates; but these forms are -comparatively rare in nature in a pure state, and have not the industrial -importance of those natural compounds of iron previously mentioned. In -small quantities iron enters into the composition of every kind of _soil_ -and all rocky formations. As ferrous oxide, FeO, is isomorphous with -magnesia, and ferric oxide, Fe_{2}O_{3}, with alumina, isomorphous -substitution is possible here, and hence minerals are not unfrequently -found in which the quantity of iron varies considerably; such, for -instance, are pyroxene, amphibole, certain varieties of mica, &c. -Although much iron oxide is deleterious to the growth of vegetation, -still plants do not flourish without iron; it enters as an indispensable -component into the composition of all higher _organisms_; in the ash of -plants we always find more or less of its compounds. It also occurs in -blood, and forms one of the colouring matters in it; 100 parts of the -blood of the highest organisms contain about 0·05 of iron. - -The _reduction_ of the ores of iron into metallic iron is in principle -very simple, because when the oxides of iron are strongly heated with -charcoal, hydrogen, carbonic oxide, and other reducing agents,[5] they -easily give metallic iron. But the matter is rendered more difficult by -the fact that the iron does not melt at the heat developed by the -combustion of the charcoal, and therefore it does not separate from those -mechanically mixed impurities which are found in the iron ore. This is -obviated by the following very remarkable property of iron: at a high -temperature it is capable of combining with a small quantity (from 2 to 5 -p.c.) of carbon, and then forms _cast iron_, which easily _melts_ in the -heat developed by the combustion of charcoal in air. For this reason -metallic iron is not obtained directly from the ore, but is only formed -after the further treatment of the cast iron; the first product extracted -from the ore being cast iron. The fused mass disposes itself in the -furnace below the slag--that is, the impurities of the ore fused by the -heat of the furnace. If these impurities did not fuse they would block up -the furnace in which the ore was being smelted, and the continuous -smelting of the cast iron would not be possible;[6] it would be necessary -periodically to cool the furnace and heat it up again, which means a -wasteful expenditure of fuel, and hence in the production of cast iron, -the object in view is to obtain all the earthy impurities of the ore in -the shape of a fused mass or slag. Only in rare cases does the ore itself -form a mass which fuses at the temperature employed, and these cases are -objectionable if much iron oxide is carried away in the slag. The -impurities of the ores most often consist of certain mixtures--for -instance, a mixture of clay and sand, or a mixture of limestone and clay, -or quartz, &c. These impurities do not separate of themselves, or do not -fuse. The difficulty of the industry lies in forming an easily-fusible -slag, into which the whole of the foreign matter of the ore would pass -and flow down to the bottom of the furnace above the heavier cast iron. -This is effected by mixing certain _fluxes_ with the ore and charcoal. A -flux is a substance which, when mixed with the foreign matter of the ore, -forms a fusible vitreous mass or slag. The flux used for silica is -limestone with clay; for limestone a definite quantity of silica is used, -the best procedure having been arrived at by experiment and by long -practice in iron smelting and other metallurgical processes.[7] - - [5] The reduction of iron oxides by hydrogen belongs to the order of - reversible reactions (Chapter II.), and is therefore determined by - a limit which is here expressed by the attainment of the same - pressure as in the case where hydrogen acts on iron oxides, and as - in the case where (at the same temperature) water is decomposed by - metallic iron. The calculations referring to this matter were made - by Henri Sainte-Claire Deville (1870). Spongy iron was placed in a - tube having a temperature _t_, one end of which was connected with - a vessel containing water at 0° (vapour tension = 4·6 mm.) and the - other end with a mercury pump and pressure gauge which determined - the limiting tension attained by the dry hydrogen _p_ (subtracting - the tension of the water vapour from the tension observed). A tube - was then taken containing an excess of iron oxide. It was filled - with hydrogen, and the tension _p__{1} observed of the residual - hydrogen when the water was condensed at 0°. - - _t_ = 200° 440° 860° 1040° - _p_ = 95·9 25·8 12·8 9·2 mm. - _p_{1} = -- -- 12·8 9·4 mm. - - The equality of the pressure (tension) of the hydrogen in the two - cases is evident. The hydrogen here behaves like the vapour of iron - or of its oxide. - - By taking ferric oxide, Fe_{2}O_{3}, Moissan observed that at 350° - it passed into magnetic oxide, Fe_{3}O_{4}, at 500° into ferrous - oxide, FeO, and at 600° into metallic iron. Wright and Luff (1878), - whilst investigating the reduction of oxides, found that (_a_) the - temperature of reaction depends on the condition of the oxide - taken--for instance, precipitated ferric oxide is reduced by - hydrogen at 85°, that obtained by oxidising the metal or from its - nitrate at 175°; (_b_) when other conditions are the same, the - reduction by carbonic oxide commences earlier than that by - hydrogen, and the reduction by hydrogen still earlier than that by - charcoal; (_c_) the reduction is effected with greater facility - when a greater quantity of heat is evolved during the reaction. - Ferric oxide obtained by heating ferrous sulphate to a red heat - begins to be reduced by carbonic oxide at 202°, by hydrogen at - 260°, by charcoal at 430°, whilst for magnetic oxide, Fe_{3}O_{4}, - the temperatures are 200°, 290°, and 450° respectively. - - [6] The primitive methods of iron manufacture were conducted by - intermittent processes in hearths resembling smiths' fires. As - evidenced by the uninterrupted action of the steam boiler, or the - process of lime burning, and the continuous preparation and - condensation of sulphuric acid or the uninterrupted smelting of - iron, every industrial process becomes increasingly profitable and - complete under the condition of the continuous action, as far as - possible, of all agencies concerned in the production. This - continuous method of production is the first condition for the - profitable production on the large scale of nearly all industrial - products. This method lessens the cost of labour, simplifies the - supervision of the work, renders the product uniform, and - frequently introduces a very great economy in the expenditure of - fuel and at the same time presents the simplicity and perfection of - an equilibrated system. Hence every manufacturing operation should - be a continuous one, and the manufacture of pig iron and sulphuric - acid, which have long since become so, may be taken as examples in - many respects. A study of these two manufactures should form the - commencement of an acquaintance with all the contemporary methods - of manufacturing both from a technical and economical point of - view. - - [7] The composition of slag suitable for iron smelting most often - approaches the following: 50 to 60 p.c. SiO_{2}, 5 to 20 - Al_{2}O_{3}, the rest of the mass consisting of MgO, CaO, MnO, FeO. - Thus the most fusible slag (according to the observations of - Bodeman) contains the alloy Al_{2}O_{3},4CaO,7SiO_{2}. On altering - the quantity of magnesia and lime, and especially of the alkalis - (which increases the fusibility) and of silica (which decreases - it), the temperature of fusion changes with the relation between - the total quantity of oxygen and that in the silica. Slags of the - composition RO,SiO_{2} are easily fusible, have a vitreous - appearance, and are very common. Basic slags approach the - composition 2RO,SiO_{2}. Hence, knowing the composition and - quantity of the foreign matter in the ore, it is at once easy to - find the quantity and quality of the flux which must be added to - form a suitable slag. The smelting of iron is rendered more complex - by the fact that the silica, SiO_{2}, which enters into the slag - and fluxes is capable of forming a slag with the iron oxides. In - order that the least quantity of iron may pass into the slag, it is - necessary for it to be reduced before the temperature is attained - at which the slags are formed (about 1000°), which is effected by - reducing the iron, not with charcoal itself, but with carbonic - oxide. From this it will be understood how the progress of the - whole treatment may be judged by the properties of the slags. - Details of this complicated and well-studied subject will be found - in works on metallurgy. - -Thus the following materials have to be introduced into the furnace -where the smelting of the iron ore is carried on: (1) the iron ore, -composed of oxide of iron and foreign matter; (2) the flux required to -form a fusible slag with the foreign matter; (3) the carbon which is -necessary (_a_) for reducing, (_b_) for combining with the reduced iron -to form cast iron, (_c_) principally for the purpose of combustion and -the heat generated thereby, necessary not only for reducing the iron and -transforming it into cast iron, but also for melting the slag, as well as -the cast iron--and (4) the air necessary for the combustion of the -charcoal. The air is introduced after a preparatory heating in order to -economise fuel and to obtain the highest temperature. The air is forced -in under pressure by means of a special blast arrangement. This permits -of an exact regulation of the heat and rate of smelting. All these -component parts necessary for the smelting of iron must be contained in a -vertical, that is, _shaft furnace_, which at the base must have a -receptacle for the accumulation of the slag and cast iron formed, in -order that the operation may proceed without interruption. The walls of -such a furnace ought to be built of fireproof materials if it be designed -to serve for the continuous production of cast iron by charging the ore, -fuel, and flux into the mouth of the furnace, forcing a blast of air into -the lower part, and running out the molten iron and slag from below. The -whole operation is conducted in furnaces known as _blast furnaces_. The -annexed illustration, fig. 93 (which is taken by kind permission from -Thorpe's Dictionary of Applied Chemistry), represents the vertical -section of such a furnace. These furnaces are generally of large -dimensions--varying from 50 to 90 feet in height. They are sometimes -built against rising ground in order to afford easy access to the top -where the ore, flux, and charcoal or coke are charged.[8] - - [8] The section of a blast furnace is represented by two truncated - cones joined at their bases, the upper cone being longer than the - lower one; the lower cone is terminated by the hearth, or almost - cylindrical cavity in which the cast iron and slag collect, one - side being provided with apertures for drawing off the iron and - slag. The air is blown into the blast furnace through special - pipes, situated over the hearth, as shown in the section. The air - previously passes through a series of cast-iron pipes, heated by - the combustion of the carbonic oxide obtained from the upper parts - of the furnace, where it is formed as in a 'gas-producer.' The - blast furnace acts continuously until it is worn out; the iron is - tapped off twice a day, and the furnace is allowed to cool a little - from time to time so as not to be spoilt by the increasing heat, - and to enable it to withstand long usage. - - Blast furnaces worked with charcoal fuel are not so high, and in - general give a smaller yield than those using coke, because the - latter are worked with heavier charges than those in which charcoal - is employed. Coke furnaces yield 20,000 tons and over of pig iron a - year. In the United States there are blast furnaces 30 metres high, - and upwards of 600 cubic metres capacity, yielding as much as - 130,000 tons of pig iron, requiring a blast of about 750 cubic - metres of air per minute, heated to 600°, and consuming about 0·85 - part of coke per 1 part of pig iron produced. At the present time - the world produces as much as 30 million tons of pig iron a year, - about 9/10 of which is converted into wrought iron and steel. The - chief producers are the United States (about 10 million tons a - year) and England (about 9 million tons a year); Russia yields - about 1-1/5 million tons a year. The world's production has doubled - during the last 20 years, and in this respect the United States - have outrun all other countries. The reason of this increase of - production must be looked for in the increased demand for iron and - steel for railway purposes, for structures (especially - ship-building), and in the fact that: (_a_) the cost of pig iron - has fallen, thanks to the erection of large furnaces and a fuller - study of the processes taking place in them, and (_b_) that every - kind of iron ore (even sulphurous and phosphoritic) can now be - converted into a homogeneous steel. - - [Illustration: FIG. 93.--Vertical section of a modern Cleveland - blast furnace capable of producing 300 to 1,000 tons of pig iron - weekly. The outer casing is of riveted iron plates, the furnace - being lined with refractory fire-brick. It is closed at the top by - a 'cap and cone' arrangement, by means of which the charge can be - fed into the furnace at suitable intervals by lowering the moveable - cone.] - - In order to more thoroughly grasp the chemical process which takes - place in blast furnaces, it is necessary to follow the course of - the material charged in at the top and of the air passing through - the furnace. From 50 to 200 parts of carbon are expended on 100 - parts of iron. The ore, flux, and coke are charged into the top of - the furnace, in layers, as the cast iron is formed in the lower - parts and flowing down to the bottom causes the whole contents of - the furnace to subside, thus forming an empty space at the top, - which is again filled up with the afore-mentioned mixture. During - its downward course this mixture is subjected to increasing heat. - This rise of temperature first drives off the moisture of the ore - mixture, and then leads to the formation of the products of the dry - distillation of coal or charcoal. Little by little the subsiding - mass attains a temperature at which the heated carbon reacts with - the carbonic anhydride passing upwards through the furnace and - transforms it into carbonic oxide. This is the reason why carbonic - anhydride is not evolved from the furnace, but only carbonic oxide. - As regards the ore itself, on being heated to about 600° to 800° it - is reduced at the expense of the carbonic oxide ascending the - furnace, and formed by the contact of the carbonic anhydride with - the incandescent charcoal, so that the reduction in the blast - furnace is without doubt brought about _by_ the formation and - decomposition of _carbonic oxide_ and not by carbon itself--thus, - Fe_{2}O_{3} + 3CO = Fe_{2} + 3CO_{2}. The reduced iron, on further - subsidence and contact with carbon, forms cast iron, which flows to - the bottom of the furnace. In these lower layers, where the - temperature is highest (about 1,300°), the foreign matter of the - ore finally forms slag, which also is fusible, with the aid of - fluxes. The air blown in from below, through the so-called - _tuyeres_, encounters carbon in the lower layers of the furnace, - and burns it, converting it into carbonic anhydride. It is evident - that this develops the highest temperature in these lower layers of - the furnace, because here the combustion of the carbon is effected - by heated and compressed air. This is very essential, for it is by - virtue of this high temperature that the process of forming the - slag and of forming and fusing the cast iron are effected - simultaneously in these lower portions of the furnace. The carbonic - acid formed in these parts rises higher, encounters incandescent - carbon, and forms with it carbonic oxide. This heated carbonic - oxide acts as a reducing agent on the iron ore, and is reconverted - by it into carbonic anhydride; this gas meets with more carbon, and - again forms carbonic oxide, which again acts as a reducing agent. - The final transformation of the carbonic anhydride into carbonic - oxide is effected in those parts of the furnace where the reduction - of the oxides of iron does not take place, but where the - temperature is still high enough to reduce the carbonic anhydride. - The ascending mixture of carbonic oxide and nitrogen, CO_{2}, &c., - is then withdrawn through special lateral apertures formed in the - upper cold parts of the furnace walls, and is conducted through - pipes to those stoves which are used for heating the air, and also - sometimes into other furnaces used for the further processes of - iron manufacture. The fuel of blast furnaces consists of wood - charcoal (this is the most expensive material, but the pig iron - produced is the purest, because charcoal does not contain any - sulphur, while coke does), anthracite (for instance, in - Pennsylvania, and in Russia at Pastouhoff's works in the Don - district), coke, coal, and even wood and peat. It must be borne in - mind that the utilisation of naphtha and naphtha refuse would - probably give very profitable results in metallurgical processes. - - The process just described is accompanied by a series of other - processes. Thus, for instance, in the blast furnace a considerable - quantity of cyanogen compounds are formed. This takes place because - the nitrogen of the air blast comes into contact with incandescent - carbon and various alkaline matters contained in the foreign matter - of the ores. A considerable quantity of potassium cyanide is formed - when wood charcoal is employed for iron smelting, as its ash is - rich in potash. - -The _cast iron_ formed in blast furnaces is not always of the same -quality. When slowly cooled it is soft, has a grey colour, and is not -completely soluble in acids. When treated with acids a residue of -graphite remains; it is known as _grey_ or soft cast iron. This is the -general form of the ordinary cast iron used for casting various objects, -because in this state it is not so brittle as in the shape of _white cast -iron_, which does not leave particles of graphite when dissolved, but -yields its carbon in the form of hydrocarbons. This white cast iron is -characterised by its whitish-grey colour, dull lustre, the crystalline -structure of its fracture (more homogeneous than that of grey iron), and -such hardness that a file will hardly cut it. When white cast iron is -produced (from manganese ore) at high temperatures (and with an excess of -lime), and containing little sulphur and silica but a considerable amount -of carbon (as much as 5 p.c.), it acquires a coarse crystalline structure -which increases in proportion to the amount of manganese, and it is then -known under the name of 'spiegeleisen' (and 'ferro-manganese').[9] - - [9] The specific gravity of white cast iron is about 7·5. Grey cast - iron has a much lower specific gravity, namely, 7·0. Grey cast iron - generally contains less manganese and more silica than white; but - both contain from 2 to 3 p.c. of carbon. The difference between the - varieties of cast iron depends on the condition of the carbon which - enters into the composition of the iron. In white cast iron the - carbon is in combination with the iron--in all probability, as the - compound CFe_{4} (Abel and Osmond and others extracted this - compound, which is sometimes called 'carbide,' from tempered steel, - which stands to unannealed steel as white cast iron does to grey), - but perhaps in the state of an indefinite chemical compound - resembling a solution. In any case the compound of the iron and - carbon in white cast iron is chemically very unstable, because when - slowly cooled it decomposes, with separation of graphite, just as a - solution when slowly cooled yields a portion of the substance - dissolved. The separation of carbon in the form of graphite on the - conversion of white cast iron into grey is never complete, however - slowly the separation be carried on; part of the carbon remains in - combination with the iron in the same state in which it exists in - white cast iron. Hence when grey cast iron is treated with acids, - the whole of the carbon does not remain in the form of graphite, - but a part of it is separated as hydrocarbons, which proves the - existence of chemically-combined carbon in grey cast iron. It is - sufficient to re-melt grey cast iron and to cool it quickly to - transform it into white cast iron. It is not carbon alone that - influences the properties of cast iron; when it contains a - considerable amount of sulphur, cast iron remains white even after - having been slowly cooled. The same is observed in cast iron very - rich in manganese (5 to 7 p.c.), and in this latter case the - fracture is very distinctly crystalline and brilliant. When cast - iron contains a large amount of manganese, the quantity of carbon - may also be increased. Crystalline varieties of cast iron rich in - manganese are in practice called ferro-manganese (p. 310), and are - prepared for the Bessemer process. Grey cast iron not having an - uniform structure is much more liable to various changes than dense - and thoroughly uniform white cast iron, and the latter oxidises - much more slowly in air than the former. White cast iron is not - only used for conversion into wrought iron and steel, but also in - those cases where great hardness is required, although it be - accompanied by a certain brittleness; for instance, for making - rollers, plough-shares, &c. - -Cast iron is a material which is either suitable for direct -application for casting in moulds or else for working up into _wrought -iron_ and _steel_. The latter principally differ from cast iron in their -containing less carbon--thus, steel contains from 1 p.c. to 0·5 p.c. of -carbon and far less silicon and manganese than cast iron; wrought iron -does not generally contain more than 0·25 p.c. of carbon and not more -than 0·25 p.c. of the other impurities. Thus the essence of the working -up of cast iron into steel and wrought iron consists in the removal of -the greater part of the carbon and other elements, S, P, Mn, Si, &c. This -is effected by means of oxidation, because the oxygen of the atmosphere, -oxidising the iron at a high temperature, forms solid oxides with it; and -the latter, coming into contact with the carbon contained in the cast -iron, are deoxidised, forming wrought iron and carbonic oxide, which is -evolved from the mass in a gaseous form. It is evident that the oxidation -must be carried on with a molten mass in a state of agitation, so that -the oxygen of the air may be brought into contact with the whole mass of -carbon contained in the cast iron, or else the operation is effected by -means of the addition of oxygen compounds of iron (oxides, ores, as in -Martin's process). Cast iron melts much more easily than wrought iron and -steel, and, therefore, as the carbon separates, the mass in the furnace -(in puddling) or hearth (in the bloomery process) becomes more and more -solid; moreover the degree of hardness forms, to a certain extent, a -measure of the amount of carbon separated, and the operation may -terminate either in the formation of steel or wrought iron.[10] In any -case, the iron used for industrial purposes contains impurities. -_Chemically pure iron_ may be obtained by precipitating iron from a -solution (a mixture of ferrous sulphate with magnesium sulphate or -ammonium chloride) by the prolonged action of a feeble galvanic current; -the iron may be then obtained as a dense mass. This method, proposed by -Böttcher and applied by Klein, gives, as R. Lenz showed, iron containing -occluded hydrogen, which is disengaged on heating. This galvanic -deposition of iron is used for making galvanoplastic _clichés_, which are -distinguished for their great hardness. Electro-deposited iron is -brittle, but if heated (after the separation of the hydrogen) it becomes -soft. If pure ferric hydroxide, which is easily prepared by the -precipitation of solutions of ferric salts by means of ammonia, be heated -in a stream of hydrogen, it forms, first of all, a dull black powder -which ignites spontaneously in air (pyrophoric iron), and then a grey -powder of pure iron. The powdery substance first obtained is an iron -suboxide; when thrown into the air it ignites, forming the oxide -Fe_{3}O_{4}. If the heating in hydrogen be continued, more water and pure -iron, which does not ignite spontaneously, will be obtained. If a small -quantity of iron be fused in the oxyhydrogen flame (with an excess of -oxygen) in a piece of lime and mixed with powdered glass, pure molten -iron will be formed, because in the oxyhydrogen flame iron melts and -burns, but the substances mixed with the iron oxidise first. The oxidised -impurities here either disappear (carbonic anhydride) in a gaseous form, -or turn into slag (silica, manganese, oxide, and others)--that is, fuse -with the glass. Pure iron has a silvery white colour and a specific -gravity of 7·84; it melts at a temperature higher than the melting-points -of silver, gold, nickel, and steel, _i.e._ about 1400°-1500° and below -the melting point of platinum (1750°).[11] But pure iron becomes soft at -a temperature considerably below that at which it melts, and may then be -easily forged, welded, and rolled or drawn into sheets and wire.[11 bis] -Pure iron may be rolled into an exceedingly thin sheet, weighing less -than a sheet of ordinary paper of the same size. This ductility is the -most important property of iron in all its forms, and is most marked with -sheet iron, and least so with cast iron, whose ductility, compared with -wrought iron, is small, but it is still very considerable when compared -with other substances--such, for instance, as rocks.[12] - - [10] This direct process of separating the carbon from cast iron is - termed _puddling_. It is conducted in reverberatory furnaces. The - cast iron is placed on the bed of the furnace and melted; through - a special aperture, the puddler stirs up the oxidising mass of - cast iron, pressing the oxides into the molten iron. This - resembles kneading dough, and the process introduced in England - became known as puddling. It is evident that the puddled mass, or - bloom, is a heterogeneous substance obtained by mixing, and hence - one part of the mass will still be rich in carbon, another will be - poor, some parts will contain oxide not reduced, &c. The further - treatment of the puddled mass consists in hammering and drawing it - out into flat pieces, which on being hammered become more - homogeneous, and when several pieces are welded together and again - hammered out a still more homogeneous mass is obtained. The - quality of the steel and iron thus formed depends principally on - their uniformity. The want of uniformity depends on the oxides - remaining inside the mass, and on the variable distribution of the - carbon throughout the mass. In order to obtain a more homogeneous - metal for manufacturing articles out of steel, it is drawn into - thin rods, which are tied together in bundles and then again - hammered out. As an example of what may be attained in this - direction, imitation Damascus steel may be cited; it consists of - twisted and plaited wire, which is then hammered into a dense - mass. (Real damascened wootz steel may be made by melting a - mixture of the best iron with graphite (1/12) and iron rust; the - article is then corroded with acid, and the carbon remains in the - form of a pattern.) - - Steel and wrought iron are manufactured from cast iron by - puddling. They are, however, obtained not only by this method but - also by the _bloomery process_, which is carried out in a fire - similar to a blacksmith's forge, fed with charcoal and provided - with a blast; a pig of cast iron is gradually pushed into the - fire, and portions of it melt and fall to the bottom of the - hearth, coming into contact with an air blast, and are thus - oxidised. The bloom thus formed is then squeezed and hammered. It - is evident that this process is only available when the charcoal - used in the fire does not contain any foreign matter which might - injure the quality of the iron or steel--for instance, sulphur or - phosphorus--and therefore only wood charcoal may be used with - impunity, from which it follows that this process can only be - carried on where the manufacture of iron can be conducted with - this fuel. Coal and coke contain the above-mentioned impurities, - and would therefore produce iron of a brittle nature, and thus it - would be necessary to have recourse to puddling, where the fuel is - burnt on a special hearth, separate from the cast iron, whereby - the impurities of the fuel do not come into contact with it. The - manufacture of steel from cast iron may also be conducted in - fires; but, in addition to this, it is also now prepared by many - other methods. One of the long-known processes is called - _cementation_, by which steel is prepared from wrought iron but - not from cast iron. For this process strips of iron are heated - red-hot for a considerable time whilst immersed in powdered - charcoal; during this operation the iron at the surface combines - with the charcoal, which however does not penetrate; after this - the iron strips are re-forged, drawn out again, and cemented anew, - repeating this process until a steel of the desired quality is - formed--that is, containing the requisite proportion of carbon. - The _Bessemer_ process occupies the front rank among the newer - methods (since 1856); it is so called from the name of its - inventor. This process consists in running melted cast iron into - converters (holding about 6 tons of cast iron)--that is, - egg-shaped receivers, fig. 94, capable of revolving on trunnions - (in order to charge in the cast iron and discharge the steel), and - forcing a stream of air through small apertures at a considerable - pressure. Combustion of the iron and carbon at an elevated - temperature then takes place, resulting from the bubbles of oxygen - thus penetrating the mass of the cast iron. The carbon, however, - burns to a greater extent than the iron, and therefore a mass is - obtained which is much poorer in carbon than cast iron. As the - combustion proceeds very rapidly in the mass of metal, the - temperature rises to such an extent that even the wrought iron - which may be formed remains in a molten condition, whilst the - steel, being more fusible than the wrought iron, remains very - liquid. In half an hour the mass is ready. The purest possible - cast iron is used in the Bessemer process, because sulphur and - phosphorus do not burn out like carbon, silicon, and manganese. - - [Illustration: FIG. 94.--Bessemer converter, constructed of iron - plate and lined with ganister. The air is carried by the tubes, L, - O, D to the bottom, M, from which it passes by a number of holes - into the converter. The converter is rotated on the trunnion _d_ - by means of the rack and pinion H, when it is required either to - receive molten cast iron from the melting furnaces or to pour out - the steel.] - - The presence of manganese enables the sulphur to be removed with - the slag, and the presence of lime or magnesia, which are - introduced into the lining of the converter, facilitates the - removal of the phosphorus. This basic Bessemer process, or _Thomas - Gilchrist process_, introduced about 1880, enables ores containing - a considerable amount of phosphorus, which had hitherto only been - used for cast iron, to be used for making wrought iron and steel. - Naturally the greatest uniformity will be obtained by re-melting - the metal. Steel is re-melted in small wind furnaces, in masses - not exceeding 30 kilos; a liquid metal is formed, which may be - cast in moulds. A mixture of wrought and cast iron is often used - for making cast steel (the addition of a small amount of metallic - Al improves the homogeneity of the castings, by facilitating the - passage of the impurities into slag). Large steel castings are - made by simultaneous fusion in several furnaces and crucibles; in - this way, castings up to 80 tons or more, such as large ordnance, - may be made. This molten, and therefore homogeneous, steel is - called _cast steel_. Of late years the _Martin's process_ for the - manufacture of steel has come largely into use; it was invented in - France about 1860, and with the use of regenerative furnaces it - enables large quantities of cast steel to be made at a time. It is - based on the melting of cast iron with iron oxides and iron - itself--for instance, pure ores, scrap, &c. There the carbon of - the cast iron and the oxygen of the oxide form carbonic oxide, and - the carbon therefore burns out, and thus cast steel is obtained - from cast iron, providing, naturally, that there is a requisite - proportion and corresponding degree of heat. The advantage of this - process is that not only do the carbon, silicon, and manganese, - but also a great part of the sulphur and phosphorus of the cast - iron burn out at the expense of the oxygen of the iron oxides. - During the last decade the manufacture of steel and its - application for rails, armour plate, guns, boilers, &c., has - developed to an enormous extent, thanks to the invention of cheap - processes for the manufacture of large masses of homogeneous cast - steel. Wrought iron may also be melted, but the heat of a blast - furnace is insufficient for this. It easily melts in the - oxyhydrogen flame. It may be obtained in a molten state directly - from cast iron, if the latter be melted with nitre and - sufficiently stirred up. Considerable oxidation then takes place - inside the mass of cast iron, and the temperature rises to such an - extent that the wrought iron formed remains liquid. A method is - also known for obtaining wrought iron directly from rich iron ores - by the action of carbonic oxide: the wrought iron is then formed - as a spongy mass (which forms an excellent filter for purifying - water), and may be worked up into wrought iron or steel either by - forging or by dissolving in molten cast iron. - - Everybody is more or less familiar with the _difference in the - properties of steel and wrought iron_. Iron is remarkable for its - softness, pliability, and small elasticity, whilst steel may be - characterised by its capability of attaining elasticity and - hardness if it be cooled suddenly after having been heated to a - definite temperature, or, as it is termed, _tempered_. But if - tempered steel be re-heated and slowly cooled, it becomes as soft - as wrought iron, and can then be cut with the file and forged, and - in general can be made to assume any shape, like wrought iron. In - this soft condition it is called _annealed steel_. The transition - from tempered to annealed steel thus takes place in a similar way - to the transition from white to grey cast iron. Steel, when - homogeneous, has considerable lustre, and such a fine granular - structure that it takes a very high polish. Its fracture clearly - shows the granular nature of its structure. The possibility of - tempering steel enables it to be used for making all kinds of - cutting instruments, because annealed steel can be forged, turned, - drawn (under rollers, for instance, for making rails, bars, &c.), - filed, &c., and it may then be tempered, ground and polished. The - method and temperature of tempering and annealing steel determine - its hardness and other qualities. Steel is generally tempered to - the required degree of hardness in the following manner: It is - first strongly heated (for instance, up to 600°), and then plunged - into water--that is, hardened by rapid cooling (it then becomes as - brittle as glass). It is then heated until the surface assumes a - definite colour, and finally cooled either quickly or slowly. When - steel is heated up to 220°, its surface acquires a yellow colour - (surgical instruments); it first of all becomes straw-coloured - (razors, &c.), and then gold-coloured; then at a temperature of - 250° it becomes brown (scissors), then red, then light blue at - 285° (springs), then indigo at 300° (files), and finally sea-green - at about 340°. These colours are only the tints of thin films, - like the hues of soap bubbles, and appear on the steel because a - thin layer of oxides is formed over its surface. Steel rusts more - slowly than wrought iron, and is more soluble in acids than cast - iron, but less so than wrought iron. Its specific gravity is about - 7·6 to 7·9. - - As regards the formation of steel, it was a long time before the - process of cementation was thoroughly understood, because in this - case infusible charcoal permeates unfused wrought iron. Caron - showed that this permeation depends on the fact that the charcoal - used in the process contains alkalis, which, in the presence of - the nitrogen of the air, form metallic cyanides; these being - volatile and fusible, permeate the iron, and, giving up their - carbon to it, serve as the material for the formation of steel. - This explanation is confirmed by the fact that charcoal without - alkalis or without nitrogen will not cement iron. The charcoal - used for cementation acts badly when used over again, as it has - lost alkali. The very volatile ammonium cyanide easily conduces to - the formation of steel. Although steel is also formed by the - action of cyanogen compounds, nevertheless it does not contain - more nitrogen than cast or wrought iron (0·01 p.c.), and these - latter contain it because their ores contain titanium, which - combines directly with nitrogen. Hence the part played by nitrogen - in steel is but an insignificant one. It may be useful here to add - some information taken from Caron's treatise concerning the - influence of foreign matter on the quality of steel. The principal - properties of steel are those of tempering and annealing. The - compounds of iron with silicon and boron have not these - properties. They are more stable than the carbon compound, and - this latter is capable of changing its properties; because the - carbon in it either enters into combination or else is disengaged, - which determines the condition of hardness or softness of steel, - as in white and grey cast iron. When slowly cooled, steel splits - up into a mixture of soft and carburetted iron; but, nevertheless, - the carbon does not separate from the iron. If such steel be again - heated, it forms a uniform compound, and hardens when rapidly - cooled. If the same steel as before be taken and heated a long - time, then, after being slowly cooled, it becomes much more - soluble in acid, and leaves a residue of pure carbon. This shows - that the combination between the carbon and iron in steel becomes - destroyed when subjected to heat, and the steel becomes iron mixed - with carbon. Such _burnt_ steel cannot be tempered, but may be - corrected by continued forging in a heated condition, which has - the effect of redistributing the carbon equally throughout the - whole mass. After the forging, if the iron is pure and the carbon - has not been burnt out, steel is again formed, which may be - tempered. If steel be repeatedly or strongly heated, it becomes - burnt through and cannot be tempered or annealed; the carbon - separates from the iron, and this is effected more easily if the - steel contains other impurities which are capable of forming - stable combinations with iron, such as silicon, sulphur, or - phosphorus. If there be much silicon, it occupies the place of the - carbon, and then continued forging will not induce the carbon once - separated to re-enter into combination. Such steel is easily burnt - through and cannot be corrected; when burnt through, it is hard - and cannot be annealed--this is tough steel, an inferior kind. - Iron which contains sulphur and phosphorus cements badly, combines - but little with carbon, and steel of this kind is brittle, both - hot and cold. Iron in combination with the above-mentioned - substances cannot be annealed by slow cooling, showing that these - compounds are more stable than those of carbon and iron, and - therefore they prevent the formation of the latter. Such metals as - tin and zinc combine with iron, but not with carbon, and form a - brittle mass which cannot be annealed and is deleterious to steel. - Manganese and tungsten, on the contrary, are capable of combining - with charcoal; they do not hinder the formation of steel, but even - remove the injurious effects of other admixtures (by transforming - these admixed substances into new compounds and slags), and are - therefore ranked with the substances which act beneficially on - steel; but, nevertheless, the best steel, which is capable of - renewing most often its primitive qualities after burning or hot - forging, is the purest. The addition of Ni, Cr, W, and certain - other metals to steel renders it very suitable for certain special - purposes, and is therefore frequently made use of. - - It is worthy of attention that steel, besides temper, possesses - many variable properties, a review of which may be made in the - classification of the _sorts of steel_ (1878, Cockerell). (1) - _Very mild steel_ contains from 0·05 to 0·20 p.c. of carbon, - breaks with a weight of 40 to 50 kilos per square millimetre, and - has an extension of 20 to 30 p.c.; it may be welded, like wrought - iron, but cannot be tempered; is used in sheets for boilers, - armour plate and bridges, nails, rivets, &c., as a substitute for - wrought iron; (2) _mild steel_, from 0·20 to 0·35 p.c. of carbon, - resistance to tension 50 to 60 kilos, extension 15 to 20 p.c., not - easily welded, and tempers badly, used for axles, rails, and - railway tyres, for cannons and guns, and for parts of machines - destined to resist bending and torsion; (3) _hard steel_, carbon - 0·35 to 0·50 p.c., breaking weight 60 to 70 kilos per square - millimetre, extension 10 to 15 p.c., cannot be welded, takes a - temper; used for rails, all kinds of springs, swords, parts of - machinery in motion subjected to friction, spindles of looms, - hammers, spades, hoes, &c.; (4) _very hard steel_, carbon 0·5 to - 0·65 p.c., tensile breaking weight 70 to 80 kilos, extension 5 to - 10 p.c., does not weld, but tempers easily; used for small - springs, saws, files, knives and similar instruments. - - The properties of ordinary _wrought iron_ are well known. The best - iron is the most tenacious--that is to say, that which does not - break up when struck with the hammer or bent, and yet at the same - time is sufficiently hard. There is, however, a distinction - between hard and soft iron. Generally the softest iron is the most - tenacious, and can best be welded, drawn into wire, sheets, &c. - Hard, especially tough, iron is often characterised by its - breaking when bent, and is therefore very difficult to work, and - objects made from it are less serviceable in many respects. Soft - iron is most adapted for making wire and sheet iron and such small - objects as nails. Soft iron is characterised by its attaining a - fibrous fracture after forging, whilst tough iron preserves its - granular structure after this operation. Certain sorts of iron, - although fairly soft at the ordinary temperature, become brittle - when heated and are difficult to weld. These sorts are less - suitable for being worked up into small objects. The variety of - the properties of iron depends on the impurities which it - contains. In general, the iron used in the arts still contains - carbon and always a certain quantity of silicon, manganese, - sulphur, phosphorus, &c. A variety in the proportion of these - component parts changes the quality of the iron. In addition to - this the change which soft wrought iron, having a fibrous - structure, undergoes when subjected to repeated blows and - vibrations is considerable; it then becomes granular and brittle. - This to a certain degree explains the want of stability of some - iron objects--such as truck axles, which must be renewed after a - certain term of service, otherwise they become brittle. It is - evident that there are innumerable intermediate transitions from - wrought iron to steel and cast iron. - - At the present day the greater part of the cast iron manufactured - is converted into steel, generally cast steel (Bessemer's and - Martin's). I may add the Urals, Donetz district, and other parts - of Russia offer the greatest advantages for the development of an - iron industry, because these localities not only contain vast - supplies of excellent iron ore, but also coal, which is necessary - for smelting it. - - [11] According to information supplied by A. T. Skinder's experiments - at the Oboukoff Steel Works, 140 volumes of liquid molten steel - give 128 volumes of solid metal. By means of a galvanic current of - great intensity and dense charcoal as one electrode, and iron as - the other, Bernadoss welded iron and fused holes through sheet - iron. Soft wrought iron, like steel and soft malleable cast iron, - may be melted in Siemens' regenerative furnaces, and in furnaces - heated with naphtha. - - [11 bis] Gore (1869), Tait, Barret, Tchernoff, Osmond, and others - observed that at a temperature approaching 600°--that is, between - dark and bright red heat--all kinds of wrought iron undergo a - peculiar change called _recalescence_, _i.e._ a spontaneous rise - of temperature. If iron be considerably heated and allowed to - cool, it may be observed that at this temperature the cooling - stops--that is, latent heat is disengaged, corresponding with a - change in condition. The specific heat, electrical conductivity, - magnetic, and other properties then also change. In tempering, the - temperature of recalescence must not be reached, and so also in - annealing, &c. It is evident that a change of the internal - condition is here encountered, exactly similar to the transition - from a solid to a liquid, although there is no evident physical - change. It is probable that attentive study would lead to the - discovery of a similar change in other substances. - - [12] The particles of steel are linked together or connected more - closely than those of the other metals; this is shown by the fact - that it only breaks with a tensile strain of 50-80 kilos per sq. - mm., whilst wrought iron only withstands about 30 kilos, cast iron - 10, copper 35, silver 23, platinum 30, wood 8. The elasticity of - iron, steel, and other metals is expressed by the so-called - _coefficient of elasticity_. Let a rod be taken whose length is L; - if a weight, P, be hung from the extremity of it, it will lengthen - to _l_. The less it lengthens under other equal conditions, the - more elastic the material, if it resumes its original length when - the weight is removed. It has been shown by experiment that the - increase in length _l_, due to elasticity, is directly - proportional to the length L and the weight P, and inversely - proportional to the section, but changes with the material. The - coefficient of elasticity expresses that weight (in kilos per sq. - mm.) under which a rod having a square section taken as 1 (we take - 1 sq. mm.) acquires double the length by tension. Naturally in - practice materials do not withstand such a lengthening, under a - certain weight they attain a limit of elasticity, _i.e._ they - stretch permanently (undergo deformation). Neglecting fractions - (as the elasticity of metals varies not only with the temperature, - but also with forging, purity, &c.), the coefficient of elasticity - of steel and iron is 20,000, copper and brass 10,000, silver - 7,000, glass 6,000, lead 2,000, and wood 1,200. - -_The chemical properties of iron_ have been already repeatedly -mentioned in preceding chapters. Iron _rusts_ in air at the ordinary -temperature--that is to say, it becomes covered with a layer of iron -oxides. Here, without doubt, the moisture of the air plays a part, -because in dry air iron does not oxidise at all, and also because, more -particularly, ammonia is always found in iron rust; the ammonia must -arise from the action of the hydrogen of the water, at the moment of its -separation, on the nitrogen of the air. Highly-polished steel does not -rust nearly so readily, but if moistened with water, it easily becomes -coated with rust. As rust depends on the access of moisture, iron may be -preserved from rust by coating it with substances which prevent the -moisture having access to it. Thus arises the practice of covering iron -objects with paraffin,[13] varnish, oil, paints, or enamelling it with a -glassy-looking flux possessing the same coefficient of expansion as iron, -or with a dense scoria (formed by the heat of superheated steam), or with -a compact coating of various metals. Wrought iron (both as sheet iron and -in other forms), cast iron, and steel are often coated with tin, copper, -lead, nickel, and similar metals, which prevent contact with the air. -These metals preserve iron very effectually from rust if they form a -completely compact surface, but in those places where the iron becomes -exposed, either accidentally or from wear, rust appears much more quickly -than on a uniform iron surface, because, towards these metals (and also -towards the rust), the iron will then behave as an electro-positive pole -in a galvanic couple, and hence will attract oxygen. A coating of zinc -does not produce this inconvenience, because iron is electro-negative -with reference to zinc, in consequence of which galvanised iron does not -easily rust, and even an iron boiler containing some lumps of zinc rusts -less than one without zinc.[14] Iron oxidises at a high temperature, -forming _iron scale_, Fe_{3}O_{4}, composed of ferrous and ferric oxides, -and, as has been seen, decomposes water and acids with the evolution of -hydrogen. It is also capable of decomposing salts and oxides of other -metals, which property is applied in the arts for the extraction of -copper, silver, lead, tin, &c. For this reason iron is soluble in the -solutions of many salts--for instance, in cupric sulphate, with -precipitation of copper and formation of ferrous sulphate.[15] When iron -_acts on acids_ it always _forms compounds_ FeX_{2}--that is, -corresponding to the suboxide FeO--and answering to magnesium -compounds--and hence two atoms of hydrogen are replaced by one atom of -iron. Strongly oxidising acids like nitric acid may transform the ferrous -salt which is forming into the higher degree of oxidation or ferric salt -(corresponding with the sesquioxide, Fe_{2}O_{3}), but this is a -secondary reaction. Iron, although easily soluble in dilute nitric acid, -loses this property when plunged into strong fuming nitric acid; after -this operation it even loses the property of solubility in other acids -until the external coating formed by the action of the strong nitric acid -is mechanically removed. This condition of iron is termed the passive -state. _The passive condition_ of iron depends on the formation, on its -surface, of a coating of oxide due to the iron being acted on by the -lower oxides of nitrogen contained in the fuming nitric acid.[16] Strong -nitric acid which does not contain these lower oxides, does not render -iron passive, but it is only necessary to add some alcohol or other -reducing agent which forms these lower oxides in the nitric acid, and the -iron will assume the passive state. - - [13] Paraffin is one of the best preservatives for iron against - oxidation in the air. I found this by experiments about 1860, and - immediately published the fact. This method is now very generally - applied. - - [14] See Chapter XVIII., Note 34 bis. Based on the rapid oxidation of - iron and its increase in volume in the presence of water and salts - of ammonium, a packing is used for water mains and steam pipes - which is tightly hammered into the socket joints. This packing - consists of a mixture of iron filings and a small quantity of - sal-ammoniac (and sulphur) moistened with water; after a certain - lapse of time, especially after the pipes have been used, this - mass swells to such an extent that it hermetically seals the - joints of the pipes. - - [15] Here, however, a ferric salt may also be formed (when all the iron - has dissolved and the cupric salt is still in excess), because the - cupric salts are reduced by ferrous salts. Cast iron is also - dissolved. - - [16] Powdery reduced iron is passive with regard to nitric acid of a - specific gravity of 1·37, but when heated the acid acts on it. - This passiveness disappears in the magnetic field. Saint-Edme - attributes the passiveness of iron (and nickel) to the formation - of nitride of iron on the surface of the metal, because he - observed that when heated in dry hydrogen ammonia is evolved by - passive iron. - - Remsen observed that if a strip of iron be immersed in acid and - placed in the magnetic field, it is principally dissolved at its - middle part--that is, the acid acts more feebly at the poles. - According to Étard (1891) strong nitric acid dissolves iron in - making it passive, although the action is a very slow one. - -Iron readily combines with non-metals--for instance, with chlorine, -iodine, bromine, sulphur, and even with phosphorus and carbon; but on the -other hand the property of combining with metals is but little developed -in it--that is to say, it does not easily form alloys. Mercury, which -acts on most metals, does not act directly on iron, and the _iron -amalgam_, or solution of iron in mercury, which is used for electrical -machines, is only obtained in a particular way--namely, with the -co-operation of a sodium amalgam, in which the iron dissolves and by -means of which it is reduced from solutions of its salts. - -When iron acts on acids it forms ferrous salts of the type FeX_{2}, and -in the presence of air and oxidising agents they change by degrees into -ferric salts of the type FeX_{3}. This faculty of passing from the -ferrous to the ferric state is still further developed in ferrous -hydroxide. If sodium hydroxide be added to a solution of ferrous sulphate -or green vitriol, FeSO_{4},[17] a white precipitate of ferrous hydroxide, -FeH_{2}O_{2}, is obtained; but on exposure to the air, even under water, -it turns green, becomes grey, and finally turns brown, which is due to -the oxidation that it undergoes. Ferrous hydroxide is very sparingly -soluble in water; the solution has, however, a distinct alkaline -reaction, which is due to its being a fairly energetic basic oxide. In -any case, ferrous oxide is far more energetic than ferric oxide, so that -if ammonia be added to a solution containing a mixture of a ferrous and -ferric salt, at first ferric hydroxide only will be precipitated. If -barium carbonate, BaCO_{3}, be shaken up in the cold with ferrous salts, -it does not precipitate them--that is, does not change them into ferrous -carbonate; but it completely separates all the iron from the ferric salts -in the cold, according to the equation Fe_{2}Cl_{6} + 3BaCO_{3} + 3H_{2}O -= Fe_{2}O_{3},3H_{2}O + 3BaCl_{2} + 3CO_{2}. If ferrous hydroxide be -boiled with a solution of potash, the water is decomposed, hydrogen is -evolved, and the ferrous hydroxide is oxidised. The ferrous salts are in -all respects similar to the salts of magnesium and zinc; they are -isomorphous with them, but differ from them in that the ferrous hydroxide -is not soluble either in aqueous potash or ammonia. In the presence of an -excess of ammonium salts, however, a certain proportion of the iron is -not precipitated by alkalis and alkali carbonates, which fact points to -the formation of double ammonium salts.[18] The ferrous salts have a dull -_greenish_ colour, and form solutions also of a pale green colour, whilst -the ferric salts have a _brown_ or reddish-brown colour. The ferrous -salts, being capable of oxidation, form very active reducing agents--for -instance, under their action gold chloride, AuCl_{3}, deposits metallic -gold, nitric acid is transformed into lower oxides, and the highest -oxides of manganese also pass into the lower forms of oxidation. All -these reactions take place with especial ease in the presence of an -excess of acid. This depends on the fact that the ferrous oxide, FeO (or -salt), acting as a reducing agent, turns into ferric oxide, Fe_{2}O_{3} -(or salt), and in the ferric state it requires more acid for the -formation of a normal salt than in the ferrous condition. Thus in the -normal ferrous sulphate, FeSO_{4}, there is one equivalent of iron to one -equivalent of sulphur (in the sulphuric radicle), but in the neutral -ferric salt, Fe_{2}(SO_{4})_{3}, there is one equivalent of iron to one -and a half of sulphur in the form of the elements of sulphuric acid.[19] - - [17] _Iron vitriol_ or _green vitriol_, sulphate of iron or ferrous - sulphate, generally crystallises from solutions, like magnesium - sulphate, with seven molecules of water, FeSO_{4},7H_{2}O. This - salt is not only formed by the action of iron on sulphuric acid, - but also by the action of moisture and air on iron pyrites, - especially when previously roasted (FeS_{2} + O_{2} = FeS + - SO_{2}), and in this condition it easily absorbs the oxygen of - damp air (FeS + O_{4} = FeSO_{4}). Green vitriol is obtained in - many processes as a by-product. Ferrous sulphate, like all the - ferrous salts, has a pale greenish colour hardly perceptible in - solution. If it be desired to preserve it without change--that is, - so as not to contain ferric compounds--it is necessary to keep it - hermetically sealed. This is best done by expelling the air by - means of sulphurous anhydride or ether; sulphurous anhydride, - SO_{2}, removes oxygen from ferric compounds, which might be - formed, and is itself changed into sulphuric acid, and hence the - oxidation of the ferrous compound does not take place in its - presence. Unless these precautions are taken, green vitriol turns - brown, partly changing into the ferric salt. When turned brown, it - is not completely soluble in water, because during its oxidation a - certain amount of free insoluble ferric oxide is formed: 6FeSO_{4} - + O_{3} = 2Fe_{2}(SO_{4})_{3} + Fe_{2}O_{3}. In order to cleanse - such mixed green vitriol from the oxide, it is necessary to add - some sulphuric acid and iron and boil the mixture; the ferric salt - is then transformed into the ferrous state: Fe_{2}(SO_{4})_{3} + - Fe = 3FeSO_{4}. - - Green vitriol is used for the manufacture of Nordhausen sulphuric - acid (Chapter XX.), for preparing ferric oxide, in many dye works - (for preparing the indigo vats and reducing blue indigo to white), - and in many other processes; it is also a very good disinfectant, - and is the cheapest salt from which other compounds of iron may be - obtained. - - The other ferrous salts (excepting the yellow prussiate, which - will be mentioned later) are but little used, and it is therefore - unnecessary to dwell upon them. We will only mention _ferrous - chloride_, which, in the crystalline state, has the composition - FeCl_{2},4H_{2}O. It is easily prepared; for instance, by the - action of hydrochloric acid on iron, and in the anhydrous state by - the action of hydrochloric acid gas on metallic iron at a red - heat. The anhydrous ferrous chloride then volatilises in the form - of colourless cubic crystals. Ferrous oxalate (or the double - potassium salt) acts as a powerful reducing agent, and is - frequently employed in photography (as a developer). - - [18] Ferrous sulphate, like magnesium sulphate, easily forms double - salts--for instance, (NH_{4})_{2}SO_{4},FeSO_{4},6H_{2}O. This - salt does not oxidise in air so readily as green vitriol, and is - therefore used for standardising KMnO_{4}. - - [19] The transformation of ferrous oxide into ferric oxide is not - completely effected in air, as then only a part of the suboxide is - converted into ferric oxide. Under these circumstances the - so-called magnetic oxide of iron is generally produced, which - contains atomic quantities of the suboxide and oxide--namely, - FeO,Fe_{2}O_{3} = Fe_{3}O_{4}. This substance, as already - mentioned, is found in nature and in iron scale. It is also formed - when most ferrous and ferric salts are heated in air; thus, for - instance, when ferrous carbonate, FeCO_{3} (native or the - precipitate given by soda in a solution of FeX_{2}), is heated it - loses the elements of carbonic anhydride, and magnetic oxide - remains. This oxide of iron is attracted by the magnet, and is on - this account called magnetic oxide, although it does not always - show magnetic properties. If magnetic oxide be dissolved in any - acid--for instance, hydrochloric--which does not act as an - oxidising agent, a ferrous salt is first formed and ferric oxide - remains, which is also capable of passing into solution. The best - way of preparing the hydrate of the magnetic oxide is by - decomposing a mixture of ferrous and ferric salts with ammonia; it - is, however, indispensable to pour this mixture into the ammonia, - and not _vice versâ_, as in that case the ferrous oxide would at - first be precipitated alone, and then the ferric oxide. The - compound thus formed has a bright green colour, and when dried - forms a black powder. Other combinations of ferrous with ferric - oxide are known, as are also compounds of ferric oxide with other - bases. Thus, for instance, compounds are known containing 4 - molecules of ferrous oxide to 1 of ferric oxide, and also 6 of - ferrous to 1 of ferric oxide. These are also magnetic, and are - formed by heating iron in air. The magnesium compound - MgO,Fe_{2}O_{3} is prepared by passing gaseous hydrochloric acid - over a heated mixture of magnesia and ferric oxide. Crystalline - magnesium oxide is then formed, and black, shiny, octahedral - crystals of the above-mentioned composition. This compound is - analogous to the aluminates--for instance, to spinel. Bernheim - (1888) and Rousseau (1891) obtained many similar compounds of - ferric oxide, and their composition apparently corresponds to the - hydrates (Note 22) known for the oxide. - -The most simple oxidising agent for transforming ferrous into ferric -salts is chlorine in the presence of water--for instance, 2FeCl_{2} + -Cl_{2} = Fe_{2}Cl_{6}, or, generally speaking, 2FeO + Cl_{2} + H_{2}O = -Fe_{2}O_{3} + 2HCl. When such a transformation is required it is best to -add potassium chlorate and hydrochloric acid to the ferrous solution; -chlorine is formed by their mutual reaction and acts as an oxidising -agent. Nitric acid produces a similar effect, although more slowly. -Ferrous salts may be completely and rapidly oxidised into ferric salts by -means of chromic acid or permanganic acid, HMnO_{4}, in the presence of -acids--for example, 10FeSO_{4} + 2KMnO_{4} + 8H_{2}SO_{4} = -5Fe_{2}(SO_{4})_{3} + 2MnSO_{4} + K_{2}SO_{4} + 8H_{2}O. This reaction is -easily observed by the change of colour, and its termination is easily -seen, because potassium permanganate forms solutions of a bright red -colour, and when added to a solution of a ferrous salt the above reaction -immediately takes place _in the presence of acid_, and the solution then -becomes colourless, because all the substances formed are only faintly -coloured in solution. Directly all the ferrous compound has passed into -the ferric state, any excess of permanganate which is added communicates -a red colour to the liquid (see Chapter XXI.) - -Thus when ferrous salts are acted on by oxidising agents, they pass into -the ferric form, and under the action of reducing agents the reverse -reaction occurs. Sulphuretted hydrogen may, for instance, be used for -this complete transformation, for under its influence ferric salts are -reduced with separation of sulphur--for example, Fe_{2}Cl_{6} + H_{2}S = -2FeCl_{2} + 2HCl + S. Sodium thiosulphate acts in a similar way: -Fe_{2}Cl_{6} + Na_{2}S_{2}O_{3} + H_{2}O = 2FeCl_{2} + Na_{2}SO_{4} + -2HCl + S. Metallic iron or zinc,[20] in the presence, of acids, or sodium -amalgam, &c., acts like hydrogen, and has also a similar reducing action, -and this furnishes the best method for reducing ferric salts to ferrous -salts--for instance, Fe_{2}Cl_{6} + Zn = 2FeCl_{2} + ZnCl_{2}. Thus _the -transition from ferrous salts to ferric salts and vice versâ is always -possible_.[21] - - [20] Copper and cuprous salts also reduce ferric oxide to ferrous - oxide, and are themselves turned into cupric salts. The essence of - the reactions is expressed by the following equations: Fe_{2}O_{3} - + Cu_{2}O = 2FeO + 2CuO; Fe_{2}O_{3} + Cu = 2FeO + CuO. This fact - is made use of in analysing copper compounds, the quantity of - copper being ascertained by the amount of ferrous salt obtained. - An excess of ferric salt is required to complete the reaction. - Here we have an example of reverse reaction; the ferrous oxide or - its salt in the presence of alkali transforms the cupric oxide - into cuprous oxide and metallic copper, as observed by Lovel, - Knopp, and others. - - [21] We will here mention the reactions by means of which it may be - ascertained whether the ferrous compound has been entirely - converted into a ferric compound or _vice versâ_. There are two - substances which are best employed for this purpose: potassium - ferricyanide, FeK_{3}C_{6}N_{6}, and potassium thiocyanate, KCNS. - The first salt gives with ferrous salts a blue precipitate of an - insoluble salt, having a composition Fe_{5}C_{12}N_{12}; but with - ferric salts it does not form any precipitate, and only gives a - brown colour, and therefore when transforming a ferrous salt into - a ferric salt, the completion of the transformation may be - detected by taking a drop of the liquid on paper or on a porcelain - plate and adding a drop of the ferricyanide solution. If a blue - precipitate be formed, then part of the ferrous salt still - remains; if there is none, the transformation is complete. The - thiocyanate does not give any marked coloration with ferrous - salts; but with ferric salts in the most diluted state it forms a - bright red soluble compound, and therefore when transforming a - ferric salt into a ferrous salt we must proceed as before, testing - a drop of the solution with thiocyanate, when the absence of a red - colour will prove the total transformation of the ferric salt into - the ferrous state, and if a red colour is apparent it shows that - the transformation is not yet complete. - -_Ferric oxide_, or _sesquioxide of iron_, Fe_{2}O_{3}, is found in -nature, and is artificially prepared in the form of a red powder by many -methods. Thus after heating green vitriol a red oxide of iron remains, -called colcothar, which is used as an oil paint, principally for painting -wood. The same substance in the form of a very fine powder (rouge) is -used for polishing glass, steel, and other objects. If a mixture of -ferrous sulphate with an excess of common salt be strongly heated, -crystalline ferric oxide will be formed, having a dark violet colour, and -resembling some natural varieties of this substance. When iron pyrites is -heated for preparing sulphurous anhydride, ferric oxide also remains -behind; it is used as a pigment. On the addition of alkalis to a solution -of ferric salts, a brown precipitate of ferric hydroxide is formed, which -when heated (even when boiled in water, that is, at about 100°, according -to Tomassi) easily parts with the water, and leaves red anhydrous ferric -oxide. Pure ferric oxide does not show any magnetic properties, but when -heated to a white heat it loses oxygen and is converted into the magnetic -oxide. Anhydrous ferric oxide which has been heated to a high temperature -is with difficulty soluble in acids (but it is soluble when heated in -strong acids, and also when fused with potassium hydrogen sulphate), -whilst ferric hydroxide, at all events that which is precipitated from -salts by means of alkalis, is very readily soluble in acids. The -precipitated _ferric hydroxide_ has the composition 2Fe_{2}O_{3}3H_{2}O, -or Fe_{4}H_{6}O_{9}. If this ordinary hydroxide be rendered anhydrous (at -100°), at a certain moment it becomes incandescent--that is, loses a -certain quantity of heat. This self-incandescence depends on internal -displacement produced by the transition of the easily-soluble (in acids) -variety into the difficultly-soluble variety, and does not depend on the -loss of water, since the anhydrous oxide undergoes the same change. In -addition to this there exists a ferric hydroxide, or hydrated oxide of -iron, which, like the strongly-heated anhydrous iron oxide, is -difficultly soluble in acids. This hydroxide on losing water, or after -the loss of water, does not undergo such self-incandescence, because no -such state of internal displacement occurs (loss of energy or heat) with -it as that which is peculiar to the ordinary oxide of iron. The ferric -hydroxide which is difficultly soluble in acids has the composition -Fe_{2}O_{3},H_{2}O. This hydroxide is obtained by a prolonged ebullition -of water in which ferric hydroxide prepared by the oxidation of ferrous -oxide is suspended, and also sometimes by similar treatment of the -ordinary hydroxide after it has been for a long time in contact with -water. The transition of one hydroxide to another is apparent by a change -of colour; the easily-soluble hydroxide is redder, and the -sparingly-soluble hydroxide more yellow in colour.[22] - - [22] The two ferric hydroxides are not only characterised by the - above-mentioned properties, but also by the fact that the first - hydroxide forms immediately with potassium ferrocyanide, - K_{4}FeC_{6}N_{6}, a blue colour depending on the formation of - Prussian blue, whilst the second hydroxide does not give any - reaction whatever with this salt. The first hydroxide is entirely - soluble in nitric, hydrochloric, and all other acids; whilst the - second sometimes (not always) forms a brick-coloured liquid, which - appears turbid and does not give the reactions peculiar to the - ferric salts (Péan de Saint-Gilles, Scheurer-Kestner). In addition - to this, when the smallest quantity of an alkaline salt is added - to this liquid, ferric oxide is precipitated. Thus a colloidal - solution is formed (hydrosol), which is exactly similar to silica - hydrosol (Chapter XVII.), according to which example the hydrosol - of ferric oxide may be obtained. - - If ordinary ferric hydroxide be dissolved in acetic acid, a - solution of the colour of red wine is obtained, which has all the - reactions characteristic of ferric salts. But if this solution - (formed in the cold) be heated to the boiling-point, its colour is - very rapidly intensified, a smell of acetic acid becomes apparent, - and the solution then contains a new variety of ferric oxide. If - the boiling of the solution be continued, acetic acid is evolved, - and the modified ferric oxide is precipitated. If the evaporation - of the acetic acid be prevented (in a closed or sealed vessel), - and the liquid be heated for some time, the whole of the ferric - hydroxide then passes into the insoluble form, and if some - alkaline salt be added (to the hydrosol formed), the whole of the - ferric oxide is then precipitated in its insoluble form. This - method may be applied for separating ferric oxide from solutions - of its salts. - - All phenomena observed respecting ferric oxide (colloidal - properties, various forms, formation of double basic salts) - demonstrate that this substance, like silica, alumina, lead - hydroxide, &c., is polymerised, that the composition is - represented by (Fe_{2}O_{3})_{_n_}. - -The normal salts of the composition Fe_{2}X_{6} or FeX_{3} correspond -with ferric oxide--for example, the exceedingly volatile _ferric -chloride_, Fe_{2}Cl_{6}, which is easily prepared in the anhydrous state -by the action of chlorine on heated iron.[23] Such also is the _normal -ferric nitrate_, Fe_{2}(NO_{3})_{6}; it is obtained by dissolving iron in -an excess of nitric acid, taking care as far as possible to prevent any -rise of temperature.[24] The normal salt separates from the brown -solution when it is concentrated under a bell jar over sulphuric acid. -This salt, Fe_{2}(NO_{3})_{6},9H_{2}O, then crystallises in well-formed -and perfectly colourless crystals,[25] which deliquesce in air, melt at -35°, and are soluble in and decomposed by water. The decomposition may be -seen from the fact that the solution is brown and does not yield the -whole of the salt again, but gives partly basic salt. The normal salt -(only stable in the presence of an excess of HNO_{3}) is completely -decomposed with great facility by heating with water, even at 130°, and -this is made use of for removing iron (and also certain other oxides of -the form R_{2}O_{3}) from many other bases (of the form RO) whose -nitrates are far more stable. The ferric salts, FeX_{3}, in passing into -ferrous salts, act as oxidising agents, as is seen from the fact that -they not only liberate S from SH_{2}, but also iodine from KI like many -oxidising agents.[25 bis] - - [23] The ferric compound which is most used in practice (for instance, - in medicine, for cauterising, stopping bleeding, &c.--Oleum - Martis) is _ferric chloride_, Fe_{2}Cl_{6}, easily obtained by - dissolving the ordinary hydrated oxide of iron in hydrochloric - acid. It is obtained in the anhydrous state by the action of - chlorine on heated iron. The experiment is carried on in a - porcelain tube, and a solid _volatile substance_ is then formed in - the shape of brilliant violet scales which very readily absorb - moisture from the air, and when heated with water decompose into - crystalline ferric oxide and hydrochloric acid: Fe_{2}Cl_{6} + - 3H_{2}O = 6HCl + Fe_{2}O_{3}. Ferric chloride is so volatile that - the density of its vapour may be determined. At 440° it is equal - to 164·0 referred to hydrogen; the formula Fe_{2}Cl_{6} - corresponds with a density of 162·5. An aqueous solution of this - salt has a brown colour. On evaporating and cooling this solution, - crystals separate containing 6 or 12 molecules of H_{2}O. Ferric - chloride is not only soluble in water, but also in alcohol - (similarly to magnesium chloride, &c.) and in ether. If the latter - solutions are exposed to the rays of the sun they become - colourless, and deposit ferrous chloride, FeCl_{2}, chlorine being - disengaged. After a certain lapse of time, the aqueous solutions - of ferric chloride decompose with precipitation of a basic salt, - thus demonstrating the instability of ferric chloride, like the - other salts of ferric oxide (Note 22). This salt is much more - stable in the form of double salts, like all the ferric salts and - also the salts of many other feeble bases. Potassium or ammonium - chloride forms with it very beautiful red crystals of a double - salt, having the composition Fe_{2}Cl_{6},4KCl,2H_{2}O. When a - solution of this salt is evaporated it decomposes, with separation - of potassium chloride. - - B. Roozeboom (1892) studied in detail (as for CaCl_{2}, Chapter - XIV., Note 50) the separation of different hydrates from saturated - solutions of Fe_{2}Cl_{6} at various concentrations and - temperatures; he found that there are 4 crystallohydrates with 12, - 7, 5, and 4 molecules of water. An orange yellow only slightly - hygroscopic hydrate, Fe_{2}Cl_{6},12H_{2}O, is most easily and - usually obtained, which melts at 37°; its solubility at different - temperatures is represented by the curve BCD in the accompanying - figure, where the point B corresponds to the formation, at -55°, - of a cryohydrate containing about Fe_{2}Cl_{6} + 36H_{2}O, the - point C corresponds to the melting-point (+37°) of the hydrate - Fe_{2}Cl_{6},12H_{2}O, and the curve CD to the fall in the - temperature of crystallisation with an increase in the amount of - salt, or decrease in the amount of water (in the figure the - temperatures are taken along the axis of abscissæ, and the amount - of _n_ in the formula _n_Fe_{2}Cl_{6} + 100H_{2}O along the axis - of ordinates). When anhydrous Fe_{2}Cl_{6} is added to the above - hydrate (12H_{2}O), or some of the water is evaporated from the - latter, very hygroscopic crystals of Fe_{2}Cl_{6},5H_{2}O - (Fritsche) are formed; they melt at 56°, their solubility is - expressed by the curve HJ, which also presents a small branch at - the end J. This again gives the fall in the temperature of - crystallisation with an increase in the amount of Fe_{2}Cl_{6}. - Besides these curves and the solubility of the anhydrous salt - expressed by the line KL (up to 100°, beyond which chlorine is - liberated), Roozeboom also gives the two curves, EFG and JK, - corresponding to the crystallohydrates, Fe_{2}Cl_{6},7H_{2}O - (melts at +32°·5, that is lower than any of the others) and - Fe_{2}Cl_{6},4H_{2}O (melts at 73°·5), which he discovered by a - systematic research on the solutions of ferric chloride. The curve - AB represents the separation of ice from dilute solutions of the - salt. - - [Illustration: FIG. 95.--Diagram of the solubility of - Fe_{2}Cl_{6}.] - - The researches of the same Dutch chemist upon the conditions of - the formation of crystals from the double salt - (NH_{4}Cl)_{4}Fe_{2}Cl_{6},2H_{2}O are even more perfect. This - salt was obtained in 1839 by Fritsche, and is easily formed from a - strong solution of Fe_{2}Cl_{6} by adding sal-ammoniac, when it - separates in crimson rhombic crystals, which, after dissolving in - water, only deposit again on evaporation, together with the - sal-ammoniac. - - Roozeboom (1892) found that when the solution contains _b_ - molecules of Fe_{2}Cl_{6}, and _a_ molecules of NH_{4}Cl, per 100 - molecules H_{2}O, then at 15° one of the following separations - takes place: (1) crystals, Fe_{2}Cl_{6},12H_{2}O, when _a_ varies - between 0 and 11, and _b_ between 4·65 and 4·8, or (2) a mixture - of these crystals and the double salt, when _a_ = 1·36, and _b_ = - 4·47, or (3) the double salt, Fe_{2}Cl_{6},4NH_{4}Cl,2H_{2}O, when - _a_ varies between 2 and 11·8, and _b_ between 3·1 and 4·56, or - (4) a mixture of sal-ammoniac with the iron salt (it crystallises - in separate cubes, Retgers, Lehmann), when _a_ varies between 7·7 - and 10·9, and _b_ is less than 3·38, or (5) sal-ammoniac, when _a_ - = 11·88. And as in the double salt, _a_ : _b_ :: 4 : 1 it is - evident that the double salt only separates out when the ratio _a_ - : _b_ is less than 4 : 1 (_i.e._ when Fe_{2}Cl_{6} predominates). - The above is seen more clearly in the accompanying figure, where - _a_, or the number of molecules of NH_{4}Cl per 100H_{2}O, is - taken along the axis of abscissæ, and _b_, or the number of - molecules of Fe_{2}Cl_{6}, along the ordinates. The curves ABCD - correspond to saturation and present an iso-therm of 15°. The - portion AB corresponds to the separation of chloride of iron (the - ascending nature of this curve shows that the solubility of - Fe_{2}Cl_{6} is increased by the presence of NH_{4}Cl, while that - of NH_{4}Cl decreases in the presence of Fe_{2}Cl_{6}), the - portion BC to the double salt, and the portion CD to a mixture of - sal-ammoniac and ferric chloride, while the straight line OF - corresponds to the ratio Fe_{2}Cl_{6},4NH_{4}Cl, or _a_ : _b_ :: 4 - : 1. The portion CE shows that more double salt may be introduced - into the solution without decomposition, but then the solution - deposits a mixture of sal-ammoniac and ferric chloride (_see_ - Chapter XXIV. Note 9 ^{bis}). If there were more such - well-investigated cases of solutions, our knowledge of double - salts, solutions, the influence of water, equilibria, isomorphous - mixtures, and such-like provinces of chemical relations might be - considerably advanced. - - [Illustration: FIG. 96.--Diagram of the formation, at 15°, of the - double salt Fe_{2}Cl_{6}4NH_{4}Cl_{2}H_{2}O or - Fe(NH_{4})_{2}Cl_{5}H_{2}O. (After Roozeboom.)] - - [24] The normal ferric salts are decomposed by heat and even by water, - forming basic salts, which may be prepared in various ways. - Generally ferric hydroxide is dissolved in solutions of ferric - nitrate; if it contains a double quantity of iron the basic salt - is formed which contains Fe_{2}O_{3} (in the form of hydroxide) + - 2Fe_{2}(NO_{3})_{6} = 3Fe_{2}O(NO_{3})_{4}, a salt of the type - Fe_{2}OX_{4}. Probably water enters into its composition. With - considerable quantities of ferric oxide, insoluble basic salts are - obtained containing various amounts of ferric hydroxide. Thus when - a solution of the above-mentioned basic acid is boiled, a - precipitate is formed containing - 4(Fe_{2}O_{3})_{8},2(N_{2}O_{5}),3H_{2}O, which probably contains - 2Fe_{2}O_{2}(NO_{3})_{2} + 2Fe_{2}O_{3},3H_{2}O. If a solution of - basic nitrate be sealed in a tube and then immersed in boiling - water, the colour of the solution changes just in the same way as - if a solution of ferric acetate had been employed (Note 22). The - solution obtained smells strongly of nitric acid, and on adding a - drop of sulphuric or hydrochloric acid the insoluble variety of - hydrated ferric oxide is precipitated. - - Normal ferric _orthophosphate_ is soluble in sulphuric, - hydrochloric, and nitric acids, but insoluble in others, such as, - for instance, acetic acid. The composition of this salt in the - anhydrous state is FePO_{4}, because in orthophosphoric acid there - are three atoms of hydrogen, and iron, in the ferric state, - replaces the three atoms of hydrogen. This salt is obtained from - ferric acetate, which, with disodium phosphate, forms a _white - precipitate_ of FePO_{4}, containing water. If a solution of - ferric chloride (yellowish-red colour) be mixed with a solution of - sodium acetate in excess, the liquid assumes an intense brown - colour which demonstrates the formation of a certain quantity of - ferric acetate; then the disodium phosphate directly forms a white - gelatinous precipitate of ferric phosphate. By this means the - whole of the iron may be precipitated, and the liquid which was - brown then becomes colourless. If this normal salt be dissolved in - orthophosphoric acid, the crystalline acid salt - FeH_{3}(PO_{4})_{2} is formed. If there be an excess of ferric - oxide in the solution, the precipitate will consist of the basic - salt. If ferric phosphate be dissolved in hydrochloric acid, and - ammonia be added, a salt is precipitated on heating which, after - continued washing in water and heating (to remove the water), has - the composition Fe_{4}P_{2}O_{11}--that is, - 2Fe_{2}O_{3},P_{2}O_{5}. In an aqueous condition this salt may be - considered as ferric hydroxide, Fe_{2}(OH)_{6}, in which (OH)_{3} - is replaced by the equivalent group PO_{4}. Whenever ammonia is - added to a solution containing an excess of ferric salt and a - certain amount of phosphoric acid, a precipitate is formed - containing the whole of the phosphoric acid in the mass of the - ferric oxide. - - Ferric oxide is characterised as a feeble base, and also by the - fact of its forming double salts--for instance, _potassium iron - alum_, which has a composition - Fe_{2}(SO_{4})_{3},K_{2}SO_{4},24H_{2}O or - FeK(SO_{4})_{2},12H_{2}O. It is obtained in the form of almost - colourless or light rose-coloured large octahedra of the regular - system by simply mixing solutions of potassium sulphate and the - ferric sulphate obtained by dissolving ferric oxide in sulphuric - acid. - - [25] It would seem that all normal ferric salts are colourless, and - that the brown colour which is peculiar to the solutions is really - due to basic ferric salts. A remarkable example of the apparent - change of colour of salts is represented by the ferrous and ferric - oxalates. The former in a dry state has a yellow colour, although - as a rule the ferrous salts are green, and the latter is - colourless or pale green. When the normal ferric salt is dissolved - in water it is, like many salts, probably decomposed by the water - into acid and basic salts, and the latter communicates a brown - colour to the solution. Iron alum is almost colourless, is easily - decomposed by water, and is the best proof of our assertion. The - study of the phenomena peculiar to ferric nitrate might, in my - opinion, give a very useful addition to our knowledge of the - aqueous solutions of salts in general. - - [25 bis] The reaction FeX_{3} + KI = FeX_{2} + KX + I proceeds - comparatively slowly in solutions, is not complete (depends upon - the mass), and is reversible. In this connection we may cite the - following data from Seubert and Rohrer's (1894) comprehensive - researches. The investigations were conducted with solutions - containing 1/10 gram--equivalent weights of Fe_{2}(SO_{4})_{3} - (_i.e._ containing 20 grams of salt per litre), and a - corresponding solution of KI; the amount of iodine liberated being - determined (after the addition of starch) by a solution (also 1/10 - normal) of Na_{2}S_{2}O_{3} (_see_ Chapter XX., Note 42). The - progress of the reaction was expressed by the amount of liberated - iodine in percentages of the theoretical amount. For instance, the - following amount of iodide of potassium was decomposed when - Fe_{2}(SO_{4})_{3} + 2_n_KI was taken: - - _n_ = 1 2 3 6 10 20 - After 15´ 11·4 26·3 40·6 73·5 91·6 96·0 - " 30´ 14·0 35·8 47·8 78·5 94·3 97·4 - " 1 hour 19·0 42·7 56·0 84·0 95·7 97·6 - " 10 " 32·6 56·0 75·7 93·2 96·5 97·6 - " 48 " 39·4 67·7 82·6 93·4 96·6 97·6 - - Similar results were obtained for FeCl_{3}, but then the amount of - iodine liberated was somewhat greater. Similar results were also - obtained by increasing the mass of FeX_{3} per KI, and by - replacing it by HI (_see_ Chapter XXI., Note 26). - -Iron forms one other oxide besides the ferric and ferrous oxides; this -contains twice as much oxygen as the former, but is so very unstable that -it can neither be obtained in the free state nor as a hydrate. Whenever -such conditions of double decomposition occur as should allow of its -separation in the free state, it decomposes into oxygen and ferric oxide. -It is known in the state of salts, and is only stable in the presence of -alkalis, and forms salts with them which have a decidedly alkaline -reaction; it is therefore a feebly acid oxide. Thus when small pieces of -iron are heated with nitre or potassium chlorate a potassium salt of the -composition K_{2}FeO_{4} is formed, and therefore the hydrate -corresponding with this salt should have the composition H_{2}FeO_{4}. It -is called _ferric acid_. Its anhydride ought to contain FeO_{3} or -Fe_{2}O_{6}--twice as much oxygen as ferric oxide. If a solution of -potassium ferrate be mixed with acid, the free hydrate ought to be -formed, but it immediately decomposes (2K_{2}FeO_{4} + 5H_{2}SO_{4} = -2K_{2}SO_{4} + Fe_{2}(SO_{4})_{3} + 5H_{2}O + O_{3}), oxygen being -evolved. If a small quantity of acid be taken, or if a solution of -potassium ferrate be heated with solutions of other metallic salts, -ferric oxide is separated--for instance: - - 2CuSO_{4} + 2K_{2}FeO_{4} = 2K_{2}SO_{4} + O_{3} + Fe_{2}O_{3} + 2CuO. - -Both these oxides are of course deposited in the form of hydrates. This -shows that not only the hydrate H_{2}FeO_{4}, but also the salts of the -heavy metals corresponding with this higher oxide of iron, are not formed -by reactions of double decomposition. The solution of potassium ferrate -naturally acts as a powerful oxidising agent; for instance, it transforms -manganous oxide into the dioxide, sulphurous into sulphuric acid, oxalic -acid into carbonic anhydride and water, &c.