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+The Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: On Laboratory Arts
+
+Author: Richard Threlfall
+
+Release Date: September 27, 2007 [EBook #22784]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ON LABORATORY ARTS ***
+
+
+
+
+Produced by Jon Richfield
+
+
+
+
+
+
+ON LABORATORY ARTS
+
+
+
+
+
+BY
+
+
+RICHARD THRELFALL, M.A.
+
+
+PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;
+
+MEMBER OF THE INSTITUTE OF ELECTRICAL ENGINEERS;
+
+ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL ENGINEERS;
+
+MEMBER OF THE PHYSICAL SOCIETY
+
+
+
+
+London
+
+
+MACMILLAN AND CO, LIMITED
+
+NEW YORK: THE MACMILLAN COMPANY
+
+
+1898
+
+
+All rights reserved
+
+
+
+
+PREFACE    5
+
+CHAPTER I    8
+
+HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
+PURPOSES    8
+
+§ 4.  Soft Soda Glass.
+
+§ 6.  Flint Glass.
+
+§ 9.  Hard or Bohemian, Glass.
+
+§ 10.  On the Choice of Sizes of Glass Tube.
+
+§ 11.  Testing Glass.
+
+§ 13.  Cleaning Glass Tubes.
+
+§ 14.  The Blow-pipe.
+
+§ 18.  The Table.
+
+§ 19.  Special Operations.
+
+§ 20.  Closing and blowing out the End of a Tube.
+
+§ 21.  To make a Weld.
+
+§ 22.  To weld two Tubes of different Sizes.
+
+§ 24.  To weld Tubes of very small Bore.
+
+§ 30.  To cut very thick Tubes.
+
+§ 31.  To blow a Bulb at the End of a Tube.
+
+§ 32.  To blow a bulb in the middle of a tube.
+
+§ 33.  To make a side Weld.
+
+§ 34.  Inserted Joints.
+
+§ 35.  Bending Tubes.
+
+§ 36.  Spiral Tubes.
+
+§ 37.  On Auxiliary Operations on Glass.
+
+§ 38.  Boring small Holes.
+
+§ 39.  For boring large holes through thick glass sheets.
+
+§ 41.  Operations depending on Grinding: Ground-in Joints.
+
+§ 42.  Use of the Lathe in Glass-working.
+
+§ 46.  Making Ground Glass.
+
+§ 47.  Glass-cutting.
+
+§ 48.  Cementing.
+
+§ 49.  Fusing Electrodes into Glass.
+
+§ 51.  The Art of making Air-light Joints.
+
+APPENDIX TO CHAPTER I.
+
+ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
+ROENTGEN'S RADIATION.
+
+CHAPTER II.
+
+GLASS-GRINDING AND OPTICIANS' WORK.
+
+§ 61.  Details of the Process of Fine Grinding.
+
+§ 62.  Polishing.
+
+§ 63.  Centering.
+
+§ 65.  Preparing Small Mirrors for Galvanometers.
+
+§ 66.  Preparation of Large Mirrors or Lenses for Telescopes.
+
+§ 69.  The Preparation of Flat Surfaces of Rock Salt.
+
+§ 70.  Casting Specula for Mirrors.
+
+§ 71.  Grinding and polishing Specula.
+
+§ 72.  Preparation of Flat Surfaces.
+
+§ 73.  Polishing Flat Surfaces on Glass or on Speculum Metal.
+
+CHAPTER III.
+
+MISCELLANEOUS PROCESSES.
+
+§ 74.  Coating Glass with Aluminium and Soldering Aluminium.
+
+§ 75.  The Use of the Diamond-cutting Wheel.
+
+§ 76.  Arming a Wheel.
+
+§ 77.  Cutting a Section.
+
+§ 78.  Grinding Rock Sections, or Thin Slips of any Hard Material.
+
+§ 79.  Cutting Sections of Soft Substances.
+
+§ 80.  On the Production of Quartz Threads.
+
+§ 84.  Drawing Quartz Threads.
+
+§ 86.  Drawing Threads by the Catapult.
+
+§ 87.  Drawing Threads by the Flame alone.
+
+§ 88.  Properties of Threads.
+
+§ 90.  On the Attachment of Quartz Fibres.
+
+§ 91.  Other Modes of soldering Quartz.
+
+§ 92.  Soldering.
+
+§ 94.  Preparing a Soldering Bit.
+
+§ 95.  Soft Soldering.
+
+§ 97.  Soldering Zinc.
+
+§ 98.  Soldering other Metals.
+
+§ 99.  Brazing.
+
+§ 100.  Silver Soldering.
+
+§ 101.  On the Construction of Electrical Apparatus--Insulators.
+
+§ 102.  Sulphur.
+
+§ 103.  Fused Quartz.
+
+§ 104.  Glass.
+
+§ 105.  Ebonite or Hard Rubber.
+
+§ 106.  Mica.
+
+§ 107.  Use of Mica in Condensers.
+
+§ 108.  Micanite.
+
+§ 109.  Celluloid.
+
+§ 110.  Paper.
+
+§ 111.  Paraffined Paper.
+
+§ 112.  Paraffin.
+
+§ 113.  Vaseline, Vaseline Oil, and Kerosene Oil.
+
+§ 114.  Imperfect Conductors.
+
+§ 116.  Conductors.
+
+§ 117.  Platinoid.
+
+§ 119.  Platinum Silver.
+
+§ 120.  Platinum Iridium.
+
+§ 121.  Manganin.
+
+§ 122.  Other Alloys.
+
+§ 123.  Nickelin.
+
+§ 124.  Patent Nickel.
+
+§ 125.  Constantin.
+
+126.  Nickel Manganese Copper.
+
+CHAPTER IV.
+
+ELECTROPLATING AND ALLIED ARTS.
+
+§ 127.  Electroplating.
+
+§ 128.  The Dipping Bath.
+
+§ 130.  Scratch-brushing.
+
+§ 131. Burnishing.
+
+§ 132.  Silver-plating.
+
+§ 133.  Cold Silvering.
+
+§ 134.  Gilding.
+
+§ 135.  Preparing Surfaces for Gilding.
+
+§ 136.  Gilding Solutions.
+
+§ 137.  Plating with Copper.
+
+§ 138.  Coppering Aluminium.
+
+§ 140.  Alkaline Coppering Solution.
+
+§ 141.  Nickel-plating.
+
+142.  Miscellaneous Notes on Electroplating.
+
+§ 143.  Blacking Brass Surfaces.
+
+§ 144.  Sieves.
+
+§ 145.  Pottery making in the Laboratory.
+
+APPENDIX.
+
+PLATINISING GLASS.
+
+PREFACE
+
+EXPERIMENTAL work in physical science rests ultimately upon the
+mechanical arts.  It is true that in a well-appointed laboratory,
+where apparatus is collected together in greater or less profusion,
+the appeal is often very indirect, and to a student carrying out a set
+experiment with apparatus provided to his hand, the temptation to
+ignore the mechanical basis of his work is often irresistible.
+
+It often happens that young physicists are to be found whose
+mathematical attainments are adequate, whose observational powers are
+perfectly trained, and whose general capacity is unquestioned, but who
+are quite unable to design or construct the simplest apparatus with
+due regard to the facility with which it ought to be constructed.
+That ultimate knowledge of materials and of processes which by long
+experience becomes intuitive in the mind of a great inventor of course
+cannot be acquired from books or from any set course of instruction.
+
+There are, however, many steps between absolute ignorance and
+consummate knowledge of the mechanical arts, and it is the object of
+the following pages to assist the young physicist in making his first
+steps towards acquiring a working knowledge of "laboratory arts."
+However humble the ambition may be, no one can be more keenly alive
+than the writer to the inadequacy of his attempt; and it is only from
+a profound sense of the necessity which exists for some  beginning to
+be made, that he has had the courage to air his views on matters about
+which there are probably hundreds or thousands of people whose
+knowledge is superior to his own.
+
+Moreover, nothing has been further from the writer's mind than any
+idea of "instructing" any one;  his desire is--if happily it may so
+befall--to be of assistance, especially to young physicists or
+inventors who wish to attain definite mechanical ends with the minimum
+expenditure of time.  Most people will agree that one condition
+essential to success in such an undertaking is brevity, and it is for
+this reason that alternative methods as a rule have not been given,
+which, of course, deprives the book of any pretence to being a
+"treatise."  The writer, therefore, is responsible for exercising a
+certain amount of discretion in the selection he has made, and it is
+hardly to be hoped that he has in all--or even in the majority of
+cases--succeeded in recommending absolutely the best method of
+procedure.
+
+This brings another point into view.  Before all things the means
+indicated must be definite and reliable.  It is for this reason that
+the writer has practically confined himself to matters lying within
+his own immediate experience, and has never recommended any process
+(with one or two minor exceptions, which he has noted) which he has
+not actually and personally carried through to a successful issue.
+This, although it is a matter which he considers of the highest
+importance, and which is his only title to a hearing, has
+unfortunately led to a very personal tone in the book.
+
+With regard to the arts treated of in the following pages, matters
+about which information is easily acquired--such as carpentering,
+blacksmithing, turning, and the arts of the watchmaker--have been
+left on one side.  With regard to the last, which is of immense use in
+the laboratory, there happen to be at least two excellent and handy
+books, viz.  Saunier's Watchmakers' Handbook, Tripplin, London, 1892;
+and Britton's Watchmakers' Dictionary and Guide.
+
+With regard to carpentering, turning, and blacksmithing, almost any
+one who so desires can obtain a little practical experience in any
+village.  A short chapter has been devoted to glass-blowing, in spite
+of there being an excellent and handy book by Mr. Shenstone (The
+Methods of Glass-blowing, Rivington) on the subject already in
+existence.  The reason for this exception lies in the fact that the
+writer's methods differ considerably from those advocated by Mr.
+Shenstone.
+
+The chapter on opticians' work has had to be compressed to an extent
+which is undesirable in dealing with so complex and delicate an art,
+but it is hoped that it will prove a sufficient introduction for
+laboratory purposes.  In this matter the writer is under great
+obligations to his friend and assistant, Mr. James Cook, F.R.A.S, who
+gave him his first lessons in lens-making some twenty years ago.  To
+Mr. John A.  Brashear of Allegheny, Pa, thanks are due for much
+miscellaneous information on optical work, which is included verbatim
+in the text, some of it contained originally in printed papers, and
+some most kindly communicated to the writer for the purpose of this
+book.  In particular, the writer would thank Mr. Brashear for his
+generously accorded information as to the production of those "flat"
+surfaces for which he is so justly famous.
+
+The writer is also indebted to Mr. A.  E.  Kennelly for some
+information as to American practice in the use of insulating material
+for electrical work, and to his friends Mr. J.  A.  Pollock and Dr. C.
+J.  Martin for many valuable suggestions.  For the illustrations
+thanks are due to Mrs.  Threlfall and Mr. James Cook.  With regard to
+matters which have come to the writer's knowledge by his being
+specifically instructed in them from time to time, due acknowledgment
+is, it is hoped, made in the text.
+
+With regard to the question as to what matters might be included and
+what omitted, the general rule has been to include information which
+the author has obtained with difficulty, and to leave on one side that
+which he has more easily attained.  All the "unities" have been
+consistently outraged by a deliberate use of the English and metric
+systems side by side.  So long as all the materials for mechanical
+processes have to be purchased to specifications in inches and feet,
+it is impossible to use the centimetre consistently without
+introducing inconvenience.  However, everybody ought to, and probably
+does, use either system with equal facility.
+
+No attempt has been made at showing how work can be done without
+tools.  Though, no doubt, a great deal can be done with inferior
+appliances where great economy of money and none of time is an object,
+the writer has long felt very strongly that English physical
+laboratory practice has gone too far in the direction of starving the
+workshop, and he does not wish, even indirectly, 'to give any
+countenance to such a mistaken policy.  Physical research is too
+difficult in itself, and students' time is too valuable, for it to be
+remunerative to work with insufficient appliances.
+
+In conclusion, the writer would ask his readers to regard the book to
+some extent as tentative, and as a means to the procuring and
+organising of information bearing upon laboratory arts.  Any
+information which can be given will be always thankfully received, and
+the author hereby requests any reader who may happen to learn
+something of value from the book to communicate any special
+information he may possess, so that it may be of use to others should
+another edition ever be called for.
+
+CHAPTER I
+
+HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
+PURPOSES
+
+§ 1.  THE art of glass-blowing has the conspicuous advantage, from the
+point of view of literary presentation, of being to a great extent
+incommunicable.  As in the case of other delightful arts--such as
+those treated of in the Badminton Library, for instance--the most
+that can be done by writing is to indicate suitable methods and to
+point out precautions which experience has shown to be necessary, and
+which are not always obvious when the art is first approached.  It is
+not the object of this work to deal with the art of glass-blowing or
+any other art after the manner befitting a complete treatise, in which
+every form of practice is rightly included.  On the contrary, it is my
+wish to avoid the presentation of alternative methods.
+
+I consider that the presentation of alternative methods would, for my
+present purpose, be a positive disadvantage, for it would swell this
+book to an outrageous size;  and to beginners--I speak from
+experience--too lavish a treatment acts rather by way of obscuring
+the points to be aimed at than as a means of enlightenment.  The
+student often does not know which particular bit of advice to follow,
+and obtains the erroneous idea that great art has to be brought to
+bear to enable him to accomplish what is, after all, most likely a
+perfectly simple and straightforward operation.
+
+This being understood, it might perhaps be expected that I should
+describe nothing but the very best methods for obtaining any proposed
+result.  Such, of course, has been my aim, but it is not likely that I
+have succeeded in every case, or even in the majority of cases, for I
+have confined myself to giving such directions as I know from my own
+personal experience will, if properly carried out, lead to the result
+claimed.  In the few cases in which I have to refer to methods of
+which I have no personal experience, I have endeavoured to give
+references (usually taking the form of an acknowledgment), so that an
+idea of their value may be formed.  All methods not particularised may
+be assumed by the reader to have come within my personal experience.
+
+§ 2.  Returning to glass-blowing, we may note that two forms of
+glass-blowing are known in the arts, "Pot" blowing and  "Table"
+blowing.  In the former case large quantities of fluid "metal"
+(technical term for melted glass) are assumed to be available, and as
+this is seldom the case in the laboratory, and as I have not yet felt
+the want of such a supply, I shall deal only with "table"  blowing.
+Fortunately there is a convenient book on this subject, by Dr.
+Shenstone (Rivingtons), so that what I have to say will be as brief as
+possible, consistent with sufficiency for everyday work.  As a matter
+of fact there is not very much to say, for if ever there was an art in
+which manual dexterity is of the first and last importance, that art
+is glass-working.
+
+I do not think that a man can become an accomplished glass-blower from
+book instructions merely--at all events, not without much unnecessary
+labour,--but he can learn to do a number of simple things which will
+make an enormous difference to him both as regards the progress of his
+work and the state of his pocket.
+
+§ 3.  The first thing is to select the glass.  In general, it will
+suffice to purchase tubes and rods;  in the case where large pieces
+(such as the bulbs of Geissler pumps) have to be specially prepared by
+pot-blowing, the student will have to observe precautions to be
+mentioned later on.  There are three kinds of glass most generally
+employed in laboratories.
+
+§ 4.  Soft Soda Glass, obtained for the most part from factories in
+Thuringia, and generally used in assembling chemical apparatus.--This
+glass is cheap, and easily obtainable from any large firm of apparatus
+dealers or chemists.  It should on no account be purchased from small
+druggists, for the following reasons:-
+
+(a) It is usually absurdly dear when obtained in this way.
+
+(b) It is generally made up of selections of different age and
+different composition, and pieces of different composition, even if
+the difference is slight, will not fuse together and remain together
+unless joined in a special manner.
+
+(c) It is generally old, and this kind of glass often devitrifies with
+age, and is then useless for blowpipe work, though it may be bent
+sufficiently for assembling chemical apparatus.  Devitrified glass
+looks frosty, or, in the earlier stages, appears to be covered by
+cobwebs, and is easily picked out and rejected.
+
+§ 5.  It might be imagined that the devitrification would disappear
+when the glass is heated to the fusing point; and so it does to a
+great extent, but for many operations one only requires to soften the
+glass, and the devitrification often persists up to this temperature.
+My experience is that denitrified glass is also more likely to crack
+in the flame than good new glass, though the difference in this
+respect is not very strongly marked with narrow tubes.
+
+§ 6.  Flint Glass.
+
+Magnificent flint glass is made both in England and France.  The
+English experimenter will probably prefer to use English glass, and,
+if he is wise, will buy a good deal at a time, since it does not
+appear to devitrify with age, and uniformity is thereby more likely to
+be secured.  I have obtained uniformly good results with glass made by
+Messrs.  Powell of Whitefriars, but I daresay equally good glass may
+be obtained elsewhere.
+
+For general purposes flint glass is vastly superior to the soft soda
+mentioned above.  In the first place, it is very much stronger, and
+also less liable to crack when heated--not alone when it is new, but
+also, and especially, after it has been partly worked.  Apparatus made
+of flint glass is less liable to crack and break at places of unequal
+thickness than if made of soda glass.  This is not of much importance
+where small pieces of apparatus only are concerned, because these can
+generally be fairly annealed; and if the work is well done, the
+thickness will not be uneven.  It is a different matter where large
+pieces of apparatus, such as connections to Geissler pumps, are
+concerned, for the glass has often to be worked partly in situ, and
+can only be imperfectly annealed.
+
+Joints made between specimens of different composition are much more
+likely to stand than when fashioned in soda glass.  Indeed, if it is
+necessary to join two bits of soda glass of different kinds, it is
+better to separate them by a short length of flint glass;  they are
+more likely to remain joined to it than to each other.  A particular
+variety of flint glass, known as white enamel, is particularly
+suitable for this purpose, and, indeed, may be used practically as a
+cement.
+
+§ 7, It is, however, when the necessity of altering or repairing
+apparatus complicated by joints arises that the advantage of flint
+glass is most apparent.  A crack anywhere near to a side, or inserted
+joint, can scarcely ever be repaired in the case of soda glass
+apparatus, even when the glass is quite thin and the dimensions small.
+
+It should also be mentioned that flint glass has a much more brilliant
+appearance than soda glass.  Of course, there is a considerable
+difference between different kinds of flint glass as to the melting
+point, and this may account for the divergency of the statements
+usually met with as to its fusibility compared with that of soda
+glass.  The kind of flint glass made by Messrs.  Powell becomes
+distinctly soft soon after it is hot enough to be appreciably luminous
+in a darkened room, and at a white heat is very fluid.  This fluidity,
+though of advantage to the practised worker, is likely to give a
+beginner some trouble.
+
+§ 8.  As against the advantages enumerated, there are some drawbacks.
+The one which will first strike the student is the tendency of the
+glass to become reduced in the flame of the blow-pipe.  This can be
+got over by proper adjustment of the flame, as will be explained later
+on.  A more serious drawback in exact work is the following.  In
+making a joint with lead glass it is quite possible to neglect to fuse
+the glass completely together at every point; in fact, the joint will
+stand perfectly well even if it be left with a hole at one side, a
+thing which is quite impossible with soft soda glass, or is at least
+exceedingly unusual.  An accident of this kind is particularly likely
+to happen if the glass be at all reduced.  Hence, if a joint does not
+crack when cold, the presumption is, in the case of soda glass, that
+the joint is perfectly made, and will not allow of any leak;  but this
+is not the case with flint glass, for which reason all joints between
+flint glass tubes require the most minute examination before they are
+passed.  If there are any air bubbles in the glass, especial care must
+be exercised.
+
+§ 9.  Hard or Bohemian, Glass.
+
+This is, of course, used where high temperatures are to be employed,
+and also in certain cases where its comparative insolubility in water
+is of importance.  It is very unusual for the investigator to have to
+make complicated apparatus from this glass.  Fused joints may be made
+between hard glass and flint glass without using enamel, and though
+they often break in the course of time, still there is no reason
+against their employment, provided the work be done properly, and they
+are not required to last too long.
+
+§ 10.  On the Choice of Sizes of Glass Tube.
+
+It will be found that for general purposes tubes about one-quarter
+inch in inside diameter, and from one-twentieth to one-fortieth of an
+inch thick, are most in demand.  Some very thin soda glass of these
+dimensions (so-called "cylinder" tubes) will be found very handy for
+many purposes.  For physico-chemical work a good supply of tubing,
+from one-half to three-quarters of an inch inside diameter, and from
+one-twentieth to one-eighth inch thick, is very necessary.  A few
+tubes up to three inches diameter, and of various thicknesses, will
+also be required for special purposes.
+
+Thermometer and "barometer" tubing is occasionally required, the
+latter, by the way, making particularly bad barometers.  The
+thermometer tubing should be of all sizes of bore, from the finest
+obtainable up to that which has a bore of about one-sixteenth of an
+inch.  Glass rods varying from about one-twentieth of an inch in
+diameter up to, say, half an inch will be required, also two or three
+sticks of white enamel glass for making joints.
+
+To facilitate choice, there is appended a diagram of sizes from the
+catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and
+the experimenter will be able to see at a glance what sizes of glass
+to order.  It is a good plan to stock the largest and smallest size of
+each material as well as the most useful working sizes.
+
+Fig.  1.
+
+§ 11.  Testing Glass.
+
+"Reject glass which has lumps or knots, is obviously conical, or has
+long drawn-out bubbles running through the substance." If a scratch be
+made on the surface of a glass tube, and one end of the scratch be
+touched by a very fine point of fused glass, say not more than
+one-sixteenth inch in diameter, the tube, however large it is (within
+reason), ought to crack in the direction of the scratch.  If a big
+crack forms and does not run straight, but tends to turn
+longitudinally, it is a sign that the glass is ill annealed, and
+nothing can be done with it.  If such glass be hit upon in the course
+of blow-pipe work, it is inadvisable to waste time upon it; the best
+plan is to reject it at once, and save it for some experiment where it
+will not have to be heated.
+
+The shortest way of selecting glass is to go to a good firm, and let
+it be understood that if the glass proves to be badly annealed it will
+be returned.  Though it was stated above that the glass should not be
+distinctly conical, of course allowance must be made for the length of
+the pieces, and, on the other hand, a few highly conical tubes will be
+of immense service in special cases, and a small supply of such should
+be included.
+
+The glass, as it is obtained, should be placed in a rack, and covered
+by a cloth to reduce the quantity of dust finding its way into the
+tubes.  It has been stated by Professor Ostwald that tubes when reared
+up on end tend to bend permanently.  I have not noticed this with lead
+glass well supported.  Each different supply should be kept by itself
+and carefully described on a label pasted on to the rack, and tubes
+from different lots should not be used for critical welds.  This
+remark is more important in the case of soda than of lead glass.
+
+In the case of very fine thermometer tubes it will be advisable to
+cover the ends with a little melted shellac, or, in special cases, to
+obtain the tubes sealed from the works.  Soda glass can generally be
+got in rather longer lengths than lead glass;  the longer the lengths
+are the better, for the waste is less.
+
+It is useful to be able to distinguish the different kinds of glass by
+the colour.  This is best observed by looking towards a bright surface
+along the whole length of the tube and through the glass.  Lead glass
+is yellow, soda glass is green, and hard glass purple in the samples
+in my laboratory, and I expect this is practically true of most
+samples.  [Footnote: Some new lead glass I have is also almost purple
+in hue.  If any doubt exists as to the kind of glass, it may be tested
+at once in the blow-pipe flame, or by a mixture of oils of different
+refractive indices, as will be explained later.]
+
+§ 12.  The question of the solubility of glass in reagents is one of
+great importance in accurate work, though it does not always meet with
+the attention it deserves.  It is impossible here to go into the
+matter with sufficient detail, and the reader is therefore referred to
+the Abstracts of the Chemical Society, particularly for the years 1889
+and 1892.  The memoir by F.  Kohlrausch, Wied.  Ann.  xliv, should be
+consulted in the original.  The following points may be noted.  A
+method of testing the quality of glass is given by Mylius (C.  S.  J.
+Abstracts, 1889, p.  549), and it is stated that the resistance of
+glass to the action of water can generally be much increased by
+leaving it in contact with cold water for several days, and then
+heating it to 300° to 400° C.  This improvement seems to be due to the
+formation of a layer of moist silica on the surface, and its
+subsequent condensation into a resisting layer by the heating.  Mylius
+(C.  S.  J.  Abstracts, 1892, p.  411), and Weber, and Sauer (C.  S.
+J.  Abstracts, 1892, p.  410) have also shown that the best glass for
+general chemical purposes consists of:
+
+Silica, 7 to 8 parts
+
+Lime, 1 part
+
+Alkali, 1.5 to 1.1 parts.
+
+This is practically "Bohemian" tube glass.
+
+The exact results are given in the Berichte of the German Chemical
+Society, vol.  xxv.  An excellent account of the properties of glass
+will be found in Grove's edition of Miller's Elements of Chemistry.
+
+§ 13.  Cleaning Glass Tubes.
+
+This is one of the most important arts in chemistry.  If the tubes are
+new, they are generally only soiled by dust, and can be cleaned fairly
+easily--first by pushing a bit of cotton waste through with a cane,
+or pulling a rag through with string--and then washing with sand and
+commercial hydrochloric acid.  I have heard of glass becoming
+scratched by this process, and breaking in consequence when heated,
+but have never myself experienced this inconvenience.  In German
+laboratories little bits of bibulous paper are sometimes used instead
+of sand; they soon break into a pulp, and this pulp has a slightly
+scouring action.
+
+As soon as the visible impurities are removed and the tube when washed
+looks bright and clean, it may be wiped on the outside and held
+perpendicularly so as to allow the water film to drain down.  If the
+tube be greasy (and perhaps in other cases) it will be observed that
+as the film gets thinner the water begins to break away and leave dry
+spots.  For accurate work this grease, or whatever it is, must be
+removed;  and after trying many plans for many years, I have come back
+to the method I first employed, viz.  boiling out with aqua regia.
+
+For this purpose, close one end of the tube by a cork (better than a
+rubber bung, because cheaper), and half fill the tube with aqua regia;
+then, having noted the greasy places, proceed to boil the liquid in
+contact with the glass at these points, and in the case of very
+obstinate dirt--such as lingers round a fused joint which has been
+made between undusted tubes--leave the whole affair for twelve hours.
+If the greasiness is only slight, then simply shaking with hot aqua
+regia will often remove it, and the aqua regia is conveniently heated
+in this case by the addition of a little strong sulphuric acid.
+
+The spent aqua regia may be put into a bottle.  It is generally quite
+good enough for the purpose of washing glass vessels with sand, as
+above explained.
+
+However carefully a tube is cleaned before being subjected to blowpipe
+operations, it will be fouled wherever there is an opening during the
+process of heating, unless the extreme tip only of an oxidising flame
+be employed.  Even this should not be trusted too implicitly unless
+an oxygas or hydrogen flame is employed.
+
+When a tube or piece of apparatus has been cleaned by acid, so that on
+clamping it vertically, dry spaces do not appear, it may be rinsed
+with platinum distilled water and left to drain, the dust being, of
+course, kept out by placing a bit of paper round the top.  For
+accurate work water thus prepared is to be preferred to anything else.
+When the glass is very clean interference colours will be noticed as
+the water dries away.
+
+Carefully-purified alcohol may in some cases be employed where it is
+desired to dry the tube or apparatus quickly.  In this case an alcohol
+wash bottle should be used, and a little alcohol squirted into the top
+of the tube all round the circumference.  The water film drags the
+alcohol after it, and by waiting a few minutes and then adding a few
+more drops of alcohol, the water may be practically entirely removed,
+especially if a bit of filter paper be held against the lower end of
+the tube.  It is customary in some laboratories to use ether for a
+final rinse, but unless the ether is freshly distilled and very pure,
+it leaves a distinct organic residue.
+
+When no more liquid can be caused to drain away, the tube may be dried
+by heating it along its length, beginning at the top (to get the
+advantage of the reduction of surface tension), and so on all down.
+It will then be possible to mop up a little more of the rinsing
+liquid.  When the tube is nearly dry a loose plug of cotton wool may
+be inserted at the bottom. The wool must be put in so that the fibres
+lie on an even surface inside the tube, and the wool must be blown
+free from dust.  Ordinary cotton wool is useless, from being dusty and
+the fibres short, and the same remark applies to wadding.  Use nothing
+but what is known as "medicated" cotton wool with a good long fibre.
+
+The tube will usually soon dry of itself when the cover is lifted an
+inch or so.  If water has been used, the air-current may be assisted
+by means of the water-pump, the air being sucked from the top, so that
+the wool has an opportunity of acting as a dust filter;  a very slow
+stream of air only must be employed.  For connecting the tube to the
+pump, a bit of India-rubber tube about an inch in diameter, with a
+bore of about one-eighth of an inch, may be employed.  The end of the
+rubber tube is merely pressed against the edge of the glass.
+
+These remarks apply, with suitable modification, to all kinds of
+finished apparatus having two openings.  For flasks and so on, it is
+convenient to employ a blowing apparatus, dust being avoided by
+inserting a permanent plug of cotton wool in one of the leading tubes.
+The efficiency of this method is greatly increased by using about one
+foot of thin copper tube, bent into a helix, and heated by means of a
+Bunsen burner; the hot air (previously filtered) is passed directly
+into the flask, bottle, or whatever the apparatus may be.  This has
+proved so convenient that a copper coil is now permanently fastened to
+the wall in one of the rooms of my laboratory.
+
+The above instructions indicate greater refinement than is in general
+necessary or proper for tubes that have to be afterwards worked by the
+blow-pipe.  In the majority of cases all that is necessary is to
+remove the dust, and this is preferably done by a wad of cotton waste
+(which does not leave shreds like cotton wool), followed by a bit of
+bibulous filter paper.  I would especially warn a beginner against
+neglecting this precaution, for in the process of blowing, the dust
+undergoes some change at the heated parts of the apparatus, and forms
+a particularly obstinate kind of dirt.
+
+In special cases the methods I have advocated for removing dirt and
+drying without covering the damp surfaces with dust are inadequate,
+but an experimenter who has got to that stage will have nothing to
+learn from such a work as this.
+
+§ 14.  The Blow-pipe.
+
+I suppose a small book might easily be written on this subject but
+what I have to say--in accordance with the limitation imposed--will
+be brief.  For working lead glass I never use anything but an oxygas
+blow-pipe, except for very large work, and should never dream of using
+anything else.  Of course, to a student who requires practice in order
+to attain dexterity this plan would be a good deal too dear.  My
+advice to such a one is--procure good soda glass, and work it by
+means of a modification of a gas blow-pipe, to be described directly.
+The Fletcher's blow-pipes on long stems are generally very
+inconvenient.  The flame should not be more than 5 or 6 inches from
+the working table at most, especially for a beginner, who needs to
+rest his arms on the edge of the table to secure steadiness.
+
+The kind of oxygas blow-pipe I find most convenient is indicated in
+the sketch.  (Fig.  2)  I like to have two nozzles, which will slip on
+and off, one with a jet of about 0.035 inch in diameter, the other of
+about double this dimension.  The oxygen is led into the main tube of
+the blow-pipe by another tube of much smaller diameter, concentric
+with the main tube (Fig.  3, at A).  The oxygen is mixed with the gas
+during its escape from the inner tube, which is pierced by a number of
+fine holes for the purpose, the extreme end being closed up.  The
+inner tube may run up to within half an inch of the point where the
+cap carrying the nozzle joins the larger tube.
+
+Fig.  2.
+
+Fig.  3.
+
+If it is desired to use the blow-pipe for working glass which is
+already fixed in position to a support, it will be found very
+advantageous to use a hooked nozzle.  The nozzle shown in the sketch
+is not hooked enough for this work, which requires that the flame be
+directed 'backwards towards the worker.  With a little practice such a
+flame may be used perfectly well for blowing operations on the table,
+as well as for getting at the back of fixed tubes.
+
+To warm up the glass, the gas supply is turned full on, and enough
+oxygen is allowed to pass in to clear the flame.  The work is held in
+front of, but not touching, the flame, until it is sufficiently hot to
+bear moving into the flame itself.  The, work is exposed to this flame
+until, in the case of lead glass, traces of reduction begin to appear.
+When this point is reached the oxygen tap is thrown wide open.  I
+generally regulate the pressure on the bags, so that under these
+circumstances the flame is rather overfed with oxygen.  This condition
+is easily recognised, as follows.  The flame shrinks down to a very
+small compass, and the inner blue cone almost disappears;  also
+flashes of yellow light begin to show themselves--a thing which does
+not occur when the proportions of the gases are adjusted for maximum
+heating effect.
+
+For many purposes the small dimensions of the flame render it very
+convenient, and the high temperature which can be attained at exact
+spots enables glass to be fused together after a certain amount of
+mixing, which is an enormous advantage in fusing lead glass on to hard
+glass.  The lead glass should not be heated hot enough to burn, but,
+short of this, the more fluid it is the better for joints between
+dissimilar samples.
+
+It will be noticed that the blow-pipe can be rotated about a vertical
+axis so as to throw the flame in various directions.  This is often
+indispensable.
+
+§ 15.  In general the oxygen flame does not require to be delivered
+under so high a pressure as for the production of a lime light.  In
+England, I presume, most experimenters will obtain their oxygen ready
+prepared in bottles, and will not have to undergo the annoyance of
+filling a bag.  If, however, a bag is used, and it has some advantages
+(the valves of bottles being generally stiff), I find that a pressure
+produced by placing about two hundredweight (conveniently divided into
+four fifty-six pound weights) on bags measuring 3' x 2'6" x 2' (at the
+thicker end) does very well.  To fill such a bag with oxygen, about
+700 grms of potassium chlorate is required.
+
+If the experimenter desires to keep his bag in good order, he must
+purify his oxygen by washing it with a solution of caustic soda, and
+then passing it through a "tower" of potash or soda in sticks, and,
+finally, through a calcium chloride tower.  This purifying apparatus
+should be permanently set up on a board, so that it may be carried
+about by the attendant to wherever it is required.  Oxygen thus
+purified does not seem to injure a good bag--at least during the
+first six or seven years:
+
+In order to reduce the annoyance of preparing oxygen, the use of the
+usual thin copper conical bottle should be avoided.  The makers of
+steel gas bottles provide retorts of wrought iron or steel for
+oxygen-making, and these do very well.  They have the incidental
+advantage of being strong enough to resist the attacks of a servant
+when a spent charge is being removed.
+
+The form of retort referred to is merely a large tube, closed at one
+end, and with a screw coupling at the other; the dimensions may be
+conveniently about 5 inches by 10.  The screw threads should be filled
+with fireclay (as recommended by Faraday) before the joint is screwed
+up.  Before purchasing a bottle the experimenter will do well to
+remember that unless it is of sufficiently small diameter to go into
+his largest vice, he will be inconvenienced in screwing the top on and
+off.  Why these affairs are not made with union joints, as they should
+be, is a question which will perhaps be answered when we learn why
+cork borers are still generally made of brass, though steel tube has
+long been available.
+
+Fig.  4.
+
+These little matters may appear very trivial--and so they are--but
+the purchaser of apparatus will generally find that unless he looks
+after details himself, they will not be attended to for him. Whether a
+union joint is provided or not, let it be seen that the end of the
+delivery tube is either small enough to fit a large rubber tube
+connection going to the wash-bottle, or large enough to allow of a
+cork carrying a bit of glass tube for the same purpose to be inserted.
+This tube should not be less than half an inch in inside diameter.
+Never use a new bottle before it has been heated sufficiently to get
+rid of grease and carbonaceous dirt.  A convenient oxygen-making
+apparatus is shown in Fig.  4, which is drawn from "life."
+
+§ 16.  For large blow-pipe work with lead glass I recommend a system
+of four simple blow-pipes, in accordance with the sketch annexed.  I
+first saw this system in operation in the lamp factory of the
+Westinghouse Electric Company at Pittsburg in 1889, and since then I
+have seen it used by an exceedingly clever "trick" glass-worker at a
+show.  After trying both this arrangement and the "brush flame"
+recommended by Mr. Shenstone, I consider the former the more
+convenient;  however, I daresay that either can be made to work in
+competent hands, but I shall here describe only my own choice.
+[Footnote: A brush flame is one which issues from the blow-pipe nozzle
+shaped like a brush, i.e.  it expands on leaving the jet.  It is
+produced by using a cylindrical air jet or a conical jet with a large
+aperture, say one-eighth of an inch.  See Fig.  25.]
+
+As will be seen, the blow-pipe really consists of four simple brass
+tube blow-pipes about three-eighths of an inch internal diameter and 3
+inches long, each with its gas and air tap and appropriate nozzle.
+Each blowpipe can turn about its support (the gas-entry pipe) to some
+extent, and this possibility of adjustment is of importance, The air
+jets are merely bits of very even three-sixteenths inch glass tubing,
+drawn down to conical points, the jets themselves being about 0.035
+inch diameter.
+
+Fig.  5.
+
+The flames produced are the long narrow blow-pipe flames used in
+blow-pipe analysis, and arranged so as to consist mostly of oxidising
+flame.  The air-supply does not require to be large, nor the pressure
+high--5 to 10 inches of water will do--but it must be very regular.
+The "trick" glass-blower I referred to employed a foot bellows in
+connection with a small weighted gasometer, the Westinghouse Company
+used their ordinary air-blast, and I have generally used a large
+gas-holder with which I am provided, which is supplied by a Roots
+blower worked by an engine.
+
+I have also used a "velocity pump" blower, which may be purchased
+amongst others from Gerhardt of Bonn.  The arrangement acts both as a
+sucking and blowing apparatus, and is furnished with two manometers
+and proper taps, etc.  As I have reason to know that arrangements of
+this kind work very ill unless really well made, I venture to add that
+the Gerhardt arrangement to which I refer is No.  239 in his
+catalogue, and costs about three pounds.  It hardly gives enough air,
+however, to work four blow-pipes, and the blast requires to be
+steadied by passing the air through a vessel covered with a rubber
+sheet.
+
+In default of any of these means being available, one of Fletcher's
+foot-blowers may be employed, but it must be worked very regularly.  A
+table mounted with one blow-pipe made on this plan, and worked by a
+double-acting bellows, is recommended for students' use.  For working
+flint glass, the air jet may be one-eighth of an inch in diameter and
+the pressure higher--this will give a brush flame.  See Fig.  25.
+
+It will be seen, on looking at the sketch of the blowpipe system, that
+the pair of blow-pipes farther from the observer can be caused to
+approach or recede at will by means of a handle working a block on a
+slide.  It often happens that after using all four blow-pipes at once
+it is necessary to have recourse to one blow-pipe only, and to do this
+conveniently and quickly is rather an object.  Now, in my arrangement
+I have to turn off both the gas and air from the farther system, and
+then put in a bit of asbestos board to prevent the nozzles being
+damaged by the flame or flames kept alight.  As I said before, when
+some experience is gained, glassblowing, becomes a very simple art,
+and work can be done under circumstances so disadvantageous that they
+would entirely frustrate the efforts of a beginner.  This is not any
+excuse, however, for recommending inferior arrangements.
+
+Consequently, I say that the pipes leading in gas and air should be
+all branches of one gas and one air pipe, in so far as the two remote
+and one proximate blow-pipe are concerned, and these pipes should come
+up to the table to the right hand of the operator, and should have
+main taps at that point, each with a handle at least 2 inches long.
+By this arrangement the operator can instantly turn down all the
+blow-pipes but one, while, if the inverse operation is required, all
+the three pipes can be started at once.  [Footnote: I find, since
+writing the above, that I have been anticipated in this recommendation
+by Mr. G.  S.  Ram, The Incandescent Lamp and its Manufacture, p.
+114.]
+
+The separate air and gas taps must be left for permanent regulation,
+and must not be used to turn the supply on or cut it off.  In some
+respects this blow-pipe will be found more easy to manage than an
+oxygas blow-pipe, for the glass is not so readily brought to the very
+fluid state, and this will often enable a beginner who proceeds
+cautiously to do more than he could with the more powerful instrument.
+
+Though I have mentioned glass nozzles for the air supply, there is no
+difficulty in making nozzles of brass.  For this purpose let the end
+of a brass tube of about one-eighth of an inch diameter be closed by a
+bit of brass wire previously turned to a section as shown (Fig.  6),
+and then bored by a drill of the required diameter, say -.035 inch.  It
+is most convenient to use too small a drill, and to gradually open the
+hole by means of that beautiful tool, the watchmaker's "broach."  The
+edges of the jet should be freed from burr by means of a watchmaker's
+chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin,
+1882, p.  232, § 342), or by the alternate use of a slip of Kansas
+stone and the broach.
+
+Fig.  6.
+
+The construction of this blow-pipe is so simple, that in case any one
+wishes to use a brush flame, he can easily produce one simply by
+changing his air jets to bits of the same size (say one-eighth to
+one-sixteenth of an inch) tubing, cut off clean.  To insure success,
+the ends of the tubes must be absolutely plane and regular; the
+slightest inequality makes all the difference in the action of the
+instrument.  If a jet is found to be defective, cut it down a little
+and try again; a clean-cut end is better than one which has been
+ground flat on a stone.  The end of a tube may, however, be turned in
+a manner hereafter to be described so as to make an efficient jet.
+Several trials by cutting will probably have to be made before success
+is attained.  For this kind of jet the air-pressure must be greatly
+increased, and a large Fletcher's foot-blower or, better still, a
+small double-action bellows worked with vigour will be found very
+suitable.  A fitting for this auxiliary blow-pipe is shown in Fig.  5
+at B.
+
+Professor Roentgen's discovery has recently made it necessary to give
+more particular attention to the working of soft soda glass, and I
+have been obliged to supplement the arrangements described by a table
+especially intended for work with glass of this character.  The
+arrangement has proved so convenient for general work that I give the
+following particulars.  The table measures 5 feet long, 2 feet 11
+inches wide, and is 2 feet 9 inches high.
+
+Fig.  7.
+
+It is provided with a single gas socket, into which either a large or
+small gas tube may be screwed.  The larger tube is 5.5 inches long and
+0.75 of an inch in diameter.  The smaller tube is the same length, and
+half an inch in diameter.  The axis of the larger tube is 3.5 inches
+above the table at the point of support, and is inclined to the
+horizontal at an angle of 12°.  The axis of the smaller tube is 2.5
+inches above the surface of the table, and is inclined to the
+horizontal at the same angle as the larger one.
+
+The air jets are simply pieces of glass tube held in position by
+corks.  The gas supply is regulated by a well-bored tap.  The air
+supply is regulated by treading the bellows--no tap is requisite.
+The bellows employed are ordinary smiths' bellows, measuring 22 inches
+long by 13 inches wide in the widest part.  They are weighted by lead
+weights, weighing 26 lbs.  The treadle is connected to the bellows by
+a small steel chain, for the length requires to be invariable.  As the
+treadle only acts in forcing air from the lower into the upper chamber
+of the bellows, a weight of 13 lbs.  is hung on to the lower cover, so
+as to open the bellows automatically.
+
+The air jets which have hitherto been found convenient are:
+for the small gas tube
+
+(1) a tube 0.12 inch diameter drawn down to a jet of 0.032 inch
+diameter for small work;
+
+(2) plain tubes not drawn down of 0.14 inch, 0.127 inch, and -0.245
+inch diameter, and for the large gas tube, plain tubes up to 0.3 inch
+in diameter.
+
+The table is placed in such a position that the operator sits with his
+back to a window and has the black calico screen in front of him and
+to his right.  The object of the screen is to protect the workman
+against draughts.  The table is purposely left unscreened to the left
+of the workman, so that long tubes may be treated.
+
+§ 17.  Other appliances which will be required for glass-blowing are
+of the simplest character.
+
+(1) Small corks for closing the ends of tubes.
+
+(2) Soft wax--a mixture of bees' wax and resin softened by linseed
+oil to the proper consistency, easily found by trial, also used for
+temporarily closing tubes.
+
+(3) A bottle of vaseline for lubricating.
+
+(4) An old biscuit tin filled with asbestos in shreds, and an asbestos
+towel or cloth for annealing glass after removal from the flame.  As
+asbestos absorbs moisture, which would defeat its use as an annealing
+material, it must be dried if necessary.
+
+(5) A Glass-Cutter's Knife. This is best made out of a fine
+three-cornered file, with the file teeth almost ground out, but not
+quite;  it should be about 2 inches long.  After the surface has been
+ground several times, it may be necessary to reharden the steel.  This
+is best done by heating to a full red and quenching in mercury.  The
+grindstone employed for sharpening the knife should be "quick," so as
+to leave a rough edge.  I have tried many so-called glass knives "made
+in Germany," but, with one exception, they were nothing like so good
+as a small French or Sheffield file.  In this matter I have the
+support of Mr. Shenstone's experience.
+
+(6) A wire nail, about 2 inches long, mounted very accurately in a
+thin cylindrical wooden handle about 5 inches long by one-quarter of
+an inch diameter, or, better still, a bit of pinion wire 6 inches
+long, of which 1.5 inches are turned down as far as the cylindrical
+core, An old dentists' chisel or filling tool is also a very good
+form of instrument.
+
+(7) A bit of charcoal about 3.5 inches long and 2 wide, and of any
+thickness, will be found very useful in helping to heat a very large
+tube.  The charcoal block is provided with a stout wire handle, bent
+in such a manner that the block can be held close above a large glass
+tube on which the flames impinge.  In some cases it is conveniently
+held by a clip stand.  By the use of such a slab of charcoal the
+temperature obtainable over a large surface can be considerably
+increased.
+
+I have seen a wine-glass (Venetian sherry-glass) worked on a table
+with four blow-pipes, such as is here described, with the help of a
+block of hard wood held over the heated glass, and helping the
+attainment of a high temperature by its own combustion.
+
+(8) Several retort stands with screw clips.
+
+(9) Some blocks of wood about 5" X 2" X 2" with V-shaped notches cut
+in from the top.
+
+(10) A strong pair of pliers.
+
+(11) An apparatus for cleaning and drying the breath, when blowing
+directly by the mouth is not allowable.  The apparatus consists of a
+solid and heavy block of wood supporting a calcium-chloride tube
+permanently connected with a tube of phosphorus pentoxide divided into
+compartments by plugs of glass wool.  Care should be taken to arrange
+these tubes so as to occupy the smallest space, and to have the stand
+particularly stable.  The exit tube from the phosphorus pentoxide
+should be drawn down to form a nozzle, from, say, half an inch to
+one-eighth of an inch in diameter, so as to easily fit almost any bit
+of rubber tube.  The entry to the calcium chloride should be
+permanently fitted to about a yard of fine soft rubber tubing, as
+light as possible.  The ends of this tube should terminate in a glass
+mouthpiece, which should not be too delicate.
+
+As an additional precaution against dust, I sometimes add a tube
+containing a long plug of glass wool, between the phosphorus pentoxide
+and the delivery tube, and also a tube containing stick potash on the
+entry side of the calcium chloride tube, but it may safely be left to
+individual judgment to determine when these additions require to be
+made.  In practice I always keep the affair set up with these
+additions.  The communication between all the parts should be
+perfectly free, and the tubes should be nearly filled with reagents,
+so as to avoid having a large volume of air to compress before a
+pressure can be got up.
+
+The arrangement will be clear by a reference to Fig.  8, which
+illustrates the apparatus in use for joining two long tubes.  I have
+tried blowing-bags, etc, but, on the whole, prefer the above
+arrangement, for, after a time, the skill one acquires in regulating
+the pressure by blowing by the mouth and lips is such an advantage
+that it is not to be lightly foregone.
+
+Fig. 8.
+
+§ 18.  The Table.
+
+The system of four blow-pipes is, of course, a fixture.  In this case
+the table may be about a yard square, and may be covered with asbestos
+mill-board neatly laid down, but this is not essential.  The table
+should have a rim running round it about a quarter of an inch high.
+The tools should be laid to the right of the worker, and for this
+purpose the blow-pipes are conveniently fixed rather to the left of
+the centre of the table, but not so far as to make the leg of the
+table come so close to the operator as to make him uncomfortable, for
+a cheerful and contented spirit ought to be part of the glass-worker's
+outfit.
+
+The most convenient height for a blow-pipe table--with the blow-pipes
+about 2 inches above the table top--is 3 feet 2 inches.  Nothing is
+so convenient to sit upon as a rough music-stool with a good range of
+adjustment.  The advantage of an adjustable seat lies in the fact that
+for some operations one wants to be well over the work, while in
+others the advantage of resting the arms against the table is more
+important.
+
+§ 19.  Special Operations.
+
+The preliminary to most operations before the blow-pipe, is to draw
+down a tube and pull it out to a fine point.  This is also the
+operation on which a beginner should exercise himself in the first
+instance.  I will suppose that it is desired to draw out a tube about
+one-quarter of an inch in diameter, with the object of closing it,
+either permanently or temporarily, and leaving a handle for future
+operations in the shape of the point, thin enough to cool quickly and
+so not delay further work.
+
+For this simple operation most of the glass-blower's skill is
+required.  The tube must be grasped between the first finger and thumb
+of both hands, and held so that the part to be operated on lies evenly
+between the two hands.  The distance between the operator's thumbs may
+conveniently vary from 2.5 to 4 inches.  Releasing the grip of the
+left hand, let the operator assure himself of his ability to easily
+rotate the tube about its axis--by the right thumb and finger--he
+will incidentally observe by the "feel" whether the tube is straight
+or not.
+
+A good deal of progress can be made from this point before the tube is
+heated at all.  The operator can acquire a habit of instinctively
+rotating the tube by both hands, however the tube itself be moved
+about in space, or however it be pushed or pulled.  The habit of
+constant and instinctive rotation is literally about one-third of the
+whole art of glassblowing.  It is unlikely, however, that the beginner
+will discover that he has not got this habit, until a few failures
+draw his attention to it.
+
+The glass tube being held in position lightly yet firmly, and the
+operator being sure that he feels comfortable and at his ease, and
+that the blow-pipe flame (a single flame in this instance) is well
+under control, the preliminary heating may be commenced.  With a tube
+of the dimensions given this is a very simple affair.  Turn the air
+partly off, or blow gently, to get a partly luminous gas flame; hold
+the tube about an inch from the end of this flame, and turn it round
+and round till it commences to soften.
+
+In the case of soda glass it is usual to employ the gas flame only,
+but I find that it is better in most cases to use the hot air of a
+gently-blown flame, rather than have the disadvantage of the soot
+deposited in the alternative operation.  When the glass begins to
+soften, or even before, it may be moved right into the blow-pipe
+flame, and the latter may be properly urged.
+
+It is not possible to give quite explicit and definite instructions,
+applicable to every case, as to when the time is ripe for passing the
+work into the flame, but the following notes will indicate the general
+rules to be observed:-
+
+(1) A thick tube must be warmed more slowly and raised to a higher
+temperature than a thin tube.
+
+(2) The same remark applies to a tube of large diameter, as compared
+with one of small diameter, whatever the thickness.
+
+(3) In the case of very large or thick tubes the hot air is
+advantageously employed at first, and to complete the preliminary
+heating, the luminous flame alone may be used.  The object of this is
+to enable the operator to judge, by the presence of soot, its
+inability to deposit--or its burning off if deposited--of the
+temperature of the glass, and of the equality of this temperature all
+over the surface, for a large and thick tube might be heated quite
+enough to enable it to be safely exposed to the full heat before it is
+appreciably yielding to the fingers.  In general, when the soot burns
+off freely, or lead glass begins to show the faintest sign of
+reduction, or soda glass begins to colour the flame, it is more than
+safe to proceed.
+
+In order to turn on the full flame the operator will form a habit of
+holding the work in the left hand only, and he will also take care not
+to let anything his right hand may be doing cause him to stop rotating
+the tube with his left thumb and finger.
+
+The preliminary adjustment of air or oxygen supply will enable the
+change to a flame of maximum power to be made very quickly.  The tube
+having been introduced with constant rotation, it will soon soften
+sufficiently to be worked.  The beginner will find it best to decide
+the convenient degree of softness by trial.
+
+With soda glass it does not much matter how soft the glass becomes,
+for it remains viscous, but with lead glass the viscosity persists for
+a longer time and then suddenly gives place to a much greater degree
+of fluidity.  [Footnote: This is only drawn from my impressions
+acquired in glass-working.  I have never explicitly tested the matter
+experimentally.]
+
+It is just at this point that a beginner will probably meet with his
+first difficulty.  As soon as the glass gets soft he will find that he
+no longer rotates the glass at the same speed by the right and left
+hand, and, moreover, he will probably unconsciously bend the tube, and
+even deform it, by pushing or pulling.
+
+The second third of the art of the glass-blower consists in being able
+to move both hands about, rotating a tube with each thumb and finger,
+and keeping the distance between the hands, and also the speed of
+rotation, constant.  Nothing but long practice can give this facility,
+but it is essential that it be acquired to some extent, or no progress
+can be made.  Some people acquire a moderate proficiency very quickly,
+others, of whom the writer is one, only become reasonably proficient
+by months, or even years, of practice.
+
+Supposing that the tube is now ready to be drawn down, the operator
+will remove it from the flame, and will gently pull the ends apart,
+interrupting his turning as little as possible.  If the tube be pulled
+too hard, or if the area heated be too small (about three-eighths of
+an inch in length in the case given would be proper), it will be found
+that the ends of the two portions of the tube will be nearly closed at
+a very sharp angle (nearly a right angle to the length of the tube),
+that the ends will be thin, and that a long length of very fine tube
+will be produced.  To heat a short length of tube and pull hard and
+suddenly is the proper way to make a very fine capillary tube, but, in
+general, this is what we want to avoid.
+
+If the operation be successfully performed, the drawn-down tube will
+have the appearance exhibited, which is suitable either for
+subsequently closing or handling by means of the drawn-down portion.
+The straightness of the point can be obtained by a little practice in
+"feeling" the glass when the tube is rotated as it cools just before
+it loses its viscous condition.
+
+When the operation is carried out properly the shoulder of the "draw"
+should be perfectly symmetrical and of even thickness, and its axis
+regarded as that of a cone should lie in the axis of the tube
+produced.  The operation should be repeated till the student finds
+that he can produce this result with certainty, and he should not be
+discouraged if this takes several days, or even weeks.  Of course, it
+is probable that within the first hour he will succeed in making a
+tolerable job, but it is his business to learn never to make anything
+else.
+
+Fig. 9.
+
+Fig. 10. Diagram of a folded end.
+
+§ 20.  Closing and blowing out the End of a Tube.
+
+When it is desired to close the end of a particular bit of tube, this
+is easily done by heating the end, and at the same time heating the
+end of a waste bit of tube or rod;  the ends, when placed in contact,
+stick together, and a point can be drawn down as before.  [Footnote:
+"Point" is here used in the technical sense, i.e.  it is a thin tail
+of glass produced by drawing down a tube.]  Having got a point, it
+will be found that the thin glass cools enough to allow of the point
+being handled after a few moments.
+
+The most convenient way of reducing the point to a suitable length
+(say 1.5 inch) is to fuse it off in the flame, but this must be done
+neatly;  if a tail is left it may cause inconvenience by catching, or
+even piercing the finger and breaking off.  The blow-pipe flame being
+turned down to a suitable size, and the shoulder of the "draw" having
+been kept warm meanwhile, let the tip of the flame impinge on a point
+where the diameter is about half that of the undrawn tube, and let the
+temperature be very high (Fig.  11).  The tube is to be inclined to
+the flame so that the latter strikes the shoulder normally, or
+nearly so.  Then, according to circumstances, little or much of the
+glass can be removed at will by drawing off the tail (Fig.  12), till,
+finally, a small drop of melted glass only, adheres to the end of the
+now closed tube (Fig.  13).
+
+Fig. 11.
+
+Fig.  12.
+
+Fig.  13.
+
+Fig. 14.
+
+When this is satisfactorily accomplished, heat the extreme end of the
+tube most carefully and equally, holding it in such a position that
+the glass will tend to flow from the bead back on to the tube, i.e.
+hold the closed end up to the flame, the tube being, say, at 45
+degrees to the horizontal.  Then when the temperature is such as to
+indicate complete softness lift the tube to the mouth, still holding
+the tube pointing with its closed end a little above the horizontal,
+and blow gently.  A beginner almost always blows too hard.
+
+What is wanted, of course, is a continued pressure, to give the
+viscous glass time to yield gradually, if it is uniform;  or else
+intermittent puffs to enable the thinner parts, if there are any, to
+cool more, and hence become more resisting than the thicker ones.  In
+any case a little practice will enable the operator to blow out a
+round and even end--neither thicker nor thinner than the rest of the
+tube.
+
+§ 21.  To make a Weld.
+
+To begin with, try on two bits of glass of the same size, i.e.  cut a
+seven-inch length of glass in half by scratching it with the knife,
+and pulling the ends apart with a slight inclination away from the
+scratch.  In other words, combine a small bending moment with a
+considerable tensional stress.  It is important to learn to do this
+properly.  If the proportions are not well observed, the tube will
+break with difficulty, and the section will not be perpendicular to
+the main length.  If the knife is in good order it will make a fine
+deep scratch--the feel of the glass under the knife will enable the
+operator to decide when the scratch is made.  The operation of cutting
+large tubes will be treated further on.  The two halves of the tube
+being held one in each hand, and one tube closed at one end, the
+extremities to be united will be warmed, and then put in the flame as
+before.
+
+Fig. 16.
+
+There are many ways of proceeding--perhaps the easiest is as follows.
+As soon as the glass shows signs of melting at the ends--and care
+should be taken that much more is not heated--take both bits out of
+the flame.  Stop rotating for a moment, and resting the arms carefully
+on the edge of the  table, raise the tubes above the flame and bring
+the ends swiftly and accurately together.  This is a case of "sudden
+death no second attempt at making the ends meet can be allowed;  if
+the tubes join in any other than a perfectly exact manner a kink more
+or less objectionable will result.  In practice the operator will
+learn to bring the ends together, commencing at one point; i.e.  the
+axes of the tubes will be inclined at first, so as to cause adherence
+at one spot only.  If this is not quite "fair", then less damage is
+done in moving one tube slightly up or down to get the contact exact.
+The tubes will then be closed upon one another as if they were hinged
+at the joint.  This must be done lightly, yet sufficiently, to ensure
+that the glass is actually in contact all round.
+
+Having gone so far, replace the tubes--now one--in the flame, and
+carefully rotating the glass, raise the temperature higher than in the
+operation just described, in fact the higher the temperature, short of
+burning the glass, the better.  Take the tube out of the flame and
+blow into the open end, turning constantly as before.  One puff is
+enough.  Then turn and pull the glass apart till it is of the same
+diameter and thickness throughout, and feel that it is straight as
+before.
+
+Though it is in general of high importance that the joint should be
+well heated, the beginner will probably find that he "ties up" his
+glass as soon as it gets really soft.
+
+If his object is to make one joint--at any cost--then let him be
+careful to use two bits of exactly the same kind of glass, and only
+get the temperature up to the viscous stage.  If the joint be then
+pulled out till it is comparatively thin, it will probably stand (if
+of soda glass); certainly, if of lead glass, though in this case it
+may not be sound.  In any case the joint should be annealed in the
+asbestos box if practicable, otherwise (unless between narrow tubes)
+with the asbestos rag.  Care must be taken that the asbestos is dry.
+
+§ 22.  To weld two Tubes of different Sizes.
+
+To do this, the diameter of the larger tube must be reduced to that of
+the smaller.  The general procedure described in drawing down must be
+followed, with the following modification.  In general, a greater
+length of the tube must be heated, and it must be made hotter.  The
+tube is to be gradually drawn in the flame with constant turning till
+the proper diameter and thickness of glass are attained.
+
+Fig. 16.
+
+For this operation time must be allowed if the operator's hands are
+steady enough to permit of it; the shoulder should form partly by the
+glass sinking in and partly by the process of drawing the hot glass
+out.  A shoulder properly prepared is shown in the sketch.  Beginners
+generally make the neck too thin on large tubes, and too thick on
+smaller ones.  There ought to be no great difference in thickness of
+glass between the neck on the larger tube, and the smaller tube.  The
+diameters should be as nearly as possible alike.
+
+Having drawn down the larger tube to a neck, take it out of the flame,
+and as it cools pull and turn till the neck is of the right thickness
+and is perfectly straight, i.e.  make the final adjustment outside the
+flame, and to that end have the neck rather too thick (as to glass)
+before it is taken out.  It is not necessary to wait till the neck
+gets cold before the end can be cut off. Make a scratch as before--this
+will probably slightly damage the temper of the file knife, but
+that must be put up with.  Hold the tube against the edge of the
+table,  so that the scratch is just above the level of the rim, and
+strike the upper part a smart blow with the handle of the glass knife
+rather in the direction of its length.  [Footnote: A bit of hoop iron
+nailed against the side of the table is a very convenient arrangement,
+and it need not project appreciably above the general level of the
+rim.]
+
+Of course this applies to a tube where economy has been exercised and
+the end is short.  If the tail is long enough to form a handle, the
+tube may be pulled apart as before.  As a rule a temporary joint
+between a tube and a rod is not strong enough to enable the shoulder
+to be broken at the scratch by mere pulling.  The ends to be welded
+must be broken off very clean and true.  Subsequent operations are to
+be carried out as already described.
+
+§ 23.  The above operations will be easily performed on tubes up to
+half an inch in diameter, if they are not too long.  It is the length
+of tube, and consequent difficulty in giving identity of motion with
+the two hands, which make the jointing of long tubes difficult.  There
+are also difficulties if the tubes are very thin, have a very fine
+bore or a very large diameter.
+
+All these difficulties merely amuse a good glass-blower, but to an
+experimenter who wants to get on to other things before sufficient
+skill is acquired (in the movement of the hands and arms) the
+following method is recommended.  First, use flint glass.  Then,
+assuming that any drawing down has to be done, do it as well as
+possible, for on this the success of the method to be described
+especially depends.  Be sure that the tubes to be welded are cut off
+clean and are as nearly as may be of the same size at the point of
+junction.
+
+To fix the description, suppose it is desired to join two tubes (see
+Fig.  8), each about one inch in diameter and a yard long.  Get four
+clip stands and place them on a level table.  Be sure that the stands
+are firm and have not warped so as to rock.  In each pair of clips
+place a tube, so that the two tubes are at the same height from the
+table, and, in fact, exactly abut, with axes in the same straight
+line.  Close one tube by a cork and then fix the blowing apparatus as
+shown to the other.
+
+In such an operation as this the drying apparatus may be dispensed
+with, and a rubber tube simply connected to one end of the system and
+brought to the mouth.  Take the oxygen blow-pipe and turn the nozzle
+till the flame issues towards you, and see that the flame is in order.
+Then turn down the oxygen till it only suffices to clear the smoky
+flame, and commence to heat the proposed joint by a current of hot
+air, moving the flame round the joint.  Finally, bring to bear the
+most powerful flame you can get out of the blow-pipe, and carry it
+round the joint so quickly that you have the latter all hot at once.
+Put down the blow-pipe, and, using both hands, press the tubes
+together (which wooden clips will readily allow), and after seeing
+that the glass has touched everywhere, pull the tubes a trifle apart.
+Apply the blow-pipe again, passing lightly over the thin parts, if
+any, and heating thicker ones; having the end of the rubber tube in
+his mouth, the operator will be able to blow out thick places.  When
+all is hot, blow out slightly, and having taken the flame away, pull
+the tubes a little apart, and see that they are straight.
+
+Throw an asbestos rag over the joint, loosen one pair of the clamps
+slightly, and leave the joint to anneal.  It is important that the
+least possible amount of glass should be heated, hence the  necessity
+of having the ends well prepared, and it is also important that the
+work should be done quickly;  otherwise glass will flow from the upper
+side downwards and no strong joint will be obtained.
+
+Fig. 17. Tube being opened at one end.
+
+§ 24.  To weld Tubes of very small Bore.
+
+If the bore is not so small as to prevent the entrance of the point of
+the iron nail, get the ends of the tubes hot, and open the bore by
+inserting the end of the nail previously smeared over with a trace of
+vaseline.  Work the nail round by holding the handle between the thumb
+and first finger of the right hand, the tube being similarly placed in
+the left.  The tube and nail should be inclined as shown in the
+sketch.
+
+Never try to force the operation;  the nail soon cools the glass, so
+that only a very short time is available after each heat;  during this
+the tube should be rotated against the nail rather than the nail
+against the tube.  Be careful not to heat a greater length of tube
+than is necessary, or the nail will, by its component of pressure
+along the tube, cause the latter to "jump up" or thicken and bulge.
+Both ends being prepared, and if possible, kept hot, the weld may be
+made as before, and the heating continued till the glass falls in to
+about its previous thickness, leaving a bore only slightly greater
+than before.
+
+It is in operations such as this that the asbestos box will be found
+of great use.  As soon as one end of the weld is ready cool it in the
+flame till soot deposits, and then plunge it into the asbestos.  This
+will cause it to cool very slowly, and renders it less likely to crack
+when again brought into the flame.  Turned-out ends, if the glass is
+at all thick, are very liable to crack off on reheating, so that they
+must be reintroduced (into the flame) with especial care.  This
+liability to breakage is reduced, but not eliminated, by the asbestos
+annealing.
+
+Figs.  18 and 19.
+
+§ 25.  When the bore is very fine, it is best to seal off the tubes,
+and blow an incipient bulb near one end of each tube.  These bulbs may
+be cooled in asbestos, and cut across when cold by means of a scratch
+touched at one end (Figs.  18 and 19) by a fine point of highly
+incandescent glass.  For details of this method see p.  46, Fig.  21.
+Time is occasionally saved by blowing off the ends of the bulbs.  The
+details of this process will be described when the operation of making
+thistle-headed tubes is dealt with.
+
+§ 26.  When the tubes are both of large diameter, long, and very thin
+(cylinder tubes), a considerable amount of difficulty will be
+experienced.  On the whole, it is best to heat each end separately
+till the glass thickens a little, anneal in the flame and in asbestos,
+and then proceed as in § 22.  If the ends are not quite true, it will
+be found that quite a thickness of glass may be "jumped" together at
+one side of the tubes, while the edges are still apart at the other.
+When this looks likely to happen, incline the tubes as if the joint
+were a hinge, and bend back quickly;  do not simply continue to push
+the tubes together in a straight line, or an unmanageable lump of
+glass will be formed on one side.
+
+If in spite of these precautions such a lump does form, proceed as
+follows.  Take a rod of glass, at least one-eighth of an inch thick,
+and warm it in the flame at one end.  Heat the imperfect joint till it
+softens all round, and then bring the flame right up to the thick
+part, and heat that as rapidly and locally as possible.  The oxygas
+flame does this magnificently.  Press the heated end of the glass rod
+against the thick part, and pull off as much of the lump as it is
+desired to remove, afterwards blowing the dint out by a judicious
+puff.  Finish off as before.
+
+§ 27.  Occasionally, when it is seen that in order to produce a joint
+closed all round, one side of the tube would be too much thickened, it
+is better to patch the open side.  For this purpose take a glass rod
+about one-sixteenth inch in diameter, and turn the flame to give its
+greatest effect, still keeping rather an excess of air or oxygen.  See
+that the side of the joint already made is kept fairly hot--it need
+not be soft;  interrupt any other work often enough to ensure this.
+Then, directing the flame chiefly on the thin rod, begin to melt and
+pull the glass over the edges of the gap.  When the gap is closed get
+the lump very hot, so that all the glass is well melted together, and
+then, if necessary, pull the excess of glass off, as before described.
+
+It must be remembered that this and the method of the previous section
+are emergency methods, and never give such nice joints as a
+manipulation which avoids them, i.e.  when the ends of the tubes are
+perfectly straight and true to begin with.  Also note that, as the
+tubes cannot be kept in rotation while being patched, it is as well to
+work at as low a temperature as possible, consistently with the other
+conditions, or the glass will tend to run down and form a drop,
+leaving a correspondingly thin place behind.
+
+Fig.  20.
+
+§ 28.  A very common fault in cutting a tube of about an inch in
+diameter is to leave it with a projecting point, as shown.  This can
+be slowly chipped off by the pliers, using the jaws to crush and grind
+away the edge of the projection;  it is fatal to attempt to break off
+large pieces of glass all at once.
+
+§ 29.  It will be convenient here to mention some methods of cutting
+large tubes.  With tubes up to an inch and a half in diameter, and
+even over this--provided that the glass is not very thick--we may
+proceed as follows:  Make a good scratch about half an inch long, and
+pretty deep, i.e.  pass the knife backwards and forwards two or three
+times.  Press a point of melted glass exactly on one end of the
+scratch;  the glass point even when pressed out of shape should not be
+as large as a button one-twelfth of an inch in diameter.  If this
+fails at first, repeat the operation two or three times.
+
+Fig.   21.
+
+If a crack does not form, touch the hot place with the cold end of the
+nail.  If no success is obtained, try the other end of the scratch.
+If failure still pursues the operator, let him make another cut on the
+opposite side of the tube and try again.  In general, the tube will
+yield the first or second time the hot drop of glass is applied.
+Never apply the drop at the centre of the scratch, or a ragged crack,
+which may run in any direction, will result.  Very often, with a large
+tube, the crack formed by a successful operation will only extend a
+short distance.  In this case it is desirable to entice the crack
+round the tube, and not trust to its running straight when the tube is
+pulled apart.
+
+On the whole, the best method in this case is to employ a flame
+pencil, which should be kept ready for use.  This merely consists of a
+bit of glass tube of about the same dimensions as an ordinary lead
+pencil, drawn down to a very fine jet at one end.  The jet must not be
+very long or thin, or the glass will soon fuse up.  A few trials will
+enable the operator to get the proper proportions, which are such that
+the tube has the general appearance of a pencil normally sharpened
+(say with a cone of 60').  This tube is best made of hard glass.
+Connect it to a gas supply by light flexible tubing, and turn down the
+gas till the flame from the end of the jet is not more than one-tenth
+of an inch long.  Then apply the jet, beginning from the end of the
+crack, and gradually draw it (the crack) round the tube.  The
+operation will be assisted if a rubber ring is slipped on the tube to
+begin with, so that the eye has some guide as to whether the flame is
+being drawn round properly or not.  The ring must, of course, be far
+enough away to escape the effect of the flame.  The crack will be
+found to follow the flame in the most docile manner, unless the tube
+is thick or badly annealed.  Some operators recommend a pencil of
+glowing charcoal, but the flame is undoubtedly better.
+
+§ 30.  To cut very thick Tubes.
+
+A large number of methods have been proposed, and nearly everybody has
+his favourite.  The following has always succeeded with me.  First
+mark on the tube, by means of a little dead black spirit paint,
+exactly where the cut is to be.  Then sharpen the glass knife and
+scratch a quite deep cut all round:  there is no difficulty in making
+the cut one-twentieth of an inch deep.  It will be proper to lubricate
+the knife with kerosene after the first mark is made.  [Footnote: The
+edge of the knife may be advantageously saved by using an old file
+moistened with kerosene for the purpose.  I find kerosene is not
+worse, but, if anything, better than the solution of camphor in
+turpentine recommended by Mr. Shenstone.]
+
+If the glass is about one-eighth of an inch thick, the scratch maybe
+conveniently about one-twentieth of an inch deep, but if the glass is
+anything like one-quarter of an inch thick, the scratch must be much
+deeper, in fact, the glass may be half cut through.  To make a very
+deep scratch, a wheel armed with diamond dust, which will be described
+later on, may be used.  However, it is not essential to use a diamond
+wheel, though it saves time.
+
+When the cut is made to a sufficient depth proceed thus:  Obtain two
+strips of bibulous paper or bits of tape and twist them round the tube
+on each side of the scratch, allowing not more than one-eighth of an
+inch between them. Then add a few drops of water to each, till it is
+thoroughly soaked, but not allowing water to run away.  Dry out the
+scratch by a shred of blotting paper.
+
+Turn down the oxygas flame to the smallest dimensions, and then boldly
+apply it with its hottest part playing right into the nick and at a
+single point.  Probably in about two seconds, or less, the tube will
+break.  If it does not, rotate the tube, but still so that the flame
+plays in the nick.  After making the tube very hot all round--if it
+has not broken--apply the flame again steadily at one point for a few
+seconds and then apply a bit of cold iron.  If the tube does not break
+at once during these processes, let it cool, and cut the groove
+deeper;  then try again. [Footnote: This method is continually being
+reinvented and published in the various journals.  It is of unknown
+antiquity.]
+
+Fig. 22.
+
+If the tube breaks after great heating and long efforts, it will
+probably leave incipient cracks running away from the break, or may
+even break irregularly.  A good break is nearly always one that was
+easily made.  If a number of rings have to be cut, or a number of cuts
+made on glass tubes of about the same size, it will be found
+economical in the end to mount a glazier's diamond for the purpose.  A
+simple but suitable apparatus is figured (Fig.  23).
+
+Fig.  23.
+
+The only difficulty is to regulate the position of the diamond so that
+it cuts.  In order to do this, carefully note its cutting angle by
+preliminary trials on sheet glass, and then adjust the diamond by
+clamps, or by wriggling it in a fork, as shown.  Weight the board very
+slightly, so as to give the small necessary pressure, and produce the
+cut by rotating the tube by hand.  When a cut is nearly completed take
+great care that the two ends join, or irregularity will result.  This
+is not always easy to do unless the tube happens to be straight.
+Having got a cut, start a crack by means of a fine light watchmaker's
+hammer, or even a bit of fused glass, and entice the crack round the
+cut by tapping with the hammer or by means of the flame pencil.
+
+If the cut is a true "cut" the tube will break at once.  As a supply
+of electrical current for lighting will, in the near future, be as
+much a matter of course for laboratory purposes as a gas supply, I add
+the following note.  To heat a tube round a scratch, nothing--not
+even the oxygas blow-pipe--is so good as a bit of platinum or iron
+wire electrically heated.  If the crack does not start by considerable
+heating of the glass, stop the current, unwind the wire, and touch the
+glass on the crack either with a bit of cold copper wire or a wet
+match stem. I prefer the copper wire, for in my experience the water
+will occasionally produce an explosion of cracks.  On the other hand,
+the cold wire frequently fails to start a crack.
+
+Judging from the appearance of thick tubes as supplied by the dealers,
+the factory method of cutting off appears to be to grind a nick almost
+through the tube, and right round;  and for really thick glass this is
+the safest but slowest way;  a thin emery wheel kept wet will do this
+perfectly.  Suitable wheels may be purchased from the "Norton" Emery
+Wheel Co.  of Bedford, Mass, U.S.A.--in England through Messrs.
+Churchill and Co.  of London, importers.
+
+§ 31.  To blow a Bulb at the End of a Tube.
+
+I must admit at once that this is a difficult operation--at all
+events, if a large bulb is required.  However, all there is to be said
+can be said in few words.  In general, when a bulb is required at the
+end of a tube it will be necessary to thicken up the glass.  A
+professional glass-worker will generally accomplish this by "jumping
+up" the tube, i.e.  by heating it where the bulb is required, and
+compressing it little by little until a sufficient amount of glass is
+collected.  The amateur will probably find that he gets a very
+irregular mass in this way, and will be tempted to begin by welding on
+a short bit of wide and thick tubing preparatory to blowing out the
+bulb.
+
+However, supposing that enough glass is assembled by-either of these
+methods, and that it is quite uniform in thickness, let the thickened
+part be heated along a circle till it becomes moderately soft, and let
+it then be expanded about one-fifth, say by gently blowing.  It is
+perhaps more important to keep turning the glass during bulb-blowing
+than in any other operation, and this both when the glass is in the
+flame and while the bulb is being blown.  It is also very important to
+avoid draughts.  In general, a bulb is best blown with the tube in a
+nearly horizontal position, but sloping slightly upwards from the
+mouth.  If it be noticed that a bulb tends to blow out more at one
+side than another, let the side of greatest protuberance be turned
+down, so that it is at the lowest point, reduce the pressure for an
+instant, and then blow again.  It will be observed that the bulb will
+now expand at the top.
+
+The reason of this is chiefly that the under side cools most rapidly
+(according to Faraday, Chemical Manipulation, § 1194), and
+consequently can expand no further;  but also it is not unlikely that
+the glass tends to flow somewhat from the upper side, which remains
+hot, and consequently the bulb, when the next puff reaches it, will
+tend to yield at this point.  By heating several zones the tube will
+become gradually expanded.
+
+Fig. 24.
+
+Fig.  25.
+
+Fig. 26.
+
+When the length of the thickened part of the tube only slightly
+exceeds its diameter (Fig.  25), let the whole be brought to a
+temperature which, with flint glass, should be just short of that of
+perfect fluidity;  and then, holding the tube horizontally and
+constantly turning it, let the bulb be blown out to its full size,
+noting the appearances and correcting too great protuberance on any
+side by the means above mentioned.  If the bulb appears pear-shaped
+turn the tube so that the melted mass is directed upwards;  if the
+bulb have the contrary fault, correct in the corresponding manner.
+
+The bulb when finished may be lightly tapped on the table, when, if
+there is any weak place owing to inequality of thickness, the bulb
+will break, and the operation may be started afresh.  "A good bulb is
+round, set truly on the tube, and free from lumps of thick glass or
+places of excessive thinness."  When the amateur has succeeded in
+blowing a bulb two inches in diameter on the end of a strong bit of
+thermometer tube--say for an air thermometer--he may well seek the
+congratulations of his friends.
+
+In case the bulb is not satisfactory on a first attempt, it may be
+melted down again, if the following precautions are taken.  Directly
+creases begin to appear in the bulb let it be withdrawn from the
+flame, and gently blown till the creases come out.  By alternate
+heating and blowing the glass can be got back to its original form, or
+nearly so, but unless the operator shows great skill and judgment, the
+probability is that the glass will be uneven.  By heating and keeping
+the thicker parts in the higher position, and blowing a little now and
+again, the glass may be got even, and a new attempt may be made.  It
+must not be supposed that this process can be carried on indefinitely,
+for the glass tends to lose its viscous properties after a time, or,
+at all events, it "perishes" in some way, especially if it has been
+allowed to get very thin;  consequently too frequent attempts on the
+same glass are unprofitable.  Two or three trials are as many as it
+generally pays to make.  As a rule the largest possible flame may be
+used with advantage in this operation.
+
+§ 32.  To blow a bulb in the middle of a tube, the procedure is much
+like that already treated, but the manipulation is, if anything,
+more difficult, for the further end of the tube must be carried and
+turned as well as the end which is held to the lips.
+
+§ 33.  To make a side Weld.
+
+This is by no means difficult, but is easier with lead glass than with
+soda glass.  The tube to which it is desired to make a side connection
+having been selected, it is closed at one end by rubber tube stops, or
+in any other suitable manner.  The zone of the proposed connection is
+noted, and the tube is brought to near softness round that circle (if
+the tube is made actually soft, inconvenience will arise from the
+bending, which is sure to occur).  Two courses are then open to the
+operator, one suitable to a thick tube, the other to a tube of
+moderate thickness.
+
+Taking the former first.  Provide a piece of glass rod and warm its
+end.  Direct a small flame against the spot on the thick tube where
+the proposed joint is to be.  When the glass becomes almost
+incandescent at this spot, put the end of the rod against it and draw
+out a thread of glass till sufficient "metal" has been removed.  Then
+fuse off the thread close to the tube.
+
+Fig.  27.
+
+The subsequent procedure is the same as for thin tubes.  In this case
+heat the spot by the smallest flame available, and get the spot very
+hot.  Blow it out gently into a bubble, perhaps extending to a height
+equal to its diameter.  Then heat the top of the bubble till it is
+incandescent and blow violently.  This will produce an opening fringed
+by glass so thin as to exhibit interference colours.  Remove the filmy
+part, and heat the frayed edges till they cohere and form an incipient
+tube.  If the flame has been of a correct size, the tube will now be
+of the same diameter as the tube to be welded on, and will project
+perhaps one-sixteenth of an inch from the surface of the main tube
+(Fig.  28).
+
+Fig. 28.
+
+Fig.  29.
+
+When this stage is reached, again heat the tube all round till it
+nearly softens, and by means of the other hand heat the end of the
+other tube which it is proposed to weld.  Just before the main tube
+actually softens, turn it so as to heat the edges of the aperture, and
+at the same time get the end of the side tube very hot.  Take both out
+of the flame for an instant, and press the parts together, instantly
+slightly withdrawing the side tube.  If the operation is well
+performed, it will be found that the point of maximum thickness of
+glass is now clear of the main tube.  The joint is then to be heated
+all round and blown out--a rather awkward operation, and one
+requiring some practice, but it can be done.
+
+Fig.  30.
+
+If great strength is wanted, heat the main tube all round the joint
+bit by bit, and blow each section slightly outwards.  If the operator
+is confident in his skill, he should then heat the whole joint to the
+softening point, blow it out slightly, and then adjust by pulling and
+pushing.  Cool first in the gas flame, and then plunge the joint into
+the asbestos and cover it up--or if too large, throw the asbestos
+cloth round it.
+
+In the case of soda glass this final "general heat" is almost
+essential, but it is not so with flint glass, and as the general heat
+is the most difficult part of the job, it will be found easier to use
+lead glass and omit the general heating.  With soda glass a very small
+irregularity will cause the joint to break when cold, but flint glass
+is much more long-suffering.  It is easy to perform the above
+operation on small tubes.  For large ones it will be found best to
+employ flint glass and use the clip stands as in the case of direct
+welds, treated above, but, of course, with suitable modifications.
+Never let the main tube cool after the hole is made until the work is
+done.
+
+§ 34.  Inserted Joints.
+
+In many instances the performance of apparatus is much improved by
+joints of this kind, even when their use is not absolutely essential.
+
+There are two ways in which inserted joints may be made.  The first
+method is the easier, and works well with flint glass; but when one
+comes to apply it to soda glass there is a danger of the glass
+becoming too thick near the joint, and this often leads to a cracking
+of the joint as the glass cools.
+
+Fig.  31.
+
+Suppose it is desired to insert the tube B into the tube A (Fig.  31).
+Begin by reducing the size of the end of tube A till B will just slip
+in quite easily.  With B about one-quarter inch in diameter, a
+clearance of about one-twentieth of an inch, or less, in all (i.e.
+one-fortieth of an inch on each side) will be proper.
+
+Heat B by itself at the proposed zone of junction, and blow out a very
+narrow ring;  then compress this slightly so that it forms an almost
+closed ring of glass.  The figure refers to the close of this
+operation (Fig.  31, B).  It does not matter much whether the ring
+remains a mere flattened bulb, or whether it is a solid ring, but it
+must be one or the other.  Some judgment must be exercised in
+preparing the ring.  In general, the beginner will collect too much
+glass in the ring, and consequently the joint, when made, will either
+be thick and liable to crack easily, or it will be blown out into an
+erratic shape in endeavours to reduce this thickness.  Accordingly,
+the operator will, if necessary, thin the tube B by drawing slightly,
+if he considers it desirable, before the little enlargement is blown
+out.  In general, two heats must be used for this operation.
+
+Fig.  32.
+
+Get the approximating parts of both A and B up to a temperature just
+below that at which they will adhere, and having closed the other end
+of A, place B carefully within it up to the ring, and if it can be
+arranged, have a mica wad in A, with a central hole through which the
+end of B can project.  This will very much facilitate the operation,
+especially if B is long, but may be dispensed with by the exercise of
+care and skill.
+
+The operation is now simple.  Fuse the junction and press the tubes
+lightly together, being careful not to collect more glass than can be
+helped;  finally, blow out the joint and reduce the thickness by mild
+drawing (Fig.  33).  In order to make a really good joint, two points
+must be particularly attended to--the rim must be thin and its plane
+perfectly perpendicular to the axis of tube B;  the end of tube A must
+be cut off quite clean and perpendicular to its axis before B is
+inserted.  So important are these conditions--especially the latter
+that the writer has even occasionally used the grindstone to get the
+end of A into a proper condition, an admission which will probably
+earn the contempt of the expert glass-worker.
+
+Fig. 33.
+
+Now for the second method, which is often practised in Germany, where
+soda glass is chiefly used.  With this glass the chief point is to get
+a very even and not too thick ring at the junction, and consequently
+the extra thickening produced by making a rim on B is rather a
+drawback.  The method consists in cutting off from B the length which
+it is desired to insert, slipping this into A (which may be an
+otherwise closed bulb, for instance), and then gradually melting up
+the open end of A till the piece of B inside will no longer fall out.
+By holding the joint downwards so that the inserted portion of B rests
+on the edges of the opening, a joint may be made with the minimum
+thickening.
+
+The external part of B, previously heated, is then applied, and the
+joint subjected to a "general" heat and blown out.  Very nice joints
+may be made by this method, and it is perhaps the better one where the
+external part of B is to be less in diameter than the inserted part.
+It was in this manner that the writer was taught to make glass
+velocity pumps, one of which, of a good design, is figured as an
+example.
+
+In all cases good annealing should follow this operation.  If the
+inserted part of the inner tube (B) is anything like an inch in
+diameter, and especially if it is of any length, as in some forms of
+ozone apparatus, or in a large Bunsen's ice calorimeter, the
+arrangements for supporting the inner part must be very good.  A
+convenient way of proceeding when the inner tube is well supported is
+to make the mouth of A only very little larger than the diameter of B,
+so that B will only just slip in.  Then the mouth of A and the zone of
+B may be heated together, and B blown out upon A.  This, of course,
+must be arranged for, if necessary, by temporarily stopping the inner
+end of B.
+
+The inner support of B should be removed as soon as practicable after
+the joint is made, or, at all events, should not be perfectly rigid; a
+tightly-fitting cork, for instance, is too rigid.  The reason is, of
+course, that in cooling there may be a tendency to set B a little to
+one side or the other, and if it is not free to take such a set, the
+joint most probably will give way.  Good annealing both with flame and
+asbestos is a sine qua non in all inserted work.
+
+Fig. 34.
+
+§ 35.  Bending Tubes.
+
+I have hitherto said nothing about bending tubes, for to bend a tube
+of a quarter of an inch in diameter, and of ordinary thickness, is
+about the first thing one learns in any laboratory, while to bend
+large tubes nicely is as difficult an operation as the practice of
+glass-blowing affords.  However, even in bending a narrow tube it is
+possible to proceed in the wrong way.  The wrong way is to heat a
+short length of the tube and then bend it rapidly, holding the plane
+of the bend horizontal.  The right way, per contra, is to use a
+batswing burner to heat, say, two inches of the tube with constant
+turning till it is very soft, and then, holding the glass so that the
+bend will be in a vertical plane passing through one eye (the other
+being shut), to make the bend rather slowly.
+
+If an exact angle is required, it is as well to have it drawn out on a
+sheet of asbestos board.  In this case bend the glass as described
+till it is approximately right, and finish by laying it on the
+asbestos board and bringing it up to the marks.  A suitable bit of
+wood may be substituted for the asbestos on occasion.
+
+N.B: The laboratory table is not a suitable piece of wood.  A
+right-angled bend is often wanted.  In this case the corner of a table
+will serve as a good guide to the eye, the glass being finished by
+being held just above it.  If great accuracy is wanted, make a wooden
+template and suspend it by a screw from the side of the table, so that
+the vertex of the gauge for the interior angle projects downwards,
+then finish by bending the tube round it.  The wood may be about half
+an inch thick.
+
+If a sharp bend is required, heat the tube in the blow-pipe, and bend
+it rapidly, blowing out the glass meanwhile.  The reason why a long
+bend should be held in a vertical plane is that the hot part tends to
+droop out of the plane of the bend if the latter be made in a
+horizontal position.  To bend a tube above half an inch in diameter is
+a more or less difficult operation, and one which increases in
+difficulty as the diameter of the tube increases.
+
+A U-tube, for instance, may be made as follows:  Use the four
+blow-pipe arrangement so as to heat a fair length of tube, and get,
+say, two inches of tube very hot--almost fluid, in fact--by means of
+the carbon block supported from a stand.  Remove the tube rapidly from
+the flame and draw the hot part out to, say, three inches.  Then,
+holding the tube so as to make the bend in a vertical plane, bend it
+and blow it out together to its proper size.
+
+This operation seems to present no difficulties to experienced
+glass-workers, even with tubes of about one inch in diameter, but to
+the amateur it is very difficult.  I always look on a large U-tube
+with feelings of envy and admiration, which the complex trick work of
+an elaborate vacuum tube does not excite in the least.  It will be
+noted that this method may, and often does, involve a preliminary
+thickening of the glass.
+
+With tubes over an inch in diameter I have no idea as to what is the
+best mode of procedure--whether, for instance, a quantity of sand or
+gas coke might not be used to stuff out the tube during bending, but
+in this case there would be the difficulty of removing the fragments,
+which would be sure to stick to the glass.
+
+Of course, if the bend need not be short, the tube could be softened
+in a tube furnace and bent in a kind of way.  I must admit that with
+tubes of even less than one inch in diameter I have generally managed
+best by proceeding little by little.  I heat as much of the glass as I
+can by means of a gigantic blow-pipe, having a nozzle of about an inch
+in diameter, and driven by a machine-blower.
+
+When I find that, in spite of blowing, the tube begins to collapse, I
+suspend operations, reheat the tube a little farther on, and so
+proceed.  If by any chance any reader knows a good laboratory method
+of performing this operation, I hope he will communicate it to me.
+After all, the difficulty chiefly arises from laboratory heating
+appliances being as a rule too limited in scope for such work.
+
+The bending of very thin tubes also is a difficulty.  I have only
+succeeded here by making very wide bends, but of course the blowing
+method is quite applicable to this case, and the effect may be
+obtained by welding in a rather thicker bit of tube, and drawing and
+blowing it till it is of the necessary thinness.  This is, however, a
+mere evasion of the difficulty.
+
+§ 36.  Spiral Tubes.
+
+These are easily made where good heating apparatus is available.  As,
+however, one constantly requires to bend tubes of about one-eighth
+inch in diameter into spirals in order to make spring connections for
+continuous glass apparatus, I will describe a method by which this is
+easily done.  Provide a bit of iron pipe about an inch and a quarter
+in outside diameter.  Cover this with a thick sheath of asbestos
+cloth, and sew the edges with iron wire.  Hammer the wire down so that
+a good cylindrical surface is obtained.  Make two wooden plugs for the
+ends of the iron pipe.  Bore one to fit a nail, which may be held in a
+small retort clip, and fasten a stout wire crank handle into the other
+one.  Support the neck of the handle by means of a second clip.  In
+this way we easily get a sort of windlass quite strong enough for our
+purpose.
+
+Fig.   35.
+
+Provide a large blow-pipe, such as the blow-pipe of a Fletcher
+crucible furnace, Select a length of tubing and clean it.  Lash one
+end to the cylinder by means of a bit of wire, and hold the other end
+out nearly horizontally.  Then start the blow-pipe to play on the tube
+just where it runs on to the asbestos cylinder, and at first right up
+to the lashing.  Get an attendant to assist in turning the handle of
+the windlass, always keeping his eye on the tube, and never turning so
+fast as to tilt the tube upwards.  By means of the blow-pipe, which
+may be moved round the tubing, heat the latter continuously as it is
+drawn through the flame, and lay it on the cylinder in even spirals.
+
+If the tubing is thin, a good deal of care will have to be exercised
+in order to prevent a collapse.  A better arrangement, which, however,
+I have not yet tried, would, I think, be to replace the blow-pipe by
+two bats-wing burners, permanently fastened to a stand, one of them
+playing its flame downwards on to the top of the flame of the other.
+The angle between the directions of the jets might be, say, 130°, or
+whatever is found convenient.  In this way the glass would not be so
+likely to get overheated in spots, and better work would doubtless
+result.  However, I have made numbers of perfectly satisfactory
+spirals as described.  Three or four turns only make a sufficiently
+springy connection for nearly all purposes.
+
+§ 37.  On Auxiliary Operations on Glass:-
+
+Boring Holes through Glass:-  This is much more easily done than is
+generally supposed.  The best mode of procedure depends on the
+circumstances.  The following three cases will be considered:-
+
+1.  Boring holes up to one-quarter inch diameter through thick glass
+(say over one-eighth inch), or rather larger holes through thin glass.
+
+2.  Boring holes of any size through thick glass.
+
+3.  Boring round holes through ordinary window glass.
+
+§ 38.  Boring small Holes.
+
+Take a three-cornered file of appropriate dimensions, and snip the
+point off by means of a hammer;  grind out most of the file marks to
+get sharp corners.  Dip the file in kerosene, and have plenty of
+kerosene at hand in a small pot.  Place the broken end of the file
+against the glass, and with considerable pressure begin to rotate it
+(the file) backwards and forwards with the fingers, very much as one
+would operate a bradawl against a hard piece of wood.  The surface of
+the glass will shortly be ground away, and then the file bradawl will
+make much quicker progress than might be expected.  Two or three
+minutes should suffice to bore a bit of sheet window-glass.
+
+The following points require attention:
+
+(1) Use any quantity of oil.
+
+(2) After getting through the skin  reduce the pressure on the file.
+
+(3) Be sure to turn the file backwards and forwards through a complete
+revolution at least.
+
+(4) When the hole is nearly through reduce the pressure.
+
+(5) When the hole is through the glass be exceedingly careful not to
+force the file through too rapidly, otherwise it will simply act as a
+wedge and cause a complete fracture.
+
+(6) In many cases it is better to harden the file in mercury before
+commencing operations; both files and glass differ so much in hardness
+that this point can only be decided by a trial.  If it is found
+necessary to harden the file, use either a large blow-pipe and a coke
+or charcoal bed, or else a small forge.  A small blowpipe, such as is
+generally found in laboratories, does more harm than good, either by
+burning the end of the file or raising it to an insufficient
+temperature.
+
+(7) To sharpen the file, which is often necessary after passing
+through the "skin" of the glass, put it in a vice so that the point
+just protrudes clear of the jaws.  Then, using a bit of waste iron as
+an intermediary anvil or punch, knock off the least bit from the
+point, so as to expose a fresh natural surface.  The same result may
+be brought about by the use of a pair of pliers.
+
+If several holes have to be bored, it is convenient to mount the file
+in the lathe and use a bit of flat hard wood to press up the glass by
+means of the back rest.  A drilling machine, if not too heavy, does
+very well, and has the advantage of allowing the glass to remain
+horizontal so that plenty of oil can be kept in the hole.
+
+Use a very slow speed in either case--much slower than would be used
+for drilling wrought iron.  It is essential that the lubricant should
+flow on to the end of the file very freely, either from a pipette or
+from the regular oil-feed.  If a little chipping where the file
+pierces the back surface is inadmissible, it is better, on the whole,
+to finish the bore by hand, using a very taper file.  It is not
+necessary to use a special file for the lathe, for a well-handled file
+can be chucked very conveniently in a three-jaw chuck by means of the
+handle.
+
+Mr. Shenstone recommends a lubricant composed of camphor dissolved in
+turpentine for general purposes.  With the object of obtaining some
+decisive information as to the use of this lubricant, and to settle
+other points, I made the following experiments.  Using an old
+three-cornered French file, I chipped off the point and adjusted the
+handle carefully.  I also ground out the file marks near the point,
+without hardening the file in mercury.  Using kerosene and turpentine
+and camphor, I began to bore holes in a hard bit of 3/32 inch window
+glass.
+
+Each hole was bored to about one-eighth inch in diameter in four
+minutes with either lubricant.  After hardening the file in mercury
+and using kerosene, I also required four minutes per hole.  After
+mounting the file in a lathe which had been speeded to turn up brass
+rods of about one-half-inch diameter, and therefore ran too fast, I
+required one and a half minutes per hole, and bored them right
+through, using kerosene.  On the whole, I think kerosene does as well
+as anything, and for filing is, I think, better than the camphor
+solution.  However, I ought to say that the camphor-turpentine
+compound has probably a good deal to recommend it, for it has survived
+from long ago.  My assistant tells me he has seen his grandfather use
+it when filing glass.
+
+I beg to acknowledge my indebtedness to Mr. Pye, of the Cambridge
+Scientific Instrument Company, for showing me in 1886 (by the courtesy
+of the Company) the file method of glass-boring;  it is also described
+by Faraday in Chemical Manipulation, 1228.
+
+It is not necessary, however, to use a file at all, for the twist
+drills made by the Morse Drill Company are quite hard enough in their
+natural state to bore glass.  The circumferential speed of the drill
+should not much exceed 10 feet per minute.  In this way the author has
+bored holes through glass an inch thick without any trouble except
+that of keeping the lubricant sufficiently supplied.  For boring very
+small holes watchmaker's drills may be used perfectly well, especially
+those tempered for boring hardened steel.  The only difficulty is in
+obtaining a sufficient supply of the lubricant, and to secure this the
+drill must be frequently withdrawn.
+
+My reason for describing the file method at such length is to be found
+in the fact that a Morse drill requires to be sharpened after drilling
+glass before it can be used in the ordinary way, and this is often a
+difficulty.
+
+I ought to say that I have never succeeded in boring the barrel of a
+glass tap by either of these methods.  [Footnote: I have been lately
+informed that it is usual to employ a splinter of diamond set in a
+steel wire holder both for tap boring and for drilling earthenware for
+riveting.  The diamond must, of course, be set so as to give
+sufficient clearance for the wire holder.
+
+For methods of using and setting diamond tools see § 55.  It will
+suffice to say here that a steel wire is softened and filed at one end
+so as to form a fork;  into this the diamond is set by squeezing with
+pliers.  The diamond is arranged so as to present a point in the axis
+of the wire, and must not project on one side of the wire more than on
+the other.  It is not always easy to get a fragment satisfying these
+conditions, and at the same time suitable for mounting.  A drop of
+solder occasionally assists the process of setting the diamond.
+
+In drilling, the diamond must be held against the work by a constant
+force, applied either by means of weight or a spring.  I made many
+trials by this method, using a watchmaker's lathe and pressing up the
+work by a weight and string, which passed over a pulley.  I used about
+40 ounces, and drilled a hole 3/32 in diameter in flint glass at a
+speed of 900 revolutions per minute to a depth of one-eighth of an
+inch in eight minutes.  I used soap and water as a lubricant, and the
+work was satisfactory.
+
+Since this was set up, I have been informed by Mr. Hicks of Hatton
+Garden that it is necessary to anneal glass rod by heating it up to
+the softening point and allowing it to cool very slowly under red-hot
+sand or asbestos before boring.  If this be done, no trouble will be
+experienced.  The annealing must be perfect.]
+
+§ 39.  For boring large holes through thick glass sheets, or, indeed,
+through anything where it is necessary to make sure that
+no accident can happen, or where great precision of position and form
+of hole is required, I find a boring tube mounted as shown in the
+picture (Fig.  36) is of great service.  Brass or iron tube borers do
+perfectly well, and the end of the spindle may be provided once for
+all with a small tube chuck, or the tubes may be separately mounted as
+shown.  A fairly high speed is desirable, and may be obtained either
+by foot, or, if power is available, is readily got by connecting to
+the speed cone of a lathe, which is presumably permanently belted to
+the motor.
+
+Fig.  36.
+
+After trying tubes armed with diamond dust, as will be presently
+explained, I find that emery and thin oil or turpentine, if liberally
+supplied below the glass, will do very nearly as well.  The tube
+should be allowed to rise from the work every few seconds, so as to
+allow of fresh emery and oil being carried into the circular grooves.
+This is done by lifting the hinged upper bearing, the drill being
+lifted by a spiral spring between the pulley and the lower bearing
+shown at B.  The glass may be conveniently supported on a few sheets
+of paper if flat, or held firm in position by wooden clamps if of any
+other shape.  In any case it should be firmly held down and should be
+well supported.  Any desired pressure upon the drill is obtained by
+weighting the hinged board A.
+
+§ 40.  The following method was shown to me by Mr. Wimshurst, but I
+have not had occasion to employ it myself.  It is suitable for boring
+large holes through such glass as the plates of Mr. Wimshurst's
+Influence machines are usually made of.  A diamond is mounted as the
+"pencil" of a compass, and with this a circle is drawn on the glass in
+the desired position.  The other leg of the compass of course rests on
+a suitable washer.
+
+To the best of my recollection the further procedure was as follows.
+A piece of steel rod about one-eighth inch in diameter was ground off
+flat and mounted in a vice vertically, so as to cause its plane end to
+form a small horizontal anvil.  The centre (approximately) of the
+diamond-cut circle of the glass was laid on this anvil so as to rest
+evenly upon it, and the upper surface (i.e.  that containing the cut)
+was then struck smartly with a hammer, completely pulverising the
+glass above the anvil.  The hole was gradually extended in a similar
+manner right up to the diamond cut, from which, of course, the glass
+broke away.
+
+A similar method has long been known to glaziers, differing from the
+preceding in that a series of diamond cuts are run across the circle
+parallel to two mutually perpendicular diameters.  A smart tap on the
+back of the scored disc will generally cause the fragments to tumble
+out.  I have never tried this myself, but I have seen it done.
+
+Large discs may easily be cut from sheet glass by drawing a circular
+diamond cut, and gradually breaking away the outer parts by the aid of
+additional cuts and a pair of pliers or "shanks" (see Fig.  44).
+
+§ 41.  Operations depending on Grinding: Ground-in Joints.
+
+The process will be perfectly understood by reference to a simple
+case.  Suppose it is desired to grind the end of a tube into the neck
+of a bottle.  If a stoppered bottle is available, the stopper must be
+taken out and measured as to its diameter at the top and bottom.
+Select a bit of tube as nearly as possible of the same diameter as the
+stopper at its thickest part.  Draw down the glass in the blow-pipe
+flame rather by allowing it to sink than by pulling it out.  After a
+few trials no difficulty will be experienced in making its taper
+nearly equal to that of the stopper, though there will in all
+probability be several ridges and inequalities.  When this stage is
+reached anneal the work carefully and see that the glass is not too
+thin.  Afterwards use emery and water, and grind the stopper into the
+bottle.
+
+There are six special directions to be note
+
+(1 )Turn the stopper through at least one revolution in each
+direction.
+
+(2) Lift it out often so as to give the fresh emery a chance of
+getting into the joint.
+
+(3) Rotate the bottle as well as the stopper in case there is any
+irregularity in the force brought to bear, which might cause one side
+of the neck to be more ground than another, or would cause the tube to
+set rather to one side or the other.
+
+(4) Use emery passing a 50 sieve, i.e.  a sieve with fifty threads to
+the inch run (see § 144) to begin with, and when the stopper nearly
+fits, wash this thoroughly away, and finish with flour emery,
+previously washed to get rid of particles of excessive size;  the
+process of washing will be fully discussed in the chapter on
+glass-grinding, which see.
+
+(5) Any degree of fineness of surface may be obtained by using graded
+emery, as will be explained, but, in general, it is unnecessary to
+attempt a finer surface than can be got with washed flour emery.  A
+superficial and imperfect polish may be given by grinding for a short
+time with powdered pumice stone.
+
+(6) If the proper taper is not attained by blowing, or if ridges are
+left on the tapered part, the process may be both hastened and
+improved by giving the taper a preliminary filing with a
+three-cornered file and kerosene, just as one would proceed with iron
+or brass.  A little filing will often save a good deal of grinding and
+make a better job.
+
+If a bottle without a tapered neck is to be employed, it is as well to
+do the preliminary grinding by means of a cone turned up from a bit of
+cast iron.  This is put in the lathe and pushed into the mouth of the
+bottle, the latter being supported by the hands.  Use about the same
+surface speed as would be employed for turning cast iron.  In this
+case the emery is better used with kerosene.
+
+If a cylindrical bit of cast iron about an inch in diameter is turned
+down conically nearly to a point, it will save a good deal of trouble
+in making separate cones.  If it gets ground into rings, and it
+becomes necessary to turn it up, use a diamond tool until the skin is
+thoroughly removed;  the embedded emery merely grinds the edge off any
+ordinary steel tool.
+
+For diamond tools see § 55.
+
+§ 42.  Use of the Lathe in Glass-working.
+
+If it is necessary to remove a good deal of glass, time may be saved
+by actually turning the glass in a lathe.  According to the direction
+given above for grinding a tube into the neck of a bottle, very little
+glass need be removed if the drawing down is well done, so that for
+this purpose turning is often unnecessary.
+
+If the taper of the stopper be small and it is permissible to use a
+thick tube, or if a solid stopper only has to be provided, or an old
+stopper quickly altered to a new form, turning is very useful.  The
+glass may be "chucked" in any suitable manner, and run at a speed not
+exceeding 10 feet per minute.  Prepare a three-cornered file by
+mercury-hardening and by grinding the end flat so as to form a cutting
+angle of about 80°, and use a moderate amount of kerosene lubrication,
+i.e.  enough to keep the glass damp, but even this is not essential.
+Use the file as an ordinary brass turning tool, and press much more
+lightly than for metal turning.  The glass will be found to scrape off
+quite pleasantly.
+
+By chucking glass tubes on wooden mandrells the ends may be nicely
+turned in this manner ready for accurate closing by glass plates.
+
+The process of grinding also is made much more rapid--at all events
+in the earlier stages--by chucking either the stopper or the bottle
+and holding the other member in the fingers, or in a wooden vice held
+in the hands.  The finishing touches are best given by hand.
+
+I ought to say that I think a good deal of glass-grinding, as
+practised in laboratories, might be advantageously replaced by glass
+turning or filing and certainly will be by any one who will give these
+methods a trial.
+
+If one tube is to be ground into another, as in grinding a retort into
+a receiver, the latter must be drawn down from a larger piece, few
+beginners being able to widen a tube by the method explained with
+sufficient ease and certainty.  The other operations are similar to
+the operations above described.
+
+§ 43.  Funnels often require to be ground to an angle of 60°.  For
+this purpose it is well to keep a cast-iron cone, tapering from
+nothing up to four inches in diameter.  This may be mounted on a
+lathe, and will be found of great use for grinding out the inside of
+funnels.  Care must be taken to work the funnel backwards and
+forwards, or it will tend to grind so as to form rings, which
+interfere with filtering.  A rough polish may be given on the lines
+explained in the next section.
+
+§ 44.  A rough polish may be easily given to a surface which has been
+finished by washed flour emery, in the following manner.  Turn up a
+disc of soft wood on the lathe, and run it at the highest wood-turning
+speed.  Rub into the periphery a paste of sifted powdered pumice stone
+and water.
+
+Any fairly smooth ground glass surface may be more or less polished by
+holding it for a moment against the revolving disc.  Exact means of
+polishing will be described later on.  Meanwhile this simple method
+will be found both quick and convenient, and is often quite sufficient
+where transparency, rather than figure, is required.  I daresay a fine
+polish may be got on the same lines, using putty powder or washed
+rouge (not jewellers' rouge, which is too soft, but glass-polishers'
+rouge) to follow the pumice powder, but I have not required to try
+this.
+
+§ 45.  It is sometimes required to give to ground glass surfaces a
+temporary transparency.  This is to be done by using a film of oil of
+the same refractive index as the glass.  Cornu has employed a varnish
+consisting of a mixture of turpentine and oil of cloves, but the
+yellow-brown colour of the latter is often a disadvantage.  It will be
+found that a mixture of nut oil and oil of bitter almonds, or of
+bromo-napthalene and acetone, can be made of only a faint yellow
+colour;  and by exact adjustment of the proportions will have the same
+refractive index for any ray as crown glass (ordinary window glass).
+
+Procure a sample of the glass and smash it up to small fragments in an
+iron mortar.  Sift out the fine dust and the larger pieces;  bits
+about as large as small beads--say one-sixteenth inch every way--do
+very well.  Boil the sifted glass with strong commercial hydrochloric
+acid to remove iron, wash with distilled water and a few drops of
+alcohol, dry on blotting paper in the sun or otherwise.  Put the dry
+glass into a bottle or beaker, and begin by adding almond oil (or
+bromo-napthalene), then add nut oil (or acetone) till the glass
+practically disappears when examined by sodium light, or light of any
+other wave-length, as may be required.
+
+The adjustment of the mixture is a matter of great delicacy, one drop
+too much of either constituent, in, say, 50 cubic centimetres, makes
+all the difference.  The final adjustment is best accomplished by
+having two mixtures of the oils, one just too rich in almond, the
+other in nut oil;  by adding one or other of these, the required
+mixture is soon obtained.
+
+It is to be noted
+
+(1) That adjustment is only perfect for light of one wave-length.
+
+(2) That adjustment is only perfect at one temperature.
+
+On examining a bottle of rather larger fragments of glass immersed in
+an adjusted mixture by ordinary daylight, a peculiarly beautiful play
+of colours is seen.
+
+Of course, if it is only desired to make ground glass fairly
+transparent, these precautions are unnecessary, but it seemed better
+to dispose of the matter once for all in this connection.
+
+M. Cornu's object was to make a varnish which would prevent reflection
+from the back of a photographic plate on to the film. I have had
+occasion to require to do the same when using a scale made by cutting
+lines through a film of black varnish on a slip of glass.  This
+succeeded perfectly by making the varnish out of Canada balsam stained
+with a black aniline dye.
+
+Mr. Russell, Government Astronomer of New South Wales, finds that the
+"halation" of star photographs can be prevented by pouring over the
+back of the plate a film of collodion suitably stained.
+
+§ 46.  Making Ground Glass.
+
+This is easily done by rubbing the surface of polished glass with a
+bit of cast iron and washed "flour of emery." Of course, if the
+fineness of grain of the surface is of importance, appropriate sizes
+of emery must be employed.  The iron may be replaced by a bit of glass
+cut with transverse grooves to allow the emery to distribute itself,
+or even by a bit of glass without such grooves, provided it does not
+measure more than one or two inches each way.  If great speed is an
+object rather than the fineness of the surface, use a bit of lead and
+coarse emery, say any that will pass a sieve with fifty threads to the
+inch.
+
+It may perhaps be mentioned here that it is a pity to throw away emery
+which has been used between glass and glass.  In the chapter dealing
+with fine optical work the use of emery of various grades of fineness
+will be treated, and the finer grades can only be obtained (to my
+knowledge) from emery which has been crushed in the process of glass
+or metal grinding, especially the former.  A large jam-pot covered
+with a cardboard lid does well as a receptacle of washings.
+
+§ 47.  Glass-cutting.
+
+This is an art about which more can be learned in five minutes by
+watching it well practised than by pages of written description.  My
+advice to any one about to commence the practice of the art would be
+to make friends with a glazier and see it done.  What follows is
+therefore on the supposition that this advice has been followed.
+
+After some experience of cutters made of especially hardened steel, I
+believe better work can generally be got out of a diamond, provided
+the cost is not an objection.  It is economy to pay a good price for a
+good diamond.  As is well known, the natural angle of the crystal
+makes the best point, and a person buying a diamond should examine the
+stone by the help of a lens, so as to see that this condition is
+fulfilled.  The natural angle is generally, if not always, bounded by
+curved edges, which have a totally different appearance from the sharp
+edges of a "splinter."
+
+When a purchase is to be made, it is as well for the student to take a
+bit of glass and a foot-rule with him, and to test the diamond before
+it is taken away.  When a good diamond has been procured, begin by
+taking cuts on bits of clean window glass until the proper angle at
+which to hold the tool is ascertained.  Never try to cut over a
+scratch, if you value your diamond, and never press hard on the glass;
+a good cut is accompanied by an unmistakable ringing sound quite
+different from the sound made when the diamond is only scratching.
+
+Perhaps the most important advice that can be given is, Never lend the
+diamond to anybody--under any circumstances.
+
+The free use of a diamond is an art which the physicist will do well
+to acquire, for quite a variety of apparatus may be made out of glass
+strips, and the accuracy with which the glass breaks along a good cut
+reduces such an operation as glass-box-making to a question of
+accurate drawing.
+
+§ 48.  Cementing.
+
+One of the matters which is generally confused by too great a
+profusion of treatment is the art of cementing glass to other
+substances.
+
+The following methods will be found to work, subject to two
+conditions:
+
+(1) The glass must be clean;
+
+(2) it must be hot enough to melt the cement.
+
+For ordinary mending purposes when the glass does not require to be
+placed in water (especially if hot) nothing is better than that kind
+of glue which is generally called "diamond cement."  This may be
+easily made by dissolving the best procurable isinglass in a mixture
+of 20 per cent water and 80 per cent glacial acetic acid--the exact
+proportions are not of consequence.
+
+First, the isinglass is to be tightly packed into a bottle with a wide
+neck, then add the water, and let the isinglass soak it up.
+Afterwards pour in the acetic acid, and keep the mixture near 100°C.
+for an hour or two on the water bath--or rather in it.  The total
+volume of acetic acid and water should not be more than about half of
+the volume of isinglass when the latter is pressed into the bottle as
+tightly as possible.
+
+The proper consistency of the cement may be ascertained by lifting a
+drop out of the bottle and allowing it to cool on a sheet of glass.
+In ten minutes it ought not to be more than slightly sticky, and the
+mass in the bottle, after standing a few hours cold, should not be
+sticky at all, and should yield, jelly-like, to the pressure of the
+finger to only a slight degree.  If the glue is too weak, more
+isinglass may be added (without any preliminary soaking).
+
+A person making the mixture for the first time almost always gets it
+too weak.  It is difficult to give exact proportions by weight, as
+isinglass and gelatine (which may replace it) differ greatly in
+quality.  This cement is applied like glue, and will cement nearly
+anything as well as glass.  Of course, as much cement as possible must
+be squeezed out of any joint where it is employed.  The addition of
+gums, as recommended in some books, is unnecessary.
+
+Ordinary glue will serve perfectly for cementing glass to wood.
+
+"Chipped glass" ware is, I understand, made by painting clean glass
+with glue.  As the glue dries and breaks by contraction, it chips off
+the surface of the glass.  I have never seen this done.  In nearly all
+cases where alcohol is not to be employed very strong joints may be
+made by shellac.  Orange shellac is stronger than the "bleached"
+variety.
+
+A sine qua non is that the glass be hot enough to melt the shellac.
+The best way is to heat the glass surfaces and rub on the shellac from
+a bit of flake;  the glass should not be so hot as to discolour the
+shellac appreciably, or its valuable properties will be partly
+destroyed.  Both glass surfaces being thus prepared, and the shellac
+being quite fluid on both, they may be brought together and clamped
+tightly together till cool.  Shellac that has been overheated, or
+dissolved in alcohol, or bleached, is of little use as compared with
+the pale orange flaky product.  Dark flakes have probably been
+overheated during the preliminary refining.
+
+For many purposes a cement is required capable of resisting carbon
+bisulphide.  This is easily made by adding a little treacle (say 20
+per cent) to ordinary glue.  Since the mixture of glue and treacle
+does not keep, i.e.  it cannot be satisfactorily melted up again after
+once it has set, no more should be made up than will be wanted at the
+time.  If the glue be thick, glass boxes for carbon disulphide may be
+easily put together, even though the edges of the glass strips are not
+quite smooth, for, unlike most cements, this mixture remains tough,
+and is fairly strong in itself.
+
+I have found by experiment that most fixed and, to a less degree,
+essential oils have little or no solvent action on shellac, and I
+suspect that the same remark applies to the treacle-glue mixture, but
+I have not tried.  Turpenes act on shellac slightly, but mineral oils
+apparently not at all.  The tests on which these statements are based
+were continued for about two years, during which time kerosene and
+mineral oils had no observable effect on shellac--fastened
+galvanometer mirrors.
+
+§ 49.  Fusing Electrodes into Glass.
+
+This art has greatly improved since the introduction of the
+incandescent lamp; however, up to the present, platinum seems to
+remain the only substance capable of giving a certainly air-tight
+result.  I have not tried the aluminium-alumina method.
+
+Many years ago it was the fashion to surround the platinum wire with a
+drop of white enamel glass in order to cause better adhesion between
+it and the ordinary glass.  [Footnote: Hittorf and Geissler (Pogg.
+Ann.  1864, § 35;  English translation, Phys.  Soc.  London, p.  138)
+found that it was impossible to make air-tight joints between platinum
+and hard potash glass, but that soft lead glass could be used with
+success as a cement.]  However, in the case of flint glass, if one may
+judge from incandescent lamps, this is not essential--a fact which
+entirely coincides with my own experience.
+
+On the other hand, when sealing electrodes into German glass I have
+often used a drop of enamel with perfect results, though this is not
+always done in Germany.  In all cases, however, in which electrodes
+have to be sealed in--especially when they are liable to heat--I
+recommend flint glass, and in this have the support of Mr. Rain (The
+Incandescent Lamp and its Manufacture, p.  131).  The exact details
+for the preparation of eudiometer tubes are given by Faraday (Chemical
+Manipulation, § 1200).
+
+In view of what has preceded, however, I will content myself with the
+following notes.  Make the hole through which the wire is to protrude
+only slightly larger than the wire itself, and be sure that the latter
+is clean.  Allow the glass to cool sufficiently not to stick to the
+wire when the latter is pushed in.  Be sure that, on heating, the
+glass does not get reduced, and that it flows up to the wire all
+round;  pull and push the wire a little with a pair of pincers, to
+ensure this.
+
+It is not a bad plan to get the glass exceedingly fluid round the
+wire--even if the lump has to be blown out a little afterwards--as it
+cools.  The seal should finally be well annealed in asbestos, but
+first by gradually moving it into the hot air in front of the flame.
+
+It was observed by Professor J.  J.  Thomson and the author some years
+ago (Proc.  Roy.  Soc.  40.  331.  1886) that when very violent
+discharges are taken through lightly sealed-in electrodes in
+lead-glass tubes--say from a large battery of Leyden jars--gas
+appears to be carried into the tube over and above that naturally
+given off by the platinum, and this without there being any apparent
+want of perfection in the seal.  This observation has since been
+confirmed by others.  Consequently in experiments on violent
+discharges in vacuo where certainty is required as to the exclusion of
+air, the seals should be protected by a guard tube or cap containing
+mercury; this must, of course, be put in hot and clean, on hot and
+clean glass, and in special cases should be boiled in situ.
+
+A well-known German physicist (Warburg, I think) recommends putting
+the seals under water, but I cannot think that this is a good plan,
+for if air can get in, why not water?  which has its surface tension
+in its favour.  The same reasoning prevents my recommending a layer of
+sulphuric acid above the mercury-a method used for securing
+air-tightness in "mercury joints" by Mr. Gimingham, Proc.  R.  S.
+1874.
+
+Further protection may be attained for many purposes by coating the
+platinum wire with a sheath of glass, say half an inch long, fused to
+the platinum wire to a depth of one-twentieth of an inch all round.
+
+In some cases the electrodes must be expected to get very hot, for
+instance, when it is desired to platinise mirrors by the device of
+Professor Wright of Yale.  In this and similar cases I have met with
+great success by using "barometer" tubes of about one-twelfth of an
+inch bore, and with walls, say, one-tenth of an inch thick.
+[Footnote: "Barometer" tube is merely very thick-walled glass tubing,
+and makes particularly bad barometers, which are sold as weather
+glasses.]
+
+This tube is drawn down to a long point--say an inch long by
+one-eighth of an inch external diameter, and the wire is fused in for
+a length, say, of three-quarters of an inch, but only in the narrow
+drawn--down part of the tube.  At different times I have tried four
+such seals, and though the electrodes were red hot for hours, I have
+never had an accident--of course they were well annealed.
+
+Fig.  37.
+
+For directions as to the making of high vacuum tubes, see the section
+dealing with that matter.
+
+§ 50.  As economy of platinum is often of importance, the following
+little art will save money and trouble.  Platinum is easily caused to
+join most firmly to copper--with which, I presume, it alloys--by the
+following method.  Hold the platinum wire against the copper wire, end
+to end, at the tip of the reducing flame of a typical blowpipe--or
+anywhere--preferably in the "reducing" part of the oxygas flame;  in
+a moment the metals will fuse together at the point of contact, when
+they may be withdrawn.
+
+Such a joint is very strong and wholly satisfactory, much better than
+a soldered joint.  If the work is not carried out successfully so that
+a considerable drop of copper-platinum alloy accumulates, cut it off
+and start again.  The essence of success is speed, so that the copper
+does not get "burned." If any considerable quantity of alloy is formed
+it dissolves the copper, and weakens it, so that we have first the
+platinum wire, then a bead of alloy, and then a copper wire fused into
+the bead, but so thin just outside the latter that the joint has no
+mechanical strength.
+
+§ 51.  The Art of making Air-light Joints.
+
+Lamp-manufacturers and others have long since learned that when glass
+is in question not only are fused joints made as easily as others, but
+that they afford the only reliable form of joint.  An experimenter who
+uses flint glass, has a little experience, an oxygas blow-pipe and a
+blowing apparatus, will prefer to make his joints in this way, simply
+from the ease with which it may be done.  When it comes to making a
+tight joint between glass and other substances the problem is by no
+means so easy.  Thus Mr. Griffiths (Phil.  Trans.  1893, p.  380)
+failed to make air-tight joints by cementing glass into steel tubes,
+using hard shellac, and the tubes fitting closely.  These joints were
+satisfactory at first, but did not last; the length of the joint is
+not stated.  The difficulty was finally got over by soldering very
+narrow platinum tubes into the steel, and fusing the former into the
+glass.
+
+Mr. Griffiths has since used an alloy with success as a cement, but I
+cannot discover what it is made from. Many years ago Professor Hittorf
+prepared good high vacuum tubes by plugging the ends of glass tubes
+with sealing wax merely, though in all cases the spaces to be filled
+with wax were long and narrow (Hittorf, Pogg.  Ann.  1869, § 5,
+English translation, Phys.  Soc.  p.  113).  Again, Regnault
+habitually used brass ferules, and cemented glass into them by means
+of his mastic, which can still be procured at a low rate from his
+instrument-makers (Golan, Paris).  Lenard also, in his investigations
+on Cathode Rays (Wied.  Ann, vol.  li.  p.  224), made use of sealing
+wax covered with marine glue.
+
+Surely in face of these facts we must admit that cement joints can be
+made with fair success.  I do not know the composition of M.
+Regnault's mastic, but Faraday (Manipulations, § 1123) gives the
+following receipt for a cement for joining ferules to retorts, etc:
+
+Resin                             5 parts.
+
+Beeswax                           1 part.
+
+Red ochre or Venetian red,
+finely powdered and sifted        1 part.
+
+I believe this to be substantially the same as Regnault's mastic,
+though I have never analysed the latter.
+
+For chemical work the possibility of evolution of gas from such a
+cement must be taken into account, and I should certainly not trust it
+for this reason in vacuum tube work, where the purity of the confined
+gas could come in question.  Otherwise it is an excellent cement, and
+does not in my experience tend to crack away from glass to the same
+extent as paraffin or pure shellac.
+
+This cracking away from glass, by the way, is probably an effect of
+difference in rate of expansion between the glass and cement which
+probably always exists, and, if the cement be not sufficiently
+viscous, must, beyond certain temperature limits, either produce
+cracks or cause separation.  Professor Wright of Yale has used a hard
+mineral pitch as a cement in vacuum work with success.
+
+My attention has been directed to a fusible metal cement containing
+mercury, and made according to the following receipt, given by Mr. S.
+G.  Rawson, Journal of the Society of Chemical Industry, vol.  ix.
+(1890), P.  150:-
+
+Bismuth         40 per cent
+
+Lead             25 per cent
+
+Tin             10 per cent
+
+Cadmium         10 per cent
+
+Mercury         15 per cent
+
+This is practically one form of Rose's fusible metal with 15 per cent
+mercury added.  It takes nearly an hour to set completely, and the
+apparatus must be clean and warm before it is applied.
+
+As the result of several trials by myself and friends, I am afraid I
+must dissent from the claim of the author that such a cement will make
+a really air-tight joint between glass tubes.  Indeed, the appearance
+of the surface as viewed through the glass is not such as to give any
+confidence, no matter what care may have been exercised in performing
+all the operations and cleaning the glass;  besides which the cement
+is rigid when cold, and the expansion difficulty comes in.
+
+On the other hand, if extreme air-tightness is not an object, the
+cement is strong and easily applied, and has many uses.  I have an
+idea that if the joints were covered with a layer of soft wax, the
+result would be satisfactory in so far as air-tightness is concerned.
+
+This anticipation has since been verified.
+
+In many cases one can resort to the device already mentioned of
+enclosing a rubber or tape-wrapped joint between two tubes in a bath
+of mercury, but in this case the glass must be clean and hot and the
+mercury also warm, dry, and pure when the joint is put together,
+otherwise an appreciable air film is left against the glass, and this
+may creep into the joint.
+
+Perhaps the easiest way of making such a joint is to use an outer tube
+of thin clean glass, and bore a narrow hole into it from one side to
+admit the mercury;  if the mercury is to be heated in vacuo, it is
+better to seal on a side joint.  It is always better, if possible, to
+boil the mercury in situ, which involves making the wrapping of
+asbestos, but, after all, we come back to the position I began by
+taking up, viz.  that the easiest and most reliable method is by
+fusion of the glass--all the rest are unsuitable for work of real
+precision.
+
+I should be ungrateful, however, were I not to devote a few lines to
+the great convenience and merit of so-called "centering cement." This
+substance has two or three very valuable properties.  It is very tough
+and strong in itself, and it remains plastic on cooling for some time
+before it really sets.  If for any reason a small tube has to be
+cemented into a larger one, which is a good deal larger, so that an
+appreciable mass of cement is necessary, and particularly if the joint
+requires to have great mechanical strength, this cement is invaluable.
+I have even used a plug of it instead of a cork for making the joint
+between a gas delivery tube and a calcium chloride tower.  (Why are
+these affairs made with such abominable tubulures?)
+
+The joint in question has never allowed the tube to sag though it
+projects horizontally to a distance of 6 inches, and has had to
+withstand nearly two years of Sydney temperature.  The cement consists
+of a mixture of shellac and 10 per cent of oil of cassia.
+
+The shellac is first melted in an iron ladle, and the oil of cassia
+quickly added and stirred in, to an extent of about 10 per cent, but
+the exact proportions are not of importance.  Great care must be taken
+not to overheat the shellac.
+
+APPENDIX TO CHAPTER I
+
+ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
+ROENTGEN'S RADIATION
+
+[Footnote: Written in May 1896.]
+
+WHEN Professor Roentgen's discovery was first announced at the end of
+1895 much difficulty was experienced in obtaining radiation of the
+requisite intensity for the repetition of his experiments.  The
+following notes on the production of vacuum tubes of the required
+quality may therefore be of use to those who desire to prepare their
+own apparatus.  It appears that flint glass is much more opaque to
+Roentgen's radiation than soda glass, and consequently the vacuum
+tubes require to be prepared from the latter material.
+
+Fig. 39.
+
+A form of vacuum tube  which has proved very successful in the
+author's hands is sketched in Fig.  38.  It is most easily constructed
+as follows.  A bit of tubing about 2 centimetres diameter, 15
+centimetres long, and 1.5 millimetre wall thickness, is drawn down to
+a point.  The larger bulb, about 5 centimetres in diameter, is blown
+at one end of this tube.  The thinner the bulb the better, provided
+that it does not collapse under atmospheric pressure.  A very good
+idea of a proper thickness may be obtained from the statement that
+about 4 centimetres length of the tubing should be blown out to form
+the bulb.  This would give a bulb of about the thickness of an
+ordinary fractionating bulb.  Before going any further it is as well
+to test the bulb by tapping on the table and by exhausting it by means
+of an ordinary water-velocity pump.
+
+The side tube is next prepared out of narrower tubing, and is provided
+with a smaller bulb, a blowing-out tube, and a terminal, to be made as
+will be described.  This side tube is next fused on to the main tube,
+special care being taken about the annealing, and the cathode terminal
+is then sealed into the main tube.  After using clean glass it is in
+general only necessary to rinse the tube out with clean alcohol, after
+which it may be dried and exhausted.
+
+The success of the operation will depend primarily on the attention
+given to the preparation and sealing-in of the electrode facing the
+large bulb.
+
+Preparation of Terminals.  Some platinum wire of about No.  26
+B.W.G--the exact size is unimportant--must be provided, also some
+sheet aluminium about 1 millimetre thick, some white enamel cement
+glass, and a "cane" of flint-glass tube of a few millimetres bore.
+
+The electrodes are prepared by cutting discs of aluminium of from 1 to
+1.5 centimetres diameter.  The discs of aluminium are bored in the
+centre, so as to admit the "stems" which are made of aluminium wire
+of about 1 millimetre diameter.  The stems are then riveted into the
+discs.  The "stems" are about I centimetre long, and are drilled to a
+depth of about 3 millimetres, the drill used being about double the
+diameter of the platinum wire to be used for making the connections.
+The faces of the electrodes--i.e.  the free surfaces of the aluminium
+discs--are then hammered flat and brought to a burnished surface by
+being placed on a bit of highly polished steel and struck by a "set"
+provided with a hole to allow of the "stem" escaping damage.  The
+operation will be obvious after a reference to Figs.  39 and 40;  it
+is referred to again on page 96.
+
+The platinum wires may be most conveniently attached by melting one
+end of the piece of platinum wire in the oxygas blow-pipe till it
+forms a bead just large enough to pass into the hole drilled up the
+stem of the electrode.  The junction between the stein and the
+platinum wire is then made permanent by squeezing the aluminium down
+upon the platinum wire with the help of a pair of pliers.  It is also
+possible to fuse the aluminium round the platinum, but as I have had
+several breakages of such joints, I prefer the mechanical connection
+described.
+
+Fig.  39. Sets for striking aluminium electrodes
+
+Fig.  40.
+
+i. Aluminium electrode.
+
+ii. Aluminium electrode connected to platinum wire.
+
+iii. Aluminium electrode connected to platinum wire and protected by
+glass.
+
+iv. Detail of fastening platinum wire.
+
+The stem and platinum wire may now be protected by covering them with
+a little flint glass.  For this purpose the flint-glass tube is pulled
+down till it will just slip over the stem and wire, and is cut off so
+as to leave about half a centimetre of platinum wire projecting.  The
+flint-glass tube is then fused down upon the platinum wire, care being
+taken to avoid the presence of air bubbles.  At the close of the
+operation a single drop of white enamel glass is fused round the
+platinum wire at a high temperature, so as to make a good joint with
+the protecting flint-glass tube.
+
+The negative electrode being nearly as large as the main tube, it must
+be introduced before the latter is drawn down for sealing.  After
+drawing down the main tube in the usual manner, taking care not to
+make it less than a millimetre in wall-thickness, it is cut off so as
+to leave a hole not quite big enough for the enamel drop to pass
+through.  By heating and opening, the aperture is got just large
+enough to allow the enamel drop to pass into it, and when this is the
+case the joint is sealed, pulled, and blown out until the electrode
+occupies the right position--viz.  in the centre of the tube and with
+its face normal to the axis of the tube.
+
+The glass walls near the negative electrode must not be less than a
+millimetre thick, and may be rather more with advantage, the glass
+must be even, and the joint between the flint glass and the soda
+glass, or between the wire and the soda glass, must be wholly through
+the enamel.  The "seal" must be well annealed.  It will be found that
+the sealing-in process is much easier when the stem of the electrode
+is short and when the glass coating is not too heavy.  Half a
+millimetre of glass thickness round the stein is quite sufficient.
+
+The diagram, of the tube shows that the main tube has been expanded
+round the edges of the cathode.  This is to reduce the heating
+consequent on the projection of cathode rays from the edges of the
+disc against the glass tube.
+
+The anode is inserted into its bulb in a quite similar manner.  If
+desired it may be made considerably smaller, and does not need the
+careful adjustment requisite in sealing-in the cathode, nor does the
+glass near the entry wire require to be so thick.
+
+More intense effects are often got by making the cathode slightly
+concave, but in this case the risk of melting the thin glass is
+considerably increased.  No doubt, Bohemian glass might be used
+throughout instead of soft soda glass, and this would not melt so
+easily; the difficulties of manipulating the glass are, however, more
+pronounced.
+
+It will be shown directly that the best Roentgen effects are got with
+a high vacuum, and it is for this reason that the glass near the
+cathode seal requires to be strong.  The potential right up to the
+cathode is strongly positive inside the tube, and this causes the
+glass to be exposed to a strong electric stress in the neighbourhood
+of the seal.
+
+Although the glass-blowing involved in the making of a so-called focus
+tube is rather more difficult than in the case just described, there
+is no reason why such a difficulty should not be overcome;  I will
+therefore explain how a focus tube may be made.
+
+Fig. 41.
+
+A bulb about 3 inches in diameter is blown from a bit of tube of a
+little more than 1 inch diameter.  Unless the walls of the tube are
+about one-eighth of an inch in thickness, this will involve a
+preliminary thickening up of the glass.  This is not difficult if care
+be taken to avoid making the glass too hot.  The larger gas jet
+described in connection with the soda-glass-blowing table must be
+employed.  In blowing a bulb of this size it must not be forgotten
+that draughts exercise a very injurious influence by causing the glass
+to cool unequally;  this leads to bulbs of irregular shape.
+
+In the method of construction shown in Fig.  41, the anode is put in
+first.  This anode simply consists of a square bit of platinum or
+platinum-iridium foil, measuring about 0.75 inch by 1 inch, and
+riveted on to a bent aluminium wire stem.
+
+As soon as the anode is fused in, and while the glass is still hot,
+the side tube is put on.  The whole of the anode end is then carefully
+annealed.  When the annealing is finished the side tube is bent as
+shown to serve as a handle when the time comes to mount the cathode.
+Before placing the cathode in position, and while the main tube is
+still wide open, the anode is adjusted by means of a tool thrust in
+through this open end.  This is necessary in view of the fact that the
+platinum foil is occasionally bent during the operation of forcing the
+anode into the bulb.
+
+The cathode is a portion of a spherical surface of polished aluminium,
+a mode of preparing which will be given directly.  The cathode having
+been placed inside the bulb, the wide glass tube is carefully drawn
+down and cut off at such a point that when the cathode is in position
+its centre of curvature will lie slightly in front of the anode plate.
+For instance, if the radius of curvature of the cathode be 1.5 inches,
+the centre of curvature may lie something like an eighth of an inch or
+less in front of the anode.
+
+The cathode as shown in Fig.  41 is rather smaller than is
+advantageous.  To make it much larger than is shown, however, the
+opening into the bulb would require to be considerably widened, and
+though this is not really a difficult operation, still it requires
+more practice than my readers are likely to have had.  The difficulty
+is not so much in widening out the entry as in closing it down again
+neatly.
+
+Now as to making the anode.  A disc of aluminium is cut from a sheet
+which must not be too thick--one twenty-fifth of an inch is quite
+thick enough.  This disc is bored at the centre to allow of the stem
+being riveted in position.  The disc is then annealed in the Bunsen
+flame and the stem riveted on.
+
+The curvature is best got by striking between steel dies (see Figs.
+39 and 40).  Two bits of tool steel are softened and turned on the
+lathe, one convex and the other concave.  The concave die has a small
+hole drilled up the centre to admit the stem. The desired radius of
+curvature is easily attained by cutting out templates from sheet zinc
+and using them to gauge the turning.  The two dies are slightly ground
+together on the lathe with emery and oil and are then polished, or
+rather the convex die is polished--the other one does not matter.
+The polishing is most easily done by using graded emery and oil and
+polishing with a rag.  The method of grading emery will be described
+in the chapter on glass-grinding.
+
+The aluminium disc is now struck between the dies by means of a
+hammer.  If the radius of curvature is anything more than one inch and
+the disc not more than one inch in diameter the cathode can be struck
+at once from the flat as described.  For very deep curves no doubt it
+will be better to make an intermediate pair of dies and to re-anneal
+the aluminium after the first striking.
+
+When the tube is successfully prepared so far as the glassblowing goes
+it may be rinsed with strong pure alcohol both inside and out, and
+dried.  The straight part of the side tube is then constricted ready
+for fusing off and the whole affair is placed on the vacuum pump.
+
+In spite of the great improvements made during recent years in the
+construction of so-called Geissler vacuum pumps--i.e.  pumps in which
+a Torricellian vacuum is continually reproduced--I am of opinion that
+Sprengel pumps are, on the whole, more convenient for exhausting
+Crooke's tubes.  A full discussion of the subject of vacuum pumps will
+be found in a work by Mr. G.  S.  Ram (The Incandescent Lamp and its
+Manufacture), published by the Electrician Publishing Company, and it
+is not my intention to deal with the matter here; the simplest kind of
+Sprengel pump will be found quite adequate for our purpose, provided
+that it is well made.
+
+Fig.  42 is intended to represent a modification of a pump based on
+the model manufactured by Hicks of Hatton Garden, and arranged to suit
+the amateur glass-blower.  The only point of importance is the
+construction of the head of the fall tube, of which a separate and
+enlarged diagram is given.  The fall tubes may have an internal
+diameter up to 2 mm. (two millimetres) and an effective length of 120
+cm.
+
+Free use is made of rubber tube connections in the part of the pump
+exposed to the passage of mercury.  The rubber employed should be
+black and of the highest quality, having the walls strengthened by a
+layer of canvas.  If such tube cannot be easily obtained, a very good
+substitute may be made by placing a bit of ordinary black tube inside
+another and rather larger bit and binding the outer tube with tape or
+ribbon.  In any case the tubing which comes in contact with the
+mercury should be boiled in strong caustic potash or soda solution for
+at least ten minutes to get rid of free sulphur, which fouls the
+mercury directly it comes in contact with it.  The tubing is well
+washed, rinsed with alcohol, and carefully dried.
+
+Fig.  42.
+
+The diagram represents what is practically a system of three Sprengel
+pumps, though they are all fed from the same mercury reservoir and run
+down into the same mercury receiver.  It is much easier to make three
+pumps, each with separate pinch cocks to regulate the mercury supply,
+than it is to make three jets, each delivering exactly the proper
+stream of mercury to three fall tubes.
+
+Sprengel pumps only work at their highest efficiency when the mercury
+supply is carefully regulated to suit the peculiarities of each fall
+tube, and this is quite easily done in the model figured.  Since on
+starting the pump the rubber connections have to stand a considerable
+pressure, the ends of the tubes must be somewhat corrugated to enable
+the rubber to be firmly wired on to them. The best binding wire is the
+purest Swedish iron wire, previously annealed in a Bunsen gas flame.
+
+The wire must never be twisted down on the bare rubber, but must
+always be separated from it by a tape binding.  By taking this
+precaution the wire maybe twisted very much more tightly than is
+otherwise possible without cutting the rubber.
+
+The only difficulty in making such a pump as is described lies in the
+bending of the heads of the fall tubes.  This bending must be done
+with perfect regularity and neatness, otherwise the drops of mercury
+will not break regularly, or will break just inside the top of the
+fall tube, and so obstruct its entrance that at high vacua no air can
+get into the tube at all.
+
+The connections at the head of the fall tubes must also be well put on
+and the joints blown out so that the mercury in dropping over the head
+is not interfered with by the upper surface of the tube.  However, a
+glance at the enlarged diagram will show what is to be aimed at better
+than any amount of description.  In preparing the fall tubes it is
+generally necessary to join at least two "canes" together.  The joint
+must be arranged to occur either in the tube leading the mercury to
+the head of the fall, or in that part of the fall tube which remains
+full of mercury when the highest vacuum is attained.  On no account
+must the joint be made at the fall itself (at least not by an
+amateur), nor in that part of the fall tube where the mercury falls
+freely, particularly at its lower end, where the drops fall on the
+head of the column of mercury.
+
+When a high vacuum is attained the efficacy of the pump depends
+chiefly on the way in which the drops fall on the head of the column.
+If the fall is too long the drops are apt to break up and allow the
+small bubble of air to escape up the tube, also any irregularity or
+dirt in the tube at this point makes it more easy for the bubbles of
+air to escape to the surface of the mercury.
+
+Any pump in which the supply of mercury to the fall tube can be
+regulated nicely will pump well until the lowest available pressures
+are being attained;  a good pump will then continue to hold the air
+bubbles, while a bad one will allow them to slip back [Footnote: For
+special methods of avoiding this difficulty see Mr. Ram's book.] ...
+
+Though three fall tubes are recommended, it must not be supposed that
+the pump will produce a Crooke's vacuum three times more rapidly than
+one fall tube.  Until the mercury commences to hammer in the pump the
+three tubes will pump approximately three times faster than one tube,
+but as soon as the major portion of the air collected begins to come
+from the layer condensed on the glass surface of the tube to be
+exhausted and from the electrodes, the rate at which exhaustion will
+go on no longer depends entirely on the pump.