[26] - - [26] If chlorine be passed through a strong solution of potassium - hydroxide in which hydrated ferric oxide is suspended, the turbid - liquid acquires a dark pomegranate-red colour and contains - potassium ferrate: 10KHO + Fe_{2}O_{3} + 3Cl_{2} = 2K_{2}FeO_{4} + - 6KCl + 5H_{2}O. The chlorine must not be in excess, otherwise the - salt is again decomposed, although the mode of decomposition is - unknown; however, ferric chloride and potassium chlorate are - probably formed. Another way in which the above-described salt is - formed is also remarkable; a galvanic current (from 6 Grove - elements) is passed through cast-iron and platinum electrodes into - a strong solution of potassium hydroxide. The cast-iron electrode - is connected with the positive pole, and the platinum electrode is - surrounded by a porous earthenware cylinder. Oxygen would be - evolved at the cast-iron electrode, but it is used up in - oxidation, and a dark solution of potassium ferrate is therefore - formed about it. It is remarkable that the cast iron cannot be - replaced by wrought iron. - -Iron thus combines with oxygen in three proportions: RO, R_{2}O_{3}, -and RO_{3}. It might have been expected that there would be intermediate -stages RO_{2} (corresponding to pyrites FeS_{2}) and R_{2}O_{5}, but for -iron these are unknown.[26 bis] The lower oxide has a distinctly basic -character, the higher is feebly acid. The only one which is stable in the -free state is ferric oxide, Fe_{2}O_{3}; the suboxide, FeO, absorbs -oxygen, and ferric anhydride, FeO_{3}, evolves it. It is also the same -for other elements; the character of each is determined by the relative -degree of stability of the known oxides. The salts FeX_{2} correspond -with the suboxide, the salts FeX_{3} or Fe_{2}X_{6} with the sesquioxide, -and FeX_{6} represents those of ferric acid, as its potassium salt is -FeO_{2}(OK)_{2}, corresponding with K_{2}SO_{4}, K_{2}MnO_{4}, -K_{2}CrO_{4}, &c. Iron therefore forms compounds of the types FeX_{2}, -FeX_{3}, and FeX_{6}, but this latter, like the type NX_{5}, does not -appear separately, but only when X represents heterogeneous elements or -groups; for instance, for nitrogen in the form of NO_{2}(OH), NH_{4}Cl, -&c., for iron in the form of FeO_{2}(OK)_{2}. But still the type FeX_{6} -exists, and therefore FeX_{2} and FeX_{3} are compounds which, like -ammonia, NH_{3}, are capable of further combinations up to FeX_{6}; this -is also seen in the property of ferrous and ferric salts of forming -compounds with water of crystallisation, besides double and basic salts, -whose stability is determined by the quality of the elements included in -the types FeX_{2} and FeX_{3}.[26 tri] It is therefore to be expected -that there should be complex compounds derived from ferrous and ferric -oxides. Amongst these the series of cyanogen compounds is particularly -interesting; their formation and character is not only determined by the -property which iron possesses of forming complex types, but also by the -similar faculty of the cyanogen compounds, which, like nitriles (Chapter -IX.), have clearly developed properties of polymerisation and in general -of forming complex compounds.[27] - - [26 bis] When Mond and his assistants obtained the remarkable volatile - compound Ni(CO)_{4} (described later, Chapter XXII.), it was shown - subsequently by Mond and Quincke (1891), and also by Berthelot, - that iron, under certain conditions, in a stream of carbonic - oxide, also volatilises and forms a compound like that given by - nickel. Roscoe and Scudder then showed that when water gas is - passed through and kept under pressure (8 atmospheres) in iron - vessels a portion of the iron volatilises from the sides of the - vessel, and that when the gas is burnt it deposits a certain - amount of oxides of iron (the same result is obtained with - ordinary coal gas which contains a small amount of CO). To obtain - the _volatile compound of iron with carbonic oxide_, Mond prepared - a finely divided iron by heating the oxalate in a stream of - hydrogen, and after cooling it to 80°-45° he passed CO over the - powder. The iron then formed (although very slowly) a volatile - compound containing Fe(CO)_{5} (as though it answered to a very - high type, FeX_{10}), which when cooled condenses into a liquid - (slightly coloured, probably owing to incipient decomposition), - sp. gr. 1·47, which solidifies at -21°, boils at about 103°, and - has a vapour density (about 6·5 with respect to air) corresponding - to the above formula; it decomposes at 180°. Water and dilute - acids do not act upon it, but it decomposes under the action of - light and forms a hard, non-volatile crystalline yellow compound - Fe_{2}(CO)_{7} which decomposes at 80° and again forms Fe(CO)_{5}. - - [26 tri] When the molecular Fe_{2}Cl_{6} is produced instead of - FeCl_{3} this complication of the type also occurs. - - [27] Some light may be thrown upon the faculty of Fe of forming various - compounds with CN, by the fact that Fe not only combines with - carbon but also with nitrogen. _Nitride of iron_ Fe_{2}N was - obtained by Fowler by heating finely powdered iron in a stream of - NH_{3} at the temperature of melting lead. - -_In the cyanogen compounds of iron_, two degrees might be expected: -Fe(CN)_{2}, corresponding with ferrous oxide, and Fe(CN)_{3}, -corresponding with ferric oxide. There are actually, however, many other -known compounds, intermediate and far more complex. They correspond with -the double salts so easily formed by metallic cyanides. The two following -double salts are particularly well known, very stable, often used, and -easily prepared. _Potassium ferrocyanide_ or _yellow prussiate of -potash_, a double salt of cyanide of potassium and ferrous cyanide, has -the composition FeC_{2}N_{2},4KCN; its crystals contain 3 mol. of water: -K_{4}FeC_{6}N_{6},3H_{2}O. The other is _potassium ferricyanide_ or _red -prussiate of potash_. It is also known as _Gmelin's salt_, and contains -cyanide of potassium with ferric cyanide; its composition is -Fe(CN)_{3},3KCN or K_{3}FeC_{6}N_{6}. Its crystals do not contain water. -It is obtained from the first by the action of chlorine, which removes -one atom of the potassium. A whole series of other ferrocyanic compounds -correspond with these ordinary salts. - -Before treating of the preparation and properties of these two -remarkable and very stable salts, it must be observed that with ordinary -reagents neither of them gives the same double decompositions as the -other ferrous and ferric salts, and they both present a series of -remarkable properties. Thus these salts have a neutral reaction, are -unchanged by air, dilute acids, or water, unlike potassium cyanide and -even some of its double salts. When solutions of these salts are treated -with caustic alkalis, they do not give a precipitate of ferrous or ferric -hydroxides, neither are they precipitated by sodium carbonate. This led -the earlier investigators to recognise special independent groupings in -them. The yellow prussiate was considered to contain the complex radicle -FeC_{6}N_{6} combined with potassium, namely with K_{4}, and K_{3} was -attributed to the red prussiate. This was confirmed by the fact that -whilst in both salts any other metal, even hydrogen, might be substituted -for potassium, the iron remained unchangeable, just as nitrogen in -cyanogen, ammonium, and nitrates does not enter into double -decomposition, being in the state of the complex radicles CN, NH_{4}, -NO_{2}. Such a representation is, however, completely superfluous for the -explanation of the peculiarities in the reactions of such compounds as -double salts. If a magnesium salt which can be precipitated by potassium -hydroxide does not form a precipitate in the presence of ammonium -chloride, it is very clear that it is owing to the formation of a soluble -double salt which is not decomposed by alkalis. And there is no necessity -to account for the peculiarity of reaction of a double salt by the -formation of a new complex radicle. In the same way also, in the presence -of an excess of tartaric acid, cupric salts do not form a precipitate -with potassium hydroxide, because a double salt is formed. These -peculiarities are more easily understood in the case of cyanogen -compounds than in all others, because all cyanogen compounds, as -unsaturated compounds, show a marked tendency to complexity. This -tendency is satisfied in double salts. The appearance of a peculiar -character in double cyanides is the more easily understood since in the -case of potassium cyanide itself, and also in hydrocyanic acid, a great -many peculiarities have been observed which are not encountered in those -haloid compounds, potassium chloride and hydrochloric acid, with which it -was usual to compare cyanogen compounds. These peculiarities become more -comprehensible on comparing cyanogen compounds with ammonium compounds. -Thus in the presence of ammonia the reactions of many compounds change -considerably. If in addition to this it is remembered that the presence -of many carbon (organic) compounds frequently completely disturbs the -reaction of salts, the peculiarities of certain double cyanides will -appear still less strange, because they contain carbon. The fact that the -presence of carbon or another element in the compound produces a change -in the reactions, may be compared to the action of oxygen, which, when -entering into a combination, also very materially changes the nature of -reactions. Chlorine is not detected by silver nitrate when it is in the -form of potassium chlorate, KClO_{3}, as it is detected in potassium -chloride, KCl. The iron in ferrous and ferric compounds varies in its -reactions. In addition to the above-mentioned facts, consideration ought -to be given to the circumstance that the easy mutability of nitric acid -undergoes modification in its alkali salts, and in general the properties -of a salt often differ much from those of the acid. Every double salt -ought to be regarded as a peculiar kind of saline compound: potassium -cyanide is, as it were, a basic, and ferrous cyanide an acid, element. -They may be unstable in the separate state, but form a stable double -compound when combined together; the act of combination disengages the -energy of the elements, and they, so to speak, saturate each other. Of -course, all this is not a definite explanation, but then the supposition -of a special complex radicle can even less be regarded as such. - -Potassium ferrocyanide, K_{4}FeC_{6}N_{6}, is very easily formed by -mixing solutions of ferrous sulphate and potassium cyanide. First, a -white precipitate of ferrous cyanide, FeC_{2}N_{2}, is formed, which -becomes blue on exposure to air, but is soluble in an excess of potassium -cyanide, forming the ferrocyanide. The same yellow prussiate is obtained -on heating animal nitrogenous charcoal or animal matters--such as horn, -leather cuttings, &c.--with potassium carbonate in iron vessels,[27 bis] -the mass formed being afterwards boiled with water with exposure to air, -potassium cyanide first appearing, which gives yellow prussiate. The -animal charcoal may be exchanged for wood charcoal, permeated with -potassium carbonate and heated in nitrogen or ammonia; the mass thus -produced is then boiled in water with ferric oxide.[28] In this manner it -is manufactured on the large scale, and is called 'yellow prussiate' -('prussiate de potasse,' Blutlaugensalz). - - [27 bis] The sulphur of the animal refuse here forms the compound - FeKS_{2}, which by the action of potassium cyanide yields - potassium sulphide, thiocyanate, and ferrocyanide. - - [28] Potassium ferrocyanide may also be obtained from Prussian blue by - boiling with a solution of potassium hydroxide, and from the - ferricyanide by the action of alkalis and reducing substances - (because the red prussiate is a product of oxidation produced by - the action of chlorine: a ferric salt is reduced to a ferrous - salt), &c. In many works (especially in Germany and France) yellow - prussiate is prepared from the mass, containing oxide of iron, and - employed for purifying coal gas (Vol. I., p. 361), which generally - contains cyanogen compounds. About 2 p.c. of the nitrogen - contained in coal is converted into cyanogen, which forms Prussian - blue and thiocyanates in the mass used for purifying the gas. On - evaporation the solution yields large yellow crystals containing 3 - molecules of water, which is easily expelled by heating above - 100°. 100 parts of water at the ordinary temperature are capable - of dissolving 25 parts of this salt; its sp. gr. is 1·83. When - ignited it forms potassium cyanide and iron carbide, FeC_{2} - (Chapter XIII., Note 12). Oxidising substances change it into - potassium ferricyanide. With strong sulphuric acid it gives - carbonic oxide, and with dilute sulphuric acid, when heated, - prussic acid is evolved according to the equation: - 2K_{4}FeC_{6}N_{6} + 3H_{2}SO_{4} = K_{2}Fe_{2}C_{6}N_{6} + - 3K_{2}SO_{4} + 6HCN; hence in the yellow prussiate K_{2} replaces - Fe. - -It is easy to substitute other metals for the potassium in the yellow -prussiate. The hydrogen salt or hydroferrocyanic acid, H_{4}FeC_{6}N_{6}, -is obtained by mixing strong solutions of yellow prussiate and -hydrochloric acid. If ether be added and the air excluded, the acid is -obtained directly in the form of a white scarcely crystalline precipitate -which becomes blue on exposure to air (as ferrous cyanide does from the -formation of blue compounds of ferrous and ferric cyanides, and it is on -this account used in cotton printing). It is soluble in water and -alcohol, but not in ether, has marked acid properties, and decomposes -carbonates, which renders it easily possible to prepare ferrocyanides of -the metals of the alkalis and alkaline earths; these are readily soluble, -have a neutral reaction, and resemble the yellow prussiate. Solutions of -these salts form precipitates with the salts of other metals, because the -ferrocyanides of the heavy metals are insoluble. Here either the whole of -the potassium of the yellow prussiate, or only a part of it, is exchanged -for an equivalent quantity of the heavy metal. Thus, when a cupric salt -is added to a solution of yellow prussiate, a red precipitate is obtained -which still contains half the potassium of the yellow prussiate: - - K_{4}FeC_{6}N_{6} + CuSO_{4} = K_{2}CuFeC_{6}N_{6} + K_{2}SO_{4}. - -But if the process be reversed (the salt of copper being then in excess) -the whole of the potassium will be exchanged for copper, forming a -reddish-brown precipitate, Cu_{2}FeC_{6}N_{6},9H_{2}O. This reaction and -those similar to it are very sensitive and may be used for detecting -metals in solution, more especially as the colour of the precipitate very -often shows a marked difference when one metal is exchanged for another. -Zinc, cadmium, lead, antimony, tin, silver, cuprous and aurous salts form -_white_ precipitates; cupric, uranium, titanium and molybdenum salts -_reddish-brown_; those of nickel, cobalt, and chromium, _green_ -precipitates; _with ferrous salts_, ferrocyanide forms, as has been -already mentioned, a _white_ precipitate--namely, Fe_{2}FeC_{6}N_{6}, or -FeC_{2}N_{2}--which turns blue on exposure to air, and with ferric salts -a _blue precipitate_ called _Prussian blue_. Here the potassium is -replaced by iron, the reaction being expressed thus: 2Fe_{2}Cl_{6} + -3K_{4}FeC_{6}N_{6} = 12KCl + Fe_{4}Fe_{3}C_{18}N_{18}, the latter formula -expressing the composition of Prussian blue. It is therefore the compound -4Fe(CN)_{3} + 3Fe(CN)_{2}. The yellow prussiate is prepared in chemical -works on a large scale especially for the manufacture of this blue -pigment, which is used for dyeing cloth and other fabrics and also as one -of the ordinary blue paints. It is insoluble in water, and the stuffs are -therefore dyed by first soaking them in a solution of a ferric salt and -then in a solution of yellow prussiate. If however an excess of yellow -prussiate be present complete substitution between potassium and iron -does not occur, and _soluble Prussian blue_ is formed; KFe_{2}(CN)_{6} = -KCN,Fe(CN)_{2},Fe(CN)_{3}. This blue salt is colloidal, is soluble in -pure water, but insoluble and precipitated when other salts--for -instance, potassium or sodium chloride--are present even in small -quantities, and is therefore first obtained as a precipitate.[29] - - [29] Skraup obtained this salt both from potassium ferrocyanide with - ferric chloride and from ferricyanide with ferrous chloride, which - evidently shows that it contains iron in both the ferric and - ferrous states. With ferrous chloride it forms Prussian blue, and - with ferric chloride Turnbull's blue. - - Prussian blue was discovered in the beginning of the last century - by a Berlin manufacturer, Diesbach. It was then prepared, as it - sometimes is also at present, directly from potassium cyanide - obtained by heating animal charcoal with potassium carbonate. The - mass thus obtained is dissolved in water, alum is added to the - solution in order to saturate the free alkali, and then a solution - of green vitriol is added which has previously been sufficiently - exposed to the air to contain both ferric and ferrous salts. If - the solution of potassium cyanide be mixed with a solution - containing both salts, Prussian blue will be formed, because it is - a compound of ferrous cyanide, FeC_{2}N_{2}, and ferric cyanide, - Fe_{2}C_{6}N_{6}. A ferric salt with potassium ferrocyanide forms - a blue colour, because ferrous cyanide is obtained from the first - salt and ferric cyanide from the second. During the preparation of - this compound alkali must be avoided, as otherwise the precipitate - would contain oxides of iron. Prussian blue has not a crystalline - structure; it forms a blue mass with a copper-red metallic lustre. - Both acids and alkalis act on it. The action is at first confined - to the ferric salt it contains. Thus alkalis form ferric oxide and - ferrocyanide in solution: 2Fe_{2}C_{6}N_{6},3FeC_{2}N_{2} + 12KHO - = 2(Fe_{2}O_{3},3H_{2}O) + 3K_{4}FeC_{6}N_{6}. Various - ferrocyanides may thus be prepared. Prussian blue is soluble in an - aqueous solution of oxalic acid, forming blue ink. In air, when - exposed to the action of light, it fades; but in the dark again - absorbs oxygen and becomes blue, which fact is also sometimes - noticed in blue cloth. An excess of potassium ferrocyanide renders - Prussian blue soluble in water, although insoluble in various - saline solutions--that is, it converts it into the soluble - variety. Strong hydrochloric acid also dissolves Prussian blue. - -Potassium ferricyanide, or _red prussiate_ of potash, K_{3}FeC_{6}N_{6}, -is called 'Gmelin's salt,' because this savant obtained it by the action -of chlorine on a solution of the yellow prussiate: K_{4}FeC_{6}N_{6} + Cl -= K_{3}FeC_{6}N_{6} + KCl. The reaction is due to the ferrous salt being -changed by the action of the chlorine into a ferric salt. It separates -from solutions in anhydrous, well-formed prisms of a red colour, but the -solution has an olive colour; 100 parts of water, at 10°, dissolve 37 -parts of the salt, and at 100°, 78 parts.[30] The red prussiate gives a -blue precipitate with ferrous salts, called _Turnbull's blue_, very much -like Prussian blue (and the soluble variety), because it also contains -ferrous cyanide and ferric cyanide, although in another proportion, being -formed according to the equation: 3FeCl_{2} + 2K_{3}FeC_{6}N_{6} = 6KCl + -Fe_{3}Fe_{2}C_{12}N_{12}, or 3FeC_{2}N_{2},Fe_{2}C_{6}N_{6}; in Prussian -blue we have Fe_{7}Cy_{18}, and here Fe_{5}Cy_{12}. A ferric salt ought -to form ferric cyanide Fe_{2}C_{6}N_{6}, with red prussiate, but ferric -cyanide is soluble, and therefore no precipitate is obtained, and the -liquid only becomes brown.[31] - - [30] An excess of chlorine must not be employed in preparing this - compound, otherwise the reaction goes further. It is easy to find - out when the action of the chlorine on potassium ferrocyanide must - cease; it is only necessary to take a sample of the liquid and add - a solution of a ferric salt to it. If a precipitate of Prussian - blue is formed, more chlorine must be added, as there is still - some undecomposed ferrocyanide, for the ferricyanide does not give - a precipitate with ferric salts. Potassium ferricyanide, like the - ferrocyanide, easily exchanges its potassium for hydrogen and - various metals by double decomposition. With the salts of tin, - silver, and mercury it forms yellow precipitates, and with those - of uranium, nickel, cobalt, copper, and bismuth brown - precipitates. The lead salt under the action of sulphuretted - hydrogen forms lead sulphide and a hydrogen salt or acid, - H_{3}FeC_{6}N_{6}, corresponding with potassium ferricyanide, - which is soluble, crystallises in red needles, and resembles - hydroferrocyanic acid, H_{4}FeC_{6}N_{6}. Under the action of - reducing agents--for instance, sulphuretted hydrogen, - copper--potassium ferricyanide is changed into ferrocyanide, - especially in the presence of alkalis, and thus forms a rather - energetic _oxidising agent_--capable, for instance, of changing - manganous oxide into dioxide, bleaching tissues, &c. - - [31] It is important to mention a series of readily crystallisable - salts formed by the action of nitric acid on potassium and other - ferrocyanides and ferricyanides. These salt contain the elements - of nitric oxide, and are therefore called _nitro-(nitroso) - ferricyanides_ (_nitroprussides_). Generally a crystalline sodium - salt is obtained, Na_{2}FeC_{5}N_{6}O,2H_{2}O. In its composition - this salt differs from the red sodium salt, Na_{3}FeC_{6}N_{6}, by - the fact that in it one molecule of sodium cyanide, NaCN, is - replaced by nitric oxide, NO. In order to prepare it, potassium - ferrocyanide in powder is mixed with five-sevenths of its weight - of nitric acid diluted with an equal volume of water. The mixture - is at first left at the ordinary temperature, and then heated on a - water-bath. Here ferricyanide is first of all formed (as shown by - the liquid giving a precipitate with ferrous chloride), which then - disappears (no precipitate with ferrous chloride), and forms a - green precipitate. The liquid, when cooled, deposits crystals of - nitre. The liquid is then strained off and mixed with sodium - carbonate, boiled, filtered, and evaporated; sodium nitrate and - the salt described are deposited in crystals. It separates in - prisms of a red colour. Alkalis and salts of the alkaline earths - do not give precipitates: they are soluble, but the salts of iron, - zinc, copper, and silver form precipitates where sodium is - exchanged with these metals. It is remarkable that the sulphides - of the alkali metals give with this salt an intense bright purple - coloration. This series of compounds was discovered by Gmelin and - studied by Playfair and others (1849). - - This series to a certain extent resembles the nitro-sulphide - series described by Roussin. Here the primary compound consists of - black crystals, which are obtained as follows:--Solutions of - potassium hydrosulphide and nitrate are mixed, and the mixture is - agitated whilst ferric chloride is added, then boiled and - filtered; on cooling, _black crystals_ are deposited, having the - composition Fe_{6}S_{3}(NO)_{10},H_{2}O (Rosenberg), or, according - to Demel, FeNO_{2},NH_{2}S. They have a slightly metallic lustre, - and are soluble in water, alcohol, and ether. They absorb the - latter as easily as calcium chloride absorbs water. In the - presence of alkalis these crystals remain unchanged, but with - acids they evolve nitric oxides. There are several compounds which - are capable of interchanging, and correspond with Roussin's salt. - Here we enter into the series of the nitrogen compounds which have - been as yet but little investigated, and will most probably in - time form most instructive material for studying the nature of - that element. These series of compounds are as unlike the usual - saline compounds of inorganic chemistry as are organic - hydrocarbons. There is no necessity to describe these series in - detail, because their connection with other compounds is not yet - clear, and they have not yet any application. - -If chlorine and sodium are representatives of independent groups of -elements, the same may also be said of iron. Its nearest analogues show, -besides a similarity in character, a likeness as regards physical -properties and a proximity in atomic weight. Iron occupies a medium -position amongst its nearest analogues, both with respect to properties -and faculty of forming saline oxides, and also as regards atomic weight. -On the one hand, cobalt, 58, and nickel, 59, approach iron, 56; they are -metals of a more basic character, they do not form stable acids or higher -degrees of oxidation, and are a transition to copper, 63, and zinc, 65. -On the other hand, manganese, 55, and chromium, 52, are the nearest to -iron; they form both basic and acid oxides, and are a transition to the -metals possessing acid properties. In addition to having atomic weights -approximately alike, chromium, manganese, iron, cobalt, nickel, and -copper have also nearly the same specific gravity, so that the atomic -volumes and the molecules of their analogous compounds are also near to -one another (see table at the beginning of this volume). Besides this, -the likeness between the above-mentioned elements is also seen from the -following: - -They form suboxides, RO, fairly energetic bases, isomorphous with -magnesia--for instance, the salt RSO_{4},7H_{2}O, akin to -MgSO_{4},7H_{2}O, and FeSO_{4},7H_{2}O, or to sulphates containing less -water; with alkali sulphates all form double salts crystallising with -6H_{2}O; all are capable of forming ammonium salts, &c. The lower oxides, -in the cases of nickel and cobalt, are tolerably stable, are not easily -oxidised (the nickel compound with more difficulty than cobalt, a -transition to copper); with manganese, and especially with chromium, they -are more easily oxidised than with iron and pass into higher oxides. They -also form oxides of the form R_{2}O_{3}, and with nickel, cobalt, and -manganese this oxide is very unstable, and is more easily reduced than -ferric oxide; but, in the case of chromium, it is very stable, and forms -the ordinary kind of salts. It is isomorphous with ferric oxide, forms -alums, is a feeble base, &c. Chromium, manganese, and iron are oxidised -by alkali and oxidising agents, forming salts like Na_{2}SO_{4}; but -cobalt and nickel are difficult to oxidise; their acids are not known -with any certainty, and are, in all probability, still less stable than -the ferrates. Cr, Mn and Fe form compounds R_{2}Cl_{6} which are like -Fe_{2}Cl_{6} in many respects; in Co this faculty is weaker and in Ni it -has almost disappeared. The cyanogen compounds, especially of manganese -and cobalt, are very near akin to the corresponding ferrocyanides. The -oxides of nickel and cobalt are more easily reduced to metal than those -of iron, but those of manganese and chromium are not reduced so easily as -iron, and the metals themselves are not easily obtained in a pure state; -they are capable of forming varieties resembling cast iron. The metals -Cr, Mn, Fe, Co, and Ni have a grey iron colour and are very difficult to -melt, but nickel and cobalt can be melted in the reverberatory furnace -and are more fusible than iron, whilst chromium is more difficult to melt -than platinum (Deville). These metals decompose water, but with greater -difficulty as the atomic weight rises, forming a transition to copper, -which does not decompose water. All the compounds of these metals have -various colours, which are sometimes very bright, especially in the -higher stages of oxidation. - -These metals of the iron group are often met with together in nature. -Manganese nearly everywhere accompanies iron, and iron is always an -ingredient in the ores of manganese. Chromium is found principally as -chrome ironstone--that is, a peculiar kind of magnetic oxide in which -Fe_{2}O_{3} is replaced by Cr_{2}O_{3}. - -Nickel and cobalt are as inseparable companions as iron and manganese. -The similarity between them even extends to such remote properties as -magnetic qualities. In this series of metals we find those which are the -most magnetic: iron, cobalt, and nickel. There is even a magnetic oxide -among the chromium compounds, such being unknown in the other series. -Nickel easily becomes passive in strong nitric acid. It absorbs hydrogen -in just the same way as iron. In short, in the series Cr, Mn, Fe, Co, and -Ni, there are many points in common although there are many differences, -as will be seen still more clearly on becoming acquainted with cobalt and -nickel. - -In nature _cobalt_ is principally found in combination with arsenic -and sulphur. _Cobalt arsenide_, or _cobalt speiss_, CoAs_{2}, is found in -brilliant crystals of the regular system, principally in Saxony. _Cobalt -glance_, CoAs_{2}CoS_{2}, resembles it very much, and also belongs to the -regular system; it is found in Sweden, Norway, and the Caucasus. -_Kupfernickel_ is a nickel ore in combination with arsenic, but of a -different composition from cobalt arsenide, having the formula NiAs; it -is found in Bohemia and Saxony. It has a copper-red colour and is rarely -crystalline; it is so called because the miners of Saxony first mistook -it for an ore of copper (_Kupfer_), but were unable to extract copper -from it. _Nickel glance_, NiS_{2},NiAs_{2}, corresponding with cobalt -glance, is also known. Nickel accompanies the ores of cobalt and cobalt -those of nickel, so that both metals are found together. The ores of -cobalt are worked in the Caucasus in the Government of Elizavetopolsk. -Nickel ores containing aqueous hydrated nickel silicate are found in the -Ural (Revdansk). Large quantities of a similar ore are exported into -Europe from New Caledonia. Both ores contain about 12 per cent. Ni. -_Garnierite_, (RO)_{5}(SiO_{2})_{4}1-1/2H_{2}O, where R = Ni and Mg, -predominates in the New Caledonian ore. Large deposits of nickel have -been discovered in Canada, where the ore (as nickelous pyrites) is free -from arsenic. Cobalt is principally worked up into cobalt compounds, but -nickel is generally reduced to the metallic state, in which it is now -often used for alloys--for instance, for coinage in many European States, -and for plating other metals, because it does not oxidise. Cobalt -arsenide and cobalt glance are principally used for the preparation of -cobalt compounds; they are first sorted by discarding the rocky matter, -and then roasted. During this process most of the sulphur and arsenic -disappears; the arsenious anhydride volatilises with the sulphurous -anhydride and the metal also oxidises.[32] It is a simple matter to -obtain nickel and cobalt from their oxides. In order to obtain the -latter, solutions of their salts are treated with sodium carbonate and -the precipitated carbonates are heated; the suboxides are thus obtained, -and these latter are reduced in a stream of hydrogen, or even by heating -with ammonium chloride. They easily oxidise when in the state of powder. -When the chlorides of nickel and cobalt are heated in a stream of -hydrogen, the metal is deposited in brilliant scales. _Nickel is always -much more easily and quickly reduced than cobalt._ Nickel melts more -easily than cobalt, and this even furnishes a means of testing the -heating powers of a reverberatory furnace. Cobalt fuses at a temperature -only a little lower than that at which iron does. In general, cobalt is -nearer to iron than nickel, nickel being nearer to copper.[32 bis] Both -nickel and cobalt have magnetic properties like iron, but Co is less -magnetic than Fe, and Ni still less so. The specific gravity of nickel -reduced by hydrogen is 9·1 and that of cobalt 8·9. Fused cobalt has a -specific gravity of 8·5, the density of ordinary nickel being almost the -same. Nickel has a greyish silvery-white colour; it is brilliant and very -ductile, so that the finest wire may be easily drawn from it. This wire -has a resistance to tension equal to iron wire. The beautiful colour of -nickel, and the high polish which it is capable of receiving and -retaining, as it does not oxidise, render it a useful metal for many -purposes, and in many ways it resembles silver.[32 tri] It is now very -common to cover other metals with a layer of nickel (nickel plating). -This is done by a process of electro-plating, using a solution of a -nickel salt. The colour of cobalt is dark and redder; it is also ductile, -and has a greater tensile resistance than iron. Dilute acids act very -slowly on nickel and cobalt; nitric acid may be considered as the best -solvent for them. The solutions in every case contain salts corresponding -with the ferrous salts--that is, the _salts_ CoX_{2}, NiX_{2}, -_correspond with the suboxides_ of these metals. These salts in their -types are similar to the magnesium salts. The salts of nickel when -crystallising with water have a green colour, and form bright green -solutions, but in the anhydrous state they most frequently have a yellow -colour. The salts of cobalt are generally rose-coloured, and generally -blue when in the anhydrous state. Their aqueous solutions are -rose-coloured. Cobaltous chloride is easily soluble in alcohol, and forms -a solution of an intense blue colour.[33] - - [32] The residue from the roasting of cobalt ores is called _zafflor_, - and is often met with in commerce. From this the purer compounds - of cobalt may be prepared. The ores of nickel are also first - roasted, and the oxides dissolved in acid, nickelous salts being - then obtained. - - The further treatment of cobalt and nickel ores is facilitated if - the arsenic can be almost entirely removed, which may be effected - by roasting the ore a second time with a small addition of nitre - and sodium carbonate; the nitre combines with the arsenic, forming - an arsenious salt, which may be extracted with water. The - remaining mass is dissolved in hydrochloric acid, mixed with a - small quantity of nitric acid. Copper, iron, manganese, nickel, - cobalt, &c., pass into solution. By passing hydrogen sulphide - through the solution, copper, bismuth, lead, and arsenic are - deposited as metallic sulphides; but iron, cobalt, nickel, and - manganese remain in solution. If an alkaline solution of bleaching - powder be then added to the remaining solution, the whole of the - manganese will first be deposited in the form of dioxide, then the - cobalt as hydrated cobaltic oxide, and finally the nickel also. It - is, however, impossible to rely on this method for effecting a - complete separation, the more so since the higher oxides of the - three above-mentioned metals have all a black colour; but, after a - few trials, it will be easy to find how much bleaching powder is - required to precipitate the manganese, and the amount which will - precipitate all the cobalt. The manganese may also be separated - from cobalt by precipitation from a mixture of the solutions of - both metals (in the form of the 'ous' salts) with ammonium - sulphide, and then treating the precipitate with acetic acid or - dilute hydrochloric acid, in which manganese sulphide is easily - soluble and cobalt sulphide almost insoluble. Further particulars - relating to the separation of cobalt from nickel may be found in - treatises on analytical chemistry. In practice it is usual to rely - on the rough method of separation founded on the fact that nickel - is more easily reduced and more difficult to oxidise than cobalt. - The New Caledonian ore is smelted with CaSO_{4} and CaCO_{3} on - coke, and a metallic regulus is obtained containing all the Ni, - Fe, and S. This is roasted with SiO_{2}, which converts all the - iron into slag, whilst the Ni remains combined with the S; this - residue on further roasting gives NiO, which is reduced by the - carbon to metallic Ni. The Canadian ore (a pyrites containing 11 - p.c. Ni) is frequently treated in America (after a preliminary - dressing) by smelting it with Na_{2}SO_{4} and charcoal; the - resultant fusible Na_{2}S then dissolves the CuS and FeS_{2}, - while the NiS is obtained in a bottom layer (Bartlett and - Thomson's process) from which Ni is obtained in the manner - described above. - - For manufacturing purposes somewhat impure cobalt compounds are - frequently used, which are converted into _smalt_. This is glass - containing a certain amount of cobalt oxide; the glass acquires a - bright blue colour from this addition, so that when powdered it - may be used as a blue pigment; it is also unaltered at high - temperatures, so that it used to take the place now occupied by - Prussian blue, ultramarine, &c. At present smalt is almost - exclusively used for colouring glass and china. To prepare smalt, - ordinary impure cobalt ore (zaffre) is fused in a crucible with - quartz and potassium carbonate. A fused mass of cobalt glass is - thus formed, containing silica, cobalt oxide, and potassium oxide, - and a metallic mass remains at the bottom of the crucible, - containing almost all the other metals, arsenic, nickel, copper, - silver, &c. This metallic mass is called _speiss_, and is used as - nickel ore for the extraction of nickel. Smalt usually contains 70 - p.c. of silica, 20 p.c. of potash and soda, and about 5 to 6 p.c. - of cobaltous oxide; the remainder consisting of other metallic - oxides. - - [32 bis] All we know respecting the relations of Co and Ni to Fe and Cu - confirms the fact that Co is more closely related to Fe and Ni to - Cu; and as the atomic weight of Fe = 56 and of Cu = 63, then - according to the principles of the periodic system it would be - expected that the atomic weight of Co would be about 59-60, whilst - that of Ni should be greater than that of Co but less than that of - Cu, _i.e._ about 50·5-60·5. However, as yet the majority of the - determinations of the atomic weights of Co and Ni give a different - result and show that a lower atomic weight is obtained for Ni than - for Co. Thus K. Winkler (1894) obtained (employing metals - deposited electrolytically and determining the amount of iodine - which combined with them) Ni = 58·72 and Co = 59·37 (if H = 1 and - I = 126·53). In my opinion this should not be regarded as proving - that the principles of the periodic system cannot be applied in - this instance, nor as a reason for altering the position of these - elements in the system (_i.e._ by placing Ni after Fe, and Co next - to Cu), because in the first place the figures given by different - chemists (for instance, Zimmermann, Krüss, and others) are - somewhat divergent, and in the second place the majority of the - latest modes of determining the atomic weights of Co and Ni aim at - finding what weights of these metals react with known weights of - other elements without taking into account the faculty they have - of absorbing hydrogen; since this faculty is more developed in Ni - than in Co the hydrogen (occluded in Ni) should lower the atomic - weight of Ni more than that of Co. On the whole, the question of - the atomic weights of Co and Ni cannot yet be considered as - decided, notwithstanding the numerous researches which have been - made; still there can be no doubt that the atomic weights of these - two metals are very nearly equal, and greater than that of Fe, but - less than that of Cu. This question is of great interest, not only - for completing our knowledge of these metals, but also for - perfecting our knowledge of the periodic system of the elements. - - [32 tri] For instance, the alkalis may be fused in nickel vessels as - well as in silver, because they have no action upon either metal. - Nickel, like silver, is not acted upon by dilute acids. Only - nitric acid dissolves both metals well. Nickel is harder, and - fuses at a higher temperature than silver. For castings, a small - quantity of magnesium (0·001 part by weight) is added to nickel to - render it more homogeneous (just as aluminium is added to steel). - Nickel forms many valuable alloys. Steel containing 3 p.c. Ni is - particularly valuable, its limit of elasticity is higher and its - hardness is greater; it is used for armour plate and other large - pieces. The alloys of nickel, especially with copper and zinc - (melchior, _see_ later), aluminium and silver, although used in - certain cases, are now replaced by nickel-plated or - nickel-deposited goods (deposited by electricity from a solution - of the ammonium salts). - - [33] The change of colour is dependent in all probability on the - combination with water, or according to others on polymeric - transformation. It enables a solution of cobalt chloride to be - used as sympathetic ink. If something be written with cobalt - chloride on white paper, it will be invisible on account of the - feeble colour of the solution, and when dry nothing can be - distinguished. If, however, the paper be heated before the fire, - the rose-coloured salt will be changed into a less hydrous blue - salt, and the writing will become quite visible, but fade away - when cool. - - The change of colour which takes place in solutions of CoCl_{2} - under the influence not only of solution in water or alcohol, but - also of a change of temperature, is a characteristic of all the - halogen salts of cobalt. Crystalline iodide of cobalt, - CoI_{2}6H_{2}O, gives a dark red solution between -22° and +20°; - above +20° the solution turns brown and passes from olive to - green, from +35° to 320° the solution remains green. According to - Étard the change of colour is due to the fact that at first the - solution contains the hydrate CoI_{2}H_{2}O, and that above 35° it - contains CoI_{2}4H_{2}O. These hydrates can be crystallised from - the solutions; the former at ordinary temperature and the latter - on heating the solution. The intermediate olive colour of the - solutions corresponds to the incipient decomposition of the - hexahydrated salt and its passage into CoI_{2}4H_{2}O. A solution - of the hexahydrated chloride of cobalt, CoCl_{2}6H_{2}O, is - rose-coloured between -22° and +25°; but the colour changes - starting from +25°, and passes through all the tints between red - and blue right up to 50°; a true blue solution is only obtained at - 55° and remains up to 300°. This true blue solution contains - another hydrate, CoCl_{2}2H_{2}O. - - The dependence between the solubility of the iodide and chloride - of cobalt and the temperature is expressed by two almost straight - lines corresponding to the hexa- and di-hydrates; the passage of - the one into the other hydrate being expressed by a curve. The - same character of phenomena is seen also in the variation of the - vapour tension of solutions of chloride of cobalt with the - temperature. We have repeatedly seen that aqueous solutions (for - instance, Chapter XXII., Note 23 for Fe_{2}Cl_{6}) deposit - different crystallo-hydrates at different temperatures, and that - the amount of water in the hydrate decreases as the temperature - _t_ rises, so that it is not surprising that CoCl_{2}2H_{2}O (or - according to Potilitzin CoCl_{2}H_{2}O) should separate out above - 55° and CoCl_{2}6H_{2}O at 25° and below. Nor is it exceptional - that the colour of a salt varies according as it contains - different amounts of H_{2}O. But in this instance it is - characteristic that the change of colour takes place in solution - in the presence of an excess of water. This apparently shows that - the actual solution may contain either CoCl_{2}6H_{2}O or - CoCl_{2}2H_{2}O. And as we know that a solution may contain both - metaphosphoric PHO_{3} and orthophosphoric acid H_{3}PO_{4} = - HPO_{3} + H_{2}O, as well as certain other anhydrides, the - question of the state of substances in solutions becomes still - more complicated. - - Nickel sulphate crystallises from neutral solutions at a - temperature of from 15° to 20° in _rhombic_ crystals containing - 7H_{2}O. Its form approaches very closely to that of the salts of - zinc and magnesium. The planes of a vertical prism for magnesium - salts are inclined at an angle of 90° 30´, for zinc salts at an - angle of 91° 7´, and for nickel salts at an angle of 91° 10´. Such - is also the form of the zinc and magnesium selenates and - chromates. Cobalt sulphate containing 7 molecules of water is - deposited in crystals of the _monoclinic_ system, like the - corresponding salts of iron and manganese. The angle of a vertical - prism for the iron salt = 82° 20´, for cobalt = 82° 22´, and the - inclination of the horizontal pinacoid to the vertical prism for - the iron salt = 99° 2´, and for the cobalt salt 99° 36´. All the - isomorphous mixtures of the salts of magnesium, iron, cobalt, - nickel and manganese have the same form if they contain 7 mol. - H_{2}O and iron or cobalt predominate, whilst if there is a - preponderance of magnesium, zinc, or nickel, the crystals have a - rhombic form like magnesium sulphate. Hence these sulphates are - _dimorphous_, but for some the one form is more stable and for - others the other. Brooke, Moss, Mitscherlich, Rammelsberg, and - Marignac have explained these relations. Brooke and Mitscherlich - also supposed that NiSO_{4},7H_{2}O is not only capable of - assuming these forms, but also that of the _tetragonal_ system, - because it is deposited in this form from acid, and especially - from slightly-heated solutions (30° to 40°). But Marignac - demonstrated that the tetragonal crystals do not contain 7, but 6, - molecules of water, NiSO_{4},6H_{2}O. He also observed that a - solution evaporated at 50° to 70° deposits monoclinic crystals, - but of a different form from ferrous sulphate, - FeSO_{4},7H_{2}O--namely, the angle of the prism is 71° 52´, that - of the pinacoid 95° 6´. This salt appears to be the same with 6 - molecules of water as the tetragonal. Marignac also obtained - magnesium and zinc salts with 6 molecules of water by evaporating - their solutions at a higher temperature, and these salts were - found to be isomorphous with the monoclinic nickel salt. In - addition to this it must be observed that the rhombic crystals of - nickel sulphate with 7H_{2}O become turbid under the influence of - heat and light, lose water, and change into the tetragonal salt. - The monoclinic crystals in time also become turbid, and change - their structure, so that the tetragonal form of this salt is the - most stable. Let us also add that nickel sulphate in all its - shapes forms very beautiful emerald green crystals, which, when - heated to 230°, assume a dirty greenish-yellow hue and then - contain one molecule of water. - - Klobb (1891) and Langlot and Lenoir obtained anhydrous CoSO_{4} - and NiSO_{4} by igniting the hydrated salt with (NH_{4})_{2}SO_{4} - until the ammonium salt had completely volatilised and decomposed. - - We may add that when equivalent aqueous solutions of NiX_{2} - (green) and CoX_{2} (red) are mixed together they give an almost - colourless (grey) solution, in which the green and red colour of - the component parts disappears owing to the combination of the - complementary colours. - - A double salt NiKF_{3} is obtained by heating NiCl_{2} with KFHF - in a platinum crucible; KCoF_{3} is formed in a similar manner. - The nickel salt occurs in fine green plates, easily soluble in - water but scarcely soluble in ethyl and methyl alcohol. They - decompose into green oxide of nickel and potassium fluoride when - heated in a current of air. The analogous salt of cobalt - crystallises in crimson flakes. - - If instead of potassium fluoride, CoCl_{2} or NiCl_{2} be fused - with ammonium fluoride, they also form double salts with the - latter. This gives the possibility of obtaining anhydrous - fluorides NiF_{2} and CoF_{2}. Crystalline fluoride of nickel, - obtained by heating the amorphous powder formed by decomposing the - double ammonium salt in a stream of hydrofluoric acid, occurs in - beautiful green prisms, sp. gr. 4·63, which are insoluble in - water, alcohol, and ether; sulphuric, hydrochloric, and nitric - acids also have no action upon them, even when heated; NiF_{2} is - decomposed by steam, with the formation of black oxide, which - retains the crystalline structure of the salt. Fluoride of cobalt, - obtained as a rose-coloured powder by decomposing the double - ammonium salt with the aid of heat in a stream of hydrofluoric - acid, fuses into a ruby-coloured mass which bears distinct signs - of a crystalline structure; sp. gr. 4·43. The molten salt only - volatilises at about 1400°, which forms a clear distinction - between CoF_{2} and the volatile NiF_{2}. Hydrochloric, sulphuric, - and nitric acids act upon CoF_{2} even in the cold, although - slowly, while when heated the reaction proceeds rapidly (Poulenc, - 1892). - -If a solution of potassium hydroxide be added to a solution of a cobalt -salt, a blue precipitate of the basic salt will be formed. If a solution -of a cobalt salt be heated almost to the boiling-point, and the solution -be then mixed with a boiling solution of an alkali hydroxide, a _pink -precipitate of cobaltous hydroxide_, CoH_{2}O_{2}, will be formed. If air -be not completely excluded during the precipitation by boiling, the -precipitate will also contain brown cobaltic hydroxide formed by the -further oxidation of the cobaltous oxide.[34] Under similar circumstances -nickel salts form _a green precipitate of nickelous hydroxide_, the -formation of which is not hindered by the presence of ammonium salts, but -in that case only requires more alkali to completely separate the nickel. -The nickelous oxide obtained by heating the hydroxide, or from the -carbonate or nitrate, is a grey powder, easily soluble in acids and -easily reduced, but the same substance may be obtained in the crystalline -form as an ordinary product from the ores; it crystallises in regular -octahedra, with a metallic lustre, and is of a grey colour. In this state -the nickelous oxide almost resists the action of acids.[34 bis] - - [34] Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the - following manner. A solution of 10 grams of CoCl_{2}6H_{2}O in 60 - c.c. of water is heated in a flask with 250 grams of caustic - potash and a stream of coal gas is passed through the solution. - When heated the hydrate of the suboxide of cobalt which separates - out, dissolves in the caustic potash and forms a dark blue - solution. This solution is allowed to stand for 24 hours in an - atmosphere of coal gas (in order to prevent oxidation). The - crystalline mass which separates out has a composition Co(OH)_{2}, - and to the naked eye appears as a violet powder, which is seen to - be crystalline under the microscope. The specific gravity of this - hydrate is 3·597 at 15°. It does not undergo change in the air; - warm acetic acid dissolves it, but it is insoluble in warm and - cold solutions of ammonia and sal-ammoniac. - - [34 bis] The following reaction may be added to those of the cobaltous - and nickelous salts: potassium cyanide forms a precipitate with - cobalt salts which is soluble in an excess of the reagent and - forms a green solution. On heating this and adding a certain - quantity of acid, a double _cobalt cyanide_ is formed which - corresponds with potassium ferricyanide. Its formation is - accompanied with the evolution of hydrogen, and is founded upon - the property which cobalt has of oxidising in an alkaline - solution, the development of which has been observed in such a - considerable measure in the cobaltamine salts. The process which - goes on here may be expressed by the following equation; - CoC_{2}N_{2} + 4KCN first forms CoK_{4}C_{6}N_{6}, which salt with - water, H_{2}O, forms potassium hydroxide, KHO, hydrogen, H, and - the salt, K_{3}CoC_{6}N_{6}. Here naturally the presence of the - acid is indispensable in consequence of its being required to - combine with the alkali. From aqueous solutions this salt - crystallises in transparent, hexagonal prisms of a yellow colour, - easily soluble in water. The reactions of double decomposition, - and even the formation of the corresponding acid, are here - completely the same as in the case of the ferricyanide. If a - nickelous salt be treated in precisely the same manner as that - just described for a salt of cobalt, decomposition will occur. - -It is interesting to note _the relation_ of the cobaltous and -nickelous hydroxides _to ammonia_; aqueous ammonia dissolves the -precipitate of cobaltous and nickelous hydroxide. The blue ammoniacal -solution of nickel resembles the same solution of cupric oxide, but has a -somewhat reddish tint. It is characterised by the fact that it dissolves -silk in the same way as the ammoniacal cupric oxide dissolves cellulose. -Ammonia likewise dissolves the precipitate of cobaltous hydroxide, -forming a brownish liquid, which becomes darker in air and finally -assumes a bright red hue, absorbing oxygen. The admixture of ammonium -chloride prevents the precipitation of cobalt salts by ammonia; when the -ammonia is added, a brown solution is obtained from which, as in the case -of the preceding solution, potassium hydroxide does not separate the -cobaltous oxide. Peculiar compounds are produced in this solution; they -are comparatively stable, containing ammonia and an excess of oxygen; -they bear the name cobaltoamine and cobaltiamine salts. They have been -principally investigated by Genth, Frémy, Jörgenson and others. Genth -found that when a cobalt salt, mixed with an excess of ammonium chloride, -is treated with ammonia and exposed to the air, after a certain lapse of -time, on adding hydrochloric acid and boiling, a red powder is -precipitated and the remaining solution contains an orange salt. The -study of these compounds led to the discovery of a whole series of -similar salts, some of which correspond with particular higher degrees of -oxidation of cobalt, which are described later.[35] Nickel does not -possess this property of absorbing the oxygen of the air when in an -ammoniacal solution. In order to understand this distinction, and in -general the relation of nickel, it is important to observe that cobalt -more easily forms a higher degree of oxidation--namely, _sesquioxide of -cobalt_, _cobaltic oxide_, Co_{2}O_{3}--than nickel, especially in the -presence of hypochlorous acid. If a solution of a cobalt salt be mixed -with barium carbonate and an excess of hypochlorous acid be added, or -chlorine gas be passed through it, then at the ordinary temperature on -shaking, the whole of the cobalt will be separated in the form of black -cobaltic oxide: 2CoSO_{4} + ClHO + 2BaCO_{3} = Co_{2}O_{3} + 2BaSO_{4} + -HCl + 2CO_{2}. Under these circumstances nickelous oxide does not -immediately form black sesquioxide, but after a considerable space of -time it also separates in the form of sesquioxide, Ni_{2}O_{3}, but -always later than cobalt. This is due to the relative difficulty of -further oxidation of the nickelous oxide. It is, however, possible to -oxidise it; if, for instance, the hydroxide NiH_{2}O_{2} be shaken in -water and chlorine gas be passed through it, then nickel chloride will be -formed, which is soluble in water, and insoluble nickelic oxide in the -form of a black precipitate: 3NiH_{2}O_{2} + Cl_{2} = NiCl_{2} + -Ni_{2}O_{3},3H_{2}O. Nickelic oxide may also be obtained by adding sodium -hypochlorite mixed with alkali to a solution of a nickel salt. Nickelic -and cobaltic hydrates are black. Nickelic oxide evolves oxygen with all -acids, and in consequence of this it is not separated as a precipitate in -the presence of acids; thus it evolves chlorine with hydrochloric acid, -exactly like manganese dioxide. When nickelic oxide is dissolved in -aqueous ammonia it liberates nitrogen, and an ammoniacal solution of -nickelous oxide is formed. When heated, nickelic oxide loses oxygen, -forming nickelous oxide. Cobaltic oxide, Co_{2}O_{3}, exhibits more -stability than nickelic oxide, and shows feeble basic properties; thus it -is dissolved in acetic acid without the evolution of oxygen.[35 bis] But -ordinary acids, especially on heating, evolve oxygen, forming a solution -of a cobaltous salt. The presence of a cobaltic salt in a solution of a -cobaltous salt may be detected by the brown colour of the solution and -the black precipitate formed by the addition of alkali, and also from the -fact that such solutions evolve chlorine when heated with hydrochloric -acid. Cobaltic oxide may not only be prepared by the above-mentioned -methods, but also by heating cobalt nitrate, after which a steel-coloured -mass remains which retains traces of nitric acid, but when heated further -to incandescence evolves oxygen, leaving a compound of cobaltic and -cobaltous oxides, similar to magnetic ironstone. Cobalt (but not nickel) -undoubtedly forms besides Co_{2}O_{3} a _dioxide_ CoO_{2}. This is -obtained[36] when the cobaltous oxide is oxidised by iodine or peroxide -of barium.[37] - - [35] The cobalt salts may be divided into at least the following - classes, which repeat themselves for Cr, Ir, Rh (we shall not stop - to consider the latter, particularly as they closely resemble the - cobalt salts):-- - - (_a_) _Ammonium cobalt salts_, which are simply direct compounds - of the cobaltous salts CoX_{2} with ammonia, similar to various - other compounds of the salts of silver, copper, and even calcium - and magnesium, with ammonia. They are easily crystallised from an - ammoniacal solution, and have a pink colour. Thus, for instance, - when cobaltous chloride in solution is mixed with sufficient - ammonia to redissolve the precipitate first formed, octahedral - crystals are deposited which have a composition - CoCl_{2},H_{2}O,6NH_{3}. These salts are nothing else but - combinations with ammonia of crystallisation--if it may be so - termed--likening them in this way to combinations with water of - crystallisation. This similarity is evident both from their - composition and from their capability of giving off ammonia at - various temperatures. The most important point to observe is that - all these salts contain 6 molecules of ammonia to 1 atom of - cobalt, and this ammonia is held in fairly stable connection. - Water decomposes these salts. (Nickel behaves similarly without - forming other compounds corresponding to the true cobaltic.) - - (_b_) The solutions of the above-mentioned salts are rendered - turbid by the action of the air; they absorb oxygen and become - covered with a crust of _oxycobaltamine salts_. The latter are - sparingly soluble in aqueous ammonia, have a brown colour, and are - characterised by the fact that with warm water _they evolve - oxygen_, forming salts of the following category: The nitrate may - be taken as an example of this kind of salt; its composition is - CoN_{2}O_{7},5NH_{3},H_{2}O. It differs from cobaltous nitrate, - Co(NO_{3})_{2}, in containing an extra atom of oxygen--that is, it - corresponds with cobalt dioxide, CoO_{2}, in the same way that the - first salts correspond with cobaltous oxide; they contain 5, and - not 6, molecules of ammonia, as if NH_{3} had been replaced by O, - but we shall afterwards meet compounds containing either 5NH_{3} - or 6NH_{3} to each atom of cobalt. - - (_c_) _The luteocobaltic salts_ are thus called because they have - a yellow (luteus) colour. They are obtained from the salts of the - first kind by submitting them in dilute solution to the action of - the air; in this case salts of the second kind are not formed, - because they are decomposed by an excess of water, with the - evolution of oxygen and the formation of luteocobaltic salts. By - the action of ammonia the salts of the fifth kind (roseocobaltic) - are also converted into luteocobaltic salts. These last-named - salts generally crystallise readily, and have a yellow colour; - they are comparatively much more stable than the preceding ones, - and even for a certain time resist the action of boiling water. - Boiling aqueous potash liberates ammonia and precipitates hydrated - cobaltic oxide, Co_{2}O_{3},3H_{2}O, from them. This shows that - the luteocobaltic salts correspond with cobaltic oxide, - Co_{2}O_{3}, and those of the second kind with the dioxide. When a - solution of luteocobaltic sulphate, - Co_{2}(SO_{4})_{3},12NH_{3},4H_{2}O, is treated with baryta, - barium sulphate is precipitated, and the solution contains - luteocobaltic hydroxide, Co(OH)_{3},6NH_{3}, which is soluble in - water, is powerfully alkaline, absorbs the oxygen of the air, and - when heated is decomposed with the evolution of ammonia. This - compound therefore corresponds to a solution of cobaltic hydroxide - in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and - 12 molecules of ammonia--that is, 6NH_{3} to each atom of cobalt, - like the salts of the first kind. The CoX_{2} salts have a - metallic taste, whilst those of luteocobalt and others have a - purely saline taste, like the salts of the alkali metals. In the - luteo-salts all the X's react (are ionised, as some chemists say) - as in ordinary salts--for instance, all the Cl_{2} is precipitated - by a solution of AgNO_{3}; all the (SO_{4})_{3} gives a - precipitate with BaX_{2}, &c. The double salt formed with PtCl_{4} - is composed in the same manner as the potassium salt, - K_{2}PtCl_{4} = 2KCl + PtCl_{4}, that is, contains - (CoCl_{3},6NH_{3})_{2},3PtCl_{4}, or the amount of chlorine in the - PtCl_{4} is double that in the alkaline salt. In the rosepentamine - (_e_), and rosetetramine (_f_), salts, also all the X's react or - are ionised, but in the (_g_) and (_h_) salts only a portion of - the X's react, and they are equal to the (_e_) and (_f_) salts - minus water; this means that although the water dissolves them it - is not combined with them, as PHO_{3} differs from PH_{3}O_{3}; - phenomena of this class correspond exactly to what has been - already (Chapter XXI., Note 7) mentioned respecting the green and - violet salts of oxide of chromium. - - (_d_) _The fuscocobaltic salts._ An ammoniacal solution of cobalt - salts acquires a brown colour in the air, due to the formation of - these salts. They are also produced by the decomposition of salts - of the second kind; they crystallise badly, and are separated from - their solutions by addition of alcohol or an excess of ammonia. - When boiled they give up the ammonia and cobaltic oxide which they - contain. Hydrochloric and nitric acids give a yellow precipitate - with these salts, which turns red when boiled, forming salts of - the next category. The following is an example of the composition - of two of the fuscocobaltic salts, - Co_{2}O(SO_{4})_{2},8NH_{3},4H_{2}O and - Co_{2}OCl_{4},8NH_{3},3H_{2}O. It is evident that the - fuscocobaltic salts are ammoniacal compounds of basic cobaltic - salts. The normal cobaltic sulphate ought to have the composition - Co_{2}(SO_{4})_{3} = Co_{2}O_{3},3SO_{3}; the simplest basic salts - will be Co_{2}O(SO_{4})_{2} = Co_{2}O_{3})2SO_{3}, and - Co_{2}O_{2}(SO_{4}) = Co_{2}O_{3},SO_{3}. The fuscocobaltic salts - correspond with the first type of basic salts. They are changed - (in concentrated solutions) into oxycobaltamine salts by - absorption of one atom of oxygen, Co_{2}O_{2}(SO_{4})_{2}. The - whole process of oxidation will be as follows: first of all - Co_{2}X_{4}, a cobaltous salt, is in the solution (X a univalent - haloid, 2 molecules of the salt being taken), then Co_{2}OX_{4}, - the basic cobaltic salt (4th series), then Co_{2}O_{2}X_{4}, the - salt of the dioxide (2nd series). The series of basic salts with - an acid, 2HX, forms water and a normal salt, Co_{2}X_{6} (in 3, 5, - 6 series). These salts are combined with various amounts of water - and ammonia. Under many conditions the salts of fuscocobalt are - easily transformed into salts of the next series. The salts of the - series that has just been described contain 4 molecules of ammonia - to 1 atom of cobalt. - - (_e_) _The roseocobaltic_ (or rosepentamine), - CoX_{2}H_{2}O,5NH_{3}, _salts_, like the luteocobaltic, correspond - with the normal cobaltic salts, but contain less ammonia, and an - extra molecule of water. Thus the sulphate is obtained from - cobaltous sulphate dissolved in ammonia and left exposed to the - air until transformed into a brown solution of the fuscocobaltic - salt; when this is treated with sulphuric acid a crystalline - powder of the roseocobaltic salt, - Co_{2}(SO_{4})_{3},10NH_{3},5H_{2}O, separates. The formation of - this salt is easily understood: cobaltous sulphate in the presence - of ammonia absorbs oxygen, and the solution of the fuscocobaltic - salt will therefore contain, like cobaltous sulphate, one part of - sulphuric acid to every part of cobalt, so that the whole process - of formation may be expressed by the equation: 10NH_{3} + - 2CoSO_{4} + H_{2}SO_{4} + 4H_{2}O + O = - Co_{2}(SO_{4})_{3},10NH_{3},5H_{2}O. This salt forms tetragonal - crystals of a red colour, slightly soluble in cold, but readily - soluble in warm water. When the sulphate is treated with baryta, - roseocobaltic hydroxide is formed in the solution, which absorbs - the carbonic anhydride of the air. It is obtained from the next - series by the action of alkalis. - - (_f_) The _rosetetramine cobaltic salts_ CoCl_{2},2H_{2}O,4NH_{3} - were obtained by Jörgenson, and belong to the type of the - luteo-salts, only with the substitution of 2NH_{3} for H_{2}O. - Like the luteo- and roseo-salts they give double salts with - PtCl_{4}, similar to the alkaline double salts, for instance - (Co_{2}H_{2}O,4NH_{3})2(SO_{4})_{2}Cl_{2}PtCl_{4}. They are darker - in colour than the preceding, but also crystallise well. They are - formed by dissolving CoCO_{3} in sulphuric acid (of a given - strength), and after NH_{3} and carbonate of ammonium have been - added, air is passed through the solution (for oxidation) until - the latter turns red. It is then evaporated with lumps of - carbonate of ammonium, filtered from the precipitate and - crystallised. A salt of the composition - Co_{2}(CO_{3})_{2}(SO_{4}),(2H_{2}O,4NH_{3})_{2} is thus obtained, - from which the other salts may be easily prepared. - - (_g_) The _purpureocobaltic salts_, CoX_{3},5NH_{3}, are also - products of the direct oxidation of ammoniacal solutions of cobalt - salts. They are easily obtained by heating the roseocobaltic and - luteo-salts with strong acids. They are to all effects the same as - the roseocobaltic salts, only anhydrous. Thus, for instance, the - purpureocobaltic chloride, Co_{2}Cl_{6},10NH_{3}, or - CoCl_{3},5NH_{3}, is obtained by boiling the oxycobaltamine salts - with ammonia. There is the same distinction between these salts - and the preceding ones as between the various compounds of - cobaltous chloride with water. In the purpureocobaltic only X_{2} - out of the X_{3} react (are ionised). To the rosetetramine salts - (_f_) there correspond the _purpureotetramine_ salts, - CoX_{3}H_{2}O,4NH_{3}. The corresponding chromium - purpureopentamine salt, CrCl_{3},5NH_{3} is obtained with - particular ease (Christensen, 1893). Dry anhydrous chromium - chloride is treated with anhydrous liquid ammonia in a freezing - mixture composed of liquid CO_{2} and chlorine, and after some - time the mixture is taken out of the freezing mixture, so that the - excess of NH_{3} boils away; the violet crystals then immediately - acquire the red colour of the salt, CrCl_{3},5NH_{3}, which is - formed. The product is washed with water (to extract the - luteo-salt, CrCl_{3},6NH_{3}), which does not dissolve the salt, - and it is then recrystallised from a hot solution of hydrochloric - acid. - - (_h_) The _praseocobaltic salts_, CoX_{3},4NH_{3}, are green, and - form, with respect to the rosetetramine salts (_f_), the products - of ultimate dehydration (for example, like metaphosphoric acid - with respect to orthophosphoric acid, but in dissolving in water - they give neither rosetetramine nor tetramine salts. (In my - opinion one should expect salts with a still smaller amount of - NH_{3}, of the blue colour proper to the low hydrated compounds of - cobalt; the green colour of the prazeo-salts already forms a step - towards the blue.) Jörgenson obtained salts for ethylene-diamine, - N_{2}H_{4}C_{2}H_{4} which replaces 2NH_{3}. After being kept a - long time in aqueous solution they give rosetetramine salts, just - as metaphosphoric acid gives orthophosphoric acid, while the - rosetetramine salts are converted into prazeo-salts by Ag_{2}O and - NaHO. Here only one X is ionised out of the X_{3}. There are also - basic salts of the same type; but the best known is the chromium - salt called the rhodozochromic salt, - Cr_{2}(OH)_{3}Cl_{3},6NH_{3},2H_{2}O, which is formed by the - prolonged action of water upon the corresponding roseo-salt. - - The cobaltamine compounds differ essentially but little from the - ammoniacal compounds of other metals. The only difference is that - here the cobaltic oxide is obtained from the cobaltous oxide in - the presence of ammonia. In any case it is a simpler question than - that of the double cyanides. Those forces in virtue of which such - a considerable number of ammonia molecules are united with a - molecule of a cobalt compound, appertain naturally to the series - of those slightly investigated forces which exist even in the - highest degrees of combination of the majority of elements. They - are the same forces which lead to the formation of compounds - containing water of crystallisation, double salts, isomorphous - mixtures and complex acids (Chapter XXI., Note 8 bis). The - simplest conception, according to my opinion, of cobalt compounds - (much more so than by assuming special complex radicles, with - Schiff, Weltzien, Claus, and others), may be formed by comparing - them with other ammoniacal products. Ammonia, like water, combines - in various proportions with a multitude of molecules. Silver - chloride and calcium chloride, just like cobalt chloride, absorb - ammonia, forming compounds which are sometimes slightly stable, - and easily dissociated, sometimes more stable, in exactly the same - way as water combines with certain substances, forming fairly - stable compounds called hydroxides or hydrates, or less stable - compounds which are called compounds with water of - crystallisation. Naturally the difference in the properties in - both cases depends on the properties of those elements which enter - into the composition of the given substance, and on those kinds of - affinity towards which chemists have not as yet turned their - attention. If boron fluoride, silicon fluoride, &c., combine with - hydrofluoric acid, if platinic chloride, and even cadmium - chloride, combine with hydrochloric acid, these compounds may be - regarded as double salts, because acids are salts of hydrogen. But - evidently water and ammonia have the same saline faculty, more - especially as they, like haloid acids, contain hydrogen, and are - both capable of further combination--for instance, ammonia with - hydrochloric acid. Hence it is simpler to compare complex - ammoniacal with double salts, hydrates, and similar compounds, but - _the ammonio-metallic salts_ present a most complete qualitative - and quantitative resemblance to _the hydrated salts of metals_. - The composition of the latter is MX_{_n_}_m_H_{2}O, where M = - metal, X = the haloid, simple or complex, and _n_ and _m_ the - quantities of the haloid and so-called water of crystallisation - respectively. The composition of the ammoniacal salts of metals is - MX_{_n_}_m_NH_{3}. The water of crystallisation is held by the - salt with more or less stability, and some salts even do not - retain it at all; some part with water easily when exposed to the - air, others when heated, and then with difficulty. In the case of - some metals all the salts combine with water, whilst with others - only a few, and the water so combined may then be easily - disengaged. All this applies equally well to the ammoniacal salts, - and therefore the combination of ammonia may be termed _the - ammonia of crystallisation_. Just as the water which is combined - with a salt is held by it with different degrees of force, so it - is with ammonia. In combining with 2NH_{3},PtCl_{2} evolves 31,000 - cals.; while CaCl_{2} only evolves 14,000 cals.; and the former - compound parts with its NH_{3} (together with HCl in this case) - with more difficulty, only above 200°, while the latter disengages - ammonia at 180°. ZnCl_{2},2NH_{3} in forming ZnCl_{2},4NH_{3} - evolves only 11,000 cals., and splits up again into its components - at 80°. The amount of combined ammonia is as variable as the - amount of water of crystallisation--for instance, SnI_{4}8NH_{3}, - CrCl_{2}8NH_{3}, CrCl_{3}6NH_{3}, - CrCl_{3}5NH_{3},PtCl_{2},4NH_{3}, &c. are known. Very often NH_{3} - is replaceable by OH_{2} and conversely. A colourless, anhydrous - cupric salt--for instance, cupric sulphate--when combined with - water forms blue and green salts, and violet when combined with - ammonia. If steam be passed through anhydrous copper sulphate the - salt absorbs water and becomes heated; if ammonia be substituted - for the water the heating becomes much more intense, and the salt - breaks up into a fine violet powder. With water CuSO_{4},5H_{2}O - is formed, and with ammonia CuSO_{4},5NH_{3}, the number of water - and ammonia molecules retained by the salt being the same in each - case, and as a proof of this, and that it is not an isolated - coincidence, the remarkable fact must be borne in mind that water - and ammonia consecutively, molecule for molecule, are capable of - supplanting each other, and forming the compounds - CuSO_{4},5H_{2}O, CuSO_{4},4H_{2}O,NH_{3}; - CuSO_{4},3H_{2}O,2NH_{3}; CuSO_{4},2H_{2}O,3NH_{3}; - CuSO_{4},H_{2}O,4NH_{3}, and CuSO_{4},5NH_{3}. The last of these - compounds was obtained by Henry Rose, and my experiments have - shown that more ammonia than this cannot be retained. By adding to - a strong solution of cupric sulphate sufficient ammonia to - dissolve the whole of the oxide precipitated, and then adding - alcohol, Berzelius obtained the compound CuSO_{4},H_{2}O,4NH_{3}, - &c. The law of substitution also assists in rendering these - phenomena clearer, because a compound of ammonia with water forms - ammonium hydroxide, NH_{4}HO, and therefore these molecules - combining with one another may also interchange, as being of equal - value. In general, those salts form stable ammoniacal compounds - which are capable of forming stable compounds with water of - crystallisation; and as ammonia is capable of combining with - acids, and as some of the salts formed by slightly energetic bases - in their properties more closely resemble acids (that is, salts of - hydrogen) than those salts containing more energetic bases, we - might expect to find more stable and more easily-formed - ammonio-metallic salts with metals and their oxides having weaker - basic properties than with those which form energetic bases. This - explains why the salts of potassium, barium, &c., do not form - ammonio-metallic salts, whilst the salts of silver, copper, zinc, - &c., easily form them, and the salts RX_{3} still more easily and - with greater stability. This consideration also accounts for the - great stability of the ammoniacal compounds of cupric oxide - compared with those of silver oxide, since the former is displaced - by the latter. It also enables us to see clearly the distinction - which exists in the stability of the cobaltamine salts containing - salts corresponding with cobaltous oxide, and those corresponding - with higher oxides of cobalt, for the latter are weaker bases than - cobaltous oxides. _The nature of the forces and quality of the - phenomena occurring during the formation of the most stable - substances, and of such compounds as crystallisable compounds, are - one and the same, although perhaps exhibited in a different - degree._ This, in my opinion, may be best confirmed by examining - the compounds of carbon, because for this element the nature of - the forces acting during the formation of its compounds is well - known. Let us take as an example two unstable compounds of carbon. - Acetic acid, C_{2}H_{4}O_{2} (specific gravity 1·06), with water - forms the hydrate, C_{2}H_{4}O_{2},H_{2}O, denser (1·07) than - either of the components, but unstable and easily decomposed, - generally simply referred to as a solution. Such also is the - crystalline compound of oxalic acid, C_{2}H_{2}O_{4}, with water, - C_{2}H_{2}O_{4},2H_{2}O. Their formation might be predicted as - starting from the hydrocarbon C_{2}H_{6}, in which, as in any - other, the hydrogen may be exchanged for chlorine, the water - residue (hydroxyl), &c. The first substitution product with - hydroxyl, C_{2}H_{5}(HO), is stable; it can be distilled without - alteration, resists a temperature higher than 100°, and then does - not give off water. This is ordinary alcohol. The second, - C_{2}H_{4}(HO)_{2}, can also be distilled without change, but can - be decomposed into water and C_{2}H_{4}O (ethylene oxide or - aldehyde); it boils at about 197°, whilst the first hydrate boils - at 78°, a difference of about 100°. The compound - C_{2}H_{3}(HO)_{3} will be the third product of such substitution; - it ought to boil at about 300°, but does not resist this - temperature--it decomposes into H_{2}O and C_{2}H_{4}O_{2}, where - only one hydroxyl group remains, and the other atom of oxygen is - left in the same condition as in ethylene oxide, C_{2}H_{4}O. - There is a proof of this. Glycol, C_{2}H_{4}(HO)_{2}, boils at - 197°, and forms water and ethylene oxide, which boils at 13° - (aldehyde, its isomeride, boils at 21°); therefore the product - disengaged by the splitting up of the hydrate boils at 184° lower - than the hydrate C_{2}H_{4}(HO)_{2}. Thus the hydrate - C_{2}H_{3}(HO)_{3}, which ought to boil at about 300°, splits up - in exactly the same way into water and the product - C_{2}H_{4}O_{2}, which boils at 117°--that is, nearly 183° lower - than the hydrate, C_{2}H_{3}(HO)_{3}. But this hydrate splits up - before distillation. The above-mentioned hydrate of acetic acid is - such a decomposable hydrate--that is to say, what is called a - solution. Still less stability may be expected from the following - hydrates. C_{2}H_{2}(HO)_{4} also splits up into water and a - hydrate (it contains two hydroxyl groups) called glycolic acid, - C_{2}H_{2}O(HO)_{2} = C_{2}H_{4}O_{3}. The next product of - substitution will be C_{2}H(HO)_{5}; it splits up into water, - H_{2}O, and glyoxylic acid, C_{2}H_{4}O_{4} (three hydroxyl - groups). The last hydrate which ought to be obtained from - C_{2}H_{6}, and ought to contain C_{2}(HO)_{6}, is the crystalline - compound of oxalic acid, C_{2}H_{2}O_{4} (two hydroxyl groups), - and water, 2H_{2}O, which has been already mentioned. The hydrate - C_{2}(HO)_{6} = C_{2}H_{2}O_{4},2H_{2}O, ought, according to the - foregoing reasoning, to boil at about 600° (because the hydrate, - C_{2}H_{4}(HO)_{2}, boils at about 200°, and the substitution of 4 - hydroxyl groups for 4 atoms of hydrogen will raise the - boiling-point 400°). It does not resist this temperature, but at a - much lower point splits up into water, 2H_{2}O, and the hydrate - C_{2}O_{2}(HO)_{2}, which is also capable of yielding water. - Without going into further discussion of this subject, it may be - observed that the formation of the hydrates, or compounds with - water of crystallisation, of acetic and oxalic acids has thus - received an accurate explanation, illustrating the point we - desired to prove in affirming that compounds with water of - crystallisation are held together by the same forces as those - which act in the formation of other complex substances, and that - the easy displaceability of the water of crystallisation is only a - peculiarity of a local character, and not a radical point of - distinction. All the above-mentioned hydrates, C_{2}X_{6}, or - products of their destruction, are actually obtained by the - oxidation of the first hydrate, C_{2}H_{3}(HO), or common alcohol, - by nitric acid (Sokoloff and others). Hence the forces which - induce salts to combine with _n_H_{2}O or with NH_{3} are - undoubtedly of the same order as the forces which govern the - formation of ordinary 'atomic' and saline compounds. (A great - impediment in the study of the former was caused by the conviction - which reigned in the sixties and seventies, that 'atomic' were - essentially different from 'molecular' compounds like - crystallohydrates, in which it was assumed that there was a - combination of entire molecules, as though without the - participation of the atomic forces.) If the bond between chlorine - and different metals is not equally strong, so also the bond - uniting _n_H_{2}O and _n_NH_{3} is exceeding variable; there is - nothing very surprising in this. And in the fact that the - combination of different amounts of NH_{3} and H_{2}O alters the - capacity of the haloids X of the salts RX_{2} for reaction (for - instance, in the luteo-salts all the X_{3}, while in the purpureo, - only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's - reacts), we should see in the first place a phenomenon similar to - what we met with in Cr_{2}Cl_{6} (Chapter XXI., Note 7 bis), for - in both instances the essence of the difference lies in the - removal of water; a molecule RCl_{3},6H_{2}O or RCl_{3},6NH_{3} - contains the halogen in a perfectly mobile (ionised) state, while - in the molecule RCl_{3},5H_{2}O or RCl_{3},5NH_{3} a portion of - the halogen has almost lost its faculty for reacting with - AgNO_{3}, just as metalepsical chlorine has lost this faculty - which is fully developed in the chloranhydride. Until the reason - of this difference be clear, we cannot expect that ordinary points - of view and generalisation can give a clear answer. However, we - may assume that here the explanation lies in the nature and kind - of motion of the atoms in the molecules, although as yet it is not - clear how. Nevertheless, I think it well to call attention again - (Chapter I.) to the fact that the combination of water, and hence, - also, of any other element, leads to most diverse consequences; - the water in the gelatinous hydrate of alumina or in the - decahydrated Glauber salt is very mobile, and easily reacts like - water in a free state; but the same water combined with oxide of - calcium, or C_{2}H_{4} (for instance, in C_{2}H_{6}O and in - C_{4}H_{10}O), or with P_{2}O_{5}, has become quite different, and - no longer acts like water in a free state. We see the same - phenomenon in many other cases--for example, the chlorine in - chlorates no longer gives a precipitate of chloride of silver with - AgNO_{3}. Thus, although the instance which is found in the - difference between the roseo- and purpureo-salts deserves to be - fully studied on account of its simplicity, still it is far from - being exceptional, and we cannot expect it to be thoroughly - explained unless a mass of similar instances, which are - exceedingly common among chemical compounds, be conjointly - explained. (Among the researches which add to our knowledge - respecting the complex ammoniacal compounds, I think it - indispensable to call the reader's attention to Prof. Kournakoff's - dissertation 'On complex metallic bases,' 1893.) - - Kournakoff (1894) showed that the solubility of the luteo-salt, - CoCl_{3},6NH_{3}, at 0° = 4·30 (per 100 of water), at 20° = 7·7, - that in passing into the roseo-salt, CoCl_{3}H_{2}O_{5}NH_{3}, the - solubility rises considerably, and at 0° = 16·4, and at 20° = - about 27, whilst the passage into the purpureo-salt, - CoCl_{3},5NH_{3}, is accompanied by a great fall in the - solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as - crystallohydrates with a smaller amount of water are usually more - soluble than the higher crystallohydrates (Le Chatelier), whilst - here we find that the solubility falls (in the purpureo-salt) with - a loss of water, that water which is contained in the roseo-salt - cannot be compared with the water of crystallisation. Kournakoff, - therefore, connects the fall in solubility (in the passage of the - roseo- into the purpureo-salts) with the accompanying loss in the - reactive capacity of the chlorine. - - In conclusion, it may be observed that the elements of the eighth - group--that is, the analogues of iron and platinum--according to - my opinion, will yield most fruitful results when studied as to - combinations with whole molecules, as already shown by the - examples of complex ammoniacal, cyanogen, nitro-, and other - compounds, which are easily formed in this eighth group, and are - remarkable for their stability. This faculty of the elements of - the eighth group for forming the complex compounds alluded to, is - in all probability connected with the position which the eighth - group occupies with regard to the others. Following the seventh, - which forms the type RX_{7}, it might be expected to contain the - most complex type, RX_{8}. This is met with in OsO_{4}. The other - elements of the eighth group, however, only form the lower types - RX_{2}, RX_{3}, RX_{4} ... and these accordingly should be - expected to aggregate themselves into the higher types, which is - accomplished in the formation of the above-mentioned complex - compounds. - - [35 bis] Marshall (1891) obtained cobaltic sulphate, - Co_{2}(SO_{4})_{3},18H_{2}O, by the action of an electric current - upon a strong solution of CoSO_{4}. - - [36] The action of an alkaline hypochlorite or hypobromite upon a - boiling solution of cobaltous salts, according to Schroederer - (1889), produces oxides, whose composition varies between - Co_{3}O_{5} (Rose's compound) and Co_{2}O_{3}, and also between - Co_{5}O_{8} and Co_{12}O_{19}. If caustic potash and then bromine - be added to the liquid, only Co_{2}O_{3} is formed. The action of - alkaline hypochlorites or hypo-bromites, or of iodine, upon - cobaltic salts, gives a highly-coloured precipitate which has a - different colour to the hydrate of the oxide Co_{2}(OH)_{6}. - According to Carnot the precipitate produced by the hypochlorites - has a composition Co_{10}O_{16}, whilst that given by iodine in - the presence of an alkali contains a larger amount of oxygen. - Fortmann (1891) reinvestigated the composition of the higher - oxygen oxide obtained by iodine in the presence of alkali, and - found that the greenish precipitate (which disengages oxygen when - heated to 100°) corresponds to the formula CoO_{2}. The reaction - must be expressed by the equation: CoX_{2} + I_{2} + 4KHO = - CoO_{2} + 2KX + 2KI + 2H_{2}O. - - [37] Prior to Fortmann, Rousseau (1889) endeavoured to solve the - question as to whether CoO_{2} was able to combine with bases. He - succeeded in obtaining a barium compound corresponding to this - oxide. Fifteen grams of BaCl_{2} or BaBr_{2} are triturated with - 5-6 grams of oxide of barium, and the mixture heated to redness in - a closed platinum crucible; 1 gram of oxide of cobalt is then - gradually added to the fused mass. Each addition of oxide is - accompanied by a violent disengagement of oxygen. After a short - time, however, the mass fuses quietly, and a salt settles at the - bottom of the crucible, which, when freed from the residue, - appears as black hexagonal, very brilliant crystals. In dissolving - in water this substance evolves chlorine; its composition - corresponds to the formula 2(CoO_{2})BaO. If the original mass be - heated for a long time (40 hours), the amount of dioxide in the - resultant mass decreases. The author obtained a neutral salt - having the composition CoO_{2}BaO (this compound = BaO_{2}CoO) by - breaking up the mass as it agglomerates together, and bringing the - pieces into contact with the more heated surface of the crucible. - This salt is formed between the somewhat narrow limits of - temperature 1,000°-1,100°; above and below these limits compounds - richer or poorer in CoO_{2} are formed. The formation of CoO_{2} - by the action of BaO_{2}, and the easy decomposition of CoO_{2} - with the evolution of oxygen, give reason for thinking that it - belongs to the class of peroxides (like Cr_{2}O_{7}, CaO_{2}, - &c.); it is not yet known whether they give peroxide of hydrogen - like the true peroxides. The fact that it is obtained by means of - iodine (probably through HIO), and its great resemblance to - MnO_{2}, leads rather to the supposition that CoO_{2} is a very - feeble saline oxide. The form CoO_{2} is repeated in the cobaltic - compounds (Note 35), and the existence of CoO_{2} should have long - ago been recognised upon this basis. - -Nickel alloys possess qualities which render them valuable for technical -purposes, the alloy of nickel with iron being particularly remarkable. -This alloy is met with in nature as _meteoric iron_. The Pallasoffsky -mass of meteoric iron, preserved in the St. Petersburg Academy, fell in -Siberia in the last century; it weighs about 15 cwt. and contains 88 p.c. -of iron and about 10 p.c. of nickel, with a small admixture of other -metals. In the arts _German silver_ is most extensively used; it is an -alloy containing nickel, copper, and zinc in various proportions. It -generally consists of about 50 parts of copper, 25 parts of zinc, and 25 -parts of nickel. This alloy is characterised by its white colour -resembling that of silver, and, like this latter metal, it does not rust, -and therefore furnishes an excellent substitute for silver in the -majority of cases where it is used. Alloys which contain silver in -addition to nickel show the properties of silver to a still greater -extent. Alloys of nickel are used for currency, and if rich deposits of -nickel are discovered a wide field of application lies before it, not -only in a pure state (because it is a beautiful metal and does not rust) -but also for use in alloys. Steel vessels (pressed or forged out of sheet -steel) covered with nickel have such practical merits that their -manufacture, which has not long commenced, will most probably be rapidly -developed, whilst nickel steel, which exceeds ordinary steel in its -tenacity, has already proved its excellent qualities for many purposes -(for instance, for armour plate). - -Until 1890 no compound of cobalt or nickel was known of sufficient -volatility to determine the molecular weights of the compounds of these -metals; but in 1890 Mr. L. Mond, in conducting (together with Langer and -Quincke) his researches on the action of nickel upon carbonic oxide -(Chapter IX., Note 24 bis), observed that nickel gradually volatilises in -a stream of carbonic oxide; this only takes place at low temperatures, -and is seen by the coloration of the flame of the carbonic oxide. This -observation led to the discovery of a remarkable volatile _compound of -nickel and carbonic oxide_, having as molecular composition -Ni(CO)_{4},[38] as determined by the vapour density and depression of the -freezing point. Cobalt and many other metals do not form volatile -compounds under these conditions, but iron gives a similar product (Note -26 bis). Ni(CO)_{4} is prepared by taking finely divided Ni (obtained by -reducing NiO by heating it in a stream of hydrogen, or by igniting the -oxalate NiC_{2}O_{4})[39] and passing (at a temperature below 50°, for -even at 60° decomposition may take place and an explosion) a stream of CO -over it; the latter carries over the vapour of the compound, which -condenses (in a well-cooled receiver) into a perfectly colourless -extremely mobile liquid, boiling without decomposition at 43°, and -crystallising in needles at -25° (Mond and Nasini, 1891). Liquid -Ni(CO)_{4} has a sp. gr. 1·356 at 0°, is insoluble in water, dissolves in -alcohol and benzene, and burns with a very smoky flame due to the -liberation of Ni. The vapour when passed through a tube heated to 180° -and above deposits a brilliant coating of metal, and disengages CO. If -the tube be strongly heated the decomposition is accompanied by an -explosion. If Ni(CO)_{4} as vapour be passed through a solution of -CuCl_{2}, it reduces the latter to metal; it has the same action upon an -ammoniacal solution of AgCl, strong nitric acid oxidises Ni(CO)_{4}, -dilute solutions of acids have no action; if the vapour be passed through -strong sulphuric acid, CO is liberated, chlorine gives NiCl and COCl_{2}; -no simple reactions of double decomposition are yet known for Ni(CO)_{4}, -however, so that its connection with other carbon compounds is not clear. -Probably the formation of this compound could be applied for extracting -nickel from its ores.[40] - - [38] This compound is known as nickel tetra-carbonyl. It appears to me - yet premature to judge of the structure of such an extraordinary - compound as Ni(CO)_{4}. It has long been known that potassium - combines with CO forming K_{_n_}(CO)_{_n_} (Chapter IX., Note 31), - but this substance is apparently saline and non-volatile, and has - as little in common with Ni(CO)_{4} as Na_{2}H has with SbH_{3}. - However, Berthelot observed that when NiC_{4}O_{4} is kept in air, - it oxidises and gives a colourless compound, - Ni_{3}C_{2}O_{3},10H_{2}O, having apparently saline properties. We - may add that Schützenberger, on reducing NiCl_{2} by heating it in - a current of hydrogen, observed that a nickel compound partially - volatilises with the HCl and gives metallic nickel when heated - again. The platinum compound, PtCl_{2}(CO)_{3} (Chapter XXIII., - Note 11), offers the greatest analogy to Ni(CO)_{4}. This compound - was obtained as a volatile substance by Schützenberger by - moderately heating (to 235°) metallic platinum in a mixture of - chlorine and carbonic oxide. If we designate CO by Y, and an atom - of chlorine by X, then taking into account that, according to the - periodic system, Ni is an analogue of Pt, a certain degree of - correspondence is seen in the composition NiY_{4} and - PtX_{2}Y_{2}. It would be interesting to compare the reactions of - the two compounds. - - [39] According to its empirical formula oxalate of nickel also contains - nickel and carbonic oxide. - - [40] The following are the thermo-chemical data (according to Thomsen, - and referred to gram weights expressed by the formula, in large - calories or thousand units of heat) for the formation of - corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies - that the reaction proceeds in an excess of water): - - R = Mn Fe Co Ni Cu - R + Cl_{2} + Aq 128 100 95 94 63 - R + Br_{2} + Aq 106 78 73 72 41 - R + I_{2} + Aq 76 48 43 41 32 - R + O + H_{2}O 95 68 63 61 38 - R + O_{2} + SO_{2} + _n_H_{2}O 193 169 163 163 130 - RCl_{2} + Aq +16 18 18 19 11 - - These examples show that for analogous reactions the amount of - heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in - regular sequences as the atomic weight increases. A similar - difference is to be found in other groups and series, and proves - that thermo-chemical phenomena are subject to the periodic law. - - - - - CHAPTER XXIII - - THE PLATINUM METALS - - -The six metals: ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium, Os, -iridium, Ir, and platinum, Pt, are met with associated together in -nature. Platinum always predominates over the others, and hence they are -known as the _platinum metals_. By their chemical character their -position in the periodic system is in the eighth group, corresponding -with iron, cobalt, and nickel. - -The natural transition from titanium and vanadium to copper and zinc by -means of the elements of the iron group is demonstrated by all the -properties of these elements, and in exactly the same manner a transition -from zirconium, niobium, and molybdenum to silver, cadmium, and indium, -through ruthenium, rhodium, and palladium, is in perfect accordance with -fact and with the magnitude of the atomic weights, as also is the -position of osmium, iridium, and platinum between tantalum and tungsten -on the one side, and gold and mercury on the other. In all these three -cases the elements of smaller atomic weight (chromium, molybdenum, and -tungsten) are able, in their higher grades of oxidation, to give acid -oxides having the properties of distinct but feebly energetic acids (in -the lower oxides they give bases), whilst the elements of greater atomic -weight (zinc, cadmium, mercury), even in their higher grades of -oxidation, only give bases, although with feebly developed basic -properties. The platinum metals present the same intermediate properties -such as we have already seen in iron and the elements of the eighth -group. - -In the platinum metals the intermediate properties _of feebly acid and -feebly basic metals_ are developed with great clearness, so that there is -not one sharply-defined acid anhydride among their oxides, although there -is a great diversity in the grades of oxidation from the type RO_{4} to -R_{2}O. The feebleness of the chemical forces observed in the platinum -metals is connected with the ready decomposability of their compounds, -with the small atomic volume of the metals themselves, and with their -large atomic weight. The oxides of platinum, iridium, and osmium can -scarcely be termed either basic or acid; they are capable of combinations -of both kinds, each of which is feeble. They are all intermediate oxides. - -The atomic weights of platinum, iridium, and osmium are nearly 191 to -196, and of palladium, rhodium, and ruthenium, 104 to 106. Thus, strictly -speaking, we have here two series of metals, which are, moreover, -perfectly parallel to each other; three members in the first series, and -three members in the second--namely, platinum presents an analogy to -palladium, iridium to rhodium, and osmium to ruthenium. As a matter of -fact, however, the whole _group_ of the platinum metals is characterised -by _a number of common properties_, both physical and chemical, and, -moreover, there are several points of resemblance between the members of -this group and those of the _iron_ group (Chapter XXII.) The atomic -volumes (Table III., column 18) of the elements of this group are _nearly -equal_ and _very small_. The iron metals have atomic volumes of nearly 7, -whilst that of the metals allied to palladium is nearly 9, and of those -adjacent to platinum (Pt, Ir, Os) nearly 9·4. This comparatively small -atomic volume corresponds with the great infusibility and tenacity proper -to all the iron and platinum metals, and to their small chemical energy, -which stands out very clearly in the heavy platinum metals. All the -platinum metals are very _easily reduced_ by ignition and by the action -of various reducing agents, in which process oxygen, or a haloid group, -is disengaged from their compounds and the metal left behind. This is a -property of the platinum metals which determines many of their reactions, -and the circumstance of their always being found in nature _in a native -state_. In Russia in the Urals (discovered in 1819) and in Brazil (1735) -platinum is obtained from alluvial deposits, but in 1892 Professor -Inostrantseff discovered a vein deposit of platinum in serpentine near -Tagil in the Urals.[1] The facility with which they are reduced is so -great that their chlorides are even decomposed by gaseous hydrogen, -especially when shaken up and heated under a certain pressure. Hence it -will be readily understood that such metals as zinc, iron, &c., separate -them from solutions with great ease, which fact is taken advantage of in -practice and in the chemical treatment of the platinum metals.[1 bis] - - [1] Wells and Penfield (1888) have described a mineral sperryllite - found in the Canadian gold-bearing quartz and consisting of - platinum diarsenide, PtAs_{2}. It is a noticeable fact that this - mineral clearly confirms the position of platinum in the same group - as iron, because it corresponds in crystalline form (regular - octahedron) and chemical composition with iron pyrites, FeS_{2}. - - [1 bis] Some light is thrown upon the facility with which the platinum - compounds decompose by Thomsen's data, showing that in an excess of - water (+ Aq) the formation from platinum, of such a double salt as - PtCl_{2},2KCl, is accompanied by a comparatively small evolution of - heat (_see_ Chapter XXI., Note 40), for instance, Pt + Cl_{2} + - 2KCl + Aq only evolves about 33,000 calories (hence the reaction, - Pt + Cl_{2} + Aq, will evidently disengage still less, because - PtCl_{2} + 2KCl evolves a certain amount of heat), whilst on the - other hand, Fe + Cl_{2} + Aq gives 100,000 calories, and even the - reaction with copper (for the formation of the double salt) evolves - 63,000 calories. - -All the platinum metals, like those of the iron group, are grey, with a -comparatively feeble metallic lustre, and are very infusible. In this -respect they stand in the same order as the metals of the iron series; -nickel is more fusible and whiter than cobalt and iron, so also palladium -is whiter and more fusible than rhodium and ruthenium, and platinum is -comparatively more fusible and whiter than iridium or osmium. The saline -compounds of these metals are red or yellow, like those of the majority -of the metals of the iron series, and like the latter, the different -forms of oxidation present different colours. Moreover, certain complex -compounds of the platinum metals, like certain complex compounds of the -iron series, either have particular characteristic tints or else are -colourless. - -The platinum metals are found _in nature associated together in alluvial -deposits_ in a few localities, from which they are washed, owing to their -very considerable density, which enables a stream of water to wash away -the sand and clay with which they are mixed. Platinum deposits are -chiefly known in the Urals, and also in Brazil and a few other -localities. The platinum ore washed from these alluvial deposits presents -the appearance of more or less coarse grains, and sometimes, as it were, -of semi-fused nuggets.[2] - - [2] The largest amount of platinum is extracted in the Urals, about - five tons annually. A certain amount of gold is extracted from the - washed platinum by means of mercury, which does not dissolve the - platinum metals but dissolves the gold accompanying the platinum in - its ores. Moreover, the ores of platinum always contain metals of - the iron series associated with them. The washed and mechanically - sorted ore in the majority of cases contains about 70 to 80 p.c. of - platinum, about 5 to 8 p.c. of iridium, and a somewhat smaller - quantity of osmium. The other platinum metals--palladium, rhodium, - and ruthenium--occur in smaller proportions than the three above - named. Sometimes grains of almost pure osmium-iridium, containing - only a small quantity of other metals, are found in platinum ores. - This _osmium-iridium_ may be easily separated from the other - platinum metals, owing to its being nearly insoluble in aqua regia, - by which the latter are easily dissolved. There are grains of - platinum which are magnetic. The grains of osmium-iridium are very - hard and malleable, and are therefore used for certain purposes, - for instance, for the tips of gold pens. - -All the platinum metals give compounds with the halogens, and the -highest haloid type of combination for all is RX_{4}. For the majority of -the platinum metals this type is exceedingly unstable; the lower -compounds corresponding to the type RX_{2}, which are formed by the -separation of X_{2}, are more stable. In the type RX_{2} the platinum -metals form more stable salts, which offer no little resemblance to the -kindred compounds of the iron series--for example, to nickelous chloride, -NiCl_{2}, cobaltous chloride, CoCl_{2}, &c. This even expresses itself in -a similarity of volume (platinous chloride, PtCl_{2}, volume, 46; -nickelous chloride, NiCl_{2} = 50), although in the type RX_{2} the true -iron metals give very stable compounds, whilst the platinum metals -frequently react after the manner of suboxides, decomposing into the -metal and higher types, 2RX_{2} = R + RX_{4}. This probably depends on -the facility with which RX_{2} decomposes into R and X_{2}, when X_{2} -combines with the remaining portion of RX_{2}. - -As in the series iron, cobalt, nickel, nickel gives NiO and Ni_{2}O_{3}, -whilst cobalt and iron give higher and varied forms of oxidation, so also -among the platinum metals, platinum and palladium only give the forms -RX_{2} and RX_{4}, whilst rhodium and iridium form another and -intermediate type, RX_{3}, also met with in cobalt, corresponding with -the oxide, having the composition R_{2}O_{3}, besides which they form an -acid oxide, like ferric acid, which is also known in the form of salts, -but is in every respect unstable. _Osmium_ and _ruthenium_, like -manganese, form still higher oxides, and in this respect exhibit the -greatest diversity. They not only give RX_{2}, RX_{3}, RX_{4}, and -RX_{6}, but also a still _higher form of oxidation_, RO_{4}, which is not -met with in any other series. This form is exceedingly characteristic, -owing to the fact that the oxides, OsO_{4} and RuO_{4}, are volatile and -have feebly acid properties. In this respect they most resemble -permanganic anhydride, which is also somewhat volatile.[3] - - [3] In characterising the platinum metals according to their relation - to the iron metals, it is very important to add two more very - remarkable points. The platinum metals are capable of forming a - sort of unstable compound with _hydrogen_; they absorb it and only - part with it when somewhat strongly heated. This faculty is - especially developed in platinum and palladium, and it is very - characteristic that nickel, which exactly corresponds with platinum - and palladium in the periodic system, should exhibit the same - faculty for retaining a considerable quantity of hydrogen (Graham's - and Raoult's experiments). Another characteristic property of the - platinum metals consists in their easily giving (like cobalt which - forms the cobaltic salts) stable and characteristic saline - _compounds with ammonia_, and like Fe and Co, double salts with the - cyanides of the alkali metals, especially in their lower forms of - combination. All the above so clearly brings the elements of the - iron series in close relation to the platinum metals, that the - eighth group acquires as natural a character as can be required, - with a certain originality or individuality for each element. - -When dissolved in aqua regia (PtCl_{4} is formed) and liberated from the -solution by sal-ammoniac ((NH_{4})_{2}PtCl_{6} is formed) and reduced by -ignition (which may be done by Zn and other reducing agents, direct from -a solution of PtCl_{4}) platinum[3 bis] forms a powdery mass, known as -spongy platinum or platinum black. If this powder of platinum be heated -and pressed, or hammered in a cylinder, the grains aggregate or forge -together, and form a continuous, though of course not entirely -homogeneous, mass. Platinum was formerly, and is even now, worked up in -this manner. The platinum money formerly used in Russia was made in this -way. Sainte-Claire Deville, in the fifties, for the first time melted -platinum in considerable quantities by employing a special furnace made -in the form of a small reverberatory furnace, and composed of two pieces -of lime, on which the heat of the oxyhydrogen flame has no action. Into -this furnace (shown in fig. 34, Vol. I. p. 175)--or, more strictly -speaking, into the cavity made in the pieces of lime--the platinum is -introduced, and two orifices are made in the lime; through one, the -upper, or side orifice, is introduced an oxyhydrogen gas burner, in which -either detonating gas or a mixture of oxygen and coal-gas is burnt, -whilst the other orifice serves for the escape of the products of -combustion and certain impurities which are more volatile than the -platinum, and especially the oxidised compounds of osmium, ruthenium, and -palladium, which are comparatively easily volatilised by heat. In this -manner the platinum is converted into a continuous metallic form by means -of fusion, and this method is now used for melting considerable masses of -platinum[4] and its alloys with iridium. - - [3 bis] Platinum was first obtained in the last century from Brazil, - where it was called silver (platinus). Watson in 1750 characterised - platinum as a separate independent metal. In 1803 Wollaston - discovered palladium and rhodium in crude platinum, and at about - the same time Tennant distinguished iridium and osmium in it. - Professor Claus, of Kazan, in his researches on the platinum metals - (about 1840) discovered ruthenium in them, and to him are due many - important discoveries with regard to these elements, such as the - indication of the remarkable analogy between the series Pd--Rh--Ru - and Pt--Ir--Os. - - _The treatment of platinum ore_ is chiefly carried on for the - extraction of the platinum itself and its alloys with iridium, - because these metals offer a greater resistance to the action of - chemical reagents and high temperatures than any of the other - malleable and ductile metals, and therefore the wire so often used - in the laboratory and for technical purposes is made from them, as - also are various vessels used for chemical purposes in the - laboratory and in works. Thus sulphuric acid is distilled in - platinum retorts, and many substances are fused, ignited, and - evaporated in the laboratory in platinum crucibles and on platinum - foil. Gold and many other substances are dissolved in dishes made - of iridium-platinum, because the alloys of platinum and iridium are - but slightly attacked when subjected to the action of aqua regia. - - The comparatively high density (about 21·5), hardness, ductility, - and infusibility (it does not melt at a furnace heat, but only in - the oxyhydrogen flame or electric furnace), as well as the fact of - its resisting the action of water, air, and other reagents, renders - an alloy of 90 parts of platinum and 10 parts of iridium (Deville's - platinum-iridium alloy) a most valuable material for making - standard weights and measures, such as the metre, kilogram, and - pound, and therefore all the newest standards of most countries are - made of this alloy. - - [4] This process has altered the technical treatment of platinum to a - considerable extent. It has in particular facilitated the - manufacture of alloys of platinum with iridium and rhodium from the - pure platinum ores, since it is sufficient to fuse the ore in order - for the greater amount of the osmium to burn off, and for the mass - to fuse into a homogeneous, malleable alloy, which can be directly - made use of. There is very little ruthenium in the ores of - platinum. If during fusion lead be added, it dissolves the platinum - (and other platinum metals) owing to its being able to form a very - characteristic alloy containing PtPb. If an alloy of the two metals - be left exposed to moist air, the excess of lead is converted into - carbonate (white lead) in the presence of the water and carbonic - acid of the air, whilst the above platinum alloy remains unchanged. - The white lead may be extracted by dilute acid, and the alloy PtPb - remains unaltered. The other platinum metals also give similar - alloys with lead. The fusibility of these alloys enables the - platinum metals to be separated from the gangue of the ore, and - they may afterwards be separated from the lead by subjecting the - alloy to oxidation in furnaces furnished with a bone ash bed, - because the lead is then oxidised and absorbed by the bone ash, - leaving the platinum metals untouched. This method of treatment was - proposed by H. Sainte-Claire Deville in the sixties, and is also - used in the analysis of these metals (_see_ further on). - -To obtain pure platinum, the ore is treated with aqua regia in which -only the osmium and iridium are insoluble. The solution contains the -platinum metals in the form RCl_{4}, and in the lower forms of -chlorination, RCl_{3} and RCl_{2}, because some of these metals--for -instance, palladium and rhodium--form such unstable chlorides of the type -RX_{4} that they partially decompose even when diluted with water, and -pass into the stable lower type of combination; in addition to which the -chlorine is very easily disengaged if it comes in contact with substances -on which it can act. In this respect platinum resists the action of heat -and reducing agents better than any of its companions--that is, it passes -with greater difficulty from PtCl_{4} to the lower compound PtCl_{2}. On -this is based the method of preparation of more or less pure platinum. -Lime or sodium hydroxide is added to the solution in aqua regia until -neutralised, or only containing a very slight excess of alkali. It is -best to first evaporate and slightly ignite the solution, in order to -remove the excess of acid, and by heating it to partially convert the -higher chlorides of the palladium, &c., into the lower. The addition of -alkalis completes the reduction, because the chlorine held in the -compounds RX_{4} acts on the alkali like free chlorine, converting it -into a hypochlorite. Thus palladium chloride, PdCl_{4}, for example, is -converted into palladious chloride, PdCl_{2}, by this means, according to -the equation PdCl_{4} + 2NaHO = PdCl_{2} + NaCl + NaClO + H_{2}O. In a -similar manner iridic chloride, IrCl_{4}, is converted into the -trichloride, IrCl_{3}, by this method. When this conversion takes place -the platinum still remains in the form of platinic chloride, PtCl_{4}. It -is then possible to take advantage of a certain difference in the -properties of the higher and lower chlorides of the platinum metals. Thus -lime precipitates the lower chlorides of the members of the platinum -metals occurring in solution without acting on the platinic chloride, -PtCl_{4}, and hence the addition of a large proportion of lime -immediately precipitates the associated metals, leaving the platinum -itself in solution in the form of a soluble double salt, -PtCl_{4},CaCl_{2}. A far better and more perfect _separation_ is effected -_by means of ammonium chloride_, which gives, with platinic chloride, an -insoluble yellow precipitate, PtCl_{4},2NH_{4}Cl, whilst it forms soluble -double salts with the lower chlorides RCl_{2} and RCl_{3}, so that -ammonium chloride precipitates the platinum only from the solution -obtained by the preceding method. These methods are employed for -preparing the platinum which is used for the manufacture of platinum -articles, because, having platinum in solution as calcium -platinochloride, PtCaCl_{6}, or as the insoluble ammonium -platinochloride, Pt(NH_{4})_{2}Cl_{6}, the platinum compound in every -case, after drying or ignition, loses all the chlorine from the platinic -chloride and leaves finely-divided metallic platinum, which may be -converted into homogeneous metal by compression and forging, or by -fusion.[5] - - [5] For the ultimate purification of platinum from palladium and - iridium the metals must be re-dissolved in aqua regia, and the - solution evaporated until the residue begins to evolve chlorine. - The residue is then re-precipitated with ammonium or potassium - chloride. The precipitate may still contain a certain amount of - iridium, which passes with greater difficulty from the - tetrachloride, IrCl_{4}, into the trichloride, IrCl_{3}, but it - will be quite free from palladium, because the latter easily loses - its chlorine and passes into palladious chloride, PdCl_{2}, which - gives an easily-soluble salt with potassium chloride. The - precipitate, containing a small quantity of iridium, is then heated - with sodium carbonate in a crucible, when the mass decomposes, - giving metallic platinum and iridium oxide. If potassium chloride - has been employed, the residue after ignition is washed with water - and treated with aqua regia. The iridium oxide remains undissolved, - and the platinum easily passes into solution. Only cold and dilute - aqua regia must be used. The solution will then contain pure - platinic chloride, which forms the starting-point for the - preparation of all platinum compounds. Pure platinum for accurate - researches (for instance, for the unit of light, according to - Violle's method) may be obtained (Mylius and Foerster, 1892) by - Finkener's method, by dissolving the impure metal in aqua regia (it - should be evaporated to drive off the nitrogen compounds), and - adding NaCl so as to form a double sodium salt, which is purified - by crystallising with a small amount of caustic soda, washing the - crystals with a strong solution of NaCl, and then dissolving them - in a hot 1 p.c. solution of soda, repeating the above and - ultimately igniting the double salt, previously dried at 120°, in a - stream of hydrogen; platinum black and NaCl are then formed. The - three following are very sensitive tests (to thousandths of a per - cent.) for the presence of Ir, Ru, Rh, Pd (osmium is not usually - present in platinum which has once been purified, since it easily - volatilises with Cl_{2} and CO_{2}, and in the first treatment of - the crude platinum either passes off as OsO_{4} or remains - undissolved), Fe, Cu, Ag, and Pb: (1) the assay is alloyed with 10 - parts of pure lead, the alloy treated with dilute nitric acid (to - remove the greater part of the Pb), and dissolved in aqua regia; - the residue will consist of Ir and Ru; the Pb is precipitated from - the nitric acid solution by sulphuric acid, whilst the remaining - platinum metals are reduced from the evaporated solution by formic - acid, and the resultant precipitate fused with KHSO_{4}; the Pd and - Rh are thus converted into soluble salts, and the former is then - precipitated by HgC_{2}N_{2}. (2) Iron may be detected by the usual - reagents, if the crude platinum be dissolved in aqua regia, and the - platinum metals precipitated from the solution by formic acid. (3) - If crude platinum (as foil or sponge) be heated in a mixture of - chlorine and carbonic oxide it volatilises (with a certain amount - of Ir, Pd, Fe, &c.) as PtCl_{2},2CO (Note 11), whilst the whole of - the Rh, Ag, and Cu it may contain remains behind. Among other - characteristic reactions for the platinum metals, we may mention: - (1) that rhodium is precipitated from the solution obtained after - fusion with KHSO_{4} (in which Pt does not dissolve) by NH_{3}, - acetic and formic acids; (2) that dilute aqua regia dissolves - precipitated Pt, but not Rh; (3) that if the insoluble residue of - the platinum metals (Ir, Ru, Os) obtained, after treating with aqua - regia, be fused with a mixture of 1 part of KNO_{3} and 3 parts of - K_{2}CO_{3} (in a gold crucible), and then treated with water, it - gives a solution containing the Ru (and a portion of the Ir), but - which throws it all down when saturated with chlorine and boiled; - (4) that if iridium be fused with a mixture of KHO and KNO_{3}, it - gives a soluble potassium salt, IrK_{2}O_{4} (the solution is - blue), which, when saturated with chlorine, gives IrCl_{4}, which - is precipitated by NH_{4}Cl (the precipitate is black), forming a - double salt, leaving metallic Ir after ignition; (5) that rhodium - mixed with NaCl and ignited in a current of chlorine gives a - soluble double salt (from which sal-ammoniac separates Pt and Ir), - which gives (according to Jörgensen) a difficultly soluble - purpureo-salt (Chapter XXII., Note 35), Rh_{2}Cl_{3},5NH_{3}, when - treated with NH_{3}; in this form the Rh may be easily purified and - obtained in a metallic form by igniting in hydrogen; and (6) that - palladium, dissolved in aqua regia and dried (NH_{4}Cl throws down - any Pt), gives soluble PdCl_{2}, which forms an easily - crystallisable yellow salt, PdCl_{2}NH_{3}, with ammonia; this salt - (Wilm) may be easily purified by crystallisation, and gives - metallic Pd when ignited. These reactions illustrate the method of - separating the platinum metals from each other. - -Metallic _platinum_ in a fused state has a specific gravity of 21; it is -grey, softer than iron but harder than copper, exceedingly ductile, and -therefore easily drawn into wire and rolled into thin sheets, and may be -hammered into crucibles and drawn into thin tubes, &c. In the state in -which it is obtained by the ignition of its compounds, it forms a spongy -mass, known as spongy platinum, or else as powder (platinum black).[6] In -either case it is dull grey, and is characterised, as we already know, by -the faculty of absorbing hydrogen and other gases. Platinum is not acted -on by hydrochloric, hydriodic, nitric, and sulphuric acids, or a mixture -of hydrofluoric and nitric acids. Aqua regia, and any liquid containing -chlorine or able to evolve chlorine or bromine, dissolves platinum. -Alkalis are decomposed by platinum at a red heat, owing to the faculty of -the platinum oxide, PtO_{2}, formed to combine with alkaline bases, -inasmuch as it has a feebly-developed acid character (_see_ Note 8). -Sulphur, phosphorus (the phosphide, PtP_{2}, is formed), arsenic and -silicon all act more or less rapidly on platinum, under the influence of -heat. Many of the metals form alloys with it. Even charcoal combines with -platinum when it is ignited with it, and therefore carbonaceous matter -cannot be subjected to prolonged and powerful ignition in platinum -vessels. Hence a platinum crucible soon becomes dull on the surface in a -smoky flame. Platinum also forms alloys with zinc, lead, tin, copper, -gold, and silver.[7] Although mercury does not directly dissolve -platinum, still it forms a solution or amalgam with spongy platinum in -the presence of sodium amalgam; a similar amalgam is also formed by the -action of sodium amalgam on a solution of platinum chloride, and is used -for physical experiments. - - [6] We have already become acquainted with the effect of finely-divided - platinum on many gaseous substances. It is best seen in the - so-called _platinum black_, which is a coal-black powder left by - the action of sulphuric acid on the alloy of zinc and platinum, or - which is precipitated by metallic zinc from a dilute solution of - platinum. In any case, finely-divided platinum absorbs gases more - powerfully and rapidly the more finely divided and porous it is. - Sulphurous anhydride, hydrogen, alcohol, and many organic - substances in the presence of such platinum are easily oxidised by - the oxygen of the air, although they do not combine with it - directly. The absorption of oxygen is as much as several hundred - volumes per one volume of platinum, and the oxidising power of such - absorbed oxygen is taken advantage of not only in the laboratory - but even in manufacturing processes. Asbestos or charcoal, soaked - in a solution of platinic chloride and ignited, is very useful for - this purpose, because by this means it becomes coated with platinum - black. If 50 grams of PtCl_{4} be dissolved in 60 c.c. of water, - and 70 c.c. of a strong (40 p.c.) solution of formic aldehyde - added, the mixture cooled, and then a solution of 50 grams of NaHO - in 50 grams of water added, the platinum is precipitated. After - washing with water the precipitate passes into solution and forms a - black liquid containing _soluble colloidal platinum_ (Loew, 1890). - If the precipitated platinum be allowed to absorb oxygen on the - filter, the temperature rises 40°, and a very porous _platinum - black_ is obtained which vigorously facilitates oxidation. - - [7] It is necessary to remark that platinum when alloyed with silver, - or as amalgam, is soluble in nitric acid, and in this respect it - differs from gold, so that it is possible, by alloying gold with - silver, and acting on the alloy with nitric acid, to recognise the - presence of platinum in the gold, because nitric acid does not act - on gold alloyed with silver. - -There are _two kinds_ of _platinum compounds_, PtX_{4} and PtX_{2}. The -former are produced by an excess of halogen in the cold, and the latter -by the aid of heat or by the splitting up of the former. The -starting-point for the platinum compounds is _platinum tetrachloride_, -_platinic chloride_, PtCl_{4}, obtained by dissolving platinum in aqua -regia.[7 bis] The solution crystallises in the cold, in a desiccator, in -the form of reddish-brown deliquescent crystals which contain -hydrochloric acid, PtCl_{4},2HCl,6H_{2}O, and behave like a true acid -whose salts correspond to the formula R_{2}PtCl_{6}--ammonium -platinochloride, for example.[7 tri] The hydrochloric acid is liberated -from these crystals by gently heating or evaporating the solution to -dryness; or, better still, after treatment with silver nitrate a -reddish-brown mass remains behind, which dissolves in water, and forms a -yellowish-red solution which on cooling deposits crystals of the -composition PtCl_{4},8H_{2}O. The _tendency_ of PtCl_{4} _to combine_ -with hydrochloric acid and water--that is, _to form higher crystalline -compounds_--is evident in the platinum compounds, and must be taken into -account in explaining the properties of platinum and the formation of -many other of its complex compounds. Dilute solutions of platinic -chloride are yellow, and are completely reduced by hydrogen, sulphurous -anhydride, and many reducing agents, which first convert the platinic -chloride into the lower compound platinous chloride, PtCl_{2}. That -faculty which reveals itself in platinum tetrachloride of combining with -water of crystallisation and hydrochloric acid is distinctly marked in -its property, with which we are already acquainted, of giving -precipitates with the salts of potassium, ammonium, rubidium, &c. In -general it _readily forms double salts_, R_{2}PtCl_{6} = PtCl_{4} + 2RCl, -where R is a univalent metal such as potassium or NH_{4}. Hence the -addition of a solution of potassium or ammonium chloride to a solution of -platinic chloride is followed by the formation of a yellow precipitate, -which is sparingly soluble in water and almost entirely insoluble in -alcohol and ether (platinic chloride is soluble in alcohol, potassium -iridiochloride, IrK_{3}Cl_{6}, _i.e._ a compound of IrCl_{3}, is soluble -in water but not in alcohol). It is especially remarkable in this case, -that the potassium compounds here, as in a number of other instances, -separate in an anhydrous form, whilst the sodium compounds, which are -soluble in water and alcohol, form red crystals containing water. The -composition Na_{2}PtCl_{6},6H_{2}O exactly corresponds with the -above-mentioned hydrochloric compound. The compounds with barium, -BaPtCl_{6},4H_{2}O, strontium, SrPtCl_{6},8H_{2}O, calcium, magnesium, -iron, manganese, and many other metals are all soluble in water.[8] - - [7 bis] PtCl_{4} is also formed by the action of a mixture of HCl - vapour and air, and by the action of gaseous chlorine upon - platinum. - - [7 tri] Pigeon (1891) obtained fine yellow crystals of - PtH_{2}Cl_{6},4H_{2}O by adding strong sulphuric acid to a strong - solution of PtH_{2}Cl_{6},6H_{2}O. If crystals of - H_{2}PtCl_{6},6H_{2}O be melted in vacuo (60°) in the presence of - anhydrous potash, a red-brown solid hydrate is obtained containing - less water and HCl, which parts with the remainder at 200°, leaving - anhydrous PtCl_{4}. The latter does not disengage chlorine before - 220°, and is perfectly soluble in water. - - [8] Nilson (1877), who investigated the platinochlorides of various - metals subsequently to Bonsdorff, Topsöe, Clève, Marignac, and - others, found that univalent and bivalent metals--such as hydrogen, - potassium, ammonium ... beryllium, calcium, barium--give compounds - of such a composition that there is always twice as much chlorine - in the platinic chloride as in the combined metallic chloride; for - example, K_{2}Cl_{2},PtCl_{4}; BeCl_{2},PtCl_{4},8H_{2}O, &c. Such - trivalent metals as aluminium, iron (ferric), chromium, didymium, - cerium (cerous) form compounds of the type RCl_{3}PtCl_{4}, in - which the amounts of chlorine are in the ratio 3:4. Only indium and - yttrium give salts of a different composition--namely, - 2InCl_{3},5PtCl_{4},36H_{2}O and 4YCl_{3},5PtCl_{4},51H_{2}O. Such - quadrivalent metals as thorium, tin, zirconium give compounds of - the type RCl_{4},PtCl_{4}, in which the ratio of the chlorine is - 1:1. In this manner the valency of a metal may, to a certain - extent, be judged from the composition of the double salts formed - with platinic chloride. - - Platinic bromide, PtBr_{4}, and iodide, PtI_{4}, are analogous to - the tetrachloride, but the iodide is decomposed still more easily - than the chloride. If sulphuric acid be added to platinic chloride, - and the solution evaporated, it forms a black porous mass like - charcoal, which deliquesces in the air, and has the composition - Pt(SO_{4})_{2}. But this, the only oxygen salt of the type PtX_{4}, - is exceedingly unstable. This is due to the fact that _platinum - oxide_, the oxide of the type PtO_{2}, has a feeble acid character. - This is shown in a number of instances. Thus if a strong solution - of platinic chloride treated with sodium carbonate be exposed to - the action of light or evaporated to dryness and then washed with - water, a sodium platinate, Pt_{3}Na_{2}O_{7},6H_{2}O, remains. The - composition of this salt, if we regard it in the same sense as we - did the salts of silicic, titanic, molybdic and other acids, will - be PtO(ONa)_{2},2PtO_{2},6H_{2}O--that is, the same type is - repeated as we saw in the crystalline compounds of platinum - tetrachloride with sodium chloride, or with hydrochloric - acid--namely, the type PtX_{4}8Y, where Y is the molecule - H_{2}O,HCl, &c. Similar compounds are also obtained with other - alkalis. They will be platinates of the alkalis in which the - platinic oxide, PtO_{2}, plays the part of an acid oxide. Rousseau - (1889) obtained different grades of combination BaOPtO_{2}, - 3(BaO)2PtO_{2}, &c., by igniting a mixture of PtCl_{4} and caustic - baryta. If such an alkaline compound of platinum be treated with - acetic acid, the alkali combines with the latter, and a _platinic - hydroxide_, Pt(OH)_{4}, remains as a brown mass, which loses water - and oxygen when ignited, and in so doing decomposes with a slight - explosion. When slightly ignited this hydroxide first loses water - and gives the very unstable oxide PtO_{2}. Platinic sulphide, - PtS_{2}, belongs to the same type; it is precipitated by the action - of sulphuretted hydrogen on a solution of platinum tetrachloride. - The moist precipitate is capable of attracting oxygen, and is then - converted into the sulphate above mentioned, which is soluble in - water. This absorption of oxygen and conversion into sulphate is - another illustration of the basic nature of PtO_{2}, so that it - clearly exhibits both basic and acid properties. The latter appear, - for instance, in the fact that platinic sulphide, PtS_{2}, gives - crystalline compounds with the alkali sulphides. - -_Platinous chloride_, PtCl_{2}, is formed when hydrogen platinochloride, -PtH_{2}Cl_{6}, is ignited at 300°, or when potassium is heated at 230° in -a stream of chlorine. The undecomposed tetrachloride is extracted from -the residue by washing it with water, and a greenish-grey or brown -insoluble mass of the dichloride (sp. gr. 5·9) is then obtained. It is -soluble in hydrochloric acid, giving an acid solution of the composition -PtCl_{2},2HCl, corresponding with the type of double salts PtR_{2}Cl_{4}. -Although platinous chloride decomposes below 500°, still it is formed to -a small extent at higher temperatures. Troost and Hautefeuille, and -Seelheim observed that when platinum was strongly ignited in a stream of -chlorine, the metal, as it were, slowly volatilised and was deposited in -crystals; a volatile chloride, probably platinous chloride, was evidently -formed in this case, and decomposed subsequently to its formation, -depositing crystals of platinum. - -The properties of platinum above-described are repeated more or -less distinctly, or sometimes with certain modifications, in the -above-mentioned associates and analogues of this metal. Thus although -palladium forms PdCl_{4}, this form passes into PdCl_{2} with extreme -ease.[9] Whilst rhodium and iridium in dissolving in aqua regia also form -RhCl_{4} and IrCl_{4}, but they pass into RhCl_{3} and IrCl_{3}[9 bis] -very easily when heated or when acted upon by substances capable of -taking up chlorine (even alkalis, which form bleaching salts). Among the -platinum metals, ruthenium and osmium have the most acid character, and -although they give RuCl_{4} and OsCl_{4} they are easily oxidised to -RuO_{4}, and OsO_{4} by the action of chlorine in the presence of water; -the latter are volatile and may be distilled with the water and -hydrochloric acid, from a solution containing other platinum metals.[9 -tri] Thus with respect to the types of combination, all the platinum -metals, under certain circumstances, give compounds of the type -RX_{4}--for instance, RO_{2}, RCl_{4}, &c. But this is the highest form -for only platinum and palladium. The remaining platinum metals further, -_like iron, give acids_ of the type RO_{3} or hydrates, H_{2}RO_{4} = -RO_{2}(HO)_{2} (the type of sulphuric acid); but they, like ferric and -manganic acids, are chiefly known in the form of salts of the composition -K_{2}RO_{4} or K_{2}R_{2}O_{7} (like the dichromate). These salts are -obtained, like the manganates and ferrates, by fusing the oxides, or even -the metals themselves, with nitric, or, better still, with potassium -peroxide. They are soluble in water, are easily deoxidised and do not -yield the acid anhydrides under the action of acids, but break up, either -(like the ferrate) forming oxygen and a basic oxide (iridium and rhodium -react in this manner, as they do not give higher forms of oxidation), or -passing into a lower and higher form of oxidation--that is, reacting like -a manganate (or partly like nitrite or phosphite). Osmium and ruthenium -react according to the latter form, as they are capable of giving _higher -forms of oxidation_, OsO_{4} and RuO_{4}, and therefore their reactions -of decomposition may be essentially represented by the equation: 2OsO_{3} -= OsO_{2} + OsO_{4}.[10] - - [9] In comparing the characteristics of the platinum metals, it must be - observed that palladium in its form of combination PdX_{2} gives - saline compounds of considerable stability. Amongst them _palladous - chloride_ is formed by the direct action of chlorine or aqua regia - (not in excess or in dilute solutions) on palladium. It forms a - brown solution, which gives a black insoluble precipitate of - _palladous iodide_, PdI_{2}, with solutions of iodides (in this - respect, as in many others, palladium resembles mercury in the - mercuric compounds HgX_{2}). With a solution of mercuric cyanide it - gives a yellowish white precipitate, palladous cyanide, - PdC_{2}N_{2}, which is soluble in potassium cyanide, and gives - other double salts, M_{2}PdC_{4}N_{4}. - - That portion of the platinum ore which dissolves in aqua regia and - is precipitated by ammonium or potassium chloride does not contain - palladium. It remains in solution, because the palladic chloride, - PdCl_{4}, is decomposed and the palladous chloride formed is not - precipitated by ammonium chloride; the same holds good for all the - other lower chlorides of the platinum metals. Zinc (and iron) - separates out all the unprecipitated platinum metals (and also - copper, &c.) from the solution. The palladium is found in these - platinum residues precipitated by zinc. If this mixture of metals - be treated with aqua regia, all the palladium will pass into - solution as palladous chloride with some platinic chloride. By this - treatment the main portion of the iridium, rhodium, &c. remains - almost undissolved, the platinum is separated from the mixture of - palladous and platinic chlorides by a solution of ammonium - chloride, and the solution of palladium is precipitated by - potassium iodide or mercuric cyanide. Wilm (1881) showed that - palladium may be separated from an impure solution by saturating it - with ammonia; all the iron present is thus precipitated, and, after - filtering, the addition of hydrochloric acid to the filtrate gives - a yellow precipitate of an ammonio-palladium compound, - PdCl_{2},2NH_{3}, whilst nearly all the other metals remain in - solution. _Metallic palladium_ is obtained by igniting the - ammonio-compound or the cyanide, PdC_{2}N_{2}. It occurs native, - although rarely, and is a metal of a whiter colour than platinum, - sp. gr. 11·4, melts at about 1,500°; it is much more volatile than - platinum, partially oxidises on the surface when heated (Wilm - obtained spongy palladium by igniting PdCl_{2},2NH_{3}, and - observed that it gives PdO when ignited in oxygen, and that on - further ignition this oxide forms a mixture of Pd_{2}O and Pd), and - loses its absorbed oxygen on a further rise of temperature. It does - not blacken or tarnish (does not absorb sulphur) in the air at the - ordinary temperature, and is therefore better suited than silver - for astronomical and other instruments in which fine divisions have - to be engraved on a white metal, in order that the fine lines - should be clearly visible. The most remarkable property of - palladium, discovered by Graham, consists in its capacity for - _absorbing_ a large amount of _hydrogen_. Ignited palladium absorbs - as much as 940 volumes of hydrogen, or about 0·7 p.c. of its own - weight, which closely approaches to the formation of the compound - Pd_{3}H_{2}, and probably indicates the formation of _palladium - hydride_, Pd_{2}H. This absorption also takes place at the ordinary - temperature--for example, when palladium serves as an electrode at - which hydrogen is evolved. In absorbing the hydrogen, the palladium - does not change in appearance, and retains all its metallic - properties, only its volume increases by about 10 p.c.--that is, - the hydrogen pushes out and separates the atoms of the palladium - from each other, and is itself compressed to 1/900 of its volume. - This compression indicates a great force of chemical attraction, - and is accompanied by the evolution of heat (Chapter II., Note 38). - The absorption of 1 grm. of hydrogen by metallic palladium (Favre) - is accompanied by the evolution of 4·2 thousand calories (for Pt - 20, for Na 13, for K 10 thousand units of heat). Troost showed that - the dissociation pressure of palladium hydride is inconsiderable at - the ordinary temperature, but reaches the atmospheric pressure at - about 140°. This subject was subsequently investigated by A. A. - Cracow of St. Petersburg (1894), who showed that at first the - absorption of hydrogen by the palladium proceeds like solution, - according to the law of Dalton and Henry, but that towards the end - it proceeds like a dissociation phenomenon in definite compounds; - this forms another link between the phenomenon of solution and of - the formation of definite atomic compounds. Cracow's observations - for a temperature 18°, showed that the electro-conductivity and - tension vary until a compound Pd_{2}H is reached, and namely, that - the tension _p_ rises with the volume _v_ of hydrogen absorbed, - according to the law of Dalton and Henry--for instance, for - - _p_ = 2·1 3·2 5·5 7·7 mm. - _v_ = 14 20 34 47 - - The maximum tension at 18° is 9 mm. At a temperature of about 140° - (in the vapour of xylene) the maximum tension is about 760 mm., and - when _v_ = 10-50 vols. the tension (according to Cracow's - experiments) stands at 90-450 mm.--that is, increases in proportion - to the volume of hydrogen absorbed. But from the point of view of - chemical mechanics it is especially important to remark that - Moutier clearly showed, through palladium hydride, the similarity - of the phenomena which proceed in evaporation and dissociation, - which fact Henri Sainte-Claire Deville placed as a fundamental - proposition in the theory of dissociation. It is possible upon the - basis of the second law of the theory of heat, according to the law - of the variation of the tension _p_ of evaporation with the - temperature T (counted from -273°), to calculate the latent heat of - evaporation L (_see_ works on physics) because 424L = T(1/_d_ - - 1/D)_dp_/_dt_, where _d_ and D are the weights of cubic measures of - the gas (vapour) and liquid. (Thus, for instance, for water, when - _t_ = 100°, T = 373, _d_ = 0·605, D = 960, _dp_/_dt_ = 0·027 m., - 13,596 = 367, L = 536, whence 424L = 227,264, and the second - portion of the equation 226,144, which is sufficiently near, within - the limits of experimental error, _see_ Chapter I., Note 11.) The - same equation is applicable to the dissociation of Na_{2}H and - K_{2}H--(Chapter XII., Note 42)--but it has only been verified in - this respect for Pd_{2}H, since Moutier, by calculating the amount - of heat L evolved, for _t_ = 20, according to the variation of the - tension (_dp_/_dt_) obtained 4·1 thousand calories, which is very - near the figure obtained experimentally by Favre (_see_ Chapter - XII., Note 44). The absorbed hydrogen is easily disengaged by - ignition or decreased pressure. The resultant compound does not - decompose at the ordinary temperature, but when exposed to air the - metal sometimes glows spontaneously, owing to the hydrogen burning - at the expense of the atmospheric oxygen. The hydrogen absorbed by - palladium acts towards many solutions as a reducing agent; in a - word, everything here points to the formation of a definite - compound and at the same time of a physically-compressed gas, and - forms one of the best examples of the bond existing between - chemical and physical processes, to which we have many times drawn - attention. It must be again remembered that the other metals of the - eighth group, even copper, are, like palladium and platinum, able - to combine with hydrogen. The permeability of iron and platinum - tubes to hydrogen is naturally due to the formation of similar - compounds, but palladium is the most permeable. - - [9 bis] _Rhodium_ is generally separated, together with iridium, from - the residues left after the treatment of native platinum, because - the palladium is entirely separated from them, and the ruthenium is - present in them in very small traces, whilst the osmium at any rate - is easily separated, as we shall soon see. The mixture of rhodium - and iridium which is left undissolved in dilute aqua regia is - dissolved in chlorine water, or by the action of chlorine on a - mixture of the metals with sodium chloride. In either case both - metals pass into solution. They may be separated by many methods. - In either case (if the action be aided by heat) the rhodium is - obtained in the form of the chloride RhCl_{3}, and the iridium as - iridious chloride, IrCl_{3}. They both form double salts with - sodium chloride which are soluble in water, but the iridium salt is - also partially soluble in alcohol, whilst the rhodium salt is not. - A mixture of the chlorides, when treated with dilute aqua regia, - gives iridic chloride, IrCl_{4}, whilst the rhodium chloride, - RhCl_{3}, remains unaltered; ammonium chloride then precipitates - the iridium as ammonium iridiochloride, Ir(NH_{4})2Cl_{6}, and on - evaporating the rose-coloured filtrate the rhodium gives a - crystalline salt, Rh(NH_{4})_{3}Cl_{6}. Rhodium and its various - oxides are dissolved when fused with potassium hydrogen sulphate, - and give a soluble double sulphate (whilst iridium remains unacted - on); this fact is very characteristic for this metal, which offers - in its properties many points of resemblance with the iron metals. - When fused with potassium hydroxide and chlorate it is oxidised - like iridium, but it is not afterwards soluble in water, in which - respect it differs from ruthenium. This is taken advantage of for - separating rhodium, ruthenium, and iridium. In any case, rhodium - under ordinary conditions always gives salts of the type RX_{3}, - and not of any other type; and not only halogen salts, but also - oxygen salts, are known in this type, which is rare among the - platinum metals. Rhodium chloride, RhCl_{3}, is known in an - insoluble anhydrous and also in a soluble form (like CrX_{3} or - salts of chromic oxides), in which it easily gives double salts, - compounds with water of crystallisation, and forms rose-coloured - solutions. In this form rhodium easily gives double salts of the - two types RhM_{3}Cl_{6} and RhM_{2}Cl_{3}--for example, - K_{5}RhCl_{6},3H_{2}O and K_{2}RhCl_{5},H_{2}O. Solutions of the - salts (at least, the ammonium salt) of the first kind give salts of - the second kind when they are boiled. If a strong solution of - potash be added to a red solution of rhodium chloride and boiled, a - black precipitate of the hydroxide Rh(OH)_{3} is formed; but if the - solution of potash is added little by little, it gives a yellow - precipitate containing more water. This yellow hydrate of rhodium - oxide gives a yellow solution when it is dissolved in acids, which - only becomes rose-coloured after being boiled. It is obvious a - change here takes place, like the transmutations of the salts of - chromic oxide. It is also a remarkable fact that the black - hydroxide, like many other oxidised compounds of the platinoid - metals, does not dissolve in the ordinary oxygen acids, whilst the - yellow hydroxide is easily soluble and gives yellow solutions, - which deposit imperfectly crystallised salts. Metallic rhodium is - easily obtained by igniting its oxygen and other compounds in - hydrogen, or by precipitation with zinc. It resembles platinum, and - has a sp. gr. of 12·1. At the ordinary temperature it decomposes - formic acid into hydrogen and carbonic anhydride, with development - of heat (Deville). With the alkali sulphites, the salts of rhodium - and iridium of the type RX_{3} give sparingly-soluble precipitates - of double sulphites of the composition R(SO_{3}Na)_{3},H_{2}O, by - means of which these metals may be separated from solution, and - also may be separated from each other, for a mixture of these salts - when treated with strong sulphuric acid gives a soluble iridium - sulphate and leaves a red insoluble double salt of rhodium and - sodium. It may be remarked that the oxides Ir_{2}O_{3} and - Rh_{2}O_{3} are comparatively stable and are easily formed, and - that they also form different double salts (for instance, - IrCl_{3},3KCl_{3}H_{2}O, RhCl_{3},2NH_{4}Cl_{4}H_{2}O, - RhCl_{3},3NH_{4}Cl1-1/2H_{2}O) and compounds like the cobaltia - compounds (for instance, luteo-salts RhX_{3},6NH_{3}, roseo-salts, - RhX_{3}H_{2}O_{5}NH_{3}, and purpureo-salts IrX_{3},5NH_{3}, &c.) - _Iridious oxide_, Ir_{2}O_{3}, is obtained by fusing iridious - chloride and its compounds with sodium carbonate, and treating the - mass with water. The oxide is then left as a black powder, which, - when strongly heated, is decomposed into iridium and oxygen; it is - easily reduced, and is insoluble in acids, which indicates the - feeble basic character of this oxide, in many respects resembling - such oxides as cobaltic oxide, ceric or lead dioxide, &c. It does - not dissolve when fused with potassium hydrogen sulphate. Rhodium - oxide, Rh_{2}O_{3}, is a far more energetic base. It dissolves when - fused with potassium hydrogen sulphate. - - From what has been said respecting the separation of platinum and - rhodium it will be understood how the compounds of _iridium_, which - is the main associate of platinum, are obtained. In describing the - treatment of osmiridium we shall again have an opportunity of - learning the method of extraction of the compounds of this metal, - which has in recent times found a technical application in the form - of its oxide, Ir_{2}O_{3}; this is obtained from many of the - compounds of iridium by ignition with water, is easily reduced by - hydrogen, and is insoluble in acids. It is used in painting on - china, for giving a black colour. Iridium itself is more - difficultly fusible than platinum, and when fused it does not - decompose acids or even aqua regia; it is extremely hard, and is - not malleable; its sp. gr. is 22·4. In the form of powder it - dissolves in aqua regia, and is even partially oxidised when heated - in air, sets fire to hydrogen, and, in a word, closely resembles - platinum. Heated in an excess of chlorine it gives iridic chloride, - IrCl_{4}, but this loses chlorine at 50°; it is, however, more - stable in the form of double salts, which have a characteristic - _black_ colour--for instance, Ir(NH_{4})_{2}Cl_{6}--but they give - iridious chloride, IrCl_{3}, when treated with sulphuric acid. - - [9 tri] We have yet to become acquainted with the two remaining - associates of platinum--ruthenium and osmium--whose most important - property is that they are oxidised even when heated in air, and - that they are able to give _volatile_ oxides of the form RuO_{4} - and OsO_{4}; these have a powerful odour (like iodine and nitrous - anhydride). Both these higher oxides are solids; they volatilise - with great ease at 100°; the former is yellow and the latter white. - They are known as _ruthenic_ and _osmic anhydrides_, although their - aqueous solutions (they both slowly dissolve in water) do not show - an acid reaction, and although they do not even expel carbonic - anhydride from potassium carbonate, do not give crystalline salts - with bases, and their alkaline solutions partially deposit them - again when boiled (an excess of water decomposes the salts). The - formulæ OsO_{4} and RuO_{4} correspond with the vapour density of - these oxides. Thus Deville found the vapour density of osmic - anhydride to be 128 (by the formula 127·5) referred to hydrogen. - Tennant and Vauquelin discovered this compound, and Berzelius, - Wöhler, Fritzsche, Struvé, Deville, Claus, Joly, and others helped - in its investigation; nevertheless there are still many questions - concerning it which remain unsolved. It should be observed that - RO_{4} is the highest known form for an oxygen compound, and RH_{4} - is the highest known form for a compound of hydrogen; whilst the - highest forms of acid hydrates contain SiH_{4}O_{4}, PH_{3}O_{4}, - SH_{2}O_{4}, ClHO_{4}--all with four atoms of oxygen, and therefore - in this number there is apparently the limit for the simple forms - of combination of hydrogen and oxygen. In combination with - _several_ atoms of an element, or several elements, there may be - more than O_{4} or H_{4}, but a molecule never contains more than - four atoms of either O or H to one atom of another element. Thus - the simplest forms of combination of hydrogen and oxygen are - exhausted by the list RH_{4}, RH_{3}, RH_{2}, RH, RO, RO_{2}, - RO_{3}, RO_{4}. The extreme members are RH_{4} and RO_{4}, and are - only met with for such elements as carbon, silicon, osmium, - ruthenium, which also give RCl_{4} with chlorine. In these extreme - forms, RH_{4} and RO_{4}, the compounds are the least stable - (compare SiH_{4}, PH_{3}, SH_{2}, ClH, or RuO_{4}, MoO_{3}, - ZrO_{2}, SrO), and easily give up part, or even all, their oxygen - or hydrogen. - - The primary source from which the compounds of ruthenium and osmium - are obtained is either _osmiridium_ (the osmium predominates, from - IrOs to IrOs_{4}, sp. gr. from 16 to 21), which occurs in platinum - ores (it is distinguished from the grains of platinum by its - crystalline structure, hardness, and insolubility in aqua regia), - or else those insoluble residues which are obtained, as we saw - above, after treating platinum with aqua regia. Osmium predominates - in these materials, which sometimes contain from 30 p.c. to 40 p.c. - of it, and rarely more than 4 p.c. to 5 p.c. of ruthenium. The - process for their treatment is as follows: they are first fused - with 6 parts of zinc, and the zinc is then extracted with dilute - hydrochloric acid. The osmiridium thus treated is, according to - Fritzsche and Struvé's method, then added to a fused mixture of - potassium hydroxide and chlorate in an iron crucible; the mass as - it begins to evolve oxygen acts on the metal, and the reaction - afterwards proceeds spontaneously. The dark product is treated with - water, and gives a solution of osmium and ruthenium in the form of - soluble salts, R_{2}OsO_{4} and R_{2}RuO_{4}, whilst the insoluble - residue contains a mixture of oxides of iridium (and some osmium, - rhodium, and ruthenium), and grains of metallic iridium still - unacted on. According to Frémy's method the lumps of osmiridium are - straightway heated to whiteness in a porcelain tube in a stream of - air or oxygen, when the very volatile osmic anhydride is obtained - directly, and is collected in a well-cooled receiver, whilst the - ruthenium gives a crystalline sublimate of the dioxide, RuO_{2}, - which is, however, very difficultly volatile (it volatilises - together with osmic anhydride), and therefore remains in the cooler - portions of the tube; this method does not give volatile ruthenic - anhydride, and the iridium and other metals are not oxidised or - give non-volatile products. This method is simple, and at once - gives dry, pure osmic anhydride in the receiver, and ruthenium - dioxide in the sublimate. The air which passes through the tube - should be previously passed through sulphuric acid, not only in - order to dry it, but also to remove the organic and reducing dust. - The vapour of osmic anhydride must be powerfully cooled, and - ultimately passed over caustic potash. A third mode of treatment, - which is most frequently employed, was proposed by Wöhler, and - consists in slightly heating (in order that the sodium chloride - should not melt) an intimate mixture of osmiridium and common salt - in a stream of moist chlorine. The metals then form compounds with - chlorine and sodium chloride, whilst the osmium forms the chloride, - OsCl_{4}, which reacts with the moisture, and gives osmic - anhydride, which is condensed. The ruthenium in this, as in the - other processes, does not directly give ruthenic anhydride, but is - always extracted as the soluble ruthenium salt, K_{2}RuO_{4}, - obtained by fusion with potassium hydroxide and chlorate or - nitrate. When the orange-coloured ruthenate, K_{2}RuO_{4}, is mixed - with acids, the liberated ruthenic acid immediately decomposes into - the volatile ruthenic anhydride and the insoluble ruthenic oxide: - 2K_{2}RuO_{4} + 4HNO_{3} = RuO_{4} + RuO_{2},2H_{2}O + 4KNO_{3}. - When once one of the above compounds of ruthenium or osmium is - procured it is easy to obtain all the remaining compounds, and by - reduction (by metals, hydrogen, formic acid, &c.) the metals - themselves. - - Osmic anhydride, OsO_{4}, is very easily deoxidised by many - methods. It blackens organic substances, owing to reduction, and is - therefore used in investigating vegetable and animal, and - especially nerve, preparations under the microscope. Although osmic - anhydride may be distilled in hydrogen, still complete reduction is - accomplished when a mixture of hydrogen and osmic anhydride is - slightly ignited (just before it inflames). If osmium be placed in - the flame it is oxidised, and gives vapours of osmic anhydride, - which become reduced, and the flame gives a brilliant light. Osmic - anhydride deflagrates like nitre on red-hot charcoal; zinc, and - even mercury and silver, reduce osmic anhydride from its aqueous - solutions into the lower oxides or metal; such reducing agents as - hydrogen sulphide, ferrous sulphate, or sulphurous anhydride, - alcohol, &c., act in the same manner with great ease. - - The lower oxides of osmium, ruthenium, and of the other elements of - the platinum series are not volatile, and it is noteworthy that the - other elements behave differently. On comparing SO_{2}, SO_{3}; - As_{2}O_{3}, As_{2}O_{5}; P_{2}O_{3}, P_{2}O_{5}; CO, CO_{2}, &c., - we observe a converse phenomenon; the higher oxides are less - volatile than the lower. In the case of osmium all the oxides, with - the exception of the highest, are non-volatile, and it may - therefore be thought that this higher form is more simply - constituted than the lower. It is possible that osmic oxide, - OsO_{2}, stands in the same relation to the anhydride as C_{2}H_{4} - to CH_{4}--_i.e._ the lower oxide is perhaps Os_{2}O_{4}, or is - still more polymerised, which would explain why the lower oxides, - having a greater molecular weight, are less volatile than the - higher oxides, just as we saw in the case of the nitrogen oxides, - N_{2}O and NO. - - _Ruthenium and osmium_, obtained by the ignition or reduction of - their compounds in the form of powder, have a density considerably - less than in the fused form, and differ in this condition in their - capacity for reaction; they are much more difficultly fused than - platinum and iridium, although ruthenium is more fusible than - osmium. Ruthenium in powder has a specific gravity of 8·5, the - fused metal of 12·2; osmium in powder has a specific gravity of - 20·0, and when semi-fused--or, more strictly speaking, - agglomerated--in the oxyhydrogen flame, of 21·4, and fused 22·5. - The powder of slightly-heated osmium oxidises very easily in the - air, and when ignited burns like tinder, directly forming the - odoriferous osmic anhydride (hence its name, from the Greek word - signifying odour); ruthenium also oxidises when heated in air, but - with more difficulty, forming the oxide RuO_{2}. The oxides of the - types RO, R_{2}O_{3}, and RO_{2} (and their hydrates) obtained by - reduction from the higher oxides, and also from the chlorides, are - analogous to those given by the other platinum metals, in which - respect osmium and ruthenium closely resemble them. We may also - remark that ruthenium has been found in the platinum deposits of - Borneo in the form of _laurite_, Ru_{2}S_{3}, in grey octahedra of - sp. gr. 7·0. - - For osmium, Moraht and Wischin (1893) obtained free osmic acid, - H_{2}OsO_{4}, by decomposing K_{2}OsO_{4} with water, and - precipitating with alcohol in a current of hydrogen (because in air - volatile OsO_{4} is formed); with H_{2}S, osmic acid gives - OsO_{3}(HS)_{2} at the ordinary temperature. - - Debray and Joly showed that ruthenic anhydride, RuO_{4}, fuses at - 25°, boils at 100°, and evolves oxygen when dissolved in potash, - forming the salt KRuO_{4} (not isomorphous with potassium - permanganate). - - Joly (1891), who studied the ruthenium compounds in greater detail, - showed that the easily-formed KRuO_{4} gives RuKO_{4}RuO_{3} when - ignited, but it resembles KMnO_{4} in many respects. In general, Ru - has much in common with Mn. Joly (1889) also showed that if KNO_{3} - be added to a solution of RuCl_{3} containing HCl, the solution - becomes hot, and a salt, RuCl_{3}NO_{2}KCl, is formed, which enters - into double decomposition and is very stable. Moreover, if RuCl_{3} - be treated with an excess of nitric acid, it forms a salt, - RuCl_{3}NOH_{2}O, after being heated (to boiling) and the addition - of HCl. The vapour density of RuO_{4}, determined by Debray and - Joly, corresponds to that formula. - - [10] Although palladium gives the same types of combination (with - chlorine) as platinum, its reduction to RX_{2} is incomparably - easier than that of platinic chloride, and in the case of iridium - it is also very easy. Iridic chloride, IrCl_{4}, acts as an - oxidising agent, readily parts with a fourth of its chlorine to a - number of substances, readily evolves chlorine when heated, and it - is only at low temperatures that chlorine and aqua regia convert - iridium into iridic chloride. In disengaging chlorine iridium more - often and easily gives the very stable iridious chloride, IrCl_{3} - (perhaps this substance is Ir_{2}Cl_{6} = IrCl_{2},IrCl_{4}, - insoluble in water, but soluble in potassium chloride, because it - forms the double salt K_{3}IrCl_{6}), than the dichloride, IrCl_2. - This compound, corresponding to IrX_{2}, is very stable, and - corresponds with the _basic oxide_, Ir_{2}O_{3}, resembling the - oxides Fe_{2}O_{3}, Co_{2}O_{3}. To this form there correspond - ammoniacal compounds similar to those given by cobaltic oxide. - Although iridium also gives an acid in the form of the salt - K_{2}Ir_{2}O_{7}, it does not, like iron (and chromium), form the - corresponding chloride, IrCl_{6}. In general, in this as in the - other elements, it is impossible to predict the chlorine compounds - from those of oxygen. Just as there is no chloride SCl_{6}, but - only SCl_{2}, so also, although IrO_{3} exists, IrCl_{6} is - wanting, the only chloride being IrCl_{4}, and this is unstable, - like SCl_{2}, and easily parts with its chlorine. In this respect - rhodium is very much like iridium (as platinum is like palladium). - For RhCl_{4} decomposes with extreme ease, whilst rhodium - chloride, RhCl_{3}, is very stable, like many of the salts of the - type RhX_{3}, although like the platinum elements these salts are - easily reduced to metal by the action of heat and powerful - reagents. There is as close a resemblance between osmium and - ruthenium. Osmium when submitted to the action of dry chlorine - gives osmic chloride, OsCl_{4}, but the latter is converted by - water (as is osmium by moist chlorine) into osmic anhydride, - although the greater portion is then decomposed into Os(HO)_{4} - and 4HCl, like a chloranhydride of an acid. In general this acid - character is more developed in osmium than in platinum and - iridium. Having parted with chlorine, osmic chloride, OsCl_{4}, - gives the unstable trichloride, OsCl_{3}, and the stable soluble - dichloride, OsCl_{2}, which corresponds with platinous chloride in - its properties and reactions. The relation of ruthenium to the - halogens is of the same nature. - -Platinum and its analogues, like iron and its analogues, are able to form -complex and comparatively stable cyanogen and ammonia compounds, -corresponding with the ferrocyanides and the ammoniacal compounds of -cobalt, which we have already considered in the preceding chapter. - -If platinous chloride, PtCl_{2} (insoluble in water), be added by degrees -to a solution of potassium cyanide, it is completely dissolved (like -silver chloride), and on evaporating the solution deposits rhombic prisms -of _potassium platinocyanide_, PtK_{2}(CN)_{4},3H_{2}O. This salt, like -all those corresponding with it, has a remarkable play of colours, due to -the phenomena of dichromism, and even polychromism, natural to all the -platinocyanides. Thus it is yellow and reflects a bright blue light. It -is easily soluble in water, effloresces in air, then turns red, and at -100° orange, when it loses all its water. The loss of water does not -destroy its stability--that is, it still remains unchanged, and its -stability is further shown by the fact that it is formed when potassium -ferrocyanide, K_{4}Fe(CN)_{6}, is heated with platinum black. This salt, -first obtained by Gmelin, shows a neutral reaction with litmus; it is -exceedingly stable under the action of air, like potassium ferrocyanide, -which it resembles in many respects. Thus the platinum in it cannot be -detected by reagents such as sulphuretted hydrogen; the potassium may be -replaced by other metals by the action of their salts, so that it -corresponds with a whole series of compounds, R_{2}Pt(CN)_{4}, and it is -stable, although the potassium cyanide and platinous salts, of which it -is composed, individually easily undergo change. When treated with -oxidising agents it passes, like the ferrocyanide, into a higher form of -combination of platinum. If salts of silver be added to its solution, it -gives a heavy white precipitate of silver platinocyanide, -PtAg_{2}(CN)_{4}, which when suspended in water and treated with -sulphuretted hydrogen, enters into double decomposition with the latter -and forms insoluble silver sulphide, Ag_{2}S, and soluble -_hydroplatinocyanic acid_, H_{2}Pt(CN)_{4}. If potassium platinocyanide -be mixed with an equivalent quantity of sulphuric acid, the -hydroplatinocyanic acid liberated may be extracted by a mixture of -alcohol and ether. The ethereal solution, when evaporated in a -desiccator, deposits bright red crystals of the composition -PtH_{2}(CN)_{4},5H_{2}O. This acid colours litmus paper, liberates -carbonic anhydride from sodium carbonate, and saturates alkalis, so that -it presents an analogy to hydroferrocyanic acid.