+
+In order that bubbles of air may not slip back up the fall tube it is
+generally desirable to allow the mercury to fall pretty briskly, and
+in this case the capacity of the pump to take air is generally far in
+excess of the air supply.  One advantage of having more than one fall
+tube is that it often happens that a fall tube gets soiled during the
+process of exhaustion and no longer works up to its best performance.
+Out of three fall tubes, however, one is pretty sure to be working
+well, and as soon as the mercury begins to hammer in the tubes the
+supply may be shut off from the two falls which are working least
+satisfactorily.
+
+Thus we are enabled to pump rapidly till a high degree of exhaustion
+is attained, having practically three pumps instead of one, whereas
+when the final stages are reached, and three pumps are only a drawback
+in that they increase the mercury flow, the apparatus is capable of
+instant modification to meet the new conditions.
+
+The thistle funnels at the head of the fall tubes are made simply by
+blowing bulbs and then blowing the heads of the bulbs into wider ones,
+and finally blowing the heads of the wider bulbs off by vigorous
+blowing.  The stoppers are ground in on the lathe before the tubes are
+attached to the fall tubes.  The stoppers require to be at least half
+an inch long where they fit the necks, and must be really well ground
+in.  The stoppers must first be turned up nicely and the necks ground
+out by a copper or iron cone and emery.  The stoppers are rotated on a
+lathe at quite a slow speed, say 30 or 40 feet per minute, and the
+necks are held against them, as described in the section dealing with
+this art.  The stoppers must in this case be finished with "two
+seconds" emery, and lastly with pumice dust and water (see chapter on
+glass-grinding).
+
+Unless the stoppers fit exceedingly well trouble will arise from the
+mercury (which is poured into the thistle heads to form a seal) being
+forced downwards into the pump by atmospheric pressure.
+
+The joints between the three fall tubes and the single exhaust main
+are easily made when the tubes are finally mounted, the hooked nozzle
+of the oxygas blow-pipe being expressly made for such work.
+
+It is, on the whole, advisable to make the pump of flint glass, or at
+all events the air-trap tube and the fall tubes.  A brush flame from
+the larger gas tube of the single blowpipe table is most suitable for
+the work of bending the tubes.  The jointing of the long, narrow bore
+fall tubes is best accomplished by the oxygas flame, for in this way
+the minimum  of irregularity is produced; the blowing tubes will of
+course be required for the job, and the narrow tubes must be well
+cleaned to begin with.
+
+The air trap is an important though simple part of the pump.  Its
+shoulder or fall should stand rather higher than the shoulders of the
+fall tubes, so that the mercury may run in a thin stream through a
+good Torricellian vacuum before it passes down to the fall tubes.
+This is easily attained by regulating the main mercury supply at the
+pinch cock situated between the tube from the upper reservoir and the
+air-trap tube, the other cocks being almost wide open.
+
+It might be thought that the mercury would tend to pick up air in
+passing through the rubber connections to the fall tubes, but I have
+not found this to be the case in practice.  There is, of course, no
+difficulty in eliminating the rubber connections between the fall
+tubes and the mercury supply from the air trap, but it impresses a
+greater rigidity on the structure and, as I say, is not in general
+necessary.  It must not be forgotten that the mercury always exercises
+considerable pressure on the rubber joints, and so there is little
+tendency for gas to come out of the rubber.
+
+The thistle funnels at the head of the fall tubes provide a simple and
+excellent means of cleaning the fall tubes.  For this purpose some
+"pure" sulphuric acid which has been boiled with pure ammonium
+sulphate is placed in each thistle funnel, and when the fall tube is
+dirty the connection to the mercury supply is cut off at the pinch
+cock so as to leave the tube between this entry and the head of the
+fall tube quite full of mercury, and the sulphuric acid is allowed to
+run down the fall tube by raising the stopper.  The fall tube should
+be allowed to stand full of acid for an hour or so, after which it
+will be found to be fairly clean.
+
+Of course the mercury reservoir thus obtains a layer of acid above the
+mercury, and as it is better not to run the risk of any acid getting
+into the pump except in the fall tubes, the reservoir is best emptied
+from the bottom, by a syphon, if a suitable vessel cannot be procured,
+so that clean mercury only is withdrawn.
+
+The phosphorus pentoxide tube is best made as shown simply from a bit
+of wide tube, with two side connections fused to the rest of the pump.
+It is no more trouble to cut the tube and fuse it up again when the
+drying material is renewed than to adjust the drying tube to two fixed
+stoppers, which is the alternative.  The practice here recommended is
+rendered possible only by the oxygas blow-pipe with hooked nozzle.
+The connection between the pump and tube to be exhausted is made
+simply by a short bit of rubber tube immersed in mercury.
+
+The phosphorus pentoxide should be pure, or rather free from
+phosphorus and lower oxides;  unless this be the case, the vapour
+arising from it is apt to soil the mercury in the pump.  The
+phosphorus pentoxide is purified by distilling with oxygen over
+red-hot platinum black; if this cannot be done, the pentoxide should
+at least be strongly heated in a tube, in a current of dry air or
+oxygen, before it is placed in the drying tube.
+
+The mercury used for the pump must be scrupulously clean.  It does
+not, however, require to have been distilled in vacuo.  It is
+sufficient to purify it by allowing it to fall in a fine spray into a
+large or rather tall jar of 25 per cent nitric acid and 75 per cent
+water.  The mercury is then to be washed and dried by heating to, say,
+110° C.  in a porcelain dish.
+
+Exhausting a Roentgen Tube.
+
+With a pump such as has been described there is seldom any advantage
+in fusing an extra connection to the vacuum tube so as to allow of a
+preliminary exhaustion by means of a water pump.  About half an hour's
+pumping may possibly be saved by making use of a water pump.
+
+The tube to be exhausted is washed and dried by careful heating over a
+Bunsen burner and by the passage of a current of air.  The exhausting
+tube is then drawn down preparatory to sealing off, and the apparatus
+placed upon the pump.  It is best held in position by a wooden clamp
+supported by a long retort stand.
+
+Exhaustion may proceed till the mercury in the fall tubes commences to
+hammer.  At this point the tube must be carefully heated by a Bunsen
+flame, the temperature being brought up to, say, 400° C.  The heating
+may be continued intermittently till little or no effect due to the
+heating is discernible at the pump.  When this stage is reached, or
+even before, the electrodes may be connected up to the coil and a
+discharge sent through the tube.
+
+Care must be taken to stop the discharge as soon as a purple glow
+begins to appear, because when this happens, the resistance of the
+tube is very low, the electrodes get very hot, and may easily get
+damaged by a powerful discharge, and the platinum of the anode (if a
+focus tube is in question) begins to be distilled on to the glass.
+The heating and sparking are to be continued till the resistance of
+the tube sharply increases.  This is tested by always having a spark
+gap, conveniently formed by the coil terminals, in parallel with the
+tube.  If the terminals are points, it is convenient to set them at
+about one quarter of an inch distance apart.
+
+As soon as sparks begin to pass between the terminals of the spark gap
+it becomes necessary to watch the process of exhaustion very
+carefully.  In the first place, stop the pump, but let the coil run,
+and note whether the sparks  continue to flow over the terminals.  If
+the glass and electrodes are getting gas free, the discharge will
+continue to pass by the spark gap, but if gas is still being freely
+given off, then in perhaps three minutes the discharge will return to
+the tube, and pumping must be recommenced.  The Roentgen effect only
+begins to appear when the tube has got to so high a state of
+exhaustion that the resistance increases rapidly.
+
+By pumping and sparking, the resistance of the tube may be gradually
+raised till the spark would rather jump over 2 inches of air than go
+through the tube.  When this state is attained the Roentgen effect as
+tested by a screen of calcium tungstate should be very brilliant.  No
+conclusion as to the equivalent resistance of the tube can be arrived
+at so long as the discharge is kept going continually.  When the spark
+would rather go over an inch of air in the spark gap than through the
+tube the pumping and sparking may be interrupted and the tube allowed
+to rest for, say, five minutes.  It will generally be found that the
+equivalent resistance of the tube will be largely increased by this
+period of quiescence.  It may even be found that the spark will now
+prefer to pass an air gap 3 inches long.
+
+In any case the sparking should now be continued, the pump being at
+rest, and the variations of tube resistance watched by adjusting the
+spark gap.  If the resistance falls below an equivalent of 2 inches of
+air in the gap the pump must be brought into action again and
+continued until the resistance as thus estimated remains fairly
+constant for, say, ten minutes.  When this occurs the narrow neck of
+the exhaust tube may be strongly heated till the blow-pipe flame
+begins to show traces of sodium light.  The flame must then be
+withdrawn and the discharge again tested.  This is necessary because
+it occasionally happens that gas is given off during the heating of
+the neck to the neighbourhood of its fusion temperature.
+
+If all is right the neck may now be fused entirely off and the tube is
+finished.  Tubes of the focus pattern with large platinum anodes are
+in general (in my experience) much more difficult to exhaust than
+tubes of the kind first described.  This is possibly to be attributed
+mainly to the gas given off by the platinum, but is also, no doubt,
+due to the tubes being much larger and exposing a larger glass
+surface.  The type of tube described first generally takes about two
+hours to exhaust by a pump made as explained, while a "focus" tube has
+taken as long as nine hours, eight of which have been consumed after
+the tube was exhausted to the hammering point.
+
+The pressure at which the maximum heating of the anode by the cathode
+rays occurs is a good deal higher than that at which the maximum
+Roentgen effect is produced.  There is little doubt that the Roentgen
+radiation changes in nature to some extent as the vacuum improves
+either as a primary or secondary effect.  It is therefore of some
+importance to test the tube for the purpose for which it is to be used
+during the actual exhaustion.  It has been stated, for instance, that
+the relative penetrability of bone and flesh to Roentgen radiation
+attains a maximum difference at a certain pressure;  this is very
+likely the case.  Whether this effect is a direct function of the
+density of the gas in the tube, or whether it is dependent on the
+voltage or time integral of the current during the discharge, are
+questions which still await a solution.
+
+The preparation of calcium tungstate for fluorescent screens is very
+simple.
+
+Commercial sodium tungstate is fused with dried calcium chloride in
+the proportion of three parts of the former to two parts of the
+latter, both constituents being in fine powder and well mixed
+together.  The fusion is conducted in a Fletcher's crucible furnace in
+a clay crucible.  The temperature is raised as rapidly as possible to
+the highest point which the furnace will attain--i.e.  a pure white
+heat.  At this temperature the mixture of salts becomes partly fluid,
+or at least pasty, and the temperature may be kept at its highest
+point for, say, a quarter of an hour.  At the end of this time the
+mass is poured and scraped on to a brick, and when cold is broken up
+and boiled with a large excess of water to dissolve out all soluble
+matter.  The insoluble part, which consists of a gray shining powder,
+is washed several times with hot water, and is finally dried on
+filter paper in a water oven.
+
+In order to prepare a screen the powder is ground slightly with very
+dilute shellac varnish, and is then floated over a glass plate so as
+to get an even covering.  Unless the covering be very even the screen
+is useless, and no pains should be spared to secure evenness.  It is
+not exactly easy to get a regular coat of the fluorescent material,
+but it may be done with a little care.
+
+CHAPTER II
+
+GLASS-GRINDING AND OPTICIANS' WORK
+
+§ 52.  As no instructions of any practical value in this art have, so
+far as I know, appeared in any book in English, though a great deal of
+valuable information has been given in the English Mechanic and
+elsewhere, I shall deal with the matter sufficiently fully for all
+practical purposes.  On the other hand, I do not propose to treat of
+all the methods which have been proposed, but only those requisite for
+the production of the results claimed.  The student is requested to
+read through the chapter before commencing any particular operation.
+
+§ 53.  The simplest way will be to describe the process of manufacture
+of some standard optical appliance, from which a general idea of the
+nature of the operations will be obtained.  After this preliminary
+account special methods may be considered in detail.  I will begin
+with an account of the construction of an achromatic object glass for
+a telescope, not because a student in a physical laboratory will often
+require to make one, but because it illustrates the usual processes
+very well; and requires to be well and accurately made.
+
+A knowledge of the ordinary principles of optics on the part of the
+reader is assumed, for there are plenty of books on the theory of
+lenses, and, in any case, it is my intention to treat of the art
+rather than of the science of the subject.  By far the best short
+statement of the principles involved which I have seen is Lord
+Rayleigh's article on Optics in the Encyclopaedia Britannica, and this
+is amply sufficient.
+
+The first question that crops up is, of course, the subject of the
+choice of glass.  It is obvious that the glass must be uniform in
+refractive index throughout, and that it must be free from air bubbles
+or bits of opaque matter.  [Footnote: The complete testing of glass
+for uniformity of refractive index can only be arrived at by grinding
+and polishing a sufficient portion of the surfaces to enable an
+examination to be made of every part.  In the case of a small disc it
+is sufficient to polish two or three facets on the edge, and to
+examine the glass in a field of uniform illumination through the
+windows thus formed.  Very slight irregularities will cause a "mirage"
+easily recognised.]
+
+The simplest procedure is to obtain glass of the desired quality from
+Messrs.  Chance of Birmingham, according to the following abbreviated
+list of names and refractive indices, which may be relied upon:-
+
+Density.   Refractive Index.
+
+       C      D        F    G
+
+Hard crown
+
+2.85 1.5146 1.5172 1.5232 1.5280
+
+Soft crown
+
+2.55 1.5119 1.5146 1.5210 1.5263
+
+Light flint
+
+3.21 1.5700 1.5740 1.5839 1.5922
+
+Dense flint
+
+3.66 1 6175 1.6224 1.6348 1.6453
+
+Extra dense flint
+
+3.85 1.6450 1.6504 1.6643 1.6761
+
+Double extra dense flint
+
+4.45 1.7036 1.7103 1.7273 ...
+
+The above glasses may be had in sheets from 0.25 to 1 inch thick, and
+6 to 12 inches square, at a cost of, say, 7s.  6d.  per pound.
+
+Discs can also be obtained of any reasonable size.  Discs 2 inches in
+diameter cost about £1 per dozen, discs 3 inches in diameter about
+10s.  each.  The price of discs increases enormously with the size.  A
+16-inch disc will cost about £100.
+
+For special purposes, where the desired quality of glass does not
+appear on the list, an application may be made to the Jena Factory of
+Herr Schott.  In order to give a definite example, I may mention that
+for ordinary telescopic objectives good results may be obtained by
+combining the hard crown and dense flint of Chance's list, using the
+crown to form a double convex, and the flint to form a double concave
+lens.  The convex lens is placed in the more outward position in the
+telescope, i.e.  the light passes first through it.
+
+The conditions to be fulfilled are:
+
+(1) The glass must be achromatic;
+
+(2) it must have a small spherical aberration for rays converging to
+the principal focus.
+
+It is impossible to discuss these matters without going into a
+complete optical discussion.  The radii of curvature of the surfaces,
+beginning with the first, i.e.  the external face of the convex lens,
+are in the ratio of 1, 2, and 3; an allowance of 15 inches focal
+length per inch of aperture is reasonable (see Optics in Ency.
+Brit.), and the focal length is the same as the greatest radius of
+curvature.  Thus, for an object glass 2 inches in diameter, the first
+surface of the convex lens would have a radius of curvature of 10
+inches, the surface common to the convex and concave lens would have a
+radius of curvature of 20 inches, and the last surface a radius of
+curvature of 30 inches.  This would also be about the focal length of
+the finished lens.  The surfaces in contact have, of course, a common
+curvature, and need not be cemented together unless a slight loss of
+light is inadmissible.
+
+I will assume that a lens of about 2 inches diameter is to be made by
+hand, i.e.  without the help of a special grinding or polishing
+machine;  this can be accomplished perfectly well, so long as the
+diameter of the glass is not above about 6 inches, after which the
+labour is rather too severe.  The two glass discs having been obtained
+from the makers, it will be found that they are slightly larger in
+diameter than the quoted size, something having been left for the
+waste of working.
+
+It is difficult to deal with the processes of lens manufacture without
+entering at every stage into rather tedious details, and, what is
+worse, without interrupting the main account for the purpose of
+describing subsidiary instruments or processes.  In order that the
+reader may have some guide in threading the maze, it is necessary that
+he should commence with a clear idea of the broad principles of
+construction which are to be carried out.  For this purpose it seems
+desirable to begin by roughly indicating the various steps which are
+to be taken.
+
+(1) The glass is to be made circular in form and of a given diameter.
+
+(2) Called Rough Grinding. The surfaces of the glass are to be made
+roughly convex, plane, or concave, as may be required; the glass is to
+be equally thick all round the edge.  In this process the glass is
+abraded by the use of sand or emery rubbed over it by properly shaped
+pieces of iron or lead called "tools."
+
+(3) The glass is ground with emery to the correct spherical figure as
+given by a spherometer.
+
+(4) Called Fine Grinding. The state of the surface is gradually
+improved by grinding with finer and finer grades of emery.
+
+(5) The glass is polished by rouge.
+
+(6) The glass is "figured." This means that it is gradually altered in
+form by a polishing tool till it gives the best results as found by
+trial.
+
+In processes 2 to 5 counterpart tool surfaces are required--as a rule
+two convex and two concave surfaces for each lens surface.  These
+subsidiary surfaces are worked (i.e.  ground) on discs of cast iron
+faced with glass, or on slate discs;  and discs thus prepared are
+called "tools."
+
+Taking these processes in the order named, the mode of manufacture is
+shortly as follows:-
+
+(1) The disc of glass, obtained in a roughly circular form, is mounted
+on an ordinary lathe, being conveniently cemented by Regnault's mastic
+to a small face plate.  The lathe is rotated slowly, and the glass is
+gradually turned down to a circular figure by means (1) of a tool with
+a diamond point;  or (2) an ordinary hand-file moistened with
+kerosene, as described in § 42;  or (3) a mass of brass or iron served
+with a mixture of emery--or sand--and water fed on to the disc, so
+that the disc is gradually ground circular.
+
+The operation of making a circular disc of given diameter does not
+differ in any important particular from the similar operation in the
+case of brass or iron, and is in fact merely a matter of turning at a
+slow speed.
+
+(2 and 3) Roughing or bringing the surfaces of the glass roughly to
+the proper convex or concave shape.  This is accomplished by
+grinding, generally with sand in large works, or with emery in the
+laboratory, where the time saved is of more importance than the value
+of the emery.
+
+Discs of iron or brass are cast and turned so as to have a diameter
+slightly less than that of the glass to be ground, and are, say, half
+an inch thick.  These discs are turned convex or concave on one face
+according as they are to be employed in the production of concave or
+convex glass surfaces.  The proper degree of convexity or concavity
+may be approximated to by turning with ordinary turning tools, using a
+circular arc cut from zinc or glass (as will be described) as a
+"template" or pattern.  This also is a mere matter of turning.
+
+The first approximation to the desired convex or concave surface of
+the glass is attained (in the case of small lenses, say up to three
+inches diameter) by rotating the glass on the lathe as described above
+(for the purpose of giving it a circular edge) and holding the tool
+against the rotating glass, a plentiful supply of coarse emery and
+water, or sand and water, being supplied between the glass and metal
+surfaces.  The tool is held by hand against the surface of the
+revolving glass, and is constantly moved about, both round its own
+axis of figure and to and fro across the glass surface.  In this way
+the glass gradually gets convex or concave.
+
+The curvature is tested from time to time by a spherometer, and the
+tool is increased or decreased in curvature by turning it on a lathe
+so as to cause it to grind the glass more at the edges or in the
+middle according to the indications of the spherometer.
+
+This instrument, by the way--so important for lens makers--consists
+essentially of a kind of three-legged stool, with an additional leg
+placed at the centre of the circle circumscribing the other three.
+This central leg is in reality a fine screw with a very large head
+graduated on the edge, so that it is easy to compute the fractions of
+a turn given to the screw.  The instrument is first placed on a flat
+plate, and the central screw turned till its end just touches the
+plate, a state of affairs which is very sharply discernible by the
+slight rocking which it enables the instrument to undergo when pushed
+by the hand.  See the sketch.
+
+On a convex or concave surface the screw has to be screwed in or out,
+and from the amount of screwing necessary to bring all four points
+into equal contact, the curvature may be ascertained.
+
+Let a be the distance between the equidistant feet, and d the distance
+through which the screw is protruded or retracted from its zero
+position on a flat surface.  Then the radius of curvature rho is given
+by the formula 2rho = a2/3d +d.
+
+Fig.  43.
+
+The process of roughing is not always carried out exactly as
+described, and will be referred to again.
+
+(4) The glass being approximately of the proper radius of curvature on
+one side, it is reversed on the chuck and the same process gone
+through on the other side.  After this the glass is usually dismounted
+from the lathe and mounted by cement on a pedestal, which is merely a
+wooden stand with a heavy foot, so that the glass may be held
+conveniently for the workman.  Sometimes a pedestal about four feet
+high is fixed in the floor of the room, so that the workman engaged in
+grinding the lens may walk round and round it to secure uniformity.
+For ordinary purposes, however, a short pedestal may be placed on a
+table and rotated from time to time by hand, the operator sitting down
+to his work.
+
+Rough iron or brass tools do not succeed for fine grinding--i.e.
+grinding with fine emery, because particles of emery become embedded
+in the metal so tightly that they cannot be got out by any ordinary
+cleaning.  If we have been using emery passing say a sieve with 60
+threads to the inch, and then go on to some passing say 100 threads to
+the inch, a few of the coarser particles will adhere to the "tool",
+and go on cutting and scratching all the time grinding by means of the
+finer emery is in progress.
+
+To get over this it is usual to use a rather different kind of
+grinding tool.  A very good kind is made by cementing small squares of
+glass (say up to half an inch on the side), on to a disc of slate
+slightly smaller than the lens surface to be formed (Fig.  51).  The
+glass-slate tool is then "roughed" just like the lens surface, but, of
+course, if the lens has been roughed "convex" the tool must be roughed
+"concave".
+
+The "roughed" tool is then used to gradually improve the fineness of
+grinding of the glass.  For this purpose grinding by hand is resorted
+to, the tool and lens being supplied continually with finer and finer
+emery.  Fig.  52 gives an idea of the way in which the tool is moved
+across the glass surface.  Very little pressure is required.  The tool
+is carried in small circular sweeps round and round the lens, so that
+the centre of the tool describes a many-looped curve on the lens
+surface.  The tool must be allowed to rotate about its own axis;  and
+the lens and pedestal must also be rotated from time to time.
+
+Every few minutes the circular strokes are interrupted, and simple,
+straight, transverse strokes taken.  In no case (except to correct a,
+defect, as will be explained) should the tool overhang the lens
+surface by more than about one quarter the diameter of the latter.
+After grinding say for an hour with one size of emery fed in by means
+of a clean stick say every five minutes, the emery is washed off, and
+everything carefully cleaned.  The process is then repeated with finer
+emery, and so on.
+
+The different grades of emery are prepared by taking advantage of the
+fact that the smaller the particles the longer do they remain
+suspended in water.  Some emery mud from a "roughing" operation is
+stirred up with plenty of water and left a few seconds to settle, the
+liquor is then decanted to a second jug and left say for double the
+time, say ten seconds;  it is decanted again, and so on till four or
+five grades of emery have been accumulated, each jug containing finer
+emery than its predecessor in the process.
+
+It is not much use using emery which takes more than half an hour to
+settle in an ordinary bedroom jug.  What remains in the liquid to be
+decanted is mostly glass mud and not emery at all.  The process of
+fine grinding is continually checked by the spherometer, and the art
+consists in knowing how to move the grinding tool so as to make the
+lens surface more or less curved.  In general it may be said that if
+the tool is moved in small sweeps, and not allowed to overhang much,
+the Centre of the lens will be more abraded, while if bold free
+strokes are taken with much overhanging, the edges of the lens will be
+more ground away.
+
+By the exercise of patience and perseverance any one will succeed in
+gradually fine grinding the lens surface and keeping it to the
+spherometer, but the skill comes in doing this rapidly by varying the
+shape of the strokes before any appreciable alteration of curvature
+has come about.
+
+Polishing.
+
+The most simple way of polishing is to coat the grinding tool with
+paper, as will be described, and then to brush some rouge into the
+paper.  The polisher is moved over the work in much the same way as
+the fine grinding tool, until the glass is polished.  Many operators
+prefer to use a tool made by squeezing a disc of slate, armed with
+squares of warm pitch, against the lens surface (finely ground), and
+then covering these squares with rouge and water instead of emery and
+water as in the fine grinding process.
+
+The final process is called "figuring."  It will in  general be
+unnecessary with a small lens.  With large lenses or mirrors the final
+touches have to be given after the optical behaviour of the lens or
+mirror has been tested with the telescope itself, and this process is
+called "figuring." A book might easily be written on the optical
+indications of various imperfections in a mirror or lens.  Suffice it
+to say here that a sufficiently skilled person will be able to decide
+from an observation of the behaviour of a telescope whether a lens
+will be improved by altering the curvature of one or all of the
+surfaces.
+
+A very small alteration will make a large difference in the optical
+properties, so that in general "figuring" is done merely by using the
+rouge polishing tool as an abrading tool, and causing it to alter the
+curves in the manner already suggested for grinding.  There are other
+methods based on knocking squares out of the pitch-polisher so that
+some parts of the glass may be more abraded than others.
+
+The "figuring" and polishing may be done by hand just like the
+grinding.  There are machines, however, which can be made to execute
+the proper motions, and a polisher is set in such a machine, and the
+mechanical work done is by no means inconsiderable.  In fact for
+surfaces above six inches in diameter few people are strong enough to
+work a polisher by hand owing to the intense adhesion between it and
+the exactly fitting glass surface.
+
+Such is a general outline of the processes required to produce a lens
+or mirror.  These processes will now be dealt with in much greater
+detail, and a certain amount of repetition of the above will
+unfortunately be necessary: the reader is asked to pardon this.  It
+will also be advisable for the reader to begin by reading the whole
+account before he commences any particular operation.  The reason for
+this is that it has been desirable to keep to the main account as far
+as possible without inserting special instructions for subsidiary
+operations, however important they may be;  consequently it may not
+always be quite clear how the steps described are to be performed.  It
+will be found, however, that all necessary information is really
+given, though perhaps not always exactly in the place the reader might
+at first expect.
+
+§ 54.  All the discs that I have seen, come from the makers already
+roughly ground on the edges to a circular figure--but occasionally the
+figure is very rough indeed--and in some cases, especially if small
+lenses have to be made, it is convenient to begin by cutting the glass
+discs out of glass sheet, which also may be purchased of suit-able
+glass.  To do this, the simplest way is to begin by cutting squares
+and then cutting off the corners with the diamond, the approximate
+circular figure being obtained by grinding the edges on an ordinary
+grindstone.
+
+If the pieces are larger, time and material may be saved by using a
+diamond compass, i.e.  an ordinary drawing compass armed with a
+diamond to cut circles on the glass, and breaking the superfluous
+glass away by means of a pair of spectacle-maker's shanks (Fig.  44),
+or what does equally well, a pair of pliers with soft iron jaws.  With
+these instruments glass can be chipped gradually up to any line,
+whether diamond-cut or not, the jaws of the pincers being worked
+against the edge of the glass, so as to gradually crush it away.
+
+Fig.  44.
+
+Assuming that the glass has been bought or made roughly circular, it
+must be finished on the lathe.  For this purpose it is necessary to
+chuck it on an iron or hardwood chuck, as shown in Fig.  46.  For a
+lens below say an inch in diameter, the centering cement may be used;
+but for a lens of a diameter greater than this, sufficient adhesion is
+easily obtained with Regnault's mastic, and its low melting point
+gives it a decided advantage over the shellac composition.
+
+The glass may be heated gradually by placing it on the water bath, or
+actually in the water, and gradually bringing the water up to the
+boiling-point.  The glass, being taken out, is rapidly wiped, and
+rubbed with a bit of waste moistened, not wet, with a little
+turpentine:  its surface is then rubbed with a stick of mastic
+previously warmed so as to melt easily.  The surface of the chuck
+being also warm, and covered with a layer of melted cement, it is
+applied to the glass.  The lathe is turned slowly by hand, and the
+glass pushed gradually into the most central position;  it is then
+pressed tight against the chuck  by the back rest, a bit of wood being
+interposed for obvious reasons.
+
+When all is cold the turning may be proceeded with.  The quickest way
+is to use the method already described (i.e.  actual turning by a file
+tool);  but if the student prefers (time being no object), he may
+accomplish the reduction to a circular form very easily by grinding.
+
+Fig.  45.
+
+Fig.  46.
+
+For this purpose he will require to make the following arrangements
+(Fig.  45).  If the lathe has a slide rest, a piece of stout iron may
+be bent and cut so as to fit the tool rest, and project beneath the
+glass.  The iron must be fairly rigid, for if it springs appreciably
+beneath the pressure of the glass, it will not grind the latter really
+round.  The lathe may run rather faster than for turning cast iron of
+the same size.  Coarse emery, passing through a sieve of 80 threads to
+the inch (run), may be fed in between the glass and iron, and the
+latter screwed up till the disc just grinds slightly as it goes round.
+
+A beginner will generally (in this as in all cases of grinding
+processes) tend to feed too fast--no grinding process can be hurried.
+If a slide rest is not available, a hinged board, carrying a bit of
+iron, may (see Fig.  45) be arranged so as to turn about its hinge at
+the back of the lathe;  and it may be screwed up readily enough by
+passing a long set-screw through the front edge, so that the point of
+the screw bears upon the lathe bed.  I may add that emery behaves as
+if it were greasy, and it is difficult to wet it with clean water.
+This is easily got over by adding a little soap or alcohol to the
+water, or exercising a little patience.
+
+A good supply of emery and water should be kept between the disc and
+the iron;  a little putty may be arranged round the point of contact
+on the iron to form a temporary trough.  In any case the resulting
+emery mud should on no account be thrown away, but should be carefully
+kept for further use.  The process is complete when the glass is
+perfectly round and of the required diameter as tested by callipers.
+
+§ 55.  The next step is to rough out the lens, and this may easily be
+done by rotating it more slowly, i.e.  with a surface speed of ten
+feet per minute, and turning the glass with a hard file, as explained
+in § 42.  If it is desired to employ the slide rest, it is quicker and
+better to use a diamond tool--an instrument quite readily made, and
+of great service for turning emery wheels and the like,--a thing, in
+fact, which no workshop should be without.  A bit of diamond bort, or
+even a clear though off-colour stone, may be employed.
+
+An ordinary lathe tool is prepared by drawing down the tool steel to a
+long cone, resembling the ordinary practice in preparing a boring
+tool.  The apex of the cone must be cut off till it is only slightly
+larger than the greatest transverse diameter of the diamond splinter.
+The latter may have almost any shape--a triangular point, one side of
+a three-sided prism is very convenient.  A hole is drilled in the
+steel (which must have been well softened), only just large enough to
+allow the diamond to enter--if the splinter is thicker in the middle
+than at either end, so much the better--the diamond is fastened in
+position by squeezing the soft steel walls tightly down upon it.
+Personally I prefer to use a tool holder, and in this case generally
+mount the diamond in a bit of brass rod of the proper diameter;  and
+instead of pinching in the sides of the cavity, I tin them, and set
+the diamond in position with a drop of soft solder.
+
+Fig.  47.
+
+In purchasing diamond bort, a good plan is to buy fragments that have
+been employed in diamond drilling, and have become too small to reset;
+in this case some idea as to the hardness of the bits may be obtained.
+Full details as to diamond tool-making are given in books on
+watch-making, and in Holtzapffell's great work on Mechanical
+Manipulation;  but the above notes are all that are really
+necessary--it is, in fact, a very simple matter. The only advantage of
+using a diamond tool for glass turning is that one does not need to be
+always taking it out of the rest to sharpen it, which generally happens
+with hard steel, especially if the work is turned a little too fast.
+
+I recommend, therefore, that the student should boldly go to work
+"free hand" with a hard file;  but if he prefer the more formal
+method, or distrust his skill (which he should not do), then let him
+use a diamond point, even if he has the trouble of making it.  When
+using a diamond it is not necessary to employ a lubricant, but there
+is some advantage in doing so.
+
+The surface of the lens can be roughly shaped by turning to a template
+or pattern made by cutting a circular arc (of the same radius as the
+required surface) out of a bit of sheet zinc.  Another very handy way
+of making templates of great accuracy is to use a beam compass
+(constructed from a light wooden bar) with a glazier's diamond instead
+of a pencil.  A bit of thin sheet glass is cut across with this
+compass to the proper curvature--which can be done with considerable
+accuracy and the two halves of the plate, after breaking along the
+cut, are ground together with a view to avoiding slight local
+irregularities, by means of a little fine emery and water laid between
+the edges.  In this process the glass is conveniently supported on a
+clean board or slate, and the bits are rubbed backwards and forwards
+against each other.
+
+§ 56.  It is not very easy for a beginner to turn a bit of
+anything--iron, wood, or glass--with great accuracy to fit a template,
+and consequently time may be saved by the following procedure, applied as
+soon as the figure of the template is roughly obtained.  A disc of
+lead or iron, of the same diameter as the glass, and of approximately
+the proper curvature, is prepared by turning, and is armed with a
+handle projecting coaxially from the back of the disc.  The glass
+revolving with moderate speed on the lathe, the lead tool, supplied
+with coarse emery and water, is held against it, care being taken to
+rotate the tool by the handle, and also to move it backwards and
+forwards across the disc, through a distance, say, up to half an inch;
+if it is allowed to overhang too much the edges of the glass disc will
+be overground.  By the use of such a tool the glass can readily be
+brought up to the template.
+
+The only thing that remains, so far as the description of this part of
+the process goes, is to give a note or two as to the best way of
+making the lead tools, and for this purpose the main narrative of
+processes must be interrupted.  The easiest way is to make a set of
+discs to begin with.  For this purpose take the mandrel out of the
+lathe, and place it nose downwards in the centre of an iron ring of
+proper diameter on a flat and level iron plate.
+
+The discs are made by pouring lead round the screw-nose of the
+mandrel.  This method, of course, leaves them with a hole in the
+centre;  but this can be stopped up by placing the hot disc (from
+which the mandrel has been unscrewed) on a hot plate, and pouring in a
+sufficiency of very hot lead; or, better still, the mandrel can be
+supported vertically at any desired distance above the plate while the
+casting is being poured.  Lead discs prepared in this way are easily
+turned so as to form very convenient chucks for brass work, and for
+use in the case now being treated, they are easily turned to a
+template, using woodturners' tools, which work better if oiled, and
+must be set to cut, not scrape.
+
+If the operator does not mind the trouble of cutting a screw, or if he
+has a jaw chuck, the lead may be replaced by iron with some advantage.
+
+The following is a neat way of making concave tools.  It is an
+application of the principle of having the cutting tool as long as the
+radius of curvature, and allowing it to move about the centre of
+curvature.  Place the disc of iron or lead on the lathe mandrel or in
+the chuck, and set the slide rest so that it is free to slide up or
+down the lathe bed.  Take a bar of tool steel and cut it a little
+longer than the radius of curvature required.  Forge and finish one
+end of the bar into a pointed turning tool of the ordinary kind.
+Measure the radius of curvature from the point of the tool along the
+bar, and bore a hole, whose centre is at this point, through the bar
+from the upper to the lower face.  I regard the upper face as the one
+whose horizontal plane contains the cutting point when the tool is in
+use.  Clamp a temporary back centre to the lathe bed, and let it carry
+a pin in the vertical plane through the lathe centres, and let this
+pin exactly fit the hole in the bar.
+
+Fig. 48.
+
+Place the "radius" tool in position for cutting, and let it be lightly
+held in the slide rest nearly at the cutting point, the centre of
+rotation of the pedestal (or its equivalent) passing through the
+central line of the bar.  Then adjust the temporary back rest, so that
+the tool will take a cut.  In the sketch the tool is shown swinging
+about the back centre instead of about a pin--there is little to
+choose between the methods unless economy of tool steel is an object.
+The tool must now be fed across the work.  The pedestal must of course
+be free to rotate, and the slide rest to slip up and down the bed.  In
+this way a better concave grinding tool can be made than would be made
+by a beginner by turning to a template--though an expert turner would
+probably carry out the latter operation so as to obtain an' accuracy
+of the same order, and would certainly do it in much less time than
+would be required in setting up the special arrangements here
+described.
+
+On the other hand, if several surfaces have to be prepared, as in the
+making of an achromatic lens, the quickest way would be by the use of
+the radius tool, bored of course to work at the several radii
+required.  I have tried both methods, and my choice would depend
+partly on the lathe at my disposal, and partly on the number of
+grinding tools that had to be prepared.
+
+Having obtained a concave tool of any given radius, it is easily
+copied--negatively, so as to make a convex tool in the following
+manner.  Adjust the concave tool already made on the back rest, so
+that if it rotated about the line of centres, it would rotate about
+its axis of figure.
+
+Arrangements for this can easily be made, but of course they will
+depend on the detailed structure of the lathe.  Use the slide rest as
+before, i.e.  let it grasp an ordinary turning tool lightly, the
+pedestal being fixed, but the rest free to slide up or down the lathe
+bed.  Push the back rest up till the butt of the turning tool (ground
+to a rounded point) rests against the concave grinding tool.  If the
+diameter of the convex tool required be very small compared with the
+radius of curvature of the surface (the most usual case), it is only
+necessary to feed the cutting tool across to "copy" the concave
+surface sufficiently nearly.
+
+Fig. 49.
+
+There seems no reason, however, why these methods should not be
+applied at once to the glass disc by means of a diamond point, and the
+rough grinding thus entirely avoided.  I am informed that this has
+been done by Sir Henry Bessemer, but that the method was found to
+present no great advantage in practice.  A reader with a taste for
+mechanical experimenting might try radius bar tools with small
+carborundum wheels rapidly driven instead of a diamond.
+
+Enough has now been said to enable any one to prepare rough convex or
+concave grinding tools of iron or lead, and of the same diameter as
+the glass to be ground.
+
+The general effect of the process of roughing the rotating lens
+surface is to alter the radius of curvature of both tool and glass;
+hence it is necessary to have for each grinding tool another to fit
+it, and enable it to be kept (by working the two together) at a
+constant figure.  After a little practice it will be found possible to
+bring the glass exactly up to the required curvature as tested by
+template or spherometer.  The art of the process consists in altering
+the shape of the grinding tool so as to take off the glass where
+required, as described in § 53, and from this point of view lead has
+some advantages;  (opinions vary as to the relative advantages of lead
+and iron tools for this purpose, however).  The subsidiary grinding
+tool is not actually needed for this preliminary operation, but it has
+to be made some time with a view to further procedure, and
+occasionally is of service here.
+
+§ 57.  'The glass disc must be ground approximately to the proper
+curvature on each side before any fine grinding is commenced.  It is
+precisely for this purpose that the previous turning of the disc is
+recommended, for it is easy to unmount and recentre a round object,
+but not so easy if the object have an indefinite shape.  Using a
+cement which is plastic before it sets, the disc may be easily taken
+off the chuck and centred by a little handicraft, i.e.  by rotating
+the lathe slowly and pushing the disc into such a position that it
+rotates about its axis.  The grinding of the second surface is
+accomplished exactly as in the former case;  of course on reversing
+the glass the chuck has to be slightly turned up to fit the convex or
+concave surface.
+
+§ 58.  There is, however, one point of interest and importance--attention
+to which will save a good deal of useless labour afterwards.
+The glass must be ground in such a manner that the thickness at the
+edge is the same all round.  In other words, the axes of figure of the
+two surfaces must coincide.  This will be the case if the recentering
+has been accurately performed, and therefore no pains should be spared
+to see that it is exactly carried out.  Any simple form of vernier
+gauge (such as Brown and Sharpe's vernier callipers) will serve to
+allow of a sufficiently accurate measurement of the edge thickness of
+the lens.  If any difference of thickness is observed as the gauge
+moves round the edge, one or other of the surfaces must be reground.
+Of course the latitude of error which may be permitted depends so much
+on the final arrangements for a special finishing process called the
+"centering of the lens"--which will be described--that it is
+difficult to fix a limit, but perhaps one-thousandth of an inch may be
+mentioned as a suitable amount for a 2-inch disc.  For rough work, of
+course; more margin may be admitted.
+
+§ 59.  In a large shop I imagine that lenses of only two inches
+diameter would be ground in nests;  or, in other words, a number would
+be worked at a time, and centering, even of a rough kind, would be
+left to the last;  but this process will be treated hereafter.  At
+present I shall assume that only one lens will be made at a time.
+Consequently we now enter on the stage of fine grinding by hand.  A
+leaden pedestal, for the sake of stability, must be provided on which
+to mount the lens, so that the surface to be operated on may be nearly
+horizontal (Fig.  50).  Before this can be done, however, fresh
+grinding tools (two for each surface) must be properly prepared.
+After trying several plans I unhesitatingly recommend that all
+fine-grinding surfaces should be made of glass.  This is easily done
+by taking two discs of lead, or iron, or slate, cut to a one-tenth
+inch smaller radius of curvature (in the case of a convex tool, and
+the opposite in the other case) than the lens surface (Fig.  51, A).
+On these, square bits of sheet glass, one-tenth of an inch thick, are
+to be cemented, so as to leave channels of about one-eighth of an inch
+between each bit of glass (Fig.  52, B).  The "mastic"    cement
+formerly described may be employed for this purpose.
+
+Fig.  50.
+
+The bits of glass ought first to have  their edges dressed smooth on
+the grind-stone.  A convex and concave glass surface having been thus
+roughly prepared, they must be mounted in turn in the lathe, and
+brought to the proper curvature by grinding with the tools formerly
+employed and tested by the template or spherometer.  It is well to
+control this process by means of a spherometer, so that the desired
+radius may be approximately reached.  The two glass-grinding tools
+are then ground together by hand (see § 53 and § 61), the spherometer
+being employed from time to time to check the progress of the work.
+In general, if large circular sweeps are taken, greatly overhanging
+the side of the glass surface to be figured, both the upper and lower
+surfaces will be more ground at the edges, while in the opposite
+event the centre will be chiefly affected.
+
+Fig.  51.
+
+A spherometer capable of measuring a 2-inch surface may be procured,
+having a screw of, say, 50 threads to the inch, and a micrometer
+surface divided into 200 parts, each part easily capable of
+subdivision--into tenths or even twentieths.  To get the full
+advantage of the spherometer it must screw exceedingly freely (i.e.
+must be well oiled with clock oil), and must not be fingered except at
+the milled head.  If one of the legs is held by the fingers the
+expansion is sufficient to throw the instrument quite out of
+adjustment.  The glass-grinding tools being brought to the proper
+figure, the next process is to transfer the same to the lens, and this
+is done by similar means, the fellow tool being used to correct the
+one employed in grinding the lens surface.  Before the grade of emery
+is changed all three surfaces must agree, as nearly, at least, as the
+spherometer will show.
+
+In order to prevent confusion the following summary of the steps
+already taken may be given.  The discs of glass are first ground or
+turned so as to be truly circular.  Four "tools" are made for each
+surface--a rough pair of iron or lead, and a finishing pair of iron,
+lead, or slate faced by glass squares.  For a small lens the iron or
+lead backing may be used, for a large one the slate.  The rough tools
+are used to give an approximate figure both to the lens and to the
+finishing tools.
+
+The final adjustment is attained by grinding one of the glass-faced
+tools alternately upon the lens and upon the fellow glass-faced tool.
+The spherometer is accepted at all stages of the process as the final
+arbiter as to curvature.  Some hints on the form of strokes used in
+grinding will be given later on (see § 61).  It suffices to state here
+that the object throughout is to secure uniformity by allowing both
+the work and the tool to rotate, and exercising no pressure by the
+fingers.  The tool backing may weigh from one to two pounds for a
+2-inch lens.
+
+§ 60.  The tools and lens being all of the same curvature, the state
+of the surface is gradually improved by grinding with finer and finer
+emery.  The best way of grading the emery is by washing it with clean
+water, and allowing the emery (at first stirred up with the water) to
+settle out.  The longer the time required for this part of the process
+the finer will be the emery deposited.  An ordinary bedroom jug is a
+very good utensil to employ during this process; a large glass jug is
+even better.  The following grades will be found sufficient, though I
+daresay every operative's practice differs a little on this point.
+
+1st grade: Flour emery, with the grit washed out, i.e. allowed to
+stand for 2" (sec.) before being poured off.
+
+2nd grade: Stand 5" (secs.), settle in 1' (min.)
+
+3rd grade: Stand 1', settle in 10'.
+
+4th grade: Stand 10', settle in 60'.
+
+It is generally advisable to repeat the washing process with each
+grade.  Thus, selecting grade 2 for illustration, the liquor for grade
+3 must be poured off without allowing any of the sediment to pass over
+with it.  If any sediment at all passes, one has no security against
+its containing perhaps the largest particle in the jug.  As soon as
+the liquor for No.  3 has been decanted, jug No.  2 is filled up again
+with clean water (filtered if necessary), and after standing 5" is
+decanted into jug No.  2b, the sediment is returned to jug No.  1, and
+the liquor, after standing 1', is transferred to jug No.  3.
+
+The greatest care is necessary at each step of the operation to
+prevent "sediment" passing over with liquor.  There is a little danger
+from the tendency which even comparatively large particles of emery
+have to float, in consequence of their refusing to get wet, and the
+emery worked up on the side of the jug is also a source of danger,
+therefore wipe the jug round inside before decanting.
+
+In order to get a uniform grade stop the currents of water in the jug,
+which may work up coarse particles, by holding a thin bit of wood in
+the rotating liquid for a moment, and then gently withdrawing it in
+its own plane.  These precautions are particularly necessary in the
+case of grades Nos.  2, 3, and 4, especially No.  4, for if a single
+coarse particle gets on the tool when the work has progressed up to
+this point it will probably necessitate a return to grinding by means
+of No.  2, and involve many hours' work.
+
+The surface of the lens will require to be ground continuously with
+each grade till it has the uniform state of roughness corresponding to
+the grade in question.  Two hours for each grade is about the usual
+time required in working such a lens as is here contemplated.
+
+The coarser grades of emery may be obtained by washing ordinary flour
+of emery, but the finer ones have to be got from emery which has been
+used in the previous processes.  It is not a good plan to wash the
+finer grades of emery out of the proceeds of very rough grinding say
+with anything coarser than flour of emery--as there is a danger of
+thereby contaminating the finer grades with comparatively coarse glass
+particles (owing to their lightness) and this may lead to scratching.
+If the finer grades are very light in colour, it may be inferred that
+a considerable portion of the dust is composed of glass, and this does
+no good.  Consequently time may be saved by stirring up the
+light-coloured mass with a little hydrofluoric acid in a platinum
+capsule;  this dissolves the finely divided glass almost
+instantaneously.  The emery and excess of hydrofluoric acid may then
+be thrown into a large beaker of clean water and washed several times.
+Fine emery thus treated has much the same dark chocolate colour as the
+coarser varieties.
+
+The operator should not wear a coat, and should have his arms bare
+while working with fine emery, for a workshop coat is sure to have
+gathered a good deal of dust, and increases the chances of coarse
+particles getting between the surfaces.
+
+§ 61.  Details of the Process of Fine Grinding.
+
+A lens of the size selected for description is mounted as before
+mentioned on a leaden pedestal, and the operator places the latter on
+a table of convenient height in a room as free from dust as possible.
+Everything should be as clean as a pin, and no splashes of emery mud
+should be allowed to lie about.  I have found it convenient to spread
+clean newspapers on the table and floor, and to wear clean linen
+clothes, which do not pick up dust.  I have an idea that in large
+workshops some simpler means of avoiding scratches must have been
+discovered, but I can only give the results of my own experience.  I
+never successfully avoided scratches till I adopted the precautions
+mentioned.
+
+Fig.  52.
+
+The left hand should be employed in rotating the pedestal either
+continuously (though slowly) or at intervals of, say, one minute.
+This point is rather important.  Some operators require two hands to
+work the grinding tool, and in any case this is the safer practice.
+Under these circumstances the pedestal may be rotated through
+one-eighth or tenth of a revolution every three minutes, or
+thereabouts.  The general motion given to the grinding tool should be
+a series of circular sweeps of about one-fourth the diameter of the
+glass disc, and gradually carried round an imaginary circle drawn on
+the surface of the lens and concentric with it (Fig.  52).
+
+The tool may overhang the lens by a quarter of the diameter of the
+latter as a maximum. The circuit may be completed in from twelve to
+thirty sweeps.  The grinding tool should be lightly held by the
+fingers and the necessary force applied parallel to the surface.  The
+tool itself must be slowly rotated about its axis of figure.  If the
+tool be lightly held, it will be found that it tends to rotate by
+itself.  I say "tends to rotate," for if the tool be touching evenly
+all over the surface it will rotate in a direction opposite to the
+direction of the circular sweep.  For instance, if the tool be carried
+round its looped path clockwise, it will tend to rotate about its own
+axis of figure counter-clockwise.  If it touch more in the middle,
+this rotation will be increased, while if it touches more along the
+edge, the rotation will be diminished, or even reversed in an extreme
+case.
+
+Every fifty sweeps or so the tool should be simply ground backwards
+and forwards along a diameter of the lens surface.  This grinding
+should consist of three or four journeys to and fro along, say, eight
+different diameters.  About one-quarter of the whole grinding should
+be accomplished by short straight strokes, during which the tool
+should only overhang about one-quarter of an inch.  The object of the
+straight strokes is to counteract the tendency to a gradual
+accumulation of the emery in the centre, which results from the
+circular grinding.
+
+A great deal of the art of the process consists in knowing how to work
+the tool to produce any given effect.  For instance, if the lens
+requires to be ground down near the centre, the epicycloidal strokes
+must be nearly central;  the tool must never overhang very much.  If,
+on the other hand, it is the edges which require attention, these must
+be dealt with by wider overhanging strokes.  The tool must be
+frequently tested on its fellow, and, indeed, ground upon it if any
+marked unevenness of action (such as that just described) is required
+for the lens.  A check by spherometer will be applied at intervals
+according to the judgment of the operator, but, in any case, the
+fellow tool and lens should be kept at very nearly the same figure.
+
+The emery should never be allowed to become anything like dry between
+the tool and the lens, for in some way (probably by capillary action
+increasing the pressure of the tool) this seems to lead to scratching
+and "rolling" of the emery.  The channels in the glass tool between
+the squares are of the greatest importance in enabling the emery to
+distribute itself.  Perhaps the best guide in enabling one to judge
+as to when it is time to wash off the emery and apply fresh is the
+"feel" of the tool; also when the mud gets light in colour we know
+that it is full of glass dust, and proportionately inoperative.
+
+New emery may be put on, say, every five minutes, but no absolute rule
+can be given, for much depends on the pressure of the tool upon the
+lens.  In the case considered a brass or lead, or even slate tool, of
+an inch, or even less, in thickness, will press quite heavily enough.
+In washing the lens and tool before new emery is introduced, a large
+enamelled iron bucket is very handy;  the whole of the tool should be
+immersed and scrubbed with a nail-brush.  The lens surface may be
+wiped with a bit of clean sponge, free from grit, or even a clean damp
+cloth.
+
+When the time comes to alter the grade of emery, a fresh lot of
+newspapers should be put down, and tools, lens, and pedestal well
+washed and brushed by the nail-brush.  The surfaces should be wiped
+dry by a fresh piece of rag, and examined for scratches and also for
+uniformity of appearance; a good opinion can be formed as to the fit
+of the surfaces by noting whether--and if so, to what degree--they
+differ in appearance from point to point when held so that the light
+falls on them obliquely.
+
+It is necessary to exercise the greatest care in the washing between
+the application of successive grades of emery, and this will be
+facilitated if the edges of the glass squares were dressed on a
+grindstone before they were mounted.  An additional precaution which
+may be of immense advantage is to allow the tool to dry between the
+application of successive grades of emery (of course, after it has
+been scrubbed), and then to brush it vigorously with a hat-brush.  It
+sometimes happens that particles of mud which have resisted the wet
+scrubbing with the nail-brush may be removed by this method.
+
+As my friend Mr. Cook informs me that his present practice differs
+slightly from the above, I will depart from the rule I laid down, and
+add a note on an alternative method.
+
+Consider a single lens surface.  This is roughed out as before by an
+iron tool, a rough fellow tool being made at the same time.  The
+squares of glass are cemented to the roughing tool, and this is ground
+to the spherometer by means of the counterpart tool.  The glass-coated
+tool is then applied to the lens surface and grinding with the first
+grade of emery commenced.  The curvature is checked by the
+spherometer.  Two auxiliary tools of, say, half the diameter of the
+lens, are prepared from slate, or glass backed with iron, and applied
+to grind down either the central part of the lens surface or tool
+surface, according to the indications of the spherometer.  Any changes
+that may occur during grinding are corrected by these tools.  The
+spherometer is accepted as the sole guide in obtaining the proper
+curvature.  A slate backing is preferred for tools of any diameter
+over, say, 2 inches.
+
+§ 62.  Polishing.
+
+After the surface has been ground with the last grade of emery, and
+commences to become translucent even when dry, the grinding may be
+considered to be accomplished, and the next step is the polishing.
+There are many ways of carrying out this process, and the relative
+suitability of these methods depends on a good many, so to speak,
+accidental circumstances.  For instance, if the intention is to finish
+the polishing at a sitting, the polishing tool may be faced with
+squares of archangel--not mineral or coal-tar--pitch and brought to
+shape simply by pressing while warm against the face of the lens.  A
+tool thus made is very convenient, accurate, and good, but it is
+difficult to keep it in shape for any length of time;  if left on the
+lens it is apt to stick, and if it overhangs ever so little will, of
+course, droop at the edges.
+
+On the whole, the following will be found a good and sufficient plan.
+The glass-grinding tool is converted into a polishing tool by pasting
+a bit of thin paper over its surface;  a bit of woven letter paper of
+medium thickness with a smooth but not glazed surface does very well.
+We have found that what is called Smith's "21 lbs.  Vellum Wove" is
+excellent.  This is steeped in water till quite pliable and almost
+free from size.  The glass tool is brushed over with a little thin
+arrowroot or starch paste, and the paper is laid upon it and squeezed
+down on the glass squares as well as possible;  if the paper is wet
+enough and of the proper quality it will expand sufficiently to
+envelop the tool without creases, unless the curvature is quite out of
+the common.
+
+This being accomplished, and the excess of water and paste removed,
+the face of the paper is (for security) washed with a little clean
+water and a bit of sponge, and, finally, the tool is slightly pressed
+on the lens so as to get the paper to take up the proper figure as
+nearly as possible.  After the polishing tool has been thus brought to
+the proper figure, it is lifted off and allowed to dry slowly.  When
+the paper is dry it may be trimmed round the edges so as not to
+project sensibly beyond the glass squares.  The next step is to brush
+the surface over very carefully with polishing rouge (prepared as is
+described at the end of this section) by means of a hat-brush.  When
+the surface of the paper is filled with rouge all excess must be
+removed by vigorous brushing.
+
+Fig.  53.
+
+The tool being placed on the lens, two or three strokes similar to
+those used in grinding may be taken, and the tool is then lifted off
+and examined.  It will be found to be dotted with a few bright points,
+produced by the adhesion of glass at the places of contact.  These
+points are then to be removed in the following manner.  An old
+three-cornered file is ground on each side till the file marks
+disappear, and sharp edges are produced (Fig.  53).  This tool is used
+as an ink eraser, and it will be found to scrape the paper of the
+polishing tool very cleanly and well.
+
+The bright spots are the objects of attention, and they must be erased
+by the old file, and the polisher reapplied to the glass.  A few
+strokes will develop other points, more numerous than before, and
+these in turn must be erased.  The process is continued till the whole
+surface of the polishing tool is evenly covered with bright specks,
+and then the polishing may be proceeded with.  The specks should not
+be more than about one-eighth of an inch apart, or the polishing will
+be irregular.
+
+The operation of polishing is similar to that of grinding.  A
+reasonable time for polishing a glass surface is twenty hours;  if
+more time is required it is a sign that the fine grinding has not been
+carried far enough.  The progress of the operation may be best watched
+by looking at the surface--not through it.  For this purpose a good
+light is requisite.  When the lens is dismounted it may be examined by
+a beam of sunlight in a dark room, under which circumstances the
+faintest signs of grayness are easily discernible.
+
+It may be mentioned here that if the surface is in any way scratched
+the rouge will lodge in the scratches with great persistence, and an
+expert can generally tell from the appearance of scratches what kind
+of polishing powder has been employed.
+
+The persistence with which rouge clings to a rough surface of glass is
+rather remarkable.  Some glass polishers prefer to use putty powder as
+a polishing material, and it is sometimes said to act more quickly
+than rouge;  from my rather limited experience I have not found this
+to be the case, but it may have merits that I do not know of.  Is it
+possible that its recommendation lies in the fact that it does not
+render scratches so obtrusively obvious as rouge does?
+
+Rouge is generally made in two or more grades.  The softer grade is
+used for polishing silver, and is called jewellers' rouge.  The harder
+grade, suitable for glass polishing, is best obtained from practical
+opticians (not mere sellers of optical instruments).  I mean people
+like Messrs.  Cook of York.  Many years ago I prepared my own hard
+rouge by precipitating ferrous sulphate solution by aqueous ammonia,
+washing the precipitate, and heating it to a red heat.  The product
+was ground up with water, and washed to get rid of large particles.
+This answered every purpose, and I could not find that it was in any
+way inferior to hard rouge as purchased.  The same precipitate heated
+to a lower temperature is said to furnish a softer variety of rouge;
+at all events, it gives one more suitable for polishing speculum
+metal.  Lord Rosso used rouge heated to a dull redness for this
+purpose.
+
+Rouge, whether made or bought, should always be washed to get rid of
+grit.  I ought to add that not the least remarkable fact about the
+polishing is the extraordinarily small quantity of the polishing
+material requisite, which suggests that the process of polishing is
+not by any means the same as that of exceptionally fine grinding.  Is
+it possible that the chief proximate cause of the utility of rouge is
+to be sought in its curious property of adhering to a rough glass
+surface, causing it, so to speak, to drag the glass off in minute
+quantities, and redeposit it after a certain thickness has been
+attained on another part of the surface?
+
+§ 63.  Centering.
+
+When a lens is ground and polished it will almost always happen that
+the axis of revolution of its cylindrical edge is inclined to the axis
+of revolution of its curved surfaces.  Since in practice lenses have
+to be adjusted by their edges, it is generally necessary to adjust the
+edge to a cylinder about the axis of figure of the active surfaces.
+This is best done on a lathe with a hollow mandrel..  The lens is
+chucked on a chuck with a central aperture--generally by means of
+pitch or Regnault's mastic, or "centering" cement for small
+lenses--and a cross wire is fixed in the axis of revolution of the lathe,
+and is illuminated by a lamp.  This cross wire is observed by an
+eye-piece (with cross wires only in the case of a convex lens, or a
+telescope similarly furnished in the case of a concave lens),
+also placed in the axis of rotation of the lathe.
+
+Both cross wires are thus in the axis of revolution of the mandrel,
+and the distant one (B in the figure) is viewed through the lens and
+referred to the fixed cross wires at A.  In general, as the lathe is
+rotated by turning the mandrel the image of the illuminated cross
+wires will be observed to rotate also.  The lens is adjusted until the
+image remains steady on rotating the mandrel and it is to give time
+for this operation that a slow-setting cement is recommended.  When
+the image remains stationary we know that the optical centre of the
+lens is in the axis of revolution, and that this axis is normal to
+both lens surfaces, i.e.  is the principal axis of the lens, or axis
+of figure.
+
+Fig.  54.
+
+A much readier method, and one, in general, good enough for most
+purposes, is to put a candle on the end of the lathe-bed where the
+back centre generally is, and observe the images of the flame by
+reflection from both the lens surfaces.  This method is very handy
+with small lenses;  the mandrel is turned, and the lens adjusted by
+hand till the images are immovable.  In both cases, of course, the
+edge of the lens is turned or ground till it is truly circular, the
+position of the lens remaining undisturbed on the chuck.  If the edge
+gauge has been properly used in the earlier stages of figuring, it
+will be found that very little turning or grinding is requisite to
+produce a true centering.
+
+The particular defect due to want of centering in a lens may be
+observed by using it as the objective of a telescope, and observing a
+star slightly out of focus.  The interference fringes will not be
+concentric circles  unless the lens is properly  centred.  I ought to
+say that I have not looked into the theory of this, but have merely
+taken it as a generally admitted fact.  The diseases of lenses and the
+modes of treating them are dealt with in a book by Messrs.  Cook of
+York, entitled On the Adjustment and Testing of Telescopic Objectives.
+
+The final process of figuring will be dealt with later on (§ § 66 and
+67), as it applies not only to lenses but to mirrors, prisms, etc.  If
+the instructions given have been carefully carried out on a 2-inch
+lens, it should perform fairly well, and possibly perfectly, without
+any further adjustment of the glass.
+
+§ 64.  Preparation of Small Lenses, where great Accuracy is not of the
+first Importance.
+
+Such lenses may generally be made out of bits of good plate or sheet
+glass, and are of constant use in the physical laboratory.  They may
+be purchased so cheaply, however, that only those who have the
+misfortune to work in out-of-the-way places need be driven to make
+them.
+
+Suitable glass having been obtained and the curves calculated from the
+index of refraction, as obtained by any of the ordinary methods
+applicable to plates (the microscope method, in general, is quite good
+enough), squares circumscribing the desired circles are cut out by the
+help of a diamond.  [Footnote: Glazebrook and Shaw's Practical
+Physics, p.  383 (4th ed.).]  The squares are roughly snipped by means
+of a pair of pliers or spectacle-maker's shanks.  The rough circles
+are then mounted on the end of a brass or iron rod of rather greater
+diameter than the finished lenses are to possess.  This mounting is
+best done by centering cement.
+
+The discs are then dressed circular on a grindstone, the rod serving
+both as a gauge and handle.  A sufficient number of these discs having
+been prepared, a pair of brass tools of the form shown in the sketch
+(Fig.  55), and of about the proper radius of curvature, are made.
+One of these tools is used as a support for the glass discs.
+
+Fig. 55.
+
+A compass being set to scribe circles of the same diameter as the
+glass discs, centre marks are made on the surface of the appropriate
+tool, circles are drawn on this, and facets are filed or milled (for
+which the spiral head of the milling machine is excellent).  In the
+case of concave supporting surfaces, i.e.  in making concave lenses, I
+apprehend filing would be difficult, and the facets would have to be
+made by a rose cutter or mill;  but if the discs are fairly round,
+then, in fact, no facets are required.
+
+The facets being ready, the glass discs are cemented to them by
+centering cement, which may be used quite generally for small lenses.
+When the cutting of facets has been omitted on a concave surface, the
+best cement is hard pitch.  The grinding tool is generally rather
+larger than the nest of lenses.  Coarse and fine grinding is
+accomplished wholly on the lathe--the tool being rotated at a fair
+speed (see infra), and the nest of lenses moved about by its handle so
+as to grind all parts equally.  It must, of course, be held anywhere
+except "dead on," for then the part round the axis would not get
+ground;  this inoperative portion of the rotating tool must therefore
+be allowed to distribute its incapable efforts evenly over the nest of
+lenses.
+
+Polishing is accomplished by means of the grinding tool, coated with
+paper and rouge as before; or the tool may be coated with very thin
+cloth and used with rouge as before--in this case the polishing goes
+on fastest when the surface of the cloth is distinctly damp.  In
+working by this method, each grade of emery need only be applied from
+five to ten minutes.  The glass does not appear to get scratched when
+the emery is changed, provided everything is well washed.  A good
+polish may be got in an hour.  The lathe is run as for turning brass
+of the same diameter as the tool.
+
+One side of the lenses being thus prepared, they are reversed, and the
+process gone through for the other side in a precisely similar manner.
+[Footnote: Unless the radius of curvature is very short and the lenses
+also convex, there is no necessity to recess the facets, provided hard
+pitch is used as the cement.  See note on hard pitch.]  To save
+trouble, it is usual, to make such lenses of equal curvature on both
+faces;  but of course this is a matter of taste.
+
+Fig.  56.
+
+For very common work, bits of good plate glass are employed, and the
+manufacturer's surface treated as flat (Fig.  56).  In this way
+plano-convex lenses are easily and cheaply made.  Finally the lenses
+have to be centred, an essential operation in this case.  This is
+easily done by the reflection method--the edge being turned off by
+the file and kerosene and the centering cement being used in making
+the preliminary adjustment on the chuck.  I presume a lens made in
+this way is worth about a shilling, so that laboratory manufacture is
+not very remunerative.  Fig.  56 shows the method of mounting small
+lenses for lathe grinding, when only one lens is required.  The tool
+is generally rotated in the lathe and the lens held against it.
+
+§ 65.  Preparing Small Mirrors for Galvanometers.
+
+To get good mirrors for galvanometers, I have found the best plan is
+to grind and polish a large number together, on a disc perhaps 8 or 10
+inches in diameter.  I was led to this after inspecting and rejecting
+four ounces of microscope cover slips, a most wearisome process.  That
+regular cover slips should be few and far between is not unlikely,
+seeing that they are made (by one eminent firm at least) simply by
+"pot" blowing a huge thin bulb, and then smashing it on the floor and
+selecting the fragments.  As in the case of large mirrors, it is of
+course only necessary to grind one side of the glass, theoretically at
+all events.  The objections to this course are:
+
+(1) A silver surface cannot, in my experience, be polished externally
+(on a minute object like a cover slip) to be anything like so bright
+as the silver surface next the glass;  and,
+
+(2) if one side only is ground, it will be found that the little
+mirror hopelessly loses its figure directly it is detached from the
+support on which it has been worked.  Consequently, I recommend that
+these small mirrors should be ground and polished on both
+sides--enough may be made at one operation to last for a very long time.
+
+A slate back is prepared of the same radius of curvature as it is
+desired to impart to the mirrors.  Bits of thin sheet glass are then
+ground circular as described in the last section and cemented to this
+surface by the smallest quantity of clean archangel pitch, allowed to
+cool slowly and even to rest for a day before the work is proceeded
+with.  The whole surface is then ground and polished as before.
+
+The mirrors are now reversed, when they ought to nearly fit the tool
+(assuming that flats are being made, and the fellow tool in all other
+cases), and are recemented by pitch to the appropriate backing ground,
+and polished.  If very excellent results are required, these processes
+may be preceded by a preliminary rough grinding of one surface, so
+that the little discs will "sit" exactly on the tool surface, and not
+run the risk of being strained by capillary forces in the pitch.  We
+have always found this necessary for really good results.
+
+On removing such mirrors from the backing, they generally, more or
+less lose their figure, becoming (in general fairly uniformly) more
+concave or convex.  About 5 per cent of the mirrors thus prepared will
+be found almost perfect if the work has been well done, and the rest
+will probably be very fair, unless the diameter is very large as
+compared with the thickness.  The best way of grinding and polishing
+such large surfaces (nests 10 inches in diameter) is on a grinding
+machine, such as will be described below.  The polishing is best done
+by means of paper, as before described.
+
+Having occasion to require hitherto unapproached lightness and optical
+accuracy in such mirrors, I got my assistant to try making them of
+fused quartz, slices being cut by a diamond wheel from a rod of that
+material.  Chips of natural quartz were also obtained from broken
+"pebble" spectacles, and these were worked at the same time.  The
+resulting mirrors were certainly superior to the best we could make
+from glass, but the labour of grinding was greater, and the labour of
+polishing less, than in the latter case.  The pebble fragments gave
+practically as good mirrors as the fused slices.  For the future it
+will be better always to make galvanometer mirrors from quartz
+crystals.  These may be easily sliced, as will be described in § 74.
+The slices are dressed on a grindstone according to instructions
+already given for small lenses.
+
+The silvering of these mirrors is a point of great importance.  After
+trying nearly every formula published, we have settled down to the
+following.
+
+A solution of pure crystallised nitrate of silver in distilled water
+is made up to a strength of 125 grams of the salt per litre.  This
+forms the stock solution and is kept in a dark bottle.
+
+Let the volume of silvering liquor required in any operation be
+denoted by 4 v.  The liquor is prepared as follows:
+
+I.  Measure out a volume v of the stock solution of silver nitrate,
+and calculate the weight of salt which it contains;  let this be w.
+In another vessel dissolve pure Rochelle salt to the amount of 2.6 w,
+and make up the solution to the volume v.  These two solutions are to
+be mixed together at a temperature of 55° C, the vessels with their
+contents being heated to this temperature on the water bath.  After
+mixing the liquids the temperature is to be kept approximately
+constant for five minutes, after which the liquor may be cooled.  The
+white precipitate which first forms will become gray or black and very
+dense as the liquid cools.  If it does not, the liquor must be
+reheated to 55° C, and kept at that temperature for a few minutes and
+then again allowed to cool.  The solution is in good order when all
+the precipitate is dense and gray or black and the liquor clear.  The
+blacker and denser the precipitate the better is the solution.  The
+liquor is decanted and filtered from the precipitate and brought up to
+the volume 2 v by addition of some of the wash water.
+
+II.  Measure out a volume 0.118 v of the stock solution into a
+separate vessel, and add to it a 5 per cent solution of ammonium
+hydrate, with proper precautions, so that the precipitate at first
+formed is all but redissolved after vigorous shaking.  It is very
+important that this condition should be exactly attained.  Therefore
+add the latter part of the ammonia very carefully.  Make up the volume
+to 2 v.
+
+Mix the solutions I.  and II.  in a separate vessel and pour the
+mixture into the depositing vessel.  The surface to be silvered should
+face downwards, and lie just beneath the free surface of the liquid.
+Bubbles must of course be removed.
+
+The silver deposit obtained in this manner is exceedingly white and,
+bright on the surface next to the glass, but the back is mat and
+requires polishing.
+
+The detail of the process described above was worked out in my
+laboratory by Mr. A. Pollock, to whom my thanks are due.
+
+This process gives good deposits when the solutions are freshly
+prepared, but the ammonia solution will not keep; The surfaces to be
+silvered require to be absolutely clean.  The process is assisted by a
+summer temperature, say 70° Fahr, and possibly by the action of
+light.  Six or seven hours at least are required for a good deposit; a
+good plan is to leave the mirrors in the bath all night.  On removal
+from the bath the mirrors require to be well washed, and allowed to
+dry thoroughly in sun heat for several hours before they are touched.
+
+Care should be taken not to pull the mirrors out of shape when they
+are mounted for the bath.  A single drop of varnish or paint (a mere
+speck) on the centre will suffice to hold them. The back of the
+deposit requires to be varnished or painted as a rule to preserve the
+silver.  All paints and varnishes thus applied tend to spoil the
+figure by expanding or contracting.  On the whole, I think boiled
+linseed oil and white or red lead--white or red paint in fact--is
+less deleterious than other things I have tried.  Shellac varnish is
+the worst.
+
+Of course, the best mirror can be easily spoiled by bad mounting.  I
+have tried a great number of methods and can recommend as fairly
+successful the following:-  A little pure white lead, i.e.  bought as
+pure as a chemical--not as a paint--is mixed with an equal quantity
+of red lead and made into a paste with a little linseed oil.  I say a
+paste, not putty.  A trace of this is then worked on to the back of
+the mirror at the centre as nearly as may be, and to this is attached
+the support.  The only objection to this is that nearly a week is
+required for the paste to set.  If people must use shellac let it be
+remembered that it will go on changing its shape for months after it
+has cooled (whether it has been dissolved in alcohol or not).
+
+§ 66.  Preparation of Large Mirrors or Lenses for Telescopes.
+
+So much has been written on this subject by astronomers, generally in
+the English Mechanic and in the Philosophical Transactions for 1840,
+that it might be thought nothing could be added.  I will only say here
+that the processes already described apply perfectly to this case; but
+of course I only refer to silver on glass mirrors.  For any size over
+6 inches in diameter, the process of grinding and polishing by hand,
+particularly the latter, will probably be found to involve too much
+labour, and a machine will be required.  A description of a
+modification of Mr. Nasmyth's machine--as made by my assistant, Mr.
+Cook--will be found below.
+
+There is no difficulty in constructing or working such a machine, and
+considered as an all round appliance, it possesses solid advantages
+over the simple double pulley and crank arrangement, which, however,
+from its simplicity deserves a note.  Two pulleys, A and B, of about
+18 inches diameter by 4 inches on the face, are arranged to rotate
+about vertical axes, and belted together.  The shaft of one of these
+pulleys is driven by a belt in any convenient manner.  Each pulley is
+provided on its upper surface with a crank of adjustable length
+carrying a vertical crank-pin.
+
+Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6" long, and
+these rods are pinned together at their farther extremities, and this
+pin carries the grinding or polishing tool, or rather engages loosely
+with the back of this tool which lies below the rod.  It is clear that
+if the pulleys are of commensurable diameters, and are rigidly
+connected--say by belting which neither stretches nor slips--the
+polishing tool will describe a closed curve.  If, however, the belt is
+arranged to slip slightly, or if the pulleys are of incommensurable
+diameters, the curve traced out by the grinding tool will be very
+complex, and in the case of the ratio of the diameters being
+incommensurable, will always remain open;  for polishing purposes the
+consummation to be wished.
+
+Mirror surfaces are ground spherical, the reduction to parabolic form
+being attained in the process of polishing.  A very interesting
+account of the practice of dealing with very large lenses will be
+found in Nature, May 1886, or the Journal of the Society of Arts, same
+date (I presume), by Sir Howard Grubb.  The author considers that the
+final adjustment of surfaces by "figuring"--of which more anon--is
+an art which cannot be learned by inspection, any more than a man
+could learn to paint by watching an artist.  This is, no doubt, the
+case to some extent;  still, a person wishing to learn how to figure a
+lens could not do better than take Sir Howard at his word, and spend a
+month at his works.  Meanwhile the following remarks must suffice; it
+is not likely that anybody to whom these notes will be of service
+would embark on such large work as is contemplated by Sir Howard
+Grubb.
+
+Fig. 57.
+
+Description of Polishing Machine.  Power is applied through belting to
+the speed cone A.  By means of a bevel pinion rotation is communicated
+to the wheel D, which is of solid metal and carries a T-slot, C.  A
+pedestal forming a crank-pin can be clamped so as to have any desired
+radius of motion by the screw E.  A train of wheels E F G H K
+(ordinary cast lathe change wheels) communicate any desired ratio of
+motion to the tool-holder, which simply consists of two pins
+projecting vertically downwards from the spokes of wheel K.
+
+These pins form a fork, and each prong engages in a corresponding hole
+in the back of the slate-grinding tool (not shown in figure).  The
+connection with the tool is purposely loose.  The wheel E, of course,
+cannot rotate about the crank-pin D.  Provision for changing the ratio
+of tool rotation is achieved by mounting the wheels composing the
+train on pins capable of sliding along a long slot in the bar
+supporting them. The farther end of this bar is caused to oscillate
+to and fro very slowly by means of an additional crank-pin S and
+crank-shaft, the projecting face of the bed-plate W being placed
+so as to allow V to slide about easily and smoothly.
+Motion is communicated to this part of the system by means of
+gears at 0 and P, and a belt working from P to Q.
+Thus the vertical shaft R is set in motion and
+communicates by gears with S.  A pulley placed on the axle of the
+wheel carrying the crank-pin S gives a slow rotation to the work which
+is mounted on the table M. A small but important feature is the tray L
+below the gear K.  This prevents dirt falling from the teeth of the
+wheel on to the work.  The motion of S is of course very much less
+than of B--say 100 times less.  The work can be conveniently adjusted
+as to height by means of the screw N.
+
+The machine must be on a steady foundation, and in a place as free
+from dust as possible.  Though it looks complicated it is quite
+straight-forward to build and to operate.
+
+It is explained in Lord Rayleigh's article on Optics in the
+Encyclopaedia Britannica that a very minute change in the form of the
+curvature of the surface of a lens will make a large difference in the
+spherical aberration.  This is to be expected, seeing that spherical
+aberration is a phenomenon of a differential sort, i.e.  a measure of
+the difference between the curvature actually attained, and the
+theoretical curvature at each point of the lens, for given positions
+of point and image.  Sir H.  Grubb gives an illustration of the
+minuteness of the abrasion required in passing from a curve of one
+sort to a curve of another, say from a spherical to a parabolic curve,
+consequently the process of figuring by the slow action of a polishing
+tool becomes quite intelligible.  In making a large mirror or lens all
+the processes hitherto described under grinding and polishing, etc,
+have to be gone through and in the manner described, and when all this
+is accomplished the final process of correcting to test commences.
+This process is called figuring.
+
+§ 67.  Of the actual operation of this process I have no personal
+knowledge, and the following brief notes are drawn from the article by
+Sir H.  Grubb, from my assistant's (Mr. Cook) experience, and from a
+small work On the Adjustment and Testing of Telescopic Objectives, by
+T.  Cook and Sons, Buckingham Works, York (printed by Ben Johnson and
+Co, Micklegate, York).  This work has excellent photographs of the
+interference rings of star images corresponding to various defects.
+It must be understood that the following is a mere sketch.  The art
+will probably hardly ever be required in laboratory practice, and
+those who wish to construct large telescopes should not be above
+looking up the references.
+
+The process is naturally divided for treatment into two parts.
+
+(1) The detection of errors, and the cause of these errors.
+
+(2) The application of a remedy.
+
+(1) A lens, being mounted with its final adjustments, is turned on to
+a star, which must not be too bright, and should be fairly overhead.
+The following appearances may be noted:-
+
+A.  In focus, the star appears as a small disc with one or two rings
+round it; inside and outside of the focus the rings increase in
+number, are round, concentric with the disc, and the bright and dark
+rings are apparently equally wide.  The appearance inside the focus
+exactly resembles that outside when allowance is made for chromatic
+effects.  Conclusion:  objective good, and correctly mounted.
+
+B.  The rings round the star in focus are not circular, nor is the
+star at the centre of the system. In bad cases the fringes are seen at
+one side only.  Effects exaggerated outside and inside the focus.
+Conclusion:  the lens is astigmatic, or the objective is not adjusted
+to be co-axial with the eyepiece.
+
+C.  When in focus the central disc is surrounded by an intermittent
+diffraction pattern, i.e.  for instance the system of rings may appear
+along, and near, three or more radii.  If these shift when the points
+of support of the lens are shifted, flexure may be suspected.
+
+D.  On observing inside and outside the focus, the rings are not
+equally bright and dark.  This may be due to uncorrected spherical
+aberration, particularly to a fault known as "zonal aberration," where
+different zones of the lens have different foci, but each zone has a
+definite focus.
+
+E.  Irregular diffraction fringes point to bad annealing of the glass.
+This may be checked by an examination of the lens in polarised light.
+
+F.  If the disc appear blurred and coloured, however the focus be
+adjusted, incomplete correction for chromatic aberration is inferred.
+If in addition the colouring is unsymmetrical (in an extreme case the
+star disc is drawn out to a coloured band), want of centering is to be
+inferred.  This will also show itself by the interference fringes
+having the characteristics described in C.
+
+(2) The following steps may be taken in applying a remedy:
+
+A.  The adjusting screws of the cell mounting the object glass may
+be worked until the best result is attained;  this requires great
+care and patience.  Any errors left over are to be attributed to
+other causes than the want of collinearity of the axes of object
+glass and eyepiece.
+
+B.  Astigmatism is detected by rotating the object glass or object
+glass cell.  If the oval fringes still persist and the longer axis
+follows the lens, astigmatism may be inferred.  Similarly, by rotating
+one lens on the other, astigmatism, or want of centering (quite a
+different thing) may be localised to the lens.
+
+C.  The presence of flexure may be confirmed by altering the position
+of the points of support with respect to the eyepiece, the lens
+maintaining its original position.  The addition of more points of
+support will in general reduce the ill effects.  How to get rid of
+them I do not know;  they are only serious with large lenses.
+
+D.  Spherical aberration may be located by using stops and zonal
+screens, and observing the effect on the image.  Sir H.  Grubb
+determines whether any point on the lens requires to be raised or
+lowered, by touching the glass at that point with a warm hand or
+cooling it by ether.  The effects so produced are the differential
+results of the change of figure and of refractive index.  By observing
+the effect of the heating or cooling of any part, the operator will
+know whether to raise or lower that part, provided that by a suitable
+preliminary experiment he has determined the relation between the
+effect produced by the change of figure, and that due to the
+temperature variation of the refractive index.  In general it is
+sufficient to consider the change of shape only and neglect the change
+in refractive power.
+
+E.  Marked astigmatism has never been noticed by me, but I should
+think that the whole lens surface would require to be repolished or
+perhaps reground in this case.
+
+F.  To decide in which surface faults exist is not easy.  By placing a
+film of oil between the two surfaces nearly in contact these may be
+easily examined.  Thus a mixture of nut and almond oil of the right
+proportion, to be found by trial, for the temperature, will have the
+same refractive index as the crown glass, and will consequently reduce
+any errors of figure in the interior crown surface, if properly
+applied between the surfaces.  Similarly the interior of the flint
+surface may have its imperfections greatly reduced in effect by using
+almond oil alone, or mixed with bisulphide of carbon.  The outer
+surfaces, I presume, must be examined by warming or cooling over
+suitable areas or zones.
+
+The defects being detected, a matter requiring a great deal of skill
+and experience according to Sir H.  Grubb, the next step is to remedy
+them; and the remedial measures as applied to the glass constitute the
+process of figuring.  There are two ways of correcting local defects,
+one by means of small paper or pitch covered tools, which according to
+Sir H.  Grubb is dangerous, and according to the experience of Mr.
+Cook, and I think of many French opticians, safe and advantageous.
+
+Pitch polishing tools are generally used for figuring.  They are made
+by covering a slate backing with squares of pitch.  The backing is
+floated with pitch say one-eighth of an inch thick.  This is then
+scored into squares by a hot iron rod.  The tool, while slightly warm,
+is laid upon the lens surface, previously slightly smeared with dilute
+glycerine, until the pitch takes the figure of the glass.  The
+polishing material is rouge and water.  Small tools are applied
+locally, and probably can only be so applied with advantage for grave
+defects.
+
+The other method is longer and probably safer.  It consists in
+furnishing the polishing tool with squares of pitch as before.  These
+being slightly warm, the lens is placed upon them so that they will
+flow to the exact figure also as before.  I presume that the lens is
+to be slightly smeared with glycerine, or some equivalent, to keep the
+pitch from sticking.  The squares are most thickly distributed where
+the abrasion is most required, i.e.  less pitch is melted out by the
+iron rod.  This may be supplemented by taking advantage of differences
+of hardness of pitch, making some squares out of harder, others out of
+softer pitch.  The aim is to produce a polishing tool which will
+polish unequally so as to remove the glass chiefly from predetermined
+parts of the lens surface.  The tool is worked over the surface of the
+lens by the polishing machine, and part of the art consists in
+adjusting the strokes to assist in the production of the local
+variations required.
+
+A source of difficulty and danger lies in the fact that the pitch
+squares are rarely of the same hardness, so that some abrade the glass
+more rapidly than others.  This is particularly likely to occur if the
+pitch has been overheated.  [Footnote: When pitch is heated till it
+evolves bubbles of gas its hardness increases with the duration of the
+process.]  The reader must be good enough to regard these remarks as
+of the barest possible kind, and not intended to convey more than a
+general idea of the nature of the process of figuring.
+
+§ 68.  A few remarks on cleaning lenses will fittingly close this part
+of the subject.  There is no need to go beyond the following
+instructions given by Mr. Brashear in Popular Astronomy, 1894, which
+are reproduced here verbatim.
+
+"The writer does not advise the use of either fine chamois skin,
+tissue paper, or an old soft silk handkerchief, nor any other such
+material to wipe the lenses, as is usually advised.  It is not,
+however, these wiping materials that do the mischief, but the dust
+particles on the lenses, many of them perhaps of a silicious nature,
+which are always harder than optical glass, and as these particles
+attach themselves to the wiping material they cut microscopic or
+greater scratches on the surfaces of the objective in the process of
+wiping.
+
+"I write this article with the hope of helping to solve this
+apparently difficult problem, but which in reality is a very simple
+one.
+
+"Let us commence by taking the object glass out of its cell.  Take out
+the screws that hold the ring in place, and lift out the ring.
+Placing the fingers of both hands so as to grasp the objective on
+opposite sides, reverse the cell, and with the thumbs gently press the
+objective squarely out of the cell on to a book, block of wood, or
+anything a little less in diameter than the objective, which has had a
+cushion of muslin or any soft substance laid upon it.  One person can
+thus handle any objective up to 12 inches in diameter.
+
+"Before separating the lenses it should be carefully noted how they
+were put together with relation to the cell, and to one another, and
+if they art not marked they should be marked on the edges
+conspicuously with a hard lead pencil, so that when separated they may
+be put together in the same way, and placed in the same relation to
+the cell.  With only ordinary precaution this should be an easy
+matter.
+
+"Setting the objective on edge the two lenses may be readily
+separated.
+
+"And now as to the cleaning of the lenses.  I have, on rare occasions,
+found the inner surfaces of an object glass covered with a curious
+film, not caused directly by moisture but by the apparent oxidation of
+the tin-foil used to keep the lenses apart.  "A year or more ago a
+7-inch objective made by Mr. Clark was brought to me to clean.  It had
+evidently been sadly neglected.  The inside of the lenses were covered
+with such a film as I have mentioned, and I feared the glass was
+ruined.  When taken apart it was found that the tin-foil had oxidised
+totally and had distributed itself all over the inner surfaces.  I
+feared the result, but was delighted to find that nitric acid and a
+tuft of absorbent cotton cut all the deposit off, leaving no stains
+after having passed through a subsequent washing with soap and water.
+
+"I mention this as others may have a similar case to deal with.
+
+"For the ordinary cleaning of an objective let a suitable sized
+vessel, always a wooden one, be thoroughly cleaned with soap and
+water, then half filled with clean water about the same temperature as
+the glass.  Slight differences of temperature are of no moment.  Great
+differences are dangerous in large objectives.
+
+"I usually put a teaspoonful of ammonia in half a pail of water, and
+it is well to let a piece of washed 'cheese cloth' lie in the pail, as
+then there is no danger if the lens slips away from the hand, and, by
+the way, every observatory, indeed every amateur owning a telescope,
+should have plenty of 'cheese cloth' handy.  It is cheap (about 3 cts.
+per yard) and is superior for wiping purposes to any 'old soft silk
+handkerchief,' chamois skin, etc.  Before using it have it thoroughly
+washed with soap and water, then rinsed in clean water, dried and laid
+away in a box or other place where it can be kept clean.  When you use
+a piece to clean an objective throw it away, it is so cheap you can
+afford to do so.
+
+"If the lenses are very dirty or 'dusty,' a tuft of cotton or a
+camel's-hair brush may be used to brush off the loose material before
+placing the lenses in the water, but no pressure other than the weight
+of the cotton or brush should be used.  The writer prefers to use the
+palms of the hand with plenty of good soap on them to rub the
+surfaces, although the cheese cloth and the soap answers nicely, and
+there seems to be absolutely no danger of scratching when using the
+hands or the cheese cloth when plenty of water is used;  indeed when I
+wish to wipe off the front surface of an objective in use, and the
+lens cannot well be taken out, I first dust off the gross particles
+and then use the cheese cloth with soap and water, and having gone
+over the surface gently with one piece of cloth, throw it away and
+take another, perhaps a third one, and then when the dirt is, as it
+were, all lifted up from the surface, a piece of dry cheese cloth will
+finish the work, leaving a clean brilliant surface, and no scratches
+of any kind.
+
+"In washing large objectives in water I generally use a 'tub' and
+stand the lenses on their edge.  When thoroughly washed they are taken
+out and laid on a bundle of cheese cloth and several pieces of the
+same used to dry them.
+
+"I think it best not to leave them to drain dry;  better take up all
+moisture with the cloth, and vigorous rubbing will do no harm if the
+surfaces have no abrading material on them. I have yet to injure a
+glass cleaned in this way.
+
+"This process may seem a rather long and tedious one, but it is not so
+in practice, and it pays.
+
+"In some places objectives must be frequently cleaned, not only
+because they become covered with an adherent dust, but because that
+dust produces so much diffused light in the field as to ruin some
+kinds of telescope work.  Mr. Hale of the Kenwood Observatory tells me
+he cannot do any good prominence photography unless his objective has
+a clean surface; indeed every observer of faint objects or delicate
+planetary markings knows full well the value of a dark field free from
+diffused light.  The object-glass maker uses his best efforts to
+produce the most perfect polish on his lenses, aside from the accuracy
+of the curves, both for high light value and freedom from diffused
+light in the field, and if the surfaces are allowed to become covered
+with dust, his good work counts for little.
+
+"If only the front surface needs cleaning, the method of cleaning
+with cheese cloth, soap and water, as described above, answers very
+well, but always throw away the first and, if necessary, the second
+cloth, then wipe dry with a third or fourth cloth;  but if the
+surfaces all need cleaning I know of no better method than that of
+taking the objective out of its cell, always using abundance of soap
+and water, and keep in a good humor."
+
+§ 69.  The Preparation of Flat Surfaces of Rock Salt.
+
+The preliminary grinding is accomplished as in the case of glass,
+except that it goes on vastly faster.  The polishing process is the
+only part of the operation which presents any difficulty.  The
+following is an extract from a paper on the subject, by Mr. J.  A.
+Brashear, Pittsburg, Pa, U.S.A, from the Proceedings of the American
+Association for the Advancement of Science, 1885.  Practically the
+same method was shown me by Mr. Cook some years earlier, so that I can
+endorse all that Mr. Brashear says, with the following exceptions.  We
+consider that for small salt surfaces the pitch is better scored into
+squares than provided with the holes recommended by Mr. Brashear.
+
+Mr. Brashear's instructions are as follows.  After alluding to the
+difficulty of drying the polished salt surface--which is of course
+wet--Mr. Brashear says:-
+
+"Happily I have no trouble in this respect now, and as my method is
+easily carried out by any physicist who desires to work with rock salt
+surfaces, it gives me pleasure to explain it.  For polishing a prism I
+make an ordinary pitch bed of about two and one-half or three times
+the area of the surface of the prism to be polished.  While the pitch
+is still warm I press upon it any approximately flat surface, such as
+a piece of ordinary plate glass.  The pitch bed is then cooled by a
+stream of water, and conical holes are then drilled in the pitch with
+an ordinary counter sink bit, say one-quarter of an inch in diameter,
+and at intervals of half an inch over the entire surface.  This is
+done to relieve the atmospheric pressure in the final work.  The upper
+surface of the pitch is now very slightly warmed and a true plane
+surface (usually a glass one, prepared by grinding and polishing three
+surfaces in the ordinary way, previously wetted) is pressed upon it
+until the pitch surface becomes an approximately true plane itself.
+Fortunately, moderately hard pitch retains its figure quite
+persistently through short periods and small changes of temperature,
+and it always pays to spend a little time in the preparation of the
+pitch bed.
+
+"The polisher being now ready, a very small quantity of rouge and
+water is taken upon a fine sponge and equally distributed over its
+surface.  The previously ground and fined salt surface (this work is
+done the same as in glass working) is now placed upon the polisher and
+motion instantly set up in diametral strokes.  I usually walk around
+the polisher while working a surface.  It is well to note that motion
+must be constant, for a moment's rest is fatal to good results, for
+the reason that the surface is quickly eaten away, and irregularly so,
+owing to the holes that are in the pitch bed.  Now comes the most
+important part of this method.  After a few minutes' work the moisture
+will begin to evaporate quite rapidly.  No new application of water is
+to be made, but a careful watch must be kept upon the pitch bed, and
+as the last vestige of moisture disappears the prism is to be slipped
+off the polisher in a perfectly horizontal direction, and if the work
+has been well done, a clean, bright, and dry surface is the result.
+The surface is now tested by the well-known method of interference
+from a perfect glass test plate (see Fig.  178).
+
+"If an error of concavity presents itself the process of polishing is
+gone over again, using short diametral strokes.  If the error is one
+of convexity, the polishing strokes are to be made along the chords,
+extending over the edge of the polisher.  The one essential feature of
+this method is the fact that the surface is wiped dry in the final
+strokes, thus getting rid of the one great difficulty of pitch
+polishing, a method undoubtedly far superior to that of polishing on
+broadcloth.  If in the final strokes the surface is not quite cleaned
+I usually breathe upon the pitch bed, and thus by condensation place
+enough moisture upon it to give a few more strokes, finishing just the
+same as before.  In ten minutes I have polished prisms of rock salt in
+this manner that have not only shown the D line double, but Professor
+Langley has informed me that his assistant, Mr. Keeler (J.  E.), has
+seen the nickel line clearly between the D lines.  This speaks for the
+superiority of the surfaces over those polished on broadcloth.
+
+"In polishing prisms I prefer to work them on top of the polisher, as
+they can be easily held, but as it is difficult to hold lenses or
+planes in this way without injuring the surfaces, I usually support
+them in a block of soft wood, turned so as to touch only at their
+edges, and work the polisher over them. Though it takes considerable
+practice to succeed at first, the results are so good that it well
+repays the few hours' work it requires to master the few difficulties
+it presents."
+
+Fig.  58.
+
+§ 70.  Casting Specula for Mirrors.
+
+According to Sir H. Grubb (loc. cit.) the best alloy is made of four
+atoms of copper and one of tin;  this gives by weight,
+copper 252, tin 117.8.
+
+The copper is melted first in a plumbago crucible;  the tin is added
+gradually.  Of course, in the process of melting, even though a little
+fine charcoal be sprinkled over the copper, some loss of that metal
+will occur from oxidation.  It is convenient in practice, therefore to
+reserve a portion of the tin and test the contents of the crucible by
+lifting a little of the alloy out and examining it.
+
+The following indications may be noted:  When the copper is in excess
+the tint of the alloy is slightly red, and the structure, as shown at
+a fractured surface, is coarsely crystalline.  As the proper
+proportions are more nearly attained, the crystalline structure
+becomes finer, the colour whiter, and the crystals brighter.  The
+alloy is ready for use when the maximum brightness is attained and the
+grain is fine.
+
+If too much tin be added, the lustre diminishes.  The correct
+proportion is, therefore, attained when a further small addition of
+tin produces no apparent increase of brightness or fineness of grain.
+About three-quarters of the tin may be added at first, and the other
+quarter added with testing as described.  The alloy is allowed to cool
+until on skimming the surface the metal appears bright and remains so
+without losing its lustre by oxidation for a sensible time;  it will
+still be quite red-hot.
+
+Fig. 59.                                            Fig.   60.
+
+As the speculum alloy is too difficult to work with ordinary tools, it
+is best to cast the speculum of exactly the required shape and size.
+This is done by means of a ring of iron turned inside (and out) and on
+one edge.  This ring is laid on a plate of  figured iron, and before
+the metal is poured the plate (G)  (Figs 59 and 61) is heated to, say,
+300° C.  In order to avoid the presence of oxide as far as possible,
+the following arrangements for pouring are made.  A portion of the
+lower surface of the ring is removed by radial filing until a notch
+equal to, say, one-twentieth of the whole circumference is produced.
+This is cut to an axial depth of, say, half an inch.
+
+A bar of iron is then dovetailed loosely into the notch (Fig.  60, B),
+so that it will rest on the iron plate, and half fill the notch.  The
+aperture thus left forms the port of ingress for the hot metal (see
+Fig.  61, M).  A bit of sheet iron is attached to the upper surface of
+the ring, and lies as a sort of flap, shaped like a deep shovel,
+against the outside of the ring overhanging the port (Figs.  59 and 61
+at F).  This flap does not quite reach the iron plate, and its sides
+are bent so as to be in contact with the ring.  A portion of a smaller
+ring is then applied in such a manner as to form a pouring lip or pool
+on the outside of the main ring at E, and the metal can only get into
+the main ring by passing under the edge of the flap and up through the
+port.  This forms an efficient skimming arrangement.  The process of
+casting is carried out by pouring steadily into the lip.
+
+To avoid air bubbles it is convenient to cause the metal to spread
+slowly over the chill, and Mr. Nasmyth's method of accomplishing this
+is shown in the figure (61).  The chill rests on three pins, A B C
+(Figs.  59 and 61).  Before pouring begins the chill is tilted up off
+C by means of the counterpoise D, which is insufficient to tilt it
+after the speculum is poured.  It is important that the chill should
+be horizontal at the close of the operation, in order that the
+speculum may be of even thickness throughout.  This is noted by means
+of levels placed on the ring (at K for instance).
+
+Fig.  61.
+
+This apparatus may appear unnecessarily complex, but it is worth while
+to set it up, for it makes the operation of casting a speculum fairly
+certain.  If the metal is at the right temperature it will form a
+uniformly liquid disc inside the ring.  The mass sets almost directly,
+and as soon as this occurs it is pushed to the edge of the plate and
+the metal in the lip broken off by a smart upward tap with a hammer.
+The dovetailed bit of iron is knocked downwards and falls off, and the
+ring may then be lifted clear of the casting.  The object of the
+dovetail will now be understood, for without it there is great risk of
+breaking into the speculum in knocking the "tail" off.
+
+A box of quite dry sawdust is prepared in readiness for the process of
+annealing before the speculum is cast.  The box must be a sound wooden
+or metal box, and must be approximately air-tight.  For a speculum a
+foot in diameter the box must measure at least 3 feet both ways in
+plan, and be 2 feet 6 inches deep.  Half the sawdust is in the box and
+is well pressed down so as to half fill it.  The other half must be
+conveniently ready to hand.  As soon as possible after casting, the
+speculum is thrown into the box, covered over with the sawdust, and
+the lid is put on.
+
+The object in having the box nearly air-tight is to avoid
+air-currents, which would increase the rate of cooling.  A speculum a
+foot in diameter may conveniently take about three days to anneal, and
+should be sensibly warm when the box is opened on the fourth day.  For
+larger sizes longer times will be required.  We will say that the
+sawdust thickness on each side must be proportional to the dimensions
+of the speculum, or may even increase faster with advantage if time is
+of no moment.
+
+The process of annealing may be considered successful if the disc does
+not fly to pieces in working;  it is to be worked on the chilled side.
+The object of giving the chill the approximate counterpart form will
+now appear; it saves some rough grinding, and causes the finished
+surface to be more homogeneous than it would be if the centre were
+sunk by grinding through the chilled surface.
+
+In 1889 I learned from Mr. Schneider, Professor Row-land's assistant
+at Baltimore, that in casting specula for concave gratings a good deal
+of trouble had been saved by carrying out the operation in an
+atmosphere consisting mostly of coal gas.  It was claimed that in this
+way the presence of specks of oxide was avoided.  I did not see the
+process in operation, but the results attained are known and admired
+by all experimenters.
+
+§ 71.  Grinding and polishing Specula.
+
+The rough grinding is accomplished by means of a lead tool and coarse
+emery; the size of grain may be such as will pass a sieve of 60
+threads to the inch.  The process of grinding is quite similar to that
+previously described, but it goes on comparatively quickly.  The rough
+grinding is checked by the spherometer, and is interrupted when that
+instrument gives accordant and correct measurements all over the
+surface.
+
+The fine grinding may be proceeded with by means of a glass-faced tool
+as before described, or the labour may be reduced in the following
+manner.  A slate tool, which must be free from green spots (a source
+of uneven hardness), is prepared, and this is brought nearly to the
+curvature of the roughly ground speculum, by turning or otherwise.  It
+is finished on the speculum itself with a little flour of emery.  The
+fine grinding is then carried on by means of slate dust and water, the
+slate tool being the grinder.  The tool is, of course, scored into
+squares on the surface.
+
+If the casting process has been carried out successfully, the rough
+grinding may take, say six hours, and the fine grinding say thirty
+hours for a disc a foot in diameter.  The greatest source of trouble
+is want of homogeneity in the casting, as evidenced by blowholes, etc.
+In general, the shortest way is to discard the disc and start afresh
+if there is any serious want of perfection in the continuity or
+homogeneity of the metal.
+
+Fig.  62.
+
+The finely ground surface must, of course, be apparently correct in so
+far as a spherometer (with 3 inches between the legs for a disc 1 foot
+in diameter) will show.  Polishing and figuring are carried out
+simultaneously.  Half an hour's polishing with a slate-backed pitch
+tool and rouge and water will enable an optical test to be made.  The
+most convenient test is that of Foucault, a simple appliance for the
+purpose being shown in the figure (62).  It essentially consists of a
+small lamp surrounded by an opaque chimney (A) through which a minute
+aperture (pin-hole) is made.  A small lens may be used, of very great
+curvature, or even a transparent marble to throw an image of the flame
+on the pin-hole.
+
+A screen (B) is placed close to the source, and is provided with a
+rocking or tilting motion (C) in its own plane.  The source and screen
+are partly independent, and each is provided with a fine adjustment
+which serves to place it in position near the centre of curvature.
+The screen is so close to the pin-hole in fact that both the source
+and a point on the edge of the screen may be said to be at the centre
+of curvature of the mirror.  The mirror is temporarily mounted so as
+to have its axis horizontal, in a cellar or other place of uniform
+temperature.
+
+The final focussing to the centre of curvature is made by the fine
+adjustment screws;  the image may be received on a bit of paper placed
+on the screen and overlapping the edge nearest the source.  The screws
+are worked till the image has its smallest dimension and is bisected
+by the edge of the screen.  The test consists in observing the
+appearance of the mirror surface while the screen is tilted to cut off
+the light, as seen by an eye placed at the edge of the screen, a
+peephole or eye lens being provided to facilitate placing the eye in a
+correct position.  The screen screws are worked so as to gradually cut
+off the light, and the observer notes the appearance of the mirror
+surface.  If the curves are perfect and spherical, the transition from
+complete illumination to darkness will be abrupt, and no part of the
+mirror will remain illuminated after the rest.
+
+For astronomical purposes a parabolic mirror is required.  In this
+case the disc may be partially screened by zonal screens, and the
+position of the image for different zones noted;  the correctness or
+otherwise of the curvature may then be ascertained by calculation.  A
+shorter way is to place the source just outside the focus, to be found
+by trial, and then, moving the extinction screen (now a separate
+appliance) to, say, five times the radius of curvature away, where the
+image should now appear, the suddenness of extinction may be
+investigated.  This, of course, involves a corresponding modification
+of the apparatus.
+
+Whether the tests indicate that a deepening of the Centre, i.e.
+increase of the curvature, or a flattening of the edges is required,
+at least two remedial processes are available.  The "chisel and
+mallet" method of altering the size of the pitch, squares of the
+polisher may be employed, or paper or small pitch tools may be used to
+deepen the centre.  The "chisel and mallet" method merely consists in
+removing pitch squares from a uniformly divided tool surface by means
+of the instruments mentioned.  This removal is effected at those
+points at which the abrasion requires to be reduced.
+
+When some practice is attained, I understand that it is usual to try
+for a parabolic form at once, as soon as the polishing commences.
+This is done by dividing the pitch surface by V-shaped grooves, the
+sides of the grooves being radii of the circular surface, so that the
+central parts of the mirror get most of the polishing action.  If
+paper tools are used they must not be allowed much overhang, or the
+edges of the mirror betray the effects of paper elasticity.  Most
+operators "sink" the middle, but the late Mr. Lassell, a most
+accomplished worker, always attained the parabolic form by reducing
+the curvature of the edges of a spherical mirror.
+
+§ 72.  Preparation of Flat Surfaces.
+
+As Sir H.  Grubb has pointed out, this operation only differs from
+those previously described in that an additional condition has to be
+satisfied.  This condition refers to the mean curvature, which must be
+exact (in the case of flats it is of course zero) to a degree which is
+quite unnecessary in the manufacture of mirrors or lenses.
+
+A little consideration will show that to get a surface flat the most
+straightforward method is to carry out the necessary and sufficient
+condition for three surfaces to fit each other impartially.  If they
+each fit each other, they must clearly all be flat.  To carry out the
+process of producing a flat surface, therefore, two tools are made,
+and the glass or speculum is ground first on one and then on the
+other, the tools being kept "in fit" by occasional mutual grinding.
+The grinding and polishing go on as usual.  If paper is employed, care
+must be taken that the polisher is about the same size as the object
+to be polished.
+
+There is a slight tendency to polish most at the edges; but if the
+sweeps are of the right shape and size, this may be corrected
+approximately.  The best surfaces which have come under my notice are
+those prepared as "test surfaces" by Mr. Brashear of Alleghany, Pa,
+U.S.A.  These I believe to be pitch polished.  A pitch bed is
+prepared, I presume, in a manner similar to that described for
+rocksalt surfaces; but the working of the glass is an immense art, and
+one which I believe--if one may judge by results--is only known to
+Mr. Brashear.
+
+In general, the effect of polishing will be to produce a convex or
+concave surface, quite good enough for most purposes, but distinctly
+faulty when tested by the interference fringes produced with the aid
+of the test plate.  The following information therefore--which I draw
+from Mr. 'Cook--will not enable a student to emulate Mr. Brashear,
+but will undoubtedly help him to get a very much better surface than
+he usually buys at a high price, as exhibited on a spectroscope prism.
+
+The only difference between this process and the one described for
+polishing lenses, lies in the fact that the rouge is put into the
+paper surface while the latter is wet with a dilute gum "mucilage."
+It is of course assumed that the object and the two tools have been
+finely ground and fit each other impartially.  The paper is rubbed
+over with rouge and weak gum water.  The tool, when dry, is applied to
+the flat ground surface (of the object), and is scraped with the
+three-cornered file chisel as formerly described.  This process must
+be very carefully carried out.  The paper must be of the quality
+mentioned, or may even be thinner and harder.  The cross strokes
+should be more employed than in the case of the curved surfaces.
+
+A good deal will depend on the method employed for supporting the
+work; it is in general better to support the tool, which may have a
+slate backing of any desired thickness, whereby the difficulty
+resulting from strains is reduced.  The work must be mounted in such a
+way as to minimise the effect of changes of temperature.  If a pitch
+bed is selected, Mr. Brashear's instructions for rock salt may be
+followed, with, of course, the obvious necessary modifications.  See
+also next section.
+
+§ 73.  Polishing Flat Surfaces on Glass or on Speculum Metal.
+
+The above process may be employed for speculum metal, or pitch may be
+used.  In the latter case a fresh tool must be prepared every hour or
+so, because the metal begins to strip and leave bits on the polisher;
+this causes a certain amount of scratching to take place.  As against
+this disadvantage, the process of polishing, in so far as the state of
+the surface is concerned, need not take an hour if the fine grinding
+has been well done.
+
+For the finest work changes of temperature, as in the case of glass,
+cause a good deal of trouble, and the operator must try to arrange his
+method of holding the object so as to give rise to the least possible
+communication of heat from the hand.
+
+The partial elasticity of paper, which is its defect as a polishing
+backing, is, I believe, partly counterbalanced by the difficulty of
+forming with pitch an exact counterpart tool without introducing a
+serious rise of temperature (i.e.  warming the pitch).  The rate of
+subsidence of the latter is very slow at temperatures where it is hard
+enough to work reliably as a polisher.
+
+A student interested in the matter of flat surfaces will do well to
+read an account of Lord Rayleigh's work on the subject, Nature, vol.
+xlviii, 1893, pp.  212, 526 (or B.  A.  Reports, 1893).  In the first
+of these communications Lord Rayleigh describes the method of using
+test plates, and shows how to obtain the interference fringes in the
+clearest manner.
+
+For the ordinary optician a dark room and a soda flame afford all
+requisite information;  and if a person succeeds in making three glass
+discs, say 6 inches in diameter, so flat that, when superposed in any
+manner, the interference fringes are parallel and equidistant, even to
+the roughest observation, he has nothing to learn from any book ever
+written on glass polishing.  Lord Rayleigh has also shown how to use
+the free clean surface of water as a natural test plate.
+
+Since the above was written the following details of his exact course
+of procedure have been sent to me by Mr. Brashear, and I hereby tender
+my thanks:-
+
+"It really takes years to know just what to do when you reach that
+point where another touch either gives you the most perfect results
+attainable, or ruins the work you have already done.  It has taken us
+a long time to find out how to make a flat surface, and when we were
+called upon to make the twenty-eight plane and parallel surfaces for
+the investigation of the value of the metre of the international
+standard, every one of which required an accuracy of one-twentieth of
+a wave length, we had a difficult task to perform. However, it was
+found that every surface had the desired accuracy, and some of them
+went far beyond it.
+
+It is an advantage in making flat surfaces to make more than one at a
+time; it is better to make at least three, and in fact we always grind
+and 'fine' three together.  In making speculum plates we get up ten or
+twelve at once on the lead lap.  These speculum plates we can test as
+we go on by means of our test plane until we get them nearly flat.  In
+polishing them we first make quite a hard polisher, forming it on a
+large test plane that is very nearly correct.  We then polish a while
+on one surface and test it, then on a second and test it, and after a
+while we accumulate plates that are slightly concave and slightly
+convex.  By working upon these alternately with the same polisher, we
+finally get our polisher into such shape that it approximates more and
+more to a flat surface, and with extreme care and slow procedure we
+finally attain the results desired.
+
+All our flats are polished on a machine which has but little virtue in
+itself, unmixed with brains.  Any machine giving a straight
+diametrical stroke will answer the purpose.  The glass should be
+mounted so as to be perfectly free to move in every direction--that
+is to say, perfectly unconstrained.  We mount all our flats on a piece
+of body Brussels carpet, so that every individual part of the woof
+acts as a yielding spring.  The flats are held in place by wooden
+clamps at the edges, which never touch, but allow the bits of glass or
+metal to move slowly around if they are circular; if they are
+rectangular we allow them to tumble about as they please within the
+frame holding them.
+
+For making speculum metal plates either plane or concave we use
+polishers so hard that they scratch the metal all over the surface
+with fine microscopic scratches.  We always work for figure, and when
+we get a hard polisher that is in proper shape, we can do ever so many
+surfaces with it if the environments of temperature are all right.  If
+we have fifty speculum flats to make, and we recently made three times
+that number, we get them all ready and of accurate surface with the
+hard polisher.  Then we prepare a very soft polisher, easily indented
+when cold with the thumb nail.  A drop of rouge and about three drops
+of water are put on the plate, and with the soft polisher about one
+minute suffices to clean up all the scratches and leave a beautiful
+black polish on the metal.  This final touch is given by hand; if we
+do not get the polish in a few minutes the surface is generally ruined
+for shape, and we have to resort to the hard polisher again.
+
+I assure you that nothing but patience and perseverance will master
+the difficulty that one has to encounter, but with these two elements
+'you are bound to get there.'"
+
+CHAPTER III
+
+MISCELLANEOUS PROCESSES
+
+§ 74.  Coating Glass with Aluminium and Soldering Aluminium.
+
+A process of coating glass with aluminium has been lately discovered,
+which, if I mistake not, may be of immense service in special cases
+where a strongly adherent deposit is required.  My attention was first
+attracted to the matter by an article in the Archives des Sciences
+physiques et naturelles de Genève, 1894, by M. Margot.  It appears
+that clean aluminium used as a pencil will leave a mark on clean damp
+glass.  If, instead of a pencil, a small wheel of aluminium--say as
+big as a halfpenny and three times as thick--is rotated on the lathe,
+and a piece of glass pressed against it, the aluminium will form an
+adherent, though not very continuous coating on the glass.
+
+Working with a disc of the size described rotating about as fast as
+for brass-turning, I covered about two square inches of glass surface
+in about five minutes.  The deposit was of very uneven thickness, but
+was nearly all thick enough to be sensibly opaque.  By burnishing the
+brilliance is improved (I used an agate burnisher and oil), but a
+little of the aluminium is rubbed off.  The fact that the burnisher
+does not entirely remove it is a sign of the strength of the adherence
+which exists between the aluminium and the glass.  In making the
+experiment, care must be taken to have the glass quite clean--or at
+all events free from grease--in order to obtain the best results.
+
+M. Margot has contributed further information to the Archives des
+Sciences physiques et naturelles (February 1895).  He finds that
+adherence between aluminium and glass is promoted by dusting the glass
+with powders, such as rouge.  There is no doubt that a considerable
+improvement is effected in this way;  both rouge and alumina have in
+my hands greatly increased the facility with which the aluminium is
+deposited.  M. Margot finds that zinc and magnesium resemble aluminium
+in having properties of adherence to glass, and, what is more, carry
+this property into their alloys with tin.  Thus an alloy of zinc and
+tin in the proportions of about 92 per cent tin and 8 per cent zinc
+may be melted on absolutely clean glass, and will adhere strongly to
+it if well rubbed by an asbestos crayon.
+
+A happy inspiration was to try whether these alloys would, under
+similar circumstances, adhere to aluminium itself, and a trial showed
+that this was indeed the case, provided that both the aluminium and
+alloy are scrupulously clean and free from oxide.  In this way M.
+Margot has solved the problem of soldering aluminium. I have satisfied
+myself by trial of the perfect ease and absolute success of this
+method.  The alloy of zinc and tin in the proportions above mentioned
+is formed at the lowest possible temperature by melting the
+constituents together.  It is then poured so as to form thin sticks.
+
+The aluminium is carefully cleaned by rubbing with a cuttle bone, or
+fine sand, and strong warm potash.  It is then washed in water and
+dried with a clean cloth.  The aluminium is now held over a clean
+flame and heated till it will melt the solder which is rubbed against
+it.  The solder sticks at once, especially if rubbed with another bit
+of aluminium (an aluminium soldering bit) similarly coated.  To solder
+two bits of aluminium together it is only necessary to tin the bits by
+this process and then sweat them together.
+
+The same process applies perfectly to aluminium caused to adhere to
+glass by the previously mentioned process, and enables strong soldered
+contacts to be made to glass.  In one case, while I was testing the
+method, the adhesion was so strong that the solder on contracting
+while cooling actually chipped the surface clean off the glass.  In
+order to get over this I have endeavoured to soften the solder by
+mixing in a little of the fusible metal mercury amalgam;  and though
+this prevents the glass from being so much strained, it reduces the
+adherence of the solder.  It is a comfort to be able to solder
+aluminium after working for so many years by way of electroplating, or
+filing under solder.  An alternative method of soldering aluminium
+will be described when the electroplating of aluminium is discussed, §
+138.
+
+Gilding Glass.  In looking over some volumes of the Journal fuer
+praktische Chemie, I came across a method of gilding glass due to
+Boettger (Journ.  f.  prakt.  Chem. 103, p.  414).  After many trials
+I believe I am in a position to give definite instructions as to the
+best way of carrying out this rather troublesome operation.  The films
+of gold obtained by the process are very thick, and the appearance of
+the gold exceedingly fine.  The difficulty lies in the exact
+apportionment of the reducing solution.  If too much of the reducing
+solution be added, the gold deposits in a fine mud, and no coating is
+obtained.  If, on the other hand, too little of the reducing solution
+be added, little or no gold is deposited.  The secret of success turns
+on exactly hitting the proper proportions.
+
+The reducing solution consists of a mixture of aldehyde and glucose,
+and the difficulty I have had in following Boettger's instructions
+arose from his specifying "commercial aldehyde" of a certain specific
+gravity which it was impossible to reproduce.  I did not wish to
+specify pure aldehyde, which is not very easily got or stored, and
+consequently I have had to determine a criterion as to when the
+proportion of reducing solution is properly adjusted.
+
+The aldehyde is best made as required.  I employed the ordinary
+process as described in Thorpe's Dictionary of Applied Chemistry, by
+distilling alcohol, water, sulphuric acid, and manganese dioxide
+together.  The crude product is mixed with a large quantity of calcium
+chloride (dry--not fused), and is rectified once.  The process is
+stopped when the specific gravity of the product reaches 0.832 at 60°
+F.  The specific gravity of pure aldehyde is 0.79 nearly.
+
+The following is a modification of Boettger's formula:-
+
+Solution I
+
+1 gram of pure gold is converted into chloride--got acid free--i.e.
+to the state represented by AuCl3, and dissolved in 120 cc.  of water.
+
+This solution is the equivalent of one containing 6.5 grains of
+trichloride to the ounce of water.
+
+Solution II.
+
+  6     grams sodium hydrate.
+
+100     grams water.
+
+Solution III.
+
+ 0.2     grams glucose (bought as pure).
+
+12.6    cubic centimetres 95 per cent alcohol.
+
+12.6    cubic centimetres water.
+
+ 2.0     cubic centimetres aldehyde, sp. gr. 0.832.
+
+To gild glass these solutions are used in the following proportions by
+volume:-
+
+16     parts of     No.  I.
+
+ 4    parts of    No.  II.
+
+ 0.8    parts of    No.  III.
+
+The glass is first cleaned well with acid and washed with water:  it
+is then rinsed with Solution No.  III.  If it is desired to gild the
+inside of a glass vessel, Solution No.  III.  may be placed in the
+vessel first, and the walls of the vessel rinsed round carefully.
+Solutions I.  and II.  are mixed separately and then added
+to III.--after about two minutes the whole is well shaken up.
+
+If it be desired to gild a mirror of glass, the glass-plate is
+suspended face downwards in a dish of the mixed solutions--care being
+taken to rinse the glass with Solution III.  first.
+
+If the mixture darkens in from 7' to 10' in diffuse daylight and at
+60°F.  it will gild well, and it generally pays to make a few trials
+in a test tube to arrive at this.  If too much reducing solution is
+present the liquid will get dark more rapidly, and vice versa.  The
+gilding will require several hours--as much as twelve hours may be
+needed.
+
+The reaction is one of great chemical interest, being one of that
+class of reactions which is greatly affected by capillarity.  Thus it
+occasionally happens that when the reducing solution is not in the
+right proportion, gold will be deposited at the surface of the liquid
+(so as to form a gilt ring on the inside of a test tube), the
+remainder of the gold going down as mud.  The gold deposited is at
+first transparent to transmitted light and is deeply blue.  I thought
+this might be due to a trace of copper or silver, but on carefully
+purifying the gold no change of colour was noted.  If the reducing
+solution is present in slightly greater proportions than that given in
+the formula, the gold comes down with a richer colour, and has a
+tendency to form a mat surface and to separate from the glass.  The
+gold which is deposited more slowly has a less rich colour but a
+brighter surface.  The operation should be interrupted when a
+sufficient deposit has been obtained, because it is found that the
+thicker the deposit, the more lightly is it held to the glass surface.
+
+§ 75.  The Use of the Diamond-cutting Wheel.
+
+A matter which is not very well known outside geological circles is
+the manipulation of the diamond-cutting wheel, and as this is often of
+great use in the physical laboratory, the following notes may not be
+out of place.  I first became acquainted with the art in connection
+with the necessity which arose for me to make galvanometer mirrors out
+of fused quartz, and it was then that I discovered with surprise how
+difficult it is to obtain information on the point.  I desire to
+express my indebtedness to my colleagues, Professor David and Mr.
+Smeeth, for the instruction they have given me.  In what follows I
+propose to describe their practice rather than my own, which has been
+of a makeshift description.  I will therefore select the process of
+cutting a slice of rock for microscopical investigation.
+
+§ 76.  Arming a Wheel.
+
+Fig. 63.
+
+A convenient wheel is made out of tin-plate, i.e.  mild steel sheet,
+about one-thirtieth of an inch thick and seven inches in diameter.
+This wheel must be quite flat and true, as well as round;  too much
+pains cannot be taken in securing these qualities.  After the wheel is
+mounted, it is better to turn it quite true by means of a
+watch-maker's "graver" or other suitable tool.  The general design of
+a rock-cutting machine will be clear from the illustration (Fig.  63).
+
+The wheel being set up correctly, the next step is to arm it with
+diamond dust.  For this purpose it is before all things necessary that
+real diamond dust should be obtained.  The best plan is to procure a
+bit of "bort" which has been used in a diamond drill, and whose
+properties have therefore been tested to some extent.  This is ground
+in a diamond mortar--or rather hammered in one--and passed through a
+sieve having at least 80 threads to the inch.  The dust may be
+conveniently kept in oil.
+
+To arm the wheel, a little dust and oil is taken on the finger, and
+laid on round the periphery of the wheel.  A bit of flint or agate is
+then held firmly against the edge of the wheel and the latter is
+rotated two or three times by hand.  The rotation must be quite
+slow--say one turn in half a minute--and the flint must be held firmly
+and steadily against the wheel.  Some operators prefer to hammer the
+diamond dust into the wheel with a lump of flint, or agate, but there
+is a risk of deforming the wheel in the process.  When a new wheel is
+set up, it may be necessary to repeat the above process once every
+half hour or so till the cutting is satisfactory, but when once a
+wheel is well armed it will work for a long time without further
+attention.
+
+§ 77.  Cutting a Section.
+
+A wheel 7 inches in diameter may be rotated about 500 times per
+minute, and will give good results at that speed.  The work, as will
+be seen from the diagram, is pressed against the edge of the wheel by
+a force, which in the case quoted was about the weight of eleven
+ounces.  This was distributed along a cutting arc of three-quarters of
+an inch.
+
+A convenient cutting lubricant is a solution of Castile soap in water,
+and this must be freely supplied;  if the wheel gets dry it is almost
+immediately spoiled owing to the diamond dust being scraped off.  In
+the figure the lubricant is supplied by a wick running into the
+reservoir.  I have used both clock oil and ordinary gas-engine oil as
+lubricants, with equally satisfactory results.  As to the speed of
+cutting, in the experiment quoted a bit of rather friable "gabbro,"
+measuring three-quarters of an inch on the face by five-eighths of an
+inch thick, was cut clean through in six minutes, or by 3000 turns of
+the wheel.  The travel of the edge was thus between 5000 and 6000
+feet, or say 9000 feet, nearly 2 miles, per inch cut.
+
+A good solid rock, like basalt, can be cut into slices of about 3/32
+inch thick.  A very loose rock is best boiled in Canada balsam, hard
+enough to set, before it is put against the wheel.
+
+Instead of a grinding machine a lathe may be employed.  The disc is,
+of course, mounted on the mandrel, and the work on the slide-rest.
+The latter must be disconnected from its feed screws, and a weight
+arranged over a pulley so as to keep the work pressed against the
+wheel by a constant force.
+
+It may, perhaps, occur to the reader to inquire whether any clearance
+in the cut is necessary.  The answer is that in all probability, and
+in spite of every care, the wheel will wobble enough to give
+clearance.  If it does not, a little diamond dust rubbed into the side
+of the wheel, as well as the edge, will do all that is required.  The
+edge also, after two or three armings, "burrs" a little, and thus
+provides a clearance naturally.  It is not unlikely that in the near
+future the electric furnace will furnish us with a number of products
+capable of replacing the diamond as abrading agents.  The cost of the
+small amount of diamond dust; required in a laboratory is so small,
+however, that it; is doubtful whether any appreciable economy will be,
+effected.
+
+§ 78.  Grinding Rock Sections, or Thin Slips of any Hard Material.
+
+A note on this is, perhaps, worth making, for the same reasons as were
+given for note, § 75, which it naturally follows.  Just as
+trout-fishing; is described by Mr. Francis as the "art of fine and far
+off," [Footnote: In the Badminton Library, volume on Fishing.]
+section grinding may be called "the art of Canada balsam cooking," as
+follows.  A section of rock having been cut from the lump as just
+described, it becomes; necessary to grind it down for purposes of
+microscopical investigation.  For this purpose it is placed on a slip
+of glass, and cemented in position by Canada, balsam. Success in the
+operation of grinding the mounted section depends almost entirely on
+the way in which the mounting is done, and this in its turn depends on
+the condition to which the Canada has been brought.
+
+To illustrate the operations, I will describe a specific case, viz.
+that of grinding the section of "gabbro"' above described, for
+microscopical purposes.  One side of the section is probably
+sufficiently smooth and plane from the operation of the diamond wheel;
+if not, it must be ground by the finger on a slab of iron or gun-metal
+with emery and water, the emery passing a sieve of 80 threads to the
+inch.  The glass base on which the section is to be mounted for
+grinding is placed on a bit of iron or copper plate over a Bunsen
+burner, and three or four drops of natural Canada balsam are placed
+upon it.  The section is placed on the plate to heat at the same time.
+
+The temperature must not rise so high as to cause any visible change
+in the Canada balsam, except a slight formation of bubbles, which rise
+to the surface, and can be blown off.  The heating may require to be
+continued, say, up to twenty minutes.  The progress of the operation
+is tested by examining the balsam as to its viscous properties.
+
+An exceedingly simple and accurate way of testing is to dip a pair of
+ordinary forceps in the balsam, which may be stirred a little to
+secure uniformity.  The forceps are introduced with the jaws in
+contact, and, as soon as withdrawn, the jaws are allowed to spring
+apart, thus drawing out a balsam thread.  In a few moments the thread
+is cold, and if the forceps be compressed, this thread will bend.
+
+The Canada must be heated until it is just in such a state that on
+bringing the jaws together the thread breaks.  The forceps may open to
+about three-quarters of an inch.  If the Canada is more viscous, so
+that the thread does not break, the section when cemented by it will
+most probably slip on the slide.  On the other hand, if the balsam is
+more brittle, it will crumble away during the grinding.
+
+Assuming that the proper point has been reached, the section is
+mounted with the usual precautions to avoid air bubbles, i.e.  by
+dropping one edge on the balsam first.  When all is cold, the surface
+of the section may be ground on an iron plate with emery passing the
+80 sieve, till it is about 1/40 inch thick.  From this point it must
+be reduced on ground glass by flours of emery and water;  the rough
+particles of the former may be washed out for fine work.
+
+The process of grinding should not take more than half an hour if the
+section is properly cut, etc.  Beyond this point the allowable
+thickness must depend on the nature of the rock; a good general rule
+is to get the section just so thin that felspars show the yellow of
+the first order in a polarising, microscope.  The section is then
+finished with, say, two minutes emery or water of Ayr-stone dust.  It
+is better not to have the surface too smooth.
+
+To transfer the section, the hard Canada round the sides is scraped
+away, and the section itself covered with some fresh Canada from the
+bottle.  It is then warmed till it will slip off when a pin, or the
+invaluable dentist's chisel, is pressed against one side.  If the
+section be very delicate, the cover slip should be placed over it
+before it is moved to the proper slide.  The Canada used for mounting
+is not quite so hard as that employed in grinding, but it should be
+hard when cold, i.e. not sticky.
+
+The art of preparing Canada balsam appears to consist in heating it
+under such conditions as will ensure its being exposed in thin layers.
+I have wasted a good deal of time in trying to bake Canada in
+evaporating basins, with the invariable result that it was either over
+or under-baked, and got dark in colour during the process.
+
+On reviewing the process of rock section-cutting and mounting as just
+described, I cannot help thinking that, if properly systematised, it
+could be made much more rapid by the introduction of proper automatic
+grinding machinery.  It also seems not improbable that a proper
+overhaul of available gums and cements would be found to lead to a
+cementing material less troublesome than Canada balsam.
+
+§ 79.  Cutting Sections of Soft Substances.
+
+Though this art is fully treated of in books on practical biology, it
+is occasionally of use to the physicist, and the following note treats
+of that part of the subject which is not distinctly biological.
+
+Soft materials, of which thin sections may be required, generally
+require to be strengthened before they are cut.  For this purpose a
+variety of materials are available.  The one most generally used is
+hard paraffin.  The only point requiring attention is the embedding.
+The material must be dry.
+
+This is accomplished by soaking in absolute alcohol, i.e.  really
+absolute alcohol made by shaking up rectified spirit with potassium
+carbonate, previously dried, and then digesting for a day with large
+excess of quick-lime, making use of an inverted condenser and finally
+distilling off the alcohol without allowing it to come in contact with
+undried air.  After soaking for some time in absolute alcohol, the
+material may be transferred to oil of bergamot, or oil of cloves, or
+almost any essential oil.  After soaking in this long enough to allow
+the alcohol to diffuse out, the material may be lifted into a bath of
+melted paraffin (melting at, say, 51° C.).  The process of soaking is
+in some cases made to go more rapidly by exhausting, and, if the
+material will stand it, by raising the temperature over 100° C.  The
+soaking process may require minutes, hours, or days, according to the
+size and density of the material;  but a few hours are usually
+sufficient.
+
+When cold, the sections may be cut in any of the ordinary forms of
+microtome.
+
+Another way of embedding is to soak in collodion, and then precipitate
+the latter in the material and around it by plunging into nearly
+absolute alcohol.  The collodion yields a harder matrix than the
+paraffin.
+
+Whatever form of cutting machine is employed, the art of sharpening
+the knife is the only one requiring any particular notice.  The
+easiest way of obtaining a knife hard enough to sharpen, is to use a
+razor of good quality.  If it has to be ground, it is best to do this
+on a fine Turkey stone which is conveniently rested on two bits of
+rubber tubing, to avoid jarring the blade.  Many stones are slightly
+cracked, but on no account must the razor be dragged across a crack,
+or the edge will suffer.
+
+The necessary and sufficient condition is that the razor must be
+worked in little sweeps over the stone, and pressed against the latter
+by little more than its own weight, and the grinding must be regular.
+The edge may be inspected under a microscope, and it must be perfectly
+smooth and even before it will cut sections.  A finishing touch may be
+given on a leather strap, but it must be done skilfully, otherwise it
+is better omitted.
+
+The necessity for providing exceptionally keen and sharp edges arose
+in the manufacture of phonographs, where the knife used to turn up the
+wax cylinders must leave a perfectly smooth surface.  In 1889 this was
+being accomplished on an ivory lap fed with a trace of very fine
+diamond dust.
+
+I have had this method in mind as a possible solution of the
+difficulty of razor-grinding, but have not tried it.  I imagine one
+would use a soft steel or ivory slip rubbed over with fine diamond
+dust and oil by means of an agate.  The lap used in the phonograph
+works was rotated at a high speed.
+
+§ 80.  On the Production of Quartz Threads.
+
+[Footnote: Since this was written an article on the same subject by
+Mr. Boys appeared in the Electrician for 1896.  The instructions
+therein given are in accordance with what I had written, and I have
+made no alteration in the text.]
+
+In 1887 the important properties of fused quartz were discovered by
+Mr. Vernon Boys (Philosophical Magazine, June 1887, p.  489, "On the
+Production, Properties, and Some Suggested Uses of the Finest
+Threads").  A detailed study of the properties of quartz threads was
+made by Mr. Boys and communicated to the Society of Arts in 1889
+(Journal of the Society of Arts, 1889).  An independent study of the
+subject was made by the present writer in 1889 (Philosophical
+Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").
+
+There is also a paper in the Philosophical Magazine for 1894 (vol.
+xxxvii.  p.  463), by Mr. Boys, on "The Attachment of Quartz Fibres."
+This paper also appeared in the Journal of the Physical Society at
+about the same date, together with an interesting discussion of the
+matter.  In the American Journal, Electric Power, for 1894, there is a
+series of articles by Professor Nichols on "Galvanometers," in which a
+particular method of producing quartz threads is recommended.  The
+method was originally discovered by Mr. Boys, but he seems to have
+made no use of it.  A hunt through French and German literature on the
+subject has disclosed nothing of interest--nothing indeed which
+cannot be found in the papers mentioned.
+
+§ 81.  Quartz fibres have two great advantages over other forms of
+suspension when employed for any kind of torsion balance, from an
+ordinary more or less "astatic" galvanometer to the Cavendish
+apparatus.  In the first place the actual strength of the fibres under
+longitudinal stress is remarkably high, ranging from fifty to seventy
+tons weight per square inch of section, and even more than this in the
+case of very fine threads;  the second and more important point in
+favour of quartz depends on the wide limits within which cylindrical
+threads of this material obey the simplest possible law of torsion,
+i.e.  the law that for a given thread carrying a given weight at a
+given temperature and having one end clamped, the twist about the axis
+of figure produced by a turning moment applied at the free end is
+proportional simply to the moment of the twisting forces, and is
+independent of the previous history of the thread.
+
+It is to be noted, however, that the torsional resilience of quartz as
+tested by the above law is not so perfect but that our instrumental
+means allow us to detect its imperfections, and thus to satisfy
+ourselves that threads made of quartz are not things standing apart
+from all other materials, except in the sense that the limits within
+which they may be twisted without deviating in their behaviour from
+the law of strict proportionality by more than some unassigned small
+quantity, are phenomenally wide.
+
+A torsion balance--if we except the case of certain spiral
+springs--is almost always called upon for information as to the magnitude
+of very small forces, and for this purpose it is not essential merely
+that some law of twisting should be exactly obeyed, but also that the
+resistance to twisting of the suspension should be small.
+
+Now, regarded merely as a substance possessing elastic rigidity,
+quartz is markedly inferior to the majority of materials, for it is
+very stiff indeed;  its utility depends as much as anything upon its
+great strength, for this allows us to, use threads of exceeding
+fineness.  In addition to this it must be possible, and moreover
+readily possible, to obtain threads of uniform section over a
+sufficient length, or the rate of twist per unit length of the thread
+will vary in practice from point to point, so that the limits of
+allowable twist averaged over the whole thread may not be exceeded,
+and yet they may be greatly overpassed at particular points of the
+thread.
+
+It is interesting to note that in the case of quartz we not only have
+a means for readily producing very uniform cylindrical threads, but
+that the limits of allowable rate of twist are so wide that a small
+departure from uniformity of section produces much less inconvenience
+than in the case of any other known substance.
+
+§ 82.  There are three methods generally in use for drawing quartz
+fibres, all depending on the fact that quartz when fused is so viscous
+that it may be drawn into threads of great length, without these
+threads breaking up into drops, or indeed without their showing any
+sign of doing so.  The surface tension of the melted quartz must,
+however, be very considerable, as may be seen by examining the shape
+of a drop of the molten material, and this suffices to impress a
+rigidly cylindrical form upon the thread, the great viscosity
+apparently damping down all oscillation.
+
+The first method is the one originally employed by Mr. Boys.  A needle
+of quartz is melted somewhere in its length and is then drawn out
+rapidly by a light arrow, to which one end of the needle is attached,
+and which is projected from a kind of crossbow.
+
+A modification of this method, which the writer has found of service
+when very thick threads are required, is to replace the bow and arrow
+by a kind of catapult.
+
+The third method, which yields threads of almost unmanageable
+fineness, depends on the experimental fact that when a fine point of
+quartz is held in a high pressure oxygen gas blow-pipe flame, the
+friction of the flame gases suffices to overcome the tendency of the
+capillary forces to produce a spherical drop, and actually causes a
+fine thread to be projected outwards in the direction of the flame.
+
+§ 83.  A preliminary operation to any method is the production of a
+stick of fused quartz.  This is managed as follows.  A rock crystal or
+quartz pebble is selected and examined.  It must be perfectly white,
+transparent, and free from dirt.  Surface impurity can of course be
+got rid of by means of a grindstone.  The crystal is placed in a
+perfectly clean Stourbridge clay crucible, furnished with a cover, and
+heated to bright redness for about an hour in a clean fire or in a
+Fletcher's gas furnace.  The contents of the crucible are turned out
+when sufficiently cool on to a clean brick or bit of slate.  It will
+be found that the crystal is completely broken up and the fragments
+must be examined in case any of them have become contaminated by the
+crucible, but this will not have happened if the temperature did not
+rise beyond a bright red heat.
+
+The heap of fragments being found satisfactory, the next thing is to
+fuse some of the pieces together.  Unless the preliminary heating has
+been efficiently carried out this will prove an annoying task, because
+a rock crystal generally contains so much water that it splinters
+under the blow-pipe in a very persistent manner.  There are two ways
+of assembling the fragments.  One is to place two tiles or bricks on
+edge about the heap of quartz lying upon a third tile, so that the
+heap occupies the angular corner or nook formed by the tiles (Fig.
+64).
+
+The oxygas blow-pipe previously described is adjusted to give its
+hottest flame, the bags being weighted by at least two hundredweight,
+if of the size described (see § 15).
+
+The tip of the inner cone of the blow-pipe is brought to bear directly
+upon one of the fragments, and if the operation is performed boldly it
+will be found that the surface of the fragment can be fused, and the
+fragment thus caused to hold together before the lower side gets hot
+enough to suffer any contamination from the tile or brick.  A second
+fragment may be treated in the same way, and then a third, and so on.
+
+Finally, the fragments may be fused together slightly at the corners,
+and a stick may thus be formed.  Of course a good deal of cracking and
+splitting of the fragments takes place in the process;  the best
+pieces to operate upon are those which are well cracked to begin with,
+and that in such a way that the little fragments are interlocked.
+
+An alternative method which has some advantages is to arm a pair of
+forceps with two stout platinum jaws, say an inch and a half long, and
+flattened a little at the ends.  The fragments are held in these
+platinum forceps and the blow-pipe applied as before.  This method
+works very well in adding to a rod which has already been partly
+formed, but the jaws require constant renewals.  The first fragment
+which is fused sufficiently to cohere may also be fused to a bit of
+tobacco pipe, or hard glass tube or rod, and the quartz stick
+gradually built up by fusing fresh pieces on to the one already in
+position.
+
+Fig. 64.
+
+Since the glass or pipeclay will contaminate the quartz which has been
+fused on to it, it is necessary to discard the end pieces at the close
+of the operation.  A string of fragments having been collected and
+stuck together, the next step is to fuse them down into a uniform rod.
+This is easily done by holding the string in the blow-pipe flame and
+allowing it to fuse down.  Twisting the fused part has a good effect
+in assisting the operation.  It is desirable to use a large jet and as
+powerful a flame as can be obtained during this part of the operation.
+
+The final result should be a rod, say two or three inches long and
+one-eighth of an inch thick, which will in most cases contain a large
+number of air bubbles.  Since the presence of drawn-out bubbles cannot
+be advantageous, it is often desirable to get rid of them, and this
+can conveniently be done at the present  stage.  The process at best
+is rather tedious;  it  consists in drawing the quartz down very fine
+before an intense flame, in order to allow the bubbles to get close
+enough to the surface to burst.  A considerable loss of material
+invariably occurs during the process;  for whenever the thin rod
+separates into two bits the process of flame-drawing of threads goes
+on, and entails a certain waste; moreover, the quartz in fine
+filaments is probably partially volatilised.
+
+Sooner or later, however, a sufficient length of bubble-free quartz
+can be obtained.  It must not be supposed that it is always necessary
+to eliminate bubbles as perfectly as is contemplated in the foregoing
+description of the treatment, but for special purposes it may be
+essential to do so, and in any case the reader's attention is directed
+to a possible source of error.
+
+It may be mentioned in connection with this matter that crystals of
+quartz may look perfectly white and clear, and yet contain impurity.
+For instance, traces of sodium are generally present, and lithium was
+found in large spectroscopic quantity in five out of six samples of
+the purest crystals in my laboratory.  The presence of lithium in rock
+crystal has also been detected by Tegetmeier (Vied.  Ann, xli.  p.
+19, 1890).
+
+After some practice in preparing rods and freeing them of bubbles the
+operator will notice a distinct difference in the fusibility of the
+samples of quartz he investigates, though all may appear equally pure
+to the unaided eye.  It should be mentioned, however, that high
+infusibility cannot always be taken as a test of purity, for the most
+infusible, or rather most viscous, sample examined by the writer
+contained more lithium than some less viscous samples.
+
+Fig.  65.
+
+During the process of freeing the quartz from bubbles the lithium and
+sodium will be found to burn away, or at all events to disappear.
+
+A rod of quartz, say three inches long, one-sixteenth of an inch in
+diameter, and free from bubbles for half an inch of its length, even
+when examined by a strong lens, is suitable for drawing into threads.
+The rod is manipulated exactly in the manner described under
+glass-blowing, and is finally drawn down at the bubble free part into
+a needle, say 0.02 inch in diameter (No.  25 on the Birmingham wire
+gauge), and 2 inches long.
+
+Fig.  66.
+
+There is one peculiarity about fused quartz which renders its
+manipulation easier than that of glass--it is impossible to break
+fused quartz, however suddenly it be thrust into the blow-pipe flame.
+A rod having a diameter of three-sixteenths of an inch--and perhaps
+much more--may be brought right up to the tip of the inner cone of
+the oxy-gas flame and held there-till one side fuses, the other being
+comparatively cool, without the slightest fear of precipitating a
+smash.  In seven years' experience I have never seen a bit of once
+fused quartz broken by sudden heating;  whether it might be done if
+sufficient precautions were taken I do not know.
+
+The reason of the fortunate peculiarity of quartz in this respect is,
+I presume, to be found in the fact that quartz once it has been fused
+is really a very strong material indeed, and is also probably the
+least expansible substance known.  From some experiments of the writer
+upon the subject, it may be concluded that at the most quartz which
+has been fused expands only about one-fifth as fast as flint-glass, at
+all events between 20° and 70° C.
+
+§ 84.  Drawing Quartz Threads.
+
+The thick end of the rod of quartz is held in the fingers or
+occasionally in a clip.  The end of the fine point is attached to a
+straw arrow by means of a little sealing-wax.  The arrow is laid on
+the stock of a crossbow in the proper position for firing.    See
+Figs.  67 and 68, which practically explain themselves.
+
+The needle is heated by the blow-pipe till a minute length is in a
+state of uniform fusion;  the arrow is then let fly, when it draws a
+thread out with it.  The arrow is preferably allowed to strike a
+wooden target placed, say, 30 feet away from the bow, and a width of
+black glazed calico is laid under the line of fire to catch the thread
+or arrow if it falls short.  The general arrangements will be obvious
+from the figure.
+
+The bow is of pine in the case where very long thin threads are
+required, though for ordinary purposes I have found a bow of
+lance-wood succeed quite as well.  The trigger of the bow consists of
+a simple pin passing through the stock and fastened at its lower end
+to a string connected with a board which can be depressed by foot.  In
+the figure an ordinary trigger is shown, but the pin does just as
+well.
+
+Fig 67.
+
+The arrow is made out of about 6 inches of straw, plugged up aft by a
+small plug of pine or willow fastened in with sealing-wax, and
+projecting backwards one-eighth of an inch.  This projection serves a
+double purpose:  it gives a point of attachment for the quartz needle,
+and on firing the bow it forms a resisting anvil on which the string
+of the bow impinges.  The head of the arrow is formed by a large
+needle stuck in with sealing-wax, and heavy enough to bring the centre
+of gravity of the arrow forward of one-third of its length, the
+condition of stability in flight.
+
+Fig.  68.
+
+It is not necessary to employ any feathering for these arrows;  though
+I have occasionally used feathers or mica to "wing the shaft" no
+advantage has resulted therefrom.
+
+To get fine threads a high velocity is essential.  This is obtained by
+considering (and acting upon) the principles involved.  The bow may be
+regarded as a doubly-tapering rod clamped at the middle.  After
+deflection it returns towards its equilibrium position at a rate
+depending in general terms on the elastic forces brought into play,
+directly, and on the effective moment of inertia of the rod, inversely
+(see Rayleigh, Sound, vol.  ii.  chap.  viii.)  If the mass of the
+arrow is negligible compared with the bow, the rate at which the arrow
+moves is practically determined by that attained by the end of the
+bow, which is a maximum in crossing its equilibrium position.
+
+The extent to which the arrow profits by this velocity depends on the
+way the bow is strung.  It will be greatest when the string is
+perpendicular to the bow when passing its equilibrium position;  or in
+other words, when the string is infinitely long.  Since the string has
+mass, however, it is not permissible to make it too long, or its
+weight begins to make itself felt, and a point is soon reached at
+which the geometrical gain in string velocity is compensated for by
+the total loss of velocity due to the inertia of the string.  In
+practice it is sufficient to use a string 10 per cent longer than the
+bow.
+
+It is well to use a light fiddle string, served with waxed silk at the
+trigger catch;  if this be omitted the gut gets worn through very
+quickly.  In order to decide how far it is permissible to bend the
+bow, the quickest way is to make a rough experiment on a bit of the
+same plank from which the bow is to be cut, and then to allow a small
+factor of safety.  In the figure the bow is of lance-wood and is more
+bent than would be suitable for pine.
+
+The bow itself is tapered from the middle outwards just like any other
+bow.  If thick threads are required, the above considerations are
+modified by the fact that quartz opposes a considerable resistance to
+drawing, and that consequently the arrow must not only have a high
+velocity, but a fair supply of energy as well;  in other words, it
+must be heavy.  A thin pine arrow instead of a straw generally does
+very well, but in this case the advantage of using pine for the bow
+vanishes;  and in fact lance-wood does better, owing to the greater
+displacement which it will stand without breaking.  This of course
+only means that a greater store of energy can be accumulated at one
+bending.
+
+I had occasion to investigate whether the unavoidable spin of an arrow
+about its axis produces any effect on the thread, and for this purpose
+made arrows with inertia bars thrust through the head, i.e.  an arrow
+with a bit of wire run through it, perpendicular to its
+length--forming a cross in fact--the arms of the cross being weighted
+at the extreme ends by shot.  This form of arrow has a considerable
+moment of inertia about its longer axis, and consequently rotates less
+than a mere straw, provided that the couples tending to produce rotation
+are not increased by the cross arm, or the velocity too much reduced.
+Shooting one of these arrows slowly, I could see that it did not
+rotate, and when fired at a high velocity, it generally arrived at the
+target (placed at varying distances front bow) with the arms nearly
+horizontal, thus showing that it probably did not rotate much.
+
+I did not succeed in this at the first trial, by any means.  The
+threads got in this way were no better than those made with a single
+straw, whence we may conclude very provisionally that the spin of the
+arrow has only a small effect, if  any, on the quality of the threads.
+
+Feathering the arrow, in my experience, tends, if anything to make it
+spin more;  for one thing, because it is practically impossible to lay
+the feathering on straight.
+
+After the arrow is shot, it remains to gather in the thread, and if
+the latter is at all thin, we have a rather troublesome job.  In a
+thread thirty or forty feet long, the most uniform part generally lies
+in the middle if the thread is thin, i.e.  of the order of a
+ten-thousandth of an inch in diameter.  If the thread is thick the
+most uniform part may be anywhere.  The part of the thread required is
+generally best isolated by passing a slip of paper under it at each
+end and cementing the thread to the paper by means of a little
+paraffin or soft wax, and then cutting off the outer portions.  One
+bit of paper may then be lifted off the calico, and the thread will
+carry the other bit.  In this way the thread may be taken to a
+blackened board, where it may be mounted for stock.
+
+By passing the two ends of the thread under a microscope, or rather by
+breaking bits off the two ends and examining them together, it is easy
+to form an Opinion as to uniformity.
+
+Mr. Boys has employed an optical method of examining threads, but the
+writer has invariably found a  high-power microscope more convenient
+and capable of giving more exact information as to the diameter of the
+threads.
+
+The beginner--or indeed the practised hand--need not expect to get a
+thread of the exact dimensions required at the first shot.  A little
+experience is necessary to enable one to judge of the right thickness
+of the needle for a thread of given diameter.  The threads are so
+easily shot, however, that a few trials take up very little time and
+generally afford quite sufficient experience to enable a thread of any
+required diameter to be prepared.
+
+It is no use attempting to heat an appreciable length of needle; if
+this be done the thread almost invariably has a thick part about the
+middle of its length..  It is sufficient to fuse at most about
+one-twentieth of an inch along the needle before firing off the bow.
+This can be done by means of the smaller oxygas blow-pipe jet
+described in the article on blow-pipes for glass-blowing, § 14.  The
+flame must of course be turned down so as to be of a suitable size.  A
+sufficiently small flame may be got from almost any jet.
+
+If the needle be not equally heated all round, the thread tends to be
+curly;  indeed by means of the catapult, threads may be pulled which,
+when broken, tend to coil up like the balance-springs of watches, if
+only care be taken to have one side of the needle much hotter than the
+other.
+
+§ 85.  When examining bits of threads, say thicker than the
+two-thousandth of an inch, under the microscope it is convenient to
+use a film of glycerine stained with some kind of dye, in order to
+render the thread more sharply visible.  The thread is mounted beneath
+a cover slip, and a drop of the stained glycerine allowed to run in.
+Such a treatment gives the image of the thread a sharply defined edge
+3 and the contrast between the whiteness of the thread and the colour
+of the background allows measurements to be made with great ease.
+
+On the whole the easiest way of measuring the diameter of a thick
+thread is to use a measuring microscope, i.e.  one in which the lens
+system can be displaced along a plane bed by means of a finely cut
+micrometer screw.  The instruments made by the Cambridge Scientific
+Instrument Company do fairly well.  Direct measurements up to 0.0001
+inch are easily made by means of a microscope provided with a Zeiss
+"A" objective, and rather smaller differences of thickness can be made
+out by it.  For thin threads the method next to be described is more
+fitting, because higher powers can be more conveniently used.
+
+In this method an ordinary microscope is employed together with a
+scale micrometer, and either an eyepiece micrometer, or a camera and
+subsidiary scale.  The eyepiece micrometer is the more convenient.  If
+a camera be employed, i.e.  such an one as is supplied by Zeiss, it is
+astonishing how the accuracy of observation may be increased by
+attending carefully to the illumination of both the subsidiary scale
+and of the thread.  The two images should be as far as possible of
+equal brightness, and for this purpose it will be found requisite to
+employ small screens.
+
+The detail of making a measurement by means of the micrometer eyepiece
+is very simple.  The thread is arranged on the stage so as to point
+towards the observer, and the apparent diameter is read off on the
+eyepiece scale.  In order to calibrate the latter it is only necessary
+to replace the thread by the stage micrometer, and to observe the
+number of stage micrometer divisions occupying the space in the
+eyepiece micrometer formerly occupied by the thread.  It is essential
+that both thread and stage micrometer should occupy the same position
+in the field, for errors due to unequal distortion may otherwise
+become of importance.  For this reason it is best to utilise the
+centre of the field only.
+
+The same remark applies to measurements by means of the camera, where
+the image of the thread is projected against the reflected image of
+the subsidiary scale laid alongside the microscope.  In this case the
+value of the subsidiary scale divisions must be obtained from the
+divisions of the stage micrometer, coinciding as nearly as possible
+with the position occupied by the thread.  Before commencing a
+measurement the screens are moved about till both images appear
+equally bright.
+
+Threads up to about one twenty-thousandth of an inch in diameter may
+be sufficiently well measured by means of a Zeiss "4 centimetre
+apochromatic object-glass" and an eyepiece "No. 6" with sixteen
+centimetre tube length. [Footnote: The objective certainly had "4 cm."
+marked on it, but the focal length appeared to be about I.5 mm. only.]
+
+§ 86.  Drawing Threads by the Catapult.
+
+The bow-and-arrow method fails when threads of a greater diameter than
+about 0.0015 inch are required--at least if any reasonable uniformity
+be demanded, and no radical change in the bow and arrow be carried
+out.
+
+Thus in the writer's laboratory a thread of about this diameter,
+within 1/10000  of an inch-13 inches long and free from air
+bubbles--was required.  A fortnight's work by a most skilful operator
+only resulted in the production of two lengths satisfying the
+conditions.
+
+The greatest loss of time occurs in the examination of the thread by
+means of the microscope.
+
+Threads for galvanometer suspensions are conveniently from 0.0001 to
+0.0004 inch in diameter, and are much more easily made and got uniform
+than thicker threads, to the production of which the catapult method
+applies.
+
+A reference to the diagram will make the construction of the
+instrument quite clear.  The moving end of the quartz is attached to a
+small boxwood slider working on a tubular girder or between wires.
+The quartz is secured in position by clamps shown at A and B, and
+motion is imparted to the slider by a stretched piece of catapult
+elastic (C).  An easy means of regulating the pull of the elastic is
+to hold it back by a loop of string whose length can be varied by
+twisting it round a pin.
+
+Fig.  69.  [Footnote: For greater clearness of drawing, the tube
+carrying the slider is shown somewhat higher above the base than is
+convenient in practice;  and the slide itself is shown too thin in the
+direction of the hole through it.]
+
+Since it is not permissible to allow the slider to rebound at the end
+of its journey, some such arrangement of breaks as is shown must be
+adopted.  In the diagram the bottom of the slider runs on to a brass
+spring between the girder and the base of the appliance, and so gets
+jammed; the spiral spring acts merely as an additional guard.  The
+diagram does not show the lower spring very clearly;  it is a mere
+strip lying in the groove.
+
+A rod of quartz, with a needle at one end, is prepared as before and
+secured in the clamps.  During the operation of fastening down the
+clamps, there is some danger of breaking the needle, and consequently
+it is advisable to soften the latter before and while adjusting the
+second clamp.
+
+The process of drawing a thread by this method is exactly similar to
+the operation already described in connection with the arrow method.
+Though short thick threads form the product generally obtained from
+the catapult, it must not be supposed that thin threads cannot be
+obtained in this way.  If a short length of a very fine needle be
+heated, it will be found to yield threads quite fine enough for
+ordinary suspension purposes, but naturally not so uniform as those
+obtained from the 40-foot lengths obtainable by the bow-and-arrow
+method.
+
+It is easy to make spiral quartz springs resembling watch
+balance-springs by means of the catapult.  All that is necessary is to
+see that the quartz is rather unequally heated before the shot is
+fired.  In the future it is by no means impossible that such springs
+may have a real value, for the rigidity of quartz is known to increase
+as temperature rises.  Hence it is probable that the springs would
+become stiffer as temperature rises, even though they work chiefly by
+bending, and little or not at all by twisting.  As this is the kind of
+temperature variation required to compensate an uncompensated watch
+balance wheel, it may turn out to have some value.
+
+§ 87.  Drawing Threads by the Flame alone.
+
+A stick of quartz is drawn down to a fine point, and the tip of this
+point is held in the blow-pipe flame in the position shown in Fig.
+70.
+
+Fig.  70.
+
+The friction of the flame gases is found to be sufficient to carry
+forward the fused quartz and to draw it into threads in spite of the
+influence of the capillary forces.  If a sheet of paper be suspended
+at a distance of two or three feet in front of the blow-pipe flame, it
+will be found to be covered with fine threads tangled together into a
+cobwebby mass.  As this method is an exceedingly simple one of
+obtaining threads, I have endeavoured to reduce it to a systematic
+operation.
+
+A sheet of cardboard, about two feet square, is painted dead black and
+suspended horizontally, painted side downwards (Fig.  70, A), at a
+height of about two feet above the blow-pipe flame.  The latter is
+adjusted so as to point almost vertically upwards and towards the
+centre of the cardboard.  A few half-inch pins are thrust through the
+card from the upper surface and pushed home; about one dozen pins
+scattered over the surface will be sufficient.  Their object is to
+prevent the threads being carried away round the edge of the screen.
+
+The flame from the jet described so often is fed from gas bags
+weighted to about eighty pounds per square foot of  (one) surface,
+i.e.  "4-foot" bags require from three to four hundredweight to give
+an advantageous pressure. [Footnote: The resulting threads were really
+too fine for convenient manipulation, so that unless extremely fine
+threads are required it will be better to reduce the pressure of the
+gases considerably.]
+
+Two sticks of quartz are introduced and caused to meet just in front
+of the inner cone--the hottest part of the flame.  They are then
+drawn apart so as to form a fine neck, which softens and is bent in
+the direction of motion of the flame gases.  When fusion is complete
+the neck separates into two parts, and a thread is drawn from each of
+them. By alternately lightly touching the rods together, and drawing
+them apart, quite a mass of threads may be obtained in two or three
+minutes, when the process should be stopped.  If too many threads get
+entangled in the pins, one gives one's self the unnecessary trouble of
+separating them. On taking down the card it will be found that the
+threads have been caught by the pins;  but the card now being laid
+black side upwards, the former easily slip off the points.
+
+Threads at least a foot long, and perhaps vastly longer, may be
+obtained by this method, and are extraordinarily fine.  When I first
+read Professor Nichols' statement (Electric Power, 1894) as to the
+value of these fibres for galvanometer purposes, I was rather
+sceptical on the ground that the threads would tend to get annealed by
+being drawn gradually, instead of suddenly, from a place of intense
+heat to regions of lower temperature.
+
+Now annealing threads by a Bunsen makes them rotten.  The threads
+being immersed in the hot flame gases could only cool at the same rate
+as the gas, and it was not--and is not--clear to me that annealing
+of the threads can be avoided.  On the other hand, it may be possible
+that a thread cooled slowly from the first does not suffer in the same
+way as a cold thread would do when annealed in a Bunsen flame.
+
+Again the velocity of the gases is beyond doubt exceedingly high, so
+that the annealing, even supposing it to be deleterious, might not be
+carried very far.  Threads drawn by this method and measured "dry,"
+i.e.  by mounting them on a slide without the addition of any liquid,
+turned out to have a diameter of about 1/20000 of an inch.
+
+I do not think I could manage to mount such fine threads without very
+special trouble.  All the threads lying on the board, however, were
+found in reality to consist of three or four separate threads, and
+there is no reason why several threads should not be mounted in
+parallel, provided, of course, that they are equally stretched and
+touching each other.  Equality of tension in the mounting could be
+secured by making one attachment good, then cementing the other
+attachment to the other end of the threads, and "drawing" the two
+attachments slightly apart at the moment the cement commences to set.
+This method may turn out to be very valuable, for, so far as I can
+see, the carrying power would be increased without an increase of
+torsional stiffness of anything like so high an order as would be the
+case were one thread only employed.  On the other hand, the law of
+torsion could hardly be quite so simple, at all events, to the second
+order of approximations.
+
+§ 88.  Properties of Threads.
+
+A large number of experiments on the numerical values of the elastic
+constants of quartz threads have been made by Mr. Boys and his
+students, and by the writer.  As the methods employed were quite
+distinct and the results wholly independent, and yet in good agreement
+with each other, a rounded average may be accepted with considerable
+confidence.
+
+TENACITY OF QUARTZ FIBRES (BOYS).
+
+Diameter of Thread.
+
+Tenacity in Tons' Weight per Square Inch of Section.
+
+Tenacity in Dynes per Square Centimetre.
+
+Inches
+
+Centimetres
+
+0.00069
+
+0.00175
+
+51.7
+
+8 X 109
+
+0.00019
+
+0.00048
+
+74.5
+
+11.5 X 109
+
+Rounded mean of Boys' and Threlfall's results:
+
+Young's Modulus at 20° C,
+
+5.6   X 1011 C.G.S.
+
+Modulus of Simple Rigidity at 20° C,
+
+2.65 X 1011 C.G.S.
+
+Modulus of Incompressibility,
+
+1.4   X 1011 C.G.S.
+
+Modulus of Torsion,
+
+3.7   X 1011 C.G.S.
+
+Approximate coefficient of linear expansion of quartz per degree
+between 80° C.  and 30° C.  is 0.0000017 (Threlfall = loc. cit.).
+
+This must be regarded with some suspicion, as the data were not
+concordant.  There is no doubt, however, about the extreme
+inexpansibility of quartz.
+
+Temperature coefficient of modulus of torsional rigidity per degree
+centigrade, 22° to 98° C, 0.000133
+
+Ditto, absolute simple rigidity, 0.000128 (Threlfall).
+
+Limit of allowable rate of twist in round numbers is, one-third turn
+per centimetre, in a fibre 0.01 cm. diameter.
+
+The limiting rate is probably roughly inversely as the diameter.
+
+Attention must be called to the rapid increase in the torsional
+rigidity of these threads as the temperature rises.  A quartz spiral
+spring-balance will be appreciably stronger in hot weather.
+
+§ 89.  In the majority of instances in which quartz threads are
+applied in the laboratory, it is desirable to keep the coefficient of
+torsion as small as possible, and hence threads are used as fine as
+possible.
+
+It is convenient to remember that a thread 0.0014 cm. or 0.0007 inch
+in diameter breaks with a weight of about ten grammes, and may
+conveniently be employed to carry, say, five grammes.  With threads
+three times finer the breaking strength per unit area increases, say,
+50 per cent.  In ordinary practice--galvanometric work for
+instance--where it is desirable to use a thread as fine and short as
+possible to sustain a weight up to, say, half a gramme, it will be
+found that fibres five centimetres long or over give no trouble
+through defect of elastic properties.  A factor of safety of two is
+a fair allowance when loading threads.
+
+No difficulty will be experienced in mounting threads having a
+diameter of 0.0002 inch or over.  With finer threads it is necessary
+to employ very dark backgrounds (Mr. Boys uses the darkness of a
+slightly opened drawer), or the threads cannot be sufficiently well
+seen.
+
+In the case of instruments in which threads remain highly twisted for
+long periods of time, the above rule as to the safe limit of twist
+does not allow of a sufficient margin;  it is only applicable to
+galvanometric and similar purposes.
+
+The cause of the increase in tenacity as the diameter diminishes is at
+present unknown.  It is due neither to an effect of annealing
+(annealed threads are rotten), nor is it a skin effect, nor is it due
+to the cooling of the thread under higher capillary pressure.  It is,
+however, possible that it may be associated with some kind of
+permanent set taken by the fibres during the stage of passage from the
+liquid to the solid state.
+
+§ 90.  On the Attachment of Quartz Fibres.
+
+For many purposes it is sufficient to cement the fibres in position by
+means of ordinary yellow shellac, but where very great accuracy is
+aimed at, the shellac (being itself imperfectly elastic and exposed to
+shearing stress) imposes its imperfections on the whole system. This
+source of error can be got over by soldering the threads in position.
+Attempts were made by the writer in this direction, with fair success,
+in 1889, but as Mr. Boys has carried the art to a high degree of
+perfection, I will suppress the description of my own method and
+describe his in preference.  It has, of course, been frequently
+repeated in my laboratory.
+
+In many cases, however, if not in all, it may be replaced by Margot
+soldering, as already described, a note on the application of which to
+this purpose will follow.
+
+A thread of the proper diameter having been selected, it is cut to the
+right length.  With fine threads this is not always a perfectly easy
+matter.  The best way is for the operator to station himself facing a
+good light, not sunlight, which is too tiring to the eye, but bright
+diffused light.  The thread will be furnished with bits of paper stuck
+on with paraffin at both ends, as already described.
+
+A rough sketch of the apparatus--or, at all events, two lines showing
+the exact length which the free part of the thread must have--are
+marked on a smooth board, and this is supported with its plane
+vertical.  The thread is held against the board, and the upper piece
+of paper is stuck lightly to the board with a trace of soft wax, so
+that the lower edge of the paper is at any desired height above the
+upper mark.  This distance is measured, and forms the length of thread
+allowed to overlap the support.  A second bit of paper is attached
+below the lower mark, a margin for the attachment of the lower end
+being measured and left as before.  The thread will be most easily
+seen if the board is painted a dead black.
+
+If it is desired to attach the thread to its supports merely by
+shellac, this is practically all that needs to be done.  The supports
+should resemble large pins.  The upper support will be a brass wire in
+most cases, and will require to be filed away as shown in the sketch
+(Fig.  71).  It is then coated with shellac by heating and rubbing
+upon the shellac.  As previously noted, the shellac must not be
+overheated.
+
+The thread is cut off below the lower slip of paper, and the upper
+support being conveniently laid in a horizontal position on another
+dead-black surface, the thread is carried to it and laid as designed
+against the shellac, which is now cold.  When the thread is in place,
+a soldering iron is put against the brass wire, and the shellac
+gradually melted till it closes over the thread.
+
+Fig.  71.
+
+The iron is then withdrawn and the thread pulled away from the point
+for one-twentieth of an inch or less.  This ensures that the thread
+makes proper contact with the cement, and also that it is free from
+kinks; of course, it must leave the cement in the proper direction.  A
+similar process is next carried out with respect to the lower
+attachment, and the ends of the thread are neatly trimmed off.
+
+Both ends of the thread being secured, the next step is to transfer
+the upper support to a clip stand, the suspended parts being held by
+hand, so that the weight comes on the thread very gradually.  In this
+way it will be easily seen whether the thread is bent where it enters
+the shellac, and should this be the case, a hot iron must be brought
+up to the shellac and the error rectified.
+
+When both the support and the suspended parts are brought nearly to
+the required bearing, the hot iron is held for a moment close up to
+each attachment, the hand being held close below but not touching the
+suspended parts, and both attachments are allowed to straighten
+themselves out naturally.
+
+These details may appear tiresome, and so they are when written out at
+length, but the time occupied in carrying them out is very short, and
+quartz threads break easily, unless the pull upon them is accurately
+in the direction of their length at all points.
+
+In the event of its being decided to attach the thread by soldering,
+the process is rather more expensive in time, but not otherwise more
+troublesome.
+
+Fig.  72.                                     Fig.  73.
+
+The thread being cut as before to the proper length, little bits of
+aluminium foil are smeared all over with melted shellac and suspended
+from the thread replacing the paper slips before described.  It is
+important that no paraffin should be allowed to touch the thread
+anywhere near a point intended to be soldered.  The thread is hung up
+from a clip stand by one of the bits of foil, and the lower end is
+washed by dipping it into strong nitric acid for a moment and thence
+into water.  The object of smearing the foils all over with shellac is
+to prevent them being acted upon by the acid.  The threads are not
+very easily washed acid free, but the process may be assisted by means
+of a fine camel's-hair pencil.
+
+Some silvering solution made as described (§ 65) is put into a test
+tube;  the thread, after rinsing with distilled water, is lowered into
+the solution so far as is required, and is allowed to receive a
+coating of silver.  It has been observed that the coating of silver
+must not be too thick--not sufficiently thick to be opaque.  A watch
+may be kept on the process by immersing a minute strip of mica
+alongside the thread.
+
+The silvered thread is rinsed with distilled water and allowed to dry.
+
+Meanwhile the other end of the thread may be silvered.  When both ends
+are silvered the process of coppering by electro deposit is commenced.
+A test tube is partially filled with a ten per cent solution of
+sulphate of copper, and several copper wires are dipped into it to
+form an anode.  The thread is lowered carefully into the solution so
+as not to introduce air bubbles, and the silvered part is allowed to
+project far enough above the surface of the solution to come in
+contact with a fine copper wire.  The circuit is closed through a
+Leclanché cell and a resistance box.
+
+It is as well to begin with a fair resistance, say 100 ohms out in the
+box, and the progress of the deposit is watched by means of a
+low-power microscope set up in front of the thread.  If the copper
+appears to come down in a granular form, the resistance is too small
+and must be increased;  if no headway appears to be made, the
+resistance must be diminished.
+
+As soon as a fair coat of copper has come down, i.e.  when the
+diameter of the thread is about doubled, the process is interrupted.
+The thread is withdrawn, washed, dipped in a solution of chloride of
+zinc, and carefully tinned by dragging it over a small clean drop of
+solder on a soldering bit.
+
+During this part of the process the shellac is apt to get melted if
+the iron is held too close, so that it is advisable to begin by making
+the thread somewhat over long.  The end of the thread must only be
+trimmed off at the conclusion of the operation, i.e.  after the thread
+is soldered up.  The thread is attached to the previously tinned
+supports much in the same way as has been described under the head of
+shellac attachments.  It does not very much matter whether both ends
+are coppered before one is soldered up or not.  At the conclusion of
+the whole process the superfluous copper and silver are dissolved off
+by a little hot strong nitric acid applied on a glass hair pencil.
+This is best done by holding the thread horizontally with the
+assistance of clip stands.
+
+If the thread is too delicate to bear brushing, the nitric acid may be
+applied by pouring out a big drop into a bit of platinum foil and
+holding this below the thread so as to touch it lightly.  The
+dissolving of the copper and silver is, of course, followed by copious
+washing with hot water.  This process is more laborious than might be
+imagined, but it may be shortened by heating the platinum foil
+supporting the water (Fig.  74).
+
+Fig.  74
+
+The washing part of the process is, in the opinion of the writer, the
+most difficult part of the whole business, and it requires to be very
+thorough, or the thread will end by drawing out of the solder.  In
+many cases it is better to try to do without any application of nitric
+acid at all, but, of course, this involves silvering and coppering to
+exact distances from the ends of the thread--at all events, in
+apparatus where the effective length of the thread is narrowly
+prescribed.
+
+It is important not to leave the active parts of the thread
+appreciably silvered, for the sake of avoiding zero changes due to the
+imperfect elasticity of the silver.  In this soldering process
+ordinary tinman's solder may be employed;  it must be applied very
+free from dust or oxide.
+
+§ 91.  Other Modes of soldering Quartz.
+
+Thick rods of quartz may be treated for attachment by solder in the
+same way as glass was treated by Professor Kundt to get a foundation
+for his electrolytically deposited prisms. [Footnote: See Appendix at
+end of book.]
+
+The application of a drop of a strong solution of platinum
+tetrachloride to the rod will, on drying, give rise to a film of the
+dry salt, and this may be reduced in the luminous gas flame.  During
+the process, however, the quartz is apt to get rotten, especially if
+the temperature has been anything approaching a full red heat.  The
+resulting platinum deposit adheres very strongly to the quartz, and
+may be soldered to as before.  This method has been employed by the
+writer with success since 1887, and may even be extended to thick
+threads.
+
+It was also found that fusible metal either stuck to or contracted
+upon clean quartz so as to make a firm joint.  In the light of M.
+Margot's researches (already described), it occurred to me that
+perhaps my experience was only a special case of the phenomena of
+adhesion investigated with so much success by M. Margot.  I therefore
+tried whether the alloy of tin and zinc used for soldering aluminium
+would stick to quartz, and instantly found that this was indeed the
+case.
+
+Adhesion between the alloy and perfectly clean quartz takes place
+almost without rubbing.  A rod of quartz thus "tinned" can be soldered
+up to anything to which solder will stick, at once.  On applying the
+method to thick quartz threads, success was instantaneous (the threads
+were some preserved for ordinary galvanometer suspensions);  but when
+the method was applied to very fine threads, great difficulty in
+tinning the threads was experienced.  The operation is best performed
+by having the alloy on the end of an aluminium soldering bit, and
+taking care that it is perfectly free from oxide before the thread is
+drawn across it.  There was no difficulty in soldering a thread
+"tinned"    in this manner to a copper wire with tinman's solder, and
+the joint appeared perfect, the thread breaking finally at about an
+inch away from the joint.
+
+I allow Mr. Boys' method to stand as I have written it, simply because
+I have not had time as yet to make thorough tests of the durability of
+"Margot" joints on the finest threads;  but I have practically no
+doubt as to its perfect applicability, provided always that the solder
+can be got clean enough when melted on the bit.  Very fine threads
+will require to be stretched before tinning, in order to enable them
+to break through the capillary barrier of the surface of the melted
+solder.
+
+§ 92.  Soldering.
+
+It is almost unfair to the arts of the glass-blower or optician to
+describe them side by side with the humble trade of soldering.
+Nevertheless, no accomplishment of a mechanical kind is so serviceable
+to the physicist as handiness with the soldering bit;  and, as a rule,
+there is no other exercise in which the average student shows such
+lamentable incapacity.  The following remarks on the subject are
+therefore addressed to persons presumably quite ignorant of the way in
+which soldering is carried out, and do not profess to be more than of
+the most elementary character.
+
+For laboratory purposes three kinds of solder are in general
+sufficient.  One is the ordinary tinman's solder composed of lead and
+tin.  The second is "spelter," or soft fusible brass, and the third is
+an alloy of silver and brass called silver solder.
+
+Tinman's solder is used for most purposes where high temperatures are
+not required, or where the apparatus is intended to be temporary.  The
+"spelter," which is really only finely granulated fusible brass, is
+used for brazing iron joints.  The silver solder is convenient for
+most purposes where permanency is required, and is especially suited
+to the joining of small objects.
+
+§ 93.  Soft tinman's solder is made by melting together two parts of
+grain tin and one of soft lead--the exact proportions are not of
+consequence--but, on the other hand, the purer the constituents the
+better the solder.  Within certain limits, the greater the proportion
+of tin the cleaner and more fusible is the solder.  It is usually
+worth while to prepare the solder in the laboratory, for in this way a
+uniform and dependable product is assured.  Good soft lead is melted
+in an iron ladle and skimmed;  the temperature is allowed to rise very
+little above the melting-point.  The tin is then added little by
+little, the alloy stirred vigorously and skimmed, and sticks of solder
+conveniently cast by sweeping the ladle over a clean iron plate, so as
+to pour out a thin stream of solder.  If the solder be properly made
+it will have a mat and bright mottled surface, and will "crackle" when
+held up to the ear and bent.
+
+Perhaps the chief precaution necessary in making solder is to exclude
+zinc.  The presence of a very small percentage of this metal entirely
+spoils the solder for tinman's work by preventing its "running" or
+flowing smoothly under the soldering bit.
+
+Fig.  75.
+
+Fig.   76.
+
+Fig.  77.
+
+§ 94.  Preparing a Soldering Bit.
+
+The wedge-shaped edge of one of the forms of bit shown in the sketch
+is filed to shape and the bit heated in a fire or on a gas heater.  A
+bit of rough sandstone, or even a clean soft brick, or a bit of tin
+plate having some sand sprinkled over it, is placed in a convenient
+position and sprinkled with resin.
+
+As soon as the bit is hot enough to melt solder it is withdrawn and a
+few drops of solder melted on to the brick or its equivalent.  The
+iron or bit is then rubbed to and fro over the solder and resin till
+the former adheres to and tins the copper head.  It will be found
+advisable to tin every side of the point of the bit and to carry the
+tinning back at least half an inch from the edge.
+
+If the solder obstinately refuses to adhere, the cause is to be sought
+in the oxidation of the copper, or of the solder, or both--in either
+case the result of too high a temperature or too prolonged heating.
+The simple remedy is to get the iron hot, and then to dress it with an
+old file, so as to expose a bright surface, which is instantly passed
+over the resin as a means of preserving it from oxidation.  If the
+process above described be now carried out, it will be found that the
+difficulty disappears.
+
+Before using the iron, wipe off any soot or coke or burned resin by
+means of an old rag.  An iron tinned in this way is much to be
+preferred to one tinned by means of chloride of zinc.
+
+A shorter and more usual method is carried out as follows:  The
+solution of chloride of zinc is prepared by adding bits of zinc to
+some commercial hydrochloric acid diluted with a little (say 25 per
+cent) of water.  The acid may conveniently be placed in a small glazed
+white jar (a jam pot does excellently), and this should only be filled
+to about one-quarter of its capacity.  An excess of zinc may be added.
+
+It may be fancy, but I prefer a soldering solution made in this way to
+a solution of chloride of zinc bought as a chemical product.  The jar
+is generally mounted on a heavy leaden base, so as to avoid any danger
+of its getting knocked over, for nothing is so nasty or bad for tools
+as a bench on which this noxious liquid has been upset (Fig.  78).
+
+Fig. 78.
+
+To tin a soldering bit, a little of the fluid is dipped out of the jar
+on to a bit of tin plate bent up at the edges--a few drops is
+sufficient--and the iron is heated and rubbed about in the liquid
+with a drop of solder.  If the iron is anything like clean it will tin
+at once and exhibit a very bright surface, but quite dirty copper may
+be tinned by dipping it for a moment in the liquid in the pot and then
+working it about over the solder.  An iron so tinned remains covered
+with chloride of zinc, and this must be carefully wiped off if it is
+intended to use the iron with a resin or tallow flux in lead
+soldering.
+
+One disadvantage of this process is that the copper bit soon gets
+eaten into holes and requires to be dressed up afresh.  On the other
+hand, an iron so tinned always presents a nice clean solder surface
+until the next time it is heated, when it generally becomes very dirty
+and requires to be carefully wiped before using.
+
+In my experience also an iron so tinned is more easily spoiled as to
+the state of its surface, "detinned," in fact, by overheating than
+when the tinning is carried out by resin and friction.  When this
+happens, the shortest way out of the difficulty is the application of
+the old file so as to obtain a perfectly fresh surface.  No one who
+knows his business ever uses an iron that is not perfectly clean and
+well tinned.
+
+The iron may be cleaned from time to time by heating it red hot and
+quenching it in water to get rid of the oxide, which scales off in the
+process.
+
+§ 95.  Soft Soldering.
+
+In the laboratory the chief application of the process is to copper
+soldering during the construction of electrical apparatus and to zinc
+soldering for general purposes.
+
+In ninety-nine cases out of every hundred where difficulties occur
+their origin is to be traced to dirt.  There seems to be some
+inexplicable kink in the human mind which renders it callous to
+repeated proofs of the necessity for cleaning surfaces which it is
+intended to solder.  The slightest trace of albuminous or gelatinous
+matter or shellac will prevent solder adhering to most metals and the
+same remark applies in a measure to the presence of oxides, although
+these may be removed by chloride of zinc or prevented from forming by
+resin or tallow.  A touch with an ordinarily dirty hand--I refer to a
+solderer's hand--will often soil work sufficiently to make the
+adherence of solder difficult.
+
+The fluxes most generally employed are tallow for lead, resin or
+Venice turpentine for copper, chloride of zinc for anything except
+lead, which never requires it.  The latter flux has the property (also
+possessed by borax at a red heat) of dissolving any traces of oxide
+which may be formed, as well as acting as a protecting layer to the
+metal.
+
+We may now turn to the consideration of a simple case of soldering,
+say the joining of two copper wires.  The wires are first cleaned
+either by dipping in a bath of sulphuric and nitric acids--a thing no
+laboratory should be without--or by any suitable mechanical means.
+The cleaned wires are then twisted together--there is a regulation
+way of doing this, but it presents no advantage in laboratory
+practice--and the joint is sprinkled over with resin, or painted
+with a solution of resin in alcohol.
+
+The iron, being heated and floated with solder, is held against the
+joint, the latter being supported on a brick, and the solder is
+allowed to "sweat" into the joint.  Enough solder must be present to
+penetrate right through the joint.  Nothing is gained by rubbing
+violently with the iron.  If the copper is clean it will tin, and if
+it is dirty it won't, and there the matter ends.
+
+Beginners generally use too small or too cold a bit, and produce a
+ragged, dirty joint in consequence.  If the saving of time be an
+object, the joint may be twisted together on ordinarily dirty oxidised
+wires and heated to, say, 200° C.  It is then painted with chloride of
+zinc and soldered with the bit.
+
+There is a difference of opinion as to the relative merits of chloride
+of zinc and of resin as a flux in soldering copper.  Thus the standing
+German practice is, or was, to employ the former flux in every case
+for soldering electric light wires, while in England the custom  used
+to be to specify that soldering should be done by resin, and this
+custom may still prevail;  it lingers in Australia at all events.
+
+However, it is agreed on all hands that when chloride of zinc is used
+it must be carefully washed off.  I have known of an electrical
+engineer insisting on his workmen "licking" joints with their tongues
+to ensure the total removal of chloride of zinc; it has a horrible
+taste; and I have occasionally pursued the same plan myself when the
+soldering of fine wires was in question.
+
+In any case, it is very certain that chloride of zinc left in a joint
+will ruin it sooner or later by loosening the contact between copper
+and solder.
+
+Very often it is requisite to solder together two extensive flat
+surfaces--for instance, in "chucking" certain kinds of brass work.
+The surfaces to be soldered must be carefully tinned, most
+conveniently by the help of the blow-pipe and chloride of zinc.  After
+tinning, the surfaces are laid together and heated so as to "sweat"
+them together; the phrase, though inelegant, is expressive.
+
+96.  Soldering Tin Plate.
+
+If the plate be new and clean, a little resin or its solution in
+alcohol is all that is necessary as a flux.  If the tin plate is rusty
+the rust must be removed and the clean iron, or rather mild steel,
+surface exposed.  The use of chloride of zinc is practically essential
+in this case.  Tin plate is often spotted with rust long before it
+becomes rusty as a whole, when, of course, it may be regarded as worn
+out, and such rust spots are most conveniently removed by means of the
+plumber's shave-hook.  The shave-hook is merely a peculiarly shaped
+hard steel scraping knife on a handle (Fig.  79).
+
+Fig.  79
+
+With tin plate the soldering of long joints is often necessary.  The
+plate must be temporarily held in position either by binding with iron
+wire, fastening by clamps, or holding by an assistant.  The flux is
+applied and the iron run slowly along the joint.  Enough solder is
+used to completely float the tip of the iron.  By arranging the joint
+so that it slopes downward slightly, and commencing at the upper end,
+the solder may be caused to flow after the iron, and will leave a
+joint with the minimum permissible amount of solder in it.  By
+regulating the slope, heat of iron, etc, any desired quantity of
+solder may be run into the joint.
+
+§ 97.  Soldering Zinc.
+
+Zinc alloys with soft solder very easily, and by so doing entirely
+spoils it, making, it "crumbly," dirty, and preventing it running.
+Consequently, in soldering up zinc great care must be taken to prevent
+the solder becoming appreciably contaminated by the zinc.  To this end
+the zinc surfaces are cleaned by means of a little hydrochloric acid,
+which is painted on instead of chloride of zinc.  Plenty of solder is
+melted on to the work, and is drawn along over the joint by a single
+slow motion of the soldering bit.  The iron must be just hot enough to
+make the solder flow freely, and it must never be rubbed violently on
+the zinc or allowed to linger in one spot;  the result of the latter
+action will be to melt a hole through the zinc, owing to the tendency
+of this metal to form an easily fusible alloy with the solder.
+
+The art of soldering zinc is a very useful one in the laboratory.  The
+majority of physicists appear to overlook the advantages of zinc
+considered as a material for apparatus construction.  It is light,
+fairly strong, cheap, easily fusible, and yet hard and elastic when
+cold.  It may be worked as easily as lead at a temperature of, say,
+150° to 200° C, and slightly below the melting-point (423° C.) it is
+brittle and may & powdered.  The property of softening at a moderate
+temperature is invaluable as a means of flattening zinc plate or
+shaping it in any way.  During the work it may be held by means of an
+old cloth.  Zinc sheet which has been heated between iron plates and
+flattened by pressure retains its flatness very fairly well after
+cooling.
+
+§ 98.  Soldering other Metals.
+
+Iron.
+
+The iron must be filed clean and then brushed with chloride of zinc
+solution.  Some people add a little sat ammoniac to the chloride of
+zinc, but the improvement thus made is practically inappreciable.  If
+the iron is clean it tins quite easily, and the process of soldering
+it is perfectly easy and requires no special comment.
+
+Brass.
+
+The same method as described for iron succeeds perfectly.  The brass,
+if not exceedingly dirty, may be cleaned by heating to the temperature
+at which solder melts (below 200° C.), and painting it over with
+chloride of zinc, or dipping it in the liquor.  If now the brass be
+heated again in the blow-pipe flame, it will be found to tin perfectly
+well when rubbed over with solder.
+
+German Silver, Platinoid, Silver, and Platinum are treated like iron.
+With regard to silver and platinum the same precautions as recommended
+in the case of zinc must be observed, for both these metals form
+fusible alloys with solder.
+
+Gold when pure requires no flux.  Standard gold, which contains copper,
+solders better with a little chloride of zinc.
+
+Lead must be pared absolutely clean and then soldered quickly with a
+hot iron, using tallow as a flux.  Since solder if over hot will
+adhere to lead almost anywhere, plumbers are in the habit of specially
+soiling those parts to which it is not intended that solder shall
+adhere. The "soiling" paint consists of very thin glue, called size,
+mixed with lampblack; on an emergency a raw potato may be cut in half,
+and the work to be soiled may be rubbed over with the cut surface of
+the potato.
+
+Hard Carbon or gas coke may be soldered after coating with copper by
+an electrolytic process, as will be described.
+
+§ 99.  Brazing.
+
+Soldering at a red heat by means of spelter is called brazing.
+Spelter is soft brass, and is generally made from zinc one part,
+copper one part; an alloy easily granulated at a red heat;  it is
+purchased in the granular form.
+
+The art of brazing is applied to metals which will withstand a red
+heat, and the joints so soldered have the strength of brass.
+
+The pieces to be jointed by this method must be carefully cleaned and
+held in their proper relative positions by means of iron wire.  It is
+generally necessary to soften iron wire as purchased by heating it red
+hot and allowing it to cool in the air;  if this is not done the wire
+is usually too hard to be employed satisfactorily for binding.
+
+Very thin wire--i.e. above No. 20 on the Birmingham wire gauge--does
+not do, for it gets burned through, and perhaps allows the work
+to fall apart at a critical moment.
+
+The work being securely fastened, the next step is to cover the
+cleaned parts with flux in order to prevent oxidation.  For this
+purpose "glass borax" is employed. "Glass" borax is simply ordinary
+borax which has been fused for the purpose of getting rid of water of
+crystallisation.  The glass borax is reduced to powder in an iron
+mortar, for it is very hard, and is then made up into a cream with a
+little water.  This cream is painted on to the parts of the work which
+are destined to receive the solder.
+
+The next step is to prepare the spelter, and this is easily done by
+mixing it with the cream, taking care to stir thoroughly with a
+flattened iron wire till each particle of spelter is perfectly covered
+with the borax.  The mixture should not be too wet to behave as a
+granular mass, and may then be lifted on to the work by means of the
+iron spatula.
+
+Care must be taken to place the spelter on those parts only which are
+intended to receive it, and when this is done, the joint may be
+lightly powdered over with the dry borax, and will then be ready for
+heating.
+
+If the object is of considerable size it is most conveniently heated
+on the forge;  if small the blowpipe is more convenient.  In the
+latter case, place the work on a firebrick, and arrange two other
+bricks on edge about it, so that it lies more or less in a corner.  A
+few bits of coke may also be placed on and about the work to increase
+the temperature by their combustion, and to concentrate the flame and
+prevent radiation.  The temperature is gradually raised to a bright
+red heat, when the spelter will be observed to fuse or "run," as it is
+technically said to do.
+
+If the cleaning and distribution of flux has been successful, the
+spelter will "run" along the joint very freely, and the work should be
+tapped gently to make sure that the spelter has really run into the
+joint.  The heating may be interrupted when the spelter is observed to
+have melted into a continuous mass.  As soon as the work has fallen
+below a red heat it may be plunged into water, a process which has the
+effect of cracking off the glass-like layer of borax.
+
+There is, however, some risk of causing the work to buckle by this
+violent treatment, which must of course be modified so as to suit the
+circumstances of the case.  If the joint is in such a position that
+the borax cannot be filed off, a very convenient instrument for its
+removal by scraping is the watchmaker's graver, a square rod of hard
+steel ground to a bevelled point (Fig.  80).
+
+Fig.  80.
+
+Several precautions require to be mentioned.  In the first place,
+spelter is merely rather soft brass, and consequently it often cannot
+be fused without endangering the rest of the work.  A good protection
+is a layer of fireclay laid upon the more delicate parts, such for
+instance as any screwed part.
+
+Gun-metal and tap-metal do not lend themselves to brazing so readily
+as iron or yellow brass, and are usually more conveniently treated by
+means of silver solder.
+
+Spelter tends to run very freely when it melts, and if the brass
+surface in the neighbourhood of the joint is at all clean, may run
+where it is not wanted.  Of course some control may be exercised by
+"soiling" with fireclay or using an oxidising flame;  but the erratic
+behaviour of spelter in this respect is the greatest drawback to its
+use in apparatus construction.  The secret of success in brazing lies
+in properly cleaning up the work to begin with, and in disposing the
+borax so as to prevent subsequent oxidation.
+
+§ 100.  Silver Soldering.
+
+This process resembles that last described, but instead of spelter an
+alloy of silver, copper, and zinc is employed.  The solder, as
+prepared by jewellers to meet special cases, varies a good deal in
+composition, but for the laboratory the usual proportions are:
+
+For soft silver solder
+
+Fine silver         2 parts
+Brass wire        1 part
+
+For hard silver solder
+
+Sterling silver     3 parts
+Brass wire      1 part
+
+The latter is, perhaps, generally the more convenient.
+
+Silver solders may, of course, be purchased at watchmakers' supply
+shops, and as thus obtained, are generally in thin sheet.  This is
+snipped fine with a pair of shears preparatory to use.
+
+As odds and ends of silver (from old anodes and silver residues)
+generally accumulate in the laboratory, it is often more convenient to
+make the solder one's self.  In this case it must be remembered in
+making hard solder by the second receipt that standard silver contains
+about one-twelfth of its weight of copper--exactly 18 parts copper to
+220 silver.
+
+The silver is first melted in a plumbago crucible in a small furnace
+together with a little borax; if any copper is required this is then
+added, and finally the brass is introduced.  When fusion is complete,
+the contents of the crucible are poured into any suitable mould.
+
+The quickest and most convenient way of preparing the alloy for use is
+to convert it into filings with the assistance of a coarse file, or by
+milling it, if a milling machine is available.
+
+Equal volumes of filings and powdered glass borax are made into a thin
+paste with water, and applied in an exactly similar manner to that
+described under the head of "brazing."  In fact all the processes
+there described may be applied equally to the case under discussion,
+the substitution of silver for spelter being the only variation.
+
+The silver solder is more manageable than spelter, and does not tend
+to run wild over the work:  a property which makes it much more
+convenient both for delicate joints and in cases where it is desired
+to restrict the solder to a single point or line.  Small objects are
+almost invariably soldered with silver solder, and are held by forceps
+or on charcoal in the pointed flame of an ordinary blow-pipe.
+
+§ 101.  On the Construction of Electrical Apparatus: Insulators.
+
+It is not intended to deal in any way with the design of special
+examples of electrical apparatus, but merely to describe a rather
+miscellaneous set of materials and processes constantly required in
+its construction.
+
+It is not known whether there is such a thing as a perfect insulator,
+even if we presuppose ideal circumstances.  Materials as they exist
+must be regarded merely as of high specific resistance, that is if we
+allow ourselves to use such a term in connection with substances,
+conduction through which is neither independent of electromotive force
+per unit length, nor of previous history.
+
+Even the best of these substances generally get coated with a layer of
+moisture when exposed to the air, and this as a rule conducts fairly
+well.  Very pure crystalline sulphur and fused quartz suffer from this
+defect less than any other substances with which the writer is
+acquainted, but even with them the surface conductivity soon grows to
+such an extent as totally to mask the internal conduction.
+
+It is proposed to give a brief account of the properties of some
+insulating substances and their application in electrical
+construction, and at the same time to indicate the appliances and
+methods requisite for working them.
+
+With regard to the specific resistances which will be quoted, the
+numbers must not be taken to mean too much, partly for the reason
+already given.  It is also in general doubtful whether sufficient care
+has been taken to distinguish the body from the surface conductivity,
+and consequently numerical estimates are to be regarded with
+suspicion.  The question of "sampling" also arises, for it must be
+remembered that a change in composition amounting to, say, 1/10000 per
+cent may be accompanied  by a million-fold change in specific
+resistance.
+
+§ 102.  Sulphur.
+
+This element exists in several allotropic forms, which have very
+different electric properties.  After melting at about 125° C, and
+annealing at 110° for several hours, the soluble crystalline
+modification is formed.  After keeping for some days--especially if
+exposed to light--the crystals lose their optical properties, but
+remain of the same melting-point, and are perfectly soluble in carbon
+bisulphide.  The change is accompanied by a change in colour, or
+rather in brightness, as the transparency changes.
+
+The "specific resistance" of sulphur in this condition is above 1028
+C.G.S.E.M. units, or 1013 megohms per cubic centimetre for an electric
+intensity of say 12,000 volts per centimetre.  This is at ordinary
+temperatures.  At 75° C.  the specific resistance falls to about 1025
+under similar conditions as to voltage.
+
+In all cases the conductivity appears to increase with the electric
+intensity, or at all events with an increase in voltage, the thickness
+of the layer of sulphur remaining the same.
+
+The specific inductive capacity is 3.162 at ordinary temperatures, and
+increases very slightly with rise of temperature. [Footnote: March
+1897.--It is now the opinion of the writer that though the specific
+inductive capacity of a given sample of a solid element is perfectly
+definite, yet it is very difficult to obtain two samples having
+exactly the same value for this constant, even in the case of a
+material so well defined as sulphur.]
+
+The total residual charge, after ten minutes' charging with an
+intensity of 12,000 volts per centimetre, is not more than 4 parts in
+10,000 of the original charge.  In making this measurement the
+discharge occupied a fraction of a second.  The electric strength for
+a homogeneous plate of crystalline sulphur is not less than 33,000
+volts per centimetre, and probably a good deal more.  If the sulphur
+is contaminated with up to 3 per cent of the amorphous variety, as is
+the case if it is cooled fairly quickly from a temperature of 170° C.
+or over, the specific resistance falls to from 10^25 to 10^26 at
+ordinary temperatures;  and the specific inductive capacity increases
+up to 3.75, according to the amount of insoluble sulphur present.
+
+The residual charge under circumstances similar to those described
+above, but with an intensity of about 4000 volts per centimetre is,
+say, 2 per cent of the initial charge.  So far as the writer is aware
+sulphur is the only solid non-conductor which can be easily obtained
+in a condition of approximate purity and in samples sufficiently
+exactly comparable with one another;  it is the only one, therefore,
+that repays any detail of description.
+
+Very pure sulphur can be bought by the ton if necessary from the
+United Alkali Company of Newcastle-on-Tyne.  It is recovered from
+sulphur waste by the Chance process, which consists in converting the
+sulphur into hydrogen sulphide, and burning the latter with
+insufficient air for complete combustion.  The sulphur is thrown out
+of combination, and forms a crystalline mass on the walls and floor of
+the chamber.
+
+The sulphur which comes into the market consists of this mass broken
+up into convenient fragments.  In order to purify it sufficiently for
+use as an insulator, the sulphur may be melted at a temperature of
+120° to 140° C, and filtered through a plug of glass wool in a zinc
+funnel; as thus prepared it is an excellent insulator.  To obtain the
+results mentioned in the table it is, however, necessary to conduct a
+further purification (chiefly from water) by distillation in a glass
+retort.
+
+The sulphur thus obtained may be cast of any desired form in zinc
+moulds, the castings and moulds being immediately removed to an
+annealing oven at a temperature of from 100° to 110° C, where they
+are left for several hours.  If the sulphur is kept melted for some
+time at 125° C.  the annealing is not so important.
+
+The castings may be removed from the mould by slightly heating the
+latter, but many breakages result.  Insulators made on this plan are
+much less affected by the condensation of moisture from the air than
+anything except fused quartz.  They are, however, very weak
+mechanically, and apt to crack by exposure to such changes of
+temperature as go on from day to day.  It is clear, however, that in
+spite of this their magnificent electrical properties fit them for
+many important uses.
+
+If the sulphur be cooled rapidly from 170° C.  or over, a mixture of
+the crystalline and amorphous varieties of sulphur is obtained.  This
+mixture is very much stronger and tougher than the purely crystalline
+substance, and may be worked with ordinary hardwood tools into fairly
+permanent plates, rods, etc.  Sheets of pure thick filter paper may
+also be dipped into sulphur at 170° C, at which temperature air and
+moisture are mostly expelled, and such sheets show a very considerable
+insulating power.  The sulphur does not penetrate the paper, which
+therefore merely forms a nucleus.
+
+Cakes of the crystalline or mixed varieties may be made by grinding up
+some purified sulphur, moistening it with redistilled carbon
+bisulphide, or toluene, or even benzene (C6H6), and pressing it in a
+suitable mould under the hydraulic press.  The plates thus formed are
+porous, but are splendid insulators, especially if made from the
+crystalline variety of sulphur, and they appear to keep their shape
+very well, and do not crack with ordinary temperature changes.
+
+The metals which resist the action of sulphur best are gold and
+aluminium;  while platinum and zinc are practically unacted upon at
+temperatures below a red heat--in the former case,--and below the
+boiling-point of sulphur in the latter.
+
+A very convenient test of the purity of sulphur is the colour assumed
+by it when suddenly cooled from the temperature at which it is
+viscous.  Quite pure sulphur remains of a pale lemon yellow under this
+treatment, but the slightest trace of impurity, such as arises from
+dust containing organic matter, stains the sulphur, and renders it
+darker in colour.
+
+§ 103.  Fused Quartz.
+
+This is on the whole the most reliable and most perfect insulator for
+general purposes.  No exact numerical data have been obtained, but the
+resistivity must certainly be of the same order as that of pure
+sulphur at its best.  The influence of the moisture of the air also
+reaches its minimum in the case of quartz, as was originally observed
+by Boys.
+
+As yet, however, the material can only be obtained in the form of rods
+or threads.  For most purposes rods of about one-eighth of an inch in
+diameter are the most convenient.  These rods may be used as
+insulating supports, and succeed perfectly even if they interpose less
+than an inch of their length to electrical conduction.  The sketch
+(Figs.  81 and 81A) shows (to a scale of about one-quarter full size)
+a complete outfit for elementary electrostatic experiments, such as
+has been in use in the writer's laboratory for five years.  With these
+appliances all the fundamental experiments may be performed, and the
+apparatus is always ready at a moment's notice.
+
+Fig.  81.
+
+Though quartz does not condense moisture or gas to form a conducting
+layer of anything like the same conductivity as in the case of glass
+or ebonite, still it is well to heat it if the best results are to be
+obtained.  For this purpose a small pointed blow-pipe flame may be
+used, and the rods may be got red-hot without the slightest danger of
+breaking them. They then remain perfectly good and satisfactory for
+several hours at least, even when exposed to damp and dusty air.
+
+The rods are conveniently held in position by small brass ferrules,
+into which they are fastened by a little plaster of Paris. Sealing-wax
+must be avoided, on account of the inconvenience it causes when the
+heating of the rods is being carried out.
+
+One useful application of fused quartz is to the insulation of
+galvanometer coils (Fig. 82), another to the manufacture of highly
+insulating keys (Fig. 83); while as an insulating suspension it has
+all the virtues.  If it is desired to render the threads conducting
+they may be lightly silvered, and will be found to conduct well enough
+for electrometer work before the silver coating is thick enough to
+sensibly impair their elastic properties.
+
+Fig.  81A.
+
+Fig.  82 is a full-size working drawing of a particular form of
+mounting for galvanometer coils.  The objects sought to be attained
+are:
+(1) high insulation of the coils,
+
+(2) easy adjustment of the coils to the suspended system.
+
+The first object is attained as follows.  The ebonite ring A is bored
+with four radial holes, through which are slipped from the inside the
+fused quartz bolt-headed pins B.  The coil already soaked in hard
+paraffin is placed concentrically in the ring A by means of a special
+temporary centering stand.  The space between the coil and the ring is
+filled up with hard paraffin, and this holds the quartz pins in
+position.  The system of ebonite ring, coil, and pins is then fastened
+into the gun-metal coil carrier, which is cut away entirely, except
+near the edges, where it carries the pin brackets C.  These brackets
+can swivel about the lower fastening at E before the latter is
+tightened up.
+
+The coil is now adjusted in the adjusting stand to be concentric with
+the axis of symmetry of the coil carrier, and the supporting pins are
+slipped into slot holes cut in the brackets, the brackets being
+swivelled as much as necessary to allow of this.  When the pins are
+all inserted the brackets are screwed up by the screws at E.  The pins
+are then cemented firmly to the brackets by a little plaster of Paris.
+The coil carrier can now be adjusted to the galvanometer frame by
+means of screws at D, which pass through wide holes in the carrier and
+bold the latter in position by their heads.  In the sectional plan the
+parts of the galvanometer frame are shown shaded.  The front of the
+frame at F F is of glass, and the back of the frame is also made of
+glass, though this is not shown in the section.
+
+A represents an ebonite ring into which the wire coil is cemented by
+means of paraffin.  B B B B are quartz pins, with heads inside the
+ebonite ring.  C C C are slotted brackets adjustable to the pins and
+capable of rotation by releasing the screws E E.  D D are the screws
+holding the coil carriage to the galvanometer framework.  These screws
+pass through large holes in the carriage so as to allow of some
+adjustment.
+
+Fig.  82.
+
+Fig.  83.
+
+§ 104.  Glass.
+
+When glass is properly chosen and perfectly dry it has insulating
+properties possibly equal to those possessed by quartz or crystalline
+sulphur.  For many purposes, however, its usefulness is seriously
+reduced by the persistence with which it exhibits the phenomena of
+residual charge, and the difficulty that is experienced in keeping it
+dry.
+
+The insulating power of white flint glass is much in excess of that of
+soft soda glass, which is a poor insulator, and of ordinary green
+bottle glass.  The jars of Lord Kelvin's electrometers, which insulate
+very well, are made of white flint glass manufactured in Glasgow, but
+it is found that occasionally a particular jar has to be rejected on
+account of its refusing to insulate, and this, if I understand aright,
+even when it exhibits no visible defects.
+
+A large number of varieties of glass were tested by Dr. Hopkinson at
+Messrs.  Chance Bros.  Works, in 1875 and 1876 (Phil.  Trans, 1877),
+and in 1887 (Proc.  Roy.  Soc.  xli.  453), chiefly with a view to the
+elucidation of the laws regulating the residual charge;  and
+incidentally some extraordinarily high insulations were noted among
+the flint glasses.  The glass which gave the smallest residual charge
+was an "opal" glass;  and flint glasses were found to insulate 105
+times as well as soda lime glasses.  The plates of Wimshurst machines
+are made of ordinary sheet window glass, but as the insulating
+property of this material appears to vary, it is generally necessary
+to clean, dry, and test a sheet before using it.  With regard to hard
+Bohemian glass, this is stated by Koeller (Wien Bericht) to insulate
+ten times as well as the ordinary Thuringian soft soda glass.
+
+On the whole the most satisfactory laboratory practice is to employ
+good white flint glass.  The only point requiring attention is the
+preparation of the glass by cleaning and drying.  Of course all grease
+or visible dirt must be removed as described in an earlier chapter (§
+13), but this is only a beginning.  The glass after being treated as
+described and got into such a state as to its surface that clean water
+no longer tends to dry off unequally, must be subjected to a further
+scrub with bibulous paper and a clear solution of oleate of soda.  The
+glass is then to be well rinsed with distilled water and allowed to
+drain on a sheet of filter paper.
+
+A very common cause of failure lies in the contamination of the glass
+with grease from the operator's fingers.  Before setting out to clean
+the glass the student will do well to wash his hands with soap and
+water, then with dilute ammonia and finally with distilled water.
+
+In the case of an electrometer jar which has become conducting but is
+not perceptibly dirty, rubbing with a little oleate of soda and a silk
+ribbon, followed, of course, by copious washing, does very well.  If
+there is any tin-foil on the jar, great care must be taken not to
+allow the glass surface to become contaminated by the shellac varnish
+or gum used to stick the tin-foil in position.
+
+Finally, the glass should be dried by radiant heat and raised to a
+temperature of 100° C.  at least, and kept at it for at least half an
+hour.  Before drying it is of course advisable to allow the water to
+drain away as far as possible, and if the water is only the ordinary
+distilled water of the laboratory, the glass is preferably wiped with
+a clean bit of filter paper; any hairs which may be left upon the
+glass will brush off easily when the glass is dry.
+
+In order to obtain satisfactory results the glass must be placed in
+dry air before it has appreciably cooled.  This is easily done in the
+case of electrometer jars, and so long as the air remains perfectly
+dry through the action of sulphuric acid or phosphorus pentoxide, the
+jar will insulate.  The slightest whiff of ordinarily damp air will,
+however, enormously reduce the insulating power of the glass, so that
+unvarnished glass surfaces must be kept for apparatus which is
+practically air-tight.
+
+For outside or imperfectly protected uses the glass does better when
+varnished.  It is a fact, however, that varnished glass is rarely if
+ever so good as unvarnished glass at its best.  Too much care cannot
+be taken over the preparation of the varnish; French polish, or
+carelessly made shellac varnish, is likely to do more harm than good.
+
+The best orange shellac must be dissolved in good cold alcohol by
+shaking the materials together in a bottle.  The alcohol is made
+sufficiently pure by starting with rectified spirit and digesting it
+in a tin flask over quick-lime for several days, a reversed condenser
+being attached.  A large excess of lime must be employed, and this
+leads to a considerable loss of alcohol, a misfortune which must be
+put up with.
+
+After, say, thirty hours' digestion, the alcohol may be distilled off
+and employed to act on the shellac.  In making varnish, time and
+trouble are saved by making a good deal at one operation--a
+Winchester full is a reasonable quantity.  The bottle may be filled
+three-quarters full of the shellac flakes and then filled up with
+alcohol; this gives a solution of a convenient strength.
+
+The solution, however, is by no means perfect, for the shellac
+contains insoluble matter, and this must be got rid off.`' One way of
+doing this is to filter the solution through the thick filtering paper
+made by Schleicher and Schuell for the purpose, but the filtering is a
+slow process, and hence requires to be conducted by a filter paper
+held in a clip (not a funnel) under a bell jar to avoid evaporation.
+
+Another and generally more convenient way in the laboratory is to
+allow the muddy varnish to settle--a process requiring at least a
+month--and to decant the clear solution off into another bottle,
+where it is kept for use.  The muddy residue  works up with the next
+lot of shellac and alcohol, which may be added at once for future use.
+
+The glass to be varnished is warmed to a temperature of, say, 50° C,
+and the varnish put on with a lacquering brush; a thin uniform coat is
+required.  The glass is left to dry long enough for the shellac to get
+nearly hard and to allow most of the alcohol to evaporate.  It is then
+heated before a fire, or even over a Bunsen, till the shellac softens
+and begins to yield its fragrant characteristic smell.
+
+If the coating is too heavy, or if the heating is commenced before the
+shellac is sufficiently dry, the latter will draw up into "tears,"
+which are unsightly and difficult to dry properly.  On no account must
+the shellac be allowed to get overheated.  If the varnish is not quite
+hard when cold it may be assumed to be doing more harm than good.
+
+In varnishing glass tubes for insulating purposes it must be
+remembered that the inside of the tube is seldom closed perfectly as
+against the external air, and consequently it also requires to be
+varnished.  This is best done by pouring in a little varnish
+considerably more dilute than that described, and allowing it to drain
+away as far as possible, after seeing that it has flooded every part
+of the tube.
+
+During this part of the process the upper end of the tube must be
+closed, or evaporation will go on so fast that moisture will be
+deposited from the air upon the varnished surface.  Afterwards the
+tube may be gently warmed and a current of air allowed to pass, so as
+to prevent alcohol distilling from one part of the tube to another.
+The tube is finally heated to the softening point of shellac, and if
+possible closed as far as is practicable at once.
+
+§ 105.  Ebonite or Hard Rubber.
+
+This exceedingly useful substance can be bought of a perfectly useless
+quality.  Much of the ebonite formerly used to cover induction coils
+for instance, deteriorates so rapidly when exposed to the air that it
+requires to have its surface renewed every few weeks.
+
+The very best quality of ebonite obtainable should be solely employed
+in constructing electric works.  It is possible to purchase good
+ebonite from the Silvertown Rubber Company (and probably from other
+firms), but the price is necessarily high, about four shillings per
+pound or over.
+
+At ordinary temperatures ebonite is hard and brittle and breaks with a
+well-marked conchoidal fracture.  At the temperature of boiling water
+the ebonite becomes somewhat softened, so that it is readily bent into
+any desired shape;  on cooling it resumes its original hardness.
+
+This property of softening at the temperature of boiling water is a
+very valuable one.  The ebonite to be bent or flattened is merely
+boiled for half an hour or so in water, taken out, brought to the
+required shape as quickly as possible, and left to cool clamped in
+position.
+
+The sheet ebonite as it comes from the makers is generally far from
+flat.  It is often necessary to flatten a sheet of ebonite, and of
+course this is the more easily accomplished the smaller the sheet.
+Consequently a bit of ebonite of about the required size is first cut
+from the stock sheet by a hack-saw such as is generally used for
+metals.  This piece is then boiled and pressed between two planed iron
+plates previously warmed to near 100° C.
+
+With pieces of ebonite such as are used for the tops of resistance
+boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is
+experienced.  The sheets when cold are found to retain the flatness
+which has been forced upon them perfectly well.  It is otherwise with
+large thin sheets such as are used for Holtz machines.  I have
+succeeded fairly, but only fairly, by pressing them in a "gluing
+press," consisting of heavy planed iron slabs previously heated to
+100° C.
+
+I do not know exactly how best to flatten very thin and large sheets.
+It is easy to make large tubes out of sheet ebonite by taking
+advantage of the softening which ebonite undergoes in boiling water.
+A wooden mandrel is prepared of the proper size and shape.  The
+ebonite is softened and bent round it; this may require two or three
+operations, for the ebonite gets stiff very quickly after it is taken
+out of the water.  Finally the tube is bound round the mandrel with
+sufficient force to bring it to the proper shape and boiled in water,
+mandrel and all.  The bath and its contents are allowed to cool
+together, so that the ebonite cools uniformly.
+
+Tubes made in this way are of course subject to the drawback of having
+an unwelded seam, but they do well enough to wind wire upon if very
+great accuracy of form is not required.  If very accurate spools are
+needed the mandrel is better made of iron or slate and the spool is
+turned up afterwards.  The seam may be strapped inside or at the ends
+by bits of ebonite acting as bridges, and the seam itself may be
+caulked with melted paraffin or anthracene.
+
+Working Ebonite.
+
+Ebonite is best worked as if it were brass, with ordinary
+brass-turning or planing tools.  These tools should be as hard as
+possible, for the edges are apt to suffer severely, and blunt tools
+leave a very undesirable woolly surface on the ebonite.  In turning or
+shaping ebonite sheets it is as well to begin by taking the skin off
+one side first, and then reversing the sheet, finishing the second
+side, and then returning to the first.  This is on account of the fact
+that ebonite sometimes springs a little out of shape when the skin is
+removed.
+
+Turned work in ebonite, if well done, requires no sand-papering, but
+may be sufficiently polished by a handful of its own shavings and a
+little vaseline.  The advantage of using a polished ebonite surface is
+that such a surface deteriorates more slowly under the influence of
+light and air than a surface left rough from the tool.  If very highly
+polished surfaces are required, the ebonite after tooling is worked
+with fine pumice dust and water, applied on felt, or where possible by
+means of a felt buff on the lathe, and finally polished with rouge and
+water, applied on felt or cloth.
+
+Ebonite works particularly well under a spiral milling cutter, and
+sufficiently well under an ordinary rounded planing tool, with cutting
+angle the same as for brass, and hardened to the lightest straw
+colour.
+
+It is not possible, on the other hand, to use the carpenter's plane
+with success, for the angle of the tool is too acute and causes the
+ebonite to chip.
+
+In boring ebonite the drill should be withdrawn from the hole pretty
+often and well lubricated, for if the borings jam, as they are apt to
+do, the heat developed is very great and the temper of the drill gets
+spoiled.  Ebonite will spoil a drill by heating as quickly as anything
+known;  on the other hand, it may be drilled very fast if proper
+precaution is taken.
+
+It is advisable to expose ebonite to the light as little as possible,
+especially if the surface is unpolished, for under the combined action
+of light and air the sulphur at the surface of the ebonite rapidly
+oxidises, and the ebonite becomes covered with a thin but highly
+conducting layer of sulphurous or sulphuric acid or their compounds.
+When this happens the ebonite may be improved by scrubbing with hot
+water, or washing freely with alcohol rubbed on with cotton waste in
+the case of apparatus that cannot be dismounted.
+
+A complete cure, however, can only be effected by scraping off the
+outer layer of ebonite so as to expose a fresh surface.  For this
+purpose a bit of sheet glass broken so as to leave a curved edge is
+very useful, and the ebonite is then scraped like a cricket bat.  In
+designing apparatus for laboratory use it is as well to bear in mind
+that sooner or later the ebonite parts will require to be taken down
+and scraped up.  Rods or tubes are, of course, most quickly treated on
+the lathe with rough glass cloth, and may be finished with fine
+sandpaper, then pumice dust and water, applied on felt.  After
+cleaning the pumice off by means of water and a rag, the final touch
+may be given by means of vaseline, applied on cloth or on ebonite
+shavings.
+
+§ 106.  Mica.
+
+A great variety of minerals go under this name.  Speaking generally,
+the Russian micas coming into commerce are potash micas, and mica
+purchased in England may be taken to be potash mica, especially if it
+is in large sheets.
+
+At ordinary temperatures "mica" of the kind found  in commerce is an
+excellent insulator.  Schultze (Wied. Ann. vol. xxxvi. p. 655) comes
+to the conclusion that both at high and at low temperatures mica (of
+all kinds?) is a better insulator than white "mirror glass," the
+composition of which is not stated.  The experiments of the author
+referred to were apparently left unfinished, and altogether too much
+stress must not be laid on the results obtained, one of which was that
+mica conducts electrolytically to some extent at high temperatures.
+
+Bouty (Journal de Physique, 1890 [9], 288) and J.  Curie (Thèse de
+Doctorat, Paris, 1888) agree in making the final conductivity of the
+mica used in Carpentier's condensers exceedingly small--at all events
+at ordinary temperatures.  Bearing in mind that for such substances
+the term specific resistance has no very definite meaning, M. Bouty
+considers it is not less than 3.19 x 1028 E.M. units at ordinary
+temperatures.  M. Bouty gives a note or illustration of what such
+numbers mean--a precaution not superfluous in cases where magnitudes
+are denoted logarithmically.  Referring to the value quoted, viz.
+3.19 x 1028, M. Bouty says, "Ce serait la resistance d'une colonne de
+mercure de 1mmq de section et de longueur telle que la lumière se
+propageant dans le vide, mettrait plus de 3000 ans A se transmettre
+d'une extrémité à I'autre de la colonne."
+
+M. Bouty returns to the study of mica (muscovite) in the Journal de
+Physique for 1892, p.  5, and there deals with the specific inductive
+capacity, which for a very small period of charge he finds has the
+value 8--an enormous value for such a good insulator, and one that it
+would be desirable to verify by some totally distinct method.  This
+remark is enforced by the fact that M. Klemencic finds the number 6
+for the same constant.  The temperature coefficient of this constant
+was too small for M. Bouty to determine.  The electric intensity was
+of the order of 100 volts per centimetre, and the experiments seem to
+indicate that the specific inductive capacity would be only slightly
+less if referred to a period of charge indefinitely short.
+
+I have found that the residual charge in a mica condenser, made
+according to Carpentier's method (to be described below), is about 1
+per cent of the original charge under the following circumstances.
+
+Voltage 300 volts on a plate 0.2 mm. thick, duration of charge ten
+minutes, temperature about 20° C.  To get this result the mica must be
+most carefully dried.  This and other facts indicate that the
+so-called residual charge on ordinary condensers is, to a very large
+extent, due to the creeping of the charge from the armatures over the
+more or less conducting varnished surfaces of the mica, and its slow
+return on discharge.
+
+This source of residual charge was carefully guarded against by
+Rowland and Nichols (Phil. Mag.  1881) in their work on quartz, and is
+referred to by M. Bouty, who adduces some experiments to show that his
+own results are not vitiated by it.  On the other hand, M. Bouty shows
+that a small rise in temperature enormously affects the state of a
+mica surface, and that the surface gets changed in such a way as to
+become very fairly conducting at 300° C.  Also anybody can easily try
+for himself whether exposing a mica condenser plate which has been
+examined in presence of phosphorus pentoxide to ordinary air for five
+minutes will not enormously increase the residual charge, as has
+always been the case in the writer's experience, and if so, it is open
+to him to suggest some cause other than surface creeping as an
+explanation.
+
+M. Bouty, using less perfectly dried mica, did not get so good a
+result as to smallness of residual charge as the one above quoted.
+
+The chief use of mica for laboratory purposes depends on the ease with
+which it can be split, and also upon the fact that it may be
+considerably crumpled and bent without breaking.  It therefore makes
+an excellent dielectric in so far as convenience of construction is
+concerned in the preparation of condensers, and lends itself freely to
+the construction of insulating washers or separators of any kind.  Its
+success as a fair insulator at moderate temperatures has led to its
+use in resistance thermometers, where it appears to have given
+satisfaction up to, at all events, 400° C.
+
+It is worth a note that according to Werner Siemens, who had immense
+experience (Wied. Ann. vol. clix.), soapstone is the only reliable
+insulator at a red heat, but, no doubt, a good deal depends on the
+particular specimen investigated.
+
+§ 107.  Use of Mica in Condensers.
+
+If good results are desired it is essential to select the mica very
+carefully.  Pieces appreciably stained,--particularly if the stain is
+not uniformly distributed,--cracked pieces, and pieces tending to
+flake off in patches should be rejected.  The best samples of mica
+that have come under the writer's observation are those sheets sold
+for the purpose of giving to silver photographic prints that hideous
+glazed surface which some years ago was so popular.
+
+Sheets of mica about 0.1 to 0.2 mm. thick form good serviceable
+condenser plates, and will certainly stand a pressure of 300 volts,
+and most likely a good deal more.  The general practice in England
+seems to have been to build up condensers of alternate sheets of
+varnished or paraffined-mica and tin-foil.
+
+This practice is open to several objections.  In the first place, the
+capacity of a condenser made in this way varies with the pressure
+binding the plates together.  In the second place, the amount of mica
+and tin-foil required is often excessive in consequence of the
+imperfect contact of these substances.  Again, the inevitable air film
+between the mica and tin-foil renders condensers so made unsuitable
+for use with alternating currents, owing to the heating set up through
+air discharges, and which is generally, though often (if not always)
+wrongly, attributed to dielectric hysteresis.
+
+These imperfections are to a great extent got over by M. Carpentier's
+method of construction, which is, however, rather more costly both in
+material and labour.  On the other hand, wonderful capacities are
+obtained with quite small amounts of mica.  M. Bouty mentions a
+condenser of one microfarad capacity weighing 1500 grms.  and
+contained in a square box measuring 12 centimetres on the side, and
+about 3 centimetres thick.
+
+The relation between the capacity and surface of doubly-coated plates
+is in electro-static units:
+
+Capacity = (sp. ind. capacity X area of one surface)/(4pi X thickness)
+
+This may be reduced to electro-magnetic units by dividing by 9x10^20,
+and to microfarads by further multiplying by 10^15.
+
+M. Carpentier begins, of course, by having his mica scrupulously clean
+and well selected.  It is then silvered by one of the silvering
+processes (§ 65) on both sides, for which purpose the sheets may be
+suspended in a paraffined wood rack, so as to lie horizontally in the
+silvering solution, a space of about half an inch being allowed
+between the sheets.  The silvering being finished, the sheets are
+dipped along two parallel edges in 75 per cent nitric acid.  With
+regard to the third and fourth edges of the sheet, the silver is
+removed on one side only, using a spun glass brush;  if we agree to
+call the two surfaces of the mica A and B respectively, and the two
+edges in question C and D, then the silver is removed from the A side
+along edge C, and from the B side along edge D.  The silvered part is
+shown shaded in Fig.  84.  By this arrangement the silver and mica
+plates may be built up together so as to form the same mutual
+arrangement of contacts as in an ordinary mica tin-foil condenser.
+
+Fig. 84.
+
+It need hardly be said that the sheets require very complete washing
+after treatment with nitric acid, followed by a varnishing of the
+edges as already described in the case of glass, and baking at a
+temperature of 140° C.  in air free from flame gases, till the shellac
+begins to emit its characteristic odour and is absolutely hard when
+cold.
+
+The plates are then built up so as to connect the sheets which require
+to be connected, and to insulate the other set.  General contact is,
+if necessary, secured by means of a little silver leaf looped across
+from plate to plate--a part of the construction which requires
+particular attention and clean hands, for it is by no means so easy to
+make an unimpeachable contact as might at first appear.
+
+The condenser, having been built up, may be clamped solid and placed
+in its case;  the capacity will not depend appreciably on the
+tightness of the clamp screws--a great feature of the construction.
+Such a condenser will not give its best results unless absolutely dry.
+I have kept one very conveniently in a vacuum desiccator over
+phosphorus pentoxide, but if of any size, the condenser deserves a box
+to itself, and this must be air-tight and provided with a drying
+reagent, so arranged that it can be removed through a manhole of some
+sort.
+
+Contact to the brass-work on the lid may be made by pressing spring
+contacts tightly down upon the ends of the silver foils and carrying
+the connections through the lid.  This also serves to secure the
+condenser in position.
+
+§ 108.  Micanite.
+
+This substance, though probably comparing somewhat unfavourably with
+the insulators already enumerated, and being subject to the
+uncertainties of manufacture, has during the last few years achieved a
+considerable success in American electrical engineering construction.
+It is composed of scrap mica and shellac varnish worked under pressure
+to the desired shape, and may be obtained in sheets, plates, and rods,
+or in any of the forms for which a die happens to have been
+constructed.
+
+Of course, in special cases it would be worth while to prepare a die,
+and then the attainable forms would be limited by moulding
+considerations only.  The writer's experience is very limited in this
+matter, but Dr. Kennelly, with whom he communicated on the subject,
+was good enough to reply in favour of micanite for engineering work.
+
+§ 109.  Celluloid.
+
+This material is composed of nitrocellulose and camphor.
+
+It has fair insulating properties, and may be obtained in a variety of
+forms, but has now been generally abandoned for electrical work on
+account of its inflammability.
+
+§ 110.  Paper.
+
+Pure white filter paper, perfectly dry, is probably a very fair
+insulator;  the misfortune is that in practice it cannot be kept dry.
+Under the most favourable circumstances its specific resistance may
+approach 1024 E.M. units.  It must therefore be considered rather as a
+partial conductor than as an insulator.  The only case of the use of
+dry paper as an insulator in machine construction which has come under
+the writer's notice is in building up the commutators of the small
+motors which used to drive the Edison phonographs.
+
+Its advantages in this connection are to be traced to the fact that a
+commutator so built up is durable and keeps a clean surface.  Of
+course, the use of paper as an insulator for telephone wires is well
+known, but its success in this direction depends less upon its
+insulating properties than upon the fact that it can be arranged in
+such a way as to allow of the wires being partially air insulated, an
+arrangement tending to reduce the electrostatic capacity of the wire
+system.
+
+At one time it was the custom of instrument makers to employ ordinary
+printed paper in the shape of leaves torn from books or the folios of
+old ledgers to form the dielectric of the condensers used in
+connection with the contact breakers of induction coils.  This
+practice has nothing but economy to recommend it, for cases often
+occur in which the paper, by gradual absorption of moisture from the
+air, comes to insulate so badly that it practically short circuits the
+spark gap, and so stops the action of the coil.  Three separate cases
+have come within the writer's experience.
+
+Some measurements of the resistance of paper have been made by F.
+Uppenborn (Centralblatt fuer Electrotechnik, Vol.  xi.  p.  215,
+1889).  There is an abstract of the paper also in Wiedemann's
+Beiblaetter (1889, vol.  xiii.  P.  711).  Uppenborn examined the
+samples of paper under normal conditions as to moisture and obtained
+the following results:-
+
+Description of Paper
+
+I
+
+Pressure Intensity
+
+II.
+
+Specific Resistance corresponding to pressures as in Column I. Ohms.
+
+III
+
+Pressure Intensity.
+
+IV.
+
+Specific Resistance corresponding to Column III. Ohms.
+
+Common cardboard 2.3 mm. thick
+
+0.05 kilo. per 6000 sq. mm.
+
+4.85 x 1015
+
+20 kg. per 6000 sq. mm.
+
+4.7 x 1014
+
+Gray paper, 0.26 mm. thick
+
+0.05 kilo. per 5000 sq. mm.
+
+3.1 x 10^15
+
+20 kg. per 5000 sq. mm.
+
+8 x 1014
+
+Yellow parchment paper-09 mm. thick
+
+0.05 kilo. per 5300 sq. mm.
+
+3.05 x 1016
+
+20 kg. per 5300 sq. mm.
+
+8 x 1016
+
+Linen tracing cloth
+
+0.05 kilo. per 6000 sq. mm.
+
+1.35 x 1016
+
+20 kg. per 33,000 sq. mm.
+
+1.86 x 10^15
+
+§ 111.  Paraffined Paper.
+
+Like wood and other semiconductors, paper can be vastly improved as an
+insulator by saturating it with melted paraffin.  To get the best
+results a pure paper free from size must be employed--gray Swedish
+filter paper does well.  This is dried at a temperature above 100° C.
+for, say, half an hour, and the sheets are then floated on the top of
+paraffin, kept melted at 140° C.  or thereabout in a baking dish.  As
+soon as the paper is placed upon the melted paraffin the latter begins
+to soak through, in virtue of capillary action, and drives before it
+the air and moisture, causing a strongly marked effervescence.
+
+After about one minute the paper may be thrust below the paraffin to
+soak.  When a sufficient number of papers have accumulated, and when
+no more gas comes off, the tray may be placed in a vacuum box (Fig.
+85), and the pressure reduced by the filter pump.  As the removal of
+the air takes time, provision must be made for keeping the bath hot.
+
+A vacuum may be maintained for about an hour, and air then readmitted.
+Repeated exhaustions and readmissions of air, which appear to improve
+wood, do not give anything like such a good result with paper.  In
+using a vacuum box provision must be made in the shape of a cool
+bottle between the air pump and the box.  If this precaution be
+omitted, and if any paraffin splashes on to the hot surface of the
+box, it volatilises with decomposition and the products go to stop up
+the pump.  Paraffin with a melting-point of 50° C. or upwards does
+well.
+
+The bath should be allowed to cool to about 60° C.  before the papers
+are removed, so that enough paraffin may be carried out to thoroughly
+coat the paper and prevent the entrance of air.
+
+Fig.  85.
+
+Fig.  85 is a section of a vacuum vessel which has been found very
+convenient.  It measures about two feet in diameter at the top.  It is
+round, because it is much easier to turn one circular surface than to
+plane up four surfaces, which has to be done if the box is square.
+Both the rim of the vessel and the approximating part of the cover
+require to be truly turned and smoothly finished.  A very good packing
+is made of solid indiarubber core about half an inch thick.  This is
+carefully spliced--cemented by means of a solution of rubber in
+naphtha, and the splice sewed by thick thread.  The lid ought to have
+a rim fitting inside the vessel, for this keeps the rubber packing in
+place;  the rim has been accidentally omitted in Fig.  85.  The bolts
+should not be more than five inches apart, and should lie at least
+half an inch in diameter, and the rim and lid should be each half an
+inch thick.
+
+Condensers may now be built up of sheets of this prepared paper
+interleaved with tin-foil in the ordinary way.  If good results are
+required, the condenser when finished is compressed between wooden or
+glass end-pieces by means of suitable clamps.  It can then be put in a
+box of melted paraffin, heated up to 140° C, and exhausted by means
+of the water pump for several hours.
+
+In this process the air rushes out from between the paper and foils
+with such vehemence that the paraffin is generally thrown entirely out
+of the box.  To guard against this the box must be provided with a
+loosely fitting and temporary lid, pierced with several holes.
+
+The real test as to when exhaustion is complete would be the cessation
+of any yield of air or water.  Since it is not generally convenient to
+make the vacuum box so air-tight that there are absolutely no leaks at
+all, and as the paraffin itself is, I think, inclined to "crack"
+slightly at the temperature of 140° C, this test or criterion cannot
+be conveniently applied.
+
+Two exhaustions, each of about two hours' duration, have, however, in
+my experience succeeded very well, provided, of course, that the
+dielectric is prepared as suggested.  At the end of the exhaustion
+process the clamping screws are tightened as far as possible, the
+condenser remaining in its bath until the paraffin is pasty.
+
+Condensers made in this way resist the application of alternating
+currents perfectly, as the following tests will show.  The dielectric
+consisted of about equal parts of hard paraffin and vaseline.  A
+condenser of about 0.123 microfarads capacity and an insulation
+resistance of 2000 megohms, [Footnote: As tested by a small voltage.]
+having a tin-foil area of 4.23 square metres (about), and double
+papers each about 0.2 mm. thick, designed to run at 2000 volts with a
+frequency of 63 complete periods, was tested at this frequency.
+
+The condenser was thoroughly packed all round in cotton-wool to a
+thickness of 6 inches, and its temperature was indicated more or less
+by a thermometer plunged through a hole in the lid of the containing
+box and of the condenser box, and resting on the upper surface of one
+set of tin-foil electrodes, from which the soft paraffin mixture had
+been purposely scraped away.  The following were the results of a four
+hours' run at a voltage 50 per cent higher than that for which the
+condenser was designed--i.e.  3000 volts.
+
+Times. Voltage Temperature   Temperature  Difference
+              in Condenser.   in Air.
+
+Hrs.  Min.
+
+2    10  2750    22.8° C.      23.0° C.      + 0.2°
+
+3    10  2700    23.0° C.      23.3° C.      + 0.3°
+
+3    18  3200    23.1° C.      23.0° C.      -0.1°
+
+4    10  3200    23.3° C.      23.7° C.      + 0.4°
+
+5    10  3100    23.6° C.      23.4° C.      -0.2°
+
+6    10  3000    23.8° C.      23.35° C.     -0.45°
+
+An idea of the order of the amount of waste may be formed from the
+following additional experiment.
+
+A condenser similar to the one described was filled with oil of a low
+insulating power.  It was tested calorimetrically, and also by the
+three voltmeter method, which, however, proved to be too insensitive.
+The temperature rise in the non-conducting box in air was about 0.3°
+C.  per hour, and the loss of power was found to be less than 0.1 per
+cent.  In the present case the actual rise was only 1° in four hours,
+and the integral give and take between the condenser and the air is
+practically nothing; consequently we may consider with safety that the
+rate of rise is certainly less than 1 degree per three hours.  The
+voltage and frequency were about the same in both experiments,
+consequently the energy passed is about proportional to the capacity
+used in the two experiments.
+
+From this it follows that since the specific heat of both condensers
+was the same (nearly), the loss in the present case is a good deal
+less than one-tenth per cent.  The residual charge is also much less
+than when the condenser is simply built up of paper paraffined in an
+unsystematic manner, and from which the air and water have been
+imperfectly extracted, as by baking the condenser first, and then
+immersing it in paraffin or oil.
+
+It is usual to consider that the phenomena of residual charge and
+heating in condensers, to which alternating voltages are applied, are
+closely allied.  This is true, but the alliance is not one between
+cause and effect--at all events, with regard to the greater part of
+the heating.  The imperfect exclusion of air and moisture,
+particularly the latter, certainly increases the residual charge by
+allowing surface creeping to occur;  but it also acts directly in
+producing heating, both by lowering the insulation of the condenser
+and by allowing of air discharges between the condenser plates.
+
+Of these causes of heating, the discharges in air or water vapour are
+probably the more important.  Long ago a theory of residual charge was
+given by Maxwell, based on the consideration of a laminated
+dielectric, the inductivity and resistance of which varied from layer
+to layer.  It was shown that such an arrangement, and hence generally
+any want of homogeneity in a direction inclined to the lines of force
+leading to a change of value of the product of specific resistance and
+specific inductive capacity, would account for residual charge.
+
+This possible explanation has been generally accepted as the actual
+explanation, and many cases of residual charge attributed to want of
+homogeneity, which are certainly to be explained in a simpler manner.
+For instance, the residual charge in a silvered mica plate condenser,
+carefully dried, can be increased at least tenfold by an exposure of a
+few minutes to ordinarily damp air.  The same result occurs with
+condensers of paraffined or sulphured paper;  and these are the
+residual changes generally observed.  The greater part must be due to
+creeping.
+
+§ 112.  Paraffin.
+
+This substance has long enjoyed great popularity in the physical
+laboratory.  Its specific resistance is given by Ayrton and Perry as
+more than 1025, but it is probably much higher in selected samples.
+The most serviceable kind of paraffin is the hardest obtainable,
+melting at a temperature of not less than 52° C.  It is a good plan to
+remelt the commercial paraffin and keep it at a temperature of, say,
+120° C.  for an hour, stirring it carefully with a glass rod so that
+it does not get overheated;  this helps to get rid of traces of water
+vapour.
+
+Hard paraffin, when melted, has an enormous rate of expansion with
+temperature, so great, indeed, that care must be taken not to overfill
+the vessels in which it is to be heated.  Castings can only be
+prepared by cooling the mould slowly from the bottom, keeping the rest
+of the mould warm, and adding-paraffin from time to time to make up
+for the contraction.  The cooling is gradually allowed to spread up to
+the free surface.
+
+The chief use of paraffin in the laboratory is in the construction of
+complicated connection boards, which are easily made by drilling holes
+in a slab of paraffin, half filling them with mercury, and using them
+as mercury cups.
+
+Since paraffin is a great collector of dust, it should be screened by
+paper, otherwise the blocks require to be scraped at frequent
+intervals, which, of course, electrifies them considerably.  This
+electrification is often difficult to remove without injuring the
+insulating power of the paraffin.  A light touch with a clean Bunsen
+flame is the readiest method, and does not appear to reduce the
+insulation so much as might be expected.  The safest way, however, is
+to leave the key covered by a clean cloth, which, however, must not
+touch the surface, for a sufficient time to allow of the charges
+getting away.
+
+The paraffin often becomes electrified itself by the friction of the
+key contacts, so that in electrometer work it is often convenient to
+form the cups by lining them with a little roll of copper foil twisted
+up at the bottom. In this case the connecting wires should, of course,
+be copper.  Small steel staples are convenient for fastening the
+collecting wires upon the paraffin;  or, in the case where these wires
+have to be often removed and changed about, drawing-pins are very
+handy.
+
+With mercury cups simply bored in paraffin great trouble will often be
+experienced in electrometer work, owing to a potential difference
+appearing between the cups--at all events when the contacts are
+inserted and however carefully this be done.  A few drops of very pure
+alcohol poured in above the mercury often cures this defect.  The
+surface of paraffin is by no means exempt from the defect of losing
+its insulating power when exposed to damp air, but it is not so
+sensitive as glass, nor does the insulating power fall so far.
+
+Two useful appliances are figured.
+
+Fig.  86.                           Fig.  87.
+
+One, in which paraffin appears as a cement, is an insulating stand
+made out of a bit of glass or ebonite tube cemented into an Erlenmeyer
+flask, having its neck protected from dust when out of use by a rubber
+washer, the other a "petticoat" insulator made by cementing a flint
+glass bottle into a glass dish with paraffin.  In course of time the
+paraffin will be found to have separated from the glass.  When this
+occurs the apparatus may be melted together again by placing it on
+the water bath for a few minutes.
+
+§ 113.  Vaseline, Vaseline Oil, and Kerosene Oil.
+
+These, when dry, insulate almost, but not quite as well as solid
+paraffin.  H. Koeller (Wien Berichte, 98, ii.  201, 1889; Beibl. Wied.
+Ann. 1890, p. 186), working with very small voltages, places the
+final(?) specific resistance of commercial petroleum, ether, and
+vaseline oil at about 2 X 1027 C.G.S.  This is ten times higher than
+the value assigned to commercial benzene (C6H6), and a hundred times
+higher than the value for commercial toluene.
+
+All these numbers mean little or nothing, but the petroleum and
+vaseline oil were the best fluid insulators examined by Koeller.  By
+mixing vaseline with paraffin a soft wax may be made of any desired
+degree of softness, and by dissolving vaseline in kerosene an
+insulating liquid of any degree of viscidity may be obtained.
+
+Hard paraffin may be softened somewhat by the addition of kerosene,
+and an apparently homogeneous mass cast from the mixture.  It will be
+found, however, that in course of time the kerosene oozes out, unless
+present in very small quantity.  Koeller has found (loc. cit.) that
+some samples of vaseline oil conducted "vollstaendig gut," but I have
+not come across such samples.  Vaseline oil, however, is sold at a
+price much above its value for insulating purposes.
+
+Kerosene oil is best obtained dry by drawing it directly from a new
+tin and exposing it to air as little as possible.  Of course, it may
+be dried by chemical means and distillation, but this is usually (or
+always) unnecessary.
+
+Fig 88.
+
+There is some danger of kerosene containing minute traces of sulphuric
+acid, and it and other oils may be conveniently tested for insulation
+in the following manner.  The quartz electroscope is taken, and the
+insulating rod heated in the blow-pipe.  The electroscope will now
+insulate well enough to show no appreciable collapse of the leaves in
+one or two hours' time.  Upon the plate of the electroscope is put a
+platinum or copper cylinder, and this is filled with kerosene (say) up
+to a fixed mark.
+
+The electroscope is placed on a surface plate, or, at all events, on a
+sheet of plate glass, and a "scribing block" is placed along side it
+and the scriber adjusted to dip into the kerosene to any required
+depth.  This is done by twisting a bit of wire round the scribing
+point and allowing it to project downwards.  The point itself serves
+to give an idea of the height to which the vessel may be filled.  The
+liquid is adjusted till its surface is in contact with the end of the
+scribing point, and the wire then projects into the liquid and forms
+an electrode of constant area of surface.  The scribing block is put
+to earth.  A charge is given to the electroscope, and the time
+required for a given degree of collapse of the leaves noted.
+
+The kerosene is then removed and its place taken by vaseline or
+paraffin, known to insulate well as a standard for comparison.  The
+experiment is then repeated, and the time noted for the same degree of
+collapse.  This test, though of course rough, is generally quite
+sufficient for workshop purposes, and is easily applied.  Moreover, it
+does not require correction for electrometer leakage, as generally
+happens when more elaborate appliances are used.
+
+The actual resistance of insulating oils depends so much on the
+electrical intensity, on the duration of that intensity, and on the
+previous history of the oil as to the direction of the voltage to
+which it has been subjected--to say nothing of the effects of traces
+of moisture--that quantitative experiments are of no value unless
+they are extremely elaborate.  I shall therefore only append the
+following numbers due to Bouty, Ann.  de Chemie et de Physique (6),
+vol. xxvii. p. 62, 1892, in which the effect of the conductivity on
+the determination of the specific inductive capacity was properly
+allowed for:-
+
+Carbon
+
+Bisulphide.
+
+Turpentine.
+
+Benzene (C6H6) at 20° C.
+
+Benzene at
+
+60° C.
+
+Specific inductive capacity
+
+2.715
+
+2.314
+
+2.21
+
+2.22
+
+Specific resistance in ohms per cubic centimetre
+
+1.5 x 1013,
+
+1.75 x 1012
+
+1.56 x 1011
+
+7.9 x 1011
+
+[Footnote: Professor J.  J.  Thomson, and Newall (Phil.  Proc.  1886)
+consider that carbon bisulphide showed traces of a "residual charge"
+effect;  hence, until this point is cleared up, we must regard Bouty's
+value as corresponding only to a very short, but not indefinitely
+short, period of charge.  On this point the paper must be consulted.
+
+March 1897--The writer has investigated this point by an independent
+method, but found no traces of "residual charge."]
+
+Information as to the specific inductive capacity of a large number of
+oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a
+paper by Quincke in Wiedemann's Annalen, 1883.
+
+§ 114.  Imperfect Conductors.
+
+Under this heading may be grouped such things as wood, slate, marble,
+etc--in fact, materials generally used for switchboard insulation.
+An examination of the insulating power of these substances has
+recently been made by B.  O.  Peirce (Electrical Review, 11th January
+1895) with quite sufficient accuracy, having in view the impossibility
+of being certain beforehand as to the character of any particular
+sample.  The tests were made by means of holes drilled in slabs of the
+material to be examined.  These holes were three-eighths of an inch in
+diameter, and from five-eighths to three-quarters of an inch deep, and
+one inch apart, centre to centre.  A voltage of about 15 volts was
+employed.  The following general results were arrived at:-
+
+(1) Heating in a paraffin bath always increases the resistance of
+wood, though only slightly if the wood be hard and dense.
+
+(2) Frequent exhaustion and readmission of air above the surface of
+the paraffin always has a good effect in increasing the resistance of
+wood.
+
+(3) When wood is once dry, impregnating it with paraffin tends to keep
+it dry.
+
+(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot
+be entirely waterproofed in this way.
+
+(5) The resistance of wood with stream lines along the grain is twenty
+to fifty per cent lower than when the stream lines cross the grain.
+
+(6) The "contact" resistance between slabs of wood pressed together is
+always very high.
+
+The following table will sufficiently illustrate the results obtained.
+The stone was dried in the sun for three weeks in the summer (United
+States), and the wood is described as having been well seasoned:-
+
+CURRENT WITH THE GRAIN
+
+Lowest Resistance  Highest Resistance Lowest Specific Highest Specific
+between two Cups   between two Cups   Resistance in   Resistance in
+in Megohms.        in Megohms.        Megohms.        Megohms.
+
+
+
+
+Ash.
+
+    550               920                 380             700
+
+Cherry
+
+   1100              4000                2800            6000
+
+Mahogany
+
+    430               730                 310             610
+
+Oak
+
+    220               420                1050            2200
+
+Pine.
+
+    330               630                  360            1470
+
+Hard pine.
+
+     10                48                   17            1050
+
+Black walnut
+
+   1100              3000                  320            2100
+
+Red fibre
+
+      2                 4                    3              60
+
+Slate
+
+                                           184             280
+
+Soapstone.
+
+                                           330             500
+
+White marble
+
+                                          2000            8800
+
+
+§ 115.  As to working the materials very little need be said.
+
+Fibre is worked like wood, but has the disadvantage of rapidly taking
+the edge off the tools.  In turning it, therefore, brass-turning
+tools, though leaving not quite such a perfect finish as wood-turning
+tools, last much longer, and really do well enough.  Fibre will not
+bear heating much above 100°C--at all events in paraffin.  At 140°
+C.  it becomes perfectly brittle.  Its chief merit lies in its great
+strength.  So far as insulation is concerned, Mr. Peirce's experiments
+show that it is far below most kinds of wood.
+
+Slate. This is a vastly more useful substance than it is generally
+credited with being.  It is very easily worked at a slow speed, either
+on the shaping machine or on the lathe, with tools adjusted for
+cutting brass, and it keeps its figure, which is more than can be said
+for most materials.  It forms a splendid base for instruments,
+especially when ground with a little emery by iron or glass grinders,
+fined with its own dust, and French polished in the ordinary way.
+Spools for coils of considerable radial dimension may be most
+conveniently made of slate instead of wood or brass, both because it
+keeps its shape, and because it insulates sufficiently well to stop
+eddy currents--at all events, sufficiently for ordinary practice.  An
+appreciable advantage is that slate may be purchased at a reasonable
+rate in large slabs of any desired thickness.  It is generally cut in
+the laboratory by means of an old cross-cut saw, but it does not do
+much damage to a hard hack saw such as is used for iron or brass.
+
+Marble. According to Holtzapffell, marble may be easily turned by
+means of simple pointed tools of good steel tempered to a straw
+colour.  The cutting point is ground on both edges like a wood-turning
+tool, and held up to the work "at an angle of twenty or thirty
+degrees" (?with the horizontal).  The marble is cut wet to save the
+tool.  As soon as the point gets, by grinding, to be about one-eighth
+of an inch broad it must either be drawn down or reground;  a flat
+tool will not turn marble at all.
+
+A convenient saw for marble is easily made on the principle of the
+frame saw.  A bit of hoop iron forms a convenient blade, and is
+sharpened by being hammered into notches along one edge, using the
+sharp end of a hammer head.  The saw is liberally supplied with sand
+and water--or emery and water, where economy of time is an object.
+The sawing of marble is thus really a grinding process, but it goes on
+rapidly.  Marble is ground very easily with sand and water;  it is
+fined with emery and polished with putty powder, which, I understand,
+is best used with water on cloth or felt.  As grinding processes have
+already been fully described, there is no need to go into them here.
+I have no personal knowledge of polishing marble.
+
+§ 116.  Conductors.
+
+The properties of conductors, more particularly of metals, have been
+so frequently examined, that the literature of the subject is
+appallingly heavy.  In what follows I have endeavoured to keep clear
+of what might properly appear in a treatise on electricity on the one
+hand, and in a wiring table on the other.  The most important work on
+the subject of the experimental resistance properties of metals has
+been done by Matthieson, Phil. Trans. 1860 and 1862, and British
+Association Reports (1864); Callender, Phil. Trans. vol. clxxiii;
+Callender and Griffiths, Phil. Trans. vol.  clxxxii;   The Committee
+of the British Association on Electrical Standards from 1862 to
+Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi.  (1893);
+
+Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p.
+838;  Feussner and St. Lindeck, Zeitsch. fuer Inst. 'Kunde, ix. 1889,
+p. 233, and B. A. Reports, 1892, p.  139.  Of these, Matthieson, and
+Dewar and Fleming treat of resistance generally, the latter
+particularly at low temperatures.
+
+[Footnote: The following is a list of Dr. Matthieson's chief papers on
+the subject of the electrical resistance of metals and alloys:  Phil.
+Mag. xvi. 1858, pp. 219-223;  Phil. Trans. 1858, pp. 383-388 Phil.
+Trans. 1860, pp. 161-176;  Phil. Trans. 1862, pp. 1-27 Phil. Mag. xxi.
+(1861), pp. 107-115;  Phil. Mag. xxiii. (1862), pp. 171-179;
+Electrician, iv. 1863, pp. 285-296;  British Association Reports,
+1863, p. 351.]
+
+Matthieson, and Matthieson and Hockin, Klemencic, Feussner, and St.
+Lindeck deal with the choice of metals for resistance standards.
+Callender's, and Callender and Griffiths' work is devoted to the study
+of platinum for thermometric purposes.
+
+The bibliography referring to special points will be given later.  The
+simplest way of exhibiting the relative resistances of metals is by
+means of a diagram published by Dewar and Fleming (loc. cit.), which
+is reproduced on a suitable scale on the opposite page.  For very
+accurate work, in which corrections for the volumes occupied by the
+metals at different temperatures are of importance, the reader is
+referred to the discussion in the original paper, which will be found
+most pleasant reading.  From this diagram both the specific resistance
+and the temperature coefficient may be deduced with sufficient
+accuracy for workshop purposes.  In interpreting the diagram the
+following notes will be of assistance.  The diagram is drawn to a
+scale of so-called "platinum temperatures"--that is to say, let R0,
+R100, Rt be the resistances of pure platinum at 0°, 100°, and t° C.
+respectively, then the platinum temperature pt is defined as
+
+pt = 100 X (Rt-R0)/(R100-R0)
+
+This amounts to making the temperature scale such that the temperature
+at any point is proportional to the resistance of platinum at that
+point.  Consequently on a resistance temperature diagram the straight
+line showing the relation between platinum resistance and platinum
+temperature will "run out" at the platinum absolute zero, which
+coincides more or less with the thermodynamic absolute zero, and also
+with the "perfect gas" absolute zero.  Platinum temperatures may be
+taken for workshop purposes over ordinary ranges as almost coinciding
+with air thermometer temperatures.  The metals used by Professors
+Dewar and Fleming were, with some exceptions, not absolutely pure, but
+in general represent the best that can be got by the most refined
+process of practical metallurgy.  We may note further that the
+specific resistance is only correct for a temperature of about 15° C,
+since no correction for the expansion or contraction of material has
+been applied.
+
+The following notes on alloys suitable for resistance coils will
+probably be found sufficient.
+
+§ 117.  Platinoid.
+
+This substance, discovered by Martino and described by Bottomley
+(Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and
+1 per cent to 2 per cent of tungsten, but I have not been able to
+obtain an analysis of its exact composition.  It appears to be
+difficult to get the tungsten to alloy, and it has to be added to part
+of the copper as phosphide of tungsten, in considerably greater
+quantity than is finally required.  The nickel is added to part of the
+copper and the phosphide of tungsten, then the zinc, and then the rest
+of the copper.  The alloy requires to be remelted several times, and a
+good deal of tungsten is lost by oxidation.
+
+The alloy is of a fine white colour, and is very little affected by
+air--in fact, it is to some extent untarnishable.  The specific
+resistance will be seen to be about one and a half times greater than
+that of German silver, and the temperature coefficient is about 0.021
+per cent per degree C.  (i.e.  about nineteen times less than copper,
+and half that of German silver).  To all intents and purposes it may
+be regarded as German silver with 1 per cent to 2 per cent of
+tungsten.  It does not appear to have been particularly examined for
+secular changes of resistance.
+
+118.  German Silver.  This material has been exhaustively examined of
+late years by Klemencic and by Feussner and St. Lindeck.  Everybody
+agrees that German silver, as ordinarily used for resistances, and
+composed of copper four parts, zinc two parts, nickel one part, is
+very ill-fitted for the purpose of making resistance standards.  This
+is due
+(1) to its experiencing a considerable increase in resistance on
+winding.  Feussner and St.  Lindeck found an increase of 1 per cent
+when German silver was wound on a core of ten wire diameters.
+
+(2) To the fact that the change goes on, though with gradually
+decreasing rate, for months or years;
+
+(3) to the fact that the resistance is permanently changed (increased)
+by heating to 40° C.  or over.  By  "artificially ageing" coils of
+German silver by heating to 150° C, say for five or six hours, its
+permanency is greatly improved, and it becomes fit for ordinary
+resistance coils where changes of, say, 1/5000 do not matter.
+
+It is a remarkable property of all nickel alloys containing zinc that
+their specific resistance is permanently increased by heating, whereas
+alloys which do not contain zinc suffer a change in the opposite
+direction.  The manufacturers of German silver appear to take very
+little care as to the uniformity of the product put on the market;
+some so-called German silver is distinctly yellow, while other samples
+are bright and white.
+
+It is noted by Price (Measurements of Electrical Resistance, p. 24)
+that German silver wire is apt to exhibit great differences of
+resistance within quite short lengths.  This has been my own
+experience as well, and is a great drawback to the use of German
+silver in the laboratory, for it makes it useless to measure off
+definite lengths of wire with a view to obtaining an approximate
+resistance.  In England German silver coils are generally soaked in
+melted hard paraffin.  In Germany, at all events at the Charlottenburg
+Institute, according to St.  Lindeck--coils are shellac-varnished and
+baked.  In any case it appears to be essential to thoroughly protect
+the metal against atmospheric influence.
+
+§ 119.  Platinum Silver.
+
+In the opinion of Matthieson and of Klemencic the 10 per cent silver,
+90 per cent platinum alloy is the one most suitable for resistance
+standards.  At all events, it has stood the test of time, for, with
+the following exceptions, all the British Association coils
+constructed of it from 1867 to the present day have continued to agree
+well together.  The exceptions were three one-ohm coils, which
+permanently increased between 1888 and 1890, probably through some
+straining when immersed in ice.  One coil changed by 0.0006 in 1
+between the years 1867 and 1891.  According to Klemencic, absolute
+permanency is not to be expected even from this alloy.
+
+Its recommendation as a standard depends on its chemical inertness,
+its small temperature coefficient (0.00027 per degree), and its small
+thermo-voltage against copper, as the following table (taken from
+Klemencic) will show:-
+
+Thermo-voltages in Micro-volts per degree against Copper
+          over the Range 0° to 17° C.
+
+Platinum iridium           7.14 micro-volts per degree C.
+
+Platinum silver            6.62 micro-volts per degree C.
+
+Nickelin                  28.5  micro-volts per degree C.
+
+German silver             10.43 micro-volts per degree C.
+
+Manganin (St.  Lindeck)    1.5  micro-volts per degree C.
+
+Mechanically, the platinum silver is weak, and is greatly affected as
+to its resistance by mechanical strains--in fact, Klemencic considers
+it the worst substance he examined from this point of view--a
+conclusion rather borne out by Mr. Glazebrook's experience with the
+British Association standards already referred to (B. A. Reports, 1891
+and 1892).
+
+Taking everything into account, it will probably be well to construct
+standards either with oil insulation only, or to bake the coils in
+shellac before testing, instead of soaking in paraffin.  Fig. 89
+illustrates a form of an oil immersed standard which is in use in my
+laboratory, and through which a considerable current may be passed.
+The oil is stirred by means of a screw propeller.
+
+Fig.  89.
+
+Fig.  89 represents a standard resistance for making Clerk cell
+comparisons by the silver voltameter method.  The framework on which
+the coils are wound consists of a base and top of slate.  The pillars
+are of flint glass tube surrounding brass bolts, and cemented to the
+latter by raw shellac.  Grooves are cut in the glass sleeves to hold
+the wires well apart.  These grooves were cut by means of a file
+working with kerosene lubrication.  A screw stirrer is provided, and
+the whole apparatus is immersed in kerosene in the glass box of a
+storage cell.  The apparatus is aged to begin with by heating to a
+temperature a good deal higher than any temperature it is expected to
+reach in actual work.  After this the rigidity of the frame is
+intended to prevent any further straining of the wire.  The apparatus
+as figured is not intended to be cooled to 0° C, so that it is put in
+as large a box as possible to gain the advantage of having a large
+volume of liquid.  The top and bottom slates measure seven inches by
+seven inches, and the distance between them is seven inches.  The
+inner coil is wound on first, and the loop which constitutes the end
+of the winding is brought up to a suitable position for adjustment.
+The insulation of the heavy copper connectors is by means of ebonite.
+
+§ 120.  Platinum Iridium.
+
+Platinum 90 per cent, iridium 10 per cent.  This material was prepared
+in some quantity at the cost of the French Government, and distributed
+for test about 1886.  Klemencic got some of it as representing
+Austria, and found it behaved very like the platinum silver alloy just
+discussed.  The temperature coefficient is, however, higher than for
+platinum silver (0.00126 as against 0.00027).  The mechanical
+properties of the alloy are, however, much better than those of the
+silver alloy; and in view of the experience with B.  A.  standards
+above quoted, it remains an open question whether, on the whole, it
+would not be the better material for standards, in spite of its
+higher price.  Improvements in absolute measurements of resistance,
+however, may render primary standards superfluous.
+
+§ 121.  Manganin.
+
+Discovered by Weston--at all events as to its application to
+resistance coils.  A glance at the diagram will exhibit its unique
+properties, on account of which it has been adopted by the
+Physikalisch Technischen Reichsanstalt for resistance standards.  The
+composition of the alloy is copper 84 per cent, manganese 12 per cent,
+nickel 4 per cent, and it is described as of a steel-gray colour.
+Unfortunately it is apt to oxidise in the air, or rather the manganese
+it contains does so, so that it wants a very perfect protection
+against the atmosphere.
+
+Like German silver, manganin changes in resistance on winding, and
+coils made of it require to be artificially aged by heating to 150°
+for five hours before final adjustment.  The annealing cannot be
+carried out in air, owing to the tendency to oxidation.  The method
+adopted by St.  Lindeck (at all events up to 1892) is to treat the
+coil with thick alcoholic shellac varnish till the insulation is
+thoroughly saturated, and then to bake the coil as described.  The
+baking not only anneals the wire, but reduces the shellac to a hard
+and highly insulating mass.
+
+Whether stresses of sufficient magnitude to produce serious mechanical
+effects can be set up by unequal expansion of wire and shellac during
+heating and cooling is not yet known, but so far as tested (and it
+must be presumed that the Reichsanstalt tests are thorough) no
+difficulty seems to have been met with.  In course of time, however,
+probably the best shellac coating will crack, and then adieu to the
+permanency of the coil!  This might, of course, be obviated by keeping
+the coil in kerosene, which has no action on shellac, but which
+decomposes somewhat itself.
+
+The method of treatment above described suffices to render coils of
+manganin constant for at least a year (in 1892 the tests had only been
+made for this time) within a few thousands per cent.  Manganin can be
+obtained in sheets, and from this material standards of 10-2, 10-3,
+and 10-4 ohms are made by soldering strips between stout copper bars,
+and these are adjusted by gradually increasing their resistance by
+boring small holes through them. The solder employed is said to be
+"silver."
+
+Mr. Griffiths (Phil.  Trans.  vol.  clxxxiv.  [1893], A, p.  390) has
+had some experience with manganin carrying comparatively heavy
+currents, under which circumstances its resistance when immersed in
+water was found to rise in spite of the varnish which coated it.
+Other experiments in which the manganin wire was immersed in paraffin
+oil did not exhibit this effect, though stronger currents were passed.
+
+On the whole, manganin appears to be the best material for coil boxes
+and "secondary" resistance standards.  Whether it is fit to rank with
+the platinum alloys as regards permanency must be treated as an open
+question.
+
+§ 122.  Other Alloys.
+
+The following tables, taken from the work of Feussner and St.
+Lindeck, Zeitschrift fuer Instrumenten Kunde, 1889, vol.  ix.  p.
+233, together with the following notes, will suffice.
+
+§ 123.  Nickelin.
+
+This is only German silver with a little less zinc, a little more
+nickel, and traces of cobalt and manganese.  It behaves like German
+silver, but is an improvement on the latter in that all the faults of
+German silver appear upon a reduced scale in nickelin.
+
+§ 124.  Patent Nickel.
+
+Practically a copper nickel alloy, used to some extent by Siemens
+and Halske.  It stands pretty well in the same relation to nickelin as
+the latter does to German silver.  After annealing as for manganin it
+can be made into serviceable standards which do not change more than a
+few thousandths per cent.  I have not come across a statement of its
+thermo-voltage against copper.
+
+§ 125.  Constantin.
+
+Another nickel copper alloy containing 50 per cent of each
+constituent.  It appears to be a serviceable substance, having a
+temperature coefficient of 0.003 per cent per degree only, but an
+exceedingly high thermo-voltage, viz.  40 micro-volts per degree
+against copper.
+
+         1        2     3       4        5         6     7       8
+       German  Nickelin made    Rheo-    Patent Nickel   Manga-  Nickel
+       Silver  by Obermaier     tane                     nese    Manga-
+               Dia-     Dia-             Dia-   Dia-     Copper  nese
+               meter    meter            meter  meter            Copper
+               1.0mm    0.1mm            0.6mm  1.0mm
+
+
+Copper 60.16    61.63    54.57  53.28     74.41  74.71    70      73
+
+Zinc   25.37    19.67    20.44  16.89     0.23   0.52     ...     ...
+
+Tin    ...      ...      ...    ...       trace  ...      ...
+
+Nickel 14.03    18.46    24.48  25.31     25.10  24.14    ...     3
+
+Iron   0.30     0.24     0.64   4.46      0.42   0.70     ...     ...
+
+Cobalt trace    0.19     ...    ...       trace  trace    ...     ...
+
+Mang-  trace    0.18    0.27    0.37      0.13   0.17     30      24
+anese.
+
+     99.86     100.37  100.40  100.31    100.24 100.24  ...      ...
+
+Specific
+resistance
+     30.0     33.2     44.8    52.5      34.2    32.8    100.6   47.7
+
+Temperature
+coefficient
+     0.00036 0.00030 0.00033 0.00041 0.00019  0.00021  0.00004  0.00003
+
+The specific resistance is in microhms, i.e. 10-6 ohms per cubic
+centimetre, and the temperature coefficient in degrees centigrade.
+
+
+126.  Nickel Manganese Copper.
+
+I can find no other reference with regard to this alloy mentioned by
+Lindeck.  Nicholls, however (Silliman's Journal [3], 39, 171, 1890),
+gives some particulars of alloys of copper and ferromanganese.  The
+following table is taken from Wiedemann's Beiblatter (abstract of
+Nicholl's paper, 1890, p.  811).  All these alloys appear to require
+annealing at a red heat before their resistances are anything like
+constant.
+
+Let x be percentage of copper, then 100--x is percentage of
+"ferromanganese."
+
+Values of x.  100   99.26  91 .88  86.98  80.4  70.65
+
+Specific
+resistance
+with respect
+to copper
+(?  pure)       1  1.19    11.28   20.4   27.5  45.1
+
+Temperature
+coefficient
+per degree
+x 10^6(hard) 3202  2167    138     16      22   -24
+
+Ditto (soft) ...   ...     184     80      66    21
+
+If nickel is added, alloys of much the same character are obtained,
+some with negative temperature coefficients--for instance, one
+containing 52.51 per cent copper, 31.27 per cent ferromanganese, and
+16.22 nickel.
+
+A detailed account of several alloys will be found in a paper by
+Griffiths (Phil.  Trans.  1894, p.  390), but as the constants were
+determined to a higher order of accuracy than the composition of the
+material--or, at all events, to a higher degree  of accuracy than
+that to which the materials can be reproduced--there is no advantage
+in quoting them here.
+
+CHAPTER IV
+
+ELECTROPLATING AND ALLIED ARTS
+
+§ 127.  Electroplating.
+
+This is an art which is usually deemed worthy of a treatise to itself,
+but for ordinary laboratory purposes it is a very simple matter--so
+simple, indeed, that the multiplicity of receipts as given in
+treatises are rather a source of embarrassment than otherwise.
+
+The fundamental principles of the art are:-
+
+(1) Dirty work cannot be electroplated.
+
+(2) Electroplated surfaces may be rougher, but will not be smoother
+than the original unplated surface.
+
+(3) The art of electroplating being in advance of the science, it is
+necessary to be careful as to carrying out instructions in detail.
+This particularly applies to the conditions which determine whether a
+metallic deposit shall come down in a reguline or in a crystalline
+manner.
+
+§ 128.  The Dipping Bath.
+
+An acid dipping bath is one of the most useful adjuncts to the
+laboratory, not only for cleansing metals for electroplating, but for
+cleaning up apparatus made out of bits of brass tube and sheet, and
+particularly for quickly cleaning binding screws, etc, where it is
+necessary to ensure good electrical contact.
+
+The cheapest and most satisfactory way in the end is to make up two or
+three rather large baths to begin with.  The glass boxes of storage
+batteries do very nicely for the purpose, and being generally ground
+pretty flat at the top, they may be covered by sheets of patent plate
+glass, and thus preserved from the action of the air.
+
+First Bath. A 30 or 40 per cent solution of commercial caustic soda.
+Objects may be cleansed from grease in this bath by heating them as
+hot as is consistent with individual circumstances, and plunging them
+into it.
+
+It is a considerable advantage to begin by removing grease from
+articles subsequently to be dipped in an acid bath, both because it
+saves time and acid, and because more uniform results are obtainable
+when this is done than when it is omitted.  It is a great advantage to
+have the caustic soda solution hot.  This is always done in factories
+where nickel-plating is carried on, but it is inconvenient in the
+laboratory.  The articles after dipping in the alkali are swilled with
+water, and may even be scrubbed with a brush, so as to remove greasy
+matters that have been softened but not entirely removed.
+
+Acid Bath. A convenient bath for laboratory purposes is made by
+mixing two volumes of strong commercial nitric acid with one of strong
+sulphuric acid in a cell measuring, say, 12 X 10 X 15 inches.
+
+Copper or brass articles are dipped in this bath for a few seconds,
+then rinsed with water, then dipped again for a second or two, or
+until they appear equally white all over, and then withdrawn as
+rapidly as possible and plunged into a large quantity of clean water.
+Care must be taken to transfer the articles from the bath to the water
+as quickly as possible, for if time be allowed for gas to be evolved,
+the surfaces become mat instead of bright.
+
+In order to save acid it is advisable to make up a third bath, using
+those odds and ends of acids which gradually accumulate in the
+laboratory.  Sulphuric acid from the balance cases, for instance,
+mixed with its own volume of commercial nitric acid, does very well.
+
+The objects to be dipped receive a preliminary cleansing by a dip in
+this bath, the strong bath being reserved for the final dip.  Sheet
+brass and drawn tube, as it comes from the makers, possesses a really
+fine surface, though this is generally obscured by grease and oxide.
+Work executed in these materials, cleaned in alkali, and dipped in
+really strong acid, will be found to present a much better appearance
+than work which has been filed, unless the latter be afterwards
+elaborately polished.
+
+On no account must paraffin be allowed to get into any of the baths.
+When the final bath gets weak it must be relegated to a subordinate
+position and a new bath set up.  A weak acid bath leaves an ugly
+mottled surface on brass work.
+
+§ 129.  A metallic surface which it is intended to electroplate must,
+as has been mentioned, be scrupulously clean.  If the metal is not too
+valuable or delicate, cleaning by dipping is easy and effectual.  The
+following notes will be found to apply to special cases which often
+occur.
+
+(1) Silver Surfaces intended to be gilt. These are first washed
+clean with soap and hot water, and polished with whitening.  They are
+then dipped for a moment in a boiling solution of potassium cyanide.
+A 20 per cent solution of common commercial cyanide does well, but the
+exact strength is quite immaterial.  The cyanide is washed away in a
+large volume of soft water, and the articles are kept under water till
+they are scratch-brushed.
+
+Mat surfaces are readily produced on standard silver by dipping in hot
+strong sulphuric acid.  The appearance of new silver coins, which is
+familiar to everybody, is obtained by this process.
+
+(2) Finely turned and finished Brass Work. If it is intended to
+nickel-plate such work, and if it is desirable to obtain brightly
+polished nickel surfaces, the work must be perfectly polished to begin
+with.  Full details as to polishing may be found in workshop books or
+treatises on watch-making.  It will suffice here to say that the brass
+work is first smoothed by the application of successive grades of
+emery and oil, or by very fine "dead" smooth files covered with chalk.
+Polishing is carried out by means of rotten stone and oil applied on
+leather.
+
+In polishing turned work care must be taken to move the file, emery,
+or rotten stone to and fro over the work with great regularity, or the
+surface will end by looking scratchy and irregular.  The first process
+of cleaning is, of course, to remove grease, and this is accomplished
+best by dipping in a bath of strong hot caustic soda solution, and
+less perfectly by heating the work and dipping it in the cold caustic
+soda bath.
+
+During this process a certain amount of chemical action often occurs
+leading to the brass surface exhibiting some discoloration.  The best
+way of remedying this is to dip the brass into a hot bath of cyanide
+of potassium solution.  If it is inconvenient to employ hot baths or
+to heat the brass work, good results may be obtained by rubbing the
+articles over with a large rough cork plentifully lubricated with a
+strong solution of an alkali.
+
+If the surfaces are very soiled or dirty, a paste of alkali and fine
+slaked lime may be applied on a cork rubber, and this in my experience
+has always been most effective and satisfactory in every way, except
+that it is difficult to get into crevices.  If the alkali stains the
+work, a little cyanide of potassium may be rubbed over the surface in
+a similar manner.
+
+Brass work treated by either of these methods is to be washed in clean
+water till the alkali is entirely removed, and may then be
+nickel-plated without any preliminary scratch-brushing.  The treatment
+in hot baths of alkali and cyanide is the method generally employed in
+American factories as a preliminary to the nickelling of small brass
+work for sewing machines, etc.
+
+(3) Copper either for use as the kathode in electrolysis calibration
+experiments or otherwise is most conveniently prepared by dipping in
+the acid bath, rinsing quickly in cold water, scratch-brushing under
+cold water, and transferring at once to the plating bath.  In the case
+where the copper plates require to be weighed they are dipped into
+very hot distilled water after scratch-brushing, and then dried at
+once by means of a clean glass cloth.
+
+(4) Aluminium (which, however, does not readily lend itself to plating
+operations [Footnote: This difficulty has now been overcome.  See
+note, section 138.] ) is best treated by alkali rubbed on with a cork,
+or by a hot alkaline carbonate where rubbing is inexpedient.  The
+clean aluminium is scratch-brushed under water, and at once
+transferred to the plating bath.
+
+(5) Iron for Nickel-plating. According to Dr. Gore
+(Electra-metallurgy, p.  319) the best bath for cleaning iron is made
+as follows: "One gallon of water and one pound of sulphuric acid are
+mixed with one or two ounces of zinc (which of course dissolves); to
+this is added half a pound of nitric acid."  The writer has been
+accustomed to clean iron by mechanical means, to deprive it of grease
+by caustic alkali, and to finish it off by, means of a hard scratch
+brush.  This process has always worked satisfactorily.
+
+(6) Articles soldered with soft solder containing lead and tin do not
+readily lend themselves to electrolytic processes, the solder
+generally becoming black and refusing to be coated with the
+electro-deposit.  Moreover, if soldered articles are boiled for any
+length of time in caustic alkali during the preliminary cleansing,
+enough tin will dissolve to form a solution of stannate of potash or
+soda--strong enough to deposit tin on brass or copper.  A method of
+coppering soldered articles will be described later on.
+
+§ 130.  Scratch-brushing.
+
+This process is generally indispensable, and to its omission is to be
+traced most laboratory failures in electroplating.  Scratch-brushes
+may be bought at those interesting shops where "watchmakers' supplies"
+are sold.  It will be well, therefore, to purchase a selection of
+scratch brushes, for they are made to suit particular kinds of work.
+They are all made of brass wire, and vary both in hardness and in the
+fineness of the wire.  The simplest kind of scratch brush consists
+merely of a bundle of wires bound up tightly by another wire, and
+somewhat "frizzed" out at the ends (Fig. 90).  A more useful kind is
+made just like a rotating brush, and has to be mounted on a lathe
+(Fig.  91).
+
+Fig.  90.                                  Fig. 91.
+
+The scratch brush is generally, if not always, applied wet;  the
+lubricant generally recommended is stale beer, but this may be
+replaced by water containing a small quantity of glue, or any other
+form of gelatine in solution--a mere trace (say .1 per cent) is quite
+sufficient.  Very fair results may be got by using either pure or
+soapy water.  The rotating brushes require to be mounted on a lathe,
+and may be run at the same speed as would be employed for turning
+wooden objects of the same dimensions.
+
+Since the brush has to be kept wet by allowing water or its equivalent
+to drip upon it, it is usual to make a tin trough over which the brush
+can revolve, and to further protect this by a tin hood to keep the
+liquid from being thrown all over the room.  In many works the brush
+is arranged to lie partly in the liquid, and this does very well if
+the hood is effective.
+
+There is a superstition that electro-deposits stick better to
+scratch-brushed surfaces than to surfaces which have not been so
+treated, and consequently it is usual to scratch-brush surfaces before
+electro-deposit.  However this may be, there is no doubt that
+adherence and solidity are promoted by frequent scratch-brushing
+during the process of depositing metal, especially when the latter
+tends to come down in a spongy manner.
+
+Gilt surfaces--if the gilding is at all heavy--are generally dull
+yellow, or even brown, when they come from the bath, and require the
+scratch brush to cause the gold to brighten, an office which it
+performs in a quite striking manner.  The same remark applies to
+silvered surfaces, which generally leave the bath a dead white--at
+all events if the deposit is thick, and if ordinary solutions are
+employed.  In either case the touch of the scratch brush is magical.
+
+§ 131. Burnishing.
+
+Burnishers of steel, agate, or bloodstone can be bought at the shops
+where scratch brushes are sold, and are used to produce the same
+brightening effect as can be got by scratch-brushing.  The same
+solutions are employed, but rather stronger, and the burnisher is
+swept over the surface so as to compress the deposited metal.
+Burnishing is rather an art, but when well done gives a harder and
+more brilliant (because smoother) surface than the scratch brush.  On
+the whole, steel burnishers are the most convenient if in constant
+use.
+
+If the burnishing tools have to lie about, steel is apt to rust,
+unless carefully protected by being plunged in quicklime or thickly
+smeared with vaseline, and the least speck of rust is fatal to a
+burnisher.  In any case the steel requires to be occasionally
+repolished by rouge and water on a bit of cloth or felt.  The process
+of burnishing is necessarily somewhat slow and tedious, and as a rule
+is not worth troubling about except in cases where great permanence is
+required.
+
+The burnisher is moved over the work somewhat like a pencil with
+considerable pressure, and care is taken to make the strokes as
+uniform in direction as possible;  otherwise the surface looks
+non-uniform, and has to be further polished by tripoli, whitening,
+etc, before it is presentable.
+
+§ 132.  Silver-plating.
+
+The most convenient solution for general purposes is an 8 to 10 per
+cent solution of the double cyanide of silver and potassium together
+with 1 or 2 per cent of "free" potassium cyanide.  Great latitude is
+permissible in the strength of solution and density of current.  As
+commercial cyanide of potassium generally contains an unknown
+percentage of other salts, which, however, do not interfere with its
+value for the purpose of silver-plating, the simplest procedure is as
+follows.
+
+For every 100 c.c.  of plating solution about 7 grms.  of dry
+crystallised silver nitrate are required.  The equivalent amount of
+potassium cyanide (if dry and pure) is 5.2 grms, but commercial
+cyanide may contain from 50 per cent upwards to 96 per cent in the
+best fused cyanide made from ferrocyanide only.  An approximate idea
+of the cyanide content can be obtained from the dealers when the salt
+is purchased, and this is all that is required.
+
+A quantity slightly in excess of the computed amount of cyanide is
+dissolved in distilled water, and this is cautiously added to the
+solution of the silver nitrate till precipitation is just complete.
+The supernatant liquors are then drained away, and the precipitate
+dissolved by adding a sufficiency of the remaining cyanide; this
+process is assisted by warming and stirring.
+
+An allowance of about one-tenth of the whole cyanide employed may be
+added to form "free" cyanide, and the solution made up to the strength
+named.  It is advisable to begin with the cyanide in a moderately
+strong solution, for the sake of ease in dissolving the precipitate.
+
+This solution will deposit silver upon articles of copper or brass
+immersed in it even without the battery, but the coat will be thin.
+The solution is used cold, with a current density of about 10 to 20
+ampères per square foot.  The articles to be silvered are
+scratch-brushed, washed, and electroplated, till they begin to look
+undesirably rough.  They are then taken out of the bath, rebrushed,
+and the process continued till a sufficiency of silver is deposited.
+Four grammes weight of silver (nearly) is deposited per ampère hour.
+It is best to use a fine silver anode, so that the solution, does not
+get contaminated by copper.
+
+In most factories it is usual to "quicken" the objects to be silvered
+before placing them in the electrolysis vats, because the deposit is
+said to adhere better in consequence of this treatment.  I have never
+found it any improvement for laboratory purposes, but it is easy to
+do.  A dilute (say 2 per cent) solution of cyanide of mercury is
+required containing a little free cyanide.  The objects to be
+"quickened" are scratch-brushed and dipped into the cyanide of mercury
+solution till they are uniformly white;  it is generally agreed that
+the less the mercury deposited the better, so long as a perfect
+coating is obtained.  The objects are rinsed after quickening, and put
+in the depositing bath at once.
+
+The mat surface of silver obtained by electrolysis of the cyanide is
+very beautiful--one of the most beautiful things in nature--shining
+with incomparable crystalline whiteness.  So delicate is it, however,
+for so great is the surface it exposes, that it is generally rapidly
+deteriorated by exposure to the air.  It may be protected to some
+extent by lacquering with pale lacquer, but it loses some of its
+brilliancy and purity in the process.  The deposit is generally
+scratch-brushed or burnished down to a regular reflecting surface.
+
+§ 133.  Cold Silvering.
+
+A thin but brilliant coat of silver may be readily applied to small
+articles of brass or copper in the following way.  A saturated
+solution of sodium sulphite (neutral) is prepared, and into this a 10
+per cent solution of nitrate of silver is poured so long as the
+precipitate formed is redissolved.  A good deal of silver may be got
+into solution in this way.  Articles to be silvered need only to be
+cleaned, brushed, and dipped in this solution till a coat of the
+required thickness is obtained.
+
+I must admit, however, that the coating thus laid on does not appear
+to be so permanent as one deposited by simple immersion from the
+cyanide solution, even though it is thicker.  The cyanide plating
+solution will itself give a good coat of silver if it is used boiling,
+and if a little potassium cyanide be added.
+
+For purposes of instrument construction, however, a thin coat of
+silver is seldom to be recommended, on account of its liability to
+tarnish and its rapid destruction when any attempt is made to repolish
+it.  For these reasons, nickel or gold plating is much to be
+preferred.
+
+§ 134.  Gilding.
+
+This art deserves to be much more widely practised than is usual in
+laboratories.  Regarded as a means of preserving brass, copper, or
+steel, it is not appreciably more "time robbing" than lacquering, and
+gives infinitely better results.  Moreover, it is not much more
+expensive.  Strange as it may seem, the costliness of gilding seldom
+lies in the value of the gold deposited;  the chief cost is in the
+chemicals employed to clean the work, and in interest on the not
+inconsiderable outlay on the solution and anode.
+
+The easiest metal to gild is silver, and it is not unusual to give
+base metals a thin coating of silver or copper, or both, one after the
+other, before gilding, in order to secure uniformity.  To illustrate
+the virtue of a thin layer of gold, I will mention the following
+experiment.  About three years ago I learned for the first time that
+to "clean" the silver used in a small household required at least an
+hour's labour per diem. I further ascertained that most of this time
+is spent on the polishing part of the process.
+
+As this seemed a waste of labour, I decided to try the effect of
+gilding.  In order to give the proposal a fair trial I gilt the
+following articles:  half a dozen table spoons and forks, a dozen
+dessert forks and spoons, and a dozen tea spoons.  These were all
+common electroplated ware.  They were weighed before and after
+gilding, and it was with difficulty that the increase of weight was
+detected, even though a fine bullion balance was employed.  On
+calculating back to money, it appeared that the value of the gold
+deposited was about threepence.  Assuming that an equal weight of
+silver had been accidentally dissolved by the free cyanide during the
+plating--which is unlikely--the total amount of gold deposited would
+be worth, say, sixpence.
+
+After three years' continuous use the gilding is still perfect, except
+at the points on which the spoons and forks rest, where it is
+certainly rather shabby.  Meanwhile the "gold" plate only requires to
+be washed with hot water and soap to keep it in perfect order, a much
+more cleanly and expeditious process than that of silver cleaning.
+
+§ 135.  Preparing Surfaces for Gilding.
+
+Ordinary brass work--rough or smooth--may for purposes of
+preservation be dipped, scratch-brushed, and gilt at once.  Seven
+years ago the writer gilt the inside of the head of a copper water
+still, and simply scratch-brushed it; it is to-day in as good order as
+when it was first done.  If it is intended to gild work from the
+first, with the view of making an exceptionally fine job of it,
+"gilding metal," i.e.  brass containing one to one and a quarter
+ounces of zinc to the pound of copper may be specified.  From its
+costliness, however, this is only desirable for small work.
+
+Iron and steel are generally given a preliminary coating of copper,
+but this may be dispensed with though with no advantage--by using a
+particular process of gilding.
+
+Base metals, zinc, pewter, lead, etc, are first coppered in a cyanide
+of copper solution, as will be described under the head of
+Copper-plating.  If it is intended to gild soldered articles,
+the preliminary coating of copper is essential.
+
+The most convenient vessel for holding a gilding solution is
+undoubtedly one formed of enamelled iron.  Particularly useful are the
+buckets and "billies" (i.e.  cylindrical cans) made of this material.
+These vessels may be heated without any fear of a smash, and do not
+appear to be appreciably affected by gilding solutions--at all events
+during several days or weeks.  The avoidance of all risk of breakage
+when twenty or thirty pounds' worth of solution is in question is a
+matter of importance.
+
+Under no circumstances is it desirable to use anything but the purest
+gold and best fused cyanide (called "gold" cyanide) in the preparation
+of the solutions.  The appearance of a pure gold deposit is far richer
+than of one containing silver, and its resistance to the atmosphere is
+perfect;  moreover, in chemico-physical processes one has the
+satisfaction of knowing what one is dealing with.
+
+§ 136.  Gilding Solutions.
+
+The strength of solution necessary for gilding brass, copper, and
+silver is not very material.  About one to two pounds of "gold"
+potassium cyanide (? 96 per cent KCN) per gallon does very well.  The
+gold is best introduced by electrolysing from a large to a small gold
+electrode.  One purchases a plate of pure gold either from the mint or
+from reliable metallurgists (say Messrs.  Johnson and Matthey of
+London), and from this electrodes are cut.
+
+The relative areas of the electrodes do not really much matter.  I
+have used an anode of four times the area of the cathode.  The
+solution is preferably heated to a temperature of about 50° C, and a
+strong current is sent through it, say twenty amperes to the square
+foot of anode.  The electrodes must be suspended below the surface of
+the solution by means of platinum wires.  If the gold plates are only
+partly immersed, they dissolve much more rapidly where they cut the
+surface, possibly on account of the effect of convection currents,
+though so far as the writer is aware no proper explanation has yet
+been given.
+
+After a time gold begins to be deposited on the cathode in a powdery
+form, for which reason it is a good plan to begin by wrapping the
+latter in filter paper.  The process has gone on for a sufficient time
+when a clean bit of platinum foil immersed in the place of the cathode
+becomes properly gilt at a current density of about ten amperes per
+square foot.
+
+The powdery gold deposited on the cathode while preparing the solution
+can be scraped off and melted for further use, or the whole cathode
+may now be used as an anode.  The platinum foil testing cathode may
+also be "stripped" by making it an anode, and is for this reason
+preferable to German silver or copper, which would contaminate the
+solution while the "stripping" process was in progress.
+
+For general purposes a current density of say ten to fifteen amperes
+per square foot may be used, but this may be considerably varied, so
+long as the upper limit is not greatly overpassed.  During
+gold-plating there is a considerable advantage in keeping the
+electrodes moving or the solution stirred.
+
+After immersing the cleaned and scratch-brushed articles, depositing
+may go on for about three minutes, after which they are removed from
+the bath and examined, in order to detect any want of uniformity in
+the deposit.
+
+The articles should be entirely immersed;  if this is not done,
+irregularity is apt to appear at the surface.  Platinum wires employed
+as suspenders, and coated along with the articles to be gilt, may also
+be cleaned without loss by making them anodes.  If, on examination,
+all is found to be going on well, reimmerse the cathodes, and continue
+plating till they appear of a dull yellowish brown (this will occur in
+about four minutes), then remove them, rinse and scratch-brush them,
+and replace them in the bath.