[11] - - [11] This acid character is explained by the influence of the platinum - on the hydrogen, and by the attachment of the cyanogen groups. - Thus cyanuric acid, H_{3}(CN)_{3}O_{3}, is an energetic acid - compared with cyanic acid, HCNO. And the formation of a compound - with five molecules of water of crystallisation, - (PtH_{2}(CN)_{4},5H_{2}O), confirms the opinion that platinum is - able to form compounds of still higher types than that expressed - in its saline compounds, and, moreover, the combination of - hydroplatinocyanic acid with water does not reach the limit of the - compounds which appears in PtCl_{4},2HCl,6H_{2}O. - - A whole series of _platinocyanides_ of the common type - PtR_{2}(CN)_{4}_n_H_{2}O is obtained by means of double - decomposition with the potassium or hydrogen or silver salts. For - example, the salts of sodium and lithium contain, like the - potassium salt, three molecules of water. The sodium salt is - soluble in water and alcohol. The ammonium salt has the - composition Pt(NH_{4})_{2}(CN)_{4},2H_{2}O and gives crystals - which reflect blue and rose-coloured light. This ammonium salt - decomposes at 300°, with evolution of water and ammonium cyanide, - leaving a greenish _platinum dicyanide_, Pt(CN)_{2}, which is - insoluble in water and acid but dissolves in potassium cyanide, - hydrocyanic acid, and other cyanides. The same platinous cyanide - is obtained by the action of sulphuric acid on the potassium salts - in the form of a reddish-brown amorphous precipitate. The most - characteristic of the platinocyanides are those of the alkaline - earths. The magnesium salt PtMg(CN)_{4},7H_{2}O crystallises in - regular prisms, whose side faces are of a metallic green colour - and terminal planes dark blue. It shows a carmine-red colour along - the main axis, and dark red along the lateral axes; it easily - loses water, (2H_{2}O), at 40°, and then turns blue (it then - contains 5H_{2}O, which is frequently the case with the - platinocyanides). Its aqueous solution is colourless, and an - alcoholic solution deposits yellow crystals. The remainder of the - water is given off at 230°. It is obtained by saturating - platinocyanic acid with magnesia, or else by double decomposition - between the barium salt and magnesium sulphate. The strontium - salt, SrPt(CN)_{4},4H_{2}O crystallises in milk-white plates - having a violet and green iridescence. When it effloresces in a - desiccator, its surfaces have a violet and metallic green - iridescence. A colourless solution of the barium salt - PtBa(CN)_{4},4H_{2}O is obtained by saturating a solution of - hydroplatinocyanic acid with baryta, or by boiling the insoluble - copper platinocyanide in baryta water. It crystallises in - monoclinic prisms of a yellow colour, with blue and green - iridescence; it loses half its water at 100°, and the whole at - 150°. The ethyl salt, Pt(C_{2}H_{5})_{2}(CN)_{4},2H_{2}0, is also - very characteristic; its crystals are isomorphous with those of - the potassium salt, and are obtained by passing hydrochloric acid - into an alcoholic solution of hydroplatinocyanic acid. The - facility with which they crystallise, the regularity of their - forms, and their remarkable play of colours, renders the - preparation of the platinocyanides one of the most attractive - lessons of the laboratory. - - By the action of chlorine or dilute nitric acid, the - platinocyanides are converted into salts of the composition - PtM_{2}(CN)_{5}, which corresponds with Pt(CN)_{3},2KCN--that is, - they express the type of a non-existent form of oxidation of - platinum, PtX_{3} (_i.e._ oxide Pt_{2}O_{3}), just as potassium - ferricyanide (FeCy_{3},3KCy) corresponds with ferric oxide, and - the ferrocyanide corresponds with the ferrous oxide. The potassium - salt of this series contains PtK_{2}(CN)_{5},3H_{2}O, and forms - brown regular prisms with a metallic lustre, and is soluble in - water but insoluble in alcohol. Alkalis re-convert this compound - into the ordinary platinocyanide K_{2}Pt(CN)_{4}, taking up the - excess of cyanogen. It is remarkable that the salts of the type - PtM_{2}Cy_{5} contain the same amount of water of crystallisation - as those of the type PtM_{2}Cy_{4}. Thus the salts of potassium - and lithium contain three, and the salt of magnesium seven, - molecules of water, like the corresponding salts of the type of - platinous oxide. Moreover, neither platinum nor any of its - associates gives any cyanogen compound corresponding with the - oxide, _i.e._ having the composition PtK_{2}Cy_{6}, just as there - are no compounds higher than those which correspond to - RCy_{3}_n_MCy_{3} for cobalt or iron. This would appear to - indicate the absence of any such cyanides, and indeed, for no - element are there yet known any poly-cyanides containing more than - three equivalents of cyanogen for one equivalent of the element. - The phenomenon is perhaps connected with the faculty of cyanogen - of giving tricyanogen polymerides, such as cyanuric acid, solid - cyanogen chloride, &c. Under the action of an excess of chlorine, - a solution of PtK_{2}(CN)_{4} gives (besides PtK_{2}Cy_{5}) a - product PtK_{2}Cy_{4}Cl_{2}, which evidently contains the form - PtX_{4}, but at first the action of the chlorine (or the - electrolysis of, or addition of dilute peroxide of hydrogen to, a - solution of PtK_{2}Cy_{4}, acidulated with hydrochloric acid) - produces an easily soluble intermediate salt which crystallises in - thin copper-red needles (Wilm, Hadow, 1889). It only contains a - small amount of chlorine, and apparently corresponds to a compound - 5PtK_{2}Cy_{4} + PtK_{2}Cy_{4}Cl_{2} + 24H_{2}O. Under the action - of an excess of ammonia both these chlorine products are converted - either completely or in part (according to Wilm ammonia does not - act upon PtK_{2}Cy_{4}) into PtCy_{2},2NH_{3}, _i.e._ a - platino-ammonia compound (_see_ further on). It is also necessary - to pay attention to the fact that ruthenium and osmium--which, as - we know, give higher forms of oxidation than platinum--are also - able to combine with a larger proportion of potassium cyanide (but - not of cyanogen) than platinum. Thus ruthenium forms a crystalline - _hydroruthenocyanic acid_, RuH_{4}(CN)_{6}, which is soluble in - water and alcohol, and corresponds with the salts M_{4}Ru(CN)_{6}. - There are exactly similar osmic compounds--for example, - K_{4}Os(CN)_{6},3H_{2}O. The latter is obtained in the form of - colourless, sparingly-soluble regular tablets on evaporating the - solution obtained from a fused mixture of potassium osmiochloride, - K_{2}OsCl_{6}, and potassium cyanide. These osmic and ruthenic - compounds fully correspond with potassium ferrocyanide, - K_{4}Fe(CN)_{6},3H_{2}O, not only in their composition but also in - their crystalline form and reactions, which again demonstrates the - close analogy between iron, ruthenium, and osmium, which we have - shown by giving these three elements a similar position (in the - eighth group) in the periodic system. For rhodium and iridium only - salts of the same type as the ferricyanides, M_{3}RCy_{6}, are - known, and for palladium only of the type M_{2}PdCy_{4}, which are - analogous to the platinum salts. In all these examples a - _constancy of the types_ of the double cyanides is apparent. In - the eighth group we have iron, cobalt, nickel, copper, and their - analogues ruthenium, rhodium, palladium, silver, and also osmium, - iridium, platinum, gold. The double cyanides of iron, ruthenium, - osmium have the type K_{4}R(CN)_{6}; of cobalt, rhodium, iridium, - the type K_{3}R(CN)_{6}; of nickel, palladium, platinum the type - K_{2}R(CN)_{4} and K_{2}R(CN)_{5}; and for copper, silver, gold - there are known KR(CN)_{2}, so that the presence of 4, 3, 2, and 1 - atoms of potassium corresponds with the order of the elements in - the periodic system. Those types which we have seen in the - ferrocyanides and ferricyanides of iron repeat themselves in all - the platinoid metals, and this naturally leads to the conclusion - that the formation of similar so-called double salts is of exactly - the same nature as that of the ordinary salts. If, in expressing - the union of the elements in the oxygen salts, the existence of an - _aqueous residue_ (hydroxyl group) be admitted, in which the - hydrogen is replaced by a metal, we have then only to apply this - mode of expression to the double salts and the analogy will be - obvious, if only we remember that Cl_{2}, (CN)_{2}, SO_{4}, &c., - are equivalent to O, as we see in RO, RCl_{2}, RSO_{4}, &c. They - all = X_{2}, and, therefore, in point of fact, wherever X (= Cl or - OH, &c.) can be placed, there (Cl_{2}H), (SO_{4}H), &c., can also - stand. And as Cl_{2}H = Cl + HCl and SO_{4}H = OH + SO_{3}, &c., - it follows that molecules HCl or SO_{3}, or, in general, whole - molecules--for instance, NH_{3}, H_{2}O, salts, &c., can annex - themselves to a compound containing X. (This is an indirect - consequence of the law of substitution which explains the origin - of double salts, ammonia compounds, compounds with water of - crystallisation, &c., by one general method.) Thus the double salt - MgSO_{4},K_{2}SO_{4}, according to this reasoning, _may be_ - considered as a substance of the same type as MgCl_{2}, namely, as - = Mg(SO_{4}K)_{2}, and the alums as derived from Al(OH)(SO_{4}), - namely, as Al(SO_{4}K)(SO_{4}). Without stopping to pursue this - digression further, we will apply these considerations to the type - of the ferrocyanides and ferricyanides and their platinum - analogues. Such a salt as K_{2}PtCy_{4} may accordingly be - regarded as Pt(Cy_{2}K)_{2}, like Pt(OH)_{2}; and such a salt as - PtK_{2}Cy_{5} as PtCy(Cy_{2}K)_{2}, the analogue of PtX(OH)_{2}, - or AlX(OH)_{2}, and other compounds of the type RX_{3}. Potassium - ferricyanide and the analogous compounds of cobalt, iridium, and - rhodium, belong to the same type, with the same difference as - there is between RX(OH)_{2} and R(OH)_{3}, since FeK_{3}Cy_{6} = - Fe(Cy_{2}K)_{3}. Limiting myself to these considerations, which - may partially elucidate the nature of double salts, I will now - pass again to the complex saline compounds known for platinum. - - (_A_) On mixing a solution of potassium thiocyanate with a - solution of potassium platinosochloride, K_{2}PtCl_{4}, they form - a double thiocyanate, PtK_{2}(CNS)_{4}, which is easily soluble in - water and alcohol, crystallises in red prisms, and gives an - orange-coloured solution, which precipitates salts of the heavy - metals. The action of sulphuric acid on the lead salt of the same - type gives the acid itself, PtH_{2}(SCN)_{4}, which corresponds - with these salts. The type of these compounds is evidently the - same as that of the cyanides. - - (_B_) _Platinous chloride_, PtCl_{2}, which is insoluble in water, - forms _double salts with the metallic chlorides_. These double - chlorides are soluble in water, and capable of crystallising. - Hence when a hydrochloric acid solution of platinous chloride is - mixed with solutions of metallic salts and evaporated it forms - crystalline salts of a red or yellow colour. Thus, for example, - the potassium salt, PtK_{2}Cl_{4}, is red, and easily soluble in - water; the sodium salt is also soluble in alcohol; the barium - salt, PtBaCl_{4},3H_{2}O, is soluble in water, but the silver - salt, PtAg_{2}Cl_{4}, is insoluble in water, and may be used for - obtaining the remaining salts by means of double decomposition - with their chlorides. - - (_C_) A remarkable example of the complex compounds of platinum - was observed by Schützenberger (1868). He showed that - finely-divided platinum in the presence of chlorine and carbonic - oxide at 250°-300° gives phosgene and a volatile compound - containing platinum. The same substance is formed by the action of - carbonic oxide on platinous chloride. It decomposes with an - explosion in contact with water. Carbon tetrachloride dissolves a - portion of this substance, and on evaporation gives crystals of - 2PtCl_{2},3CO, whilst the compound PtCl_{2},2CO remains - undissolved. When fused and sublimed it gives yellow needles of - PtCl_{2},CO, and in the presence of an excess of carbonic oxide - PtCl_{2},2CO is formed. These compounds are fusible (the first at - 250°, the second at 142°, and the third at 195°). In this case (as - in the double cyanides) combination takes place, because both - carbonic oxide and platinous chloride are unsaturated compounds - capable of further combination. The carbon tetrachloride solution - absorbs NH_{3} and gives PtCl_{2},CO,2NH_{3}, and - PtCl_{2},2CO,2NH_{3}, and these substances are analogous - (Foerster, Zeisel, Jörgensen) to similar compounds containing - complex amines (for instance, pyridine, C_{5}H_{5}N), instead of - NH_{3}, and ethylene, &c., instead of CO, so that here we have a - whole series of complex platino-compounds. The compound PtCl_{2}CO - dissolves in hydrochloric acid without change, and the solution - disengages all the carbonic oxide when KCN is added to it, which - shows that those forces which bind 2 molecules of KCN to PtCl_{2} - can also bind the molecule CO, or 2 molecules of CO. When the - hydrochloric acid solution of PtCl_{2}CO is mixed with a solution - of sodium acetate or acetic acid, it gives a precipitate of PtOCO, - _i.e._ the Cl_{2} is replaced by oxygen (probably because the - acetate is decomposed by water). This oxide, PtOCO, splits up into - Pt + CO_{2} at 350°. PtSCO is obtained by the action of - sulphuretted hydrogen upon PtCl_{2}CO. All this leads to the - conclusion that the group PtCO is able to assimilate X_{2} = - Cl_{2}, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by - igniting spongy platinum at 250°, first in a stream of chlorine, - and then in a stream of carbonic oxide, obtained (besides volatile - products) a non-volatile yellow substance which remained unchanged - in air and disengaged chlorine and phosgene gas when ignited; its - composition was PtCl_{6}(CO)_{2}, which apparently proves it to be - a compound of PtCl_{2} and 2COCl_{2}, as PtCl_{2} is able to - combine with oxychlorides, and forms somewhat stable compounds. - - (_D_) The faculty of platinous chloride for forming stable - compounds with divers substances shows itself in the formation of - the compound PtCl_{2},PCl_{3} by the action of phosphorus - pentachloride at 250° on platinum powder (Pd reacts in a similar - manner, according to Fink, 1892). The product contains both - phosphorus pentachloride and platinum, whilst the presence of - PtCl_{2} is shown in the fact that the action of water produces - _chlorplatino-phosphorous acid_, PtCl_{2}P(OH)_{3}. - - (_E_) After the cyanides, the _double salts_ of platinum _formed - by sulphurous acid_ are most distinguished for their stability and - characteristic properties. This is all the more instructive, as - sulphurous acid is only feebly energetic, and, moreover, in these, - as in all its compounds, it exhibits a dual reaction. The salts of - sulphurous acid, R_{2}SO_{3}, either react as salts of a feeble - bibasic acid, where the group SO_{3} presents itself as bivalent, - and consequently equal to X_{2}, or else they react after the - manner of salts of a monobasic acid containing the same residue, - RSO_{3}, as occurs in the salts of sulphuric acid. In sulphurous - acid this residue is combined with hydrogen, H(SO_{3}H), whilst in - sulphuric acid it is united with the aqueous residue (hydroxyl), - OH(SO_{3}H). These two forms of action of the sulphites appear in - their reactions with the platinum salts--that is to say, salts of - both kinds are formed, and they both correspond with the type - PtH_{2}X_{4}. The one series of salts contain PtH_{2}(SO_{3})_{2}, - and their reactions are due to the bivalent residue of sulphurous - acid, which replaces X_{2}. The others, which have the composition - PtR_{2}(SO_{3}H)_{4}, contain sulphoxyl. The latter salts will - evidently react like acids; they are formed simultaneously with - the salts of the first kind, and pass into them. These salts are - obtained either by directly dissolving platinous oxide in water - containing sulphurous acid, or by passing sulphurous anhydride - into a solution of platinous chloride in hydrochloric acid. If a - solution of platinous chloride or platinous oxide in sulphurous - acid be saturated with sodium carbonate, it forms a white, - sparingly soluble precipitate containing - PtNa_{2}(SO_{3}Na)_{4},7H_{2}O. If this precipitate be dissolved - in a small quantity of hydrochloric acid and left to evaporate at - the ordinary temperature, it deposits a salt of the other type, - PtNa_{2}(SO_{3})_{2},H_{2}O, in the form of a yellow powder, which - is sparingly soluble in water. The potassium salt analogous to the - first salt, PtK_{2}(SO_{3}K)_{4},2H_{2}O, is precipitated by - passing sulphurous anhydride into a solution of potassium sulphite - in which platinous oxide is suspended. A similar salt is known for - ammonium, and with hydrochloric acid it gives a salt of the second - kind, Pt(NH_{4})_{2}(SO_{3})_{2},H_{2}O. If ammonio-chloride of - platinum be added to an aqueous solution of sulphurous anhydride, - it is first deoxidised, and chlorine is evolved, forming a salt of - the type PtX_{2}; a double decomposition then takes place with the - ammonium sulphite, and a salt of the composition - Pt(NH_{4})_{2}Cl_{3}(SO_{3}H) is formed (in a desiccator). The - acid character of this substance is explained by the fact that it - contains the elements SO_{3}H--sulphoxyl, with the hydrogen not - yet displaced by a metal. On saturating a solution of this acid - with potassium carbonate it gives orange-coloured crystals of a - potassium salt of the composition Pt(NH_{4})_{2}Cl_{3}(SO_{3}K). - Here it is evident that an equivalent of chlorine in - Pt(NH_{4})_{2})Cl_{4} is replaced by the univalent residue of - sulphurous acid. Among these salts, that of the composition - Pt(NH_{4})_{2})Cl_{2}(SO_{3}H)_{2},H_{2}O is very readily formed, - and crystallises in well-formed colourless crystals; it is - obtained by dissolving ammonium platinosochloride, - Pt(NH_{4})_{2}Cl_{4}, in an aqueous solution of sulphurous acid. - The difficulty with which sulphurous anhydride and platinum are - separated from these salts indicates the same basic character in - these compounds as is seen in the double cyanides of platinum. In - their passage into a complex salt, the metal platinum and the - group SO_{2} modify their relations (compared with those of - PtX_{2} or SO_{2}X_{2}), just as the chlorine in the salts KClO, - KClO_{3}, and KClO_{4} is modified in its relations as compared - with hydrochloric acid or potassium chloride. - - (_F_) No less characteristic are the _platinonitrites_ formed by - platinous oxide. They correspond with nitrous acid, whose salts, - RNO_{2}, contain the univalent radicle, NO_{2}, which is capable - of replacing chlorine, and therefore the salts of this kind should - form a common type PtR_{2}(NO_{2})_{4}, and such a salt of - potassium has actually been obtained by mixing a solution of - potassium platinosochloride with a solution of potassium nitrite, - when the liquid becomes colourless, especially if it be heated, - which indicates the change in the chemical distribution of the - elements. As the liquid decolorises it gradually deposits - sparingly soluble, colourless prisms of the potassium salt - K_{2}Pt(NO_{2})_{4}, which does not contain any water. With silver - nitrate a solution of this salt gives a precipitate of silver - platinonitrite, PtAg_{2}(NO_{2})_{4}. The silver of this salt may - be replaced by other metals by means of double decomposition with - metallic chlorides. The sparingly soluble barium salt, when - treated with an equivalent quantity of sulphuric acid, gives a - soluble acid, which separates, under the receiver of an air-pump, - in red crystals; this acid has the composition - PtH_{2}(NO_{2})_{4}. To the potassium salt, K_{2}Pt(NO_{2})_{4}, - there correspond (Vèzes, 1892) K_{2}Pt(NO_{2})_{4}Br_{2} and - K_{2}Pt(NO_{2})_{4}Cl_{2} and other compounds of the same type - K_{2}PtX_{6}, where X is partly replaced by Cl or Br and partly by - (NO_{2}), showing a transition towards the type of the double - salts like the platino-ammoniacal salts. (The corresponding double - sodium nitrite salt of cobalt is soluble in water, while the - K,NH_{4} and many other salts are insoluble in water, as I was - informed by Prof. K. Winkler in 1894). - - In all the preceding complex compounds of Pt we see a common type - PtX_{2},2MX (_i.e._ of double salts corresponding to PtO) or - PtM_{2}X_{4} = Pt(MX_{2})_{2}, corresponding to Pt(HO)_{2} with - the replacement of O by its equivalent X_{2}. Two other facts must - also be noted. In the first place these X's generally correspond - to elements (like chlorine) or groups (like CN, NO_{2}, SO_{3}, - &c.), which are capable of further combination. In the second - place all the compounds of the type PtM_{2}X_{4} are capable of - combining with chlorine or similar elements, and thus passing into - compounds of the types PtX_{3} or PtX_{4}. - -Ammonia, like potassium cyanide, has the faculty of easily reacting -with platinum dichloride, forming compounds similar to the platinocyanide -and cobaltia compounds, which are comparatively stable. But as ammonia -does not contain any hydrogen easily replaceable by metals, and as -ammonia itself is able to combine with acids, the PtX_{2} plays, as it -were, the part of an acid with reference to the ammonia. Owing to the -influence of the ammonia, the X_{2} in the resultant compound will -represent the same character as it has in ammoniacal salts; consequently, -the ammoniacal compounds produced from PtX_{2} will be salts in which X -will be replaceable by various other haloids, just as the metal is -replaced in the cyanogen salts; such is the nature of the -_platino-ammonium compounds_. PtX_{2} forms compounds with 2NH_{3} and -with 4NH_{3}, and so also PtX_{4} gives (not directly from PtX_{4} and -ammonia, but from the compounds of PtX_{2} by the action of chlorine, -&c.) similar compounds with 2NH_{3} and with 4NH_{3}.[12] - - [12] The platinum salt and ammonia, when once combined together, are no - longer subject to their ordinary reactions but form compounds - which are comparatively very stable. The question at once suggests - itself to all who are acquainted with these phenomena, as to what - is the relation of the elements contained in these compounds. The - first explanation is that these compounds are salts of ammonium in - which the hydrogen is partially replaced by platinum. This is the - view, with certain shades of difference, held by many respecting - the platino-ammonium compounds. They were regarded in this light - by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we - suppose the hydrogen in 2NH_{4}X to be replaced by bivalent - platinum (as in the salts PtX_{2}), we shall obtain - NH_{3} X - Pt - NH_{3} X - --that is, the compound PtX_{2},2NH_{3}. The compound with 4NH_{3} - will then be represented by a further substitution of the hydrogen - in ammonia by ammonium itself--_i.e._ as NH_{2}(NH_{4}X)_{2}Pt or - PtX_{2},4NH_{3}. A modification of this view is found in that - representation of compounds of this kind which is based on - atomicity. As platinum in PtX_{2} is bivalent, has two affinities, - and ammonia, NH_{3}, is also bivalent, because nitrogen is - quinquivalent and is here only combined with H_{3}, it is evident - what bonds should be represented in PtX_{2},2NH_{3} and in - PtX_{2},4NH_{3}. In the former, Pt(NH_{3}Cl)_{2}, the nitrogen of - each atom of ammonia is united by three affinities with H_{3}, by - one with platinum, and by the fifth with chlorine. The other - compound is Pt(NH_{3}.NH_{3}Cl)_{2}--that is, the N is united by - one affinity with the other N, whilst the remaining bonds are the - same as in the first salt. It is evident that this union or chain - of ammonias has no obvious limit, and the most essential fault of - such a mode of representation is that it does not indicate at all - what number of ammonias are capable of being retained by platinum. - Moreover, it is hardly possible to admit the bond between nitrogen - and platinum in such stable compounds, for these kinds of - affinities are, at all events, feeble, and cannot lead to - stability, but would rather indicate explosive and - easily-decomposed compounds. Moreover, it is not clear why this - platinum, which is capable of giving PtX_{4}, does not act with - its remaining affinities when the addition of ammonia to PtX_{2} - takes place. These, and certain other considerations which - indicate the imperfection of this representation of the structure - of the platino-ammonium salts, cause many chemists to incline more - to the representations of Berzelius, Claus, Gibbs, and others, who - suppose that NH_{3} is able to combine with substances, to adjoin - itself or pair itself with them (this kind of combination is - called 'Paarung') without altering the fundamental capacity of a - substance for further combinations. Thus, in PtX_{2},2NH_{3}, the - ammonia is the associate of PtX_{2}, as is expressed by the - formula N_{2}H_{6}PtX_{2}. Without enlarging on the exposition of - the details of this doctrine, we will only mention that it, like - the first, does not render it possible to foresee a limit to the - compounds with ammonia; it isolates compounds of this kind into a - special and artificial class; does not show the connection between - compounds of this and of other kinds, and therefore it essentially - only expresses the fact of the combination with ammonia and the - modification in its ordinary reactions. For these reasons we do - not hold to either of these proposed representations of the - ammonio-platinum compounds, but regard them from the point of view - cited above with reference to double salts and water of - crystallisation--that is, we embrace all these compounds under the - representation of compounds of complex types. The type of the - compound PtX_{2},2NH_{3} is far more probably the same as that of - PtX_{2},2Z--_i.e._ as PtX_{4}, or, still more accurately and - truly, it is a compound of the same type as PtX_{2},2KX or - PtX_{2},2H_{2}O, &c. Although the platinum first entered into - PtK_{2}X_{4} as the type PtX_{2}, yet its character has changed in - the same manner as the character of sulphur changes when from - SO_{2} the compound SO_{2}(OH)_{2} is obtained, or when KClO_{4}, - the higher form, is obtained from KCl. For us as yet there is no - question as to _what_ affinities hold X_{2} and what hold 2NH_{3}, - because this is a question which arises from the supposition of - the existence of different affinities in the atoms, which there is - no reason for taking as a common phenomenon. It seems to me that - it is most important _as a commencement_ to render clear the - analogy in the formation of various complex compounds, and it is - this analogy of the ammonia compounds with those of water of - crystallisation and double salts that forms the main object of the - primary generalisation. We recognise in platinum, at all events, - not only the four affinities expressed in the compound PtCl_{4}, - but a much larger number of them, if only the _summation of - affinities_ is actually possible. Thus, in sulphur we recognise - not two but a much greater number of affinities; it is clear that - at least six affinities can act. So also among the analogues of - platinum: osmic anhydride, OsO_{4}, Ni(CO)_{4}, PtH_{2}Cl_{6}, &c. - indicate the existence of at least eight affinities; whilst, in - chlorine, judging from the compound KClO_{4} = ClO_{3}(OK) = - ClX_{7}, we must recognise at least seven affinities, instead of - the one which is accepted. The latter mode of calculating - affinities is a tribute to that period of the development of - science when only the simplest hydrogen compounds were considered, - and when all complex compounds were entirely neglected (they were - placed under the class of molecular compounds). This is - insufficient for the present state of knowledge, because we find - that, in complex compounds as in the most simple, the same - constant types or modes of equilibrium are repeated, and the - character of certain elements is greatly modified in the passage - from the most simple into very complex compounds. - - Judging from the most complex platino-ammonium compounds - PtCl_{4},4NH_{3}, we should admit the possibility of the formation - of compounds of the type PtX_{4}Y_{4}, where Y_{4} = 4X_{2} = - 4NH_{3}, and this shows that those forces which form such a - characteristic series of double platinocyanides - PtK_{2}(CN)_{4},3H_{2}O, probably also determine the formation of - the higher ammonia derivatives, as is seen on comparing-- - - PtCl_{2} NH_{3} Cl_{2} 3NH_{3} - Pt(CN)_{2} KCN KCN 3H_{2}O. - - Moreover, it is obviously much more natural to ascribe the faculty - for combination with _n_Y to the whole of the acting - elements--that is, to PtX_{2} or PtX_{4}, and not to platinum - alone. Naturally such compounds are not produced with any Y. With - certain X's there only combine certain Y's. The best known and - most frequently-formed compounds of this kind are those with - water--that is, compounds with water of crystallisation. Compounds - with salts are double salts; also we know that similar compounds - are also frequently formed by means of ammonia. Salts of zinc, - ZnX_{2}, copper, CuX_{2}, silver, AgX, and many others give - similar compounds, but these and many other _ammonio-metallic_ - saline compounds are unstable, and readily part with their - combined ammonia, and it is only in the elements of the platinum - group and in the group of the analogues of iron, that we observe - the faculty to form stable ammonio-metallic compounds. It must be - remembered that the metals of the platinum and iron groups are - able to form several high grades of oxidation which have an acid - character, and consequently in the lower degrees of combination - there yet remain affinities capable of retaining other elements, - and they probably retain ammonia, and hold it the more stably, - because all the properties of the platinum compounds are rather - acid than basic--that is, PtX_{n} recalls rather HX or SnX_{n} or - CX_{n} than KX, CaX_{2}, BaX_{2}, &c., and ammonia naturally will - rather combine with an acid than with a basic substance. Further, - a dependence, or certain connection of the forms of oxidation with - the ammonia compounds, is seen on comparing the following - compounds: - - PdCl_{2},2NH_{3},H_{2}O PdCl_{2},4NH_{3},H_{2}O - PtCl_{2},2NH_{3} PtCl_{4},4NH_{3} - RhCl_{3},5NH_{3} RuCl_{2},4NH_{3},3H_{2}O - IrCl_{3},5NH_{3} OsCl_{2},4NH_{3},2H_{2}O - - We know that platinum and palladium give compounds of lower types - than iridium and rhodium, whilst ruthenium and osmium give the - highest forms of oxidation; this shows itself in this case also. - We have purposely cited the same compounds with 4NH_{3} for osmium - and ruthenium as we have for platinum and palladium, and it is - then seen that Ru and Os are capable of retaining 2H_{2}O and - 3H_{2}O, besides Cl_{2} and NH_{3}, which the compounds of - platinum and palladium are unable to do. The same ideas which were - developed in Note 35, Chapter XXII. respecting the cobaltia - compounds are perfectly applicable to the present case, _i.e._ to - the _platinia_ compounds or ammonia compounds of the platinum - metals, among which Rh and Ir give compounds which are perfectly - analogous to the cobaltia compounds. - - Iridium and rhodium, which easily give compounds of the type - RX_{3}, give compounds (Claus) of the type IrX_{3},5NH_{3}, of a - rose colour, and RhX_{3},5NH_{3}, of a yellow colour. Jörgensen, - in his researches on these compounds, showed their entire analogy - with the cobalt compounds, as was to be expected from the periodic - system. - -If ammonia acts on a boiling solution of platinous chloride in -hydrochloric acid, it produces the green _salt of Magnus_ (1829), -PtCl_{2},2NH_{3}, insoluble in water and hydrochloric acid. But, judging -by its reactions, this salt has twice this formula. Thus, Gros (1837), on -boiling Magnus's salt with nitric acid, observed that half the chlorine -was replaced by the residue of nitric acid and half the -platinum was disengaged: 2PtCl_{2}(NH_{3})_{2} + 2HNO_{3} = -PtCl_{2}(NO_{3})_{2}(NH_{3})_{4} + 2PtCl_{2}. The Gros's salt thus -obtained, PtCl_{2}(NO_{3})_{2}4NH_{3} (if Magnus's salt belongs to the -type PtX_{2}, then Gros's salt belongs to the type PtX_{4}), is soluble -in water, and the elements of nitric acid, but not the chlorine, -contained in it are capable of easily submitting themselves to double -saline decomposition. Thus silver nitrate does not enter into double -decomposition with the chlorine of Gros's salt. Most instructive was the -circumstance that Gros, by acting on his salt with hydrochloric acid, -succeeded in substituting the residue of nitric acid in it by chlorine, -and the chlorine thus introduced, easily reacted with silver nitrate. -Thus it appeared that Gros's salt contained two varieties of -chlorine--one which reacts readily, and the other which reacts with -difficulty. The composition of Gros's first salt is -PtCl_{2}(NH_{3})_{4}(NO_{3})_{2}; it may be converted into -PtCl_{2}(NH_{3})_{4}(SO_{4}), and in general into -PtCl_{2}(NH_{3})_{4}X_{2}.[13] - - [13] Subsequently, a whole series of such compounds was obtained with - various elements in the place of the (non-reacting) chlorine, and - nevertheless they, like the chlorine, reacted with difficulty, - whilst the second portion of the X's introduced into such salts - easily underwent reaction. This formed the most important reason - for the interest which the study of the composition and structure - of the platino-ammonium salts subsequently presented to many - chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt, - Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and - others. The salts PtX_{4},2NH_{3}, discovered by Gerhardt, also - exhibited several different properties in the two pairs of X's. In - the remaining platino-ammonium salts all the X's appear to react - alike. - - The quality of the X's, retainable in the platino-ammonium salts, - may be considerably modified, and they may frequently be wholly or - partially replaced by hydroxyl. For example, the action of ammonia - on the nitrate of Gerhardt's base, Pt(NO_{3})_{4},2NH_{3}, in a - boiling solution, gradually produces a yellow crystalline - precipitate which is nothing else than a _basic hydrate_ or - _alkali_, Pt(OH)_{4},2NH_{3}. It is sparingly soluble in water, - but gives directly soluble salts PtX_{4},2NH_{3} with acids. The - stability of this hydroxide is such that potash does not expel - ammonia from it, even on boiling, and it does not change below - 130°. Similar properties are shown by the hydroxide - Pt(OH)_{2},2NH_{3} and the oxide PtO,2NH_{3} of Reiset's second - base. But the hydroxides of the compounds containing 4NH_{3} are - particularly remarkable. The presence of ammonia renders them - soluble and energetic. The brevity of this work does not permit - us, however, to mention many interesting particulars in connection - with this subject. - -The salt of Magnus when boiled with a solution of ammonia gives the -salt (of Reiset's first base) PtCl_{2}(NH_{3})_{4}, and this, when -treated with bromine, forms the salt PtCl_{2}Br_{2}(NH_{3})_{4}, which -has the same composition and reactions as Gros's salt. To Reiset's salts -there corresponds a soluble, colourless, crystalline _hydroxide_, -Pt(OH)_{2}(NH_{3})_{4}, having the properties of a powerful and very -energetic _alkali_; it attracts carbonic anhydride from the atmosphere, -precipitates metallic salts like potash, saturates active acids, even -sulphuric, forming colourless (with nitric, carbonic, and hydrochloric -acids), or yellow (with sulphuric acid), salts of the type -PtX_{2}(NH_{3})_{4}.[14] The comparative stability (for instance, as -compared with AgCl and NH_{3}) of such compounds, and the existence of -many other compounds analogous to them, endows them with a particular -chemical interest. Thus Kournakoff (1889) obtained a series of -corresponding compounds containing thiocarbamide, CSN_{2}H_{4}, in the -place of ammonia, PtCl_{2},4CSN_{2}H_{4}, and others corresponding with -Reiset's salts. Hydroxylamine, and other substances corresponding with -ammonia, also give similar compounds. The common properties and -composition of such compounds show their entire analogy to the cobaltia -compounds (especially for ruthenium and iridium) and correspond to the -fact that both the platinum metals and cobalt occur in the same, eighth, -group. - - [14] Hydroxides are known corresponding with Gros's salts, which - contain one hydroxyl group in the place of that chlorine or haloid - which in Gros's salts reacts with difficulty, and these hydroxides - do not at once show the properties of alkalis, just as the - chlorine which stands in the same place does not react distinctly; - but still, after the prolonged action of acids, this hydroxyl - group is also replaced by acids. Thus, for example, the action of - nitric acid on Pt(NO_{3})_{2}Cl_{2},4NH_{3} causes the non-active - chlorine to react, but in the product all the chlorine is not - replaced by NO_{3}, but only half, and the other half is replaced - by the hydroxyl group: Pt(NO_{3})_{2}Cl_{2},4NH_{3} + HNO_{3} + - H_{2}O = Pt(NO_{3})_{3}(OH),4NH_{3} + 2HCl; and this is - particularly characteristic, because here the hydroxyl group has - not reacted with the acid--an evident sign of the non-alkaline - character of this residue. I think it may be well to call - attention to the fact that the composition of the - ammonio-metallosalts very often exhibits a correspondence between - the amount of X's and the amount of NH_{3}, of such a nature that - we find they contain either XNH_{3} or the grouping X_{2}NH_{3}; - for example, Pt(XNH_{3})_{2} and Pt(X_{2}NH_{3})_{2}, - Co(X_{2}NH_{3})_{3}, Pt(XNH_{3})_{4}, &c. Judging from this, the - view of the constitution of the double cyanides of platinum given - in Note 11 finds some confirmation here, but, in my opinion, all - questions respecting the composition (and structure) of the - ammoniacal, double, complex, and crystallisation compounds stand - connected with the solution of questions respecting the formation - of compounds of various degrees of stability, among which a theory - of solutions must be included, and therefore I think that the time - has not yet come for a complete generalisation of the data which - exist for these compounds; and here I again refer the reader to - Prof. Kournakoff's work cited in Chapter XXII., Note 35. However, - we may add a few individual remarks concerning the platinia - compounds. - - To the common properties of the platino-ammonium salts, we must - add not only their _stability_ (feeble acids and alkalis do not - decompose them, the ammonia is not evolved by heating, &c.), but - also the fact that the ordinary reactions of platinum are - concealed in them to as great an extent as those of iron in the - ferricyanides. Thus neither alkalis nor hydrogen sulphide will - separate the platinum from them. For example, sulphuretted - hydrogen in acting on Gros's salts gives sulphur, removes half the - chlorine by means of its hydrogen, and forms salts of Reiset's - first base. This may be understood or explained by considering the - platinum in the molecule as covered, walled up by the ammonia, or - situated in the centre of the molecule, and therefore inaccessible - to reagents. On this assumption, however, we should expect to find - clearly-expressed ammoniacal properties, and this is not the case. - Thus ammonia is easily decomposed by chlorine, whilst in acting on - the platino-ammonium salts containing PtX_{2} and 2NH_{3} or - 4NH_{3}, chlorine combines and does not destroy the ammonia; it - converts Reiset's salts into those of Gros and Gerhardt. Thus from - PtX_{2},2NH_{3} there is formed PtX_{2}Cl_{2},2NH_{3}, and from - PtX_{2},4NH_{3} the salt of Gros's base PtX_{2}Cl_{2},4NH_{3}. - This shows that the amount of chlorine which combines is not - dependent on the amount of ammonia present, but is due to the - basic properties of platinum. Owing to this some chemists suppose - the ammonia to be inactive or passive in certain compounds. It - appears to me that these relations, these modifications, in the - usual properties of ammonia and platinum are explained directly by - their mutual combination. Sulphur, in sulphurous anhydride, - SO_{2}, and hydrogen sulphide, SH_{2}, is naturally one and the - same, but if we only knew of it in the form of hydrogen sulphide, - then, having obtained it in the form of sulphurous anhydride, we - should consider its properties as hidden. The oxygen in magnesia, - MgO, and in nitric peroxide, NO_{2}, is so different that there is - no resemblance. Arsenic no longer reacts in its compounds with - hydrogen as it reacts in its compounds with chlorine, and in their - compounds with nitrogen all metals modify both their reactions and - their physical properties. We are accustomed to judge the metals - by their saline compounds with haloid groups, and ammonia by its - compounds with acid substances, and here, in the - platino-compounds, if we assume the platinum to be bound to the - entire mass of the ammonia--to its hydrogen and nitrogen--we shall - understand that both the platinum and ammonia modify their - characters. Far more complicated is the question why a portion of - the chlorine (and other haloid simple and complex groups) in - Gros's salts acts in a different manner from the other portion, - and why only half of it acts in the usual way. But this also is - not an exclusive case. The chlorine in potassium chlorate or in - carbon tetrachloride does not react with the same ease with metals - as the chlorine in the salts corresponding with hydrochloric acid. - In this case it is united to oxygen and carbon, whilst in the - platino-ammonium compounds it is united partly to platinum and - partly to the platino-ammonium group. Many chemists, moreover, - suppose that a part of the chlorine is united directly to the - platinum and the other part to the nitrogen of the ammonia, and - thus explain the difference of the reactions; but chlorine united - to platinum reacts as well with a silver salt as the chlorine of - ammonium chloride, NH_{4}Cl, or nitrosyl chloride, NOCl, although - there is no doubt that in this case there is a union between the - chlorine and nitrogen. Hence it is necessary to explain the - absence of a facile reactive capacity in a portion of the chlorine - by the conjoint influence of the platinum and ammonia on it, - whilst the other portion may be admitted as being under the - influence of the platinum only, and therefore as reacting as in - other salts. By admitting a certain kind of stable union in the - platino-ammonium grouping, it is possible to imagine that the - chlorine does not react with its customary facility, because - access to a portion of the atoms of chlorine in this complex - grouping is difficult, and the chlorine union is not the same as - we usually meet in the saline compounds of chlorine. These are the - grounds on which we, in refuting the now accepted explanations of - the reactions and formation of the platino-compounds, pronounce - the following opinion as to their structure. - - In characterising the platino-ammonium compounds, it is necessary - to bear in mind that compounds which already contain PtX_{4} do - not combine directly with NH_{3}, and that such compounds as - PtX_{4},4NH_{3} only proceed from PtX_{2}, and therefore it is - natural to conclude that those affinities and forces which cause - PtX_{2} to combine with X_{2} also cause it to combine with - 2NH_{3}. And having the compound PtX_{2},2NH_{3}, and supposing - that in subsequently combining with Cl_{2} it reacts with those - affinities which produce the compounds of platinic chloride, - PtCl_{4}, with water, potassium chloride, potassium cyanide, - hydrochloric acid, and the like, we explain not only the fact of - combination, but also many of the reactions occurring in the - transition of one kind of platino-ammonium salts into another. - Thus by this means we explain the fact that (1) PtX_{2},2NH_{3} - combines with 2NH_{3}, forming salts of Reiset's first base; (2) - and the fact that this compound (represented as follows for - distinctness), PtX_{2},2NH_{3},2NH_{3}, when heated, or even when - boiled in solution, again passes into PtX_{2},2NH_{3} (which - resembles the easy disengagement of water of crystallisation, - &c.); (3) the fact that PtX_{2},2NH_{3} is capable of absorbing, - under the action of the same forces, a molecule of chlorine, - PtX_{2},2NH_{3},Cl_{2}, which it then retains with energy, because - it is attracted, not only by the platinum, but also by the - hydrogen of the ammonia; (4) the fact that this chlorine held in - this compound (of Gerhardt) will have a position unusual in salts, - which will explain a certain (although very feebly-marked) - difficulty of reaction; (5) the fact that this does not exhaust - the faculty of platinum for further combination (we need only - recall the compound PtCl_{4},2HCl,16H_{2}O), and that therefore - both PtX_{2},2NH_{3},Cl_{2} and PtX_{2},2NH_{3},2NH_{3} are still - capable of combination, whence the latter, with chlorine, gives - PtX_{2},2NH_{3},2NH_{3},Cl_{2}, after the type of PtX_{4}Y_{4} - (and perhaps higher); (6) the fact that Gros's compounds thus - formed are readily reconverted into the salts of Reiset's first - base when acted on by reducing agents; (7) the fact that in Gros's - salts, PtX_{2},2NH_{3}(NH_{3}X)_{2}, the newly-attached chlorine - or haloid will react with difficulty with salts of silver, &c., - because it is attached both to the platinum and to the ammonia, - for both of which it has an attraction; (8) the fact that the - faculty for further combination is not even yet exhausted in the - type of Gros's salts, and that we actually have a compound of - Gros's chlorine salt with platinous chloride and with platinic - chloride; the salt PtSO_{4},2NH_{3},2NH_{3},SO_{4} combines - further also with H_{2}O; (9) the fact that such a faculty of - combination with new molecules is naturally more developed in the - lower forms of combination than in the higher. Hence the salts of - Reiset's first base--for example, PtCl_{2},2NH_{3},2NH_{3}--both - combine with water and give precipitates (soluble in water but not - in hydrochloric acid) of double salts with many salts of the heavy - metals--for example, with lead chloride, cupric chloride, and also - with platinic and platinous chlorides (Buckton's salts). The - latter compounds will have the composition - PtCl_{2},2NH_{3},2NH_{3},PtCl_{2}--that is, the same composition - as the salts of Reiset's second base, but it cannot be identical - with it. Such an interesting case does actually exist. The first - salt, PtCl_{2},4NH_{3},PtCl_{2}, is green, insoluble in water and - in hydrochloric acid, and is known as _Magnus's salt_, and the - second, PtCl_{2},2NH_{3}, is Reiset's yellow, sparingly soluble - (in water). They are polymeric, namely, the first contains twice - the number of elements held in the second, and at the same time - they easily pass into each other. If ammonia be added to a hot - hydrochloric acid solution of platinous chloride, it forms the - salt PtCl_{2},4NH_{3}, but in the presence of an excess of - platinous chloride it gives Magnus's salt. On boiling the latter - in ammonia it gives a colourless soluble salt of Reiset's first - base, PtCl_{2},4NH_{3}, and if this be boiled with water, ammonia - is disengaged, and a salt of Reiset's second base, - PtCl_{2},2NH_{3}, is obtained. - - A class of platino-ammonium isomerides (obtained by Millon and - Thomsen) are also known. Buckton's salts--for example, the copper - salt--were obtained by them from the salts of Reiset's first base, - PtCl_{2},4NH_{3}, by treatment with a solution of cupric chloride, - &c., and therefore, according to our method of expression, - Buckton's copper salt will be PtCl_{2},4NH_{3},CuCl_{2}. This salt - is soluble in water, but not in hydrochloric acid. In it the - ammonia must be considered as united to the platinum. But if - cupric chloride be dissolved in ammonia, and a solution of - platinous chloride in ammonium chloride is added to it, a violet - precipitate is obtained of the same composition as Buckton's salt, - which, however, is insoluble in water, but soluble in hydrochloric - acid. In this a portion, if not all, of the ammonia must be - regarded as united to the copper, and it must therefore be - represented as CuCl_{2},4NH_{3},PtCl_{2}. This form is identical - in composition but different in properties (is isomeric) with the - preceding salt (Buckton's). The salt of Magnus is intermediate - between them, PtCl_{2},4NH_{3},PtCl_{2}; it is insoluble in water - and hydrochloric acid. These and certain other instances of - isomeric compounds in the series of the platino-ammonium salts - throw a light on the nature of the compounds in question, just as - the study of the isomerides of the carbon compounds has served and - still serves as the chief cause of the rapid progress of organic - chemistry. In conclusion, we may add that (according to the law of - substitution) we must necessarily expect all kinds of intermediate - compounds between the platino and analogous ammonia derivatives on - the one hand, and the complex compounds of nitrous acid on the - other. Perhaps the instance of the reaction of ammonia upon osmic - anhydride, OsO_{4}, observed by Fritsche, Frémy, and others, and - more fully studied by Joly (1891), belongs to this class. The - latter showed that when ammonia acts upon an alkaline solution of - OsO_{4} the reaction proceeds according to the equation: OsO_{4} + - KHO + NH_{3} = OsNKO_{3} + 2H_{2}O. It might be imagined that in - this case the ammonia is oxidised, probably forming the residue of - nitrous acid (NO), while the type OsO_{4} is deoxidised into - OsO_{2}, and a salt, OsO(NO)(KO), of the type OsX_{4} is formed. - This salt crystallises well in light yellow octahedra. It - corresponds to _osmiamic acid_, OsO(ON)(HO), whose anhydride - [OsO(NO)]_{2}, has the composition Os_{2}N_{2}O_{5}, which equals - 2Os + N_{2}O_{5} to the same extent as the above-mentioned - compound PtCO_{2} equals Pt + CO_{2} (_see_ Note 11). - - - - - CHAPTER XXIV - - COPPER, SILVER, AND GOLD - - -That degree of analogy and difference which exists between iron, cobalt, -and nickel repeats itself in the corresponding triad ruthenium, rhodium, -and palladium, and also in the heavy platinum metals, osmium, iridium, -and platinum. These nine metals form Group VIII. of the elements in the -periodic system, being the intermediate group between the even elements -of the large periods and the uneven, among which we know zinc, cadmium, -and mercury in Group II. Copper, silver, and gold complete[1] this -transition, because their properties place them in proximity to nickel, -palladium, and platinum on the one hand, and to zinc, cadmium, and -mercury on the other. Just as Zn, Cd, and Hg; Fe, Ru, and Os; Co, Rh, and -Ir; Ni, Pd, and Pt, resemble each other in many respects, so also do Cu, -Ag, and Au. Thus, for example, the atomic weight of copper Cu = 63, and -in all its properties it stands between Ni = 59 and Zn = 65. But as the -transition from Group VIII. to Group II., where zinc is situated, cannot -be otherwise than through Group I., so in copper there are certain -properties of the elements of Group I. Thus it gives a suboxide, Cu_{2}O, -and salts, CuX, like the elements of Group I., although at the same time -it forms an oxide, CuO, and salts CuX_{2}, like nickel and zinc. In the -state of the oxide, CuO, and the salts, CuX_{2}, copper is analogous to -zinc, judging from the insolubility of the carbonates, phosphates, and -similar salts, and by the isomorphism, and other characters.[2] In the -cuprous salts there is undoubtedly a great resemblance to the silver -salts--thus, for example, silver chloride, AgCl, is characterised by its -insolubility and capacity of combining with ammonia, and in this respect -cuprous chloride closely resembles it, for it is also insoluble in water, -and combines with ammonia and dissolves in it, &c. Its composition is -also RCl, the same as AgCl, NaCl, KCl, &c., and silver in many compounds -resembles, and is even isomorphous with, sodium, so that this again -justifies their being brought together. Silver chloride, cuprous -chloride, and sodium chloride crystallise in the regular system. Besides -which, the specific heats of copper and silver require that they should -have the atomic weights ascribed to them. To the oxides Cu_{2}O and -Ag_{2}O there are corresponding sulphides Ag_{2}S and Cu_{2}S. They both -occur in nature in crystals of the rhombic system, and, what is most -important, copper glance contains an isomorphous mixture of them both, -and retains the form of copper glance with various proportions of copper -and silver, and therefore has the composition R_{2}S where R = Cu, Ag. - - [1] The perfectly unique position held by copper, silver, and gold in - the periodic system of the elements, and the degree of affinity - which is found between them, is all the more remarkable, as nature - and practice have long isolated these metals from all others by - having employed them--for example, for coinage--and determined - their relative importance and value in conformity with the order - (silver between copper and gold) of their atomic weights, &c. - - [2] Cupric sulphate contains 5 molecules of water, CuSO_{4},5H_{2}O, - and the isomorphous mixtures with ZnSO_{4},7H_{2}O contain either 5 - or 7 equivalents, according to whether copper or zinc predominates - (Vol. II. p. 6). If there be a large proportion of copper, and if - the mixture contain 5H_{2}O, the form of the isomorphous mixture - (triclinic) will be isomorphous with cupric sulphate, - CuSO_{4},5H_{2}O, but if a large amount of zinc (or magnesium, - iron, nickel, or cobalt) be present the form (rhombic or - monoclinic) will be nearly the same as that of zinc sulphate, - ZnSO_{4},7H_{2}O. Supersaturated solutions of each of these salts - crystallise in that form and with that amount of water which is - contained in a crystal of one or other of the salts brought in - contact with the solution (Chapter XIV., Note 27). - -Notwithstanding the resemblance in the atomic composition of the cuprous -compounds, CuX, and silver compounds, AgX, with the compounds of the -alkali metals KX, NaX, there is a considerable degree of difference -between these two series of elements. This difference is clearly seen in -the fact that the alkali metals belong to those elements which combine -with extreme facility with oxygen, decompose water, and form the most -alkaline bases; whilst silver and copper are oxidised with difficulty, -form less energetic oxides, and do not decompose water, even at a rather -high temperature. Moreover, they only displace hydrogen from very few -acids. The difference between them is also seen in the dissimilarity of -the properties of many of the corresponding compounds. Thus cuprous -oxide, Cu_{2}O, and silver oxide, Ag_{2}O, are insoluble in water: the -cuprous and silver carbonates, chlorides, and sulphates are also -sparingly soluble in water. The oxides of silver and copper are also -easily reduced to metal. This difference in properties is in intimate -relation with that difference in the density of the metals which exists -in this case. The alkali metals belong to the lightest, and copper and -silver to the heaviest, and therefore the distance between the molecules -in these metals is very dissimilar--it is greater for the former than the -latter (tables in Chapter XV.). From the point of view of the periodic -law, this difference between copper and silver and such elements of Group -I. as potassium and rubidium, is clearly seen from the fact that copper -and silver stand in the middle of those large periods (for example, K, -Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br) which -start with the true metals of the alkalis--that is to say, the analogy -and difference between potassium and copper are of the same nature as -that between chromium and selenium, or vanadium and arsenic. - -_Copper_ is one of the few metals which have long been known in a -metallic form. The Greeks and Romans imported copper chiefly from the -island of Cyprus--whence its Latin name, _cuprum_. It was known to the -ancients before iron, and was used, especially when alloyed with other -metals, for arms and domestic utensils. That copper was known to the -ancients will be understood from the fact that it occurs, although -rarely, in a _native state_, and is easily extracted from its other -natural compounds. Among the latter are the oxygen compounds of copper. -When ignited with charcoal, they easily give up their oxygen to it, and -yield metallic copper; hydrogen also easily takes up the oxygen from -copper oxide when heated. Copper occurs in a native state, sometimes in -association with other ores, in many parts of the Urals and in Sweden, -and in considerable masses in America, especially in the neighbourhood of -the great American lakes; and also in Chili, Japan, and China. The oxygen -compounds of copper are also of somewhat common occurrence in certain -localities; in this respect certain deposits of the Urals are especially -famous. The geological period of the Urals (Permian) is characterised by -a considerable distribution of copper ores. Copper is met with in the -form of _cuprous oxide_, or _suboxide of copper_, Cu_{2}O, and is then -known as _red copper ore_, because it forms red masses which not -unfrequently are crystallised in the regular system. It is found much -more rarely in the state of _cupric oxide_, CuO, and is then called -_black copper ore_. The most common of the oxygenised compounds of copper -are the _basic carbonates_ corresponding with the oxides. That these -compounds are undoubtedly of aqueous origin is apparent, not only from -the fact that specimens are frequently found of a gradual transition from -the metallic, sulphuretted, and oxidised copper into its various -carbonates, but also from the presence of water in their composition, and -from the laminar, reniform structure which many of them present. In this -respect _malachite_ is particularly well known; it is used as a green -paint and also for ornaments, owing to the diversity of the shades of -colour presented by the different layers of deposited malachite. The -composition of malachite corresponds with the basic carbonate containing -one molecule of cupric carbonate to one of hydroxide: -CuCO_{3},CuH_{2}O_{2}. In this form the copper frequently occurs in -admixture with various sedimentary rocks, forming large strata, which -confirms the aqueous origin of these compounds. There are many such -localities in the Perm and other Governments bounding the Urals. _Blue -carbonate of copper_, or _azurite_, is also often met with in the same -localities; it contains the same ingredients as malachite, but in a -different proportion, its composition being CuH_{2}O_{2},2CuCO_{3}. Both -these substances may be obtained artificially by the action of the alkali -carbonates on solutions of cupric salts at various temperatures. These -native carbonates are often used for the extraction of copper, all the -more as they very readily give metallic copper, evolving water and -carbonic anhydride when ignited, and leaving the easily-reducible cupric -oxide. Copper is, however, still more often met with in the form of the -sulphides. The sulphides of copper generally occur in chemical -combination with the sulphides of iron.[3] These copper-sulphur compounds -(copper pyrites CuFeS_{2}, variegated copper ore Cu_{3}FeS_{3}, &c.) -generally occur in veins in a rock gangue. - - [3] Iron pyrites, FeS_{2}, very often contain a small quantity of - copper sulphide (_see_ Chapter XXII., Note 2 bis), and on burning - the iron pyrites for sulphurous anhydride the copper oxide remains - in the residue, from which the copper is often extracted with - profit. For this purpose the whole of the sulphur is not burnt off - from the iron pyrites, but a portion is left behind in the ore, - which is then slowly ignited (roasted) with access of air. Cupric - sulphate is then formed, and is extracted by water; or what is - better and more frequently done, the residue from the roasting of - the pyrites is roasted with common salt, and the solution of cupric - chloride obtained by lixiviating is precipitated with iron. A far - greater amount of copper is obtained from other sulphuretted ores. - Among these _copper glance_, Cu_{2}S, is more rarely met with. It - has a metallic lustre, is grey, generally crystalline, and is - obtained in admixture with organic matter; so that there is no - doubt that its origin is due to the reducing action of the latter - on solutions of cupric sulphate. _Variegated copper ore_, which - crystallises in octahedra, not infrequently forms an admixture in - copper glance; it has a metallic lustre, and is reddish-brown; it - has a superficial play of colours, due to oxidation proceeding on - its surface. Its composition is Cu_{3}FeS_{3}. But the most common - and widely-distributed copper ore is _copper pyrites_, which - crystallises in regular octahedra; it has a metallic lustre, a sp. - gr. of 4·0, and yellow colour. Its composition is CuFeS_{2}. It - must be remarked that the sulphurous ores of copper are oxidised in - the presence of water containing oxygen in solution, and form - cupric sulphate, blue vitriol, which is easily soluble in water. If - this water contains calcium carbonate, gypsum and cupric carbonate - are formed by double decomposition: CuSO_{4} + CaCO_{3} = CuCO_{3} - + CaSO_{4}. Hence copper sulphide in the form of different ores - must be considered as the primary product, and the many other - copper ores as secondary products, formed by water. This is - confirmed by the fact that at the present time the water extracted - from many copper mines contains cupric sulphate in solution. From - this liquid it is easy to extract cupric oxide by the action of - organic matter and various impurities of water. Hence metallic - copper is sometimes found in natural products of the modification - of copper sulphide and is probably deposited by the action of - organic matter present in the water. - -_The extraction of copper from its oxide ores_ does not present any -difficulty, because the copper, when ignited with charcoal and melted, is -reduced from the impurities which accompany it. This mode of smelting -copper ores is carried on in cupola or cylindrical furnaces, fluxes -forming a slag being added to the mixture of ore and charcoal. The -smelted copper still contains sulphur, iron, and other metallic -impurities, from which it is freed by fusion in reverberatory furnaces, -with access of air to the surface of the molten metal, as the iron and -sulphur are more easily oxidised than the copper. The iron then separates -as oxides, which collect in the slag.[4] - - [4] Copper ores rich in oxygen are very rare; the sulphur ores are of - more common occurrence, but the extraction of the copper from them - is much more difficult. The problem here not only consists in the - removal of the sulphur, but also in the removal of the iron - combined with the sulphur and copper. This is attained by a whole - series of operations, after which there still sometimes remains the - extraction of the metallic silver which generally accompanies the - copper, although in but small quantity. These processes commence - with the roasting--_i.e._ calcination--of the ore with access of - air, by which means the sulphur is converted into sulphurous - anhydride. It should here be remarked that iron sulphide is more - easily oxidised than copper sulphide, and therefore the greater - part of the iron in the residue from roasting is no longer in the - form of sulphide but of oxide of iron. The roasted ore is mixed - with charcoal, and siliceous fluxes, and smelted in a cupola - furnace. The iron then passes into the slag, because its oxide - gives an easily-fusible mass with the silica, whilst the copper, in - the form of sulphide, fuses and collects under the slag. The - greater part of the iron is removed from the mass by this smelting. - The resultant _coarse metal_ is again roasted in order to remove - the greater part of the sulphur from the copper sulphide, and to - convert the metal into oxide, after which the mass is again - smelted. These processes are repeated several times, according to - the richness of the ore. During these smeltings a portion of the - copper is already obtained in a metallic form, because copper - sulphide gives metallic copper with the oxide (CuS + 2CuO = 3Cu + - SO_{2}). We will not here describe the furnaces used or the details - of this process, but the above remarks include the explanation of - those chemical processes which are accomplished in the various - technical operations which are made use of in the process (for - details _see_ works on metallurgy). - - Besides the smelting of copper there also exist methods for its - extraction from solutions in the wet way, as it is called. Recourse - is generally had to these methods for poor copper ores. The copper - is brought into solution, from which it is separated by means of - metallic iron or by other methods (by the action of an electric - current). The sulphides are roasted in such a manner that the - greater part of the copper is oxidised into cupric sulphate, whilst - at the same time the corresponding iron salts are as far as - possible decomposed. This process is based on the fact that the - copper sulphides absorb oxygen when they are calcined in the - presence of air, forming cupric sulphate. The roasted ore is - treated with water, to which acid is sometimes added, and after - lixiviation the resultant solution containing copper is treated - either with metallic iron or with milk of lime, which precipitates - cupric hydroxide from the solution. Copper oxide ores poor in metal - may be treated with dilute acids in order to obtain the copper - oxides in solution, from which the copper is then easily - precipitated either by iron or as hydroxide by lime. According to - Hunt and Douglas's method, the copper in the ore is converted by - calcination into the cupric oxide, which is brought into solution - by the action of a mixture of solutions of ferrous sulphate and - sodium chloride; the oxide converts the ferrous chloride into - ferric oxide, forming copper chlorides, according to the equation - 3CuO + 2FeCl_{2} = CuCl_{2} + 2CuCl + Fe_{2}O_{3}. The cupric - chloride is soluble in water, whilst the cuprous chloride is - dissolved in the solution of sodium chloride, and therefore all the - copper passes into solution, from which it is precipitated by iron. - - The same American metallurgists give the following wet method for - extracting the Ag and Au occurring in many copper ores, especially - in sulphurous ores: (1) The Cu_{2}S is first converted into oxide - by roasting in a calciner; (2) the CuO is extracted by the dilute - sulphuric acid obtained in the fourth process, the Cu then passes - into solution, while the Ag, Au and oxides of iron remain behind in - the residue (from which the noble metals may be extracted); (3) a - portion of the copper in solution is converted into CuCl_{2} (and - CaSO_{4} precipitated) by means of the CaCl_{2} obtained in the - fifth process; (4) the mixture of solutions of CuSO_{4} and - CuCl_{2} is converted into the insoluble CuCl (salt of the - suboxide) by the action of the SO_{2} obtained by roasting the ore - (in the first operation), sulphuric acid is then formed in the - solution, according to the equation: CuSO_{4} + CuCl_{2} + SO_{2} + - 2H_{2}O = 2H_{2}SO_{4} + 2CuCl; (5) the precipitated CuCl is - treated with lime and water, and gives CuCl_{2} in solution and CuO - in the residue; and lastly (6) the Cu_{2}O is reduced to metallic - Cu by carbon in a furnace. According to Crooke's method the impure - copper regulus obtained by roasting and smelting the ore is broken - up and immersed repeatedly in molten lead, which extracts the Ag - and Au occurring in the regulus. The regulus is then heated in a - reverberatory furnace to run off the lead, and is then smelted for - Cu. - - The copper brought into the market often contains small quantities - of various impurities. Among these there are generally present - iron, lead, silver, arsenic, and sometimes small quantities of - oxides of copper. As copper, when mixed with a small amount of - foreign substances, loses its tenacity to a certain degree, the - manufacture of very thin sheet copper requires the use of Chili - copper, which is distinguished for its great softness, and - therefore when it is desired to have pure copper, it is best to - take thin sheet copper, like that which is used in the manufacture - of cartridges. But the purest copper is electrolytic copper--that - is, that which is deposited from a solution by the action of an - electric current. - - If the copper contains silver, as is often the case, it is used in - gold refineries for the precipitation of silver from its solutions - in sulphuric acid. Iron and zinc reduce copper salts, but copper - reduces mercury and silver salts. The precipitate contains not only - the silver which was previously in solution, but also all that - which was in the copper. The silver solutions in sulphuric acid are - obtained in the separation of silver from gold by treating their - alloys with sulphuric acid, which only dissolves the silver. - -Copper is characterised by its red colour, which distinguishes it from -all other metals. Pure copper is soft, and may be beaten out by a hammer -at the ordinary temperature, and when hot may be rolled into very thin -sheets. Extremely thin leaves of copper transmit a green light. The -tenacity of copper is also considerable, and next to iron it is one of -the most durable metals in this respect. Copper wire of 1 sq. millimetre -in section only breaks under a weight of 45 kilograms. The specific -gravity of copper is 8·8, unless it contains cavities due to the fact -that molten copper absorbs oxygen from the air, which is disengaged on -cooling, and therefore gives a porous mass whose density is much less. -Rolled copper, and also that which is deposited by the electric current, -has a comparatively high density. Copper melts at a bright red heat, -about 1050°, although below the temperature at which many kinds of cast -iron melt. At a high temperature it is converted into vapour, which -communicates a green colour to the flame. Both native copper and that -cooled from a molten state crystallise in regular octahedra. Copper is -not oxidised in dry air at the ordinary temperature, but when calcined it -becomes coated with a layer of oxide, and it does not burn even at the -highest temperature. Copper, when calcined in air, forms either the red -cuprous oxide or the black cupric oxide, according to the temperature and -quantity of air supplied. In air at the ordinary temperature, copper--as -everyone knows--becomes coated with a brown layer of oxides or a green -coating of basic salts, due to the action of the damp air containing -carbonic acid. If this action continue for a prolonged time, the copper -is covered with a thick coating of basic carbonate, or the so-called -verdigris (the _ærugo nobilis_ of ancient statues). This is due to the -fact that copper, although scarcely capable of oxidising by itself,[5] -_in the presence of water and acids_--even very feeble acids, like -carbonic acid--_absorbs oxygen from the air and forms salts_, which is a -very characteristic property of it (and of lead).[6] _Copper does not -decompose water_, and therefore does not disengage hydrogen from it -either at the ordinary or at high temperatures. Nor does copper liberate -hydrogen from the oxygen acids; these act on it in two ways: they either -give up a portion of their oxygen, forming lower grades of oxidation, or -else only react in the presence of air. Thus, when nitric acid acts on -copper it evolves nitric oxide, the copper being oxidised at the expense -of the nitric acid. In the same way copper converts sulphuric acid into -the lower grade of oxidation--into sulphurous anhydride, SO_{2}. In these -cases the copper is oxidised to copper oxide, which combines with the -excess of acid taken, and therefore forms a cupric salt, CuX_{2}. Dilute -nitric acid does not act on copper at the ordinary temperature, but when -heated it reacts with great ease; dilute sulphuric acid does not act on -copper except in presence of air. - - [5] Schützenberger showed that when the basic carbonate of copper is - decomposed by an electric current it gives, besides the ordinary - copper, an allotropic form which grows on the negative platinum - electrode, if its surface be smaller than that of the positive - copper electrode, in the form of brittle crystalline growths of sp. - gr. 8·1. It differs from ordinary copper by giving not nitric oxide - but nitrous oxide when treated with nitric acid, and in being very - easily oxidised in air, and coated with red shades of colour. It is - possible that this is copper hydride, or copper which has occluded - hydrogen. Spring (1892) observed that copper reduced from the oxide - by hydrogen at the lowest possible temperature was pulverulent, - while that reduced from CuCl_{2} at a somewhat high temperature - appeared in bright crystals. The same difference occurs with many - other metals, and is probably partly due to the volatility of the - metallic chlorides. - - [6] This is taken advantage of in practice; for instance, by pouring - dilute acids over copper turnings on revolving tables in the - preparation of copper salts, such as verdigris, or the basic - acetate 2C_{4}H_{6}CuO_{4},CuH_{2}O_{2},5H_{2}O, which is so much - used as an oil paint (_i.e._ with boiled oil). The capacity of - copper for absorbing oxygen in the presence of acids is so great - that it is possible by this means (by taking, for example, thin - copper shavings moistened with sulphuric acid) to take up all the - oxygen from a given volume of air, and this is even employed for - the analysis of air. - - The combination of copper with oxygen is not only aided by acids - but also by alkalis, although cupric oxide does not appear to have - an acid character. Alkalis do not act on copper except in the - presence of air, when they produce cupric oxide, which does not - appear to combine with such alkalis as caustic potash or soda. But - the _action of ammonia_ is particularly distinct (Chapter V., Note - 2). In the action of a solution of ammonia not only is oxygen - absorbed by the copper, but it also acts on the ammonia, and a - definite quantity of ammonia is always acted on simultaneously with - the passage of the copper into solution. The ammonia is then - converted into nitrous acid, according to the reaction: NH_{3} + - O_{3} = NHO_{2} + H_{2}O, and the nitrous acid thus formed passes - into the state of ammonium nitrite, NH_{4}NO_{2}. In this manner - three equivalents of oxygen are expended on the oxidation of the - ammonia, and six equivalents of oxygen pass over to the copper, - forming six atoms of cupric oxide. The latter does not remain in - the state of oxide, but combines with the ammonia. - - A strong solution of common salt does not act on copper, but a - dilute solution of the salt corrodes copper, converting it into - oxychloride--that is, in the presence of air. This action of salt - water is evident in those cases where the bottoms of ships are - coated with sheet copper. From what has been said above it will be - evident that copper vessels should not be employed in the - preparation of food, because this contains salts and acids which - act on copper in the presence of air, and give copper salts, which - are poisonous, and therefore the food prepared in untinned copper - vessels may be poisonous. Hence tinned vessels are employed for - this purpose--that is, copper vessels coated with a thin layer of - tin, on which acid and saline solutions do not act. - -Both the oxides of copper, Cu_{2}O and CuO, are unacted on by air, and, -as already mentioned, they both occur in nature.[6 bis] However, in the -majority of cases copper is obtained in the form of cupric oxide and its -salts--and the copper compounds used industrially generally belong to -this type. This is due to the fact that the _cuprous compounds absorb -oxygen_ from the air and pass into cupric compounds. The cupric compounds -may serve as the source for the preparation of cuprous oxide, because -many reducing agents are capable of deoxidising the oxide into the -suboxide. Organic substances are most generally employed for this -purpose, and especially saccharine substances, which are able, in the -presence of alkalis, to undergo oxidation at the expense of the oxygen of -the cupric oxide, and to give acids which combine with the alkali: 2CuO - -O = Cu_{2}O. In this case the deoxidation of the copper may be carried -further and metallic copper obtained, if only the reaction be aided by -heat. Thus, for example, a fine powder of metallic copper may be obtained -by heating an ammoniacal solution of cupric oxide with caustic potash and -grape sugar. But if the reducing action of the saccharine substance -proceed in the presence of a sufficient quantity of alkali in solution, -and at not too high a temperature, cuprous oxide is obtained. To see this -reaction clearly, it is not sufficient to take any cupric salt, because -the alkali necessary for the reaction might precipitate cupric oxide--it -is necessary to add previously some substance which will prevent this -precipitation. Among such substances, tartaric acid, C_{4}H_{6}O_{6}, is -one of the best. In the presence of a sufficient quantity of tartaric -acid, any amount of alkali may be added to a solution of cupric salt -without producing a precipitate, because a soluble double salt of cupric -oxide and alkali is then formed. If glucose (for instance, honey or -molasses) be added to such an alkaline tartaric solution, and the -temperature be slightly raised, it first gives a yellow precipitate (this -is cuprous hydroxide, CuHO), and then, on boiling, a red precipitate of -(anhydrous) cuprous oxide. If such a mixture be left for a long time at -the ordinary temperature, it deposits well-formed crystals of anhydrous -cuprous oxide belonging to the regular system.[7] - - [6 bis] Copper, besides the cuprous oxide, Cu_{2}O, and cupric oxide, - CuO, gives two known higher forms of oxidation, but they have - scarcely been investigated, and even their composition is not well - known. _Copper dioxide_ (CuO_{2}, or CuO_{2},H_{2}O, perhaps - CuOH_{2}O_{2}) is obtained by the action of hydrogen peroxide on - cupric hydroxide, when the green colour of the latter is changed to - yellow. It is very unstable, and is decomposed even by boiling - water, with the evolution of oxygen, whilst the action of acids - gives cupric salts, oxygen being also disengaged. A still higher - _copper peroxide_ is formed by heating a mixture of caustic potash, - nitre, and metallic copper to a red heat, and by dissolving cupric - hydroxide in solutions of the hypochlorites of the alkali metals. A - slight heating of the soluble salt formed is enough for it to be - decomposed into oxygen and copper dioxide, which is precipitated. - Judging from Frémy's researches, the composition of the - copper-potassic compound should be K_{2}CuO_{4}. Perhaps this is a - compound of the peroxides of potassium, K_{2}O_{2}, and of copper, - CuO_{2}. - - [7] Colourless solutions of cuprous salts may also be obtained by the - action of sulphurous or phosphorous acid and similar lower grades - of oxidation on the blue solutions of the cupric salts. This is - very clearly and easily effected by means of sodium thiosulphate, - Na_{2}S_{2}O_{3}, which is oxidised in the process. Cuprous oxide - can not only be obtained by the deoxidation of cupric oxide, but - also directly from metallic copper itself, because the latter, in - oxidising at a red heat in air, first gives cuprous oxide. It is - prepared in this manner on a large scale by heating sheet copper - rolled into spirals in reverberatory furnaces. Care must be taken - that the air is not in great excess, and that the coating of red - cuprous oxide formed does not begin to pass into the black cupric - oxide. If the oxidised spiral sheet is then unbent, the brittle - cuprous oxide falls away from the soft metal. The suboxide obtained - in this manner fuses with ease. It is necessary to prevent the - access of air during the fusion, and if the mass contains cupric - oxide it must be mixed with charcoal, which reduces the latter. - Cuprous chloride, CuCl, corresponding with cuprous oxide (as sodium - chloride corresponds with sodium oxide), when calcined with sodium - carbonate, gives sodium chloride and cuprous oxide, carbonic - anhydride being evolved, because it does not combine with the - cuprous oxide under these conditions. The reaction can be expressed - by the following equation: 2CuCl + Na_{2}CO_{3} = Cu_{2}O + 2NaCl + - CO_{2}. The cupric oxide itself, when calcined with finely-divided - copper, this copper powder may be obtained by many methods--for - instance, by immersing zinc in a solution of a copper salt, or by - igniting cupric oxide in hydrogen), gives the fusible cuprous - oxide: Cu + CuO = Cu_{2}O. Both the native and artificial cuprous - oxide have a sp. gr. of 5·6. It is insoluble in water, and is not - acted on by (dry) air. When heated with acids the suboxide forms a - solution of a cupric salt and metallic copper--for example, Cu_{2}O - + H_{2}SO_{4} = Cu + CuSO_{4} + H_{2}O. However, strong - hydrochloric acid does not separate metallic copper on dissolving - cuprous oxide, which is due to the fact that the cuprous chloride - formed is soluble in strong hydrochloric acid. Cuprous oxide also - dissolves in a solution of ammonia, and in the absence of air gives - a colourless solution, which turns blue in the air, absorbing - oxygen, owing to the conversion of the cuprous oxide into cupric - oxide. The blue solution thus formed may be again reconverted into - a colourless cuprous solution by immersing a copper strip in it, - because the metallic copper then deoxidises the cupric oxide in the - solution into cuprous oxide. Cuprous oxide is characterised by the - fact that it gives red glasses when fused with glass or with salts - forming vitreous alloys. Glass tinted with cuprous oxide is used - for ornaments. The access of air must be avoided during its - preparation, because the colour then becomes green, owing to the - formation of cupric oxide, which colours glass blue. This may even - be taken advantage of in testing for copper under the blow-pipe by - heating the copper compound with borax in the flame of a blow-pipe; - a red glass is obtained in the reducing flame, and a blue glass in - the oxidising flame, owing to the conversion of the cuprous into - cupric oxide. - - Étard (1882), by passing sulphurous anhydride into a solution of - cupric acetate, obtained a white precipitate of cuprous sulphite, - Cu_{2}SO_{3},H_{2}O, whilst he obtained the same salt, of a red - colour, from the double salt of sodium and copper; but there are - not any convincing proofs of isomerism in this case. - -Cupric chloride, CuCl_{2}, when ignited, gives _cuprous chloride_, -CuCl--_i.e._ the salt corresponding with suboxide of copper--and -therefore cuprous chloride is always formed when copper enters into -reaction with chlorine at a high temperature. Thus, for example, when -copper is calcined with mercuric chloride, it forms cuprous chloride and -vapours of mercury. The same substance is obtained on heating metallic -copper in hydrochloric acid, hydrogen being disengaged; but this reaction -only proceeds with finely-divided copper, as hydrochloric acid acts very -feebly on compact masses of copper, and, in the presence of air, gives -cupric chloride. The green solution of cupric chloride is decolorised by -metallic copper, cuprous chloride being formed; but this reaction is only -accomplished with ease when the solution is very concentrated and in the -presence of an excess of hydrochloric acid to dissolve the cuprous -chloride. The addition of water to the solution precipitates the cuprous -chloride, because it is less soluble in dilute than in strong -hydrochloric acid. Many reducing agents which are able to take up half -the oxygen from cupric oxide are able, in the presence of hydrochloric -acid, to form cuprous chloride. Stannous salts, sulphurous anhydride, -alkali sulphites, phosphorous and hypophosphorous acids, and many similar -reducing agents, act in this manner. The usual method of preparing -cuprous chloride consists in passing sulphurous anhydride into a very -strong solution of cupric chloride: 2CuCl_{2} + SO_{2} + 2H_{2}O = 2CuCl -+ 2HCl + H_{2}SO_{4}. Cuprous chloride forms colourless cubic crystals -which are insoluble in water. It is easily fusible, and even volatile. -Under the action of oxidising agents, it passes into the cupric salt, and -it absorbs oxygen from moist air, forming cupric oxychloride, -Cu_{2}Cl_{2}O. _Aqueous ammonia_ easily _dissolves_ cuprous chloride as -well as cuprous oxide; the solution also turns blue on exposure to the -air. Thus an ammoniacal solution of cuprous chloride serves as an -excellent absorbent for oxygen; but this solution absorbs not only -oxygen, but also certain other gases--for example, carbonic oxide and -acetylene.[8] - - [8] The solubility of cuprous chloride in ammonia is due to the - formation of compounds between the ammonia and the chloride. In a - warm solution the compound NH_{3},2CuCl is formed, and at the - ordinary temperature CuCl,NH_{3}. This salt is soluble in - hydrochloric acid, and then forms a corresponding double salt of - cuprous chloride and ammonium chloride. By the action of a certain - excess of ammonia on a hydrochloric acid solution of cuprous - chloride, very well formed crystals, having the composition - CuCl,NH_{3},H_{2}O, are obtained. Cuprous chloride is not only - soluble in ammonia and hydrochloric acid, but it also dissolves in - solutions of certain other salts--for example, in sodium chloride, - potassium chloride, sodium thiosulphate, and certain others. All - the solutions of cuprous chloride act in many cases as very - powerful deoxidising substances; for example, it is easy, by means - of these solutions, to precipitate gold from its solutions in a - metallic form, according to the equation AuCl_{3} + 3CuCl = Au + - 3CuCl_{2}. - - Among the other compounds corresponding with cuprous oxide, - _cuprous iodide_, CuI, is worthy of remark. It is a colourless - substance which is insoluble in water and sparingly soluble in - ammonia (like silver iodide), but capable of absorbing it, and in - this respect it resembles cuprous chloride. It is remarkable from - the fact that it is exceedingly easily formed from the - corresponding cupric compound CuI_{2}. A solution of cupric iodide - easily decomposes into iodine and cuprous iodide, even at the - ordinary temperature, whilst cupric chloride only suffers a similar - change on ignition. If a solution of a cupric salt be mixed with a - solution of potassium iodide the cupric iodide formed immediately - decomposes into free iodine and cuprous iodide, which separates out - as a precipitate. In this case the cupric salt acts in an oxidising - manner, like, for example, nitrous acid, ozone, and other - substances which liberate iodine from iodides, but with this - difference, that it only liberates half, whilst they set free the - whole of the iodine from potassium iodide: 2KI + CuCl_{2} = 2KCl + - CuI + I. - - It must also be remarked that cuprous oxide, when treated with - hydrofluoric acid, gives an insoluble cuprous fluoride, CuF. - Cuprous cyanide is also insoluble in water, and is obtained by the - addition of hydrocyanic acid to a solution of cupric chloride - saturated with sulphurous anhydride. This cuprous cyanide, like - silver cyanide, gives a double soluble salt with potassium cyanide. - The double cyanide of copper and potassium is tolerably stable in - the air, and enters into double decompositions with various other - salts, like those double cyanides of iron with which we are already - acquainted. - - _Copper hydride_, CuH, also belongs to the number of the cuprous - compounds. It was obtained by Würtz by mixing a hot (70°) solution - of cupric sulphate with a solution of hypophosphorous acid, - H_{3}PO_{2}. The addition of the reducing hypophosphorous acid must - be stopped when a brown precipitate makes its appearance, and when - gas begins to be evolved. The brown precipitate is the hydrated - cuprous hydride. When gently heated it disengages hydrogen; it - gives cuprous oxide when exposed to the air, burns in a stream of - chlorine, and liberates hydrogen with hydrochloric acid: CuH + HCl - = CuCl + H_{2}. Zinc, silver, mercury, lead, and many other heavy - metals do not form such a compound with hydrogen, neither under - these circumstances nor under the action of hydrogen at the moment - of the decomposition of salts by a galvanic current. The greatest - resemblance is seen between cuprous hydride and the hydrogen - compounds of potassium, sodium, Pd, Ca, and Ba. - -When copper is oxidised with a considerable quantity of oxygen at a -high temperature, or at the ordinary temperature in the presence of -acids, and also when it decomposes acids, converting them into lower -grades of oxidation (for example, when submitted to the action of nitric -and sulphuric acids), it forms _cupric oxide_, CuO, or, in the presence -of acids, cupric salts. Copper rust, or that black mass which forms on -the surface of copper when it is calcined, consists of cupric oxide. The -coating of the oxidised copper is very easily separated from the metallic -copper, because it is brittle and very easily peels off, when it is -struck or immersed in water. Many copper salts (for instance, the nitrite -and carbonate) leave oxide of copper[8 bis] in the form of friable black -powder, after being ignited. If the ignition be carried further, Cu_{2}O -may be formed from the CuO.[8 tri] Anhydrous cupric oxide is very easily -dissolved in acids, forming cupric salts, CuX_{2}. They are analogous to -the salts MgX_{2}, ZnX_{2}, NiX_{2}, FeX_{2}, in many respects. On adding -potassium or ammonium hydroxide to a solution of a cupric salt, it forms -a gelatinous blue precipitate of the hydrated oxide of copper, -CuH_{2}O_{2}, insoluble in water. The resultant precipitate _is -redissolved by an excess of ammonia_, and gives a very beautiful azure -blue solution, of so intense a colour that the presence of small traces -of cupric salts may be discovered by this means.[9] An excess of -potassium or sodium hydroxide does not dissolve cupric hydroxide. A hot -solution gives a black precipitate of the anhydrous oxide instead of the -blue precipitate, and the precipitate of the hydroxide of copper becomes -granular, and turns black when the solution is heated. This is due to the -fact that the blue hydroxide is exceedingly unstable, and when slightly -heated it loses the elements of water and gives the black anhydrous -cupric oxide: CuH_{2}O_{2} = CuO + H_{2}O. - - [8 bis] The oxide of copper obtained by igniting the nitrate is - frequently used for organic analyses. It is hygroscopic and retains - nitrogen (1·5 c.c. per gram) when the nitrate is heated in vacuo - (Richards and Rogers, 1893). - - [8 tri] Oxide of copper is also capable of dissociating when heated. - Debray and Joannis showed that it then disengages oxygen, whose - maximum tension is constant for a given temperature, providing that - fusion does not take place (the CuO then dissolves in the molten - Cu_{2}O); that this loss of oxygen is followed by the formation of - suboxide, and that on cooling, the oxygen is again absorbed, - forming CuO. - - [9] Cupric oxide and many of its salts are able to give definite, - although unstable, _compounds with ammonia_. This faculty already - shows itself in the fact that cupric oxide, as well as the salts of - copper, dissolves in aqueous ammonia, and also in the fact that - salts of copper absorb ammonia gas. If ammonia be added to a - solution of any cupric salt, it first forms a precipitate of cupric - hydroxide, which then dissolves in an excess of ammonia. The - solution thus formed, when evaporated or on the addition of - alcohol, frequently deposits crystals of salts containing both the - elements of the salt of copper taken and of ammonia. Several such - compounds are generally formed. Thus cupric chloride, CuCl_{2}, - according to Deherain, forms four compounds with ammonia--namely, - with one, two, four, and six molecules of ammonia. Thus, for - example, if ammonia gas be passed into a boiling saturated solution - of cupric chloride, on cooling, small octahedral crystals of a blue - colour separate out, containing CuCl_{2},2NH_{3},H_{2}O. At 150° - this substance loses half the ammonia and all the water contained - in it, leaving the compound CuCl_{2},NH_{3}. Nitrate of copper - forms the compound Cu(NO_{3})_{2},2NH_{3}· This compound remains - unchanged on evaporation. Dry cupric sulphate absorbs ammonia gas, - and gives a compound containing five molecules of ammonia to one of - sulphate (Vol. I., p. 257, and Chapter XXII., Note 35). If this - compound is dissolved in aqueous ammonia, on evaporation it - deposits a crystalline substance containing - CuSO_{4},4NH_{3},H_{2}O. At 150° this substance loses the molecule - of water and one-fourth of its ammonia. On ignition all these - compounds part with the remaining ammonia in the form of an - ammoniacal salt, so that the residue consists of cupric oxide. Both - the hydrated and anhydrous cupric oxide are soluble in aqueous - ammonia. - - The solution obtained by the action of aqueous ammonia and air on - copper turnings (Note 6) is remarkable for its faculty of - _dissolving cellulose_, which is insoluble in water, dilute acids, - and alkalis. Paper soaked in such a solution acquires the property - of not rotting, of being difficultly combustible, and waterproof, - &c. It has therefore been applied, especially in England, to many - practical purposes--for example, to the construction of temporary - buildings, for covering roofs, &c. The composition of the substance - held in solution is Cu(HO)_{2},4NH_{3}. - - If dry ammonia gas be passed over cupric oxide heated to 265°, a - portion of the oxide of copper remains unaltered, whilst the other - portion gives _copper nitride_, the oxygen of the copper oxide - combining with the hydrogen and forming water. The oxide of copper - which remains unchanged is easily removed by washing the resultant - product with aqueous ammonia. Copper nitride is very stable, and is - insoluble; it has the composition Cu_{3}N (_i.e._ the copper is - monatomic here as in Cu_{2}O), and is an amorphous green powder, - which is decomposed when strongly ignited, and gives cuprous - chloride and ammonium chloride when treated with hydrochloric acid. - Like the other nitrides, copper nitride, Cu_{3}N, has scarcely been - investigated. Granger (1892), by heating copper in the vapour of - phosphorus, obtained hexagonal prisms of Cu_{5}P, which passed into - Cu_{6}P (previously obtained by Abel) when heated in nitrogen. - Arsenic is easily absorbed by copper, and its presence (like P), - even in small quantities, has a great influence upon the properties - of copper--for instance, pure copper wire 1 sq. mm. in section - breaks under a load of 35 kilos, while the presence of O·22 p.c. of - arsenic raises the breaking load to 42 kilos. - -Cupric oxide fuses at a strong heat, and on cooling forms a heavy -crystalline mass, which is black, opaque, and somewhat tenacious. It is a -feebly energetic base, so that not only do the oxides of the metals of -the alkalis and alkaline earths displace it from its compounds, but even -such oxides as those of lead and silver precipitate it from solutions, -which is partially due to these oxides being soluble, although but -slightly so, in water. However, cupric oxide, and especially the -hydroxide, easily combines with even the least energetic acids, and does -not give any compounds with bases; but, on the other hand, _it easily -forms basic salts_,[9 bis] and in this respect outstrips magnesium and -recalls the oxides of lead or mercury. Hence the hydroxide of copper -dissolves in solutions of neutral cupric salts. The cupric salts are -generally blue or green, because cupric hydroxide itself is coloured. But -some of the salts in the anhydrous state are colourless.