+
+When a second coat appears to be getting rather brown than yellowish
+brown, i.e.  of the colour of wet wash-leather, the removal, followed
+by scratch-brushing, may be repeated, and for nearly all laboratory
+purposes, the articles are now fully gilt.
+
+The coating of gold deposited from a hot cyanide solution is spongy in
+the extreme, and if the maximum wear-resisting effect is to be
+obtained, it is advisable to burnish the gold rather than to rely upon
+the scratch brush alone.
+
+If the area of the cathode exceeds that of the anode the solution is
+said to grow weaker, and vice versa.  This may be remedied in the
+former case by an obvious readjustment;  the latter introduces no
+difficulty so far as I know except when plating iron or steel.
+
+The student need not be troubled at the poor appearance of the deposit
+before it is scratch-brushed.  Heavy gold deposits are almost always
+dull, not to say dirty, in appearance till the burnisher or scratch
+brush is applied.  On the other hand, the deposit ought not to get
+anything like black in colour.
+
+The following indications of defects may be noted--they are taken from
+Gore.  I have never been really troubled with them.
+
+The deposit is blackish.  This is caused by too strong a current in
+too weak a bath.  This may be remedied to some extent by stirring or
+keeping the cathode in motion.  The obvious remedy is to add a little
+cyanide of gold.
+
+The gold anode gets incrusted.  This is a sign that the bath is
+deficient in potassium cyanide.  The gold anode gets black and gives
+off gas.  The solution is deficient in cyanide, and too large a
+current is being passed.
+
+If a bright surface is desired direct from the bath, some caustic
+potash (say 2 per cent) may, according to Gore, be added, or the
+articles may be plated only slightly by using a weak current and
+taking them out directly they show signs of getting dull.  By a weak
+current I mean one of about five amperes per square foot.
+
+The deposit is said to be denser if the solution be heated as
+directed;  but the bath will gild, though not quite so freely when
+cold.
+
+To gild iron or steel directly, dilute the bath as above recommended
+some five or six times, add about 1 per cent of potassium cyanide, and
+gild with a very weak current (say two or three amperes per square
+foot) in the cold.  Frequent scratch-brushing will be found requisite
+to secure proper adherence.
+
+It is generally recommended to gild brass or German silver in
+solutions which are rather weak, but in the small practice which
+occurs in the laboratory a solution prepared as suggested does
+perfectly for everything except iron or steel.  The scratch-brushing
+should be done over a large photographic developing dish to avoid loss
+of gold.  It is a good plan to rinse the articles after leaving the
+bath in a limited quantity of distilled water, which is afterwards
+placed in a "residue" bottle, and then to scratch-brush them by hand
+over the dish to catch fine gold.  When any loose dust is removed the
+articles may be scratched in the lathe without appreciable further
+loss.
+
+Silver-gilt articles tend to get discoloured by use, but this
+discoloration can be removed by soap and water.  After long use a gold
+cyanide bath tends to alter greatly in composition, In general, the
+bath tends to grow weaker, from the fact that there is a strong
+temptation to gild as many articles at once as possible.
+
+It is therefore a good plan to keep a rough profit and loss account of
+the gold in order to find the quantity in solution.  Fifty dwts.  per
+gallon (or 78 grms. per 4.5 litres) is recommended.  A gallon of
+solution of this strength is worth about eleven pounds sterling in
+gold and cyanide, and a serviceable anode will be worth about 10
+pounds.  (Fine gold is worth nominally four pounds four shillings and
+eleven pence ha'penny per oz.) Gold may be easily obtained containing
+less impurity than one part in ten thousand.
+
+§ 137.  Plating with Copper.
+
+Copper may be deposited from almost any of its salts in reguline form,
+the sulphate and nitrate being most usually employed.  In the
+laboratory a nearly saturated solution of sulphate of copper with 1 or
+2 per cent of sulphuric acid will answer most purposes.  A current
+density of, at most, fifteen amperes per square foot may be used,
+either for obtaining solid deposits for constructional purposes or for
+calibrating current measuring instruments by electrolysis.  A copper
+anode is of course employed.
+
+When coppering with a view to obtaining thick deposits it is a good
+plan to place the electrodes several inches apart, and, if possible,
+to keep the liquid stirred, as there is a considerable tendency on the
+part of copper deposits to grow out into mossy masses wherever the
+current density exceeds the limit mentioned.  As the masses grow
+towards the anode the defect naturally tends to increase of itself,
+hence the necessity for care.  The phenomenon is particularly marked
+at the edges and corners of the cathode.
+
+If the deposit becomes markedly irregular, the best plan is to stop
+the process and file the face of the deposit down to approximate
+smoothness.  In coppering it is of the utmost importance that the
+cathode be clean and free from grease; it must never be touched (by
+the finger, for instance) from the time it is scratch-brushed till it
+is immersed in the plating bath.  Any grease or oxidation tends to
+prevent the copper deposit adhering properly.
+
+A copper deposit oxidises very easily when exposed to the air.
+Consequently if the surface be required free from oxide, as, for
+instance, when it is to be silvered or gilt, it must be quickly washed
+when withdrawn from the coppering bath, scratch-brushed, and
+transferred immediately to the silvering or gilding bath.
+
+If the surface is to be dried, as in electrolysis calibrations, it
+must be rinsed quickly with boiling water and pressed between sheets
+of filter paper.  Another method which has been recommended is to
+rinse the copper in--water slightly acidulated with sulphuric acid
+(which prevents oxidation), then in distilled water, and to dry by
+blotting paper and in front of a fire, taking care not to make the
+plate too hot.  The wash water is sufficiently acidulated by the
+addition of two or three drops of acid per litre.  So far as I know,
+the method of washing in acidulated water was first proposed by Mr. T.
+Gray.
+
+§ 138.  Coppering Aluminium.
+
+A good adherent deposit of copper on aluminium used to be considered a
+desideratum in the days when it afforded the only means of soldering
+the latter.  Many receipts have been published from time to time, and
+I have tried, I think, most of them.  On no occasion, however, till
+this year (1896), have I succeeded in obtaining a deposit which would
+not strip after it was tinned and soldered, though it is not difficult
+to get apparently adherent deposits so long as they are not operated
+upon by the soldering iron.  The best of the many solutions which have
+been proposed in years gone by is very dilute cupric nitrate with
+about 5 per cent of free nitric acid.
+
+The problem of electroplating aluminium which I have indicated as
+awaiting a solution has at last found one.  In the Archives des
+Sciences physiques et naturelles de Genève for December 1895 (vol.
+xxxiv.  p.  563) there is a paper by M. Margot on the subject, which
+discloses a perfectly successful method of plating aluminium with
+copper.  The paper itself deals in an interesting way with the theory
+of the matter--however, the result is as follows.
+
+(1) The aluminium articles are boiled for a few minutes in a strong
+solution of ordinary washing soda.  The aluminium surface is thus
+corroded somewhat, and rendered favourable to the deposit of an
+adherent film of copper.  After removal from the soda solution the
+aluminium is well washed and brushed in running water.
+
+(2) The articles are dipped for thirty seconds or so in a hot 5 per
+cent solution of pure hydrochloric acid.
+
+(3) After dipping in the hydrochloric acid, the work is instantly
+plunged into clean water for about one second, so as to remove nearly,
+but not quite, all of the aluminium chloride.
+
+(4) The work is transferred to a cold dilute (say 5 per cent) solution
+of cupric sulphate slightly acidulated with sulphuric acid.  The
+degree of acidulation does not appear to be very important, but about
+one-tenth per cent of strong acid does well.
+
+If the preliminary processes have been properly carried out the
+aluminium will become coated with copper, and the process is
+accompanied by the disengagement of gas.  It appears to be a rule that
+if gas is not given off, the film of copper deposited is non-adherent.
+The work must be left in the copper sulphate solution till it has
+received a uniform coating of copper.
+
+(5) When this is the case the work is removed--well washed so as to
+get rid of the rest of the aluminium chloride, and then electroplated
+by the battery in the ordinary copper sulphate bath.
+
+If the operation (4) does not appear to give a uniform coat, or if gas
+is not evolved from every part of the aluminium surface, I find that
+operations (2) and (3) may be repeated without danger, provided that
+the dip in the hydrochloric acid is shortened to two or three
+seconds.
+
+The copper layer obtained by Margot's method is perfectly
+adherent--even when used as a base for ordinary solder--though in
+this case it can be stripped if sufficient force is applied.
+
+Since the solder recommended by M. Margot for aluminium contains zinc,
+it does not run well when used to unite aluminium to copper, brass,
+iron, etc.  In this case, therefore, I have found the most
+advantageous method of soldering to be by way of a preliminary
+copper-plating.
+
+The success of M. Margot's method depends in my experience on
+obtaining just the proper amount of aluminium chloride in contact with
+the aluminium when the latter is immersed in the copper sulphate
+solution.
+
+§ 139.  The process of copper-plating from sulphate or nitrate may,
+according to Mr. Swan (Journal of the Royal Institution, 1892, p.
+630), be considerably accelerated by the addition of a trace of
+gelatine to the solution.  As success appears to depend upon hitting
+the exact percentage amount of the gelatine, which must in any case be
+but a fraction of one per cent, and as Mr. Swan refrains from stating
+what the amount is, I am unable to give more precise instructions.  A
+few experiments made on the subject failed, doubtless through the
+gelatine content not having been rightly adjusted.  Mr. Swan claims to
+be able to get a hard deposit of copper with a current density of 1000
+amperes per square foot, but seems to recommend about one-tenth of
+that amount for general use.
+
+The solution employed is a mixture of nitrate of copper and ammonium
+chloride--proportions not stated.  Electrolytic copper, as generally
+prepared, is very pure, but this is a mere accident depending on the
+impurities which, as a rule, have to be got rid of.  Electrolysis
+seems to have no effect in purifying from arsenic, for instance.
+
+Roughly speaking, about 11 grms.  of copper are deposited per ampere
+hour from cupric salt solutions.  When the current density is too high
+the anode suffers by oxidation, and this introduces a large and very
+variable resistance into the circuit.
+
+§ 140.  Alkaline Coppering Solution
+
+Coppering Base Metals. It is often desirable to coat lead, zinc,
+pewter, iron, etc, with a firm and uniform layer of copper
+preparatory to gilding or silvering.  If copper or brass articles are
+soldered with soft solder it is found that the solder does not become
+silvered or gilt along with the rest of the material, but remains
+uncoated and of an ugly dark colour.  This defect is got over by
+giving a preliminary coating of copper.
+
+This is done in an alkaline solution, generally containing cyanogen
+and ammonia.  The following method has succeeded remarkably well with
+me.  The receipt was taken originally from Gore's Electro-metallurgy,
+p.  208.  A solution is made of 50 grms.  of potassium cyanide
+(ordinary commercial, say, 75 per cent) and 30 grms.  of sodium
+bisulphite in I.5 litres of water.  Thirty-five grammes of cupric
+acetate are dissolved in a litre of water, and 20 cubic centimetres of
+the strongest liquid ammonia are added.  The precipitate formed must
+be more or less dissolved to a strong blue solution.  The cyanide and
+bisulphite solution is then added with warming till the blue colour is
+destroyed.  This usually requires the exact amount of cyanide and
+bisulphite mentioned, but I have not found it essential to entirely
+destroy the colour.
+
+The solution contains cuprocyanide of sodium and ammonium (?), which
+is not very soluble, and this salt tends to be deposited in granular
+crystalline masses on standing.  However, at a temperature of 50° C.
+the above receipt gives an excellent coppering liquid, which will coat
+zinc with a fine reguline deposit.  Brass or copper partly smeared
+with solder will receive a deposit of copper on the latter as well as
+on the former, and, moreover, a deposit which appears to be perfectly
+uniform.
+
+In using the bath the anode tends, as a rule, to become incrusted, and
+this rapidly increases the resistance of the cell, so that the current
+falls off quickly.  The articles should be scratch-brushed and plated
+for about two minutes with a current density of about ten ampères per
+square foot.
+
+As soon as the deposit begins to look red the articles are to be
+removed and rebrushed, after which the process may be continued.
+About five minutes' plating will give a copper deposit quite thick
+enough after scratch-brushing to allow of a very even gilding or
+silvering.
+
+Aluminium appears to be fairly coated, but, as usual, the copper
+strips after soldering.  Iron receives an excellent and adherent coat.
+
+I do not think that the formation of a crust upon the anode can be
+entirely prevented.  According to Gore, its formation is due to the
+solution being too poor in copper, but I have added a solution of the
+acetate of copper and ammonium till the colour was bright blue without
+in any way reducing the incrustation.  If the solutions become
+violently blue it is perhaps as well to add a little more cyanide and
+bisulphite, but I have not found such an addition necessary.  The
+process is one of the easiest and most satisfactory in
+electro-metallurgy.
+
+§ 141.  Nickel-plating.
+
+An examination of several American samples of nickel-plated goods has
+disclosed that the coating of nickel is, as a rule, exceedingly thin.
+This is what one would expect from laboratory repetition of the
+processes employed.
+
+Commercial practice in the matter of the composition of nickelling
+solutions appears to vary a good deal.  Thin coatings of nickel may be
+readily given in a solution of the double sulphate of nickel and
+ammonia, which does rather better if slightly alkaline.  Deposits from
+this solution, however, become gray if of any thickness, and,
+moreover, are-apt to flake off the work.  The following solution has
+given very good results with me.  It is mentioned, together with
+others, in the Electrical Review, 7th June 1895.
+
+The ingredients are:-
+
+Nickel sulphate            5 parts
+
+Ammonia sufficient to neutralise the nickel salt.
+
+Ammonium tartrate        3.75 parts
+
+Tannin                0.025 parts
+
+Water                    100 parts
+
+The nickel sulphate and ammonia are dissolved in half the water, the
+ammonium tartrate in the other half with the tannin.  The solutions
+are mixed and filtered at about 40° C.  This solution works well at
+ordinary temperatures, or slightly warm, with a current density of ten
+ampères per square foot.  In an experiment made for the purpose I
+found that plating may go on for an hour in this solution before the
+deposit begins to show signs of flaking off.  The deposit is of a fine
+white colour.
+
+The resistance of the bath is rather high and rather variable,
+consequently it is as well to have a current indicator in circuit, and
+it may well happen that five or six volts will be found requisite to
+get the current up to the value stated.  For nickelling small objects
+of brass, such as binding screws, etc, it is very necessary to be
+careful as to the state of polish and uniformity of their surfaces
+before placing them in the plating bath.  A polished surface will
+appear when coated as a polished surface, and a mat surface as a mat
+surface;  moreover, any local irregularity, such as a speck of a
+foreign metal, will give rise to an ugly spot in the nickelling bath.
+For this reason it is often advisable to commence with a coat of
+copper laid on in an alkaline solution and scratch-brushed to absolute
+uniformity.
+
+An examination of the work will, however, disclose whether such a
+course is desirable or not;  it is not done in American practice, at
+all events for small brass objects.  These are cleaned in alkali and
+in boiling cyanide, which does not render a polished surface mat, as
+weak acid is apt to do, and are then coated with a current density of
+about ten ampères per square foot.
+
+In spite of what is to be found in books as to the ease with which
+nickel deposits may be polished, I find that the mat surface obtained
+by plating on an imperfectly polished cathode of iron is by no means
+easily polished either by fine emery, tripoli, or rouge.
+Consequently, as in the case of brass, if a polished surface is
+desired, it must be first prepared on the unplated cathode.  In this
+case, even if the deposit appears dull, but not gray, it may be easily
+polished by tripoli and water, using a cork as the polisher.
+Scratch-brushing with brass wire, however, though possibly not with
+German silver wire, brightens the deposit, but discolours it.  When
+the deposit becomes gray I have not succeeded in polishing it
+satisfactorily.
+
+Soldered brass or iron may be satisfactorily coated with nickel by
+giving it a preliminary coating of copper in the cyanide bath.  On the
+whole, I recommend in general that iron be first coated with copper in
+the alkaline bath, scratch-brushed, and then nickel-plated, and this
+whether the iron appears to be uniform or not.  Much smoother,
+thicker, and stronger coats of nickel are obtained upon the
+copper-plated surface than on the iron one, and the coating does not
+become discoloured (?  by iron rust) in the same way that a coating on
+bare iron does.  The copper surface may be plated for at least an hour
+at a density of ten ampères per square foot without scaling.
+
+Scales or circles divided on brass may be greatly improved in
+durability by nickel--plating.  For this purpose the brass must be
+highly polished and divided before it is nickelled.
+
+The plating should be continued for a few minutes only, when a very
+bright but thin coat of nickel will be deposited;  it then only
+remains to wash and dry the work, and this must be done at once.  If
+the nickel is deposited before the scale or circle is engraved, very
+fine and legible divisions are obtained, but there is a risk that
+flakes of nickel may become detached here and there in the process of
+engraving.
+
+142.  Miscellaneous Notes on Electroplating.
+
+Occasionally it is desirable to make a metallic mould or other object
+of complex shape.  The quickest way to do this is to carve the object
+out of hard paraffin, and then copy it by electrotyping.  Electrotype
+moulds can be made in many ways.  The easiest way perhaps is to take a
+casting in plaster of Paris, or by means of pressure in warm
+gutta-percha.
+
+In cases where the mould will not draw, recourse must be had to the
+devices of iron-founders, i.e.  the plaster cast must be made in
+suitable pieces, and these afterwards fitted together.  This process
+can occasionally be replaced by another in which the moulding material
+is a mixture of treacle and glue.  The glue is soaked in cold water
+till it is completely soft.  The superfluous water thrown away,
+one-fourth part by volume of thick treacle is added, and the mixture
+is melted on the water bath;  during which process stirring has to be
+resorted to, to produce a uniform mixture.
+
+This liquid forms the moulding mixture, and it is allowed to flow
+round the object to be copied, contained in a suitable box, whose
+sides have been slightly oiled.  The object to be copied should also
+be oiled.  After some hours, when the glue mixture has set, it will be
+found to be highly elastic, so that it may be pulled away from the
+mould, and afterwards resume very nearly its original form.
+
+One drawback to the use of these moulds lies in the fact that the
+gelatine will rarely stand the plating solution without undergoing
+change, but this may be partially obviated by dipping it for a few
+seconds in a 10 per cent solution of bichromate of potash, exposing it
+to the sunlight for a few minutes, and then rinsing it.
+
+In order to render the surface conducting, it is washed over with a
+solution of a gold or silver salt, and the latter reduced in situ to
+metal by a suitable reagent.  A solution of phosphorus is the most
+usual one (see Gore, Electro-metallurgy, p.  216).  Such a mould may
+be copper-plated in the sulphate bath, connection being made by wires
+suitably thrust into the material.
+
+Plaster of Paris moulds require to be dried and waxed by standing on a
+hot plate in melted wax before they are immersed in the plating bath.
+In this case the surface is best made conducting either by silvering
+it by the silvering process used for mirrors, or by brushing it over
+with good black lead rendered more conducting by moistening with an
+ethereal solution of chloride of gold and then drying in the sun.
+
+The brushing requires a stiff camel's-hair pencil of large size cut so
+that the hairs project to a distance of about a quarter of an inch
+from the holder.  The brushing must continue till the surface is
+bright, and is often a lengthy process.
+
+The same process of blackleading may be employed to get a coat of
+deposited metal which will strip easily from the cathode.
+
+In all cases where extensive deposits of copper are required, the
+growth takes place too rapidly at the corners.  Consequently it is
+often desirable to localise the action of the deposit.  A "stopping"
+of ordinary copal varnish seems to be the usual thing, but a thin coat
+of wax or paraffin or photographic (black) varnish does practically as
+well.
+
+I do not propose to deal with the subject of electrotyping to any
+extent, for if practised as an art, a good many little precautions are
+required, as the student may read in Gore's Electro-metallurgy.  The
+above instructions will be found sufficient for the occasional use of
+the process in the construction of apparatus, etc.  There is no
+advantage in attempting to hurry the process, a current density of
+about ten ampères per square foot being quite suitable and
+sufficiently low to ensure a solid deposit.
+
+§ 143.  Blacking Brass Surfaces.
+
+A really uniform dead-black surface is difficult to produce on brass
+by chemical means.  A paste of nitrate of copper and nitrate of silver
+heated on the brass is said to give a dead-black surface, but I have
+not succeeded in making it act uniformly.  For optical purposes the
+best plan is to use a paint made up of  "drop" black, ground very fine
+with a little shellac varnish, and diluted for use with alcohol.  No
+more varnish than is necessary to cause the black to hold together
+should be employed.
+
+In general, if the paint be ground to the consistency of very thick
+cream with ordinary shellac varnish it will be found to work well when
+reduced by alcohol to a free painting consistency.
+
+A very fine gray and black finish, with a rather metallic lustre, may
+be easily given to brass work.  For this purpose a dilute solution of
+platinum tetrachloride (not stronger than 1 per cent) is prepared by
+dissolving the salt in distilled water.  The polished brass work is
+cleaned by rubbing with a cork and strong potash till all grease has
+disappeared, as shown by water standing uniformly on the metal and
+draining away without gathering into drops.
+
+After copious washing the work is wholly immersed in a considerable
+volume of the platinum tetrachloride solution at the ordinary
+temperature.  After about a quarter of an hour the brass may be taken
+out and washed.  The surface will be found to be nicely and uniformly
+coated if the above instructions have been carried out, but any
+finger-marks or otherwise dirty places will cause irregularity of
+deposit.  If the process has been successful it will be found that the
+deposit adheres perfectly, hardly any of it being removed by vigorous
+rubbing with a cloth.  If the deposit is allowed to thicken--either
+by leaving the articles in the solution too long or heating the
+solution, or having it too strong--it will merely rub off and leave
+an irregular surface.
+
+This process succeeds well with yellow brass and Muntz metal, either
+cast or rolled, but it does not give quite such uniform (though still
+good) results with gun-metal, on which, however, the deposit is darker
+and deader in appearance.
+
+A book might be written (several have been written) on the art of
+metal colouring, but though doubtless a beautiful and delicate art, it
+is of little service in the laboratory.  For further information the
+reader may consult a work by Hiorns.
+
+§ 144.  Sieves.
+
+Properly graded sieves with meshes of a reliable size are often of
+great use.  They should be made out of proper "bolting" cloth, a
+beautiful material made for flour-millers.  Messrs. Henry Simon and
+Company of Manchester have kindly furnished me with the following
+table of materials used in flour-milling.
+
+Sieves made of these materials will be found to work much more quickly
+and satisfactorily than those made from ordinary muslin or wire gauze.
+
+Relative Bolting Value of Silk, Wire, and Grit Gauze
+
+Threads per inch   Trade  No.   Trade  No.  Trade No. of
+Approximate.        of Silk.     of Wire.    Grit Gauze.
+
+18                  0000            18            16
+
+22                   000            20            20
+
+28                    00            26            26
+
+38                     0            32            34
+
+48                     1            40            44
+
+52                     2            45            50
+
+56                     3            50            54
+
+60                     4            56            58
+
+64                     5            60            60
+
+72                     6            64            66
+
+80                     7            70            70
+
+84                     8            80            80
+
+94                     9
+
+106                   10
+
+114                   11
+
+124                   12
+
+130                   13
+
+139                   14
+
+148                   15
+
+156                   16
+
+163                   17
+
+167                   18
+
+170                   19
+
+173                   20
+
+
+§ 145.  Pottery making in the Laboratory.
+
+When large pieces of earthenware of any special design are required,
+recourse must be had to a pottery.  Small vessels, plates, parts of
+machines, etc, can often be made in the laboratory in less time than
+it would take to explain to the potter what is required.  For this
+purpose any good pipeclay may be employed.  I have used a white
+pipe-clay dug up in the laboratory garden with complete success.
+
+The clay should be kneaded with water and squeezed through a cloth to
+separate grit.  It is then mixed with its own volume or thereabouts of
+powdered porcelain evaporating basins, broken basins being kept for
+this purpose.  The smoothness of the resulting earthenware will depend
+on the fineness to which the porcelain fragments have been reduced.  I
+have found that fragments passing a sieve of sixty threads to the inch
+run, do very well, though the resulting earthenware is decidedly
+rough.
+
+The porcelain and clay being thoroughly incorporated by kneading, the
+articles are moulded, it being borne in mind that they will contract
+somewhat on firing. [Footnote: The contraction depends on the
+temperature attained as well as on the time.  An allowance of one part
+in twelve will be suitable in the case considered.]  The clay should
+be as stiff as is convenient to work, and after moulding must be
+allowed to get thoroughly dry by standing in an airy place;  the
+drying must not be forced, especially at first, or the clay will
+crack.
+
+Small articles are readily fired in a Fletcher's crucible furnace
+supplied with a gas blow-pipe;  the furnace is heated gradually to
+begin with.  When a dull red heat is attained, the full power of the
+blast may be turned on, and the furnace kept at its maximum
+temperature for three or four hours at least, though on an emergency
+shorter periods may be made to do.
+
+The articles are supported on a bed of white sand;  after firing, the
+crucible furnace must be allowed to cool slowly.  It must be
+remembered that the furnace walls will get hot externally after the
+first few hours, consequently the furnace must be supported on bricks,
+to protect the bench.
+
+The pottery when cold may be dressed on a grindstone if necessary.
+This amateur pottery will be found of service in making small fittings
+for switch-boards, commutators, and in electrical work generally.
+
+Pottery made as described is very hard and strong, the hardness and
+strength depending in a great degree  on the proportion of powdered
+porcelain added to the clay, as well, of course, as on the quality of
+both of these materials.
+
+It is a good plan to knead a considerable quantity of the mixture,
+which may then be placed in a well-covered jar, and kept damp by the
+addition of a little water.
+
+Pottery thus made does not require to be glazed, but, of course, a
+glaze can be obtained by any of the methods described in works on
+pottery manufacture.  The following glaze has been recommended to me
+by a very competent potter:-
+
+Litharge
+
+7 parts by weight
+
+Ground flint
+
+2 parts by weight
+
+Cornish stone or felspar
+
+1 parts by weight
+
+These ingredients are to be ground up till they will pass the finest
+sieve--say 180 threads to the inch.  They are then mixed with water
+till they form a paste of the consistency of cream. They must, of
+course, be mixed together perfectly.  The ware to be glazed is dipped
+into the cream after the first firing; it is then dried as before and
+refired.  The glaze will melt at a bright red heat, but it will crack
+if not fired harder;  the harder it is fired the less likely is it to
+crack.
+
+If colouring matters are added they must be ground in a mill free from
+iron till they are so fine that a thick blanket filter will not filter
+them when suspended in water.  This remark applies particularly to
+oxide of cobalt.
+
+APPENDIX
+
+PLATINISING GLASS
+
+IN the Philosophical Magazine for July 1888 (vol.  xxvi.  p.  1) there
+is a paper by Professor Kundt translated from the Sitzungsberichte of
+the Prussian Academy.  This paper deals with the indices of refraction
+of metals.  Thin prisms were obtained by depositing metals
+electrolytically on glass surfaces coated with platinum. The
+preparation of these surfaces is troublesome.  Kundt recounts that no
+less than two thousand trials were made before success was attained.
+A detailed account of the preparation of these surfaces is not given
+by Kundt, but one is promised--a promise unfortunately unfulfilled so
+far as I am able to discover.  A hunt through the literature led to
+the discovery of the following references:  Central Zeitung fuer Optik
+und Mechanik, p.  142 (1888); Dingler's Polytechnik Journal, Vol.
+cxcv.  p.  464; Comptes Rendus, vol.  lxx.  (1870).
+
+The original communication is a paper by Jouglet in the Comptes
+Rendus, of which the other references are abstracts.  The account in
+Dingier is a literal translation of the original paper, and the note
+in the Central Zeitung is abbreviated sufficiently to be of no value.
+The details are briefly as follows:-
+
+One hundred grams of platinum are dissolved in aqua regia and the
+solution is dried on the sand bath, without, however, producing
+decomposition.  Though the instructions are not definite, I presume
+that the formation of PtCl4 is contemplated.
+
+The dried salt is added little by little to rectified oil of lavender,
+placed on a glass paint-grinding plate, and the salt and oil are
+ground together with a muller.  Care is required to prevent any
+appreciable rise of temperature which would decompose the compound
+aimed at, and it is for this reason that the salt is to be added
+gradually.  Of course the absorption of water from the air must be
+prevented from taking place as far as possible.  Finally, the compound
+is diluted by adding oil of lavender up to a total weight of 1400
+grams (of oil).
+
+The liquid is poured into a porcelain dish and left absolutely at rest
+for eight days.  It is then decanted and filtered, left six days at
+rest, and again decanted (if necessary).  The liquid should have a
+specific gravity of 5° on the acid hydrometer.  (If by this the Baumé
+scale is intended, the corresponding specific gravity would be 1.037.)
+A second liquid is prepared by grinding up 25 grams of litharge with
+25 grams of borate of lead and 8 to 10 grams of oil of lavender.  The
+grinding must be thoroughly carried out.
+
+This liquid is to be added to the one first described, and the whole
+well mixed.  The resulting fluid constitutes the platinising liquid,
+and is applied as follows:-
+
+A sheet of clean glass is held vertically, and the liquid is painted
+over it, carrying the brush from the lower to the upper edge.  The
+layer of oil dries slowly, and when it is dry the painting is again
+proceeded with, moving the brush this time from right to left; and
+similarly the process is repeated twice, the brush being carried from
+top to bottom and left to right.  This is with the object of securing
+great uniformity in the coating.  Nothing is said as to the manner in
+which the glass is to be dried.
+
+The dried glass is finally heated to a temperature of dull redness in
+a muffle furnace.  The resinous layer burns away without running or
+bubbling, and leaves a dull metallic surface.  As the temperature
+rises this suddenly brightens, and we obtain the desired surface
+(which probably consists of an alloy of lead and platinum).  It is
+bright only on the surface away from the glass.
+
+I have not had an opportunity of trying this process since I
+discovered the detailed account given by Jouglet;  but many
+modifications have been tried in the laboratory of the Sydney
+University by Mr. Pollock, starting from the imperfect note in the
+Central Zeitung, which led to no real success.
+
+It was found that it is perfectly easy to obtain brilliant films of
+platinum by the following process, provided that the presence of a few
+pin-holes does not matter.
+
+The platinum salt employed is what is bought under the name of
+platinic chloride;  it is, however, probably a mixture of this salt
+and hydro-chloro-platinic acid, and has all the appearance of having
+been obtained by evaporating a solution of platinum in aqua regia to
+dryness on the water bath.  A solution of this salt in distilled water
+is prepared;  the strength does not seem to matter very much, but
+perhaps one of salt to ninety-nine water may be regarded as a standard
+proportion.  To this solution is added a few drops of ordinary gum
+water (i.e.  a solution of dextrin).  The exact quantity does not
+matter, but perhaps about 2 per cent may be mentioned as giving good
+results.
+
+The glass is painted over with this solution and dried slowly on the
+water bath.  When the glass is dry, and covered with a uniform hard
+film of gum and platinum salt free from bubble holes, it is heated to
+redness in a muffle furnace.  The necessary and sufficient temperature
+is reached as soon as the glass is just sensibly red-hot.
+
+The mirrors obtained in this way are very brilliant on the free
+platinum surface.  If the gum be omitted, the platinum will have a mat
+surface;  and if too much gum be used, the platinum will get spotty by
+bubbles bursting.  There does not appear to be any advantage in using
+lead.
+
+It is quite essential that the film be dry and hard before the glass
+is fired.
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
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+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
+<html>
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
+<title>ON LABORATORY ARTS</title>
+</head>
+<body>
+
+
+<pre>
+
+The Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: On Laboratory Arts
+
+Author: Richard Threlfall
+
+Release Date: September 27, 2007 [EBook #22784]
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ON LABORATORY ARTS ***
+
+
+
+
+Produced by Jon Richfield
+
+
+
+
+
+</pre>
+
+<p align="center"><b><font face="Bookman Old Style" size="6">ON
+LABORATORY ARTS</font></b></p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"><font face="Bookman Old Style" size=
+"2">BY</font></p>
+<p align="center"> </p>
+<p align="center"><b><font face="Bookman Old Style" size=
+"4">RICHARD THRELFALL, M.A.</font></b></p>
+<p align="center"> </p>
+<p align="center"><font face="Bookman Old Style" size=
+"2">PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;</font></p>
+<p align="center">MEMBER OF THE INSTITUTE OF ELECTRICAL
+ENGINEERS;</p>
+<p align="center">ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL
+ENGINEERS;</p>
+<p align="center">MEMBER OF THE PHYSICAL SOCIETY</p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center"> </p>
+<p align="center">London</p>
+<p align="center">MACMILLAN AND CO., LIMITED</p>
+<p align="center"><font face="Bookman Old Style" size="2">NEW
+YORK: THE MACMILLAN COMPANY</font></p>
+<p align="center"><font face="Bookman Old Style">1898</font></p>
+<p align="center"><font face="Bookman Old Style" size="1">All
+rights reserved</font></p>
+<p><b><font size="2">PREFACE</font></b> <a href=
+"#Toc158108886">*</a></p>
+<p><b><font size="2">CHAPTER I</font></b> <a href=
+"#Toc158108887">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">HINTS ON THE MANIPULATION OF GLASS AND ON
+GLASS-BLOWING FOR LABORATORY PURPOSES</font> <a href=
+"#Toc158108888">*</a></p>
+<p><font size="2">&sect; 4. Soft Soda Glass,</font> <a href=
+"#Toc158108889">*</a></p>
+<p><font size="2">&sect; 6. Flint Glass. &mdash;</font> <a href=
+"#Toc158108890">*</a></p>
+<p><font size="2">&sect; 9. Hard or Bohemian, Glass.
+&mdash;</font> <a href="#Toc158108891">*</a></p>
+<p><font size="2">&sect; 10. On the Choice of Sizes of Glass
+Tube. &mdash;</font> <a href="#Toc158108892">*</a></p>
+<p><font size="2">&sect; 11. Testing Glass. &mdash;</font>
+<a href="#Toc158108893">*</a></p>
+<p><font size="2">&sect; 13. Cleaning Glass Tubes. &mdash;</font>
+<a href="#Toc158108894">*</a></p>
+<p><font size="2">&sect; 14. The Blow-pipe. &mdash;</font>
+<a href="#Toc158108895">*</a></p>
+<p><font size="2">&sect; 18. <i>The Table</i>. &mdash;</font>
+<a href="#Toc158108896">*</a></p>
+<p><font size="2">&sect; 19. Special Operations. &mdash;</font>
+<a href="#Toc158108897">*</a></p>
+<p><font size="2">&sect; 20. Closing and blowing out the End of a
+Tube. &mdash;</font> <a href="#Toc158108898">*</a></p>
+<p><font size="2">&sect; 21. To make a Weld. &mdash;</font>
+<a href="#Toc158108899">*</a></p>
+<p><font size="2">&sect; 22. To weld two Tubes of different
+Sizes. &mdash;</font> <a href="#Toc158108900">*</a></p>
+<p><font size="2">&sect; 24. To weld Tubes of very small Bore.
+&mdash;</font> <a href="#Toc158108901">*</a></p>
+<p><font size="2">&sect; 30. To cut very thick Tubes.</font>
+<a href="#Toc158108902">*</a></p>
+<p><font size="2">&sect; 31. To blow a Bulb at the End of a Tube.
+&mdash;</font> <a href="#Toc158108903">*</a></p>
+<p><font size="2">&sect; 32. To blow a bulb in the middle of a
+tube,</font> <a href="#Toc158108904">*</a></p>
+<p><font size="2">&sect; 33. To make a side Weld. &mdash;</font>
+<a href="#Toc158108905">*</a></p>
+<p><font size="2">&sect; 34. Inserted Joints. &mdash;</font>
+<a href="#Toc158108906">*</a></p>
+<p><font size="2">&sect; 35. Bending Tubes. &mdash;</font>
+<a href="#Toc158108907">*</a></p>
+<p><font size="2">&sect; 36. Spiral Tubes. &mdash;</font>
+<a href="#Toc158108908">*</a></p>
+<p><font size="2">&sect; 37. On Auxiliary Operations on Glass:-
+</font> <a href="#Toc158108909">*</a></p>
+<p><font size="2">&sect; 38. Boring small Holes. &mdash;</font>
+<a href="#Toc158108910">*</a></p>
+<p><font size="2">&sect; 39. For boring large holes through thick
+glass sheets,</font> <a href="#Toc158108911">*</a></p>
+<p><font size="2">&sect; 41. Operations depending on Grinding:
+Ground-in Joints. &mdash;</font> <a href=
+"#Toc158108912">*</a></p>
+<p><font size="2">&sect; 42. Use of the Lathe in Glass-working.
+&mdash;</font> <a href="#Toc158108913">*</a></p>
+<p><font size="2">&sect; 46. Making Ground Glass. &mdash;</font>
+<a href="#Toc158108914">*</a></p>
+<p><font size="2">&sect; 47. Glass-cutting. &mdash;</font>
+<a href="#Toc158108915">*</a></p>
+<p><font size="2">&sect; 48. <i>Cementing</i>. &mdash;</font>
+<a href="#Toc158108916">*</a></p>
+<p><font size="2">&sect; 49. Fusing Electrodes into Glass.
+&mdash;</font> <a href="#Toc158108917">*</a></p>
+<p><font size="2">&sect; 51. The Art of making Air-tight Joints.
+&mdash;</font> <a href="#Toc158108918">*</a></p>
+</div>
+<p><b><font size="2">APPENDIX TO CHAPTER I</font></b> <a href=
+"#Toc158108919">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">ON THE PREPARATION OF VACUUM TUBES FOR THE
+PRODUCTION OF PROFESSOR ROENTGEN'S RADIATION</font> <a href=
+"#Toc158108920">*</a></p>
+</div>
+<p><b><font size="2">CHAPTER II</font></b> <a href=
+"#Toc158108921">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">GLASS-GRINDING AND OPTICIANS' WORK</font>
+<a href="#Toc158108922">*</a></p>
+<p><font size="2">&sect; 61. Details of the Process of Fine
+Grinding. &mdash;</font> <a href="#Toc158108923">*</a></p>
+<p><font size="2">&sect; 62. <i>Polishing</i>. &mdash;</font>
+<a href="#Toc158108924">*</a></p>
+<p><font size="2">&sect; 63. <i>Centering</i>. &mdash;</font>
+<a href="#Toc158108925">*</a></p>
+<p><font size="2">&sect; 65. Preparing Small Mirrors for
+Galvanometers. &mdash;</font> <a href="#Toc158108926">*</a></p>
+<p><font size="2">&sect; 66. Preparation of Large Mirrors or
+Lenses for Telescopes. &mdash;</font> <a href=
+"#Toc158108927">*</a></p>
+<p><font size="2">&sect; 69. The Preparation of Flat Surfaces of
+Rock Salt. &mdash;</font> <a href="#Toc158108928">*</a></p>
+<p><font size="2">&sect; 70. Casting Specula for Mirrors.
+&mdash;</font> <a href="#Toc158108929">*</a></p>
+<p><font size="2">&sect; 71. Grinding and polishing Specula.
+&mdash;</font> <a href="#Toc158108930">*</a></p>
+<p><font size="2">&sect; 72. Preparation of Flat Surfaces.
+&mdash;</font> <a href="#Toc158108931">*</a></p>
+<p><font size="2">&sect; 73. Polishing Flat Surfaces on Glass or
+on Speculum Metal. &mdash;</font> <a href=
+"#Toc158108932">*</a></p>
+</div>
+<p><b><font size="2">CHAPTER III</font></b> <a href=
+"#Toc158108933">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">MISCELLANEOUS PROCESSES</font> <a href=
+"#Toc158108934">*</a></p>
+<p><font size="2">&sect; 74. Coating Glass with Aluminium and
+Soldering Aluminium. &mdash;</font> <a href=
+"#Toc158108935">*</a></p>
+<p><font size="2">&sect; 75. The Use of the Diamond-cutting
+Wheel. &mdash;</font> <a href="#Toc158108936">*</a></p>
+<p><font size="2">&sect; 76. Arming a Wheel. &mdash;</font>
+<a href="#Toc158108937">*</a></p>
+<p><font size="2">&sect; 77. Cutting a Section. &mdash;</font>
+<a href="#Toc158108938">*</a></p>
+<p><font size="2">&sect; 78. Grinding Rock Sections, or Thin
+Slips of any Hard Material.&mdash;</font> <a href=
+"#Toc158108939">*</a></p>
+<p><font size="2">&sect; 79. Cutting Sections of Soft Substances.
+&mdash;</font> <a href="#Toc158108940">*</a></p>
+<p><font size="2">&sect; 80. On the Production of Quartz
+Threads.' &mdash;</font> <a href="#Toc158108941">*</a></p>
+<p><font size="2">&sect; 84. Drawing Quartz Threads.
+&mdash;</font> <a href="#Toc158108942">*</a></p>
+<p><font size="2">&sect; 86. Drawing Threads by the Catapult.
+&mdash;</font> <a href="#Toc158108943">*</a></p>
+<p><font size="2">&sect; 87. Drawing Threads by the Flame alone.
+&mdash;</font> <a href="#Toc158108944">*</a></p>
+<p><font size="2">&sect; 88. Properties of Threads.
+&mdash;</font> <a href="#Toc158108945">*</a></p>
+<p><font size="2">&sect; 90. On the Attachment of Quartz Fibres.
+&mdash;</font> <a href="#Toc158108946">*</a></p>
+<p><font size="2">&sect; 91. Other Modes of soldering Quartz.
+&mdash;</font> <a href="#Toc158108947">*</a></p>
+<p><font size="2">&sect; 92. Soldering. &mdash;</font> <a href=
+"#Toc158108948">*</a></p>
+<p><font size="2">&sect; 94. Preparing a Soldering Bit.
+&mdash;</font> <a href="#Toc158108949">*</a></p>
+<p><font size="2">&sect; 95. Soft Soldering. &mdash;</font>
+<a href="#Toc158108950">*</a></p>
+<p><font size="2">&sect; 97. Soldering Zinc. &mdash;</font>
+<a href="#Toc158108951">*</a></p>
+<p><font size="2">&sect; 98. Soldering other Metals
+&mdash;</font> <a href="#Toc158108952">*</a></p>
+<p><font size="2">&sect; 99. <i>Brazing</i>.</font> <a href=
+"#Toc158108953">*</a></p>
+<p><font size="2">&sect; 100. Silver Soldering. &mdash;</font>
+<a href="#Toc158108954">*</a></p>
+<p><font size="2">&sect; 101. On the Construction of Electrical
+Apparatus - Insulators. &mdash;</font> <a href=
+"#Toc158108955">*</a></p>
+<p><font size="2">&sect; 102. Sulphur. &mdash;</font> <a href=
+"#Toc158108956">*</a></p>
+<p><font size="2">&sect; 103. <i>Fused Quartz</i>. &mdash;</font>
+<a href="#Toc158108957">*</a></p>
+<p><font size="2">&sect; 104. <i>Glass.</i> &mdash;</font>
+<a href="#Toc158108958">*</a></p>
+<p><font size="2">&sect; 105. Ebonite or Hard Rubber.
+&mdash;</font> <a href="#Toc158108959">*</a></p>
+<p><font size="2">&sect; 106. <i>Mica</i>. &mdash;</font>
+<a href="#Toc158108960">*</a></p>
+<p><font size="2">&sect; 107. Use of Mica in Condensers.
+&mdash;</font> <a href="#Toc158108961">*</a></p>
+<p><font size="2">&sect; 108. <i>Micanite</i>. &mdash;</font>
+<a href="#Toc158108962">*</a></p>
+<p><font size="2">&sect; 109. <i>Celluloid</i>. &mdash;</font>
+<a href="#Toc158108963">*</a></p>
+<p><font size="2">&sect; 110. <i>Paper</i>.</font> <a href=
+"#Toc158108964">*</a></p>
+<p><font size="2">&sect; 111. Paraffined Paper. &mdash;</font>
+<a href="#Toc158108965">*</a></p>
+<p><font size="2">&sect; 112. <i>Paraffin</i> &mdash;</font>
+<a href="#Toc158108966">*</a></p>
+<p><font size="2">&sect; 113. Vaseline, Vaseline Oil, and
+Kerosene Oil. &mdash;</font> <a href="#Toc158108967">*</a></p>
+<p><font size="2">&sect; 114. Imperfect Conductors.
+&mdash;</font> <a href="#Toc158108968">*</a></p>
+<p><font size="2">&sect; 116. <i>Conductors</i>. &mdash;</font>
+<a href="#Toc158108969">*</a></p>
+<p><font size="2">&sect; 117. <i>Platinoid</i>. &mdash;</font>
+<a href="#Toc158108970">*</a></p>
+<p><font size="2">&sect; 119. Platinum Silver. &mdash;</font>
+<a href="#Toc158108971">*</a></p>
+<p><font size="2">&sect; 120. Platinum Iridium. &mdash;</font>
+<a href="#Toc158108972">*</a></p>
+<p><font size="2">&sect; 121. <i>Manganin</i>. &mdash;</font>
+<a href="#Toc158108973">*</a></p>
+<p><font size="2">&sect; 122. <i>Other Alloys</i>. &mdash;</font>
+<a href="#Toc158108974">*</a></p>
+<p><font size="2">&sect; 123. <i>Nickelin</i>. &mdash;</font>
+<a href="#Toc158108975">*</a></p>
+<p><font size="2">&sect; 124. Patent Nickel. &mdash;</font>
+<a href="#Toc158108976">*</a></p>
+<p><font size="2">&sect; 125. <i>Constantin</i>. &mdash;</font>
+<a href="#Toc158108977">*</a></p>
+<p><font size="2">126. Nickel Manganese Copper. &mdash;</font>
+<a href="#Toc158108978">*</a></p>
+</div>
+<p><b><font size="2">CHAPTER IV</font></b> <a href=
+"#Toc158108979">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">ELECTROPLATING AND ALLIED ARTS</font> <a href=
+"#Toc158108980">*</a></p>
+<p><font size="2">&sect; 127. Electroplating. &mdash;</font>
+<a href="#Toc158108981">*</a></p>
+<p><font size="2">&sect; 128. The Dipping Bath. &mdash;</font>
+<a href="#Toc158108982">*</a></p>
+<p><font size="2">&sect; 130. Scratch-brushing. &mdash;</font>
+<a href="#Toc158108983">*</a></p>
+<p><font size="2">&sect; 131. Burnishing. &mdash;</font> <a href=
+"#Toc158108984">*</a></p>
+<p><font size="2">&sect; 132. Silver-plating. &mdash;</font>
+<a href="#Toc158108985">*</a></p>
+<p><font size="2">&sect; 133. Cold Silvering. &mdash;</font>
+<a href="#Toc158108986">*</a></p>
+<p><font size="2">&sect; 134. <i>Gilding</i>. &mdash;</font>
+<a href="#Toc158108987">*</a></p>
+<p><font size="2">&sect; 135. Preparing Surfaces for Gilding.
+&mdash;</font> <a href="#Toc158108988">*</a></p>
+<p><font size="2">&sect; 136. Gilding Solutions. &mdash;</font>
+<a href="#Toc158108989">*</a></p>
+<p><font size="2">&sect; 137. Plating with Copper. &mdash;</font>
+<a href="#Toc158108990">*</a></p>
+<p><font size="2">&sect; 138. Coppering Aluminium. &mdash;</font>
+<a href="#Toc158108991">*</a></p>
+<p><font size="2">&sect; 140. Alkaline Coppering Solution
+&mdash;</font> <a href="#Toc158108992">*</a></p>
+<p><font size="2">&sect; 141. Nickel-plating.&mdash;</font>
+<a href="#Toc158108993">*</a></p>
+<p><font size="2">142. Miscellaneous Notes on Electroplating.
+</font> <a href="#Toc158108994">*</a></p>
+<p><font size="2">&sect; 143. Blacking Brass Surfaces.
+&mdash;</font> <a href="#Toc158108995">*</a></p>
+<p><font size="2">&sect; 144. <i>Sieves</i>. &mdash;</font>
+<a href="#Toc158108996">*</a></p>
+<p><font size="2">&sect; 145. Pottery making in the Laboratory.
+&mdash;</font> <a href="#Toc158108997">*</a></p>
+</div>
+<p><b><font size="2">APPENDIX</font></b> <a href=
+"#Toc158108998">*</a></p>
+<div style="margin-left: 2em">
+<p><font size="2">PLATINISING GLASS</font> <a href=
+"#Toc158108999">*</a></p>
+</div>
+<p><b><a name="Toc158108886" id="Toc158108886"><font face=
+"Bookman Old Style" size="4">PREFACE</font></a></b></p>
+<p><font face="Bookman Old Style">EXPERIMENTAL work in physical
+science rests ultimately upon the mechanical arts. It is true
+that in a well-appointed laboratory, where apparatus is collected
+together in greater or less profusion, the appeal is often very
+indirect, and to a student carrying out a set experiment with
+apparatus provided to his hand, the temptation to ignore the
+mechanical basis of his work is often irresistible.</font></p>
+<p>It often happens that young physicists are to be found whose
+mathematical attainments are adequate, whose observational powers
+are perfectly trained, and whose general capacity is
+unquestioned, but who are quite unable to design or construct the
+simplest apparatus with due regard to the facility with which it
+ought to be constructed. That ultimate knowledge of materials and
+of processes which by long experience becomes intuitive in the
+mind of a great inventor of course cannot be acquired from books
+or from any set course of instruction.</p>
+<p>There are, however, many steps between absolute ignorance and
+consummate knowledge of the mechanical arts, and it is the object
+of the following pages to assist the young physicist in making
+his first steps towards acquiring a working knowledge of
+"laboratory arts." However humble the ambition may be, no one can
+be more keenly alive than the writer to the inadequacy of his
+attempt; and it is only from a profound sense of the necessity
+which exists for some beginning to be made, that he has had the
+courage to air his views on matters about which there are
+probably hundreds or thousands of people whose knowledge is
+superior to his own.</p>
+<p>Moreover, nothing has been further from the writer's mind than
+any idea of "instructing" any one; his desire is &mdash; if
+happily it may so befall &mdash; to be of assistance, especially
+to young physicists or inventors who wish to attain definite
+mechanical ends with the minimum expenditure of time. Most people
+will agree that one condition essential to success in such an
+undertaking is brevity, and it is for this reason that
+alternative methods as a rule have not been given, which, of
+course, deprives the book of any pretence to being a "treatise."
+The writer, therefore, is responsible for exercising a certain
+amount of discretion in the selection he has made, and it is
+hardly to be hoped that he has in all &mdash; or even in the
+majority of cases &mdash; succeeded in recommending absolutely
+the best method of procedure.</p>
+<p>This brings another point into view. Before all things the
+means indicated must be definite and reliable. It is for this
+reason that the writer has practically confined himself to
+matters lying within his own immediate experience, and has never
+recommended any process (with one or two minor exceptions, which
+he has noted) which he has not actually and personally carried
+through to a successful issue. This, although it is a matter
+which he considers of the highest importance, and which is his
+only title to a hearing, has unfortunately led to a very personal
+tone in the book.</p>
+<p>With regard to the arts treated of in the following pages,
+matters about which information is easily acquired &mdash; such
+as carpentering, blacksmithing, turning, and the arts of the
+watchmaker &mdash; have been left on one side. With regard to the
+last, which is of immense use in the laboratory, there happen to
+be at least two excellent and handy books, viz. Saunier's
+Watchmakers' Handbook, Tripplin, London, 1892; and Britton's
+Watchmakers' Dictionary and Guide.</p>
+<p>With regard to carpentering, turning, and blacksmithing,
+almost any one who so desires can obtain a little practical
+experience in any village. A short chapter has been devoted to
+GLASS-BLOWING, in spite of there being an excellent and handy
+book by Mr. Shenstone (The Methods of GLASS-BLOWING, Rivington)
+on the subject already in existence. The reason for this
+exception lies in the fact that the writer's methods differ
+considerably from those advocated by Mr. Shenstone.</p>
+<p>The chapter on opticians' work has had to be compressed to an
+extent which is undesirable in dealing with so complex and
+delicate an art, but it is hoped that it will prove a sufficient
+introduction for laboratory purposes. In this matter the writer
+is under great obligations to his friend and assistant, Mr. James
+Cook, F.R.A.S., who gave him his first lessons in lens-making
+some twenty years ago. To Mr. John A. Brashear of Allegheny, Pa.,
+thanks are due for much miscellaneous information on optical
+work, which is included verbatim in the text, some of it
+contained originally in printed papers, and some most kindly
+communicated to the writer for the purpose of this book. In
+particular, the writer would thank Mr. Brashear for his
+generously accorded information as to the production of those
+"flat" surfaces for which he is so justly famous.</p>
+<p>The writer is also indebted to Mr. A. E. Kennelly for some
+information as to American practice in the use of insulating
+material for electrical work, and to his friends Mr. J. A.
+Pollock and Dr. C. J. Martin for many valuable suggestions. For
+the illustrations thanks are due to Mrs. Threlfall and Mr. James
+Cook. With regard to matters which have come to the writer's
+knowledge by his being specifically instructed in them from time
+to time, due acknowledgment is, it is hoped, made in the
+text.</p>
+<p>With regard to the question as to what matters might be
+included and what omitted, the general rule has been to include
+information which the author has obtained with difficulty, and to
+leave on one side that which he has more easily attained. All the
+"unities" have been consistently outraged by a deliberate use of
+the English and metric systems side by side. So long as all the
+materials for mechanical processes have to be purchased to
+specifications in inches and feet, it is impossible to use the
+centimetre consistently without introducing inconvenience.
+However, everybody ought to, and probably does, use either system
+with equal facility.</p>
+<p>No attempt has been made at showing how work can be done
+without tools. Though, no doubt, a great deal can be done with
+inferior appliances where great economy of money and none of time
+is an object, the writer has long felt very strongly that English
+physical laboratory practice has gone too far in the direction of
+starving the workshop, and he does not wish, even indirectly, 'to
+give any countenance to such a mistaken policy. Physical research
+is too difficult in itself, and students' time is too valuable,
+for it to be remunerative to work with insufficient
+appliances.</p>
+<p>In conclusion, the writer would ask his readers to regard the
+book to some extent as tentative, and as a means to the procuring
+and organising of information bearing upon laboratory arts. Any
+information which can be given will be always thankfully
+received, and the author hereby requests any reader who may
+happen to learn something of value from the book to communicate
+any special information he may possess, so that it may be of use
+to others should another edition ever be called for.</p>
+<p><b><a name="Toc158108887" id="Toc158108887"><font face=
+"Bookman Old Style" size="4">CHAPTER I</font></a></b></p>
+<p><a name="Toc158108888" id="Toc158108888">HINTS ON THE
+MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
+PURPOSES</a></p>
+<p>&sect; 1. THE art of GLASS-BLOWING has the conspicuous
+advantage, from the point of view of literary presentation, of
+being to a great extent incommunicable. As in the case of other
+delightful arts &mdash; such as those treated of in the Badminton
+Library, for instance &mdash; the most that can be done by
+writing is to indicate suitable methods and to point out
+precautions which experience has shown to be necessary, and which
+are not always obvious when the art is first approached. It is
+not the object of this work to deal with the art of GLASS-BLOWING
+or any other art after the manner befitting a complete treatise,
+in which every form of practice is rightly included. On the
+contrary, it is my wish to avoid the presentation of alternative
+methods.</p>
+<p>I consider that the presentation of alternative methods would,
+for my present purpose, be a positive disadvantage, for it would
+swell this book to an outrageous size; and to beginners &mdash; I
+speak from experience &mdash; too lavish a treatment acts rather
+by way of obscuring the points to be aimed at than as a means of
+enlightenment. The student often does not know which particular
+bit of advice to follow, and obtains the erroneous idea that
+great art has to be brought to bear to enable him to accomplish
+what is, after all, most likely a perfectly simple and
+straightforward operation.</p>
+<p>This being understood, it might perhaps be expected that I
+should describe nothing but the very best methods for obtaining
+any proposed result. Such, of course, has been my aim, but it is
+not likely that I have succeeded in every case, or even in the
+majority of cases, for I have confined myself to giving such
+directions as I know from my own personal experience will, if
+properly carried out, lead to the result claimed. In the few
+cases in which I have to refer to methods of which I have no
+personal experience, I have endeavoured to give references
+(usually taking the form of an acknowledgment), so that an idea
+of their value may be formed. All methods not particularised may
+be assumed by the reader to have come within my personal
+experience.</p>
+<p>&sect; 2. Returning to GLASS-BLOWING, we may note that two
+forms of GLASS-BLOWING are known in the arts, "Pot" blowing and
+"Table" blowing. In the former case large quantities of fluid
+"metal" (technical term for melted glass) are assumed to be
+available, and as this is seldom the case in the laboratory, and
+as I have not yet felt the want of such a supply, I shall deal
+only with "table" blowing. Fortunately there is a convenient book
+on this subject, by Dr. Shenstone (Rivingtons), so that what I
+have to say will be as brief as possible, consistent with
+sufficiency for everyday work. As a matter of fact there is not
+very much to say, for if ever there was an art in which manual
+dexterity is of the first and last importance, that art is
+glass-working.</p>
+<p>I do not think that a man can become an accomplished
+glass-blower from book instructions merely &mdash; at all events,
+not without much unnecessary labour, &mdash; but he can learn to
+do a number of simple things which will make an enormous
+difference to him both as regards the progress of his work and
+the state of his pocket.</p>
+<p>&sect; 3. The first thing is to select the glass. In general,
+it will suffice to purchase tubes and rods; in the case where
+large pieces (such as the bulbs of Geissler pumps) have to be
+specially prepared by pot-blowing, the student will have to
+observe precautions to be mentioned later on. There are three
+kinds of glass most generally employed in laboratories.</p>
+<p><b><a name="Toc158108889" id="Toc158108889"><font face=
+"Bookman Old Style" size="4">&sect; 4. Soft Soda
+Glass,</font></a></b></p>
+<p><font face="Bookman Old Style">obtained for the most part from
+factories in Thuringia, and generally used in assembling chemical
+apparatus. &mdash; This glass is cheap, and easily obtainable
+from any large firm of apparatus dealers or chemists. It should
+on no account be purchased from small druggists, for the
+following reasons:-</font></p>
+<p>(a) It is usually absurdly dear when obtained in this way.</p>
+<p>(b) It is generally made up of selections of different age and
+different composition, and pieces of different composition, even
+if the difference is slight, will not fuse together and remain
+together unless joined in a special manner.</p>
+<p>(c) It is generally old, and this kind of glass often
+devitrifies with age, and is then useless for blowpipe work,
+though it may be bent sufficiently for assembling chemical
+apparatus. Devitrified glass looks frosty, or, in the earlier
+stages, appears to be covered by cobwebs, and is easily picked
+out and rejected.</p>
+<p>&sect; 5. It might be imagined that the devitrification would
+disappear when the glass is heated to the fusing point; and so it
+does to a great extent, but for many operations one only requires
+to soften the glass, and the devitrification often persists up to
+this temperature. My experience is that denitrified glass is also
+more likely to crack in the flame than good new glass, though the
+difference in this respect is not very strongly marked with
+narrow tubes.</p>
+<p><b><a name="Toc158108890" id="Toc158108890"><font face=
+"Bookman Old Style" size="4">&sect; 6. Flint Glass.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Magnificent flint glass is made
+both in England and France. The English experimenter will
+probably prefer to use English glass, and, if he is wise, will
+buy a good deal at a time, since it does not appear to devitrify
+with age, and uniformity is thereby more likely to be secured. I
+have obtained uniformly good results with glass made by Messrs.
+Powell of Whitefriars, but I daresay equally good glass may be
+obtained elsewhere.</font></p>
+<p>For general purposes flint glass is vastly superior to the
+soft soda mentioned above. In the first place, it is very much
+stronger, and also less liable to crack when heated &mdash; not
+alone when it is new, but also, and especially, after it has been
+partly worked. Apparatus made of flint glass is less liable to
+crack and break at places of unequal thickness than if made of
+soda glass. This is not of much importance where small pieces of
+apparatus only are concerned, because these can generally be
+fairly annealed; and if the work is well done, the thickness will
+not be uneven. It is a different matter where large pieces of
+apparatus, such as connections to Geissler pumps, are concerned,
+for the glass has often to be worked partly in situ, and can only
+be imperfectly annealed.</p>
+<p>Joints made between specimens of different composition are
+much more likely to stand than when fashioned in soda glass.
+Indeed, if it is necessary to join two bits of soda glass of
+different kinds, it is better to separate them by a short length
+of flint glass; they are more likely to remain joined to it than
+to each other. A particular variety of flint glass, known as
+white enamel, is particularly suitable for this purpose, and,
+indeed, may be used practically as a cement.</p>
+<p>&sect; 7, It is, however, when the necessity of altering or
+repairing apparatus complicated by joints arises that the
+advantage of flint glass is most apparent. A crack anywhere near
+to a side, or inserted joint, can scarcely ever be repaired in
+the case of soda glass apparatus, even when the glass is quite
+thin and the dimensions small.</p>
+<p>It should also be mentioned that flint glass has a much more
+brilliant appearance than soda glass. Of course, there is a
+considerable difference between different kinds of flint glass as
+to the melting point, and this may account for the divergency of
+the statements usually met with as to its fusibility compared
+with that of soda glass. The kind of flint glass made by Messrs.
+Powell becomes distinctly soft soon after it is hot enough to be
+appreciably luminous in a darkened room, and at a white heat is
+very fluid. This fluidity, though of advantage to the practised
+worker, is likely to give a beginner some trouble.</p>
+<p>&sect; 8. As against the advantages enumerated, there are some
+drawbacks. The one which will first strike the student is the
+tendency of the glass to become reduced in the flame of the
+blow-pipe. This can be got over by proper adjustment of the
+flame, as will be explained later on. A more serious drawback in
+exact work is the following. In making a joint with lead glass it
+is quite possible to neglect to fuse the glass completely
+together at every point; in fact, the joint will stand perfectly
+well even if it be left with a hole at one side, a thing which is
+quite impossible with soft soda glass, or is at least exceedingly
+unusual. An accident of this kind is particularly likely to
+happen if the glass be at all reduced. Hence, if a joint does not
+crack when cold, the presumption is, in the case of soda glass,
+that the joint is perfectly made, and will not allow of any leak;
+but this is not the case with flint glass, for which reason all
+joints between flint glass tubes require the most minute
+examination before they are passed. If there are any air bubbles
+in the glass, especial care must be exercised.</p>
+<p><b><a name="Toc158108891" id="Toc158108891"><font face=
+"Bookman Old Style" size="4">&sect; 9. Hard or Bohemian, Glass.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is, of course, used where
+high temperatures are to be employed, and also in certain cases
+where its comparative insolubility in water is of importance. It
+is very unusual for the investigator to have to make complicated
+apparatus from this glass. Fused joints may be made between hard
+glass and flint glass without using enamel, and though they often
+break in the course of time, still there is no reason against
+their employment, provided the work be done properly, and they
+are not required to last too long.</font></p>
+<p><b><a name="Toc158108892" id="Toc158108892"><font face=
+"Bookman Old Style" size="4">&sect; 10. On the Choice of Sizes of
+Glass Tube. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">It will be found that for
+general purposes tubes about one-quarter inch in inside diameter,
+and from one-twentieth to one-fortieth of an inch thick, are most
+in demand. Some very thin soda glass of these dimensions
+(so-called "cylinder" tubes) will be found very handy for many
+purposes. For physico-chemical work a good supply of tubing, from
+one-half to three-quarters of an inch inside diameter, and from
+one-twentieth to one-eighth inch thick, is very necessary. A few
+tubes up to three inches diameter, and of various thicknesses,
+will also be required for special purposes.</font></p>
+<p>Thermometer and "barometer" tubing is occasionally required,
+the latter, by the way, making particularly bad barometers. The
+thermometer tubing should be of all sizes of bore, from the
+finest obtainable up to that which has a bore of about
+one-sixteenth of an inch. Glass rods varying from about
+one-twentieth of an inch in diameter up to, say, half an inch
+will be required, also two or three sticks of white enamel glass
+for making joints.</p>
+<p>To facilitate choice, there is appended a diagram of sizes
+from the catalogue of a reliable German firm, Messrs. Desaga of
+Heidelberg, and the experimenter will be able to see at a glance
+what sizes of glass to order. It is a good plan to stock the
+largest and smallest size of each material as well as the most
+useful working sizes.  </p>
+<p><img src="images/Image27.gif" alt="images/Image27.gif" width=
+"505" height="818">Fig. 1.</p>
+<p><b><a name="Toc158108893" id="Toc158108893"><font face=
+"Bookman Old Style" size="4">&sect; 11. Testing Glass.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">"Reject glass which has lumps
+or knots, is obviously conical, or has long drawn-out bubbles
+running through the substance." If a scratch be made on the
+surface of a glass tube, and one end of the scratch be touched by
+a very fine point of fused glass, say not more than one-sixteenth
+inch in diameter, the tube, however large it is (within reason),
+ought to crack in the direction of the scratch. If a big crack
+forms and does not run straight, but tends to turn
+longitudinally, it is a sign that the glass is ill annealed, and
+nothing can be done with it. If such glass be hit upon in the
+course of blow-pipe work, it is inadvisable to waste time upon
+it; the best plan is to reject it at once, and save it for some
+experiment where it will not have to be heated.</font></p>
+<p>The shortest way of selecting glass is to go to a good firm,
+and let it be understood that if the glass proves to be badly
+annealed it will be returned. Though it was stated above that the
+glass should not be distinctly conical, of course allowance must
+be made for the length of the pieces, and, on the other hand, a
+few highly conical tubes will be of immense service in special
+cases, and a small supply of such should be included.</p>
+<p>The glass, as it is obtained, should be placed in a rack, and
+covered by a cloth to reduce the quantity of dust finding its way
+into the tubes. It has been stated by Professor Ostwald that
+tubes when reared up on end tend to bend permanently. I have not
+noticed this with lead glass well supported. Each different
+supply should be kept by itself and carefully described on a
+label pasted on to the rack, and tubes from different lots should
+not be used for critical welds. This remark is more important in
+the case of soda than of lead glass.</p>
+<p>In the case of very fine thermometer tubes it will be
+advisable to cover the ends with a little melted shellac, or, in
+special cases, to obtain the tubes sealed from the works. Soda
+glass can generally be got in rather longer lengths than lead
+glass; the longer the lengths are the better, for the waste is
+less.</p>
+<p>It is useful to be able to distinguish the different kinds of
+glass by the colour. This is best observed by looking towards a
+bright surface along the whole length of the tube and through the
+glass. Lead glass is yellow, soda glass is green, and hard glass
+purple in the samples in my laboratory, and I expect this is
+practically true of most samples. <i>[Footnote:</i> Some new lead
+glass I have is also almost purple in hue. If any doubt exists as
+to the kind of glass, it may be tested at once in the blow-pipe
+flame, or by a mixture of oils of different refractive indices,
+as will be explained later.<i>]</i></p>
+<p>&sect; 12. The question of the solubility of glass in reagents
+is one of great importance in accurate work, though it does not
+always meet with the attention it deserves. It is impossible here
+to go into the matter with sufficient detail, and the reader is
+therefore referred to the <i>Abstracts</i> of the Chemical
+Society, particularly for the years 1889 and 1892. The memoir by
+F. Kohlrausch, <i>Wied. Ann. xliv</i>., should be consulted in
+the original. The following points may be noted. A method of
+testing the quality of glass is given by Mylius (<i>C. S. J.
+Abstracts</i>, 1889, p. 549), and it is stated that the
+resistance of glass to the action of water can generally be much
+increased by leaving it in contact with cold water for several
+days, and then heating it to 300&deg; to 400&deg; C. This
+improvement seems to be due to the formation of a layer of moist
+silica on the surface, and its subsequent condensation into a
+resisting layer by the heating. Mylius (<i>C. S. J.
+Abstracts,</i> 1892, p. 411), and Weber, and Sauer (<i>C. S. J.
+Abstracts</i>, 1892, p. 410) have also shown that the best glass
+for general chemical purposes consists of</p>
+<div style="margin-left: 4em">
+<p>Silica, 7 to 8 parts</p>
+<p>Lime, 1 part</p>
+<p>Alkali, 1.5 to 1.1 parts.</p>
+</div>
+<p>This is practically "Bohemian" tube glass.</p>
+<p>The exact results are given in the <i>Berichte</i> of the
+German Chemical Society, vol. xxv. An excellent account of the
+properties of glass will be found in Grove's edition of Miller's
+<i>Elements of Chemistry</i>.</p>
+<p><b><a name="Toc158108894" id="Toc158108894"><font face=
+"Bookman Old Style" size="4">&sect; 13. Cleaning Glass Tubes.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is one of the most
+important arts in chemistry. If the tubes are new, they are
+generally only soiled by dust, and can be cleaned fairly easily
+&mdash; first by pushing a bit of cotton waste through with a
+cane, or pulling a rag through with string &mdash; and then
+washing with sand and commercial hydrochloric acid. I have heard
+of glass becoming scratched by this process, and breaking in
+consequence when heated, but have never myself experienced this
+inconvenience. In German laboratories little bits of bibulous
+paper are sometimes used instead of sand; they soon break into a
+pulp, and this pulp has a slightly scouring action.</font></p>
+<p>As soon as the visible impurities are removed and the tube
+when washed looks bright and clean, it may be wiped on the
+outside and held perpendicularly so as to allow the water film to
+drain down. If the tube be greasy (and perhaps in other cases) it
+will be observed that as the film gets thinner the water begins
+to break away and leave dry spots. For accurate work this grease,
+or whatever it is, must be removed; and after trying many plans
+for many years, I have come back to the method I first employed,
+viz. boiling out with aqua regia.</p>
+<p>For this purpose, close one end of the tube by a cork (better
+than a rubber bung, because cheaper), and half fill the tube with
+aqua regia; then, having noted the greasy places, proceed to boil
+the liquid in contact with the glass at these points, and in the
+case of very obstinate dirt &mdash; such as lingers round a fused
+joint which has been made between undusted tubes &mdash; leave
+the whole affair for twelve hours. If the greasiness is only
+slight, then simply shaking with hot aqua regia will often remove
+it, and the aqua regia is conveniently heated in this case by the
+addition of a little strong sulphuric acid.</p>
+<p>The spent aqua regia may be put into a bottle. It is generally
+quite good enough for the purpose of washing glass vessels with
+sand, as above explained.</p>
+<p>However carefully a tube is cleaned before being subjected to
+blowpipe operations, it will be fouled wherever there is an
+opening during the process of heating, unless the extreme tip
+only of an oxidising flame be employed. Even this should not be
+trusted too implicitly unless an oxygas or hydrogen flame is
+employed.</p>
+<p>When a tube or piece of apparatus has been cleaned by acid, so
+that on clamping it vertically, dry spaces do not appear, it may
+be rinsed with platinum distilled water and left to drain, the
+dust being, of course, kept out by placing a bit of paper round
+the top. For accurate work water thus prepared is to be preferred
+to anything else. When the glass is very clean interference
+colours will be noticed as the water dries away.</p>
+<p>Carefully-purified alcohol may in some cases be employed where
+it is desired to dry the tube or apparatus quickly. In this case
+an alcohol wash bottle should be used, and a little alcohol
+squirted into the top of the tube all round the circumference.
+The water film drags the alcohol after it, and by waiting a few
+minutes and then adding a few more drops of alcohol, the water
+may be practically entirely removed, especially if a bit of
+filter paper be held against the lower end of the tube. It is
+customary in some laboratories to use ether for a final rinse,
+but unless the ether is freshly distilled and very pure, it
+leaves a distinct organic residue.</p>
+<p>When no more liquid can be caused to drain away, the tube may
+be dried by heating it along its length, beginning at the top (to
+get the advantage of the reduction of surface tension), and so on
+all down. It will then be possible to mop up a little more of the
+rinsing liquid. When the tube is nearly dry a loose plug of
+cotton wool may be inserted at the bottom. The wool must be put
+in so that the fibres lie on an even surface inside the tube, and
+the wool must be blown free from dust. Ordinary cotton wool is
+useless, from being dusty and the fibres short, and the same
+remark applies to wadding. Use nothing but what is known as
+"medicated" cotton wool with a good long fibre.</p>
+<p>The tube will usually soon dry of itself when the cover is
+lifted an inch or so. If water has been used, the air-current may
+be assisted by means of the water-pump, the air being sucked from
+the top, so that the wool has an opportunity of acting as a dust
+filter; a very slow stream of air only must be employed. For
+connecting the tube to the pump, a bit of India-rubber tube about
+an inch in diameter, with a bore of about one-eighth of an inch,
+may be employed. The end of the rubber tube is merely pressed
+against the edge of the glass.</p>
+<p>These remarks apply, with suitable modification, to all kinds
+of finished apparatus having two openings. For flasks and so on,
+it is convenient to employ a blowing apparatus, dust being
+avoided by inserting a permanent plug of cotton wool in one of
+the leading tubes. The efficiency of this method is greatly
+increased by using about one foot of thin copper tube, bent into
+a helix, and heated by means of a Bunsen burner; the hot air
+(previously filtered) is passed directly into the flask, bottle,
+or whatever the apparatus may be. This has proved so convenient
+that a copper coil is now permanently fastened to the wall in one
+of the rooms of my laboratory.</p>
+<p>The above instructions indicate greater refinement than is in
+general necessary or proper for tubes that have to be afterwards
+worked by the blow-pipe. In the majority of cases all that is
+necessary is to remove the dust, and this is preferably done by a
+wad of cotton waste (which does not leave shreds like cotton
+wool), followed by a bit of bibulous filter paper. I would
+especially warn a beginner against neglecting this precaution,
+for in the process of blowing, the dust undergoes some change at
+the heated parts of the apparatus, and forms a particularly
+obstinate kind of dirt.</p>
+<p>In special cases the methods I have advocated for removing
+dirt and drying without covering the damp surfaces with dust are
+inadequate, but an experimenter who has got to that stage will
+have nothing to learn from such a work as this.</p>
+<p><b><a name="Toc158108895" id="Toc158108895"><font face=
+"Bookman Old Style" size="4">&sect; 14. The Blow-pipe.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">I suppose a small book might
+easily be written on this subject but what I have to say &mdash;
+in accordance with the limitation imposed &mdash; will be brief.
+For working lead glass I never use anything but an oxygas
+blow-pipe, except for very large work, and should never dream of
+using anything else. Of course, to a student who requires
+practice in order to attain dexterity this plan would be a good
+deal too dear. My advice to such a one is &mdash; procure good
+soda glass, and work it by means of a modification of a gas
+blow-pipe, to be described directly. The Fletcher's blow-pipes on
+long stems are generally very inconvenient. The flame should not
+be more than 5 or 6 inches from the working table at most,
+especially for a beginner, who needs to rest his arms on the edge
+of the table to secure steadiness.</font></p>
+<p>The kind of oxygas blow-pipe I find most convenient is
+indicated in the sketch. (Fig. 2) I like to have two nozzles,
+which will slip on and off, one with a jet of about 0.035 inch in
+diameter, the other of about double this dimension. The oxygen is
+led into the main tube of the blow-pipe by another tube of much
+smaller diameter, concentric with the main tube (Fig. 3, at A).
+The oxygen is mixed with the gas during its escape from the inner
+tube, which is pierced by a number of fine holes for the purpose,
+the extreme end being closed up. The inner tube may run up to
+within half an inch of the point where the cap carrying the
+nozzle joins the larger tube.</p>
+<p><img src="images/Image28.gif" alt="images/Image28.gif" width=
+"358" height="243"><img src="images/Image29.gif" alt=
+"images/Image29.gif" width="470" height="372">Fig. 2.</p>
+<p>Fig. 3.</p>
+<p>If it is desired to use the blow-pipe for working glass which
+is already fixed in position to a support, it will be found very
+advantageous to use a hooked nozzle. The nozzle shown in the
+sketch is not hooked enough for this work, which requires that
+the flame be directed 'backwards towards the worker. With a
+little practice such a flame may be used perfectly well for
+blowing operations on the table, as well as for getting at the
+back of fixed tubes.</p>
+<p>To warm up the glass, the gas supply is turned full on, and
+enough oxygen is allowed to pass in to clear the flame. The work
+is held in front of, but not touching, the flame, until it is
+sufficiently hot to bear moving into the flame itself. The, work
+is exposed to this flame until, in the case of lead glass, traces
+of reduction begin to appear. When this point is reached the
+oxygen tap is thrown wide open. I generally regulate the pressure
+on the bags, so that under these circumstances the flame is
+rather overfed with oxygen. This condition is easily recognised,
+as follows. The flame shrinks down to a very small compass, and
+the inner blue cone almost disappears; also flashes of yellow
+light begin to show themselves &mdash; a thing which does not
+occur when the proportions of the gases are adjusted for maximum
+heating effect.</p>
+<p>For many purposes the small dimensions of the flame render it
+very convenient, and the high temperature which can be attained
+at exact spots enables glass to be fused together after a certain
+amount of mixing, which is an enormous advantage in fusing lead
+glass on to hard glass. The lead glass should not be heated hot
+enough to burn, but, short of this, the more fluid it is the
+better for joints between dissimilar samples.</p>
+<p>It will be noticed that the blow-pipe can be rotated about a
+vertical axis so as to throw the flame in various directions.
+This is often indispensable.</p>
+<p>&sect; 15. In general the oxygen flame does not require to be
+delivered under so high a pressure as for the production of a
+lime light. In England, I presume, most experimenters will obtain
+their oxygen ready prepared in bottles, and will not have to
+undergo the annoyance of filling a bag. If, however, a bag is
+used, and it has some advantages (the valves of bottles being
+generally stiff), I find that a pressure produced by placing
+about two hundredweight (conveniently divided into four fifty-six
+pound weights) on bags measuring 3' x 2'6" x 2' (at the thicker
+end) does very well. To fill such a bag with oxygen, about 700
+grms of potassium chlorate is required.</p>
+<p>If the experimenter desires to keep his bag in good order, he
+must purify his oxygen by washing it with a solution of caustic
+soda, and then passing it through a "tower" of potash or soda in
+sticks, and, finally, through a calcium chloride tower. This
+purifying apparatus should be permanently set up on a board, so
+that it may be carried about by the attendant to wherever it is
+required. Oxygen thus purified does not seem to injure a good bag
+&mdash; at least during the first six or seven years:</p>
+<p>In order to reduce the annoyance of preparing oxygen, the use
+of the usual thin copper conical bottle should be avoided. The
+makers of steel gas bottles provide retorts of wrought iron or
+steel for oxygen-making, and these do very well. They have the
+incidental advantage of being strong enough to resist the attacks
+of a servant when a spent charge is being removed.</p>
+<p>The form of retort referred to is merely a large tube, closed
+at one end, and with a screw coupling at the other; the
+dimensions may be conveniently about 5 inches by 10. The screw
+threads should be filled with fireclay (as recommended by
+Faraday) before the joint is screwed up. Before purchasing a
+bottle the experimenter will do well to remember that unless it
+is of sufficiently small diameter to go into his largest vice, he
+will be inconvenienced in screwing the top on and off. Why these
+affairs are not made with union joints, as they should be, is a
+question which will perhaps be answered when we learn why cork
+borers are still generally made of brass, though steel tube has
+long been available.</p>
+<p><img src="images/Image30.gif" alt="images/Image30.gif" width=
+"526" height="318">Fig. 4.</p>
+<p>These little matters may appear very trivial &mdash; and so
+they are &mdash; but the purchaser of apparatus will generally
+find that unless he looks after details himself, they will not be
+attended to for him. Whether a union joint is provided or not,
+let it be seen that the end of the delivery tube is either small
+enough to fit a large rubber tube connection going to the
+wash-bottle, or large enough to allow of a cork carrying a bit of
+glass tube for the same purpose to be inserted. This tube should
+not be less than half an inch in inside diameter. Never use a new
+bottle before it has been heated sufficiently to get rid of
+grease and carbonaceous dirt. A convenient oxygen-making
+apparatus is shown in Fig. 4, which is drawn from "life."</p>
+<p>&sect; 16. For large blow-pipe work with lead glass I
+recommend a system of four simple blow-pipes, in accordance with
+the sketch annexed. I first saw this system in operation in the
+lamp factory of the Westinghouse Electric Company at Pittsburg in
+1889, and since then I have seen it used by an exceedingly clever
+"trick" glass-worker at a show. After trying both this
+arrangement and the "brush flame" recommended by Mr. Shenstone, I
+consider the former the more convenient; however, I daresay that
+either can be made to work in competent hands, but I shall here
+describe only my own choice. <i>[Footnote:</i> A brush flame is
+one which issues from the blow-pipe nozzle shaped like a brush,
+i.e. it expands on leaving the jet. It is produced by using a
+cylindrical air jet or a conical jet with a large aperture, say
+one-eighth of an inch. See Fig. 25.<i>]</i></p>
+<p>As will be seen, the blow-pipe really consists of four simple
+brass tube blow-pipes about three-eighths of an inch internal
+diameter and 3 inches long, each with its gas and air tap and
+appropriate nozzle. Each blowpipe can turn about its support (the
+gas-entry pipe) to some extent, and this possibility of
+adjustment is of importance, The air jets are merely bits of very
+even three-sixteenths inch glass tubing, drawn down to conical
+points, the jets themselves being about 0.035 inch diameter.</p>
+<p>Fig. 5 <img src="images/Image31.gif" alt="images/Image31.gif"
+width="458" height="318">.</p>
+<p>The flames produced are the long narrow blow-pipe flames used
+in blow-pipe analysis, and arranged so as to consist mostly of
+oxidising flame. The air-supply does not require to be large, nor
+the pressure high &mdash; 5 to 10 inches of water will do &mdash;
+but it must be very regular. The "trick" glass-blower I referred
+to employed a foot bellows in connection with a small weighted
+gasometer, the Westinghouse Company used their ordinary
+air-blast, and I have generally used a large gas-holder with
+which I am provided, which is supplied by a Roots blower worked
+by an engine.</p>
+<p>I have also used a "velocity pump" blower, which may be
+purchased amongst others from Gerhardt of Bonn. The arrangement
+acts both as a sucking and blowing apparatus, and is furnished
+with two manometers and proper taps, etc. As I have reason to
+know that arrangements of this kind work very ill unless really
+well made, I venture to add that the Gerhardt arrangement to
+which I refer is No. 239 in his catalogue, and costs about three
+pounds. It hardly gives enough air, however, to work four
+blow-pipes, and the blast requires to be steadied by passing the
+air through a vessel covered with a rubber sheet.</p>
+<p>In default of any of these means being available, one of
+Fletcher's foot-blowers may be employed, but it must be worked
+very regularly. A table mounted with one blow-pipe made on this
+plan, and worked by a double-acting bellows, is recommended for
+students' use. For working flint glass, the air jet may be
+one-eighth of an inch in diameter and the pressure higher &mdash;
+this will give a brush flame. See Fig. 25.</p>
+<p>It will be seen, on looking at the sketch of the blowpipe
+system, that the pair of blow-pipes farther from the observer can
+be caused to approach or recede at will by means of a handle
+working a block on a slide. It often happens that after using all
+four blow-pipes at once it is necessary to have recourse to one
+blow-pipe only, and to do this conveniently and quickly is rather
+an object. Now, in my arrangement I have to turn off both the gas
+and air from the farther system, and then put in a bit of
+asbestos board to prevent the nozzles being damaged by the flame
+or flames kept alight. As I said before, when some experience is
+gained, glassblowing, becomes a very simple art, and work can be
+done under circumstances so disadvantageous that they would
+entirely frustrate the efforts of a beginner. This is not any
+excuse, however, for recommending inferior arrangements.</p>
+<p>Consequently, I say that the pipes leading in gas and air
+should be all branches of one gas and one air pipe, in so far as
+the two remote and one proximate blow-pipe are concerned, and
+these pipes should come up to the table to the right hand of the
+operator, and should have main taps at that point, each with a
+handle at least 2 inches long. By this arrangement the operator
+can instantly turn down all the blow-pipes but one, while, if the
+inverse operation is required, all the three pipes can be started
+at once. <i>[Footnote:</i> I find, since writing the above, that
+I have been anticipated in this recommendation by Mr. G. S. Ram,
+The Incandescent Lamp and its Manufacture, p. 114.<i>]</i></p>
+<p>The separate air and gas taps must be left for permanent
+regulation, and must not be used to turn the supply on or cut it
+off. In some respects this blow-pipe will be found more easy to
+manage than an oxygas blow-pipe, for the glass is not so readily
+brought to the very fluid state, and this will often enable a
+beginner who proceeds cautiously to do more than he could with
+the more powerful instrument.</p>
+<p>Though I have mentioned glass nozzles for the air supply,
+there is no difficulty in making nozzles of brass. For this
+purpose let the end of a brass tube of about one-eighth of an
+inch diameter be closed by a bit of brass wire previously turned
+to a section as shown (Fig. 6), and then bored by a drill of the
+required diameter, say .035 inch. It is most convenient to use
+too small a drill, and to gradually open the hole by means of
+that beautiful tool, the watchmaker's "broach." The edges of the
+jet should be freed from burr by means of a watchmaker's
+chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin,
+1882, p. 232, &sect; 342), or by the alternate use of a slip of
+Kansas stone and the broach.</p>
+<p>Fig. 6 <img src="images/Image32.gif" alt="images/Image32.gif"
+width="325" height="60">.</p>
+<p>The construction of this blow-pipe is so simple, that in case
+any one wishes to use a brush flame, he can easily produce one
+simply by changing his air jets to bits of the same size (say
+one-eighth to one-sixteenth of an inch) tubing, cut off clean. To
+insure success, the ends of the tubes must be absolutely plane
+and regular; the slightest inequality makes all the difference in
+the action of the instrument. If a jet is found to be defective,
+cut it down a little and try again; a clean-cut end is better
+than one which has been ground flat on a stone. The end of a tube
+may, however, be turned in a manner hereafter to be described so
+as to make an efficient jet. Several trials by cutting will
+probably have to be made before success is attained. For this
+kind of jet the air-pressure must be greatly increased, and a
+large Fletcher's foot-blower or, better still, a small
+double-action bellows worked with vigour will be found very
+suitable. A fitting for this auxiliary blow-pipe is shown in Fig.
+5 at B.</p>
+<p>Professor Roentgen's discovery has recently made it necessary
+to give more particular attention to the working of soft soda
+glass, and I have been obliged to supplement the arrangements
+described by a table especially intended for work with glass of
+this character. The arrangement has proved so convenient for
+general work that I give the following particulars. The table
+measures 5 feet long, 2 feet 11 inches wide, and is 2 feet 9
+inches high.</p>
+<p><img src="images/Image33.gif" alt="images/Image33.gif" width=
+"474" height="364">Fig. 7.</p>
+<p>It is provided with a single gas socket, into which either a
+large or small gas tube may be screwed. The larger tube is 5.5
+inches long and 0.75 of an inch in diameter. The smaller tube is
+the same length, and half an inch in diameter. The axis of the
+larger tube is 3.5 inches above the table at the point of
+support, and is inclined to the horizontal at an angle of
+12&deg;. The axis of the smaller tube is 2.5 inches above the
+surface of the table, and is inclined to the horizontal at the
+same angle as the larger one.</p>
+<p>The air jets are simply pieces of glass tube held in position
+by corks. The gas supply is regulated by a well-bored tap. The
+air supply is regulated by treading the bellows &mdash; no tap is
+requisite. The bellows employed are ordinary smiths' bellows,
+measuring 22 inches long by 13 inches wide in the widest part.
+They are weighted by lead weights, weighing 26 lbs. The treadle
+is connected to the bellows by a small steel chain, for the
+length requires to be invariable. As the treadle only acts in
+forcing air from the lower into the upper chamber of the bellows,
+a weight of 13 lbs. is hung on to the lower cover, so as to open
+the bellows automatically.</p>
+<p>The air jets which have hitherto been found convenient
+are:</p>
+<p>for the small gas tube</p>
+<div style="margin-left: 2em">
+<ol>
+<li>a tube 0.12 inch diameter drawn down to a jet of 0.032 inch
+diameter for small work;</li>
+<li>plain tubes not drawn down of 0.14 inch, 0.127 inch, and
+&ndash;0.245 inch diameter, and for the large gas tube, plain
+tubes up to 0.3 inch in diameter.</li>
+</ol>
+</div>
+<p>The table is placed in such a position that the operator sits
+with his back to a window and has the black calico screen in
+front of him and to his right. The object of the screen is to
+protect the workman against draughts. The table is purposely left
+unscreened to the left of the workman, so that long tubes may be
+treated.</p>
+<p>&sect; 17. Other appliances which will be required for
+GLASS-BLOWING are of the simplest character.</p>
+<p>(1) Small corks for closing the ends of tubes.</p>
+<p>(2) Soft wax &mdash; a mixture of bees' wax and resin softened
+by linseed oil to the proper consistency, easily found by trial,
+also used for temporarily closing tubes.</p>
+<p>(3) A bottle of vaseline for lubricating.</p>
+<p>(4) An old biscuit tin filled with asbestos in shreds, and an
+asbestos towel or cloth for annealing glass after removal from
+the flame. As asbestos absorbs moisture, which would defeat its
+use as an annealing material, it must be dried if necessary.</p>
+<p>(5) <i>A Glass-Cutter's Knife</i>. &mdash; This is best made
+out of a fine three-cornered file, with the file teeth almost
+ground out, but not quite; it should be about 2 inches long.
+After the surface has been ground several times, it may be
+necessary to reharden the steel. This is best done by heating to
+a full red and quenching in mercury. The grindstone employed for
+sharpening the knife should be "quick," so as to leave a rough
+edge. I have tried many so-called glass knives "made in Germany,"
+but, with one exception, they were nothing like so good as a
+small French or Sheffield file. In this matter I have the support
+of Mr. Shenstone's experience.</p>
+<p>(6) A wire nail, about 2 inches long, mounted very accurately
+in a thin cylindrical wooden handle about 5 inches long by
+one-quarter of an inch diameter, or, better still, a bit of
+pinion wire 6 inches long, of which 1.5 inches are turned down as
+far as the cylindrical core., An old dentists' chisel or filling
+tool is also a very good form of instrument.</p>
+<p>(7) A bit of charcoal about 3.5 inches long and 2 wide, and of
+any thickness, will be found very useful in helping to heat a
+very large tube. The charcoal block is provided with a stout wire
+handle, bent in such a manner that the block can be held close
+above a large glass tube on which the flames impinge. In some
+cases it is conveniently held by a clip stand. By the use of such
+a slab of charcoal the temperature obtainable over a large
+surface can be considerably increased.</p>
+<p>I have seen a wine-glass (Venetian sherry-glass) worked on a
+table with four blow-pipes, such as is here described, with the
+help of a block of hard wood held over the heated glass, and
+helping the attainment of a high temperature by its own
+combustion.</p>
+<p>(8) Several retort stands with screw clips.</p>
+<p>(9) Some blocks of wood about 5" X 2" X 2" with V-shaped
+notches cut in from the top.</p>
+<p>(10) A strong pair of pliers.</p>
+<p>(11) An apparatus for cleaning and drying the breath, when
+blowing directly by the mouth is not allowable. The apparatus
+consists of a solid and heavy block of wood supporting a
+calcium-chloride tube permanently connected with a tube of
+phosphorus pentoxide divided into compartments by plugs of glass
+wool. Care should be taken to arrange these tubes so as to occupy
+the smallest space, and to have the stand particularly stable.
+The exit tube from the phosphorus pentoxide should be drawn down
+to form a nozzle, from, say, half an inch to one-eighth of an
+inch in diameter, so as to easily fit almost any bit of rubber
+tube. The entry to the calcium chloride should be permanently
+fitted to about a yard of fine soft rubber tubing, as light as
+possible. The ends of this tube should terminate in a glass
+mouthpiece, which should not be too delicate.</p>
+<p>As an additional precaution against dust, I sometimes add a
+tube containing a long plug of glass wool, between the phosphorus
+pentoxide and the delivery tube, and also a tube containing stick
+potash on the entry side of the calcium chloride tube, but it may
+safely be left to individual judgment to determine when these
+additions require to be made. In practice I always keep the
+affair set up with these additions. The communication between all
+the parts should be perfectly free, and the tubes should be
+nearly filled with reagents, so as to avoid having a large volume
+of air to compress before a pressure can be got up.</p>
+<p>The arrangement will be clear by a reference to Fig. 8, which
+illustrates the apparatus in use for joining two long tubes. I
+have tried blowing-bags, etc., but, on the whole, prefer the
+above arrangement, for, after a time, the skill one acquires in
+regulating the pressure by blowing by the mouth and lips is such
+an advantage that it is not to be lightly foregone.</p>
+<p><img src="images/Image34.gif" alt="images/Image34.gif" width=
+"446" height="277">Fig. 8.</p>
+<p><b><a name="Toc158108896" id="Toc158108896"><font face=
+"Bookman Old Style" size="4">&sect; 18. <i>The Table</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The system of four blow-pipes
+is, of course, a fixture. In this case the table may be about a
+yard square, and may be covered with asbestos mill-board neatly
+laid down, but this is not essential. The table should have a rim
+running round it about a quarter of an inch high. The tools
+should be laid to the right of the worker, and for this purpose
+the blow-pipes are conveniently fixed rather to the left of the
+centre of the table, but not so far as to make the leg of the
+table come so close to the operator as to make him uncomfortable,
+for a cheerful and contented spirit ought to be part of the
+glass-worker's outfit.</font></p>
+<p>The most convenient height for a blow-pipe table &mdash; with
+the blow-pipes about 2 inches above the table top &mdash; is 3
+feet 2 inches. Nothing is so convenient to sit upon as a rough
+music-stool with a good range of adjustment. The advantage of an
+adjustable seat lies in the fact that for some operations one
+wants to be well over the work, while in others the advantage of
+resting the arms against the table is more important.</p>
+<p><b><a name="Toc158108897" id="Toc158108897"><font face=
+"Bookman Old Style" size="4">&sect; 19. Special Operations.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The preliminary to most
+operations before the blow-pipe, is to draw down a tube and pull
+it out to a fine point. This is also the operation on which a
+beginner should exercise himself in the first instance. I will
+suppose that it is desired to draw out a tube about one-quarter
+of an inch in diameter, with the object of closing it, either
+permanently or temporarily, and leaving a handle for future
+operations in the shape of the point, thin enough to cool quickly
+and so not delay further work.</font></p>
+<p>For this simple operation most of the glass-blower's skill is
+required. The tube must be grasped between the first finger and
+thumb of both hands, and held so that the part to be operated on
+lies evenly between the two hands. The distance between the
+operator's thumbs may conveniently vary from 2.5 to 4 inches.
+Releasing the grip of the left hand, let the operator assure
+himself of his ability to easily rotate the tube about its axis
+&mdash; by the right thumb and finger &mdash; he will
+incidentally observe by the "feel" whether the tube is straight
+or not.</p>
+<p>A good deal of progress can be made from this point before the
+tube is heated at all. The operator can acquire a habit of
+instinctively rotating the tube by both hands, however the tube
+itself be moved about in space, or however it be pushed or
+pulled. The habit of constant and instinctive rotation is
+literally about one-third of the whole art of glassblowing. It is
+unlikely, however, that the beginner will discover that he has
+not got this habit, until a few failures draw his attention to
+it.</p>
+<p>The glass tube being held in position lightly yet firmly, and
+the operator being sure that he feels comfortable and at his
+ease, and that the blow-pipe flame (a single flame in this
+instance) is well under control, the preliminary heating may be
+commenced. With a tube of the dimensions given this is a very
+simple affair. Turn the air partly off, or blow gently, to get a
+partly luminous gas flame; hold the tube about an inch from the
+end of this flame, and turn it round and round till it commences
+to soften.</p>
+<p>In the case of soda glass it is usual to employ the gas flame
+only, but I find that it is better in most cases to use the hot
+air of a gently-blown flame, rather than have the disadvantage of
+the soot deposited in the alternative operation. When the glass
+begins to soften, or even before, it may be moved right into the
+blow-pipe flame, and the latter may be properly urged.</p>
+<p>It is not possible to give quite explicit and definite
+instructions, applicable to every case, as to when the time is
+ripe for passing the work into the flame, but the following notes
+will indicate the general rules to be observed:&mdash;</p>
+<p>(1) A thick tube must be warmed more slowly and raised to a
+higher temperature than a thin tube.</p>
+<p>(2) The same remark applies to a tube of large diameter, as
+compared with one of small diameter, whatever the thickness.</p>
+<p>(3) In the case of very large or thick tubes the hot air is
+advantageously employed at first, and to complete the preliminary
+heating, the luminous flame alone may be used. The object of this
+is to enable the operator to judge, by the presence of soot, its
+inability to deposit &mdash; or its burning off if deposited
+&mdash; of the temperature of the glass, and of the equality of
+this temperature all over the surface, for a large and thick tube
+might be heated quite enough to enable it to be safely exposed to
+the full heat before it is appreciably yielding to the fingers.
+In general, when the soot burns off freely, or lead glass begins
+to show the faintest sign of reduction, or soda glass begins to
+colour the flame, it is more than safe to proceed.</p>
+<p>In order to turn on the full flame the operator will form a
+habit of holding the work in the left hand only, and he will also
+take care not to let anything his right hand may be doing cause
+him to stop rotating the tube with his left thumb and finger.</p>
+<p>The preliminary adjustment of air or oxygen supply will enable
+the change to a flame of maximum power to be made very quickly.
+The tube having been introduced with constant rotation, it will
+soon soften sufficiently to be worked. The beginner will find it
+best to decide the convenient degree of softness by trial.</p>
+<p>With soda glass it does not much matter how soft the glass
+becomes, for it remains viscous, but with lead glass the
+viscosity persists for a longer time and then suddenly gives
+place to a much greater degree of fluidity. <i>[Footnote:</i>
+This is only drawn from my impressions acquired in glass-working.
+I have never explicitly tested the matter
+experimentally.<i>]</i></p>
+<p>It is just at this point that a beginner will probably meet
+with his first difficulty. As soon as the glass gets soft he will
+find that he no longer rotates the glass at the same speed by the
+right and left hand, and, moreover, he will probably
+unconsciously bend the tube, and even deform it, by pushing or
+pulling.</p>
+<p>The second third of the art of the glass-blower consists in
+being able to move both hands about, rotating a tube with each
+thumb and finger, and keeping the distance between the hands, and
+also the speed of rotation, constant. Nothing but long practice
+can give this facility, but it is essential that it be acquired
+to some extent, or no progress can be made. Some people acquire a
+moderate proficiency very quickly, others, of whom the writer is
+one, only become reasonably proficient by months, or even years,
+of practice.</p>
+<p>Supposing that the tube is now ready to be drawn down, the
+operator will remove it from the flame, and will gently pull the
+ends apart, interrupting his turning as little as possible. If
+the tube be pulled too hard, or if the area heated be too small
+(about three-eighths of an inch in length in the case given would
+be proper), it will be found that the ends of the two portions of
+the tube will be nearly closed at a very sharp angle (nearly a
+right angle to the length of the tube), that the ends will be
+thin, and that a long length of very fine tube will be produced.
+To heat a short length of tube and pull hard and suddenly is the
+proper way to make a very fine capillary tube, but, in general,
+this is what we want to avoid.</p>
+<p>If the operation be successfully performed, the drawn-down
+tube will have the appearance exhibited, which is suitable either
+for subsequently closing or handling by means of the drawn-down
+portion. The straightness of the point can be obtained by a
+little practice in "feeling" the glass when the tube is rotated
+as it cools just before it loses its viscous condition.</p>
+<p>When the operation is carried out properly the shoulder of the
+"draw" should be perfectly symmetrical and of even thickness, and
+its axis regarded as that of a cone should lie in the axis of the
+tube produced. The operation should be repeated till the student
+finds that he can produce this result with certainty, and he
+should not be discouraged if this takes several days, or even
+weeks. Of course, it is probable that within the first hour he
+will succeed in making a tolerable job, but it is his business to
+learn never to make anything else.</p>
+<p><img src="images/Image35.gif" alt="images/Image35.gif" width=
+"486" height="32">Fig. 9.</p>
+<p>Fig. <img src="images/Image36.gif" alt="images/Image36.gif"
+width="523" height="63">10. &mdash; Diagram of a folded end.</p>
+<p><b><a name="Toc158108898" id="Toc158108898"><font face=
+"Bookman Old Style" size="4">&sect; 20. Closing and blowing out
+the End of a Tube. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">When it is desired to close the
+end of a particular bit of tube, this is easily done by heating
+the end, and at the same time heating the end of a waste bit of
+tube or rod; the ends, when placed in contact, stick together,
+and a point can be drawn down as before. <i>[Footnote:</i>
+"Point" is here used in the technical sense, i.e. it is a thin
+tail of glass produced by drawing down a tube.<i>]</i> Having got
+a point, it will be found that the thin glass cools enough to
+allow of the point being handled after a few moments.</font></p>
+<p>The most convenient way of reducing the point to a suitable
+length (say 1.5 inch) is to fuse it off in the flame, but this
+must be done neatly; if a tail is left it may cause inconvenience
+by catching, or even piercing the finger and breaking off. The
+blow-pipe flame being turned down to a suitable size, and the
+shoulder of the "draw" having been kept warm meanwhile, let the
+tip of the flame impinge on a point where the diameter is about
+half that of the undrawn tube, and let the temperature be very
+high (Fig. 11). The tube is to be inclined to the flame so that
+the latter strikes the shoulder normally, or nearly so. Then,
+according to circumstances, little or much of the glass can be
+removed at will by drawing off the tail (Fig. 12), till, finally,
+a small drop of melted glass only, adheres to the end of the now
+closed tube (Fig. 13).</p>
+<p>Fig. <img src="images/Image37.gif" alt="images/Image37.gif"
+width="510" height="113">11.</p>
+<p><img src="images/Image38.gif" alt="images/Image38.gif" width=
+"493" height="51">Fig. 12.</p>
+<p><img src="images/Image39.gif" alt="images/Image39.gif" width=
+"365" height="54">Fig. 13.</p>
+<p>Fig. <img src="images/Image40.gif" alt="images/Image40.gif"
+width="314" height="50">14.</p>
+<p>When this is satisfactorily accomplished, heat the extreme end
+of the tube most carefully and equally, holding it in such a
+position that the glass will tend to flow from the bead back on
+to the tube, i.e. hold the closed end up to the flame, the tube
+being, say, at 45 degrees to the horizontal. Then when the
+temperature is such as to indicate complete softness lift the
+tube to the mouth, still holding the tube pointing with its
+closed end a little above the horizontal, and blow gently. A
+beginner almost always blows too hard.</p>
+<p>What is wanted, of course, is a continued pressure, to give
+the viscous glass time to yield gradually, if it is uniform; or
+else intermittent puffs to enable the thinner parts, if there are
+any, to cool more, and hence become more resisting than the
+thicker ones. In any case a little practice will enable the
+operator to blow out a round and even end &mdash; neither thicker
+nor thinner than the rest of the tube.</p>
+<p><b><a name="Toc158108899" id="Toc158108899"><font face=
+"Bookman Old Style" size="4">&sect; 21. To make a Weld.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">To begin with, try on two bits
+of glass of the same size, i.e. cut a seven-inch length of glass
+in half by scratching it with the knife, and pulling the ends
+apart with a slight inclination away from the scratch. In other
+words, combine a small bending moment with a considerable
+tensional stress. It is important to learn to do this properly.
+If the proportions are not well observed, the tube will break
+with difficulty, and the section will not be perpendicular to the
+main length. If the knife is in good order it will make a fine
+deep scratch &mdash; the feel of the glass under the knife will
+enable the operator to decide when the scratch is made. The
+operation of cutting large tubes will be treated further on. The
+two halves of the tube being held one in each hand, and one tube
+closed at one end, the extremities to be united will be warmed,
+and then put in the flame as before.</font></p>
+<p>Fig. <img src="images/Image41.gif" alt="images/Image41.gif"
+width="487" height="113">16.</p>
+<p>There are many ways of proceeding - perhaps the easiest is as
+follows. As soon as the glass shows signs of melting at the ends
+&mdash; and care should be taken that much more is not heated
+&mdash; take both bits out of the flame. Stop rotating for a
+moment, and resting the arms carefully on the edge of the table,
+raise the tubes above the flame and bring the ends swiftly and
+accurately together. This is a case of "sudden death no second
+attempt at making the ends meet can be allowed; if the tubes join
+in any other than a perfectly exact manner a kink more or less
+objectionable will result. In practice the operator will learn to
+bring the ends together, commencing at one point; i.e. the axes
+of the tubes will be inclined at first, so as to cause adherence
+at one spot only. If this is not quite "fair," then less damage
+is done in moving one tube slightly up or down to get the contact
+exact. The tubes will then be closed upon one another as if they
+were hinged at the joint. This must be done lightly, yet
+sufficiently, to ensure that the glass is actually in contact all
+round.</p>
+<p>Having gone so far, replace the tubes &mdash; now one &mdash;
+in the flame, and carefully rotating the glass, raise the
+temperature higher than in the operation just described, in fact
+the higher the temperature, short of burning the glass, the
+better. Take the tube out of the flame and blow into the open
+end, turning constantly as before. One puff is enough. Then turn
+and pull the glass apart till it is of the same diameter and
+thickness throughout, and feel that it is straight as before.</p>
+<p>Though it is in general of high importance that the joint
+should be well heated, the beginner will probably find that he
+"ties up" his glass as soon as it gets really soft.</p>
+<p>If his object is to make one joint &mdash; at any cost &mdash;
+then let him be careful to use two bits of exactly the same kind
+of glass, and only get the temperature up to the viscous stage.
+If the joint be then pulled out till it is comparatively thin, it
+will probably stand (if of soda glass); certainly, if of lead
+glass, though in this case it may not be sound. In any case the
+joint should be annealed in the asbestos box if practicable,
+otherwise (unless between narrow tubes) with the asbestos rag.
+Care must be taken that the asbestos is dry.</p>
+<p><b><a name="Toc158108900" id="Toc158108900"><font face=
+"Bookman Old Style" size="4">&sect; 22. To weld two Tubes of
+different Sizes. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">To do this, the diameter of the
+larger tube must be reduced to that of the smaller. The general
+procedure described in drawing down must be followed, with the
+following modification. In general, a greater length of the tube
+must be heated, and it must be made hotter. The tube is to be
+gradually drawn in the flame with constant turning till the
+proper diameter and thickness of glass are attained.</font></p>
+<p>Fig. <img src="images/Image42.gif" alt="images/Image42.gif"
+width="455" height="79">16.</p>
+<p>For this operation time must be allowed if the operator's
+hands are steady enough to permit of it; the shoulder should form
+partly by the glass sinking in and partly by the process of
+drawing the hot glass out. A shoulder properly prepared is shown
+in the sketch. Beginners generally make the neck too thin on
+large tubes, and too thick on smaller ones. There ought to be no
+great difference in thickness of glass between the neck on the
+larger tube, and the smaller tube. The diameters should be as
+nearly as possible alike.</p>
+<p>Having drawn down the larger tube to a neck, take it out of
+the flame, and as it cools pull and turn till the neck is of the
+right thickness and is perfectly straight, i.e. make the final
+adjustment outside the flame, and to that end have the neck
+rather too thick (as to glass) before it is taken out. It is not
+necessary to wait till the neck gets cold before the end can be
+cut off. Make a scratch as before &mdash; this will probably
+slightly damage the temper of the file knife, but that must be
+put up with. Hold the tube against the edge of the table, so that
+the scratch is just above the level of the rim, and strike the
+upper part a smart blow with the handle of the glass knife rather
+in the direction of its length. <i>[Footnote:</i> A bit of hoop
+iron nailed against the side of the table is a very convenient
+arrangement, and it need not project appreciably above the
+general level of the rim.<i>]</i></p>
+<p>Of course this applies to a tube where economy has been
+exercised and the end is short. If the tail is long enough to
+form a handle, the tube may be pulled apart as before. As a rule
+a temporary joint between a tube and a rod is not strong enough
+to enable the shoulder to be broken at the scratch by mere
+pulling. The ends to be welded must be broken off very clean and
+true. Subsequent operations are to be carried out as already
+described.</p>
+<p>&sect; 23. The above operations will be easily performed on
+tubes up to half an inch in diameter, if they are not too long.
+It is the length of tube, and consequent difficulty in giving
+identity of motion with the two hands, which make the jointing of
+long tubes difficult. There are also difficulties if the tubes
+are very thin, have a very fine bore or a very large
+diameter.</p>
+<p>All these difficulties merely amuse a good glass-blower, but
+to an experimenter who wants to get on to other things before
+sufficient skill is acquired (in the movement of the hands and
+arms) the following method is recommended. First, use flint
+glass. Then, assuming that any drawing down has to be done, do it
+as well as possible, for on this the success of the method to be
+described especially depends. Be sure that the tubes to be welded
+are cut off clean and are as nearly as may be of the same size at
+the point of junction.</p>
+<p>To fix the description, suppose it is desired to join two
+tubes (see Fig. 8), each about one inch in diameter and a yard
+long. Get four clip stands and place them on a level table. Be
+sure that the stands are firm and have not warped so as to rock.
+In each pair of clips place a tube, so that the two tubes are at
+the same height from the table, and, in fact, exactly abut, with
+axes in the same straight line. Close one tube by a cork and then
+fix the blowing apparatus as shown to the other.</p>
+<p>In such an operation as this the drying apparatus may be
+dispensed with, and a rubber tube simply connected to one end of
+the system and brought to the mouth. Take the oxygen blow-pipe
+and turn the nozzle till the flame issues towards you, and see
+that the flame is in order. Then turn down the oxygen till it
+only suffices to clear the smoky flame, and commence to heat the
+proposed joint by a current of hot air, moving the flame round
+the joint. Finally, bring to bear the most powerful flame you can
+get out of the blow-pipe, and carry it round the joint so quickly
+that you have the latter all hot at once. Put down the blow-pipe,
+and, using both hands, press the tubes together (which wooden
+clips will readily allow), and after seeing that the glass has
+touched everywhere, pull the tubes a trifle apart. Apply the
+blow-pipe again, passing lightly over the thin parts, if any, and
+heating thicker ones; having the end of the rubber tube in his
+mouth, the operator will be able to blow out thick places. When
+all is hot, blow out slightly, and having taken the flame away,
+pull the tubes a little apart, and see that they are
+straight.</p>
+<p>Throw an asbestos rag over the joint, loosen one pair of the
+clamps slightly, and leave the joint to anneal. It is important
+that the least possible amount of glass should be heated, hence
+the necessity of having the ends well prepared, and it is also
+important that the work should be done quickly; otherwise glass
+will flow from the upper side downwards and no strong joint will
+be obtained.</p>
+<p>Fig. <img src="images/Image43.gif" alt="images/Image43.gif"
+width="382" height="146">17. &mdash; Tube being opened at one
+end.</p>
+<p><b><a name="Toc158108901" id="Toc158108901"><font face=
+"Bookman Old Style" size="4">&sect; 24. To weld Tubes of very
+small Bore. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">If the bore is not so small as
+to prevent the entrance of the point of the iron nail, get the
+ends of the tubes hot, and open the bore by inserting the end of
+the nail previously smeared over with a trace of vaseline. Work
+the nail round by holding the handle between the thumb and first
+finger of the right hand, the tube being similarly placed in the
+left. The tube and nail should be inclined as shown in the
+sketch.</font></p>
+<p>Never try to force the operation; the nail soon cools the
+glass, so that only a very short time is available after each
+heat; during this the tube should be rotated against the nail
+rather than the nail against the tube. Be careful not to heat a
+greater length of tube than is necessary, or the nail will, by
+its component of pressure along the tube, cause the latter to
+"jump up" or thicken and bulge. Both ends being prepared, and if
+possible, kept hot, the weld may be made as before, and the
+heating continued till the glass falls in to about its previous
+thickness, leaving a bore only slightly greater than before.</p>
+<p>It is in operations such as this that the asbestos box will be
+found of great use. As soon as one end of the weld is ready cool
+it in the flame till soot deposits, and then plunge it into the
+asbestos. This will cause it to cool very slowly, and renders it
+less likely to crack when again brought into the flame.
+Turned-out ends, if the glass is at all thick, are very liable to
+crack off on reheating, so that they must be reintroduced (into
+the flame) with especial care. This liability to breakage is
+reduced, but not eliminated, by the asbestos annealing.</p>
+<p><img src="images/Image44.gif" alt="images/Image44.gif" width=
+"515" height="121">Figs. 18 and 19.</p>
+<p>&sect; 25. When the bore is very fine, it is best to seal off
+the tubes, and blow an incipient bulb near one end of each tube.
+These bulbs may be cooled in asbestos, and cut across when cold
+by means of a scratch touched at one end (Figs. 18 and 19) by a
+fine point of highly incandescent glass. For details of this
+method see p. 46, Fig. 21. Time is occasionally saved by blowing
+off the ends of the bulbs. The details of this process will be
+described when the operation of making thistle-headed tubes is
+dealt with.</p>
+<p>&sect; 26. When the tubes are both of large diameter, long,
+and very thin (cylinder tubes), a considerable amount of
+difficulty will be experienced. On the whole, it is best to heat
+each end separately till the glass thickens a little, anneal in
+the flame and in asbestos, and then proceed as in &sect; 22. If
+the ends are not quite true, it will be found that quite a
+thickness of glass may be "jumped" together at one side of the
+tubes, while the edges are still apart at the other. When this
+looks likely to happen, incline the tubes as if the joint were a
+hinge, and bend back quickly; do not simply continue to push the
+tubes together in a straight line, or an unmanageable lump of
+glass will be formed on one side.</p>
+<p>If in spite of these precautions such a lump does form,
+proceed as follows. Take a rod of glass, at least one-eighth of
+an inch thick, and warm it in the flame at one end. Heat the
+imperfect joint till it softens all round, and then bring the
+flame right up to the thick part, and heat that as rapidly and
+locally as possible. The oxygas flame does this magnificently.
+Press the heated end of the glass rod against the thick part, and
+pull off as much of the lump as it is desired to remove,
+afterwards blowing the dint out by a judicious puff. Finish off
+as before.</p>
+<p>&sect; 27. Occasionally, when it is seen that in order to
+produce a joint closed all round, one side of the tube would be
+too much thickened, it is better to patch the open side. For this
+purpose take a glass rod about one-sixteenth inch in diameter,
+and turn the flame to give its greatest effect, still keeping
+rather an excess of air or oxygen. See that the side of the joint
+already made is kept fairly hot &mdash; it need not be soft;
+interrupt any other work often enough to ensure this. Then,
+directing the flame chiefly on the thin rod, begin to melt and
+pull the glass over the edges of the gap. When the gap is closed
+get the lump very hot, so that all the glass is well melted
+together, and then, if necessary, pull the excess of glass off,
+as before described.</p>
+<p>It must be remembered that this and the method of the previous
+section are emergency methods, and never give such nice joints as
+a manipulation which avoids them, i.e. when the ends of the tubes
+are perfectly straight and true to begin with. Also note that, as
+the tubes cannot be kept in rotation while being patched, it is
+as well to work at as low a temperature as possible, consistently
+with the other conditions, or the glass will tend to run down and
+form a drop, leaving a correspondingly thin place behind.</p>
+<p><img src="images/Image45.gif" alt="images/Image45.gif" width=
+"381" height="51">Fig. 20.</p>
+<p>&sect; 28. A very common fault in cutting a tube of about an
+inch in diameter is to leave it with a projecting point, as
+shown. This can be slowly chipped off by the pliers, using the
+jaws to crush and grind away the edge of the projection; it is
+fatal to attempt to break off large pieces of glass all at
+once.</p>
+<p>&sect; 29. It will be convenient here to mention some methods
+of cutting large tubes. With tubes up to an inch and a half in
+diameter, and even over this &mdash; provided that the glass is
+not very thick &mdash; we may proceed as follows: Make a good
+scratch about half an inch long, and pretty deep, i.e. pass the
+knife backwards and forwards two or three times. Press a point of
+melted glass exactly on one end of the scratch; the glass point
+even when pressed out of shape should not be as large as a button
+one-twelfth of an inch in diameter. If this fails at first,
+repeat the operation two or three times.</p>
+<p><img src="images/Image46.gif" alt="images/Image46.gif" width=
+"476" height="195">Fig. 21.</p>
+<p>If a crack does not form, touch the hot place with the cold
+end of the nail. If no success is obtained, try the other end of
+the scratch. If failure still pursues the operator, let him make
+another cut on the opposite side of the tube and try again. In
+general, the tube will yield the first or second time the hot
+drop of glass is applied. Never apply the drop at the centre of
+the scratch, or a ragged crack, which may run in any direction,
+will result. Very often, with a large tube, the crack formed by a
+successful operation will only extend a short distance. In this
+case it is desirable to entice the crack round the tube, and not
+trust to its running straight when the tube is pulled apart.</p>
+<p>On the whole, the best method in this case is to employ a
+flame pencil, which should be kept ready for use. This merely
+consists of a bit of glass tube of about the same dimensions as
+an ordinary lead pencil, drawn down to a very fine jet at one
+end. The jet must not be very long or thin, or the glass will
+soon fuse up. A few trials will enable the operator to get the
+proper proportions, which are such that the tube has the general
+appearance of a pencil normally sharpened (say with a cone of
+60'). This tube is best made of hard glass. Connect it to a gas
+supply by light flexible tubing, and turn down the gas till the
+flame from the end of the jet is not more than one-tenth of an
+inch long. Then apply the jet, beginning from the end of the
+crack, and gradually draw it (the crack) round the tube. The
+operation will be assisted if a rubber ring is slipped on the
+tube to begin with, so that the eye has some guide as to whether
+the flame is being drawn round properly or not. The ring must, of
+course, be far enough away to escape the effect of the flame. The
+crack will be found to follow the flame in the most docile
+manner, unless the tube is thick or badly annealed. Some
+operators recommend a pencil of glowing charcoal, but the flame
+is undoubtedly better.</p>
+<p><b><a name="Toc158108902" id="Toc158108902"><font face=
+"Bookman Old Style" size="4">&sect; 30. To cut very thick Tubes.
+-</font></a></b></p>
+<p><font face="Bookman Old Style">A large number of methods have
+been proposed, and nearly everybody has his favourite. The
+following has always succeeded with me. First mark on the tube,
+by means of a little dead black spirit paint, exactly where the
+cut is to be. Then sharpen the glass knife and scratch a quite
+deep cut all round: there is no difficulty in making the cut
+one-twentieth of an inch deep. It will be proper to lubricate the
+knife with kerosene after the first mark is made.
+<i>[Footnote:</i> The edge of the knife may be advantageously
+saved by using an old file moistened with kerosene for the
+purpose. I find kerosene is not worse, but, if anything, better
+than the solution of camphor in turpentine recommended by Mr.
+Shenstone.<i>]</i></font></p>
+<p>If the glass is about one-eighth of an inch thick, the scratch
+maybe conveniently about one-twentieth of an inch deep, but if
+the glass is anything like one-quarter of an inch thick, the
+scratch must be much deeper, in fact, the glass may be half cut
+through. To make a very deep scratch, a wheel armed with diamond
+dust, which will be described later on, may be used. However, it
+is not essential to use a diamond wheel, though it saves
+time.</p>
+<p>When the cut is made to a sufficient depth proceed thus:
+Obtain two strips of bibulous paper or bits of tape and twist
+them round the tube on each side of the scratch, allowing not
+more than one-eighth of an inch between them. Then add a few
+drops of water to each, till it is thoroughly soaked, but not
+allowing water to run away. Dry out the scratch by a shred of
+blotting paper.</p>
+<p>Turn down the oxygas flame to the smallest dimensions, and
+then boldly apply it with its hottest part playing right into the
+nick and at a single point. Probably in about two seconds, or
+less, the tube will break. If it does not, rotate the tube, but
+still so that the flame plays in the nick. After making the tube
+very hot all round &mdash; if it has not broken &mdash; apply the
+flame again steadily at one point for a few seconds and then
+apply a bit of cold iron. If the tube does not break at once
+during these processes, let it cool, and cut the groove deeper;
+then try again. <i>[Footnote:</i> This method is continually
+being reinvented and published in the various journals. It is of
+unknown antiquity.<i>]</i></p>
+<p><img src="images/Image47.gif" alt="images/Image47.gif" width=
+"416" height="187">Fig. 22.</p>
+<p>If the tube breaks after great heating and long efforts, it
+will probably leave incipient cracks running away from the break,
+or may even break irregularly. A good break is nearly always one
+that was easily made. If a number of rings have to be cut, or a
+number of cuts made on glass tubes of about the same size, it
+will be found economical in the end to mount a glazier's diamond
+for the purpose. A simple but suitable apparatus is figured (Fig.
+23).</p>
+<p><img src="images/Image48.gif" alt="images/Image48.gif" width=
+"355" height="316">Fig. 23.</p>
+<p>The only difficulty is to regulate the position of the diamond
+so that it cuts. In order to do this, carefully note its cutting
+angle by preliminary trials on sheet glass, and then adjust the
+diamond by clamps, or by wriggling it in a fork, as shown. Weight
+the board very slightly, so as to give the small necessary
+pressure, and produce the cut by rotating the tube by hand. When
+a cut is nearly completed take great care that the two ends join,
+or irregularity will result. This is not always easy to do unless
+the tube happens to be straight. Having got a cut, start a crack
+by means of a fine light watchmaker's hammer, or even a bit of
+fused glass, and entice the crack round the cut by tapping with
+the hammer or by means of the flame pencil.</p>
+<p>If the cut is a true "cut" the tube will break at once. As a
+supply of electrical current for lighting will, in the near
+future, be as much a matter of course for laboratory purposes as
+a gas supply, I add the following note. To heat a tube round a
+scratch, nothing &mdash; not even the oxygas blow-pipe &mdash; is
+so good as a bit of platinum or iron wire electrically heated. If
+the crack does not start by considerable heating of the glass,
+stop the current, unwind the wire, and touch the glass on the
+crack either with a bit of cold copper wire or a wet match stem.
+I prefer the copper wire, for in my experience the water will
+occasionally produce an explosion of cracks. On the other hand,
+the cold wire frequently fails to start a crack.</p>
+<p>Judging from the appearance of thick tubes as supplied by the
+dealers, the factory method of cutting off appears to be to grind
+a nick almost through the tube, and right round; and for really
+thick glass this is the safest but slowest way; a thin emery
+wheel kept wet will do this perfectly. Suitable wheels may be
+purchased from the "Norton" Emery Wheel Co. of Bedford, Mass.,
+U.S.A. &mdash; in England through Messrs. Churchill and Co. of
+London, importers.</p>
+<p><b><a name="Toc158108903" id="Toc158108903"><font face=
+"Bookman Old Style" size="4">&sect; 31. To blow a Bulb at the End
+of a Tube. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">I must admit at once that this
+is a difficult operation &mdash; at all events, if a large bulb
+is required. However, all there is to be said can be said in few
+words. In general, when a bulb is required at the end of a tube
+it will be necessary to thicken up the glass. A professional
+glass-worker will generally accomplish this by "jumping up" the
+tube, i.e. by heating it where the bulb is required, and
+compressing it little by little until a sufficient amount of
+glass is collected. The amateur will probably find that he gets a
+very irregular mass in this way, and will be tempted to begin by
+welding on a short bit of wide and thick tubing preparatory to
+blowing out the bulb.</font></p>
+<p>However, supposing that enough glass is assembled by-either of
+these methods, and that it is quite uniform in thickness, let the
+thickened part be heated along a circle till it becomes
+moderately soft, and let it then be expanded about one-fifth, say
+by gently blowing. It is perhaps more important to keep turning
+the glass during bulb-blowing than in any other operation, and
+this both when the glass is in the flame and while the bulb is
+being blown. It is also very important to avoid draughts. In
+general, a bulb is best blown with the tube in a nearly
+horizontal position, but sloping slightly upwards from the mouth.
+If it be noticed that a bulb tends to blow out more at one side
+than another, let the side of greatest protuberance be turned
+down, so that it is at the lowest point, reduce the pressure for
+an instant, and then blow again. It will be observed that the
+bulb will now expand at the top.</p>
+<p>The reason of this is chiefly that the under side cools most
+rapidly (according to Faraday, <i>Chemical Manipulation</i>,
+&sect; 1194), and consequently can expand no further; but also it
+is not unlikely that the glass tends to flow somewhat from the
+upper side, which remains hot, and consequently the bulb, when
+the next puff reaches it, will tend to yield at this point. By
+heating several zones the tube will become gradually
+expanded.</p>
+<p>Fig. <img src="images/Image49.gif" alt="images/Image49.gif"
+width="433" height="31">24.</p>
+<p><img src="images/Image50.gif" alt="images/Image50.gif" width=
+"405" height="72">Fig. 25.</p>
+<p>Fig. <img src="images/Image51.gif" alt="images/Image51.gif"
+width="399" height="92">26.</p>
+<p>When the length of the thickened part of the tube only
+slightly exceeds its diameter (Fig. 25), let the whole be brought
+to a temperature which, with flint glass, should be just short of
+that of perfect fluidity; and then, holding the tube horizontally
+and constantly turning it, let the bulb be blown out to its full
+size, noting the appearances and correcting too great
+protuberance on any side by the means above mentioned. If the
+bulb appears pear-shaped turn the tube so that the melted mass is
+directed upwards; if the bulb have the contrary fault, correct in
+the corresponding manner.</p>
+<p>The bulb when finished may be lightly tapped on the table,
+when, if there is any weak place owing to inequality of
+thickness, the bulb will break, and the operation may be started
+afresh. "A good bulb is round, set truly on the tube, and free
+from lumps of thick glass or places of excessive thinness." When
+the amateur has succeeded in blowing a bulb two inches in
+diameter on the end of a strong bit of thermometer tube &mdash;
+say for an air thermometer &mdash; he may well seek the
+congratulations of his friends.</p>
+<p>In case the bulb is not satisfactory on a first attempt, it
+may be melted down again, if the following precautions are taken.
+Directly creases begin to appear in the bulb let it be withdrawn
+from the flame, and gently blown till the creases come out. By
+alternate heating and blowing the glass can be got back to its
+original form, or nearly so, but unless the operator shows great
+skill and judgment, the probability is that the glass will be
+uneven. By heating and keeping the thicker parts in the higher
+position, and blowing a little now and again, the glass may be
+got even, and a new attempt may be made. It must not be supposed
+that this process can be carried on indefinitely, for the glass
+tends to lose its viscous properties after a time, or, at all
+events, it "perishes" in some way, especially if it has been
+allowed to get very thin; consequently too frequent attempts on
+the same glass are unprofitable. Two or three trials are as many
+as it generally pays to make. As a rule the largest possible
+flame may be used with advantage in this operation.</p>
+<p><b><a name="Toc158108904" id="Toc158108904"><font face=
+"Bookman Old Style" size="4">&sect; 32. To blow a bulb in the
+middle of a tube,</font></a></b></p>
+<p><font face="Bookman Old Style">the procedure is much like that
+already treated, but the manipulation is, if anything, more
+difficult, for the further end of the tube must be carried and
+turned as well as the end which is held to the lips.</font></p>
+<p><b><a name="Toc158108905" id="Toc158108905"><font face=
+"Bookman Old Style" size="4">&sect; 33. To make a side Weld.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is by no means difficult,
+but is easier with lead glass than with soda glass. The tube to
+which it is desired to make a side connection having been
+selected, it is closed at one end by rubber tube stops, or in any
+other suitable manner. The zone of the proposed connection is
+noted, and the tube is brought to near softness round that circle
+(if the tube is made actually soft, inconvenience will arise from
+the bending, which is sure to occur). Two courses are then open
+to the operator, one suitable to a thick tube, the other to a
+tube of moderate thickness.</font></p>
+<p>Taking the former first. Provide a piece of glass rod and warm
+its end. Direct a small flame against the spot on the thick tube
+where the proposed joint is to be. When the glass becomes almost
+incandescent at this spot, put the end of the rod against it and
+draw out a thread of glass till sufficient "metal" has been
+removed. Then fuse off the thread close to the tube.</p>
+<p><img src="images/Image52.gif" alt="images/Image52.gif" width=
+"467" height="118">Fig. 27.</p>
+<p>The subsequent procedure is the same as for thin tubes. In
+this case heat the spot by the smallest flame available, and get
+the spot very hot. Blow it out gently into a bubble, perhaps
+extending to a height equal to its diameter. Then heat the top of
+the bubble till it is incandescent and blow violently. This will
+produce an opening fringed by glass so thin as to exhibit
+interference colours. Remove the filmy part, and heat the frayed
+edges till they cohere and form an incipient tube. If the flame
+has been of a correct size, the tube will now be of the same
+diameter as the tube to be welded on, and will project perhaps
+one-sixteenth of an inch from the surface of the main tube (Fig.
+28).</p>
+<p><img src="images/Image53.gif" alt="images/Image53.gif" width=
+"423" height="68">Fig. 28.</p>
+<p>Fig. <img src="images/Image54.gif" alt="images/Image54.gif"
+width="453" height="160"> 29.</p>
+<p>When this stage is reached, again heat the tube all round till
+it nearly softens, and by means of the other hand heat the end of
+the other tube which it is proposed to weld. Just before the main
+tube actually softens, turn it so as to heat the edges of the
+aperture, and at the same time get the end of the side tube very
+hot. Take both out of the flame for an instant, and press the
+parts together, instantly slightly withdrawing the side tube. If
+the operation is well performed, it will be found that the point
+of maximum thickness of glass is now clear of the main tube. The
+joint is then to be heated all round and blown out &mdash; a
+rather awkward operation, and one requiring some practice, but it
+can be done.</p>
+<p>Fig. <img src="images/Image55.gif" alt="images/Image55.gif"
+width="362" height="139"> 30.</p>
+<p>If great strength is wanted, heat the main tube all round the
+joint bit by bit, and blow each section slightly outwards. If the
+operator is confident in his skill, he should then heat the whole
+joint to the softening point, blow it out slightly, and then
+adjust by pulling and pushing. Cool first in the gas flame, and
+then plunge the joint into the asbestos and cover it up &mdash;
+or if too large, throw the asbestos cloth round it.</p>
+<p>In the case of soda glass this final "general heat" is almost
+essential, but it is not so with flint glass, and as the general
+heat is the most difficult part of the job, it will be found
+easier to use lead glass and omit the general heating. With soda
+glass a very small irregularity will cause the joint to break
+when cold, but flint glass is much more long-suffering. It is
+easy to perform the above operation on small tubes. For large
+ones it will be found best to employ flint glass and use the clip
+stands as in the case of direct welds, treated above, but, of
+course, with suitable modifications. Never let the main tube cool
+after the hole is made until the work is done.</p>
+<p><b><a name="Toc158108906" id="Toc158108906"><font face=
+"Bookman Old Style" size="4">&sect; 34. Inserted Joints.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">In many instances the
+performance of apparatus is much improved by joints of this kind,
+even when their use is not absolutely essential.</font></p>
+<p>There are two ways in which inserted joints may be made. The
+first method is the easier, and works well with flint glass; but
+when one comes to apply it to soda glass there is a danger of the
+glass becoming too thick near the joint, and this often leads to
+a cracking of the joint as the glass cools.</p>
+<p>Fig. 31. <img src="images/Image56.gif" alt=
+"images/Image56.gif" width="407" height="202"></p>
+<p>Suppose it is desired to insert the tube B into the tube A
+(Fig. 31). Begin by reducing the size of the end of tube A till B
+will just slip in quite easily. With B about one-quarter inch in
+diameter, a clearance of about one-twentieth of an inch, or less,
+in all (i.e. one-fortieth of an inch on each side) will be
+proper.</p>
+<p>Heat B by itself at the proposed zone of junction, and blow
+out a very narrow ring; then compress this slightly so that it
+forms an almost closed ring of glass. The figure refers to the
+close of this operation (Fig. 31, B). It does not matter much
+whether the ring remains a mere flattened bulb, or whether it is
+a solid ring, but it must be one or the other. Some judgment must
+be exercised in preparing the ring. In general, the beginner will
+collect too much glass in the ring, and consequently the joint,
+when made, will either be thick and liable to crack easily, or it
+will be blown out into an erratic shape in endeavours to reduce
+this thickness. Accordingly, the operator will, if necessary,
+thin the tube B by drawing slightly, if he considers it
+desirable, before the little enlargement is blown out. In
+general, two heats must be used for this operation.</p>
+<p>Fig. <img src="images/Image57.gif" alt="images/Image57.gif"
+width="498" height="63"> 32.</p>
+<p>Get the approximating parts of both A and B up to a
+temperature just below that at which they will adhere, and having
+closed the other end of A, place B carefully within it up to the
+ring, and if it can be arranged, have a mica wad in A, with a
+central hole through which the end of B can project. This will
+very much facilitate the operation, especially if B is long, but
+may be dispensed with by the exercise of care and skill.</p>
+<p>The operation is now simple. Fuse the junction and press the
+tubes lightly together, being careful not to collect more glass
+than can be helped; finally, blow out the joint and reduce the
+thickness by mild drawing (Fig. 33). In order to make a really
+good joint, two points must be particularly attended to &mdash;
+the rim must be thin and its plane perfectly perpendicular to the
+axis of tube B; the end of tube A must be cut off quite clean and
+perpendicular to its axis before B is inserted. So important are
+these conditions &mdash; especially the latter that the writer
+has even occasionally used the grindstone to get the end of A
+into a proper condition, an admission which will probably earn
+the contempt of the expert glass-worker.</p>
+<p><img src="images/Image58.gif" alt="images/Image58.gif" width=
+"467" height="139">Fig. 33.</p>
+<p>Now for the second method, which is often practised in
+Germany, where soda glass is chiefly used. With this glass the
+chief point is to get a very even and not too thick ring at the
+junction, and consequently the extra thickening produced by
+making a rim on B is rather a drawback. The method consists in
+cutting off from B the length which it is desired to insert,
+slipping this into A (which may be an otherwise closed bulb, for
+instance), and then gradually melting up the open end of A till
+the piece of B inside will no longer fall out. By holding the
+joint downwards so that the inserted portion of B rests on the
+edges of the opening, a joint may be made with the minimum
+thickening.</p>
+<p>The external part of B, previously heated, is then applied,
+and the joint subjected to a "general" heat and blown out. Very
+nice joints may be made by this method, and it is perhaps the
+better one where the external part of B is to be less in diameter
+than the inserted part. It was in this manner that the writer was
+taught to make glass velocity pumps, one of which, of a good
+design, is figured as an example.</p>
+<p>In all cases good annealing should follow this operation. If
+the inserted part of the inner tube (B) is anything like an inch
+in diameter, and especially if it is of any length, as in some
+forms of ozone apparatus, or in a large Bunsen's ice calorimeter,
+the arrangements for supporting the inner part must be very good.
+A convenient way of proceeding when the inner tube is well
+supported is to make the mouth of A only very little larger than
+the diameter of B, so that B will only just slip in. Then the
+mouth of A and the zone of B may be heated together, and B blown
+out upon A. This, of course, must be arranged for, if necessary,
+by temporarily stopping the inner end of B.</p>
+<p><img src="images/Image59.gif" alt="images/Image59.gif" width=
+"497" height="125">The inner support of B should be removed as
+soon as practicable after the joint is made, or, at all events,
+should not be perfectly rigid; a tightly-fitting cork, for
+instance, is too rigid. The reason is, of course, that in cooling
+there may be a tendency to set B a little to one side or the
+other, and if it is not free to take such a set, the joint most
+probably will give way. Good annealing both with flame and
+asbestos is a <i>sine qua non</i> in all inserted work.</p>
+<p>Fig. 34.</p>
+<p><b><a name="Toc158108907" id="Toc158108907"><font face=
+"Bookman Old Style" size="4">&sect; 35. Bending Tubes.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">I have hitherto said nothing
+about bending tubes, for to bend a tube of a quarter of an inch
+in diameter, and of ordinary thickness, is about the first thing
+one learns in any laboratory, while to bend large tubes nicely is
+as difficult an operation as the practice of GLASS-BLOWING
+affords. However, even in bending a narrow tube it is possible to
+proceed in the wrong way. The wrong way is to heat a short length
+of the tube and then bend it rapidly, holding the plane of the
+bend horizontal. The right way, <i>per contra</i>, is to use a
+batswing burner to heat, say, two inches of the tube with
+constant turning till it is very soft, and then, holding the
+glass so that the bend will be in a vertical plane passing
+through one eye (the other being shut), to make the bend rather
+slowly.</font></p>
+<p>If an exact angle is required, it is as well to have it drawn
+out on a sheet of asbestos board. In this case bend the glass as
+described till it is approximately right, and finish by laying it
+on the asbestos board and bringing it up to the marks. A suitable
+bit of wood may be substituted for the asbestos on occasion.</p>
+<p>N.B. &mdash; The laboratory table is not a suitable piece of
+wood. A right-angled bend is often wanted. In this case the
+corner of a table will serve as a good guide to the eye, the
+glass being finished by being held just above it. If great
+accuracy is wanted, make a wooden template and suspend it by a
+screw from the side of the table, so that the vertex of the gauge
+for the interior angle projects downwards, then finish by bending
+the tube round it. The wood may be about half an inch thick.</p>
+<p>If a sharp bend is required, heat the tube in the blow-pipe,
+and bend it rapidly, blowing out the glass meanwhile. The reason
+why a long bend should be held in a vertical plane is that the
+hot part tends to droop out of the plane of the bend if the
+latter be made in a horizontal position. To bend a tube above
+half an inch in diameter is a more or less difficult operation,
+and one which increases in difficulty as the diameter of the tube
+increases.</p>
+<p>A U-tube, for instance, may be made as follows: Use the four
+blow-pipe arrangement so as to heat a fair length of tube, and
+get, say, two inches of tube very hot--almost fluid, in fact
+&mdash; by means of the carbon block supported from a stand.
+Remove the tube rapidly from the flame and draw the hot part out
+to, say, three inches. Then, holding the tube so as to make the
+bend in a vertical plane, bend it and blow it out together to its
+proper size.</p>
+<p>This operation seems to present no difficulties to experienced
+glass-workers, even with tubes of about one inch in diameter, but
+to the amateur it is very difficult. I always look on a large
+U-tube with feelings of envy and admiration, which the complex
+trick work of an elaborate vacuum tube does not excite in the
+least. It will be noted that this method may, and often does,
+involve a preliminary thickening of the glass.</p>
+<p>With tubes over an inch in diameter I have no idea as to what
+is the best mode of procedure &mdash; whether, for instance, a
+quantity of sand or gas coke might not be used to stuff out the
+tube during bending, but in this case there would be the
+difficulty of removing the fragments, which would be sure to
+stick to the glass.</p>
+<p>Of course, if the bend need not be short, the tube could be
+softened in a tube furnace and bent in a kind of way. I must
+admit that with tubes of even less than one inch in diameter I
+have generally managed best by proceeding little by little. I
+heat as much of the glass as I can by means of a gigantic
+blow-pipe, having a nozzle of about an inch in diameter, and
+driven by a machine-blower.</p>
+<p>When I find that, in spite of blowing, the tube begins to
+collapse, I suspend operations, reheat the tube a little farther
+on, and so proceed. If by any chance any reader knows a good
+laboratory method of performing this operation, I hope he will
+communicate it to me. After all, the difficulty chiefly arises
+from laboratory heating appliances being as a rule too limited in
+scope for such work.</p>
+<p>The bending of very thin tubes also is a difficulty. I have
+only succeeded here by making very wide bends, but of course the
+blowing method is quite applicable to this case, and the effect
+may be obtained by welding in a rather thicker bit of tube, and
+drawing and blowing it till it is of the necessary thinness. This
+is, however, a mere evasion of the difficulty.</p>
+<p><b><a name="Toc158108908" id="Toc158108908"><font face=
+"Bookman Old Style" size="4">&sect; 36. Spiral Tubes.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">These are easily made where
+good heating apparatus is available. As, however, one constantly
+requires to bend tubes of about one-eighth inch in diameter into
+spirals in order to make spring connections for continuous glass
+apparatus, I will describe a method by which this is easily done.
+Provide a bit of iron pipe about an inch and a quarter in outside
+diameter. Cover this with a thick sheath of asbestos cloth, and
+sew the edges with iron wire. Hammer the wire down so that a good
+cylindrical surface is obtained. Make two wooden plugs for the
+ends of the iron pipe. Bore one to fit a nail, which may be held
+in a small retort clip, and fasten a stout wire crank handle into
+the other one. Support the neck of the handle by means of a
+second clip. In this way we easily get a sort of windlass quite
+strong enough for our purpose.</font></p>
+<p>Fig. <img src="images/Image60.gif" alt="images/Image60.gif"
+width="419" height="215"> 35.</p>
+<p>Provide a large blow-pipe, such as the blow-pipe of a Fletcher
+crucible furnace, Select a length of tubing and clean it. Lash
+one end to the cylinder by means of a bit of wire, and hold the
+other end out nearly horizontally. Then start the blow-pipe to
+play on the tube just where it runs on to the asbestos cylinder,
+and at first right up to the lashing. Get an attendant to assist
+in turning the handle of the windlass, always keeping his eye on
+the tube, and never turning so fast as to tilt the tube upwards.
+By means of the blow-pipe, which may be moved round the tubing,
+heat the latter continuously as it is drawn through the flame,
+and lay it on the cylinder in even spirals.</p>
+<p>If the tubing is thin, a good deal of care will have to be
+exercised in order to prevent a collapse. A better arrangement,
+which, however, I have not yet tried, would, I think, be to
+replace the blow-pipe by two bats-wing burners, permanently
+fastened to a stand, one of them playing its flame downwards on
+to the top of the flame of the other. The angle between the
+directions of the jets might be, say, 130&deg;, or whatever is
+found convenient. In this way the glass would not be so likely to
+get overheated in spots, and better work would doubtless result.
+However, I have made numbers of perfectly satisfactory spirals as
+described. Three or four turns only make a sufficiently springy
+connection for nearly all purposes.</p>
+<p><b><a name="Toc158108909" id="Toc158108909"><font face=
+"Bookman Old Style" size="4">&sect; 37. On Auxiliary Operations
+on Glass:-</font></a></b></p>
+<p><font face="Bookman Old Style">Boring Holes through Glass.
+&mdash; This is much more easily done than is generally supposed.
+The best mode of procedure depends on the circumstances. The
+following three cases will be considered:-</font></p>
+<p>1. Boring holes up to one-quarter inch diameter through thick
+glass (say over one-eighth inch), or rather larger holes through
+thin glass.</p>
+<p>2. Boring holes of any size through thick glass.</p>
+<p>3. Boring round holes through ordinary window glass.</p>
+<p><b><a name="Toc158108910" id="Toc158108910"><font face=
+"Bookman Old Style" size="4">&sect; 38. Boring small Holes.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Take a three-cornered file of
+appropriate dimensions, and snip the point off by means of a
+hammer; grind out most of the file marks to get sharp corners.
+Dip the file in kerosene, and have plenty of kerosene at hand in
+a small pot. Place the broken end of the file against the glass,
+and with considerable pressure begin to rotate it (the file)
+backwards and forwards with the fingers, very much as one would
+operate a bradawl against a hard piece of wood. The surface of
+the glass will shortly be ground away, and then the file bradawl
+will make much quicker progress than might be expected. Two or
+three minutes should suffice to bore a bit of sheet
+window-glass.</font></p>
+<p>The following points require attention:</p>
+<p>(1) Use any quantity of oil.</p>
+<p>(2) After getting through the skin reduce the pressure on the
+file.</p>
+<p>(3) Be sure to turn the file backwards and forwards through a
+complete revolution at least.</p>
+<p>(4) When the hole is nearly through reduce the pressure.</p>
+<p>(5) When the hole is through the glass be exceedingly careful
+not to force the file through too rapidly, otherwise it will
+simply act as a wedge and cause a complete fracture.</p>
+<p>(6) In many cases it is better to harden the file in mercury
+before commencing operations; both files and glass differ so much
+in hardness that this point can only be decided by a trial. If it
+is found necessary to harden the file, use either a large
+blow-pipe and a coke or charcoal bed, or else a small forge. A
+small blowpipe, such as is generally found in laboratories, does
+more harm than good, either by burning the end of the file or
+raising it to an insufficient temperature.</p>
+<p>(7) To sharpen the file, which is often necessary after
+passing through the "skin" of the glass, put it in a vice so that
+the point just protrudes clear of the jaws. Then, using a bit of
+waste iron as an intermediary anvil or punch, knock off the least
+bit from the point, so as to expose a fresh natural surface. The
+same result may be brought about by the use of a pair of
+pliers.</p>
+<p>If several holes have to be bored, it is convenient to mount
+the file in the lathe and use a bit of flat hard wood to press up
+the glass by means of the back rest. A drilling machine, if not
+too heavy, does very well, and has the advantage of allowing the
+glass to remain horizontal so that plenty of oil can be kept in
+the hole.</p>
+<p>Use a very slow speed in either case &mdash; much slower than
+would be used for drilling wrought iron. It is essential that the
+lubricant should flow on to the end of the file very freely,
+either from a pipette or from the regular oil-feed. If a little
+chipping where the file pierces the back surface is inadmissible,
+it is better, on the whole, to finish the bore by hand, using a
+very taper file. It is not necessary to use a special file for
+the lathe, for a well-handled file can be chucked very
+conveniently in a three-jaw chuck by means of the handle.</p>
+<p>Mr. Shenstone recommends a lubricant composed of camphor
+dissolved in turpentine for general purposes. With the object of
+obtaining some decisive information as to the use of this
+lubricant, and to settle other points, I made the following
+experiments. Using an old three-cornered French file, I chipped
+off the point and adjusted the handle carefully. I also ground
+out the file marks near the point, without hardening the file in
+mercury. Using kerosene and turpentine and camphor, I began to
+bore holes in a hard bit of 3/32 inch window glass.</p>
+<p>Each hole was bored to about one-eighth inch in diameter in
+four minutes with either lubricant. After hardening the file in
+mercury and using kerosene, I also required four minutes per
+hole. After mounting the file in a lathe which had been speeded
+to turn up brass rods of about one-half-inch diameter, and
+therefore ran too fast, I required one and a half minutes per
+hole, and bored them right through, using kerosene. On the whole,
+I think kerosene does as well as anything, and for filing is, I
+think, better than the camphor solution. However, I ought to say
+that the camphor-turpentine compound has probably a good deal to
+recommend it, for it has survived from long ago. My assistant
+tells me he has seen his grandfather use it when filing
+glass.</p>
+<p>I beg to acknowledge my indebtedness to Mr. Pye, of the
+Cambridge Scientific Instrument Company, for showing me in 1886
+(by the courtesy of the Company) the file method of glass-boring;
+it is also described by Faraday in <i>Chemical Manipulation</i>,
+1228.</p>
+<p>It is not necessary, however, to use a file at all, for the
+twist drills made by the Morse Drill Company are quite hard
+enough in their natural state to bore glass. The circumferential
+speed of the drill should not much exceed 10 feet per minute. In
+this way the author has bored holes through glass an inch thick
+without any trouble except that of keeping the lubricant
+sufficiently supplied. For boring very small holes watchmaker's
+drills may be used perfectly well, especially those tempered for
+boring hardened steel. The only difficulty is in obtaining a
+sufficient supply of the lubricant, and to secure this the drill
+must be frequently withdrawn.</p>
+<p>My reason for describing the file method at such length is to
+be found in the fact that a Morse drill requires to be sharpened
+after drilling glass before it can be used in the ordinary way,
+and this is often a difficulty.</p>
+<p>I ought to say that I have never succeeded in boring the
+<i>barrel of a glass tap</i> by either of these methods.
+<i>[Footnote:</i> I have been lately informed that it is usual to
+employ a splinter of diamond set in a steel wire holder both for
+tap boring and for drilling earthenware for riveting. The diamond
+must, of course, be set so as to give sufficient clearance for
+the wire holder.</p>
+<p>For methods of using and setting diamond tools see &sect; 55.
+It will suffice to say here that a steel wire is softened and
+filed at one end so as to form a fork; into this the diamond is
+set by squeezing with pliers. The diamond is arranged so as to
+present a point in the axis of the wire, and must not project on
+one side of the wire more than on the other. It is not always
+easy to get a fragment satisfying these conditions, and at the
+same time suitable for mounting. A drop of solder occasionally
+assists the process of setting the diamond.</p>
+<p>In drilling, the diamond must be held against the work by a
+constant force, applied either by means of weight or a spring. I
+made many trials by this method, using a watchmaker's lathe and
+pressing up the work by a weight and string, which passed over a
+pulley. I used about 40 ounces, and drilled a hole 3/32 in
+diameter in flint glass at a speed of 900 revolutions per minute
+to a depth of one-eighth of an inch in eight minutes. I used soap
+and water as a lubricant, and the work was satisfactory.</p>
+<p>Since this was set up, I have been informed by Mr. Hicks of
+Hatton Garden that it is necessary to anneal glass rod by heating
+it up to the softening point and allowing it to cool very slowly
+under red-hot sand or asbestos before boring. If this be done, no
+trouble will be experienced. The annealing must be
+perfect.<i>]</i></p>
+<p><b><a name="Toc158108911" id="Toc158108911"><font face=
+"Bookman Old Style" size="4">&sect; 39. For boring large holes
+through thick glass sheets,</font></a></b></p>
+<p><font face="Bookman Old Style">or, indeed, through anything
+where it is necessary to make sure that no accident can happen,
+or where great precision of position and form of hole is
+required, I find a boring tube mounted as shown in the picture
+(Fig. 36) is of great service. Brass or iron tube borers do
+perfectly well, and the end of the spindle may be provided once
+for all with a small tube chuck, or the tubes may be separately
+mounted as shown. A fairly high speed is desirable, and may be
+obtained either by foot, or, if power is available, is readily
+got by connecting to the speed cone of a lathe, which is
+presumably permanently belted to the motor.</font></p>
+<p>Fig. 36. <img src="images/Image61.gif" alt=
+"images/Image61.gif" width="352" height="285"></p>
+<p>After trying tubes armed with diamond dust, as will be
+presently explained, I find that emery and thin oil or
+turpentine, if liberally supplied below the glass, will do very
+nearly as well. The tube should be allowed to rise from the work
+every few seconds, so as to allow of fresh emery and oil being
+carried into the circular grooves. This is done by lifting the
+hinged upper bearing, the drill being lifted by a spiral spring
+between the pulley and the lower bearing shown at B. The glass
+may be conveniently supported on a few sheets of paper if flat,
+or held firm in position by wooden clamps if of any other shape.
+In any case it should be firmly held down and should be well
+supported. Any desired pressure upon the drill is obtained by
+weighting the hinged board A.</p>
+<p>&sect; 40. The following method was shown to me by Mr.
+Wimshurst, but I have not had occasion to employ it myself. It is
+suitable for boring large holes through such glass as the plates
+of Mr. Wimshurst's Influence machines are usually made of. A
+diamond is mounted as the "pencil" of a compass, and with this a
+circle is drawn on the glass in the desired position. The other
+leg of the compass of course rests on a suitable washer.</p>
+<p>To the best of my recollection the further procedure was as
+follows. A piece of steel rod about one-eighth inch in diameter
+was ground off flat and mounted in a vice vertically, so as to
+cause its plane end to form a small horizontal anvil. The centre
+(approximately) of the diamond-cut circle of the glass was laid
+on this anvil so as to rest evenly upon it, and the upper surface
+(i.e. that containing the cut) was then struck smartly with a
+hammer, completely pulverising the glass above the anvil. The
+hole was gradually extended in a similar manner right up to the
+diamond cut, from which, of course, the glass broke away.</p>
+<p>A similar method has long been known to glaziers, differing
+from the preceding in that a series of diamond cuts are run
+across the circle parallel to two mutually perpendicular
+diameters. A smart tap on the back of the scored disc will
+generally cause the fragments to tumble out. I have never tried
+this myself, but I have seen it done.</p>
+<p>Large discs may easily be cut from sheet glass by drawing a
+circular diamond cut, and gradually breaking away the outer parts
+by the aid of additional cuts and a pair of pliers or "shanks"
+(see Fig. 44).</p>
+<p><b><a name="Toc158108912" id="Toc158108912"><font face=
+"Bookman Old Style" size="4">&sect; 41. Operations depending on
+Grinding: Ground-in Joints. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The process will be perfectly
+understood by reference to a simple case. Suppose it is desired
+to grind the end of a tube into the neck of a bottle. If a
+stoppered bottle is available, the stopper must be taken out and
+measured as to its diameter at the top and bottoM. Select a bit
+of tube as nearly as possible of the same diameter as the stopper
+at its thickest part. Draw down the glass in the blow-pipe flame
+rather by allowing it to sink than by pulling it out. After a few
+trials no difficulty will be experienced in making its taper
+nearly equal to that of the stopper, though there will in all
+probability be several ridges and inequalities. When this stage
+is reached anneal the work carefully and see that the glass is
+not too thin. Afterwards use emery and water, and grind the
+stopper into the bottle.</font></p>
+<p>There are six special directions to be note</p>
+<p>(1 )Turn the stopper through at least one revolution in each
+direction.</p>
+<p>(2) Lift it out often so as to give the fresh emery a chance
+of getting into the joint.</p>
+<p>(3) Rotate the bottle as well as the stopper in case there is
+any irregularity in the force brought to bear, which might cause
+one side of the neck to be more ground than another, or would
+cause the tube to set rather to one side or the other.</p>
+<p>(4) Use emery passing a 50 sieve, i.e. a sieve with fifty
+threads to the inch run (see &sect; 144) to begin with, and when
+the stopper nearly fits, wash this thoroughly away, and finish
+with flour emery, previously washed to get rid of particles of
+excessive size; the process of washing will be fully discussed in
+the chapter on glass-grinding, which see.</p>
+<p>(5) Any degree of fineness of surface may be obtained by using
+graded emery, as will be explained, but, in general, it is
+unnecessary to attempt a finer surface than can be got with
+washed flour emery. A superficial and imperfect polish may be
+given by grinding for a short time with powdered pumice
+stone.</p>
+<p>(6) If the proper taper is not attained by blowing, or if
+ridges are left on the tapered part, the process may be both
+hastened and improved by giving the taper a preliminary filing
+with a three-cornered file and kerosene, just as one would
+proceed with iron or brass. A little filing will often save a
+good deal of grinding and make a better job.</p>
+<p>If a bottle without a tapered neck is to be employed, it is as
+well to do the preliminary grinding by means of a cone turned up
+from a bit of cast iron. This is put in the lathe and pushed into
+the mouth of the bottle, the latter being supported by the hands.
+Use about the same surface speed as would be employed for turning
+cast iron. In this case the emery is better used with
+kerosene.</p>
+<p>If a cylindrical bit of cast iron about an inch in diameter is
+turned down conically nearly to a point, it will save a good deal
+of trouble in making separate cones. If it gets ground into
+rings, and it becomes necessary to turn it up, use a diamond tool
+until the skin is thoroughly removed; the embedded emery merely
+grinds the edge off any ordinary steel tool.</p>
+<p>For diamond tools see &sect; 55.</p>
+<p><b><a name="Toc158108913" id="Toc158108913"><font face=
+"Bookman Old Style" size="4">&sect; 42. Use of the Lathe in
+Glass-working. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">If it is necessary to remove a
+good deal of glass, time may be saved by actually turning the
+glass in a lathe. According to the direction given above for
+grinding a tube into the neck of a bottle, very little glass need
+be removed if the drawing down is well done, so that for this
+purpose turning is often unnecessary.</font></p>
+<p>If the taper of the stopper be small and it is permissible to
+use a thick tube, or if a solid stopper only has to be provided,
+or an old stopper quickly altered to a new form, turning is very
+useful. The glass may be "chucked" in any suitable manner, and
+run at a speed not exceeding 10 feet per minute. Prepare a
+three-cornered file by mercury-hardening and by grinding the end
+flat so as to form a cutting angle of about 80&deg;, and use a
+moderate amount of kerosene lubrication, i.e. enough to keep the
+glass damp, but even this is not essential. Use the file as an
+ordinary brass turning tool, and press much more lightly than for
+metal turning. The glass will be found to scrape off quite
+pleasantly.</p>
+<p>By chucking glass tubes on wooden mandrells the ends may be
+nicely turned in this manner ready for accurate closing by glass
+plates.</p>
+<p>The process of grinding also is made much more rapid &mdash;
+at all events in the earlier stages &mdash; by chucking either
+the stopper or the bottle and holding the other member in the
+fingers, or in a wooden vice held in the hands. The finishing
+touches are best given by hand.</p>
+<p>I ought to say that I think a good deal of glass-grinding, as
+practised in laboratories, might be advantageously replaced by
+glass turning or filing and certainly will be by any one who will
+give these methods a trial.</p>
+<p>If one tube is to be ground into another, as in grinding a
+retort into a receiver, the latter must be drawn down from a
+larger piece, few beginners being able to widen a tube by the
+method explained with sufficient ease and certainty. The other
+operations are similar to the operations above described.</p>
+<p>&sect; 43. Funnels often require to be ground to an angle of
+60&deg;. For this purpose it is well to keep a cast-iron cone,
+tapering from nothing up to four inches in diameter. This may be
+mounted on a lathe, and will be found of great use for grinding
+out the inside of funnels. Care must be taken to work the funnel
+backwards and forwards, or it will tend to grind so as to form
+rings, which interfere with filtering. A rough polish may be
+given on the lines explained in the next section.</p>
+<p>&sect; 44. A rough polish may be easily given to a surface
+which has been finished by washed flour emery, in the following
+manner. Turn up a disc of soft wood on the lathe, and run it at
+the highest wood-turning speed. Rub into the periphery a paste of
+sifted powdered pumice stone and water.</p>
+<p>Any fairly smooth ground glass surface may be more or less
+polished by holding it for a moment against the revolving disc.
+Exact means of polishing will be described later on. Meanwhile
+this simple method will be found both quick and convenient, and
+is often quite sufficient where transparency, rather than figure,
+is required. I daresay a fine polish may be got on the same
+lines, using putty powder or washed rouge (not jewellers' rouge,
+which is too soft, but glass-polishers' rouge) to follow the
+pumice powder, but I have not required to try this.</p>
+<p>&sect; 45. It is sometimes required to give to ground glass
+surfaces a temporary transparency. This is to be done by using a
+film of oil of the same refractive index as the glass. Cornu has
+employed a varnish consisting of a mixture of turpentine and oil
+of cloves, but the yellow-brown colour of the latter is often a
+disadvantage. It will be found that a mixture of nut oil and oil
+of bitter almonds, or of bromo-napthalene and acetone, can be
+made of only a faint yellow colour; and by exact adjustment of
+the proportions will have the same refractive index for any ray
+as crown glass (ordinary window glass).</p>
+<p>Procure a sample of the glass and smash it up to small
+fragments in an iron mortar. Sift out the fine dust and the
+larger pieces; bits about as large as small beads &mdash; say
+one-sixteenth inch every way &mdash; do very well. Boil the
+sifted glass with strong commercial hydrochloric acid to remove
+iron, wash with distilled water and a few drops of alcohol, dry
+on blotting paper in the sun or otherwise. Put the dry glass into
+a bottle or beaker, and begin by adding almond oil (or
+bromo-napthalene), then add nut oil (or acetone) till the glass
+practically disappears when examined by sodium light, or light of
+any other wave-length, as may be required.</p>
+<p>The adjustment of the mixture is a matter of great delicacy,
+one drop too much of either constituent, in, say, 50 cubic
+centimetres, makes all the difference. The final adjustment is
+best accomplished by having two mixtures of the oils, one just
+too rich in almond, the other in nut oil; by adding one or other
+of these, the required mixture is soon obtained.</p>
+<p><i>It is to be noted</i></p>
+<p>(1) That adjustment is only perfect for light of one
+wave-length.</p>
+<p>(2) That adjustment is only perfect at one temperature.</p>
+<p>On examining a bottle of rather larger fragments of glass
+immersed in an adjusted mixture by ordinary daylight, a
+peculiarly beautiful play of colours is seen.</p>
+<p>Of course, if it is only desired to make ground glass fairly
+transparent, these precautions are unnecessary, but it seemed
+better to dispose of the matter once for all in this
+connection.</p>
+<p>M. Cornu's object was to make a varnish which would prevent
+reflection from the back of a photographic plate on to the film.
+I have had occasion to require to do the same when using a scale
+made by cutting lines through a film of black varnish on a slip
+of glass. This succeeded perfectly by making the varnish out of
+Canada balsam stained with a black aniline dye.</p>
+<p>Mr. Russell, Government Astronomer of New South Wales, finds
+that the "halation" of star photographs can be prevented by
+pouring over the back of the plate a film of collodion suitably
+stained.</p>
+<p><b><a name="Toc158108914" id="Toc158108914"><font face=
+"Bookman Old Style" size="4">&sect; 46. Making Ground Glass.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is easily done by rubbing
+the surface of polished glass with a bit of cast iron and washed
+"flour of emery." Of course, if the fineness of grain of the
+surface is of importance, appropriate sizes of emery must be
+employed. The iron may be replaced by a bit of glass cut with
+transverse grooves to allow the emery to distribute itself, or
+even by a bit of glass without such grooves, provided it does not
+measure more than one or two inches each way. If great speed is
+an object rather than the fineness of the surface, use a bit of
+lead and coarse emery, say any that will pass a sieve with fifty
+threads to the inch.</font></p>
+<p>It may perhaps be mentioned here that it is a pity to throw
+away emery which has been used between glass and glass. In the
+chapter dealing with fine optical work the use of emery of
+various grades of fineness will be treated, and the finer grades
+can only be obtained (to my knowledge) from emery which has been
+crushed in the process of glass or metal grinding, especially the
+former. A large jam-pot covered with a cardboard lid does well as
+a receptacle of washings.</p>
+<p><b><a name="Toc158108915" id="Toc158108915"><font face=
+"Bookman Old Style" size="4">&sect; 47. Glass-cutting.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is an art about which more
+can be learned in five minutes by watching it well practised than
+by pages of written description. My advice to any one about to
+commence the practice of the art would be to make friends with a
+glazier and see it done. What follows is therefore on the
+supposition that this advice has been followed.</font></p>
+<p>After some experience of cutters made of especially hardened
+steel, I believe better work can generally be got out of a
+diamond, provided the cost is not an objection. It is economy to
+pay a good price for a good diamond. As is well known, the
+natural angle of the crystal makes the best point, and a person
+buying a diamond should examine the stone by the help of a lens,
+so as to see that this condition is fulfilled. The natural angle
+is generally, if not always, bounded by curved edges, which have
+a totally different appearance from the sharp edges of a
+"splinter."</p>
+<p>When a purchase is to be made, it is as well for the student
+to take a bit of glass and a foot-rule with him, and to test the
+diamond before it is taken away. When a good diamond has been
+procured, begin by taking cuts on bits of clean window glass
+until the proper angle at which to hold the tool is ascertained.
+Never try to cut over a scratch, if you value your diamond, and
+never press hard on the glass; a good cut is accompanied by an
+unmistakable ringing sound quite different from the sound made
+when the diamond is only scratching.</p>
+<p>Perhaps the most important advice that can be given is,
+<i>Never lend the diamond to anybody &mdash; under any
+circumstances</i>.</p>
+<p>The free use of a diamond is an art which the physicist will
+do well to acquire, for quite a variety of apparatus may be made
+out of glass strips, and the accuracy with which the glass breaks
+along a good cut reduces such an operation as glass-box-making to
+a question of accurate drawing.</p>
+<p><b><a name="Toc158108916" id="Toc158108916"><font face=
+"Bookman Old Style" size="4">&sect; 48. <i>Cementing</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">One of the matters which is
+generally confused by too great a profusion of treatment is the
+art of cementing glass to other substances.</font></p>
+<p>The following methods will be found to work, subject to two
+conditions:</p>
+<p>(1) The glass must be clean;</p>
+<p>(2) it must be hot enough to melt the cement.</p>
+<p>For ordinary mending purposes when the glass does not require
+to be placed in water (especially if hot) nothing is better than
+that kind of glue which is generally called "diamond cement."
+This may be easily made by dissolving the best procurable
+isinglass in a mixture of 20 per cent water and 80 per cent
+glacial acetic acid &mdash; the exact proportions are not of
+consequence.</p>
+<p>First, the isinglass is to be tightly packed into a bottle
+with a wide neck, then add the water, and let the isinglass soak
+it up. Afterwards pour in the acetic acid, and keep the mixture
+near 100&deg;C. for an hour or two on the water bath &mdash; or
+rather in it. The total volume of acetic acid and water should
+not be more than about half of the volume of isinglass when the
+latter is pressed into the bottle as tightly as possible.</p>
+<p>The proper consistency of the cement may be ascertained by
+lifting a drop out of the bottle and allowing it to cool on a
+sheet of glass. In ten minutes it ought not to be more than
+slightly sticky, and the mass in the bottle, after standing a few
+hours cold, should not be sticky at all, and should yield,
+jelly-like, to the pressure of the finger to only a slight
+degree. If the glue is too weak, more isinglass may be added
+(without any preliminary soaking).</p>
+<p>A person making the mixture for the first time almost always
+gets it too weak. It is difficult to give exact proportions by
+weight, as isinglass and gelatine (which may replace it) differ
+greatly in quality. This cement is applied like glue, and will
+cement nearly anything as well as glass. Of course, as much
+cement as possible must be squeezed out of any joint where it is
+employed. The addition of gums, as recommended in some books, is
+unnecessary.</p>
+<p>Ordinary glue will serve perfectly for cementing glass to
+wood.</p>
+<p>"Chipped glass" ware is, I understand, made by painting clean
+glass with glue. As the glue dries and breaks by contraction, it
+chips off the surface of the glass. I have never seen this done.
+In nearly all cases where alcohol is not to be employed very
+strong joints may be made by shellac. Orange shellac is stronger
+than the "bleached" variety.</p>
+<p>A <i>sine qua non</i> is that the glass be hot enough to melt
+the shellac. The best way is to heat the glass surfaces and rub
+on the shellac from a bit of flake; the glass should not be so
+hot as to discolour the shellac appreciably, or its valuable
+properties will be partly destroyed. Both glass surfaces being
+thus prepared, and the shellac being quite fluid on both, they
+may be brought together and clamped tightly together till cool.
+Shellac that has been overheated, or dissolved in alcohol, or
+bleached, is of little use as compared with the pale orange flaky
+product. Dark flakes have probably been overheated during the
+preliminary refining.</p>
+<p>For many purposes a cement is required capable of resisting
+carbon bisulphide. This is easily made by adding a little treacle
+(say 20 per cent) to ordinary glue. Since the mixture of glue and
+treacle does not keep, i.e. it cannot be satisfactorily melted up
+again after once it has set, no more should be made up than will
+be wanted at the time. If the glue be thick, glass boxes for
+carbon disulphide may be easily put together, even though the
+edges of the glass strips are not quite smooth, for, unlike most
+cements, this mixture remains tough, and is fairly strong in
+itself.</p>
+<p>I have found by experiment that most fixed and, to a less
+degree, essential oils have little or no solvent action on
+shellac, and I suspect that the same remark applies to the
+treacle-glue mixture, but I have not tried. Turpenes act on
+shellac slightly, but mineral oils apparently not at all. The
+tests on which these statements are based were continued for
+about two years, during which time kerosene and mineral oils had
+no observable effect on shellac-fastened galvanometer
+mirrors.</p>
+<p><b><a name="Toc158108917" id="Toc158108917"><font face=
+"Bookman Old Style" size="4">&sect; 49. Fusing Electrodes into
+Glass. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This art has greatly improved
+since the introduction of the incandescent lamp; however, up to
+the present, platinum seems to remain the only substance capable
+of giving a certainly air-tight result. I have not tried the
+aluminium-alumina method.</font></p>
+<p>Many years ago it was the fashion to surround the platinum
+wire with a drop of white enamel glass in order to cause better
+adhesion between it and the ordinary glass. <i>[Footnote:</i>
+Hittorf and Geissler (Pogg. Ann. 1864, &sect; 35; English
+translation, Phys. Soc. London, p. 138) found that it was
+impossible to make air-tight joints between platinum and hard
+potash glass, but that soft lead glass could be used with success
+as a cement.<i>]</i> However, in the case of flint glass, if one
+may judge from incandescent lamps, this is not essential &mdash;
+a fact which entirely coincides with my own experience.</p>
+<p>On the other hand, when sealing electrodes into German glass I
+have often used a drop of enamel with perfect results, though
+this is not always done in Germany. In all cases, however, in
+which electrodes have to be sealed in &mdash; especially when
+they are liable to heat &mdash; I recommend flint glass, and in
+this have the support of Mr. Rain (<i>The Incandescent Lamp and
+its Manufacture</i>, p. 131). The exact details for the
+preparation of eudiometer tubes are given by Faraday (<i>Chemical
+Manipulation</i>, &sect; 1200).</p>
+<p>In view of what has preceded, however, I will content myself
+with the following notes. Make the hole through which the wire is
+to protrude only slightly larger than the wire itself, and be
+sure that the latter is clean. Allow the glass to cool
+sufficiently not to stick to the wire when the latter is pushed
+in. Be sure that, on heating, the glass does not get reduced, and
+that it flows up to the wire all round; pull and push the wire a
+little with a pair of pincers, to ensure this.</p>
+<p>It is not a bad plan to get the glass exceedingly fluid round
+the wire &mdash; even if the lump has to be blown out a little
+afterwards--as it cools. The seal should finally be well annealed
+in asbestos, but first by gradually moving it into the hot air in
+front of the flame.</p>
+<p>It was observed by Professor J. J. Thomson and the author some
+years ago (<i>Proc</i>. Roy. Soc. 40. 331. 1886) that when very
+violent discharges are taken through lightly sealed-in electrodes
+in lead-glass tubes &mdash; say from a large battery of Leyden
+jars &mdash; gas appears to be carried into the tube over and
+above that naturally given off by the platinum, and this without
+there being any apparent want of perfection in the seal. This
+observation has since been confirmed by others. Consequently in
+experiments on violent discharges in vacuo where certainty is
+required as to the exclusion of air, the seals should be
+protected by a guard tube or cap containing mercury; this must,
+of course, be put in hot and clean, on hot and clean glass, and
+in special cases should be boiled in situ.</p>
+<p>A well-known German physicist (Warburg, I think) recommends
+putting the seals under water, but I cannot think that this is a
+good plan, for if air can get in, why not water? which has its
+surface tension in its favour. The same reasoning prevents my
+recommending a layer of sulphuric acid above the mercury-a method
+used for securing air-tightness in "mercury joints" by Mr.
+Gimingham, Proc. R. S. 1874.</p>
+<p>Further protection may be attained for many purposes by
+coating the platinum wire with a sheath of glass, say half an
+inch long, fused to the platinum wire to a depth of one-twentieth
+of an inch all round.</p>
+<p>In some cases the electrodes must be expected to get very hot,
+for instance, when it is desired to platinise mirrors by the
+device of Professor Wright of Yale. In this and similar cases I
+have met with great success by using "barometer" tubes of about
+one-twelfth of an inch bore, and with walls, say, one-tenth of an
+inch thick. <i>[Footnote:</i> "Barometer" tube is merely very
+thick-walled glass tubing, and makes particularly bad barometers,
+which are sold as weather glasses.<i>]</i></p>
+<p>This tube is drawn down to a long point &mdash; say an inch
+long by one-eighth of an inch external diameter, and the wire is
+fused in for a length, say, of three-quarters of an inch, but
+only in the narrow drawn-down part of the tube. At different
+times I have tried four such seals, and though the electrodes
+were red hot for hours, I have never had an accident &mdash; of
+course they were well annealed.</p>
+<p>Fig. <img src="images/Image62.gif" alt="images/Image62.gif"
+width="439" height="52"> 37.</p>
+<p>For directions as to the making of high vacuum tubes, see the
+section dealing with that matter.</p>
+<p>&sect; 50. As economy of platinum is often of importance, the
+following little art will save money and trouble. Platinum is
+easily caused to join most firmly to copper &mdash; with which, I
+presume, it alloys &mdash; by the following method. Hold the
+platinum wire against the copper wire, end to end, at the tip of
+the reducing flame of a typical blowpipe &mdash; or anywhere
+&mdash; preferably in the "reducing" part of the oxygas flame; in
+a moment the metals will fuse together at the point of contact,
+when they may be withdrawn.</p>
+<p>Such a joint is very strong and wholly satisfactory, much
+better than a soldered joint. If the work is not carried out
+successfully so that a considerable drop of copper-platinum alloy
+accumulates, cut it off and start again. The essence of success
+is speed, so that the copper does not get "burned." If any
+considerable quantity of alloy is formed it dissolves the copper,
+and weakens it, so that we have first the platinum wire, then a
+bead of alloy, and then a copper wire fused into the bead, but so
+thin just outside the latter that the joint has no mechanical
+strength.</p>
+<p><b><a name="Toc158108918" id="Toc158108918"><font face=
+"Bookman Old Style" size="4">&sect; 51. The Art of making
+Air-light Joints. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Lamp-manufacturers and others
+have long since learned that when glass is in question not only
+are fused joints made as easily as others, but that they afford
+the only reliable form of joint. An experimenter who uses flint
+glass, has a little experience, an oxygas blow-pipe and a blowing
+apparatus, will prefer to make his joints in this way, simply
+from the ease with which it may be done. When it comes to making
+a tight joint between glass and other substances the problem is
+by no means so easy. Thus Mr. Griffiths (Phil. Trans. 1893, p.
+380) failed to make air-tight joints by cementing glass into
+steel tubes, using hard shellac, and the tubes fitting closely.
+These joints were satisfactory at first, but did not last; the
+length of the joint is not stated. The difficulty was finally got
+over by soldering very narrow platinum tubes into the steel, and
+fusing the former into the glass.</font></p>
+<p>Mr. Griffiths has since used an alloy with success as a
+cement, but I cannot discover what it is made from. Many years
+ago Professor Hittorf prepared good high vacuum tubes by plugging
+the ends of glass tubes with sealing wax merely, though in all
+cases the spaces to be filled with wax were long and narrow
+(Hittorf, Pogg. Ann. 1869, &sect; 5, English translation, Phys.
+Soc. p. 113). Again, Regnault habitually used brass ferules, and
+cemented glass into them by means of his mastic, which can still
+be procured at a low rate from his instrument-makers (Golan,
+Paris). Lenard also, in his investigations on Cathode Rays (Wied.
+Ann., vol. li. p. 224), made use of sealing wax covered with
+marine glue.</p>
+<p>Surely in face of these facts we must admit that cement joints
+can be made with fair success. I do not know the composition of
+M. Regnault's mastic, but Faraday (Manipulations, &sect; 1123)
+gives the following receipt for a cement for joining ferules to
+retorts, etc. &mdash;</p>
+<div style="margin-left: 4em">
+<p>Resin 5 parts</p>
+<p>Beeswax 1 part</p>
+<p>Red ochre or Venetian red,</p>
+<p>finely powdered and sifted 1 part</p>
+</div>
+<p>I believe this to be substantially the same as Regnault's
+mastic, though I have never analysed the latter.</p>
+<p>For chemical work the possibility of evolution of gas from
+such a cement must be taken into account, and I should certainly
+not trust it for this reason in vacuum tube work, where the
+purity of the confined gas could come in question. Otherwise it
+is an excellent cement, and does not in my experience tend to
+crack away from glass to the same extent as paraffin or pure
+shellac.</p>
+<p>This cracking away from glass, by the way, is probably an
+effect of difference in rate of expansion between the glass and
+cement which probably always exists, and, if the cement be not
+sufficiently viscous, must, beyond certain temperature limits,
+either produce cracks or cause separation. Professor Wright of
+Yale has used a hard mineral pitch as a cement in vacuum work
+with success.</p>
+<p>My attention has been directed to a fusible metal cement
+containing mercury, and made according to the following receipt,
+given by Mr. S. G. Rawson, Journal of the Society of Chemical
+Industry, vol. ix. (1890), P. 150:&mdash;</p>
+<div style="margin-left: 4em">
+<p>Bismuth 40 per cent</p>
+<p>Lead 25 per cent</p>
+<p>Tin 10 per cent</p>
+<p>Cadmium 10 per cent</p>
+<p>Mercury 15 per cent</p>
+</div>
+<p>This is practically one form of Rose's fusible metal with 15
+per cent mercury added. It takes nearly an hour to set
+completely, and the apparatus must be clean and warm before it is
+applied.</p>
+<p>As the result of several trials by myself and friends, I am
+afraid I must dissent from the claim of the author that such a
+cement will make a really air-tight joint between glass tubes.
+Indeed, the appearance of the surface as viewed through the glass
+is not such as to give any confidence, no matter what care may
+have been exercised in performing all the operations and cleaning
+the glass; besides which the cement is rigid when cold, and the
+expansion difficulty comes in.</p>
+<p>On the other hand, if extreme air-tightness is not an object,
+the cement is strong and easily applied, and has many uses. I
+have an idea that if the joints were covered with a layer of soft
+wax, the result would be satisfactory in so far as air-tightness
+is concerned.</p>
+<p>This anticipation has since been verified.</p>
+<p>In many cases one can resort to the device already mentioned
+of enclosing a rubber or tape-wrapped joint between two tubes in
+a bath of mercury, but in this case the glass must be clean and
+hot and the mercury also warm, dry, and pure when the joint is
+put together, otherwise an appreciable air film is left against
+the glass, and this may creep into the joint.</p>
+<p>Perhaps the easiest way of making such a joint is to use an
+outer tube of thin clean glass, and bore a narrow hole into it
+from one side to admit the mercury; if the mercury is to be
+heated in vacuo, it is better to seal on a side joint. It is
+always better, if possible, to boil the mercury in situ, which
+involves making the wrapping of asbestos, but, after all, we come
+back to the position I began by taking up, viz. that the easiest
+and most reliable method is by fusion of the glass &mdash; all
+the rest are unsuitable for work of real precision.</p>
+<p>I should be ungrateful, however, were I not to devote a few
+lines to the great convenience and merit of so-called "centering
+cement." This substance has two or three very valuable
+properties. It is very tough and strong in itself, and it remains
+plastic on cooling for some time before it really sets. If for
+any reason a small tube has to be cemented into a larger one,
+which is a good deal larger, so that an appreciable mass of
+cement is necessary, and particularly if the joint requires to
+have great mechanical strength, this cement is invaluable. I have
+even used a plug of it instead of a cork for making the joint
+between a gas delivery tube and a calcium chloride tower. (Why
+are these affairs made with such abominable tubulures?)</p>
+<p>The joint in question has never allowed the tube to sag though
+it projects horizontally to a distance of 6 inches, and has had
+to withstand nearly two years of Sydney temperature. The cement
+consists of a mixture of shellac and 10 per cent of oil of
+cassia.</p>
+<p>The shellac is first melted in an iron ladle, and the oil of
+cassia quickly added and stirred in, to an extent of about 10 per
+cent, but the exact proportions are not of importance. Great care
+must be taken not to overheat the shellac.</p>
+<p><b><a name="Toc158108919" id="Toc158108919"><font face=
+"Bookman Old Style" size="4">APPENDIX TO CHAPTER
+I</font></a></b></p>
+<p><a name="Toc158108920" id="Toc158108920">ON THE PREPARATION OF
+VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR ROENTGEN'S
+RADIATION</a></p>
+<p><i><font face="Bookman Old Style">[Footnote:</font> Written in
+May 1896.]</i></p>
+<p>WHEN Professor Roentgen's discovery was first announced at the
+end of 1895 much difficulty was experienced in obtaining
+radiation of the requisite intensity for the repetition of his
+experiments. The following notes on the production of vacuum
+tubes of the required quality may therefore be of use to those
+who desire to prepare their own apparatus. It appears that flint
+glass is much more opaque to Roentgen's radiation than soda
+glass, and consequently the vacuum tubes require to be prepared
+from the latter material.</p>
+<p>Fig. <img src="images/Image63.gif" alt="images/Image63.gif"
+width="412" height="262">39.</p>
+<p>A form of vacuum tube which has proved very successful in the
+author's hands is sketched in Fig. 38. It is most easily
+constructed as follows. A bit of tubing about 2 centimetres
+diameter, 15 centimetres long, and 1.5 millimetre wall thickness,
+is drawn down to a point. The larger bulb, about 5 centimetres in
+diameter, is blown at one end of this tube. The thinner the bulb
+the better, provided that it does not collapse under atmospheric
+pressure. A very good idea of a proper thickness may be obtained
+from the statement that about 4 centimetres length of the tubing
+should be blown out to form the bulb. This would give a bulb of
+about the thickness of an ordinary fractionating bulb. Before
+going any further it is as well to test the bulb by tapping on
+the table and by exhausting it by means of an ordinary
+water-velocity pump.</p>
+<p>The side tube is next prepared out of narrower tubing, and is
+provided with a smaller bulb, a blowing-out tube, and a terminal,
+to be made as will be described. This side tube is next fused on
+to the main tube, special care being taken about the annealing,
+and the cathode terminal is then sealed into the main tube. After
+using clean glass it is in general only necessary to rinse the
+tube out with clean alcohol, after which it may be dried and
+exhausted.</p>
+<p>The success of the operation will depend primarily on the
+attention given to the preparation and sealing-in of the
+electrode facing the large bulb.</p>
+<p><i>Preparation of Terminals</i>. &mdash; Some platinum wire of
+about No. 26 B.W.G. &mdash; the exact size is unimportant &mdash;
+must be provided, also some sheet aluminium about 1 millimetre
+thick, some white enamel cement glass, and a "cane" of
+flint-glass tube of a few millimetres bore.</p>
+<p>The electrodes are prepared by cutting discs of aluminium of
+from 1 to 1.5 centimetres diameter. The discs of aluminium are
+bored in the centre, so as to admit the "stems" which are made
+of aluminium wire of about 1 millimetre diameter. The stems are
+then riveted into the discs. The "stems" are about I centimetre
+long, and are drilled to a depth of about 3 millimetres, the
+drill used being about double the diameter of the platinum wire
+to be used for making the connections. The faces of the
+electrodes &mdash; i.e. the free surfaces of the aluminium discs
+&mdash; are then hammered flat and brought to a burnished surface
+by being placed on a bit of highly polished steel and struck by a
+"set" provided with a hole to allow of the "stem" escaping
+damage. The operation will be obvious after a reference to Figs.
+39 and 40; it is referred to again on page 96.</p>
+<p>The platinum wires may be most conveniently attached by
+melting one end of the piece of platinum wire in the oxygas
+blow-pipe till it forms a bead just large enough to pass into the
+hole drilled up the stem of the electrode. The junction between
+the stein and the platinum wire is then made permanent by
+squeezing the aluminium down upon the platinum wire with the help
+of a pair of pliers. It is also possible to fuse the aluminium
+round the platinum, but as I have had several breakages of such
+joints, I prefer the mechanical connection described.</p>
+<p><img src="images/Image64.gif" alt="images/Image64.gif" width=
+"245" height="159">Fig. 39. &mdash; Sets for striking aluminium
+electrodes</p>
+<p>Fig. <img src="images/Image65.gif" alt="images/Image65.gif"
+width="287" height="139"> 40.-</p>
+<ol type="i">
+<li>Aluminium electrode.</li>
+<li>Aluminium electrode connected to platinum wire.</li>
+<li>Aluminium electrode connected to platinum wire and protected
+by glass.</li>
+<li>Detail of fastening platinum wire.</li>
+</ol>
+<p>The stem and platinum wire may now be protected by covering
+them with a little flint glass. For this purpose the flint-glass
+tube is pulled down till it will just slip over the stem and
+wire, and is cut off so as to leave about half a centimetre of
+platinum wire projecting. The flint-glass tube is then fused down
+upon the platinum wire, care being taken to avoid the presence of
+air bubbles. At the close of the operation a single drop of white
+enamel glass is fused round the platinum wire at a high
+temperature, so as to make a good joint with the protecting
+flint-glass tube.</p>
+<p>The negative electrode being nearly as large as the main tube,
+it must be introduced before the latter is drawn down for
+sealing. After drawing down the main tube in the usual manner,
+taking care not to make it less than a millimetre in
+wall-thickness, it is cut off so as to leave a hole not quite big
+enough for the enamel drop to pass through. By heating and
+opening, the aperture is got just large enough to allow the
+enamel drop to pass into it, and when this is the case the joint
+is sealed, pulled, and blown out until the electrode occupies the
+right position &mdash; viz. in the centre of the tube and with
+its face normal to the axis of the tube.</p>
+<p>The glass walls near the negative electrode must not be less
+than a millimetre thick, and may be rather more with advantage,
+the glass must be even, and the joint between the flint glass and
+the soda glass, or between the wire and the soda glass, must be
+wholly through the enamel. The "seal" must be well annealed. It
+will be found that the sealing-in process is much easier when the
+stem of the electrode is short and when the glass coating is not
+too heavy. Half a millimetre of glass thickness round the stein
+is quite sufficient.</p>
+<p>The diagram, of the tube shows that the main tube has been
+expanded round the edges of the cathode. This is to reduce the
+heating consequent on the projection of cathode rays from the
+edges of the disc against the glass tube.</p>
+<p>The anode is inserted into its bulb in a quite similar manner.
+If desired it may be made considerably smaller, and does not need
+the careful adjustment requisite in sealing-in the cathode, nor
+does the glass near the entry wire require to be so thick.</p>
+<p>More intense effects are often got by making the cathode
+slightly concave, but in this case the risk of melting the thin
+glass is considerably increased. No doubt, Bohemian glass might
+be used throughout instead of soft soda glass, and this would not
+melt so easily; the difficulties of manipulating the glass are,
+however, more pronounced.</p>
+<p>It will be shown directly that the best Roentgen effects are
+got with a high vacuum, and it is for this reason that the glass
+near the cathode seal requires to be strong. The potential right
+up to the cathode is strongly positive inside the tube, and this
+causes the glass to be exposed to a strong electric stress in the
+neighbourhood of the seal.</p>
+<p>Although the GLASS-BLOWING involved in the making of a
+so-called focus tube is rather more difficult than in the case
+just described, there is no reason why such a difficulty should
+not be overcome; I will therefore explain how a focus tube may be
+made.</p>
+<p>Fig. <img src="images/Image66.gif" alt="images/Image66.gif"
+width="403" height="386">41.</p>
+<p>A bulb about 3 inches in diameter is blown from a bit of tube
+of a little more than 1 inch diameter. Unless the walls of the
+tube are about one-eighth of an inch in thickness, this will
+involve a preliminary thickening up of the glass. This is not
+difficult if care be taken to avoid making the glass too hot. The
+larger gas jet described in connection with the
+soda-GLASS-BLOWING table must be employed. In blowing a bulb of
+this size it must not be forgotten that draughts exercise a very
+injurious influence by causing the glass to cool unequally; this
+leads to bulbs of irregular shape.</p>
+<p>In the method of construction shown in Fig. 41, the anode is
+put in first. This anode simply consists of a square bit of
+platinum or platinum-iridium foil, measuring about 0.75 inch by 1
+inch, and riveted on to a bent aluminium wire stem.</p>
+<p>As soon as the anode is fused in, and while the glass is still
+hot, the side tube is put on. The whole of the anode end is then
+carefully annealed. When the annealing is finished the side tube
+is bent as shown to serve as a handle when the time comes to
+mount the cathode. Before placing the cathode in position, and
+while the main tube is still wide open, the anode is adjusted by
+means of a tool thrust in through this open end. This is
+necessary in view of the fact that the platinum foil is
+occasionally bent during the operation of forcing the anode into
+the bulb.</p>
+<p>The cathode is a portion of a spherical surface of polished
+aluminium, a mode of preparing which will be given directly. The
+cathode having been placed inside the bulb, the wide glass tube
+is carefully drawn down and cut off at such a point that when the
+cathode is in position its centre of curvature will lie slightly
+in front of the anode plate. For instance, if the radius of
+curvature of the cathode be 1.5 inches, the centre of curvature
+may lie something like an eighth of an inch or less in front of
+the anode.</p>
+<p>The cathode as shown in Fig. 41 is rather smaller than is
+advantageous. To make it much larger than is shown, however, the
+opening into the bulb would require to be considerably widened,
+and though this is not really a difficult operation, still it
+requires more practice than my readers are likely to have had.
+The difficulty is not so much in widening out the entry as in
+closing it down again neatly.</p>
+<p>Now as to making the anode. A disc of aluminium is cut from a
+sheet which must not be too thick &mdash; one twenty-fifth of an
+inch is quite thick enough. This disc is bored at the centre to
+allow of the stem being riveted in position. The disc is then
+annealed in the Bunsen flame and the stem riveted on.</p>
+<p>The curvature is best got by striking between steel dies (see
+Figs. 39 and 40). Two bits of tool steel are softened and turned
+on the lathe, one convex and the other concave. The concave die
+has a small hole drilled up the centre to admit the stem. The
+desired radius of curvature is easily attained by cutting out
+templates from sheet zinc and using them to gauge the turning.
+The two dies are slightly ground together on the lathe with emery
+and oil and are then polished, or rather the convex die is
+polished &mdash; the other one does not matter. The polishing is
+most easily done by using graded emery and oil and polishing with
+a rag. The method of grading emery will be described in the
+chapter on glass-grinding.</p>
+<p>The aluminium disc is now struck between the dies by means of
+a hammer. If the radius of curvature is anything more than one
+inch and the disc not more than one inch in diameter the cathode
+can be struck at once from the flat as described. For very deep
+curves no doubt it will be better to make an intermediate pair of
+dies and to re-anneal the aluminium after the first striking.</p>
+<p>When the tube is successfully prepared so far as the
+glassblowing goes it may be rinsed with strong pure alcohol both
+inside and out, and dried. The straight part of the side tube is
+then constricted ready for fusing off and the whole affair is
+placed on the vacuum pump.</p>
+<p>In spite of the great improvements made during recent years in
+the construction of so-called Geissler vacuum pumps &mdash; i.e.
+pumps in which a Torricellian vacuum is continually reproduced
+&mdash; I am of opinion that Sprengel pumps are, on the whole,
+more convenient for exhausting Crooke's tubes. A full discussion
+of the subject of vacuum pumps will be found in a work by Mr. G.
+S. Ram (The Incandescent Lamp and its Manufacture), published by
+the Electrician Publishing Company, and it is not my intention to
+deal with the matter here; the simplest kind of Sprengel pump
+will be found quite adequate for our purpose, provided that it is
+well made.</p>
+<p>Fig. 42 is intended to represent a modification of a pump
+based on the model manufactured by Hicks of Hatton Garden, and
+arranged to suit the amateur glass-blower. The only point of
+importance is the construction of the head of the fall tube, of
+which a separate and enlarged diagram is given. The fall tubes
+may have an internal diameter up to 2 mm. (two millimetres) and
+an effective length of 120 cm.</p>
+<p>Free use is made of rubber tube connections in the part of the
+pump exposed to the passage of mercury. The rubber employed
+should be black and of the highest quality, having the walls
+strengthened by a layer of canvas. If such tube cannot be easily
+obtained, a very good substitute may be made by placing a bit of
+ordinary black tube inside another and rather larger bit and
+binding the outer tube with tape or ribbon. In any case the
+tubing which comes in contact with the mercury should be boiled
+in strong caustic potash or soda solution for at least ten
+minutes to get rid of free sulphur, which fouls the mercury
+directly it comes in contact with it. The tubing is well washed,
+rinsed with alcohol, and carefully dried.</p>
+<p><img src="images/Image67.gif" alt="images/Image67.gif" width=
+"152" height="545">Fig. 42.</p>
+<p>The diagram represents what is practically a system of three
+Sprengel pumps, though they are all fed from the same mercury
+reservoir and run down into the same mercury receiver. It is much
+easier to make three pumps, each with separate pinch cocks to
+regulate the mercury supply, than it is to make three jets, each
+delivering exactly the proper stream of mercury to three fall
+tubes.</p>
+<p>Sprengel pumps only work at their highest efficiency when the
+mercury supply is carefully regulated to suit the peculiarities
+of each fall tube, and this is quite easily done in the model
+figured. Since on starting the pump the rubber connections have
+to stand a considerable pressure, the ends of the tubes must be
+somewhat corrugated to enable the rubber to be firmly wired on to
+them. The best binding wire is the purest Swedish iron wire,
+previously annealed in a Bunsen gas flame.</p>
+<p>The wire must never be twisted down on the bare rubber, but
+must always be separated from it by a tape binding. By taking
+this precaution the wire maybe twisted very much more tightly
+than is otherwise possible without cutting the rubber.</p>
+<p>The only difficulty in making such a pump as is described lies
+in the bending of the heads of the fall tubes. This bending must
+be done with perfect regularity and neatness, otherwise the drops
+of mercury will not break regularly, or will break just inside
+the top of the fall tube, and so obstruct its entrance that at
+high vacua no air can get into the tube at all.</p>
+<p>The connections at the head of the fall tubes must also be
+well put on and the joints blown out so that the mercury in
+dropping over the head is not interfered with by the upper
+surface of the tube. However, a glance at the enlarged diagram
+will show what is to be aimed at better than any amount of
+description. In preparing the fall tubes it is generally
+necessary to join at least two "canes" together. The joint must
+be arranged to occur either in the tube leading the mercury to
+the head of the fall, or in that part of the fall tube which
+remains full of mercury when the highest vacuum is attained. On
+no account must the joint be made at the fall itself (at least
+not by an amateur), nor in that part of the fall tube where the
+mercury falls freely, particularly at its lower end, where the
+drops fall on the head of the column of mercury.</p>
+<p>When a high vacuum is attained the efficacy of the pump
+depends chiefly on the way in which the drops fall on the head of
+the column. If the fall is too long the drops are apt to break up
+and allow the small bubble of air to escape up the tube, also any
+irregularity or dirt in the tube at this point makes it more easy
+for the bubbles of air to escape to the surface of the
+mercury.</p>
+<p>Any pump in which the supply of mercury to the fall tube can
+be regulated nicely will pump well until the lowest available
+pressures are being attained; a good pump will then continue to
+hold the air bubbles, while a bad one will allow them to slip
+back <i>[Footnote:</i> For special methods of avoiding this
+difficulty see Mr. Ram's book.<i>]</i> &hellip;</p>
+<p>Though three fall tubes are recommended, it must not be
+supposed that the pump will produce a Crooke's vacuum three times
+more rapidly than one fall tube. Until the mercury commences to
+hammer in the pump the three tubes will pump approximately three
+times faster than one tube, but as soon as the major portion of
+the air collected begins to come from the layer condensed on the
+glass surface of the tube to be exhausted and from the
+electrodes, the rate at which exhaustion will go on no longer
+depends entirely on the pump.</p>
+<p>In order that bubbles of air may not slip back up the fall
+tube it is generally desirable to allow the mercury to fall
+pretty briskly, and in this case the capacity of the pump to take
+air is generally far in excess of the air supply. One advantage
+of having more than one fall tube is that it often happens that a
+fall tube gets soiled during the process of exhaustion and no
+longer works up to its best performance. Out of three fall tubes,
+however, one is pretty sure to be working well, and as soon as
+the mercury begins to hammer in the tubes the supply may be shut
+off from the two falls which are working least
+satisfactorily.</p>
+<p>Thus we are enabled to pump rapidly till a high degree of
+exhaustion is attained, having practically three pumps instead of
+one, whereas when the final stages are reached, and three pumps
+are only a drawback in that they increase the mercury flow, the
+apparatus is capable of instant modification to meet the new
+conditions.</p>
+<p>The thistle funnels at the head of the fall tubes are made
+simply by blowing bulbs and then blowing the heads of the bulbs
+into wider ones, and finally blowing the heads of the wider bulbs
+off by vigorous blowing. The stoppers are ground in on the lathe
+before the tubes are attached to the fall tubes. The stoppers
+require to be at least half an inch long where they fit the
+necks, and must be really well ground in. The stoppers must first
+be turned up nicely and the necks ground out by a copper or iron
+cone and emery. The stoppers are rotated on a lathe at quite a
+slow speed, say 30 or 40 feet per minute, and the necks are held
+against them, as described in the section dealing with this art.
+The stoppers must in this case be finished with "two seconds"
+emery, and lastly with pumice dust and water (see chapter on
+glass-grinding).</p>
+<p>Unless the stoppers fit exceedingly well trouble will arise
+from the mercury (which is poured into the thistle heads to form
+a seal) being forced downwards into the pump by atmospheric
+pressure.</p>
+<p>The joints between the three fall tubes and the single exhaust
+main are easily made when the tubes are finally mounted, the
+hooked nozzle of the oxygas blow-pipe being expressly made for
+such work.</p>
+<p>It is, on the whole, advisable to make the pump of flint
+glass, or at all events the air-trap tube and the fall tubes. A
+brush flame from the larger gas tube of the single blowpipe table
+is most suitable for the work of bending the tubes. The jointing
+of the long, narrow bore fall tubes is best accomplished by the
+oxygas flame, for in this way the minimum of irregularity is
+produced; the blowing tubes will of course be required for the
+job, and the narrow tubes must be well cleaned to begin with.</p>
+<p>The air trap is an important though simple part of the pump.
+Its shoulder or fall should stand rather higher than the
+shoulders of the fall tubes, so that the mercury may run in a
+thin stream through a good Torricellian vacuum before it passes
+down to the fall tubes. This is easily attained by regulating the
+main mercury supply at the pinch cock situated between the tube
+from the upper reservoir and the air-trap tube, the other cocks
+being almost wide open.</p>
+<p>It might be thought that the mercury would tend to pick up air
+in passing through the rubber connections to the fall tubes, but
+I have not found this to be the case in practice. There is, of
+course, no difficulty in eliminating the rubber connections
+between the fall tubes and the mercury supply from the air trap,
+but it impresses a greater rigidity on the structure and, as I
+say, is not in general necessary. It must not be forgotten that
+the mercury always exercises considerable pressure on the rubber
+joints, and so there is little tendency for gas to come out of
+the rubber.</p>
+<p>The thistle funnels at the head of the fall tubes provide a
+simple and excellent means of cleaning the fall tubes. For this
+purpose some "pure" sulphuric acid which has been boiled with
+pure ammonium sulphate is placed in each thistle funnel, and when
+the fall tube is dirty the connection to the mercury supply is
+cut off at the pinch cock so as to leave the tube between this
+entry and the head of the fall tube quite full of mercury, and
+the sulphuric acid is allowed to run down the fall tube by
+raising the stopper. The fall tube should be allowed to stand
+full of acid for an hour or so, after which it will be found to
+be fairly clean.</p>
+<p>Of course the mercury reservoir thus obtains a layer of acid
+above the mercury, and as it is better not to run the risk of any
+acid getting into the pump except in the fall tubes, the
+reservoir is best emptied from the bottom, by a syphon, if a
+suitable vessel cannot be procured, so that clean mercury only is
+withdrawn.</p>
+<p>The phosphorus pentoxide tube is best made as shown simply
+from a bit of wide tube, with two side connections fused to the
+rest of the pump. It is no more trouble to cut the tube and fuse
+it up again when the drying material is renewed than to adjust
+the drying tube to two fixed stoppers, which is the alternative.
+The practice here recommended is rendered possible only by the
+oxygas blow-pipe with hooked nozzle. The connection between the
+pump and tube to be exhausted is made simply by a short bit of
+rubber tube immersed in mercury.</p>
+<p>The phosphorus pentoxide should be pure, or rather free from
+phosphorus and lower oxides; unless this be the case, the vapour
+arising from it is apt to soil the mercury in the pump. The
+phosphorus pentoxide is purified by distilling with oxygen over
+red-hot platinum black; if this cannot be done, the pentoxide
+should at least be strongly heated in a tube, in a current of dry
+air or oxygen, before it is placed in the drying tube.</p>
+<p>The mercury used for the pump must be scrupulously clean. It
+does not, however, require to have been distilled in vacuo. It is
+sufficient to purify it by allowing it to fall in a fine spray
+into a large or rather tall jar of 25 per cent nitric acid and 75
+per cent water. The mercury is then to be washed and dried by
+heating to, say, 110&deg; C. in a porcelain dish.</p>
+<p><i>Exhausting a Roentgen Tube.</i> &mdash;</p>
+<p>With a pump such as has been described there is seldom any
+advantage in fusing an extra connection to the vacuum tube so as
+to allow of a preliminary exhaustion by means of a water pump.
+About half an hour's pumping may possibly be saved by making use
+of a water pump.</p>
+<p>The tube to be exhausted is washed and dried by careful
+heating over a Bunsen burner and by the passage of a current of
+air. The exhausting tube is then drawn down preparatory to
+sealing off, and the apparatus placed upon the pump. It is best
+held in position by a wooden clamp supported by a long retort
+stand.</p>
+<p>Exhaustion may proceed till the mercury in the fall tubes
+commences to hammer. At this point the tube must be carefully
+heated by a Bunsen flame, the temperature being brought up to,
+say, 400&deg; C. The heating may be continued intermittently till
+little or no effect due to the heating is discernible at the
+pump. When this stage is reached, or even before, the electrodes
+may be connected up to the coil and a discharge sent through the
+tube.</p>
+<p>Care must be taken to stop the discharge as soon as a purple
+glow begins to appear, because when this happens, the resistance
+of the tube is very low, the electrodes get very hot, and may
+easily get damaged by a powerful discharge, and the platinum of
+the anode (if a focus tube is in question) begins to be distilled
+on to the glass. The heating and sparking are to be continued
+till the resistance of the tube sharply increases. This is tested
+by always having a spark gap, conveniently formed by the coil
+terminals, in parallel with the tube. If the terminals are
+points, it is convenient to set them at about one quarter of an
+inch distance apart.</p>
+<p>As soon as sparks begin to pass between the terminals of the
+spark gap it becomes necessary to watch the process of exhaustion
+very carefully. In the first place, stop the pump, but let the
+coil run, and note whether the sparks continue to flow over the
+terminals. If the glass and electrodes are getting gas free, the
+discharge will continue to pass by the spark gap, but if gas is
+still being freely given off, then in perhaps three minutes the
+discharge will return to the tube, and pumping must be
+recommenced. The Roentgen effect only begins to appear when the
+tube has got to so high a state of exhaustion that the resistance
+increases rapidly.</p>
+<p>By pumping and sparking, the resistance of the tube may be
+gradually raised till the spark would rather jump over 2 inches
+of air than go through the tube. When this state is attained the
+Roentgen effect as tested by a screen of calcium tungstate should
+be very brilliant. No conclusion as to the equivalent resistance
+of the tube can be arrived at so long as the discharge is kept
+going continually. When the spark would rather go over an inch of
+air in the spark gap than through the tube the pumping and
+sparking may be interrupted and the tube allowed to rest for,
+say, five minutes. It will generally be found that the equivalent
+resistance of the tube will be largely increased by this period
+of quiescence. It may even be found that the spark will now
+prefer to pass an air gap 3 inches long.</p>
+<p>In any case the sparking should now be continued, the pump
+being at rest, and the variations of tube resistance watched by
+adjusting the spark gap. If the resistance falls below an
+equivalent of 2 inches of air in the gap the pump must be brought
+into action again and continued until the resistance as thus
+estimated remains fairly constant for, say, ten minutes. When
+this occurs the narrow neck of the exhaust tube may be strongly
+heated till the blow-pipe flame begins to show traces of sodium
+light. The flame must then be withdrawn and the discharge again
+tested. This is necessary because it occasionally happens that
+gas is given off during the heating of the neck to the
+neighbourhood of its fusion temperature.</p>
+<p>If all is right the neck may now be fused entirely off and the
+tube is finished. Tubes of the focus pattern with large platinum
+anodes are in general (in my experience) much more difficult to
+exhaust than tubes of the kind first described. This is possibly
+to be attributed mainly to the gas given off by the platinum, but
+is also, no doubt, due to the tubes being much larger and
+exposing a larger glass surface. The type of tube described first
+generally takes about two hours to exhaust by a pump made as
+explained, while a "focus" tube has taken as long as nine hours,
+eight of which have been consumed after the tube was exhausted to
+the hammering point.</p>
+<p>The pressure at which the maximum heating of the anode by the
+cathode rays occurs is a good deal higher than that at which the
+maximum Roentgen effect is produced. There is little doubt that
+the Roentgen radiation changes in nature to some extent as the
+vacuum improves either as a primary or secondary effect. It is
+therefore of some importance to test the tube for the purpose for
+which it is to be used during the actual exhaustion. It has been
+stated, for instance, that the relative penetrability of bone and
+flesh to Roentgen radiation attains a maximum difference at a
+certain pressure; this is very likely the case. Whether this
+effect is a direct function of the density of the gas in the
+tube, or whether it is dependent on the voltage or time integral
+of the current during the discharge, are questions which still
+await a solution.</p>
+<p>The preparation of calcium tungstate for fluorescent screens
+is very simple.</p>
+<p>Commercial sodium tungstate is fused with dried calcium
+chloride in the proportion of three parts of the former to two
+parts of the latter, both constituents being in fine powder and
+well mixed together. The fusion is conducted in a Fletcher's
+crucible furnace in a clay crucible. The temperature is raised as
+rapidly as possible to the highest point which the furnace will
+attain &mdash; i.e. a pure white heat. At this temperature the
+mixture of salts becomes partly fluid, or at least pasty, and the
+temperature may be kept at its highest point for, say, a quarter
+of an hour. At the end of this time the mass is poured and scraped
+on to a brick, and when cold is broken up and boiled with a
+large excess of water to dissolve out all soluble matter. The
+insoluble part, which consists of a gray shining powder, is
+washed several times with hot water, and is finally dried on
+filter paper in a water oven.</p>
+<p>In order to prepare a screen the powder is ground slightly
+with very dilute shellac varnish, and is then floated over a
+glass plate so as to get an even covering. Unless the covering be
+very even the screen is useless, and no pains should be spared to
+secure evenness. It is not exactly easy to get a regular coat of
+the fluorescent material, but it may be done with a little
+care.</p>
+<p><b><a name="Toc158108921" id="Toc158108921"><font face=
+"Bookman Old Style" size="4">CHAPTER II</font></a></b></p>
+<p><a name="Toc158108922" id="Toc158108922">GLASS-GRINDING AND
+OPTICIANS' WORK</a></p>
+<p><font face="Bookman Old Style">&sect; 52. As no instructions
+of any practical value in this art have, so far as I know,
+appeared in any book in English, though a great deal of valuable
+information has been given in the <i>English Mechanic</i> and
+elsewhere, I shall deal with the matter sufficiently fully for
+all practical purposes. On the other hand, I do not propose to
+treat of all the methods which have been proposed, but only those
+requisite for the production of the results claimed. The student
+is requested to read through the chapter before commencing any
+particular operation.</font></p>
+<p>&sect; 53. The simplest way will be to describe the process of
+manufacture of some standard optical appliance, from which a
+general idea of the nature of the operations will be obtained.
+After this preliminary account special methods may be considered
+in detail. I will begin with an account of the construction of an
+achromatic object glass for a telescope, not because a student in
+a physical laboratory will often require to make one, but because
+it illustrates the usual processes very well; and requires to be
+well and accurately made.</p>
+<p>A knowledge of the ordinary principles of optics on the part
+of the reader is assumed, for there are plenty of books on the
+theory of lenses, and, in any case, it is my intention to treat
+of the art rather than of the science of the subject. By far the
+best short statement of the principles involved which I have seen
+is Lord Rayleigh's article on Optics in the Encyclopaedia
+Britannica, and this is amply sufficient.</p>
+<p>The first question that crops up is, of course, the subject of
+the choice of glass. It is obvious that the glass must be uniform
+in refractive index throughout, and that it must be free from air
+bubbles or bits of opaque matter. <i>[Footnote:</i> The complete
+testing of glass for uniformity of refractive index can only be
+arrived at by grinding and polishing a sufficient portion of the
+surfaces to enable an examination to be made of every part. In
+the case of a small disc it is sufficient to polish two or three
+facets on the edge, and to examine the glass in a field of
+uniform illumination through the windows thus formed. Very slight
+irregularities will cause a "mirage" easily
+recognised.<i>]</i></p>
+<p>The simplest procedure is to obtain glass of the desired
+quality from Messrs. Chance of Birmingham, according to the
+following abbreviated list of names and refractive indices, which
+may be relied upon:&mdash;</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"568">
+<tr>
+<td width="17%" valign="top"></td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Density.</font></p>
+</td>
+<td width="67%" valign="top" colspan="4">
+<p><font face="Bookman Old Style">Refractive Index.</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top"></td>
+<td width="17%" valign="top"></td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">C</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">D</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">F</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">G</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Hard crown</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">2.85</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5146</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5172</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5232</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5280</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Soft crown</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">2.55</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5119</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5146</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5210</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5263</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Light flint</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">3.21</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5700</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5740</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5839</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.5922</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Dense flint</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">3.66</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1 6175</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6224</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6348</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6453</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Extra dense flint</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">3.85</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6450</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6504</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6643</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.6761</font></p>
+</td>
+</tr>
+<tr>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Double extra dense
+flint</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">4.45</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.7036</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.7103</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">1.7273</font></p>
+</td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">...</font></p>
+</td>
+</tr>
+</table>
+<p><font face="Bookman Old Style">The above glasses may be had in
+sheets from 0.25 to 1 inch thick, and 6 to 12 inches square, at a
+cost of, say, 7s. 6d. per pound.</font></p>
+<p>Discs can also be obtained of any reasonable size. Discs 2
+inches in diameter cost about &pound;1 per dozen, discs 3 inches
+in diameter about 10s. each. The price of discs increases
+enormously with the size. A 16-inch disc will cost about
+&pound;100.</p>
+<p>For special purposes, where the desired quality of glass does
+not appear on the list, an application may be made to the Jena
+Factory of Herr Schott. In order to give a definite example, I
+may mention that for ordinary telescopic objectives good results
+may be obtained by combining the hard crown and dense flint of
+Chance's list, using the crown to form a double convex, and the
+flint to form a double concave lens. The convex lens is placed in
+the more outward position in the telescope, i.e. the light passes
+first through it.</p>
+<p>The conditions to be fulfilled are:</p>
+<ol>
+<li>The glass must be achromatic;</li>
+<li>it must have a small spherical aberration for rays converging
+to the principal focus.</li>
+</ol>
+<p>It is impossible to discuss these matters without going into a
+complete optical discussion. The radii of curvature of the
+surfaces, beginning with the first, i.e. the external face of the
+convex lens, are in the ratio of 1, 2, and 3; an allowance of 15
+inches focal length per inch of aperture is reasonable (see
+Optics in Ency. Brit.), and the focal length is the same as the
+greatest radius of curvature. Thus, for an object glass 2 inches
+in diameter, the first surface of the convex lens would have a
+radius of curvature of 10 inches, the surface common to the
+convex and concave lens would have a radius of curvature of 20
+inches, and the last surface a radius of curvature of 30 inches.
+This would also be about the focal length of the finished lens.
+The surfaces in contact have, of course, a common curvature, and
+need not be cemented together unless a slight loss of light is
+inadmissible.</p>
+<p>I will assume that a lens of about 2 inches diameter is to be
+made by hand, i.e. without the help of a special grinding or
+polishing machine; this can be accomplished perfectly well, so
+long as the diameter of the glass is not above about 6 inches,
+after which the labour is rather too severe. The two glass discs
+having been obtained from the makers, it will be found that they
+are slightly larger in diameter than the quoted size, something
+having been left for the waste of working.</p>
+<p>It is difficult to deal with the processes of lens manufacture
+without entering at every stage into rather tedious details, and,
+what is worse, without interrupting the main account for the
+purpose of describing subsidiary instruments or processes. In
+order that the reader may have some guide in threading the maze,
+it is necessary that he should commence with a clear idea of the
+broad principles of construction which are to be carried out. For
+this purpose it seems desirable to begin by roughly indicating
+the various steps which are to be taken.</p>
+<p>(1) The glass is to be made circular in form and of a given
+diameter.</p>
+<p>(2) <i>Called Rough Grinding</i>. &mdash; The surfaces of the
+glass are to be made roughly convex, plane, or concave, as may be
+required; the glass is to be equally thick all round the edge. In
+this process the glass is abraded by the use of sand or emery
+rubbed over it by properly shaped pieces of iron or lead called
+"tools."</p>
+<p>(3) The glass is ground with emery to the correct spherical
+figure as given by a spherometer.</p>
+<p>(4) <i>Called Fine Grinding</i>. &mdash; The state of the
+surface is gradually improved by grinding with finer and finer
+grades of emery.</p>
+<p>(5) The glass is polished by rouge.</p>
+<p>(6) The glass is "figured." This means that it is gradually
+altered in form by a polishing tool till it gives the best
+results as found by trial.</p>
+<p>In processes 2 to 5 counterpart tool surfaces are required
+&mdash; as a rule two convex and two concave surfaces for each
+lens surface. These subsidiary surfaces are worked (i.e. ground)
+on discs of cast iron faced with glass, or on slate discs; and
+discs thus prepared are called "tools."</p>
+<p>Taking these processes in the order named, the mode of
+manufacture is shortly as follows:&mdash;</p>
+<p>(1) The disc of glass, obtained in a roughly circular form, is
+mounted on an ordinary lathe, being conveniently cemented by
+Regnault's mastic to a small face plate. The lathe is rotated
+slowly, and the glass is gradually turned down to a circular
+figure by means (1) of a tool with a diamond point; or (2) an
+ordinary hand-file moistened with kerosene, as described in
+&sect; 42; or (3) a mass of brass or iron served with a mixture
+of emery &mdash; or sand &mdash; and water fed on to the disc, so
+that the disc is gradually ground circular.</p>
+<p>The operation of making a circular disc of given diameter does
+not differ in any important particular from the similar operation
+in the case of brass or iron, and is in fact merely a matter of
+turning at a slow speed.</p>
+<p>(2 and 3) Roughing or bringing the surfaces of the glass
+roughly to the proper convex or concave shape. &mdash; This is
+accomplished by grinding, generally with sand in large works, or
+with emery in the laboratory, where the time saved is of more
+importance than the value of the emery.</p>
+<p>Discs of iron or brass are cast and turned so as to have a
+diameter slightly less than that of the glass to be ground, and
+are, say, half an inch thick. These discs are turned convex or
+concave on one face according as they are to be employed in the
+production of concave or convex glass surfaces. The proper degree
+of convexity or concavity may be approximated to by turning with
+ordinary turning tools, using a circular arc cut from zinc or
+glass (as will be described) as a "template" or pattern. This
+also is a mere matter of turning.</p>
+<p>The first approximation to the desired convex or concave
+surface of the glass is attained (in the case of small lenses,
+say up to three inches diameter) by rotating the glass on the
+lathe as described above (for the purpose of giving it a circular
+edge) and holding the tool against the rotating glass, a
+plentiful supply of coarse emery and water, or sand and water,
+being supplied between the glass and metal surfaces. The tool is
+held by hand against the surface of the revolving glass, and is
+constantly moved about, both round its own axis of figure and to
+and fro across the glass surface. In this way the glass gradually
+gets convex or concave.</p>
+<p>The curvature is tested from time to time by a spherometer,
+and the tool is increased or decreased in curvature by turning it
+on a lathe so as to cause it to grind the glass more at the edges
+or in the middle according to the indications of the
+spherometer.</p>
+<p>This instrument, by the way &mdash; so important for lens
+makers &mdash; consists essentially of a kind of three-legged
+stool, with an additional leg placed at the centre of the circle
+circumscribing the other three. This central leg is in reality a
+fine screw with a very large head graduated on the edge, so that
+it is easy to compute the fractions of a turn given to the screw.
+The instrument is first placed on a flat plate, and the central
+screw turned till its end just touches the plate, a state of
+affairs which is very sharply discernible by the slight rocking
+which it enables the instrument to undergo when pushed by the
+hand. See the sketch.</p>
+<p>On a convex or concave surface the screw has to be screwed in
+or out, and from the amount of screwing necessary to bring all
+four points into equal contact, the curvature may be
+ascertained.</p>
+<p>Let a be the distance between the equidistant feet, and d the
+distance through which the screw is protruded or retracted from
+its zero position on a flat surface. Then the radius of curvature
+<font face="Bookman Old Style">&rho;</font> <font face=
+"Bookman Old Style">is given by the formula 2</font><font face=
+"Microsoft Sans Serif">&rho;</font><font face=
+"Bookman Old Style">= a<sup>2</sup>/3d +d.</font></p>
+<p>Fig. <img src="images/Image86.gif" alt="images/Image86.gif"
+width="161" height="211"> 43.</p>
+<p>The process of roughing is not always carried out exactly as
+described, and will be referred to again.</p>
+<p>(4) The glass being approximately of the proper radius of
+curvature on one side, it is reversed on the chuck and the same
+process gone through on the other side. After this the glass is
+usually dismounted from the lathe and mounted by cement on a
+pedestal, which is merely a wooden stand with a heavy foot, so
+that the glass may be held conveniently for the workman.
+Sometimes a pedestal about four feet high is fixed in the floor
+of the room, so that the workman engaged in grinding the lens may
+walk round and round it to secure uniformity. For ordinary
+purposes, however, a short pedestal may be placed on a table and
+rotated from time to time by hand, the operator sitting down to
+his work.</p>
+<p>Rough iron or brass tools do not succeed for fine grinding
+&mdash; i.e. grinding with fine emery, because particles of emery
+become embedded in the metal so tightly that they cannot be got
+out by any ordinary cleaning. If we have been using emery passing
+say a sieve with 60 threads to the inch, and then go on to some
+passing say 100 threads to the inch, a few of the coarser
+particles will adhere to the "tool", and go on cutting and
+scratching all the time grinding by means of the finer emery is
+in progress.</p>
+<p>To get over this it is usual to use a rather different kind of
+grinding tool. A very good kind is made by cementing small
+squares of glass (say up to half an inch on the side), on to a
+disc of slate slightly smaller than the lens surface to be formed
+(Fig. 51). The glass-slate tool is then "roughed" just like the
+lens surface, but, of course, if the lens has been roughed
+"convex" the tool must be roughed "concave".</p>
+<p>The "roughed" tool is then used to gradually improve the
+fineness of grinding of the glass. For this purpose grinding by
+hand is resorted to, the tool and lens being supplied continually
+with finer and finer emery. Fig. 52 gives an idea of the way in
+which the tool is moved across the glass surface. Very little
+pressure is required. The tool is carried in small circular
+sweeps round and round the lens, so that the centre of the tool
+describes a many-looped curve on the lens surface. The tool must
+be allowed to rotate about its own axis; and the lens and
+pedestal must also be rotated from time to time.</p>
+<p>Every few minutes the circular strokes are interrupted, and
+simple, straight, transverse strokes taken. In no case (except to
+correct a, defect, as will be explained) should the tool overhang
+the lens surface by more than about one quarter the diameter of
+the latter. After grinding say for an hour with one size of emery
+fed in by means of a clean stick say every five minutes, the
+emery is washed off, and everything carefully cleaned. The
+process is then repeated with finer emery, and so on.</p>
+<p>The different grades of emery are prepared by taking advantage
+of the fact that the smaller the particles the longer do they
+remain suspended in water. Some emery mud from a "roughing"
+operation is stirred up with plenty of water and left a few
+seconds to settle, the liquor is then decanted to a second jug
+and left say for double the time, say ten seconds; it is decanted
+again, and so on till four or five grades of emery have been
+accumulated, each jug containing finer emery than its predecessor
+in the process.</p>
+<p>It is not much use using emery which takes more than half an
+hour to settle in an ordinary bedroom jug. What remains in the
+liquid to be decanted is mostly glass mud and not emery at all.
+The process of fine grinding is continually checked by the
+spherometer, and the art consists in knowing how to move the
+grinding tool so as to make the lens surface more or less curved.
+In general it may be said that if the tool is moved in small
+sweeps, and not allowed to overhang much, the Centre of the lens
+will be more abraded, while if bold free strokes are taken with
+much overhanging, the edges of the lens will be more ground
+away.</p>
+<p>By the exercise of patience and perseverance any one will
+succeed in gradually fine grinding the lens surface and keeping
+it to the spherometer, but the skill comes in doing this rapidly
+by varying the shape of the strokes before any appreciable
+alteration of curvature has come about.</p>
+<p><i>Polishing</i>. &mdash;</p>
+<p>The most simple way of polishing is to coat the grinding tool
+with paper, as will be described, and then to brush some rouge
+into the paper. The polisher is moved over the work in much the
+same way as the fine grinding tool, until the glass is polished.
+Many operators prefer to use a tool made by squeezing a disc of
+slate, armed with squares of warm pitch, against the lens surface
+(finely ground), and then covering these squares with rouge and
+water instead of emery and water as in the fine grinding
+process.</p>
+<p>The final process is called "figuring." It will in general be
+unnecessary with a small lens. With large lenses or mirrors the
+final touches have to be given after the optical behaviour of the
+lens or mirror has been tested with the telescope itself, and
+this process is called "figuring." A book might easily be written
+on the optical indications of various imperfections in a mirror
+or lens. Suffice it to say here that a sufficiently skilled
+person will be able to decide from an observation of the
+behaviour of a telescope whether a lens will be improved by
+altering the curvature of one or all of the surfaces.</p>
+<p>A very small alteration will make a large difference in the
+optical properties, so that in general "figuring" is done merely
+by using the rouge polishing tool as an abrading tool, and
+causing it to alter the curves in the manner already suggested
+for grinding. There are other methods based on knocking squares
+out of the pitch-polisher so that some parts of the glass may be
+more abraded than others.</p>
+<p>The "figuring" and polishing may be done by hand just like the
+grinding. There are machines, however, which can be made to
+execute the proper motions, and a polisher is set in such a
+machine, and the mechanical work done is by no means
+inconsiderable. In fact for surfaces above six inches in diameter
+few people are strong enough to work a polisher by hand owing to
+the intense adhesion between it and the exactly fitting glass
+surface.</p>
+<p>Such is a general outline of the processes required to produce
+a lens or mirror. These processes will now be dealt with in much
+greater detail, and a certain amount of repetition of the above
+will unfortunately be necessary: the reader is asked to pardon
+this. It will also be advisable for the reader to begin by
+reading the whole account before he commences any particular
+operation. The reason for this is that it has been desirable to
+keep to the main account as far as possible without inserting
+special instructions for subsidiary operations, however important
+they may be; consequently it may not always be quite clear how
+the steps described are to be performed. It will be found,
+however, that all necessary information is really given, though
+perhaps not always exactly in the place the reader might at first
+expect.</p>
+<p>&sect; 54. All the discs that I have seen, come from the
+makers already roughly ground on the edges to a circular
+figure--but occasionally the figure is very rough indeed &mdash;
+and in some cases, especially if small lenses have to be made, it
+is convenient to begin by cutting the glass discs out of glass
+sheet, which also may be purchased of suit-able glass. To do
+this, the simplest way is to begin by cutting squares and then
+cutting off the corners with the diamond, the approximate
+circular figure being obtained by grinding the edges on an
+ordinary grindstone.</p>
+<p>If the pieces are larger, time and material may be saved by
+using a diamond compass, i.e. an ordinary drawing compass armed
+with a diamond to cut circles on the glass, and breaking the
+superfluous glass away by means of a pair of spectacle-maker's
+shanks (Fig. 44), or what does equally well, a pair of pliers
+with soft iron jaws. With these instruments glass can be chipped
+gradually up to any line, whether diamond-cut or not, the jaws of
+the pincers being worked against the edge of the glass, so as to
+gradually crush it away.</p>
+<p>Fig. <img src="images/Image87.gif" alt="images/Image87.gif"
+width="314" height="146"> 44.</p>
+<p>Assuming that the glass has been bought or made roughly
+circular, it must be finished on the lathe. For this purpose it
+is necessary to chuck it on an iron or hardwood chuck, as shown
+in Fig. 46. For a lens below say an inch in diameter, the
+centering cement may be used; but for a lens of a diameter
+greater than this, sufficient adhesion is easily obtained with
+Regnault's mastic, and its low melting point gives it a decided
+advantage over the shellac composition.</p>
+<p>The glass may be heated gradually by placing it on the water
+bath, or actually in the water, and gradually bringing the water
+up to the boiling-point. The glass, being taken out, is rapidly
+wiped, and rubbed with a bit of waste moistened, not wet, with a
+little turpentine: its surface is then rubbed with a stick of
+mastic previously warmed so as to melt easily. The surface of the
+chuck being also warm, and covered with a layer of melted cement,
+it is applied to the glass. The lathe is turned slowly by hand,
+and the glass pushed gradually into the most central position; it
+is then pressed tight against the chuck by the back rest, a bit
+of wood being interposed for obvious reasons.</p>
+<p>When all is cold the turning may be proceeded with. The
+quickest way is to use the method already described (i.e. actual
+turning by a file tool); but if the student prefers (time being
+no object), he may accomplish the reduction to a circular form
+very easily by grinding.</p>
+<p><img src="images/Image88.gif" alt="images/Image88.gif" width=
+"439" height="404">Fig. 45.</p>
+<p><img src="images/Image89.gif" alt="images/Image89.gif" width=
+"272" height="372">Fig. 46.</p>
+<p>For this purpose he will require to make the following
+arrangements (Fig. 45). If the lathe has a slide rest, a piece of
+stout iron may be bent and cut so as to fit the tool rest, and
+project beneath the glass. The iron must be fairly rigid, for if
+it springs appreciably beneath the pressure of the glass, it will
+not grind the latter really round. The lathe may run rather
+faster than for turning cast iron of the same size. Coarse emery,
+passing through a sieve of 80 threads to the inch (run), may be
+fed in between the glass and iron, and the latter screwed up till
+the disc just grinds slightly as it goes round.</p>
+<p>A beginner will generally (in this as in all cases of grinding
+processes) tend to feed too fast &mdash; no grinding process can
+be hurried. If a slide rest is not available, a hinged board,
+carrying a bit of iron, may (see Fig. 45) be arranged so as to
+turn about its hinge at the back of the lathe; and it may be
+screwed up readily enough by passing a long set-screw through the
+front edge, so that the point of the screw bears upon the lathe
+bed. I may add that emery behaves as if it were greasy, and it is
+difficult to wet it with clean water. This is easily got over by
+adding a little soap or alcohol to the water, or exercising a
+little patience.</p>
+<p>A good supply of emery and water should be kept between the
+disc and the iron; a little putty may be arranged round the point
+of contact on the iron to form a temporary trough. In any case
+the resulting emery mud should on no account be thrown away, but
+should be carefully kept for further use. The process is complete
+when the glass is perfectly round and of the required diameter as
+tested by callipers.</p>
+<p>&sect; 55. The next step is to rough out the lens, and this
+may easily be done by rotating it more slowly, i.e. with a
+surface speed of ten feet per minute, and turning the glass with
+a hard file, as explained in &sect; 42. If it is desired to
+employ the slide rest, it is quicker and better to use a diamond
+tool &mdash; an instrument quite readily made, and of great
+service for turning emery wheels and the like, &mdash; a thing,
+in fact, which no workshop should be without. A bit of diamond
+bort, or even a clear though off-colour stone, may be
+employed.</p>
+<p>An ordinary lathe tool is prepared by drawing down the tool
+steel to a long cone, resembling the ordinary practice in
+preparing a boring tool. The apex of the cone must be cut off
+till it is only slightly larger than the greatest transverse
+diameter of the diamond splinter. The latter may have almost any
+shape &mdash; a triangular point, one side of a three-sided prism
+is very convenient. A hole is drilled in the steel (which must
+have been well softened), only just large enough to allow the
+diamond to enter &mdash; if the splinter is thicker in the middle
+than at either end, so much the better &mdash; the diamond is
+fastened in position by squeezing the soft steel walls tightly
+down upon it. Personally I prefer to use a tool holder, and in
+this case generally mount the diamond in a bit of brass rod of
+the proper diameter; and instead of pinching in the sides of the
+cavity, I tin them, and set the diamond in position with a drop
+of soft solder.</p>
+<p><img src="images/Image90.gif" alt="images/Image90.gif" width=
+"434" height="62">Fig. 47.</p>
+<p>In purchasing diamond bort, a good plan is to buy fragments
+that have been employed in diamond drilling, and have become too
+small to reset; in this case some idea as to the hardness of the
+bits may be obtained. Full details as to diamond tool-making are
+given in books on watch-making, and in Holtzapffell's great work
+on Mechanical Manipulation; but the above notes are all that are
+really necessary &mdash; it is, in fact, a very simple matter.
+The only advantage of using a diamond tool for glass turning is
+that one does not need to be always taking it out of the rest to
+sharpen it, which generally happens with hard steel, especially
+if the work is turned a little too fast.</p>
+<p>I recommend, therefore, that the student should boldly go to
+work "free hand" with a hard file; but if he prefer the more
+formal method, or distrust his skill (which he should not do),
+then let him use a diamond point, even if he has the trouble of
+making it. When using a diamond it is not necessary to employ a
+lubricant, but there is some advantage in doing so.</p>
+<p>The surface of the lens can be roughly shaped by turning to a
+template or pattern made by cutting a circular arc (of the same
+radius as the required surface) out of a bit of sheet zinc.
+Another very handy way of making templates of great accuracy is
+to use a beam compass (constructed from a light wooden bar) with
+a glazier's diamond instead of a pencil. A bit of thin sheet
+glass is cut across with this compass to the proper
+curvature--which can be done with considerable accuracy and the
+two halves of the plate, after breaking along the cut, are ground
+together with a view to avoiding slight local irregularities, by
+means of a little fine emery and water laid between the edges. In
+this process the glass is conveniently supported on a clean board
+or slate, and the bits are rubbed backwards and forwards against
+each other.</p>
+<p>&sect; 56. It is not very easy for a beginner to turn a bit of
+anything &mdash; iron, wood, or glass - with great accuracy to
+fit a template, and consequently time may be saved by the
+following procedure, applied as soon as the figure of the
+template is roughly obtained. A disc of lead or iron, of the same
+diameter as the glass, and of approximately the proper curvature,
+is prepared by turning, and is armed with a handle projecting
+coaxially from the back of the disc. The glass revolving with
+moderate speed on the lathe, the lead tool, supplied with coarse
+emery and water, is held against it, care being taken to rotate
+the tool by the handle, and also to move it backwards and
+forwards across the disc, through a distance, say, up to half an
+inch; if it is allowed to overhang too much the edges of the
+glass disc will be overground. By the use of such a tool the
+glass can readily be brought up to the template.</p>
+<p>The only thing that remains, so far as the description of this
+part of the process goes, is to give a note or two as to the best
+way of making the lead tools, and for this purpose the main
+narrative of processes must be interrupted. The easiest way is to
+make a set of discs to begin with. For this purpose take the
+mandrel out of the lathe, and place it nose downwards in the
+centre of an iron ring of proper diameter on a flat and level
+iron plate.</p>
+<p>The discs are made by pouring lead round the screw-nose of the
+mandrel. This method, of course, leaves them with a hole in the
+centre; but this can be stopped up by placing the hot disc (from
+which the mandrel has been unscrewed) on a hot plate, and pouring
+in a sufficiency of very hot lead; or, better still, the mandrel
+can be supported vertically at any desired distance above the
+plate while the casting is being poured. Lead discs prepared in
+this way are easily turned so as to form very convenient chucks
+for brass work, and for use in the case now being treated, they
+are easily turned to a template, using woodturners' tools, which
+work better if oiled, and must be set to cut, not scrape.</p>
+<p>If the operator does not mind the trouble of cutting a screw,
+or if he has a jaw chuck, the lead may be replaced by iron with
+some advantage.</p>
+<p>The following is a neat way of making concave tools. It is an
+application of the principle of having the cutting tool as long
+as the radius of curvature, and allowing it to move about the
+centre of curvature. Place the disc of iron or lead on the lathe
+mandrel or in the chuck, and set the slide rest so that it is
+free to slide up or down the lathe bed. Take a bar of tool steel
+and cut it a little longer than the radius of curvature required.
+Forge and finish one end of the bar into a pointed turning tool
+of the ordinary kind. Measure the radius of curvature from the
+point of the tool along the bar, and bore a hole, whose centre is
+at this point, through the bar from the upper to the lower face.
+I regard the upper face as the one whose horizontal plane
+contains the cutting point when the tool is in use. Clamp a
+temporary back centre to the lathe bed, and let it carry a pin in
+the vertical plane through the lathe centres, and let this pin
+exactly fit the hole in the bar.</p>
+<p><img src="images/Image91.gif" alt="images/Image91.gif" width=
+"128" height="383">Fig. 48.</p>
+<p>Place the "radius" tool in position for cutting, and let it be
+lightly held in the slide rest nearly at the cutting point, the
+centre of rotation of the pedestal (or its equivalent) passing
+through the central line of the bar. Then adjust the temporary
+back rest, so that the tool will take a cut. In the sketch the
+tool is shown swinging about the back centre instead of about a
+pin &mdash; there is little to choose between the methods unless
+economy of tool steel is an object. The tool must now be fed
+across the work. The pedestal must of course be free to rotate,
+and the slide rest to slip up and down the bed. In this way a
+better concave grinding tool can be made than would be made by a
+beginner by turning to a template &mdash; though an expert turner
+would probably carry out the latter operation so as to obtain an'
+accuracy of the same order, and would certainly do it in much
+less time than would be required in setting up the special
+arrangements here described.</p>
+<p>On the other hand, if several surfaces have to be prepared, as
+in the making of an achromatic lens, the quickest way would be by
+the use of the radius tool, bored of course to work at the
+several radii required. I have tried both methods, and my choice
+would depend partly on the lathe at my disposal, and partly on
+the number of grinding tools that had to be prepared.</p>
+<p>Having obtained a concave tool of any given radius, it is
+easily copied &mdash; negatively, so as to make a convex tool in
+the following manner. Adjust the concave tool already made on the
+back rest, so that if it rotated about the line of centres, it
+would rotate about its axis of figure.</p>
+<p>Arrangements for this can easily be made, but of course they
+will depend on the detailed structure of the lathe. Use the slide
+rest as before, i.e. let it grasp an ordinary turning tool
+lightly, the pedestal being fixed, but the rest free to slide up
+or down the lathe bed. Push the back rest up till the butt of the
+turning tool (ground to a rounded point) rests against the
+concave grinding tool. If the diameter of the convex tool
+required be very small compared with the radius of curvature of
+the surface (the most usual case), it is only necessary to feed
+the cutting tool across to "copy" the concave surface
+sufficiently nearly.</p>
+<p>Fig. <img src="images/Image92.gif" alt="images/Image92.gif"
+width="396" height="176">49.</p>
+<p>There seems no reason, however, why these methods should not
+be applied at once to the glass disc by means of a diamond point,
+and the rough grinding thus entirely avoided. I am informed that
+this has been done by Sir Henry Bessemer, but that the method was
+found to present no great advantage in practice. A reader with a
+taste for mechanical experimenting might try radius bar tools
+with small carborundum wheels rapidly driven instead of a
+diamond.</p>
+<p>Enough has now been said to enable any one to prepare rough
+convex or concave grinding tools of iron or lead, and of the same
+diameter as the glass to be ground.</p>
+<p>The general effect of the process of roughing the rotating
+lens surface is to alter the radius of curvature of both tool and
+glass; hence it is necessary to have for each grinding tool
+another to fit it, and enable it to be kept (by working the two
+together) at a constant figure. After a little practice it will
+be found possible to bring the glass exactly up to the required
+curvature as tested by template or spherometer. The art of the
+process consists in altering the shape of the grinding tool so as
+to take off the glass where required, as described in &sect; 53,
+and from this point of view lead has some advantages; (opinions
+vary as to the relative advantages of lead and iron tools for
+this purpose, however). The subsidiary grinding tool is not
+actually needed for this preliminary operation, but it has to be
+made some time with a view to further procedure, and occasionally
+is of service here.</p>
+<p>&sect; 57. 'The glass disc must be ground approximately to the
+proper curvature on each side before any fine grinding is
+commenced. It is precisely for this purpose that the previous
+turning of the disc is recommended, for it is easy to unmount and
+recentre a round object, but not so easy if the object have an
+indefinite shape. Using a cement which is plastic before it sets,
+the disc may be easily taken off the chuck and centred by a
+little handicraft, i.e. by rotating the lathe slowly and pushing
+the disc into such a position that it rotates about its axis. The
+grinding of the second surface is accomplished exactly as in the
+former case; of course on reversing the glass the chuck has to be
+slightly turned up to fit the convex or concave surface.</p>
+<p>&sect; 58. There is, however, one point of interest and
+importance &mdash; attention to which will save a good deal of
+useless labour afterwards. The glass must be ground in such a
+manner that the thickness at the edge is the same all round. In
+other words, the axes of figure of the two surfaces must
+coincide. This will be the case if the recentering has been
+accurately performed, and therefore no pains should be spared to
+see that it is exactly carried out. Any simple form of vernier
+gauge (such as Brown and Sharpe's vernier callipers) will serve
+to allow of a sufficiently accurate measurement of the edge
+thickness of the lens. If any difference of thickness is observed
+as the gauge moves round the edge, one or other of the surfaces
+must be reground. Of course the latitude of error which may be
+permitted depends so much on the final arrangements for a special
+finishing process called the "centering of the lens" &mdash;
+which will be described &mdash; that it is difficult to fix a
+limit, but perhaps one-thousandth of an inch may be mentioned as
+a suitable amount for a 2-inch disc. For rough work, of course;
+more margin may be admitted.</p>
+<p>&sect; 59. In a large shop I imagine that lenses of only two
+inches diameter would be ground in nests; or, in other words, a
+number would be worked at a time, and centering, even of a rough
+kind, would be left to the last; but this process will be treated
+hereafter. At present I shall assume that only one lens will be
+made at a time. Consequently we now enter on the stage of fine
+grinding by hand. A leaden pedestal, for the sake of stability,
+must be provided on which to mount the lens, so that the surface
+to be operated on may be nearly horizontal (Fig. 50). Before this
+can be done, however, fresh grinding tools (two for each surface)
+must be properly prepared. After trying several plans I
+unhesitatingly recommend that all fine-grinding surfaces should
+be made of glass. This is easily done by taking two discs of
+lead, or iron, or slate, cut to a one-tenth inch smaller radius
+of curvature (in the case of a convex tool, and the opposite in
+the other case) than the lens surface (Fig. 51, A). On these,
+square bits of sheet glass, one-tenth of an inch thick, are to be
+cemented, so as to leave channels of about one-eighth of an inch
+between each bit of glass (Fig. 52, B). The "mastic" cement
+formerly described may be employed for this purpose.</p>
+<p><img src="images/Image93.gif" alt="images/Image93.gif" width=
+"229" height="87">Fig. 50.</p>
+<p>The bits of glass ought first to have their edges dressed
+smooth on the grind-stone. A convex and concave glass surface
+having been thus roughly prepared, they must be mounted in turn
+in the lathe, and brought to the proper curvature by grinding
+with the tools formerly employed and tested by the template or
+spherometer. It is well to control this process by means of a
+spherometer, so that the desired radius may be approximately
+reached. The two glass-grinding tools are then ground together
+by hand (see &sect; 53 and &sect; 61), the spherometer being
+employed from time to time to check the progress of the work. In
+general, if large circular sweeps are taken, greatly overhanging
+the side of the glass surface to be figured, both the upper and
+lower surfaces will be more ground at the edges, while in the
+opposite event the centre will be chiefly affected.</p>
+<p><img src="images/Image94.gif" alt="images/Image94.gif" width=
+"381" height="235">Fig. 51.</p>
+<p>A spherometer capable of measuring a 2-inch surface may be
+procured, having a screw of, say, 50 threads to the inch, and a
+micrometer surface divided into 200 parts, each part easily
+capable of subdivision--into tenths or even twentieths. To get
+the full advantage of the spherometer it must screw exceedingly
+freely (i.e. must be well oiled with clock oil), and must not be
+fingered except at the milled head. If one of the legs is held by
+the fingers the expansion is sufficient to throw the instrument
+quite out of adjustment. The glass-grinding tools being brought
+to the proper figure, the next process is to transfer the same to
+the lens, and this is done by similar means, the fellow tool
+being used to correct the one employed in grinding the lens
+surface. Before the grade of emery is changed all three surfaces
+must agree, as nearly, at least, as the spherometer will
+show.</p>
+<p>In order to prevent confusion the following summary of the
+steps already taken may be given. The discs of glass are first
+ground or turned so as to be truly circular. Four "tools" are
+made for each surface &mdash; a rough pair of iron or lead, and a
+finishing pair of iron, lead, or slate faced by glass squares.
+For a small lens the iron or lead backing may be used, for a
+large one the slate. The rough tools are used to give an
+approximate figure both to the lens and to the finishing
+tools.</p>
+<p>The final adjustment is attained by grinding one of the
+glass-faced tools alternately upon the lens and upon the fellow
+glass-faced tool. The spherometer is accepted at all stages of
+the process as the final arbiter as to curvature. Some hints on
+the form of strokes used in grinding will be given later on (see
+&sect; 61). It suffices to state here that the object throughout
+is to secure uniformity by allowing both the work and the tool to
+rotate, and exercising no pressure by the fingers. The tool
+backing may weigh from one to two pounds for a 2-inch lens.</p>
+<p>&sect; 60. The tools and lens being all of the same curvature,
+the state of the surface is gradually improved by grinding with
+finer and finer emery. The best way of grading the emery is by
+washing it with clean water, and allowing the emery (at first
+stirred up with the water) to settle out. The longer the time
+required for this part of the process the finer will be the emery
+deposited. An ordinary bedroom jug is a very good utensil to
+employ during this process; a large glass jug is even better. The
+following grades will be found sufficient, though I daresay every
+operative's practice differs a little on this point.</p>
+<p>1st grade. &mdash; Flour emery, with the grit washed out, i.e.
+allowed to stand for 2" (sec.) before being poured off.</p>
+<p>2nd grade.--Stand 5" (secs.), settle in 1&rsquo; (min.)</p>
+<p>3rd grade. &mdash; Stand 1', settle in 10'.</p>
+<p>4th grade. &mdash; Stand 10', settle in 60'.</p>
+<p>It is generally advisable to repeat the washing process with
+each grade. Thus, selecting grade 2 for illustration, the liquor
+for grade 3 must be poured off without allowing any of the
+sediment to pass over with it. If any sediment at all passes, one
+has no security against its containing perhaps the largest
+particle in the jug. As soon as the liquor for No. 3 has been
+decanted, jug No. 2 is filled up again with clean water (filtered
+if necessary), and after standing 5" is decanted into jug No. 2b,
+the sediment is returned to jug No. 1, and the liquor, after
+standing 1', is transferred to jug No. 3.</p>
+<p>The greatest care is necessary at each step of the operation
+to prevent "sediment" passing over with liquor. There is a little
+danger from the tendency which even comparatively large particles
+of emery have to float, in consequence of their refusing to get
+wet, and the emery worked up on the side of the jug is also a
+source of danger, therefore wipe the jug round inside before
+decanting.</p>
+<p>In order to get a uniform grade stop the currents of water in
+the jug, which may work up coarse particles, by holding a thin
+bit of wood in the rotating liquid for a moment, and then gently
+withdrawing it in its own plane. These precautions are
+particularly necessary in the case of grades Nos. 2, 3, and 4,
+especially No. 4, for if a single coarse particle gets on the
+tool when the work has progressed up to this point it will
+probably necessitate a return to grinding by means of No. 2, and
+involve many hours' work.</p>
+<p>The surface of the lens will require to be ground continuously
+with each grade till it has the uniform state of roughness
+corresponding to the grade in question. Two hours for each grade
+is about the usual time required in working such a lens as is
+here contemplated.</p>
+<p>The coarser grades of emery may be obtained by washing
+ordinary flour of emery, but the finer ones have to be got from
+emery which has been used in the previous processes. It is not a
+good plan to wash the finer grades of emery out of the proceeds
+of very rough grinding say with anything coarser than flour of
+emery &mdash; as there is a danger of thereby contaminating the
+finer grades with comparatively coarse glass particles (owing to
+their lightness) and this may lead to scratching. If the finer
+grades are very light in colour, it may be inferred that a
+considerable portion of the dust is composed of glass, and this
+does no good. Consequently time may be saved by stirring up the
+light-coloured mass with a little hydrofluoric acid in a platinum
+capsule; this dissolves the finely divided glass almost
+instantaneously. The emery and excess of hydrofluoric acid may
+then be thrown into a large beaker of clean water and washed
+several times. Fine emery thus treated has much the same dark
+chocolate colour as the coarser varieties.</p>
+<p>The operator should not wear a coat, and should have his arms
+bare while working with fine emery, for a workshop coat is sure
+to have gathered a good deal of dust, and increases the chances
+of coarse particles getting between the surfaces.</p>
+<p><b><a name="Toc158108923" id="Toc158108923"><font face=
+"Bookman Old Style" size="4">&sect; 61. Details of the Process of
+Fine Grinding. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A lens of the size selected for
+description is mounted as before mentioned on a leaden pedestal,
+and the operator places the latter on a table of convenient
+height in a room as free from dust as possible. Everything should
+be as clean as a pin, and no splashes of emery mud should be
+allowed to lie about. I have found it convenient to spread clean
+newspapers on the table and floor, and to wear clean linen
+clothes, which do not pick up dust. I have an idea that in large
+work-shops some simpler means of avoiding scratches must have
+been discovered, but I can only give the results of my own
+experience. I never successfully avoided scratches till I adopted
+the precautions mentioned.</font></p>
+<p>Fig. 52. <img src="images/Image95.gif" alt=
+"images/Image95.gif" width="155" height="162"></p>
+<p>The left hand should be employed in rotating the pedestal
+either continuously (though slowly) or at intervals of, say, one
+minute. This point is rather important. Some operators require
+two hands to work the grinding tool, and in any case this is the
+safer practice. Under these circumstances the pedestal may be
+rotated through one-eighth or tenth of a revolution every three
+minutes, or thereabouts. The general motion given to the grinding
+tool should be a series of circular sweeps of about one-fourth
+the diameter of the glass disc, and gradually carried round an
+imaginary circle drawn on the surface of the lens and concentric
+with it (Fig. 52).</p>
+<p>The tool may overhang the lens by a quarter of the diameter of
+the latter as a maximum. The circuit may be completed in from
+twelve to thirty sweeps. The grinding tool should be lightly held
+by the fingers and the necessary force applied parallel to the
+surface. The tool itself must be slowly rotated about its axis of
+figure. If the tool be lightly held, it will be found that it
+tends to rotate by itself. I say "tends to rotate," for if the
+tool be touching evenly all over the surface it will rotate in a
+direction opposite to the direction of the circular sweep. For
+instance, if the tool be carried round its looped path clockwise,
+it will tend to rotate about its own axis of figure
+counter-clockwise. If it touch more in the middle, this rotation
+will be increased, while if it touches more along the edge, the
+rotation will be diminished, or even reversed in an extreme
+case.</p>
+<p>Every fifty sweeps or so the tool should be simply ground
+backwards and forwards along a diameter of the lens surface. This
+grinding should consist of three or four journeys to and fro
+along, say, eight different diameters. About one-quarter of the
+whole grinding should be accomplished by short straight strokes,
+during which the tool should only overhang about one-quarter of
+an inch. The object of the straight strokes is to counteract the
+tendency to a gradual accumulation of the emery in the centre,
+which results from the circular grinding.</p>
+<p>A great deal of the art of the process consists in knowing how
+to work the tool to produce any given effect. For instance, if
+the lens requires to be ground down near the centre, the
+epicycloidal strokes must be nearly central; the tool must never
+overhang very much. If, on the other hand, it is the edges which
+require attention, these must be dealt with by wider overhanging
+strokes. The tool must be frequently tested on its fellow, and,
+indeed, ground upon it if any marked unevenness of action (such
+as that just described) is required for the lens. A check by
+spherometer will be applied at intervals according to the
+judgment of the operator, but, in any case, the fellow tool and
+lens should be kept at very nearly the same figure.</p>
+<p>The emery should never be allowed to become anything like dry
+between the tool and the lens, for in some way (probably by
+capillary action increasing the pressure of the tool) this seems
+to lead to scratching and "rolling" of the emery. The channels in
+the glass tool between the squares are of the greatest importance
+in enabling the emery to distribute itself. Perhaps the best
+guide in enabling one to judge as to when it is time to wash off
+the emery and apply fresh is the "feel" of the tool; also when
+the mud gets light in colour we know that it is full of glass
+dust, and proportionately inoperative.</p>
+<p>New emery may be put on, say, every five minutes, but no
+absolute rule can be given, for much depends on the pressure of
+the tool upon the lens. In the case considered a brass or lead,
+or even slate tool, of an inch, or even less, in thickness, will
+press quite heavily enough. In washing the lens and tool before
+new emery is introduced, a large enamelled iron bucket is very
+handy; the whole of the tool should be immersed and scrubbed with
+a nail-brush. The lens surface may be wiped with a bit of clean
+sponge, free from grit, or even a clean damp cloth.</p>
+<p>When the time comes to alter the grade of emery, a fresh lot
+of newspapers should be put down, and tools, lens, and pedestal
+well washed and brushed by the nail-brush. The surfaces should be
+wiped dry by a fresh piece of rag, and examined for scratches and
+also for uniformity of appearance; a good opinion can be formed
+as to the fit of the surfaces by noting whether &mdash; and if
+so, to what degree &mdash; they differ in appearance from point
+to point when held so that the light falls on them obliquely.</p>
+<p>It is necessary to exercise the greatest care in the washing
+between the application of successive grades of emery, and this
+will be facilitated if the edges of the glass squares were
+dressed on a grindstone before they were mounted. An additional
+precaution which may be of immense advantage is to allow the tool
+to dry between the application of successive grades of emery (of
+course, after it has been scrubbed), and then to brush it
+vigorously with a hat-brush. It sometimes happens that particles
+of mud which have resisted the wet scrubbing with the nail-brush
+may be removed by this method.</p>
+<p>As my friend Mr. Cook informs me that his present practice
+differs slightly from the above, I will depart from the rule I
+laid down, and add a note on an alternative method.</p>
+<p>Consider a single lens surface. This is roughed out as before
+by an iron tool, a rough fellow tool being made at the same time.
+The squares of glass are cemented to the roughing tool, and this
+is ground to the spherometer by means of the counterpart tool.
+The glass-coated tool is then applied to the lens surface and
+grinding with the first grade of emery commenced. The curvature
+is checked by the spherometer. Two auxiliary tools of, say, half
+the diameter of the lens, are prepared from slate, or glass
+backed with iron, and applied to grind down either the central
+part of the lens surface or tool surface, according to the
+indications of the spherometer. Any changes that may occur during
+grinding are corrected by these tools. The spherometer is
+accepted as the sole guide in obtaining the proper curvature. A
+slate backing is preferred for tools of any diameter over, say, 2
+inches.</p>
+<p><b><a name="Toc158108924" id="Toc158108924"><font face=
+"Bookman Old Style" size="4">&sect; 62. <i>Polishing</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">After the surface has been
+ground with the last grade of emery, and commences to become
+translucent even when dry, the grinding may be considered to be
+accomplished, and the next step is the polishing. There are many
+ways of carrying out this process, and the relative suitability
+of these methods depends on a good many, so to speak, accidental
+circumstances. For instance, if the intention is to finish the
+polishing at a sitting, the polishing tool may be faced with
+squares of archangel &mdash; not mineral or coal-tar &mdash;
+pitch and brought to shape simply by pressing while warm against
+the face of the lens. A tool thus made is very convenient,
+accurate, and good, but it is difficult to keep it in shape for
+any length of time; if left on the lens it is apt to stick, and
+if it overhangs ever so little will, of course, droop at the
+edges.</font></p>
+<p>On the whole, the following will be found a good and
+sufficient plan. The glass-grinding tool is converted into a
+polishing tool by pasting a bit of thin paper over its surface; a
+bit of woven letter paper of medium thickness with a smooth but
+not glazed surface does very well. We have found that what is
+called Smith's "21 lbs. Vellum Wove" is excellent. This is
+steeped in water till quite pliable and almost free from size.
+The glass tool is brushed over with a little thin arrowroot or
+starch paste, and the paper is laid upon it and squeezed down on
+the glass squares as well as possible; if the paper is wet enough
+and of the proper quality it will expand sufficiently to envelop
+the tool without creases, unless the curvature is quite out of
+the common.</p>
+<p>This being accomplished, and the excess of water and paste
+removed, the face of the paper is (for security) washed with a
+little clean water and a bit of sponge, and, finally, the tool is
+slightly pressed on the lens so as to get the paper to take up
+the proper figure as nearly as possible. After the polishing tool
+has been thus brought to the proper figure, it is lifted off and
+allowed to dry slowly. When the paper is dry it may be trimmed
+round the edges so as not to project sensibly beyond the glass
+squares. The next step is to brush the surface over very
+carefully with polishing rouge (prepared as is described at the
+end of this section) by means of a hat-brush. When the surface of
+the paper is filled with rouge all excess must be removed by
+vigorous brushing.</p>
+<p>Fig. <img src="images/Image96.gif" alt="images/Image96.gif"
+width="444" height="52"> 53.</p>
+<p>The tool being placed on the lens, two or three strokes
+similar to those used in grinding may be taken, and the tool is
+then lifted off and examined. It will be found to be dotted with
+a few bright points, produced by the adhesion of glass at the
+places of contact. These points are then to be removed in the
+following manner. An old three-cornered file is ground on each
+side till the file marks disappear, and sharp edges are produced
+(Fig. 53). This tool is used as an ink eraser, and it will be
+found to scrape the paper of the polishing tool very cleanly and
+well.</p>
+<p>The bright spots are the objects of attention, and they must
+be erased by the old file, and the polisher reapplied to the
+glass. A few strokes will develop other points, more numerous
+than before, and these in turn must be erased. The process is
+continued till the whole surface of the polishing tool is evenly
+covered with bright specks, and then the polishing may be
+proceeded with. The specks should not be more than about
+one-eighth of an inch apart, or the polishing will be
+irregular.</p>
+<p>The operation of polishing is similar to that of grinding. A
+reasonable time for polishing a glass surface is twenty hours; if
+more time is required it is a sign that the fine grinding has not
+been carried far enough. The progress of the operation may be
+best watched by looking at the surface &mdash; not through it.
+For this purpose a good light is requisite. When the lens is
+dismounted it may be examined by a beam of sunlight in a dark
+room, under which circumstances the faintest signs of grayness
+are easily discernible.</p>
+<p>It may be mentioned here that if the surface is in any way
+scratched the rouge will lodge in the scratches with great
+persistence, and an expert can generally tell from the appearance
+of scratches what kind of polishing powder has been employed.</p>
+<p>The persistence with which rouge clings to a rough surface of
+glass is rather remarkable. Some glass polishers prefer to use
+putty powder as a polishing material, and it is sometimes said to
+act more quickly than rouge; from my rather limited experience I
+have not found this to be the case, but it may have merits that I
+do not know of. Is it possible that its recommendation lies in
+the fact that it does not render scratches so obtrusively obvious
+as rouge does?</p>
+<p>Rouge is generally made in two or more grades. The softer
+grade is used for polishing silver, and is called jewellers'
+rouge. The harder grade, suitable for glass polishing, is best
+obtained from practical opticians (not mere sellers of optical
+instruments). I mean people like Messrs. Cook of York. Many years
+ago I prepared my own hard rouge by precipitating ferrous
+sulphate solution by aqueous ammonia, washing the precipitate,
+and heating it to a red heat. The product was ground up with
+water, and washed to get rid of large particles. This answered
+every purpose, and I could not find that it was in any way
+inferior to hard rouge as purchased. The same precipitate heated
+to a lower temperature is said to furnish a softer variety of
+rouge; at all events, it gives one more suitable for polishing
+speculum metal. Lord Rosso used rouge heated to a dull redness
+for this purpose.</p>
+<p>Rouge, whether made or bought, should always be washed to get
+rid of grit. I ought to add that not the least remarkable fact
+about the polishing is the extraordinarily small quantity of the
+polishing material requisite, which suggests that the process of
+polishing is not by any means the same as that of exceptionally
+fine grinding. Is it possible that the chief proximate cause of
+the utility of rouge is to be sought in its curious property of
+adhering to a rough glass surface, causing it, so to speak, to
+drag the glass off in minute quantities, and redeposit it after a
+certain thickness has been attained on another part of the
+surface?</p>
+<p><b><a name="Toc158108925" id="Toc158108925"><font face=
+"Bookman Old Style" size="4">&sect; 63. <i>Centering</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">When a lens is ground and
+polished it will almost always happen that the axis of revolution
+of its cylindrical edge is inclined to the axis of revolution of
+its curved surfaces. Since in practice lenses have to be adjusted
+by their edges, it is generally necessary to adjust the edge to a
+cylinder about the axis of figure of the active surfaces. This is
+best done on a lathe with a hollow mandrel.. The lens is chucked
+on a chuck with a central aperture &mdash; generally by means of
+pitch or Regnault's mastic, or "centering" cement for small
+lenses &mdash; and a cross wire is fixed in the axis of
+revolution of the lathe, and is illuminated by a lamp. This cross
+wire is observed by an eye-piece (with cross wires only in the
+case of a convex lens, or a telescope similarly furnished in the
+case of a concave lens), also placed in the axis of rotation of
+the lathe.</font></p>
+<p>Both cross wires are thus in the axis of revolution of the
+mandrel, and the distant one (B in the figure) is viewed through
+the lens and referred to the fixed cross wires at A. In general,
+as the lathe is rotated by turning the mandrel the image of the
+illuminated cross wires will be observed to rotate also. The lens
+is adjusted until the image remains steady on rotating the
+mandrel and it is to give time for this operation that a
+slow-setting cement is recommended. When the image remains
+stationary we know that the optical centre of the lens is in the
+axis of revolution, and that this axis is normal to both lens
+surfaces, i.e. is the principal axis of the lens, or axis of
+figure.</p>
+<p>Fig. 54. <img src="images/Image97.gif" alt=
+"images/Image97.gif" width="442" height="175"></p>
+<p>A much readier method, and one, in general, good enough for
+most purposes, is to put a candle on the end of the lathe-bed
+where the back centre generally is, and observe the images of the
+flame by reflection from both the lens surfaces. This method is
+very handy with small lenses; the mandrel is turned, and the lens
+adjusted by hand till the images are immovable. In both cases, of
+course, the edge of the lens is turned or ground till it is truly
+circular, the position of the lens remaining undisturbed on the
+chuck. If the edge gauge has been properly used in the earlier
+stages of figuring, it will be found that very little turning or
+grinding is requisite to produce a true centering.</p>
+<p>The particular defect due to want of centering in a lens may
+be observed by using it as the objective of a telescope, and
+observing a star slightly out of focus. The interference fringes
+will not be concentric circles unless the lens is properly
+centred. I ought to say that I have not looked into the theory of
+this, but have merely taken it as a generally admitted fact. The
+diseases of lenses and the modes of treating them are dealt with
+in a book by Messrs. Cook of York, entitled <i>On the Adjustment
+and Testing of Telescopic Objectives</i>.</p>
+<p>The final process of figuring will be dealt with later on
+(&sect; &sect; 66 and 67), as it applies not only to lenses but
+to mirrors, prisms, etc. If the instructions given have been
+carefully carried out on a 2-inch lens, it should perform fairly
+well, and possibly perfectly, without any further adjustment of
+the glass.</p>
+<p><b><font face="Bookman Old Style" size="4">&sect; 64.
+<i>Preparation of Small Lenses, where great Accuracy is not of
+the first Importance. &mdash;</i></font></b></p>
+<p><font face="Bookman Old Style">Such lenses may generally be
+made out of bits of good plate or sheet glass, and are of
+constant use in the physical laboratory. They may be purchased so
+cheaply, however, that only those who have the misfortune to work
+in out-of-the-way places need be driven to make them.</font></p>
+<p>Suitable glass having been obtained and the curves calculated
+from the index of refraction, as obtained by any of the ordinary
+methods applicable to plates (the microscope method, in general,
+is quite good enough), squares circumscribing the desired circles
+are cut out by the help of a diamond. <i>[Footnote:</i>
+Glazebrook and Shaw's Practical Physics, p. 383 (4th
+ed.).<i>]</i> The squares are roughly snipped by means of a pair
+of pliers or spectacle-maker's shanks. The rough circles are then
+mounted on the end of a brass or iron rod of rather greater
+diameter than the finished lenses are to possess. This mounting
+is best done by centering cement.</p>
+<p>The discs are then dressed circular on a grindstone, the rod
+serving both as a gauge and handle. A sufficient number of these
+discs having been prepared, a pair of brass tools of the form
+shown in the sketch (Fig. 55), and of about the proper radius of
+curvature, are made. One of these tools is used as a support for
+the glass discs.</p>
+<p>Fig. <img src="images/Image98.gif" alt="images/Image98.gif"
+width="401" height="116">55.</p>
+<p>A compass being set to scribe circles of the same diameter as
+the glass discs, centre marks are made on the surface of the
+appropriate tool, circles are drawn on this, and facets are filed
+or milled (for which the spiral head of the milling machine is
+excellent). In the case of concave supporting surfaces, i.e. in
+making concave lenses, I apprehend filing would be difficult, and
+the facets would have to be made by a rose cutter or mill; but if
+the discs are fairly round, then, in fact, no facets are
+required.</p>
+<p>The facets being ready, the glass discs are cemented to them
+by centering cement, which may be used quite generally for small
+lenses. When the cutting of facets has been omitted on a concave
+surface, the best cement is hard pitch. The grinding tool is
+generally rather larger than the nest of lenses. Coarse and fine
+grinding is accomplished wholly on the lathe &mdash; the tool
+being rotated at a fair speed (see infra), and the nest of lenses
+moved about by its handle so as to grind all parts equally. It
+must, of course, be held anywhere except "dead on," for then the
+part round the axis would not get ground; this inoperative
+portion of the rotating tool must therefore be allowed to
+distribute its incapable efforts evenly over the nest of
+lenses.</p>
+<p>Polishing is accomplished by means of the grinding tool,
+coated with paper and rouge as before; or the tool may be coated
+with very thin cloth and used with rouge as before &mdash; in
+this case the polishing goes on fastest when the surface of the
+cloth is distinctly damp. In working by this method, each grade
+of emery need only be applied from five to ten minutes. The glass
+does not appear to get scratched when the emery is changed,
+provided everything is well washed. A good polish may be got in
+an hour. The lathe is run as for turning brass of the same
+diameter as the tool.</p>
+<p>One side of the lenses being thus prepared, they are reversed,
+and the process gone through for the other side in a precisely
+similar manner. <i>[Footnote:</i> Unless the radius of curvature
+is very short and the lenses also convex, there is no necessity
+to recess the facets, provided hard pitch is used as the cement.
+See note on hard pitch.<i>]</i> To save trouble, it is usual, to
+make such lenses of equal curvature on both faces; but of course
+this is a matter of taste.</p>
+<p>Fig. <img src="images/Image99.gif" alt="images/Image99.gif"
+width="401" height="68"> 56.</p>
+<p>For very common work, bits of good plate glass are employed,
+and the manufacturer's surface treated as flat (Fig. 56). In this
+way plano-convex lenses are easily and cheaply made. Finally the
+lenses have to be centred, an essential operation in this case.
+This is easily done by the reflection method &mdash; the edge
+being turned off by the file and kerosene and the centering
+cement being used in making the preliminary adjustment on the
+chuck. I presume a lens made in this way is worth about a
+shilling, so that laboratory manufacture is not very
+remunerative. Fig. 56 shows the method of mounting small lenses
+for lathe grinding, when only one lens is required. The tool is
+generally rotated in the lathe and the lens held against it.</p>
+<p><b><a name="Toc158108926" id="Toc158108926"><font face=
+"Bookman Old Style" size="4">&sect; 65. Preparing Small Mirrors
+for Galvanometers. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">To get good mirrors for
+galvanometers, I have found the best plan is to grind and polish
+a large number together, on a disc perhaps 8 or 10 inches in
+diameter. I was led to this after inspecting and rejecting four
+ounces of microscope cover slips, a most wearisome process. That
+regular cover slips should be few and far between is not
+unlikely, seeing that they are made (by one eminent firm at
+least) simply by "pot" blowing a huge thin bulb, and then
+smashing it on the floor and selecting the fragments. As in the
+case of large mirrors, it is of course only necessary to grind
+one side of the glass, theoretically at all events. The
+objections to this course are:</font></p>
+<ol>
+<li>A silver surface cannot, in my experience, be polished
+externally (on a minute object like a cover slip) to be anything
+like so bright as the silver surface next the glass; and,</li>
+<li>if one side only is ground, it will be found that the little
+mirror hopelessly loses its figure directly it is detached from
+the support on which it has been worked. Consequently, I
+recommend that these small mirrors should be ground and polished
+on both sides &mdash; enough may be made at one operation to last
+for a very long time.</li>
+</ol>
+<p>A slate back is prepared of the same radius of curvature as it
+is desired to impart to the mirrors. Bits of thin sheet glass are
+then ground circular as described in the last section and
+cemented to this surface by the smallest quantity of clean
+archangel pitch, allowed to cool slowly and even to rest for a
+day before the work is proceeded with. The whole surface is then
+ground and polished as before.</p>
+<p>The mirrors are now reversed, when they ought to nearly fit
+the tool (assuming that flats are being made, and the fellow tool
+in all other cases), and are recemented by pitch to the
+appropriate backing ground, and polished. If very excellent
+results are required, these processes may be preceded by a
+preliminary rough grinding of one surface, so that the little
+discs will "sit" exactly on the tool surface, and not run the
+risk of being strained by capillary forces in the pitch. We have
+always found this necessary for really good results.</p>
+<p>On removing such mirrors from the backing, they generally,
+more or less lose their figure, becoming (in general fairly
+uniformly) more concave or convex. About 5 per cent of the
+mirrors thus prepared will be found almost perfect if the work
+has been well done, and the rest will probably be very fair,
+unless the diameter is very large as compared with the thickness.
+The best way of grinding and polishing such large surfaces (nests
+10 inches in diameter) is on a grinding machine, such as will be
+described below. The polishing is best done by means of paper, as
+before described.</p>
+<p>Having occasion to require hitherto unapproached lightness and
+optical accuracy in such mirrors, I got my assistant to try
+making them of fused quartz, slices being cut by a diamond wheel
+from a rod of that material. Chips of natural quartz were also
+obtained from broken "pebble" spectacles, and these were worked
+at the same time. The resulting mirrors were certainly superior
+to the best we could make from glass, but the labour of grinding
+was greater, and the labour of polishing less, than in the latter
+case. The pebble fragments gave practically as good mirrors as
+the fused slices. For the future it will be better always to make
+galvanometer mirrors from quartz crystals. These may be easily
+sliced, as will be described in &sect; 74. The slices are dressed
+on a grindstone according to instructions already given for small
+lenses.</p>
+<p>The silvering of these mirrors is a point of great importance.
+After trying nearly every formula published, we have settled down
+to the following.</p>
+<p>A solution of pure crystallised nitrate of silver in distilled
+water is made up to a strength of 125 grams of the salt per
+litre. This forms the stock solution and is kept in a dark
+bottle.</p>
+<p>Let the volume of silvering liquor required in any operation
+be denoted by 4 <i>v</i>. The liquor is prepared as follows:</p>
+<p>I. Measure out a volume <i>v</i> of the stock solution of
+silver nitrate, and calculate the weight of salt which it
+contains; let this be <i>w</i>. In another vessel dissolve pure
+Rochelle salt to the amount of 2.6 <i>w</i>, and make up the
+solution to the volume <i>v</i>. These two solutions are to be
+mixed together at a temperature of 55&deg; C., the vessels with
+their contents being heated to this temperature on the water
+bath. After mixing the liquids the temperature is to be kept
+approximately constant for five minutes, after which the liquor
+may be cooled. The white precipitate which first forms will
+become gray or black and very dense as the liquid cools. If it
+does not, the liquor must be reheated to 55&deg; C., and kept at
+that temperature for a few minutes and then again allowed to
+cool. The solution is in good order when all the precipitate is
+dense and gray or black and the liquor clear. The blacker and
+denser the precipitate the better is the solution. The liquor is
+decanted and filtered from the precipitate and brought up to the
+volume 2 <i>v</i> by addition of some of the wash water.</p>
+<p>II. Measure out a volume 0.118 <i>v</i> of the stock solution
+into a separate vessel, and add to it a 5 per cent solution of
+ammonium hydrate, with proper precautions, so that the
+precipitate at first formed is all but redissolved after vigorous
+shaking. It is very important that this condition should be
+exactly attained. Therefore add the latter part of the ammonia
+very carefully. Make up the volume to 2 <i>v</i>.</p>
+<p>Mix the solutions I. and II. in a separate vessel and pour the
+mixture into the depositing vessel. The surface to be silvered
+should face downwards, and lie just beneath the free surface of
+the liquid. Bubbles must of course be removed.</p>
+<p>The silver deposit obtained in this manner is exceedingly
+white and, bright on the surface next to the glass, but the back
+is mat and requires polishing.</p>
+<p>The detail of the process described above was worked out in my
+laboratory by Mr. A. Pollock, to whom my thanks are due.</p>
+<p>This process gives good deposits when the solutions are
+freshly prepared, but the ammonia solution will not keep; The
+surfaces to be silvered require to be absolutely clean. The
+process is assisted by a summer temperature, say 70&deg; Fahr.,
+and possibly by the action of light. Six or seven hours at least
+are required for a good deposit; a good plan is to leave the
+mirrors in the bath all night. On removal from the bath the
+mirrors require to be well washed, and allowed to dry thoroughly
+in sun heat for several hours before they are touched.</p>
+<p>Care should be taken not to pull the mirrors out of shape when
+they are mounted for the bath. A single drop of varnish or paint
+(a mere speck) on the centre will suffice to hold them. The back
+of the deposit requires to be varnished or painted as a rule to
+preserve the silver. All paints and varnishes thus applied tend
+to spoil the figure by expanding or contracting. On the whole, I
+think boiled linseed oil and white or red lead &mdash; white or
+red paint in fact &mdash; is less deleterious than other things I
+have tried. Shellac varnish is the worst.</p>
+<p>Of course, the best mirror can be easily spoiled by bad
+mounting. I have tried a great number of methods and can
+recommend as fairly successful the following:&mdash; A little
+pure white lead, i.e. bought as pure as a chemical &mdash; not as
+a paint &mdash; is mixed with an equal quantity of red lead and
+made into a paste with a little linseed oil. I say a paste, not
+putty. A trace of this is then worked on to the back of the
+mirror at the centre as nearly as may be, and to this is attached
+the support. The only objection to this is that nearly a week is
+required for the paste to set. If people must use shellac let it
+be remembered that it will go on changing its shape for months
+after it has cooled (whether it has been dissolved in alcohol or
+not).</p>
+<p><b><a name="Toc158108927" id="Toc158108927"><font face=
+"Bookman Old Style" size="4">&sect; 66. Preparation of Large
+Mirrors or Lenses for Telescopes. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">So much has been written on
+this subject by astronomers, generally in the <i>English
+Mechanic</i> and in the <i>Philosophical Transactions</i> for
+1840, that it might be thought nothing could be added. I will
+only say here that the processes already described apply
+perfectly to this case; but of course I only refer to silver on
+glass mirrors. For any size over 6 inches in diameter, the
+process of grinding and polishing by hand, particularly the
+latter, will probably be found to involve too much labour, and a
+machine will be required. A description of a modification of Mr.
+Nasmyth's machine &mdash; as made by my assistant, Mr. Cook
+&mdash; will be found below.</font></p>
+<p>There is no difficulty in constructing or working such a
+machine, and considered as an all round appliance, it possesses
+solid advantages over the simple double pulley and crank
+arrangement, which, however, from its simplicity deserves a note.
+Two pulleys, A and B, of about 18 inches diameter by 4 inches on
+the face, are arranged to rotate about vertical axes, and belted
+together. The shaft of one of these pulleys is driven by a belt
+in any convenient manner. Each pulley is provided on its upper
+surface with a crank of adjustable length carrying a vertical
+crank-pin.</p>
+<p>Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6"
+long, and these rods are pinned together at their farther
+extremities, and this pin carries the grinding or polishing tool,
+or rather engages loosely with the back of this tool which lies
+below the rod. It is clear that if the pulleys are of
+commensurable diameters, and are rigidly connected &mdash; say by
+belting which neither stretches nor slips &mdash; the polishing
+tool will describe a closed curve. If, however, the belt is
+arranged to slip slightly, or if the pulleys are of
+incommensurable diameters, the curve traced out by the grinding
+tool will be very complex, and in the case of the ratio of the
+diameters being incommensurable, will always remain open; for
+polishing purposes the consummation to be wished.</p>
+<p>Mirror surfaces are ground spherical, the reduction to
+parabolic form being attained in the process of polishing. A very
+interesting account of the practice of dealing with very large
+lenses will be found in Nature, May 1886, or the Journal of the
+Society of Arts, same date (I presume), by Sir Howard Grubb. The
+author considers that the final adjustment of surfaces by
+"figuring" &mdash; of which more anon &mdash; is an art which
+cannot be learned by inspection, any more than a man could learn
+to paint by watching an artist. This is, no doubt, the case to
+some extent; still, a person wishing to learn how to figure a
+lens could not do better than take Sir Howard at his word, and
+spend a month at his works. Meanwhile the following remarks must
+suffice; it is not likely that anybody to whom these notes will
+be of service would embark on such large work as is contemplated
+by Sir Howard Grubb.</p>
+<p>Fig. <img src="images/Image100.gif" alt="images/Image100.gif"
+width="264" height="314">57.</p>
+<p><i><font face="Bookman Old Style" size="3">Description of
+Polishing Machine. </font></i>Power is applied through belting to the speed
+cone A. By means of a bevel pinion rotation is communicated to
+the wheel D, which is of solid metal and carries a T-slot, C. A
+pedestal forming a crank-pin can be clamped so as to have any
+desired radius of motion by the screw E. A train of wheels E F G
+H K (ordinary cast lathe change wheels) communicate any desired
+ratio of motion to the tool-holder, which simply consists of two
+pins projecting vertically downwards from the spokes of wheel
+K.</p>
+<p>These pins form a fork, and each prong engages in a
+corresponding hole in the back of the slate-grinding tool (not
+shown in figure). The connection with the tool is purposely
+loose. The wheel E, of course, cannot rotate about the crank-pin
+D. Provision for changing the ratio of tool rotation is achieved
+by mounting the wheels composing the train on pins capable of
+sliding along a long slot in the bar supporting them. </p>
+<p><font face="Bookman Old Style" size="3">The farther
+end of this bar is caused to oscillate to and fro very slowly by
+means of an additional crank-pin S and crank-shaft, the
+projecting face of the bed-plate W being placed so as to allow V
+to slide about easily and smoothly. Motion is communicated to
+this part of the system by means of gears at 0 and P, and a belt
+working from P to Q. Thus the vertical shaft R is set in motion
+and communicates by gears with S. A pulley placed on the axle of
+the wheel carrying the crank-pin S gives a slow rotation to the
+work which is mounted on the table M. A small but important
+feature is the tray L below the gear K. This prevents dirt
+falling from the teeth of the wheel on to the work. The motion of
+S is of course very much less than of B &mdash; say 100 times
+less. The work can be conveniently adjusted as to height by means
+of the screw N.</font></p>
+<p>The machine must be on a steady foundation, and in a place as
+free from dust as possible. Though it looks complicated it is
+quite straight-forward to build and to operate.</p>
+<p>It is explained in Lord Rayleigh's article on Optics in the
+<i>Encyclopaedia Britannica</i> that a very minute change in the
+form of the curvature of the surface of a lens will make a large
+difference in the spherical aberration. This is to be expected,
+seeing that spherical aberration is a phenomenon of a
+differential sort, i.e. a measure of the difference between the
+curvature actually attained, and the theoretical curvature at
+each point of the lens, for given positions of point and image.
+Sir H. Grubb gives an illustration of the minuteness of the
+abrasion required in passing from a curve of one sort to a curve
+of another, say from a spherical to a parabolic curve,
+consequently the process of figuring by the slow action of a
+polishing tool becomes quite intelligible. In making a large
+mirror or lens all the processes hitherto described under
+grinding and polishing, etc., have to be gone through and in the
+manner described, and when all this is accomplished the final
+process of correcting to test commences. This process is called
+figuring.</p>
+<p>&sect; 67. Of the actual operation of this process I have no
+personal knowledge, and the following brief notes are drawn from
+the article by Sir H. Grubb, from my assistant's (Mr. Cook)
+experience, and from a small work On the Adjustment and Testing
+of Telescopic Objectives, by T. Cook and Sons, Buckingham Works,
+York (printed by Ben Johnson and Co., Micklegate, York). This
+work has excellent photographs of the interference rings of star
+images corresponding to various defects. It must be understood
+that the following is a mere sketch. The art will probably hardly
+ever be required in laboratory practice, and those who wish to
+construct large telescopes should not be above looking up the
+references.</p>
+<p>The process is naturally divided for treatment into two
+parts.</p>
+<p>(1) The detection of errors, and the cause of these
+errors.</p>
+<p>(2) The application of a remedy.</p>
+<p>(1) A lens, being mounted with its final adjustments, is
+turned on to a star, which must not be too bright, and should be
+fairly overhead. The following appearances may be noted:-</p>
+<p>A. In focus, the star appears as a small disc with one or two
+rings round it; inside and outside of the focus the rings
+increase in number, are round, concentric with the disc, and the
+bright and dark rings are apparently equally wide. The appearance
+inside the focus exactly resembles that outside when allowance is
+made for chromatic effects. Conclusion: objective good, and
+correctly mounted.</p>
+<p>B. The rings round the star in focus are not circular, nor is
+the star at the centre of the system. In bad cases the fringes
+are seen at one side only. Effects exaggerated outside and inside
+the focus. Conclusion: the lens is astigmatic, or the objective
+is not adjusted to be co-axial with the eyepiece.</p>
+<p>C. When in focus the central disc is surrounded by an
+intermittent diffraction pattern, i.e. for instance the system of
+rings may appear along, and near, three or more radii. If these
+shift when the points of support of the lens are shifted, flexure
+may be suspected.</p>
+<p>D. On observing inside and outside the focus, the rings are
+not equally bright and dark. This may be due to uncorrected
+spherical aberration, particularly to a fault known as "zonal
+aberration," where different zones of the lens have different
+foci, but each zone has a definite focus.</p>
+<p>E. Irregular diffraction fringes point to bad annealing of the
+glass. This may be checked by an examination of the lens in
+polarised light.</p>
+<p>F. If the disc appear blurred and coloured, however the focus
+be adjusted, incomplete correction for chromatic aberration is
+inferred. If in addition the colouring is unsymmetrical (in an
+extreme case the star disc is drawn out to a coloured band), want
+of centering is to be inferred. This will also show itself by the
+interference fringes having the characteristics described in
+C.</p>
+<p>(2) The following steps may be taken in applying a remedy:</p>
+<p>A. The adjusting screws of the cell mounting the object glass
+may be worked until the best result is attained; this requires
+great care and patience. Any errors left over are to be
+attributed to other causes than the want of collinearity of the
+axes of object glass and eyepiece.</p>
+<p>B. Astigmatism is detected by rotating the object glass or
+object glass cell. If the oval fringes still persist and the
+longer axis follows the lens, astigmatism may be inferred.
+Similarly, by rotating one lens on the other, astigmatism, or
+want of centering (quite a different thing) may be localised to
+the lens.</p>
+<p>C. The presence of flexure may be confirmed by altering the
+position of the points of support with respect to the eyepiece,
+the lens maintaining its original position. The addition of more
+points of support will in general reduce the ill effects. How to
+get rid of them I do not know; they are only serious with large
+lenses.</p>
+<p>D. Spherical aberration may be located by using stops and
+zonal screens, and observing the effect on the image. Sir H.