[10] - - [9 bis] As a comparatively feeble base, oxide of copper easily forms - both basic and double salts. As an instance we may mention the - double salts composed of the dichloride CuCl_{2},2H_{2}O and - potassium chloride. The double salt CuK_{2}Cl_{4},2H_{2}O - crystallises from solutions in _blue_ plates, but when heated alone - or with substances taking up water easily gives _brown_ needles - CuKCl_{3} and at the same time KCl, and this reaction is reversible - at 92° as Meyerhoffer (1889) showed (_i.e._ above 92° the simpler - double salt is formed and below 92° the more complex salt). With an - excess of the copper salt, KCl gives another double salt, - Cu_{2}KCl_{5},4H_{2}O, the transition temperature of which is 55°. - The instances of equilibria which are encountered in such complex - relations (_see_ Chapter XIV., Note 25, astrakhanite, and Chapter - XXII., Note 23) are embraced by the _law of phases_ given by Gibbs - (Transactions of the Connecticut Academy of Sciences, 1875-1878, in - J. Willard Gibbs' memoir 'On the equilibrium of heterogeneous - substances:' and in a clearer and more accessible form in H. W. - Bakhuis Roozeboom's papers, Rec. trav. chim., Vol. VI., and in W. - Meyerhoffer's memoir _Die Phasenregel und ihre Anwendungen_, 1893, - to which sources we refer those desiring fuller information - respecting this law). Gibbs calls '_bodies_' substances (simple or - compound) capable of forming homogeneous complexes (for instance, - solutions or intercombinations) of a varied composition; a - _phase_--a mechanically separable portion of such bodies or of - their homogeneous complexes (for instance, a vapour, liquid or - precipitated solid), _perfect equilibrium_--such a state of bodies - and of their complexes as is characterised by a constant pressure - at a constant temperature even under a change in the amount of one - of the component parts (for instance, of a salt in a saturated - solution), while an _imperfect equilibrium_ is such a one for which - such a change corresponds with a change of pressure (for instance, - an unsaturated solution). The law of phases consists in the fact - that: _n bodies only give a perfect equilibrium when n + 1 phases - participate in that equilibrium_--for example, in the equilibrium - of a salt in its saturated solution in water there are two bodies - (the salt and water) and three phases, namely, the salt, solution, - and vapour, which can be mechanically separated from each other, - and to this equilibrium there corresponds a definite tension. At - the same time, _n bodies may occur in n + 2 phases, but only at one - definite temperature and one pressure_; a change of one of these - may bring about another state (perfect or not--equilibrium stable - or unstable). Thus water when liquid at the ordinary temperature - offers two phases (liquid and vapour) and is in perfect equilibrium - (as also is ice below 0°), but water, ice, and vapour (three phases - and only one body) can only be in equilibrium at 0°, and at the - ordinary pressure; with a change of _t_ there will remain either - only ice and vapour or only liquid water and vapour; whilst with a - rise of pressure not only will the vapour pass into the liquid - (there again only remain two phases) but also the temperature of - the formation of ice will fall (by about 7° per 1000 atmospheres). - The same laws of phases are applicable to the consideration of the - formation of simple or double salts from saturated solutions and to - a number of other purely chemical relations. Thus, for example, in - the above-mentioned instance, when the bodies are KCl, CuCl_{2}, - and H_{2}O, perfect equilibrium (which here has reference to the - solubility) consisting of four phases, corresponds to the following - seven cases, considering only the phases (above 0°) A = - CuCl_{2},2KCl,2H_{2}O; B = CuCl_{2}KCl; C = CuCl_{2},2H_{2}O,KCl, - solution and vapour: (1) A + B + solution + vapour; (2) A + C + - solution + vapour; (3) A + KCl + solution + vapour; (4) A + B + C + - vapour (it follows that B + KCl + solution gives A); (5) A + C + - KCl + vapour; (6) B + C + solution + vapour; and (7) B + KCl + - solution + vapour. Thus above 92° A gives B + KCl. The law of - phases by bringing complex instances of chemical reaction under - simple physical schemes, facilitates their study in detail and - gives the means of seeking the simplest chemical relations dealing - with solutions, dissociation, double decompositions and similar - cases, and therefore deserves consideration, but a detailed - exposition of this subject must be looked for in works on physical - chemistry. - - [10] The normal _cupric nitrate_, CuN_{2}O_{6},3H_{2}O, is obtained as - a deliquescent salt of a blue colour (soluble in water and in - alcohol) by dissolving copper or cupric oxide in nitric acid. It - is so easily decomposed by the action of heat that it is - impossible to drive off the water of crystallisation from it - before it begins to decompose. During the ignition of the normal - salt the cupric oxide formed enters into combination with the - remaining undecomposed normal salt, and gives a basic salt, - CuN_{2}O_{6},2CuH_{2}O_{2}. The same basic salt is obtained if a - certain quantity of alkali or cupric hydroxide or carbonate be - added to the solution of the normal salt, which is even decomposed - when boiled with metallic copper, and forms the basic salt as a - green powder, which easily decomposes under the action of heat and - leaves a residue of cupric oxide. The basic salt, having the - composition CuN_{2}O_{6},3CuH_{2}O_{2}, is nearly insoluble in - water. - - The normal _carbonate of copper_, CuCO_{3}, occurs in nature, - although extremely rarely. If solutions of cupric salts be mixed - with solutions of alkali carbonates, then, as in the case of - magnesium, carbonic anhydride is evolved and basic salts are - formed, which vary in composition according to the temperature and - conditions of the reaction. By mixing cold solutions, a voluminous - blue precipitate is formed, containing an equivalent proportion of - cupric hydroxide and carbonate (after standing or heating, its - composition is the same as malachite, sp. gr. 3·51: 2CuSO_{4} + - 2Na_{2}CO_{3} + H_{2}O = CuCO_{3},CuH_{2}O_{2} + 2Na_{2}SO_{4} + - CO_{2}. If the resultant blue precipitate be heated in the liquid, - it loses water and is transformed into a granular green mass of - the composition Cu_{2}CO_{4}--_i.e._ into a compound of the normal - salt with anhydrous cupric oxide. This salt of the oxide - corresponds with orthocarbonic acid, C(OH)_{4} = CH_{4}O_{4} where - 4H is replaced by 2Cu. On further boiling this salt loses a - portion of the carbonic acid, forming black cupric oxide, so - unstable is the compound of copper with carbonic anhydride. - Another basic salt which occurs in nature, 2CuCO_{3},CuH_{2}O_{2}, - is known as azurite, or blue carbonate of copper; it also loses - carbonic acid when boiled with water. On mixing a solution of - cupric sulphate with sodium sesquicarbonate no precipitate is at - first obtained, but after boiling a precipitate is formed having - the composition of malachite. Debray obtained artificial azurite - by heating cupric nitrate with chalk. - -The commonest normal salt is _blue vitriol_--_i.e._ the normal cupric -sulphate. It generally contains five molecules of water of -crystallisation, CuSO_{4},5H_{2}O. It forms the product of the action of -strong sulphuric acid on copper, sulphurous anhydride being evolved. The -same salt is obtained in practice by carefully roasting sulphuretted ores -of copper, and also by the action of water holding oxygen in solution on -them: CuS + O_{4} = CuSO_{4}. This salt forms a by-product, obtained in -gold refineries, when the silver is precipitated from the sulphuric acid -solution by means of copper. It is also obtained by pouring dilute -sulphuric acid over sheet copper in the presence of air, or by heating -cupric oxide or carbonate in sulphuric acid. The crystals of this salt -belong to the triclinic system, have a specific gravity of 2·19, are of a -beautiful blue colour, and give a solution of the same colour. 100 parts -of water at 0° dissolve 15, at 25° 23, and at 100° about 45 parts of -cupric sulphate, CuSO_{4}.[10 bis] At 100° this salt loses a portion of -its water of crystallisation, which it only parts with entirely at a high -temperature (220°) and then gives a white powder of the anhydrous -sulphate; and the latter, on further calcination, loses the elements of -sulphuric anhydride, leaving cupric oxide, like all the cupric salts. The -anhydrous (colourless) cupric sulphate is sometimes used for absorbing -water; it turns blue in the process. It offers the advantage that it -retains both hydrochloric acid and water, but not carbonic anhydride.[11] -Cupric sulphate is used for steeping seed corn; this is said to prevent -the growth of certain parasites on the plants. In the arts a considerable -quantity of cupric sulphate is also used in the preparation of other -copper salts--for instance, of certain pigments[11 bis]--and a -particularly large quantity is used _in the galvanoplastic process_, -which consists in the deposition of copper from a solution of cupric -sulphate by the action of a galvanic current, when the metallic copper is -deposited on the negative pole and takes the shape of the latter. The -description of the processes of galvanoplastic art introduced by Jacobi -in St. Petersburg forms a part of applied physics, and will not be -touched on here, and we will only mention that, although first introduced -for small articles, it is now used for such articles as type moulds -(_clichés_), for maps, prints, &c., and also for large statues, and for -the deposition of iron, zinc, nickel, gold, silver, &c. on other metals -and materials. The beginning of the application of the galvanic current -to the practical extraction of metals from solutions has also been -established, especially since the dynamo-electric machines of Gramme, -Siemens, and others have rendered it possible to cheaply convert the -mechanical motion of the steam engine into an electric current. It is to -be expected that the application of the electric current, which has long -since given such important results in chemistry, will, in the near -future, play an important part in technical processes, the example being -shown by electric lighting. - - [10 bis] Although sulphate of copper usually crystallises with 5H_{2}O, - that is, differently to the sulphates of Mg, Fe, and Mn, it is - nevertheless perfectly isomorphous with them, as is seen not only - in the fact that it gives isomorphous mixtures with them, - containing a similar amount of water of crystallisation, but also - in the ease with which it forms, like all bases analogous to MgO, - double salts, R_{2}Cu(SO_{4})_{2},6H_{2}O, where R = K, Rb, Cs, of - the monoclinic system. - - Salts of this kind, like CuCl_{2},2KCl,2H_{2}O,PtK_{2}Cy_{4}, &c., - present a composition CuX_{2} if the representation of double - salts given in Chapter XXIII., Note 11, be admitted, because they, - like Cu(HO)_{2}, contain Cu(X_{2}K)_{2}, where X_{2} = SO_{4}, - _i.e._ the residue of sulphuric acid, which combines with H_{2}, - and is therefore able to replace the H_{2} by X_{2} or O. A - detailed study of the crystalline forms of these salts, made by - Tutton (1893) (_see_ Chapter XIII., Note 1), showed: (1) that 22 - investigated salts of the composition R_{2}M(SO_{4}),6H_{2}O, - where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni, Cu, - present a complete crystallographic resemblance; (2) that in all - respects the Rb salts present a transition between the K and Cs - salts; (3) that the Cs salts form crystals most easily, and the K - salts the most difficultly, and that for the K salts of Cd and Mn - it was even impossible to obtain well-formed crystals; (4) that - notwithstanding the closeness of their angles, the general - appearance (habit) of the potassium compound differs very clearly - from the Cs salts, while the Rb salts present a distinct - transition in this respect; (5) that the angle of the inclination - of one of the axes to the plane of the two other axes showed that - in the K salts (angle from 75° to 75° 38´) the inclination is - least, in the Cs salts (from 72° 52´ to 73° 50´) greatest, and in - the Rb salts (from 73° 57´ to 74° 42´) intermediate between the - two; the replacement of Mg ... Cu produces but a very small change - in this angle; (6) that the other angles and the ratio of the axes - of the crystals exhibit a similar variation; and (7) that thus the - variation of the form is chiefly determined by the atomic weight - of the alkaline metal. As an example we cite the magnitude of the - inclination of the axes of R_{2}M(SO_{4})_{2},6H_{2}O. - - R = K Rb Cs - M = Mg 75° 12´ 74° 1´ 72° 54´ - Zn 75° 12´ 74° 7´ 72° 59´ - Cd -- 74° 7´ 72° 49´ - Mn -- 73° 3´ 72° 53´ - Fe 75° 28´ 74° 16´ 73° 8´ - Co 75° 5´ 73° 59´ 72° 52´ - Ni 75° 0´ 73° 57´ 72° 58´ - Cu 75° 32´ 74° 42´ 73° 50´ - - This shows clearly (within the limits of possible error, which may - be as much as 30´) the almost perfect identity of the independent - crystalline forms notwithstanding the difference of the atomic - weights of the diatomic elements, M = Mg, Cu. - - [11] In addition to what has been said (Chapter I., Note 65, and - Chapter XXII., Note 35) respecting the combination of CuSO_{4} - with water and ammonia, we may add that Lachinoff (1893) showed - that CuSO_{4},5H_{2}O loses 4-3/4H_{2}O at 180°, that - CuSO_{4},5NH_{3} also loses 4-3/4NH_{3} at 320°, and that only - 1/4H_{2}O and 1/4NH_{3} remain in combination with the CuSO_{4}. - The last 1/4H_{2}O can only be driven off by heating to 200°, and - the last 1/4NH_{3} by heating to 360°. Ammonia displaces water - from CuSO_{4},5H_{2}O, but water cannot displace the ammonia from - CuSO_{4},5NH_{3}. If hydrochloric acid gas be passed over - CuSO_{4},5H_{2}O at the ordinary temperature, it first forms - CuSO_{4},5H_{2}O,3HCl, and then CuSO_{4},2H_{2}O,2HCl. When air is - passed over the latter compound it passes into CuSO_{4}H_{2}O with - a small amount of HCl (about 1/8HCl). At 100° CuSO_{4},5H_{2}O in - a stream of hydrochloric acid gas gives CuSO_{4},1/4H_{2}O,2HCl, - and then CuSO_{4},1/4H_{2}O,HCl, whilst after prolonged heating - CuSO_{4} remains, which rapidly passes into CuSO_{4},5H_{2}O when - placed under a bell jar over water. Over sulphuric acid, however, - CuSO_{4},5H_{2}O only parts with 3H_{2}O, and if CuSO_{4},2H_{2}O - be placed over water it again forms CuSO_{4},5H_{2}O, and so on. - - [11 bis] Commercial blue vitriol generally contains ferrous sulphate. - The salt is purified by converting the ferrous salt into a ferric - salt by heating the solution with chlorine or nitric acid. The - solution is then evaporated to dryness, and the unchanged cupric - sulphate extracted from the residue, which will contain the larger - portion of the ferric oxide. The remainder will be separated if - cupric hydroxide is added to the solution and boiled; the cupric - oxide, CuO, then precipitates the ferric oxide, Fe_{2}O_{3}, just - as it is itself precipitated by silver oxide. But the solution - will contain a small proportion of a basic salt of copper, and - therefore sulphuric acid must be added to the filtered solution, - and the salt allowed to crystallise. Acid salts are not formed, - and cupric sulphate itself has an acid reaction on litmus paper. - -_The alloys of copper_ with certain metals, and especially with zinc and -tin, are easily formed by directly melting the metals together. They are -easily cast into moulds, forged, and worked like copper, whilst they are -much more durable in the air, and are therefore frequently used in the -arts. Even the ancients used exclusively alloys of copper, and not pure -copper, but its alloys with tin or different kinds of bronze (Chapter -XVIII., Note 35). The alloys of copper with zinc are called _brass_ or -'yellow metal.' Brass contains about 32 p.c. of zinc; generally, however, -it does not contain more than 65 p.c. of copper. The remainder is -composed of lead and tin, which usually occur, although in small -quantities, in brass. Yellow metal contains about 40 p.c. of zinc.[12] -The addition of zinc to copper changes the colour of the latter to a -considerable degree; with a certain amount of zinc the colour of the -copper becomes yellow, and with a still larger proportion of zinc an -alloy is formed which has a greenish tint. In those alloys of zinc and -copper which contain a larger amount of zinc than of copper, the yellow -colour disappears and is replaced by a greyish colour. But when the -amount of zinc is diminished to about 20 p.c., the alloy is red and hard, -and is called 'tombac.' A contraction takes place in alloying copper with -zinc, so that the volume of the alloy is less than that of either metal -individually. The zinc volatilises on prolonged heating at a high -temperature and the excess of metallic copper remains behind. When heated -in the air, the zinc oxidises before the copper, so that all the zinc -alloyed with copper may be removed from the copper by this means. An -important property of brass containing about 30 p.c. of zinc is that it -is soft and malleable in the cold, but becomes somewhat brittle when -heated. We may also mention that ordinary copper coins contain, in order -to render them hard, tin, zinc, and iron (Cu = 95 p.c.); that it is now -customary to add a small amount of phosphorus to copper and bronze, for -the same purpose; and also that copper is added to silver and gold in -coining, &c. to render it hard; moreover, in Germany, Switzerland, and -Belgium, and other countries, a silver-white alloy (melchior, German -silver, &c.), for base coinage and other purposes, is prepared from brass -and nickel (from 10 to 20 p.c. of nickel; 20 to 30 p.c. zinc: 50 to 70 -p.c. copper), or directly from copper and nickel, or, more rarely, from -an alloy containing silver, nickel, and copper.[12 bis] - - [12] Among the alloys of copper resembling brass, _delta metal_, - invented by A. Dick (London) is largely used (since 1883). It - contains 55 p.c. Cu, and 41 p.c. Zn, the remaining 4 p.c. being - composed of iron (as much as 3-1/2 p.c., which is first alloyed - with zinc), or of cobalt, and manganese, and certain other metals. - The sp. gr. of delta metal is 8·4. It melts at 950°, and then - becomes so fluid that it fills up all the cavities in a mould and - forms excellent castings. It has a tensile strength of 70 kilos - per sq. mm. (gun metal about 20, phosphor bronze about 30). It is - very soft, especially when heated to 600°, but after forging and - rolling it becomes very hard; it is more difficultly acted upon by - air and water than other kinds of brass, and preserves its golden - yellow colour for any length of time, especially if well polished. - It is used for making bearings, screw propellers, valves, and many - other articles. In general the alloys of Cu and Zn containing - about 2/3 p.c. by weight of copper were for a long time almost - exclusively made in Sweden and England (Bristol, Birmingham). - These alloys for the most part are cheaper, harder, and more - fusible than copper alone, and form good castings. The alloys - containing 45-80 p.c. Cu crystallise in cubes if slowly cooled (Bi - also gives crystals). By washing the surface of brass with dilute - sulphuric acid, Zn is removed and the article acquires the colour - of copper. The alloys approaching Zn_{2}Cu_{3} in their - composition exhibit the greatest resistance (under other equal - conditions; of purity, forging, rolling, &c.) The addition of 3 - p.c. Al, or 5 p.c. Sn, improves the quality of brass. Respecting - aluminium bronze _see_ Chapter XVII. p. 88. - - [12 bis] Ball (also Kamensky), 1888, by investigating the electrical - conductivity of the alloys of antimony and copper with lead, came - to the conclusion that only two definite compounds of antimony and - copper exist, whilst the other alloys are either alloys of these - two together or with antimony or with copper. These compounds are - Cu_{2}Sb and Cu_{4}Sb--one corresponds with the maximum, and the - other with the minimum, electrical resistance. In general, the - resistance offered to an electrical current forms one of the - methods by which the composition of definite alloys (for example, - Pb_{2}Zn_{7}) is often established, whilst the electromotive force - of alloys affords (Laurie, 1888) a still more accurate method--for - instance, several definite compounds were discovered by this - method among the alloys of copper with zinc and tin; but we will - not enter into any details of this subject, because we avoid all - references to electricity, although the reader is recommended to - make himself acquainted with this branch of science, which has - many points in common with chemistry. The study of alloys regarded - as solid solutions should, in my opinion, throw much light upon - the question of solutions, which is still obscure in many aspects - and in many branches of chemistry. - -Copper, in its cuprous compounds, is so analogous to _silver_, that were -there no cupric compounds, or if silver gave stable compounds of the -higher oxide, AgO, the resemblance would be as close as that between -chlorine and bromine or zinc and cadmium; but silver compounds -corresponding to AgO are quite unknown. Although silver peroxide--which -was regarded as AgO, but which Berthelot (1880) recognised as the -sesquioxide Ag_{2}O_{3}--is known, still it does not form any true salts, -and consequently cannot be placed along with cupric oxide. In distinction -to copper, silver as a metal does not oxidise under the influence of -heat; and its oxides, Ag_{2}O and Ag_{2}O_{3}, easily lose oxygen (_see_ -Note 8 tri). Silver does _not oxidise_ in air at the ordinary pressure, -and is therefore classed among the so-called _noble metals_. It has a -white colour, which is much purer than that of any other known metal, -especially when the metal is chemically pure. In the arts silver is -always used alloyed, because chemically-pure silver is so soft that it -wears exceedingly easily, whilst when fused with a small amount of -copper, it becomes very hard, without losing its colour.[13] - - [13] There are not many soft metals; lead, tin, copper, silver, iron, - and gold are somewhat soft, and potassium and sodium very soft. - The metals of the alkaline earths are sonorous and hard, and many - other metals are even brittle, especially bismuth and antimony. - But the very slight significance which these properties have in - determining the fundamental chemical properties of substances - (although, however, of immense importance in the practical - applications of metals) is seen from the example shown by zinc, - which is hard at the ordinary temperature, soft at 100°, and - brittle at 200°. - - [Illustration: FIG. 95.--Cupel for silver assaying.] - - [Illustration: FIG. 96.--Clay muffle.] - - [Illustration: FIG. 97.--Portable muffle furnace.] - - As the value of silver depends exclusively on its purity, and as - there is no possibility of telling the amount of impurities - alloyed with it from its external appearance, it is customary in - most countries to mark an article with the amount of pure silver - it contains after an accurately-made analysis known as the assay - of the silver. In France the assay of silver shows the amount of - pure silver in 1,000 parts by weight; in Russia the amount of pure - silver in 96 parts--that is, the assay shows the number of - zolotniks (4·26 grams) of pure silver in one pound (410 grams) of - alloyed silver. Russian silver is generally 84 assay--that is, - contains 84 parts by weight of pure silver and 12 parts of copper - and other metals. French money contains 90 p.c. (in the Russian - system this will be 86·4 assay) by weight of silver [English coins - and jewellery contain 92·5 p.c. of silver]; the silver rouble is - of 83-1/3 assay--that is, it contains 86·8 p.c. of silver--and the - smaller Russian silver coinage is of 48 assay, and therefore - contains 50 p.c. of silver. Silver ornaments and articles are - usually made in Russia of 84 and 72 assay. As the alloys of silver - and copper, especially after being subjected to the action of - heat, are not so white as pure silver, they generally undergo a - process known as 'blanching' (or 'pickling') after being worked - up. This consists in removing the copper from the surface of the - article by subjecting it to a dark-red heat and then immersing it - in dilute acid. During the calcination the copper on the surface - is oxidised, whilst the silver remains unchanged; the dilute acid - then dissolves the copper oxides formed, and pure silver is left - on the surface. The surface is dull after this treatment, owing to - the removal of a portion of the metal by the acid. After being - polished the article acquires the desired lustre and colour, so as - to be indistinguishable from a pure silver object. In order to - test a silver article, a portion of its mass must be taken, not - from the surface, but to a certain depth. The methods of assay - used in practice are very varied. The commonest and most often - used is that known as _cupellation_. It is based on the difference - in the oxidisability of copper, lead, and silver. The cupel is a - porous cup with thick sides, made by compressing bone ash. The - porous mass of bone ash absorbs the fused oxides, especially the - lead oxide, which is easily fusible, but it does not absorb the - unoxidised metal. The latter collects into a globule under the - action of a strong heat in the cupel, and on cooling solidifies - into a button, which may then be weighed. Several cupels are - placed in a muffle. A muffle is a semi-cylindrical clay vessel, - shown in the accompanying drawing. The sides of the muffle are - pierced with several orifices, which allow the access of air into - it. The muffle is placed in a furnace, where it is strongly - heated. Under the action of the air entering the muffle the copper - of the silver alloy is oxidised, but as the oxide of copper is - infusible, or, more strictly speaking, difficultly fusible, a - certain quantity of lead is added to the alloy; the lead is also - oxidised by the air at the high temperature of the muffle, and - gives the very fusible lead oxide. The copper oxide then fuses - with the lead oxide, and is absorbed by the cupel, whilst the - silver remains as a bright white globule. If the weight of the - alloy taken and of the silver left on the cupel be determined, it - is possible to calculate the composition of the alloy. Thus the - essence of cupellation consists in the separation of the - oxidisable metals from silver, which does not oxidise under the - action of heat. A more accurate method, based on the precipitation - of silver from its solutions in the form of silver chloride, is - described in detail in works on analytical chemistry. - -Silver occurs in _nature_, both in a native state and in certain -compounds. Native silver, however, is of rather rare occurrence. A far -greater quantity of silver occurs in combination with sulphur, and -especially in the form of _silver sulphide_, Ag_{2}S, with lead sulphide -or copper sulphide, or the ores of various other metals. The largest -amount of silver is extracted from the lead in which it occurs. If this -lead be calcined in the presence of air, it oxidises, and the resultant -lead oxide, PbO ('litharge' or 'silberglätte,' as it is called), melts -into a mobile liquid, which is easily removed. The silver remains in an -unoxidised metallic state.[14] This process is called _cupellation_. - - [14] In America, whence the largest amount of silver is now obtained, - ores are worked containing not more than 1/5 p.c. of silver, - whilst at 1/2 p.c. its extraction is very profitable. Moreover, - the extraction of silver from ores containing not more than 0·01 - p.c. of this metal is sometimes profitable. The majority of the - lead smelted from galena contains silver, which is extracted from - it. Thus near Arras, in France, an ore is worked which contains - about 65 parts of lead and 0·088 part of silver in 100 parts of - ore, which corresponds with 136 parts of silver in 100,000 parts - of lead. At Freiberg, in Saxony, the ore used (enriched by - mechanical dressing) contains about 0·9 of silver, 160 of lead, - and 2 of copper in 10,000 parts. In every case the lead is first - extracted in the manner described in Chapter XVIII., and this lead - will contain all the silver. Not unfrequently other ores of silver - are mixed with lead ores, in order to obtain an argentiferous lead - as the product. The extraction of small quantities of silver from - lead is facilitated by the fact (Pattinson's process) that molten - argentiferous lead in cooling first deposits crystals of pure - lead, which fall to the bottom of the cooling vessel, whilst the - proportion of silver in the unsolidified mass increases owing to - the removal of the crystals of lead. The lead is enriched in this - manner until it contains 1/400 part of silver, and is then - subjected to cupellation on a larger scale. According to Park's - process, zinc is added to the molten argentiferous lead, and the - alloy of Pb and Zn, which first separates out on cooling, is - collected. This alloy is found to contain all the silver - previously contained in the lead. The addition of 0·5 p.c. of - aluminium to the zinc (Rossler and Edelman) facilitates the - extraction of the Ag from the resultant alloy besides preventing - oxidation; for, after re-melting, nearly all the lead easily runs - off (remains fluid), and leaves an alloy containing about 30 p.c. - Ag and about 70 p.c. Zn. This alloy may be used as an anode in a - solution of ZnCl_{2}, when the Zn is deposited on the cathode, - leaving the silver with a small amount of Pb, &c. behind. The - silver can be easily obtained pure by treating it with dilute - acids and cupelling. - - The ores of silver which contain a larger amount of it are: silver - glance, Ag_{2}S (sp. gr. 7·2); argentiferous-copper glance, CuAgS; - horn silver or chloride of silver, AgCl; argentiferous grey copper - ore; polybasite, M_{9}RS_{6} (where M = Ag, Cu, and R = Sb, As), - and argentiferous gold. The latter is the usual form in which gold - is found in alluvial deposits and ores. The crystals of gold from - the Berezoffsky mines in the Urals contain 90 to 95 of gold and 5 - to 9 of silver, and the Altai gold contains 50 to 65 of gold and - 36 to 38 of silver. The proportion of silver in native gold varies - between these limits in other localities. Silver ores, which - generally occur in veins, usually contain native silver and - various sulphur compounds. The most famous mines in Europe are in - Saxony (Freiberg), which has a yearly output of as much as 26 tons - of silver, Hungary, and Bohemia (41 tons). In Russia, silver is - extracted in the Altai and at Nerchinsk (17 tons). The richest - silver mines known are in America, especially in Chili (as much as - 70 tons), Mexico (200 tons), and more particularly in the Western - States of North America. The richness of these mines may be judged - from the fact that one mine in the State of Nevada (Comstock, near - Washoe and the cities of Gold Hill and Virginia), which was - discovered in 1859, gave an output of 400 tons in 1866. In place - of cupellation, chlorination may also be employed for extracting - silver from its ores. The method of chlorination consists in - converting the silver in an ore into silver chloride. This is - either done by a wet or by a dry method, roasting the ore with - NaCl. When the silver chloride is formed, the extraction of the - metal is also done by two methods. The first consists in the - silver chloride being reduced to metal by means of iron in - rotating barrels, with the subsequent addition of mercury which - dissolves the silver, but does not act on the other metals. The - mercury holding the silver in solution is distilled, when the - silver remains behind. This method is called _amalgamation_. The - other method is less frequently used, and consists in dissolving - the silver chloride in sodium chloride or in sodium thiosulphate, - and then precipitating the silver from the solution. The - amalgamation is then carried on in rotating barrels containing the - roasted ore mixed with water, iron, and mercury. The iron reduces - the silver chloride by taking up the chlorine from it. The - technical details of these processes are described in works on - metallurgy. The extraction of AgCl by the wet method is carried on - (Patera's process) by means of a solution of hyposulphite of - sodium which dissolves AgCl (_see_ Note 23), or by lixiviating - with a 2 p.c. solution of a double hyposulphite of Na and Cu - (obtained by adding CuSO_{4} to Na_{2}S_{2}O_{3}). The resultant - solution of AgCl is first treated with soda to precipitate - PbCO_{3}, and then with Na_{2}S, which precipitates the Ag and Au. - The process should be carried on rapidly to prevent the - precipitation of Cu_{2}S from the solution of CuSO_{4} and - Na_{2}S_{2}O_{3}. - -Commercial silver generally contains copper, and, more rarely, other -metallic impurities also. Chemically _pure silver_ is obtained either by -cupellation or by subjecting ordinary silver to the following treatment. -The silver is first dissolved in nitric acid, which converts it and the -copper into nitrates, Cu(NO_{3})_{2} and AgNO_{3}; hydrochloric acid is -then added to the resultant solution (green, owing to the presence of the -cupric salt), which is considerably diluted with water in order to retain -the lead chloride in solution if the silver contained lead. The copper -and many other metals remain in solution, whilst the silver is -precipitated as silver chloride. The precipitate is allowed to settle, -and the liquid is decanted off; the precipitate is then washed and fused -with sodium carbonate. A double decomposition then takes place, sodium -chloride and silver carbonate being formed; but the latter decomposes -into metallic silver, because the silver oxide is decomposed by heat: -Ag_{2}CO_{3} = Ag_{2} + O + CO_{2}. The silver chloride may also be mixed -with metallic zinc, sulphuric acid, and water, and left for some time, -when the zinc removes the chlorine from the silver chloride and -precipitates the silver as a powder. This finely-divided silver is called -'molecular silver.'[15] - - [15] There is another practical method which is also suitable for - separating the silver from the solutions obtained in photography, - and consists in precipitating the silver by oxalic acid. In this - case the amount of silver in the solution must be known, and 23 - grams of oxalic acid dissolved in 400 grams of water must be added - for every 60 grams of silver in solution in a litre of water. A - precipitate of silver oxalate, Ag_{2}C_{2}O_{4}, is then obtained, - which is insoluble in water but soluble in acids. Hence, if the - liquid contain any free acid it must be previously freed from it - by the addition of sodium carbonate. The resultant precipitate of - silver oxalate is dried, mixed with an equal weight of dry sodium - carbonate, and thrown into a gently-heated crucible. The - separation of the silver then proceeds without an explosion, - whilst the silver oxalate if heated alone decomposes with - explosion. - - According to Stas, the best method for obtaining silver from its - solutions is by the reduction of silver chloride dissolved in - ammonia by means of an ammoniacal solution of cuprous - thiosulphate; the silver is then precipitated in a crystalline - form. A solution of ammonium sulphite may be used instead of the - cuprous salt. - -Chemically-pure silver has an exceeding pure white colour, and a specific -gravity of 10·5. Solid silver is lighter than the molten metal, and -therefore a piece of silver floats on the latter. The fusing-point of -silver is about 950° C., and at the high temperature attained by the -combustion of detonating gas it volatilises.[16] By employing silver -reduced from silver chloride by milk sugar and caustic potash, and -distilling it, Stas obtained silver purer than that obtained by any other -means; in fact, this was perfectly pure silver. The vapour of silver has -a very beautiful green colour, which is seen when a silver wire is placed -in an oxyhydrogen flame.[17] - - [16] Silver is very malleable and ductile; it may be beaten into leaves - 0·002 mm. in thickness. Silver wire may be made so fine that 1 - gram is drawn into a wire 2-1/2 kilometres long. In this respect - silver is second only to gold. A wire of 2 mm. diameter breaks - under a strain of 20 kilograms. - - [17] In melting, silver absorbs a considerable amount of oxygen, which - is disengaged on solidifying. One volume of molten silver absorbs - as much as 22 volumes of oxygen. In solidifying, the silver forms - cavities like the craters of a volcano, and throws off metal, - owing to the evolution of the gas; all these phenomena recall a - volcano on a miniature scale (Dumas). Silver which contains a - small quantity of copper or gold, &c., does not show this property - of dissolving oxygen. - - The absorption of oxygen by molten silver is, however, an - oxidation, but it is at the same time a phenomenon of solution. - One cubic centimetre of molten silver can dissolve twenty-two - cubic centimetres of oxygen, which, even at 0°, only weighs 0·03 - gram, whilst 1 cubic centimetre of silver weighs at least 10 - grams, and therefore it is impossible to suppose that the - absorption of the oxygen is attended by the formation of any - definite compound (rich in oxygen) of silver and oxygen (about 45 - atoms of silver to 1 of oxygen) in any other but a dissociated - form, and this is the state in which substances in solution must - be regarded (Chapter I.) - - Le Chatelier showed that at 300° and 15 atmospheres pressure - silver absorbs so much oxygen that it may be regarded as having - formed the compound Ag_{4}O, or a mixture of Ag_{2} and Ag_{2}O. - Moreover, silver oxide, Ag_{2}O, only decomposes at 300° under low - pressures, whilst at pressures above 10 atmospheres there is no - decomposition at 300° but only at 400°. - - Stas showed that silver is oxidised by air in the presence of - acids. V. d. Pfordten confirmed this, and showed that an acidified - solution of potassium permanganate rapidly dissolves silver in the - presence of air. - -It has long been known (Wöhler) that when nitrate of silver, AgNO_{3}, -reacts as an oxidising agent upon citrates and tartrates, it is able -under certain conditions to give either a salt of suboxide of silver (see -Note 19) or a red solution, or to give a precipitate of metallic silver -reduced at the expense of the organic substances. In 1889 Carey Lea, in -his researches on this class of reactions, showed that _soluble silver_ -is here formed, which he called _allotropic silver_. It may be obtained -by taking 200 c.c. of a 10 per cent. solution of AgNO_{3} and quickly -adding a mixture (neutralised with NaHO) of 200 c.c. of a 30 per cent. -solution of FeSO_{4} and 200 c.c. of a 40 per cent. solution of sodium -citrate. A lilac precipitate is obtained, which is collected on a filter -(the precipitate becomes blue) and washed with a solution of -NH_{4}NO_{3}. It then becomes soluble in pure water, forming a red -perfectly transparent solution from which the dissolved silver is -precipitated on the addition of many soluble foreign bodies. Some of the -latter--for instance, NH_{4}NO_{3}, alkaline sulphates, nitrates, and -citrates--give a precipitate which redissolves in pure water, whilst -others--for instance, MgSO_{4}, FeSO_{4}, K_{2}Cr_{2}O_{7}, AgNO_{3}, -Ba(NO_{3})_{2} and many others--convert the precipitated silver into a -new variety, which, although no longer soluble in water, regains its -solubility in a solution of borax and is soluble in ammonia. Both the -soluble and insoluble silver are rapidly converted into the ordinary -grey-metallic variety by sulphuric acid, although nothing is given off in -the reaction; the same change takes place on ignition, but in this case -CO_{2} is disengaged; the latter is formed from the organic substances -which remain (to the amount of 3 per cent.) in the modified silver (they -are not removed by soaking in alcohol or water). If the precipitated -silver be slightly washed and laid in a smooth thin layer on paper or -glass, it is seen that the soluble variety is red when moist and a fine -blue colour when dry, whilst the insoluble variety has a blue reflex. -Besides these, under special conditions[18] a golden yellow variety may -be obtained, which gives a brilliant golden yellow coating on glass; but -it is easily converted into the ordinary grey-metallic state by friction -or trituration. There is no doubt[18 bis] that there is the same relation -between ordinary silver which is perfectly insoluble in water and the -varieties of silver obtained by Carey Lea[18 tri] as there is between -quartz and soluble silica or between CuS and As_{2}S_{2} in their -ordinary insoluble forms and in the state of the colloid solution of -their hydrosols (_see_ Chapter I., Note 57, and Chapter XVII., Note 25 -bis). Here, however, an important step in advance has been made in this -respect, that we are dealing with the solution of a simple body, and -moreover of a metal--_i.e._ of a particularly characteristic state of -matter. And as boron, gold, and certain other simple bodies have already -been obtained in a soluble (colloid) form, and as numerous organic -compounds (albuminous substances, gum, cellulose, starch, &c.) and -inorganic substances are also known in this form, it might be said that -the colloid state (of hydrogels and hydrosols) can be acquired, if not by -every substance, at all events by substances of most varied chemical -character under particular conditions of formation from solutions. And -this being the case, we may hope that a further study of soluble colloid -compounds, which apparently present various transitions towards -emulsions, may throw a new light upon the complex question of solutions, -which forms one of the problems of the present epoch of chemical science. -Moreover, we may remark that Spring (1890) clearly proved the colloid -state of soluble silver by means of dialysis as it did not pass through -the membrane. - - [18] When solutions of AgNO_{3}, FeSO_{4}, sodium citrate, and NaHO are - mixed together in the manner described above, they throw down a - precipitate of a beautiful lilac colour; when transferred to a - filter paper the precipitate soon changes colour, and becomes dark - blue. To obtain the substance as pure as possible it is washed - with a 5-10 p.c. solution of ammonium nitrate; the liquid is - decanted, and 150 c.c. of water poured over the precipitate. It - then dissolves entirely in the water. A small quantity of a - saturated solution of ammonium nitrate is added to the solution, - and the silver in solution again separates out as a precipitate. - These alternate solutions and precipitations are repeated seven or - eight times, after which the precipitate is transferred to a - filter and washed with 95 p.c. alcohol until the filtrate gives no - residue on evaporation. An analysis of the substance so obtained - showed that it contained from 97·18 p.c. to 97·31 p.c. of metallic - silver. It remained to discover what the remaining 2-3 p.c. were - composed of. Are they merely impurities, or is the substance some - compound of silver with oxygen or hydrogen, or does it contain - citric acid in combination which might account for its solubility? - The first supposition is set aside by the fact that no gases are - disengaged by the precipitate of silver, either under the action - of gases or when heated. The second supposition is shown to be - impossible by the fact that there is no definite relation between - the silver and citric acid. A determination of the amount of - silver in solution showed that the amount of citric acid varies - greatly for one and the same amount of silver, and there is no - simple ratio between them. Among other methods of preparing - soluble silver given by Carey Lea, we may mention the method - published by him in 1891. AgNO_{3} is added to a solution of - dextrine in caustic soda or potash; at first a precipitate of - brown oxide of silver is thrown down, but the brown colour then - changes into a reddish chocolate, owing to the reduction of the - silver by the dextrine, and the solution turns a deep red. A few - drops of this solution turn water bright red, and give a perfectly - transparent liquid. The dextrine solution is prepared by - dissolving 40 grams of caustic soda and the same amount of - ordinary brown dextrine in two litres of water. To this solution - is gradually added 28 grams of AgNO_{3} dissolved in a small - quantity of water. - - The insoluble allotropic silver is obtained, as was mentioned - above, from a solution of silver prepared in the manner described, - by the addition of sulphate of copper, iron, barium, magnesium, - &c. In one experiment Lea succeeded in obtaining the insoluble - allotropic Ag in a crystalline form. The red solution, described - above, after standing several weeks, deposits crystals - spontaneously in the form of short black needles and thin prisms, - the liquid becoming colourless. This insoluble variety, when - rubbed upon paper, has the appearance of bright shining green - flakes, which polarise light. - - The gold variety is obtained in a different manner to the two - other varieties. A solution is prepared containing 200 c.c. of a - 10 p.c. solution of nitrate of silver, 200 c.c. of a 20 p.c. - solution of Rochelle salt, and 800 c.c. of water. Just as in the - previous case the reaction consisted in the reduction of the - citrate of silver, so in this case it consists in the reduction of - the tartrate, which here first forms a red, and then a black - precipitate of allotropic Ag, which, when transferred to the - filter, appears of a beautiful bronze colour. After washing and - drying, this precipitate acquires the lustre and colour peculiar - to polished gold, and this is especially remarked where the - precipitate comes into contact with glass or china. An analysis of - the golden variety gave a percentage composition of 98·750 to - 98·749 Ag. Both the insoluble varieties (the blue and gold) have a - different specific gravity from ordinary silver. Whilst that of - fused silver is 10·50, and of finely-divided silver 10·62, the - specific gravity of the blue insoluble variety is 9·58, and of the - gold variety 8·51. The gold variety passes into ordinary Ag with - great ease. This transition may even be remarked on the filter in - those places which have accidentally not been moistened with - water. A simple shock, and therefore friction of one particle upon - another, is enough to convert the gold variety into normal white - silver. Carey Lea sent samples of the gold variety for a long - distance by rail packed in three tubes, in which the silver - occupied about the quarter of their volume; in one tube only he - filled up this space with cotton-wool. It was afterwards found - that the shaking of the particles of Ag had completely converted - it into ordinary white silver, and that only the tube containing - the cotton-wool had preserved the golden variety intact. - - The soluble variety of Ag also passes into the ordinary state with - great ease, the heat of conversion being, as Prange showed in - 1890, about +60 calories. - - [18 bis] The opinion of the nature of soluble silver given below was - first enunciated in the _Journal of the Russian Chemical Society_, - February 1, 1890, Vol. XXII., Note 73. This view is, at the - present time, generally accepted, and this silver is frequently - known as the 'colloid' variety. I may add that Carey Lea observed - the solution of ordinary molecular silver in ammonia without the - access of air. - - [18 tri] It is, however, noteworthy that ordinary metallic lead has - long been considered soluble in water, that boron has been - repeatedly obtained in a brown solution, and that observations - upon the development of certain bacteria have shown that the - latter die in water which has been for some time in contact with - metals. This seems to indicate the passage of small quantities of - metals into water (however, the formation of peroxide of hydrogen - may be supposed to have some influence in these cases). - -As regards the capacity of silver for chemical reactions, it is -remarkable for its small capacity for combination with oxygen and for its -considerable energy of combination with sulphur, iodine, and certain -kindred non-metals. _Silver does not oxidise_ at any temperature, and its -oxide, Ag_{2}O, is decomposed by heat. It is also a very important fact -that silver is not oxidised by oxygen either in the presence of alkalis, -even at exceedingly high temperatures, or in the presence of acids--at -least, of dilute acids--which properties render it a very important metal -in chemical industry for the fusion of alkalis, and also for many -purposes in everyday life; for instance, for making spoons, salt-cellars, -&c. Ozone, however, oxidises it. Of all acids nitric acid has the -greatest action on silver. The reaction is accompanied by the formation -of oxides of nitrogen and silver nitrate, AgNO_{3}, which dissolves in -water and does not, therefore, hinder the further action of the acid on -the metal. The halogen acids, especially hydriodic acid, act on silver, -hydrogen being evolved; but this action soon stops, owing to the halogen -compounds of silver being insoluble in water and only very slightly -soluble in acids; they therefore preserve the remaining mass of metal -from the further action of the acid; in consequence of this the action of -the halogen acids is only distinctly seen with finely-divided silver. -Sulphuric acid acts on silver in the same manner that it does on copper, -only it must be concentrated and at a higher temperature. Sulphurous -anhydride, and not hydrogen, is then evolved, but there is no action at -the ordinary temperature, even in the presence of air. Among the various -salts, sodium chloride (in the presence of moisture, air, and carbonic -acid) and potassium cyanide (in the presence of air) act on silver more -decidedly than any others, converting it respectively into silver -chloride and a double cyanide. - -Although silver does not directly combine with oxygen, still three -different grades of combination with oxygen may be obtained indirectly -from the salts of silver. They are all, however, unstable, and decompose -into oxygen and metallic silver when ignited. These three oxides of -silver have the following composition: _silver suboxide_, Ag_{4}O,[19] -corresponding with the (little investigated) suboxides of the alkali -metals; _silver oxide_, Ag_{2}O, corresponding with the oxides of the -alkali metals and the ordinary salts of silver, AgX; and _silver -peroxide_, AgO,[19 bis] or, judging from Berthelot's researches, -Ag_{2}O_{3}. _Silver oxide_ is obtained as a brown precipitate (which -when dried does not contain water) by adding potassium hydroxide to a -solution of a silver salt--for example, of silver nitrate. The -precipitate formed seems, however, to be an hydroxide, AgHO, _i.e._ -AgNO_{3} + KHO = KNO_{3} + AgHO, and the formation of the anhydrous -oxide, 2AgHO = Ag_{2}O + H_{2}O, may be compared with the formation of -the anhydrous cupric oxide by the action of potassium hydroxide on hot -cupric solutions. Silver hydroxide decomposes into water and silver -oxide, even at low temperatures; at least, the hydroxide no longer exists -at 60°, but forms the anhydrous oxide, Ag_{2}O.[19 tri] Silver oxide is -almost insoluble in water; but, nevertheless, it is undoubtedly a rather -powerful basic oxide, because it displaces the oxides of many metals from -their soluble salts, and saturates such acids as nitric acid, forming -with them neutral salts, which do not act on litmus paper.[20] -Undoubtedly water dissolves a small quantity of silver oxide, which -explains the possibility of its action on solutions of salts--for -example, on solutions of cupric salts. Water in which silver oxide is -shaken up has a distinctly alkaline reaction. The oxide is distinguished -by its great instability when heated, so that it loses all its oxygen -when slightly heated. Hydrogen reduces it at about 80°.[20 bis] The -feebleness of the affinity of silver for oxygen is shown by the fact that -silver oxide decomposes under the action of light, so that it must be -kept in opaque vessels. The silver _salts_ are colourless and decompose -when heated, leaving metallic silver if the elements of the acid are -volatile.[20 tri] They have a peculiar metallic taste, and are -exceedingly poisonous; the majority of them are acted on by light, -especially in the presence of organic substances, which are then -oxidised. The alkaline carbonates give a white precipitate of silver -carbonate, Ag_{2}CO_{3}, which is insoluble in water, but soluble in -ammonia and ammonium carbonate. Aqueous ammonia, added to a solution of a -normal silver salt, first acts like potassium hydroxide, but the -precipitate dissolves in an excess of the reagent, like the precipitate -of cupric hydroxide.[21] Silver oxalate and the halogen compounds of -silver are insoluble in water; hydrochloric acid and soluble chlorides -give, as already repeatedly observed, a white precipitate of silver -chloride in solutions of silver salts. Potassium iodide gives a yellowish -precipitate of silver iodide. Zinc separates all the silver in a metallic -form from solutions of silver salts. Many other metals and reducing -agents--for example, organic substances--also reduce silver from the -solutions of its salts. - - [19] Silver suboxide (Ag_{4}O) or argentous oxide is obtained from - argentic citrate by heating it to 100° in a stream of hydrogen. - Water and argentous citrate are then formed, and the latter, - although but slightly soluble in water, gives a reddish-brown - solution of colloid silver (Note 18), and when boiled this - solution becomes colourless and deposits metallic silver, the - argentic salt being again formed. Wöhler, who discovered this - oxide, obtained it as a black precipitate by adding potassium - hydroxide to the above solution of argentous citrate. With - hydrochloric acid the suboxide gives a brown compound, Ag_{2}Cl. - Since the discovery of soluble silver the above data cannot be - regarded as perfectly trustworthy; it is probable that a mixture - of Ag_{2} and Ag_{2}O was being dealt with, so that the actual - existence of Ag_{4}O is now doubtful, but there can be no doubt as - to the formation of a subchloride, Ag_{2}Cl (_see_ Note 25), - corresponding to the suboxide. The same compound is obtained by - the action of light on the higher chloride. Other acids do not - combine with silver suboxide, but convert it into an argentic salt - and metallic silver. In this respect cuprous oxide presents a - certain resemblance to these suboxides. But copper forms a - suboxide of the composition Cu_{4}O, which is obtained by the - action of an alkaline solution of stannous oxide on cupric - hydroxide, and is decomposed by acids into cupric salts and - metallic copper. The problems offered by the suboxides, as well as - by the peroxides, cannot be considered as fully solved. - - [19 bis] _Silver peroxide_, AgO or Ag_{2}O_{3}, is obtained by the - decomposition of a dilute (10 p.c.) solution of silver nitrate by - the action of a galvanic current (Ritter). On the positive pole, - where oxygen is usually evolved in the decomposition of salts, - brittle grey needles with a metallic lustre, which occasionally - attain a somewhat considerable size, are then formed. They are - insoluble in water, and decompose with the evolution of oxygen - when they are dried and heated, especially up to 150°, and, like - lead dioxide, barium peroxide, &c., their action is strongly - oxidising. When treated with acids, oxygen is evolved and a salt - of the oxide formed. Silver peroxide absorbs sulphurous anhydride - and forms silver sulphate. Hydrochloric acid evolves chlorine; - ammonia reduces the silver, and is itself oxidised, forming water - and gaseous nitrogen. Analyses of the above-mentioned crystals - show that they contain silver nitrate, peroxide, and water. - According to Fisher, they have the composition - 4AgO,AgNO_{3},H_{2}O, and, according to Berthelot, - 4Ag_{2}O_{5},2AgNO_{3},H_{2}O. - - [19 tri] According to Carey Lea, however, oxide of silver still retains - water even at 100°, and only parts with it together with the - oxygen. Oxide of silver is used for colouring glass yellow. - - [20] The reaction of Pb(OH)_{2} upon AgHO in the presence of NaHO leads - to the formation of a compound of both oxides, PbO_n_Ag_{2}O, from - which the oxide of lead cannot be removed by alkalies (Wöhler, - Leton). Wöhler, Welch, and others obtained crystalline double - salts, R_{2}AgX_{3}, by the action of strong solutions of RX of - the halogen salts of the alkaline metals upon AgX, where R = Cs, - Rb, K. - - [20 bis] According to Müller, ferric oxide is reduced by hydrogen - (_see_ Chapter XXII., Note 5) at 295° (into what ?), cupric oxide - at 140°, Ni_{2}O_{3} at 150°; nickelous oxide, NiO, is reduced to - the suboxide, Ni_{2}O, at 195°, and to nickel at 270°; zinc oxide - requires so high a temperature for its reduction that the glass - tube in which Müller conducted the experiment did not stand the - heat; antimony oxide requires a temperature of 215° for its - reduction; yellow mercuric oxide is reduced at 130° and the red - oxide at 230°; silver oxide at 85°, and platinum oxide even at the - ordinary temperature. - - [20 tri] A silica compound, Ag_{2}OSiO_{2} is obtained by fusing - AgNO_{3} with silica; this salt is able to decompose with the - evolution of oxygen, leaving Ag + SiO_{2}. - - [21] If a solution of a silver salt be precipitated by sodium - hydroxide, and aqueous ammonia is added drop by drop until the - precipitate is completely dissolved, the liquid when evaporated - deposits a violet mass of crystalline silver oxide. If moist - silver oxide be left in a strong solution of ammonia it gives a - black mass, which easily decomposes with a loud explosion, - especially when struck. This black substance is called fulminating - silver. Probably this is a compound like the other compounds of - oxides with ammonia, and in exploding the oxygen of the silver - oxide forms water with the hydrogen of the ammonia, which is - naturally accompanied by the evolution of heat and formation of - gaseous nitrogen, or, as Raschig states, fulminating silver - contains NAg_{3} or one of the amides (for instance, NHAg_{2} = - NH_{3} + Ag_{2}O - H_{2}O). Fulminating silver is also formed when - potassium hydroxide is added to a solution of silver nitrate in - ammonia. The dangerous explosions which are produced by this - compound render it needful that great care be taken when salts of - silver come into contact with ammonia and alkalis (_see_ Chapter - XVI., Note 26). - -_Silver nitrate_, AgNO_{3}, is known by the name of lunar caustic (or -_lapis infernalis_); it is obtained by dissolving metallic silver in -nitric acid. If the silver be impure, the resultant solution will contain -a mixture of the nitrates of copper and silver. If this mixture be -evaporated to dryness and the residue carefully fused at an incipient red -heat, all the cupric nitrate is decomposed, whilst the greater part of -the silver nitrate remains unchanged. On treating the fused mass with -water the latter is dissolved, whilst the cupric oxide remains insoluble. -If a certain amount of silver oxide be added to the solution containing -the nitrates of silver and copper, it displaces all the cupric oxide. In -this case it is of course not necessary to take pure silver oxide, but -only to pour off some of the solution and to add potassium hydroxide to -one portion, and to mix the resultant precipitate of the hydroxides, -Cu(OH)_{2} and AgOH, with the remaining portion.[22] By these methods all -the copper can be easily removed and pure silver nitrate obtained (its -solution is colourless, while the presence of Cu renders it blue), which -may be ultimately purified by crystallisation. It crystallises in -colourless transparent prismatic plates, which are not acted on by air. -They are anhydrous. Its sp. gr. is 4·34; it dissolves in half its weight -of water at the ordinary temperature.[22 bis] The pure salt is not acted -on by light, but it easily acts in an oxidising manner on the majority of -organic substances, which it generally blackens. This is due to the fact -that the organic substance is oxidised by the silver nitrate, which is -reduced to metallic silver; the latter is thus obtained in a -finely-divided state, which causes the black stain. This peculiarity is -taken advantage of for marking linen. Silver nitrate is for the same -reason used for _cauterising wounds_ and various growths on the body. -Here again it acts by virtue of its oxidising capacity in destroying the -organic matter, which it oxidises, as is seen from the separation of a -coating of black metallic powdery silver from the part cauterised.[22 -tri] From the description of the preparation of silver nitrate it will -have been seen that this salt, which fuses at 218°, does not decompose at -an incipient red heat; when cast into sticks it is usually employed for -cauterising. On further heating, the fused salt undergoes decomposition, -first forming silver nitrite and then metallic silver. With ammonia, -silver nitrate forms, on evaporation of the solution, colourless crystals -containing AgNO_{3},2HN_{3} (Marignac). In general the salts of silver, -like cuprous, cupric, zinc, &c. salts, are able to give several compounds -with ammonia; for example, silver nitrate in a dry state absorbs three -molecules (Rose). The ammonia is generally easily expelled from these -compounds by the action of heat. - - [22] So that we here encounter the following phenomena: copper - displaces silver from the solutions of its salts, and silver oxide - displaces copper oxide from cupric salts. Guided by the - conceptions enunciated in Chapter XV., we can account for this in - the following manner: The atomic volume of silver = 10·3, and of - copper = 7·2, of silver oxide = 32, and of copper oxide = 13. A - greater contraction has taken place in the formation of cupric - oxide, CuO, than in the formation of silver oxide, Ag_{2}O, since - in the former (13 - 7 = 6) the volume after combination with the - oxygen has increased by very little, whilst the volume of silver - oxide is considerably greater than that of the metal it contains - [32 - (2 × 10·3) = 11·4]. Hence silver oxide is less compact than - cupric oxide, and is therefore less stable; but, on the other - hand, there are greater intervals between the atoms in silver - oxide than in cupric oxide, and therefore silver oxide is able to - give more stable compounds than those of copper oxide. This is - verified by the figures and data of their reactions. It is - impossible to calculate for cupric nitrate, because this salt has - not yet been obtained in an anhydrous state; but the sulphates of - both oxides are known. The specific gravity of copper sulphate in - an anhydrous state is 3·53, and of silver sulphate 5·36; the - molecular volume of the former is 45, and of the latter 58. The - group SO_{3} in the copper occupies, as it were, a volume 45 - 13 - = 32, and in the silver salt a volume 58 - 32 = 26; hence a - smaller contraction has taken place in the formation of the copper - salt from the oxide than in the formation of the silver salt, and - consequently the latter should be more stable than the former. - Hence silver oxide is able to decompose the salt of copper oxide, - whilst with respect to the metals both salts have been formed with - an almost identical contraction, since 58 volumes of the silver - salt contain 21 volumes of metal (difference = 37), and 45 volumes - of the copper salt contain 7 volumes of copper (difference = 38). - Besides which, it must be observed that copper oxide displaces - iron oxide, just as silver oxide displaces copper oxide. Silver, - copper, and iron, in the form of oxides, displace each other in - the above order, but in the form of metals in a reverse order - (iron, copper, silver). The cause of this order of the - displacement of the oxides lies, amongst other things, in their - composition. They have the composition Ag_{2}O, Cu_{2}O_{2}, - Fe_{2}O_{3}; the oxide containing a less proportion of oxygen - displaces that containing a larger proportion, because the basic - character diminishes with the increase of contained oxygen. - - Copper also displaces mercury from its salts. It may here be - remarked that Spring (1888), on leaving a mixture of dry mercurous - chloride and copper for two hours, observed a distinct reduction, - which belongs to the category of those phenomena which demonstrate - the existence of a mobility of parts (_i.e._ atoms and molecules) - in solid substances. - - [22 bis] The reaction of 1 part by weight of AgNO_{3} requires - (according to Kremers) the following amounts of water: at 0°, 0·82 - part, at 19°·5, 0·41 part, at 54°, 0·20 part, at 110°, 0·09 part, - and, according to Tilden, at 125°, 0·0617 part, and at 133°, - 0·0515 part. - - [22 tri] It may be remarked that the black stain produced by the - reduction of metallic silver disappears under the action of a - solution of mercuric chloride or of potassium cyanide, because - these salts act on finely-divided silver. - -Nitrate of silver easily forms double salts like AgNO_{3}2NaNO_{3} and -AgNO_{3}KNO_{3}. Silver nitrate under the action of water and a halogen -gives nitric acid (_see_ Vol. I. p. 280, formation of N_{2}O_{5}), a -halogen salt of silver, and a silver salt of an oxygen acid of the -halogen. Thus, for example, a solution of chlorine in water, when mixed -with a solution of silver nitrate, gives silver chloride and chlorate. It -is here evident that the reaction of the silver nitrate is identical with -the reaction of the caustic alkalis, as the nitric acid is all set free -and the silver oxide only reacts in exactly the same way in which aqueous -potash acts on free chlorine. Hence the reaction may be expressed in the -following manner: 6AgNO_{3} + 3Cl_{2} + 3H_{2}O = 5AgCl + AgClO_{3} + -6NHO_{3}. - -Silver nitrate, like the nitrates of the alkalis, does not contain any -water of crystallisation. Moreover the other salts of silver almost -always separate out without any water of crystallisation. The silver -salts are further characterised by the fact that they _give neither basic -nor acid salts_, owing to which the formation of silver salts generally -forms the means of determining the true composition of acids--thus, to -any acid H_{n}X there corresponds a salt Ag_{n}X--for instance, -Ag_{3}PO_{4} (Chapter XIX., Note 15). - -_Silver_ gives insoluble and exceedingly stable _compounds with the -halogens_. They are obtained by double decomposition with great facility -whenever a silver salt comes in contact with halogen salts. Solutions of -nitrate, sulphate, and all other kindred salts of silver give a -precipitate of silver chloride or iodide in solutions of chlorides and -iodides and of the halogen acids, because the halogen salts of silver are -insoluble both in water[23] and in other acids. _Silver chloride_, AgCl, -is then obtained as a white flocculent precipitate, silver bromide forms -a yellowish precipitate, and silver iodide has a very distinct yellow -colour. These halogen compounds sometimes occur in nature; they are -formed by a dry method--by the action of halogen compounds on silver -compounds, especially under the influence of heat. Silver chloride easily -fuses at 451° on cooling from a molten state; it forms a somewhat soft -horn-like mass which can be cut with a knife and is known as _horn -silver_. It volatilises at a higher temperature. Its ammoniacal solution, -on the evaporation of the ammonia, deposits crystalline chloride of -silver, in octahedra. Bromide and iodide of silver also appear in forms -of the regular system, so that in this respect the halogen salts of -silver resemble the halogen salts of the alkali metals.[24] - - [23] Silver chloride is almost perfectly insoluble in water, but is - somewhat soluble in water containing sodium chloride or - hydrochloric acid, or other chlorides, and many salts, in - solution. Thus at 100°, 100 parts of water saturated with sodium - chloride dissolve 0·4 part of silver chloride. Bromide and iodide - of silver are less soluble in this respect, as also in regard to - other solvents. It should be remarked that _silver chloride - dissolves in solutions of ammonia, potassium cyanide, and of - sodium thiosulphate_, Na_{2}S_{2}O_{3}. Silver bromide is almost - perfectly analogous to the chloride, but silver iodide is nearly - insoluble in a solution of ammonia. Silver chloride even absorbs - dry ammonia gas, forming very unstable ammoniacal compounds. When - heated, these compounds (Vol. I. p. 250, Note 8) evolve the - ammonia, as they also do under the action of all acids. Silver - chloride enters into double decomposition with potassium cyanide, - forming a soluble double cyanide, which we shall presently - describe; it also forms a soluble double salt, NaAgS_{2}O_{3}, - with sodium thiosulphate. - - Silver chloride offers different modifications in the structure of - its molecule, as is seen in the variations in the consistency of - the precipitate, and in the differences in the action of light - which partially decomposes AgCl (_see_ Note 25). Stas and Carey - Lea investigated this subject, which has a particular importance - in photography, because silver bromide also gives _photo-salts_. - There is still much to be discovered in this respect, since Abney - showed that perfectly dry AgCl placed in a vacuum in the dark is - not in the least acted upon when subsequently exposed to light. - - [24] _Silver bromide_ and _iodide_ (which occur as the minerals bromite - and iodite) resemble the chloride in many respects, but the degree - of affinity of silver for iodine is greater than that for chlorine - and bromine, although less heat is evolved (_see_ Note 28 bis). - Deville deduced this fact from a number of experiments. Thus - silver chloride, when treated with hydriodic acid, evolves - hydrochloric acid, and forms silver iodide. Finely-divided silver - easily liberates hydrogen when treated with hydriodic acid; it - produces the same decomposition with hydrochloric acid, but in a - considerably less degree and only on the surface. The difference - between silver chloride and iodide is especially remarkable, since - the formation of the former is attended with a greater contraction - than that of the latter. The volume of AgCl = 26; of chlorine 27, - of silver 10, the sum = 37, hence a contraction has ensued; and in - the formation of silver iodide an expansion takes place, for the - volume of Ag is 10, of I 26, and of AgI 39 instead of 36 (density, - AgCl, 5·59; AgI, 5·67). The atoms of chlorine have united with the - atoms of silver without moving asunder, whilst the atoms of iodine - must have moved apart in combining with the silver. It is - otherwise with respect to the metal; the distance between its - atoms in the metal = 2·2, in silver chloride = 3·0, and in silver - iodide = 3·5; hence its atoms have moved asunder considerably in - both cases. It is also very remarkable, as Fizeau observed, that - the density of silver iodide increases with a rise of - temperature--that is, a contraction takes place when it is heated - and an expansion when it is cooled. - - In order to explain the fact that in silver compounds the iodide - is more stable than the chloride and oxide, Professor N. N. - Beketoff, in his 'Researches on the Phenomena of Substitutions' - (Kharkoff, 1865), proposed the following original hypothesis, - which we will give in almost the words of the author:--In the case - of aluminium, the oxide, Al_{2}O_{3}, is more stable than the - chloride, Al_{2}Cl_{6}, and the iodide, Al_{2}I_{6}. In the oxide - the amount of the metal is to the amount of the element combined - with it as 54·8 (Al = 27·3) is to 48, or in the ratio 112 : 100; - for the chloride the ratio is = 25 : 100; for the iodide it = 7 : - 100. In the case of silver the oxide (ratio = 1350 : 100) is less - stable than the chloride (ratio = 304 : 100), and the iodide - (ratio of the weight of metal to the weight of the halogen = 85 : - 100) is the most stable. From these and similar examples it - follows that the most stable compounds are those in which the - weights of the combined substances are equal. This may be partly - explained by the attraction of similar molecules even after their - having passed into combination with others. This attraction is - proportional to the product of the acting masses. In silver oxide - the attraction of Ag_{2} for Ag_{2} = 216 × 216 = 46,656, and the - attraction of Ag_{2} for O = 216 × 16 = 3,456. The attraction of - like molecules thus counteracts the attraction of the unlike - molecules. The former naturally does not overcome the latter, - otherwise there would be a disruption, but it nevertheless - diminishes the stability. In the case of an equality or proximity - of the magnitude of the combining masses, the attraction of the - like parts will counteract the stability of the compound to the - least extent--in other words, with an inequality of the combined - masses, the molecules have an inclination to return to an - elementary state, to decompose, which does not exist to such an - extent where the combined masses are equal. There is, therefore, a - tendency for large masses to combine with large, and for small - masses to combine with small. Hence Ag_{2}O + 2KI gives K_{2}O + - 2AgI. The influence of an equality of masses on the stability is - seen particularly clearly in the effect of a rise of temperature. - Argentic, mercuric, auric and other oxides composed of unequal - masses, are somewhat readily decomposed by heat, whilst the oxides - of the lighter metals (like water) are not so easily decomposed by - heat. Silver chloride and iodide approach the condition of - equality, and are not decomposed by heat. The most stable oxides - under the action of heat are those of magnesium, calcium, silicon, - and aluminium, since they also approach the condition of equality. - For the same reason hydriodic acid decomposes with greater - facility than hydrochloric acid. Chlorine does not act on magnesia - or alumina, but it acts on lime and silver oxide, &c. This is - partially explained by the fact that by considering heat as a mode - of motion, and knowing that the atomic heats of the free elements - are equal, it must be supposed that the amount of the motion of - atoms (their _vis viva_) is equal, and as it is equal to the - product of the mass (atomic weight) into the square of the - velocity, it follows that the greater the combining weight the - smaller will be the square of the velocity, and if the combining - weights be nearly equal, then the velocities also will be nearly - equal. Hence the greater the difference between the weights of the - combined atoms the greater will be the difference between their - velocities. The difference between the velocities will increase - with the temperature, and therefore the temperature of - decomposition will be the sooner attained the greater be the - original difference--that is, the greater the difference of the - weights of the combined substances. The nearer these weights are - to each other, the more analogous the motion of the unlike atoms, - and consequently, the more stable the resultant compound. - - The instability of cupric chloride and nitric oxide, the absence - of compounds of fluorine with oxygen, whilst there are compounds - of oxygen with chlorine, the greater stability of the oxygen - compounds of iodine than those of chlorine, the stability of boron - nitride, and the instability of cyanogen, and a number of similar - instances, where, judging from the above argument, one would - expect (owing to the closeness of the atomic weights) a stability, - show that Beketoff's addition to the mechanical theory of chemical - phenomena is still far from sufficient for explaining the true - relations of affinities. Nevertheless, in his mode of explaining - the relative stabilities of compounds, we find an exceedingly - interesting treatment of questions of primary importance. Without - such efforts it would be impossible to generalise the complex data - of experimental knowledge. - - _Fluoride of silver_, AgF, is obtained by dissolving Ag_{2}O or - Ag_{2}CO_{3} in hydrofluoric acid. It differs from the other - halogen salts of silver in being soluble in water (1 part of salt - in 0·55 of water). It crystallises from its solution in prisms, - AgFH_{2}O (Marignac), or AgF_{2}H_{2}O (Pfaundler), which lose - their water in vacuo. Güntz (1891), by electrolising a saturated - solution of Ag_{2}F, obtained _polyfluoride of silver_, Ag_{2}F, - which is decomposed by water into AgF + Ag. It is also formed by - the action of a strong solution of AgF upon finely-divided - (precipitated) silver. - -Silver chloride may be decomposed, with the separation of silver oxide, -by heating it with a solution of an alkali, and if an organic substance -be added to the alkali the chloride can easily be reduced o metallic -silver, the silver oxide being reduced in the oxidation of the organic -substance. Iron, zinc, and many other metals reduce silver chloride in -the presence of water. Cuprous and mercurous chlorides and many organic -substances are also able to reduce the silver from chloride of silver. -This shows the rather easy decomposability of the halogen compounds of -silver. Silver iodide is much more stable in this respect than the -chloride. The same is also observed with respect to the _action of light_ -upon moist AgCl. White silver chloride soon acquires a violet colour when -exposed to the action of light, and especially under the direct action of -the sun's rays. After being acted upon by light it is no longer entirely -soluble in ammonia, but leaves metallic silver undissolved, from which it -might be assumed that the action of light consisted in the decomposition -of the silver chloride into chlorine and metallic silver and in fact the -silver chloride becomes in time darker and darker. Silver bromide and -iodide are much more slowly acted on by light, and, according to certain -observations, when pure they are even quite unacted on; at least they do -not change in weight,[24 bis] so that if they are acted on by light, the -change they undergo must be one of a change in the structure of their -parts and not of decomposition, as it is in silver chloride. The silver -chloride under the action of light changes in weight, which indicates the -formation of a volatile product, and the deposition of metallic silver on -dissolving in ammonia shows the loss of chlorine. The change does -actually occur under the action of light, but the decomposition does not -go as far as into chlorine and silver, but only to the formation of a -subchloride of silver, Ag_{2}Cl, which is of a brown colour and is easily -decomposed into metallic silver and silver chloride, Ag_{2}Cl = AgCl + -Ag. This change of the chemical composition and structure of the halogen -salts of silver under the action of light forms the basis of -_photography_, because the halogen compounds of silver, after having been -exposed to light, give a precipitate of finely-divided silver, of a black -colour, when treated with reducing agents.[25] - - [24 bis] The changes brought about by the action of light necessitate - distinguishing the photo-salts of silver. - - [25] In photography these are called 'developers.' The most common - developers are: solutions of ferrous sulphate, pyrogallol, ferrous - oxalate, hydroxylamine, potassium sulphite, hydroquinone (the last - acts particularly well and is very convenient to use), &c. The - chemical processes of photography are of great practical and - theoretical interest; but it would be impossible in this work to - enter into this special branch of chemistry, which has as yet been - very little worked out from a theoretical point of view. - Nevertheless, we will pause to consider certain aspects of this - subject which are of a purely chemical interest, and especially - the facts concerning _subchloride of silver_, Ag_{2}Cl (_see_ Note - 19), and the photo-salts (Note 23). There is no doubt that under - the action of light, AgCl becomes darker in colour, decreases in - weight, and probably forms a mixture of AgCl, Ag_{2}Cl, and Ag. - But the isolation of the subchloride has only been recently - accomplished by Güntz by means of the Ag_{2}F, discovered by him - (_see_ Note 24). Many chemists (and among them Hodgkinson) assumed - that an oxychloride of silver was formed by the decomposition of - AgCl under the action of light. Carey Lea's (1889) and A. - Richardson's (1891) experiments showed that the product formed - does not, however, contain any oxygen at all, and the change in - colour produced by the action of light upon AgCl is most probably - due to the formation of Ag_{2}Cl. This substance was isolated by - Güntz (1891) by passing HCl over crystals of Ag_{2}F. He also - obtained Ag_{2}I in a similar manner by passing HI, and Ag_{2}S by - passing H_{2}S over Ag_{2}F. Ag_{2}Cl is best prepared by the - action of phosphorus trichloride upon Ag_{2}F. At the temperature - of its formation Ag_{2}Cl has an easily changeable tint, with - shades of violet red to violet black. Under the action of light a - similar (isomeric) substance is obtained, which splits up into - AgCl + Ag when heated. With potassium cyanide Ag_{2}Cl gives Ag + - AgCN + KCl, whence it is possible to calculate the heat of - formation of Ag_{2}Cl; it = 29·7, whilst the heat of formation of - AgCl = 29·2--_i.e._ the reaction 2AgCl = Ag_{2}Cl + Cl corresponds - to an absorption of 28·7 major calories. If we admit the formation - of such a compound by the action of light, it is evident that the - energy of the light is consumed in the above reaction. Carey Lea - (1892) subjected AgCl, AgBr, and AgI to a pressure (of course in - the dark) of 3,000 atmospheres, and to trituration with water in a - mortar, and observed a change of colour indicating incipient - decomposition, which is facilitated under the action of light by - the molecular currents set up (Lermontoff, Egoroff). The change of - colour of the halogen salts of silver under the action of light, - and their faculty of subsequently giving a visible photographic - image under the action of 'developers,' must now be regarded as - connected with the decomposition of AgX, leading to the formation - of Ag_{2}X, and the different tinted photo-salts must be - considered as systems containing such Ag_{2}X's. Carey Lea - obtained photo-salts of this kind not only by the action of light - but also in many other ways, which we will enumerate to prove that - they contain the products of an incomplete combination of Ag with - the halogens, (for the salts Ag_{2}X must be regarded as such). - The photo-salts have been obtained (1) by the imperfect - chlorination of silver; (2) by the incomplete decomposition of - Ag_{2}O or Ag_{2}CO_{3} by alternately heating and treating with a - halogen acid; (3) by the action of nitric acid or Na_{2}S_{2}O_{3} - upon Ag_{2}Cl; (4) by mixing a solution of AgNO_{3} with the - hydrates of FeO, MnO and CrO, and precipitating by HCl; (5) by the - action of HCl upon the product obtained by the reduction of - citrate of silver in hydrogen (Note 19), and (6) by the action of - milk sugar upon AgNO_{3} together with soda and afterwards - acidulating with HCl. All these reactions should lead to the - formation of products of imperfect combination with the halogens - and give photo-salts of a similar diversity of colour to those - produced by the action of developers upon the halogen salts of - silver after exposure to light. - -The insolubility of the halogen compounds of silver forms the basis of -many methods used in practical chemistry. Thus by means of this reaction -it is possible to obtain salts of other acids from a halogen salt of a -given metal, for instance, RCl_{2} + 2AgNO_{3} = R(NO_{3})_{2} + 2AgCl. -The formation of the halogen compounds of silver is very frequently used -in the investigation of organic substances; for example, if any product -of metalepsis containing iodine or chlorine be heated with a silver salt -or silver oxide, the silver combines with the halogen and gives a halogen -salt, whilst the elements previously combined with the silver replace the -halogen. For instance, ethylene dibromide, C_{2}H_{4}Br_{2}, is -transformed into ethylene diacetate, C_{2}H_{4}(C_{2}H_{3}O_{2})_{2}, and -silver bromide by heating it with silver acetate, 2C_{2}H_{3}O_{2}Ag. The -insolubility of the halogen compounds of silver is still more frequently -taken advantage of in determining the amount of silver and halogen in a -given solution. If it is required, for instance, to determine the -quantity of chlorine present in the form of a metallic chloride in a -given solution, a solution of silver nitrate is added to it so long as it -gives a precipitate. On _shaking or stirring_ the liquid, the silver -chloride easily settles in the form of heavy flakes. It is possible in -this way to precipitate the whole of the chlorine from a solution, -without adding an excess of silver nitrate, since it can be easily seen -whether the addition of a fresh quantity of silver nitrate produces a -precipitate in the clear liquid. In this manner it is possible to add to -a solution containing chlorine, as much silver as is required for its -entire precipitation, and to calculate the amount of chlorine previously -in solution from the amount of the solution of silver nitrate consumed, -if the quantity of silver nitrate in this solution has been previously -determined.[25 bis] The atomic proportions and preliminary experiments -with a pure salt--for example, with sodium chloride--will give the amount -of chlorine from the quantity of silver nitrate. Details of these methods -will be found in works on analytical chemistry.[25 tri] - - [25 bis] In order to determine when the reaction is at an end, a few - drops of a solution of K_{2}CrO_{4} are added to the solution of - the chloride. Before all the chlorine is precipitated as AgCl, the - precipitate (after shaking) is white (since Ag_{2}CrO_{4} with - 2RCl gives 2AgCl); but when all the chlorine is thrown down - Ag_{2}CrO_{4} is formed, which colours the precipitate - reddish-brown. In order to obtain accurate results the liquid - should be neutral to litmus. - - [25 tri] _Silver cyanide_, AgCN, is closely analogous to the haloid - salts of silver. It is obtained, in similar manner to silver - chloride, by the addition of potassium cyanide to silver nitrate. - A white precipitate is then formed, which is almost insoluble in - boiling water. It is also, like silver chloride, insoluble in - dilute acids. However, it is dissolved when heated with nitric - acid, and both hydriodic and hydrochloric acids act on it, - converting it into silver chloride and iodide. Alkalis, however, - do not act on silver cyanide, although they act on the other - haloid salts of silver. Ammonia and solutions of the cyanides of - the alkali metals dissolve silver cyanide, as they do the - chloride. In the latter case double cyanides are formed--for - example, KAgC_{2}N_{2}. This salt is obtained in a crystalline - state on evaporating a solution of silver cyanide in potassium - cyanide. It is much more stable than silver cyanide itself. It has - a neutral reaction, does not change in the air, and does not smell - of hydrocyanic acid. Many acids, in acting on a solution of this - double salt, precipitate the insoluble silver cyanide. Metallic - silver dissolves in a solution of potassium cyanide in the - presence of air, with formation of the same double salt and - potassium hydroxide, and when silver chloride dissolves in - potassium cyanide it forms potassium chloride, besides the salt - KAgC_{2}N_{2}. This double salt of silver is used in silver - plating. For this purpose potassium cyanide is added to its - solution, as otherwise silver cyanide, and not metallic silver, is - deposited by the electric current. If two electrodes--one positive - (silver) and the other negative (copper)--be immersed in such a - solution, silver will be deposited upon the latter, and the silver - of the positive electrode will be dissolved by the liquid, which - will thus preserve the same amount of metal in solution as it - originally contained. If instead of the negative electrode a - copper object be taken, well cleaned from all dirt, the silver - will be deposited in an even coating; this, indeed, forms the mode - of _silver plating by the wet method_, which is most often used in - practice. A solution of one part of silver nitrate in 30 to 50 - parts of water, and mixed with a sufficient quantity of a solution - of potassium cyanide to redissolve the precipitate of silver - cyanide formed, gives a dull coating of silver, but if twice as - much water be used the same mixture gives a bright coating. - - Silver plating in the wet way has now replaced to a considerable - extent the old process of _dry silvering_, because this process, - which consists in dissolving silver in mercury and applying the - amalgam to the surface of the objects, and then vaporising the - mercury, offers the great disadvantage of the poisonous mercury - fumes. Besides these, there is another method of silver plating, - based on the direct displacement of silver from its salts by other - metals--for example, by copper. The copper reduces the silver from - its compounds, and the silver separated is deposited upon the - copper. Thus a solution of silver chloride in sodium thiosulphate - deposits a coating of silver upon a strip of copper immersed in - it. It is best for this purpose to take pure _silver sulphite_. - This is prepared by mixing a solution of silver nitrate with an - excess of ammonia, and adding a saturated solution of sodium - sulphite and then alcohol, which precipitates silver sulphite from - the solution. The latter and its solutions are very easily - decomposed by copper. Metallic iron produces the same - decomposition, and iron and steel articles may be very readily - silver-plated by means of the thiosulphate solution of silver - chloride. Indeed, copper and similar metals may even be - silver-plated by means of silver chloride; if the chloride of - silver, with a small amount of acid, be rubbed upon the surface of - the copper, the latter becomes covered with a coating of silver, - which it has reduced. - - Silver plating is not only applicable to metallic objects, but - also to glass, china, &c. Glass is silvered for various - purposes--for example, glass globes silvered internally are used - for ornamentation, and have a mirrored surface. Common - looking-glass silvered upon one side forms a mirror which is - better than the ordinary mercury mirrors, owing to the truer - colours of the image due to the whiteness of the silver. For - optical instruments--for example, telescopes--concave mirrors are - now made of silvered glass, which has first been ground and - polished into the required form. The _silvering of glass_ is based - on the fact that silver which is reduced from certain solutions - deposits itself uniformly in a perfectly homogeneous and - continuous but very thin layer, forming a bright reflecting - surface. Certain organic substances have the property of reducing - silver in this form. The best known among these are certain - aldehydes--for instance, ordinary acetaldehyde, C_{2}H_{4}O, which - easily oxidises in the air and forms acetic acid, C_{2}H_{4}O_{2}. - This oxidation also easily takes place at the expense of silver - oxide, when a certain amount of ammonia is added to the mixture. - The oxide of silver gives up its oxygen to the aldehyde, and the - silver reduced from it is deposited in a metallic state in a - uniform bright coating. The same action is produced by certain - saccharine substances and certain organic acids, such as tartaric - acid, &c. - -Accurate experiments, and more especially the _researches of Stas_ at -Brussels, show the proportion in which silver reacts with metallic -chlorides. These researches have led to the determination of the -_combining weights_ of silver, sodium, potassium, chlorine, bromine, -iodine, and other elements, and are distinguished for their model -exactitude, and we will therefore describe them in some detail. As sodium -chloride is the chloride most generally used for the precipitation of -silver, since it can most easily be obtained in a pure state, we will -here cite the quantitative observations made by Stas for showing the -co-relation between the quantities of chloride of sodium and silver which -react together. In order to obtain perfectly pure sodium chloride, he -took pure rock salt, containing only a small quantity of magnesium and -calcium compounds and a small amount of potassium salts. This salt was -dissolved in water, and the saturated solution evaporated by boiling. The -sodium chloride separated out during the boiling, and the mother liquor -containing the impurities was poured off. Alcohol of 65 p.c. strength and -platinic chloride were added to the resultant salt, in order to -precipitate all the potassium and a certain part of the sodium salts. The -resultant alcoholic solution, containing the sodium and platinum -chlorides, was then mixed with a solution of pure ammonium chloride in -order to remove the platinic chloride. After this precipitation, the -solution was evaporated in a platinum retort, and then separate portions -of this purified sodium chloride were collected as they crystallised. The -same salt was prepared from sodium sulphate, tartrate, nitrate, and from -the platinochloride, in order to have sodium chloride prepared by -different methods and from different sources, and in this manner ten -samples of sodium chloride thus prepared were purified and investigated -in their relation to silver. After being dried, weighed quantities of all -ten samples of sodium chloride were dissolved in water and mixed with a -solution in nitric acid of a weighed quantity of perfectly pure silver. A -slightly greater quantity of silver was taken than would be required for -the decomposition of the sodium chloride, and when, after pouring in all -the silver solution, the silver chloride had settled, the amount of -silver remaining in excess was determined by means of a solution of -sodium chloride of known strength. This solution of sodium chloride was -added so long as it formed a precipitate. In this manner Stas determined -how many parts of sodium chloride correspond to 100 parts by weight of -silver. The result of ten determinations was that for the entire -precipitation of 100 parts of silver, from 54·2060 to 54·2093 parts of -sodium chloride were required. The difference is so inconsiderable that -it has no perceptible influence on the subsequent calculations. The mean -of ten experiments was that 100 parts of silver react with 54·2078 parts -of sodium chloride. In order to learn from this the relation between the -chlorine and silver, it was necessary to determine the quantity of -chlorine contained in 54·2078 parts of sodium chloride, or, what is the -same thing, the quantity of chlorine which combines with 100 parts of -silver. For this purpose Stas made a series of observations on the -quantity of silver chloride obtained from 100 parts of silver. Four -syntheses were made by him for this purpose. The first synthesis -consisted in the formation of silver chloride by the action of chlorine -on silver at a red heat. This experiment showed that 100 parts of silver -give 132·841, 132·843 and 132·843 of silver chloride. The second method -consisted in dissolving a given quantity of silver in nitric acid and -precipitating it by means of gaseous hydrochloric acid passed over the -surface of the liquid; the resultant mass was evaporated in the dark to -drive off the nitric acid and excess of hydrochloric acid, and the -remaining silver chloride was fused first in an atmosphere of -hydrochloric acid gas and then in air. In this process the silver -chloride was not washed, and therefore there could be no loss from -solution. Two experiments made by this method showed that 100 parts of -silver give 132·849 and 132·846 parts of silver chloride. A third series -of determinations was also made by precipitating a solution of silver -nitrate with a certain excess of gaseous hydrochloric acid. The amount of -silver chloride obtained was altogether 132·848. Lastly, a fourth -determination was made by precipitating dissolved silver with a solution -of ammonium chloride, when it was found that a considerable amount of -silver (0·3175) had passed into solution in the washing; for 100 parts of -silver there was obtained altogether 132·8417 of silver chloride. Thus -from the mean of seven determinations it appears that 100 parts of silver -give 132·8445 parts of silver chloride--that is, that 32·8445 parts of -chlorine are able to combine with 100 parts of silver and with that -quantity of sodium which is contained in 54·2078 parts of sodium -chloride. These observations show that 32·8445 parts of chlorine combine -with 100 parts of silver and with 21·3633 parts of sodium. From these -figures expressing the relation between the combining weights of -chlorine, silver, and sodium, it would be possible to determine their -atomic weights--that is, the combining quantity of these elements with -respect to one part by weight of hydrogen or 16 parts of oxygen, if there -existed a series of similarly accurate determinations for the reactions -between hydrogen or oxygen and one of these elements--chlorine, sodium, -or silver. If we determine the quantity of silver chloride which is -obtained from silver chlorate, AgClO_{3}, we shall know the relation -between the combining weights of silver chloride and oxygen, so that, -taking the quantity of oxygen as a constant magnitude, we can learn from -this reaction the combining weight of silver chloride, and from the -preceding numbers the combining weights of chlorine and silver. For this -purpose it was first necessary to obtain pure silver chlorate. This Stas -did by acting on silver oxide or carbonate, suspended in water, with -gaseous chlorine.[26] - - [26] The phenomenon which then takes place is described by Stas as - follows, in a manner which is perfect in its clearness and - accuracy: if silver oxide or carbonate be suspended in water, and - an excess of water saturated with chlorine be added, all the - silver is converted into chloride, just as is the case with oxide - or carbonate of mercury, and the water then contains, besides the - excess of chlorine, only pure hypochlorous acid without the least - trace of chloric or chlorous acid. If a stream of chlorine be - passed into water containing _an excess of silver oxide_ or silver - carbonate while the liquid is continually agitated, the reaction - is the same as the preceding; silver chloride and hypochlorous - acid are formed. But this acid does not long remain in a free - state: it gradually acts on the silver oxide and gives silver - hypochlorite, _i.e._ AgClO. If, after some time, the current of - chlorine be stopped but the shaking continued, the liquid loses - its characteristic odour of hypochlorous acid, while preserving - its energetic decolorising property, because the silver - hypochlorite which is formed is easily soluble in water. In the - presence of an excess of silver oxide this salt can be kept for - several days without decomposition, but it is exceedingly unstable - when no excess of silver oxide or carbonate is present. So long as - the solution of silver hypochlorite is shaken up with the silver - oxide, it preserves its transparency and bleaching property, but - directly it is allowed to stand, and the silver oxide settles, it - becomes rapidly cloudy and deposits large flakes of silver - chloride, so that the black silver oxide which had settled becomes - covered with the white precipitate. The liquid then loses its - bleaching properties and contains silver chlorate, _i.e._ - AgClO_{3}, in solution, which has a slightly alkaline reaction, - owing to the presence of a small amount of dissolved oxide. In - this manner the reactions which are consecutively accomplished may - be expressed by the equations: - - 6Cl_{2} + 3Ag_{2}O + 3H_{2}O = 6AgCl + 6HClO; - 6HClO + 3Ag_{2}O = 3H_{2}O + 6AgClO; - 6AgClO = 4AgCl + 2AgClO_{3}. - - Hence, Stas gives the following method for the preparation of - silver chlorate: A slow current of chlorine is caused to act on - oxide of silver, suspended in water which is kept in a state of - continual agitation. The shaking is continued after the supply of - chlorine has been stopped, in order that the free hypochlorous - acid should pass into silver hypochlorite, and the resultant - solution of the hypochlorite is drawn off from the sediment of the - excess of silver oxide. This solution decomposes spontaneously - into silver chloride and chlorate. The pure silver chlorate, - AgClO_{3}, does not change under the action of light. The salt is - prepared for further use by drying it in dry air at 150°. It is - necessary during drying to prevent the access of any organic - matter; this is done by filtering the air through cotton wool, and - passing it over a layer of red-hot copper oxide. - -The decomposition of the silver chlorate thus obtained was accomplished -by the action of a solution of sulphurous anhydride on it. The salt was -first fused by carefully heating it at 243°. The solution of sulphurous -anhydride used was one saturated at 0°. Sulphurous anhydride in dilute -solutions is oxidised at the expense of silver chlorate, even at low -temperatures, with great ease if the liquid be continually shaken, -sulphuric acid and silver chloride being formed: AgClO_{3} + 3SO_{2} + -3H_{2}O = AgCl + 3H_{2}SO_{4}. After decomposition, the resultant liquid -was evaporated, and the residue of silver chloride weighed. Thus the -process consisted in taking a known weight of silver chlorate, converting -it into silver chloride, and determining the weight of the latter. The -analysis conducted in this manner gave the following results, which, like -the preceding, designate the weight in a vacuum calculated from the -weights obtained in air: In the first experiment it appeared that -138·7890 grams of silver chlorate gave 103·9795 parts of silver chloride, -and in the second experiment that 259·5287 grains of chlorate gave -194·44515 grams of silver chloride, and after fusion 194·4435 grams. The -mean result of both experiments, converted into percentages, shows that -100 parts of silver chlorate contain 74·9205 of silver chloride and -25·0795 parts of oxygen. From this it is possible to calculate the -combining weight of silver chloride, because in the decomposition of -silver chlorate there are obtained three atoms of oxygen and one molecule -of silver chloride: AgClO_{3} = AgCl + 3O. Taking the weight of an atom -of oxygen to be 16, we find from the mean result that the equivalent -weight of silver chloride is equal to 143·395. Thus if O = 16, AgCl = -143·395, and as the preceding experiments show that silver chloride -contains 32·8445 parts of chlorine per 100 parts of silver, the weight of -the atom of silver[26 bis] must be 107·94 and that of chlorine 35·45. The -weight of the atom of sodium is determined from the fact that 21·3633 -parts of sodium chloride combine with 32·8445 parts of chlorine; -consequently Na = 23·05. This conclusion, arrived at by the analysis of -silver chlorate, was verified by means of the analysis of potassium -chlorate by decomposing it by heat and determining the weight of the -potassium chloride formed, and also by effecting the same decomposition -by igniting the chlorate in a stream of hydrochloric acid. The combining -weight of potassium chloride was thus determined, and another series of -determinations confirmed the relation between chlorine, potassium, and -silver, in the same manner as the relation between sodium, chlorine, and -silver was determined above. Consequently, the combining weights of -sodium, chlorine, and potassium could be deduced by combining these data -with the analysis of silver chlorate and the synthesis of silver -chloride. The agreement between the results showed that the -determinations made by the last method were perfectly correct, and did -not depend in any considerable degree on the methods which were employed -in the preceding determinations, as the combining weights of chlorine and -silver obtained were the same as before. There was naturally a -difference, but so small a one that it undoubtedly depended on the errors -incidental to every process of weighing and experiment. The atomic weight -of silver was also determined by Stas by means of the synthesis of silver -sulphide and the analysis of silver sulphate. The combining weight -obtained by this method was 107·920. The synthesis of silver iodide and -the analysis of silver iodate gave the figure 107·928. The synthesis of -silver bromide with the analysis of silver bromate gave the figure -107·921. The synthesis of silver chloride and the analysis of silver -chlorate gave a mean result of 107·937. Hence there is no doubt that the -combining weight of silver is at least as much as 107·9--greater than -107·90 and less than 107·95, and probably equal to the mean = 107·92. -Stas determined the combining weights of many other elements in this -manner, such as lithium, potassium, sodium, bromine, chlorine, iodine, -and also nitrogen, for the determination of the amount of silver nitrate -obtained from a given amount of silver gives directly the combining -weight of nitrogen. Taking that of oxygen as 16, he obtained the -following combining weights for these elements: nitrogen 14·04, silver -107·93, chlorine 35·46, bromine 79·95, iodine 126·85, lithium 7·02, -sodium 23·04, potassium 39·15. These figures differ slightly from those -which are usually employed in chemical investigations. They must be -regarded as the result of the best observations, whilst the figures -usually used in practical chemistry are only approximate--are, so to -speak, round numbers for the atomic weights which differ so little from -the exact figures (for instance, for Ag 108 instead of 107·92, for Na 23 -instead of 23·04) that in ordinary determinations and calculations the -difference falls within the limits of experimental error inseparable from -such determinations. - - [26 bis] The results given by Stas' determinations have recently - been recalculated and certain corrections have been introduced. We - give in the context the average results of van der Plaats and - Thomsen's calculations, as well as in Table III. neglecting the - doubtful thousandths. - -The exhaustive investigations conducted by Stas on the atomic weights -of the above-named elements have great significance in the solution of -the problem as to whether the atomic weights of the elements can be -expressed in whole numbers if the unit taken be the atomic weight of -hydrogen. Prout, at the beginning of this century, stated that this was -the case, and held that the atomic weights of the elements are multiples -of the atomic weight of hydrogen. The subsequent determinations of -Berzelius, Penny, Marchand, Marignac, Dumas, and more especially of Stas, -proved this conclusion to be untenable; since a whole series of elements -proved to have fractional atomic weights--for example, chlorine, about -35·5. On account of this, Marignac and Dumas stated that the atomic -weights of the elements are expressed in relation to hydrogen, either by -whole numbers or by numbers with simple fractions of the magnitudes 1/2 -and 1/4. But Stas's researches refute this supposition also. Even between -the combining weight of hydrogen and oxygen, there is not, so far as is -yet known, that simple relation which is required by _Prout's -hypothesis_,[27] _i.e._, taking O = 16, the atomic weight of hydrogen is -equal not to 1 but to a greater number somewhere between 1·002 and 1·008 -or mean 1·005. Such a conclusion arrived at by direct experiment cannot -but be regarded as having greater weight than Prout's supposition -(hypothesis) that the atomic weights of the elements are in multiple -proportion to each other, which would give reason for surmising (but not -asserting) a complexity of nature in the elements, and their common -origin from a single primary material, and for expecting their mutual -conversion into each other. All such ideas and hopes must now, thanks -more especially to Stas, be placed in a region void of any experimental -support whatever, and therefore not subject to the discipline of the -positive data of science. - - [27] This hypothesis, for the establishment or refutation of which so - many researches have been made, is exceedingly important, and - fully deserves the attention which has been given to it. Indeed, - if it appeared that the atomic weights of all the elements could - be expressed in whole numbers with reference to hydrogen, or if - they at least proved to be commensurable with one another, then it - could be affirmed with confidence that the elements, with all - their diversity, were formed of one material condensed or grouped - in various manners into the stable, and, under known conditions, - undecomposable groups which we call the atoms of the elements. At - first it was supposed that all the elements were nothing else but - condensed hydrogen, but when it appeared that the atomic weights - of the elements could not be expressed in whole numbers in - relation to hydrogen, it was still possible to imagine the - existence of a certain material from which both hydrogen and all - the other elements were formed. If it should transpire that four - atoms of this material form an atom of hydrogen, then the atom of - chlorine would present itself as consisting of 142 atoms of this - substance, the weight of whose atom would be equal to 0·25. But in - this case the atoms of all the elements should be expressed in - whole numbers with respect to the weight of the atom of this - original material. Let us suppose that the atomic weight of this - material is equal to unity, then all the atomic weights should be - expressible in whole numbers relatively to this unit. Thus the - atom of one element, let us suppose, would weigh _m_, and of - another _n_, but, as both _m_ and _n_ must be whole numbers, it - follows that the atomic weights of all the elements would be - commensurable. But it is sufficient to glance over the results - obtained by Stas, and to be assured of their accuracy, especially - for silver, in order to entirely destroy, or at least strongly - undermine, this attractive hypothesis. We must therefore refuse - our assent to the doctrine of the building up from a single - substance of the elements known to us. This hypothesis is not - supported either by any known transformation (for one element has - never been converted into another element), or by the - commensurability of the atomic weights of the elements. Although - the hypothesis of the formation of all the elements from a single - substance (for which Crookes has suggested the name protyle) is - most attractive in its comprehensiveness, it can neither be denied - nor accepted for want of sufficient data. Marignac endeavoured, - however, to overcome Stas's conclusions as to the - incommensurability of the atomic weights by supposing that in his, - as in the determinations of all other observers, there were - unperceived errors which were quite independent of the mode of - observation--for example, silver nitrate might be supposed to be - an unstable substance which changes, under the heatings, - evaporations, and other processes to which it is subjected in the - reactions for the determination of the combining weight of silver. - It might be supposed, for instance, that silver nitrate contains - some impurity which cannot be removed by any means; it might also - be supposed that a portion of the elements of the nitric acid are - disengaged in the evaporation of the solution of silver nitrate - (owing to the decomposing action of water), and in its fusion, and - that we have not to deal with normal silver nitrate, but with a - slightly basic salt, or perhaps an excess of nitric acid which - cannot be removed from the salt. In this case the observed - combining weight will not refer to an actually definite chemical - compound, but to some mixture for which there does not exist any - perfectly exact combining relations. Marignac upholds this - proposition by the fact that the conclusions of Stas and other - observers respecting the combining weights determined with the - greatest exactitude very nearly agree with the proposition of the - commensurability of the atomic weights--for example, the combining - weight of silver was shown to be equal to 107·93, so that it only - differs by 0·08 from the whole number 108, which is generally - accepted for silver. The combining weight of iodine proved to be - equal to 126·85--that is, it differs from 127 by 0·15. The - combining weights of sodium, nitrogen, bromine, chlorine, and - lithium are still nearer to the whole or round numbers which are - generally accepted. But Marignac's proposition will hardly bear - criticism. Indeed if we express the combining weights of the - elements determined by Stas in relation to hydrogen, the - approximation of these weights to whole numbers disappears, - because one part of hydrogen in reality does not combine with 16 - parts of oxygen, but with 15·92 parts, and therefore we shall - obtain, taking H = 1, not the above-cited figures, but for silver - 107·38, for bromine 79·55, magnitudes which are still further - removed from whole numbers. Besides which, if Marignac's - proposition were true the combining weight of silver determined by - one method--_e.g._ by the analysis of silver chlorate combined - with the synthesis of silver chloride--would not agree well with - the combining weight determined by another method--_e.g._ by means - of the analysis of silver iodate and the synthesis of silver - iodide. If in one case a basic salt could be obtained, in the - other case an acid salt might be obtained. Then the analysis of - the acid salt would give different results from that of the basic - salt. Thus Marignac's arguments cannot serve as a support for the - vindication of Prout's hypothesis. - - In conclusion, I think it will not be out of place to cite the - following passage from a paper I read before the Chemical Society - of London in 1889 (Appendix II.), referring to the hypothesis of - the complexity of the elements recognised in chemistry, owing to - the fact that many have endeavoured to apply the periodic law to - the justification of this idea 'dating from a remote antiquity, - when it was found convenient to admit the existence of many gods - but only one matter.' - - 'When we try to explain the origin of the idea of a unique primary - matter, we easily trace that, in the absence of deductions from - experiment, it derives its origin from the scientifically - philosophical attempt at discovering some kind of unity in the - immense diversity of individualities which we see around. In - classical times such a tendency could only be satisfied by - conceptions about the immaterial world. As to the material world, - our ancestors were compelled to resort to some hypothesis, and - they adopted the idea of unity in the formative material, because - they were not able to evolve the conception of any other possible - unity in order to connect the multifarious relations of matter. - Responding to the same legitimate scientific tendency, natural - science has discovered throughout the universe a unity of plan, a - unity of forces, and a unity of matter; and the convincing - conclusions of modern science compel every one to admit these - kinds of unity. But while we admit unity in many things, we none - the less must also explain the individuality and the apparent - diversity which we cannot fail to trace everywhere. It was said of - old "Give us a fulcrum and it will become easy to displace the - earth." So also we must say, "Give us something that is - individualised, and the apparent diversity will be easily - understood." Otherwise, how could unity result in a multitude. - - 'After a long and painstaking research, natural science has - discovered the individualities of the chemical elements, and - therefore it is now capable, not only of analysing, but also of - synthesising; it can understand and grasp generality and unity, as - well as the individualised and multifarious. The general and - universal, like time and space, like force and motion, vary - uniformly. The uniform admit of interpolations, revealing every - intermediate phase; but the multitudinous, the - individualised--such as ourselves, or the chemical elements, or - the members of a peculiar periodic function of the elements, or - Dalton's multiple proportions--is characterised in another way. We - see in it--side by side with a general connecting - principle--leaps, breaks of continuity, points which escape from - the analysis of the infinitely small--an absence of complete - intermediate links. Chemistry has found an answer to the question - as to the causes of multitudes, and while retaining the conception - of many elements, all submitted to the discipline of a general - law, it offers an escape from the Indian Nirvana--the absorption - in the universal--replacing it by the individualised. However, the - place for individuality is so limited by the all-grasping, - all-powerful universal, that it is merely a point of support for - the understanding of multitude in unity.' - -Among the platinum metals ruthenium, rhodium, and palladium, by their -atomic weights and properties, approach silver, just as iron and its -analogues (cobalt and nickel) approach copper in all respects. _Gold_ -stands in exactly the same position in relation to the heavy platinum -metals, osmium, iridium, and platinum, as copper and silver do to the two -preceding series. The atomic weight of gold is nearly equal to their -atomic weights;[28] it is dense like these metals. It also gives various -grades of oxidation, which are feeble, both in a basic and an acid sense. -Whilst near to osmium, iridium, and platinum, gold at the same time is -able, like copper and silver, to form compounds which answer to the type -RX--that is, oxides of the composition R_{2}O. Cuprous chloride, CuCl, -silver chloride, AgCl, and aurous chloride, AuCl, are substances which -are very much alike in their physical and chemical properties.[28 bis] -They are insoluble in water, but dissolve in hydrochloric acid and -ammonia, in potassium cyanide, sodium thiosulphate, &c. Just as copper -forms a link between the iron metals and zinc, and as silver unites the -light platinum metals with cadmium, so also gold presents a transition -from the heavy platinum metals to mercury. Copper gives saline compounds -of the types CuX and CuX_{2}, silver of the type AgX, whilst gold, -besides compounds of the type AuX, very easily and most frequently forms -those of the type AuCl_{3}. The compounds of this type frequently pass -into those of the lower type, just as PtX_{4} passes into PtX_{2}, and -the same is observable in the elements which, in their atomic weights, -follow gold. Mercury gives HgX_{2} and HgX, thallium gives TlX_{3} and -TlX, lead gives PbX_{4} and PbX_{2}. On the other hand, gold in a -qualitative respect differs from silver and copper in the _extreme ease_ -with which all its compounds are _reduced to metal_ by many means. This -is not only accomplished by many reducing agents, but also by the action -of heat. Thus its chlorides and oxides lose their chlorine and oxygen -when heated, and, if the temperature be sufficiently high, these elements -are entirely expelled and metallic gold alone remains. Its compounds, -therefore, act as oxidising agents.[29] - - [28] It might be expected from the periodic law and analogies with the - series iron, cobalt, nickel, copper, zinc, that the atomic weights - of the elements of the series osmium, iridium, platinum, gold, - mercury, would rise in this order, and at the time of the - establishment of the periodic law (1869), the determinations of - Berzelius, Rose, and others gave the following values for the - atomic weights: Os = 200, Ir = 197, Pt = 198, Au = 196, Hg = 200. - The fulfilment of the expectations of the periodic law was given - in the first place by the fresh determinations (Seubert, Dittmar, - and Arthur) of the atomic weight of platinum, which proved to be - nearly 196, if O = 16 (as Marignac, Brauner, and others propose); - in the second place, by the fact that Seubert proved that the - atomic weight of osmium is really less than that of platinum, and - approximately Os = 191; and, in the third place, by the fact that - after the researches of Krüss, Thorpe, and Laurie there was no - doubt that the atomic weight of gold is greater than that of - platinum--namely, nearly 197. - - [28 bis] In Chapter XXII., Note 40, we gave the thermal data for - certain of the compounds of copper of the type CuX_{2}; we will - now cite certain data for the cuprous compounds of the type CuX, - which present an analogy to the corresponding compounds AgX and - AuX, some of which were investigated by Thomsen in his classical - work, 'Thermochemische Untersuchungen' (Vol. iii., 1883). The data - are given in the same manner as in the above-mentioned note: - - R = Cu Ag Au - R + Cl +33 +29 +6 - R + Br +25 +23 0 - R + I +16 +14 -6 - R + O +41 + 6 -? - - Thus we see in the first place that gold, which possesses a much - smaller affinity than Ag, evolves far less heat than an equivalent - amount of copper, giving the same compound, and in the second - place that the combination of copper with one atom of oxygen - disengages more heat than its combination with one atom of a - halogen, whilst with silver the reverse is the case. This is - connected with the fact that Cu_{2}O is more stable under the - action of heat than Ag_{2}O. - - [29] Heavy atoms and molecules, although they may present many points - of analogy, are more easily isolated; thus C_{16}H_{32}, although, - like C_{2}H_{4}, it combines with Br_{2}, and has a similar - composition, yet reacts with much greater difficulty than - C_{2}H_{4}, and in this it resembles gold; the heavy atoms and - molecules are, so to say, inert, and already saturated by - themselves. Gold in its higher grade of oxidation, Au_{2}O_{3}, - presents feeble basic properties and weakly-developed acid - properties, so that this oxide of gold, Au_{2}O_{3}, may be - referred to the class of feeble acid oxides, like platinic oxide. - This is not the case in the highest known oxides of copper and - silver. But in the lower grade of oxidation, aurous oxide, - Au_{2}O, gold, like silver and copper, presents basic properties, - although they are not very pronounced. In this respect it stands - very close in its properties, although not in its types of - combination (AuX and AuX_{3}), to platinum (PtX_{2} and PtX_{4}) - and its analogues. - - As yet the general chemical characteristics of gold and its - compounds have not been fully investigated. This is partly due to - the fact that very few researches have been undertaken on the - compounds of this metal, owing to its inaccessibility for working - in large quantities. As the atomic weight of gold is high (Au = - 197), the preparation of its compounds requires that it should be - taken in large quantities, which forms an obstacle to its being - fully studied. Hence the facts concerning the history of this - metal are rarely distinguished by that exactitude with which many - facts have been established concerning other elements more - accessible, and long known in use. - -_In nature_ gold occurs in the primary and chiefly in quartzose rocks, -and especially in quartz veins, as in the Urals (at Berezoffsk), in -Australia, and in California. The native gold is extracted from these -rocks by subjecting them to a mechanical treatment consisting of crushing -and washing.[29 bis] Nature has already accomplished a similar -disintegration of the hard rocky matter containing gold.[30] These -disintegrated rocks, washed by rain and other water, have formed -gold-bearing deposits, which are known as _alluvial gold deposits_. -Gold-bearing soil is sometimes met with on the surface and sometimes -under the upper soil, but more frequently along the banks of dried-up -water-courses and running streams. The sand of many rivers contains, -however, a very small amount of gold, which it is not profitable to work; -for example, that of the Alpine rivers contains 5 parts of gold in -10,000,000 parts of sand. The richest gold deposits are those of Siberia, -especially in the southern parts of the Government of Yeniseisk, the -South Urals, Mexico, California, South Africa, and Australia, and then -the comparatively poorer alluvial deposits of many countries (Hungary, -the Alps, and Spain in Europe). The extraction of the gold from alluvial -deposits is based on the principle of levigation; the earth is washed, -while constantly agitated, by a stream of water, which carries away the -lighter portion of the earth, and leaves the coarser particles of the -rock and heavier particles of the gold, together with certain substances -which accompany it, in the washing apparatus. The extraction of this -_washed_ gold only necessitates mechanical appliances,[31] and it is not -therefore surprising that gold was known to savages and in the most -remote period of history. It sometimes occurs in crystals belonging to -the regular system, but in the majority of cases in nuggets or grains of -greater or less magnitude. It always contains silver (from very small -quantities up to 30 p.c., when it is called 'electrum') and certain other -metals, among which lead and rhodium are sometimes found. - - [29 bis] Sonstadt (1872) showed that sea water, besides silver, always - contains gold. Munster (1892) showed that the water of the - Norwegian fiords contains about 5 milligrams of gold per ton (or 5 - milliardths)--_i.e._ a quantity deserving practical attention, and - I think it may be already said that, considering the immeasurable - amount of sea water, in time means will be discovered for - profitably extracting gold from sea water by bringing it into - contact with substances capable of depositing gold upon their - surface. The first efforts might be made upon the extraction of - salt from sea water, and as the total amount of sea water may be - taken as about 2,000,000,000,000,000,000 tons, it follows that it - contains about 10,000 million tons of gold. The yearly production - of gold is about 200 tons for the whole world, of which about one - quarter is extracted in Russia. It is supposed that gold is - dissolved in sea water owing to the presence of iodides, which, - under the action of animal organisms, yield free iodine. It is - thought (as Professor Konovaloff mentions in his work upon 'The - Industries of the United States,' 1894) that iodine facilitates - the solution of the gold, and the organic matter its - precipitation. These facts and considerations to a certain extent - explain the distribution of gold in veins or rock fissures, - chiefly filled with quartz, because there is sufficient reason for - supposing that these rocks once formed the ocean bottom. R. - Dentrie, and subsequently Wilkinson, showed that organic - matter--for instance, cork--and pyrites are able to precipitate - gold from its solutions in that metallic form and state in which - it occurs in quartz veins, where (especially in the deeper parts - of vein deposits) gold is frequently found on the surface of - pyrites, chiefly arsenical pyrites. Kazantseff (in Ekaterinburg, - 1891) even supposes, from the distribution of the gold in these - pyrites, that it occurred in solution as a compound of sulphide of - gold and sulphide of arsenic when it penetrated into the veins. It - is from such considerations that the origin of vein and pyritic - gold is, at the present time, attributed to the reaction of - solutions of this metal, the remains of which are seen in the gold - still present in sea water. - - [30] However, in recent times, especially since about 1870, when - chlorine (either as a solution of the gas or as bleaching powder) - and bromine began to be applied to the extraction of - finely-divided gold from poor ores (previously roasted in order to - drive off arsenic and sulphur, and oxidise the iron), the - extraction of gold from quartz and pyrites, by the wet method, - increases from year to year, and begins to equal the amount - extracted from alluvial deposits. Since the nineties the _cyanide - process_ (Chapter XIII., Note 13 bis) has taken an important place - among the wet methods for extracting gold from its ores. It - consists in pouring a dilute solution of cyanide of potassium - (about 500 parts of water and 1 to 4 parts of cyanide of potassium - per 1,000 parts of ore, the amount of cyanide depending upon the - richness of the ore) and a mixture of it with NaCN, (_see_ Chapter - XIII., Note 12) over the crushed ore (which need not be roasted, - whilst roasting is indispensable in the chlorination process, as - otherwise the chlorine is used up in oxidising the sulphur, - arsenic, &c.) The gold is dissolved very rapidly even from - pyrites, where it generally occurs on the surface in such fine and - adherent particles that it either cannot be mechanically washed - away, or, more frequently is carried away by the stream of water, - and cannot be caught by mechanical means or by the mercury used - for catching the gold in the sluices. Chlorination had already - given the possibility of extracting the finest particles of gold; - but the cyanide process enables such pyrites to be treated as - could be scarcely worked by other means. The treatment of the - crushed ore by the KCN is carried on in simple wooden vats (coated - with paraffin or tar) with the greatest possible rapidity (in - order that the KCN solution should not have time to change) by a - method of systematic lixiviation, and is completed in 10 to 12 - hours. The resultant solution of gold, containing AuK(CN)_{2}, is - decomposed either with freshly-made zinc filings (but when the - gold settles on the Zn, the cyanide solution reacts upon the Zn - with the evolution of H_{2} and formation of ZnH_{2}O_{2}) or by - sodium amalgam prepared at the moment of reaction by the action of - an electric current upon a solution of NaHO poured into a vessel - partially immersed in mercury (the NaCN is renewed continually by - this means). The silver in the ore passes into solution, together - with the gold, as in amalgamation. - - [31] But the particles of gold are sometimes so small that a large - amount is lost during the washing. It is then profitable to have - recourse to the extraction by chlorine and KCN (Note 30). - - In speaking of the extraction of gold the following remarks may - not be out of place: - - In California advantage is taken of water supplied from high - altitudes in order to have a powerful head of water, with which - the rocks are directly washed away, thus avoiding the greater - portion of the mechanical labour required for the exploitation of - these deposits. - - The last residues of gold are sometimes extracted from sand by - washing them with mercury, which dissolves the gold. The sand - mixed with water is caused to come into contact with mercury - during the washing. The mercury is then distilled. - - Many sulphurous ores, even pyrites, contain a small amount of - gold. Compounds of gold with bismuth, BiAu_{2}, tellurium, - AuTe_{2} (calverite), &c., have been found, although rarely. - - Among the minerals which accompany gold, and from which the - presence of gold may be expected, we may mention white quartz, - titanic and magnetic iron ores, and also the following, which are - of rarer occurrence: zircon, topaz, garnet, and such like. The - concentrated gold washings first undergo a mechanical treatment, - and the impure gold obtained is treated for pure gold by various - methods. If the gold contain a considerable amount of foreign - metals, especially lead and copper, it is sometimes cupelled, like - silver, so that the oxidisable metals may be absorbed by the cupel - in the form of oxides, but in every case the gold is obtained - together with silver, because the latter metal also is not - oxidised. Sometimes the gold is extracted by means of mercury, - that is, by amalgamation (and the mercury subsequently driven off - by distillation), or by smelting it with lead (which is afterwards - removed by oxidation) and processes like those employed for the - extraction of silver, because gold, like silver, does not oxidise, - is dissolved by lead and mercury, and is non-volatile. If copper - or any other metal contain gold and it be employed as an anode, - pure copper will be deposited upon the cathode, while all the gold - will remain at the anode as a slime. This method often amply - repays the whole cost of the process, since it gives, besides the - gold, a pure electrolytic copper. - -_The separation of the silver_ from gold is generally carried on with -great precision, as the presence of the silver in the gold does not -increase its value for exchange, and it can be substituted by other less -valuable metals, so that the extraction of the silver, as a precious -metal, from its alloy with gold, is a profitable operation. This -separation is conducted by different methods. Sometimes the argentiferous -gold is melted in crucibles, together with a mixture of common salt and -powdered bricks. The greater portion of the silver is thus converted into -the chloride, which fuses and is absorbed by the slags, from which it may -be extracted by the usual methods. The silver is also extracted from gold -by treating it with boiling sulphuric acid, which does not act on the -gold but dissolves the silver. But if the alloy does not contain a large -proportion of silver it cannot be extracted by this method or at all -events the separation will be imperfect, and therefore a fresh amount of -silver is added (by fusion) to the gold, in such quantity that the alloy -contains twice as much silver as gold. The silver which is added is -preferably such as contains gold, which is very frequently the case. The -alloy thus formed is poured in a thin stream into water, by which means -it is obtained in a granulated form; it is then boiled with strong -sulphuric acid, three parts of acid being used to one part of alloy. The -sulphuric acid extracts all the silver without acting on the gold. It is -best, however, to pour off the first portion of the acid, which has -dissolved the silver, and then treat the residue of still imperfectly -pure gold with a fresh quantity of sulphuric acid. The gold is thus -obtained in the form of powder, which is washed with water until it is -quite free from silver. The silver is precipitated from the solution by -means of copper, so that cupric sulphate and metallic silver are -obtained. This process is carried out in many countries, as in Russia, at -the Government mints. - -Gold is generally used alloyed with copper; since pure gold, like pure -silver, is very soft, and therefore soon worn away. In assaying or -determining the amount of pure gold in such an alloy it is usual to add -silver to the gold in order to make up an alloy containing three parts of -silver to one of gold (this is known as quartation because the alloy -contains 1/4 of gold), and the resultant alloy is treated with nitric -acid. If the silver be not in excess over the gold, it is not all -dissolved by the nitric acid, and this is the reason for the quartation. -The amount of pure gold (assay) is determined by weighing the gold which -remains after this treatment. English gold (= 22 carats) coinage is -composed of an alloy containing 91·66 p.c. of gold, but for many articles -gold is frequently used containing a larger amount of foreign metals. - -_Pure gold_ may be obtained from gold alloys by dissolving in aqua -regia, and then adding ferrous sulphate to the solution or heating it -with a solution of oxalic acid. These deoxidising agents reduce the gold, -but not the other metals. The chlorine combined with the gold then acts -like free chlorine. The gold, thus reduced, is precipitated as an -exceedingly fine brown powder.[31 bis] It is then washed with water, and -fused with nitre or borax. Pure gold reflects a yellow light, and in the -form of very thin sheets (gold leaf), into which it can be hammered and -rolled,[31 tri] it transmits a bluish-green light. The specific gravity -of gold is about 19·5, the sp. gr. of gold coin is about 17·1. It fuses -at 1090°--at a higher temperature than silver--and can be drawn into -exceedingly fine wires or hammered into thin sheets. With its softness -and ductility, gold is distinguished for its tenacity, and a gold wire -two millimetres thick breaks only under a load of 68 kilograms. Gold -vaporises even at a furnace heat, and imparts a greenish colour to a -flame passing over it in a furnace. Gold alloys with copper almost -without changing its volume.[32] In its chemical aspect, gold presents, -as is already seen from its general characteristics given above, an -example of the so-called noble metals--_i.e._ it is incapable of being -oxidised at any temperature, and its oxide is decomposed when calcined. -Only chlorine and bromine combine directly with it at the ordinary -temperature, but many other metals and non-metals combine with it at a -red heat--for example, sulphur, phosphorus, and arsenic. Mercury -dissolves it with great ease. It dissolves in potassium cyanide in the -presence of air; a mixture of sulphuric acid with nitric acid dissolves -it with the aid of heat, although in small quantity. It is also soluble -in aqua regia and in selenic acid. Sulphuric, hydrochloric, nitric, and -hydrofluoric acids and the caustic alkalis do not act on gold, but a -mixture of hydrochloric acid with such oxidising agents as evolve -chlorine naturally dissolves it like aqua regia.[32 bis] - - [31 bis] Schottländer (1893) obtained gold in a soluble colloid form - (the solution is violet) by the action of a mixture of solutions - of cerium acetate and NaHO upon a solution of AuCl_{3}. The gold - separates out from such a solution in exactly the same manner as - Ag does from the solution of colloid silver mentioned above. There - always remains a certain amount of a higher oxide of cerium, - CeO_{2}, in the solution--_i.e._ the gold is reduced by converting - the cerium into a higher grade of oxidation. Besides which Krüss - and Hofmann showed that sulphide of gold precipitated by the - action of H_{2}S upon a solution of AuKCy_{2} mixed with HCl - easily passes into a colloid solution after being properly washed - (like As_{2}S_{3}, CuS, &c., Chapter I., Note 57). - - [31 tri] Gold-leaf is used for gilding wood (leather, cardboard, and - suchlike, upon which it is glued by means of varnish, &c.), and is - about 0·003 millimetre thick. It is obtained from thin sheets - (weighing at first about 1/4 grm. to a square inch), rolled - between gold rollers, by gradually hammering them (in packets of a - number at once) between sheets of moist (but not wet) parchment, - and then, after cutting them into four pieces, between a specially - prepared membrane, which, when at the right degree of moisture, - does not tear or stick together under the blows of the hammer. - - [32] The formation of the alloys Cu + Zn, Cu + Sn, Cu + Bi, Cu + Sb, - Pb + Sb, Ag + Pb, Ag + Sn, Au + Zn, Au + Sn, &c., is accompanied - by a contraction (and evolution of heat). The formation of the - alloys Fe + Sb, Fe + Pb, Cu + Pb, Pb + Sn, Pb + Sb, Zn + Sb, Ag + - Cu, Au + Cu, Au + Pb, takes place with a certain increase in - volume. With regard to the alloys of gold, it may be mentioned - that gold is only slightly dissolved by mercury (about 0·06 p.c., - Dudley, 1890); the remaining portion forms a granular alloy, whose - composition has not been definitely determined. Aluminium (and - silicon) also have the capacity of forming alloys with gold. The - presence of a small amount of aluminium lowers the melting point - of gold considerably (Roberts-Austen, 1892); thus the addition of - 4 p.c. of aluminium lowers it by 14°·28, the addition of 10 p.c. - Al by 41°·7. The latter alloy is white. The alloy AuAl_{2} has a - characteristic purple colour, and its melting point is 32°·5 above - that of gold, which shows it to be a definite compound of the two - metals. The melting points of alloys richer in Al gradually fall - to 660°--that is, below that of aluminium (665°). - - Heycock and Neville (1892), in studying the triple alloys of Au, - Cd, and Sn, observed a tendency in the gold to give compounds with - Cd, and by sealing a mixture of Au and Cd in a tube, from which - the air had been exhausted, and heating it, they obtained a grey - crystalline brittle definite alloy AuCd. - - [32 bis] Calderon (1892), at the request of some jewellers, - investigated the cause of a peculiar alteration sometimes found on - the surface of dead-gold articles, there appearing brownish and - blackish spots, which widen and alter their form in course of - time. He came to the conclusion that these spots are due to the - appearance and development of peculiar micro-organisms - (Aspergillus niger and Micrococcus cimbareus) on the gold, spores - of which were found in abundance on the cotton-wool in which the - gold articles had been kept. - -As regards the compounds of gold, they belong, as was said above, to the -types AuX_{3} and AuX. _Auric chloride_ or _gold trichloride_, AuCl_{3}, -which is formed when gold is dissolved in aqua regia, belongs to the -former and higher of these types. The solution of this substance in water -has a yellow colour, and it may be obtained pure by evaporating the -solution in aqua regia to dryness, but not to the point of decomposition. -If the evaporation proceed to the point of crystallisation, a compound of -gold chloride and hydrochloric acid, AuHCl_{4}, is obtained, like the -allied compounds of platinum; but it easily parts with the acid and -leaves auric chloride, which fuses into a red-brown liquid, and then -solidifies to a crystalline mass. If dry chlorine be passed over gold in -powder it forms a mixture of aurous and auric chlorides, but the aurous -chloride is also decomposed by water into gold and auric chloride. Auric -chloride crystallises from its solutions as AuCl_{3},2H_{2}O, which -easily loses water, and the dry chloride loses two-thirds of its chlorine -at 185°, forming aurous chloride, whilst above 300° the latter chloride -also loses its chlorine and leaves metallic gold. Auric chloride is the -usual form in which gold occurs in solutions, and in which its salts are -used in the arts and for chemical purposes. It is soluble in water, -alcohol, and ether. Light has a reducing action on these solutions, and -after a time metallic gold is deposited upon the sides of vessels -containing the solution. Hydrogen when nascent, and even in a gaseous -form, reduces gold from this solution to a metallic state. The reduction -is more conveniently and usually effected by ferrous sulphate, and in -general by the action of ferrous salts.[33] - - [33] Stannous chloride as a reducing agent also acts on auric chloride, - and gives a red precipitate known as _purple of Cassius_. This - substance, which probably contains a mixture or compound of aurous - oxide and tin oxide, is used as a red pigment for china and glass. - Oxalic acid, on heating, reduces metallic gold from its salts, and - this property may be taken advantage of for separating it from its - solutions. The oxidation which then takes place in the presence of - water may be expressed by the following equation: 2AuCl_{3} + - 3C_{2}H_{2}O_{4} = 2Au + 6HCl + 6CO_{2}. Nearly all organic - substances have a reducing action on gold, and solutions of gold - leave a violet stain on the skin. - - Auric chloride, like platinic chloride, is distinguished for its - clearly-developed property of forming double salts. These double - salts, as a rule, belong to the type AuMCl_{4}. The compound of - auric chloride with hydrochloric acid mentioned above evidently - belongs to the same type. The compounds 2KAuCl_{4},5H_{2}O, - NaAuCl_{4},2H_{2}O, AuNH_{4}Cl_{4},H_{2}O, - Mg(AuCl_{4})_{2},2H_{2}O, and the like are easily crystallised in - well-formed crystals. Wells, Wheeler, and Penfield (1892) obtained - RbAuCl_{4} (reddish yellow) and CsAuCl_{4} (golden yellow), and - corresponding bromides (dark coloured). AuBr_{3} is extremely like - the chloride. Auric cyanide is obtained easily in the form of a - double salt of potassium, KAu(CN)_{4} by mixing saturated and hot - solutions of potassium cyanide with auric chloride and then - cooling. - -If a solution of potassium hydroxide be added to a solution of auric -chloride, a precipitate is first formed, which re-dissolves in an excess -of the alkali. On being evaporated under the receiver of an air-pump, -this solution yields yellow crystals, which present the same composition -as the double salts AuMCl_{4}, with the substitution of the chlorine by -oxygen--that is to say, _potassium aurate_, AuKO_{2}, is formed in -crystals containing 3H_{2}O. The solution has a distinctly alkaline -reaction. _Auric oxide_, Au_{2}O_{3}, separates when this alkaline -solution is boiled with an excess of sulphuric acid. But it then still -retains some alkali; however, it may be obtained in a pure state as a -brown powder by dissolving in nitric acid and diluting with water. The -brown powder decomposes below 250° into gold and oxygen. It is insoluble -in water and in many acids, but it dissolves in alkalis, which shows the -acid character of this oxide. An hydroxide, Au(OH)_{3} may be obtained as -a brown powder by adding magnesium oxide to a solution of auric chloride -and treating the resultant precipitate of magnesium aurate with nitric -acid. This hydroxide loses water at 100°, and gives auric oxide.[34] - - [34] If ammonia be added to a solution of auric chloride, it forms a - yellow precipitate of the so-called fulminating gold, which - contains gold, chlorine, hydrogen, nitrogen, and oxygen, but its - formula is not known with certainty. It is probably a sort of - ammonio-metallic compound, Au_{2}O_{3},4NH_{3}, or amide (like the - mercury compound). This precipitate explodes at 140°, but when - left in the presence of solutions containing ammonia it loses all - its chlorine and becomes non-explosive. In this form the - composition Au_{2}O_{3},2NH_{3},H_{2}O is ascribed to it, but this - is uncertain. Auric sulphide, Au_{2}S_{3}, is obtained by the - action of hydrogen sulphide on a solution of auric chloride, and - also directly by fusing sulphur with gold. It has an acid - character, and therefore dissolves in sodium and ammonium - sulphides. - -The starting-point of the compounds of the type AuX[35] is _gold -monochloride_ or _aurous chloride_, AuCl, which is formed, as mentioned -above, by heating auric chloride at 185°. Aurous chloride forms a -yellowish-white powder; this, when heated with water, is decomposed into -metallic gold and auric chloride, which passes into solution: 3AuCl = -AuCl_{3} + 2Au. This decomposition is accelerated by the action of light. -Hence it is obvious that the compounds corresponding with aurous oxide -are comparatively unstable. But this only refers to the simple compounds -AuX; some of the complex compounds, on the contrary, form the most stable -compounds of gold. Such, for example, is the cyanide of gold and -potassium, AuK(CN)_{2}. It is formed, for instance, when finely-divided -gold dissolves in the presence of air in a solution of potassium cyanide: -4KCN + 2Au + H_{2}O + O = 2KAu(CN)_{2} + 2KHO (this reaction also -proceeds with solid pieces of gold, although very slowly). The same -compound is formed in solution when many compounds of gold are mixed with -potassium cyanide, because if a higher compound of gold be taken, it is -reduced by the potassium cyanide into aurous oxide, which dissolves in -potassium cyanide and forms KAu(CN)_{2}. This substance is soluble in -water, and gives a colourless solution, which can be kept for a long -time, and is employed in electro-gilding--that is, for coating other -metallic objects with a layer of gold, which is deposited if the object -be connected with the negative pole of a battery and the positive pole -consist of a gold plate. When an electric current is passed between them, -the gold from the latter will dissolve, whilst a coating of gold from the -solution will be deposited on the object. - - [35] Many double salts of suboxide of gold belong to the type AuX--for - instance, the cyanide corresponding to the type AuKX_{2}, like - PtK_{2}X_{4}, with which we became acquainted in the last chapter. - We will enumerate several of the representatives of this class of - compounds. If auric chloride, AuCl_{3}, be mixed with a solution - of sodium thiosulphate, the gold passes into a colourless - solution, which deposits colourless crystals, containing a double - thiosulphate of gold and sodium, which are easily soluble in water - but are precipitated by alcohol. The composition of this salt is - Na_{3}Au(S_{2}O_{3})_{2},2H_{2}O. If the sodium thiosulphate be - represented as NaS_{2}O_{3}Na, the double salt in question will be - AuNa(S_{2}O_{3}Na)_{2},2H_{2}O, according to the type AuNaX_{2}. - The solution of this colourless and easily crystallisable salt has - a sweet taste, and the gold is not separated from it either by - ferrous sulphate or oxalic acid. This salt, which is known as - _Fordos and Gelis's salt_, is used in medicine and photography. In - general, aurous oxide exhibits a distinct inclination to the - formation of similar double salts, as we saw also with - PtX_{2}--for example, it forms similar salts with sulphurous acid. - Thus if a solution of sodium sulphite be gradually added to a - solution of oxide of gold in sodium hydroxide, the precipitate at - first formed re-dissolves to a colourless solution, which contains - the double salt Na_{3}Au(SO_{3})_{2} = AuNa(SO_{3}Na)_{2}. The - solution of this salt, when mixed with barium chloride, first - forms a precipitate of barium sulphite, and then a red barium - double salt which corresponds with the above sodium salt. - - The oxygen compound of the type AuX, _aurous oxide_, Au_{2}O, is - obtained as a greenish violet powder on mixing aurous chloride - with potassium chloride in the cold. With hydrochloric acid this - oxide gives gold and auric chloride, and when heated it easily - splits up into oxygen and metallic gold. - - - - - APPENDIX I - - AN ATTEMPT TO APPLY TO CHEMISTRY ONE OF THE PRINCIPLES - OF NEWTON'S NATURAL PHILOSOPHY - - BY PROFESSOR MENDELÉEFF - - A LECTURE DELIVERED AT THE ROYAL INSTITUTION OF GREAT BRITAIN - ON FRIDAY, MAY 31, 1889 - - -Nature, inert to the eyes of the ancients, has been revealed to us as -full of life and activity. The conviction that motion pervaded all -things, which was first realised with respect to the stellar universe, -has now extended to the unseen world of atoms. No sooner had the human -understanding denied to the earth a fixed position and launched it along -its path in space, than it was sought to fix immovably the sun and the -stars. But astronomy has demonstrated that the sun moves with unswerving -regularity through the star-set universe at the rate of about 50 -kilometres per second. Among the so-called fixed stars are now discerned -manifold changes and various orders of movement. Light, heat, -electricity--like sound--have been proved to be modes of motion; to the -realisation of this fact modern science is indebted for powers which have -been used with such brilliant success, and which have been expounded so -clearly at this lecture table by Faraday and by his successors. As, in -the imagination of Dante, the invisible air became peopled with spiritual -beings, so before the eyes of earnest investigators, and especially -before those of Clerk Maxwell, the invisible mass of gases became peopled -with particles: their rapid movements, their collisions, and impacts -became so manifest that it seemed almost possible to count the impacts -and determine many of the peculiarities or laws of their collisions. The -fact of the existence of these invisible motions may at once be made -apparent by demonstrating the difference in the rate of diffusion through -porous bodies of the light and rapidly moving atoms of hydrogen and the -heavier and more sluggish particles of air. Within the masses of liquid -and of solid bodies we have been forced to acknowledge the existence of -persistent though limited motion of their ultimate particles, for -otherwise it would be impossible to explain, for example, the celebrated -experiments of Graham on diffusion through liquid and colloidal -substances. If there were, in our times, no belief in the molecular -motion in solid bodies, could the famous Spring have hoped to attain any -result by mixing carefully-dried powders of potash, saltpetre and sodium -acetate, in order to produce, by pressure, a chemical reaction between -these substances through the interchange of their metals, and have -derived, for the conviction of the incredulous, a mixture of two -hygroscopic though solid salts--sodium nitrate and potassium acetate? - -In these invisible and apparently chaotic movements, reaching from the -stars to the minutest atoms, there reigns, however, a harmonious order -which is commonly mistaken for complete rest, but which is really a -consequence of the conservation of that dynamic equilibrium which was -first discerned by the genius of Newton, and which has been traced by his -successors in the detailed analysis of the particular consequences of the -great generalisation, namely, relative immovability in the midst of -universal and active movement. - -But the unseen world of chemical changes is closely analogous to the -visible world of the heavenly bodies, since our atoms form distinct -portions of an invisible world, as planets, satellites, and comets form -distinct portions of the astronomer's universe; our atoms may therefore -be compared to the solar systems, or to the systems of double or of -single stars: for example, ammonia (NH_{3}) may be represented in the -simplest manner by supposing the sun, nitrogen, surrounded by its planets -of hydrogen; and common salt (NaCl) may be looked on as a double star -formed of sodium and chlorine. Besides, now that the indestructibility of -the elements has been acknowledged, chemical changes cannot otherwise be -explained than as changes of motion, and the production by chemical -reactions of galvanic currents, of light, of heat, of pressure, or of -steam power, demonstrates visibly that the processes of chemical reaction -are inevitably connected with enormous though unseen displacements, -originating in the movements of atoms in molecules. Astronomers and -natural philosophers, in studying the visible motions of the heavenly -bodies and of matter on the earth, have understood and have estimated the -value of this store of energy. But the chemist has had to pursue a -contrary course. Observing in the physical and mechanical phenomena which -accompany chemical reactions the quantity of energy manifested by the -atoms and molecules, he is constrained to acknowledge that within the -molecules there exist atoms in motion, endowed with an energy which, like -matter itself, is neither being created nor capable of being destroyed. -Therefore, in chemistry, we must seek dynamic equilibrium not only -between the molecules, but also in their midst among their component -atoms. Many conditions of such equilibrium have been determined, but much -remains to be done, and it is not uncommon, even in these days, to find -that some chemists forget that there is the possibility of motion in the -interior of molecules, and therefore represent them as being in a -condition of death-like inactivity. - -Chemical combinations take place with so much ease and rapidity, possess -so many special characteristics, and are so numerous, that their -simplicity and order were for a long time hidden from investigators. -Sympathy, relationship, all the caprices or all the fancifulness of human -intercourse, seemed to have found complete analogies in chemical -combinations, but with this difference, that the characteristics of the -material substances--such as silver, for example, or of any other -body--remain unchanged in every subdivision from the largest masses to -the smallest particles, and consequently these characteristics must be -properties of the particles. But the world of heavenly luminaries -appeared equally fanciful at man's first acquaintance with it, so much -so, that the astrologers imagined a connection between the -individualities of men and the conjunctions of planets. Thanks to the -genius of Lavoisier and of Dalton, man has been able, in the unseen world -of chemical combinations, to recognise laws of the same simple order as -those which Copernicus and Kepler proved to exist in the planetary -universe. Man discovered, and continues every hour to discover, _what_ -remains unchanged in chemical evolution, and _how_ changes take place in -combinations of the unchangeable. He has learned to predict, not only -what possible combinations may take place, but also the very existence of -atoms of unknown elementary substances, and has besides succeeded in -making innumerable practical applications of his knowledge to the great -advantage of his race, and has accomplished this notwithstanding that -notions of sympathy and affinity still preserve a strong vitality in -science. At present we cannot apply Newton's principles to chemistry, -because the soil is only being now prepared. The invisible world of -chemical atoms is still waiting for the creator of chemical mechanics. -For him our age is collecting a mass of materials, the inductions of -well-digested facts, and many-sided inferences similar to those which -existed for Astronomy and Mechanics in the days of Newton. It is well -also to remember that Newton devoted much time to chemical experiments, -and while considering questions of celestial mechanics, persistently kept -in view the mutual action of those infinitely small worlds which are -concerned in chemical evolutions. For this reason, and also to maintain -the unity of laws, it seems to me that we must, in the first instance, -seek to harmonise the various phases of contemporary chemical theories -with the immortal principles of the Newtonian natural philosophy, and so -hasten the advent of true chemical mechanics. Let the above -considerations serve as my justification for the attempt which I propose -to make to act as a champion of the universality of the Newtonian -principles, which I believe are competent to embrace every phenomenon in -the universe, from the rotation of the fixed stars to the interchanges of -chemical atoms. - -In the first place I consider it indispensable to bear in mind that, up -to quite recent times, only a one-sided affinity has been recognised in -chemical reactions. Thus, for example, from the circumstance that red-hot -iron decomposes water with the evolution of hydrogen, it was concluded -that oxygen had a greater affinity for iron than for hydrogen. But -hydrogen, in presence of red-hot iron scale, appropriates its oxygen and -forms water, whence an exactly opposite conclusion may be formed. - -During the last ten years a gradual, scarcely perceptible, but most -important change has taken place in the views, and consequently in the -researches, of chemists. They have sought everywhere, and have always -found, systems of conservation or dynamic equilibrium substantially -similar to those which natural philosophers have long since discovered in -the visible world, and in virtue of which the position of the heavenly -bodies in the universe is determined. There where one-sided affinities -only were at first detected, not only secondary or lateral ones have been -found, but even those which are diametrically opposite; yet among these, -dynamical equilibrium establishes itself not by excluding one or other of -the forces, but regulating them all. So the chemist finds in the flame of -the blast furnace, in the formation of every salt, and, with especial -clearness, in double salts and in the crystallisation of solutions, not a -fight ending in the victory of one side, as used to be supposed, but the -conjunction of forces; the peace of dynamic equilibrium resulting from -the action of many forces and affinities. Carbonaceous matters, for -example, burn at the expense of the oxygen of the air, yielding a -quantity of heat, and forming products of combustion, in which it was -thought that the affinities of the oxygen with the combustible elements -were satisfied. But it appeared that the heat of combustion was competent -to decompose these products, to dissociate the oxygen from the -combustible elements, and therefore to explain combustion fully it is -necessary to take into account the equilibrium between opposite -reactions, between those which evolve and those which absorb heat. - -In the same way, in the case of the solution of common salt in water, it -is necessary to take into account, on the one hand, the formation of -compound particles generated by the combination of salt with water, and, -on the other, the disintegration or scattering of the new particles -formed, as well as of these originally contained. At present we find two -currents of thought, apparently antagonistic to each other, dominating -the study of solutions: according to the one, solution seems a mere act -of building up or association; according to the other, it is only -dissociation or disintegration. The truth lies, evidently, between these -views; it lies, as I have endeavoured to prove by my investigations into -aqueous solutions, in the dynamic equilibrium of particles tending to -combine and also to fall asunder. The large majority of chemical -reactions which appeared to act victoriously along one line have been -proved capable of acting as victoriously even along an exactly opposite -line. Elements which utterly decline to combine directly may often be -formed into comparatively stable compounds by indirect means, as, for -example, in the case of chlorine and carbon; and consequently the -sympathies and antipathies which it was thought to transfer from human -relations to those of atoms should be laid aside until the mechanism of -chemical relations is explained. Let us remember, however, that chlorine, -which does not form with carbon the chloride of carbon, is strongly -absorbed, or, as it were, dissolved, by carbon, which leads us to suspect -incipient chemical action even in an external and purely surface contact, -and involuntarily gives rise to conceptions of that unity of the forces -of nature which has been so energetically insisted on by Sir William -Grove and formulated in his famous paradox. Grove noticed that platinum, -when fused in the oxyhydrogen flame, during which operation water is -formed, when allowed to drop into water decomposes the latter and -produces the explosive oxyhydrogen mixture. The explanation of this -paradox, as of many others which arose during the period of chemical -renaissance, has led, in our time, to the promulgation by Henri -Sainte-Claire Deville of the conception of dissociation and of -equilibrium, and has recalled the teaching of Berthollet, which, -notwithstanding its brilliant confirmation by Heinrich Rose and Dr. -Gladstone, had not, up to that period, been included in received chemical -views. - -Chemical equilibrium in general, and dissociation in particular, are now -being so fully worked out in detail, and supplied in such various ways, -that I do not allude to them to develop, but only use them as examples by -which to indicate the correctness of a tendency to regard chemical -combinations from points of view differing from those expressed by the -term hitherto appropriated to define chemical forces, namely, 'affinity.' -Chemical equilibria, dissociation, the speed of chemical reactions, -thermochemistry, spectroscopy, and, more than all, the determination of -the influence of masses and the search for a connection between the -properties and weights of atoms and molecules--in one word, the vast mass -of the most important chemical researches of the present day--clearly -indicate the near approach of the time when chemical doctrines will -submit fully and completely to the doctrine which was first announced in -the _Principia_ of Newton. - -In order that the application of these principles may bear fruit it is -evidently insufficient to assume that statical equilibrium reigns alone -in chemical systems or chemical molecules: it is necessary to grasp the -conditions of possible states of dynamical equilibria, and to apply to -them kinetic principles. Numerous considerations compel us to renounce -the idea of statical equilibrium in molecules, and the recent yet -strongly-supported appeals to dynamic principles constitute, in my -opinion, the foundation of the modern teaching relating to atomicity, or -the valency of the elements, which usually forms the basis of -investigations into organic or carbon compounds. - -This teaching has led to brilliant explanations of very many chemical -relations and to cases of isomerism, or the difference in the properties -of substances having the same composition. It has been so fruitful in its -many applications and in the foreshadowing of remote consequences, -especially respecting carbon compounds, that it is impossible to deny its -claims to be ranked as a great achievement of chemical science. Its -practical application to the synthesis of many substances of the most -complicated composition entering into the structure of organised bodies, -and to the creation of an unlimited number of carbon compounds, among -which the colours derived from coal tar stand prominently forward, -surpass the synthetical powers of Nature itself. Yet this teaching, as -applied to the structure of carbon compounds, is not on the face of it -directly applicable to the investigation of other elements, because in -examining the first it is possible to assume that the atoms of carbon -have always a definite and equal number of affinities, whilst in the -combinations of other elements this is evidently inadmissible. Thus, for -example, an atom of carbon yields only one compound with four atoms of -hydrogen and one with four atoms of chlorine in the molecule, whilst the -atoms of chlorine and hydrogen unite only in the proportions of one to -one. Simplicity is here evident, and forms a point of departure from -which it is easy to move forward with firm and secure tread. Other -elements are of a different nature. Phosphorus unites with three and with -five atoms of chlorine, and consequently the simplicity and sharpness of -the application of structural conceptions are lost. Sulphur unites only -with two atoms of hydrogen, but with oxygen it enters into higher orders -of combination. The periodic relationship which exists among all the -properties of the elements--such, for example, as their ability to enter -into various combinations--and their atomic weights, indicate that this -variation in atomicity is subject to one perfectly exact and general law, -and it is only carbon and its near analogues which constitute cases of -permanently preserved atomicity. It is impossible to recognise as -constant and fundamental properties of atoms, powers which, in substance, -have proved to be variable. But by abandoning the idea of permanence, and -of the constant saturation of affinities--that is to say, by -acknowledging the possibility of free affinities--many retain a -comprehension of the atomicity of the elements 'under given conditions;' -and on this frail foundation they build up structures composed of -chemical molecules, evidently only because the conception of manifold -affinities gives, at once, a simple statical method of estimating the -composition of the most complicated molecules. - -I shall enter neither into details, nor into the various consequences -following from these views, nor into the disputes which have sprung up -respecting them (and relating especially to the number of isomerides -possible on the assumption of free affinities), because the foundation or -origin of theories of this nature suffers from the radical defect of -being in opposition to dynamics. The molecule, as even Laurent expressed -himself, is represented as an architectural structure, the style of which -is determined by the fundamental arrangement of a few atoms, whilst the -decorative details, which are capable of being varied by the same forces, -are formed by the elements entering into the combination. It is on this -account that the term 'structural' is so appropriate to the contemporary -views of the above order, and that the 'structuralists' seek to justify -the tetrahedric, plane, or prismatic disposition of the atoms of carbon -in benzene. It is evident that the consideration relates to the statical -position of atoms and molecules and not to their kinetic relations. The -atoms of the structural type are like the lifeless pieces on a chess -board: they are endowed but with the voices of living beings, and are not -those living beings themselves; acting, indeed, according to laws, yet -each possessed of a store of energy which, in the present state of our -knowledge, must be taken into account. - -In the days of Haüy, crystals were considered in the same statical and -structural light, but modern crystallographers, having become more -thoroughly acquainted with their physical properties and their actual -formation, have abandoned the earlier views, and have made their -doctrines dependent on dynamics. - -The immediate object of this lecture is to show that, starting with -Newton's third law of motion, it is possible to preserve to chemistry all -the advantages arising from structural teaching, without being obliged to -build up molecules in solid and motionless figures, or to ascribe to -atoms definite limited valencies, directions of cohesion, or affinities. -The wide extent of the subject obliges me to treat only a small portion -of it, namely of _substitutions_, without specially considering -combinations and decompositions, and even then limiting myself to the -simplest examples, which, however, will throw open prospects embracing -all the natural complexity of chemical relations. For this reason, if it -should prove possible to form groups similar, for example, to H_{4} or -CH_{6} as the remnants of molecules CH_{4} or C_{2}H_{7} we shall not -pause to consider them, because, as far as we know, they fall asunder -into two parts, H_{2} + H_{2} or CH_{4} + H_{2}, as soon as they are even -temporarily formed, and are incapable of separate existence, and -therefore can take no part in the elementary act of substitution. With -respect to the simplest molecules which we shall select--that is to say, -those of which the parts have no separate existence, and therefore cannot -appear in substitutions--we shall consider them according to the periodic -law, arranging them in direct dependence on the atomic weight of the -elements. - -Thus, for example, the molecules of the simplest hydrogen compounds-- - - HF H_{2}O H_{3}N H_{4}C - hydrofluoric acid water ammonia methane - -correspond with elements the atomic weights of which decrease -consecutively - - F = 19, O = 16, N = 14, C = 12. - -Neither the arithmetical order (1, 2, 3, 4 atoms of hydrogen) nor the -total information we possess respecting the elements will permit us to -interpolate into this typical series one more additional element; and -therefore we have here, for hydrogen compounds, a natural base on which -are built up those simple chemical combinations which we take as typical. -But even they are competent to unite with each other, as we see, for -instance, in the property which hydrofluoric acid has of forming a -hydrate--that is, of combining with water; and a similar attribute of -ammonia, resulting in the formation of a caustic alkali, NH_{3},H_{2}O, -or NH_{4}OH. - -Having made these indispensable preliminary observations, I may now -attack the problem itself and attempt to explain the so-called structure -or rather construction, of molecules--that is to say, their constitution -and transformations--without having recourse to the teaching of -'structuralists,' but on Newton's dynamical principles. - -Of Newton's three laws of motion, only the third can be applied directly -to chemical molecules when regarded as systems of atoms among which it -must be supposed that there exist common influences or forces, and -resulting compounded relative motions. Chemical reactions of every kind -are undoubtedly accomplished by changes in these internal movements, -respecting the nature of which nothing is known at present, but the -existence of which the mass of evidence collected in modern times forces -us to acknowledge as forming part of the common motion of the universe, -and as a fact further established by the circumstance that chemical -reactions are always characterised by changes of volume or the relations -between the atoms or the molecules. Newton's third law, which is -applicable to every system, declares that, 'action is also associated -with reaction, and is equal to it.' The brevity of conciseness of this -axiom was, however, qualified by Newton in a more expanded statement, -'the action of bodies one upon another are always equal, and in opposite -directions.' This simple fact constitutes the point of departure for -explaining dynamic equilibrium--that is to say, systems of conservancy. -It is capable of satisfying even the dualists, and of explaining, without -additional assumptions, the preservation of those chemical types which -Dumas, Laurent, and Gerhardt created unit types, and those views of -atomic combinations which the structuralists express by atomicity or the -valency of the elements, and, in connection with them, the various -numbers of affinities. In reality, if a system of atoms or a molecule be -given, then in it, according to the third law of Newton, each portion of -atoms acts on the remaining portion in the same manner, and with the same -force as the second set of atoms acts on the first. We infer directly -from this consideration that both sets of atoms, forming a molecule, are -not only equivalent with regard to themselves, as they must be according -to Dalton's law, but also that they may, if united, replace each other. -Let there be a molecule containing atoms A B C, it is clear that, -according to Newton's law, the action of A on B C must be equal to the -action of B C on A, and if the first action is directed on B C, then the -second must be directed on A, and consequently then, where A can exist in -dynamic equilibrium, B C may take its place and act in a like manner. In -the same way the action of C is equal to the action of A B. In one word -every two sets of atoms forming a molecule are equivalent to each other, -and may take each other's place in other molecules, or, having the power -of balancing each other, the atoms or their complements are endowed with -the power of replacing each other. Let us call this consequence of an -evident axiom 'the principle of substitution,' and let us apply it to -those typical forms of hydrogen compounds which we have already -discussed, and which, on account of their simplicity, and regularity, -have served as starting-points of chemical argument long before the -appearance of the doctrine of structure. - -In the type of hydrofluoric acid, HF, or in systems of double stars, are -included a multitude of the simplest molecules. It will be sufficient for -our purpose to recall a few: for example, the molecules of chlorine, -Cl_{2}, and of hydrogen, H_{2}, and hydrochloric acid, HCl, which is -familiar to all in aqueous solution as spirits of salt, and which has -many points of resemblance with HF, HBr, HI. In these cases division into -two parts can only be made in one way, and therefore the principle of -substitution renders it probable that exchanges between the chlorine and -the hydrogen can take place, if they are competent to unite with each -other. There was a time when no chemist would even admit the idea of any -such action; it was then thought that the power of combination indicated -a polar difference of the molecules in combination, and this thought set -aside all idea of the substitution of one component element by another. - -Thanks to the observations and experiments of Dumas and Laurent fifty -years ago, such fallacies were dispelled, and in this manner the -principle of substitution was exhibited. Chlorine and bromine acting on -many hydrogen compounds, occupy immediately the place of their hydrogen, -and the displaced hydrogen, with another atom of chlorine or bromine, -forms hydrochloric acid or bromide of hydrogen. This takes place in all -typical hydrogen compounds. Thus chlorine acts on this principle on -gaseous hydrogen--reaction, under the influence of light, resulting in -the formation of hydrochloric acid. Chlorine acting on the alkalis, -constituted similarly to water, and even on water itself--only, however, -under the influence of light and only partially because of the -instability of HClO--forms by this principle bleaching salts, which are -the same as the alkalis, but with their hydrogen replaced by chlorine. In -ammonia and in methane, chlorine can also replace the hydrogen. From -ammonia is formed in this manner the so-called chloride of nitrogen, -NCl_{3}, which decomposes very readily with violent explosion on account -of the evolved gases, and falls asunder as chlorine and nitrogen. Out of -marsh gas, or methane, CH_{4}, may be obtained consecutively, by this -method, every possible substitution, of which chloroform, CHCl_{3}, is -the best known, and carbon tetrachloride, CCl_{4}, the most instructive. -But by virtue of the fact that chlorine and bromine act, in the manner -shown, on the simplest typical hydrogen compounds, their action on the -more complicated ones may be assumed to be the same. This can be easily -demonstrated. The hydrogen of benzene, C_{6}H_{6}, reacts feebly under -the influence of light on liquid bromine, but Gustavson has shown that -the addition of the smallest quantity of metallic aluminium causes -energetic action and the evolution of large volumes of hydrogen bromide. - -If we pass on to the second typical hydrogen compound--that is to say, -water--its molecule, HOH, may be split up in two ways: either into an -atom of hydrogen and a semi-molecule of hydrogen peroxide, HO, or into -oxygen, O, and two atoms of hydrogen, H; and therefore, according to the -principle of substitution, it is evident that one atom of hydrogen can -exchange with hydrogen oxide, HO, and two atoms of hydrogen, H, with one -atom of oxygen, O. - -Both these forms of substitution will constitute methods of -oxidation--that is to say, of the entrance of oxygen into the compound--a -reaction which is so common in nature as well as in the arts, taking -place at the expense of the oxygen of the air or by the aid of various -oxidising substances or bodies which part easily with their oxygen. There -is no occasion to reckon up the unlimited number of cases of such -oxidising reactions. It is sufficient to state that in the first of these -oxygen is directly transferred, and the position, the chemical function, -which hydrogen originally occupied, is, after the substitution, occupied -by the hydroxyl. Thus ammonia, NH_{3}, yields hydroxylamine, NH_{2}(OH), -a substance which retains many of the properties of ammonia. - -Methane and a number of other hydrocarbons yield, by substitution of the -hydrogen by its oxide, methyl alcohol, CH_{3}(OH), and other alcohols. -The substitution of one atom of oxygen for two atoms of hydrogen is -equally common with hydrogen compounds. By this means alcoholic liquids -containing ethyl alcohol, or spirits of wine, C_{2}H_{5}(OH), are -oxidised until they become vinegar, or acetic acid, C_{2}H_{3}O(OH). In -the same way caustic ammonia, or the combination of ammonia with water, -NH_{3},H_{2}O, or NH_{4}(OH), which contains a great deal of hydrogen, by -oxidation exchanges four atoms of hydrogen for two atoms of oxygen, and -becomes converted into nitric acid, NO_{2}(OH). This process of -conversion of ammonium salts into saltpetre goes on in the fields every -summer, and with especial rapidity in tropical countries. The method by -which this is accomplished, though complex, though involving the agency -of all-permeating micro-organisms, is, in substance, the same as that by -which alcohol is converted into acetic acid, or glycol, -C_{2}H_{4}(OH)_{2}, into oxalic acid, if we view the process of oxidation -in the light of the Newtonian principles. - -But while speaking of the application of the principle of substitution -to water, we need not multiply instances, but must turn our attention to -two special circumstances which are closely connected with the very -mechanism of substitutions. - -In the first place, the replacement of two atoms of hydrogen by one atom -of oxygen may take place in two ways, because the hydrogen molecule is -composed of two atoms, and therefore, under the influence of oxygen, the -molecule forming water may separate before the oxygen has time to take -its place. It is for this reason that we find, during the conversion of -alcohol into acetic acid, that there is an interval during which is -formed aldehyde, C_{2}H_{4}O, which, as its very name implies, is -'alcohol dehydrogenatum,' or alcohol deprived of hydrogen. Hence aldehyde -combined with hydrogen yields alcohol; and united to oxygen, acetic acid. - -For the same reason there should be, and there actually are, intermediate -products between ammonia and nitric acid, NO_{2}(HO), containing either -less hydrogen than ammonia, less oxygen than nitric acid, or less water -than caustic ammonia. Accordingly we find, among the products of the -deoxidation of nitric acid and the oxidation of ammonia, not only -hydroxylamine, but also nitrous oxide, nitrous and nitric anhydrides. -Thus, the production of nitrous acid results from the removal of two -atoms of hydrogen from caustic ammonia and the substitution of the oxygen -for the hydrogen, NO(OH); or by the substitution, in ammonia, of three -atoms of hydrogen by hydroxyl, N(OH)_{3}, and by the removal of water: -N(OH)_{3} - H_{2}O = NO(OH). The peculiarities and properties of nitrous -acid--as, for instance, its action on ammonia and its conversion, by -oxidation, into nitric acid--are thus clearly revealed. - -On the other hand, in speaking of the principle of substitution as -applied to water, it is necessary to observe that hydrogen and hydroxyl, -H and OH, are not only competent to unite, but also to form combinations -with themselves, and thus become H_{2} and H_{2}O_{2}; and such are -hydrogen and the peroxide thereof. In general, if a molecule A B exists, -then molecules A A and B B can exist also. A direct reaction of this kind -does not, however, take place in water, therefore undoubtedly, at the -moment of formation, hydrogen reacts on hydrogen peroxide, as we can show -at once by experiment; and further because hydrogen peroxide, H_{2}O_{2}, -exhibits a structure containing a molecule of hydrogen, H_{2}, and one of -oxygen, O_{2}, either of which is capable of separate existence. The -fact, however, may now be taken as thoroughly established, that, at the -moment of combustion of hydrogen or of the hydrogen compounds, hydrogen -peroxide is always formed, and not only so, but in all probability its -formation invariably precedes the formation of water. This was to be -expected as a consequence of the law of Avogadro and Gerhardt, which -leads us to expect this sequence in the case of equal interactions of -volumes of vapours and gases; and in hydrogen peroxide we actually have -such equal volumes of the elementary gases. - -The instability of hydrogen peroxide--that is to say, the ease with -which it decomposes into water and oxygen, even at the mere contact of -porous substances--accounts for the circumstance that it does not form a -permanent product of combustion, and is not produced during the -decomposition of water. I may mention this additional consideration that, -with respect to hydrogen peroxide, we may look for its effecting still -further substitutions of hydrogen by means of which we may expect to -obtain still more highly oxidised water compounds, such as H_{2}O_{3} and -H_{2}O_{4}. These Schönbein and Bunsen have long been seeking, and -Berthelot is investigating them at present. It is probable, however, that -the reaction will stop at the last compound, because we find that, in a -number of cases, the addition of four atoms of oxygen seems to form a -limit. Thus, OsO_{4}, KClO_{4}, KMnO_{4}, K_{2}SO_{4}, Na_{3}PO_{4}, and -such like, represent the highest grades of oxidation.