+Grubb determines whether any point on the lens requires to be
+raised or lowered, by touching the glass at that point with a
+warm hand or cooling it by ether. The effects so produced are the
+differential results of the change of figure and of refractive
+index. By observing the effect of the heating or cooling of any
+part, the operator will know whether to raise or lower that part,
+provided that by a suitable preliminary experiment he has
+determined the relation between the effect produced by the change
+of figure, and that due to the temperature variation of the
+refractive index. In general it is sufficient to consider the
+change of shape only and neglect the change in refractive
+power.</p>
+<p>E. Marked astigmatism has never been noticed by me, but I
+should think that the whole lens surface would require to be
+repolished or perhaps reground in this case.</p>
+<p>F. To decide in which surface faults exist is not easy. By
+placing a film of oil between the two surfaces nearly in contact
+these may be easily examined. Thus a mixture of nut and almond
+oil of the right proportion, to be found by trial, for the
+temperature, will have the same refractive index as the crown
+glass, and will consequently reduce any errors of figure in the
+interior crown surface, if properly applied between the surfaces.
+Similarly the interior of the flint surface may have its
+imperfections greatly reduced in effect by using almond oil
+alone, or mixed with bisulphide of carbon. The outer surfaces, I
+presume, must be examined by warming or cooling over suitable
+areas or zones.</p>
+<p>The defects being detected, a matter requiring a great deal of
+skill and experience according to Sir H. Grubb, the next step is
+to remedy them; and the remedial measures as applied to the glass
+constitute the process of figuring. There are two ways of
+correcting local defects, one by means of small paper or pitch
+covered tools, which according to Sir H. Grubb is dangerous, and
+according to the experience of Mr. Cook, and I think of many
+French opticians, safe and advantageous.</p>
+<p>Pitch polishing tools are generally used for figuring. They
+are made by covering a slate backing with squares of pitch. The
+backing is floated with pitch say one-eighth of an inch thick.
+This is then scored into squares by a hot iron rod. The tool,
+while slightly warm, is laid upon the lens surface, previously
+slightly smeared with dilute glycerine, until the pitch takes the
+figure of the glass. The polishing material is rouge and water.
+Small tools are applied locally, and probably can only be so
+applied with advantage for grave defects.</p>
+<p>The other method is longer and probably safer. It consists in
+furnishing the polishing tool with squares of pitch as before.
+These being slightly warm, the lens is placed upon them so that
+they will flow to the exact figure also as before. I presume that
+the lens is to be slightly smeared with glycerine, or some
+equivalent, to keep the pitch from sticking. The squares are most
+thickly distributed where the abrasion is most required, i.e.
+less pitch is melted out by the iron rod. This may be
+supplemented by taking advantage of differences of hardness of
+pitch, making some squares out of harder, others out of softer
+pitch. The aim is to produce a polishing tool which will polish
+unequally so as to remove the glass chiefly from predetermined
+parts of the lens surface. The tool is worked over the surface of
+the lens by the polishing machine, and part of the art consists
+in adjusting the strokes to assist in the production of the local
+variations required.</p>
+<p>A source of difficulty and danger lies in the fact that the
+pitch squares are rarely of the same hardness, so that some
+abrade the glass more rapidly than others. This is particularly
+likely to occur if the pitch has been overheated.
+<i>[Footnote:</i> When pitch is heated till it evolves bubbles of
+gas its hardness increases with the duration of the
+process.<i>]</i> The reader must be good enough to regard these
+remarks as of the barest possible kind, and not intended to
+convey more than a general idea of the nature of the process of
+figuring.</p>
+<p>&sect; 68. A few remarks on cleaning lenses will fittingly
+close this part of the subject. There is no need to go beyond the
+following instructions given by Mr. Brashear in Popular
+Astronomy, 1894, which are reproduced here verbatim.</p>
+<p>"The writer does not advise the use of either fine chamois
+skin, tissue paper, or an old soft silk handkerchief, nor any
+other such material to wipe the lenses, as is usually advised. It
+is not, however, these wiping materials that do the mischief, but
+the dust particles on the lenses, many of them perhaps of a
+silicious nature, which are always harder than optical glass, and
+as these particles attach themselves to the wiping material they
+cut microscopic or greater scratches on the surfaces of the
+objective in the process of wiping.</p>
+<p>"I write this article with the hope of helping to solve this
+apparently difficult problem, but which in reality is a very
+simple one.</p>
+<p>"Let us commence by taking the object glass out of its cell.
+Take out the screws that hold the ring in place, and lift out the
+ring. Placing the fingers of both hands so as to grasp the
+objective on opposite sides, reverse the cell, and with the
+thumbs gently press the objective <i>squarely</i> out of the cell
+on to a book, block of wood, or anything a little less in
+diameter than the objective, which has had a cushion of muslin or
+any soft substance laid upon it. One person can thus handle any
+objective up to 12 inches in diameter.</p>
+<p>"Before separating the lenses it should be carefully noted how
+they were put together with relation to the cell, and to one
+another, and if they art not marked they should be marked on the
+edges conspicuously with a hard lead pencil, so that when
+separated they may be put together in the same way, and placed in
+the same relation to the cell. With only ordinary precaution this
+should be an easy matter.</p>
+<p>"Setting the objective on edge the two lenses may be readily
+separated.</p>
+<p>"And now as to the cleaning of the lenses. I have, on rare
+occasions, found the inner surfaces of an object glass covered
+with a curious film, not caused directly by moisture but by the
+apparent oxidation of the tin-foil used to keep the lenses apart.
+"A year or more ago a 7-inch objective made by Mr. Clark was
+brought to me to clean. It had evidently been sadly neglected.
+The inside of the lenses were covered with such a film as I have
+mentioned, and I feared the glass was ruined. When taken apart it
+was found that the tin-foil had oxidised totally and had
+distributed itself all over the inner surfaces. I feared the
+result, but was delighted to find that nitric acid and a tuft of
+absorbent cotton cut all the deposit off, leaving no stains after
+having passed through a subsequent washing with soap and
+water.</p>
+<p>"I mention this as others may have a similar case to deal
+with.</p>
+<p>"For the ordinary cleaning of an objective let a suitable
+sized vessel, always a wooden one, be thoroughly cleaned with
+soap and water, then half filled with clean water about the same
+temperature as the glass. Slight differences of temperature are
+of no moment. Great differences are dangerous in large
+objectives.</p>
+<p>"I usually put a teaspoonful of ammonia in half a pail of
+water, and it is well to let a piece of washed 'cheese cloth' lie
+in the pail, as then there is no danger if the lens slips away
+from the hand, and, by the way, every observatory, indeed every
+amateur owning a telescope, should have plenty of 'cheese cloth'
+handy. It is cheap (about 3 cts. per yard) and is superior for
+wiping purposes to any 'old soft silk handkerchief,' chamois
+skin, etc. Before using it have it thoroughly washed with soap
+and water, then rinsed in clean water, dried and laid away in a
+box or other place where it can be kept clean. When you use a
+piece to clean an objective throw it away, it is so cheap you can
+afford to do so.</p>
+<p>"If the lenses are very dirty or 'dusty,' a tuft of cotton or
+a camel's-hair brush may be used to brush off the loose material
+before placing the lenses in the water, but no pressure other
+than the weight of the cotton or brush should be used. The writer
+prefers to use the palms of the hand with plenty of good soap on
+them to rub the surfaces, although the cheese cloth and the soap
+answers nicely, and there seems to be absolutely no danger of
+scratching when using the hands or the cheese cloth when plenty
+of water is used; indeed when I wish to wipe off the front
+surface of an objective in use, and the lens cannot well be taken
+out, I first dust off the gross particles and then use the cheese
+cloth with soap and water, and having gone over the surface
+gently with one piece of cloth, throw it away and take another,
+perhaps a third one, and then when the dirt is, as it were, all
+lifted up from the surface, a piece of dry cheese cloth will
+finish the work, leaving a clean brilliant surface, and no
+scratches of any kind.</p>
+<p>"In washing large objectives in water I generally use a 'tub'
+and stand the lenses on their edge. When thoroughly washed they
+are taken out and laid on a bundle of cheese cloth and several
+pieces of the same used to dry them.</p>
+<p>"I think it best not to leave them to drain dry; better take
+up all moisture with the cloth, and vigorous rubbing will do no
+harm if the surfaces have no abrading material on them. I have
+yet to injure a glass cleaned in this way.</p>
+<p>"This process may seem a rather long and tedious one, but it
+is not so in practice, and it pays.</p>
+<p>"In some places objectives must be frequently cleaned, not
+only because they become covered with an adherent dust, but
+because that dust produces so much diffused light in the field as
+to ruin some kinds of telescope work. Mr. Hale of the Kenwood
+Observatory tells me he cannot do any good prominence photography
+unless his objective has a clean surface; indeed every observer
+of faint objects or delicate planetary markings knows full well
+the value of a dark field free from diffused light. The
+object-glass maker uses his best efforts to produce the most
+perfect polish on his lenses, aside from the accuracy of the
+curves, both for high light value and freedom from diffused light
+in the field, and if the surfaces are allowed to become covered
+with dust, his good work counts for little.</p>
+<p>"If only the front surface needs cleaning, the method of
+cleaning with cheese cloth, soap and water, as described above,
+answers very well, but always throw away the first and, if
+necessary, the second cloth, then wipe dry with a third or fourth
+cloth; but if the surfaces all need cleaning I know of no better
+method than that of taking the objective out of its cell, always
+using abundance of soap and water, and keep in a good humor."</p>
+<p><b><a name="Toc158108928" id="Toc158108928"><font face=
+"Bookman Old Style" size="4">&sect; 69. The Preparation of Flat
+Surfaces of Rock Salt. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The preliminary grinding is
+accomplished as in the case of glass, except that it goes on
+vastly faster. The polishing process is the only part of the
+operation which presents any difficulty. The following is an
+extract from a paper on the subject, by Mr. J. A. Brashear,
+Pittsburg, Pa., U.S.A., from the Proceedings of the American
+Association for the Advancement of Science, 1885. Practically the
+same method was shown me by Mr. Cook some years earlier, so that
+I can endorse all that Mr. Brashear says, with the following
+exceptions. We consider that for small salt surfaces the pitch is
+better scored into squares than provided with the holes
+recommended by Mr. Brashear.</font></p>
+<p>Mr. Brashear's instructions are as follows. After alluding to
+the difficulty of drying the polished salt surface &mdash; which
+is of course wet &mdash; Mr. Brashear says:-</p>
+<p>"Happily I have no trouble in this respect now, and as my
+method is easily carried out by any physicist who desires to work
+with rock salt surfaces, it gives me pleasure to explain it. For
+polishing a prism I make an ordinary pitch bed of about two and
+one-half or three times the area of the surface of the prism to
+be polished. While the pitch is still warm I press upon it any
+approximately flat surface, such as a piece of ordinary plate
+glass. The pitch bed is then cooled by a stream of water, and
+conical holes are then drilled in the pitch with an ordinary
+counter sink bit, say one-quarter of an inch in diameter, and at
+intervals of half an inch over the entire surface. This is done
+to relieve the atmospheric pressure in the final work. The upper
+surface of the pitch is now very slightly warmed and a true plane
+surface (usually a glass one, prepared by grinding and polishing
+three surfaces in the ordinary way, previously wetted) is pressed
+upon it until the pitch surface becomes an approximately true
+plane itself. Fortunately, moderately hard pitch retains its
+figure quite persistently through short periods and small changes
+of temperature, and it always pays to spend a little time in the
+preparation of the pitch bed.</p>
+<p>"The polisher being now ready, a very small quantity of rouge
+and water is taken upon a fine sponge and equally distributed
+over its surface. The previously ground and fined salt surface
+(this work is done the same as in glass working) is now placed
+upon the polisher and motion instantly set up in diametral
+strokes. I usually walk around the polisher while working a
+surface. It is well to note that motion must be constant, for a
+moment's rest is fatal to good results, for the reason that the
+surface is quickly eaten away, and irregularly so, owing to the
+holes that are in the pitch bed. Now comes the most important
+part of this method. After a few minutes' work the moisture will
+begin to evaporate quite rapidly. No new application of water is
+to be made, but a careful watch must be kept upon the pitch bed,
+and as the last vestige of moisture disappears the prism is to be
+slipped off the polisher in a perfectly horizontal direction, and
+if the work has been well done, a clean, bright, and dry surface
+is the result. The surface is now tested by the well-known method
+of interference from a perfect glass test plate (see Fig.
+178).</p>
+<p>"If an error of concavity presents itself the process of
+polishing is gone over again, using short diametral strokes. If
+the error is one of convexity, the polishing strokes are to be
+made along the chords, extending over the edge of the polisher.
+The one essential feature of this method is the fact that the
+surface is wiped dry in the final strokes, thus getting rid of
+the one great difficulty of pitch polishing, a method undoubtedly
+far superior to that of polishing on broadcloth. If in the final
+strokes the surface is not quite cleaned I usually breathe upon
+the pitch bed, and thus by condensation place enough moisture
+upon it to give a few more strokes, finishing just the same as
+before. In ten minutes I have polished prisms of rock salt in
+this manner that have not only shown the D line double, but
+Professor Langley has informed me that his assistant, Mr. Keeler
+(J. E.), has seen the nickel line clearly between the D lines.
+This speaks for the superiority of the surfaces over those
+polished on broadcloth.</p>
+<p>"In polishing prisms I prefer to work them on top of the
+polisher, as they can be easily held, but as it is difficult to
+hold lenses or planes in this way without injuring the surfaces,
+I usually support them in a block of soft wood, turned so as to
+touch only at their edges, and work the polisher over them.
+Though it takes considerable practice to succeed at first, the
+results are so good that it well repays the few hours' work it
+requires to master the few difficulties it presents."</p>
+<p><img src="images/Image101.gif" alt="images/Image101.gif"
+width="468" height="144">Fig. 58.</p>
+<p><b><a name="Toc158108929" id="Toc158108929"><font face=
+"Bookman Old Style" size="4">&sect; 70. Casting Specula for
+Mirrors. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">According to Sir H. Grubb (loc.
+cit.) the best alloy is made of four atoms of copper and one of
+tin; this gives by weight, copper 252, tin 117.8.</font></p>
+<p>The copper is melted first in a plumbago crucible; the tin is
+added gradually. Of course, in the process of melting, even
+though a little fine charcoal be sprinkled over the copper, some
+loss of that metal will occur from oxidation. It is convenient in
+practice, therefore to reserve a portion of the tin and test the
+contents of the crucible by lifting a little of the alloy out and
+examining it.</p>
+<p>The following indications may be noted: When the copper is in
+excess the tint of the alloy is slightly red, and the structure,
+as shown at a fractured surface, is coarsely crystalline. As the
+proper proportions are more nearly attained, the crystalline
+structure becomes finer, the colour whiter, and the crystals
+brighter. The alloy is ready for use when the maximum brightness
+is attained and the grain is fine.</p>
+<p>If too much tin be added, the lustre diminishes. The correct
+proportion is, therefore, attained when a further small addition
+of tin produces no apparent increase of brightness or fineness of
+grain. About three-quarters of the tin may be added at first, and
+the other quarter added with testing as described. The alloy is
+allowed to cool until on skimming the surface the metal appears
+bright and remains so without losing its lustre by oxidation for
+a sensible time; it will still be quite red-hot.</p>
+<p><img src="images/Image102.gif" alt="images/Image102.gif"
+width="298" height="94">Fig. <img src="images/Image103.gif" alt=
+"images/Image103.gif" width="280" height="288">59. Fig. 60.</p>
+<p>As the speculum alloy is too difficult to work with ordinary
+tools, it is best to cast the speculum of exactly the required
+shape and size. This is done by means of a ring of iron turned
+inside (and out) and on one edge. This ring is laid on a plate of
+figured iron, and before the metal is poured the plate (G) (Figs
+59 and 61) is heated to, say, 300&deg; C. In order to avoid the
+presence of oxide as far as possible, the following arrangements
+for pouring are made. A portion of the lower surface of the ring
+is removed by radial filing until a notch equal to, say,
+one-twentieth of the whole circumference is produced. This is cut
+to an axial depth of, say, half an inch.</p>
+<p>A bar of iron is then dovetailed loosely into the notch (Fig.
+60, B), so that it will rest on the iron plate, and half fill the
+notch. The aperture thus left forms the port of ingress for the
+hot metal (see Fig. 61, M). A bit of sheet iron is attached to
+the upper surface of the ring, and lies as a sort of flap, shaped
+like a deep shovel, against the outside of the ring overhanging
+the port (Figs. 59 and 61 at F). This flap does not quite reach
+the iron plate, and its sides are bent so as to be in contact
+with the ring. A portion of a smaller ring is then applied in
+such a manner as to form a pouring lip or pool on the outside of
+the main ring at E, and the metal can only get into the main ring
+by passing under the edge of the flap and up through the port.
+This forms an efficient skimming arrangement. The process of
+casting is carried out by pouring steadily into the lip.</p>
+<p>To avoid air bubbles it is convenient to cause the metal to
+spread slowly over the chill, and Mr. Nasmyth's method of
+accomplishing this is shown in the figure (61). The chill rests
+on three pins, A B C (Figs. 59 and 61). Before pouring begins the
+chill is tilted up off C by means of the counterpoise D, which is
+insufficient to tilt it after the speculum is poured. It is
+important that the chill should be horizontal at the close of the
+operation, in order that the speculum may be of even thickness
+throughout. This is noted by means of levels placed on the ring
+(at K for instance).</p>
+<p><img src="images/Image104.gif" alt="images/Image104.gif"
+width="452" height="169">Fig. 61.</p>
+<p>This apparatus may appear unnecessarily complex, but it is
+worth while to set it up, for it makes the operation of casting a
+speculum fairly certain. If the metal is at the right temperature
+it will form a uniformly liquid disc inside the ring. The mass
+sets almost directly, and as soon as this occurs it is pushed to
+the edge of the plate and the metal in the lip broken off by a
+smart upward tap with a hammer. The dovetailed bit of iron is
+knocked downwards and falls off, and the ring may then be lifted
+clear of the casting. The object of the dovetail will now be
+understood, for without it there is great risk of breaking into
+the speculum in knocking the "tail" off.</p>
+<p>A box of quite dry sawdust is prepared in readiness for the
+process of annealing before the speculum is cast. The box must be
+a sound wooden or metal box, and must be approximately air-tight.
+For a speculum a foot in diameter the box must measure at least 3
+feet both ways in plan, and be 2 feet 6 inches deep. Half the
+sawdust is in the box and is well pressed down so as to half fill
+it. The other half must be conveniently ready to hand. As soon as
+possible after casting, the speculum is thrown into the box,
+covered over with the sawdust, and the lid is put on.</p>
+<p>The object in having the box nearly air-tight is to avoid
+air-currents, which would increase the rate of cooling. A
+speculum a foot in diameter may conveniently take about three
+days to anneal, and should be sensibly warm when the box is
+opened on the fourth day. For larger sizes longer times will be
+required. We will say that the sawdust thickness on each side
+must be proportional to the dimensions of the speculum, or may
+even increase faster with advantage if time is of no moment.</p>
+<p>The process of annealing may be considered successful if the
+disc does not fly to pieces in working; it is to be worked on the
+chilled side. The object of giving the chill the approximate
+counterpart form will now appear; it saves some rough grinding,
+and causes the finished surface to be more homogeneous than it
+would be if the centre were sunk by grinding through the chilled
+surface.</p>
+<p>In 1889 I learned from Mr. Schneider, Professor Row-land's
+assistant at Baltimore, that in casting specula for concave
+gratings a good deal of trouble had been saved by carrying out
+the operation in an atmosphere consisting mostly of coal gas. It
+was claimed that in this way the presence of specks of oxide was
+avoided. I did not see the process in operation, but the results
+attained are known and admired by all experimenters.</p>
+<p><b><a name="Toc158108930" id="Toc158108930"><font face=
+"Bookman Old Style" size="4">&sect; 71. Grinding and polishing
+Specula. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The rough grinding is
+accomplished by means of a lead tool and coarse emery; the size
+of grain may be such as will pass a sieve of 60 threads to the
+inch. The process of grinding is quite similar to that previously
+described, but it goes on comparatively quickly. The rough
+grinding is checked by the spherometer, and is interrupted when
+that instrument gives accordant and correct measurements all over
+the surface.</font></p>
+<p>The fine grinding may be proceeded with by means of a
+glass-faced tool as before described, or the labour may be
+reduced in the following manner. A slate tool, which must be free
+from green spots (a source of uneven hardness), is prepared, and
+this is brought nearly to the curvature of the roughly ground
+speculum, by turning or otherwise. It is finished on the speculum
+itself with a little flour of emery. The fine grinding is then
+carried on by means of slate dust and water, the slate tool being
+the grinder. The tool is, of course, scored into squares on the
+surface.</p>
+<p>If the casting process has been carried out successfully, the
+rough grinding may take, say six hours, and the fine grinding say
+thirty hours for a disc a foot in diameter. The greatest source
+of trouble is want of homogeneity in the casting, as evidenced by
+blowholes, etc. In general, the shortest way is to discard the
+disc and start afresh if there is any serious want of perfection
+in the continuity or homogeneity of the metal.</p>
+<p>Fig. <img src="images/Image105.gif" alt="images/Image105.gif"
+width="383" height="243"> 62.</p>
+<p>The finely ground surface must, of course, be apparently
+correct in so far as a spherometer (with 3 inches between the
+legs for a disc 1 foot in diameter) will show. Polishing and
+figuring are carried out simultaneously. Half an hour's polishing
+with a slate-backed pitch tool and rouge and water will enable an
+optical test to be made. The most convenient test is that of
+Foucault, a simple appliance for the purpose being shown in the
+figure (62). It essentially consists of a small lamp surrounded
+by an opaque chimney (A) through which a minute aperture
+(pin-hole) is made. A small lens may be used, of very great
+curvature, or even a transparent marble to throw an image of the
+flame on the pin-hole.</p>
+<p>A screen (B) is placed close to the source, and is provided
+with a rocking or tilting motion (C) in its own plane. The source
+and screen are partly independent, and each is provided with a
+fine adjustment which serves to place it in position near the
+centre of curvature. The screen is so close to the pin-hole in
+fact that both the source and a point on the edge of the screen
+may be said to be at the centre of curvature of the mirror. The
+mirror is temporarily mounted so as to have its axis horizontal,
+in a cellar or other place of uniform temperature.</p>
+<p>The final focussing to the centre of curvature is made by the
+fine adjustment screws; the image may be received on a bit of
+paper placed on the screen and overlapping the edge nearest the
+source. The screws are worked till the image has its smallest
+dimension and is bisected by the edge of the screen. The test
+consists in observing the appearance of the mirror surface while
+the screen is tilted to cut off the light, as seen by an eye
+placed at the edge of the screen, a peephole or eye lens being
+provided to facilitate placing the eye in a correct position. The
+screen screws are worked so as to gradually cut off the light,
+and the observer notes the appearance of the mirror surface. If
+the curves are perfect and spherical, the transition from
+complete illumination to darkness will be abrupt, and no part of
+the mirror will remain illuminated after the rest.</p>
+<p>For astronomical purposes a parabolic mirror is required. In
+this case the disc may be partially screened by zonal screens,
+and the position of the image for different zones noted; the
+correctness or otherwise of the curvature may then be ascertained
+by calculation. A shorter way is to place the source just outside
+the focus, to be found by trial, and then, moving the extinction
+screen (now a separate appliance) to, say, five times the radius
+of curvature away, where the image should now appear, the
+suddenness of extinction may be investigated. This, of course,
+involves a corresponding modification of the apparatus.</p>
+<p>Whether the tests indicate that a deepening of the Centre,
+i.e. increase of the curvature, or a flattening of the edges is
+required, at least two remedial processes are available. The
+"chisel and mallet" method of altering the size of the pitch,
+squares of the polisher may be employed, or paper or small pitch
+tools may be used to deepen the centre. The "chisel and mallet"
+method merely consists in removing pitch squares from a uniformly
+divided tool surface by means of the instruments mentioned. This
+removal is effected at those points at which the abrasion
+requires to be reduced.</p>
+<p>When some practice is attained, I understand that it is usual
+to try for a parabolic form at once, as soon as the polishing
+commences. This is done by dividing the pitch surface by V-shaped
+grooves, the sides of the grooves being radii of the circular
+surface, so that the central parts of the mirror get most of the
+polishing action. If paper tools are used they must not be
+allowed much overhang, or the edges of the mirror betray the
+effects of paper elasticity. Most operators "sink" the middle,
+but the late Mr. Lassell, a most accomplished worker, always
+attained the parabolic form by reducing the curvature of the
+edges of a spherical mirror.</p>
+<p><b><a name="Toc158108931" id="Toc158108931"><font face=
+"Bookman Old Style" size="4">&sect; 72. Preparation of Flat
+Surfaces. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">As Sir H. Grubb has pointed
+out, this operation only differs from those previously described
+in that an additional condition has to be satisfied. This
+condition refers to the mean curvature, which must be exact (in
+the case of flats it is of course zero) to a degree which is
+quite unnecessary in the manufacture of mirrors or
+lenses.</font></p>
+<p>A little consideration will show that to get a surface flat
+the most straightforward method is to carry out the necessary and
+sufficient condition for three surfaces to fit each other
+impartially. If they each fit each other, they must clearly all
+be flat. To carry out the process of producing a flat surface,
+therefore, two tools are made, and the glass or speculum is
+ground first on one and then on the other, the tools being kept
+"in fit" by occasional mutual grinding. The grinding and
+polishing go on as usual. If paper is employed, care must be
+taken that the polisher is about the same size as the object to
+be polished.</p>
+<p>There is a slight tendency to polish most at the edges; but if
+the sweeps are of the right shape and size, this may be corrected
+approximately. The best surfaces which have come under my notice
+are those prepared as "test surfaces" by Mr. Brashear of
+Alleghany, Pa., U.S.A. These I believe to be pitch polished. A
+pitch bed is prepared, I presume, in a manner similar to that
+described for rocksalt surfaces; but the working of the glass is
+an immense art, and one which I believe &mdash; if one may judge
+by results &mdash; is only known to Mr. Brashear.</p>
+<p>In general, the effect of polishing will be to produce a
+convex or concave surface, quite good enough for most purposes,
+but distinctly faulty when tested by the interference fringes
+produced with the aid of the test plate. The following
+information therefore &mdash; which I draw from Mr. 'Cook &mdash;
+will not enable a student to emulate Mr. Brashear, but will
+undoubtedly help him to get a very much better surface than he
+usually buys at a high price, as exhibited on a spectroscope
+prism.</p>
+<p>The only difference between this process and the one described
+for polishing lenses, lies in the fact that the rouge is put into
+the paper surface while the latter is wet with a dilute gum
+"mucilage." It is of course assumed that the object and the two
+tools have been finely ground and fit each other impartially. The
+paper is rubbed over with rouge and weak gum water. The tool,
+when dry, is applied to the flat ground surface (of the object),
+and is scraped with the three-cornered file chisel as formerly
+described. This process must be very carefully carried out. The
+paper must be of the quality mentioned, or may even be thinner
+and harder. The cross strokes should be more employed than in the
+case of the curved surfaces.</p>
+<p>A good deal will depend on the method employed for supporting
+the work; it is in general better to support the tool, which may
+have a slate backing of any desired thickness, whereby the
+difficulty resulting from strains is reduced. The work must be
+mounted in such a way as to minimise the effect of changes of
+temperature. If a pitch bed is selected, Mr. Brashear's
+instructions for rock salt may be followed, with, of course, the
+obvious necessary modifications. See also next section.</p>
+<p><b><a name="Toc158108932" id="Toc158108932"><font face=
+"Bookman Old Style" size="4">&sect; 73. Polishing Flat Surfaces
+on Glass or on Speculum Metal. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The above process may be
+employed for speculum metal, or pitch may be used. In the latter
+case a fresh tool must be prepared every hour or so, because the
+metal begins to strip and leave bits on the polisher; this causes
+a certain amount of scratching to take place. As against this
+disadvantage, the process of polishing, in so far as the state of
+the surface is concerned, need not take an hour if the fine
+grinding has been well done.</font></p>
+<p>For the finest work changes of temperature, as in the case of
+glass, cause a good deal of trouble, and the operator must try to
+arrange his method of holding the object so as to give rise to
+the least possible communication of heat from the hand.</p>
+<p>The partial elasticity of paper, which is its defect as a
+polishing backing, is, I believe, partly counterbalanced by the
+difficulty of forming with pitch an exact counterpart tool
+without introducing a serious rise of temperature (i.e. warming
+the pitch). The rate of subsidence of the latter is very slow at
+temperatures where it is hard enough to work reliably as a
+polisher.</p>
+<p>A student interested in the matter of flat surfaces will do
+well to read an account of Lord Rayleigh's work on the subject,
+Nature, vol. xlviii., 1893, pp. 212, 526 (or B. A. Reports,
+1893). In the first of these communications Lord Rayleigh
+describes the method of using test plates, and shows how to
+obtain the interference fringes in the clearest manner.</p>
+<p>For the ordinary optician a dark room and a soda flame afford
+all requisite information; and if a person succeeds in making
+three glass discs, say 6 inches in diameter, so flat that, when
+superposed in any manner, the interference fringes are parallel
+and equidistant, even to the roughest observation, he has nothing
+to learn from any book ever written on glass polishing. Lord
+Rayleigh has also shown how to use the free clean surface of
+water as a natural test plate.</p>
+<p>Since the above was written the following details of his exact
+course of procedure have been sent to me by Mr. Brashear, and I
+hereby tender my thanks:&mdash;</p>
+<p>"It really takes years to know just what to do when you reach
+that point where another touch either gives you the most perfect
+results attainable, or ruins the work you have already done. It
+has taken us a long time to find out how to make a flat surface,
+and when we were called upon to make the twenty-eight plane and
+parallel surfaces for the investigation of the value of the metre
+of the international standard, every one of which required an
+accuracy of one-twentieth of a wave length, we had a difficult
+task to perform. However, it was found that every surface had the
+desired accuracy, and some of them went far beyond it.</p>
+<p>It is an advantage in making flat surfaces to make more than
+one at a time; it is better to make at least three, and in fact
+we always grind and 'fine' three together. In making speculum
+plates we get up ten or twelve at once on the lead lap. These
+speculum plates we can test as we go on by means of our test
+plane until we get them nearly flat. In polishing them we first
+make quite a hard polisher, forming it on a large test plane that
+is very nearly correct. We then polish a while on one surface and
+test it, then on a second and test it, and after a while we
+accumulate plates that are slightly concave and slightly convex.
+By working upon these alternately with the same polisher, we
+finally get our polisher into such shape that it approximates
+more and more to a flat surface, and with extreme care and slow
+procedure we finally attain the results desired.</p>
+<p>All our flats are polished on a machine which has but little
+virtue in itself, unmixed with brains. Any machine giving a
+straight diametrical stroke will answer the purpose. The glass
+should be mounted so as to be perfectly free to move in every
+direction &mdash; that is to say, perfectly unconstrained. We
+mount all our flats on a piece of body Brussels carpet, so that
+every individual part of the woof acts as a yielding spring. The
+flats are held in place by wooden clamps at the edges, which
+never touch, but allow the bits of glass or metal to move slowly
+around if they are circular; if they are rectangular we allow
+them to tumble about as they please within the frame holding
+them.</p>
+<p>For making speculum metal plates either plane or concave we
+use polishers so hard that they scratch the metal all over the
+surface with fine microscopic scratches. We always work for
+figure, and when we get a hard polisher that is in proper shape,
+we can do ever so many surfaces with it if the environments of
+temperature are all right. If we have fifty speculum flats to
+make, and we recently made three times that number, we get them
+all ready and of accurate surface with the hard polisher. Then we
+prepare a very soft polisher, easily indented when cold with the
+thumb nail. A drop of rouge and about three drops of water are
+put on the plate, and with the soft polisher about one minute
+suffices to clean up all the scratches and leave a beautiful
+black polish on the metal. This final touch is given by hand; if
+we do not get the polish in a few minutes the surface is
+generally ruined for shape, and we have to resort to the hard
+polisher again.</p>
+<p>I assure you that nothing but patience and perseverance will
+master the difficulty that one has to encounter, but with these
+two elements &lsquo;you are bound to get there.&rsquo;"</p>
+<p><a name="Toc158108933" id="Toc158108933">CHAPTER III</a></p>
+<p><a name="Toc158108934" id="Toc158108934">MISCELLANEOUS
+PROCESSES</a></p>
+<p><a name="Toc158108935" id="Toc158108935"><font face=
+"Bookman Old Style" size="4">&sect; 74. Coating Glass with
+Aluminium and Soldering Aluminium. &mdash;</font></a></p>
+<p><font face="Bookman Old Style">A process of coating glass with
+aluminium has been lately discovered, which, if I mistake not,
+may be of immense service in special cases where a strongly
+adherent deposit is required. My attention was first attracted to
+the matter by an article in the <i>Archives des Sciences
+physiques et naturelles de Gen&egrave;ve,</i> 1894, by M. Margot.
+It appears that clean aluminium used as a pencil will leave a
+mark on clean damp glass. If, instead of a pencil, a small wheel
+of aluminium &mdash; say as big as a halfpenny and three times as
+thick &mdash; is rotated on the lathe, and a piece of glass
+pressed against it, the aluminium will form an adherent, though
+not very continuous coating on the glass.</font></p>
+<p>Working with a disc of the size described rotating about as
+fast as for brass-turning, I covered about two square inches of
+glass surface in about five minutes. The deposit was of very
+uneven thickness, but was nearly all thick enough to be sensibly
+opaque. By burnishing the brilliance is improved (I used an agate
+burnisher and oil), but a little of the aluminium is rubbed off.
+The fact that the burnisher does not entirely remove it is a sign
+of the strength of the adherence which exists between the
+aluminium and the glass. In making the experiment, care must be
+taken to have the glass quite clean &mdash; or at all events free
+from grease &mdash; in order to obtain the best results.</p>
+<p>M. Margot has contributed further information to the Archives
+des Sciences physiques et naturelles (February 1895). He finds
+that adherence between aluminium and glass is promoted by dusting
+the glass with powders, such as rouge. There is no doubt that a
+considerable improvement is effected in this way; both rouge and
+alumina have in my hands greatly increased the facility with
+which the aluminium is deposited. M. Margot finds that zinc and
+magnesium resemble aluminium in having properties of adherence to
+glass, and, what is more, carry this property into their alloys
+with tin. Thus an alloy of zinc and tin in the proportions of
+about 92 per cent tin and 8 per cent zinc may be melted on
+absolutely clean glass, and will adhere strongly to it if well
+rubbed by an asbestos crayon.</p>
+<p>A happy inspiration was to try whether these alloys would,
+under similar circumstances, adhere to aluminium itself, and a
+trial showed that this was indeed the case, provided that both
+the aluminium and alloy are scrupulously clean and free from
+oxide. In this way M. Margot has solved the problem of soldering
+aluminium. I have satisfied myself by trial of the perfect ease
+and absolute success of this method. The alloy of zinc and tin in
+the proportions above mentioned is formed at the lowest possible
+temperature by melting the constituents together. It is then
+poured so as to form thin sticks.</p>
+<p>The aluminium is carefully cleaned by rubbing with a cuttle
+bone, or fine sand, and strong warm potash. It is then washed in
+water and dried with a clean cloth. The aluminium is now held
+over a clean flame and heated till it will melt the solder which
+is rubbed against it. The solder sticks at once, especially if
+rubbed with another bit of aluminium (an aluminium soldering bit)
+similarly coated. To solder two bits of aluminium together it is
+only necessary to tin the bits by this process and then sweat
+them together.</p>
+<p>The same process applies perfectly to aluminium caused to
+adhere to glass by the previously mentioned process, and enables
+strong soldered contacts to be made to glass. In one case, while
+I was testing the method, the adhesion was so strong that the
+solder on contracting while cooling actually chipped the surface
+clean off the glass. In order to get over this I have endeavoured
+to soften the solder by mixing in a little of the fusible metal
+mercury amalgam; and though this prevents the glass from being so
+much strained, it reduces the adherence of the solder. It is a
+comfort to be able to solder aluminium after working for so many
+years by way of electroplating, or filing under solder. An
+alternative method of soldering aluminium will be described when
+the electroplating of aluminium is discussed, &sect; 138.</p>
+<p><i>Gilding Glass</i>. &mdash; In looking over some volumes of
+the Journal fuer praktische Chemie, I came across a method of
+gilding glass due to Boettger (Journ. f. prakt. Chem. 103, p.
+414). After many trials I believe I am in a position to give
+definite instructions as to the best way of carrying out this
+rather troublesome operation. The films of gold obtained by the
+process are very thick, and the appearance of the gold
+exceedingly fine. The difficulty lies in the exact apportionment
+of the reducing solution. If too much of the reducing solution be
+added, the gold deposits in a fine mud, and no coating is
+obtained. If, on the other hand, too little of the reducing
+solution be added, little or no gold is deposited. The secret of
+success turns on exactly hitting the proper proportions.</p>
+<p>The reducing solution consists of a mixture of aldehyde and
+glucose, and the difficulty I have had in following Boettger's
+instructions arose from his specifying "commercial aldehyde" of a
+certain specific gravity which it was impossible to reproduce. I
+did not wish to specify pure aldehyde, which is not very easily
+got or stored, and consequently I have had to determine a
+criterion as to when the proportion of reducing solution is
+properly adjusted.</p>
+<p>The aldehyde is best made as required. I employed the ordinary
+process as described in Thorpe's Dictionary of Applied Chemistry,
+by distilling alcohol, water, sulphuric acid, and manganese
+dioxide together. The crude product is mixed with a large
+quantity of calcium chloride (dry &mdash; not fused), and is
+rectified once. The process is stopped when the specific gravity
+of the product reaches 0.832 at 60&deg; F. The specific gravity
+of pure aldehyde is 0.79 nearly.</p>
+<p>The following is a modification of Boettger's
+formula:&mdash;</p>
+<p><i>Solution I</i></p>
+<p>1 gram of pure gold is converted into chloride &mdash; got
+acid free &mdash; i.e. to the state represented by AuCl<sub>3</sub>, and
+dissolved in 120 cc. of water.</p>
+<p>This solution is the equivalent of one containing 6.5 grains
+of trichloride to the ounce of water.</p>
+<p><i>Solution II.</i></p>
+<p>6 grams sodium hydrate.</p>
+<p>100 grams water.</p>
+<p><i>Solution III.</i></p>
+<p>0.2 grams glucose (bought as pure).</p>
+<p>12.6 cubic centimetres 95 per cent alcohol.</p>
+<p>12.6 cubic centimetres water.</p>
+<p>2.0 cubic centimetres aldehyde, sp. gr. 0.832.</p>
+<p>To gild glass these solutions are used in the following
+proportions by volume:-</p>
+<p>16 parts of No. I.</p>
+<p>4 parts of No. II.</p>
+<p>0.8 parts of No. III.</p>
+<p>The glass is first cleaned well with acid and washed with
+water: it is then rinsed with Solution No. III. If it is desired
+to gild the inside of a glass vessel, Solution No. III. may be
+placed in the vessel first, and the walls of the vessel rinsed
+round carefully. Solutions I. and II. are mixed separately and
+then added to III. &mdash; after about two minutes the whole is
+well shaken up.</p>
+<p>If it be desired to gild a mirror of glass, the glass-plate is
+suspended face downwards in a dish of the mixed solutions &mdash;
+care being taken to rinse the glass with Solution III. first.</p>
+<p>If the mixture darkens in from 7' to 10' in diffuse daylight
+and at 60&deg;F. it will gild well, and it generally pays to make
+a few trials in a test tube to arrive at this. If too much
+reducing solution is present the liquid will get dark more
+rapidly, and vice versa. The gilding will require several hours
+&mdash; as much as twelve hours may be needed.</p>
+<p>The reaction is one of great chemical interest, being one of
+that class of reactions which is greatly affected by capillarity.
+Thus it occasionally happens that when the reducing solution is
+not in the right proportion, gold will be deposited at the
+surface of the liquid (so as to form a gilt ring on the inside of
+a test tube), the remainder of the gold going down as mud. The
+gold deposited is at first transparent to transmitted light and
+is deeply blue. I thought this might be due to a trace of copper
+or silver, but on carefully purifying the gold no change of
+colour was noted. If the reducing solution is present in slightly
+greater proportions than that given in the formula, the gold
+comes down with a richer colour, and has a tendency to form a mat
+surface and to separate from the glass. The gold which is
+deposited more slowly has a less rich colour but a brighter
+surface. The operation should be interrupted when a sufficient
+deposit has been obtained, because it is found that the thicker
+the deposit, the more lightly is it held to the glass
+surface.</p>
+<p><b><a name="Toc158108936" id="Toc158108936"><font face=
+"Bookman Old Style" size="4">&sect; 75. The Use of the
+Diamond-cutting Wheel. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A matter which is not very well
+known outside geological circles is the manipulation of the
+diamond-cutting wheel, and as this is often of great use in the
+physical laboratory, the following notes may not be out of place.
+I first became acquainted with the art in connection with the
+necessity which arose for me to make galvanometer mirrors out of
+fused quartz, and it was then that I discovered with surprise how
+difficult it is to obtain information on the point. I desire to
+express my indebtedness to my colleagues, Professor David and Mr.
+Smeeth, for the instruction they have given me. In what follows I
+propose to describe their practice rather than my own, which has
+been of a makeshift description. I will therefore select the
+process of cutting a slice of rock for microscopical
+investigation.</font></p>
+<p><b><a name="Toc158108937" id="Toc158108937"><font face=
+"Bookman Old Style" size="4">&sect; 76. Arming a Wheel.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Fig. <img src=
+"images/Image106.gif" alt="images/Image106.gif" width="430" height="310">63.</font></p>
+<p>A convenient wheel is made out of tin-plate, i.e. mild steel
+sheet, about one-thirtieth of an inch thick and seven inches in
+diameter. This wheel must be quite flat and true, as well as
+round; too much pains cannot be taken in securing these
+qualities. After the wheel is mounted, it is better to turn it
+quite true by means of a watch-maker's "graver", or other
+suitable tool. The general design of a rock-cutting machine will
+be clear from the illustration (Fig. 63).</p>
+<p>The wheel being set up correctly, the next step is to arm it
+with diamond dust. For this purpose it is before all things
+necessary that real diamond dust should be obtained. The best
+plan is to procure a bit of "bort" which has been used in a
+diamond drill, and whose properties have therefore been tested to
+some extent. This is ground in a diamond mortar &mdash; or rather
+hammered in one &mdash; and passed through a sieve having at
+least 80 threads to the inch. The dust may be conveniently kept
+in oil.</p>
+<p>To arm the wheel, a little dust and oil is taken on the
+finger, and laid on round the periphery of the wheel. A bit of
+flint or agate is then held firmly against the edge of the wheel
+and the latter is rotated two or three times by hand. The
+rotation must be quite slow &mdash; say one turn in half a minute
+&mdash; and the flint must be held firmly and steadily against
+the wheel. Some operators prefer to hammer the diamond dust into
+the wheel with a lump of flint, or agate, but there is a risk of
+deforming the wheel in the process. When a new wheel is set up,
+it may be necessary to repeat the above process once every half
+hour or so till the cutting is satisfactory, but when once a
+wheel is well armed it will work for a long time without further
+attention.</p>
+<p><b><a name="Toc158108938" id="Toc158108938"><font face=
+"Bookman Old Style" size="4">&sect; 77. Cutting a Section.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A wheel 7 inches in diameter
+may be rotated about 500 times per minute, and will give good
+results at that speed. The work, as will be seen from the
+diagram, is pressed against the edge of the wheel by a force,
+which in the case quoted was about the weight of eleven ounces.
+This was distributed along a cutting arc of three-quarters of an
+inch.</font></p>
+<p>A convenient cutting lubricant is a solution of Castile soap
+in water, and this must be freely supplied; if the wheel gets dry
+it is almost immediately spoiled owing to the diamond dust being
+scraped off. In the figure the lubricant is supplied by a wick
+running into the reservoir. I have used both clock oil and
+ordinary gas-engine oil as lubricants, with equally satisfactory
+results. As to the speed of cutting, in the experiment quoted a
+bit of rather friable "gabbro," measuring three-quarters of an
+inch on the face by five-eighths of an inch thick, was cut clean
+through in six minutes, or by 3000 turns of the wheel. The travel
+of the edge was thus between 5000 and 6000 feet, or say 9000
+feet, nearly 2 miles, per inch cut.</p>
+<p>A good solid rock, like basalt, can be cut into slices of
+about 3/32 inch thick. A very loose rock is best boiled in Canada
+balsam, hard enough to set, before it is put against the
+wheel.</p>
+<p>Instead of a grinding machine a lathe may be employed. The
+disc is, of course, mounted on the mandrel, and the work on the
+slide-rest. The latter must be disconnected from its feed screws,
+and a weight arranged over a pulley so as to keep the work
+pressed against the wheel by a constant force.</p>
+<p>It may, perhaps, occur to the reader to inquire whether any
+clearance in the cut is necessary. The answer is that in all
+probability, and in spite of every care, the wheel will wobble
+enough to give clearance. If it does not, a little diamond dust
+rubbed into the side of the wheel, as well as the edge, will do
+all that is required. The edge also, after two or three armings,
+"burrs" a little, and thus provides a clearance naturally. It is
+not unlikely that in the near future the electric furnace will
+furnish us with a number of products capable of replacing the
+diamond as abrading agents. The cost of the small amount of
+diamond dust; required in a laboratory is so small, however, that
+it; is doubtful whether any appreciable economy will be,
+effected.</p>
+<p><b><a name="Toc158108939" id="Toc158108939"><font face=
+"Bookman Old Style" size="4">&sect; 78. Grinding Rock Sections,
+or Thin Slips of any Hard Material.&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A note on this is, perhaps,
+worth making, for the same reasons as were given for note, &sect;
+75, which it naturally follows. Just as trout-fishing; is
+described by Mr. Francis as the "art of fine and far off,"
+<i>[Footnote:</i> In the Badminton Library, volume on
+Fishing.<i>]</i> section grinding may be called "the art of
+Canada balsam cooking," as follows. A section of rock having been
+cut from the lump as just described, it becomes; necessary to
+grind it down for purposes of microscopical investigation. For
+this purpose it is placed on a slip of glass, and cemented in
+position by Canada, balsam. Success in the operation of grinding
+the mounted section depends almost entirely on the way in which
+the mounting is done, and this in its turn depends on the
+condition to which the Canada has been brought.</font></p>
+<p>To illustrate the operations, I will describe a specific case,
+viz. that of grinding the section of "gabbro"' above described,
+for microscopical purposes. One side of the section is probably
+sufficiently smooth and plane from the operation of the diamond
+wheel; if not, it must be ground by the finger on a slab of iron
+or gun-metal with emery and water, the emery passing a sieve of
+80 threads to the inch. The glass base on which the section is to
+be mounted for grinding is placed on a bit of iron or copper
+plate over a Bunsen burner, and three or four drops of natural
+Canada balsam are placed upon it. The section is placed on the
+plate to heat at the same time.</p>
+<p>The temperature must not rise so high as to cause any visible
+change in the Canada balsam, except a slight formation of
+bubbles, which rise to the surface, and can be blown off. The
+heating may require to be continued, say, up to twenty minutes.
+The progress of the operation is tested by examining the balsam
+as to its viscous properties.</p>
+<p>An exceedingly simple and accurate way of testing is to dip a
+pair of ordinary forceps in the balsam, which may be stirred a
+little to secure uniformity. The forceps are introduced with the
+jaws in contact, and, as soon as withdrawn, the jaws are allowed
+to spring apart, thus drawing out a balsam thread. In a few
+moments the thread is cold, and if the forceps be compressed,
+this thread will bend.</p>
+<p>The Canada must be heated until it is just in such a state
+that on bringing the jaws together the thread breaks. The forceps
+may open to about three-quarters of an inch. If the Canada is
+more viscous, so that the thread does not break, the section when
+cemented by it will most probably slip on the slide. On the other
+hand, if the balsam is more brittle, it will crumble away during
+the grinding.</p>
+<p>Assuming that the proper point has been reached, the section
+is mounted with the usual precautions to avoid air bubbles, i.e.
+by dropping one edge on the balsam first. When all is cold, the
+surface of the section may be ground on an iron plate with emery
+passing the 80 sieve, till it is about 1/40 inch thick. From this
+point it must be reduced on ground glass by flours of emery and
+water; the rough particles of the former may be washed out for
+fine work.</p>
+<p>The process of grinding should not take more than half an hour
+if the section is properly cut, etc. Beyond this point the
+allowable thickness must depend on the nature of the rock; a good
+general rule is to get the section just so thin that felspars
+show the yellow of the first order in a polarising, microscope.
+The section is then finished with, say, two minutes emery or
+water of Ayr-stone dust. It is better not to have the surface too
+smooth.</p>
+<p>To transfer the section, the hard Canada round the sides is
+scraped away, and the section itself covered with some fresh
+Canada from the bottle. It is then warmed till it will slip off
+when a pin, or the invaluable dentist's chisel, is pressed
+against one side. If the section be very delicate, the cover slip
+should be placed over it before it is moved to the proper slide.
+The Canada used for mounting is not quite so hard as that
+employed in grinding, but it should be hard when cold, i.e. not
+sticky.</p>
+<p>The art of preparing Canada balsam appears to consist in
+heating it under such conditions as will ensure its being exposed
+in thin layers. I have wasted a good deal of time in trying to
+bake Canada in evaporating basins, with the invariable result
+that it was either over or under-baked, and got dark in colour
+during the process.</p>
+<p>On reviewing the process of rock section-cutting and mounting
+as just described, I cannot help thinking that, if properly
+systematised, it could be made much more rapid by the
+introduction of proper automatic grinding machinery. It also
+seems not improbable that a proper overhaul of available gums and
+cements would be found to lead to a cementing material less
+troublesome than Canada balsam.</p>
+<p><b><a name="Toc158108940" id="Toc158108940"><font face=
+"Bookman Old Style" size="4">&sect; 79. Cutting Sections of Soft
+Substances. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Though this art is fully
+treated of in books on practical biology, it is occasionally of
+use to the physicist, and the following note treats of that part
+of the subject which is not distinctly biological.</font></p>
+<p>Soft materials, of which thin sections may be required,
+generally require to be strengthened before they are cut. For
+this purpose a variety of materials are available. The one most
+generally used is hard paraffin. The only point requiring
+attention is the embedding. The material must be dry.</p>
+<p>This is accomplished by soaking in absolute alcohol, i.e.
+really absolute alcohol made by shaking up rectified spirit with
+potassium carbonate, previously dried, and then digesting for a
+day with large excess of quick-lime, making use of an inverted
+condenser and finally distilling off the alcohol without allowing
+it to come in contact with undried air. After soaking for some
+time in absolute alcohol, the material may be transferred to oil
+of bergamot, or oil of cloves, or almost any essential oil. After
+soaking in this long enough to allow the alcohol to diffuse out,
+the material may be lifted into a bath of melted paraffin
+(melting at, say, 51&deg; C.). The process of soaking is in some
+cases made to go more rapidly by exhausting, and, if the material
+will stand it, by raising the temperature over 100&deg; C. The
+soaking process may require minutes, hours, or days, according to
+the size and density of the material; but a few hours are usually
+sufficient.</p>
+<p>When cold, the sections may be cut in any of the ordinary
+forms of microtome.</p>
+<p>Another way of embedding is to soak in collodion, and then
+precipitate the latter in the material and around it by plunging
+into nearly absolute alcohol. The collodion yields a harder
+matrix than the paraffin.</p>
+<p>Whatever form of cutting machine is employed, the art of
+sharpening the knife is the only one requiring any particular
+notice. The easiest way of obtaining a knife hard enough to
+sharpen, is to use a razor of good quality. If it has to be
+ground, it is best to do this on a fine Turkey stone which is
+conveniently rested on two bits of rubber tubing, to avoid
+jarring the blade. Many stones are slightly cracked, but on no
+account must the razor be dragged across a crack, or the edge
+will suffer.</p>
+<p>The necessary and sufficient condition is that the razor must
+be worked in little sweeps over the stone, and pressed against
+the latter by little more than its own weight, and the grinding
+must be regular. The edge may be inspected under a microscope,
+and it must be perfectly smooth and even before it will cut
+sections. A finishing touch may be given on a leather strap, but
+it must be done skilfully, otherwise it is better omitted.</p>
+<p>The necessity for providing exceptionally keen and sharp edges
+arose in the manufacture of phonographs, where the knife used to
+turn up the wax cylinders must leave a perfectly smooth surface.
+In 1889 this was being accomplished on an ivory lap fed with a
+trace of very fine diamond dust.</p>
+<p>I have had this method in mind as a possible solution of the
+difficulty of razor-grinding, but have not tried it. I imagine
+one would use a soft steel or ivory slip rubbed over with fine
+diamond dust and oil by means of an agate. The lap used in the
+phonograph works was rotated at a high speed.</p>
+<p><b><a name="Toc158108941" id="Toc158108941"><font face=
+"Bookman Old Style" size="4">&sect; 80. On the Production of
+Quartz Threads.' &mdash;</font></a></b></p>
+<p><i><font face="Bookman Old Style">[Footnote:</font> Since this
+was written an article on the same subject by Mr. Boys appeared
+in the Electrician for 1896. The instructions therein given are
+in accordance with what I had written, and I have made no
+alteration in the text.]</i></p>
+<p>In 1887 the important properties of fused quartz were
+discovered by Mr. Vernon Boys (Philosophical Magazine, June 1887,
+p. 489, "On the Production, Properties, and Some Suggested Uses
+of the Finest Threads"). A detailed study of the properties of
+quartz threads was made by Mr. Boys and communicated to the
+Society of Arts in 1889 (Journal of the Society of Arts, 1889).
+An independent study of the subject was made by the present
+writer in 1889 (Philosophical Magazine, July 1890, "On the
+Elastic Constants of Quartz Threads ").</p>
+<p>There is also a paper in the Philosophical Magazine for 1894
+(vol. xxxvii. p. 463), by Mr. Boys, on "The Attachment of Quartz
+Fibres." This paper also appeared in the Journal of the Physical
+Society at about the same date, together with an interesting
+discussion of the matter. In the American Journal, Electric
+Power, for 1894, there is a series of articles by Professor
+Nichols on "Galvanometers," in which a particular method of
+producing quartz threads is recommended. The method was
+originally discovered by Mr. Boys, but he seems to have made no
+use of it. A hunt through French and German literature on the
+subject has disclosed nothing of interest &mdash; nothing indeed
+which cannot be found in the papers mentioned.</p>
+<p>&sect; 81. Quartz fibres have two great advantages over other
+forms of suspension when employed for any kind of torsion
+balance, from an ordinary more or less "astatic" galvanometer to
+the Cavendish apparatus. In the first place the actual strength
+of the fibres under longitudinal stress is remarkably high,
+ranging from fifty to seventy tons weight per square inch of
+section, and even more than this in the case of very fine
+threads; the second and more important point in favour of quartz
+depends on the wide limits within which cylindrical threads of
+this material obey the simplest possible law of torsion, i.e. the
+law that for a given thread carrying a given weight at a given
+temperature and having one end clamped, the twist about the axis
+of figure produced by a turning moment applied at the free end is
+proportional simply to the moment of the twisting forces, and is
+independent of the previous history of the thread.</p>
+<p>It is to be noted, however, that the torsional resilience of
+quartz as tested by the above law is not so perfect but that our
+instrumental means allow us to detect its imperfections, and thus
+to satisfy ourselves that threads made of quartz are not things
+standing apart from all other materials, except in the sense that
+the limits within which they may be twisted without deviating in
+their behaviour from the law of strict proportionality by more
+than some unassigned small quantity, are phenomenally wide.</p>
+<p>A torsion balance &mdash; if we except the case of certain
+spiral springs &mdash; is almost always called upon for
+information as to the magnitude of very small forces, and for
+this purpose it is not essential merely that some law of twisting
+should be exactly obeyed, but also that the resistance to
+twisting of the suspension should be small.</p>
+<p>Now, regarded merely as a substance possessing elastic
+rigidity, quartz is markedly inferior to the majority of
+materials, for it is very stiff indeed; its utility depends as
+much as anything upon its great strength, for this allows us to,
+use threads of exceeding fineness. In addition to this it must be
+possible, and moreover readily possible, to obtain threads of
+uniform section over a sufficient length, or the rate of twist
+per unit length of the thread will vary in practice from point to
+point, so that the limits of allowable twist averaged over the
+whole thread may not be exceeded, and yet they may be greatly
+overpassed at particular points of the thread.</p>
+<p>It is interesting to note that in the case of quartz we not
+only have a means for readily producing very uniform cylindrical
+threads, but that the limits of allowable rate of twist are so
+wide that a small departure from uniformity of section produces
+much less inconvenience than in the case of any other known
+substance.</p>
+<p>&sect; 82. There are three methods generally in use for
+drawing quartz fibres, all depending on the fact that quartz when
+fused is so viscous that it may be drawn into threads of great
+length, without these threads breaking up into drops, or indeed
+without their showing any sign of doing so. The surface tension
+of the melted quartz must, however, be very considerable, as may
+be seen by examining the shape of a drop of the molten material,
+and this suffices to impress a rigidly cylindrical form upon the
+thread, the great viscosity apparently damping down all
+oscillation.</p>
+<p>The first method is the one originally employed by Mr. Boys. A
+needle of quartz is melted somewhere in its length and is then
+drawn out rapidly by a light arrow, to which one end of the
+needle is attached, and which is projected from a kind of
+crossbow.</p>
+<p>A modification of this method, which the writer has found of
+service when very thick threads are required, is to replace the
+bow and arrow by a kind of catapult.</p>
+<p>The third method, which yields threads of almost unmanageable
+fineness, depends on the experimental fact that when a fine point
+of quartz is held in a high pressure oxygen gas blow-pipe flame,
+the friction of the flame gases suffices to overcome the tendency
+of the capillary forces to produce a spherical drop, and actually
+causes a fine thread to be projected outwards in the direction of
+the flame.</p>
+<p>&sect; 83. A preliminary operation to any method is the
+production of a stick of fused quartz. This is managed as
+follows. A rock crystal or quartz pebble is selected and
+examined. It must be perfectly white, transparent, and free from
+dirt. Surface impurity can of course be got rid of by means of a
+grindstone. The crystal is placed in a perfectly clean
+Stourbridge clay crucible, furnished with a cover, and heated to
+bright redness for about an hour in a clean fire or in a
+Fletcher's gas furnace. The contents of the crucible are turned
+out when sufficiently cool on to a clean brick or bit of slate.
+It will be found that the crystal is completely broken up and the
+fragments must be examined in case any of them have become
+contaminated by the crucible, but this will not have happened if
+the temperature did not rise beyond a bright red heat.</p>
+<p>The heap of fragments being found satisfactory, the next thing
+is to fuse some of the pieces together. Unless the preliminary
+heating has been efficiently carried out this will prove an
+annoying task, because a rock crystal generally contains so much
+water that it splinters under the blow-pipe in a very persistent
+manner. There are two ways of assembling the fragments. One is to
+place two tiles or bricks on edge about the heap of quartz lying
+upon a third tile, so that the heap occupies the angular corner
+or nook formed by the tiles (Fig. 64).</p>
+<p>The oxygas blow-pipe previously described is adjusted to give
+its hottest flame, the bags being weighted by at least two
+hundredweight, if of the size described (see &sect; 15).</p>
+<p>The tip of the inner cone of the blow-pipe is brought to bear
+directly upon one of the fragments, and if the operation is
+performed boldly it will be found that the surface of the
+fragment can be fused, and the fragment thus caused to hold
+together before the lower side gets hot enough to suffer any
+contamination from the tile or brick. A second fragment may be
+treated in the same way, and then a third, and so on.</p>
+<p>Finally, the fragments may be fused together slightly at the
+corners, and a stick may thus be formed. Of course a good deal of
+cracking and splitting of the fragments takes place in the
+process; the best pieces to operate upon are those which are well
+cracked to begin with, and that in such a way that the little
+fragments are interlocked.</p>
+<p>An alternative method which has some advantages is to arm a
+pair of forceps with two stout platinum jaws, say an inch and a
+half long, and flattened a little at the ends. The fragments are
+held in these platinum forceps and the blow-pipe applied as
+before. This method works very well in adding to a rod which has
+already been partly formed, but the jaws require constant
+renewals. The first fragment which is fused sufficiently to
+cohere may also be fused to a bit of tobacco pipe, or hard glass
+tube or rod, and the quartz stick gradually built up by fusing
+fresh pieces on to the one already in position.</p>
+<p>Fig. <img src="images/Image107.gif" alt="images/Image107.gif"
+width="415" height="200">64.</p>
+<p>Since the glass or pipeclay will contaminate the quartz which
+has been fused on to it, it is necessary to discard the end
+pieces at the close of the operation. A string of fragments
+having been collected and stuck together, the next step is to
+fuse them down into a uniform rod. This is easily done by holding
+the string in the blow-pipe flame and allowing it to fuse down.
+Twisting the fused part has a good effect in assisting the
+operation. It is desirable to use a large jet and as powerful a
+flame as can be obtained during this part of the operation.</p>
+<p>The final result should be a rod, say two or three inches long
+and one-eighth of an inch thick, which will in most cases contain
+a large number of air bubbles. Since the presence of drawn-out
+bubbles cannot be advantageous, it is often desirable to get rid
+of them, and this can conveniently be done at the present stage.
+The process at best is rather tedious; it consists in drawing the
+quartz down very fine before an intense flame, in order to allow
+the bubbles to get close enough to the surface to burst. A
+considerable loss of material invariably occurs during the
+process; for whenever the thin rod separates into two bits the
+process of flame-drawing of threads goes on, and entails a
+certain waste; moreover, the quartz in fine filaments is probably
+partially volatilised.</p>
+<p>Sooner or later, however, a sufficient length of bubble-free
+quartz can be obtained. It must not be supposed that it is always
+necessary to eliminate bubbles as perfectly as is contemplated in
+the foregoing description of the treatment, but for special
+purposes it may be essential to do so, and in any case the
+reader's attention is directed to a possible source of error.</p>
+<p>It may be mentioned in connection with this matter that
+crystals of quartz may look perfectly white and clear, and yet
+contain impurity. For instance, traces of sodium are generally
+present, and lithium was found in large spectroscopic quantity in
+five out of six samples of the purest crystals in my laboratory.
+The presence of lithium in rock crystal has also been detected by
+Tegetmeier (Vied. Ann., xli. p. 19, 1890).</p>
+<p>After some practice in preparing rods and freeing them of
+bubbles the operator will notice a distinct difference in the
+fusibility of the samples of quartz he investigates, though all
+may appear equally pure to the unaided eye. It should be
+mentioned, however, that high infusibility cannot always be taken
+as a test of purity, for the most infusible, or rather most
+viscous, sample examined by the writer contained more lithium
+than some less viscous samples.</p>
+<p>Fig. <img src="images/Image108.gif" alt="images/Image108.gif"
+width="237" height="21"> 65.</p>
+<p>During the process of freeing the quartz from bubbles the
+lithium and sodium will be found to burn away, or at all events
+to disappear.</p>
+<p>A rod of quartz, say three inches long, one-sixteenth of an
+inch in diameter, and free from bubbles for half an inch of its
+length, even when examined by a strong lens, is suitable for
+drawing into threads. The rod is manipulated exactly in the
+manner described under GLASS-BLOWING, and is finally drawn down
+at the bubble free part into a needle, say 0.02 inch in diameter
+(No. 25 on the Birmingham wire gauge), and 2 inches long.</p>
+<p>Fig. <img src="images/Image109.gif" alt="images/Image109.gif"
+width="343" height="24"> 66.</p>
+<p>There is one peculiarity about fused quartz which renders its
+manipulation easier than that of glass &mdash; it is impossible
+to break fused quartz, however suddenly it be thrust into the
+blow-pipe flame. A rod having a diameter of three-sixteenths of
+an inch &mdash; and perhaps much more &mdash; may be brought
+right up to the tip of the inner cone of the oxy-gas flame and
+held there-till one side fuses, the other being comparatively
+cool, without the slightest fear of precipitating a smash. In
+seven years' experience I have never seen a bit of once fused
+quartz broken by sudden heating; whether it might be done if
+sufficient precautions were taken I do not know.</p>
+<p>The reason of the fortunate peculiarity of quartz in this
+respect is, I presume, to be found in the fact that quartz once
+it has been fused is really a very strong material indeed, and is
+also probably the least expansible substance known. From some
+experiments of the writer upon the subject, it may be concluded
+that at the most quartz which has been fused expands only about
+one-fifth as fast as flint-glass, at all events between 20&deg;
+and 70&deg; C.</p>
+<p><b><a name="Toc158108942" id="Toc158108942"><font face=
+"Bookman Old Style" size="4">&sect; 84. Drawing Quartz Threads.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The thick end of the rod of
+quartz is held in the fingers or occasionally in a clip. The end
+of the fine point is attached to a straw arrow by means of a
+little sealing-wax. The arrow is laid on the stock of a crossbow
+in the proper position for firing. See Figs. 67 and 68, which
+practically explain themselves.</font></p>
+<p>The needle is heated by the blow-pipe till a minute length is
+in a state of uniform fusion; the arrow is then let fly, when it
+draws a thread out with it. The arrow is preferably allowed to
+strike a wooden target placed, say, 30 feet away from the bow,
+and a width of black glazed calico is laid under the line of fire
+to catch the thread or arrow if it falls short. The general
+arrangements will be obvious from the figure.</p>
+<p>The bow is of pine in the case where very long thin threads
+are required, though for ordinary purposes I have found a bow of
+lance-wood succeed quite as well. The trigger of the bow consists
+of a simple pin passing through the stock and fastened at its
+lower end to a string connected with a board which can be
+depressed by foot. In the figure an ordinary trigger is shown,
+but the pin does just as well.</p>
+<p>Fig 67<img src="images/Image110.gif" alt=
+"images/Image110.gif" width="299" height="382">.</p>
+<p>The arrow is made out of about 6 inches of straw, plugged up
+aft by a small plug of pine or willow fastened in with
+sealing-wax, and projecting backwards one-eighth of an inch. This
+projection serves a double purpose: it gives a point of
+attachment for the quartz needle, and on firing the bow it forms
+a resisting anvil on which the string of the bow impinges. The
+head of the arrow is formed by a large needle stuck in with
+sealing-wax, and heavy enough to bring the centre of gravity of
+the arrow forward of one-third of its length, the condition of
+stability in flight.</p>
+<p>Fig. 68<img src="images/Image111.gif" alt=
+"images/Image111.gif" width="396" height="252">.</p>
+<p>It is not necessary to employ any feathering for these arrows;
+though I have occasionally used feathers or mica to "wing the
+shaft" no advantage has resulted therefrom.</p>
+<p>To get fine threads a high velocity is essential. This is
+obtained by considering (and acting upon) the principles
+involved. The bow may be regarded as a doubly-tapering rod
+clamped at the middle. After deflection it returns towards its
+equilibrium position at a rate depending in general terms on the
+elastic forces brought into play, directly, and on the effective
+moment of inertia of the rod, inversely (see Rayleigh, Sound,
+vol. ii. chap. viii.) If the mass of the arrow is negligible
+compared with the bow, the rate at which the arrow moves is
+practically determined by that attained by the end of the bow,
+which is a maximum in crossing its equilibrium position.</p>
+<p>The extent to which the arrow profits by this velocity depends
+on the way the bow is strung. It will be greatest when the string
+is perpendicular to the bow when passing its equilibrium
+position; or in other words, when the string is infinitely long.
+Since the string has mass, however, it is not permissible to make
+it too long, or its weight begins to make itself felt, and a
+point is soon reached at which the geometrical gain in string
+velocity is compensated for by the total loss of velocity due to
+the inertia of the string. In practice it is sufficient to use a
+string 10 per cent longer than the bow.</p>
+<p>It is well to use a light fiddle string, served with waxed
+silk at the trigger catch; if this be omitted the gut gets worn
+through very quickly. In order to decide how far it is
+permissible to bend the bow, the quickest way is to make a rough
+experiment on a bit of the same plank from which the bow is to be
+cut, and then to allow a small factor of safety. In the figure
+the bow is of lance-wood and is more bent than would be suitable
+for pine.</p>
+<p>The bow itself is tapered from the middle outwards just like
+any other bow. If thick threads are required, the above
+considerations are modified by the fact that quartz opposes a
+considerable resistance to drawing, and that consequently the
+arrow must not only have a high velocity, but a fair supply of
+energy as well; in other words, it must be heavy. A thin pine
+arrow instead of a straw generally does very well, but in this
+case the advantage of using pine for the bow vanishes; and in
+fact lance-wood does better, owing to the greater displacement
+which it will stand without breaking. This of course only means
+that a greater store of energy can be accumulated at one
+bending.</p>
+<p>I had occasion to investigate whether the unavoidable spin of
+an arrow about its axis produces any effect on the thread, and
+for this purpose made arrows with inertia bars thrust through the
+head, i.e. an arrow with a bit of wire run through it,
+perpendicular to its length &mdash; forming a cross in fact
+&mdash; the arms of the cross being weighted at the extreme ends
+by shot. This form of arrow has a considerable moment of inertia
+about its longer axis, and consequently rotates less than a mere
+straw, provided that the couples tending to produce rotation are
+not increased by the cross arm, or the velocity too much reduced.
+Shooting one of these arrows slowly, I could see that it did not
+rotate, and when fired at a high velocity, it generally arrived
+at the target (placed at varying distances front bow) with the
+arms nearly horizontal, thus showing that it probably did not
+rotate much.</p>
+<p>I did not succeed in this at the first trial, by any means.
+The threads got in this way were no better than those made with a
+single straw, whence we may conclude very provisionally that the
+spin of the arrow has only a small effect, if any, on the quality
+of the threads.</p>
+<p>Feathering the arrow, in my experience, tends, if anything to
+make it spin more; for one thing, because it is practically
+impossible to lay the feathering on straight.</p>
+<p>After the arrow is shot, it remains to gather in the thread,
+and if the latter is at all thin, we have a rather troublesome
+job. In a thread thirty or forty feet long, the most uniform part
+generally lies in the middle if the thread is thin, i.e. of the
+order of a ten-thousandth of an inch in diameter. If the thread
+is thick the most uniform part may be anywhere. The part of the
+thread required is generally best isolated by passing a slip of
+paper under it at each end and cementing the thread to the paper
+by means of a little paraffin or soft wax, and then cutting off
+the outer portions. One bit of paper may then be lifted off the
+calico, and the thread will carry the other bit. In this way the
+thread may be taken to a blackened board, where it may be mounted
+for stock.</p>
+<p>By passing the two ends of the thread under a microscope, or
+rather by breaking bits off the two ends and examining them
+together, it is easy to form an Opinion as to uniformity.</p>
+<p>Mr. Boys has employed an optical method of examining threads,
+but the writer has invariably found a high-power microscope more
+convenient and capable of giving more exact information as to the
+diameter of the threads.</p>
+<p>The beginner &mdash; or indeed the practised hand &mdash; need
+not expect to get a thread of the exact dimensions required at
+the first shot. A little experience is necessary to enable one to
+judge of the right thickness of the needle for a thread of given
+diameter. The threads are so easily shot, however, that a few
+trials take up very little time and generally afford quite
+sufficient experience to enable a thread of any required diameter
+to be prepared.</p>
+<p>It is no use attempting to heat an appreciable length of
+needle; if this be done the thread almost invariably has a thick
+part about the middle of its length.. It is sufficient to fuse at
+most about one-twentieth of an inch along the needle before
+firing off the bow. This can be done by means of the smaller
+oxygas blow-pipe jet described in the article on blow-pipes for
+GLASS-BLOWING, &sect; 14. The flame must of course be turned down
+so as to be of a suitable size. A sufficiently small flame may be
+got from almost any jet.</p>
+<p>If the needle be not equally heated all round, the thread
+tends to be curly; indeed by means of the catapult, threads may
+be pulled which, when broken, tend to coil up like the
+balance-springs of watches, if only care be taken to have one
+side of the needle much hotter than the other.</p>
+<p>&sect; 85. When examining bits of threads, say thicker than
+the two-thousandth of an inch, under the microscope it is
+convenient to use a film of glycerine stained with some kind of
+dye, in order to render the thread more sharply visible. The
+thread is mounted beneath a cover slip, and a drop of the stained
+glycerine allowed to run in. Such a treatment gives the image of
+the thread a sharply defined edge 3 and the contrast between the
+whiteness of the thread and the colour of the background allows
+measurements to be made with great ease.</p>
+<p>On the whole the easiest way of measuring the diameter of a
+thick thread is to use a measuring microscope, i.e. one in which
+the lens system can be displaced along a plane bed by means of a
+finely cut micrometer screw. The instruments made by the
+Cambridge Scientific Instrument Company do fairly well. Direct
+measurements up to 0.0001 inch are easily made by means of a
+microscope provided with a Zeiss "A" objective, and rather
+smaller differences of thickness can be made out by it. For thin
+threads the method next to be described is more fitting, because
+higher powers can be more conveniently used.</p>
+<p>In this method an ordinary microscope is employed together
+with a scale micrometer, and either an eyepiece micrometer, or a
+camera and subsidiary scale. The eyepiece micrometer is the more
+convenient. If a camera be employed, i.e. such an one as is
+supplied by Zeiss, it is astonishing how the accuracy of
+observation may be increased by attending carefully to the
+illumination of both the subsidiary scale and of the thread. The
+two images should be as far as possible of equal brightness, and
+for this purpose it will be found requisite to employ small
+screens.</p>
+<p>The detail of making a measurement by means of the micrometer
+eyepiece is very simple. The thread is arranged on the stage so
+as to point towards the observer, and the apparent diameter is
+read off on the eyepiece scale. In order to calibrate the latter
+it is only necessary to replace the thread by the stage
+micrometer, and to observe the number of stage micrometer
+divisions occupying the space in the eyepiece micrometer formerly
+occupied by the thread. It is essential that both thread and
+stage micrometer should occupy the same position in the field,
+for errors due to unequal distortion may otherwise become of
+importance. For this reason it is best to utilise the centre of
+the field only.</p>
+<p>The same remark applies to measurements by means of the
+camera, where the image of the thread is projected against the
+reflected image of the subsidiary scale laid alongside the
+microscope. In this case the value of the subsidiary scale
+divisions must be obtained from the divisions of the stage
+micrometer, coinciding as nearly as possible with the position
+occupied by the thread. Before commencing a measurement the
+screens are moved about till both images appear equally
+bright.</p>
+<p>Threads up to about one twenty-thousandth of an inch in
+diameter may be sufficiently well measured by means of a Zeiss "4
+centimetre apochromatic object-glass" and an eyepiece "No. 6"
+with sixteen centimetre tube length. <i>[Footnote:</i> The
+objective certainly had "4 cm." marked on it, but the focal
+length appeared to be about I.5 mm. only.<i>]</i></p>
+<p><b><a name="Toc158108943" id="Toc158108943"><font face=
+"Bookman Old Style" size="4">&sect; 86. Drawing Threads by the
+Catapult. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The bow-and-arrow method fails
+when threads of a greater diameter than about 0.0015 inch are
+required &mdash; at least if any reasonable uniformity be
+demanded, and no radical change in the bow and arrow be carried
+out.</font></p>
+<p>Thus in the writer's laboratory a thread of about this
+diameter, within 1/10000 of an inch-13 inches long and free from
+air bubbles &mdash; was required. A fortnight's work by a most
+skilful operator only resulted in the production of two lengths
+satisfying the conditions.</p>
+<p>The greatest loss of time occurs in the examination of the
+thread by means of the microscope.</p>
+<p>Threads for galvanometer suspensions are conveniently from
+0.0001 to 0.0004 inch in diameter, and are much more easily made
+and got uniform than thicker threads, to the production of which
+the catapult method applies.</p>
+<p>A reference to the diagram will make the construction of the
+instrument quite clear. The moving end of the quartz is attached
+to a small boxwood slider working on a tubular girder or between
+wires. The quartz is secured in position by clamps shown at A and
+B, and motion is imparted to the slider by a stretched piece of
+catapult elastic (C). An easy means of regulating the pull of the
+elastic is to hold it back by a loop of string whose length can
+be varied by twisting it round a pin.</p>
+<p>Fig. 69. <img src="images/Image112.gif" alt=
+"images/Image112.gif" width="490" height="223"> <i>[Footnote:</i>
+For greater clearness of drawing, the tube carrying the slider is
+shown somewhat higher above the base than is convenient in
+practice; and the slide itself is shown too thin in the direction
+of the hole through it.<i>]</i></p>
+<p>Since it is not permissible to allow the slider to rebound at
+the end of its journey, some such arrangement of breaks as is
+shown must be adopted. In the diagram the bottom of the slider
+runs on to a brass spring between the girder and the base of the
+appliance, and so gets jammed; the spiral spring acts merely as
+an additional guard. The diagram does not show the lower spring
+very clearly; it is a mere strip lying in the groove.</p>
+<p>A rod of quartz, with a needle at one end, is prepared as
+before and secured in the clamps. During the operation of
+fastening down the clamps, there is some danger of breaking the
+needle, and consequently it is advisable to soften the latter
+before and while adjusting the second clamp.</p>
+<p>The process of drawing a thread by this method is exactly
+similar to the operation already described in connection with the
+arrow method. Though short thick threads form the product
+generally obtained from the catapult, it must not be supposed
+that thin threads cannot be obtained in this way. If a short
+length of a very fine needle be heated, it will be found to yield
+threads quite fine enough for ordinary suspension purposes, but
+naturally not so uniform as those obtained from the 40-foot
+lengths obtainable by the bow-and-arrow method.</p>
+<p>It is easy to make spiral quartz springs resembling watch
+balance-springs by means of the catapult. All that is necessary
+is to see that the quartz is rather unequally heated before the
+shot is fired. In the future it is by no means impossible that
+such springs may have a real value, for the rigidity of quartz is
+known to increase as temperature rises. Hence it is probable that
+the springs would become stiffer as temperature rises, even
+though they work chiefly by bending, and little or not at all by
+twisting. As this is the kind of temperature variation required
+to compensate an uncompensated watch balance wheel, it may turn
+out to have some value.</p>
+<p><b><a name="Toc158108944" id="Toc158108944"><font face=
+"Bookman Old Style" size="4">&sect; 87. Drawing Threads by the
+Flame alone. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A stick of quartz is drawn down
+to a fine point, and the tip of this point is held in the
+blow-pipe flame in the position shown in Fig. 70.</font></p>
+<p><img src="images/Image113.gif" alt="images/Image113.gif"
+width="348" height="469">Fig. 70.</p>
+<p>The friction of the flame gases is found to be sufficient to
+carry forward the fused quartz and to draw it into threads in
+spite of the influence of the capillary forces. If a sheet of
+paper be suspended at a distance of two or three feet in front of
+the blow-pipe flame, it will be found to be covered with fine
+threads tangled together into a cobwebby mass. As this method is
+an exceedingly simple one of obtaining threads, I have
+endeavoured to reduce it to a systematic operation.</p>
+<p>A sheet of cardboard, about two feet square, is painted dead
+black and suspended horizontally, painted side downwards (Fig.
+70, A), at a height of about two feet above the blow-pipe flame.
+The latter is adjusted so as to point almost vertically upwards
+and towards the centre of the cardboard. A few half-inch pins are
+thrust through the card from the upper surface and pushed home;
+about one dozen pins scattered over the surface will be
+sufficient. Their object is to prevent the threads being carried
+away round the edge of the screen.</p>
+<p>The flame from the jet described so often is fed from gas bags
+weighted to about eighty pounds per square foot of (one) surface,
+i.e. "4-foot" bags require from three to four hundredweight to
+give an advantageous pressure. <i>[Footnote:</i> The resulting
+threads were really too fine for convenient manipulation, so that
+unless extremely fine threads are required it will be better to
+reduce the pressure of the gases considerably.<i>]</i></p>
+<p>Two sticks of quartz are introduced and caused to meet just in
+front of the inner cone &mdash; the hottest part of the flame.
+They are then drawn apart so as to form a fine neck, which
+softens and is bent in the direction of motion of the flame
+gases. When fusion is complete the neck separates into two parts,
+and a thread is drawn from each of them. By alternately lightly
+touching the rods together, and drawing them apart, quite a mass
+of threads may be obtained in two or three minutes, when the
+process should be stopped. If too many threads get entangled in
+the pins, one gives one's self the unnecessary trouble of
+separating them. On taking down the card it will be found that
+the threads have been caught by the pins; but the card now being
+laid black side upwards, the former easily slip off the
+points.</p>
+<p>Threads at least a foot long, and perhaps vastly longer, may
+be obtained by this method, and are extraordinarily fine. When I
+first read Professor Nichols' statement (Electric Power, 1894) as
+to the value of these fibres for galvanometer purposes, I was
+rather sceptical on the ground that the threads would tend to get
+annealed by being drawn gradually, instead of suddenly, from a
+place of intense heat to regions of lower temperature.</p>
+<p>Now annealing threads by a Bunsen makes them rotten. The
+threads being immersed in the hot flame gases could only cool at
+the same rate as the gas, and it was not &mdash; and is not
+&mdash; clear to me that annealing of the threads can be avoided.
+On the other hand, it may be possible that a thread cooled slowly
+from the first does not suffer in the same way as a cold thread
+would do when annealed in a Bunsen flame.</p>
+<p>Again the velocity of the gases is beyond doubt exceedingly
+high, so that the annealing, even supposing it to be deleterious,
+might not be carried very far. Threads drawn by this method and
+measured "dry," i.e. by mounting them on a slide without the
+addition of any liquid, turned out to have a diameter of about
+1/20000 of an inch.</p>
+<p>I do not think I could manage to mount such fine threads
+without very special trouble. All the threads lying on the board,
+however, were found in reality to consist of three or four
+separate threads, and there is no reason why several threads
+should not be mounted in parallel, provided, of course, that they
+are equally stretched and touching each other. Equality of
+tension in the mounting could be secured by making one attachment
+good, then cementing the other attachment to the other end of the
+threads, and "drawing" the two attachments slightly apart at the
+moment the cement commences to set. This method may turn out to
+be very valuable, for, so far as I can see, the carrying power
+would be increased without an increase of torsional stiffness of
+anything like so high an order as would be the case were one
+thread only employed. On the other hand, the law of torsion could
+hardly be quite so simple, at all events, to the second order of
+approximations.</p>
+<p><b><a name="Toc158108945" id="Toc158108945"><font face=
+"Bookman Old Style" size="4">&sect; 88. Properties of Threads.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A large number of experiments
+on the numerical values of the elastic constants of quartz
+threads have been made by Mr. Boys and his students, and by the
+writer. As the methods employed were quite distinct and the
+results wholly independent, and yet in good agreement with each
+other, a rounded average may be accepted with considerable
+confidence.</font></p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"593">
+<tr>
+<td valign="top" colspan="4">
+<p><font face="Bookman Old Style">TENACITY OF QUARTZ FIBRES
+(BOYS).</font></p>
+</td>
+</tr>
+<tr>
+<td width="35%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">Diameter of Thread.</font></p>
+</td>
+<td width="29%" valign="top" rowspan="2">
+<p><font face="Bookman Old Style">Tenacity in Tons' Weight per
+Square Inch of Section.</font></p>
+</td>
+<td width="36%" valign="top" rowspan="2">
+<p><font face="Bookman Old Style">Tenacity in Dynes per Square
+Centimetre.</font></p>
+</td>
+</tr>
+<tr>
+<td width="16%" valign="top" height="54">
+<p><font face="Bookman Old Style">Inches</font></p>
+</td>
+<td width="19%" valign="top" height="54">
+<p><font face="Bookman Old Style">Centimetres</font></p>
+</td>
+</tr>
+<tr>
+<td width="16%" valign="top">
+<p align="right"><font face=
+"Bookman Old Style">0.00069</font></p>
+</td>
+<td width="19%" valign="top">
+<p align="right"><font face=
+"Bookman Old Style">0.00175</font></p>
+</td>
+<td width="29%" valign="top">
+<p align="right"><font face="Bookman Old Style">51.7</font></p>
+</td>
+<td width="36%" valign="top">
+<p align="right"><font face="Bookman Old Style">8 X
+10<sup>9</sup></font></p>
+</td>
+</tr>
+<tr>
+<td width="16%" valign="top">
+<p align="right"><font face=
+"Bookman Old Style">0.00019</font></p>
+</td>
+<td width="19%" valign="top">
+<p align="right"><font face=
+"Bookman Old Style">0.00048</font></p>
+</td>
+<td width="29%" valign="top">
+<p align="right"><font face="Bookman Old Style">74.5</font></p>
+</td>
+<td width="36%" valign="top">
+<p align="right"><font face="Bookman Old Style">11.5 X
+10<sup>9</sup></font></p>
+</td>
+</tr>
+</table>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"568">
+<tr>
+<td valign="top" colspan="2">
+<p><font face="Bookman Old Style">Rounded mean of Boys' and
+Threlfall's results:</font></p>
+</td>
+</tr>
+<tr>
+<td width="58%" valign="top">
+<p><font face="Bookman Old Style">Young's Modulus at 20&deg;
+C.,</font></p>
+</td>
+<td width="42%" valign="top">
+<p align="right"><font face="Bookman Old Style">5.6 X
+10<sup>11</sup> C.G.S.</font></p>
+</td>
+</tr>
+<tr>
+<td width="58%" valign="top">
+<p><font face="Bookman Old Style">Modulus of Simple Rigidity at
+20&deg; C.,</font></p>
+</td>
+<td width="42%" valign="top">
+<p align="right"><font face="Bookman Old Style">2.65 X
+10<sup>11</sup> C.G.S.</font></p>
+</td>
+</tr>
+<tr>
+<td width="58%" valign="top">
+<p><font face="Bookman Old Style">Modulus of
+Incompressibility,</font></p>
+</td>
+<td width="42%" valign="top">
+<p align="right"><font face="Bookman Old Style">1.4 X
+10<sup>11</sup> C.G.S.</font></p>
+</td>
+</tr>
+<tr>
+<td width="58%" valign="top">
+<p><font face="Bookman Old Style">Modulus of Torsion,</font></p>
+</td>
+<td width="42%" valign="top">
+<p align="right"><font face="Bookman Old Style">3.7 X
+10<sup>11</sup> C.G.S.</font></p>
+</td>
+</tr>
+</table>
+<p>Approximate coefficient of linear expansion of quartz per
+degree between 80&deg; C. and 30&deg; C. is 0.0000017
+(<i>Threlfall = loc. cit</i>.).</p>
+<p>This must be regarded with some suspicion, as the data were
+not concordant. There is no doubt, however, about the extreme
+inexpansibility of quartz.</p>
+<p>Temperature coefficient of modulus of torsional rigidity per
+degree centigrade, 22&deg; to 98&deg; C., 0.000133</p>
+<p>Ditto, absolute simple rigidity, 0.000128
+(<i>Threlfall</i>).</p>
+<p>Limit of allowable rate of twist in round numbers is,
+one-third turn per centimetre, in a fibre 0.01 cm. diameter.</p>
+<p>The limiting rate is probably roughly inversely as the
+diameter.</p>
+<p>Attention must be called to the rapid increase in the
+torsional rigidity of these threads as the temperature rises. A
+quartz spiral spring-balance will be appreciably stronger in hot
+weather.</p>
+<p>&sect; 89. In the majority of instances in which quartz
+threads are applied in the laboratory, it is desirable to keep
+the coefficient of torsion as small as possible, and hence
+threads are used as fine as possible.</p>
+<p>It is convenient to remember that a thread 0.0014 cm. or
+0.0007 inch in diameter breaks with a weight of about ten
+grammes, and may conveniently be employed to carry, say, five
+grammes. With threads three times finer the breaking strength per
+unit area increases, say, 50 per cent. In ordinary practice
+&mdash; galvanometric work for instance &mdash; where it is
+desirable to use a thread as fine and short as possible to
+sustain a weight up to, say, half a gramme, it will be found that
+fibres five centimetres long or over give no trouble through
+defect of elastic properties. A factor of safety of two is a fair
+allowance when loading threads.</p>
+<p>No difficulty will be experienced in mounting threads having a
+diameter of 0.0002 inch or over. With finer threads it is
+necessary to employ very dark backgrounds (Mr. Boys uses the
+darkness of a slightly opened drawer), or the threads cannot be
+sufficiently well seen.</p>
+<p>In the case of instruments in which threads remain highly
+twisted for long periods of time, the above rule as to the safe
+limit of twist does not allow of a sufficient margin; it is only
+applicable to galvanometric and similar purposes.</p>
+<p>The cause of the increase in tenacity as the diameter
+diminishes is at present unknown. It is due neither to an effect
+of annealing (annealed threads are rotten), nor is it a skin
+effect, nor is it due to the cooling of the thread under higher
+capillary pressure. It is, however, possible that it may be
+associated with some kind of permanent set taken by the fibres
+during the stage of passage from the liquid to the solid
+state.</p>
+<p><b><a name="Toc158108946" id="Toc158108946"><font face=
+"Bookman Old Style" size="4">&sect; 90. On the Attachment of
+Quartz Fibres. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">For many purposes it is
+sufficient to cement the fibres in position by means of ordinary
+yellow shellac, but where very great accuracy is aimed at, the
+shellac (being itself imperfectly elastic and exposed to shearing
+stress) imposes its imperfections on the whole system. This
+source of error can be got over by soldering the threads in
+position. Attempts were made by the writer in this direction,
+with fair success, in 1889, but as Mr. Boys has carried the art
+to a high degree of perfection, I will suppress the description
+of my own method and describe his in preference. It has, of
+course, been frequently repeated in my laboratory.</font></p>
+<p>In many cases, however, if not in all, it may be replaced by
+Margot soldering, as already described, a note on the application
+of which to this purpose will follow.</p>
+<p>A thread of the proper diameter having been selected, it is
+cut to the right length. With fine threads this is not always a
+perfectly easy matter. The best way is for the operator to
+station himself facing a good light, not sunlight, which is too
+tiring to the eye, but bright diffused light. The thread will be
+furnished with bits of paper stuck on with paraffin at both ends,
+as already described.</p>
+<p>A rough sketch of the apparatus &mdash; or, at all events, two
+lines showing the exact length which the free part of the thread
+must have &mdash; are marked on a smooth board, and this is
+supported with its plane vertical. The thread is held against the
+board, and the upper piece of paper is stuck lightly to the board
+with a trace of soft wax, so that the lower edge of the paper is
+at any desired height above the upper mark. This distance is
+measured, and forms the length of thread allowed to overlap the
+support. A second bit of paper is attached below the lower mark,
+a margin for the attachment of the lower end being measured and
+left as before. The thread will be most easily seen if the board
+is painted a dead black.</p>
+<p>If it is desired to attach the thread to its supports merely
+by shellac, this is practically all that needs to be done. The
+supports should resemble large pins. The upper support will be a
+brass wire in most cases, and will require to be filed away as
+shown in the sketch (Fig. 71). It is then coated with shellac by
+heating and rubbing upon the shellac. As previously noted, the
+shellac must not be overheated.</p>
+<p>The thread is cut off below the lower slip of paper, and the
+upper support being conveniently laid in a horizontal position on
+another dead-black surface, the thread is carried to it and laid
+as designed against the shellac, which is now cold. When the
+thread is in place, a soldering iron is put against the brass
+wire, and the shellac gradually melted till it closes over the
+thread.</p>
+<p><img src="images/Image114.gif" alt="images/Image114.gif"
+width="209" height="33">Fig. 71.</p>
+<p>The iron is then withdrawn and the thread pulled away from the
+point for one-twentieth of an inch or less. This ensures that the
+thread makes proper contact with the cement, and also that it is
+free from kinks; of course, it must leave the cement in the
+proper direction. A similar process is next carried out with
+respect to the lower attachment, and the ends of the thread are
+neatly trimmed off.</p>
+<p>Both ends of the thread being secured, the next step is to
+transfer the upper support to a clip stand, the suspended parts
+being held by hand, so that the weight comes on the thread very
+gradually. In this way it will be easily seen whether the thread
+is bent where it enters the shellac, and should this be the case,
+a hot iron must be brought up to the shellac and the error
+rectified.</p>
+<p>When both the support and the suspended parts are brought
+nearly to the required bearing, the hot iron is held for a moment
+close up to each attachment, the hand being held close below but
+not touching the suspended parts, and both attachments are
+allowed to straighten themselves out naturally.</p>
+<p>These details may appear tiresome, and so they are when
+written out at length, but the time occupied in carrying them out
+is very short, and quartz threads break easily, unless the pull
+upon them is accurately in the direction of their length at all
+points.</p>
+<p>In the event of its being decided to attach the thread by
+soldering, the process is rather more expensive in time, but not
+otherwise more troublesome.</p>
+<p><img src="images/Image115.gif" alt="images/Image115.gif"
+width="152" height="296">Fig. 7<img src="images/Image116.gif"
+alt="images/Image116.gif" width="161" height="286">2. Fig.
+73.</p>
+<p>The thread being cut as before to the proper length, little
+bits of aluminium foil are smeared all over with melted shellac
+and suspended from the thread replacing the paper slips before
+described. It is important that no paraffin should be allowed to
+touch the thread anywhere near a point intended to be soldered.
+The thread is hung up from a clip stand by one of the bits of
+foil, and the lower end is washed by dipping it into strong
+nitric acid for a moment and thence into water. The object of
+smearing the foils all over with shellac is to prevent them being
+acted upon by the acid. The threads are not very easily washed
+acid free, but the process may be assisted by means of a fine
+camel's-hair pencil.</p>
+<p>Some silvering solution made as described (&sect; 65) is put
+into a test tube; the thread, after rinsing with distilled water,
+is lowered into the solution so far as is required, and is
+allowed to receive a coating of silver. It has been observed that
+the coating of silver must not be too thick &mdash; not
+sufficiently thick to be opaque. A watch may be kept on the
+process by immersing a minute strip of mica alongside the
+thread.</p>
+<p>The silvered thread is rinsed with distilled water and allowed
+to dry.</p>
+<p>Meanwhile the other end of the thread may be silvered. When
+both ends are silvered the process of coppering by electro
+deposit is commenced. A test tube is partially filled with a ten
+per cent solution of sulphate of copper, and several copper wires
+are dipped into it to form an anode. The thread is lowered
+carefully into the solution so as not to introduce air bubbles,
+and the silvered part is allowed to project far enough above the
+surface of the solution to come in contact with a fine copper
+wire. The circuit is closed through a Leclanch&eacute; cell and a
+resistance box.</p>
+<p>It is as well to begin with a fair resistance, say 100 ohms
+out in the box, and the progress of the deposit is watched by
+means of a low-power microscope set up in front of the thread. If
+the copper appears to come down in a granular form, the
+resistance is too small and must be increased; if no headway
+appears to be made, the resistance must be diminished.</p>
+<p>As soon as a fair coat of copper has come down, i.e. when the
+diameter of the thread is about doubled, the process is
+interrupted. The thread is withdrawn, washed, dipped in a
+solution of chloride of zinc, and carefully tinned by dragging it
+over a small clean drop of solder on a soldering bit.</p>
+<p>During this part of the process the shellac is apt to get
+melted if the iron is held too close, so that it is advisable to
+begin by making the thread somewhat over long. The end of the
+thread must only be trimmed off at the conclusion of the
+operation, i.e. after the thread is soldered up. The thread is
+attached to the previously tinned supports much in the same way
+as has been described under the head of shellac attachments. It
+does not very much matter whether both ends are coppered before
+one is soldered up or not. At the conclusion of the whole process
+the superfluous copper and silver are dissolved off by a little
+hot strong nitric acid applied on a glass hair pencil. This is
+best done by holding the thread horizontally with the assistance
+of clip stands.</p>
+<p>If the thread is too delicate to bear brushing, the nitric
+acid may be applied by pouring out a big drop into a bit of
+platinum foil and holding this below the thread so as to touch it
+lightly. The dissolving of the copper and silver is, of course,
+followed by copious washing with hot water. This process is more
+laborious than might be imagined, but it may be shortened by
+heating the platinum foil supporting the water (Fig. 74).</p>
+<p>Fig. <img src="images/Image117.gif" alt="images/Image117.gif"
+width="209" height="259"> 74</p>
+<p>The washing part of the process is, in the opinion of the
+writer, the most difficult part of the whole business, and it
+requires to be very thorough, or the thread will end by drawing
+out of the solder. In many cases it is better to try to do
+without any application of nitric acid at all, but, of course,
+this involves silvering and coppering to exact distances from the
+ends of the thread &mdash; at all events, in apparatus where the
+effective length of the thread is narrowly prescribed.</p>
+<p>It is important not to leave the active parts of the thread
+appreciably silvered, for the sake of avoiding zero changes due
+to the imperfect elasticity of the silver. In this soldering
+process ordinary tinman's solder may be employed; it must be
+applied very free from dust or oxide.</p>
+<p><b><a name="Toc158108947" id="Toc158108947"><font face=
+"Bookman Old Style" size="4">&sect; 91. Other Modes of soldering
+Quartz. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Thick rods of quartz may be
+treated for attachment by solder in the same way as glass was
+treated by Professor Kundt to get a foundation for his
+electrolytically deposited prisms. <i>[Footnote:</i> See Appendix
+at end of book.<i>]</i></font></p>
+<p>The application of a drop of a strong solution of platinum
+tetrachloride to the rod will, on drying, give rise to a film of
+the dry salt, and this may be reduced in the luminous gas flame.
+During the process, however, the quartz is apt to get rotten,
+especially if the temperature has been anything approaching a
+full red heat. The resulting platinum deposit adheres very
+strongly to the quartz, and may be soldered to as before. This
+method has been employed by the writer with success since 1887,
+and may even be extended to thick threads.</p>
+<p>It was also found that fusible metal either stuck to or
+contracted upon clean quartz so as to make a firm joint. In the
+light of M. Margot's researches (already described), it occurred
+to me that perhaps my experience was only a special case of the
+phenomena of adhesion investigated with so much success by M.
+Margot. I therefore tried whether the alloy of tin and zinc used
+for soldering aluminium would stick to quartz, and instantly
+found that this was indeed the case.</p>
+<p>Adhesion between the alloy and perfectly clean quartz takes
+place almost without rubbing. A rod of quartz thus "tinned" can
+be soldered up to anything to which solder will stick, at once.
+On applying the method to thick quartz threads, success was
+instantaneous (the threads were some preserved for ordinary
+galvanometer suspensions); but when the method was applied to
+very fine threads, great difficulty in tinning the threads was
+experienced. The operation is best performed by having the alloy
+on the end of an aluminium soldering bit, and taking care that it
+is <i>perfectly free from oxide</i> before the thread is drawn
+across it. There was no difficulty in soldering a thread "tinned"
+in this manner to a copper wire with tinman's solder, and the
+joint appeared perfect, the thread breaking finally at about an
+inch away from the joint.</p>
+<p>I allow Mr. Boys' method to stand as I have written it, simply
+because I have not had time as yet to make thorough tests of the
+durability of "Margot" joints on the finest threads; but I have
+practically no doubt as to its perfect applicability, provided
+always that the solder can be got clean enough when melted on the
+bit. Very fine threads will require to be stretched before
+tinning, in order to enable them to break through the capillary
+barrier of the surface of the melted solder.</p>
+<p><b><a name="Toc158108948" id="Toc158108948"><font face=
+"Bookman Old Style" size="4">&sect; 92. Soldering.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">It is almost unfair to the arts
+of the glass-blower or optician to describe them side by side
+with the humble trade of soldering. Nevertheless, no
+accomplishment of a mechanical kind is so serviceable to the
+physicist as handiness with the soldering bit; and, as a rule,
+there is no other exercise in which the average student shows
+such lamentable incapacity. The following remarks on the subject
+are therefore addressed to persons presumably quite ignorant of
+the way in which soldering is carried out, and do not profess to
+be more than of the most elementary character.</font></p>
+<p>For laboratory purposes three kinds of solder are in general
+sufficient. One is the ordinary tinman's solder composed of lead
+and tin. The second is "spelter," or soft fusible brass, and the
+third is an alloy of silver and brass called silver solder.</p>
+<p>Tinman's solder is used for most purposes where high
+temperatures are not required, or where the apparatus is intended
+to be temporary. The "spelter," which is really only finely
+granulated fusible brass, is used for brazing iron joints. The
+silver solder is convenient for most purposes where permanency is
+required, and is especially suited to the joining of small
+objects.</p>
+<p>&sect; 93. Soft tinman's solder is made by melting together
+two parts of grain tin and one of soft lead &mdash; the exact
+proportions are not of consequence &mdash; but, on the other
+hand, the purer the constituents the better the solder. Within
+certain limits, the greater the proportion of tin the cleaner and
+more fusible is the solder. It is usually worth while to prepare
+the solder in the laboratory, for in this way a uniform and
+dependable product is assured. Good soft lead is melted in an
+iron ladle and skimmed; the temperature is allowed to rise very
+little above the melting-point. The tin is then added little by
+little, the alloy stirred vigorously and skimmed, and sticks of
+solder conveniently cast by sweeping the ladle over a clean iron
+plate, so as to pour out a thin stream of solder. If the solder
+be properly made it will have a mat and bright mottled surface,
+and will "crackle" when held up to the ear and bent.</p>
+<p>Perhaps the chief precaution necessary in making solder is to
+exclude zinc. The presence of a very small percentage of this
+metal entirely spoils the solder for tinman's work by preventing
+its "running" or flowing smoothly under the soldering bit.</p>
+<p>Fig. <img src="images/Image118.gif" alt="images/Image118.gif"
+width="499" height="84"> 75.</p>
+<p>Fig. <img src="images/Image119.gif" alt="images/Image119.gif"
+width="499" height="70"> 76.</p>
+<p>Fig. <img src="images/Image120.gif" alt="images/Image120.gif"
+width="463" height="89"> 77.</p>
+<p><b><a name="Toc158108949" id="Toc158108949"><font face=
+"Bookman Old Style" size="4">&sect; 94. Preparing a Soldering
+Bit. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The wedge-shaped edge of one of
+the forms of bit shown in the sketch is filed to shape and the
+bit heated in a fire or on a gas heater. A bit of rough
+sandstone, or even a clean soft brick, or a bit of tin plate
+having some sand sprinkled over it, is placed in a convenient
+position and sprinkled with resin.</font></p>
+<p>As soon as the bit is hot enough to melt solder it is
+withdrawn and a few drops of solder melted on to the brick or its
+equivalent. The iron or bit is then rubbed to and fro over the
+solder and resin till the former adheres to and tins the copper
+head. It will be found advisable to tin every side of the point
+of the bit and to carry the tinning back at least half an inch
+from the edge.</p>
+<p>If the solder obstinately refuses to adhere, the cause is to
+be sought in the oxidation of the copper, or of the solder, or
+both &mdash; in either case the result of too high a temperature
+or too prolonged heating. The simple remedy is to get the iron
+hot, and then to dress it with an old file, so as to expose a
+bright surface, which is instantly passed over the resin as a
+means of preserving it from oxidation. If the process above
+described be now carried out, it will be found that the
+difficulty disappears.</p>
+<p>Before using the iron, wipe off any soot or coke or burned
+resin by means of an old rag. An iron tinned in this way is much
+to be preferred to one tinned by means of chloride of zinc.</p>
+<p>A shorter and more usual method is carried out as follows: The
+solution of chloride of zinc is prepared by adding bits of zinc
+to some commercial hydrochloric acid diluted with a little (say
+25 per cent) of water. The acid may conveniently be placed in a
+small glazed white jar (a jam pot does excellently), and this
+should only be filled to about one-quarter of its capacity. An
+excess of zinc may be added.</p>
+<p>It may be fancy, but I prefer a soldering solution made in
+this way to a solution of chloride of zinc bought as a chemical
+product. The jar is generally mounted on a heavy leaden base, so
+as to avoid any danger of its getting knocked over, for nothing
+is so nasty or bad for tools as a bench on which this noxious
+liquid has been upset (Fig. 78).</p>
+<p>Fig. <img src="images/Image121.gif" alt="images/Image121.gif"
+width="161" height="146">78.</p>
+<p>To tin a soldering bit, a little of the fluid is dipped out of
+the jar on to a bit of tin plate bent up at the edges--a few
+drops is sufficient &mdash; and the iron is heated and rubbed
+about in the liquid with a drop of solder. If the iron is
+anything like clean it will tin at once and exhibit a very bright
+surface, but quite dirty copper may be tinned by dipping it for a
+moment in the liquid in the pot and then working it about over
+the solder. An iron so tinned remains covered with chloride of
+zinc, and this must be carefully wiped off if it is intended to
+use the iron with a resin or tallow flux in lead soldering.</p>
+<p>One disadvantage of this process is that the copper bit soon
+gets eaten into holes and requires to be dressed up afresh. On
+the other hand, an iron so tinned always presents a nice clean
+solder surface until the next time it is heated, when it
+generally becomes very dirty and requires to be carefully wiped
+before using.</p>
+<p>In my experience also an iron so tinned is more easily spoiled
+as to the state of its surface, "detinned," in fact, by
+overheating than when the tinning is carried out by resin and
+friction. When this happens, the shortest way out of the
+difficulty is the application of the old file so as to obtain a
+perfectly fresh surface. No one who knows his business ever uses
+an iron that is not perfectly clean and well tinned.</p>
+<p>The iron may be cleaned from time to time by heating it red
+hot and quenching it in water to get rid of the oxide, which
+scales off in the process.</p>
+<p><b><a name="Toc158108950" id="Toc158108950"><font face=
+"Bookman Old Style" size="4">&sect; 95. Soft Soldering.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">In the laboratory the chief
+application of the process is to copper soldering during the
+construction of electrical apparatus and to zinc soldering for
+general purposes.</font></p>
+<p>In ninety-nine cases out of every hundred where difficulties
+occur their origin is to be traced to dirt. There seems to be
+some inexplicable kink in the human mind which renders it callous
+to repeated proofs of the necessity for cleaning surfaces which
+it is intended to solder. The slightest trace of albuminous or
+gelatinous matter or shellac will prevent solder adhering to most
+metals and the same remark applies in a measure to the presence
+of oxides, although these may be removed by chloride of zinc or
+prevented from forming by resin or tallow. A touch with an
+ordinarily dirty hand &mdash; I refer to a solderer's hand
+&mdash; will often soil work sufficiently to make the adherence
+of solder difficult.</p>
+<p>The fluxes most generally employed are tallow for lead, resin
+or Venice turpentine for copper, chloride of zinc for anything
+except lead, which never requires it. The latter flux has the
+property (also possessed by borax at a red heat) of dissolving
+any traces of oxide which may be formed, as well as acting as a
+protecting layer to the metal.</p>
+<p>We may now turn to the consideration of a simple case of
+soldering, say the joining of two copper wires. The wires are
+first cleaned either by dipping in a bath of sulphuric and nitric
+acids &mdash; a thing no laboratory should be without &mdash; or
+by any suitable mechanical means. The cleaned wires are then
+twisted together &mdash; there is a regulation way of doing this,
+but it presents no advantage in laboratory practice &mdash; and
+the joint is sprinkled over with resin, or painted with a
+solution of resin in alcohol.</p>
+<p>The iron, being heated and floated with solder, is held
+against the joint, the latter being supported on a brick, and the
+solder is allowed to "sweat" into the joint. Enough solder must
+be present to penetrate right through the joint. Nothing is
+gained by rubbing violently with the iron. If the copper is clean
+it will tin, and if it is dirty it won't, and there the matter
+ends.</p>
+<p>Beginners generally use too small or too cold a bit, and
+produce a ragged, dirty joint in consequence. If the saving of
+time be an object, the joint may be twisted together on
+ordinarily dirty oxidised wires and heated to, say, 200&deg; C.
+It is then painted with chloride of zinc and soldered with the
+bit.</p>
+<p>There is a difference of opinion as to the relative merits of
+chloride of zinc and of resin as a flux in soldering copper. Thus
+the standing German practice is, or was, to employ the former
+flux in every case for soldering electric light wires, while in
+England the custom used to be to specify that soldering should be
+done by resin, and this custom may still prevail; it lingers in
+Australia at all events.</p>
+<p>However, it is agreed on all hands that when chloride of zinc
+is used it must be carefully washed off. I have known of an
+electrical engineer insisting on his workmen "licking" joints
+with their tongues to ensure the total removal of chloride of
+zinc; it has a horrible taste; and I have occasionally pursued
+the same plan myself when the soldering of fine wires was in
+question.</p>
+<p>In any case, it is very certain that chloride of zinc left in
+a joint will ruin it sooner or later by loosening the contact
+between copper and solder.</p>
+<p>Very often it is requisite to solder together two extensive
+flat surfaces &mdash; for instance, in "chucking" certain kinds
+of brass work. The surfaces to be soldered must be carefully
+tinned, most conveniently by the help of the blow-pipe and
+chloride of zinc. After tinning, the surfaces are laid together
+and heated so as to "sweat" them together; the phrase, though
+inelegant, is expressive.</p>
+<p>96. <i>Soldering Tin Plate.</i> &mdash;</p>
+<p>If the plate be new and clean, a little resin or its solution
+in alcohol is all that is necessary as a flux. If the tin plate
+is rusty the rust must be removed and the clean iron, or rather
+mild steel, surface exposed. The use of chloride of zinc is
+practically essential in this case. Tin plate is often spotted
+with rust long before it becomes rusty as a whole, when, of
+course, it may be regarded as worn out, and such rust spots are
+most conveniently removed by means of the plumber's shave-hook.
+The shave-hook is merely a peculiarly shaped hard steel scraping
+knife on a handle (Fig. 79).</p>
+<p>Fig. <a name="LastCursor" id="LastCursor"></a><img src=
+"images/Image122.gif" alt="images/Image122.gif" width="279" height="80"> 79</p>
+<p>With tin plate the soldering of long joints is often
+necessary. The plate must be temporarily held in position either
+by binding with iron wire, fastening by clamps, or holding by an
+assistant. The flux is applied and the iron run slowly along the
+joint. Enough solder is used to completely float the tip of the
+iron. By arranging the joint so that it slopes downward slightly,
+and commencing at the upper end, the solder may be caused to flow
+after the iron, and will leave a joint with the minimum
+permissible amount of solder in it. By regulating the slope, heat
+of iron, etc., any desired quantity of solder may be run into the
+joint.</p>
+<p><b><a name="Toc158108951" id="Toc158108951"><font face=
+"Bookman Old Style" size="4">&sect; 97. Soldering Zinc.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Zinc alloys with soft solder
+very easily, and by so doing entirely spoils it, making, it
+"crumbly," dirty, and preventing it running. Consequently, in
+soldering up zinc great care must be taken to prevent the solder
+becoming appreciably contaminated by the zinc. To this end the
+zinc surfaces are cleaned by means of a little hydrochloric acid,
+which is painted on instead of chloride of zinc. Plenty of solder
+is melted on to the work, and is drawn along over the joint by a
+single slow motion of the soldering bit. The iron must be just
+hot enough to make the solder flow freely, and it must never be
+rubbed violently on the zinc or allowed to linger in one spot;
+the result of the latter action will be to melt a hole through
+the zinc, owing to the tendency of this metal to form an easily
+fusible alloy with the solder.</font></p>
+<p>The art of soldering zinc is a very useful one in the
+laboratory. The majority of physicists appear to overlook the
+advantages of zinc considered as a material for apparatus
+construction. It is light, fairly strong, cheap, easily fusible,
+and yet hard and elastic when cold. It may be worked as easily as
+lead at a temperature of, say, 150&deg; to 200&deg; C., and
+slightly below the melting-point (423&deg; C.) it is brittle and
+may &amp; powdered. The property of softening at a moderate
+temperature is invaluable as a means of flattening zinc plate or
+shaping it in any way. During the work it may be held by means of
+an old cloth. Zinc sheet which has been heated between iron
+plates and flattened by pressure retains its flatness very fairly
+well after cooling.</p>
+<p><b><a name="Toc158108952" id="Toc158108952"><font face=
+"Bookman Old Style" size="4">&sect; 98. Soldering other Metals
+&mdash;</font></a></b></p>
+<p><i><font face="Bookman Old Style">Iron</font>. &mdash;</i></p>
+<p>The iron must be filed clean and then brushed with chloride of
+zinc solution. Some people add a little sat ammoniac to the
+chloride of zinc, but the improvement thus made is practically
+inappreciable. If the iron is clean it tins quite easily, and the
+process of soldering it is perfectly easy and requires no special
+comment.</p>
+<p><i>Brass. &mdash;</i></p>
+<p>The same method as described for iron succeeds perfectly. The
+brass, if not exceedingly dirty, may be cleaned by heating to the
+temperature at which solder melts (below 200&deg; C.), and
+painting it over with chloride of zinc, or dipping it in the
+liquor. If now the brass be heated again in the blow-pipe flame,
+it will be found to tin perfectly well when rubbed over with
+solder.</p>
+<p><i>German Silver, Platinoid, Silver, and Platinum</i></p>
+<p>are treated like iron. With regard to silver and platinum the
+same precautions as recommended in the case of zinc must be
+observed, for both these metals form fusible alloys with
+solder.</p>
+<p><i>Gold</i></p>
+<p>when pure requires no flux. Standard gold, which contains
+copper, solders better with a little chloride of zinc.</p>
+<p><i>Lead</i></p>
+<p>must be pared absolutely clean and then soldered quickly with
+a hot iron, using tallow as a flux. Since solder if over hot will
+adhere to lead almost anywhere, plumbers are in the habit of
+specially soiling those parts to which it is not intended that
+solder shall adhere. The "soiling" paint consists of very thin
+glue, called size, mixed with lampblack; on an emergency a raw
+potato may be cut in half, and the work to be soiled may be
+rubbed over with the cut surface of the potato.</p>
+<p><i>Hard Carbon or gas coke</i></p>
+<p>may be soldered after coating with copper by an electrolytic
+process, as will be described.</p>
+<p><b><a name="Toc158108953" id="Toc158108953"><font face=
+"Bookman Old Style" size="4">&sect; 99.
+<i>Brazing</i>.</font></a></b></p>
+<p><font face="Bookman Old Style">Soldering at a red heat by
+means of spelter is called brazing. Spelter is soft brass, and is
+generally made from zinc one part, copper one part; an alloy
+easily granulated at a red heat; it is purchased in the granular
+form.</font></p>
+<p>The art of brazing is applied to metals which will withstand a
+red heat, and the joints so soldered have the strength of
+brass.</p>
+<p>The pieces to be jointed by this method must be carefully
+cleaned and held in their proper relative positions by means of
+iron wire. It is generally necessary to soften iron wire as
+purchased by heating it red hot and allowing it to cool in the
+air; if this is not done the wire is usually too hard to be
+employed satisfactorily for binding.</p>
+<p>Very thin wire--i.e. above No. 20 on the Birmingham wire gauge
+&mdash; does not do, for it gets burned through, and perhaps
+allows the work to fall apart at a critical moment.</p>
+<p>The work being securely fastened, the next step is to cover
+the cleaned parts with flux in order to prevent oxidation. For
+this purpose "glass borax" is employed.  "Glass" borax is simply
+ordinary borax which has been fused for the purpose of getting
+rid of water of crystallisation. The glass borax is reduced to
+powder in an iron mortar, for it is very hard, and is then made
+up into a cream with a little water. This cream is painted on to
+the parts of the work which are destined to receive the
+solder.</p>
+<p>The next step is to prepare the spelter, and this is easily
+done by mixing it with the cream, taking care to stir thoroughly
+with a flattened iron wire till each particle of spelter is
+perfectly covered with the borax. The mixture should not be too
+wet to behave as a granular mass, and may then be lifted on to
+the work by means of the iron spatula.</p>
+<p>Care must be taken to place the spelter on those parts only
+which are intended to receive it, and when this is done, the
+joint may be lightly powdered over with the dry borax, and will
+then be ready for heating.</p>
+<p>If the object is of considerable size it is most conveniently
+heated on the forge; if small the blowpipe is more convenient. In
+the latter case, place the work on a firebrick, and arrange two
+other bricks on edge about it, so that it lies more or less in a
+corner. A few bits of coke may also be placed on and about the
+work to increase the temperature by their combustion, and to
+concentrate the flame and prevent radiation. The temperature is
+gradually raised to a bright red heat, when the spelter will be
+observed to fuse or "run," as it is technically said to do.</p>
+<p>If the cleaning and distribution of flux has been successful,
+the spelter will "run" along the joint very freely, and the work
+should be tapped gently to make sure that the spelter has really
+run into the joint. The heating may be interrupted when the
+spelter is observed to have melted into a continuous mass. As
+soon as the work has fallen below a red heat it may be plunged
+into water, a process which has the effect of cracking off the
+glass-like layer of borax.</p>
+<p>There is, however, some risk of causing the work to buckle by
+this violent treatment, which must of course be modified so as to
+suit the circumstances of the case. If the joint is in such a
+position that the borax cannot be filed off, a very convenient
+instrument for its removal by scraping is the watchmaker's
+graver, a square rod of hard steel ground to a bevelled point
+(Fig. 80).</p>
+<p>Fig. <img src="images/Image123.gif" alt="images/Image123.gif"
+width="300" height="72"> 80.</p>
+<p>Several precautions require to be mentioned. In the first
+place, spelter is merely rather soft brass, and consequently it
+often cannot be fused without endangering the rest of the work. A
+good protection is a layer of fireclay laid upon the more
+delicate parts, such for instance as any screwed part.</p>
+<p>Gun-metal and tap-metal do not lend themselves to brazing so
+readily as iron or yellow brass, and are usually more
+conveniently treated by means of silver solder.</p>
+<p>Spelter tends to run very freely when it melts, and if the
+brass surface in the neighbourhood of the joint is at all clean,
+may run where it is not wanted. Of course some control may be
+exercised by "soiling" with fireclay or using an oxidising flame;
+but the erratic behaviour of spelter in this respect is the
+greatest drawback to its use in apparatus construction. The
+secret of success in brazing lies in properly cleaning up the
+work to begin with, and in disposing the borax so as to prevent
+subsequent oxidation.</p>
+<p><b><a name="Toc158108954" id="Toc158108954"><font face=
+"Bookman Old Style" size="4">&sect; 100. Silver Soldering.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This process resembles that
+last described, but instead of spelter an alloy of silver,
+copper, and zinc is employed. The solder, as prepared by
+jewellers to meet special cases, varies a good deal in
+composition, but for the laboratory the usual proportions are
+&mdash;</font></p>
+<p>For soft silver solder</p>
+<div style="margin-left: 4em">
+<p>Fine silver 2 parts</p>
+<p>Brass wire 1 part</p>
+</div>
+<p>For hard silver solder</p>
+<div style="margin-left: 4em">
+<p>Sterling silver 3 parts</p>
+<p>Brass wire   1 part</p>
+</div>
+<p>The latter is, perhaps, generally the more convenient.</p>
+<p>Silver solders may, of course, be purchased at watchmakers'
+supply shops, and as thus obtained, are generally in thin sheet.
+This is snipped fine with a pair of shears preparatory to
+use.</p>
+<p>As odds and ends of silver (from old anodes and silver
+residues) generally accumulate in the laboratory, it is often
+more convenient to make the solder one's self. In this case it
+must be remembered in making hard solder by the second receipt
+that standard silver contains about one-twelfth of its weight of
+copper &mdash; exactly 18 parts copper to 220 silver.</p>
+<p>The silver is first melted in a plumbago crucible in a small
+furnace together with a little borax; if any copper is required
+this is then added, and finally the brass is introduced. When
+fusion is complete, the contents of the crucible are poured into
+any suitable mould.</p>
+<p>The quickest and most convenient way of preparing the alloy
+for use is to convert it into filings with the assistance of a
+coarse file, or by milling it, if a milling machine is
+available.</p>
+<p>Equal volumes of filings and powdered glass borax are made
+into a thin paste with water, and applied in an exactly similar
+manner to that described under the head of "brazing." In fact all
+the processes there described may be applied equally to the case
+under discussion, the substitution of silver for spelter being
+the only variation.</p>
+<p>The silver solder is more manageable than spelter, and does
+not tend to run wild over the work: a property which makes it
+much more convenient both for delicate joints and in cases where
+it is desired to restrict the solder to a single point or line.
+Small objects are almost invariably soldered with silver solder,
+and are held by forceps or on charcoal in the pointed flame of an
+ordinary blow-pipe.</p>
+<p><b><a name="Toc158108955" id="Toc158108955"><font face=
+"Bookman Old Style" size="4">&sect; 101. On the Construction of
+Electrical Apparatus - Insulators. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">It is not intended to deal in
+any way with the design of special examples of electrical
+apparatus, but merely to describe a rather miscellaneous set of
+materials and processes constantly required in its
+construction.</font></p>
+<p>It is not known whether there is such a thing as a perfect
+insulator, even if we presuppose ideal circumstances. Materials
+as they exist must be regarded merely as of high specific
+resistance, that is if we allow ourselves to use such a term in
+connection with substances, conduction through which is neither
+independent of electromotive force per unit length, nor of
+previous history.</p>
+<p>Even the best of these substances generally get coated with a
+layer of moisture when exposed to the air, and this as a rule
+conducts fairly well. Very pure crystalline sulphur and fused
+quartz suffer from this defect less than any other substances
+with which the writer is acquainted, but even with them the
+surface conductivity soon grows to such an extent as totally to
+mask the internal conduction.</p>
+<p>It is proposed to give a brief account of the properties of
+some insulating substances and their application in electrical
+construction, and at the same time to indicate the appliances and
+methods requisite for working them.</p>
+<p>With regard to the specific resistances which will be quoted,
+the numbers must not be taken to mean too much, partly for the
+reason already given. It is also in general doubtful whether
+sufficient care has been taken to distinguish the body from the
+surface conductivity, and consequently numerical estimates are to
+be regarded with suspicion. The question of "sampling" also
+arises, for it must be remembered that a change in composition
+amounting to, say, 1/10000 per cent may be accompanied by a
+million-fold change in specific resistance.</p>
+<p><b><a name="Toc158108956" id="Toc158108956"><font face=
+"Bookman Old Style" size="4">&sect; 102. Sulphur.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This element exists in several
+allotropic forms, which have very different electric properties.
+After melting at about 125&deg; C., and annealing at 110&deg; for
+several hours, the soluble crystalline modification is formed.
+After keeping for some days &mdash; especially if exposed to
+light &mdash; the crystals lose their optical properties, but
+remain of the same melting-point, and are perfectly soluble in
+carbon bisulphide. The change is accompanied by a change in
+colour, or rather in brightness, as the transparency
+changes.</font></p>
+<p>The "specific resistance" of sulphur in this condition is
+above 10<sup>28</sup> C.G.S.E.M. units, or 10<sup>13</sup>
+megohms per cubic centimetre for an electric intensity of say
+12,000 volts per centimetre. This is at ordinary temperatures. At
+75&deg; C. the specific resistance falls to about 10<sup>25</sup>
+under similar conditions as to voltage.</p>
+<p>In all cases the conductivity appears to increase with the
+electric intensity, or at all events with an increase in voltage,
+the thickness of the layer of sulphur remaining the same.</p>
+<p>The specific inductive capacity is 3.162 at ordinary
+temperatures, and increases very slightly with rise of
+temperature. <i>[Footnote:</i> March 1897. &mdash; It is now the
+opinion of the writer that though the specific inductive capacity
+of a given sample of a solid element is perfectly definite, yet
+it is very difficult to obtain two samples having exactly the
+same value for this constant, even in the case of a material so
+well defined as sulphur.<i>]</i></p>
+<p>The total residual charge, after ten minutes' charging with an
+intensity of 12,000 volts per centimetre, is not more than 4
+parts in 10,000 of the original charge. In making this
+measurement the discharge occupied a fraction of a second. The
+electric strength for a homogeneous plate of crystalline sulphur
+is not less than 33,000 volts per centimetre, and probably a good
+deal more. If the sulphur is contaminated with up to 3 per cent
+of the amorphous variety, as is the case if it is cooled fairly
+quickly from a temperature of 170&deg; C. or over, the specific
+resistance falls to from 10^25 to 10^26 at ordinary temperatures;
+and the specific inductive capacity increases up to 3.75,
+according to the amount of insoluble sulphur present.</p>
+<p>The residual charge under circumstances similar to those
+described above, but with an intensity of about 4000 volts per
+centimetre is, say, 2 per cent of the initial charge. So far as
+the writer is aware sulphur is the only solid non-conductor which
+can be easily obtained in a condition of approximate purity and
+in samples sufficiently exactly comparable with one another; it
+is the only one, therefore, that repays any detail of
+description.</p>
+<p>Very pure sulphur can be bought by the ton if necessary from
+the United Alkali Company of Newcastle-on-Tyne. It is recovered
+from sulphur waste by the Chance process, which consists in
+converting the sulphur into hydrogen sulphide, and burning the
+latter with insufficient air for complete combustion. The sulphur
+is thrown out of combination, and forms a crystalline mass on the
+walls and floor of the chamber.</p>
+<p>The sulphur which comes into the market consists of this mass
+broken up into convenient fragments. In order to purify it
+sufficiently for use as an insulator, the sulphur may be melted
+at a temperature of 120&deg; to 140&deg; C., and filtered through
+a plug of glass wool in a zinc funnel; as thus prepared it is an
+excellent insulator. To obtain the results mentioned in the table
+it is, however, necessary to conduct a further purification
+(chiefly from water) by distillation in a glass retort.</p>
+<p>The sulphur thus obtained may be cast of any desired form in
+zinc moulds, the castings and moulds being immediately removed to
+an annealing oven at a temperature of from 100&deg; to 110&deg;
+C., where they are left for several hours. If the sulphur is kept
+melted for some time at 125&deg; C. the annealing is not so
+important.</p>
+<p>The castings may be removed from the mould by slightly heating
+the latter, but many breakages result. Insulators made on this
+plan are much less affected by the condensation of moisture from
+the air than anything except fused quartz. They are, however,
+very weak mechanically, and apt to crack by exposure to such
+changes of temperature as go on from day to day. It is clear,
+however, that in spite of this their magnificent electrical
+properties fit them for many important uses.</p>
+<p>If the sulphur be cooled rapidly from 170&deg; C. or over, a
+mixture of the crystalline and amorphous varieties of sulphur is
+obtained. This mixture is very much stronger and tougher than the
+purely crystalline substance, and may be worked with ordinary
+hardwood tools into fairly permanent plates, rods, etc. Sheets of
+pure thick filter paper may also be dipped into sulphur at
+170&deg; C., at which temperature air and moisture are mostly
+expelled, and such sheets show a very considerable insulating
+power. The sulphur does not penetrate the paper, which therefore
+merely forms a nucleus.</p>
+<p>Cakes of the crystalline or mixed varieties may be made by
+grinding up some purified sulphur, moistening it with redistilled
+carbon bisulphide, or toluene, or even benzene
+(C<sub>6</sub>H<sub>6</sub>), and pressing it in a suitable mould
+under the hydraulic press. The plates thus formed are porous, but
+are splendid insulators, especially if made from the crystalline
+variety of sulphur, and they appear to keep their shape very
+well, and do not crack with ordinary temperature changes.</p>
+<p>The metals which resist the action of sulphur best are gold
+and aluminium; while platinum and zinc are practically unacted
+upon at temperatures below a red heat &mdash; in the former case,
+&mdash; and below the boiling-point of sulphur in the latter.</p>
+<p>A very convenient test of the purity of sulphur is the colour
+assumed by it when suddenly cooled from the temperature at which
+it is viscous. Quite pure sulphur remains of a pale lemon yellow
+under this treatment, but the slightest trace of impurity, such
+as arises from dust containing organic matter, stains the
+sulphur, and renders it darker in colour.</p>
+<p><b><a name="Toc158108957" id="Toc158108957"><font face=
+"Bookman Old Style" size="4">&sect; 103. <i>Fused Quartz</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This is on the whole the most
+reliable and most perfect insulator for general purposes. No
+exact numerical data have been obtained, but the resistivity must
+certainly be of the same order as that of pure sulphur at its
+best. The influence of the moisture of the air also reaches its
+minimum in the case of quartz, as was originally observed by
+Boys.</font></p>
+<p>As yet, however, the material can only be obtained in the form
+of rods or threads. For most purposes rods of about one-eighth of
+an inch in diameter are the most convenient. These rods may be
+used as insulating supports, and succeed perfectly even if they
+interpose less than an inch of their length to electrical
+conduction. The sketch (Figs. 81 and 81A) shows (to a scale of
+about one-quarter full size) a complete outfit for elementary
+electrostatic experiments, such as has been in use in the
+writer's laboratory for five years. With these appliances all the
+fundamental experiments may be performed, and the apparatus is
+always ready at a moment's notice.</p>
+<p>Fig. <img src="images/Image124.gif" alt="images/Image124.gif"
+width="283" height="304"> 81.</p>
+<p>Though quartz does not condense moisture or gas to form a
+conducting layer of anything like the same conductivity as in the
+case of glass or ebonite, still it is well to heat it if the best
+results are to be obtained. For this purpose a small pointed
+blow-pipe flame may be used, and the rods may be got red-hot
+without the slightest danger of breaking them. They then remain
+perfectly good and satisfactory for several hours at least, even
+when exposed to damp and dusty air.</p>
+<p>The rods are conveniently held in position by small brass
+ferrules, into which they are fastened by a little plaster of
+Paris. Sealing-wax must be avoided, on account of the
+inconvenience it causes when the heating of the rods is being
+carried out.</p>
+<p>One useful application of fused quartz is to the insulation of
+galvanometer coils (Fig. 82), another to the manufacture of
+highly insulating keys (Fig. 83); while as an insulating
+suspension it has all the virtues. If it is desired to render the
+threads conducting they may be lightly silvered, and will be
+found to conduct well enough for electrometer work before the
+silver coating is thick enough to sensibly impair their elastic
+properties.</p>
+<p>Fig. <img src="images/Image125.gif" alt="images/Image125.gif"
+width="449" height="373"> 81A.</p>
+<p>Fig. 82 is a full-size working drawing of a particular form of
+mounting for galvanometer coils. The objects sought to be
+attained are</p>
+<p>(1) high insulation of the coils,</p>
+<p>(2) easy adjustment of the coils to the suspended system.</p>
+<p>The first object is attained as follows. The ebonite ring A is
+bored with four radial holes, through which are slipped from the
+inside the fused quartz bolt-headed pins B. The coil already
+soaked in hard paraffin is placed concentrically in the ring A by
+means of a special temporary centering stand. The space between
+the coil and the ring is filled up with hard paraffin, and this
+holds the quartz pins in position. The system of ebonite ring,
+coil, and pins is then fastened into the gun-metal coil carrier,
+which is cut away entirely, except near the edges, where it
+carries the pin brackets C. These brackets can swivel about the
+lower fastening at E before the latter is tightened up.</p>
+<p>The coil is now adjusted in the adjusting stand to be
+concentric with the axis of symmetry of the coil carrier, and the
+supporting pins are slipped into slot holes cut in the brackets,
+the brackets being swivelled as much as necessary to allow of
+this. When the pins are all inserted the brackets are screwed up
+by the screws at E. The pins are then cemented firmly to the
+brackets by a little plaster of Paris. The coil carrier can now
+be adjusted to the galvanometer frame by means of screws at D,
+which pass through wide holes in the carrier and bold the latter
+in position by their heads. In the sectional plan the parts of
+the galvanometer frame are shown shaded. The front of the frame
+at F F is of glass, and the back of the frame is also made of
+glass, though this is not shown in the section.</p>
+<p>A represents an ebonite ring into which the wire coil is
+cemented by means of paraffin. B B B B are quartz pins, with
+heads inside the ebonite ring. C C C are slotted brackets
+adjustable to the pins and capable of rotation by releasing the
+screws E E. D D are the screws holding the coil carriage to the
+galvanometer framework. These screws pass through large holes in
+the carriage so as to allow of some adjustment.</p>
+<p>Fig. <img src="images/Image126.gif" alt="images/Image126.gif"
+width="549" height="124"> 82.</p>
+<p>Fig. <img src="images/Image127.gif" alt="images/Image127.gif"
+width="261" height="255"> 83.</p>
+<p><b><a name="Toc158108958" id="Toc158108958"><font face=
+"Bookman Old Style" size="4">&sect; 104. <i>Glass.</i>
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">When glass is properly chosen
+and perfectly dry it has insulating properties possibly equal to
+those possessed by quartz or crystalline sulphur. For many
+purposes, however, its usefulness is seriously reduced by the
+persistence with which it exhibits the phenomena of residual
+charge, and the difficulty that is experienced in keeping it
+dry.</font></p>
+<p>The insulating power of white flint glass is much in excess of
+that of soft soda glass, which is a poor insulator, and of
+ordinary green bottle glass. The jars of Lord Kelvin's
+electrometers, which insulate very well, are made of white flint
+glass manufactured in Glasgow, but it is found that occasionally
+a particular jar has to be rejected on account of its refusing to
+insulate, and this, if I understand aright, even when it exhibits
+no visible defects.</p>
+<p>A large number of varieties of glass were tested by Dr.
+Hopkinson at Messrs. Chance Bros. Works, in 1875 and 1876 (Phil.
+Trans., 1877), and in 1887 (Proc. Roy. Soc. xli. 453), chiefly
+with a view to the elucidation of the laws regulating the
+residual charge; and incidentally some extraordinarily high
+insulations were noted among the flint glasses. The glass which
+gave the smallest residual charge was an "opal" glass; and flint
+glasses were found to insulate 10<sup>5</sup> times as well as
+soda lime glasses. The plates of Wimshurst machines are made of
+ordinary sheet window glass, but as the insulating property of
+this material appears to vary, it is generally necessary to
+clean, dry, and test a sheet before using it. With regard to hard
+Bohemian glass, this is stated by Koeller (Wien Bericht) to
+insulate ten times as well as the ordinary Thuringian soft soda
+glass.</p>
+<p>On the whole the most satisfactory laboratory practice is to
+employ good white flint glass. The only point requiring attention
+is the preparation of the glass by cleaning and drying. Of course
+all grease or visible dirt must be removed as described in an
+earlier chapter (&sect; 13), but this is only a beginning. The
+glass after being treated as described and got into such a state
+as to its surface that clean water no longer tends to dry off
+unequally, must be subjected to a further scrub with bibulous
+paper and a clear solution of oleate of soda. The glass is then
+to be well rinsed with distilled water and allowed to drain on a
+sheet of filter paper.</p>
+<p>A very common cause of failure lies in the contamination of
+the glass with grease from the operator's fingers. Before setting
+out to clean the glass the student will do well to wash his hands
+with soap and water, then with dilute ammonia and finally with
+distilled water.</p>
+<p>In the case of an electrometer jar which has become conducting
+but is not perceptibly dirty, rubbing with a little oleate of
+soda and a silk ribbon, followed, of course, by copious washing,
+does very well. If there is any tin-foil on the jar, great care
+must be taken not to allow the glass surface to become
+contaminated by t<a name="lastbookmark" id="lastbookmark"></a>he
+shellac varnish or gum used to stick the tin-foil in
+position.</p>
+<p>Finally, the glass should be dried by radiant heat and raised
+to a temperature of 100&deg; C. at least, and kept at it for at
+least half an hour. Before drying it is of course advisable to
+allow the water to drain away as far as possible, and if the
+water is only the ordinary distilled water of the laboratory, the
+glass is preferably wiped with a clean bit of filter paper; any
+hairs which may be left upon the glass will brush off easily when
+the glass is dry.</p>
+<p>In order to obtain satisfactory results the glass must be
+placed in dry air before it has appreciably cooled. This is
+easily done in the case of electrometer jars, and so long as the
+air remains perfectly dry through the action of sulphuric acid or
+phosphorus pentoxide, the jar will insulate. The slightest whiff
+of ordinarily damp air will, however, enormously reduce the
+insulating power of the glass, so that unvarnished glass surfaces
+must be kept for apparatus which is practically air-tight.</p>
+<p>For outside or imperfectly protected uses the glass does
+better when varnished. It is a fact, however, that varnished
+glass is rarely if ever so good as unvarnished glass at its best.
+Too much care cannot be taken over the preparation of the
+varnish; French polish, or carelessly made shellac varnish, is
+likely to do more harm than good.</p>
+<p>The best orange shellac must be dissolved in good cold alcohol
+by shaking the materials together in a bottle. The alcohol is
+made sufficiently pure by starting with rectified spirit and
+digesting it in a tin flask over quick-lime for several days, a
+reversed condenser being attached. A large excess of lime must be
+employed, and this leads to a considerable loss of alcohol, a
+misfortune which must be put up with.</p>
+<p>After, say, thirty hours' digestion, the alcohol may be
+distilled off and employed to act on the shellac. In making
+varnish, time and trouble are saved by making a good deal at one
+operation &mdash; a Winchester full is a reasonable quantity. The
+bottle may be filled three-quarters full of the shellac flakes
+and then filled up with alcohol; this gives a solution of a
+convenient strength.</p>
+<p>The solution, however, is by no means perfect, for the shellac
+contains insoluble matter, and this must be got rid off.`' One
+way of doing this is to filter the solution through the thick
+filtering paper made by Schleicher and Schuell for the purpose,
+but the filtering is a slow process, and hence requires to be
+conducted by a filter paper held in a clip (not a funnel) under a
+bell jar to avoid evaporation.</p>
+<p>Another and generally more convenient way in the laboratory is
+to allow the muddy varnish to settle &mdash; a process requiring
+at least a month &mdash; and to decant the clear solution off
+into another bottle, where it is kept for use. The muddy residue
+works up with the next lot of shellac and alcohol, which may be
+added at once for future use.</p>
+<p>The glass to be varnished is warmed to a temperature of, say,
+50&deg; C., and the varnish put on with a lacquering brush; a
+thin uniform coat is required. The glass is left to dry long
+enough for the shellac to get nearly hard and to allow most of
+the alcohol to evaporate. It is then heated before a fire, or
+even over a Bunsen, till the shellac softens and begins to yield
+its fragrant characteristic smell.</p>
+<p>If the coating is too heavy, or if the heating is commenced
+before the shellac is sufficiently dry, the latter will draw up
+into "tears," which are unsightly and difficult to dry properly.