[1] - - [1] Because more than four atoms of hydrogen never unite with one atom - of the elements, and because the hydrogen compounds (_e.g._ HCl, - H_{2}S, H_{3}P, H_{4}Si) always form their highest oxides with four - atoms of oxygen, and as the highest forms of oxides (OsO_{4}, - RuO_{4}) also contain four of oxygen, and eight groups of the - periodic system, corresponding to the highest basic oxides R_{2}O, - RO, R_{2}O_{3}, RO_{2}, R_{2}O_{5}, RO_{3}, R_{2}O_{7}, and RO_{4}, - imply the above relationship, and because of the nearest analogues - among the elements--such as Mg, Zn, Cd, and Hg; or Cr, Mo, W, and - U; or Si, Ge, Sn, and Pt; or F, Cl, Br, and I, and so forth--not - more than four are known, it seems to me that in these - relationships there lies a deep interest and meaning with regard to - chemical mechanics. But because, to my imagination, the idea of - unity of design in Nature, either acting in complex celestial - systems or among chemical molecules, is very attractive, especially - because the atomic teaching at once acquires its true meaning, I - will recall the following facts relating to the solar system. There - are eight major planets, of which the four inner ones are not only - separated from the four outer by asteroids, but differ from them in - many respects, as, for example, in the smallness of their diameters - and their greater density. Saturn with his ring has eight - satellites, Jupiter and Uranus have each four. It is evident that - in the solar systems also we meet with these higher numbers four - and eight which appear in the combination of chemical molecules. - -As for the last forty years, from the times of Berzelius, Dumas, Liebig, -Gerhardt, Williamson, Frankland, Kolbe, Kekulé, and Butleroff, most -theoretical generalisations have centred round organic or carbon -compounds, we will, for the sake of brevity, leave out the discussion of -ammonia derivatives, notwithstanding their simplicity with respect to the -doctrine of substitutions; we will dwell more especially on its -application to carbon compounds, starting from methane, CH_{4}, as the -simplest of the hydrocarbons, containing in its molecule one atom of -carbon. According to the principles enumerated we may derive from CH_{4} -every combination of the form CH_{3}X, CH_{2}X_{2}, CHX_{3}, and CX_{4}, -in which X is an element, or radicle, equivalent to hydrogen--that is to -say, competent to take its place or to combine with it. Such are the -chlorine substitutes already mentioned, such is wood-spirit, CH_{3}(OH), -in which X is represented by the residue of water, and such are numerous -other carbon derivatives. If we continue, with the aid of hydroxyl, -further substitutions of the hydrogen of methane we shall obtain -successively CH_{2}(OH)_{2}, CH(OH)_{3}, and C(OH)_{4}. But if, in -proceeding thus, we bear in mind that CH_{2}(OH)_{2} contains two -hydroxyls in the same form as hydrogen peroxide, H_{2}O_{2} or (OH)_{2}, -contains them--and moreover not only in one molecule, but together, -attached to one and the same atom of carbon--so here we must look for the -same decomposition as that which we find in hydrogen peroxide, and -accompanied also by the formation of water as an independently existing -molecule; therefore CH_{2}(OH)_{2} should yield, as it actually does, -immediately water and the oxide of methylene, CH_{2}O, which is methane -with oxygen substituted for two atoms of hydrogen. Exactly in the same -manner out of CH(OH)_{3} are formed water and formic acid, CHO(OH), and -out of C(OH)_{4} is produced water and carbonic acid, or directly -carbonic anhydride, CO_{2}, which will therefore be nothing else than -methane with the double replacement of pairs of hydrogen by oxygen. As -nothing leads to the supposition that the four atoms of hydrogen in -methane differ one from the other, so it does not matter by what means we -obtain any one of the combinations indicated--they will be identical; -that is to say, there will be no case of actual isomerism, although there -may easily be such cases of isomerism as have been distinguished by the -term metamerism. - -Formic acid, for example, has two atoms of hydrogen, one attached to the -carbon left from the methane, and the other attached to the oxygen which -has entered in the form of hydroxyl, and if one of them be replaced by -some substance X it is evident that we shall obtain substances of the -same composition, but of different construction, or of different orders -of movement among the molecules, and therefore endowed with other -properties and reactions. If X be methyl, CH_{4}--that is to say, a group -capable of replacing hydrogen because it is actually contained with -hydrogen in methane itself--then by substituting this group for the -original hydrogen we obtain acetic acid, CCH_{3}O(OH), out of formic, and -by substitution of the hydrogen in its oxide or hydroxyl we obtain methyl -formate, CHO(OCH_{3}). These substances differ so much from each other -physically and chemically that at first sight it is hardly possible to -admit that they contain the same atoms in identically the same -proportions. Acetic acid, for example, boils at a higher temperature than -water, and has a higher specific gravity than it, whilst its metameride, -methyl formate, is lighter than water, and boils at 30°--that is to say, -it evaporates very easily. - -Let us now turn to carbon compounds containing two atoms of carbon to the -molecule, as in acetic acid, and proceed to evolve them from methane by -the principle of substitution. This principle declares at once that -methane can only be split up in the four following ways:-- - -1. Into a group CH_{3} equivalent with H. Let us call changes of this -nature methylation. - -2. Into a group CH_{2} and H_{2}. We will call this order of -substitutions methylenation. - -3. Into CH and H_{3}, which commutations we will call acetylenation. - -4. Into C and H_{4}, which may be called carbonation. - -It is evident that hydrocarbon compounds containing two atoms of carbon -can only proceed from methane, CH_{4}, which contains four atoms of -hydrogen by the first three methods of substitution; carbonation would -yield free carbon if it could take place directly, and if the molecule of -free carbon--which is in reality very complex, that is to say strongly -polyatomic, as I have long since been proving by various means--could -contain only C_{2} like the molecules O_{2}, H_{2}, N_{2}, and so on. - -By methylation we should evidently obtain from marsh gas, ethane, -CH_{3}CH_{3} = C_{2}H_{6}. - -By methylenation--that is, by substituting group CH_{2} for -H_{2}--methane forms ethylene, CH_{2}CH_{2} = C_{2}H_{4}. - -By acetylenation--that is, by substituting three atoms of hydrogen, -H_{3}, in methane--by the remnant CH, we get acetylene, CHCH = -C_{2}H_{2}. - -If we have applied the principles of Newton correctly, there should not -be any other hydrocarbons containing two atoms of carbon in the molecule. -All these combinations have long been known, and in each of them we can -not only produce those substitutions of which an example has been given -in the case of methane, but also all the phases of other substitutions, -as we shall find from a few more instances, by the aid of which I trust -that I shall be able to show the great complexity of those derivatives -which, on the principle of substitution, can be obtained from each -hydrocarbon. Let us content ourselves with the case of ethane, -CH_{3}CH_{3}, and the substitution of the hydrogen by hydroxyl. The -following are the possible changes:-- - -1. CH_{3}CH_{2}(OH): this is nothing more than spirit of wine, or ethyl -alcohol, C_{2}H_{5}(OH) or C_{2}H_{6}O. - -2. CH_{2}(OH)CH_{2}(OH): this is the glycol of Würtz, which has shed so -much light on the history of alcohol. Its isomeride may be -CH_{3}CH(OH)_{2}, but as we have seen in the case of CH(OH)_{2}, it -decomposes, giving off water, and forming aldehyde, CH_{3}CHO, a -substance capable of yielding alcohol by uniting with hydrogen, and of -yielding acetic acid by uniting with oxygen. - -If glycol, CH_{2}(OH)CH_{2}(OH), loses its water, it may be seen at once -that it will not now yield aldehyde, CH_{3}CHO, but its isomeride, -CH_{2}CH_{2}/O, the oxide of ethylene. I have here indicated in a special -manner the oxygen which has taken the place of two atoms of the hydrogen -of ethane taken from different atoms of the carbon. - -3. CH_{3}C(OH)_{3} decomposed as CH(OH)_{3}, forming water and acetic -acid, CH_{3}CO(OH). It is evident that this acid is nothing else than -formic acid, CHO(OH), with its hydrogen replaced by methyl. Without -examining further the vast number of possible derivatives, I will direct -your attention to the circumstance that in dissolving acetic acid in -water we obtain the maximum contraction and the greatest viscosity when -to the molecule CH_{3}CO(OH) is added a molecule of water, which is the -proportion which would form the hydrate CH_{3}C(OH)_{3}. It is probable -that the doubling of the molecule of acetic acid at temperatures -approaching its boiling-point has some connection with this power of -uniting with one molecule of water. - -4. CH_{2}(OH)C(OH)_{3} is evidently an alcoholic acid, and indeed this -compound, after losing water, answers to glycolic acid, CH_{2}(OH)CO(OH). -Without investigating all the possible isomerides, we will note only that -the hydrate CH(OH)_{2}CH(OH)_{2} has the same composition as -CH_{2}(OH)C(OH)_{3}, and although corresponding to glycol, and being a -symmetrical substance, it becomes, on parting with its water, the -aldehyde of oxalic acid, or the glyoxal of Debus, CHOCHO. - -5. CH(OH)_{2}C(OH_{3}), from the tendency of all the preceding, -corresponds with glyoxylic acid, an aldehyde acid, CHOCO(OH), because the -group CO(OH), or carboxyl, enters into the compositions of organic acids, -and the group CHO defines the aldehyde function. - -6. C(OH)_{3}C(OH)_{3} through the loss of 2H_{2}O yields the bibasic -oxalic acid CO(OH)CO(OH), which generally crystallises with 2H_{2}O, -following thus the normal type of hydration characteristic of ethane.[2] - - [2] One more isomeride, CH_{2}CH(OH), is possible--that is, secondary - vinyl alcohol, which is related to ethylene, CH_{2}CH_{2}, but - derived by the principle of substitution from CH_{4}. Other - isomerides, of the composition C_{2}H_{4}O, such, for example, as - CCH_{3}(OH), are impossible, because it would correspond with the - hydrocarbon CHCH_{3} = C_{2}H_{4}, which is isomeric with ethylene, - and it cannot be derived from methane. If such an isomeride existed - it would be derived from CH_{2}, but such products are, up to the - present, unknown. In such cases the insufficiency of the points of - departure of the statical structural teaching is shown. It first - admits constant atomicity and then rejects it, the facts serving to - establish either one or the other view; and therefore it seems to - me that we must come to the conclusion that the structural method - of reasoning, having done a service to science, has outlived the - age, and must be regenerated, as in their time was the teaching of - the electro-chemists, the radicalists, and the adherents of the - doctrine of types. As we cannot now lean on the views above stated, - it is time to abandon the structural theory. They will all be - united in chemical mechanics, and the principle of substitution - must be looked on only as a preparation for the coming epoch in - chemistry, where such cases as the isomerism of fumaric and maleic - acids, when explained dynamically, as proposed by Le Bel and Van't - Hoff, may yield points of departure. - -Thus, by applying the principle of substitution, we can, in the simplest -manner, derive not only every kind of hydrocarbon compound, such as the -alcohols, the aldehyde-alcohols, aldehydes, alcohol-acids, and the acids, -but also combinations analogous to hydrated crystals which usually are -disregarded. - -But even those unsaturated substances, of which ethylene, CH_{2}CH_{2}, -and acetylene, CHCH, are types, may be evolved with equal simplicity. -With respect to the phenomena of isomerism, there are many possibilities -among the hydrocarbon compounds containing two atoms of carbon, and -without going into details it will be sufficient to indicate that the -following formulæ, though not identical, will be isomeric substantially -among themselves:--CH_{3}CHX_{2} and CH_{2}XCH_{2}X, although both -contain C_{2}H_{4}X_{2}; or CH_{2}CX_{2} and CHXCHX, although both -contain C_{2}H_{2}X_{2}, if by X we indicate chlorine or generally an -element capable of replacing one atom of hydrogen, or capable of uniting -with it. To isomerism of this kind belongs the case of aldehyde and the -oxide of ethylene, to which we have already referred, because both have -the composition C_{2}H_{4}O. - -What I have said appears to me sufficient to show that the principle of -substitution adequately explains the composition, the isomerism, and all -the diversity of combination of the hydrocarbons, and I shall limit the -further development of these views to preparing a complete list of every -possible hydrocarbon compound containing three atoms of carbon in the -molecule. There are eight in all, of which only five are known at -present.[3] - - [3] Conceding variable atomicity, the structuralists must expect an - incomparably larger number of isomerides, and they cannot now - decline to acknowledge the change of atomicity, were it only for - the examples HgCl and HgCl_{2}, CO and CO_{2}, PCl_{3} and PCl_{5}. - -Among those possible for C_{3}H_{6} there should be two isomerides, -propylene and trimethylene, and they are both already known. For -C_{3}H_{4} there should be three isomerides: allylene and allene are -known, but the third has not yet been discovered; and for C_{3}H_{2} -there should be two isomerides, though neither of them is known as yet. -Their composition and structure are easily deduced from ethane, ethylene, -and acetylene, by methylation, by methylenation, by acetylenation and by -carbonation. - -1. C_{3}H_{8} = CH_{3}CH_{2}CH_{3} out of CH_{3}CH_{3} by methylation. -This hydrocarbon is named propane. - -2. C_{3}H_{6} = CH_{3}CHCH_{2} out of CH_{3}CH_{3} by methylenation. This -substance is propylene. - -3. C_{3}H_{6} = CH_{2}CH_{2}CH_{2} out of CH_{3}CH_{3} by methylenation. -This substance is trimethylene. - -4. C_{3}H_{4} = CH_{3}CCH out of CH_{3}CH_{3} by acetylenation or from -CHCH by methylation. This hydrocarbon is named allylene. - -5. C_{3}H_{4} = CHCH/CH_{2} out of CH_{3}CH_{3} by acetylenation, or from -CH_{2}CH_{2} by methylenation, because CH_{2}CH/CH = CHCH/CH_{2}. This -body is as yet unknown. - -6. C_{3}H_{4} = CH_{2}CCH_{2} out of CH_{2}CH_{2} by methylenation. This -hydrocarbon is named allene, or iso-allylene. - -7. C_{3}H_{2} = CHCH/C out of CH_{3}CH_{3} by symmetrical carbonation, or -out of CH_{2}CH_{2} by acetylenation. This compound is unknown. - -8. C_{3}H_{2} = CC/CH_{2} out of CH_{3}CH_{3} by carbonation, or out of -CHCH by methylenation. This compound is unknown. - -If we bear in mind that for each hydrocarbon serving as a type in the -above tables there are a number of corresponding derivatives, and that -every compound obtained may, by further methylation, methylenation, -acetylenation, and carbonation, produce new hydrocarbons, and these may -be followed by a numerous suite of derivatives and an immense number of -isomeric substances, it is possible to understand the limitless number of -carbon compounds, although they all have the one substance, methane, for -their origin. The number of substances is so enormous that it is no -longer a question of enlarging the possibilities of discovery, but rather -of finding some means of testing them analogous to the well-known two -which for a long time have served as gauges for all carbon compounds. - -I refer to the law of even numbers and to that of limits, the first -enunciated by Gerhardt some forty years ago, with respect to -hydrocarbons, namely, that their molecules always contain an even number -of atoms of hydrogen. But by the method which I have used of deriving all -the hydrocarbons from methane, CH_{4}, this law may be deduced as a -direct consequence of the principle of substitutions. Accordingly, in -methylation, CH_{3} takes the place of H, and therefore CH_{2} is added. -In methylenation the number of atoms of hydrogen remains unchanged, and -at each acetylenation it is reduced by two, and in carbonation by four, -atoms--that is to say, an even number of atoms of hydrogen is always -added or removed. And because the fundamental hydrocarbon, methane, -CH_{4}, contains an even number of atoms of hydrogen, all its derivative -hydrocarbons will also contain even numbers of hydrogen, and this -constitutes the law of even numbers. - -The principle of substitutions explains with equal simplicity the -conception of the limiting compositions of hydrocarbons C_{_n_}H_{2_n_ + -2}, which I derived, in 1861,[4] in an empirical manner from accumulated -materials available at that time, and on the basis of the limits to -combinations worked out by Dr. Frankland for other elements. - - [4] 'Essai d'une théorie sur les limites des combinaisons organiques,' - par D. Mendeléeff, 2/11 août 1861, _Bulletin de l'Académie i. d. - Sc. de St. Pétersbourg_, t. v - -Of all the various substitutions the highest proportion of hydrogen is -yielded by methylation, because in that operation alone does the quantity -of hydrogen increase; hence, taking methane as a point of departure, if -we imagine methylation effected (_n_ - 1) times we obtain hydrocarbon -compounds containing the highest quantities of hydrogen. It is evident -that they will contain CH_{4} + (_n_ - 1)CH_{2}, or C_{_n_}H_{2_n_} + -{2}, because methylation leads to the addition of CH_{2} to the compound. - -It will thus be seen that by the principle of substitution--that is to -say, by the third law of Newton--we are able to deduce, in the simplest -manner, not only the individual composition, the isomerism, and relations -of substances, but also the general laws which govern their most complex -combinations without having recourse either to statical constructions, to -the definition of atomicities, to the exclusion of free affinities, or to -the recognition of those single, double or treble bonds which are so -indispensable to structuralists in the explanation of the composition and -construction of hydrocarbon compounds. And yet, by the application of the -dynamical principles of Newton, we can attain to that chief and -fundamental object, the comprehension of isomerism in hydrocarbon -compounds, and the forecasting of the existence of combinations as yet -unknown, by which the edifice raised by structural teaching is -strengthened and supported. Besides--and I count this for a circumstance -of special importance--the process which I advocate will make no -difference in those special cases which have been already so well worked -out, such as, for example, the isomerism of the hydrocarbons and -alcohols, even to the extent of not interfering with the nomenclature -which has been adopted, and the structural system will retain all the -glory of having worked up, in a thoroughly scientific manner, the store -of information which Gerhardt had accumulated about the middle of the -fifties, and the still higher glory of establishing the rational -synthesis of organic substances. Nothing will be lost to the structural -doctrine except its statical origin; and as soon as it will embrace the -dynamic principles of Newton, and suffer itself to be guided by them, I -believe that we shall attain for chemistry that unity of principle which -is now wanting. Many an adept will be attracted to that brilliant and -fascinating enterprise, the penetration into the unseen world of the -kinetic relations of atoms, to the study of which the last twenty-five -years have contributed so much labour and such high inventive faculties. - -D'Alembert found in mechanics that if inertia be taken to represent -force, dynamic equations may be applied to statical questions, which are -thereby rendered more simple and more easily understood. - -The structural doctrine in chemistry has unconsciously followed the same -course, and therefore its terms are easily adopted; they may retain their -present forms provided that a truly dynamical--that is to say, -Newtonian--meaning be ascribed to them. - -Before finishing my task and demonstrating the possibility of adapting -structural doctrines to the dynamics of Newton, I consider it -indispensable to touch on one question which naturally arises, and which -I have heard discussed more than once. If bromine, the atom of which is -eighty times heavier than that of hydrogen, takes the place of hydrogen, -it would seem that the whole system of dynamic equilibrium must be -destroyed. - -Without entering into the minute analysis of this question, I think it -will be sufficient to examine it by the light of two well-known -phenomena, one of which will be found in the department of chemistry and -the other in that of celestial mechanics, and both will serve to -demonstrate the existence of that unity in the plan of creation which is -a consequence of the Newtonian doctrines. Experiments demonstrate that -when a heavy element is substituted for a light one in a chemical -compound--for example, for magnesium, in the oxide of that metal, an atom -of mercury, which is 8-1/3 times heavier--the chief chemical -characteristics or properties are generally, though not always, -preserved. - -The substitution of silver for hydrogen, than which it is 108 times -heavier, does not affect all the properties of the substance, though it -does some. Therefore chemical substitutions of this kind--the -substitution of light for heavy atoms--need not necessarily entail -changes in the original equilibrium; and this point is still further -elucidated by the consideration that the periodic law indicates the -degree of influence of an increment of weight in the atom as affecting -the possible equilibria, and also what degree of increase in the weight -of the atoms reproduces some, though not all, of the properties of the -substance. - -This tendency to repetition--these periods--may be likened to those -annual or diurnal periods with which we are so familiar on the earth. -Days and years follow each other, but, as they do so, many things change; -and in like manner chemical evolutions, changes in the masses of the -elements, permit of much remaining undisturbed, though many properties -undergo alteration. The system is maintained according to the laws of -conservation in nature, but the motions are altered in consequence of the -change of parts. - -Next, let us take an astronomical case--such, for example, as the earth -and the moon--and let us imagine that the mass of the latter is -constantly increasing. The question is, what will then occur? The path of -the moon in space is a wave-line similar to that which geometricians have -named epicycloidal, or the locus of a point in a circle rolling round -another circle. But in consequence of the influence of the moon it is -evident that the path of the earth itself cannot be a geometric ellipse, -even supposing the sun to be immovably fixed; it must be an epicycloidal -curve, though not very far removed from the true ellipse--that is to say, -it will be impressed with but faint undulations. It is only the common -centre of gravity of the earth and the moon which describes a true -ellipse round the sun. If the moon were to increase, the relative -undulations of the earth's path would increase in amplitude, those of the -moon would also change, and when the mass of the moon had increased to an -equality with that of the earth, the path would consist of epicycloidal -curves crossing each other, and having opposite phases. But a similar -relation exists between the sun and the earth, because the former is also -moving in space. We may apply these views to the world of atoms, and -suppose that in their movements, when heavy ones take the place of those -that are lighter, similar changes take place, provided that the system or -the molecule is preserved throughout the change. - -It seems probable that in the heavenly systems, during incalculable -astronomical periods, changes have taken place and are still going on -similar to those which pass rapidly before our eyes during the chemical -reaction of molecules, and the progress of molecular mechanics may--we -hope will--in course of time permit us to explain those changes in the -stellar world which have more than once been noticed by astronomers, and -which are now so carefully studied. A coming Newton will discover the -laws of these changes. Those laws, when applied to chemistry, may exhibit -peculiarities, but these will certainly be mere variations on the grand -harmonious theme which reigns in nature. The discovery of the laws which -produce this harmony in chemical evolution will only be possible, it -seems to me, under the banner of Newtonian dynamics, which has so long -waved over the domains of mechanics, astronomy, and physics. In calling -chemists to take their stand under its peaceful and catholic shadow I -imagine that I am aiding in establishing that scientific union which the -managers of the Royal Institution wish to effect, who have shown their -desire to do so by the flattering invitation which has given me--a -Russian--the opportunity of laying before the countrymen of Newton an -attempt to apply to chemistry one of his immortal principles. - - - - - APPENDIX II - - THE PERIODIC LAW OF THE CHEMICAL ELEMENTS - - BY PROFESSOR MENDELÉEFF - - FARADAY LECTURE DELIVERED BEFORE THE FELLOWS OF THE - CHEMICAL SOCIETY IN THE THEATRE OF THE ROYAL INSTITUTION, - ON TUESDAY, JUNE 4, 1889 - - -The high honour bestowed by the Chemical Society in inviting me to pay a -tribute to the world-famed name of Faraday by delivering this lecture has -induced me to take for its subject the Periodic Law of the Elements--this -being a generalisation in chemistry which has of late attracted much -attention. - -While science is pursuing a steady onward movement, it is convenient -from time to time to cast a glance back on the route already traversed, -and especially to consider the new conceptions which aim at discovering -the general meaning of the stock of facts accumulated from day to day in -our laboratories. Owing to the possession of laboratories, modern science -now bears a new character, quite unknown, not only to antiquity, but even -to the preceding century. Bacon's and Descartes' idea of submitting the -mechanism of science simultaneously to experiment and reasoning has been -fully realised in the case of chemistry, it having become not only -possible but always customary to experiment. Under the all-penetrating -control of experiment, a new theory, even if crude, is quickly -strengthened, provided it be founded on a sufficient basis; the -asperities are removed, it is amended by degrees, and soon loses the -phantom light of a shadowy form or of one founded on mere prejudice; it -is able to lead to logical conclusions, and to submit to experimental -proof. Willingly or not, in science we all must submit not to what seems -to us attractive from one point of view or from another, but to what -represents an agreement between theory and experiment; in other words, to -demonstrated generalisation and to the approved experiment. Is it long -since many refused to accept the generalisations involved in the law of -Avogadro and Ampère, so widely extended by Gerhardt? We still may hear -the voices of its opponents; they enjoy perfect freedom, but vainly will -their voices rise so long as they do not use the language of demonstrated -facts The striking observations with the spectroscope which have -permitted us to analyse the chemical constitution of distant worlds, -seemed, at first, applicable to the task of determining the nature of the -atoms themselves; but the working out of the idea in the laboratory soon -demonstrated that the characters of spectra are determined, not directly -by the atoms, but by the molecules into which the atoms are packed; and -so it became evident that more verified facts must be collected before it -will be possible to formulate new generalisations capable of taking their -place beside those ordinary ones based upon the conception of simple -substances and atoms. But as the shade of the leaves and roots of living -plants, together with the relics of a decayed vegetation, favour the -growth of the seedling and serve to promote its luxurious development, in -like manner sound generalisations--together with the relics of those -which have proved to be untenable--promote scientific productivity, and -ensure the luxurious growth of science under the influence of rays -emanating from the centres of scientific energy. Such centres are -scientific associations and societies. Before one of the oldest and most -powerful of these I am about to take the liberty of passing in review the -twenty years' life of a generalisation which is known under the name of -the Periodic Law. It was in March 1869 that I ventured to lay before the -then youthful Russian Chemical Society the ideas upon the same subject -which I had expressed in my just written 'Principles of Chemistry.' - -Without entering into details, I will give the conclusions I then arrived -at in the very words I used:-- - -'1. The elements, if arranged according to their atomic weights, exhibit -an evident _periodicity_ of properties. - -'2. Elements which are similar as regards their chemical properties have -atomic weights which are either of nearly the same value (_e.g._ -platinum, iridium, osmium) or which increase regularly (_e.g._ potassium, -rubidium, cæsium). - -'3. The arrangement of the elements, or of groups of elements, in the -order of their atomic weights, corresponds to their so-called _valencies_ -as well as, to some extent, to their distinctive chemical properties--as -is apparent, among other series, in that of lithium, beryllium, barium, -carbon, nitrogen, oxygen, and iron. - -'4. The elements which are the most widely diffused have _small_ atomic -weights. - -'5. The _magnitude_ of the atomic weight determines the character of the -element, just as the magnitude of the molecule determines the character -of a compound. - -'6. We must expect the discovery of many yet _unknown_ elements--for -example, elements analogous to aluminium and silicon, whose atomic weight -would be between 65 and 75. - -'7. The atomic weight of an element may sometimes be amended by a -knowledge of those of the contiguous elements. Thus, the atomic weight of -tellurium must lie between 123 and 126, and cannot be 128. - -'8. Certain characteristic properties of the elements can be foretold -from their atomic weights. - -'The aim of this communication will be fully attained if I succeed in -drawing the attention of investigators to those relations which exist -between the atomic weights of dissimilar elements, which, so far as I -know, have hitherto been almost completely neglected. I believe that the -solution of some of the most important problems of our science lies in -researches of this kind.' - -To-day, twenty years after the above conclusions were formulated, they -may still be considered as expressing the essence of the now well-known -periodic law. - -Reverting to the epoch terminating with the sixties, it is proper to -indicate three series of data without the knowledge of which the periodic -law could not have been discovered, and which rendered its appearance -natural and intelligible. - -In the first place, it was at that time that the numerical value of -atomic weights became definitely known. Ten years earlier such knowledge -did not exist, as may be gathered from the fact that in 1860 chemists -from all parts of the world met at Karlsruhe in order to come to some -agreement, if not with respect to views relating to atoms, at any rate as -regards their definite representation. Many of those present probably -remember how vain were the hopes of coming to an understanding, and how -much ground was gained at that Congress by the followers of the unitary -theory so brilliantly represented by Cannizzaro. I vividly remember the -impression produced by his speeches, which admitted of no compromise, and -seemed to advocate truth itself, based on the conceptions of Avogadro, -Gerhardt, and Regnault, which at that time were far from being generally -recognised. And though no understanding could be arrived at, yet the -objects of the meeting were attained, for the ideas of Cannizzaro proved, -after a few years, to be the only ones which could stand criticism, and -which represented an atom as--'the smallest portion of an element which -enters into a molecule of its compound.' Only such real atomic -weights--not conventional ones--could afford a basis for generalisation. -It is sufficient, by way of example, to indicate the following cases in -which the relation is seen at once and is perfectly clear:-- - - K = 39 Rb = 85 Cs = 133 - Ca = 40 Sr = 87 Ba = 137 - -whereas with the equivalents then in use-- - - K = 39 Rb = 85 Cs = 133 - Ca = 20 Sr = 43·5 Ba = 68·5 - -the consecutiveness of change in atomic weight, which with the true -values is so evident, completely disappears. - -Secondly, it had become evident during the period 1860-70, and even -during the preceding decade, that the relations between the atomic -weights of analogous elements were governed by some general and simple -laws. Cooke, Cremers, Gladstone, Gmelin, Lenssen, Pettenkofer, and -especially Dumas, had already established many facts bearing on that -view. Thus Dumas compared the following groups of analogous elements with -organic radicles:-- - - Diff. Diff. Diff. Diff. - Mg = 12} P = 31} O = 8} - }8 }44 }8 - Li = 7 } Ca = 20} As= 75} S = 16} - }16 }3 × 8 }44 }3 × 8 - Na = 23} Sr = 44} Sb = 119} Se = 40} - }16 }3 × 8 }2 × 44 }3 × 8 - K = 39 } Ba = 68} Bi = 207} Te = 64} - -and pointed out some really striking relationships, such as the -following:-- - - F = 19. - Cl = 35·5 = 19 + 16·5. - Br = 80 = 19 + 2 × 16·5 + 28. - I = 127 = 2 x 19 + 2 × 16·5 + 2 × 28. - -A. Strecker, in his work 'Theorien und Experimente zur Bestimmung der -Atomgewichte der Elemente' (Braunschweig, 1859), after summarising the -data relating to the subject, and pointing out the remarkable series of -equivalents-- - - Cr = 26·2 Mn = 27·6 Fe = 28 Ni = 29 Co = 30 Cu = 31·7 Zn = 32·5 - -remarks that: 'It is hardly probable that all the above-mentioned -relations between the atomic weights (or equivalents) of chemically -analogous elements are merely accidental. We must, however, leave to the -future the discovery of the _law_ of the relations which appears in these -figures.'[1] - - [1] 'Es ist wohl kaum anzunehmen, dass alle im Vorhergehenden - hervorgehobenen Beziehungen zwischen den Atomgewichten (oder - Aequivalenten) in chemischen Verhältnissen einander ähnliche - Elemente bloss zufällig sind. Die Auffindung der in diesen Zahlen - _gesetzlichen_ Beziehungen müssen wir jedoch der Zukunft - überlassen.' - -In such attempts at arrangement and in such views are to be recognised -the real forerunners of the periodic law; the ground was prepared for it -between 1860 and 1870, and that it was not expressed in a determinate -form before the end of the decade may, I suppose, be ascribed to the fact -that only analogous elements had been compared. The idea of seeking for a -relation between the atomic weights of all the elements was foreign to -the ideas then current, so that neither the _vis tellurique_ of De -Chancourtois, nor the _law of octaves_ of Newlands, could secure -anybody's attention. And yet both De Chancourtois and Newlands like Dumas -and Strecker, more than Lenssen and Pettenkofer, had made an approach to -the periodic law and had discovered its germs. The solution of the -problem advanced but slowly, because the facts, but not the law, stood -foremost in all attempts; and the law could not awaken a general interest -so long as elements, having no apparent connection with each other, were -included in the same octave, as for example:-- - - 1st octave of | | | | | | | | - Newlands | H | F | Cl | Co & Ni | Br | Pd | I | Pt & Ir - 7th Ditto | O | S | Fe | Se | Rh & Ru | Te | Au | Os or Th - -Analogies of the above order seemed quite accidental, and the more so as -the octave contained occasionally ten elements instead of eight, and when -two such elements as Ba and V, Co and Ni, or Rh and Ru, occupied one -place in the octave.[2] Nevertheless, the fruit was ripening, and I now -see clearly that Strecker, De Chancourtois, and Newlands stood foremost -in the way towards the discovery of the periodic law, and that they -merely wanted the boldness necessary to place the whole question at such -a height that its reflection on the facts could be clearly seen. - - [2] To judge from J. A. R. Newlands's work, _On the Discovery of the - Periodic Law_, London, 1884, p. 149; 'On the Law of Octaves' (from - the _Chemical News_, 12, 83, August 18, 1865). - -A third circumstance which revealed the periodicity of chemical elements -was the accumulation, by the end of the sixties, of new information -respecting the rare elements, disclosing their many-sided relations to -the other elements and to each other. The researches of Marignac on -niobium, and those of Roscoe on vanadium, were of special moment. The -striking analogies between vanadium and phosphorus on the one hand, and -between vanadium and chromium on the other, which became so apparent in -the investigations connected with that element, naturally induced the -comparison of V = 51 with Cr = 52, Nb = 94 with Mo = 96, and Ta = 192 -with W = 194; while, on the other hand, P = 31 could be compared with S = -32, As = 75 with Se = 79, and Sb = 120 with Te = 125. From such -approximations there remained but one step to the discovery of the law of -periodicity. - -The law of periodicity was thus a direct outcome of the stock of -generalisations and established facts which had accumulated by the end of -the decade 1860-1870; it is an embodiment of those data in a more or less -systematic expression. Where, then, lies the secret of the special -importance which has since been attached to the periodic law, and has -raised it to the position of a generalisation which has already given to -chemistry unexpected aid, and which promises to be far more fruitful in -the future and to impress upon several branches of chemical research a -peculiar and original stamp? The remaining part of my communication will -be an attempt to answer this question. - -In the first place we have the circumstance that, as soon as the law made -its appearance, it demanded a revision of many facts which were -considered by chemists as fully established by existing experience. I -shall return, later on, briefly to this subject, but I wish now to remind -you that the periodic law, by insisting on the necessity for a revision -of supposed facts, exposed itself at once to destruction in its very -origin. Its first requirements, however, have been almost entirely -satisfied during the last 20 years; the supposed facts have yielded to -the law, thus proving that the law itself was a legitimate induction from -the verified facts. But our inductions from data have often to do with -such details of a science so rich in facts, that only generalisations -which cover a wide range of important phenomena can attract general -attention. What were the regions touched on by the periodic law? This is -what we shall now consider. - -The most important point to notice is, that periodic functions, used for -the purpose of expressing changes which are dependent on variations of -time and space, have been long known. They are familiar to the mind when -we have to deal with motion in closed cycles, or with any kind of -deviation from a stable position, such as occurs in -pendulum-oscillations. A like periodic function became evident in the -case of the elements, depending on the mass of the atom. The primary -conception of the masses of bodies, or of the masses of atoms, belongs to -a category which the present state of science forbids us to discuss, -because as yet we have no means of dissecting or analysing the -conception. All that was known of functions dependent on masses derived -its origin from Galileo and Newton, and indicated that such functions -either decrease or increase with the increase of mass, like the -attraction of celestial bodies. The numerical expression of the phenomena -was always found to be proportional to the mass, and in no case was an -increase of mass followed by a recurrence of properties such as is -disclosed by the periodic law of the elements. This constituted such a -novelty in the study of the phenomena of nature that, although it did not -lift the veil which conceals the true conception of mass, it nevertheless -indicated that the explanation of that conception must be searched for in -the masses of the atoms; the more so, as all masses are nothing but -aggregations, or additions, of chemical atoms which would be best -described as chemical individuals. Let me remark, by the way, that though -the Latin word 'individual' is merely a translation of the Greek word -'atom,' nevertheless history and custom have drawn a sharp distinction -between the two words, and the present chemical conception of atoms is -nearer to that defined by the Latin word than by the Greek, although this -latter also has acquired a special meaning which was unknown to the -classics. The periodic law has shown that our chemical individuals -display a harmonic periodicity of properties dependent on their masses. -Now natural science has long been accustomed to deal with periodicities -observed in nature, to seize them with the vice of mathematical analysis, -to submit them to the rasp of experiment. And these instruments of -scientific thought would surely, long since, have mastered the problem -connected with the chemical elements, were it not for a new feature which -was brought to light by the periodic law, and which gave a peculiar and -original character to the periodic function. - -If we mark on an axis of abscissæ a series of lengths proportional to -angles, and trace ordinates which are proportional to sines or other -trigonometrical functions, we get periodic curves of a harmonic -character. So it might seem, at first sight, that with the increase of -atomic weights the function of the properties of the elements should also -vary in the same harmonious way. But in this case there is no such -continuous change as in the curves just referred to, because the periods -do not contain the infinite number of points constituting a curve, but a -_finite_ number only of such points. An example will better illustrate -this view. The atomic weights-- - - Ag = 108 Cd = 112 In = 113 Sn = 118 Sb = 120 - Te = 125 I = 127 - -steadily increase, and their increase is accompanied by a modification of -many properties which constitutes the essence of the periodic law. Thus, -for example, the densities of the above elements decrease steadily, being -respectively-- - - 10·5 8·6 7·4 7·2 6·7 6·4 4·9 - -while their oxides contain an increasing quantity of oxygen-- - - Ag_{2}O Cd_{2}O_{2} In_{2}O_{3} Sn_{2}O_{4} Sb_{2}O_{5} - Te_{2}O_{6} I_{2}O_{7} - -But to connect by a curve the summits of the ordinates expressing any of -these properties would involve the rejection of Dalton's law of multiple -proportions. Not only are there no intermediate elements between silver, -which gives AgCl, and cadmium, which gives CdCl_{2}, but, according to -the very essence of the periodic law, there can be none; in fact a -uniform curve would be inapplicable in such a case, as it would lead us -to expect elements possessed of special properties at any point of the -curve. The periods of the elements have thus a character very different -from those which are so simply represented by geometers. They correspond -to points, to numbers, to sudden changes of the masses, and not to a -continuous evolution. In these sudden changes destitute of intermediate -steps or positions, in the absence of elements intermediate between, say, -silver and cadmium, or aluminium and silicon, we must recognise a problem -to which no direct application of the analysis of the infinitely small -can be made. Therefore, neither the trigonometrical functions proposed by -Ridberg and Flavitzky, nor the pendulum-oscillations suggested by -Crookes, nor the cubical curves of the Rev. Mr. Haughton, which have been -proposed for expressing the periodic law, from the nature of the case, -can represent the periods of the chemical elements. If geometrical -analysis is to be applied to this subject, it will require to be modified -in a special manner. It must find the means of representing in a special -way, not only such long periods as that comprising - - K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br, - -but short periods like the following:-- - - Na Mg Al Si P S Cl. - -In the theory of numbers only do we find problems analogous to ours, and -two attempts at expressing the atomic weights of the elements by -algebraic formulæ seem to be deserving of attention, although neither of -them can be considered as a complete theory, nor as promising finally to -solve the problem of the periodic law. The attempt of E. J. Mills (1886) -does not even aspire to attain this end. He considers that all atomic -weights can be expressed by a logarithmic function, - - 15(_n_ - 0·9375^_t_), - -in which the variables _n_ and _t_ are _whole numbers_. Thus, for oxygen, -_n_ = 2, and _t_ = 1, whence its atomic weight is = 15·94; in the case of -chlorine, bromine, and iodine, _n_ has respective values of 3, 6, and 9, -whilst _t_ = 7, 6, and 9; in the case of potassium, rubidium, and cæsium, -_n_ = 4, 6, and 9, and _t_ = 14, 18, and 20. - -Another attempt was made in 1888 by B. N. Tchitchérin. Its author places -the problem of the periodic law in the first rank, but as yet he has -investigated the alkali metals only. Tchitchérin first noticed the simple -relations existing between the atomic volumes of all alkali metals; they -can be expressed, according to his views, by the formula - - A(2 - 0·00535A_n_), - -where A is the atomic weight, and _n_ is equal to 8 for lithium and -sodium, to 4 for potassium, to 3 for rubidium, and to 2 for cæsium. If -_n_ remained equal to 8 during the increase of A, the volume would become -zero at A = 46-2/3, and it would reach its maximum at A = 23-1/3. The -close approximation of the number 46-2/3 to the differences between the -atomic weights of analogous elements (such as Cs - Rb, I - Br, and so -on); the close correspondence of the number 23-1/3 to the atomic weight -of sodium; the fact of _n_ being necessarily a whole number, and several -other aspects of the question, induce Tchitchérin to believe that they -afford a clue to the understanding of the nature of the elements; we -must, however, await the full development of his theory before -pronouncing judgment on it. What we can at present only be certain of is -this: that attempts like the two above named must be repeated and -multiplied, because the periodic law has clearly shown that the masses of -the atoms increase abruptly, by steps, which are clearly connected in -some way with Dalton's law of multiple proportions; and because the -periodicity of the elements finds expression in the transition from RX to -RX_{2}, RX_{3}, RX_{4}, and so on till RX_{8}, at which point, the energy -of the combining forces being exhausted, the series begins anew from RX -to RX_{2}, and so on. - -While connecting by new bonds the theory of the chemical elements with -Dalton's theory of multiple proportions, or atomic structure of bodies, -the periodic law opened for natural philosophy a new and wide field for -speculation. Kant said that there are in the world 'two things which -never cease to call for the admiration and reverence of man: the moral -law within ourselves, and the stellar sky above us.' But when we turn our -thoughts towards the nature of the elements and the periodic law, we must -add a third subject, namely, 'the nature of the elementary individuals -which we discover everywhere around us.' Without them the stellar sky -itself is inconceivable; and in the atoms we see at once their peculiar -individualities, the infinite multiplicity of the individuals, and the -submission of their seeming freedom to the general harmony of Nature. - -Having thus indicated a new mystery of Nature, which does not yet yield -to rational conception, the periodic law, together with the revelations -of spectrum analysis, have contributed to again revive an old but -remarkably long-lived hope--that of discovering, if not by experiment, at -least by a mental effort, the _primary matter_--which had its genesis in -the minds of the Grecian philosophers, and has been transmitted, together -with many other ideas of the classic period, to the heirs of their -civilisation. Having grown, during the times of the alchemists up to the -period when experimental proof was required, the idea has rendered good -service; it induced those careful observations and experiments which -later on called into being the works of Scheele, Lavoisier, Priestley, -and Cavendish. It then slumbered awhile, but was soon awakened by the -attempts either to confirm or to refute the ideas of Prout as to the -multiple proportion relationship of the atomic weights of all the -elements. And once again the inductive or experimental method of studying -Nature gained a direct advantage from the old Pythagorean idea: because -atomic weights were determined with an accuracy formerly unknown. But -again the idea could not stand the ordeal of experimental test, yet the -prejudice remains and has not been uprooted, even by Stas; nay, it has -gained a new vigour, for we see that all which is imperfectly worked out, -new and unexplained, from the still scarcely studied rare metals to the -hardly perceptible nebulæ, have been used to justify it. As soon as -spectrum analysis appears as a new and powerful weapon of chemistry, the -idea of a primary matter is immediately attached to it. From all sides we -see attempts to constitute the imaginary substance _helium_[3] the so -much longed for primary matter. No attention is paid to the circumstance -that the helium line is only seen in the spectrum of the solar -protuberances, so that its universality in Nature remains as problematic -as the primary matter itself; nor to the fact that the helium line is -wanting amongst the Fraunhofer lines of the solar spectrum, and thus does -not answer to the brilliant fundamental conception which gives its real -force to spectrum analysis. - - [3] That is, a substance having a wave-length equal to 0·0005875 - millimetre. - -And finally, no notice is even taken of the indubitable fact that the -brilliancies of the spectral lines of the simple substances vary under -different temperatures and pressures; so that all probabilities are in -favour of the helium line simply belonging to some long since known -element placed under such conditions of temperature, pressure, and -gravity as have not yet been realised in our experiments. Again, the idea -that the excellent investigations of Lockyer of the spectrum of iron can -be interpreted in favour of the compound nature of that element, -evidently must have arisen from some misunderstanding. The spectrum of a -compound certainly does not appear as a sum of the spectra of its -components; and therefore the observations of Lockyer can be considered -precisely as a proof that iron undergoes no other changes at the -temperature of the sun than those which it experiences in the voltaic -arc--provided the spectrum of iron is preserved. As to the shifting of -some of the lines of the spectrum of iron while the other lines maintain -their positions, it can be explained, as shown by M. Kleiber ('Journal of -the Russian Chemical and Physical Society,' 1885, 147), by the relative -motion of the various strata of the sun's atmosphere, and by Zöllner's -laws of the relative brilliancies of different lines of the spectrum. -Moreover, it ought not to be forgotten that if iron were really proved to -consist of two or more unknown elements, we should simply have an -increase in the number of our elements--not a reduction, and still less a -reduction of all of them to one single primary matter. - -Feeling that spectrum analysis will not yield a support to the -Pythagorean conception, its modern promoters are so bent upon its being -confirmed by the periodic law, that the illustrious Berthelot, in his -work 'Les origines de l'Alchimie,' 1885, 313, has simply mixed up the -fundamental idea of the law of periodicity with the ideas of Prout, the -alchemists, and Democritus about primary matter.[4] But the periodic law, -based as it is on the solid and wholesome ground of experimental -research, has been evolved independently of any conception as to the -nature of the elements; it does not in the least originate in the idea of -a unique matter; and it has no historical connection with that relic of -the torments of classical thought, and therefore it affords no more -indication of the unity of matter or of the compound character of our -elements, than the law of Avogadro, or the law of specific heats, or even -the conclusions of spectrum analysis. None of the advocates of a unique -matter have ever tried to explain the law from the standpoint of ideas -taken from a remote antiquity when it was found convenient to admit the -existence of many gods--and of a unique matter. - - [4] He maintains (on p. 309) that the periodic law requires two new - analogous elements, having atomic weights of 48 and 64, occupying - positions between sulphur and selenium, although nothing of the - kind results from any of the different readings of the law. - -When we try to explain the origin of the idea of a unique primary matter, -we easily trace that in the absence of inductions from experiment it -derives its origin from the scientifically philosophical attempt at -discovering some kind of unity in the immense diversity of -individualities which we see around. In classical times such a tendency -could only be satisfied by conceptions about the immaterial world. As to -the material world, our ancestors were compelled to resort to some -hypothesis, and they adopted the idea of unity in the formative material, -because they were not able to evolve the conception of any other possible -unity in order to connect the multifarious relations of matter. -Responding to the same legitimate scientific tendency, natural science -has discovered throughout the universe a unity of plan, a unity of -forces, and a unity of matter, and the convincing conclusions of modern -science compel every one to admit these kinds of unity. But while we -admit unity in many things, we none the less must also explain the -individuality and the apparent diversity which we cannot fail to trace -everywhere. It has been said of old, 'Give us a fulcrum, and it will -become easy to displace the earth.' So also we must say, 'Give us -something that is individualised, and the apparent diversity will be -easily understood.' Otherwise, how could unity result in a multitude? - -After a long and painstaking research, natural science has discovered -the individualities of the chemical elements, and therefore it is now -capable not only of analysing, but also of synthesising; it can -understand and grasp generality and unity, as well as the individualised -and the multifarious. The general and universal, like time and space, -like force and motion, vary uniformly; the uniform admit of -interpolations, revealing every intermediate phase. But the -multitudinous, the individualised--such as ourselves, or the chemical -elements, or the members of a peculiar periodic function of the elements, -or Dalton's multiple proportions--is characterised in another way: we see -in it, side by side with a connecting general principle, leaps, breaks of -continuity, points which escape from the analysis of the infinitely -small--an absence of complete intermediate links. Chemistry has found an -answer to the question as to the causes of multitudes; and while -retaining the conception of many elements, all submitted to the -discipline of a general law, it offers an escape from the Indian -Nirvana--the absorption in the universal, replacing it by the -individualised. However, the place for individuality is so limited by the -all-grasping, all-powerful universal, that it is merely a point of -support for the understanding of multitude in unity. - -Having touched upon the metaphysical bases of the conception of a unique -matter which is supposed to enter into the composition of all bodies. I -think it necessary to dwell upon another theory, akin to the above -conception--the theory of the compound character of the elements now -admitted by some--and especially upon one particular circumstance which, -being related to the periodic law, is considered to be an argument in -favour of that hypothesis. - -Dr. Pelopidas, in 1883, made a communication to the Russian Chemical and -Physical Society on the periodicity of the hydrocarbon radicles, pointing -out the remarkable parallelism which was to be noticed in the change of -properties of hydrocarbon radicles and elements when classed in groups. -Professor Carnelley, in 1886, developed a similar parallelism. The idea -of M. Pelopidas will be easily understood if we consider the series of -hydrocarbon radicles which contain, say, 6 atoms of carbon:-- - - I. II. III. IV. V. - C_{6}H_{13} C_{6}H_{12} C_{6}H_{11} C_{6}H_{10} C_{6}H_{9} - VI. VII. VIII. - C_{6}H_{8} C_{6}H_{7} C_{6}H_{6} - -The first of these radicles, like the elements of the 1st group, combines -with Cl, OH, and so on, and gives the derivatives of hexyl alcohol, -C_{6}H_{13}(OH); but, in proportion as the number of hydrogen atoms -decreases, the capacity of the radicles of combining with, say, the -halogens increases. C_{6}H_{12} already combines with 2 atoms of -chlorine; C_{6}H_{11}, with 3 atoms, and so on. The last members of the -series comprise the radicles of acids: thus C_{6}H_{8}, which belongs to -the 6th group, gives, like sulphur, a bibasic acid, -C_{6}H_{8}O_{2}(OH)_{2}, which is homologous with oxalic acid. The -parallelism can be traced still further, because C_{6}H_{5} appears as a -monovalent radicle of benzene, and with it begins a new series of -aromatic derivatives, so analogous to the derivatives of the aliphatic -series. Let me also mention another example from among those which have -been given by M. Pelopidas. Starting from the alkaline radicle of -monomethylammonium, N(CH_{3})H_{3}, or NCH_{6}, which presents many -analogies with the alkaline metals of the 1st group, he arrives, by -successively diminishing the number of the atoms of hydrogen, at a 7th -group which contains cyanogen, CN, which has long since been compared to -the halogens of the 7th group. - -The most important consequence which, in my opinion, can be drawn from -the above comparison is that the periodic law, so apparent in the -elements, has a wider application than might appear at first sight; it -opens up a new vista of chemical evolutions. But, while admitting the -fullest parallelism between the periodicity of the elements and that of -the compound radicles, we must not forget that in the periods of the -hydrocarbon radicles we have a _decrease_ of mass as we pass from the -representatives of the first group to the next, while in the periods of -the elements the mass _increases_ during the progression. It thus becomes -evident that we cannot speak of an identity of periodicity in both cases, -unless we put aside the ideas of mass and attraction, which are the real -corner-stones of the whole of natural science, and even enter into those -very conceptions of simple substances which came to light a full hundred -years later than the immortal principles of Newton.[5] - - [5] It is noteworthy that the year in which Lavoisier was born - (1743)--the author of the idea of elements and of the - indestructibility of matter--is later by exactly one century than - the year in which the author of the theory of gravitation and mass - was born (1643 N.S.). The affiliation of the ideas of Lavoisier and - those of Newton is beyond doubt. - -From the foregoing, as well as from the failures of so many attempts at -finding in experiment and speculation a proof of the compound character -of the elements and of the existence of primordial matter, it is evident, -in my opinion, that this theory must be classed among mere utopias. But -utopias can only be combated by freedom of opinion, by experiment, and by -new utopias. In the republic of scientific theories freedom of opinions -is guaranteed. It is precisely that freedom which permits me to criticise -openly the widely-diffused idea as to the unity of matter in the -elements. Experiments and attempts at confirming that idea have been so -numerous that it really would be instructive to have them all collected -together, if only to serve as a warning against the repetition of old -failures. And now as to new utopias which may be helpful in the struggle -against the old ones, I do not think it quite useless to mention a -_phantasy_ of one of my students who imagined that the weight of bodies -does not depend upon their mass, but upon the character of the motion of -their atoms. The atoms, according to this new utopian, may all be -homogeneous or heterogeneous, we know not which; we know them in motion -only, and that motion they maintain with the same persistence as the -stellar bodies maintain theirs. The weights of atoms differ only in -consequence of their various modes and quantity of motion; the heaviest -atoms may be much simpler than the lighter ones: thus an atom of mercury -may be simpler than an atom of hydrogen--the manner in which it moves -causes it to be heavier. My interlocutor even suggested that the view -which attributes the greater complexity to the lighter elements finds -confirmation in the fact that the hydrocarbon radicles mentioned by -Pelopidas, while becoming lighter as they lose hydrogen, change their -properties periodically in the same manner as the elements change theirs, -according as the atoms grow heavier. - -The French proverb, _La critique est facile, mais l'art est difficile_, -however, may well be reversed in the case of all such ideal views, as it -is much easier to formulate than to criticise them. Arising from the -virgin soil of newly-established facts, the knowledge relating to the -elements, to their masses, and to the periodic changes of their -properties has given a motive for the formation of utopian hypotheses, -probably because they could not be foreseen by the aid of any of the -various metaphysical systems, and exist, like the idea of gravitation, as -an independent outcome of natural science, requiring the acknowledgment -of general laws, when these have been established with the same degree of -persistency as is indispensable for the acceptance of a thoroughly -established fact. Two centuries have elapsed since the theory of -gravitation was enunciated, and although we do not understand its cause, -we still must regard gravitation as a fundamental conception of natural -philosophy, a conception which has enabled us to perceive much more than -the metaphysicians did or could with their seeming omniscience. A hundred -years later the conception of the elements arose; it made chemistry what -it now is; and yet we have advanced as little in our comprehension of -simple substances since the times of Lavoisier and Dalton as we have in -our understanding of gravitation. The periodic law of the elements is -only twenty years old; it is not surprising, therefore, that, knowing -nothing about the causes of gravitation and mass, or about the nature of -the elements, we do not comprehend the _rationale_ of the periodic law. -It is only by collecting established laws--that is, by working at the -acquirement of truth--that we can hope gradually to lift the veil which -conceals from us the causes of the mysteries of Nature and to discover -their mutual dependency. Like the telescope and the microscope, laws -founded on the basis of experiment are the instruments and means of -enlarging our mental horizon. - -In the remaining part of my communication I shall endeavour to show, and -as briefly as possible, in how far the periodic law contributes to -enlarge our range of vision. Before the promulgation of this law the -chemical elements were mere fragmentary, incidental facts in Nature; -there was no special reason to expect the discovery of new elements, and -the new ones which were discovered from time to time appeared to be -possessed of quite novel properties. The law of periodicity first enabled -us to perceive undiscovered elements at a distance which formerly was -inaccessible to chemical vision; and long ere they were discovered new -elements appeared before our eyes possessed of a number of well-defined -properties. We now know three cases of elements whose existence and -properties were foreseen by the instrumentality of the periodic law. I -need but mention the brilliant discovery of _gallium_, which proved to -correspond to eka-aluminium of the periodic law, by Lecoq de Boisbaudran; -of _scandium_, corresponding to ekaboron, by Nilson; and of _germanium_, -which proved to correspond in all respects to ekasilicon, by Winkler. -When, in 1871, I described to the Russian Chemical Society the -properties, clearly defined by the periodic law, which such elements -ought to possess, I never hoped that I should live to mention their -discovery to the Chemical Society of Great Britain as a confirmation of -the exactitude and the generality of the periodic law. Now that I have -had the happiness of doing so, I unhesitatingly say that, although -greatly enlarging our vision, even now the periodic law needs further -improvements in order that it may become a trustworthy instrument in -further discoveries.[6] - - [6] I foresee some more new elements, but not with the same certitude - as before. I shall give one example, and yet I do not see it quite - distinctly. In the series which contains Hg = 204, Pb = 206, and Bi - = 208, we can imagine the existence (at the place VI-11) of an - element analogous to tellurium, which we can describe as - dvi-tellurium, Dt, having an atomic weight of 212, and the property - of forming the oxide DtO_{3}. If this element really exists, it - ought in the free state to be an easily fusible, crystalline, - non-volatile metal of a grey colour, having a density of about 9·3, - capable of giving a dioxide, DtO_{2}, equally endowed with feeble - acid and basic properties. This dioxide must give on active - oxidation an unstable higher oxide, DtO_{3}, which should resemble - in its properties PbO_{2} and Bi_{2}O_{5}. Dvi-tellurium hydride, - if it be found to exist, will be a less stable compound than even - H_{2}Te. The compounds of dvi-tellurium will be easily reduced, and - it will form characteristic definite alloys with other metals. - -I will venture to allude to some other matters which chemistry has -discerned by means of its new instrument, and which it could not have -made out without a knowledge of the law of periodicity, and I will -confine myself to simple substances and to oxides. - -Before the periodic law was formulated the atomic weights of the elements -were purely empirical numbers, so that the magnitude of the equivalent, -and the atomicity, or the value in substitution possessed by an atom, -could only be tested by critically examining the methods of -determination, but never directly by considering the numerical values -themselves; in short, we were compelled to move in the dark, to submit to -the facts, instead of being masters of them. I need not recount the -methods which permitted the periodic law at last to master the facts -relating to atomic weights, and I would merely call to mind that it -compelled us to modify the valencies of _indium_ and _cerium_, and to -assign to their compounds a different molecular composition. -Determinations of the specific heats of these two metals fully confirmed -the change. The trivalency of _yttrium_, which makes us now represent its -oxide as Y_{2}O_{3} instead of as YO, was also foreseen (in 1870) by the -periodic law, and it has now become so probable that Clève, and all other -subsequent investigators of the rare metals, have not only adopted it, -but have also applied it without any new demonstration to substances so -imperfectly known as those of the cerite and gadolinite group, especially -since Hillebrand determined the specific heats of lanthanum and didymium -and confirmed the expectations suggested by the periodic law. But here, -especially in the case of didymium, we meet with a series of difficulties -long since foreseen through the periodic law, but only now becoming -evident, and chiefly arising from the relative rarity and insufficient -knowledge of the elements which usually accompany didymium. - -Passing to the results obtained in the case of the rare elements -_beryllium_, _scandium_, and _thorium_, it is found that these have many -points of contact with the periodic law. Although Avdéeff long since -proposed the magnesia formula to represent beryllium oxide, yet there was -so much to be said in favour of the alumina formula, on account of the -specific heat of the metals and the isomorphism of the two oxides, that -it became generally adopted and seemed to be well established. The -periodic law, however, as Brauner repeatedly insisted ('Berichte,' 1878, -872; 1881, 53), was against the formula Be_{2}O_{3}; it required the -magnesia formula BeO--that is, an atomic weight of 9--because there was -no place in the system for an element like beryllium having an atomic -weight of 13·5. This divergence of opinion lasted for years, and I often -heard that the question as to the atomic weight of beryllium threatened -to disturb the generality of the periodic law, or, at any rate, to -require some important modifications of it. Many forces were operating in -the controversy regarding beryllium, evidently because a much more -important question was at issue than merely that involved in the -discussion of the atomic weight of a relatively rare element: and during -the controversy the periodic law became better understood, and the mutual -relations of the elements became more apparent than ever before. It is -most remarkable that the victory of the periodic law was won by the -researches of the very observers who previously had discovered a number -of facts in support of the trivalency of beryllium. Applying the higher -law of Avogadro, Nilson and Petterson have finally shown that the density -of the vapour of the beryllium chloride, BeCl_{2}, obliges us to regard -beryllium as bivalent in conformity with the periodic law.[7] I consider -the confirmation of Avdéeff's and Brauner's view as important in the -history of the periodic law as the discovery of scandium, which, in -Nilson's hands, confirmed the existence of ekaboron. - - [7] Let me mention another proof of the bivalency of beryllium which - may have passed unnoticed, as it was only published in the Russian - chemical literature. Having remarked (in 1884) that the density of - such solutions of chlorides of metals, MCl_{_n_}, as contain 200 - mols. of water (or a large and constant amount of water) regularly - increases as the molecular weight of the dissolved salt increases, - I proposed to one of our young chemists, M. Burdakoff, that he - should investigate beryllium chloride. If its molecule be BeCl_{2} - its weight must be = 80; and in such a case it must be heavier than - the molecule of KCl = 74·5, and lighter than that of MgCl_{2}, = - 93. On the contrary, if beryllium chloride is a trichloride, - BeCl_{3} = 120, its molecule must be heavier than that of CaCl_{2} - = 111, and lighter than that of MnCl_{2} = 126. Experiment has - shown the correctness of the former formula, the solution BeCl_{2} - + 200H_{2}O having (at 15°/4°) a density of 1·0138, this being a - higher density than that of the solution KCl + 200H_{2}O (= - 1·0121), and lower than that of MgCl_{2} + 200H_{2}O (= 1·0203). - The bivalency of beryllium was thus confirmed in the case both of - the dissolved and the vaporised chloride. - -The circumstance that _thorium_ proved to be quadrivalent, and Th = 232, -in accordance with the views of Chydenius and the requirements of the -periodic law, passed almost unnoticed, and was accepted without -opposition, and yet both thorium and uranium are of great importance in -the periodic system, as they are its last members, and have the highest -atomic weights of all the elements. - -The alteration of the atomic weight of _uranium_ from U = 120 into U = -240 attracted more attention, the change having been made on account of -the periodic law, and for no other reason. Now that Roscoe, Rammelsberg, -Zimmermann, and several others have admitted the various claims of the -periodic law in the case of uranium, its high atomic weight is received -without objection, and it endows that element with a special interest. - -While thus demonstrating the necessity for modifying the atomic weights -of several insufficiently known elements, the periodic law enabled us -also to detect errors in the determination of the atomic weights of -several elements whose valencies and true position among other elements -were already well known. Three such cases are especially noteworthy: -those of tellurium, titanium and platinum. Berzelius had determined the -atomic weight of _tellurium_ to be 128, while the periodic law claimed -for it an atomic weight below that of iodine, which had been fixed by -Stas at 126·5, and which was certainly not higher than 127. Brauner then -undertook the investigation, and he has shown that the true atomic weight -of tellurium is lower than that of iodine, being near to 125. For -_titanium_ the extensive researches of Thorpe have confirmed the atomic -weight of Ti = 48, indicated by the law, and already foreseen by Rose, -but contradicted by the analyses of Pierre and several other chemists. An -equally brilliant confirmation of the expectations based on the periodic -law has been given in the case of the series osmium, iridium, platinum, -and gold. At the time of the promulgation of the periodic law, the -determinations of Berzelius, Rose, and many others gave the following -figures:-- - - Os = 200; Ir = 197; Pt = 198; Au = 196. - -The expectations of the periodic law[8] have been confirmed, first, by -new determinations of the atomic weight of _platinum_ (by Seubert, -Dittmar, and M'Arthur, which proved to be near to 196 (taking O = 16, as -proposed by Marignac, Brauner, and others); secondly, by Seubert having -proved that the atomic weight of _osmium_ is really lower than that of -platinum, being near to 191; and thirdly, by the investigations of Krüss, -Thorpe and Laurie, proving that the atomic weight of _gold_ exceeds that -of platinum, and approximates to 197. The atomic weights which were thus -found to require correction were precisely those which the periodic law -had indicated as affected with errors; and it has been proved, therefore, -that the periodic law affords a means of testing experimental results. If -we succeed in discovering the exact character of the periodic -relationships between the increments in atomic weights of allied elements -discussed by Ridberg in 1885, and again by Bazaroff in 1887, we may -expect that our instrument will give us the means of still more closely -controlling the experimental data relating to atomic weights. - - [8] I pointed them out in the _Liebig's Annalen_, Supplement Band., - viii. 1871, p. 211. - -Let me next call to mind that, while disclosing the variation of chemical -properties,[9] the periodic law, has also enabled us to systematically -discuss many of the physical properties of elementary bodies, and to show -that these properties are also subject to the law of periodicity. At the -Moscow Congress of Russian Naturalists in August, 1869, I dwelt upon the -relations which existed between density and the atomic weight of the -elements. The following year Professor Lothar Meyer, in his well-known -paper,[10] studied the same subject in more detail, and thus contributed -to spread information about the periodic law. Later on, Carnelley, -Laurie, L. Meyer, Roberts-Austen, and several others applied the periodic -system to represent the order in the changes of the magnetic properties -of the elements, their melting points, the heats of formation of their -haloid compounds, and even of such mechanical properties as the -coefficient of elasticity, the breaking stress, &c., &c. These -deductions, which have received further support in the discovery of new -elements endowed not only with chemical but even with physical -properties, which were foreseen by the law of periodicity, are well -known; so I need not dwell upon the subject, and may pass to the -consideration of oxides.[11] - - [9] Thus, in the typical small period of - - Li, Be, B, C, N, O, F, - - we see at once the progression from the alkali metals to the acid - non-metals, such as are the halogens. - - [10] _Liebig's Annalen_, Supplement Band., vii. 1870. - - [11] A distinct periodicity can also be discovered in the spectra of - the elements. Thus the researches of Hartley, Ciamician, and - others have disclosed, first, the homology of the spectra of - analogous elements: secondly, that the alkali metals have simpler - spectra than the metals of the following groups; and thirdly, that - there is a certain likeness between the complicated spectra of - manganese and iron on the one hand, and the no less complicated - spectra of chlorine and bromine on the other hand, and their - likeness corresponds to the degree of analogy between those - elements which is indicated by the periodic law. - -In indicating that the gradual increase of the power of elements of -combining with oxygen is accompanied by a corresponding decrease in their -power of combining with hydrogen, the periodic law has shown that there -is a limit of oxidation, just as there is a well-known limit to the -capacity of elements for combining with hydrogen. A single atom of an -element combines with at most four atoms of either hydrogen or oxygen; -and while CH_{4} and SiH_{4} represent the highest hydrides, so RuO_{4} -and OsO_{4} are the highest oxides. We are thus led to recognise types of -oxides, just as we have had to recognise types of hydrides.