+On no account must the shellac be allowed to get overheated. If
+the varnish is not quite hard when cold it may be assumed to be
+doing more harm than good.</p>
+<p>In varnishing glass tubes for insulating purposes it must be
+remembered that the inside of the tube is seldom closed perfectly
+as against the external air, and consequently it also requires to
+be varnished. This is best done by pouring in a little varnish
+considerably more dilute than that described, and allowing it to
+drain away as far as possible, after seeing that it has flooded
+every part of the tube.</p>
+<p>During this part of the process the upper end of the tube must
+be closed, or evaporation will go on so fast that moisture will
+be deposited from the air upon the varnished surface. Afterwards
+the tube may be gently warmed and a current of air allowed to
+pass, so as to prevent alcohol distilling from one part of the
+tube to another. The tube is finally heated to the softening
+point of shellac, and if possible closed as far as is practicable
+at once.</p>
+<p><b><a name="Toc158108959" id="Toc158108959"><font face=
+"Bookman Old Style" size="4">&sect; 105. Ebonite or Hard Rubber.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This exceedingly useful
+substance can be bought of a perfectly useless quality. Much of
+the ebonite formerly used to cover induction coils for instance,
+deteriorates so rapidly when exposed to the air that it requires
+to have its surface renewed every few weeks.</font></p>
+<p>The very best quality of ebonite obtainable should be solely
+employed in constructing electric works. It is possible to
+purchase good ebonite from the Silvertown Rubber Company (and
+probably from other firms), but the price is necessarily high,
+about four shillings per pound or over.</p>
+<p>At ordinary temperatures ebonite is hard and brittle and
+breaks with a well-marked conchoidal fracture. At the temperature
+of boiling water the ebonite becomes somewhat softened, so that
+it is readily bent into any desired shape; on cooling it resumes
+its original hardness.</p>
+<p>This property of softening at the temperature of boiling water
+is a very valuable one. The ebonite to be bent or flattened is
+merely boiled for half an hour or so in water, taken out, brought
+to the required shape as quickly as possible, and left to cool
+clamped in position.</p>
+<p>The sheet ebonite as it comes from the makers is generally far
+from flat. It is often necessary to flatten a sheet of ebonite,
+and of course this is the more easily accomplished the smaller
+the sheet. Consequently a bit of ebonite of about the required
+size is first cut from the stock sheet by a hack-saw such as is
+generally used for metals. This piece is then boiled and pressed
+between two planed iron plates previously warmed to near 100&deg;
+C.</p>
+<p>With pieces of ebonite such as are used for the tops of
+resistance boxes, measuring, say, 20 X 8 X 11 inches, very little
+trouble is experienced. The sheets when cold are found to retain
+the flatness which has been forced upon them perfectly well. It
+is otherwise with large thin sheets such as are used for Holtz
+machines. I have succeeded fairly, but only fairly, by pressing
+them in a "gluing press," consisting of heavy planed iron slabs
+previously heated to 100&deg; C.</p>
+<p>I do not know exactly how best to flatten very thin and large
+sheets. It is easy to make large tubes out of sheet ebonite by
+taking advantage of the softening which ebonite undergoes in
+boiling water. A wooden mandrel is prepared of the proper size
+and shape. The ebonite is softened and bent round it; this may
+require two or three operations, for the ebonite gets stiff very
+quickly after it is taken out of the water. Finally the tube is
+bound round the mandrel with sufficient force to bring it to the
+proper shape and boiled in water, mandrel and all. The bath and
+its contents are allowed to cool together, so that the ebonite
+cools uniformly.</p>
+<p>Tubes made in this way are of course subject to the drawback
+of having an unwelded seam, but they do well enough to wind wire
+upon if very great accuracy of form is not required. If very
+accurate spools are needed the mandrel is better made of iron or
+slate and the spool is turned up afterwards. The seam may be
+strapped inside or at the ends by bits of ebonite acting as
+bridges, and the seam itself may be caulked with melted paraffin
+or anthracene.</p>
+<p><i>Working Ebonite.</i> &mdash;</p>
+<p>Ebonite is best worked as if it were brass, with ordinary
+brass-turning or planing tools. These tools should be as hard as
+possible, for the edges are apt to suffer severely, and blunt
+tools leave a very undesirable woolly surface on the ebonite. In
+turning or shaping ebonite sheets it is as well to begin by
+taking the skin off one side first, and then reversing the sheet,
+finishing the second side, and then returning to the first. This
+is on account of the fact that ebonite sometimes springs a little
+out of shape when the skin is removed.</p>
+<p>Turned work in ebonite, if well done, requires no
+sand-papering, but may be sufficiently polished by a handful of
+its own shavings and a little vaseline. The advantage of using a
+polished ebonite surface is that such a surface deteriorates more
+slowly under the influence of light and air than a surface left
+rough from the tool. If very highly polished surfaces are
+required, the ebonite after tooling is worked with fine pumice
+dust and water, applied on felt, or where possible by means of a
+felt buff on the lathe, and finally polished with rouge and
+water, applied on felt or cloth.</p>
+<p>Ebonite works particularly well under a spiral milling cutter,
+and sufficiently well under an ordinary rounded planing tool,
+with cutting angle the same as for brass, and hardened to the
+lightest straw colour.</p>
+<p>It is not possible, on the other hand, to use the carpenter's
+plane with success, for the angle of the tool is too acute and
+causes the ebonite to chip.</p>
+<p>In boring ebonite the drill should be withdrawn from the hole
+pretty often and well lubricated, for if the borings jam, as they
+are apt to do, the heat developed is very great and the temper of
+the drill gets spoiled. Ebonite will spoil a drill by heating as
+quickly as anything known; on the other hand, it may be drilled
+very fast if proper precaution is taken.</p>
+<p>It is advisable to expose ebonite to the light as little as
+possible, especially if the surface is unpolished, for under the
+combined action of light and air the sulphur at the surface of
+the ebonite rapidly oxidises, and the ebonite becomes covered
+with a thin but highly conducting layer of sulphurous or
+sulphuric acid or their compounds. When this happens the ebonite
+may be improved by scrubbing with hot water, or washing freely
+with alcohol rubbed on with cotton waste in the case of apparatus
+that cannot be dismounted.</p>
+<p>A complete cure, however, can only be effected by scraping off
+the outer layer of ebonite so as to expose a fresh surface. For
+this purpose a bit of sheet glass broken so as to leave a curved
+edge is very useful, and the ebonite is then scraped like a
+cricket bat. In designing apparatus for laboratory use it is as
+well to bear in mind that sooner or later the ebonite parts will
+require to be taken down and scraped up. Rods or tubes are, of
+course, most quickly treated on the lathe with rough glass cloth,
+and may be finished with fine sandpaper, then pumice dust and
+water, applied on felt. After cleaning the pumice off by means of
+water and a rag, the final touch may be given by means of
+vaseline, applied on cloth or on ebonite shavings.</p>
+<p><b><a name="Toc158108960" id="Toc158108960"><font face=
+"Bookman Old Style" size="4">&sect; 106. <i>Mica</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A great variety of minerals go
+under this name. Speaking generally, the Russian micas coming
+into commerce are potash micas, and mica purchased in England may
+be taken to be potash mica, especially if it is in large
+sheets.</font></p>
+<p>At ordinary temperatures "mica" of the kind found in commerce
+is an excellent insulator. Schultze (<i>Wied. Ann</i>. vol.
+xxxvi. p. 655) comes to the conclusion that both at high and at
+low temperatures mica (of all kinds?) is a better insulator than
+white "mirror glass," the composition of which is not stated. The
+experiments of the author referred to were apparently left
+unfinished, and altogether too much stress must not be laid on
+the results obtained, one of which was that mica conducts
+electrolytically to some extent at high temperatures.</p>
+<p>Bouty (<i>Journal de Physique</i>, 1890 [9], 288) and J. Curie
+(<i>Th&egrave;se de Doctorat</i>, Paris, 1888) agree in making
+the final conductivity of the mica used in Carpentier's
+condensers exceedingly small &mdash; at all events at ordinary
+temperatures. Bearing in mind that for such substances the term
+specific resistance has no very definite meaning, M. Bouty
+considers it is not less than 3.19 x 10<sup>28</sup> E.M. units
+at ordinary temperatures. M. Bouty gives a note or illustration
+of what such numbers mean &mdash; a precaution not superfluous in
+cases where magnitudes are denoted logarithmically. Referring to
+the value quoted, viz. 3.19 x 10<sup>28</sup>, M. Bouty says, "Ce
+serait la resistance d'une colonne de mercure de 1<sup>mmq</sup>
+de section et de longueur telle que la lumi&egrave;re se
+propageant dans le vide, mettrait plus de 3000 ans A se
+transmettre d'une extr&eacute;mit&eacute; &agrave; I'autre de la
+colonne."</p>
+<p>M. Bouty returns to the study of mica (muscovite) in the
+<i>Journal de Physique</i> for 1892, p. 5, and there deals with
+the specific inductive capacity, which for a very small period of
+charge he finds has the value 8 &mdash; an enormous value for
+such a good insulator, and one that it would be desirable to
+verify by some totally distinct method. This remark is enforced
+by the fact that M. Klemencic finds the number 6 for the same
+constant. The temperature coefficient of this constant was too
+small for M. Bouty to determine. The electric intensity was of
+the order of 100 volts per centimetre, and the experiments seem
+to indicate that the specific inductive capacity would be only
+slightly less if referred to a period of charge indefinitely
+short.</p>
+<p>I have found that the residual charge in a mica condenser,
+made according to Carpentier's method (to be described below), is
+about 1 per cent of the original charge under the following
+circumstances.</p>
+<p>Voltage 300 volts on a plate 0.2 mm. thick, duration of charge
+ten minutes, temperature about 20&deg; C. To get this result the
+mica must be most carefully dried. This and other facts indicate
+that the so-called residual charge on ordinary condensers is, to
+a very large extent, due to the creeping of the charge from the
+armatures over the more or less conducting varnished surfaces of
+the mica, and its slow return on discharge.</p>
+<p>This source of residual charge was carefully guarded against
+by Rowland and Nichols (<i>Phil. Mag</i>. 1881) in their work on
+quartz, and is referred to by M. Bouty, who adduces some
+experiments to show that his own results are not vitiated by it.
+On the other hand, M. Bouty shows that a small rise in
+temperature enormously affects the state of a mica surface, and
+that the surface gets changed in such a way as to become very
+fairly conducting at 300&deg; C. Also anybody can easily try for
+himself whether exposing a mica condenser plate which has been
+examined in presence of phosphorus pentoxide to ordinary air for
+five minutes will not enormously increase the residual charge, as
+has always been the case in the writer's experience, and if so,
+it is open to him to suggest some cause other than surface
+creeping as an explanation.</p>
+<p>M. Bouty, using less perfectly dried mica, did not get so good
+a result as to smallness of residual charge as the one above
+quoted.</p>
+<p>The chief use of mica for laboratory purposes depends on the
+ease with which it can be split, and also upon the fact that it
+may be considerably crumpled and bent without breaking. It
+therefore makes an excellent dielectric in so far as convenience
+of construction is concerned in the preparation of condensers,
+and lends itself freely to the construction of insulating washers
+or separators of any kind. Its success as a fair insulator at
+moderate temperatures has led to its use in resistance
+thermometers, where it appears to have given satisfaction up to,
+at all events, 400&deg; C.</p>
+<p>It is worth a note that according to Werner Siemens, who had
+immense experience (<i>Wied. Ann</i>. vol. clix.), soapstone is
+the only reliable insulator at a red heat, but, no doubt, a good
+deal depends on the particular specimen investigated.</p>
+<p><b><a name="Toc158108961" id="Toc158108961"><font face=
+"Bookman Old Style" size="4">&sect; 107. Use of Mica in
+Condensers. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">If good results are desired it
+is essential to select the mica very carefully. Pieces
+appreciably stained, &mdash; particularly if the stain is not
+uniformly distributed, &mdash; cracked pieces, and pieces tending
+to flake off in patches should be rejected. The best samples of
+mica that have come under the writer's observation are those
+sheets sold for the purpose of giving to silver photographic
+prints that hideous glazed surface which some years ago was so
+popular.</font></p>
+<p>Sheets of mica about 0.1 to 0.2 mm. thick form good
+serviceable condenser plates, and will certainly stand a pressure
+of 300 volts, and most likely a good deal more. The general
+practice in England seems to have been to build up condensers of
+alternate sheets of varnished or paraffined-mica and
+tin-foil.</p>
+<p>This practice is open to several objections. In the first
+place, the capacity of a condenser made in this way varies with
+the pressure binding the plates together. In the second place,
+the amount of mica and tin-foil required is often excessive in
+consequence of the imperfect contact of these substances. Again,
+the inevitable air film between the mica and tin-foil renders
+condensers so made unsuitable for use with alternating currents,
+owing to the heating set up through air discharges, and which is
+generally, though often (if not always) wrongly, attributed to
+dielectric hysteresis.</p>
+<p>These imperfections are to a great extent got over by M.
+Carpentier's method of construction, which is, however, rather
+more costly both in material and labour. On the other hand,
+wonderful capacities are obtained with quite small amounts of
+mica. M. Bouty mentions a condenser of one microfarad capacity
+weighing 1500 grms. and contained in a square box measuring 12
+centimetres on the side, and about 3 centimetres thick.</p>
+<p>The relation between the capacity and surface of doubly-coated
+plates is in electro-static units &mdash;</p>
+<p>Capacity = (sp. ind. capacity X area of one surface)/(4pi X
+thickness)</p>
+<p>This may be reduced to electro-magnetic units by dividing by
+9x10^20, and to microfarads by further multiplying by 10^15.</p>
+<p>M. Carpentier begins, of course, by having his mica
+scrupulously clean and well selected. It is then silvered by one
+of the silvering processes (&sect; 65) on both sides, for which
+purpose the sheets may be suspended in a paraffined wood rack, so
+as to lie horizontally in the silvering solution, a space of
+about half an inch being allowed between the sheets. The
+silvering being finished, the sheets are dipped along two
+parallel edges in 75 per cent nitric acid. With regard to the
+third and fourth edges of the sheet, the silver is removed on one
+side only, using a spun glass brush; if we agree to call the two
+surfaces of the mica A and B respectively, and the two edges in
+question C and D, then the silver is removed from the A side
+along edge C, and from the B side along edge D. The silvered part
+is shown shaded in Fig. 84. By this arrangement the silver and
+mica plates may be built up together so as to form the same
+mutual arrangement of contacts as in an ordinary mica tin-foil
+condenser.</p>
+<p><img src="images/Image128.gif" alt="images/Image128.gif"
+width="396" height="237">Fig. 84.</p>
+<p>It need hardly be said that the sheets require very complete
+washing after treatment with nitric acid, followed by a
+varnishing of the edges as already described in the case of
+glass, and baking at a temperature of 140&deg; C. in air free
+from flame gases, till the shellac begins to emit its
+characteristic odour and is absolutely hard when cold.</p>
+<p>The plates are then built up so as to connect the sheets which
+require to be connected, and to insulate the other set. General
+contact is, if necessary, secured by means of a little silver
+leaf looped across from plate to plate &mdash; a part of the
+construction which requires particular attention and clean hands,
+for it is by no means so easy to make an unimpeachable contact as
+might at first appear.</p>
+<p>The condenser, having been built up, may be clamped solid and
+placed in its case; the capacity will not depend appreciably on
+the tightness of the clamp screws &mdash; a great feature of the
+construction. Such a condenser will not give its best results
+unless absolutely dry. I have kept one very conveniently in a
+vacuum desiccator over phosphorus pentoxide, but if of any size,
+the condenser deserves a box to itself, and this must be
+air-tight and provided with a drying reagent, so arranged that it
+can be removed through a manhole of some sort.</p>
+<p>Contact to the brass-work on the lid may be made by pressing
+spring contacts tightly down upon the ends of the silver foils
+and carrying the connections through the lid. This also serves to
+secure the condenser in position.</p>
+<p><b><a name="Toc158108962" id="Toc158108962"><font face=
+"Bookman Old Style" size="4">&sect; 108. <i>Micanite</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This substance, though probably
+comparing somewhat unfavourably with the insulators already
+enumerated, and being subject to the uncertainties of
+manufacture, has during the last few years achieved a
+considerable success in American electrical engineering
+construction. It is composed of scrap mica and shellac varnish
+worked under pressure to the desired shape, and may be obtained
+in sheets, plates, and rods, or in any of the forms for which a
+die happens to have been constructed.</font></p>
+<p>Of course, in special cases it would be worth while to prepare
+a die, and then the attainable forms would be limited by moulding
+considerations only. The writer's experience is very limited in
+this matter, but Dr. Kennelly, with whom he communicated on the
+subject, was good enough to reply in favour of micanite for
+engineering work.</p>
+<p><b><a name="Toc158108963" id="Toc158108963"><font face=
+"Bookman Old Style" size="4">&sect; 109. <i>Celluloid</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This material is composed of
+nitrocellulose and camphor.</font></p>
+<p>It has fair insulating properties, and may be obtained in a
+variety of forms, but has now been generally abandoned for
+electrical work on account of its inflammability.</p>
+<p><b><a name="Toc158108964" id="Toc158108964"><font face=
+"Bookman Old Style" size="4">&sect; 110.
+<i>Paper</i>.</font></a></b></p>
+<p><font face="Bookman Old Style">Pure white filter paper,
+perfectly dry, is probably a very fair insulator; the misfortune
+is that in practice it cannot be kept dry. Under the most
+favourable circumstances its specific resistance may approach
+10<sup>24</sup> E.M. units. It must therefore be considered
+rather as a partial conductor than as an insulator. The only case
+of the use of dry paper as an insulator in machine construction
+which has come under the writer's notice is in building up the
+commutators of the small motors which used to drive the Edison
+phonographs.</font></p>
+<p>Its advantages in this connection are to be traced to the fact
+that a commutator so built up is durable and keeps a clean
+surface. Of course, the use of paper as an insulator for
+telephone wires is well known, but its success in this direction
+depends less upon its insulating properties than upon the fact
+that it can be arranged in such a way as to allow of the wires
+being partially air insulated, an arrangement tending to reduce
+the electrostatic capacity of the wire system.</p>
+<p>At one time it was the custom of instrument makers to employ
+ordinary printed paper in the shape of leaves torn from books or
+the folios of old ledgers to form the dielectric of the
+condensers used in connection with the contact breakers of
+induction coils. This practice has nothing but economy to
+recommend it, for cases often occur in which the paper, by
+gradual absorption of moisture from the air, comes to insulate so
+badly that it practically short circuits the spark gap, and so
+stops the action of the coil. Three separate cases have come
+within the writer's experience.</p>
+<p>Some measurements of the resistance of paper have been made by
+F. Uppenborn (<i>Centralblatt fuer Electrotechnik</i>, Vol. xi.
+p. 215, 1889). There is an abstract of the paper also in
+<i>Wiedemann's Beiblaetter</i> (1889, vol. xiii. P. 711).
+Uppenborn examined the samples of paper under normal conditions
+as to moisture and obtained the following results: &mdash;</p>
+<table border summary="" cellspacing="1" cellpadding="1" width=
+"567">
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">Description of
+Paper</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">I</font></p>
+<p>Pressure Intensity</p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">II.</font></p>
+<p>Specific Resistance corresponding to pressures as in Column I.
+Ohms.</p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">III</font></p>
+<p>Pressure Intensity.</p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">IV.</font></p>
+<p>Specific Resistance corresponding to Column III. Ohms.</p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">Common cardboard 2.3
+mm. thick</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.05 kilo. per 6000
+sq. mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">4.85 x
+10<sup>15</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">20 kg. per 6000 sq.
+mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">4.7 x
+10<sup>14</sup></font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">Gray paper, 0.26 mm.
+thick</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.05 kilo. per 5000
+sq. mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">3.1 x
+10^<sup>15</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">20 kg. per 5000 sq.
+mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">8 x
+10<sup>14</sup></font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">Yellow parchment
+paper-09 mm. thick</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.05 kilo. per 5300
+sq. mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">3.05 x
+10<sup>16</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">20 kg. per 5300 sq.
+mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">8 x
+10<sup>16</sup></font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">Linen tracing
+cloth</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.05 kilo. per 6000
+sq. mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">1.35 x
+10<sup>16</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">20 kg. per 33,000 sq.
+mm.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="1">1.86 x
+10^<sup>15</sup></font></p>
+</td>
+</tr>
+</table>
+<p><b><a name="Toc158108965" id="Toc158108965"><font face=
+"Bookman Old Style" size="4">&sect; 111. Paraffined Paper.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Like wood and other
+semiconductors, paper can be vastly improved as an insulator by
+saturating it with melted paraffin. To get the best results a
+pure paper free from size must be employed &mdash; gray Swedish
+filter paper does well. This is dried at a temperature above
+100&deg; C. for, say, half an hour, and the sheets are then
+floated on the top of paraffin, kept melted at 140&deg; C. or
+thereabout in a baking dish. As soon as the paper is placed upon
+the melted paraffin the latter begins to soak through, in virtue
+of capillary action, and drives before it the air and moisture,
+causing a strongly marked effervescence.</font></p>
+<p>After about one minute the paper may be thrust below the
+paraffin to soak. When a sufficient number of papers have
+accumulated, and when no more gas comes off, the tray may be
+placed in a vacuum box (Fig. 85), and the pressure reduced by the
+filter pump. As the removal of the air takes time, provision must
+be made for keeping the bath hot.</p>
+<p>A vacuum may be maintained for about an hour, and air then
+readmitted. Repeated exhaustions and readmissions of air, which
+appear to improve wood, do not give anything like such a good
+result with paper. In using a vacuum box provision must be made
+in the shape of a cool bottle between the air pump and the box.
+If this precaution be omitted, and if any paraffin splashes on to
+the hot surface of the box, it volatilises with decomposition and
+the products go to stop up the pump. Paraffin with a
+melting-point of 50&deg; C. or upwards does well.</p>
+<p>The bath should be allowed to cool to about 60&deg; C. before
+the papers are removed, so that enough paraffin may be carried
+out to thoroughly coat the paper and prevent the entrance of
+air.</p>
+<p>Fig. <img src="images/Image129.gif" alt="images/Image129.gif"
+width="259" height="238"> 85.</p>
+<p>Fig. 85 is a section of a vacuum vessel which has been found
+very convenient. It measures about two feet in diameter at the
+top. It is round, because it is much easier to turn one circular
+surface than to plane up four surfaces, which has to be done if
+the box is square. Both the rim of the vessel and the
+approximating part of the cover require to be truly turned and
+smoothly finished. A very good packing is made of solid
+indiarubber core about half an inch thick. This is carefully
+spliced &mdash; cemented by means of a solution of rubber in
+naphtha, and the splice sewed by thick thread. The lid ought to
+have a rim fitting inside the vessel, for this keeps the rubber
+packing in place; the rim has been accidentally omitted in Fig.
+85. The bolts should not be more than five inches apart, and
+should lie at least half an inch in diameter, and the rim and lid
+should be each half an inch thick.</p>
+<p>Condensers may now be built up of sheets of this prepared
+paper interleaved with tin-foil in the ordinary way. If good
+results are required, the condenser when finished is compressed
+between wooden or glass end-pieces by means of suitable clamps.
+It can then be put in a box of melted paraffin, heated up to
+140&deg; C., and exhausted by means of the water pump for several
+hours.</p>
+<p>In this process the air rushes out from between the paper and
+foils with such vehemence that the paraffin is generally thrown
+entirely out of the box. To guard against this the box must be
+provided with a loosely fitting and temporary lid, pierced with
+several holes.</p>
+<p>The real test as to when exhaustion is complete would be the
+cessation of any yield of air or water. Since it is not generally
+convenient to make the vacuum box so air-tight that there are
+absolutely no leaks at all, and as the paraffin itself is, I
+think, inclined to "crack" slightly at the temperature of
+140&deg; C., this test or criterion cannot be conveniently
+applied.</p>
+<p>Two exhaustions, each of about two hours' duration, have,
+however, in my experience succeeded very well, provided, of
+course, that the dielectric is prepared as suggested. At the end
+of the exhaustion process the clamping screws are tightened as
+far as possible, the condenser remaining in its bath until the
+paraffin is pasty.</p>
+<p>Condensers made in this way resist the application of
+alternating currents perfectly, as the following tests will show.
+The dielectric consisted of about equal parts of hard paraffin
+and vaseline. A condenser of about 0.123 microfarads capacity and
+an insulation resistance of 2000 megohms, <i>[Footnote:</i> As
+tested by a small voltage.<i>]</i> having a tin-foil area of 4.23
+square metres (about), and double papers each about 0.2 mm.
+thick, designed to run at 2000 volts with a frequency of 63
+complete periods, was tested at this frequency.</p>
+<p>The condenser was thoroughly packed all round in cotton-wool
+to a thickness of 6 inches, and its temperature was indicated
+more or less by a thermometer plunged through a hole in the lid
+of the containing box and of the condenser box, and resting on
+the upper surface of one set of tin-foil electrodes, from which
+the soft paraffin mixture had been purposely scraped away. The
+following were the results of a four hours' run at a voltage 50
+per cent higher than that for which the condenser was designed
+&mdash; i.e. 3000 volts.</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"525">
+<tr>
+<td width="22%" valign="top" colspan="2">
+<p align="center"><font face=
+"Bookman Old Style">Times.</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face=
+"Bookman Old Style">Voltage</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">Temperature in
+Condenser.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">Temperature in Air.</font></p>
+</td>
+<td width="18%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">Difference</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">Hrs.</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">Min.</font></p>
+</td>
+<td width="14%" valign="top" colspan="2"></td>
+<td width="23%" valign="top" colspan="2"></td>
+<td width="23%" valign="top" colspan="2"></td>
+<td width="18%" valign="top"></td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">2</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">2750</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">22.8&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.0&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">+ 0.2&deg;</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">3</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">2700</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.0&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.3&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">+ 0.3&deg;</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">3</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">18</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">3200</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.1&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.0&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">-0.1&deg;</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">4</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">3200</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.3&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.7&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">+ 0.4&deg;</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">5</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">3100</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.6&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.4&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">-0.2&deg;</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style">6</font></p>
+</td>
+<td width="11%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="14%" valign="top" colspan="2">
+<p align="center"><font face="Bookman Old Style">3000</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.8&deg; C.</font></p>
+</td>
+<td width="23%" valign="top" colspan="2">
+<p><font face="Bookman Old Style">23.35&deg; C.</font></p>
+</td>
+<td width="18%" valign="top">
+<p><font face="Bookman Old Style">-0.45&deg;</font></p>
+</td>
+</tr>
+</table>
+<p><font face="Bookman Old Style">An idea of the order of the
+amount of waste may be formed from the following additional
+experiment.</font></p>
+<p>A condenser similar to the one described was filled with oil
+of a low insulating power. It was tested calorimetrically, and
+also by the three voltmeter method, which, however, proved to be
+too insensitive. The temperature rise in the non-conducting box
+in air was about 0.3&deg; C. per hour, and the loss of power was
+found to be less than 0.1 per cent. In the present case the
+actual rise was only 1&deg; in four hours, and the integral give
+and take between the condenser and the air is practically
+nothing; consequently we may consider with safety that the rate
+of rise is certainly less than 1 degree per three hours. The
+voltage and frequency were about the same in both experiments,
+consequently the energy passed is about proportional to the
+capacity used in the two experiments.</p>
+<p>From this it follows that since the specific heat of both
+condensers was the same (nearly), the loss in the present case is
+a good deal less than one-tenth per cent. The residual charge is
+also much less than when the condenser is simply built up of
+paper paraffined in an unsystematic manner, and from which the
+air and water have been imperfectly extracted, as by baking the
+condenser first, and then immersing it in paraffin or oil.</p>
+<p>It is usual to consider that the phenomena of residual charge
+and heating in condensers, to which alternating voltages are
+applied, are closely allied. This is true, but the alliance is
+not one between cause and effect &mdash; at all events, with
+regard to the greater part of the heating. The imperfect
+exclusion of air and moisture, particularly the latter, certainly
+increases the residual charge by allowing surface creeping to
+occur; but it also acts directly in producing heating, both by
+lowering the insulation of the condenser and by allowing of air
+discharges between the condenser plates.</p>
+<p>Of these causes of heating, the discharges in air or water
+vapour are probably the more important. Long ago a theory of
+residual charge was given by Maxwell, based on the consideration
+of a laminated dielectric, the inductivity and resistance of
+which varied from layer to layer. It was shown that such an
+arrangement, and hence generally any want of homogeneity in a
+direction inclined to the lines of force leading to a change of
+value of the product of specific resistance and specific
+inductive capacity, would account for residual charge.</p>
+<p>This possible explanation has been generally accepted as the
+actual explanation, and many cases of residual charge attributed
+to want of homogeneity, which are certainly to be explained in a
+simpler manner. For instance, the residual charge in a silvered
+mica plate condenser, carefully dried, can be increased at least
+tenfold by an exposure of a few minutes to ordinarily damp air.
+The same result occurs with condensers of paraffined or sulphured
+paper; and these are the residual changes generally observed. The
+greater part must be due to creeping.</p>
+<p><b><a name="Toc158108966" id="Toc158108966"><font face=
+"Bookman Old Style" size="4">&sect; 112. <i>Paraffin</i>
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This substance has long enjoyed
+great popularity in the physical laboratory. Its specific
+resistance is given by Ayrton and Perry as more than
+10<sup>25</sup>, but it is probably much higher in selected
+samples. The most serviceable kind of paraffin is the hardest
+obtainable, melting at a temperature of not less than 52&deg; C.
+It is a good plan to remelt the commercial paraffin and keep it
+at a temperature of, say, 120&deg; C. for an hour, stirring it
+carefully with a glass rod so that it does not get overheated;
+this helps to get rid of traces of water vapour.</font></p>
+<p>Hard paraffin, when melted, has an enormous rate of expansion
+with temperature, so great, indeed, that care must be taken not
+to overfill the vessels in which it is to be heated. Castings can
+only be prepared by cooling the mould slowly from the bottom,
+keeping the rest of the mould warm, and adding-paraffin from time
+to time to make up for the contraction. The cooling is gradually
+allowed to spread up to the free surface.</p>
+<p>The chief use of paraffin in the laboratory is in the
+construction of complicated connection boards, which are easily
+made by drilling holes in a slab of paraffin, half filling them
+with mercury, and using them as mercury cups.</p>
+<p>Since paraffin is a great collector of dust, it should be
+screened by paper, otherwise the blocks require to be scraped at
+frequent intervals, which, of course, electrifies them
+considerably. This electrification is often difficult to remove
+without injuring the insulating power of the paraffin. A light
+touch with a clean Bunsen flame is the readiest method, and does
+not appear to reduce the insulation so much as might be expected.
+The safest way, however, is to leave the key covered by a clean
+cloth, which, however, must not touch the surface, for a
+sufficient time to allow of the charges getting away.</p>
+<p>The paraffin often becomes electrified itself by the friction
+of the key contacts, so that in electrometer work it is often
+convenient to form the cups by lining them with a little roll of
+copper foil twisted up at the bottom. In this case the connecting
+wires should, of course, be copper. Small steel staples are
+convenient for fastening the collecting wires upon the paraffin;
+or, in the case where these wires have to be often removed and
+changed about, drawing-pins are very handy.</p>
+<p>With mercury cups simply bored in paraffin great trouble will
+often be experienced in electrometer work, owing to a potential
+difference appearing between the cups &mdash; at all events when
+the contacts are inserted and however carefully this be done. A
+few drops of very pure alcohol poured in above the mercury often
+cures this defect. The surface of paraffin is by no means exempt
+from the defect of losing its insulating power when exposed to
+damp air, but it is not so sensitive as glass, nor does the
+insulating power fall so far.</p>
+<p>Two useful appliances are figured.</p>
+<p><img src="images/Image130.gif" alt="images/Image130.gif"
+width="95" height="150">Fig. <img src="images/Image131.gif" alt=
+"images/Image131.gif" width="142" height="283"> 86. Fig. 87.</p>
+<p>One, in which paraffin appears as a cement, is an insulating
+stand made out of a bit of glass or ebonite tube cemented into an
+Erlenmeyer flask, having its neck protected from dust when out of
+use by a rubber washer, the other a "petticoat" insulator made by cementing a flint glass bottle into a glass
+dish with paraffin. In course of time the paraffin will be found
+to have separated from the glass. When this occurs the apparatus
+may be melted together again by placing it on the water bath for
+a few minutes.</p>
+<p><b><a name="Toc158108967" id="Toc158108967"><font face=
+"Bookman Old Style" size="4">&sect; 113. Vaseline, Vaseline Oil,
+and Kerosene Oil. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">These, when dry, insulate
+almost, but not quite as well as solid paraffin. H. Koeller
+(<i>Wien Berichte</i>, 98, ii. 201, 1889; <i>Beibl</i>.
+<i>Wied</i>. <i>Ann</i>. 1890, p. 186), working with very small
+voltages, places the final(?) specific resistance of commercial
+petroleum, ether, and vaseline oil at about 2 X 10<sup>27</sup>
+C.G.S. This is ten times higher than the value assigned to
+commercial benzene (C<sub>6</sub>H<sub>6</sub>), and a hundred
+times higher than the value for commercial toluene.</font></p>
+<p>All these numbers mean little or nothing, but the petroleum
+and vaseline oil were the best fluid insulators examined by
+Koeller. By mixing vaseline with paraffin a soft wax may be made
+of any desired degree of softness, and by dissolving vaseline in
+kerosene an insulating liquid of any degree of viscidity may be
+obtained.</p>
+<p>Hard paraffin may be softened somewhat by the addition of
+kerosene, and an apparently homogeneous mass cast from the
+mixture. It will be found, however, that in course of time the
+kerosene oozes out, unless present in very small quantity.
+Koeller has found (<i>loc. cit</i>.) that some samples of
+vaseline oil conducted "vollstaendig gut," but I have not come
+across such samples. Vaseline oil, however, is sold at a price
+much above its value for insulating purposes.</p>
+<p>Kerosene oil is best obtained dry by drawing it directly from
+a new tin and exposing it to air as little as possible. Of
+course, it may be dried by chemical means and distillation, but
+this is usually (or always) unnecessary.</p>
+<p>Fig <img src="images/Image132.gif" alt="images/Image132.gif"
+width="282" height="253">88.</p>
+<p>There is some danger of kerosene containing minute traces of
+sulphuric acid, and it and other oils may be conveniently tested
+for insulation in the following manner. The quartz electroscope
+is taken, and the insulating rod heated in the blow-pipe. The
+electroscope will now insulate well enough to show no appreciable
+collapse of the leaves in one or two hours' time. Upon the plate
+of the electroscope is put a platinum or copper cylinder, and
+this is filled with kerosene (say) up to a fixed mark.</p>
+<p>The electroscope is placed on a surface plate, or, at all
+events, on a sheet of plate glass, and a "scribing block" is
+placed along side it and the scriber adjusted to dip into the
+kerosene to any required depth. This is done by twisting a bit of
+wire round the scribing point and allowing it to project
+downwards. The point itself serves to give an idea of the height
+to which the vessel may be filled. The liquid is adjusted till
+its surface is in contact with the end of the scribing point, and
+the wire then projects into the liquid and forms an electrode of
+constant area of surface. The scribing block is put to earth. A
+charge is given to the electroscope, and the time required for a
+given degree of collapse of the leaves noted.</p>
+<p>The kerosene is then removed and its place taken by vaseline
+or paraffin, known to insulate well as a standard for comparison.
+The experiment is then repeated, and the time noted for the same
+degree of collapse. This test, though of course rough, is
+generally quite sufficient for workshop purposes, and is easily
+applied. Moreover, it does not require correction for
+electrometer leakage, as generally happens when more elaborate
+appliances are used.</p>
+<p>The actual resistance of insulating oils depends so much on
+the electrical intensity, on the duration of that intensity, and
+on the previous history of the oil as to the direction of the
+voltage to which it has been subjected &mdash; to say nothing of
+the effects of traces of moisture &mdash; that quantitative
+experiments are of no value unless they are extremely elaborate.
+I shall therefore only append the following numbers due to Bouty,
+<i>Ann. de Chemie et de Physique</i> (6), vol. xxvii. p. 62,
+1892, in which the effect of the conductivity on the
+determination of the specific inductive capacity was properly
+allowed for:&mdash;</p>
+<table border summary="" cellspacing="1" cellpadding="3" width=
+"661">
+<tr>
+<td width="23%" valign="top"></td>
+<td width="17%" valign="top">
+<p><font face="Bookman Old Style">Carbon<br>
+Bisulphide.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style">Turpentine.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style">Benzene
+(C<sub>6</sub>H<sub>6</sub>) at 20&deg; C.</font></p>
+</td>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style">Benzene at<br>
+60&deg; C.</font></p>
+</td>
+</tr>
+<tr>
+<td width="23%" valign="top">
+<p><font face="Bookman Old Style">Specific inductive
+capacity</font></p>
+</td>
+<td width="17%" valign="top">
+<p align="right"><font face="Bookman Old Style">2.715</font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">2.314</font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">2.21</font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">2.22</font></p>
+</td>
+</tr>
+<tr>
+<td width="23%" valign="top">
+<p><a name="lastbookmark1" id="lastbookmark1"><font face=
+"Bookman Old Style">Specific resistance in ohms per cubic
+centimetre</font></a></p>
+</td>
+<td width="17%" valign="top">
+<p align="right"><font face="Bookman Old Style">1.5 x
+10<sup>13</sup>,</font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">1.75 x
+10<sup>12</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">1.56 x
+10<sup>11</sup></font></p>
+</td>
+<td width="20%" valign="top">
+<p align="right"><font face="Bookman Old Style">7.9 x
+10<sup>11</sup></font></p>
+</td>
+</tr>
+</table>
+<p><i><font face="Bookman Old Style">[Footnote:</font> Professor
+J. J. Thomson, and Newall (Phil. Proc. 1886) consider that carbon
+bisulphide showed traces of a "residual charge" effect; hence,
+until this point is cleared up, we must regard Bouty's value as
+corresponding only to a very short, but not indefinitely short,
+period of charge. On this point the paper must be
+consulted.</i></p>
+<p>March 1897 &mdash; The writer has investigated this point by
+an independent method, but found no traces of "residual
+charge."<i>]</i></p>
+<p>Information as to the specific inductive capacity of a large
+number of oils may be found in a paper by Hopkinson, <i>Phil.
+Proc</i>. 1887, and in a paper by Quincke in Wiedemann's
+<i>Annalen</i>, 1883.</p>
+<p><b><a name="Toc158108968" id="Toc158108968"><font face=
+"Bookman Old Style" size="4">&sect; 114. Imperfect Conductors.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Under this heading may be
+grouped such things as wood, slate, marble, etc. &mdash; in fact,
+materials generally used for switchboard insulation. An
+examination of the insulating power of these substances has
+recently been made by B. O. Peirce (<i>Electrical Review</i>,
+11th January 1895) with quite sufficient accuracy, having in view
+the impossibility of being certain beforehand as to the character
+of any particular sample. The tests were made by means of holes
+drilled in slabs of the material to be examined. These holes were
+three-eighths of an inch in diameter, and from five-eighths to
+three-quarters of an inch deep, and one inch apart, centre to
+centre. A voltage of about 15 volts was employed. The following
+general results were arrived at:-</font></p>
+<p>(1) Heating in a paraffin bath always increases the resistance
+of wood, though only slightly if the wood be hard and dense.</p>
+<p>(2) Frequent exhaustion and readmission of air above the
+surface of the paraffin always has a good effect in increasing
+the resistance of wood.</p>
+<p>(3) When wood is once dry, impregnating it with paraffin tends
+to keep it dry.</p>
+<p>(4) Red vulcanised fibre, like wood, absorbs paraffin, but it
+cannot be entirely waterproofed in this way.</p>
+<p>(5) The resistance of wood with stream lines along the grain
+is twenty to fifty per cent lower than when the stream lines
+cross the grain.</p>
+<p>(6) The "contact" resistance between slabs of wood pressed
+together is always very high.</p>
+<p>The following table will sufficiently illustrate the results
+obtained. The stone was dried in the sun for three weeks in the
+summer (United States), and the wood is described as having been
+well seasoned:&mdash;</p>
+<p align="center"><b>CURRENT WITH THE GRAIN</b></p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"567">
+<tr>
+<td width="18%" valign="top" height="4"></td>
+<td width="22%" valign="top" height="4">
+<p><font face="Bookman Old Style">Lowest Resistance between two
+Cups in Megohms.</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p><font face="Bookman Old Style">Highest Resistance between two
+Cups in Megohms.</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p><font face="Bookman Old Style">Lowest Specific Resistance in
+Megohms.</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p><font face="Bookman Old Style">Highest Specific Resistance in
+Megohms.</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Ash.</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">550</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">920</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">380</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">700</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Cherry</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">1100</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">4000</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">2800</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">6000</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Mahogany</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">430</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">730</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">310</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">610</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Oak</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">220</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">420</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">1050</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">2200</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Pine.</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">330</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">630</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">360</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">1470</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Hard pine.</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">48</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">17</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">1050</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Black walnut</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">1100</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">3000</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">320</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">2100</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Red fibre</font></p>
+</td>
+<td width="22%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">2</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">4</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">3</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">60</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Slate</font></p>
+</td>
+<td width="22%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">184</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">280</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">Soapstone.</font></p>
+</td>
+<td width="22%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">330</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">500</font></p>
+</td>
+</tr>
+<tr>
+<td width="18%" valign="top" height="4">
+<p><font face="Bookman Old Style">White marble</font></p>
+</td>
+<td width="22%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4"></td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">2000</font></p>
+</td>
+<td width="20%" valign="top" height="4">
+<p align="right"><font face="Bookman Old Style">8800</font></p>
+</td>
+</tr>
+</table>
+<p>&sect; 115. As to working the materials very little need be
+said.</p>
+<p>Fibre is worked like wood, but has the disadvantage of rapidly
+taking the edge off the tools. In turning it, therefore,
+brass-turning tools, though leaving not quite such a perfect
+finish as wood-turning tools, last much longer, and really do
+well enough. Fibre will not bear heating much above 100&deg; C.
+&mdash; at all events in paraffin. At 140&deg; C. it becomes
+perfectly brittle. Its chief merit lies in its great strength. So
+far as insulation is concerned, Mr. Peirce's experiments show
+that it is far below most kinds of wood.</p>
+<p><i>Slate</i>. &mdash; This is a vastly more useful substance
+than it is generally credited with being. It is very easily
+worked at a slow speed, either on the shaping machine or on the
+lathe, with tools adjusted for cutting brass, and it keeps its
+figure, which is more than can be said for most materials. It
+forms a splendid base for instruments, especially when ground
+with a little emery by iron or glass grinders, fined with its own
+dust, and French polished in the ordinary way. Spools for coils
+of considerable radial dimension may be most conveniently made of
+slate instead of wood or brass, both because it keeps its shape,
+and because it insulates sufficiently well to stop eddy currents
+&mdash; at all events, sufficiently for ordinary practice. An
+appreciable advantage is that slate may be purchased at a
+reasonable rate in large slabs of any desired thickness. It is
+generally cut in the laboratory by means of an old cross-cut saw,
+but it does not do much damage to a hard hack saw such as is used
+for iron or brass.</p>
+<p><i>Marble</i>. &mdash; According to Holtzapffell, marble may
+be easily turned by means of simple pointed tools of good steel
+tempered to a straw colour. The cutting point is ground on both
+edges like a wood-turning tool, and held up to the work "at an
+angle of twenty or thirty degrees" (?with the horizontal). The
+marble is cut wet to save the tool. As soon as the point gets, by
+grinding, to be about one-eighth of an inch broad it must either
+be drawn down or reground; a flat tool will not turn marble at
+all.</p>
+<p>A convenient saw for marble is easily made on the principle of
+the frame saw. A bit of hoop iron forms a convenient blade, and
+is sharpened by being hammered into notches along one edge, using
+the sharp end of a hammer head. The saw is liberally supplied
+with sand and water &mdash; or emery and water, where economy of
+time is an object. The sawing of marble is thus really a grinding
+process, but it goes on rapidly. Marble is ground very easily
+with sand and water; it is fined with emery and polished with
+putty powder, which, I understand, is best used with water on
+cloth or felt. As grinding processes have already been fully
+described, there is no need to go into them here. I have no
+personal knowledge of polishing marble.</p>
+<p><b><a name="Toc158108969" id="Toc158108969"><font face=
+"Bookman Old Style" size="4">&sect; 116. <i>Conductors</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The properties of conductors,
+more particularly of metals, have been so frequently examined,
+that the literature of the subject is appallingly heavy. In what
+follows I have endeavoured to keep clear of what might properly
+appear in a treatise on electricity on the one hand, and in a
+wiring table on the other. The most important work on the subject
+of the experimental resistance properties of metals has been done
+by Matthieson, <i>Phil. Trans</i>. 1860 and 1862, and <i>British
+Association Reports</i> (1864); Callender, <i>Phil. Trans</i>.
+vol. clxxiii.; Callender and Griffiths, <i>Phil. Trans</i>. vol.
+clxxxii.; <i>The Committee of the British Association on
+Electrical Standards from 1862 to Present Time;</i> Dewar and
+Fleming, <i>Phil. Mag</i>. vol. xxxvi. (1893);</font></p>
+<p>Klemencic, <i>Wiener Sitzungsberichte</i> (Denkschrift), 1888,
+vol. xcvii. p. 838; Feussner and St. Lindeck, <i>Zeitsch. fuer
+Inst. 'Kunde</i>, ix. 1889, p. 233, and <i>B. A. Reports</i>,
+1892, p. 139. Of these, Matthieson, and Dewar and Fleming treat
+of resistance generally, the latter particularly at low
+temperatures.</p>
+<p><i>[Footnote:</i> The following is a list of Dr. Matthieson's
+chief papers on the subject of the electrical resistance of
+metals and alloys: <i>Phil. Mag</i>. xvi. 1858, pp. 219-223;
+<i>Phil. Trans</i>. 1858, pp. 383-388 <i>Phil. Trans</i>. 1860,
+pp. 161-176; <i>Phil. Trans</i>. 1862, pp. 1-27 <i>Phil. Mag</i>.
+xxi. (1861), pp. 107-115; <i>Phil. Mag</i>. xxiii. (1862), pp.
+171-179; <i>Electrician</i>, iv. 1863, pp. 285-296; <i>British
+Association Reports</i>, 1863, p. 351.<i>]</i></p>
+<p>Matthieson, and Matthieson and Hockin, Klemencic, Feussner,
+and St. Lindeck deal with the choice of metals for resistance
+standards. Callender's, and Callender and Griffiths' work is
+devoted to the study of platinum for thermometric purposes.</p>
+<p>The bibliography referring to special points will be given
+later. The simplest way of exhibiting the relative resistances of
+metals is by means of a diagram published by Dewar and Fleming
+(<i>loc. cit</i>.), which is reproduced on a suitable scale on
+the opposite page. For very accurate work, in which corrections
+for the volumes occupied by the metals at different temperatures
+are of importance, the reader is referred to the discussion in
+the original paper, which will be found most pleasant reading.
+From this diagram both the specific resistance and the
+temperature coefficient may be deduced with sufficient accuracy
+for workshop purposes. In interpreting the diagram the following
+notes will be of assistance. The diagram is drawn to a scale of
+so-called "platinum temperatures" &mdash; that is to say, let
+R<sub>0</sub>, R<sub>100</sub>, R<sub>t</sub> be the resistances
+of pure platinum at 0&deg;, 100&deg;, and t&deg; C. respectively,
+then the platinum temperature p<sub>t</sub> is defined as</p>
+<p>p<sub>t</sub> = 100 X
+(R<sub>t</sub>-R<sub>0</sub>)/(R<sub>100</sub>-R<sub>0</sub>)</p>
+<p>This amounts to making the temperature scale such that the
+temperature at any point is proportional to the resistance of
+platinum at that point. Consequently on a resistance temperature
+diagram the straight line showing the relation between platinum
+resistance and platinum temperature will "run out" at the
+platinum absolute zero, which coincides more or less with the
+thermodynamic absolute zero, and also with the "perfect gas"
+absolute zero. Platinum temperatures may be taken for workshop
+purposes over ordinary ranges as almost coinciding with air
+thermometer temperatures. The metals used by Professors Dewar and
+Fleming were, with some exceptions, not absolutely pure, but in
+general represent the best that can be got by the most refined
+process of practical metallurgy. We may note further that the
+specific resistance is only correct for a temperature of about
+15&deg; C., since no correction for the expansion or contraction
+of material has been applied.</p>
+<p><img src="images/Image133.gif" alt="images/Image133.gif"
+width="410" height="910">  </p><p>The following notes on alloys suitable
+for resistance coils will probably be found sufficient.</p>
+<p><b><a name="Toc158108970" id="Toc158108970"><font face=
+"Bookman Old Style" size="4">&sect; 117. <i>Platinoid</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This substance, discovered by
+Martino and described by Bottomley (<i>Phil. Proc. Roy. Soc</i>.
+1885), is an alloy of nickel, zinc, copper, and 1 per cent to 2
+per cent of tungsten, but I have not been able to obtain an
+analysis of its exact composition. It appears to be difficult to
+get the tungsten to alloy, and it has to be added to part of the
+copper as phosphide of tungsten, in considerably greater quantity
+than is finally required. The nickel is added to part of the
+copper and the phosphide of tungsten, then the zinc, and then the
+rest of the copper. The alloy requires to be remelted several
+times, and a good deal of tungsten is lost by
+oxidation.</font></p>
+<p>The alloy is of a fine white colour, and is very little
+affected by air &mdash; in fact, it is to some extent
+untarnishable. The specific resistance will be seen to be about
+one and a half times greater than that of German silver, and the
+temperature coefficient is about 0.021 per cent per degree C.
+(i.e. about nineteen times less than copper, and half that of
+German silver). To all intents and purposes it may be regarded as
+German silver with 1 per cent to 2 per cent of tungsten. It does
+not appear to have been particularly examined for secular changes
+of resistance.</p>
+<p>118. <i>German Silver</i>. &mdash; This material has been
+exhaustively examined of late years by Klemencic and by Feussner
+and St. Lindeck. Everybody agrees that German silver, as
+ordinarily used for resistances, and composed of copper four
+parts, zinc two parts, nickel one part, is very ill-fitted for
+the purpose of making resistance standards. This is due</p>
+<p>(1) to its experiencing a considerable increase in resistance
+on winding. Feussner and St. Lindeck found an increase of 1 per
+cent when German silver was wound on a core of ten wire
+diameters.</p>
+<p>(2) To the fact that the change goes on, though with gradually
+decreasing rate, for months or years;</p>
+<p>(3) to the fact that the resistance is permanently changed
+(increased) by heating to 40&deg; C. or over. By "artificially
+ageing" coils of German silver by heating to 150&deg; C., say for
+five or six hours, its permanency is greatly improved, and it
+becomes fit for ordinary resistance coils where changes of, say,
+1/5000 do not matter.</p>
+<p>It is a remarkable property of all nickel alloys containing
+zinc that their specific resistance is permanently increased by
+heating, whereas alloys which do not contain zinc suffer a change
+in the opposite direction. The manufacturers of German silver
+appear to take very little care as to the uniformity of the
+product put on the market; some so-called German silver is
+distinctly yellow, while other samples are bright and white.</p>
+<p>It is noted by Price (<i>Measurements of Electrical
+Resistance</i>, p. 24) that German silver wire is apt to exhibit
+great differences of resistance within quite short lengths. This
+has been my own experience as well, and is a great drawback to
+the use of German silver in the laboratory, for it makes it
+useless to measure off definite lengths of wire with a view to
+obtaining an approximate resistance. In England German silver
+coils are generally soaked in melted hard paraffin. In Germany,
+at all events at the Charlottenburg Institute, according to St.
+Lindeck &mdash; coils are shellac-varnished and baked. In any
+case it appears to be essential to thoroughly protect the metal
+against atmospheric influence.</p>
+<p><b><a name="Toc158108971" id="Toc158108971"><font face=
+"Bookman Old Style" size="4">&sect; 119. Platinum Silver.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">In the opinion of Matthieson
+and of Klemencic the 10 per cent silver, 90 per cent platinum
+alloy is the one most suitable for resistance standards. At all
+events, it has stood the test of time, for, with the following
+exceptions, all the British Association coils constructed of it
+from 1867 to the present day have continued to agree well
+together. The exceptions were three one-ohm coils, which
+permanently increased between 1888 and 1890, probably through
+some straining when immersed in ice. One coil changed by 0.0006
+in 1 between the years 1867 and 1891. According to Klemencic,
+absolute permanency is not to be expected even from this
+alloy.</font></p>
+<p>Its recommendation as a standard depends on its chemical
+inertness, its small temperature coefficient (0.00027 per
+degree), and its small thermo-voltage against copper, as the
+following table (taken from Klemencic) will show:&mdash;</p>
+<p align="center">Thermo-voltages in Micro-volts per degree
+against Copper<br>
+over the Range 0&deg; to 17&deg; C.</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"568">
+<tr>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">Platinum iridium</font></p>
+</td>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">7.14 micro-volts per degree
+C.</font></p>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">Platinum silver</font></p>
+</td>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">6.62 micro-volts per degree
+C.</font></p>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">Nickelin .</font></p>
+</td>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">28.5 micro-volts per degree
+C.</font></p>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">German silver</font></p>
+</td>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">10.43 micro-volts per degree
+C.</font></p>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">Manganin (St.
+Lindeck)</font></p>
+</td>
+<td width="50%" valign="top">
+<p><font face="Bookman Old Style">1.5 micro-volts per degree
+C.</font></p>
+</td>
+</tr>
+</table>
+<p><font face="Bookman Old Style">Mechanically, the platinum
+silver is weak, and is greatly affected as to its resistance by
+mechanical strains &mdash; in fact, Klemencic considers it the
+worst substance he examined from this point of view &mdash; a
+conclusion rather borne out by Mr. Glazebrook's experience with
+the British Association standards already referred to (<i>B. A.
+Reports</i>, 1891 and 1892).</font></p>
+<p>Taking everything into account, it will probably be well to
+construct standards either with oil insulation only, or to bake
+the coils in shellac before testing, instead of soaking in
+paraffin. Fig. 89 illustrates a form of an oil immersed standard
+which is in use in my laboratory, and through which a
+considerable current may be passed. The oil is stirred by means
+of a screw propeller.</p>
+<p>Fig. <img src="images/Image134.gif" alt="images/Image134.gif"
+width="552" height="323"> 89.</p>
+<p>Fig. 89 represents a standard resistance for making Clerk cell
+comparisons by the silver voltameter method. The framework on
+which the coils are wound consists of a base and top of slate.
+The pillars are of flint glass tube surrounding brass bolts, and
+cemented to the latter by raw shellac. Grooves are cut in the
+glass sleeves to hold the wires well apart. These grooves were
+cut by means of a file working with kerosene lubrication. A screw
+stirrer is provided, and the whole apparatus is immersed in
+kerosene in the glass box of a storage cell. The apparatus is
+aged to begin with by heating to a temperature a good deal higher
+than any temperature it is expected to reach in actual work.
+After this the rigidity of the frame is intended to prevent any
+further straining of the wire. The apparatus as figured is not
+intended to be cooled to 0&deg; C., so that it is put in as large
+a box as possible to gain the advantage of having a large volume
+of liquid. The top and bottom slates measure seven inches by
+seven inches, and the distance between them is seven inches. The
+inner coil is wound on first, and the loop which constitutes the
+end of the winding is brought up to a suitable position for
+adjustment. The insulation of the heavy copper connectors is by
+means of ebonite.</p>
+<p><b><a name="Toc158108972" id="Toc158108972"><font face=
+"Bookman Old Style" size="4">&sect; 120. Platinum Iridium.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Platinum 90 per cent, iridium
+10 per cent. This material was prepared in some quantity at the
+cost of the French Government, and distributed for test about
+1886. Klemencic got some of it as representing Austria, and found
+it behaved very like the platinum silver alloy just discussed.
+The temperature coefficient is, however, higher than for platinum
+silver (0.00126 as against 0.00027). The mechanical properties of
+the alloy are, however, much better than those of the silver
+alloy; and in view of the experience with B. A. standards above
+quoted, it remains an open question whether, on the whole, it
+would not be the better material for standards, in spite of its
+higher price. Improvements in absolute measurements of
+resistance, however, may render primary standards
+superfluous.</font></p>
+<p><b><a name="Toc158108973" id="Toc158108973"><font face=
+"Bookman Old Style" size="4">&sect; 121. <i>Manganin</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Discovered by Weston &mdash; at
+all events as to its application to resistance coils. A glance at
+the diagram will exhibit its unique properties, on account of
+which it has been adopted by the Physikalisch Technischen
+Reichsanstalt for resistance standards. The composition of the
+alloy is copper 84 per cent, manganese 12 per cent, nickel 4 per
+cent., and it is described as of a steel-gray colour.
+Unfortunately it is apt to oxidise in the air, or rather the
+manganese it contains does so, so that it wants a very perfect
+protection against the atmosphere.</font></p>
+<p>Like German silver, manganin changes in resistance on winding,
+and coils made of it require to be artificially aged by heating
+to 150&deg; for five hours before final adjustment. The annealing
+cannot be carried out in air, owing to the tendency to oxidation.
+The method adopted by St. Lindeck (at all events up to 1892) is
+to treat the coil with thick alcoholic shellac varnish till the
+insulation is thoroughly saturated, and then to bake the coil as
+described. The baking not only anneals the wire, but reduces the
+shellac to a hard and highly insulating mass.</p>
+<p>Whether stresses of sufficient magnitude to produce serious
+mechanical effects can be set up by unequal expansion of wire and
+shellac during heating and cooling is not yet known, but so far
+as tested (and it must be presumed that the Reichsanstalt tests
+are thorough) no difficulty seems to have been met with. In
+course of time, however, probably the best shellac coating will
+crack, and then adieu to the permanency of the coil! This might,
+of course, be obviated by keeping the coil in kerosene, which has
+no action on shellac, but which decomposes somewhat itself.</p>
+<p>The method of treatment above described suffices to render
+coils of manganin constant for at least a year (in 1892 the tests
+had only been made for this time) within a few thousands per
+cent. Manganin can be obtained in sheets, and from this material
+standards of 10<sup>-2</sup>, 10<sup>-3</sup>, and
+10<sup>-4</sup> ohms are made by soldering strips between stout
+copper bars, and these are adjusted by gradually increasing their
+resistance by boring small holes through them. The solder
+employed is said to be "silver."</p>
+<p>Mr. Griffiths (<i>Phil. Trans</i>. vol. clxxxiv. [1893], A, p.
+390) has had some experience with manganin carrying comparatively
+heavy currents, under which circumstances its resistance when
+immersed in water was found to rise in spite of the varnish which
+coated it. Other experiments in which the manganin wire was
+immersed in paraffin oil did not exhibit this effect, though
+stronger currents were passed.</p>
+<p>On the whole, manganin appears to be the best material for
+coil boxes and "secondary" resistance standards. Whether it is
+fit to rank with the platinum alloys as regards permanency must
+be treated as an open question.</p>
+<p><b><a name="Toc158108974" id="Toc158108974"><font face=
+"Bookman Old Style" size="4">&sect; 122. <i>Other Alloys</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The following tables, taken
+from the work of Feussner and St. Lindeck, <i>Zeitschrift fuer
+Instrumenten Kunde</i>, 1889, vol. ix. p. 233, together with the
+following notes, will suffice.</font></p>
+<p><b><a name="Toc158108975" id="Toc158108975"><font face=
+"Bookman Old Style" size="4">&sect; 123. <i>Nickelin</i>.
+&mdash;</font></a></b></p>
+<p>This is only German silver with a little less zinc, a little
+more nickel, and traces of cobalt and manganese. It behaves like
+German silver, but is an improvement on the latter in that all
+the faults of German silver appear upon a reduced scale in
+nickelin.</p>
+<p><b><a name="Toc158108976" id="Toc158108976"><font face=
+"Bookman Old Style" size="4">&sect; 124. Patent Nickel.
+&mdash;</font></a></b></p>
+<p>Practically a copper nickel alloy, used to some extent by
+Siemens and Halske. It stands pretty well in the same relation to
+nickelin as the latter does to German silver. After annealing as
+for manganin it can be made into serviceable standards which do
+not change more than a few thousandths per cent. I have not come
+across a statement of its thermo-voltage against copper.</p>
+<p><a name="Toc158108977" id="Toc158108977">&sect; 125.
+<i>Constantin</i>. &mdash;</a></p>
+<p><font face="Bookman Old Style">Another nickel copper alloy
+containing 50 per cent of each constituent. It appears to be a
+serviceable substance, having a temperature coefficient of 0.003
+per cent per degree only, but an exceedingly high thermo-voltage,
+viz. 40 micro-volts per degree against copper.</font></p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"612">
+<tr>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">1</font></p>
+<p>German Silver.</p>
+</td>
+<td width="22%" valign="top" colspan="2">
+<p><font face="Bookman Old Style" size="2">2</font> <font face=
+"Bookman Old Style"> __________ 3</font></p>
+<p><font face="Bookman Old Style" size="2">Nickelin made by
+Obermaier</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">4</font></p>
+<p>Rheo-tane.</p>
+</td>
+<td width="22%" valign="top" colspan="2">
+<p><font face="Bookman Old Style" size="2">5</font> <font face=
+"Bookman Old Style"> ___________6</font></p>
+<p><font face="Bookman Old Style" size="2">Patent
+Nickel</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">7. Manga-nese
+Copper.</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">8. Nickel Manga-nese
+Copper.</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">Dia-meter 1.0
+mm</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">Dia-meter 0.1
+mm</font></p>
+</td>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">Dia-meter 0.6
+mm</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">Dia-meter 1.0
+mm</font></p>
+</td>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top"></td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Copper</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">60.16</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">61.63</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">54.57</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">53.28</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">74.41</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">74.71</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">70</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">73</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Zinc</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">25.37</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">19.67</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">20.44</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">16.89</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.23</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.52</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Tin</font></p>
+</td>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">trace</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Nickel</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">14.03</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">18.46</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">24.48</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">25.31</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">25.10</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">24.14</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">3</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Iron</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.30</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.24</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.64</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">4.46</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.42</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.70</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Cobalt</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">trace</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.19</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">trace</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">trace</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Mang-anese</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">trace</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.18</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.27</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.37</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.13</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">0.17</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">30</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">24</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top"></td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">99.86</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.37</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.40</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.31</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.24</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.24</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">...</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Specific resist-ance</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">30.0</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">33.2</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">44.8</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">52.5</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">34.2</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">32.8</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">100.6</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="2">47.7</font></p>
+</td>
+</tr>
+<tr>
+<td width="11%" valign="top">
+<p align="justify"><font face="Bookman Old Style" size=
+"2">Temp-erature co-efficient</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00036</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00030</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00033</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00041</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00019</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00021</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00004</font></p>
+</td>
+<td width="11%" valign="top">
+<p><font face="Bookman Old Style" size="1">0.00003</font></p>
+</td>
+</tr>
+</table>
+<p>The specific resistance is in "microhms, i.e. 10<sup>-6</sup>
+ohms per cubic centimetre, and the temperature coefficient in
+degrees centigrade.</p>
+<p><a name="Toc158108978" id="Toc158108978">126. Nickel Manganese
+Copper. &mdash;</a></p>
+<p><font face="Bookman Old Style">I can find no other reference
+with regard to this alloy mentioned by Lindeck. Nicholls, however
+(Silliman's Journal [3], 39, 171, 1890), gives some particulars
+of alloys of copper and ferromanganese. The following table is
+taken from Wiedemann's <i>Beiblatter</i> (abstract of Nicholl's
+paper, 1890, p. 811). All these alloys appear to require
+annealing at a red heat before their resistances are anything
+like constant.</font></p>
+<p>Let x be percentage of copper, then 100 &mdash; x is
+percentage of "ferromanganese."</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"567">
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="2">Values of
+x.</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">100</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">99.26</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size="2">91
+.88</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">86.98</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">80.4</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">70.65</font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="2">Specific resistance
+with respect to copper (? pure)</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">1</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">1.19</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">11.28</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">20.4</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">27.5</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">45.1</font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="2">Temperature
+coefficient per degree x 10<sup>6</sup> (hard)</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">3202</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">2167</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">138</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">16</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">22</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">-24</font></p>
+</td>
+</tr>
+<tr>
+<td width="20%" valign="top">
+<p><font face="Bookman Old Style" size="2">Ditto
+(soft)</font></p>
+</td>
+<td width="13%" valign="top"></td>
+<td width="13%" valign="top"></td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">184</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">80</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">66</font></p>
+</td>
+<td width="13%" valign="top">
+<p align="right"><font face="Bookman Old Style" size=
+"2">21</font></p>
+</td>
+</tr>
+</table>
+<p>If nickel is added, alloys of much the same character are
+obtained, some with negative temperature coefficients &mdash; for
+instance, one containing 52.51 per cent copper, 31.27 per cent
+ferromanganese, and 16.22 nickel.</p>
+<p>A detailed account of several alloys will be found in a paper
+by Griffiths (Phil. Trans. 1894, p. 390), but as the constants
+were determined to a higher order of accuracy than the
+composition of the material &mdash; or, at all events, to a
+higher degree of accuracy than that to which the materials can be
+reproduced &mdash; there is no advantage in quoting them
+here.</p>
+<p><a name="Toc158108979" id="Toc158108979">CHAPTER IV</a></p>
+<p><a name="Toc158108980" id="Toc158108980">ELECTROPLATING AND
+ALLIED ARTS</a></p>
+<p><a name="Toc158108981" id="Toc158108981"><font face=
+"Bookman Old Style" size="4">&sect; 127. Electroplating.
+&mdash;</font></a></p>
+<p><font face="Bookman Old Style">This is an art which is usually
+deemed worthy of a treatise to itself, but for ordinary
+laboratory purposes it is a very simple matter &mdash; so simple,
+indeed, that the multiplicity of receipts as given in treatises
+are rather a source of embarrassment than otherwise.</font></p>
+<p>The fundamental principles of the art are:-</p>
+<p>(1) Dirty work cannot be electroplated.</p>
+<p>(2) Electroplated surfaces may be rougher, but will not be
+smoother than the original unplated surface.</p>
+<p>(3) The art of electroplating being in advance of the science,
+it is necessary to be careful as to carrying out instructions in
+detail. This particularly applies to the conditions which
+determine whether a metallic deposit shall come down in a
+reguline or in a crystalline manner.</p>
+<p><b><a name="Toc158108982" id="Toc158108982"><font face=
+"Bookman Old Style" size="4">&sect; 128. The Dipping Bath.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">An acid dipping bath is one of
+the most useful adjuncts to the laboratory, not only for
+cleansing metals for electroplating, but for cleaning up
+apparatus made out of bits of brass tube and sheet, and
+particularly for quickly cleaning binding screws, etc., where it
+is necessary to ensure good electrical contact.</font></p>
+<p>The cheapest and most satisfactory way in the end is to make
+up two or three rather large baths to begin with. The glass boxes
+of storage batteries do very nicely for the purpose, and being
+generally ground pretty flat at the top, they may be covered by
+sheets of patent plate glass, and thus preserved from the action
+of the air.</p>
+<p><i>First Bath</i>. &mdash; A 30 or 40 per cent solution of
+commercial caustic soda. Objects may be cleansed from grease in
+this bath by heating them as hot as is consistent with individual
+circumstances, and plunging them into it.</p>
+<p>It is a considerable advantage to begin by removing grease
+from articles subsequently to be dipped in an acid bath, both
+because it saves time and acid, and because more uniform results
+are obtainable when this is done than when it is omitted. It is a
+great advantage to have the caustic soda solution hot. This is
+always done in factories where nickel-plating is carried on, but
+it is inconvenient in the laboratory. The articles after dipping
+in the alkali are swilled with water, and may even be scrubbed
+with a brush, so as to remove greasy matters that have been
+softened but not entirely removed.</p>
+<p><i>Acid Bath</i>. &mdash; A convenient bath for laboratory
+purposes is made by mixing two volumes of strong commercial
+nitric acid with one of strong sulphuric acid in a cell
+measuring, say, 12 X 10 X 15 inches.</p>
+<p>Copper or brass articles are dipped in this bath for a few
+seconds, then rinsed with water, then dipped again for a second
+or two, or until they appear equally white all over, and then
+withdrawn as rapidly as possible and plunged into a large
+quantity of clean water. Care must be taken to transfer the
+articles from the bath to the water as quickly as possible, for
+if time be allowed for gas to be evolved, the surfaces become mat
+instead of bright.</p>
+<p>In order to save acid it is advisable to make up a third bath,
+using those odds and ends of acids which gradually accumulate in
+the laboratory. Sulphuric acid from the balance cases, for
+instance, mixed with its own volume of commercial nitric acid,
+does very well.</p>
+<p>The objects to be dipped receive a preliminary cleansing by a
+dip in this bath, the strong bath being reserved for the final
+dip. Sheet brass and drawn tube, as it comes from the makers,
+possesses a really fine surface, though this is generally
+obscured by grease and oxide. Work executed in these materials,
+cleaned in alkali, and dipped in really strong acid, will be
+found to present a much better appearance than work which has
+been filed, unless the latter be afterwards elaborately
+polished.</p>
+<p>On no account must paraffin be allowed to get into any of the
+baths. When the final bath gets weak it must be relegated to a
+subordinate position and a new bath set up. A weak acid bath
+leaves an ugly mottled surface on brass work.</p>
+<p>&sect; 129. A metallic surface which it is intended to
+electroplate must, as has been mentioned, be scrupulously clean.
+If the metal is not too valuable or delicate, cleaning by dipping
+is easy and effectual. The following notes will be found to apply
+to special cases which often occur.</p>
+<p>(1) <i>Silver Surfaces intended to be gilt</i>. &mdash; These
+are first washed clean with soap and hot water, and polished with
+whitening. They are then dipped for a moment in a boiling
+solution of potassium cyanide. A 20 per cent solution of common
+commercial cyanide does well, but the exact strength is quite
+immaterial. The cyanide is washed away in a large volume of soft
+water, and the articles are kept under water till they are
+scratch-brushed.</p>
+<p>Mat surfaces are readily produced on standard silver by
+dipping in hot strong sulphuric acid. The appearance of new
+silver coins, which is familiar to everybody, is obtained by this
+process.</p>
+<p>(2) <i>Finely turned and finished Brass Work</i>. &mdash; If
+it is intended to nickel-plate such work, and if it is desirable
+to obtain brightly polished nickel surfaces, the work must be
+perfectly polished to begin with. Full details as to polishing
+may be found in workshop books or treatises on watch-making. It
+will suffice here to say that the brass work is first smoothed by
+the application of successive grades of emery and oil, or by very
+fine "dead" smooth files covered with chalk. Polishing is carried
+out by means of rotten stone and oil applied on leather.</p>
+<p>In polishing turned work care must be taken to move the file,
+emery, or rotten stone to and fro over the work with great
+regularity, or the surface will end by looking scratchy and
+irregular. The first process of cleaning is, of course, to remove
+grease, and this is accomplished best by dipping in a bath of
+strong hot caustic soda solution, and less perfectly by heating
+the work and dipping it in the cold caustic soda bath.</p>
+<p>During this process a certain amount of chemical action often
+occurs leading to the brass surface exhibiting some
+discoloration. The best way of remedying this is to dip the brass
+into a hot bath of cyanide of potassium solution. If it is
+inconvenient to employ hot baths or to heat the brass work, good
+results may be obtained by rubbing the articles over with a large
+rough cork plentifully lubricated with a strong solution of an
+alkali.</p>
+<p>If the surfaces are very soiled or dirty, a paste of alkali
+and fine slaked lime may be applied on a cork rubber, and this in
+my experience has always been most effective and satisfactory in
+every way, except that it is difficult to get into crevices. If
+the alkali stains the work, a little cyanide of potassium may be
+rubbed over the surface in a similar manner.</p>
+<p>Brass work treated by either of these methods is to be washed
+in clean water till the alkali is entirely removed, and may then
+be nickel-plated without any preliminary scratch-brushing. The
+treatment in hot baths of alkali and cyanide is the method
+generally employed in American factories as a preliminary to the
+nickelling of small brass work for sewing machines, etc.</p>
+<p>(3) <i>Copper</i> either for use as the kathode in
+electrolysis calibration experiments or otherwise is most
+conveniently prepared by dipping in the acid bath, rinsing
+quickly in cold water, scratch-brushing under cold water, and
+transferring at once to the plating bath. In the case where the
+copper plates require to be weighed they are dipped into very hot
+distilled water after scratch-brushing, and then dried at once by
+means of a clean glass cloth.</p>
+<p>(4) <i>Aluminium</i> (which, however, does not readily lend
+itself to plating operations <i>[Footnote:</i> This difficulty
+has now been overcome. See note, section 138.<i>]</i> ) is best
+treated by alkali rubbed on with a cork, or by a hot alkaline
+carbonate where rubbing is inexpedient. The clean aluminium is
+scratch-brushed under water, and at once transferred to the
+plating bath.</p>
+<p>(5) <i>Iron for Nickel-plating</i>. &mdash; According to Dr.
+Gore (Electra-metallurgy, p. 319) the best bath for cleaning iron
+is made as follows: "One gallon of water and one pound of
+sulphuric acid are mixed with one or two ounces of zinc (which of
+course dissolves); to this is added half a pound of nitric acid."
+The writer has been accustomed to clean iron by mechanical means,
+to deprive it of grease by caustic alkali, and to finish it off
+by, means of a hard scratch brush. This process has always worked
+satisfactorily.</p>
+<p>(6) <i>Articles soldered with soft solder containing lead and
+tin</i> do not readily lend themselves to electrolytic processes,
+the solder generally becoming black and refusing to be coated
+with the electro-deposit. Moreover, if soldered articles are
+boiled for any length of time in caustic alkali during the
+preliminary cleansing, enough tin will dissolve to form a
+solution of stannate of potash or soda &mdash; strong enough to
+deposit tin on brass or copper. A method of coppering soldered
+articles will be described later on.</p>
+<p><b><a name="Toc158108983" id="Toc158108983"><font face=
+"Bookman Old Style" size="4">&sect; 130. Scratch-brushing.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This process is generally
+indispensable, and to its omission is to be traced most
+laboratory failures in electroplating. Scratch-brushes may be
+bought at those interesting shops where "watchmakers' supplies"
+are sold. It will be well, therefore, to purchase a selection of
+scratch brushes, for they are made to suit particular kinds of
+work. They are all made of brass wire, and vary both in hardness
+and in the fineness of the wire. The simplest kind of scratch
+brush consists merely of a bundle of wires bound up tightly by
+another wire, and somewhat "frizzed" out at the ends (Fig. 90). A
+more useful kind is made just like a rotating brush, and has to
+be mounted on a lathe (Fig. 91).</font></p>
+<p><img src="images/Image135.gif" alt="images/Image135.gif"
+width="225" height="213"><img src="images/Image136.gif" alt=
+"images/Image136.gif" width="54" height="329">Fig. 90. Fig.
+91.</p>
+<p>The scratch brush is generally, if not always, applied wet;
+the lubricant generally recommended is stale beer, but this may
+be replaced by water containing a small quantity of glue, or any
+other form of gelatine in solution &mdash; a mere trace (say .1
+per cent) is quite sufficient. Very fair results may be got by
+using either pure or soapy water. The rotating brushes require to
+be mounted on a lathe, and may be run at the same speed as would
+be employed for turning wooden objects of the same
+dimensions.</p>
+<p>Since the brush has to be kept wet by allowing water or its
+equivalent to drip upon it, it is usual to make a tin trough over
+which the brush can revolve, and to further protect this by a tin
+hood to keep the liquid from being thrown all over the room. In
+many works the brush is arranged to lie partly in the liquid, and
+this does very well if the hood is effective.</p>
+<p>There is a superstition that electro-deposits stick better to
+scratch-brushed surfaces than to surfaces which have not been so
+treated, and consequently it is usual to scratch-brush surfaces
+before electro-deposit. However this may be, there is no doubt
+that adherence and solidity are promoted by frequent
+scratch-brushing during the process of depositing metal,
+especially when the latter tends to come down in a spongy
+manner.</p>
+<p><i>Gilt surfaces</i> &mdash; if the gilding is at all heavy
+&mdash; are generally dull yellow, or even brown, when they come
+from the bath, and require the scratch brush to cause the gold to
+brighten, an office which it performs in a quite striking manner.
+The same remark applies to silvered surfaces, which generally
+leave the bath a dead white &mdash; at all events if the deposit
+is thick, and if ordinary solutions are employed. In either case
+the touch of the scratch brush is magical.</p>
+<p><b><a name="Toc158108984" id="Toc158108984"><font face=
+"Bookman Old Style" size="4">&sect; 131. Burnishing.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Burnishers of steel, agate, or
+bloodstone can be bought at the shops where scratch brushes are
+sold, and are used to produce the same brightening effect as can
+be got by scratch-brushing. The same solutions are employed, but
+rather stronger, and the burnisher is swept over the surface so
+as to compress the deposited metal. Burnishing is rather an art,
+but when well done gives a harder and more brilliant (because
+smoother) surface than the scratch brush. On the whole, steel
+burnishers are the most convenient if in constant use.</font></p>
+<p>If the burnishing tools have to lie about, steel is apt to
+rust, unless carefully protected by being plunged in quicklime or
+thickly smeared with vaseline, and the least speck of rust is
+fatal to a burnisher. In any case the steel requires to be
+occasionally repolished by rouge and water on a bit of cloth or
+felt. The process of burnishing is necessarily somewhat slow and
+tedious, and as a rule is not worth troubling about except in
+cases where great permanence is required.</p>
+<p>The burnisher is moved over the work somewhat like a pencil
+with considerable pressure, and care is taken to make the strokes
+as uniform in direction as possible; otherwise the surface looks
+non-uniform, and has to be further polished by tripoli,
+whitening, etc., before it is presentable.</p>
+<p><b><a name="Toc158108985" id="Toc158108985"><font face=
+"Bookman Old Style" size="4">&sect; 132. Silver-plating.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The most convenient solution
+for general purposes is an 8 to 10 per cent solution of the
+double cyanide of silver and potassium together with 1 or 2 per
+cent of "free" potassium cyanide. Great latitude is permissible
+in the strength of solution and density of current. As commercial
+cyanide of potassium generally contains an unknown percentage of
+other salts, which, however, do not interfere with its value for
+the purpose of silver-plating, the simplest procedure is as
+follows.</font></p>
+<p>For every 100 c.c. of plating solution about 7 grms. of dry
+crystallised silver nitrate are required. The equivalent amount
+of potassium cyanide (if dry and pure) is 5.2 grms., but
+commercial cyanide may contain from 50 per cent upwards to 96 per
+cent in the best fused cyanide made from ferrocyanide only. An
+approximate idea of the cyanide content can be obtained from the
+dealers when the salt is purchased, and this is all that is
+required.</p>
+<p>A quantity slightly in excess of the computed amount of
+cyanide is dissolved in distilled water, and this is cautiously
+added to the solution of the silver nitrate till precipitation is
+just complete. The supernatant liquors are then drained away, and
+the precipitate dissolved by adding a sufficiency of the
+remaining cyanide; this process is assisted by warming and
+stirring.</p>
+<p>An allowance of about one-tenth of the whole cyanide employed
+may be added to form "free" cyanide, and the solution made up to
+the strength named. It is advisable to begin with the cyanide in
+a moderately strong solution, for the sake of ease in dissolving
+the precipitate.</p>
+<p>This solution will deposit silver upon articles of copper or
+brass immersed in it even without the battery, but the coat will
+be thin. The solution is used cold, with a current density of
+about 10 to 20 amp&egrave;res per square foot. The articles to be
+silvered are scratch-brushed, washed, and electroplated, till
+they begin to look undesirably rough. They are then taken out of
+the bath, rebrushed, and the process continued till a sufficiency
+of silver is deposited. Four grammes weight of silver (nearly) is
+deposited per amp&egrave;re hour. It is best to use a fine silver
+anode, so that the solution, does not get contaminated by
+copper.</p>
+<p>In most factories it is usual to "quicken" the objects to be
+silvered before placing them in the electrolysis vats, because
+the deposit is said to adhere better in consequence of this
+treatment. I have never found it any improvement for laboratory
+purposes, but it is easy to do. A dilute (say 2 per cent)
+solution of cyanide of mercury is required containing a little
+free cyanide. The objects to be "quickened" are scratch-brushed
+and dipped into the cyanide of mercury solution till they are
+uniformly white; it is generally agreed that the less the mercury
+deposited the better, so long as a perfect coating is obtained.
+The objects are rinsed after quickening, and put in the
+depositing bath at once.</p>
+<p>The mat surface of silver obtained by electrolysis of the
+cyanide is very beautiful &mdash; one of the most beautiful
+things in nature &mdash; shining with incomparable crystalline
+whiteness. So delicate is it, however, for so great is the
+surface it exposes, that it is generally rapidly deteriorated by
+exposure to the air. It may be protected to some extent by
+lacquering with pale lacquer, but it loses some of its brilliancy
+and purity in the process. The deposit is generally
+scratch-brushed or burnished down to a regular reflecting
+surface.</p>
+<p><b><a name="Toc158108986" id="Toc158108986"><font face=
+"Bookman Old Style" size="4">&sect; 133. Cold Silvering.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A thin but brilliant coat of
+silver may be readily applied to small articles of brass or
+copper in the following way. A saturated solution of sodium
+sulphite (neutral) is prepared, and into this a 10 per cent
+solution of nitrate of silver is poured so long as the
+precipitate formed is redissolved. A good deal of silver may be
+got into solution in this way. Articles to be silvered need only
+to be cleaned, brushed, and dipped in this solution till a coat
+of the required thickness is obtained.</font></p>
+<p>I must admit, however, that the coating thus laid on does not
+appear to be so permanent as one deposited by simple immersion
+from the cyanide solution, even though it is thicker. The cyanide
+plating solution will itself give a good coat of silver if it is
+used boiling, and if a little potassium cyanide be added.</p>
+<p>For purposes of instrument construction, however, a thin coat
+of silver is seldom to be recommended, on account of its
+liability to tarnish and its rapid destruction when any attempt
+is made to repolish it. For these reasons, nickel or gold plating
+is much to be preferred.</p>
+<p><b><a name="Toc158108987" id="Toc158108987"><font face=
+"Bookman Old Style" size="4">&sect; 134. <i>Gilding</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">This art deserves to be much
+more widely practised than is usual in laboratories. Regarded as
+a means of preserving brass, copper, or steel, it is not
+appreciably more "time robbing" than lacquering, and gives
+infinitely better results. Moreover, it is not much more
+expensive. Strange as it may seem, the costliness of gilding
+seldom lies in the value of the gold deposited; the chief cost is
+in the chemicals employed to clean the work, and in interest on
+the not inconsiderable outlay on the solution and
+anode.</font></p>
+<p>The easiest metal to gild is silver, and it is not unusual to
+give base metals a thin coating of silver or copper, or both, one
+after the other, before gilding, in order to secure uniformity.
+To illustrate the virtue of a thin layer of gold, I will mention
+the following experiment. About three years ago I learned for the
+first time that to "clean" the silver used in a small household
+required at least an hour's labour <i>per diem</i>. I further
+ascertained that most of this time is spent on the polishing part
+of the process.</p>
+<p>As this seemed a waste of labour, I decided to try the effect
+of gilding. In order to give the proposal a fair trial I gilt the
+following articles: half a dozen table spoons and forks, a dozen
+dessert forks and spoons, and a dozen tea spoons. These were all
+common electroplated ware. They were weighed before and after
+gilding, and it was with difficulty that the increase of weight
+was detected, even though a fine bullion balance was employed. On
+calculating back to money, it appeared that the value of the gold
+deposited was about threepence. Assuming that an equal weight of
+silver had been accidentally dissolved by the free cyanide during
+the plating &mdash; which is unlikely &mdash; the total amount of
+gold deposited would be worth, say, sixpence.</p>
+<p>After three years' continuous use the gilding is still
+perfect, except at the points on which the spoons and forks rest,
+where it is certainly rather shabby. Meanwhile the "gold" plate
+only requires to be washed with hot water and soap to keep it in
+perfect order, a much more cleanly and expeditious process than
+that of silver cleaning.</p>
+<p><b><a name="Toc158108988" id="Toc158108988"><font face=
+"Bookman Old Style" size="4">&sect; 135. Preparing Surfaces for
+Gilding. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Ordinary brass work &mdash;
+rough or smooth &mdash; may for purposes of preservation be
+dipped, scratch-brushed, and gilt at once. Seven years ago the
+writer gilt the inside of the head of a copper water still, and
+simply scratch-brushed it; it is to-day in as good order as when
+it was first done. If it is intended to gild work from the first,
+with the view of making an exceptionally fine job of it, "gilding
+metal," i.e. brass containing one to one and a quarter ounces of
+zinc to the pound of copper may be specified. From its
+costliness, however, this is only desirable for small
+work.</font></p>
+<p>Iron and steel are generally given a preliminary coating of
+copper, but this may be dispensed with though with no advantage
+&mdash; by using a particular process of gilding.</p>
+<p>Base metals, zinc, pewter, lead, etc., are first coppered in a
+cyanide of copper solution, as will be described under the head
+of Copper-plating. If it is intended to gild soldered articles,
+the preliminary coating of copper is essential.</p>
+<p>The most convenient vessel for holding a gilding solution is
+undoubtedly one formed of enamelled iron. Particularly useful are
+the buckets and "billies" (i.e. cylindrical cans) made of this
+material. These vessels may be heated without any fear of a
+smash, and do not appear to be appreciably affected by gilding
+solutions &mdash; at all events during several days or weeks. The
+avoidance of all risk of breakage when twenty or thirty pounds'
+worth of solution is in question is a matter of importance.</p>
+<p>Under no circumstances is it desirable to use anything but the
+purest gold and best fused cyanide (called "gold" cyanide) in the
+preparation of the solutions. The appearance of a pure gold
+deposit is far richer than of one containing silver, and its
+resistance to the atmosphere is perfect; moreover, in
+chemico-physical processes one has the satisfaction of knowing
+what one is dealing with.</p>
+<p><b><a name="Toc158108989" id="Toc158108989"><font face=
+"Bookman Old Style" size="4">&sect; 136. Gilding Solutions.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">The strength of solution
+necessary for gilding brass, copper, and silver is not very
+material. About one to two pounds of "gold" potassium cyanide (?
+96 per cent KCN) per gallon does very well. The gold is best
+introduced by electrolysing from a large to a small gold
+electrode. One purchases a plate of pure gold either from the
+mint or from reliable metallurgists (say Messrs. Johnson and
+Matthey of London), and from this electrodes are cut.</font></p>
+<p>The relative areas of the electrodes do not really much
+matter. I have used an anode of four times the area of the
+cathode. The solution is preferably heated to a temperature of
+about 50&deg; C., and a strong current is sent through it, say
+twenty amperes to the square foot of anode. The electrodes must
+be suspended below the surface of the solution by means of
+platinum wires. If the gold plates are only partly immersed, they
+dissolve much more rapidly where they cut the surface, possibly
+on account of the effect of convection currents, though so far as
+the writer is aware no proper explanation has yet been given.</p>
+<p>After a time gold begins to be deposited on the cathode in a
+powdery form, for which reason it is a good plan to begin by
+wrapping the latter in filter paper. The process has gone on for
+a sufficient time when a clean bit of platinum foil immersed in
+the place of the cathode becomes properly gilt at a current
+density of about ten amperes per square foot.</p>
+<p>The powdery gold deposited on the cathode while preparing the
+solution can be scraped off and melted for further use, or the
+whole cathode may now be used as an anode. The platinum foil
+testing cathode may also be "stripped" by making it an anode, and
+is for this reason preferable to German silver or copper, which
+would contaminate the solution while the "stripping" process was
+in progress.</p>
+<p>For general purposes a current density of say ten to fifteen
+amperes per square foot may be used, but this may be considerably
+varied, so long as the upper limit is not greatly overpassed.
+During gold-plating there is a considerable advantage in keeping
+the electrodes moving or the solution stirred.</p>
+<p>After immersing the cleaned and scratch-brushed articles,
+depositing may go on for about three minutes, after which they
+are removed from the bath and examined, in order to detect any
+want of uniformity in the deposit.</p>
+<p>The articles should be entirely immersed; if this is not done,
+irregularity is apt to appear at the surface. Platinum wires
+employed as suspenders, and coated along with the articles to be
+gilt, may also be cleaned without loss by making them anodes. If,
+on examination, all is found to be going on well, reimmerse the
+cathodes, and continue plating till they appear of a dull
+yellowish brown (this will occur in about four minutes), then
+remove them, rinse and scratch-brush them, and replace them in
+the bath.</p>
+<p>When a second coat appears to be getting rather brown than
+yellowish brown, i.e. of the colour of wet wash-leather, the
+removal, followed by scratch-brushing, may be repeated, and for
+nearly all laboratory purposes, the articles are now fully
+gilt.</p>
+<p>The coating of gold deposited from a hot cyanide solution is
+spongy in the extreme, and if the maximum wear-resisting effect
+is to be obtained, it is advisable to burnish the gold rather
+than to rely upon the scratch brush alone.</p>
+<p>If the area of the cathode exceeds that of the anode the
+solution is said to grow weaker, and vice versa. This may be
+remedied in the former case by an obvious readjustment; the
+latter introduces no difficulty so far as I know except when
+plating iron or steel.</p>
+<p>The student need not be troubled at the poor appearance of the
+deposit before it is scratch-brushed. Heavy gold deposits are
+almost always dull, not to say dirty, in appearance till the
+burnisher or scratch brush is applied. On the other hand, the
+deposit ought not to get anything like black in colour.</p>
+<p>The following indications of defects may be noted--they are
+taken from Gore. I have never been really troubled with them.</p>
+<p>The deposit is blackish. This is caused by too strong a
+current in too weak a bath. This may be remedied to some extent
+by stirring or keeping the cathode in motion. The obvious remedy
+is to add a little cyanide of gold.</p>
+<p>The gold anode gets incrusted. This is a sign that the bath is
+deficient in potassium cyanide. The gold anode gets black and
+gives off gas. The solution is deficient in cyanide, and too
+large a current is being passed.</p>
+<p>If a bright surface is desired direct from the bath, some
+caustic potash (say 2 per cent) may, according to Gore, be added,
+or the articles may be plated only slightly by using a weak
+current and taking them out directly they show signs of getting
+dull. By a weak current I mean one of about five amperes per
+square foot.</p>
+<p>The deposit is said to be denser if the solution be heated as
+directed; but the bath will gild, though not quite so freely when
+cold.</p>
+<p>To gild iron or steel directly, dilute the bath as above
+recommended some five or six times, add about 1 per cent of
+potassium cyanide, and gild with a very weak current (say two or
+three amperes per square foot) in the cold. Frequent
+scratch-brushing will be found requisite to secure proper
+adherence.</p>
+<p>It is generally recommended to gild brass or German silver in
+solutions which are rather weak, but in the small practice which
+occurs in the laboratory a solution prepared as suggested does
+perfectly for everything except iron or steel. The
+scratch-brushing should be done over a large photographic
+developing dish to avoid loss of gold. It is a good plan to rinse
+the articles after leaving the bath in a limited quantity of
+distilled water, which is afterwards placed in a "residue"
+bottle, and then to scratch-brush them by hand over the dish to
+catch fine gold. When any loose dust is removed the articles may
+be scratched in the lathe without appreciable further loss.</p>
+<p>Silver-gilt articles tend to get discoloured by use, but this
+discoloration can be removed by soap and water. After long use a
+gold cyanide bath tends to alter greatly in composition, In
+general, the bath tends to grow weaker, from the fact that there
+is a strong temptation to gild as many articles at once as
+possible.</p>
+<p>It is therefore a good plan to keep a rough profit and loss
+account of the gold in order to find the quantity in solution.
+Fifty dwts. per gallon (or 78 grms. per 4.5 litres) is
+recommended. A gallon of solution of this strength is worth about
+eleven pounds sterling in gold and cyanide, and a serviceable
+anode will be worth about 10 pounds. (Fine gold is worth
+nominally four pounds four shillings and eleven pence ha'penny
+per oz.) Gold may be easily obtained containing less impurity
+than one part in ten thousand.</p>
+<p><b><a name="Toc158108990" id="Toc158108990"><font face=
+"Bookman Old Style" size="4">&sect; 137. Plating with Copper.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Copper may be deposited from
+almost any of its salts in reguline form, the sulphate and
+nitrate being most usually employed. In the laboratory a nearly
+saturated solution of sulphate of copper with 1 or 2 per cent of
+sulphuric acid will answer most purposes. A current density of,
+at most, fifteen amperes per square foot may be used, either for
+obtaining solid deposits for constructional purposes or for
+calibrating current measuring instruments by electrolysis. A
+copper anode is of course employed.</font></p>
+<p>When coppering with a view to obtaining thick deposits it is a
+good plan to place the electrodes several inches apart, and, if
+possible, to keep the liquid stirred, as there is a considerable
+tendency on the part of copper deposits to grow out into mossy
+masses wherever the current density exceeds the limit mentioned.
+As the masses grow towards the anode the defect naturally tends
+to increase of itself, hence the necessity for care. The
+phenomenon is particularly marked at the edges and corners of the
+cathode.</p>
+<p>If the deposit becomes markedly irregular, the best plan is to
+stop the process and file the face of the deposit down to
+approximate smoothness. In coppering it is of the utmost
+importance that the cathode be clean and free from grease; it
+must never be touched (by the finger, for instance) from the time
+it is scratch-brushed till it is immersed in the plating bath.
+Any grease or oxidation tends to prevent the copper deposit
+adhering properly.</p>
+<p>A copper deposit oxidises very easily when exposed to the air.
+Consequently if the surface be required free from oxide, as, for
+instance, when it is to be silvered or gilt, it must be quickly
+washed when withdrawn from the coppering bath, scratch-brushed,
+and transferred immediately to the silvering or gilding bath.</p>
+<p>If the surface is to be dried, as in electrolysis
+calibrations, it must be rinsed quickly with boiling water and
+pressed between sheets of filter paper. Another method which has
+been recommended is to rinse the copper in water slightly
+acidulated with sulphuric acid (which prevents oxidation), then
+in distilled water, and to dry by blotting paper and in front of
+a fire, taking care not to make the plate too hot. The wash water
+is sufficiently acidulated by the addition of two or three drops
+of acid per litre. So far as I know, the method of washing in
+acidulated water was first proposed by Mr. T. Gray.</p>
+<p><b><a name="Toc158108991" id="Toc158108991"><font face=
+"Bookman Old Style" size="4">&sect; 138. Coppering Aluminium.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A good adherent deposit of
+copper on aluminium used to be considered a desideratum in the
+days when it afforded the only means of soldering the latter.
+Many receipts have been published from time to time, and I have
+tried, I think, most of them. On no occasion, however, till this
+year (1896), have I succeeded in obtaining a deposit which would
+not strip after it was tinned and soldered, though it is not
+difficult to get apparently adherent deposits so long as they are
+not operated upon by the soldering iron. The best of the many
+solutions which have been proposed in years gone by is very
+dilute cupric nitrate with about 5 per cent of free nitric
+acid.</font></p>
+<p>The problem of electroplating aluminium which I have indicated
+as awaiting a solution has at last found one. In the <i>Archives
+des Sciences physiques et naturelles de Gen&egrave;ve</i> for
+December 1895 (vol. xxxiv. p. 563) there is a paper by M. Margot
+on the subject, which discloses a perfectly successful method of
+plating aluminium with copper. The paper itself deals in an
+interesting way with the theory of the matter &mdash; however,
+the result is as follows.</p>
+<p>(1) The aluminium articles are boiled for a few minutes in a
+strong solution of ordinary washing soda. The aluminium surface
+is thus corroded somewhat, and rendered favourable to the deposit
+of an adherent film of copper. After removal from the soda
+solution the aluminium is well washed and brushed in running
+water.</p>
+<p>(2) The articles are dipped for thirty seconds or so in a hot
+5 per cent solution of pure hydrochloric acid.</p>
+<p>(3) After dipping in the hydrochloric acid, the work is
+instantly plunged into clean water for about one second, so as to
+remove nearly, but not quite, all of the aluminium chloride.</p>
+<p>(4) The work is transferred to a cold dilute (say 5 per cent)
+solution of cupric sulphate slightly acidulated with sulphuric
+acid. The degree of acidulation does not appear to be very
+important, but about one-tenth per cent of strong acid does
+well.</p>
+<p>If the preliminary processes have been properly carried out
+the aluminium will become coated with copper, and the process is
+accompanied by the disengagement of gas. It appears to be a rule
+that if gas is not given off, the film of copper deposited is
+non-adherent. The work must be left in the copper sulphate
+solution till it has received a uniform coating of copper.</p>
+<p>(5) When this is the case the work is removed &mdash; well
+washed so as to get rid of the rest of the aluminium chloride,
+and then electroplated by the battery in the ordinary copper
+sulphate bath.</p>
+<p>If the operation (4) does not appear to give a uniform coat,
+or if gas is not evolved from every part of the aluminium
+surface, I find that operations (2) and (3) may be repeated
+without danger, provided that the dip in the hydrochloric acid
+is shortened to two or three seconds.</p>
+<p>The copper layer obtained by Margot's method is perfectly
+adherent &mdash; even when used as a base for ordinary solder
+&mdash; though in this case it can be stripped if sufficient
+force is applied.</p>
+<p>Since the solder recommended by M. Margot for aluminium
+contains zinc, it does not run well when used to unite aluminium
+to copper, brass, iron, etc. In this case, therefore, I have
+found the most advantageous method of soldering to be by way of a
+preliminary copper-plating.</p>
+<p>The success of M. Margot's method depends in my experience on
+obtaining just the proper amount of aluminium chloride in contact
+with the aluminium when the latter is immersed in the copper
+sulphate solution.</p>
+<p>&sect; 139. The process of copper-plating from sulphate or
+nitrate may, according to Mr. Swan (Journal of the Royal
+Institution, 1892, p. 630), be considerably accelerated by the
+addition of a trace of gelatine to the solution. As success
+appears to depend upon hitting the exact percentage amount of the
+gelatine, which must in any case be but a fraction of one per
+cent, and as Mr. Swan refrains from stating what the amount is, I
+am unable to give more precise instructions. A few experiments
+made on the subject failed, doubtless through the gelatine
+content not having been rightly adjusted. Mr. Swan claims to be
+able to get a hard deposit of copper with a current density of
+1000 amperes per square foot, but seems to recommend about
+one-tenth of that amount for general use.</p>
+<p>The solution employed is a mixture of nitrate of copper and
+ammonium chloride &mdash; proportions not stated. Electrolytic
+copper, as generally prepared, is very pure, but this is a mere
+accident depending on the impurities which, as a rule, have to be
+got rid of. Electrolysis seems to have no effect in purifying
+from arsenic, for instance.</p>
+<p>Roughly speaking, about 11 grms. of copper are deposited per
+ampere hour from cupric salt solutions. When the current density
+is too high the anode suffers by oxidation, and this introduces a
+large and very variable resistance into the circuit.</p>
+<p><b><a name="Toc158108992" id="Toc158108992"><font face=
+"Bookman Old Style" size="4">&sect; 140. Alkaline Coppering
+Solution &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Coppering Base Metals. &mdash;
+It is often desirable to coat lead, zinc, pewter, iron, etc.,
+with a firm and uniform layer of copper preparatory to gilding or
+silvering. If copper or brass articles are soldered with soft
+solder it is found that the solder does not become silvered or
+gilt along with the rest of the material, but remains uncoated
+and of an ugly dark colour. This defect is got over by giving a
+preliminary coating of copper.</font></p>
+<p>This is done in an alkaline solution, generally containing
+cyanogen and ammonia. The following method has succeeded
+remarkably well with me. The receipt was taken originally from
+Gore's <i>Electro-metallurgy</i>, p. 208. A solution is made of
+50 grms. of potassium cyanide (ordinary commercial, say, 75 per
+cent) and 30 grms. of sodium bisulphite in I.5 litres of water.
+Thirty-five grammes of cupric acetate are dissolved in a litre of
+water, and 20 cubic centimetres of the strongest liquid ammonia
+are added. The precipitate formed must be more or less dissolved
+to a strong blue solution. The cyanide and bisulphite solution is
+then added with warming till the blue colour is destroyed. This
+usually requires the exact amount of cyanide and bisulphite
+mentioned, but I have not found it essential to entirely destroy
+the colour.</p>
+<p>The solution contains cuprocyanide of sodium and ammonium (?),
+which is not very soluble, and this salt tends to be deposited in
+granular crystalline masses on standing. However, at a
+temperature of 50&deg; C. the above receipt gives an excellent
+coppering liquid, which will coat zinc with a fine reguline
+deposit. Brass or copper partly smeared with solder will receive
+a deposit of copper on the latter as well as on the former, and,
+moreover, a deposit which appears to be perfectly uniform.</p>
+<p>In using the bath the anode tends, as a rule, to become
+incrusted, and this rapidly increases the resistance of the cell,
+so that the current falls off quickly. The articles should be
+scratch-brushed and plated for about two minutes with a current
+density of about ten amp&egrave;res per square foot.</p>
+<p>As soon as the deposit begins to look red the articles are to
+be removed and rebrushed, after which the process may be
+continued. About five minutes' plating will give a copper deposit
+quite thick enough after scratch-brushing to allow of a very even
+gilding or silvering.</p>
+<p>Aluminium appears to be fairly coated, but, as usual, the
+copper strips after soldering. Iron receives an excellent and
+adherent coat.</p>
+<p>I do not think that the formation of a crust upon the anode
+can be entirely prevented. According to Gore, its formation is
+due to the solution being too poor in copper, but I have added a
+solution of the acetate of copper and ammonium till the colour
+was bright blue without in any way reducing the incrustation. If
+the solutions become violently blue it is perhaps as well to add
+a little more cyanide and bisulphite, but I have not found such
+an addition necessary. The process is one of the easiest and most
+satisfactory in electro-metallurgy.</p>
+<p><b><a name="Toc158108993" id="Toc158108993"><font face=
+"Bookman Old Style" size="4">&sect; 141.
+Nickel-plating.&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">An examination of several
+American samples of nickel-plated goods has disclosed that the
+coating of nickel is, as a rule, exceedingly thin. This is what
+one would expect from laboratory repetition of the processes
+employed.</font></p>
+<p>Commercial practice in the matter of the composition of
+nickelling solutions appears to vary a good deal. Thin coatings
+of nickel may be readily given in a solution of the double
+sulphate of nickel and ammonia, which does rather better if
+slightly alkaline. Deposits from this solution, however, become
+gray if of any thickness, and, moreover, are-apt to flake off the
+work. The following solution has given very good results with me.
+It is mentioned, together with others, in the <i>Electrical
+Review</i>, 7th June 1895.</p>
+<p>The ingredients are:-</p>
+<div style="margin-left: 4em">
+<p>Nickel sulphate 5 parts</p>
+<p>Ammonia sufficient to neutralise the nickel salt.</p>
+<p>Ammonium tartrate 3.75 parts</p>
+<p>Tannin 0.025 parts</p>
+<p>Water 100 parts</p>
+</div>
+<p>The nickel sulphate and ammonia are dissolved in half the
+water, the ammonium tartrate in the other half with the tannin.
+The solutions are mixed and filtered at about 40<a name=
+"lastbookmark2" id="lastbookmark2"></a>&deg; C. This solution
+works well at ordinary temperatures, or slightly warm, with a
+current density of ten amp&egrave;res per square foot. In an
+experiment made for the purpose I found that plating may go on
+for an hour in this solution before the deposit begins to show
+signs of flaking off. The deposit is of a fine white colour.</p>
+<p>The resistance of the bath is rather high and rather variable,
+consequently it is as well to have a current indicator in
+circuit, and it may well happen that five or six volts will be
+found requisite to get the current up to the value stated. For
+nickelling small objects of brass, such as binding screws, etc.,
+it is very necessary to be careful as to the state of polish and
+uniformity of their surfaces before placing them in the plating
+bath. A polished surface will appear when coated as a polished
+surface, and a mat surface as a mat surface; moreover, any local
+irregularity, such as a speck of a foreign metal, will give rise
+to an ugly spot in the nickelling bath. For this reason it is
+often advisable to commence with a coat of copper laid on in an
+alkaline solution and scratch-brushed to absolute uniformity.</p>
+<p>An examination of the work will, however, disclose whether
+such a course is desirable or not; it is not done in American
+practice, at all events for small brass objects. These are
+cleaned in alkali and in boiling cyanide, which does not render a
+polished surface mat, as weak acid is apt to do, and are then
+coated with a current density of about ten amp&egrave;res per
+square foot.</p>
+<p>In spite of what is to be found in books as to the ease with
+which nickel deposits may be polished, I find that the mat
+surface obtained by plating on an imperfectly polished cathode of
+iron is by no means easily polished either by fine emery,
+tripoli, or rouge. Consequently, as in the case of brass, if a
+polished surface is desired, it must be first prepared on the
+unplated cathode. In this case, even if the deposit appears dull,
+but not gray, it may be easily polished by tripoli and water,
+using a cork as the polisher. Scratch-brushing with brass wire,
+however, though possibly not with German silver wire, brightens
+the deposit, but discolours it. When the deposit becomes gray I
+have not succeeded in polishing it satisfactorily.</p>
+<p>Soldered brass or iron may be satisfactorily coated with
+nickel by giving it a preliminary coating of copper in the
+cyanide bath. On the whole, I recommend in general that iron be
+first coated with copper in the alkaline bath, scratch-brushed,
+and then nickel-plated, and this whether the iron appears to be
+uniform or not. Much smoother, thicker, and stronger coats of
+nickel are obtained upon the copper-plated surface than on the
+iron one, and the coating does not become discoloured (? by iron
+rust) in the same way that a coating on bare iron does. The
+copper surface may be plated for at least an hour at a density of
+ten amp&egrave;res per square foot without scaling.</p>
+<p>Scales or circles divided on brass may be greatly improved in
+durability by nickel-plating. For this purpose the brass must
+be highly polished and divided before it is nickelled.</p>
+<p>The plating should be continued for a few minutes only, when a
+very bright but thin coat of nickel will be deposited; it then
+only remains to wash and dry the work, and this must be done at
+once. If the nickel is deposited before the scale or circle is
+engraved, very fine and legible divisions are obtained, but there
+is a risk that flakes of nickel may become detached here and
+there in the process of engraving.</p>
+<p><b><a name="Toc158108994" id="Toc158108994"><font face=
+"Bookman Old Style" size="4">142. Miscellaneous Notes on
+Electroplating.</font></a></b></p>
+<p><font face="Bookman Old Style">Occasionally it is desirable to
+make a metallic mould or other object of complex shape. The
+quickest way to do this is to carve the object out of hard
+paraffin, and then copy it by electrotyping. Electrotype moulds
+can be made in many ways. The easiest way perhaps is to take a
+casting in plaster of Paris, or by means of pressure in warm
+gutta-percha.</font></p>
+<p>In cases where the mould will not draw, recourse must be had
+to the devices of iron-founders, i.e. the plaster cast must be
+made in suitable pieces, and these afterwards fitted together.
+This process can occasionally be replaced by another in which the
+moulding material is a mixture of treacle and glue. The glue is
+soaked in cold water till it is completely soft. The superfluous
+water thrown away, one-fourth part by volume of thick treacle is
+added, and the mixture is melted on the water bath; during which
+process stirring has to be resorted to, to produce a uniform
+mixture.</p>
+<p>This liquid forms the moulding mixture, and it is allowed to
+flow round the object to be copied, contained in a suitable box,
+whose sides have been slightly oiled. The object to be copied
+should also be oiled. After some hours, when the glue mixture has
+set, it will be found to be highly elastic, so that it may be
+pulled away from the mould, and afterwards resume very nearly its
+original form.</p>
+<p>One drawback to the use of these moulds lies in the fact that
+the gelatine will rarely stand the plating solution without
+undergoing change, but this may be partially obviated by dipping
+it for a few seconds in a 10 per cent solution of bichromate of
+potash, exposing it to the sunlight for a few minutes, and then
+rinsing it.</p>
+<p>In order to render the surface conducting, it is washed over
+with a solution of a gold or silver salt, and the latter reduced
+in situ to metal by a suitable reagent. A solution of phosphorus
+is the most usual one (see Gore, Electro-metallurgy, p. 216).
+Such a mould may be copper-plated in the sulphate bath,
+connection being made by wires suitably thrust into the
+material.</p>
+<p>Plaster of Paris moulds require to be dried and waxed by
+standing on a hot plate in melted wax before they are immersed in
+the plating bath. In this case the surface is best made
+conducting either by silvering it by the silvering process used
+for mirrors, or by brushing it over with good black lead rendered
+more conducting by moistening with an ethereal solution of
+chloride of gold and then drying in the sun.</p>
+<p>The brushing requires a stiff camel's-hair pencil of large
+size cut so that the hairs project to a distance of about a
+quarter of an inch from the holder. The brushing must continue
+till the surface is bright, and is often a lengthy process.</p>
+<p>The same process of blackleading may be employed to get a coat
+of deposited metal which will strip easily from the cathode.</p>
+<p>In all cases where extensive deposits of copper are required,
+the growth takes place too rapidly at the corners. Consequently
+it is often desirable to localise the action of the deposit. A
+"stopping" of ordinary copal varnish seems to be the usual thing,
+but a thin coat of wax or paraffin or photographic (black)
+varnish does practically as well.</p>
+<p>I do not propose to deal with the subject of electrotyping to
+any extent, for if practised as an art, a good many little
+precautions are required, as the student may read in Gore's
+Electro-metallurgy. The above instructions will be found
+sufficient for the occasional use of the process in the
+construction of apparatus, etc. There is no advantage in
+attempting to hurry the process, a current density of about ten
+amp&egrave;res per square foot being quite suitable and
+sufficiently low to ensure a solid deposit.</p>
+<p><b><a name="Toc158108995" id="Toc158108995"><font face=
+"Bookman Old Style" size="4">&sect; 143. Blacking Brass Surfaces.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">A really uniform dead-black
+surface is difficult to produce on brass by chemical means. A
+paste of nitrate of copper and nitrate of silver heated on the
+brass is said to give a dead-black surface, but I have not
+succeeded in making it act uniformly. For optical purposes the
+best plan is to use a paint made up of "drop" black, ground very
+fine with a little shellac varnish, and diluted for use with
+alcohol. No more varnish than is necessary to cause the black to
+hold together should be employed.</font></p>
+<p>In general, if the paint be ground to the consistency of very
+thick cream with ordinary shellac varnish it will be found to
+work well when reduced by alcohol to a free painting
+consistency.</p>
+<p>A very fine gray and black finish, with a rather metallic
+lustre, may be easily given to brass work. For this purpose a
+dilute solution of platinum tetrachloride (not stronger than 1
+per cent) is prepared by dissolving the salt in distilled water.
+The polished brass work is cleaned by rubbing with a cork and
+strong potash till all grease has disappeared, as shown by water
+standing uniformly on the metal and draining away without
+gathering into drops.</p>
+<p>After copious washing the work is wholly immersed in a
+considerable volume of the platinum tetrachloride solution at the
+ordinary temperature. After about a quarter of an hour the brass
+may be taken out and washed. The surface will be found to be
+nicely and uniformly coated if the above instructions have been
+carried out, but any finger-marks or otherwise dirty places will
+cause irregularity of deposit. If the process has been successful
+it will be found that the deposit adheres perfectly, hardly any
+of it being removed by vigorous rubbing with a cloth. If the
+deposit is allowed to thicken &mdash; either by leaving the
+articles in the solution too long or heating the solution, or
+having it too strong &mdash; it will merely rub off and leave an
+irregular surface.</p>
+<p>This process succeeds well with yellow brass and Muntz metal,
+either cast or rolled, but it does not give quite such uniform
+(though still good) results with gun-metal, on which, however,
+the deposit is darker and deader in appearance.</p>
+<p>A book might be written (several have been written) on the art
+of metal colouring, but though doubtless a beautiful and delicate
+art, it is of little service in the laboratory. For further
+information the reader may consult a work by Hiorns.</p>
+<p><b><a name="Toc158108996" id="Toc158108996"><font face=
+"Bookman Old Style" size="4">&sect; 144. <i>Sieves</i>.
+&mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">Properly graded sieves with
+meshes of a reliable size are often of great use. They should be
+made out of proper "bolting" cloth, a beautiful material made for
+flour-millers. Messrs. Henry Simon and Company of Manchester have
+kindly furnished me with the following table of materials used in
+flour-milling.</font></p>
+<p>Sieves made of these materials will be found to work much more
+quickly and satisfactorily than those made from ordinary muslin
+or wire gauze.</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"568">
+<tr>
+<td valign="top" colspan="4">
+<p align="center"><b><font face="Bookman Old Style">Relative
+Bolting Value of Silk, Wire, and Grit Gauze</font></b></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="center"><font face="Bookman Old Style" size="2">Threads
+per inch Approximate.</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="center"><font face="Bookman Old Style" size="2">Trade
+No. of Silk.</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="center"><font face="Bookman Old Style" size="2">Trade
+No. of Wire.</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="center"><font face="Bookman Old Style" size="2">Trade
+No. of Grit Gauze.</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">18</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">0000</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">18</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">16</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">22</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">000</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">20</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">20</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">28</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">00</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">26</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">26</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">38</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">0</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">32</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">34</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">48</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">1</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">40</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">44</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">52</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">2</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">45</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">50</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">56</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">3</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">50</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">54</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">60</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">4</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">56</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">58</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">64</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">5</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">60</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">60</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">72</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">6</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">64</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">66</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">80</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">7</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">70</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">70</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">84</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">8</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">80</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">80</font></p>
+</td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">94</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">9</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">106</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">10</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">114</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">11</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">124</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">12</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">130</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">13</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">139</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">14</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">148</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">15</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">156</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">16</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">163</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">17</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">167</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">18</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">170</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">19</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+<tr>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">173</font></p>
+</td>
+<td width="25%" valign="top">
+<p align="right"><font face="Bookman Old Style">20</font></p>
+</td>
+<td width="25%" valign="top"></td>
+<td width="25%" valign="top"></td>
+</tr>
+</table>
+<p><b><a name="Toc158108997" id="Toc158108997"><font face=
+"Bookman Old Style" size="4">&sect; 145. Pottery making in the
+Laboratory. &mdash;</font></a></b></p>
+<p><font face="Bookman Old Style">When large pieces of
+earthenware of any special design are required, recourse must be
+had to a pottery. Small vessels, plates, parts of machines, etc.,
+can often be made in the laboratory in less time than it would
+take to explain to the potter what is required. For this purpose
+any good pipeclay may be employed. I have used a white pipe-clay
+dug up in the laboratory garden with complete success.</font></p>
+<p>The clay should be kneaded with water and squeezed through a
+cloth to separate grit. It is then mixed with its own volume or
+thereabouts of powdered porcelain evaporating basins, broken
+basins being kept for this purpose. The smoothness of the
+resulting earthenware will depend on the fineness to which the
+porcelain fragments have been reduced. I have found that
+fragments passing a sieve of sixty threads to the inch run, do
+very well, though the resulting earthenware is decidedly
+rough.</p>
+<p>The porcelain and clay being thoroughly incorporated by
+kneading, the articles are moulded, it being borne in mind that
+they will contract somewhat on firing. <i>[Footnote:</i> The
+contraction depends on the temperature attained as well as on the
+time. An allowance of one part in twelve will be suitable in the
+case considered.<i>]</i> The clay should be as stiff as is
+convenient to work, and after moulding must be allowed to get
+thoroughly dry by standing in an airy place; the drying must not
+be forced, especially at first, or the clay will crack.</p>
+<p>Small articles are readily fired in a Fletcher's crucible
+furnace supplied with a gas blow-pipe; the furnace is heated
+gradually to begin with. When a dull red heat is attained, the
+full power of the blast may be turned on, and the furnace kept at
+its maximum temperature for three or four hours at least, though
+on an emergency shorter periods may be made to do.</p>
+<p>The articles are supported on a bed of white sand; after
+firing, the crucible furnace must be allowed to cool slowly. It
+must be remembered that the furnace walls will get hot externally
+after the first few hours, consequently the furnace must be
+supported on bricks, to protect the bench.</p>
+<p>The pottery when cold may be dressed on a grindstone if
+necessary. This amateur pottery will be found of service in
+making small fittings for switch-boards, commutators, and in
+electrical work generally.</p>
+<p>Pottery made as described is very hard and strong, the
+hardness and strength depending in a great degree on the
+proportion of powdered porcelain added to the clay, as well, of
+course, as on the quality of both of these materials.</p>
+<p>It is a good plan to knead a considerable quantity of the
+mixture, which may then be placed in a well-covered jar, and kept
+damp by the addition of a little water.</p>
+<p>Pottery thus made does not require to be glazed, but, of
+course, a glaze can be obtained by any of the methods described
+in works on pottery manufacture. The following glaze has been
+recommended to me by a very competent potter:&mdash;</p>
+<table border summary="" cellspacing="1" cellpadding="7" width=
+"375">
+<tr>
+<td width="38%" valign="top">
+<p><font face="Bookman Old Style">Litharge</font></p>
+</td>
+<td width="62%" valign="top">
+<p align="right"><font face="Bookman Old Style">7 parts by
+weight</font></p>
+</td>
+</tr>
+<tr>
+<td width="38%" valign="top">
+<p><font face="Bookman Old Style">Ground flint</font></p>
+</td>
+<td width="62%" valign="top">
+<p align="right"><font face="Bookman Old Style">2 parts by
+weight</font></p>
+</td>
+</tr>
+<tr>
+<td width="38%" valign="top">
+<p><font face="Bookman Old Style">Cornish stone or
+felspar</font></p>
+</td>
+<td width="62%" valign="top">
+<p align="right"><font face="Bookman Old Style">1 parts by
+weight</font></p>
+</td>
+</tr>
+</table>
+<p><font face="Bookman Old Style">These ingredients are to be
+ground up till they will pass the finest sieve &mdash; say 180
+threads to the inch. They are then mixed with water till they
+form a paste of the consistency of cream. They must, of course,
+be mixed together perfectly. The ware to be glazed is dipped into
+the cream after the first firing; it is then dried as before and
+refired. The glaze will melt at a bright red heat, but it will
+crack if not fired harder; the harder it is fired the less likely
+is it to crack.</font></p>
+<p>If colouring matters are added they must be ground in a mill
+free from iron till they are so fine that a thick blanket filter
+will not filter them when suspended in water. This remark applies
+particularly to oxide of cobalt.</p>
+<p><b><a name="Toc158108998" id="Toc158108998"><font face=
+"Bookman Old Style" size="4">APPENDIX</font></a></b></p>
+<p><a name="Toc158108999" id="Toc158108999">PLATINISING
+GLASS</a></p>
+<p><font face="Bookman Old Style">IN the <i>Philosophical
+Magazine</i> for July 1888 (vol. xxvi. p. 1) there is a paper by
+Professor Kundt translated from the <i>Sitzungsberichte</i> of
+the Prussian Academy. This paper deals with the indices of
+refraction of metals. Thin prisms were obtained by depositing
+metals electrolytically on glass surfaces coated with platinum.
+The preparation of these surfaces is troublesome. Kundt recounts
+that no less than two thousand trials were made before success
+was attained. A detailed account of the preparation of these
+surfaces is not given by Kundt, but one is promised &mdash; a
+promise unfortunately unfulfilled so far as I am able to
+discover. A hunt through the literature led to the discovery of
+the following references: <i>Central Zeitung fuer Optik und
+Mechanik</i>, p. 142 (1888); Dingler's <i>Polytechnik
+Journal</i>, Vol. cxcv. p. 464; <i>Comptes Rendus</i>, vol. lxx.
+(1870).</font></p>
+<p>The original communication is a paper by Jouglet in the
+Comptes Rendus, of which the other references are abstracts. The
+account in Dingier is a literal translation of the original
+paper, and the note in the <i>Central Zeitung</i> is abbreviated
+sufficiently to be of no value. The details are briefly as
+follows:-</p>
+<p>One hundred grams of platinum are dissolved in aqua regia and
+the solution is dried on the sand bath, without, however,
+producing decomposition. Though the instructions are not
+definite, I presume that the formation of PtCl<sub>4</sub> is
+contemplated.</p>
+<p>The dried salt is added little by little to rectified oil of
+lavender, placed on a glass paint-grinding plate, and the salt
+and oil are ground together with a muller. Care is required to
+prevent any appreciable rise of temperature which would decompose
+the compound aimed at, and it is for this reason that the salt is
+to be added gradually. Of course the absorption of water from the
+air must be prevented from taking place as far as possible.
+Finally, the compound is diluted by adding oil of lavender up to
+a total weight of 1400 grams (of oil).</p>
+<p>The liquid is poured into a porcelain dish and left absolutely
+at rest for eight days. It is then decanted and filtered, left
+six days at rest, and again decanted (if necessary). The liquid
+should have a specific gravity of 5&deg; on the acid hydrometer.
+(If by this the Baum&eacute; scale is intended, the corresponding
+specific gravity would be 1.037.) A second liquid is prepared by
+grinding up 25 grams of litharge with 25 grams of borate of lead
+and 8 to 10 grams of oil of lavender. The grinding must be
+thoroughly carried out.</p>
+<p>This liquid is to be added to the one first described, and the
+whole well mixed. The resulting fluid constitutes the platinising
+liquid, and is applied as follows:-</p>
+<p>A sheet of clean glass is held vertically, and the liquid is
+painted over it, carrying the brush from the lower to the upper
+edge. The layer of oil dries slowly, and when it is dry the
+painting is again proceeded with, moving the brush this time from
+right to left; and similarly the process is repeated twice, the
+brush being carried from top to bottom and left to right. This is
+with the object of securing great uniformity in the coating.
+Nothing is said as to the manner in which the glass is to be
+dried.</p>
+<p>The dried glass is finally heated to a temperature of dull
+redness in a muffle furnace. The resinous layer burns away
+without running or bubbling, and leaves a dull metallic surface.
+As the temperature rises this suddenly brightens, and we obtain
+the desired surface (which probably consists of an alloy of lead
+and platinum). It is bright only on the surface away from the
+glass.</p>
+<p>I have not had an opportunity of trying this process since I
+discovered the detailed account given by Jouglet; but many
+modifications have been tried in the laboratory of the Sydney
+University by Mr. Pollock, starting from the imperfect note in
+the Central Zeitung, which led to no real success.</p>
+<p>It was found that it is perfectly easy to obtain brilliant
+films of platinum by the following process, provided that the
+presence of a few pin-holes does not matter.</p>
+<p>The platinum salt employed is what is bought under the name of
+platinic chloride; it is, however, probably a mixture of this
+salt and hydro-chloro-platinic acid, and has all the appearance
+of having been obtained by evaporating a solution of platinum in
+aqua regia to dryness on the water bath. A solution of this salt
+in distilled water is prepared; the strength does not seem to
+matter very much, but perhaps one of salt to ninety-nine water
+may be regarded as a standard proportion. To this solution is
+added a few drops of ordinary gum water (i.e. a solution of
+dextrin). The exact quantity does not matter, but perhaps about 2
+per cent may be mentioned as giving good results.</p>
+<p>The glass is painted over with this solution and dried slowly
+on the water bath. When the glass is dry, and covered with a
+uniform hard film of gum and platinum salt free from bubble
+holes, it is heated to redness in a muffle furnace. The necessary
+and sufficient temperature is reached as soon as the glass is
+just sensibly red-hot.</p>
+<p>The mirrors obtained in this way are very brilliant on the
+free platinum surface. If the gum be omitted, the platinum will
+have a mat surface; and if too much gum be used, the platinum
+will get spotty by bubbles bursting. There does not appear to be
+any advantage in using lead.</p>
+<p>It is quite essential that the film be dry and hard before the
+glass is fired.</p>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
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+The Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: On Laboratory Arts
+
+Author: Richard Threlfall
+
+Release Date: September 27, 2007 [EBook #22784]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ON LABORATORY ARTS ***
+
+
+
+
+Produced by Jon Richfield
+
+
+
+
+
+
+ON LABORATORY ARTS
+
+
+
+
+
+BY
+
+
+RICHARD THRELFALL, M.A.
+
+
+PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;
+
+MEMBER OF THE INSTITUTE OF ELECTRICAL ENGINEERS;
+
+ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL ENGINEERS;
+
+MEMBER OF THE PHYSICAL SOCIETY
+
+
+
+
+London
+
+
+MACMILLAN AND CO, LIMITED
+
+NEW YORK: THE MACMILLAN COMPANY
+
+
+1898
+
+
+All rights reserved
+
+
+
+
+PREFACE    5
+
+CHAPTER I    8
+
+HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
+PURPOSES    8
+
+Sec. 4.  Soft Soda Glass.
+
+Sec. 6.  Flint Glass.
+
+Sec. 9.  Hard or Bohemian, Glass.
+
+Sec. 10.  On the Choice of Sizes of Glass Tube.
+
+Sec. 11.  Testing Glass.
+
+Sec. 13.  Cleaning Glass Tubes.
+
+Sec. 14.  The Blow-pipe.
+
+Sec. 18.  The Table.
+
+Sec. 19.  Special Operations.
+
+Sec. 20.  Closing and blowing out the End of a Tube.
+
+Sec. 21.  To make a Weld.
+
+Sec. 22.  To weld two Tubes of different Sizes.
+
+Sec. 24.  To weld Tubes of very small Bore.
+
+Sec. 30.  To cut very thick Tubes.
+
+Sec. 31.  To blow a Bulb at the End of a Tube.
+
+Sec. 32.  To blow a bulb in the middle of a tube.
+
+Sec. 33.  To make a side Weld.
+
+Sec. 34.  Inserted Joints.
+
+Sec. 35.  Bending Tubes.
+
+Sec. 36.  Spiral Tubes.
+
+Sec. 37.  On Auxiliary Operations on Glass.
+
+Sec. 38.  Boring small Holes.
+
+Sec. 39.  For boring large holes through thick glass sheets.
+
+Sec. 41.  Operations depending on Grinding: Ground-in Joints.
+
+Sec. 42.  Use of the Lathe in Glass-working.
+
+Sec. 46.  Making Ground Glass.
+
+Sec. 47.  Glass-cutting.
+
+Sec. 48.  Cementing.
+
+Sec. 49.  Fusing Electrodes into Glass.
+
+Sec. 51.  The Art of making Air-light Joints.
+
+APPENDIX TO CHAPTER I.
+
+ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
+ROENTGEN'S RADIATION.
+
+CHAPTER II.
+
+GLASS-GRINDING AND OPTICIANS' WORK.
+
+Sec. 61.  Details of the Process of Fine Grinding.
+
+Sec. 62.  Polishing.
+
+Sec. 63.  Centering.
+
+Sec. 65.  Preparing Small Mirrors for Galvanometers.
+
+Sec. 66.  Preparation of Large Mirrors or Lenses for Telescopes.
+
+Sec. 69.  The Preparation of Flat Surfaces of Rock Salt.
+
+Sec. 70.  Casting Specula for Mirrors.
+
+Sec. 71.  Grinding and polishing Specula.
+
+Sec. 72.  Preparation of Flat Surfaces.
+
+Sec. 73.  Polishing Flat Surfaces on Glass or on Speculum Metal.
+
+CHAPTER III.
+
+MISCELLANEOUS PROCESSES.
+
+Sec. 74.  Coating Glass with Aluminium and Soldering Aluminium.
+
+Sec. 75.  The Use of the Diamond-cutting Wheel.
+
+Sec. 76.  Arming a Wheel.
+
+Sec. 77.  Cutting a Section.
+
+Sec. 78.  Grinding Rock Sections, or Thin Slips of any Hard Material.
+
+Sec. 79.  Cutting Sections of Soft Substances.
+
+Sec. 80.  On the Production of Quartz Threads.
+
+Sec. 84.  Drawing Quartz Threads.
+
+Sec. 86.  Drawing Threads by the Catapult.
+
+Sec. 87.  Drawing Threads by the Flame alone.
+
+Sec. 88.  Properties of Threads.
+
+Sec. 90.  On the Attachment of Quartz Fibres.
+
+Sec. 91.  Other Modes of soldering Quartz.
+
+Sec. 92.  Soldering.
+
+Sec. 94.  Preparing a Soldering Bit.
+
+Sec. 95.  Soft Soldering.
+
+Sec. 97.  Soldering Zinc.
+
+Sec. 98.  Soldering other Metals.
+
+Sec. 99.  Brazing.
+
+Sec. 100.  Silver Soldering.
+
+Sec. 101.  On the Construction of Electrical Apparatus--Insulators.
+
+Sec. 102.  Sulphur.
+
+Sec. 103.  Fused Quartz.
+
+Sec. 104.  Glass.
+
+Sec. 105.  Ebonite or Hard Rubber.
+
+Sec. 106.  Mica.
+
+Sec. 107.  Use of Mica in Condensers.
+
+Sec. 108.  Micanite.
+
+Sec. 109.  Celluloid.
+
+Sec. 110.  Paper.
+
+Sec. 111.  Paraffined Paper.
+
+Sec. 112.  Paraffin.
+
+Sec. 113.  Vaseline, Vaseline Oil, and Kerosene Oil.
+
+Sec. 114.  Imperfect Conductors.
+
+Sec. 116.  Conductors.
+
+Sec. 117.  Platinoid.
+
+Sec. 119.  Platinum Silver.
+
+Sec. 120.  Platinum Iridium.
+
+Sec. 121.  Manganin.
+
+Sec. 122.  Other Alloys.
+
+Sec. 123.  Nickelin.
+
+Sec. 124.  Patent Nickel.
+
+Sec. 125.  Constantin.
+
+126.  Nickel Manganese Copper.
+
+CHAPTER IV.
+
+ELECTROPLATING AND ALLIED ARTS.
+
+Sec. 127.  Electroplating.
+
+Sec. 128.  The Dipping Bath.
+
+Sec. 130.  Scratch-brushing.
+
+Sec. 131. Burnishing.
+
+Sec. 132.  Silver-plating.
+
+Sec. 133.  Cold Silvering.
+
+Sec. 134.  Gilding.
+
+Sec. 135.  Preparing Surfaces for Gilding.
+
+Sec. 136.  Gilding Solutions.
+
+Sec. 137.  Plating with Copper.
+
+Sec. 138.  Coppering Aluminium.
+
+Sec. 140.  Alkaline Coppering Solution.
+
+Sec. 141.  Nickel-plating.
+
+142.  Miscellaneous Notes on Electroplating.
+
+Sec. 143.  Blacking Brass Surfaces.
+
+Sec. 144.  Sieves.
+
+Sec. 145.  Pottery making in the Laboratory.
+
+APPENDIX.
+
+PLATINISING GLASS.
+
+PREFACE
+
+EXPERIMENTAL work in physical science rests ultimately upon the
+mechanical arts.  It is true that in a well-appointed laboratory,
+where apparatus is collected together in greater or less profusion,
+the appeal is often very indirect, and to a student carrying out a set
+experiment with apparatus provided to his hand, the temptation to
+ignore the mechanical basis of his work is often irresistible.
+
+It often happens that young physicists are to be found whose
+mathematical attainments are adequate, whose observational powers are
+perfectly trained, and whose general capacity is unquestioned, but who
+are quite unable to design or construct the simplest apparatus with
+due regard to the facility with which it ought to be constructed.
+That ultimate knowledge of materials and of processes which by long
+experience becomes intuitive in the mind of a great inventor of course
+cannot be acquired from books or from any set course of instruction.
+
+There are, however, many steps between absolute ignorance and
+consummate knowledge of the mechanical arts, and it is the object of
+the following pages to assist the young physicist in making his first
+steps towards acquiring a working knowledge of "laboratory arts."
+However humble the ambition may be, no one can be more keenly alive
+than the writer to the inadequacy of his attempt; and it is only from
+a profound sense of the necessity which exists for some  beginning to
+be made, that he has had the courage to air his views on matters about
+which there are probably hundreds or thousands of people whose
+knowledge is superior to his own.
+
+Moreover, nothing has been further from the writer's mind than any
+idea of "instructing" any one;  his desire is--if happily it may so
+befall--to be of assistance, especially to young physicists or
+inventors who wish to attain definite mechanical ends with the minimum
+expenditure of time.  Most people will agree that one condition
+essential to success in such an undertaking is brevity, and it is for
+this reason that alternative methods as a rule have not been given,
+which, of course, deprives the book of any pretence to being a
+"treatise."  The writer, therefore, is responsible for exercising a
+certain amount of discretion in the selection he has made, and it is
+hardly to be hoped that he has in all--or even in the majority of
+cases--succeeded in recommending absolutely the best method of
+procedure.
+
+This brings another point into view.  Before all things the means
+indicated must be definite and reliable.  It is for this reason that
+the writer has practically confined himself to matters lying within
+his own immediate experience, and has never recommended any process
+(with one or two minor exceptions, which he has noted) which he has
+not actually and personally carried through to a successful issue.
+This, although it is a matter which he considers of the highest
+importance, and which is his only title to a hearing, has
+unfortunately led to a very personal tone in the book.
+
+With regard to the arts treated of in the following pages, matters
+about which information is easily acquired--such as carpentering,
+blacksmithing, turning, and the arts of the watchmaker--have been
+left on one side.  With regard to the last, which is of immense use in
+the laboratory, there happen to be at least two excellent and handy
+books, viz.  Saunier's Watchmakers' Handbook, Tripplin, London, 1892;
+and Britton's Watchmakers' Dictionary and Guide.
+
+With regard to carpentering, turning, and blacksmithing, almost any
+one who so desires can obtain a little practical experience in any
+village.  A short chapter has been devoted to glass-blowing, in spite
+of there being an excellent and handy book by Mr. Shenstone (The
+Methods of Glass-blowing, Rivington) on the subject already in
+existence.  The reason for this exception lies in the fact that the
+writer's methods differ considerably from those advocated by Mr.
+Shenstone.
+
+The chapter on opticians' work has had to be compressed to an extent
+which is undesirable in dealing with so complex and delicate an art,
+but it is hoped that it will prove a sufficient introduction for
+laboratory purposes.  In this matter the writer is under great
+obligations to his friend and assistant, Mr. James Cook, F.R.A.S, who
+gave him his first lessons in lens-making some twenty years ago.  To
+Mr. John A.  Brashear of Allegheny, Pa, thanks are due for much
+miscellaneous information on optical work, which is included verbatim
+in the text, some of it contained originally in printed papers, and
+some most kindly communicated to the writer for the purpose of this
+book.  In particular, the writer would thank Mr. Brashear for his
+generously accorded information as to the production of those "flat"
+surfaces for which he is so justly famous.
+
+The writer is also indebted to Mr. A.  E.  Kennelly for some
+information as to American practice in the use of insulating material
+for electrical work, and to his friends Mr. J.  A.  Pollock and Dr. C.
+J.  Martin for many valuable suggestions.  For the illustrations
+thanks are due to Mrs.  Threlfall and Mr. James Cook.  With regard to
+matters which have come to the writer's knowledge by his being
+specifically instructed in them from time to time, due acknowledgment
+is, it is hoped, made in the text.
+
+With regard to the question as to what matters might be included and
+what omitted, the general rule has been to include information which
+the author has obtained with difficulty, and to leave on one side that
+which he has more easily attained.  All the "unities" have been
+consistently outraged by a deliberate use of the English and metric
+systems side by side.  So long as all the materials for mechanical
+processes have to be purchased to specifications in inches and feet,
+it is impossible to use the centimetre consistently without
+introducing inconvenience.  However, everybody ought to, and probably
+does, use either system with equal facility.
+
+No attempt has been made at showing how work can be done without
+tools.  Though, no doubt, a great deal can be done with inferior
+appliances where great economy of money and none of time is an object,
+the writer has long felt very strongly that English physical
+laboratory practice has gone too far in the direction of starving the
+workshop, and he does not wish, even indirectly, 'to give any
+countenance to such a mistaken policy.  Physical research is too
+difficult in itself, and students' time is too valuable, for it to be
+remunerative to work with insufficient appliances.
+
+In conclusion, the writer would ask his readers to regard the book to
+some extent as tentative, and as a means to the procuring and
+organising of information bearing upon laboratory arts.  Any
+information which can be given will be always thankfully received, and
+the author hereby requests any reader who may happen to learn
+something of value from the book to communicate any special
+information he may possess, so that it may be of use to others should
+another edition ever be called for.
+
+CHAPTER I
+
+HINTS ON THE MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY
+PURPOSES
+
+Sec. 1.  THE art of glass-blowing has the conspicuous advantage, from the
+point of view of literary presentation, of being to a great extent
+incommunicable.  As in the case of other delightful arts--such as
+those treated of in the Badminton Library, for instance--the most
+that can be done by writing is to indicate suitable methods and to
+point out precautions which experience has shown to be necessary, and
+which are not always obvious when the art is first approached.  It is
+not the object of this work to deal with the art of glass-blowing or
+any other art after the manner befitting a complete treatise, in which
+every form of practice is rightly included.  On the contrary, it is my
+wish to avoid the presentation of alternative methods.
+
+I consider that the presentation of alternative methods would, for my
+present purpose, be a positive disadvantage, for it would swell this
+book to an outrageous size;  and to beginners--I speak from
+experience--too lavish a treatment acts rather by way of obscuring
+the points to be aimed at than as a means of enlightenment.  The
+student often does not know which particular bit of advice to follow,
+and obtains the erroneous idea that great art has to be brought to
+bear to enable him to accomplish what is, after all, most likely a
+perfectly simple and straightforward operation.
+
+This being understood, it might perhaps be expected that I should
+describe nothing but the very best methods for obtaining any proposed
+result.  Such, of course, has been my aim, but it is not likely that I
+have succeeded in every case, or even in the majority of cases, for I
+have confined myself to giving such directions as I know from my own
+personal experience will, if properly carried out, lead to the result
+claimed.  In the few cases in which I have to refer to methods of
+which I have no personal experience, I have endeavoured to give
+references (usually taking the form of an acknowledgment), so that an
+idea of their value may be formed.  All methods not particularised may
+be assumed by the reader to have come within my personal experience.
+
+Sec. 2.  Returning to glass-blowing, we may note that two forms of
+glass-blowing are known in the arts, "Pot" blowing and  "Table"
+blowing.  In the former case large quantities of fluid "metal"
+(technical term for melted glass) are assumed to be available, and as
+this is seldom the case in the laboratory, and as I have not yet felt
+the want of such a supply, I shall deal only with "table"  blowing.
+Fortunately there is a convenient book on this subject, by Dr.
+Shenstone (Rivingtons), so that what I have to say will be as brief as
+possible, consistent with sufficiency for everyday work.  As a matter
+of fact there is not very much to say, for if ever there was an art in
+which manual dexterity is of the first and last importance, that art
+is glass-working.
+
+I do not think that a man can become an accomplished glass-blower from
+book instructions merely--at all events, not without much unnecessary
+labour,--but he can learn to do a number of simple things which will
+make an enormous difference to him both as regards the progress of his
+work and the state of his pocket.
+
+Sec. 3.  The first thing is to select the glass.  In general, it will
+suffice to purchase tubes and rods;  in the case where large pieces
+(such as the bulbs of Geissler pumps) have to be specially prepared by
+pot-blowing, the student will have to observe precautions to be
+mentioned later on.  There are three kinds of glass most generally
+employed in laboratories.
+
+Sec. 4.  Soft Soda Glass, obtained for the most part from factories in
+Thuringia, and generally used in assembling chemical apparatus.--This
+glass is cheap, and easily obtainable from any large firm of apparatus
+dealers or chemists.  It should on no account be purchased from small
+druggists, for the following reasons:-
+
+(a) It is usually absurdly dear when obtained in this way.
+
+(b) It is generally made up of selections of different age and
+different composition, and pieces of different composition, even if
+the difference is slight, will not fuse together and remain together
+unless joined in a special manner.
+
+(c) It is generally old, and this kind of glass often devitrifies with
+age, and is then useless for blowpipe work, though it may be bent
+sufficiently for assembling chemical apparatus.  Devitrified glass
+looks frosty, or, in the earlier stages, appears to be covered by
+cobwebs, and is easily picked out and rejected.
+
+Sec. 5.  It might be imagined that the devitrification would disappear
+when the glass is heated to the fusing point; and so it does to a
+great extent, but for many operations one only requires to soften the
+glass, and the devitrification often persists up to this temperature.
+My experience is that denitrified glass is also more likely to crack
+in the flame than good new glass, though the difference in this
+respect is not very strongly marked with narrow tubes.
+
+Sec. 6.  Flint Glass.
+
+Magnificent flint glass is made both in England and France.  The
+English experimenter will probably prefer to use English glass, and,
+if he is wise, will buy a good deal at a time, since it does not
+appear to devitrify with age, and uniformity is thereby more likely to
+be secured.  I have obtained uniformly good results with glass made by
+Messrs.  Powell of Whitefriars, but I daresay equally good glass may
+be obtained elsewhere.
+
+For general purposes flint glass is vastly superior to the soft soda
+mentioned above.  In the first place, it is very much stronger, and
+also less liable to crack when heated--not alone when it is new, but
+also, and especially, after it has been partly worked.  Apparatus made
+of flint glass is less liable to crack and break at places of unequal
+thickness than if made of soda glass.  This is not of much importance
+where small pieces of apparatus only are concerned, because these can
+generally be fairly annealed; and if the work is well done, the
+thickness will not be uneven.  It is a different matter where large
+pieces of apparatus, such as connections to Geissler pumps, are
+concerned, for the glass has often to be worked partly in situ, and
+can only be imperfectly annealed.
+
+Joints made between specimens of different composition are much more
+likely to stand than when fashioned in soda glass.  Indeed, if it is
+necessary to join two bits of soda glass of different kinds, it is
+better to separate them by a short length of flint glass;  they are
+more likely to remain joined to it than to each other.  A particular
+variety of flint glass, known as white enamel, is particularly
+suitable for this purpose, and, indeed, may be used practically as a
+cement.
+
+Sec. 7, It is, however, when the necessity of altering or repairing
+apparatus complicated by joints arises that the advantage of flint
+glass is most apparent.  A crack anywhere near to a side, or inserted
+joint, can scarcely ever be repaired in the case of soda glass
+apparatus, even when the glass is quite thin and the dimensions small.