[12] - - [12] Formerly it was supposed that, being a bivalent element, oxygen - can enter into any grouping of the atoms, and there was no limit - foreseen as to the extent to which it could further enter into - combination. We could not explain why bivalent sulphur, which - forms compounds such as - - O O - / \ / \ - S | and S O, - \ / \ / - O O - - could not also form oxides such as-- - - O--O O--O - / \ / \ - S | or S O, - \ / \ / - O--O O--O - - while other elements, as, for instance, chlorine, form compounds - such as-- - - Cl--O--O--O--O--K - -The periodic law has demonstrated that the maximum extent to which -different non-metals enter into combination with oxygen is determined by -the extent to which they combine with hydrogen, and that the sum of the -number of equivalents of both must be equal to 8. Thus chlorine, which -combines with 1 atom or 1 equivalent of hydrogen, cannot fix more than 7 -equivalents of oxygen, giving Cl_{2}O_{7}; while sulphur, which fixes 2 -equivalents of hydrogen, cannot combine with more than 6 equivalents or 3 -atoms of oxygen. It thus becomes evident that we cannot recognise as a -fundamental property of the elements the atomic valencies deduced from -their hydrides; and that we must modify, to a certain extent, the theory -of atomicity if we desire to raise it to the dignity of a general -principle capable of affording an insight into the constitution of all -compound molecules. In other words, it is only to carbon, which is -quadrivalent with regard both to oxygen and hydrogen, that we can apply -the theory of constant valency and of bond, by means of which so many -still endeavour to explain the structure of compound molecules. But I -should go too far if I ventured to explain in detail the conclusions -which can be drawn from the above considerations. Still, I think it -necessary to dwell upon one particular fact which must be explained from -the point of view of the periodic law in order to clear the way to its -extension in that particular direction. - -The higher oxides yielding salts the formation of which was foreseen by -the periodic system--for instance, in the short series beginning with -sodium-- - - Na_{2}O, MgO, Al_{2}O_{3}, SiO_{2}, P_{2}O_{5}, SO_{3}, Cl_{2}O_{7}, - -must be clearly distinguished from the higher degrees of oxidation which -correspond to hydrogen peroxide and bear the true character of peroxides. -Peroxides such as Na_{2}O_{2}, BaO_{2}, and the like have long been -known. Similar peroxides have also recently become known in the case of -chromium, sulphur, titanium, and many other elements, and I have -sometimes heard it said that discoveries of this kind weaken the -conclusions of the periodic law in so far as it concerns the oxides. I do -not think so in the least, and I may remark, in the first place, that all -these peroxides are endowed with certain properties obviously common to -all of them, which distinguish them from the actual, higher, salt-forming -oxides, especially their easy decomposition by means of simple contact -agencies; their incapability of forming salts of the common type; and -their capability of combining with other peroxides (like the faculty -which hydrogen peroxide possesses of combining with barium peroxide, -discovered by Schoene). Again, we remark that some groups are especially -characterised by their capacity of generating peroxides. Such is, for -instance, the case in the sixth group, where we find the well-known -peroxides of sulphur, chromium, and uranium; so that further -investigation of peroxides will probably establish a new periodic -function, foreshadowing that molybdenum and tungsten will assume peroxide -forms with comparative readiness. To appreciate the constitution of such -peroxides, it is enough to notice that the peroxide form of sulphur -(so-called persulphuric acid) stands in the same relation to sulphuric -acid as hydrogen peroxide stands to water:-- - - H(OH), or H_{2}O, responds to (OH)(OH), or H_{2}O_{2}, - -and so also-- - - H(HSO_{4}), or H_{2}SO_{4}, responds to - (HSO_{4})(HSO_{4}), or H_{2}S_{2}O_{8}. - -Similar relations are seen everywhere, and they correspond to the -principle of substitutions which I long since endeavoured to represent as -one of the chemical generalisations called into life by the periodic law. -So also sulphuric acid, if considered with reference to hydroxyl, and -represented as follows:-- - - HO(SO_{2}OH), - -has its corresponding compound in dithionic acid-- - - (SO_{2}OH)(SO_{2}OH), or H_{2}S_{2}O_{6}. - -Therefore, also, phosphoric acid, HO(POH_{2}O_{2}), has, in the same -sense, its corresponding compound in the subphosphoric acid of Saltzer:-- - - (POH_{2}O_{2})(POH_{2}O_{2}), or H_{4}P_{2}O_{6}; - -and we must suppose that the peroxide compound corresponding to -phosphoric acid, if it be discovered, will have the following -structure:-- - - (H_{2}PO_{4})_{2} or H_{4}P_{2}O_{8} = 2H_{2}O + 2PO_{3}.[13] - -So far as is known at present, the highest form of peroxides is met with -in the peroxide of uranium, UO_{4}, prepared by Fairley;[14] while -OsO_{4} is the highest oxide giving salts. The line of argument which is -inspired by the periodic law, so far from being weakened by the discovery -of peroxides, is thus actually strengthened, and we must hope that a -further exploration of the region under consideration will confirm the -applicability to chemistry generally of the principles deduced from the -periodic law. - - [13] In this sense, oxalic acid, (COOH)_{2}, also corresponds to - carbonic acid, OH(COOH), in the same way that dithionic acid - corresponds to sulphuric acid, and subphosphoric acid to - phosphoric; hence, if a peroxide corresponding to carbonic acid be - obtained, it will have the structure of (HCO_{3})_{2}, or - H_{2}C_{2}O_{6} = H_{2}O + C_{2}O_{5}. So also lead must have a - real peroxide, Pb_{2}O_{5}. - - [14] The compounds of uranium prepared by Fairley seem to me especially - instructive in understanding the peroxides. By the action of - hydrogen peroxide on uranium oxide, UO_{3}, a peroxide of uranium, - UO_{4},4H_{2}O, is obtained (U = 240) if the solution be acid; but - if hydrogen peroxide act on uranium oxide in the presence of - caustic soda, a crystalline deposit is obtained which has the - composition Na_{4}UO_{8},4H_{2}O, and evidently is a combination - of sodium peroxide, Na_{2}O_{2}, with uranium peroxide, UO_{4}. It - is possible that the former peroxide, UO_{4},4H_{2}O, contains the - elements of hydrogen peroxide and uranium peroxide, U_{2}O_{7}, or - even U(OH)_{6},H_{2}O_{2}, like the peroxide of tin recently - discovered by Spring, which has the constitution - Sn_{2}O_{5},H_{2}O_{2}. - -Permit me now to conclude my rapid sketch of the oxygen compounds by the -observation that the periodic law is especially brought into evidence in -the case of the oxides which constitute the immense majority of bodies at -our disposal on the surface of the earth. - -The oxides are evidently subject to the law, both as regards their -chemical and their physical properties, especially if we take into -account the cases of polymerism which are so obvious when comparing -CO_{2}, with Si_{_n_}O_{2_n_}. In order to prove this I give the -densities s and the specific volumes v of the higher oxides of two short -periods. To render comparison easier, the oxides are all represented as -of the form R_{2}O_{_n_}. In the column headed [Delta] the differences -are given between the volume of the oxygen compound and that of the -parent element, divided by _n_--that is, by the number of atoms of oxygen -in the compound:--[15] - - _s._ _v._ [Delta] - Na_{2}O 2·6 24 -22 - Mg_{2}O_{2} 3·6 22 -3 - Al_{2}O_{3} 4·0 26 +1·3 - Si_{2}O_{4} 2·65 45 5·2 - P_{2}O_{5} 2·39 59 6·2 - S_{2}O_{6} 1·96 82 8·7 - K_{2}O 2·7 35 -55 - Sc_{2}O_{3} 3·86 35 0 - Li_{2}O_{4} 4·2 38 +5 - V_{2}O_{5} 3·49 52 6·7 - Cr_{2}O_{6} 2·74 73 9·5 - - [15] [Delta] thus represents the average increase of volume for each - atom of oxygen contained in the higher salt-forming oxide. The - acid oxides give, as a rule, a higher value of [Delta], while in - the case of the strongly alkaline oxides its value is usually - negative. - -I have nothing to add to these figures, except that like relations appear -in other periods as well. The above relations were precisely those which -made it possible for me to be certain that the relative density of -ekasilicon oxide would be about 4·7; germanium oxide, actually obtained -by Winkler, proved, in fact, to have the relative density 4·703. - -The foregoing account is far from being an exhaustive one of all that -has already been discovered by means of the periodic law telescope in the -boundless realms of chemical evolution. Still less is it an exhaustive -account of all that may yet be seen, but I trust that the little which I -have said will account for the philosophical interest attached in -chemistry to this law. Although but a recent scientific generalisation, -it has already stood the test of laboratory verification, and appears as -an instrument of thought which has not yet been compelled to undergo -modification; but it needs not only new applications, but also -improvements, further development, and plenty of fresh energy. All this -will surely come, seeing that such an assembly of men of science as the -Chemical Society of Great Britain has expressed the desire to have the -history of the periodic law described in a lecture dedicated to the -glorious name of Faraday. - - - - - APPENDIX III - - ARGON, A NEW CONSTITUENT OF THE ATMOSPHERE - - WRITTEN BY PROFESSOR MENDELÉEFF IN FEBRUARY 1895 - - -The remarks made in Chapter V., Note 16 bis respecting the newly -discovered constituent of the atmosphere are here supplemented by data -(taken from the publications of the Royal Society of London) given by the -discoverers Lord Rayleigh and Professor Ramsay in January 1895, together -with observations made by Crookes and Olszewsky upon the same subject. - -This gas, which was discovered by Rayleigh and Ramsay in atmospheric -nitrogen, was named _argon_[1] by them, and upon the supposition of its -being an element, they gave it the symbol A. But its true chemical nature -is not yet fully known, for not only has no compound of it been yet -obtained, but it has not even been brought into any reaction. From all -that is known about it at the present time, we may conclude with the -discoverers that argon belongs to those gases which are permanent -constituents of the atmosphere, and that it is a new element. The latter -statement, however, requires confirmation. We shall presently see, -however, that the negative chemical character of argon (its incapacity to -react with any substance), and the small amount of it present in the -atmosphere (about 1-1/4 per cent. by volume in the nitrogen of air, and -consequently about 1 per cent. by volume in air), as well as the recent -date of its discovery (1894) and the difficulty of its preparation, are -quite sufficient reasons for the incompleteness of the existing knowledge -respecting this element. But since, so far as is yet known, we are -dealing with a normal constituent of the atmosphere[1 bis], the existing -data, notwithstanding their insufficiently definite nature, should find a -place even in such an elementary work as the present, all the more as the -names of Rayleigh, Ramsay, Crookes and Olszewsky, who have worked upon -argon, are among the highest in our science, and their researches among -the most difficult.[2] These researches, moreover, were directed straight -to the goal, which was only partly reached owing to the unusual -properties of argon itself. - - [1] From the Greek [Greek: Argon]--inert. - - [1 bis] In Note 16 bis, Chapter V., I mentioned that, judging from the - specific gravity of argon, it might possibly be polymerised - nitrogen, N_{3}, bearing the same relationship to nitrogen, N_{2}, - that ozone, O_{3}, bears to ordinary oxygen. If this idea were - confirmed, still one would not imagine that argon was formed from - the atmospheric nitrogen by those reactions by which it was - obtained by Rayleigh and Ramsay, but rather that it arises from the - nitrogen of the atmosphere under natural conditions. Although this - proposition is not quite destroyed by the more recent results, - still it is contradicted by the fact that the ratio of the specific - heats of argon was found to be 1·66, which, as far as is now known, - could not be the case for a gas containing 3 atoms in its molecule, - since such gases (_see_ Chapter XIV., Note 7) give the ratio - approximately 1·3 (for example, CO_{2}). In abstaining from further - conclusions, for they must inevitably be purely conjectural, I - consider it advisable to suggest that in conducting further - researches upon argon it might be well to subject it to as high a - temperature as possible. And the possibility of nitrogen - polymerising is all the more admissible from the fact that the - aggregation of its atoms in the molecule is not at all unlikely, - and that polymerised nitrogen, judging from many examples, might be - inert if the polymerisation were accompanied by the evolution of - heat. In the following footnotes I frequently return to this - hypothesis, not only because I have not yet met any facts - definitely contradictory to it, but also because the chief - properties of argon agree with it to a certain extent. - - [2] The chief difficulty in investigating argon lies in the fact that - its preparation requires the employment of a large quantity of air, - which has to be treated with a number of different reagents, whose - perfect purity (especially that of magnesium) will always be - doubtful, and argon has not yet been transferred to a substance in - which it could be easily purified. Perhaps the considerable - solubility of argon in water (or in other suitable liquids, which - have not apparently yet been tried) may give the means of doing so, - and it may be possible, by collecting the air expelled from boiling - water, to obtain a richer source of argon than ordinary air. - -When it became known (Chapter V., Note 4 bis) that the nitrogen -obtained from air (by removing the oxygen, moisture and CO_{2}, by -various reagents) has a greater density than that obtained from the -various (oxygen, hydrogen and metallic) compounds of nitrogen, it was a -plausible explanation that the latter contained an admixture of hydrogen, -or of some other light gas lowering the density of the mixture. But such -an assumption is refuted not only by the fact that the nitrogen obtained -from its various compounds (after purification) has always the same -density (although the supposed impurities mixed with it should vary), but -also by Rayleigh and Ramsay's experiment of artificially adding hydrogen -to nitrogen, and then passing the mixture over red-hot oxide of copper, -when it was found that the nitrogen regained its original density, _i.e._ -that the whole of the hydrogen was removed by this treatment. Therefore -the difference in the density of the two varieties of nitrogen had to be -explained by the presence of a heavier gas in admixture with the nitrogen -obtained from the atmosphere. This hypothesis was confirmed by the fact -that Rayleigh and Ramsay having obtained purified nitrogen (by removing -the O_{2}, CO_{2}, and H_{2}O), both from ordinary air and from air which -had been previously subjected to atmolysis, that is which had been passed -through porous tubes (of burnt clay, _e.g._ pipe-stem), surrounded by a -rarefied space, and so deprived of its lighter constituents (chiefly -nitrogen), found that the nitrogen from the air which had been subjected -to atmolysis was heavier than that obtained from air which had not been -so treated. This experiment showed that the nitrogen of air contains an -admixture of a gas which, being heavier than nitrogen itself,[3] diffuses -more slowly than nitrogen through the porous material. It remained, -therefore, to separate this impurity from the nitrogen. To do this -Rayleigh and Ramsay adopted two methods, converting the nitrogen into -solid and liquid substances, either by absorbing the nitrogen by heated -magnesium (Chapter V., Note 6, and Chapter XIV., Note 14), with the -formation of nitride of magnesium, or else by converting it into nitric -acid by the action of electric sparks or the presence of an excess of air -and alkali, as in Cavendish's method.[3 bis] In both cases the nitrogen -entered into reaction, while the heavier gas mixed with it remained -inert, and was thus able to be isolated. That is, the argon could be -separated by these means from the excess of atmospheric nitrogen -accompanying it.[4] As an illustration we will describe how argon was -obtained from the atmospheric nitrogen by means of magnesium.[5] To begin -with, it was discovered that when atmospheric nitrogen was passed through -a tube containing metallic magnesium heated to redness, its specific -gravity rose to 14·88. As this showed that part of the gas was absorbed -by the magnesium, a mercury gasometer filled with atmospheric nitrogen -was taken, and the gas drawn over soda-lime, P_{2}O_{5}, heated -magnesium[6] and then through tubes containing red-hot copper oxide, -soda-lime and phosphoric anhydride to a second mercury gasometer. Every -time the gas was repassed through the tubes, it decreased in volume and -increased in density. After repeating this for ten days 1,500 c.c. of gas -were reduced to 200 cc., and the density increased to 16·1 (if that of -H_{2} = 1 and N_{2} = 14). Further treatment of the remainder brought the -density up to 19·09. After adding a small quantity of oxygen and -repassing the gas through the apparatus, the density rose to 20·0. To -obtain argon by this process Ramsay and Rayleigh (employing a mercury air -pump and mercury gasometers) once treated about 150 litres of atmospheric -nitrogen. On another occasion they treated 7,925 c.c. of air by the -oxidation method and obtained 65 c.c. of argon, which corresponds to 0·82 -per cent. The density of the argon obtained by this means was nearly -19·7, while that obtained by the magnesium method varied between 19·09 -and 20·38. - - [3] It might also be supposed that this heavy gas is separated by the - copper when the latter absorbs the oxygen of the air; but such a - supposition is not only improbable in itself, but does not agree - with the fact that nitrogen may be obtained from air by absorbing - the oxygen by various other substances in solution (for instance, - by the lower oxides of the metals, like FeO) besides red-hot - copper, and that the nitrogen obtained is always just as heavy. - Besides which, nitrogen is also set free from its oxides by copper, - and the nitrogen thus obtained is lighter. Therefore it is not the - copper which produces the heavy gas--_i.e._ argon. - - [3 bis] It is worthy of note that Cavendish obtained a small residue of - gas in converting nitrogen into nitric acid; but he paid no - attention to it, although probably he had in his hands the very - argon recently discovered. - - [4] When in these experiments, instead of atmospheric nitrogen the gas - obtained from its compound was taken, an inert residue of a heavy - gas, having the properties of argon, was also remarked, but its - amount was very small. Rayleigh and Ramsay ascribe the formation of - this residue to the fact that the gas in these experiments was - collected over water, and a portion of the dissolved argon in it - might have passed into the nitrogen. As the authors of this - supposition did not prove it by any special experiments, it forms a - weak point in their classical research. If it be admitted that - argon is N_{3}, the fact of its being obtained from the nitrogen of - compounds might be explained by the polymerisation of a portion of - the nitrogen in the act of reaction, although it is impossible to - refute Rayleigh and Ramsay's hypothesis of its being evolved from - the water employed in the manipulation of the gases. Three thousand - volumes of nitrogen extracted from its compounds gave about three - volumes of argon, while thirty volumes were yielded by the same - amount of atmospheric nitrogen. - - [5] The preparation of argon by the conversion of nitrogen into nitric - acid is complicated by the necessity of adding a large proportion - of oxygen and alkali, of passing an electric discharge through the - mixture for a long period, and then removing the remaining oxygen. - All this was repeatedly done by the authors, but this method is far - more complex, both in practice and theory, than the preparation of - argon by means of magnesium. From 100 volumes of air subjected to - conversion into HNO_{3}, 0·76 volume of argon were obtained after - absorbing the excess of oxygen. - - [6] In these and the following experiments the magnesium was placed in - an ordinary hard glass tube, and heated in a gas furnace to a - temperature almost sufficient to soften the glass. The current of - gas must be very slow (a tube containing a small quantity of - sulphuric acid served as a meter), as otherwise the heat evolved in - the formation of the Mg_{3}N_{2} (Chapter XIV., Note 14) will melt - the tube. - -Thus the first positive and very important fact respecting argon is that -its specific gravity is nearly 20--that is, that it is 20 times heavier -than hydrogen, while nitrogen is only 14 times and oxygen 16 times -heavier than hydrogen. This explains the difference observed by Rayleigh -between the densities of nitrogen obtained from its compounds and from -the atmosphere (Chapter V., Note 4 bis). At 0° and 760 mm. a litre of the -former gas weighs 1·2505 grm., while a litre of the latter weighs 1·2572, -or taking H = 1, the density of the first = 13·916, and of the latter = -13·991. If the density of argon be taken as 20, it is contained in -atmospheric nitrogen to the extent of about 1·23 per cent. by volume, -whilst air contains about 0·97 per cent. by volume. - -When argon had been isolated the question naturally arose, was it a new -homogeneous substance having definite properties or was it a mixture of -gases? The former may now be positively asserted, namely, that argon is a -peculiar gas previously unknown to chemistry. Such a conviction is in the -first place established by the fact that argon has a greater number of -negative properties, a smaller capacity for reaction, than any other -simple or compound body known. The most inert gas known is nitrogen, but -argon far exceeds it in this respect. Thus nitrogen is absorbed at a red -heat by many metals, with the formation of nitrides, while argon, as is -seen in the mode of its preparation and by direct experiment, does not -possess this property. Nitrogen, under the action of electric sparks, -combines with hydrogen in the presence of acids and with oxygen in the -presence of alkalis, while argon is unable to do so, as is seen from the -method of separation from nitrogen. Rayleigh and Ramsay also proved that -argon is unable to react with chlorine (dry or moist) either directly or -under the action of an electric discharge, or with phosphorus or sulphur, -at a red heat. Sodium, potassium, and tellurium may be distilled in an -atmosphere of argon without change. Fused caustic soda, incandescent -soda-lime, molten nitre, red-hot peroxide of sodium, and the -polysulphides of calcium and sodium also do not react with argon. -Platinum black does not absorb it, and spongy platinum is unable to -excite its reaction with oxygen or chlorine. Aqua regia, bromine water, -and a mixture of hydrochloric acid and KMnO_{4} were also without action -upon argon. Besides which it is evident from the method of its -preparation that it is not acted upon by red-hot oxide of copper. All -these facts exclude any possibility of argon containing any already known -body, and prove it to be the most inert of all the gases known. But -besides these negative points, the independency of argon is confirmed by -four observed positive properties possessed by it, which are:-- - -1. The spectrum of argon observed by Crookes under a low pressure (in -Geissler-Plücker tubes) distinguishes it from other gases.[7] It was -proved by this means that the argon obtained by means of magnesium is -identical with that which remains after the conversion of the atmospheric -nitrogen into nitric acid. Like nitrogen, argon presents two spectra -produced at different potentials of the induced current, one being -orange-red, the other steel-blue; the latter is obtained under a higher -degree of rarefaction and with a battery of Leyden jars. Both the spectra -of argon (in contradistinction to those of nitrogen) are distinguished by -clearly defined lines.[8] The red (ordinary) spectrum of argon has two -particularly brilliant and characteristic red lines (not far from the -bright red line of lithium, on the opposite side to the orange band) -having wave-lengths 705·64 and 696·56 (_see_ Vol. I., p. 565). Between -these bright lines there are in addition lines with wave lengths 603·8, -565·1, 561·0, 555·7, 518·58, 516·5, 450·95, 420·10, 415·95 and 394·85. -Altogether 80 lines have been observed in this spectrum and 119 in the -blue spectrum, of which 26 are common to both spectra.[9] - - [7] The greatest brilliancy of the spectrum of argon is obtained at a - tension of 3 mm., while for nitrogen it is about 75 mm. (Crookes). - In Chapter V., Note 16 bis, it is said that the same blue line - observed in the spectrum of argon is also observed in the spectrum - of nitrogen. This is a mistake, since there is no coincidence - between the blue lines of the argon and nitrogen spectra. However, - we may add that for nitrogen the following moderately bright lines - are known of wave-lengths 585, 574, 544, 516, 457, 442, 436, and - 426, which are repeated in the spectra (red and blue) of argon, - judging by Crookes' researches (1895); but it is naturally - impossible to assert that there is perfect identity until some - special comparative work has been done in this subject, which is - very desirable, and more especially for the bluish-violet portion - of the spectrum, more particularly between the lines 442-436, as - these lines are distinguished by their brilliancy in both the argon - and nitrogen spectra. The above-mentioned supposition of argon - being polymerised nitrogen (N_{3}), formed from nitrogen (N_{2}), - with the evolution of heat, might find some support should it be - found after careful comparison that even a limited number of - spectral lines coincided. - - [8] At first the spectrum of argon exhibits the nitrogen lines, but - after a certain time these lines disappear (under the influence of - the platinum, and also of Al and Mg, but with the latter the - spectrum of hydrogen appears) and leave a pure argon spectrum. It - does not appear clear to me whether a polymerisation here takes - place or a simple absorption. Perhaps the elucidation of this - question would prove important in the history of argon. It would be - desirable to know, for instance, whether the volume of argon - changes when it is first subjected to the action of the electric - discharge. - - [9] Crookes supposes that argon contains a mixture of two gases, but as - he gives no reasons for this, beyond certain peculiarities of a - spectroscopic character, we will not consider this hypothesis - further. - -2. According to Rayleigh and Ramsay the solubility of argon in water is -approximately 4 volumes in 100 volumes of water at 13°. Thus argon is -nearly 2-1/2 times more soluble than nitrogen, and its solubility -approaches that of oxygen. Direct experiment proves that nitrogen -obtained from air from boiled water is heavier than that obtained -straight from the atmosphere. This again is an indirect proof of the -presence of argon in air. - -3. The ratio _k_ of the two specific heats (at a constant pressure and -at a constant volume) of argon was determined by Rayleigh and Ramsay by -the method of the velocity of sound (_see_ Chapter XIV., Note 7 and -Chapter VII., Note 26) and was found to be nearly 1·66, that is greater -than for those gases whose molecules contain two atoms (for instance, CO, -H_{2}, N_{2}, air, &c., for which _k_ is nearly 1·4) or those whose -molecules contain three atoms (for instance, CO_{2}, N_{2}O, &c., for -which _k_ is about 1·3), but closely approximate to the ratio of the -specific heats of mercury vapour (Kundt and Warburg, _k_ = 1·67). And as -the molecule of mercury vapour contains one atom, so it may be said that -argon is a simple gaseous body whose molecule contains one atom.[10] A -compound body should give a smaller ratio. The experiments upon the -liquefaction of argon, which we shall presently describe, speak against -the supposition that argon is a mixture of two gases. The importance of -the results in question makes one wish that the determinations of the -ratio of the specific heats (and other physical properties) might be -confirmed with all possible accuracy.[11] If we admit, as we are obliged -to do for the present, that argon is a new element, its density shows -that its atomic weight must be nearly 40, that is, near to that of K = 39 -and Ca = 40, which does not correspond to the existing data respecting -the periodicity of the properties of the elements in dependence upon -their atomic weights, for there is no reason on the basis of existing -data for admitting any intermediate elements between Cl = 35·5 and K = -39, and all the positions above potassium in the periodic system are -occupied. This renders it very desirable that the velocity of sound in -argon should be re-determined.[12] - - [10] This portion of Rayleigh and Ramsay's researches deserves - particular attention as, so far, no gaseous substance is known - whose molecule contains but one atom. Were it not for the above - determinations, it might be thought that argon, having a density - 20, has a complex molecule, and may be a compound or polymerised - body, for instance, N_{3} or NX_{_n_}, or in general X_{_n_}; but - as the matter stands, it can only be said that either (1) argon is - a new, peculiar, and quite unusual elementary substance, since - there is no reason for assuming it to contain two simple gases, or - (2) the magnitude, _k_ (the ratio of the specific heats) does not - only depend upon the number of atoms contained in the molecules, - but also upon the store of internal energy (internal motion of the - atoms in the molecule). Should the latter be admitted, it would - follow that the molecules of very active gaseous elements would - correspond to a smaller _k_ than those of other gases having an - equal number of atoms in their molecule. Such a gas is chlorine, - for which _k_ = 1·33 (Chapter XIV., Note 7). For gases having a - small chemical energy, on the contrary, a larger magnitude would - be expected for _k_. I think these questions might be partially - settled by determining _k_ for ozone (O_{3}) and sulphur (S_{6}) - (at about 500°). In other words, I would suggest, though only - provisionally, that the magnitude, _k_ = 1·6, obtained for argon - might prove to agree with the hypothesis that argon is N_{3}, - formed from N_{2} with the evolution of heat or loss of energy. - Here argon gives rise to questions of primary importance, and it - is to be hoped that further research will throw some light upon - them. In making these remarks, I only wish to clear the road for - further progress in the study of argon, and of the questions - depending on it. I may also remark that if argon is N_{3} formed - with the evolution of heat, its conversion into nitrogen, N_{2}, - and into nitride compounds (for instance, boron nitride or nitride - of titanium) might only take place at a very high temperature. - - [11] Without having the slightest reason for doubting the accuracy of - Rayleigh and Ramsay's determinations, I think it necessary to say - that as yet (February 1895) I am only acquainted with the short - memoir of the above chemists in the 'Proceedings of the Royal - Society,' which does not give any description of the methods - employed and results obtained, while at the end (in the general - conclusions) the authors themselves express some doubt as to the - simple nature of argon. Moreover, it seems to me that (Note 10) - there must be a dependence of _k_ upon the chemical energy. - Besides which, it is not clear what density of the gas Rayleigh - and Ramsay took in determining _k_. (If argon be N_{3}, its - density would be near to 21.) Hence I permit myself to express - some doubt as to whether the molecule of argon contains but one - atom. - - [12] If it should be found that _k_ for argon is less than 1·4, or that - _k_ is dependent upon the chemical energy, it would be possible to - admit that the molecule of argon contains not one, but several - atoms--for instance, either N_{3} (then the density would be 21, - which is near to the observed density) or X_{6}, if X stand for an - element with an atomic weight near to 6·7. No elements are known - between H = 1 and Li = 7, but perhaps they may exist. The - hypothesis A = 40 does not admit argon into the periodic system. - If the molecule of argon be taken as A_{2}--_i.e._ the atomic - weight as A = 20--argon apparently finds a place in Group VIII., - between F = 19 and Na = 23; but such a position could only be - justified by the consideration that elements of small atomic - weight belong to the category of typical elements which offer many - peculiarities in their properties, as is seen on comparing N with - the other elements of Group V., or O with those of Group VI. Apart - from this there appears to me to be little probability, in the - light of the periodic law, in the position of an inert substance - like argon in Group VIII., between such active elements as - fluorine and sodium, as the representatives of this group by their - atomic weights and also by their properties show distinct - transitions from the elements of the last groups of the uneven - series to the elements of the first groups of the even series--for - instance, - - Group VI. VII. VIII. I. II. - Cr Mn Fe, Co, Ni Cu Zn - - While if we place argon in a similar manner, - - VI. VII. VIII. I. II. - O = 16 F = 19 A = 20 Na = 23 Mg = 24 - - although from a numerical point of view there is a similar - sequence to the above, still from a chemical and physical point of - view the result is quite different, as there is no such - resemblance between the properties of O, F and Na, Mg, as between - Cr, Mn, and Cu, Zn. I repeat that only the typical character of - the elements with small atomic weights can justify the atomic - weight A = 20, and the placing of argon in Group VIII. amongst the - typical elements; then N, O, F, A are a series of gases. - - It appears to me simpler to assume that argon contains N_{3}, - especially as argon is present in nitrogen and accompanies it, - and, as a matter of fact, none of the observed properties of argon - are contradictory to this hypothesis. - - These observations were written by me in the beginning of February - 1895, and on the 29th of that month I received a letter, dated - February 25, from Professor Ramsay informing me that 'the periodic - classification entirely corresponds to its (argon's) atomic - weight, and that it even gives a fresh proof of the periodic law,' - judging from the researches of my English friends. But in what - these researches consisted, and how the above agreement between - the atomic weight of argon and the periodic system was arrived at, - is not referred to in the letter, and we remain in expectation of - a first publication of the work of Lord Rayleigh and Professor - Ramsey. [For more complete information see papers read before the - Royal Society, January 31, 1895, February 13, March 10, and May - 21, 1896, and a paper published in the Chemical Society's - Transactions, 1895, p. 684. For abstracts of these and other - papers on argon and helium, and correspondence, see 'Nature,' 1895 - and 1896. - -4. Argon was liquefied by Professor Olszewsky, who is well known for -his classical researches upon liquefied gases. These researches have an -especial interest since they show that argon exhibits a perfect constancy -in its properties in the liquid and critical states, which almost[13] -disposes of the supposition that it contains a mixture of two or more -unknown gases. As the first experiments showed, argon remains a gas under -a pressure of 100 atmospheres and at a temperature of -90°; this -indicated that its critical temperature was probably below this -temperature, as was indeed found to be the case when the temperature was -lowered to -128°·6[14] by means of liquid ethylene. At this temperature -argon easily liquefies to a colourless liquid under 38 atmospheres. The -meniscus begins to disappear at between -119°·8 and -121°·6, mean -121° -at a pressure of 50·6 atmospheres. The vapour tension of liquid argon at --128°·6, is 38·0 atmospheres, at -187° it is one atmosphere, and at --189°·6 it solidifies to a colourless substance like ice. The specific -gravity of liquid argon at about -187° is nearly 1·5, which is far above -that of other liquefied gases of very low absolute boiling point. - -The discovery of argon is one of the most remarkable chemical -acquisitions of recent times, and we trust that Lord Rayleigh and -Professor Ramsay, who made this wonderful discovery, will further -elucidate the true nature of argon, as this should widen the fundamental -principles of chemistry, to which the chemists of Great Britain have from -early times made such valuable contributions. It would be premature now -to give any definite opinions upon so new a subject. Only one thing can -be said; argon is so inert that its rôle in nature cannot be -considerable, notwithstanding its presence in the atmosphere. But as the -atmosphere itself plays such a vast part in the life of the surface of -the earth, every addition to our knowledge of its composition must -directly or indirectly react upon the sum total of our knowledge of -nature. - - [13] There only remains the very remote possibility that argon consists - of a mixture of two gases having very nearly the same properties. - - [14] The following data, given by Olszewsky, supplement the data given - in Chapter II., Note 29, upon liquefied gases. - - (_tc_) (_pc_) _t_ _t__{1} _s_ - N_{2} -146° 35 -194°·4 -214° 0·885 - CO -139°·5 35·5 -190° -207 ? - A -121° 50·6 -187° -189°·6 1·5 - O_{2} -118°·8 50·8 -182°·7 ? 1·124 - NO -93°·5 71·2 -153°·6 -167° ? - CH_{4} -81°·8 54·9 -164° -158°·8 0·415 - - where _tc_ is the absolute (critical) boiling point, _pc_ the - pressure (critical) in atmospheres corresponding to it, _t_ the - boiling point (under a pressure of 760 mm.), _t_{1}_ the melting - point, and _s_ the specific gravity in a liquid state at _t_. - - The above shows that argon in its properties in a liquid state - stands near to oxygen (as it also does in its solubility), but - that all the temperatures relating to it (_tc_, _t_, and _t_{1}_) - are higher than for nitrogen. This fully answers, not only to the - higher density of argon, but also to the hypothesis that it - contains N_{3}. And as the boiling point of argon differs from - that of nitrogen and oxygen by less than 10°, and its amount is - small, it is easy to understand how Dewar (1894), who tried to - separate it from liquid air and nitrogen by fractional - distillation, was unable to do so. The first and last portions - were identical, and nitrogen from air showed no difference in its - liquefaction from that obtained from its compounds, or from that - which had been passed through a tube containing incandescent - magnesium. Still, it is not quite clear why both kinds of - nitrogen, after being passed over the magnesium in Dewar's - experiments, exhibited an almost similar alteration in their - properties, independent of the appearance of a small quantity of - hydrogen in them. - - _Concluding Remarks_ (March 31, 1895).--The 'Comptes rendus' of - the Paris Academy of Sciences of March 18, 1895, contains a memoir - by Berthelot upon the reaction of argon with the vapour of benzene - under the action of a silent discharge. In his experiments, - Berthelot succeeded in treating 83 per cent. of the argon taken - for the purpose, and supplied to him by Ramsay (37 c.c. in all). - The composition of the product could not be determined owing to - the small amount obtained, but in its outward appearance it quite - resembled the product formed under similar conditions by nitrogen. - This observation of the famous French chemist to some extent - supports the supposition that argon is a polymerised variety of - nitrogen whose molecule contains N_{3}, while ordinary nitrogen - contains N_{2}. Should this supposition be eventually verified, - the interest in argon will not only not lessen, but become - greater. For this, however, we must wait for further observations - and detailed experimental data from Rayleigh and Ramsay. - - The latest information obtained by me from London is that - Professor Ramsay, by treating cleveite (containing PbO, UO_{3}, - Y_{2}O_{3}, &c.) with sulphuric acid, obtained argon, and, judging - by the spectrum, helium also. The accumulation of similar data - may, after detailed and diversified research, considerably - increase the stock of chemical knowledge which, constantly - widening, cannot be exhaustively treated in these 'Principles of - Chemistry,' although very probably furnishing fresh proof of the - 'periodicity of the elements.' - - * * * * * - - - - - INDEX OF AUTHORITIES - - - Abasheff, i. 75 - Abel, ii. 56, 326, 410 - Acheson, ii. 107 - Adie, ii. 186 - Alexéeff, i. 75, 94 - Alluard, i. 458 - Amagat, i. 132, 135, 140 - Amat, ii. 171 - Ammermüller, i. 504 - Ampère, i. 309 - Andréeff, i. 251 - Andrews, i. 136, 203 - Angeli, i. 266 - Ansdell, i. 451 - Arfvedson, i. 575 - Arrhenius, i. 89, 92, 389 - Aschoff, ii. 313 - Askenasy, i. 508 - Aubel, ii. 45 - Aubin, i. 238 - Avdéeff, i. 618; ii. 484 - Avogadro, i. 309 - - Babo, v., i. 93, 200, 203 - Bach, i. 394 - Bachmetieff, ii. 31 - Baeyer, v., i. 507 - Bagouski, i. 384 - Bailey, i. 449; ii. 29 - Baker, i. 318, 403 - Balard, i. 480, 494, 495, 505 - Ball, ii. 414 - Bannoff, i. 506 - Barfoed, ii. 53 - Baroni, i. 331 - Barreswill, ii. 282 - Baudrimont, ii. 35 - Baumé, i. 193 - Baumgauer, ii. 20 - Baumhauer, i. 495 - Bayer, ii. 76, 159 - Bazaroff, i. 409; ii. 24, 68, 486 - Becher, i. 17 - Becker, i. 16 - Beckmann, i. 91, 496; ii. 156 - Becquerel, i. 228; ii. 97, 220 - Beilby, i. 71 - Beilstein, i. 373; ii. 188 - Beketoff, i. 120, 122, 124, 146, 403, 459, 466, 534, 541, 574, 577; - ii. 87, 102, 289, 429 - Bender, i. 476 - Benedict, ii. 65 - Berglund, ii. 229 - Bergman, i. 27, 435; ii. 100 - Berlin, i. 95 - Bernouilli, i. 81 - Bernthsen, ii. 228 - Bert, i. 86, 153 - Bertheim, ii. 337 - Berthelot, i. 171, 173, 189, 199, 229, 230, 258, 264, 266, 267, 272, - 83, 289, 351, 372, 393, 394, 405, 415, 424, 438, 457, 463, 502, - 506, 507, 518, 529, 537, 582; ii. 23, 57, 207, 209, 251, 253, 259, - 345, 367 - Berthier, ii. 8 - Berthollet, i. 27, 31, 105, 433, 434, 459, 470, 502, 609 - Berzelius, i. 131, 148, 194, 255, 379; ii. 8, 100, 102, 147, 148, - 219, 281, 300, 485 - Besson, i. 288; ii. 67, 70, 105, 179 - Beudant, ii. 7, 8 - Bineau, i. 100, 271, 452, 504; ii. 239 - Binget, i. 75 - Blaese, ii. 188 - Blagden, i. 91, 428 - Blake, ii. 30 - Blitz, ii. 184 - Blomstrand, ii. 299 - Boerwald, ii. 279 - Böttger, i. 595 - Bogorodsky, i. 574 - Boilleau, i. 415 - Boisbaudran, L. de, i. 97, 102, 572, 600; ii. 6, 26, 90, 82, 284, 483 - Bornemann, i. 509 - Botkin, ii. 30 - Bouchardat, ii. 45 - Boullay, ii. 55 - Bourdiakoff, i. 584, 617 - Boussingault, i. 131, 157, 233, 235, 525, 615 - Boyle, i. 124 - Brand, ii. 150 - Brandau, i. 481 - Brandes, i. 72 - Bravais, i. 233 - Brauner, i. 490, 491; ii. 26, 59, 94, 96, 97, 134, 144, 194, 271, 483 - Brewster, i. 569 - Brigham, ii. 193 - Brodie, i. 212, 351, 405; ii. 252 - Brooke, ii. 357 - Brown, i. 81, 88 - Brugellmann, i. 616 - Brunn, ii. 182, 189 - Bruyn, i. 262 - Brühl, i. 263, 337 - Brunner, i. 124, 146, 263; ii. 230, 309, 534 - Buchner, i. 615 - Buckton, ii. 143 - Buff, ii. 103 - Bunge, i. 288 - Bunsen, i. 43, 69, 78, 117, 180, 465, 568, 575, 576, 577; ii. 27, 289 - Bussy, i. 75, 594, 619 - Butleroff, i. 143 - Bystrom, i. 585 - - Cagniard de Latour, i. 135, 345 - Cahours, ii. 143, 173 - Cailletet, i. 132, 138; ii. 45 - Calderon, i. 596 - Callender, i. 134 - Calvert, i. 484; ii. 45 - Cannizzaro, i. 584, 587 - Carey-Lea, ii. 420, 424, 425, 432 - Carius, i. 69, 481 - Carnelley, i. 483, 515, 555; ii. 22, 29, 30, 31, 64, 143, 486 - Carnot, ii. 294, 361 - Caron, i. 595, 604, 610; ii. 336 - Carrara, i. 213 - Cass, ii. 85 - Castner, i. 431, 535, 541 - Cavazzi, ii. 160, 172, 182 - Cavendish, i. 113, 125, 228; ii. 493 - Chabrié, i. 229 - Chappuis, i. 50, 199, 205, 264 - Chapuy, i. 59 - Cheltzoff, i. 393, 457, 582; ii. 41, 247 - Cherikoff, ii. 102 - Chertel, ii. 245 - Chevillot, ii. 311 - Chevreul, i. 530 - Chigeffsky, ii. 62 - Christomanos, i. 511 - Chroustchoff, i. 353, 444; ii. 122 - Chydenius, ii. 148, 485 - Ciamician, i. 565, 573; ii. 486 - Clark, i. 26 - Classen, ii. 146 - Clausius, i. 81, 93, 140, 142, 212, 309, 491 - Clement, i. 494 - Clève, ii. 26, 94, 97, 484 - Cloez, i. 207, 246, 377 - Clowes, i. 242 - Collendar, i. 134 - Comaille, i. 596 - Comb, ii. 81 - Connell, i. 508 - Coppet, i. 91, 428, 601 - Corenwinder, i. 501 - Cornu, i. 565 - Courtois, i. 494 - Cracow, ii. 380 - Crafts, i. 380 ; ii. 80, 83 - Cremers, ii. 100 - Croissier, i. 251 - Crompton, i. 247 - Crookes, i. 229, 617; ii. 20, 91, 96, 440, 491 - Crum, ii. 79, 311 - Cundall, i. 611 - Curtius, i. 258, 265 - - Dahl, ii. 59 - Dalton, i. 29, 78, 81, 109, 206, 271, 322 - Dana, ii. 8 - Davies, i. 484 - Davy, i. 37, 114, 195, 255, 364, 460, 463, 484, 489, 494, 533, 541, - 594, 604, 617 - Deacon, i. 599 - Debray, i. 609; ii. 45, 122, 291, 293, 384, 385 - De Chancourtois, ii. 20, 26 - De Forcrand, ii. 106, 211 - De Haën, ii. 189 - De Heen, i. 140 - Delafontaine, ii. 97, 148, 198 - De la Rive, i. 198; ii. 226 - Del-Rio, ii. 197 - De Saussure, i. 235, 240 - De Schulten, ii. 48 - Deville, St.-Claire, i. 4, 36, 118, 143, 179, 180, 227, 239, 280, 281, - 301, 320, 392, 393, 399, 459, 467, 476, 477, 500, 534, 595, 608, 609; - ii. 48, 80, 83, 85, 102, 147, 156, 198, 289, 309, 321, 352, 373, 374, - 429 - De Vries, i. 62, 64, 429 - Dewar, i. 3, 5, 135, 139, 163, 297, 563, 565, 569, 585; ii. 176, 220 - Dick, ii. 414 - Dingwall, i. 486 - Ditte, i. 72, 403, 430, 457, 509, 539, 618; ii. 64, 65, 85, 189, 249 - Dittmar, i. 100, 452; ii. 240 - Divers, i. 274, 294; ii. 54 - Dixon, i. 171 - Döbereiner, i. 145 - Dokouchaeff, i. 344 - Donny, i. 534 - Dossios, i. 502 - Draper, i. 465 - Drawe, ii. 161 - Drebbel, i. 294 - Dulong, i. 131, 148, 437 - Dumas, i. 28, 131, 148, 150, 233, 302, 320, 379, 471, 476, 584, 586, - 604; ii. 22, 37, 62, 101, 156, 420 - Dumont, ii. 197 - - Ebelmann, ii. 65 - Eder, i. 566 - Edron, ii. 95 - Edwards, ii. 311 - Egoreff, i. 569 - Eissler, i. 553 - Elbers, ii. 221 - Emich, i. 286, 287 - Emilianoff, ii. 126 - Engel, i. 457; ii. 130, 132, 189, 206 - Engelhardt, i. 530 - Eötvös, i. 333 - Erdmann, i. 150 - Ernst, i. 399 - Eroféeff, i. 352 - Esson, ii. 314 - Étard, i. 72, 516, 615; ii. 288, 335, 356 - Ettinger, i. 53, 312 - - Famintzin, i. 611 - Faraday, i. 134, 177, 296, 385, 463, 464 - Favorsky, i. 373 - Favre, i. 120, 172, 267, 582; ii. 83, 259, 284, 380 - Fick, i. 62 - Fisher, ii. 424 - Fizeau, ii. 31, 429 - Flavitzky, i. 21 - Fleitmann, ii. 170 - Foerster, ii. 375, 389 - Forchhammer, ii. 311 - Fordos, ii. 257 - Fortmann, ii. 230, 366 - Fourcroy, i. 114 - Fowler, i. 449 - Frank, ii. 88 - Franke, ii. 311, 313 - Frankel, ii. 294 - Frankenheim, ii. 7 - Frankland, i. 178, 357, 486; ii. 16, 143 - Fraunhofer, i. 563 - Frémy, i. 228, 489, 492; ii. 74, 131, 133, 142, 229, 290, 359 - Freyer, i. 171, 488 - Friedheim, ii. 197, 294 - Friedel, i. 353, 472; ii. 80, 83, 103, 122 - Friedrich, i. 49; ii. 144 - Fritzsche, i. 94, 285, 600, 612; ii. 125, 218, 280, 341 - Fromherz, ii. 313 - Fürst, i. 484 - - Galileo, i. 7 - Garni, i. 582 - Garzarolli-Thurnlackh, i. 481 - Gattermann, i. 596; ii. 102, 104 - Gautier, i. 585 - Gavaloffsky, i. 160 - Gay-Lussac, i. 40, 61, 71, 93, 170, 302, 307, 406, 412, 460, 463, 464, - 467, 500, 506, 508, 511, 515, 534, 539; ii. 8, 56, 256 - Geber, i. 17 - Gélis, ii. 257 - Genth, ii. 359 - Georgi, ii. 197 - Georgiewics, ii. 64 - Gerberts, i. 528 - Gerhardt, i. 196, 309, 357, 388 - Gerlach, i. 525 - Gernez, i. 97; ii. 205 - Geuther, i. 281, 283, 285; ii. 176 - Gibbs, i. 140, 464; ii. 293, 410 - Girault, i. 498 - Gladstone, i. 337, 438, 573; ii. 213 - Glatzel, ii. 213, 289, 309 - Glauber, i. 17, 26, 193, 432 - Glinka, i. 607 - Goldberg, i. 93 - Gooch, i. 484 - Gore, i. 489, 492, 493 - Graham, i. 62, 63, 98, 143, 155, 388, 429, 518, 601; ii. 77, 114, 131, - 163, 170, 296, 307 - Granger, ii. 157, 410 - Grassi, i. 88 - Green, ii. 310 - Greshoff, i. 403 - Griffiths, i. 135 - Grimaldi, i. 537 - Groth, ii. 10 - Grouven, i. 615 - Grove, i. 118, 119 - Grünwald, i. 573 - Grützner, ii. 296 - Guckelberger, ii. 84 - Guibourt, ii. 53 - Guldberg, i. 439, 464 - Güntz, i. 575; ii. 430 - Gustavson, i. 443, 444, 472, 505, 547; ii. 29, 175 - Guthrie, i. 99, 428, 601 - Guy, i. 136 - - Habermann, ii. 210 - Hagebach, i. 573 - Hagen, i. 337 - Haitinger, i. 593 - Hammerl, i. 613 - Hanisch, ii. 233 - Hannay, i. 352; ii. 135 - Harcourt, ii. 314 - Hargreaves, i. 515 - Harris, ii. 52 - Hartley, i. 573; ii. 486 - Hartog, ii. 268 - Hasselberg, i. 566 - Haüy, ii. 7 - Haughton, ii. 20 - Häussermann, i. 483 - Hautefeuille, i. 199, 205, 264, 409, 414, 476, 477, 501, 538; ii. 102, - 122, 379 - Hayter, ii. 175 - Hemilian, i. 132 - Hempel, i. 59, 524 - Henkoff, i. 530 - Henneberg, ii. 170 - Henning, ii. 3 - Henry, i. 78, 81 - Hérard, ii. 191 - Hermann, ii. 8, 47, 197 - Hermes, i. 529 - Hertz, ii. 156 - Hess, i. 178, 588 - Heycock, i. 537; ii. 128, 448 - Hillebrand, ii. 26, 93, 94, 484 - Hintze, ii. 10 - Hirtzel, ii. 55 - Hittorf, ii. 155 - Hodgkinson, ii. 432 - Höglund, ii. 94 - Hofmann, i. 302; ii. 146, 218, 447 - Holtzmann, i. 505 - Hoppé-Seyler, i. 611 - Horstmann, i. 408 - Houzeau, i. 202 - Hughes, ii. 212 - Hugo, ii. 21 - Humboldt, i. 170 - Humbly, i. 493; ii. 311 - Hutchinson, i. 491 - Huth, ii. 20 - Huyghens, i. 569 - - Ikeda, ii. 152 - Ilosva, i. 202 - Inostrantzeff, i. 345; ii. 4 - Isambert, i. 250, 257, 408; ii. 41 - Ittner, i. 412 - - Janssen, i. 569 - Jawein, ii. 170 - Jay, i. 258 - Jeannel, i. 104 - Joannis, i. 251, 255, 405, 537, 559 - Jörgensen, i. 498; ii. 359, 361, 376 - Johnson, ii. 45 - Jolly, i. 233 - Joly, ii. 384, 385 - - Kamensky, ii. 414 - Kammerer, i. 286, 462, 509; ii. 297 - Kane, ii. 57 - Kapoustin, i. 403 - Karsten, i. 427, 428, 541, 599 - Kassner, i. 158 - Kayander, i. 133, 384; ii. 46 - Keiser, i. 150 - Kekulé, i. 358, 369, 507; ii. 294 - Keyser, ii. 33 - Khichinsky, i. 440 - Kimmins, i. 510 - Kirchhoff, i. 567 - Kirmann, ii. 268 - Kirpicheff, i. 132 - Kjeldahl, i. 249; ii. 249 - Klaproth, ii. 7, 145, 147, 301 - Kleiber, i. 570 - Klimenko, i. 465 - Klobb, ii. 357 - Klodt, i. 426 - Knopp, ii. 338 - Knox, i. 489 - Kobb, ii. 125 - Kobell, ii. 197 - Koch, i. 44 - Kohlrausch, i. 245, 525 - Kolbe, i. 506 - Kolotoff, i. 263 - Konovaloff, i. 39, 65, 90, 93, 100, 140, 142, 172, 322; ii. 235, 268 - Kopp, i. 586, 587, 612; ii. 3, 37 - Koucheroff, i. 373 - Kouriloff, i. 209, 247, 274; ii. 41 - Kournakoff, i. 393; ii. 294, 365, 396 - Kraevitch, i. 133, 134 - Kraft, i. 65, 88, 537 - Krafts, i. 393 - Kreisler, i. 233 - Kremers, i. 87, 443; ii. 244, 427 - Kreider, i. 484 - Krönig, i. 81 - Krüger, ii. 282, 284 - Krüss, ii. 355, 442, 447, 486 - Kubierschky, ii. 213 - Kühlmann, i. 608 - Kuhnheim, i. 612 - Kundt, i. 328, 589; ii. 496 - Kvasnik, ii. 57 - Kynaston, i. 522 - Lachinoff, i. 116, 457; ii. 410 - Ladenburg, ii, 103 - Lamy, ii. 91 - Landolt, i. 7, 337 - Lang, i. 399 - Langer, i. 226, 459, 462 - Langlois, i. 570 ; ii. 257 - Latchinoff (_see_ Lachinoff), i. 103, 352 - Laurent, i. 28, 196, 388, 471, 526; ii. 7, 9, 10, 117, 292 - Laurie, i. 106; ii. 32, 442, 486 - Lavenig, i. 140 - Lavoisier, i. 7, 29, 49, 114, 131, 155, 379, 459 - Leblanc, ii. 8 - Le Chatelier, i. 158, 172, 350, 393, 399, 585, 588, 611; ii. 51, 65, - 420 - Le Duc, i. 131, 170 - Lémery, i. 125 - Lemoine, i. 501; ii. 155 - Lerch, i. 405 - Leroy, i. 285 - Lesc[oe]ur, i. 103 - Leton, ii. 425 - Levy, ii. 102 - Lewes, i. 371 - Lewy, i. 232 - Lidoff, ii. 209 - Liebig, i. 195, 388, 495, 527; ii. 56 - Linder, ii. 223 - Liés-Bodart, i. 604, 612 - Lisenko, i. 373 - Liveing, i. 563, 569 - Lockyer, i. 565, 569 - Loew, ii. 376 - Löwel, i. 525, 600; ii. 45, 284, 286 - Loewig, i. 528; ii. 77 - Loewitz, i. 96 - Lossen, i. 262 - Louget, i. 489 - Louguinine, i. 360 - Louise, ii. 81 - Lovel, i. 515; ii. 338 - Lubavin, i. 593 - Lubbert, ii. 85, 170 - Ludwig, i. 463 - Luedeking, ii. 194 - Luff, ii. 321 - Lunge, ii. 244, 246 - Lüpke, ii. 157 - Lvoff, i. 358 - - Maack, i. 596 - Mac Cobb, i. 612 - Mac Laurin, i. 553 - McLeod, ii. 180 - Magnus, i. 93, 510 - Mailfert, i. 199 - Malaguti, i. 437; ii. 300 - Mallard, i. 172, 393, 588; ii. 4 - Mallet, i. 493 - Maquenne, i. 349, 620, 621 - Marchand, i. 150 - Marchetti, ii. 288 - Maresca, i. 534 - Marguerite, ii. 292 - Marignac, i. 198, 233, 428, 430, 453, 454, 518, 525, 600, 601; ii. 6, - 9, 95, 101, 194, 197, 198, 199, 234, 239, 241, 244, 292, 293, 295, - 357, 440, 486 - Markleffsky, i. 273 - Markovnikoff, i. 373 - Maroffsky, ii. 138 - Marshall, ii. 253, 365 - Matigon, i. 258, 266 - Maumené, i. 258 - Maxwell, i. 81 - Mayow, i. 17 - Mendeléeff, i. 99, 132, 133, 136, 141, 275, 321, 357, 373, 377, 406, - 426, 427, 428, 506, 587, 596; ii. 27, 33, 93, 94 - Menschutkin, i. 171 - Mente, ii. 270 - Mermé, i. 462 - Merz, i. 505 - Meselan, i. 463 - Metzner, ii. 189 - Metchikoff, i. 44 - Meusnier, i. 114 - Meyer (Lothar), i. 226, 321, 403; ii. 21, 24, 26, 29, 33, 486 - Meyer (Victor), i. 135, 171, 294, 303, 320, 427, 459, 462, 467, 488, - 506, 508, 558; ii. 43, 48, 52, 80, 129, 184 - Meyerhoffer, ii. 410 - Miasnikoff, i. 372 - Michaelis, ii. 175 - Michel, i. 65, 88 - Millon, i. 481, 484, 508 - Mills, ii. 20 - Mitchell, i. 156 - Mitscherlich, i. 428, 527; ii. 1, 5, 8, 156, 184, 311, 313 - Moissan, i. 202, 349, 353, 490, 564, 585, 621; ii. 66, 67, 70, 88, 100, - 107, 147, 174, 196, 289, 295, 309, 311, 313, 321 - Mond, i. 129, 400, 405; ii. 345, 367 - Monge, i. 114 - Monnier, i. 611 - Montemartini, i. 279 - Moraht, ii. 384 - Moreau, ii. 298 - Morel, i. 549 - Mosander, ii. 97 - Mühlhäuser, ii. 66, 107 - Muir, ii. 193 - Mulder, i. 515 - Müller-Erzbach, i. 103 - Müller, i. 427; ii. 425 - Munster, ii. 443 - Müntz, i. 238, 241, 420, 553 - Muthmann, ii. 273 - Mylius, ii. 375, 389 - - Naschold, i. 483 - Nasini, i. 496; ii. 156 - Natanson, i. 282, 409 - Natterer, i. 132, 135, 141, 385 - Naumann, i. 399, 408 - Nernst, i. 62, 148; ii. 3, 50 - Nensky, i. 245 - Neville, i. 537; ii. 128, 448 - Newlands, ii. 21, 26 - Newth, i. 505 - Newton, i. 7, 29 - Nicklès, ii. 10 - Nikolukin, i. 491; ii. 144 - Nilson, i. 618; ii. 26, 37, 80, 83, 91, 94, 95, 271, 378, 483 - Nordenskiöld, i. 241 - Norton, i. 76; ii. 94 - Nuricsán, ii. 264 - - Odling, ii. 52 - Offer, i. 99 - Ogier, i. 321, 509; ii. 159, 182 - Olszewski, i. 139, 569; ii. 491, 497 - Oppenheim, i. 506 - Ordway, ii. 80 - Osmond, ii. 326 - Ossovetsky, ii. 137 - Ostwald, i. 89, 92, 389, 441, 443 - Oumoff, i. 62 - - Pallard, i. 491; ii. 83 - Panfeloff, i. 603 - Paracelsus, i. 17, 125, 129, 379 - Parkinson, i. 596, - Pashkoffsky, i. 595 - Pasteur, i. 44, 241, 242 - Paterno, i. 496; ii. 156 - Pattison Muir, i. 436 - Pebal, i. 315, 484 - Péchard, ii. 282, 294, 296, 297 - Pekatoros, i. 465 - Peligot, ii. 299, 301 - Pelopidas, ii. 22, 481 - Pelouze, i. 463, 464, 480, 610; ii. 229 - Penfield, i. 545; ii. 370 - Perkin, i. 558; ii. 244 - Perman, i. 537 - Personne, i. 75, 506, 537 - Petit, i. 584, 586 - Petrieff, i. 440 - Pettenkofer, ii. 22 - Pettersson, i. 618, 619; ii. 37, 80, 83, 91, 197, 484 - Pfaundler, i. 445; ii. 241, 430 - Pfeiffer, i. 64 - Pfordten, V. der, ii. 420 - Phipson, i. 596; ii. 59 - Piccini, ii. 23, 146, 197, 288, 298 - Pici, ii. 57 - Pickering, i. 88, 91, 99, 104, 106, 272, 333, 452, 517, 525, 529, 613; - ii. 241, 245, 246, 247 - Pictet, i. 81, 129, 137; ii. 31, 241 - Picton, ii. 223 - Pierre, i. 452, 495; ii. 226, 485 - Pierson, i. 93 - Pigeon, ii. 377 - Pionchon, i. 585 - Pistor, i. 399 - Plantamour, ii. 5 - Plaset, ii. 289 - Plessy, ii. 257 - Plücker, i. 572 - Poggiale, i. 427 - Poiseuille, i. 355 - Poleck, ii. 296 - Poluta, ii. 30 - Popp, ii. 232 - Potilitzin, i. 96, 97, 98, 445, 486, 499, 502, 509, 612; ii. 29, 357 - Pott, ii. 100 - Poulenc, ii. 174, 289 - Prange, ii. 422 - Prelinger, ii. 310 - Priestley, i. 17, 154, 159, 297, 379, 402 - Pringsheim, i. 465 - Prost, i. 98, 486 - Prout, i. 31; ii. 439 - Puchot, i. 452 - Pullinger, ii. 389 - - Quincke, i. 427, 495 - - Rammelsberg, i. 430, 510, 525; ii. 26, 161, 485 - Ramsay, i. 133, 140, 141, 232, 247, 333, 495, 496, 581; ii. 128, 491 - Rantsheff, ii. 20 - Raoult, i. 91, 274, 330, 331, 332, 429 - Rascher, ii. 85 - Raschig, i. 263; ii. 229 - Rathke, i. 399 - Ray, i. 17 - Rayleigh, i. 226, 232, 491 - Rebs, ii. 213, 217 - Recoura, i. 332; ii. 289 - Regnault, i. 40, 53, 54, 90, 93, 131, 133, 297, 443, 495, 584, 587, - 588; ii. 50, 208, 238 - Reich, ii. 91 - Reiset, i. 238 - Remsen, ii. 335 - Retgers, ii. 157, 158, 180 - Reychler, ii. 65 - Reynolds, i. 581 - Richards, i. 526, 585; ii. 32, 432 - Riche, i. 509; ii. 127, 292 - Richter, i. 193, 194; ii. 91 - Ridberg, ii. 21, 24, 486 - Riddle, i. 135 - Rideal, ii. 297 - Roberts-Austen, ii. 486 - Robinson, i. 515 - Rodger, ii. 213, 263 - Rodwell, i. 17 - Roebuck, i. 294 - Röggs, ii. 119 - Rohrer, ii. 343 - Roozeboom, i. 106, 452, 453, 464, 496, 506, 511, 599, 613; ii. 3, 226, - 341, 410 - Roscoe, i. 80, 100, 101, 379, 452, 463, 485, 486, 568, 572; ii. 26, - 194, 196, 197, 297, 303, 485 - Rose, i. 436, 437, 518, 525, 608, 612; ii. 8, 230, 235, 248, 281, 363, - 428, 485 - Rosenberg, ii. 351 - Rossetti, i. 428 - Rouart, Le, ii. 86 - Rousseau, i. 354; ii. 337, 366, 378 - Roux, ii. 81 - Rudberg, ii. 136 - Rücker, i. 142 - Rüdorff, i. 91, 428, 598, 601 - Rybalkin, i. 455 - - Sabanéeff, i. 371 - Sabatier, i. 284; ii. 66 - Saint Edmé, ii. 335 - Saint Gilles, i. 431 - Sakurai, i. 331 - Salzer, ii. 161 - Sarasin, ii. 122 - Sarrau, i. 140, 142 - Saunders, ii. 189 - Scharples, i. 576 - Scheele, i. 155, 161, 412, 459, 462, 608; ii. 100, 150, 291 - Scheffer, i. 453 - Scheibler, ii. 292, 296 - Scherer, ii. 8 - Schiaparelli, ii. 318 - Schidloffsky, i. 238 - Schiloff, i. 212 - Schlamp, i. 332 - Schiff, i. 430, 588; ii. 106, 267 - Schloesing, i. 238, 239, 240, 553, 610 - Schmidt, i. 539 - Schneider, i. 89 - Schöne, i. 208, 209, 211, 394, 617; ii. 15, 72, 219, 251, 488 - Schönebein, i. 198, 202, 208, 212, 509; ii. 228, 463 - Schottländer, ii. 447 - Schröder, i. 75 - Schroederer, ii. 366 - Schrötter, ii. 153, 284 - Schützenberger, i. 511, 579; ii. 102, 107, 228, 367, 389 - Schuliachenko, i. 608 - Schuller, ii. 180 - Schultz, i. 518; ii. 273 - Schulze, i. 98; ii. 215 - Schuster, i. 572 - Schwicker, ii. 227, 230 - Scott, i. 405, 537, 558 - Sechenoff, i. 80, 86 - Seelheim, ii. 379 - Sefström, ii. 197 - Selivanoff, i. 476, 507, 508 - Senderens, i. 284 - Serullas, i. 485 - Setterberg, i. 576 - Seubert, ii. 27, 83, 343, 442 - Sewitsch, i. 372 - Shaffgotsch, i. 555 - Shapleigh, ii. 95 - Shenstone, i. 611 - Shields, i. 333 - Shishkoff, i. 276; ii. 56 - Silberman, i. 120, 172; ii. 259 - Sims, ii. 268 - Skraup, ii. 346 - Smith, i. 271 - Smithson, ii. 100 - Snyders, ii. 100 - Sokoloff, ii. 85, 122 - Solet, i. 509 - Sonstadt, ii. 443 - Sorby, i. 88 - Soret, i. 66, 202, 203, 427 - Spring, i. 38, 98, 434, 486; ii. 45, 50, 133, 223, 258, 288, 314, 423, - 427 - Stadion, i. 485 - Stahl, i. 16 - Stas, i. 7, 233, 379, 428, 498, 581; ii. 420, 434, 485 - Staudenmaier, ii. 168 - Stcherbakoff, i. 97, 428, 458, 601 - Stohmann, i. 359, 360, 396 - Stokes, i. 355 - Stortenbeker, i. 511 - Stromeyer, ii. 47 - Struvé, i. 208, 612 - - Tait, i. 203 - Tammann, i. 91, 148; ii. 170, 247 - Tanatar, i. 511 - Tchitchérin, ii. 21 - Terreil, ii. 313 - Than, i. 317 - Thénard, i. 207, 229, 460, 464, 534, 539; ii. 251 - Thillot, ii. 170 - Thilorier, i. 385 - Thomsen, i. 111, 120, 124, 131, 173, 189, 267, 359, 389, 396, 441, 453, - 466, 472, 494, 502, 515, 529, 555, 582; ii. 9, 32, 50, 55, 105, 165, - 208, 224, 264, 368, 370, 438, 442 - Thorpe, i. 142, 285, 445, 493; ii. 27, 160, 173, 213, 259, 263, 268, - 301, 313, 442, 486 - Thoune, i. 294, 295 - Tiemman, i. 213 - Tilden, i. 516 - Timeraséeff, i. 170 - Timoféeff, i. 78 - Tessié du Motay, i. 158 - Tissandier, i. 78 - Titherley, i. 539 - Tivoli, ii. 183 - Tomassi, ii. 339 - Topsöe, i. 506 - Tourbaba, i. 88; ii. 247 - Trapp, i. 511 - Traubé, i. 312, 611; ii. 270 - Troost, i. 64, 274, 281, 320, 409, 414, 500, 538; ii. 80, 83, 102, 147, - 156, 254, 379 - Tscherbacheff, i. 577 - Tutton, i. 543; ii. 160, 174, 412 - - Umoff, i. 429 - Unverdorben, ii. 280 - Urlaub, ii. 301 - - Valentine, i. 17 - Van der Heyd, i. 599 - Van der Plaats, i. 496; ii. 438 - Van der Waals, i. 82, 140 - Van Deventer, i. 599 - Van Helmont, i. 379 - Van Marum, i. 198 - Van't Hoff, i. 64, 65, 331, 599; ii. 3 - Vare, ii. 55 - Vauquelin, i. 114, 619; ii. 7 - Veeren, i. 612; ii. 45 - Veley, i. 279 - Verneuille, ii. 225 - Vernon, ii. 151 - Vèzes, ii. 391 - Viard, ii. 285 - Vignon, ii. 126, 131 - Villard, i. 106, 296, 297 - Villiers, ii. 259 - Violette, i. 342, 345 - Violle, i. 301 - Vogt, i. 611 - Volkovitch, ii. 201 - Voskresensky, i. 345 - - Waage, i. 439 - Wachter, i. 508 - Wagner, i. 357 - Wahl, ii. 310 - Walden, ii. 57 - Walker, ii. 143 - Walmer, i. 573 - Walter, ii. 256 - Walters, ii. 234 - Wanklyn, i. 100, 539 - Warburg, i. 589; ii. 496 - Warder, i. 450 - Warren, ii. 102 - Watson, i. 527; ii. 169 - Watts, i. 526 - Weber, i. 280, 583; ii. 83, 129, 131, 186, 230, 233, 234, 249 - Weith, i. 502 - Weitz, ii. 57 - Welch, ii. 425 - Weller, ii. 146 - Wells, i. 477, 545; ii. 57, 370 - Welsbach, ii. 96, 97 - Weltzien, i. 204, 595 - Wenzel, i. 193 - Weruboff (_see_ Wyruboff), ii. 4 - Weselski, i. 507 - Weyl, i. 255 - Wheeler, i. 545 - Wichelhaus, ii. 179 - Wiedemann, i. 439, 588 - Wilhelmj, ii. 315 - Willgerodt, i. 508; ii. 29 - Williamson, ii. 268 - Wilm, ii. 376, 388 - Winkler, i. 78, 79, 577, 594, 621; ii. 25, 30, 66, 97, 102, 124, 125, - 147, 234, 246, 355, 483 - Wischin, ii. 384 - Wislicenus, i. 267, 294 - Witt, ii. 3 - Wöhler, i. 410, 619; ii. 85, 103, 107, 146, 285, 289, 420, 425 - Wollaston, i. 8 - Wreden, i. 507 - Wright, ii. 321 - Wroblewski, i. 79, 80, 106, 139, 387; ii. 226 - Wülfing, ii. 119 - Wülner, i. 91, 572 - Würtz, i. 301, 476; ii. 171, 173, 213, 267 - Wyruboff, ii. 4, 9 - - Young, i. 134, 136, 140, 141, 247, 495, 496 - - Zaboudsky, i. 354 - Zaencheffsky, i. 140 - Zimmermann, ii. 26, 303, 355, 485 - Zinin, i. 276 - Zörensen, i. 284 - Zorn, i. 295 - - * * * * * - - - - - SUBJECT INDEX - - - Acid, acetic sp. gr. of solutions of, i. 59 - -- arsenic, ii. 181 - -- bismuthic, ii. 190 - -- boric, ii. 64 - -- carbamic, i. 408 - -- chamber, i. 294 - -- chloric, i. 482 - -- chloroplatino-phosphorous, ii. 390 - -- chlorosulphonic, ii. 268 - -- chlorous, i. 481 - -- chromic, i. 208; ii. 282 - -- chromo-sulphuric, ii. 288 - -- cyanic, i. 409 - -- cyanuric, i. 409 - -- dithionic, ii. 256 - -- ferric, ii. 344 - -- fluoboric, ii. 69 - -- graphitic, i. 351 - -- hydriodic, i. 501, 503, 505, 506 - -- hydroborofluoric, ii. 69 - -- hydrobromic, i. 80, 503, 505, 506 - -- hydrochloric, i. 448, 451, 453 - -- hydrocyanic, i. 406, 411 - -- hydroferrocyanic, ii. 348 - -- hydrofluoric, i. 49 - -- hydrofluosilic, ii. 106 - -- hydroplatinocyanic, ii. 386 - -- hydrosulphurous, ii. 228 - -- hydroruthenocyanic, ii. 388 - -- hypochlorous, i. 479, 481 - -- hyponitrous, i. 265, 294 - -- hypophosphoric, ii. 161 - -- hypophosphorous, ii. 172 - -- iodic, i. 100, 508 - -- isethionic, ii. 250 - -- metantimonic, ii. 188 - -- metaphosphoric, ii. 162, 169 - -- metastannic, ii. 131 - -- molybdic, ii. 292 - -- nitric, i. 268, 272 - -- Nordhausen, ii. 233 - -- orthophosphoric, ii. 162 - -- osmic, ii. 384 - -- pentathionic, ii. 257 - -- percarbonic, i. 394 - -- perchloric, i. 484 - -- periodic, i. 510 - -- permanganic, ii. 313 - -- permolybdic, ii. 297 - -- pernitric, i. 264 - -- persulphuric, ii. 251 - -- pertungstic, ii. 297 - -- phosphamic, ii. 179 - -- phosphamolybdic, ii. 293 - -- phosphorous, ii. 171 - -- polysilicic, ii. 117 - -- pyrophosphoric, ii. 169 - -- pyrosulphuric, ii. 234 - -- silenic, ii. 272 - -- silicotungstic, ii. 295 - -- stannic, ii. 130 - -- sulphonic, ii. 249 - -- sulphuric, i. 76, 77, 89, 111, 290, 294; ii. 235, 238, 241 - -- telluric, ii. 272 - -- tetrathionic, ii. 257 - -- thiocarbonic, ii. 263 - -- thiocyanic, ii. 263 - -- thionic, ii. 255 - -- thiosulphuric, ii. 230 - -- trithionic, ii. 257 - -- tungstic, ii. 292, 294 - -- vanadic, ii. 196 - Acids, i. 185 - -- avidity of, i. 389, 442 - -- basicity of, i. 387 - -- complex, i. 197; ii. 293 - -- fuming, i. 102 - -- organic, i. 394, 396, 405 - Acetylene, i. 372 - Actinium, ii. 59 - Affinity, chemical, i. 26, 389 - Air, i. 131, 231, 233 - Alchemy, i. 14 - Alcohol, i. 53, 88 - Alkali, metals, i. 558, 577 - -- waste, ii. 204 - Alkalis, i. 186 - Allotropism, i. 207 - Alloys, i. 537; ii. 128 - Alumina, ii. 75 - Aluminium, ii. 70, 85 - -- bromide, ii. 84 - -- bronze, ii. 88 - -- carbide, ii. 88 - -- chloride, ii. 80, 83 - -- double chlorides, ii. 84 - -- fluoride, ii. 83 - -- hydroxide, ii. 75 - -- iodide, ii. 85 - -- nitrate, ii. 80 - -- sulphate, ii. 82 - Alums, ii. 5, 82, 343 - Alunite, ii. 80 - Amalgams, ii. 58 - Amides, i. 258, 406 - Amidogen, i. 258 - -- hydrate, i. 258 - Amines, i. 416 - Ammonia, i. 229, 246 - -- of crystallisation, i. 257 - -- heat of solution of, i. 74 - -- in air, i. 240 - -- liquefaction of, i. 250 - -- salts, i. 254 - -- soda process, i. 524 - -- solutions of, i. 80, 252 - Ammonium, i. 254 - -- amalgam, i. 255 - -- bicarbonate, i. 527 - -- carbamate, i. 407, 408 - -- carbonate, i. 407 - -- cobalt salts, ii. 359 - -- dichromate, ii. 279 - -- molybdate, ii. 292 - -- nitrate, i. 273, 274 - -- nitrite, i. 284 - -- phosphates, ii. 167 - -- sulphate, ii. 269 - -- sulphide, ii. 218 - Analogy of elements, i. 573, 578 - Anthracite, i. 345 - Antimoniuretted hydrogen, ii. 189 - Antimony, ii. 186 - -- chlorides, ii. 189 - -- oxides, ii. 187, 188 - -- sulphides, ii. 221 - Aqua Regia, i. 467 - Aqueous radicle, i. 213 - Argon, i. 226, 232; App. III. - Arsenic, ii. 179 - -- anhydride, ii. 181 - -- sulphides, ii. 221 - -- tribromide, ii. 181 - -- trichloride, ii. 180 - -- trifluoride, ii. 181 - Arsenious anhydride, ii. 184 - -- oxychloride, ii. 180 - Arsenites, ii. 185 - Arseniuretted hydrogen, ii. 182 - Astrakhanite, i. 59 - Atmolysis, i. 156 - Atomic theory, i. 216 - -- volumes, ii. 33 - -- weights, i. 21 - Atoms and molecules, i. 322 - - Barium, i. 614, 617 - -- chlorate, i. 483 - -- chloride, i. 615 - -- hydroxide, i. 616 - -- metatungstate, ii. 295 - -- nitrate, i. 615 - -- oxide, i. 616 - -- peroxide, i. 157, 160, 209, 617 - -- sulphate, i. 614, 615 - Bauxite, ii. 76 - Benzalazine, i. 258 - Berthollet's doctrine, i. 433 - Beryllium, i. 618 - -- atomic weight of, i. 325, 618 - -- chloride, i. 584 - -- oxide, i. 619 - Binary theory, i. 195 - Bismuth, ii. 189 - -- nitrates, ii. 192 - -- oxides, ii. 190, 191 - Blast furnace, ii. 324 - Bleaching, i. 469 - -- powder, i. 162, 477 - Boiling point, absolute, i. 130 - Borates, ii. 63 - Borax, ii. 61 - Boric anhydride, ii. 64 - Boron, ii. 60, 66 - -- chloride, ii. 69 - -- fluoride, ii. 67, 68 - -- iodide, ii. 70 - -- nitride, i. 227; ii. 67 - -- oxide, ii. 60 - -- specific heat of, i. 585 - -- sulphide, ii. 62 - Bromides, ii. 32 - Bromine, i. 494 - Bronze, ii. 127 - Butyl alcohol, solubility of, i. 75 - - Cadmium, ii. 47 - -- iodide, ii. 48 - -- oxide, ii. 48 - -- sulphide, ii. 47 - Cæsium, i. 576 - Calcium, i. 590, 604 - -- carbonate, i. 592, 608, 609, 610 - -- chloride, i. 237, 612 - -- -- crystallohydrates of, i. 613 - -- fluoride, i. 491 - -- hypochlorite, i. 162 - -- iodide, i. 604 - -- peroxide, i. 607 - -- phosphate, ii. 167 - -- sulphate, i. 611 - -- sulphide, ii. 220 - Calomel, ii. 54 - Carbamide, i. 409 - Carbides, i. 349, 353 - Carbon, i. 338 - -- bisulphide, ii. 258 - -- molecule of, i. 354 - -- oxysulphide, ii. 264 - -- tetrachloride, i. 473 - Carbonic anhydride, i. 379 - -- -- assimilation of by plants, i. 393 - -- -- dissociation of, i. 392, 393, 399 - -- -- in air, i. 238, 242 - -- -- liquid, i. 385 - -- -- solutions of, i. 80, 86 - -- -- specific heat of, i. 393 - Carbonic oxide, i. 396 - -- -- and nickel, i. 405 - Carborundum, ii. 107 - Carboxyl, i. 395 - Carnallite, i. 421, 544, 560 - Catalytic phenomena, i. 211 - Caustic potash, i. 550 - -- soda, i. 529 - Cements, ii. 122 - Cerite metals, ii. 93 - Cerium, ii. 93 - Chamber crystals, i. 290; ii. 230 - Charcoal, i. 343 - Chemical change, rate of, ii. 314 - -- transformations, i. 3 - Chloranhydrides, i. 468; ii. 174, 175, 177 - Chlorates, i. 482 - Chlorides, i. 455, 466; ii. 31 - Chlorine, i. 463 - -- compounds, heat of formation of, i. 44 - -- crystallohydrates of, i. 464 - -- oxides, i. 479 - -- preparation of, i. 460 - -- solubility of, i. 463 - Chloroform, i. 473 - Chlorophosphamide, ii. 179 - Chloryl compounds, i. 476 - Chrome alum, ii. 283 - Chromic acid, i. 208 - -- anhydride, ii. 280 - -- oxide, ii. 284, 285 - Chromium, ii. 276, 289 - -- chlorides, ii. 285 - -- fluorides, ii. 280, 289 - Chromyl chloride, ii. 281 - Chryseone, ii. 108 - Clay, ii. 70 - Coal, i. 345 - Cobalt, ii. 353 - -- dioxide, ii. 366 - -- fluoride, ii. 358 - Cobaltamine salts, ii. 359 - Cobaltic oxide, ii. 362 - Cobalto-amine, ii. 359 - Cobaltous hydroxide, ii. 358 - Cohesion of liquids, i. 52 - Coke, i. 345 - Collodion cotton, i. 275 - Colloids, i. 63; ii. 77, 423 - Combination, chemical, i. 3 - Combining weights, i. 21; ii. 439 - Combustion, imperfect, i. 341 - -- heat of, i. 172, 176, 399, 400 - Compounds, definite and indefinite, i. 31 - -- types of, ii. 10 - Compressibility of solutions, i. 88 - Conductivity, electro-molecular, i. 389 - Contact reactions, i. 163, 290 - Copper, ii. 400 - -- carbonate, ii. 411 - -- complex salts of, ii. 412 - -- nitrate, ii. 411 - -- nitride, ii. 409 - -- sulphate, ii. 413 - Corundum, ii. 75 - Critical points, i. 141 - Cryohydrates, i. 99 - Cryoscopic investigations of solutions, i. 90, 332 - Crystals, i. 51 - Crystalline form, ii. 7 - Crystallo-hydrates, i. 102 - Crystalloids, i. 63 - Cupellation, ii. 417 - Cyanides, i. 406 - Cyanogen, i. 406, 414 - -- chloride, ii. 176 - - Decomposition, chemical, i. 4 - Deliquescence, i. 104 - Delta metal, ii. 414 - Desiccator, i. 58 - Detonating gas, i. 115, 170, 173 - Depression of freezing point of solutions, i. 90, 92, 330 - Dialysis, i. 63; ii. 114 - Diamond, i. 350, 353 - Didymium, ii. 93 - Diffusion, rate of, i. 63 - Dimorphism, i. 610, ii. 178 - Disinfectants, i. 245 - Disodium orthophosphate, ii. 166 - Dissociation, i. 36, 282, 608 - Distillation, dry, i. 4, 247, 342 - Dust, atmospheric, i. 241 - - Efflorescence, i. 103 - Ekacadmium, ii. 59 - Ekasilicon, ii. 25 - Electro-chemical theory, i. 195 - Electric energy and thermal units, i. 582 - Electrolysis, i. 116 - Elements, i. 20 - -- grouping of, ii. 1 - -- typical, ii. 19 - Emulsions, i. 98 - Energy, chemical, i. 29 - Equations, chemical, i. 278 - Equivalents, law of, i. 194 - Equivalent weights, i. 581 - Ethane, i. 366 - Ether, critical points of, i. 141 - Ethylene, i. 370 - Ethyl silicates, i. 104 - Euchlorine, i. 484 - Eudiometer, i. 169 - Expansion, linear, of elements, ii. 31 - Explosion, rate of transmission of, i. 171 - Explosives, i. 275, 276 - - Felspar, ii. 122 - Fermentation, i. 242 - Ferric chloride, i. 558; ii. 340 - -- hydrates, ii. 339 - -- nitrate, ii. 340 - -- orthophosphate, ii. 342 - -- oxide, ii. 339 - Ferrous chloride, ii. 335 - -- sulphate, ii. 335 - -- -- solubility of, i. 72 - -- sulphide, ii. 210 - Flame, i. 177, 179 - Fluoborates, ii. 69 - Fluorides, i. 491, 493 - Fluorine, i. 203, 489 - Fluorspar, i. 491 - Formula, chemical, i. 151, 326 - Freezing mixtures, i. 76 - Fuel, calorific capacity of, i. 360 - Furnace, electrical, i. 352 - Fusco-cobaltic salts, ii. 360 - - Gadolinite metals, ii. 93 - Gallium, ii. 88, 90 - Gas, illuminating, i. 361 - -- producers, i. 397 - Gases, absorption of, i. 348 - -- diffusion of, i. 83 - -- expansion of, i. 133 - -- liquefaction of, i. 134, 135, 137 - -- measurement of, i. 78, 300 - -- solution of, i. 68, 78, 86 - -- theory of, i. 81, 83, 140 - Germanium, ii. 26, 124 - -- chloride, ii. 125 - -- oxide, ii. 125 - Glass, i. 123 - -- soluble, ii. 110 - Glauber's salt, i. 517 - Glycols, ii. 117 - Gold, ii. 442 - -- alloys, i. 446, 447 - -- chlorides, ii. 448, 450 - -- colloid, ii. 447 - -- cyanide, ii. 450 - -- extraction of, ii. 444, 445 - -- fulminating, ii. 450 - -- oxides, ii. 448 - -- refining, ii. 446 - Graduators, i. 424 - Graphite, i. 350, 351 - Gros' salt, ii. 393 - Guignet's green, ii. 285 - Gunpowder, i. 557 - Gypsum, i. 593, 611 - - Halogens, i. 445, 487, 499 - Halogen compounds, heat of formation of, i. 494, 502; ii. 32 - -- -- boiling-points of, i. 502 - Hausmannite, ii. 10 - Helium, i. 570; ii. 498 - Hemimorphism, ii. 9 - Homeomorphism, ii. 8 - Homologous compounds, i. 358 - Humus, i. 344 - Hydrates, i. 109, 185 - Hydrazine, i. 258 - Hydrides, i. 621; ii. 23 - Hydrocarbons, i. 355, 359 - Hydrogen, i. 123, 129, 130, 142, 143, 146 - -- pentasulphide, ii. 217 - -- peroxide, i. 207, 312 - Hydrosols, i. 98 - Hydroxyl, i. 192, 213 - Hydroxylamine, i. 262 - Hypochlorites, i. 481 - Hyponitrites, i. 294 - - Imides, i. 258 - Indium, ii. 27, 37, 88, 97 - Iodates, i. 509 - Iodides, ii. 32 - -- of nitrogen, i. 507 - Iodine, i. 320, 321, 496, 497, 498 - -- chlorides of, i. 511 - Iodosobenzol, i. 508 - Iridious oxide, ii. 382 - Iridium, ii. 382 - Iron, i. 585; ii. 317, 322 - -- and carbonic oxide, ii. 345 - -- cast, ii. 325 - -- nitride, ii. 346 - -- ores, ii. 319 - -- sulphate, ii. 335 - Isethionic acid, ii. 250 - Isomorphism, i. 203, 368; ii. 1, 4, 8 - - Kaolin, ii. 70 - - Lakes, ii. 77 - Lanthanum, ii. 93 - Laughing gas, ii. 297 - Law of Avogadro-Gerhardt, i. 309 - -- -- Berthollet, i. 445 - -- -- Boyle and Mariotte, i. 132 - -- -- combining weights, i. 221 - -- -- Dulong and Petit, i. 584 - -- -- equivalents, i. 194 - -- -- even numbers, i. 357 - -- -- Gay Lussac, i. 133, 304, 307 - -- -- Guldberg and Waage, i. 441 - -- -- Henry and Dalton, i. 78 - -- -- indestructibility of matter, i. 6 - -- -- Kirchoff, i. 568 - -- -- limits, i. 357 - -- -- maximum work, i. 120 - -- -- multiple proportions, i. 109, 214 - -- -- partial pressures, i. 82 - -- -- periodic, ii. 17 - -- -- phases, ii. 410 - -- -- reversed spectra, i. 568 - -- -- specific heats, i. 584 - -- -- substitution, i. 260, 365 - -- -- volumes, i. 304 - Lead, ii. 134 - -- acetate, ii. 137 - -- carbonate, ii. 140 - -- chloride, ii. 139 - -- chromate, ii. 136, 279 - -- dioxide, ii. 142 - -- nitrate, ii. 139 - -- oxide, ii. 137 - -- red, ii. 142 - -- salts of, i. 491 - -- tetrachloride, ii. 144 - -- tetrafluoride, ii. 144 - -- white, ii. 140 - Leucone, ii. 107 - Levigation, ii. 72 - Light, chemical action of, i. 465 - Lime, i. 605 - Liquids, boiling points of, i. 135 - Lithium, i. 574 - -- carbonate, i. 575 - Litharge, ii. 137 - Litmus, i. 185 - Lixiviation, methodical, i. 521 - Luteocobaltic salts, ii. 359 - - Magnus' salt, ii. 392 - Magnesia, i. 597 - Magnesium, i. 590, 594 - -- carbonate, i. 592, 602 - -- chloride, i. 602 - -- crystallohydrates of, i. 601 - -- double salts of, i. 597 - -- nitride, i. 595 - -- silicide, ii. 102 - -- sulphate, i. 600 - Manganese, ii. 303 - -- nitrides, ii. 310 - -- oxides, ii. 306, 307, 308, 313 - -- peroxide, i. 159; ii. 305 - -- sulphate, ii. 307 - Mass, influence of, i. 32, 436 - Matches, ii. 154 - Matter, primary, ii. 440 - -- transmutability of, i. 14 - Mercury, ii. 48 - -- ammonia compounds, ii. 57 - -- basic salts of, ii. 54 - -- chlorides, ii. 52, 53, 54 - -- compounds, heat of formation, ii. 50 - -- cyanide, ii. 55 - -- fulminating, ii. 56 - -- iodide, ii. 55 - -- nitrates, ii. 51 - -- nitrides, ii. 56 - -- oxides, ii. 53 - -- sulphate, ii. 57 - -- sulphides, ii. 221 - Metalepsis, i. 28, 471 - Metalloids, i. 23 - Metals, i. 23 - -- of alkaline earths, i. 64, 590, 591 - -- of alkalis, i. 543 - -- displacement of, ii. 427 - Methane, i. 360 - Moisture, determination of, in gases, i. 40 - -- influence upon reaction, i. 403 - Molecular volumes, ii. 37 - -- weight and boiling point, i. 331 - -- -- -- coefficient of refraction, i. 336 - -- -- -- latent heat, i. 329 - -- -- -- specific gravity of solutions, i. 335 - -- -- -- surface tension, i. 334 - Molecules, i. 319, 322 - Molybdates, ii. 292 - Molybdenum, ii. 290 - -- anhydride, ii. 291 - -- fluo-compounds, ii. 298 - -- sulphides, ii. 297 - Monophosphamide, ii. 178 - Monosodium orthophosphate, ii. 167 - Morphotropy, ii. 10 - - Naphtha, i. 373, 377 - Nascent state, i. 33, 145, 146 - Neodymium, ii. 97 - Nickel, ii. 353 - -- alloys, ii. 367 - -- and carbonic oxide, ii. 367 - -- fluoride, ii. 358 - -- hydroxide, ii. 358 - -- oxide, ii. 365 - -- sulphate, i. 97; ii. 359 - -- tetra-carboxyl, ii. 367 - Niobium, i. 199; ii. 194, 198 - Nitrates, i. 273 - Nitres, i. 268, 555 - Nitric anhydride, i. 280 - -- oxide, i. 286 - Nitrides, i. 227, 258, 620 - Nitriles, i. 406 - Nitrites, i. 284 - Nitro-cellulose, i. 275 - Nitro-compounds, i. 274 - Nitrogen, i. 223, 225, 475 - -- chloride, i. 476 - -- iodide, i. 507 - -- oxides of, i. 267, 280, 284, 294, 295 - -- sulphide, ii. 270 - Nitro-prussides, ii. 351 - Nitroso-compounds, i. 288 - Nitrosulphates, ii. 229 - Nitrosyl chloride, ii. 176 - Norwegium, ii. 59 - - Occlusion, i. 143 - Olefiant gas, i. 370 - Organo-metallic compounds, i. 356 - Osmium, ii. 372, 382, 384 - Osmotic pressure, i. 64 - Osmuridium, ii. 383 - Oxamide, i. 406 - Oxidation, i. 16 - Oxides, i. 183; ii. 36 - Oxycobaltamine salts, ii. 359 - Oxygen, i. 152, 157, 158, 163 - -- compounds, heat of formation of, i. 120, 466 - Ozone, i. 198, 229 - - Palladium, ii. 369 - -- hydride, i. 143; ii. 380 - Palladous chloride, ii. 379 - -- iodide, ii. 379 - Paracyanogen, i. 414 - Paramorphism, ii. 9 - Parasulphatammon, ii. 269 - Peat, i. 344 - Peligot's salt, ii. 281 - Percentage composition, i. 326 - Perchloric anhydride, ii. 282 - Periodates, i. 510 - Permanganic anhydride, ii. 313 - Permolybdates, ii. 297 - Peroxide, chloric, i. 484 - Peroxides, i. 159; ii. 15, 23 - Perstannic oxide, ii. 133 - Persulphates, ii. 253 - Petroleum, i. 373 - Phenol, solubility of, i. 75 - Phlogiston, i. 17 - Phosgene gas, ii. 175 - Phospham, ii. 178 - Phosphides, ii. 157 - Phosphine, ii. 158, 160 - Phosphonium iodide, ii. 159 - Phosphoric anhydride, ii. 161 - Phosphorous anhydride, ii. 160 - Phosphorus, ii. 149 - -- ammonium compounds, ii. 178 - -- chlorides, ii. 174 - -- fluorides, ii. 173 - -- iodides, i. 505, 506; ii. 172 - -- oxychlorides, ii. 175 - -- sulphides, ii. 213 - -- sulphochloride, ii. 213 - -- thermo-chemical data for, ii. 153 - Phosphuretted hydrogen, ii. 158, 160 - Photography, ii. 431 - Photo-salts, ii. 432 - Plants, chemical reactions in, i. 547 - -- and nitrogen, i. 230 - Platinic chloride, ii. 377 - -- hydroxide, ii. 379 - Platino-ammonium compounds, ii. 391 - -- chlorides, i. 467; ii. 378 - -- cyanides, ii. 386 - -- nitrites, ii. 390 - -- sulphites, ii. 390 - Platinous chloride, ii. 379 - Platinum, ii. 376 - -- alloys, ii. 373 - -- black, ii. 376 - -- metals, ii. 369, 375 - -- oxide, ii. 378 - Poly-haloid salts, i. 545 - Polymerism, i. 207, 367 - Polysulphides, ii. 217 - Potassium, i. 544, 558 - -- aurate, ii. 449 - -- bromide, i. 550 - -- carbonate, i. 549 - -- chlorate, i. 161, 482 - -- chloride, i. 72, 543 - -- chromate, ii. 280 - -- cyanide, i. 412, 551 - -- dichromate, ii. 278 - -- ferricyanide, ii. 346 - -- ferrocyanide, i. 346, 412 - -- hydrosulphide, ii. 219 - -- hydroxide, i. 548 - -- iodide, i. 550 - -- manganate, ii. 310 - -- nitrate, i. 553 - -- oxides, i. 559 - -- permanganate, ii. 311 - -- stannate, ii. 133 - -- sulphate, i. 72, 549 - -- sulphide, ii. 219 - -- telluride, ii. 274 - Praseocobaltic salts, ii. 361 - Praseodidymium, ii. 97 - Proteid substances, i. 224 - Prout's hypothesis, ii. 439 - Prussian blue, i. 419; ii. 349 - Purpureocobaltic salts, ii. 361 - Purpureotetramine salts, ii. 361 - Pyrocollodion, i. 275 - Pyronaphtha, i. 375 - Pyrosulphuryl chloride, i. 321; ii. 235 - - Reactions, chemical, i. 3 - -- -- conditions for, i. 34 - -- -- contact, i. 39 - -- -- endothermal, i. 30 - -- -- exothermal, i. 30 - -- -- limit of, i. 437 - -- -- rate of, ii. 152 - Recalescence, ii. 333 - Reduction, i. 16 - Refraction equivalent, i. 336 - Regenerative furnaces, i. 398 - Reiset's salts, ii. 394 - Respiration, i. 152, 154, 387 - Rhodium, ii. 381 - Rock salt, i. 421 - Roseocobaltic salts, ii. 360 - Rosetetramine salts, ii. 361 - Rubidium, i. 576 - Ruthenium, ii. 372, 382, 384 - - Sal-ammoniac, i. 248, 318, 457 - -- solubility of, i. 458 - -- vapour density of, i. 317 - Salts, i. 187, 419 - -- acid, i. 193, 533 - -- basic, i. 193, 533; ii. 54 - -- double, i. 598 - -- electrolysis of, i. 191 - -- heat of formation, i. 189 - -- melting points of, i. 135 - -- pyro, i. 193 - -- theory of, i. 193 - Saponification, i. 530 - Scandium, ii. 94 - Selenium, ii. 273 - -- chlorides, ii. 275 - Selenious anhydride, ii. 271 - Silica, i. 100; ii. 108 - -- soluble, ii. 113 - Silicates, i. 544; ii. 116 - Silicon, ii. 99 - -- chloride, ii. 103, 104 - -- chloroform, ii. 103 - -- bromide, ii. 104 - -- fluoride, ii. 105 - -- hydride, ii. 102, 103 - -- iodide, ii. 105 - -- iodoform, ii. 105 - Silver, ii. 418 - -- allotropic varieties of, ii. 421 - -- bromide, ii. 429 - -- chlorate, ii. 437 - -- chloride, ii. 429 - -- cyanide, ii. 433 - -- fluoride, ii. 430 - -- fulminating, ii. 426 - -- hyponitrite, i. 294 - -- iodide, ii. 429 - -- nitrate, ii. 426 - -- nitrite, i. 284 - -- orthophosphate, ii. 164 - -- oxides, ii. 424 - -- peroxide, ii. 422 - -- plating, ii. 434 - -- soluble, ii. 420 - -- subchloride, ii. 432 - Slags, ii. 323 - Smalt, ii. 354 - Soaps, i. 531 - Soda ash, i. 519 - -- caustic, i. 527 - -- manufacture of, i. 459 - -- waste, i. 522 - Soda lime, i. 237 - Sodamide, i. 539 - Sodium, i. 513, 533 - -- alloys, i. 559 - -- amalgams, i. 537 - -- bicarbonate, i. 526 - -- carbonate, i. 519, 525 - -- -- crystallohydrates of, i. 108 - -- -- manufacture of, i. 523 - -- -- solutions of, i. 525 - -- chloride, i. 419 - -- -- double salts of, i. 430 - -- -- solutions of, i. 88, 99, 429 - -- hydride, i. 537 - -- hydroxide, i. 528, 529 - -- -- solutions of, i. 529 - -- nitrate, i. 269 - -- -- solutions of, i. 72 - -- organo compounds of, i. 540 - -- oxides, i. 540, 541 - -- phosphates, ii. 166 - -- platinate, ii. 378 - -- pyrosulphate, i. 518 - -- sesquicarbonate, i. 526 - -- stannate, ii. 133 - -- subchloride, i. 540 - -- sulphate, i. 513 - -- -- acid salt, i. 518 - -- -- crystallohydrates of, i. 515 - -- -- solutions of, i. 73, 515, 516 - -- sulphite, ii. 226 - -- thiosulphate, ii. 230 - -- -- solutions of, i. 74 - -- tungstate, ii. 294 - Soils, i. 344; ii. 73 - Solubility, coefficient of, i. 67, 71 - Solutions, i. 330 - -- aqueous, i. 59 - -- boiling points of, i. 94, 100 - -- crystallisation of, i. 427 - -- colour of, i. 95 - -- diffusion of, i. 61, 429 - -- of double salts, i. 599 - -- formation of ice from, i. 91, 428 - -- heat of formation of, i. 74, 75, 76 - -- of gases, i. 68 - -- isotonic, i. 64 - -- saturated, i. 65 - -- specific gravity of, i. 429, 584 - -- supersaturated, i. 96 - -- theory of, i. 64, 89, 92, 97, 106, 215, 323, 608; ii. 3, 164 - -- vapour tension of, i. 90, 92 - -- volumes of, i. 87 - -- Specific heat, i. 585, 586, 588 - Spectra absorption, i. 566 - Spectrum analysis, i. 560, 561 - Stannic chloride, ii. 132 - -- fluoride, ii. 132 - -- oxide, ii. 130 - -- sulphide, ii. 132 - Stannous chloride, ii. 130 - -- oxide, ii. 129 - -- salts, ii. 129 - Steam, vapour tension of, i. 54 - Steel, ii. 327, 328, 330 - Strontium, i. 615 - -- chloride, i. 615 - -- hydroxide, i. 615 - -- nitrate, i. 615 - -- oxide, i. 617 - Substitution chemical, i. 5 - Sulphamide, ii. 270 - Sulphatammon, ii. 269 - Sulphates, ii. 248 - Sulphides, i. 98; ii. 213 - Sulphonitrites, ii. 229 - Sulphoxyl, ii. 250 - Sulphur, ii. 200 - -- chlorides of, ii. 264 - Sulphuretted hydrogen, ii. 208 - Sulphuric anhydride, ii. 232 - -- peroxide, ii. 251 - Sulphurous anhydride, ii. 224 - Sulphuryl chloride, ii. 268 - Superphosphates, ii. 168 - - Tantalum, ii. 194, 198 - Tellurium, ii. 274 - -- bromide, ii. 275 - -- chlorides, ii. 275 - Tellurous anhydride, ii. 271 - Temperature, critical, i. 131 - Test papers, i. 185 - Thallium, ii. 88, 91 - Thallic oxide, ii. 93 - Thallous hydroxide, ii. 92 - -- oxide, ii. 92 - Thiocarbonates, ii. 262 - Thionyl chloride, ii. 267 - Thiophosgene, ii. 262 - Thiophosphoryl fluoride, ii. 263 - Theory, atomic, i. 216 - -- unitary, i. 195 - -- vortex, i. 217 - Thermochemistry, i. 173 - Thorium, ii. 148 - Tin, ii. 125 - -- alloys, ii. 127 - Titanium, ii. 144 - -- chloride, ii. 145 - -- nitride, ii. 146 - -- nitrocyanide, ii. 146 - -- oxides, ii. 145 - Tripoli, ii. 110 - Trisodium orthophosphate, ii. 166 - Tungstates, ii. 292 - Tungsten, ii. 290 - -- anhydride, ii. 291 - -- nitride, ii. 297 - -- sulphide, ii. 297 - Turnbull's blue, ii. 350 - Types of combination, ii. 10 - - Ultramarine, ii. 84 - Uranium, ii. 30, 297 - -- atomic weight of, ii. 26 - -- dioxide, ii. 301 - -- oxides, ii. 298 - -- tetrachloride, ii. 301 - Urano-alkali compounds, ii. 298 - Uranyl, ii. 301 - -- ammonium carbonate, ii. 300 - -- nitrate, ii. 300 - -- phosphate, ii. 300 - Urea, i. 409 - - Valency of elements, i. 404, 418, 581 - Van der Waal's formula, i. 82, 140 - Vanadic anhydride, ii. 196 - Vanadium, ii. 194 - -- oxychloride, ii. 195 - Vapour density, determination of, i. 301 - Ventilation, i. 244 - Viscosity, i. 355 - Volumes, molecular, ii. 4 - -- gases, i. 300 - - Water, i. 40 - -- composition of, i. 114, 118, 148, 169, 305, 333 - -- compressibility of, i. 53 - -- of constitution, i. 109 - -- of crystallisation, i. 95, 510 - -- dissociation of, i. 118 - -- expansion of, i. 53 - -- gas, i. 129, 400, 401 - -- hard, i. 47 - -- hygroscopic, i. 56 - -- mineral, i. 45 - -- rain, i. 43 - -- river, i. 43 - -- sea, i. 46 - -- specific heat of, i. 52 - -- -- gravity of, i. 50 - -- spring, i. 44 - Wave lengths, i. 564 - Wood, i. 339 - - Ytterbium, ii. 93 - Yttrium, ii. 93 - - Zinc, ii. 39 - -- ammonia-chlorides, ii. 41 - -- chloride, ii. 40, 41 - -- compounds, heat of formation of, ii. 51 - -- oxide, ii. 39, 40 - -- sulphate, ii. 39 - Zirconium, ii. 146 - -- chloride, ii. 147 - -- hydroxide, ii. 147 - -- oxide, ii. 147 - - * * * * * - - PRINTED BY - SPOTTISWOODE AND CO., NEW-STREET SQUARE - LONDON - - * * * * * - - - - - A - - CLASSIFIED CATALOGUE - - OF - - SCIENTIFIC WORKS - - PUBLISHED BY - - MESSRS. 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