+
+It should also be mentioned that flint glass has a much more brilliant
+appearance than soda glass.  Of course, there is a considerable
+difference between different kinds of flint glass as to the melting
+point, and this may account for the divergency of the statements
+usually met with as to its fusibility compared with that of soda
+glass.  The kind of flint glass made by Messrs.  Powell becomes
+distinctly soft soon after it is hot enough to be appreciably luminous
+in a darkened room, and at a white heat is very fluid.  This fluidity,
+though of advantage to the practised worker, is likely to give a
+beginner some trouble.
+
+Sec. 8.  As against the advantages enumerated, there are some drawbacks.
+The one which will first strike the student is the tendency of the
+glass to become reduced in the flame of the blow-pipe.  This can be
+got over by proper adjustment of the flame, as will be explained later
+on.  A more serious drawback in exact work is the following.  In
+making a joint with lead glass it is quite possible to neglect to fuse
+the glass completely together at every point; in fact, the joint will
+stand perfectly well even if it be left with a hole at one side, a
+thing which is quite impossible with soft soda glass, or is at least
+exceedingly unusual.  An accident of this kind is particularly likely
+to happen if the glass be at all reduced.  Hence, if a joint does not
+crack when cold, the presumption is, in the case of soda glass, that
+the joint is perfectly made, and will not allow of any leak;  but this
+is not the case with flint glass, for which reason all joints between
+flint glass tubes require the most minute examination before they are
+passed.  If there are any air bubbles in the glass, especial care must
+be exercised.
+
+Sec. 9.  Hard or Bohemian, Glass.
+
+This is, of course, used where high temperatures are to be employed,
+and also in certain cases where its comparative insolubility in water
+is of importance.  It is very unusual for the investigator to have to
+make complicated apparatus from this glass.  Fused joints may be made
+between hard glass and flint glass without using enamel, and though
+they often break in the course of time, still there is no reason
+against their employment, provided the work be done properly, and they
+are not required to last too long.
+
+Sec. 10.  On the Choice of Sizes of Glass Tube.
+
+It will be found that for general purposes tubes about one-quarter
+inch in inside diameter, and from one-twentieth to one-fortieth of an
+inch thick, are most in demand.  Some very thin soda glass of these
+dimensions (so-called "cylinder" tubes) will be found very handy for
+many purposes.  For physico-chemical work a good supply of tubing,
+from one-half to three-quarters of an inch inside diameter, and from
+one-twentieth to one-eighth inch thick, is very necessary.  A few
+tubes up to three inches diameter, and of various thicknesses, will
+also be required for special purposes.
+
+Thermometer and "barometer" tubing is occasionally required, the
+latter, by the way, making particularly bad barometers.  The
+thermometer tubing should be of all sizes of bore, from the finest
+obtainable up to that which has a bore of about one-sixteenth of an
+inch.  Glass rods varying from about one-twentieth of an inch in
+diameter up to, say, half an inch will be required, also two or three
+sticks of white enamel glass for making joints.
+
+To facilitate choice, there is appended a diagram of sizes from the
+catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and
+the experimenter will be able to see at a glance what sizes of glass
+to order.  It is a good plan to stock the largest and smallest size of
+each material as well as the most useful working sizes.
+
+Fig.  1.
+
+Sec. 11.  Testing Glass.
+
+"Reject glass which has lumps or knots, is obviously conical, or has
+long drawn-out bubbles running through the substance." If a scratch be
+made on the surface of a glass tube, and one end of the scratch be
+touched by a very fine point of fused glass, say not more than
+one-sixteenth inch in diameter, the tube, however large it is (within
+reason), ought to crack in the direction of the scratch.  If a big
+crack forms and does not run straight, but tends to turn
+longitudinally, it is a sign that the glass is ill annealed, and
+nothing can be done with it.  If such glass be hit upon in the course
+of blow-pipe work, it is inadvisable to waste time upon it; the best
+plan is to reject it at once, and save it for some experiment where it
+will not have to be heated.
+
+The shortest way of selecting glass is to go to a good firm, and let
+it be understood that if the glass proves to be badly annealed it will
+be returned.  Though it was stated above that the glass should not be
+distinctly conical, of course allowance must be made for the length of
+the pieces, and, on the other hand, a few highly conical tubes will be
+of immense service in special cases, and a small supply of such should
+be included.
+
+The glass, as it is obtained, should be placed in a rack, and covered
+by a cloth to reduce the quantity of dust finding its way into the
+tubes.  It has been stated by Professor Ostwald that tubes when reared
+up on end tend to bend permanently.  I have not noticed this with lead
+glass well supported.  Each different supply should be kept by itself
+and carefully described on a label pasted on to the rack, and tubes
+from different lots should not be used for critical welds.  This
+remark is more important in the case of soda than of lead glass.
+
+In the case of very fine thermometer tubes it will be advisable to
+cover the ends with a little melted shellac, or, in special cases, to
+obtain the tubes sealed from the works.  Soda glass can generally be
+got in rather longer lengths than lead glass;  the longer the lengths
+are the better, for the waste is less.
+
+It is useful to be able to distinguish the different kinds of glass by
+the colour.  This is best observed by looking towards a bright surface
+along the whole length of the tube and through the glass.  Lead glass
+is yellow, soda glass is green, and hard glass purple in the samples
+in my laboratory, and I expect this is practically true of most
+samples.  [Footnote: Some new lead glass I have is also almost purple
+in hue.  If any doubt exists as to the kind of glass, it may be tested
+at once in the blow-pipe flame, or by a mixture of oils of different
+refractive indices, as will be explained later.]
+
+Sec. 12.  The question of the solubility of glass in reagents is one of
+great importance in accurate work, though it does not always meet with
+the attention it deserves.  It is impossible here to go into the
+matter with sufficient detail, and the reader is therefore referred to
+the Abstracts of the Chemical Society, particularly for the years 1889
+and 1892.  The memoir by F.  Kohlrausch, Wied.  Ann.  xliv, should be
+consulted in the original.  The following points may be noted.  A
+method of testing the quality of glass is given by Mylius (C.  S.  J.
+Abstracts, 1889, p.  549), and it is stated that the resistance of
+glass to the action of water can generally be much increased by
+leaving it in contact with cold water for several days, and then
+heating it to 300 deg. to 400 deg. C.  This improvement seems to be due to the
+formation of a layer of moist silica on the surface, and its
+subsequent condensation into a resisting layer by the heating.  Mylius
+(C.  S.  J.  Abstracts, 1892, p.  411), and Weber, and Sauer (C.  S.
+J.  Abstracts, 1892, p.  410) have also shown that the best glass for
+general chemical purposes consists of:
+
+Silica, 7 to 8 parts
+
+Lime, 1 part
+
+Alkali, 1.5 to 1.1 parts.
+
+This is practically "Bohemian" tube glass.
+
+The exact results are given in the Berichte of the German Chemical
+Society, vol.  xxv.  An excellent account of the properties of glass
+will be found in Grove's edition of Miller's Elements of Chemistry.
+
+Sec. 13.  Cleaning Glass Tubes.
+
+This is one of the most important arts in chemistry.  If the tubes are
+new, they are generally only soiled by dust, and can be cleaned fairly
+easily--first by pushing a bit of cotton waste through with a cane,
+or pulling a rag through with string--and then washing with sand and
+commercial hydrochloric acid.  I have heard of glass becoming
+scratched by this process, and breaking in consequence when heated,
+but have never myself experienced this inconvenience.  In German
+laboratories little bits of bibulous paper are sometimes used instead
+of sand; they soon break into a pulp, and this pulp has a slightly
+scouring action.
+
+As soon as the visible impurities are removed and the tube when washed
+looks bright and clean, it may be wiped on the outside and held
+perpendicularly so as to allow the water film to drain down.  If the
+tube be greasy (and perhaps in other cases) it will be observed that
+as the film gets thinner the water begins to break away and leave dry
+spots.  For accurate work this grease, or whatever it is, must be
+removed;  and after trying many plans for many years, I have come back
+to the method I first employed, viz.  boiling out with aqua regia.
+
+For this purpose, close one end of the tube by a cork (better than a
+rubber bung, because cheaper), and half fill the tube with aqua regia;
+then, having noted the greasy places, proceed to boil the liquid in
+contact with the glass at these points, and in the case of very
+obstinate dirt--such as lingers round a fused joint which has been
+made between undusted tubes--leave the whole affair for twelve hours.
+If the greasiness is only slight, then simply shaking with hot aqua
+regia will often remove it, and the aqua regia is conveniently heated
+in this case by the addition of a little strong sulphuric acid.
+
+The spent aqua regia may be put into a bottle.  It is generally quite
+good enough for the purpose of washing glass vessels with sand, as
+above explained.
+
+However carefully a tube is cleaned before being subjected to blowpipe
+operations, it will be fouled wherever there is an opening during the
+process of heating, unless the extreme tip only of an oxidising flame
+be employed.  Even this should not be trusted too implicitly unless
+an oxygas or hydrogen flame is employed.
+
+When a tube or piece of apparatus has been cleaned by acid, so that on
+clamping it vertically, dry spaces do not appear, it may be rinsed
+with platinum distilled water and left to drain, the dust being, of
+course, kept out by placing a bit of paper round the top.  For
+accurate work water thus prepared is to be preferred to anything else.
+When the glass is very clean interference colours will be noticed as
+the water dries away.
+
+Carefully-purified alcohol may in some cases be employed where it is
+desired to dry the tube or apparatus quickly.  In this case an alcohol
+wash bottle should be used, and a little alcohol squirted into the top
+of the tube all round the circumference.  The water film drags the
+alcohol after it, and by waiting a few minutes and then adding a few
+more drops of alcohol, the water may be practically entirely removed,
+especially if a bit of filter paper be held against the lower end of
+the tube.  It is customary in some laboratories to use ether for a
+final rinse, but unless the ether is freshly distilled and very pure,
+it leaves a distinct organic residue.
+
+When no more liquid can be caused to drain away, the tube may be dried
+by heating it along its length, beginning at the top (to get the
+advantage of the reduction of surface tension), and so on all down.
+It will then be possible to mop up a little more of the rinsing
+liquid.  When the tube is nearly dry a loose plug of cotton wool may
+be inserted at the bottom. The wool must be put in so that the fibres
+lie on an even surface inside the tube, and the wool must be blown
+free from dust.  Ordinary cotton wool is useless, from being dusty and
+the fibres short, and the same remark applies to wadding.  Use nothing
+but what is known as "medicated" cotton wool with a good long fibre.
+
+The tube will usually soon dry of itself when the cover is lifted an
+inch or so.  If water has been used, the air-current may be assisted
+by means of the water-pump, the air being sucked from the top, so that
+the wool has an opportunity of acting as a dust filter;  a very slow
+stream of air only must be employed.  For connecting the tube to the
+pump, a bit of India-rubber tube about an inch in diameter, with a
+bore of about one-eighth of an inch, may be employed.  The end of the
+rubber tube is merely pressed against the edge of the glass.
+
+These remarks apply, with suitable modification, to all kinds of
+finished apparatus having two openings.  For flasks and so on, it is
+convenient to employ a blowing apparatus, dust being avoided by
+inserting a permanent plug of cotton wool in one of the leading tubes.
+The efficiency of this method is greatly increased by using about one
+foot of thin copper tube, bent into a helix, and heated by means of a
+Bunsen burner; the hot air (previously filtered) is passed directly
+into the flask, bottle, or whatever the apparatus may be.  This has
+proved so convenient that a copper coil is now permanently fastened to
+the wall in one of the rooms of my laboratory.
+
+The above instructions indicate greater refinement than is in general
+necessary or proper for tubes that have to be afterwards worked by the
+blow-pipe.  In the majority of cases all that is necessary is to
+remove the dust, and this is preferably done by a wad of cotton waste
+(which does not leave shreds like cotton wool), followed by a bit of
+bibulous filter paper.  I would especially warn a beginner against
+neglecting this precaution, for in the process of blowing, the dust
+undergoes some change at the heated parts of the apparatus, and forms
+a particularly obstinate kind of dirt.
+
+In special cases the methods I have advocated for removing dirt and
+drying without covering the damp surfaces with dust are inadequate,
+but an experimenter who has got to that stage will have nothing to
+learn from such a work as this.
+
+Sec. 14.  The Blow-pipe.
+
+I suppose a small book might easily be written on this subject but
+what I have to say--in accordance with the limitation imposed--will
+be brief.  For working lead glass I never use anything but an oxygas
+blow-pipe, except for very large work, and should never dream of using
+anything else.  Of course, to a student who requires practice in order
+to attain dexterity this plan would be a good deal too dear.  My
+advice to such a one is--procure good soda glass, and work it by
+means of a modification of a gas blow-pipe, to be described directly.
+The Fletcher's blow-pipes on long stems are generally very
+inconvenient.  The flame should not be more than 5 or 6 inches from
+the working table at most, especially for a beginner, who needs to
+rest his arms on the edge of the table to secure steadiness.
+
+The kind of oxygas blow-pipe I find most convenient is indicated in
+the sketch.  (Fig.  2)  I like to have two nozzles, which will slip on
+and off, one with a jet of about 0.035 inch in diameter, the other of
+about double this dimension.  The oxygen is led into the main tube of
+the blow-pipe by another tube of much smaller diameter, concentric
+with the main tube (Fig.  3, at A).  The oxygen is mixed with the gas
+during its escape from the inner tube, which is pierced by a number of
+fine holes for the purpose, the extreme end being closed up.  The
+inner tube may run up to within half an inch of the point where the
+cap carrying the nozzle joins the larger tube.
+
+Fig.  2.
+
+Fig.  3.
+
+If it is desired to use the blow-pipe for working glass which is
+already fixed in position to a support, it will be found very
+advantageous to use a hooked nozzle.  The nozzle shown in the sketch
+is not hooked enough for this work, which requires that the flame be
+directed 'backwards towards the worker.  With a little practice such a
+flame may be used perfectly well for blowing operations on the table,
+as well as for getting at the back of fixed tubes.
+
+To warm up the glass, the gas supply is turned full on, and enough
+oxygen is allowed to pass in to clear the flame.  The work is held in
+front of, but not touching, the flame, until it is sufficiently hot to
+bear moving into the flame itself.  The, work is exposed to this flame
+until, in the case of lead glass, traces of reduction begin to appear.
+When this point is reached the oxygen tap is thrown wide open.  I
+generally regulate the pressure on the bags, so that under these
+circumstances the flame is rather overfed with oxygen.  This condition
+is easily recognised, as follows.  The flame shrinks down to a very
+small compass, and the inner blue cone almost disappears;  also
+flashes of yellow light begin to show themselves--a thing which does
+not occur when the proportions of the gases are adjusted for maximum
+heating effect.
+
+For many purposes the small dimensions of the flame render it very
+convenient, and the high temperature which can be attained at exact
+spots enables glass to be fused together after a certain amount of
+mixing, which is an enormous advantage in fusing lead glass on to hard
+glass.  The lead glass should not be heated hot enough to burn, but,
+short of this, the more fluid it is the better for joints between
+dissimilar samples.
+
+It will be noticed that the blow-pipe can be rotated about a vertical
+axis so as to throw the flame in various directions.  This is often
+indispensable.
+
+Sec. 15.  In general the oxygen flame does not require to be delivered
+under so high a pressure as for the production of a lime light.  In
+England, I presume, most experimenters will obtain their oxygen ready
+prepared in bottles, and will not have to undergo the annoyance of
+filling a bag.  If, however, a bag is used, and it has some advantages
+(the valves of bottles being generally stiff), I find that a pressure
+produced by placing about two hundredweight (conveniently divided into
+four fifty-six pound weights) on bags measuring 3' x 2'6" x 2' (at the
+thicker end) does very well.  To fill such a bag with oxygen, about
+700 grms of potassium chlorate is required.
+
+If the experimenter desires to keep his bag in good order, he must
+purify his oxygen by washing it with a solution of caustic soda, and
+then passing it through a "tower" of potash or soda in sticks, and,
+finally, through a calcium chloride tower.  This purifying apparatus
+should be permanently set up on a board, so that it may be carried
+about by the attendant to wherever it is required.  Oxygen thus
+purified does not seem to injure a good bag--at least during the
+first six or seven years:
+
+In order to reduce the annoyance of preparing oxygen, the use of the
+usual thin copper conical bottle should be avoided.  The makers of
+steel gas bottles provide retorts of wrought iron or steel for
+oxygen-making, and these do very well.  They have the incidental
+advantage of being strong enough to resist the attacks of a servant
+when a spent charge is being removed.
+
+The form of retort referred to is merely a large tube, closed at one
+end, and with a screw coupling at the other; the dimensions may be
+conveniently about 5 inches by 10.  The screw threads should be filled
+with fireclay (as recommended by Faraday) before the joint is screwed
+up.  Before purchasing a bottle the experimenter will do well to
+remember that unless it is of sufficiently small diameter to go into
+his largest vice, he will be inconvenienced in screwing the top on and
+off.  Why these affairs are not made with union joints, as they should
+be, is a question which will perhaps be answered when we learn why
+cork borers are still generally made of brass, though steel tube has
+long been available.
+
+Fig.  4.
+
+These little matters may appear very trivial--and so they are--but
+the purchaser of apparatus will generally find that unless he looks
+after details himself, they will not be attended to for him. Whether a
+union joint is provided or not, let it be seen that the end of the
+delivery tube is either small enough to fit a large rubber tube
+connection going to the wash-bottle, or large enough to allow of a
+cork carrying a bit of glass tube for the same purpose to be inserted.
+This tube should not be less than half an inch in inside diameter.
+Never use a new bottle before it has been heated sufficiently to get
+rid of grease and carbonaceous dirt.  A convenient oxygen-making
+apparatus is shown in Fig.  4, which is drawn from "life."
+
+Sec. 16.  For large blow-pipe work with lead glass I recommend a system
+of four simple blow-pipes, in accordance with the sketch annexed.  I
+first saw this system in operation in the lamp factory of the
+Westinghouse Electric Company at Pittsburg in 1889, and since then I
+have seen it used by an exceedingly clever "trick" glass-worker at a
+show.  After trying both this arrangement and the "brush flame"
+recommended by Mr. Shenstone, I consider the former the more
+convenient;  however, I daresay that either can be made to work in
+competent hands, but I shall here describe only my own choice.
+[Footnote: A brush flame is one which issues from the blow-pipe nozzle
+shaped like a brush, i.e.  it expands on leaving the jet.  It is
+produced by using a cylindrical air jet or a conical jet with a large
+aperture, say one-eighth of an inch.  See Fig.  25.]
+
+As will be seen, the blow-pipe really consists of four simple brass
+tube blow-pipes about three-eighths of an inch internal diameter and 3
+inches long, each with its gas and air tap and appropriate nozzle.
+Each blowpipe can turn about its support (the gas-entry pipe) to some
+extent, and this possibility of adjustment is of importance, The air
+jets are merely bits of very even three-sixteenths inch glass tubing,
+drawn down to conical points, the jets themselves being about 0.035
+inch diameter.
+
+Fig.  5.
+
+The flames produced are the long narrow blow-pipe flames used in
+blow-pipe analysis, and arranged so as to consist mostly of oxidising
+flame.  The air-supply does not require to be large, nor the pressure
+high--5 to 10 inches of water will do--but it must be very regular.
+The "trick" glass-blower I referred to employed a foot bellows in
+connection with a small weighted gasometer, the Westinghouse Company
+used their ordinary air-blast, and I have generally used a large
+gas-holder with which I am provided, which is supplied by a Roots
+blower worked by an engine.
+
+I have also used a "velocity pump" blower, which may be purchased
+amongst others from Gerhardt of Bonn.  The arrangement acts both as a
+sucking and blowing apparatus, and is furnished with two manometers
+and proper taps, etc.  As I have reason to know that arrangements of
+this kind work very ill unless really well made, I venture to add that
+the Gerhardt arrangement to which I refer is No.  239 in his
+catalogue, and costs about three pounds.  It hardly gives enough air,
+however, to work four blow-pipes, and the blast requires to be
+steadied by passing the air through a vessel covered with a rubber
+sheet.
+
+In default of any of these means being available, one of Fletcher's
+foot-blowers may be employed, but it must be worked very regularly.  A
+table mounted with one blow-pipe made on this plan, and worked by a
+double-acting bellows, is recommended for students' use.  For working
+flint glass, the air jet may be one-eighth of an inch in diameter and
+the pressure higher--this will give a brush flame.  See Fig.  25.
+
+It will be seen, on looking at the sketch of the blowpipe system, that
+the pair of blow-pipes farther from the observer can be caused to
+approach or recede at will by means of a handle working a block on a
+slide.  It often happens that after using all four blow-pipes at once
+it is necessary to have recourse to one blow-pipe only, and to do this
+conveniently and quickly is rather an object.  Now, in my arrangement
+I have to turn off both the gas and air from the farther system, and
+then put in a bit of asbestos board to prevent the nozzles being
+damaged by the flame or flames kept alight.  As I said before, when
+some experience is gained, glassblowing, becomes a very simple art,
+and work can be done under circumstances so disadvantageous that they
+would entirely frustrate the efforts of a beginner.  This is not any
+excuse, however, for recommending inferior arrangements.
+
+Consequently, I say that the pipes leading in gas and air should be
+all branches of one gas and one air pipe, in so far as the two remote
+and one proximate blow-pipe are concerned, and these pipes should come
+up to the table to the right hand of the operator, and should have
+main taps at that point, each with a handle at least 2 inches long.
+By this arrangement the operator can instantly turn down all the
+blow-pipes but one, while, if the inverse operation is required, all
+the three pipes can be started at once.  [Footnote: I find, since
+writing the above, that I have been anticipated in this recommendation
+by Mr. G.  S.  Ram, The Incandescent Lamp and its Manufacture, p.
+114.]
+
+The separate air and gas taps must be left for permanent regulation,
+and must not be used to turn the supply on or cut it off.  In some
+respects this blow-pipe will be found more easy to manage than an
+oxygas blow-pipe, for the glass is not so readily brought to the very
+fluid state, and this will often enable a beginner who proceeds
+cautiously to do more than he could with the more powerful instrument.
+
+Though I have mentioned glass nozzles for the air supply, there is no
+difficulty in making nozzles of brass.  For this purpose let the end
+of a brass tube of about one-eighth of an inch diameter be closed by a
+bit of brass wire previously turned to a section as shown (Fig.  6),
+and then bored by a drill of the required diameter, say -.035 inch.  It
+is most convenient to use too small a drill, and to gradually open the
+hole by means of that beautiful tool, the watchmaker's "broach."  The
+edges of the jet should be freed from burr by means of a watchmaker's
+chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin,
+1882, p.  232, Sec. 342), or by the alternate use of a slip of Kansas
+stone and the broach.
+
+Fig.  6.
+
+The construction of this blow-pipe is so simple, that in case any one
+wishes to use a brush flame, he can easily produce one simply by
+changing his air jets to bits of the same size (say one-eighth to
+one-sixteenth of an inch) tubing, cut off clean.  To insure success,
+the ends of the tubes must be absolutely plane and regular; the
+slightest inequality makes all the difference in the action of the
+instrument.  If a jet is found to be defective, cut it down a little
+and try again; a clean-cut end is better than one which has been
+ground flat on a stone.  The end of a tube may, however, be turned in
+a manner hereafter to be described so as to make an efficient jet.
+Several trials by cutting will probably have to be made before success
+is attained.  For this kind of jet the air-pressure must be greatly
+increased, and a large Fletcher's foot-blower or, better still, a
+small double-action bellows worked with vigour will be found very
+suitable.  A fitting for this auxiliary blow-pipe is shown in Fig.  5
+at B.
+
+Professor Roentgen's discovery has recently made it necessary to give
+more particular attention to the working of soft soda glass, and I
+have been obliged to supplement the arrangements described by a table
+especially intended for work with glass of this character.  The
+arrangement has proved so convenient for general work that I give the
+following particulars.  The table measures 5 feet long, 2 feet 11
+inches wide, and is 2 feet 9 inches high.
+
+Fig.  7.
+
+It is provided with a single gas socket, into which either a large or
+small gas tube may be screwed.  The larger tube is 5.5 inches long and
+0.75 of an inch in diameter.  The smaller tube is the same length, and
+half an inch in diameter.  The axis of the larger tube is 3.5 inches
+above the table at the point of support, and is inclined to the
+horizontal at an angle of 12 deg..  The axis of the smaller tube is 2.5
+inches above the surface of the table, and is inclined to the
+horizontal at the same angle as the larger one.
+
+The air jets are simply pieces of glass tube held in position by
+corks.  The gas supply is regulated by a well-bored tap.  The air
+supply is regulated by treading the bellows--no tap is requisite.
+The bellows employed are ordinary smiths' bellows, measuring 22 inches
+long by 13 inches wide in the widest part.  They are weighted by lead
+weights, weighing 26 lbs.  The treadle is connected to the bellows by
+a small steel chain, for the length requires to be invariable.  As the
+treadle only acts in forcing air from the lower into the upper chamber
+of the bellows, a weight of 13 lbs.  is hung on to the lower cover, so
+as to open the bellows automatically.
+
+The air jets which have hitherto been found convenient are:
+for the small gas tube
+
+(1) a tube 0.12 inch diameter drawn down to a jet of 0.032 inch
+diameter for small work;
+
+(2) plain tubes not drawn down of 0.14 inch, 0.127 inch, and -0.245
+inch diameter, and for the large gas tube, plain tubes up to 0.3 inch
+in diameter.
+
+The table is placed in such a position that the operator sits with his
+back to a window and has the black calico screen in front of him and
+to his right.  The object of the screen is to protect the workman
+against draughts.  The table is purposely left unscreened to the left
+of the workman, so that long tubes may be treated.
+
+Sec. 17.  Other appliances which will be required for glass-blowing are
+of the simplest character.
+
+(1) Small corks for closing the ends of tubes.
+
+(2) Soft wax--a mixture of bees' wax and resin softened by linseed
+oil to the proper consistency, easily found by trial, also used for
+temporarily closing tubes.
+
+(3) A bottle of vaseline for lubricating.
+
+(4) An old biscuit tin filled with asbestos in shreds, and an asbestos
+towel or cloth for annealing glass after removal from the flame.  As
+asbestos absorbs moisture, which would defeat its use as an annealing
+material, it must be dried if necessary.
+
+(5) A Glass-Cutter's Knife. This is best made out of a fine
+three-cornered file, with the file teeth almost ground out, but not
+quite;  it should be about 2 inches long.  After the surface has been
+ground several times, it may be necessary to reharden the steel.  This
+is best done by heating to a full red and quenching in mercury.  The
+grindstone employed for sharpening the knife should be "quick," so as
+to leave a rough edge.  I have tried many so-called glass knives "made
+in Germany," but, with one exception, they were nothing like so good
+as a small French or Sheffield file.  In this matter I have the
+support of Mr. Shenstone's experience.
+
+(6) A wire nail, about 2 inches long, mounted very accurately in a
+thin cylindrical wooden handle about 5 inches long by one-quarter of
+an inch diameter, or, better still, a bit of pinion wire 6 inches
+long, of which 1.5 inches are turned down as far as the cylindrical
+core, An old dentists' chisel or filling tool is also a very good
+form of instrument.
+
+(7) A bit of charcoal about 3.5 inches long and 2 wide, and of any
+thickness, will be found very useful in helping to heat a very large
+tube.  The charcoal block is provided with a stout wire handle, bent
+in such a manner that the block can be held close above a large glass
+tube on which the flames impinge.  In some cases it is conveniently
+held by a clip stand.  By the use of such a slab of charcoal the
+temperature obtainable over a large surface can be considerably
+increased.
+
+I have seen a wine-glass (Venetian sherry-glass) worked on a table
+with four blow-pipes, such as is here described, with the help of a
+block of hard wood held over the heated glass, and helping the
+attainment of a high temperature by its own combustion.
+
+(8) Several retort stands with screw clips.
+
+(9) Some blocks of wood about 5" X 2" X 2" with V-shaped notches cut
+in from the top.
+
+(10) A strong pair of pliers.
+
+(11) An apparatus for cleaning and drying the breath, when blowing
+directly by the mouth is not allowable.  The apparatus consists of a
+solid and heavy block of wood supporting a calcium-chloride tube
+permanently connected with a tube of phosphorus pentoxide divided into
+compartments by plugs of glass wool.  Care should be taken to arrange
+these tubes so as to occupy the smallest space, and to have the stand
+particularly stable.  The exit tube from the phosphorus pentoxide
+should be drawn down to form a nozzle, from, say, half an inch to
+one-eighth of an inch in diameter, so as to easily fit almost any bit
+of rubber tube.  The entry to the calcium chloride should be
+permanently fitted to about a yard of fine soft rubber tubing, as
+light as possible.  The ends of this tube should terminate in a glass
+mouthpiece, which should not be too delicate.
+
+As an additional precaution against dust, I sometimes add a tube
+containing a long plug of glass wool, between the phosphorus pentoxide
+and the delivery tube, and also a tube containing stick potash on the
+entry side of the calcium chloride tube, but it may safely be left to
+individual judgment to determine when these additions require to be
+made.  In practice I always keep the affair set up with these
+additions.  The communication between all the parts should be
+perfectly free, and the tubes should be nearly filled with reagents,
+so as to avoid having a large volume of air to compress before a
+pressure can be got up.
+
+The arrangement will be clear by a reference to Fig.  8, which
+illustrates the apparatus in use for joining two long tubes.  I have
+tried blowing-bags, etc, but, on the whole, prefer the above
+arrangement, for, after a time, the skill one acquires in regulating
+the pressure by blowing by the mouth and lips is such an advantage
+that it is not to be lightly foregone.
+
+Fig. 8.
+
+Sec. 18.  The Table.
+
+The system of four blow-pipes is, of course, a fixture.  In this case
+the table may be about a yard square, and may be covered with asbestos
+mill-board neatly laid down, but this is not essential.  The table
+should have a rim running round it about a quarter of an inch high.
+The tools should be laid to the right of the worker, and for this
+purpose the blow-pipes are conveniently fixed rather to the left of
+the centre of the table, but not so far as to make the leg of the
+table come so close to the operator as to make him uncomfortable, for
+a cheerful and contented spirit ought to be part of the glass-worker's
+outfit.
+
+The most convenient height for a blow-pipe table--with the blow-pipes
+about 2 inches above the table top--is 3 feet 2 inches.  Nothing is
+so convenient to sit upon as a rough music-stool with a good range of
+adjustment.  The advantage of an adjustable seat lies in the fact that
+for some operations one wants to be well over the work, while in
+others the advantage of resting the arms against the table is more
+important.
+
+Sec. 19.  Special Operations.
+
+The preliminary to most operations before the blow-pipe, is to draw
+down a tube and pull it out to a fine point.  This is also the
+operation on which a beginner should exercise himself in the first
+instance.  I will suppose that it is desired to draw out a tube about
+one-quarter of an inch in diameter, with the object of closing it,
+either permanently or temporarily, and leaving a handle for future
+operations in the shape of the point, thin enough to cool quickly and
+so not delay further work.
+
+For this simple operation most of the glass-blower's skill is
+required.  The tube must be grasped between the first finger and thumb
+of both hands, and held so that the part to be operated on lies evenly
+between the two hands.  The distance between the operator's thumbs may
+conveniently vary from 2.5 to 4 inches.  Releasing the grip of the
+left hand, let the operator assure himself of his ability to easily
+rotate the tube about its axis--by the right thumb and finger--he
+will incidentally observe by the "feel" whether the tube is straight
+or not.
+
+A good deal of progress can be made from this point before the tube is
+heated at all.  The operator can acquire a habit of instinctively
+rotating the tube by both hands, however the tube itself be moved
+about in space, or however it be pushed or pulled.  The habit of
+constant and instinctive rotation is literally about one-third of the
+whole art of glassblowing.  It is unlikely, however, that the beginner
+will discover that he has not got this habit, until a few failures
+draw his attention to it.
+
+The glass tube being held in position lightly yet firmly, and the
+operator being sure that he feels comfortable and at his ease, and
+that the blow-pipe flame (a single flame in this instance) is well
+under control, the preliminary heating may be commenced.  With a tube
+of the dimensions given this is a very simple affair.  Turn the air
+partly off, or blow gently, to get a partly luminous gas flame; hold
+the tube about an inch from the end of this flame, and turn it round
+and round till it commences to soften.
+
+In the case of soda glass it is usual to employ the gas flame only,
+but I find that it is better in most cases to use the hot air of a
+gently-blown flame, rather than have the disadvantage of the soot
+deposited in the alternative operation.  When the glass begins to
+soften, or even before, it may be moved right into the blow-pipe
+flame, and the latter may be properly urged.
+
+It is not possible to give quite explicit and definite instructions,
+applicable to every case, as to when the time is ripe for passing the
+work into the flame, but the following notes will indicate the general
+rules to be observed:-
+
+(1) A thick tube must be warmed more slowly and raised to a higher
+temperature than a thin tube.
+
+(2) The same remark applies to a tube of large diameter, as compared
+with one of small diameter, whatever the thickness.
+
+(3) In the case of very large or thick tubes the hot air is
+advantageously employed at first, and to complete the preliminary
+heating, the luminous flame alone may be used.  The object of this is
+to enable the operator to judge, by the presence of soot, its
+inability to deposit--or its burning off if deposited--of the
+temperature of the glass, and of the equality of this temperature all
+over the surface, for a large and thick tube might be heated quite
+enough to enable it to be safely exposed to the full heat before it is
+appreciably yielding to the fingers.  In general, when the soot burns
+off freely, or lead glass begins to show the faintest sign of
+reduction, or soda glass begins to colour the flame, it is more than
+safe to proceed.
+
+In order to turn on the full flame the operator will form a habit of
+holding the work in the left hand only, and he will also take care not
+to let anything his right hand may be doing cause him to stop rotating
+the tube with his left thumb and finger.
+
+The preliminary adjustment of air or oxygen supply will enable the
+change to a flame of maximum power to be made very quickly.  The tube
+having been introduced with constant rotation, it will soon soften
+sufficiently to be worked.  The beginner will find it best to decide
+the convenient degree of softness by trial.
+
+With soda glass it does not much matter how soft the glass becomes,
+for it remains viscous, but with lead glass the viscosity persists for
+a longer time and then suddenly gives place to a much greater degree
+of fluidity.  [Footnote: This is only drawn from my impressions
+acquired in glass-working.  I have never explicitly tested the matter
+experimentally.]
+
+It is just at this point that a beginner will probably meet with his
+first difficulty.  As soon as the glass gets soft he will find that he
+no longer rotates the glass at the same speed by the right and left
+hand, and, moreover, he will probably unconsciously bend the tube, and
+even deform it, by pushing or pulling.
+
+The second third of the art of the glass-blower consists in being able
+to move both hands about, rotating a tube with each thumb and finger,
+and keeping the distance between the hands, and also the speed of
+rotation, constant.  Nothing but long practice can give this facility,
+but it is essential that it be acquired to some extent, or no progress
+can be made.  Some people acquire a moderate proficiency very quickly,
+others, of whom the writer is one, only become reasonably proficient
+by months, or even years, of practice.
+
+Supposing that the tube is now ready to be drawn down, the operator
+will remove it from the flame, and will gently pull the ends apart,
+interrupting his turning as little as possible.  If the tube be pulled
+too hard, or if the area heated be too small (about three-eighths of
+an inch in length in the case given would be proper), it will be found
+that the ends of the two portions of the tube will be nearly closed at
+a very sharp angle (nearly a right angle to the length of the tube),
+that the ends will be thin, and that a long length of very fine tube
+will be produced.  To heat a short length of tube and pull hard and
+suddenly is the proper way to make a very fine capillary tube, but, in
+general, this is what we want to avoid.
+
+If the operation be successfully performed, the drawn-down tube will
+have the appearance exhibited, which is suitable either for
+subsequently closing or handling by means of the drawn-down portion.
+The straightness of the point can be obtained by a little practice in
+"feeling" the glass when the tube is rotated as it cools just before
+it loses its viscous condition.
+
+When the operation is carried out properly the shoulder of the "draw"
+should be perfectly symmetrical and of even thickness, and its axis
+regarded as that of a cone should lie in the axis of the tube
+produced.  The operation should be repeated till the student finds
+that he can produce this result with certainty, and he should not be
+discouraged if this takes several days, or even weeks.  Of course, it
+is probable that within the first hour he will succeed in making a
+tolerable job, but it is his business to learn never to make anything
+else.
+
+Fig. 9.
+
+Fig. 10. Diagram of a folded end.
+
+Sec. 20.  Closing and blowing out the End of a Tube.
+
+When it is desired to close the end of a particular bit of tube, this
+is easily done by heating the end, and at the same time heating the
+end of a waste bit of tube or rod;  the ends, when placed in contact,
+stick together, and a point can be drawn down as before.  [Footnote:
+"Point" is here used in the technical sense, i.e.  it is a thin tail
+of glass produced by drawing down a tube.]  Having got a point, it
+will be found that the thin glass cools enough to allow of the point
+being handled after a few moments.
+
+The most convenient way of reducing the point to a suitable length
+(say 1.5 inch) is to fuse it off in the flame, but this must be done
+neatly;  if a tail is left it may cause inconvenience by catching, or
+even piercing the finger and breaking off.  The blow-pipe flame being
+turned down to a suitable size, and the shoulder of the "draw" having
+been kept warm meanwhile, let the tip of the flame impinge on a point
+where the diameter is about half that of the undrawn tube, and let the
+temperature be very high (Fig.  11).  The tube is to be inclined to
+the flame so that the latter strikes the shoulder normally, or
+nearly so.  Then, according to circumstances, little or much of the
+glass can be removed at will by drawing off the tail (Fig.  12), till,
+finally, a small drop of melted glass only, adheres to the end of the
+now closed tube (Fig.  13).
+
+Fig. 11.
+
+Fig.  12.
+
+Fig.  13.
+
+Fig. 14.
+
+When this is satisfactorily accomplished, heat the extreme end of the
+tube most carefully and equally, holding it in such a position that
+the glass will tend to flow from the bead back on to the tube, i.e.
+hold the closed end up to the flame, the tube being, say, at 45
+degrees to the horizontal.  Then when the temperature is such as to
+indicate complete softness lift the tube to the mouth, still holding
+the tube pointing with its closed end a little above the horizontal,
+and blow gently.  A beginner almost always blows too hard.
+
+What is wanted, of course, is a continued pressure, to give the
+viscous glass time to yield gradually, if it is uniform;  or else
+intermittent puffs to enable the thinner parts, if there are any, to
+cool more, and hence become more resisting than the thicker ones.  In
+any case a little practice will enable the operator to blow out a
+round and even end--neither thicker nor thinner than the rest of the
+tube.
+
+Sec. 21.  To make a Weld.
+
+To begin with, try on two bits of glass of the same size, i.e.  cut a
+seven-inch length of glass in half by scratching it with the knife,
+and pulling the ends apart with a slight inclination away from the
+scratch.  In other words, combine a small bending moment with a
+considerable tensional stress.  It is important to learn to do this
+properly.  If the proportions are not well observed, the tube will
+break with difficulty, and the section will not be perpendicular to
+the main length.  If the knife is in good order it will make a fine
+deep scratch--the feel of the glass under the knife will enable the
+operator to decide when the scratch is made.  The operation of cutting
+large tubes will be treated further on.  The two halves of the tube
+being held one in each hand, and one tube closed at one end, the
+extremities to be united will be warmed, and then put in the flame as
+before.
+
+Fig. 16.
+
+There are many ways of proceeding--perhaps the easiest is as follows.
+As soon as the glass shows signs of melting at the ends--and care
+should be taken that much more is not heated--take both bits out of
+the flame.  Stop rotating for a moment, and resting the arms carefully
+on the edge of the  table, raise the tubes above the flame and bring
+the ends swiftly and accurately together.  This is a case of "sudden
+death no second attempt at making the ends meet can be allowed;  if
+the tubes join in any other than a perfectly exact manner a kink more
+or less objectionable will result.  In practice the operator will
+learn to bring the ends together, commencing at one point; i.e.  the
+axes of the tubes will be inclined at first, so as to cause adherence
+at one spot only.  If this is not quite "fair", then less damage is
+done in moving one tube slightly up or down to get the contact exact.
+The tubes will then be closed upon one another as if they were hinged
+at the joint.  This must be done lightly, yet sufficiently, to ensure
+that the glass is actually in contact all round.
+
+Having gone so far, replace the tubes--now one--in the flame, and
+carefully rotating the glass, raise the temperature higher than in the
+operation just described, in fact the higher the temperature, short of
+burning the glass, the better.  Take the tube out of the flame and
+blow into the open end, turning constantly as before.  One puff is
+enough.  Then turn and pull the glass apart till it is of the same
+diameter and thickness throughout, and feel that it is straight as
+before.
+
+Though it is in general of high importance that the joint should be
+well heated, the beginner will probably find that he "ties up" his
+glass as soon as it gets really soft.
+
+If his object is to make one joint--at any cost--then let him be
+careful to use two bits of exactly the same kind of glass, and only
+get the temperature up to the viscous stage.  If the joint be then
+pulled out till it is comparatively thin, it will probably stand (if
+of soda glass); certainly, if of lead glass, though in this case it
+may not be sound.  In any case the joint should be annealed in the
+asbestos box if practicable, otherwise (unless between narrow tubes)
+with the asbestos rag.  Care must be taken that the asbestos is dry.
+
+Sec. 22.  To weld two Tubes of different Sizes.
+
+To do this, the diameter of the larger tube must be reduced to that of
+the smaller.  The general procedure described in drawing down must be
+followed, with the following modification.  In general, a greater
+length of the tube must be heated, and it must be made hotter.  The
+tube is to be gradually drawn in the flame with constant turning till
+the proper diameter and thickness of glass are attained.
+
+Fig. 16.
+
+For this operation time must be allowed if the operator's hands are
+steady enough to permit of it; the shoulder should form partly by the
+glass sinking in and partly by the process of drawing the hot glass
+out.  A shoulder properly prepared is shown in the sketch.  Beginners
+generally make the neck too thin on large tubes, and too thick on
+smaller ones.  There ought to be no great difference in thickness of
+glass between the neck on the larger tube, and the smaller tube.  The
+diameters should be as nearly as possible alike.
+
+Having drawn down the larger tube to a neck, take it out of the flame,
+and as it cools pull and turn till the neck is of the right thickness
+and is perfectly straight, i.e.  make the final adjustment outside the
+flame, and to that end have the neck rather too thick (as to glass)
+before it is taken out.  It is not necessary to wait till the neck
+gets cold before the end can be cut off. Make a scratch as before--this
+will probably slightly damage the temper of the file knife, but
+that must be put up with.  Hold the tube against the edge of the
+table,  so that the scratch is just above the level of the rim, and
+strike the upper part a smart blow with the handle of the glass knife
+rather in the direction of its length.  [Footnote: A bit of hoop iron
+nailed against the side of the table is a very convenient arrangement,
+and it need not project appreciably above the general level of the
+rim.]
+
+Of course this applies to a tube where economy has been exercised and
+the end is short.  If the tail is long enough to form a handle, the
+tube may be pulled apart as before.  As a rule a temporary joint
+between a tube and a rod is not strong enough to enable the shoulder
+to be broken at the scratch by mere pulling.  The ends to be welded
+must be broken off very clean and true.  Subsequent operations are to
+be carried out as already described.
+
+Sec. 23.  The above operations will be easily performed on tubes up to
+half an inch in diameter, if they are not too long.  It is the length
+of tube, and consequent difficulty in giving identity of motion with
+the two hands, which make the jointing of long tubes difficult.  There
+are also difficulties if the tubes are very thin, have a very fine
+bore or a very large diameter.
+
+All these difficulties merely amuse a good glass-blower, but to an
+experimenter who wants to get on to other things before sufficient
+skill is acquired (in the movement of the hands and arms) the
+following method is recommended.  First, use flint glass.  Then,
+assuming that any drawing down has to be done, do it as well as
+possible, for on this the success of the method to be described
+especially depends.  Be sure that the tubes to be welded are cut off
+clean and are as nearly as may be of the same size at the point of
+junction.
+
+To fix the description, suppose it is desired to join two tubes (see
+Fig.  8), each about one inch in diameter and a yard long.  Get four
+clip stands and place them on a level table.  Be sure that the stands
+are firm and have not warped so as to rock.  In each pair of clips
+place a tube, so that the two tubes are at the same height from the
+table, and, in fact, exactly abut, with axes in the same straight
+line.  Close one tube by a cork and then fix the blowing apparatus as
+shown to the other.
+
+In such an operation as this the drying apparatus may be dispensed
+with, and a rubber tube simply connected to one end of the system and
+brought to the mouth.  Take the oxygen blow-pipe and turn the nozzle
+till the flame issues towards you, and see that the flame is in order.
+Then turn down the oxygen till it only suffices to clear the smoky
+flame, and commence to heat the proposed joint by a current of hot
+air, moving the flame round the joint.  Finally, bring to bear the
+most powerful flame you can get out of the blow-pipe, and carry it
+round the joint so quickly that you have the latter all hot at once.
+Put down the blow-pipe, and, using both hands, press the tubes
+together (which wooden clips will readily allow), and after seeing
+that the glass has touched everywhere, pull the tubes a trifle apart.
+Apply the blow-pipe again, passing lightly over the thin parts, if
+any, and heating thicker ones; having the end of the rubber tube in
+his mouth, the operator will be able to blow out thick places.  When
+all is hot, blow out slightly, and having taken the flame away, pull
+the tubes a little apart, and see that they are straight.
+
+Throw an asbestos rag over the joint, loosen one pair of the clamps
+slightly, and leave the joint to anneal.  It is important that the
+least possible amount of glass should be heated, hence the  necessity
+of having the ends well prepared, and it is also important that the
+work should be done quickly;  otherwise glass will flow from the upper
+side downwards and no strong joint will be obtained.
+
+Fig. 17. Tube being opened at one end.
+
+Sec. 24.  To weld Tubes of very small Bore.
+
+If the bore is not so small as to prevent the entrance of the point of
+the iron nail, get the ends of the tubes hot, and open the bore by
+inserting the end of the nail previously smeared over with a trace of
+vaseline.  Work the nail round by holding the handle between the thumb
+and first finger of the right hand, the tube being similarly placed in
+the left.  The tube and nail should be inclined as shown in the
+sketch.
+
+Never try to force the operation;  the nail soon cools the glass, so
+that only a very short time is available after each heat;  during this
+the tube should be rotated against the nail rather than the nail
+against the tube.  Be careful not to heat a greater length of tube
+than is necessary, or the nail will, by its component of pressure
+along the tube, cause the latter to "jump up" or thicken and bulge.
+Both ends being prepared, and if possible, kept hot, the weld may be
+made as before, and the heating continued till the glass falls in to
+about its previous thickness, leaving a bore only slightly greater
+than before.
+
+It is in operations such as this that the asbestos box will be found
+of great use.  As soon as one end of the weld is ready cool it in the
+flame till soot deposits, and then plunge it into the asbestos.  This
+will cause it to cool very slowly, and renders it less likely to crack
+when again brought into the flame.  Turned-out ends, if the glass is
+at all thick, are very liable to crack off on reheating, so that they
+must be reintroduced (into the flame) with especial care.  This
+liability to breakage is reduced, but not eliminated, by the asbestos
+annealing.
+
+Figs.  18 and 19.
+
+Sec. 25.  When the bore is very fine, it is best to seal off the tubes,
+and blow an incipient bulb near one end of each tube.  These bulbs may
+be cooled in asbestos, and cut across when cold by means of a scratch
+touched at one end (Figs.  18 and 19) by a fine point of highly
+incandescent glass.  For details of this method see p.  46, Fig.  21.
+Time is occasionally saved by blowing off the ends of the bulbs.  The
+details of this process will be described when the operation of making
+thistle-headed tubes is dealt with.
+
+Sec. 26.  When the tubes are both of large diameter, long, and very thin
+(cylinder tubes), a considerable amount of difficulty will be
+experienced.  On the whole, it is best to heat each end separately
+till the glass thickens a little, anneal in the flame and in asbestos,
+and then proceed as in Sec. 22.  If the ends are not quite true, it will
+be found that quite a thickness of glass may be "jumped" together at
+one side of the tubes, while the edges are still apart at the other.
+When this looks likely to happen, incline the tubes as if the joint
+were a hinge, and bend back quickly;  do not simply continue to push
+the tubes together in a straight line, or an unmanageable lump of
+glass will be formed on one side.
+
+If in spite of these precautions such a lump does form, proceed as
+follows.  Take a rod of glass, at least one-eighth of an inch thick,
+and warm it in the flame at one end.  Heat the imperfect joint till it
+softens all round, and then bring the flame right up to the thick
+part, and heat that as rapidly and locally as possible.  The oxygas
+flame does this magnificently.  Press the heated end of the glass rod
+against the thick part, and pull off as much of the lump as it is
+desired to remove, afterwards blowing the dint out by a judicious
+puff.  Finish off as before.
+
+Sec. 27.  Occasionally, when it is seen that in order to produce a joint
+closed all round, one side of the tube would be too much thickened, it
+is better to patch the open side.  For this purpose take a glass rod
+about one-sixteenth inch in diameter, and turn the flame to give its
+greatest effect, still keeping rather an excess of air or oxygen.  See
+that the side of the joint already made is kept fairly hot--it need
+not be soft;  interrupt any other work often enough to ensure this.
+Then, directing the flame chiefly on the thin rod, begin to melt and
+pull the glass over the edges of the gap.  When the gap is closed get
+the lump very hot, so that all the glass is well melted together, and
+then, if necessary, pull the excess of glass off, as before described.
+
+It must be remembered that this and the method of the previous section
+are emergency methods, and never give such nice joints as a
+manipulation which avoids them, i.e.  when the ends of the tubes are
+perfectly straight and true to begin with.  Also note that, as the
+tubes cannot be kept in rotation while being patched, it is as well to
+work at as low a temperature as possible, consistently with the other
+conditions, or the glass will tend to run down and form a drop,
+leaving a correspondingly thin place behind.
+
+Fig.  20.
+
+Sec. 28.  A very common fault in cutting a tube of about an inch in
+diameter is to leave it with a projecting point, as shown.  This can
+be slowly chipped off by the pliers, using the jaws to crush and grind
+away the edge of the projection;  it is fatal to attempt to break off
+large pieces of glass all at once.
+
+Sec. 29.  It will be convenient here to mention some methods of cutting
+large tubes.  With tubes up to an inch and a half in diameter, and
+even over this--provided that the glass is not very thick--we may
+proceed as follows:  Make a good scratch about half an inch long, and
+pretty deep, i.e.  pass the knife backwards and forwards two or three
+times.  Press a point of melted glass exactly on one end of the
+scratch;  the glass point even when pressed out of shape should not be
+as large as a button one-twelfth of an inch in diameter.  If this
+fails at first, repeat the operation two or three times.
+
+Fig.   21.
+
+If a crack does not form, touch the hot place with the cold end of the
+nail.  If no success is obtained, try the other end of the scratch.
+If failure still pursues the operator, let him make another cut on the
+opposite side of the tube and try again.  In general, the tube will
+yield the first or second time the hot drop of glass is applied.
+Never apply the drop at the centre of the scratch, or a ragged crack,
+which may run in any direction, will result.  Very often, with a large
+tube, the crack formed by a successful operation will only extend a
+short distance.  In this case it is desirable to entice the crack
+round the tube, and not trust to its running straight when the tube is
+pulled apart.
+
+On the whole, the best method in this case is to employ a flame
+pencil, which should be kept ready for use.  This merely consists of a
+bit of glass tube of about the same dimensions as an ordinary lead
+pencil, drawn down to a very fine jet at one end.  The jet must not be
+very long or thin, or the glass will soon fuse up.  A few trials will
+enable the operator to get the proper proportions, which are such that
+the tube has the general appearance of a pencil normally sharpened
+(say with a cone of 60').  This tube is best made of hard glass.
+Connect it to a gas supply by light flexible tubing, and turn down the
+gas till the flame from the end of the jet is not more than one-tenth
+of an inch long.  Then apply the jet, beginning from the end of the
+crack, and gradually draw it (the crack) round the tube.  The
+operation will be assisted if a rubber ring is slipped on the tube to
+begin with, so that the eye has some guide as to whether the flame is
+being drawn round properly or not.  The ring must, of course, be far
+enough away to escape the effect of the flame.  The crack will be
+found to follow the flame in the most docile manner, unless the tube
+is thick or badly annealed.  Some operators recommend a pencil of
+glowing charcoal, but the flame is undoubtedly better.
+
+Sec. 30.  To cut very thick Tubes.
+
+A large number of methods have been proposed, and nearly everybody has
+his favourite.  The following has always succeeded with me.  First
+mark on the tube, by means of a little dead black spirit paint,
+exactly where the cut is to be.  Then sharpen the glass knife and
+scratch a quite deep cut all round:  there is no difficulty in making
+the cut one-twentieth of an inch deep.  It will be proper to lubricate
+the knife with kerosene after the first mark is made.  [Footnote: The
+edge of the knife may be advantageously saved by using an old file
+moistened with kerosene for the purpose.  I find kerosene is not
+worse, but, if anything, better than the solution of camphor in
+turpentine recommended by Mr. Shenstone.]
+
+If the glass is about one-eighth of an inch thick, the scratch maybe
+conveniently about one-twentieth of an inch deep, but if the glass is
+anything like one-quarter of an inch thick, the scratch must be much
+deeper, in fact, the glass may be half cut through.  To make a very
+deep scratch, a wheel armed with diamond dust, which will be described
+later on, may be used.  However, it is not essential to use a diamond
+wheel, though it saves time.
+
+When the cut is made to a sufficient depth proceed thus:  Obtain two
+strips of bibulous paper or bits of tape and twist them round the tube
+on each side of the scratch, allowing not more than one-eighth of an
+inch between them. Then add a few drops of water to each, till it is
+thoroughly soaked, but not allowing water to run away.  Dry out the
+scratch by a shred of blotting paper.
+
+Turn down the oxygas flame to the smallest dimensions, and then boldly
+apply it with its hottest part playing right into the nick and at a
+single point.  Probably in about two seconds, or less, the tube will
+break.  If it does not, rotate the tube, but still so that the flame
+plays in the nick.  After making the tube very hot all round--if it
+has not broken--apply the flame again steadily at one point for a few
+seconds and then apply a bit of cold iron.  If the tube does not break
+at once during these processes, let it cool, and cut the groove
+deeper;  then try again. [Footnote: This method is continually being
+reinvented and published in the various journals.  It is of unknown
+antiquity.]
+
+Fig. 22.
+
+If the tube breaks after great heating and long efforts, it will
+probably leave incipient cracks running away from the break, or may
+even break irregularly.  A good break is nearly always one that was
+easily made.  If a number of rings have to be cut, or a number of cuts
+made on glass tubes of about the same size, it will be found
+economical in the end to mount a glazier's diamond for the purpose.  A
+simple but suitable apparatus is figured (Fig.  23).
+
+Fig.  23.
+
+The only difficulty is to regulate the position of the diamond so that
+it cuts.  In order to do this, carefully note its cutting angle by
+preliminary trials on sheet glass, and then adjust the diamond by
+clamps, or by wriggling it in a fork, as shown.  Weight the board very
+slightly, so as to give the small necessary pressure, and produce the
+cut by rotating the tube by hand.  When a cut is nearly completed take
+great care that the two ends join, or irregularity will result.  This
+is not always easy to do unless the tube happens to be straight.
+Having got a cut, start a crack by means of a fine light watchmaker's
+hammer, or even a bit of fused glass, and entice the crack round the
+cut by tapping with the hammer or by means of the flame pencil.
+
+If the cut is a true "cut" the tube will break at once.  As a supply
+of electrical current for lighting will, in the near future, be as
+much a matter of course for laboratory purposes as a gas supply, I add
+the following note.  To heat a tube round a scratch, nothing--not
+even the oxygas blow-pipe--is so good as a bit of platinum or iron
+wire electrically heated.  If the crack does not start by considerable
+heating of the glass, stop the current, unwind the wire, and touch the
+glass on the crack either with a bit of cold copper wire or a wet
+match stem. I prefer the copper wire, for in my experience the water
+will occasionally produce an explosion of cracks.  On the other hand,
+the cold wire frequently fails to start a crack.
+
+Judging from the appearance of thick tubes as supplied by the dealers,
+the factory method of cutting off appears to be to grind a nick almost
+through the tube, and right round;  and for really thick glass this is
+the safest but slowest way;  a thin emery wheel kept wet will do this
+perfectly.  Suitable wheels may be purchased from the "Norton" Emery
+Wheel Co.  of Bedford, Mass, U.S.A.--in England through Messrs.
+Churchill and Co.  of London, importers.
+
+Sec. 31.  To blow a Bulb at the End of a Tube.
+
+I must admit at once that this is a difficult operation--at all
+events, if a large bulb is required.  However, all there is to be said
+can be said in few words.  In general, when a bulb is required at the
+end of a tube it will be necessary to thicken up the glass.  A
+professional glass-worker will generally accomplish this by "jumping
+up" the tube, i.e.  by heating it where the bulb is required, and
+compressing it little by little until a sufficient amount of glass is
+collected.  The amateur will probably find that he gets a very
+irregular mass in this way, and will be tempted to begin by welding on
+a short bit of wide and thick tubing preparatory to blowing out the
+bulb.
+
+However, supposing that enough glass is assembled by-either of these
+methods, and that it is quite uniform in thickness, let the thickened
+part be heated along a circle till it becomes moderately soft, and let
+it then be expanded about one-fifth, say by gently blowing.  It is
+perhaps more important to keep turning the glass during bulb-blowing
+than in any other operation, and this both when the glass is in the
+flame and while the bulb is being blown.  It is also very important to
+avoid draughts.  In general, a bulb is best blown with the tube in a
+nearly horizontal position, but sloping slightly upwards from the
+mouth.  If it be noticed that a bulb tends to blow out more at one
+side than another, let the side of greatest protuberance be turned
+down, so that it is at the lowest point, reduce the pressure for an
+instant, and then blow again.  It will be observed that the bulb will
+now expand at the top.
+
+The reason of this is chiefly that the under side cools most rapidly
+(according to Faraday, Chemical Manipulation, Sec. 1194), and
+consequently can expand no further;  but also it is not unlikely that
+the glass tends to flow somewhat from the upper side, which remains
+hot, and consequently the bulb, when the next puff reaches it, will
+tend to yield at this point.  By heating several zones the tube will
+become gradually expanded.
+
+Fig. 24.
+
+Fig.  25.
+
+Fig. 26.
+
+When the length of the thickened part of the tube only slightly
+exceeds its diameter (Fig.  25), let the whole be brought to a
+temperature which, with flint glass, should be just short of that of
+perfect fluidity;  and then, holding the tube horizontally and
+constantly turning it, let the bulb be blown out to its full size,
+noting the appearances and correcting too great protuberance on any
+side by the means above mentioned.  If the bulb appears pear-shaped
+turn the tube so that the melted mass is directed upwards;  if the
+bulb have the contrary fault, correct in the corresponding manner.
+
+The bulb when finished may be lightly tapped on the table, when, if
+there is any weak place owing to inequality of thickness, the bulb
+will break, and the operation may be started afresh.  "A good bulb is
+round, set truly on the tube, and free from lumps of thick glass or
+places of excessive thinness."  When the amateur has succeeded in
+blowing a bulb two inches in diameter on the end of a strong bit of
+thermometer tube--say for an air thermometer--he may well seek the
+congratulations of his friends.
+
+In case the bulb is not satisfactory on a first attempt, it may be
+melted down again, if the following precautions are taken.  Directly
+creases begin to appear in the bulb let it be withdrawn from the
+flame, and gently blown till the creases come out.  By alternate
+heating and blowing the glass can be got back to its original form, or
+nearly so, but unless the operator shows great skill and judgment, the
+probability is that the glass will be uneven.  By heating and keeping
+the thicker parts in the higher position, and blowing a little now and
+again, the glass may be got even, and a new attempt may be made.  It
+must not be supposed that this process can be carried on indefinitely,
+for the glass tends to lose its viscous properties after a time, or,
+at all events, it "perishes" in some way, especially if it has been
+allowed to get very thin;  consequently too frequent attempts on the
+same glass are unprofitable.  Two or three trials are as many as it
+generally pays to make.  As a rule the largest possible flame may be
+used with advantage in this operation.
+
+Sec. 32.  To blow a bulb in the middle of a tube, the procedure is much
+like that already treated, but the manipulation is, if anything,
+more difficult, for the further end of the tube must be carried and
+turned as well as the end which is held to the lips.
+
+Sec. 33.  To make a side Weld.
+
+This is by no means difficult, but is easier with lead glass than with
+soda glass.  The tube to which it is desired to make a side connection
+having been selected, it is closed at one end by rubber tube stops, or
+in any other suitable manner.  The zone of the proposed connection is
+noted, and the tube is brought to near softness round that circle (if
+the tube is made actually soft, inconvenience will arise from the
+bending, which is sure to occur).  Two courses are then open to the
+operator, one suitable to a thick tube, the other to a tube of
+moderate thickness.
+
+Taking the former first.  Provide a piece of glass rod and warm its
+end.  Direct a small flame against the spot on the thick tube where
+the proposed joint is to be.  When the glass becomes almost
+incandescent at this spot, put the end of the rod against it and draw
+out a thread of glass till sufficient "metal" has been removed.  Then
+fuse off the thread close to the tube.
+
+Fig.  27.
+
+The subsequent procedure is the same as for thin tubes.  In this case
+heat the spot by the smallest flame available, and get the spot very
+hot.  Blow it out gently into a bubble, perhaps extending to a height
+equal to its diameter.  Then heat the top of the bubble till it is
+incandescent and blow violently.  This will produce an opening fringed
+by glass so thin as to exhibit interference colours.  Remove the filmy
+part, and heat the frayed edges till they cohere and form an incipient
+tube.  If the flame has been of a correct size, the tube will now be
+of the same diameter as the tube to be welded on, and will project
+perhaps one-sixteenth of an inch from the surface of the main tube
+(Fig.  28).
+
+Fig. 28.
+
+Fig.  29.
+
+When this stage is reached, again heat the tube all round till it
+nearly softens, and by means of the other hand heat the end of the
+other tube which it is proposed to weld.  Just before the main tube
+actually softens, turn it so as to heat the edges of the aperture, and
+at the same time get the end of the side tube very hot.  Take both out
+of the flame for an instant, and press the parts together, instantly
+slightly withdrawing the side tube.  If the operation is well
+performed, it will be found that the point of maximum thickness of
+glass is now clear of the main tube.  The joint is then to be heated
+all round and blown out--a rather awkward operation, and one
+requiring some practice, but it can be done.
+
+Fig.  30.
+
+If great strength is wanted, heat the main tube all round the joint
+bit by bit, and blow each section slightly outwards.  If the operator
+is confident in his skill, he should then heat the whole joint to the
+softening point, blow it out slightly, and then adjust by pulling and
+pushing.  Cool first in the gas flame, and then plunge the joint into
+the asbestos and cover it up--or if too large, throw the asbestos
+cloth round it.
+
+In the case of soda glass this final "general heat" is almost
+essential, but it is not so with flint glass, and as the general heat
+is the most difficult part of the job, it will be found easier to use
+lead glass and omit the general heating.  With soda glass a very small
+irregularity will cause the joint to break when cold, but flint glass
+is much more long-suffering.  It is easy to perform the above
+operation on small tubes.  For large ones it will be found best to
+employ flint glass and use the clip stands as in the case of direct
+welds, treated above, but, of course, with suitable modifications.
+Never let the main tube cool after the hole is made until the work is
+done.
+
+Sec. 34.  Inserted Joints.
+
+In many instances the performance of apparatus is much improved by
+joints of this kind, even when their use is not absolutely essential.
+
+There are two ways in which inserted joints may be made.  The first
+method is the easier, and works well with flint glass; but when one
+comes to apply it to soda glass there is a danger of the glass
+becoming too thick near the joint, and this often leads to a cracking
+of the joint as the glass cools.
+
+Fig.  31.
+
+Suppose it is desired to insert the tube B into the tube A (Fig.  31).
+Begin by reducing the size of the end of tube A till B will just slip
+in quite easily.  With B about one-quarter inch in diameter, a
+clearance of about one-twentieth of an inch, or less, in all (i.e.
+one-fortieth of an inch on each side) will be proper.
+
+Heat B by itself at the proposed zone of junction, and blow out a very
+narrow ring;  then compress this slightly so that it forms an almost
+closed ring of glass.  The figure refers to the close of this
+operation (Fig.  31, B).  It does not matter much whether the ring
+remains a mere flattened bulb, or whether it is a solid ring, but it
+must be one or the other.  Some judgment must be exercised in
+preparing the ring.  In general, the beginner will collect too much
+glass in the ring, and consequently the joint, when made, will either
+be thick and liable to crack easily, or it will be blown out into an
+erratic shape in endeavours to reduce this thickness.  Accordingly,
+the operator will, if necessary, thin the tube B by drawing slightly,
+if he considers it desirable, before the little enlargement is blown
+out.  In general, two heats must be used for this operation.
+
+Fig.  32.
+
+Get the approximating parts of both A and B up to a temperature just
+below that at which they will adhere, and having closed the other end
+of A, place B carefully within it up to the ring, and if it can be
+arranged, have a mica wad in A, with a central hole through which the
+end of B can project.  This will very much facilitate the operation,
+especially if B is long, but may be dispensed with by the exercise of
+care and skill.
+
+The operation is now simple.  Fuse the junction and press the tubes
+lightly together, being careful not to collect more glass than can be
+helped;  finally, blow out the joint and reduce the thickness by mild
+drawing (Fig.  33).  In order to make a really good joint, two points
+must be particularly attended to--the rim must be thin and its plane
+perfectly perpendicular to the axis of tube B;  the end of tube A must
+be cut off quite clean and perpendicular to its axis before B is
+inserted.  So important are these conditions--especially the latter
+that the writer has even occasionally used the grindstone to get the
+end of A into a proper condition, an admission which will probably
+earn the contempt of the expert glass-worker.
+
+Fig. 33.
+
+Now for the second method, which is often practised in Germany, where
+soda glass is chiefly used.  With this glass the chief point is to get
+a very even and not too thick ring at the junction, and consequently
+the extra thickening produced by making a rim on B is rather a
+drawback.  The method consists in cutting off from B the length which
+it is desired to insert, slipping this into A (which may be an
+otherwise closed bulb, for instance), and then gradually melting up
+the open end of A till the piece of B inside will no longer fall out.
+By holding the joint downwards so that the inserted portion of B rests
+on the edges of the opening, a joint may be made with the minimum
+thickening.
+
+The external part of B, previously heated, is then applied, and the
+joint subjected to a "general" heat and blown out.  Very nice joints
+may be made by this method, and it is perhaps the better one where the
+external part of B is to be less in diameter than the inserted part.
+It was in this manner that the writer was taught to make glass
+velocity pumps, one of which, of a good design, is figured as an
+example.
+
+In all cases good annealing should follow this operation.  If the
+inserted part of the inner tube (B) is anything like an inch in
+diameter, and especially if it is of any length, as in some forms of
+ozone apparatus, or in a large Bunsen's ice calorimeter, the
+arrangements for supporting the inner part must be very good.  A
+convenient way of proceeding when the inner tube is well supported is
+to make the mouth of A only very little larger than the diameter of B,
+so that B will only just slip in.  Then the mouth of A and the zone of
+B may be heated together, and B blown out upon A.  This, of course,
+must be arranged for, if necessary, by temporarily stopping the inner
+end of B.
+
+The inner support of B should be removed as soon as practicable after
+the joint is made, or, at all events, should not be perfectly rigid; a
+tightly-fitting cork, for instance, is too rigid.  The reason is, of
+course, that in cooling there may be a tendency to set B a little to
+one side or the other, and if it is not free to take such a set, the
+joint most probably will give way.  Good annealing both with flame and
+asbestos is a sine qua non in all inserted work.
+
+Fig. 34.
+
+Sec. 35.  Bending Tubes.
+
+I have hitherto said nothing about bending tubes, for to bend a tube
+of a quarter of an inch in diameter, and of ordinary thickness, is
+about the first thing one learns in any laboratory, while to bend
+large tubes nicely is as difficult an operation as the practice of
+glass-blowing affords.  However, even in bending a narrow tube it is
+possible to proceed in the wrong way.  The wrong way is to heat a
+short length of the tube and then bend it rapidly, holding the plane
+of the bend horizontal.  The right way, per contra, is to use a
+batswing burner to heat, say, two inches of the tube with constant
+turning till it is very soft, and then, holding the glass so that the
+bend will be in a vertical plane passing through one eye (the other
+being shut), to make the bend rather slowly.
+
+If an exact angle is required, it is as well to have it drawn out on a
+sheet of asbestos board.  In this case bend the glass as described
+till it is approximately right, and finish by laying it on the
+asbestos board and bringing it up to the marks.  A suitable bit of
+wood may be substituted for the asbestos on occasion.
+
+N.B: The laboratory table is not a suitable piece of wood.  A
+right-angled bend is often wanted.  In this case the corner of a table
+will serve as a good guide to the eye, the glass being finished by
+being held just above it.  If great accuracy is wanted, make a wooden
+template and suspend it by a screw from the side of the table, so that
+the vertex of the gauge for the interior angle projects downwards,
+then finish by bending the tube round it.  The wood may be about half
+an inch thick.
+
+If a sharp bend is required, heat the tube in the blow-pipe, and bend
+it rapidly, blowing out the glass meanwhile.  The reason why a long
+bend should be held in a vertical plane is that the hot part tends to
+droop out of the plane of the bend if the latter be made in a
+horizontal position.  To bend a tube above half an inch in diameter is
+a more or less difficult operation, and one which increases in
+difficulty as the diameter of the tube increases.
+
+A U-tube, for instance, may be made as follows:  Use the four
+blow-pipe arrangement so as to heat a fair length of tube, and get,
+say, two inches of tube very hot--almost fluid, in fact--by means of
+the carbon block supported from a stand.  Remove the tube rapidly from
+the flame and draw the hot part out to, say, three inches.  Then,
+holding the tube so as to make the bend in a vertical plane, bend it
+and blow it out together to its proper size.
+
+This operation seems to present no difficulties to experienced
+glass-workers, even with tubes of about one inch in diameter, but to
+the amateur it is very difficult.  I always look on a large U-tube
+with feelings of envy and admiration, which the complex trick work of
+an elaborate vacuum tube does not excite in the least.  It will be
+noted that this method may, and often does, involve a preliminary
+thickening of the glass.
+
+With tubes over an inch in diameter I have no idea as to what is the
+best mode of procedure--whether, for instance, a quantity of sand or
+gas coke might not be used to stuff out the tube during bending, but
+in this case there would be the difficulty of removing the fragments,
+which would be sure to stick to the glass.
+
+Of course, if the bend need not be short, the tube could be softened
+in a tube furnace and bent in a kind of way.  I must admit that with
+tubes of even less than one inch in diameter I have generally managed
+best by proceeding little by little.  I heat as much of the glass as I
+can by means of a gigantic blow-pipe, having a nozzle of about an inch
+in diameter, and driven by a machine-blower.
+
+When I find that, in spite of blowing, the tube begins to collapse, I
+suspend operations, reheat the tube a little farther on, and so
+proceed.  If by any chance any reader knows a good laboratory method
+of performing this operation, I hope he will communicate it to me.
+After all, the difficulty chiefly arises from laboratory heating
+appliances being as a rule too limited in scope for such work.
+
+The bending of very thin tubes also is a difficulty.  I have only
+succeeded here by making very wide bends, but of course the blowing
+method is quite applicable to this case, and the effect may be
+obtained by welding in a rather thicker bit of tube, and drawing and
+blowing it till it is of the necessary thinness.  This is, however, a
+mere evasion of the difficulty.
+
+Sec. 36.  Spiral Tubes.
+
+These are easily made where good heating apparatus is available.  As,
+however, one constantly requires to bend tubes of about one-eighth
+inch in diameter into spirals in order to make spring connections for
+continuous glass apparatus, I will describe a method by which this is
+easily done.  Provide a bit of iron pipe about an inch and a quarter
+in outside diameter.  Cover this with a thick sheath of asbestos
+cloth, and sew the edges with iron wire.  Hammer the wire down so that
+a good cylindrical surface is obtained.  Make two wooden plugs for the
+ends of the iron pipe.  Bore one to fit a nail, which may be held in a
+small retort clip, and fasten a stout wire crank handle into the other
+one.  Support the neck of the handle by means of a second clip.  In
+this way we easily get a sort of windlass quite strong enough for our
+purpose.
+
+Fig.   35.
+
+Provide a large blow-pipe, such as the blow-pipe of a Fletcher
+crucible furnace, Select a length of tubing and clean it.  Lash one
+end to the cylinder by means of a bit of wire, and hold the other end
+out nearly horizontally.  Then start the blow-pipe to play on the tube
+just where it runs on to the asbestos cylinder, and at first right up
+to the lashing.  Get an attendant to assist in turning the handle of
+the windlass, always keeping his eye on the tube, and never turning so
+fast as to tilt the tube upwards.  By means of the blow-pipe, which
+may be moved round the tubing, heat the latter continuously as it is
+drawn through the flame, and lay it on the cylinder in even spirals.
+
+If the tubing is thin, a good deal of care will have to be exercised
+in order to prevent a collapse.  A better arrangement, which, however,
+I have not yet tried, would, I think, be to replace the blow-pipe by
+two bats-wing burners, permanently fastened to a stand, one of them
+playing its flame downwards on to the top of the flame of the other.
+The angle between the directions of the jets might be, say, 130 deg., or
+whatever is found convenient.  In this way the glass would not be so
+likely to get overheated in spots, and better work would doubtless
+result.  However, I have made numbers of perfectly satisfactory
+spirals as described.  Three or four turns only make a sufficiently
+springy connection for nearly all purposes.
+
+Sec. 37.  On Auxiliary Operations on Glass:-
+
+Boring Holes through Glass:-  This is much more easily done than is
+generally supposed.  The best mode of procedure depends on the
+circumstances.  The following three cases will be considered:-
+
+1.  Boring holes up to one-quarter inch diameter through thick glass
+(say over one-eighth inch), or rather larger holes through thin glass.
+
+2.  Boring holes of any size through thick glass.
+
+3.  Boring round holes through ordinary window glass.
+
+Sec. 38.  Boring small Holes.
+
+Take a three-cornered file of appropriate dimensions, and snip the
+point off by means of a hammer;  grind out most of the file marks to
+get sharp corners.  Dip the file in kerosene, and have plenty of
+kerosene at hand in a small pot.  Place the broken end of the file
+against the glass, and with considerable pressure begin to rotate it
+(the file) backwards and forwards with the fingers, very much as one
+would operate a bradawl against a hard piece of wood.  The surface of
+the glass will shortly be ground away, and then the file bradawl will
+make much quicker progress than might be expected.  Two or three
+minutes should suffice to bore a bit of sheet window-glass.
+
+The following points require attention:
+
+(1) Use any quantity of oil.
+
+(2) After getting through the skin  reduce the pressure on the file.
+
+(3) Be sure to turn the file backwards and forwards through a complete
+revolution at least.
+
+(4) When the hole is nearly through reduce the pressure.
+
+(5) When the hole is through the glass be exceedingly careful not to
+force the file through too rapidly, otherwise it will simply act as a
+wedge and cause a complete fracture.
+
+(6) In many cases it is better to harden the file in mercury before
+commencing operations; both files and glass differ so much in hardness
+that this point can only be decided by a trial.  If it is found
+necessary to harden the file, use either a large blow-pipe and a coke
+or charcoal bed, or else a small forge.  A small blowpipe, such as is
+generally found in laboratories, does more harm than good, either by
+burning the end of the file or raising it to an insufficient
+temperature.
+
+(7) To sharpen the file, which is often necessary after passing
+through the "skin" of the glass, put it in a vice so that the point
+just protrudes clear of the jaws.  Then, using a bit of waste iron as
+an intermediary anvil or punch, knock off the least bit from the
+point, so as to expose a fresh natural surface.  The same result may
+be brought about by the use of a pair of pliers.
+
+If several holes have to be bored, it is convenient to mount the file
+in the lathe and use a bit of flat hard wood to press up the glass by
+means of the back rest.  A drilling machine, if not too heavy, does
+very well, and has the advantage of allowing the glass to remain
+horizontal so that plenty of oil can be kept in the hole.
+
+Use a very slow speed in either case--much slower than would be used
+for drilling wrought iron.  It is essential that the lubricant should
+flow on to the end of the file very freely, either from a pipette or
+from the regular oil-feed.  If a little chipping where the file
+pierces the back surface is inadmissible, it is better, on the whole,
+to finish the bore by hand, using a very taper file.  It is not
+necessary to use a special file for the lathe, for a well-handled file
+can be chucked very conveniently in a three-jaw chuck by means of the
+handle.
+
+Mr. Shenstone recommends a lubricant composed of camphor dissolved in
+turpentine for general purposes.  With the object of obtaining some
+decisive information as to the use of this lubricant, and to settle
+other points, I made the following experiments.  Using an old
+three-cornered French file, I chipped off the point and adjusted the
+handle carefully.  I also ground out the file marks near the point,
+without hardening the file in mercury.  Using kerosene and turpentine
+and camphor, I began to bore holes in a hard bit of 3/32 inch window
+glass.
+
+Each hole was bored to about one-eighth inch in diameter in four
+minutes with either lubricant.  After hardening the file in mercury
+and using kerosene, I also required four minutes per hole.  After
+mounting the file in a lathe which had been speeded to turn up brass
+rods of about one-half-inch diameter, and therefore ran too fast, I
+required one and a half minutes per hole, and bored them right
+through, using kerosene.  On the whole, I think kerosene does as well
+as anything, and for filing is, I think, better than the camphor
+solution.  However, I ought to say that the camphor-turpentine
+compound has probably a good deal to recommend it, for it has survived
+from long ago.  My assistant tells me he has seen his grandfather use
+it when filing glass.
+
+I beg to acknowledge my indebtedness to Mr. Pye, of the Cambridge
+Scientific Instrument Company, for showing me in 1886 (by the courtesy
+of the Company) the file method of glass-boring;  it is also described
+by Faraday in Chemical Manipulation, 1228.
+
+It is not necessary, however, to use a file at all, for the twist
+drills made by the Morse Drill Company are quite hard enough in their
+natural state to bore glass.  The circumferential speed of the drill
+should not much exceed 10 feet per minute.  In this way the author has
+bored holes through glass an inch thick without any trouble except
+that of keeping the lubricant sufficiently supplied.  For boring very
+small holes watchmaker's drills may be used perfectly well, especially
+those tempered for boring hardened steel.  The only difficulty is in
+obtaining a sufficient supply of the lubricant, and to secure this the
+drill must be frequently withdrawn.
+
+My reason for describing the file method at such length is to be found
+in the fact that a Morse drill requires to be sharpened after drilling
+glass before it can be used in the ordinary way, and this is often a
+difficulty.
+
+I ought to say that I have never succeeded in boring the barrel of a
+glass tap by either of these methods.  [Footnote: I have been lately
+informed that it is usual to employ a splinter of diamond set in a
+steel wire holder both for tap boring and for drilling earthenware for
+riveting.  The diamond must, of course, be set so as to give
+sufficient clearance for the wire holder.
+
+For methods of using and setting diamond tools see Sec. 55.  It will
+suffice to say here that a steel wire is softened and filed at one end
+so as to form a fork;  into this the diamond is set by squeezing with
+pliers.  The diamond is arranged so as to present a point in the axis
+of the wire, and must not project on one side of the wire more than on
+the other.  It is not always easy to get a fragment satisfying these
+conditions, and at the same time suitable for mounting.  A drop of
+solder occasionally assists the process of setting the diamond.
+
+In drilling, the diamond must be held against the work by a constant
+force, applied either by means of weight or a spring.  I made many
+trials by this method, using a watchmaker's lathe and pressing up the
+work by a weight and string, which passed over a pulley.  I used about
+40 ounces, and drilled a hole 3/32 in diameter in flint glass at a
+speed of 900 revolutions per minute to a depth of one-eighth of an
+inch in eight minutes.  I used soap and water as a lubricant, and the
+work was satisfactory.
+
+Since this was set up, I have been informed by Mr. Hicks of Hatton
+Garden that it is necessary to anneal glass rod by heating it up to
+the softening point and allowing it to cool very slowly under red-hot
+sand or asbestos before boring.  If this be done, no trouble will be
+experienced.  The annealing must be perfect.]
+
+Sec. 39.  For boring large holes through thick glass sheets, or, indeed,
+through anything where it is necessary to make sure that
+no accident can happen, or where great precision of position and form
+of hole is required, I find a boring tube mounted as shown in the
+picture (Fig.  36) is of great service.  Brass or iron tube borers do
+perfectly well, and the end of the spindle may be provided once for
+all with a small tube chuck, or the tubes may be separately mounted as
+shown.  A fairly high speed is desirable, and may be obtained either
+by foot, or, if power is available, is readily got by connecting to
+the speed cone of a lathe, which is presumably permanently belted to
+the motor.
+
+Fig.  36.
+
+After trying tubes armed with diamond dust, as will be presently
+explained, I find that emery and thin oil or turpentine, if liberally
+supplied below the glass, will do very nearly as well.  The tube
+should be allowed to rise from the work every few seconds, so as to
+allow of fresh emery and oil being carried into the circular grooves.
+This is done by lifting the hinged upper bearing, the drill being
+lifted by a spiral spring between the pulley and the lower bearing
+shown at B.  The glass may be conveniently supported on a few sheets
+of paper if flat, or held firm in position by wooden clamps if of any
+other shape.  In any case it should be firmly held down and should be
+well supported.  Any desired pressure upon the drill is obtained by
+weighting the hinged board A.
+
+Sec. 40.  The following method was shown to me by Mr. Wimshurst, but I
+have not had occasion to employ it myself.  It is suitable for boring
+large holes through such glass as the plates of Mr. Wimshurst's
+Influence machines are usually made of.  A diamond is mounted as the
+"pencil" of a compass, and with this a circle is drawn on the glass in
+the desired position.  The other leg of the compass of course rests on
+a suitable washer.
+
+To the best of my recollection the further procedure was as follows.
+A piece of steel rod about one-eighth inch in diameter was ground off
+flat and mounted in a vice vertically, so as to cause its plane end to
+form a small horizontal anvil.  The centre (approximately) of the
+diamond-cut circle of the glass was laid on this anvil so as to rest
+evenly upon it, and the upper surface (i.e.  that containing the cut)
+was then struck smartly with a hammer, completely pulverising the
+glass above the anvil.  The hole was gradually extended in a similar
+manner right up to the diamond cut, from which, of course, the glass
+broke away.
+
+A similar method has long been known to glaziers, differing from the
+preceding in that a series of diamond cuts are run across the circle
+parallel to two mutually perpendicular diameters.  A smart tap on the
+back of the scored disc will generally cause the fragments to tumble
+out.  I have never tried this myself, but I have seen it done.
+
+Large discs may easily be cut from sheet glass by drawing a circular
+diamond cut, and gradually breaking away the outer parts by the aid of
+additional cuts and a pair of pliers or "shanks" (see Fig.  44).
+
+Sec. 41.  Operations depending on Grinding: Ground-in Joints.
+
+The process will be perfectly understood by reference to a simple
+case.  Suppose it is desired to grind the end of a tube into the neck
+of a bottle.  If a stoppered bottle is available, the stopper must be
+taken out and measured as to its diameter at the top and bottom.
+Select a bit of tube as nearly as possible of the same diameter as the
+stopper at its thickest part.  Draw down the glass in the blow-pipe
+flame rather by allowing it to sink than by pulling it out.  After a
+few trials no difficulty will be experienced in making its taper
+nearly equal to that of the stopper, though there will in all
+probability be several ridges and inequalities.  When this stage is
+reached anneal the work carefully and see that the glass is not too
+thin.  Afterwards use emery and water, and grind the stopper into the
+bottle.
+
+There are six special directions to be note
+
+(1 )Turn the stopper through at least one revolution in each
+direction.
+
+(2) Lift it out often so as to give the fresh emery a chance of
+getting into the joint.
+
+(3) Rotate the bottle as well as the stopper in case there is any
+irregularity in the force brought to bear, which might cause one side
+of the neck to be more ground than another, or would cause the tube to
+set rather to one side or the other.
+
+(4) Use emery passing a 50 sieve, i.e.  a sieve with fifty threads to
+the inch run (see Sec. 144) to begin with, and when the stopper nearly
+fits, wash this thoroughly away, and finish with flour emery,
+previously washed to get rid of particles of excessive size;  the
+process of washing will be fully discussed in the chapter on
+glass-grinding, which see.
+
+(5) Any degree of fineness of surface may be obtained by using graded
+emery, as will be explained, but, in general, it is unnecessary to
+attempt a finer surface than can be got with washed flour emery.  A
+superficial and imperfect polish may be given by grinding for a short
+time with powdered pumice stone.
+
+(6) If the proper taper is not attained by blowing, or if ridges are
+left on the tapered part, the process may be both hastened and
+improved by giving the taper a preliminary filing with a
+three-cornered file and kerosene, just as one would proceed with iron
+or brass.  A little filing will often save a good deal of grinding and
+make a better job.
+
+If a bottle without a tapered neck is to be employed, it is as well to
+do the preliminary grinding by means of a cone turned up from a bit of
+cast iron.  This is put in the lathe and pushed into the mouth of the
+bottle, the latter being supported by the hands.  Use about the same
+surface speed as would be employed for turning cast iron.  In this
+case the emery is better used with kerosene.
+
+If a cylindrical bit of cast iron about an inch in diameter is turned
+down conically nearly to a point, it will save a good deal of trouble
+in making separate cones.  If it gets ground into rings, and it
+becomes necessary to turn it up, use a diamond tool until the skin is
+thoroughly removed;  the embedded emery merely grinds the edge off any
+ordinary steel tool.
+
+For diamond tools see Sec. 55.
+
+Sec. 42.  Use of the Lathe in Glass-working.
+
+If it is necessary to remove a good deal of glass, time may be saved
+by actually turning the glass in a lathe.  According to the direction
+given above for grinding a tube into the neck of a bottle, very little
+glass need be removed if the drawing down is well done, so that for
+this purpose turning is often unnecessary.
+
+If the taper of the stopper be small and it is permissible to use a
+thick tube, or if a solid stopper only has to be provided, or an old
+stopper quickly altered to a new form, turning is very useful.  The
+glass may be "chucked" in any suitable manner, and run at a speed not
+exceeding 10 feet per minute.  Prepare a three-cornered file by
+mercury-hardening and by grinding the end flat so as to form a cutting
+angle of about 80 deg., and use a moderate amount of kerosene lubrication,
+i.e.  enough to keep the glass damp, but even this is not essential.
+Use the file as an ordinary brass turning tool, and press much more
+lightly than for metal turning.  The glass will be found to scrape off
+quite pleasantly.
+
+By chucking glass tubes on wooden mandrells the ends may be nicely
+turned in this manner ready for accurate closing by glass plates.
+
+The process of grinding also is made much more rapid--at all events
+in the earlier stages--by chucking either the stopper or the bottle
+and holding the other member in the fingers, or in a wooden vice held
+in the hands.  The finishing touches are best given by hand.
+
+I ought to say that I think a good deal of glass-grinding, as
+practised in laboratories, might be advantageously replaced by glass
+turning or filing and certainly will be by any one who will give these
+methods a trial.
+
+If one tube is to be ground into another, as in grinding a retort into
+a receiver, the latter must be drawn down from a larger piece, few
+beginners being able to widen a tube by the method explained with
+sufficient ease and certainty.  The other operations are similar to
+the operations above described.
+
+Sec. 43.  Funnels often require to be ground to an angle of 60 deg..  For
+this purpose it is well to keep a cast-iron cone, tapering from
+nothing up to four inches in diameter.  This may be mounted on a
+lathe, and will be found of great use for grinding out the inside of
+funnels.  Care must be taken to work the funnel backwards and
+forwards, or it will tend to grind so as to form rings, which
+interfere with filtering.  A rough polish may be given on the lines
+explained in the next section.
+
+Sec. 44.  A rough polish may be easily given to a surface which has been
+finished by washed flour emery, in the following manner.  Turn up a
+disc of soft wood on the lathe, and run it at the highest wood-turning
+speed.  Rub into the periphery a paste of sifted powdered pumice stone
+and water.
+
+Any fairly smooth ground glass surface may be more or less polished by
+holding it for a moment against the revolving disc.  Exact means of
+polishing will be described later on.  Meanwhile this simple method
+will be found both quick and convenient, and is often quite sufficient
+where transparency, rather than figure, is required.  I daresay a fine
+polish may be got on the same lines, using putty powder or washed
+rouge (not jewellers' rouge, which is too soft, but glass-polishers'
+rouge) to follow the pumice powder, but I have not required to try
+this.
+
+Sec. 45.  It is sometimes required to give to ground glass surfaces a
+temporary transparency.  This is to be done by using a film of oil of
+the same refractive index as the glass.  Cornu has employed a varnish
+consisting of a mixture of turpentine and oil of cloves, but the
+yellow-brown colour of the latter is often a disadvantage.  It will be
+found that a mixture of nut oil and oil of bitter almonds, or of
+bromo-napthalene and acetone, can be made of only a faint yellow
+colour;  and by exact adjustment of the proportions will have the same
+refractive index for any ray as crown glass (ordinary window glass).
+
+Procure a sample of the glass and smash it up to small fragments in an
+iron mortar.  Sift out the fine dust and the larger pieces;  bits
+about as large as small beads--say one-sixteenth inch every way--do
+very well.  Boil the sifted glass with strong commercial hydrochloric
+acid to remove iron, wash with distilled water and a few drops of
+alcohol, dry on blotting paper in the sun or otherwise.  Put the dry
+glass into a bottle or beaker, and begin by adding almond oil (or
+bromo-napthalene), then add nut oil (or acetone) till the glass
+practically disappears when examined by sodium light, or light of any
+other wave-length, as may be required.
+
+The adjustment of the mixture is a matter of great delicacy, one drop
+too much of either constituent, in, say, 50 cubic centimetres, makes
+all the difference.  The final adjustment is best accomplished by
+having two mixtures of the oils, one just too rich in almond, the
+other in nut oil;  by adding one or other of these, the required
+mixture is soon obtained.
+
+It is to be noted
+
+(1) That adjustment is only perfect for light of one wave-length.
+
+(2) That adjustment is only perfect at one temperature.
+
+On examining a bottle of rather larger fragments of glass immersed in
+an adjusted mixture by ordinary daylight, a peculiarly beautiful play
+of colours is seen.
+
+Of course, if it is only desired to make ground glass fairly
+transparent, these precautions are unnecessary, but it seemed better
+to dispose of the matter once for all in this connection.
+
+M. Cornu's object was to make a varnish which would prevent reflection
+from the back of a photographic plate on to the film. I have had
+occasion to require to do the same when using a scale made by cutting
+lines through a film of black varnish on a slip of glass.  This
+succeeded perfectly by making the varnish out of Canada balsam stained
+with a black aniline dye.
+
+Mr. Russell, Government Astronomer of New South Wales, finds that the
+"halation" of star photographs can be prevented by pouring over the
+back of the plate a film of collodion suitably stained.
+
+Sec. 46.  Making Ground Glass.
+
+This is easily done by rubbing the surface of polished glass with a
+bit of cast iron and washed "flour of emery." Of course, if the
+fineness of grain of the surface is of importance, appropriate sizes
+of emery must be employed.  The iron may be replaced by a bit of glass
+cut with transverse grooves to allow the emery to distribute itself,
+or even by a bit of glass without such grooves, provided it does not
+measure more than one or two inches each way.  If great speed is an
+object rather than the fineness of the surface, use a bit of lead and
+coarse emery, say any that will pass a sieve with fifty threads to the
+inch.
+
+It may perhaps be mentioned here that it is a pity to throw away emery
+which has been used between glass and glass.  In the chapter dealing
+with fine optical work the use of emery of various grades of fineness
+will be treated, and the finer grades can only be obtained (to my
+knowledge) from emery which has been crushed in the process of glass
+or metal grinding, especially the former.  A large jam-pot covered
+with a cardboard lid does well as a receptacle of washings.
+
+Sec. 47.  Glass-cutting.
+
+This is an art about which more can be learned in five minutes by
+watching it well practised than by pages of written description.  My
+advice to any one about to commence the practice of the art would be
+to make friends with a glazier and see it done.  What follows is
+therefore on the supposition that this advice has been followed.
+
+After some experience of cutters made of especially hardened steel, I
+believe better work can generally be got out of a diamond, provided
+the cost is not an objection.  It is economy to pay a good price for a
+good diamond.  As is well known, the natural angle of the crystal
+makes the best point, and a person buying a diamond should examine the
+stone by the help of a lens, so as to see that this condition is
+fulfilled.  The natural angle is generally, if not always, bounded by
+curved edges, which have a totally different appearance from the sharp
+edges of a "splinter."
+
+When a purchase is to be made, it is as well for the student to take a
+bit of glass and a foot-rule with him, and to test the diamond before
+it is taken away.  When a good diamond has been procured, begin by
+taking cuts on bits of clean window glass until the proper angle at
+which to hold the tool is ascertained.  Never try to cut over a
+scratch, if you value your diamond, and never press hard on the glass;
+a good cut is accompanied by an unmistakable ringing sound quite
+different from the sound made when the diamond is only scratching.
+
+Perhaps the most important advice that can be given is, Never lend the
+diamond to anybody--under any circumstances.
+
+The free use of a diamond is an art which the physicist will do well
+to acquire, for quite a variety of apparatus may be made out of glass
+strips, and the accuracy with which the glass breaks along a good cut
+reduces such an operation as glass-box-making to a question of
+accurate drawing.
+
+Sec. 48.  Cementing.
+
+One of the matters which is generally confused by too great a
+profusion of treatment is the art of cementing glass to other
+substances.
+
+The following methods will be found to work, subject to two
+conditions:
+
+(1) The glass must be clean;
+
+(2) it must be hot enough to melt the cement.
+
+For ordinary mending purposes when the glass does not require to be
+placed in water (especially if hot) nothing is better than that kind
+of glue which is generally called "diamond cement."  This may be
+easily made by dissolving the best procurable isinglass in a mixture
+of 20 per cent water and 80 per cent glacial acetic acid--the exact
+proportions are not of consequence.
+
+First, the isinglass is to be tightly packed into a bottle with a wide
+neck, then add the water, and let the isinglass soak it up.
+Afterwards pour in the acetic acid, and keep the mixture near 100 deg.C.
+for an hour or two on the water bath--or rather in it.  The total
+volume of acetic acid and water should not be more than about half of
+the volume of isinglass when the latter is pressed into the bottle as
+tightly as possible.
+
+The proper consistency of the cement may be ascertained by lifting a
+drop out of the bottle and allowing it to cool on a sheet of glass.
+In ten minutes it ought not to be more than slightly sticky, and the
+mass in the bottle, after standing a few hours cold, should not be
+sticky at all, and should yield, jelly-like, to the pressure of the
+finger to only a slight degree.  If the glue is too weak, more
+isinglass may be added (without any preliminary soaking).
+
+A person making the mixture for the first time almost always gets it
+too weak.  It is difficult to give exact proportions by weight, as
+isinglass and gelatine (which may replace it) differ greatly in
+quality.  This cement is applied like glue, and will cement nearly
+anything as well as glass.  Of course, as much cement as possible must
+be squeezed out of any joint where it is employed.  The addition of
+gums, as recommended in some books, is unnecessary.
+
+Ordinary glue will serve perfectly for cementing glass to wood.
+
+"Chipped glass" ware is, I understand, made by painting clean glass
+with glue.  As the glue dries and breaks by contraction, it chips off
+the surface of the glass.  I have never seen this done.  In nearly all
+cases where alcohol is not to be employed very strong joints may be
+made by shellac.  Orange shellac is stronger than the "bleached"
+variety.
+
+A sine qua non is that the glass be hot enough to melt the shellac.
+The best way is to heat the glass surfaces and rub on the shellac from
+a bit of flake;  the glass should not be so hot as to discolour the
+shellac appreciably, or its valuable properties will be partly
+destroyed.  Both glass surfaces being thus prepared, and the shellac
+being quite fluid on both, they may be brought together and clamped
+tightly together till cool.  Shellac that has been overheated, or
+dissolved in alcohol, or bleached, is of little use as compared with
+the pale orange flaky product.  Dark flakes have probably been
+overheated during the preliminary refining.
+
+For many purposes a cement is required capable of resisting carbon
+bisulphide.  This is easily made by adding a little treacle (say 20
+per cent) to ordinary glue.  Since the mixture of glue and treacle
+does not keep, i.e.  it cannot be satisfactorily melted up again after
+once it has set, no more should be made up than will be wanted at the
+time.  If the glue be thick, glass boxes for carbon disulphide may be
+easily put together, even though the edges of the glass strips are not
+quite smooth, for, unlike most cements, this mixture remains tough,
+and is fairly strong in itself.
+
+I have found by experiment that most fixed and, to a less degree,
+essential oils have little or no solvent action on shellac, and I
+suspect that the same remark applies to the treacle-glue mixture, but
+I have not tried.  Turpenes act on shellac slightly, but mineral oils
+apparently not at all.  The tests on which these statements are based
+were continued for about two years, during which time kerosene and
+mineral oils had no observable effect on shellac--fastened
+galvanometer mirrors.
+
+Sec. 49.  Fusing Electrodes into Glass.
+
+This art has greatly improved since the introduction of the
+incandescent lamp; however, up to the present, platinum seems to
+remain the only substance capable of giving a certainly air-tight
+result.  I have not tried the aluminium-alumina method.
+
+Many years ago it was the fashion to surround the platinum wire with a
+drop of white enamel glass in order to cause better adhesion between
+it and the ordinary glass.  [Footnote: Hittorf and Geissler (Pogg.
+Ann.  1864, Sec. 35;  English translation, Phys.  Soc.  London, p.  138)
+found that it was impossible to make air-tight joints between platinum
+and hard potash glass, but that soft lead glass could be used with
+success as a cement.]  However, in the case of flint glass, if one may
+judge from incandescent lamps, this is not essential--a fact which
+entirely coincides with my own experience.
+
+On the other hand, when sealing electrodes into German glass I have
+often used a drop of enamel with perfect results, though this is not
+always done in Germany.  In all cases, however, in which electrodes
+have to be sealed in--especially when they are liable to heat--I
+recommend flint glass, and in this have the support of Mr. Rain (The
+Incandescent Lamp and its Manufacture, p.  131).  The exact details
+for the preparation of eudiometer tubes are given by Faraday (Chemical
+Manipulation, Sec. 1200).
+
+In view of what has preceded, however, I will content myself with the
+following notes.  Make the hole through which the wire is to protrude
+only slightly larger than the wire itself, and be sure that the latter
+is clean.  Allow the glass to cool sufficiently not to stick to the
+wire when the latter is pushed in.  Be sure that, on heating, the
+glass does not get reduced, and that it flows up to the wire all
+round;  pull and push the wire a little with a pair of pincers, to
+ensure this.
+
+It is not a bad plan to get the glass exceedingly fluid round the
+wire--even if the lump has to be blown out a little afterwards--as it
+cools.  The seal should finally be well annealed in asbestos, but
+first by gradually moving it into the hot air in front of the flame.
+
+It was observed by Professor J.  J.  Thomson and the author some years
+ago (Proc.  Roy.  Soc.  40.  331.  1886) that when very violent
+discharges are taken through lightly sealed-in electrodes in
+lead-glass tubes--say from a large battery of Leyden jars--gas
+appears to be carried into the tube over and above that naturally
+given off by the platinum, and this without there being any apparent
+want of perfection in the seal.  This observation has since been
+confirmed by others.  Consequently in experiments on violent
+discharges in vacuo where certainty is required as to the exclusion of
+air, the seals should be protected by a guard tube or cap containing
+mercury; this must, of course, be put in hot and clean, on hot and
+clean glass, and in special cases should be boiled in situ.
+
+A well-known German physicist (Warburg, I think) recommends putting
+the seals under water, but I cannot think that this is a good plan,
+for if air can get in, why not water?  which has its surface tension
+in its favour.  The same reasoning prevents my recommending a layer of
+sulphuric acid above the mercury-a method used for securing
+air-tightness in "mercury joints" by Mr. Gimingham, Proc.  R.  S.
+1874.
+
+Further protection may be attained for many purposes by coating the
+platinum wire with a sheath of glass, say half an inch long, fused to
+the platinum wire to a depth of one-twentieth of an inch all round.
+
+In some cases the electrodes must be expected to get very hot, for
+instance, when it is desired to platinise mirrors by the device of
+Professor Wright of Yale.  In this and similar cases I have met with
+great success by using "barometer" tubes of about one-twelfth of an
+inch bore, and with walls, say, one-tenth of an inch thick.
+[Footnote: "Barometer" tube is merely very thick-walled glass tubing,
+and makes particularly bad barometers, which are sold as weather
+glasses.]
+
+This tube is drawn down to a long point--say an inch long by
+one-eighth of an inch external diameter, and the wire is fused in for
+a length, say, of three-quarters of an inch, but only in the narrow
+drawn--down part of the tube.  At different times I have tried four
+such seals, and though the electrodes were red hot for hours, I have
+never had an accident--of course they were well annealed.
+
+Fig.  37.
+
+For directions as to the making of high vacuum tubes, see the section
+dealing with that matter.
+
+Sec. 50.  As economy of platinum is often of importance, the following
+little art will save money and trouble.  Platinum is easily caused to
+join most firmly to copper--with which, I presume, it alloys--by the
+following method.  Hold the platinum wire against the copper wire, end
+to end, at the tip of the reducing flame of a typical blowpipe--or
+anywhere--preferably in the "reducing" part of the oxygas flame;  in
+a moment the metals will fuse together at the point of contact, when
+they may be withdrawn.
+
+Such a joint is very strong and wholly satisfactory, much better than
+a soldered joint.  If the work is not carried out successfully so that
+a considerable drop of copper-platinum alloy accumulates, cut it off
+and start again.  The essence of success is speed, so that the copper
+does not get "burned." If any considerable quantity of alloy is formed
+it dissolves the copper, and weakens it, so that we have first the
+platinum wire, then a bead of alloy, and then a copper wire fused into
+the bead, but so thin just outside the latter that the joint has no
+mechanical strength.
+
+Sec. 51.  The Art of making Air-light Joints.
+
+Lamp-manufacturers and others have long since learned that when glass
+is in question not only are fused joints made as easily as others, but
+that they afford the only reliable form of joint.  An experimenter who
+uses flint glass, has a little experience, an oxygas blow-pipe and a
+blowing apparatus, will prefer to make his joints in this way, simply
+from the ease with which it may be done.  When it comes to making a
+tight joint between glass and other substances the problem is by no
+means so easy.  Thus Mr. Griffiths (Phil.  Trans.  1893, p.  380)
+failed to make air-tight joints by cementing glass into steel tubes,
+using hard shellac, and the tubes fitting closely.  These joints were
+satisfactory at first, but did not last; the length of the joint is
+not stated.  The difficulty was finally got over by soldering very
+narrow platinum tubes into the steel, and fusing the former into the
+glass.
+
+Mr. Griffiths has since used an alloy with success as a cement, but I
+cannot discover what it is made from. Many years ago Professor Hittorf
+prepared good high vacuum tubes by plugging the ends of glass tubes
+with sealing wax merely, though in all cases the spaces to be filled
+with wax were long and narrow (Hittorf, Pogg.  Ann.  1869, Sec. 5,
+English translation, Phys.  Soc.  p.  113).  Again, Regnault
+habitually used brass ferules, and cemented glass into them by means
+of his mastic, which can still be procured at a low rate from his
+instrument-makers (Golan, Paris).  Lenard also, in his investigations
+on Cathode Rays (Wied.  Ann, vol.  li.  p.  224), made use of sealing
+wax covered with marine glue.
+
+Surely in face of these facts we must admit that cement joints can be
+made with fair success.  I do not know the composition of M.
+Regnault's mastic, but Faraday (Manipulations, Sec. 1123) gives the
+following receipt for a cement for joining ferules to retorts, etc:
+
+Resin                             5 parts.
+
+Beeswax                           1 part.
+
+Red ochre or Venetian red,
+finely powdered and sifted        1 part.
+
+I believe this to be substantially the same as Regnault's mastic,
+though I have never analysed the latter.
+
+For chemical work the possibility of evolution of gas from such a
+cement must be taken into account, and I should certainly not trust it
+for this reason in vacuum tube work, where the purity of the confined
+gas could come in question.  Otherwise it is an excellent cement, and
+does not in my experience tend to crack away from glass to the same
+extent as paraffin or pure shellac.
+
+This cracking away from glass, by the way, is probably an effect of
+difference in rate of expansion between the glass and cement which
+probably always exists, and, if the cement be not sufficiently
+viscous, must, beyond certain temperature limits, either produce
+cracks or cause separation.  Professor Wright of Yale has used a hard
+mineral pitch as a cement in vacuum work with success.
+
+My attention has been directed to a fusible metal cement containing
+mercury, and made according to the following receipt, given by Mr. S.
+G.  Rawson, Journal of the Society of Chemical Industry, vol.  ix.
+(1890), P.  150:-
+
+Bismuth         40 per cent
+
+Lead             25 per cent
+
+Tin             10 per cent
+
+Cadmium         10 per cent
+
+Mercury         15 per cent
+
+This is practically one form of Rose's fusible metal with 15 per cent
+mercury added.  It takes nearly an hour to set completely, and the
+apparatus must be clean and warm before it is applied.
+
+As the result of several trials by myself and friends, I am afraid I
+must dissent from the claim of the author that such a cement will make
+a really air-tight joint between glass tubes.  Indeed, the appearance
+of the surface as viewed through the glass is not such as to give any
+confidence, no matter what care may have been exercised in performing
+all the operations and cleaning the glass;  besides which the cement
+is rigid when cold, and the expansion difficulty comes in.
+
+On the other hand, if extreme air-tightness is not an object, the
+cement is strong and easily applied, and has many uses.  I have an
+idea that if the joints were covered with a layer of soft wax, the
+result would be satisfactory in so far as air-tightness is concerned.
+
+This anticipation has since been verified.
+
+In many cases one can resort to the device already mentioned of
+enclosing a rubber or tape-wrapped joint between two tubes in a bath
+of mercury, but in this case the glass must be clean and hot and the
+mercury also warm, dry, and pure when the joint is put together,
+otherwise an appreciable air film is left against the glass, and this
+may creep into the joint.
+
+Perhaps the easiest way of making such a joint is to use an outer tube
+of thin clean glass, and bore a narrow hole into it from one side to
+admit the mercury;  if the mercury is to be heated in vacuo, it is
+better to seal on a side joint.  It is always better, if possible, to
+boil the mercury in situ, which involves making the wrapping of
+asbestos, but, after all, we come back to the position I began by
+taking up, viz.  that the easiest and most reliable method is by
+fusion of the glass--all the rest are unsuitable for work of real
+precision.
+
+I should be ungrateful, however, were I not to devote a few lines to
+the great convenience and merit of so-called "centering cement." This
+substance has two or three very valuable properties.  It is very tough
+and strong in itself, and it remains plastic on cooling for some time
+before it really sets.  If for any reason a small tube has to be
+cemented into a larger one, which is a good deal larger, so that an
+appreciable mass of cement is necessary, and particularly if the joint
+requires to have great mechanical strength, this cement is invaluable.
+I have even used a plug of it instead of a cork for making the joint
+between a gas delivery tube and a calcium chloride tower.  (Why are
+these affairs made with such abominable tubulures?)
+
+The joint in question has never allowed the tube to sag though it
+projects horizontally to a distance of 6 inches, and has had to
+withstand nearly two years of Sydney temperature.  The cement consists
+of a mixture of shellac and 10 per cent of oil of cassia.
+
+The shellac is first melted in an iron ladle, and the oil of cassia
+quickly added and stirred in, to an extent of about 10 per cent, but
+the exact proportions are not of importance.  Great care must be taken
+not to overheat the shellac.
+
+APPENDIX TO CHAPTER I
+
+ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR
+ROENTGEN'S RADIATION
+
+[Footnote: Written in May 1896.]
+
+WHEN Professor Roentgen's discovery was first announced at the end of
+1895 much difficulty was experienced in obtaining radiation of the
+requisite intensity for the repetition of his experiments.  The
+following notes on the production of vacuum tubes of the required
+quality may therefore be of use to those who desire to prepare their
+own apparatus.  It appears that flint glass is much more opaque to
+Roentgen's radiation than soda glass, and consequently the vacuum
+tubes require to be prepared from the latter material.
+
+Fig. 39.
+
+A form of vacuum tube  which has proved very successful in the
+author's hands is sketched in Fig.  38.  It is most easily constructed
+as follows.  A bit of tubing about 2 centimetres diameter, 15
+centimetres long, and 1.5 millimetre wall thickness, is drawn down to
+a point.  The larger bulb, about 5 centimetres in diameter, is blown
+at one end of this tube.  The thinner the bulb the better, provided
+that it does not collapse under atmospheric pressure.  A very good
+idea of a proper thickness may be obtained from the statement that
+about 4 centimetres length of the tubing should be blown out to form
+the bulb.  This would give a bulb of about the thickness of an
+ordinary fractionating bulb.  Before going any further it is as well
+to test the bulb by tapping on the table and by exhausting it by means
+of an ordinary water-velocity pump.
+
+The side tube is next prepared out of narrower tubing, and is provided
+with a smaller bulb, a blowing-out tube, and a terminal, to be made as
+will be described.  This side tube is next fused on to the main tube,
+special care being taken about the annealing, and the cathode terminal
+is then sealed into the main tube.  After using clean glass it is in
+general only necessary to rinse the tube out with clean alcohol, after
+which it may be dried and exhausted.
+
+The success of the operation will depend primarily on the attention
+given to the preparation and sealing-in of the electrode facing the
+large bulb.
+
+Preparation of Terminals.  Some platinum wire of about No.  26
+B.W.G--the exact size is unimportant--must be provided, also some
+sheet aluminium about 1 millimetre thick, some white enamel cement
+glass, and a "cane" of flint-glass tube of a few millimetres bore.
+
+The electrodes are prepared by cutting discs of aluminium of from 1 to
+1.5 centimetres diameter.  The discs of aluminium are bored in the
+centre, so as to admit the "stems" which are made of aluminium wire
+of about 1 millimetre diameter.  The stems are then riveted into the
+discs.  The "stems" are about I centimetre long, and are drilled to a
+depth of about 3 millimetres, the drill used being about double the
+diameter of the platinum wire to be used for making the connections.
+The faces of the electrodes--i.e.  the free surfaces of the aluminium
+discs--are then hammered flat and brought to a burnished surface by
+being placed on a bit of highly polished steel and struck by a "set"
+provided with a hole to allow of the "stem" escaping damage.  The
+operation will be obvious after a reference to Figs.  39 and 40;  it
+is referred to again on page 96.
+
+The platinum wires may be most conveniently attached by melting one
+end of the piece of platinum wire in the oxygas blow-pipe till it
+forms a bead just large enough to pass into the hole drilled up the
+stem of the electrode.  The junction between the stein and the
+platinum wire is then made permanent by squeezing the aluminium down
+upon the platinum wire with the help of a pair of pliers.  It is also
+possible to fuse the aluminium round the platinum, but as I have had
+several breakages of such joints, I prefer the mechanical connection
+described.
+
+Fig.  39. Sets for striking aluminium electrodes
+
+Fig.  40.
+
+i. Aluminium electrode.
+
+ii. Aluminium electrode connected to platinum wire.
+
+iii. Aluminium electrode connected to platinum wire and protected by
+glass.
+
+iv. Detail of fastening platinum wire.
+
+The stem and platinum wire may now be protected by covering them with
+a little flint glass.  For this purpose the flint-glass tube is pulled
+down till it will just slip over the stem and wire, and is cut off so
+as to leave about half a centimetre of platinum wire projecting.  The
+flint-glass tube is then fused down upon the platinum wire, care being
+taken to avoid the presence of air bubbles.  At the close of the
+operation a single drop of white enamel glass is fused round the
+platinum wire at a high temperature, so as to make a good joint with
+the protecting flint-glass tube.
+
+The negative electrode being nearly as large as the main tube, it must
+be introduced before the latter is drawn down for sealing.  After
+drawing down the main tube in the usual manner, taking care not to
+make it less than a millimetre in wall-thickness, it is cut off so as
+to leave a hole not quite big enough for the enamel drop to pass
+through.  By heating and opening, the aperture is got just large
+enough to allow the enamel drop to pass into it, and when this is the
+case the joint is sealed, pulled, and blown out until the electrode
+occupies the right position--viz.  in the centre of the tube and with
+its face normal to the axis of the tube.
+
+The glass walls near the negative electrode must not be less than a
+millimetre thick, and may be rather more with advantage, the glass
+must be even, and the joint between the flint glass and the soda
+glass, or between the wire and the soda glass, must be wholly through
+the enamel.  The "seal" must be well annealed.  It will be found that
+the sealing-in process is much easier when the stem of the electrode
+is short and when the glass coating is not too heavy.  Half a
+millimetre of glass thickness round the stein is quite sufficient.
+
+The diagram, of the tube shows that the main tube has been expanded
+round the edges of the cathode.  This is to reduce the heating
+consequent on the projection of cathode rays from the edges of the
+disc against the glass tube.
+
+The anode is inserted into its bulb in a quite similar manner.  If
+desired it may be made considerably smaller, and does not need the
+careful adjustment requisite in sealing-in the cathode, nor does the
+glass near the entry wire require to be so thick.
+
+More intense effects are often got by making the cathode slightly
+concave, but in this case the risk of melting the thin glass is
+considerably increased.  No doubt, Bohemian glass might be used
+throughout instead of soft soda glass, and this would not melt so
+easily; the difficulties of manipulating the glass are, however, more
+pronounced.
+
+It will be shown directly that the best Roentgen effects are got with
+a high vacuum, and it is for this reason that the glass near the
+cathode seal requires to be strong.  The potential right up to the
+cathode is strongly positive inside the tube, and this causes the
+glass to be exposed to a strong electric stress in the neighbourhood
+of the seal.
+
+Although the glass-blowing involved in the making of a so-called focus
+tube is rather more difficult than in the case just described, there
+is no reason why such a difficulty should not be overcome;  I will
+therefore explain how a focus tube may be made.
+
+Fig. 41.
+
+A bulb about 3 inches in diameter is blown from a bit of tube of a
+little more than 1 inch diameter.  Unless the walls of the tube are
+about one-eighth of an inch in thickness, this will involve a
+preliminary thickening up of the glass.  This is not difficult if care
+be taken to avoid making the glass too hot.  The larger gas jet
+described in connection with the soda-glass-blowing table must be
+employed.  In blowing a bulb of this size it must not be forgotten
+that draughts exercise a very injurious influence by causing the glass
+to cool unequally;  this leads to bulbs of irregular shape.
+
+In the method of construction shown in Fig.  41, the anode is put in
+first.  This anode simply consists of a square bit of platinum or
+platinum-iridium foil, measuring about 0.75 inch by 1 inch, and
+riveted on to a bent aluminium wire stem.
+
+As soon as the anode is fused in, and while the glass is still hot,
+the side tube is put on.  The whole of the anode end is then carefully
+annealed.  When the annealing is finished the side tube is bent as
+shown to serve as a handle when the time comes to mount the cathode.
+Before placing the cathode in position, and while the main tube is
+still wide open, the anode is adjusted by means of a tool thrust in
+through this open end.  This is necessary in view of the fact that the
+platinum foil is occasionally bent during the operation of forcing the
+anode into the bulb.
+
+The cathode is a portion of a spherical surface of polished aluminium,
+a mode of preparing which will be given directly.  The cathode having
+been placed inside the bulb, the wide glass tube is carefully drawn
+down and cut off at such a point that when the cathode is in position
+its centre of curvature will lie slightly in front of the anode plate.
+For instance, if the radius of curvature of the cathode be 1.5 inches,
+the centre of curvature may lie something like an eighth of an inch or
+less in front of the anode.
+
+The cathode as shown in Fig.  41 is rather smaller than is
+advantageous.  To make it much larger than is shown, however, the
+opening into the bulb would require to be considerably widened, and
+though this is not really a difficult operation, still it requires
+more practice than my readers are likely to have had.  The difficulty
+is not so much in widening out the entry as in closing it down again
+neatly.
+
+Now as to making the anode.  A disc of aluminium is cut from a sheet
+which must not be too thick--one twenty-fifth of an inch is quite
+thick enough.  This disc is bored at the centre to allow of the stem
+being riveted in position.  The disc is then annealed in the Bunsen
+flame and the stem riveted on.
+
+The curvature is best got by striking between steel dies (see Figs.
+39 and 40).  Two bits of tool steel are softened and turned on the
+lathe, one convex and the other concave.  The concave die has a small
+hole drilled up the centre to admit the stem. The desired radius of
+curvature is easily attained by cutting out templates from sheet zinc
+and using them to gauge the turning.  The two dies are slightly ground
+together on the lathe with emery and oil and are then polished, or
+rather the convex die is polished--the other one does not matter.
+The polishing is most easily done by using graded emery and oil and
+polishing with a rag.  The method of grading emery will be described
+in the chapter on glass-grinding.
+
+The aluminium disc is now struck between the dies by means of a
+hammer.  If the radius of curvature is anything more than one inch and
+the disc not more than one inch in diameter the cathode can be struck
+at once from the flat as described.  For very deep curves no doubt it
+will be better to make an intermediate pair of dies and to re-anneal
+the aluminium after the first striking.
+
+When the tube is successfully prepared so far as the glassblowing goes
+it may be rinsed with strong pure alcohol both inside and out, and
+dried.  The straight part of the side tube is then constricted ready
+for fusing off and the whole affair is placed on the vacuum pump.
+
+In spite of the great improvements made during recent years in the
+construction of so-called Geissler vacuum pumps--i.e.  pumps in which
+a Torricellian vacuum is continually reproduced--I am of opinion that
+Sprengel pumps are, on the whole, more convenient for exhausting
+Crooke's tubes.  A full discussion of the subject of vacuum pumps will
+be found in a work by Mr. G.  S.  Ram (The Incandescent Lamp and its
+Manufacture), published by the Electrician Publishing Company, and it
+is not my intention to deal with the matter here; the simplest kind of
+Sprengel pump will be found quite adequate for our purpose, provided
+that it is well made.
+
+Fig.  42 is intended to represent a modification of a pump based on
+the model manufactured by Hicks of Hatton Garden, and arranged to suit
+the amateur glass-blower.  The only point of importance is the
+construction of the head of the fall tube, of which a separate and
+enlarged diagram is given.  The fall tubes may have an internal
+diameter up to 2 mm. (two millimetres) and an effective length of 120
+cm.
+
+Free use is made of rubber tube connections in the part of the pump
+exposed to the passage of mercury.  The rubber employed should be
+black and of the highest quality, having the walls strengthened by a
+layer of canvas.  If such tube cannot be easily obtained, a very good
+substitute may be made by placing a bit of ordinary black tube inside
+another and rather larger bit and binding the outer tube with tape or
+ribbon.  In any case the tubing which comes in contact with the
+mercury should be boiled in strong caustic potash or soda solution for
+at least ten minutes to get rid of free sulphur, which fouls the
+mercury directly it comes in contact with it.  The tubing is well
+washed, rinsed with alcohol, and carefully dried.
+
+Fig.  42.
+
+The diagram represents what is practically a system of three Sprengel
+pumps, though they are all fed from the same mercury reservoir and run
+down into the same mercury receiver.  It is much easier to make three
+pumps, each with separate pinch cocks to regulate the mercury supply,
+than it is to make three jets, each delivering exactly the proper
+stream of mercury to three fall tubes.
+
+Sprengel pumps only work at their highest efficiency when the mercury
+supply is carefully regulated to suit the peculiarities of each fall
+tube, and this is quite easily done in the model figured.  Since on
+starting the pump the rubber connections have to stand a considerable
+pressure, the ends of the tubes must be somewhat corrugated to enable
+the rubber to be firmly wired on to them. The best binding wire is the
+purest Swedish iron wire, previously annealed in a Bunsen gas flame.
+
+The wire must never be twisted down on the bare rubber, but must
+always be separated from it by a tape binding.  By taking this
+precaution the wire maybe twisted very much more tightly than is
+otherwise possible without cutting the rubber.
+
+The only difficulty in making such a pump as is described lies in the
+bending of the heads of the fall tubes.  This bending must be done
+with perfect regularity and neatness, otherwise the drops of mercury
+will not break regularly, or will break just inside the top of the
+fall tube, and so obstruct its entrance that at high vacua no air can
+get into the tube at all.
+
+The connections at the head of the fall tubes must also be well put on
+and the joints blown out so that the mercury in dropping over the head
+is not interfered with by the upper surface of the tube.  However, a
+glance at the enlarged diagram will show what is to be aimed at better
+than any amount of description.  In preparing the fall tubes it is
+generally necessary to join at least two "canes" together.  The joint
+must be arranged to occur either in the tube leading the mercury to
+the head of the fall, or in that part of the fall tube which remains
+full of mercury when the highest vacuum is attained.  On no account
+must the joint be made at the fall itself (at least not by an
+amateur), nor in that part of the fall tube where the mercury falls
+freely, particularly at its lower end, where the drops fall on the
+head of the column of mercury.
+
+When a high vacuum is attained the efficacy of the pump depends
+chiefly on the way in which the drops fall on the head of the column.
+If the fall is too long the drops are apt to break up and allow the
+small bubble of air to escape up the tube, also any irregularity or
+dirt in the tube at this point makes it more easy for the bubbles of
+air to escape to the surface of the mercury.
+
+Any pump in which the supply of mercury to the fall tube can be
+regulated nicely will pump well until the lowest available pressures
+are being attained;  a good pump will then continue to hold the air
+bubbles, while a bad one will allow them to slip back [Footnote: For
+special methods of avoiding this difficulty see Mr. Ram's book.] ...
+
+Though three fall tubes are recommended, it must not be supposed that
+the pump will produce a Crooke's vacuum three times more rapidly than
+one fall tube.  Until the mercury commences to hammer in the pump the
+three tubes will pump approximately three times faster than one tube,
+but as soon as the major portion of the air collected begins to come
+from the layer condensed on the glass surface of the tube to be
+exhausted and from the electrodes, the rate at which exhaustion will
+go on no longer depends entirely on the pump.
+
+In order that bubbles of air may not slip back up the fall tube it is
+generally desirable to allow the mercury to fall pretty briskly, and
+in this case the capacity of the pump to take air is generally far in
+excess of the air supply.  One advantage of having more than one fall
+tube is that it often happens that a fall tube gets soiled during the
+process of exhaustion and no longer works up to its best performance.
+Out of three fall tubes, however, one is pretty sure to be working
+well, and as soon as the mercury begins to hammer in the tubes the
+supply may be shut off from the two falls which are working least
+satisfactorily.
+
+Thus we are enabled to pump rapidly till a high degree of exhaustion
+is attained, having practically three pumps instead of one, whereas
+when the final stages are reached, and three pumps are only a drawback
+in that they increase the mercury flow, the apparatus is capable of
+instant modification to meet the new conditions.
+
+The thistle funnels at the head of the fall tubes are made simply by
+blowing bulbs and then blowing the heads of the bulbs into wider ones,
+and finally blowing the heads of the wider bulbs off by vigorous
+blowing.  The stoppers are ground in on the lathe before the tubes are
+attached to the fall tubes.  The stoppers require to be at least half
+an inch long where they fit the necks, and must be really well ground
+in.  The stoppers must first be turned up nicely and the necks ground
+out by a copper or iron cone and emery.  The stoppers are rotated on a
+lathe at quite a slow speed, say 30 or 40 feet per minute, and the
+necks are held against them, as described in the section dealing with
+this art.  The stoppers must in this case be finished with "two
+seconds" emery, and lastly with pumice dust and water (see chapter on
+glass-grinding).
+
+Unless the stoppers fit exceedingly well trouble will arise from the
+mercury (which is poured into the thistle heads to form a seal) being
+forced downwards into the pump by atmospheric pressure.
+
+The joints between the three fall tubes and the single exhaust main
+are easily made when the tubes are finally mounted, the hooked nozzle
+of the oxygas blow-pipe being expressly made for such work.
+
+It is, on the whole, advisable to make the pump of flint glass, or at
+all events the air-trap tube and the fall tubes.  A brush flame from
+the larger gas tube of the single blowpipe table is most suitable for
+the work of bending the tubes.  The jointing of the long, narrow bore
+fall tubes is best accomplished by the oxygas flame, for in this way
+the minimum  of irregularity is produced; the blowing tubes will of
+course be required for the job, and the narrow tubes must be well
+cleaned to begin with.
+
+The air trap is an important though simple part of the pump.  Its
+shoulder or fall should stand rather higher than the shoulders of the
+fall tubes, so that the mercury may run in a thin stream through a
+good Torricellian vacuum before it passes down to the fall tubes.
+This is easily attained by regulating the main mercury supply at the
+pinch cock situated between the tube from the upper reservoir and the
+air-trap tube, the other cocks being almost wide open.
+
+It might be thought that the mercury would tend to pick up air in
+passing through the rubber connections to the fall tubes, but I have
+not found this to be the case in practice.  There is, of course, no
+difficulty in eliminating the rubber connections between the fall
+tubes and the mercury supply from the air trap, but it impresses a
+greater rigidity on the structure and, as I say, is not in general
+necessary.  It must not be forgotten that the mercury always exercises
+considerable pressure on the rubber joints, and so there is little
+tendency for gas to come out of the rubber.
+
+The thistle funnels at the head of the fall tubes provide a simple and
+excellent means of cleaning the fall tubes.  For this purpose some
+"pure" sulphuric acid which has been boiled with pure ammonium
+sulphate is placed in each thistle funnel, and when the fall tube is
+dirty the connection to the mercury supply is cut off at the pinch
+cock so as to leave the tube between this entry and the head of the
+fall tube quite full of mercury, and the sulphuric acid is allowed to
+run down the fall tube by raising the stopper.  The fall tube should
+be allowed to stand full of acid for an hour or so, after which it
+will be found to be fairly clean.
+
+Of course the mercury reservoir thus obtains a layer of acid above the
+mercury, and as it is better not to run the risk of any acid getting
+into the pump except in the fall tubes, the reservoir is best emptied
+from the bottom, by a syphon, if a suitable vessel cannot be procured,
+so that clean mercury only is withdrawn.
+
+The phosphorus pentoxide tube is best made as shown simply from a bit
+of wide tube, with two side connections fused to the rest of the pump.
+It is no more trouble to cut the tube and fuse it up again when the
+drying material is renewed than to adjust the drying tube to two fixed
+stoppers, which is the alternative.  The practice here recommended is
+rendered possible only by the oxygas blow-pipe with hooked nozzle.
+The connection between the pump and tube to be exhausted is made
+simply by a short bit of rubber tube immersed in mercury.
+
+The phosphorus pentoxide should be pure, or rather free from
+phosphorus and lower oxides;  unless this be the case, the vapour
+arising from it is apt to soil the mercury in the pump.  The
+phosphorus pentoxide is purified by distilling with oxygen over
+red-hot platinum black; if this cannot be done, the pentoxide should
+at least be strongly heated in a tube, in a current of dry air or
+oxygen, before it is placed in the drying tube.
+
+The mercury used for the pump must be scrupulously clean.  It does
+not, however, require to have been distilled in vacuo.  It is
+sufficient to purify it by allowing it to fall in a fine spray into a
+large or rather tall jar of 25 per cent nitric acid and 75 per cent
+water.  The mercury is then to be washed and dried by heating to, say,
+110 deg. C.  in a porcelain dish.
+
+Exhausting a Roentgen Tube.
+
+With a pump such as has been described there is seldom any advantage
+in fusing an extra connection to the vacuum tube so as to allow of a
+preliminary exhaustion by means of a water pump.  About half an hour's
+pumping may possibly be saved by making use of a water pump.
+
+The tube to be exhausted is washed and dried by careful heating over a
+Bunsen burner and by the passage of a current of air.  The exhausting
+tube is then drawn down preparatory to sealing off, and the apparatus
+placed upon the pump.  It is best held in position by a wooden clamp
+supported by a long retort stand.
+
+Exhaustion may proceed till the mercury in the fall tubes commences to
+hammer.  At this point the tube must be carefully heated by a Bunsen
+flame, the temperature being brought up to, say, 400 deg. C.  The heating
+may be continued intermittently till little or no effect due to the
+heating is discernible at the pump.  When this stage is reached, or
+even before, the electrodes may be connected up to the coil and a
+discharge sent through the tube.
+
+Care must be taken to stop the discharge as soon as a purple glow
+begins to appear, because when this happens, the resistance of the
+tube is very low, the electrodes get very hot, and may easily get
+damaged by a powerful discharge, and the platinum of the anode (if a
+focus tube is in question) begins to be distilled on to the glass.
+The heating and sparking are to be continued till the resistance of
+the tube sharply increases.  This is tested by always having a spark
+gap, conveniently formed by the coil terminals, in parallel with the
+tube.  If the terminals are points, it is convenient to set them at
+about one quarter of an inch distance apart.
+
+As soon as sparks begin to pass between the terminals of the spark gap
+it becomes necessary to watch the process of exhaustion very
+carefully.  In the first place, stop the pump, but let the coil run,
+and note whether the sparks  continue to flow over the terminals.  If
+the glass and electrodes are getting gas free, the discharge will
+continue to pass by the spark gap, but if gas is still being freely
+given off, then in perhaps three minutes the discharge will return to
+the tube, and pumping must be recommenced.  The Roentgen effect only
+begins to appear when the tube has got to so high a state of
+exhaustion that the resistance increases rapidly.
+
+By pumping and sparking, the resistance of the tube may be gradually
+raised till the spark would rather jump over 2 inches of air than go
+through the tube.  When this state is attained the Roentgen effect as
+tested by a screen of calcium tungstate should be very brilliant.  No
+conclusion as to the equivalent resistance of the tube can be arrived
+at so long as the discharge is kept going continually.  When the spark
+would rather go over an inch of air in the spark gap than through the
+tube the pumping and sparking may be interrupted and the tube allowed
+to rest for, say, five minutes.  It will generally be found that the
+equivalent resistance of the tube will be largely increased by this
+period of quiescence.  It may even be found that the spark will now
+prefer to pass an air gap 3 inches long.
+
+In any case the sparking should now be continued, the pump being at
+rest, and the variations of tube resistance watched by adjusting the
+spark gap.  If the resistance falls below an equivalent of 2 inches of
+air in the gap the pump must be brought into action again and
+continued until the resistance as thus estimated remains fairly
+constant for, say, ten minutes.  When this occurs the narrow neck of
+the exhaust tube may be strongly heated till the blow-pipe flame
+begins to show traces of sodium light.  The flame must then be
+withdrawn and the discharge again tested.  This is necessary because
+it occasionally happens that gas is given off during the heating of
+the neck to the neighbourhood of its fusion temperature.
+
+If all is right the neck may now be fused entirely off and the tube is
+finished.  Tubes of the focus pattern with large platinum anodes are
+in general (in my experience) much more difficult to exhaust than
+tubes of the kind first described.  This is possibly to be attributed
+mainly to the gas given off by the platinum, but is also, no doubt,
+due to the tubes being much larger and exposing a larger glass
+surface.  The type of tube described first generally takes about two
+hours to exhaust by a pump made as explained, while a "focus" tube has
+taken as long as nine hours, eight of which have been consumed after
+the tube was exhausted to the hammering point.
+
+The pressure at which the maximum heating of the anode by the cathode
+rays occurs is a good deal higher than that at which the maximum
+Roentgen effect is produced.  There is little doubt that the Roentgen
+radiation changes in nature to some extent as the vacuum improves
+either as a primary or secondary effect.  It is therefore of some
+importance to test the tube for the purpose for which it is to be used
+during the actual exhaustion.  It has been stated, for instance, that
+the relative penetrability of bone and flesh to Roentgen radiation
+attains a maximum difference at a certain pressure;  this is very
+likely the case.  Whether this effect is a direct function of the
+density of the gas in the tube, or whether it is dependent on the
+voltage or time integral of the current during the discharge, are
+questions which still await a solution.
+
+The preparation of calcium tungstate for fluorescent screens is very
+simple.
+
+Commercial sodium tungstate is fused with dried calcium chloride in
+the proportion of three parts of the former to two parts of the
+latter, both constituents being in fine powder and well mixed
+together.  The fusion is conducted in a Fletcher's crucible furnace in
+a clay crucible.  The temperature is raised as rapidly as possible to
+the highest point which the furnace will attain--i.e.  a pure white
+heat.  At this temperature the mixture of salts becomes partly fluid,
+or at least pasty, and the temperature may be kept at its highest
+point for, say, a quarter of an hour.  At the end of this time the
+mass is poured and scraped on to a brick, and when cold is broken up
+and boiled with a large excess of water to dissolve out all soluble
+matter.  The insoluble part, which consists of a gray shining powder,
+is washed several times with hot water, and is finally dried on
+filter paper in a water oven.
+
+In order to prepare a screen the powder is ground slightly with very
+dilute shellac varnish, and is then floated over a glass plate so as
+to get an even covering.  Unless the covering be very even the screen
+is useless, and no pains should be spared to secure evenness.  It is
+not exactly easy to get a regular coat of the fluorescent material,
+but it may be done with a little care.
+
+CHAPTER II
+
+GLASS-GRINDING AND OPTICIANS' WORK
+
+Sec. 52.  As no instructions of any practical value in this art have, so
+far as I know, appeared in any book in English, though a great deal of
+valuable information has been given in the English Mechanic and
+elsewhere, I shall deal with the matter sufficiently fully for all
+practical purposes.  On the other hand, I do not propose to treat of
+all the methods which have been proposed, but only those requisite for
+the production of the results claimed.  The student is requested to
+read through the chapter before commencing any particular operation.
+
+Sec. 53.  The simplest way will be to describe the process of manufacture
+of some standard optical appliance, from which a general idea of the
+nature of the operations will be obtained.  After this preliminary
+account special methods may be considered in detail.  I will begin
+with an account of the construction of an achromatic object glass for
+a telescope, not because a student in a physical laboratory will often
+require to make one, but because it illustrates the usual processes
+very well; and requires to be well and accurately made.
+
+A knowledge of the ordinary principles of optics on the part of the
+reader is assumed, for there are plenty of books on the theory of
+lenses, and, in any case, it is my intention to treat of the art
+rather than of the science of the subject.  By far the best short
+statement of the principles involved which I have seen is Lord
+Rayleigh's article on Optics in the Encyclopaedia Britannica, and this
+is amply sufficient.
+
+The first question that crops up is, of course, the subject of the
+choice of glass.  It is obvious that the glass must be uniform in
+refractive index throughout, and that it must be free from air bubbles
+or bits of opaque matter.  [Footnote: The complete testing of glass
+for uniformity of refractive index can only be arrived at by grinding
+and polishing a sufficient portion of the surfaces to enable an
+examination to be made of every part.  In the case of a small disc it
+is sufficient to polish two or three facets on the edge, and to
+examine the glass in a field of uniform illumination through the
+windows thus formed.  Very slight irregularities will cause a "mirage"
+easily recognised.]
+
+The simplest procedure is to obtain glass of the desired quality from
+Messrs.  Chance of Birmingham, according to the following abbreviated
+list of names and refractive indices, which may be relied upon:-
+
+Density.   Refractive Index.
+
+       C      D        F    G
+
+Hard crown
+
+2.85 1.5146 1.5172 1.5232 1.5280
+
+Soft crown
+
+2.55 1.5119 1.5146 1.5210 1.5263
+
+Light flint
+
+3.21 1.5700 1.5740 1.5839 1.5922
+
+Dense flint
+
+3.66 1 6175 1.6224 1.6348 1.6453
+
+Extra dense flint
+
+3.85 1.6450 1.6504 1.6643 1.6761
+
+Double extra dense flint
+
+4.45 1.7036 1.7103 1.7273 ...
+
+The above glasses may be had in sheets from 0.25 to 1 inch thick, and
+6 to 12 inches square, at a cost of, say, 7s.  6d.  per pound.
+
+Discs can also be obtained of any reasonable size.  Discs 2 inches in
+diameter cost about L1 per dozen, discs 3 inches in diameter about
+10s.  each.  The price of discs increases enormously with the size.  A
+16-inch disc will cost about L100.
+
+For special purposes, where the desired quality of glass does not
+appear on the list, an application may be made to the Jena Factory of
+Herr Schott.  In order to give a definite example, I may mention that
+for ordinary telescopic objectives good results may be obtained by
+combining the hard crown and dense flint of Chance's list, using the
+crown to form a double convex, and the flint to form a double concave
+lens.  The convex lens is placed in the more outward position in the
+telescope, i.e.  the light passes first through it.
+
+The conditions to be fulfilled are:
+
+(1) The glass must be achromatic;
+
+(2) it must have a small spherical aberration for rays converging to
+the principal focus.
+
+It is impossible to discuss these matters without going into a
+complete optical discussion.  The radii of curvature of the surfaces,
+beginning with the first, i.e.  the external face of the convex lens,
+are in the ratio of 1, 2, and 3; an allowance of 15 inches focal
+length per inch of aperture is reasonable (see Optics in Ency.
+Brit.), and the focal length is the same as the greatest radius of
+curvature.  Thus, for an object glass 2 inches in diameter, the first
+surface of the convex lens would have a radius of curvature of 10
+inches, the surface common to the convex and concave lens would have a
+radius of curvature of 20 inches, and the last surface a radius of
+curvature of 30 inches.  This would also be about the focal length of
+the finished lens.  The surfaces in contact have, of course, a common
+curvature, and need not be cemented together unless a slight loss of
+light is inadmissible.
+
+I will assume that a lens of about 2 inches diameter is to be made by
+hand, i.e.  without the help of a special grinding or polishing
+machine;  this can be accomplished perfectly well, so long as the
+diameter of the glass is not above about 6 inches, after which the
+labour is rather too severe.  The two glass discs having been obtained
+from the makers, it will be found that they are slightly larger in
+diameter than the quoted size, something having been left for the
+waste of working.
+
+It is difficult to deal with the processes of lens manufacture without
+entering at every stage into rather tedious details, and, what is
+worse, without interrupting the main account for the purpose of
+describing subsidiary instruments or processes.  In order that the
+reader may have some guide in threading the maze, it is necessary that
+he should commence with a clear idea of the broad principles of
+construction which are to be carried out.  For this purpose it seems
+desirable to begin by roughly indicating the various steps which are
+to be taken.
+
+(1) The glass is to be made circular in form and of a given diameter.
+
+(2) Called Rough Grinding. The surfaces of the glass are to be made
+roughly convex, plane, or concave, as may be required; the glass is to
+be equally thick all round the edge.  In this process the glass is
+abraded by the use of sand or emery rubbed over it by properly shaped
+pieces of iron or lead called "tools."
+
+(3) The glass is ground with emery to the correct spherical figure as
+given by a spherometer.
+
+(4) Called Fine Grinding. The state of the surface is gradually
+improved by grinding with finer and finer grades of emery.
+
+(5) The glass is polished by rouge.
+
+(6) The glass is "figured." This means that it is gradually altered in
+form by a polishing tool till it gives the best results as found by
+trial.
+
+In processes 2 to 5 counterpart tool surfaces are required--as a rule
+two convex and two concave surfaces for each lens surface.  These
+subsidiary surfaces are worked (i.e.  ground) on discs of cast iron
+faced with glass, or on slate discs;  and discs thus prepared are
+called "tools."
+
+Taking these processes in the order named, the mode of manufacture is
+shortly as follows:-
+
+(1) The disc of glass, obtained in a roughly circular form, is mounted
+on an ordinary lathe, being conveniently cemented by Regnault's mastic
+to a small face plate.  The lathe is rotated slowly, and the glass is
+gradually turned down to a circular figure by means (1) of a tool with
+a diamond point;  or (2) an ordinary hand-file moistened with
+kerosene, as described in Sec. 42;  or (3) a mass of brass or iron served
+with a mixture of emery--or sand--and water fed on to the disc, so
+that the disc is gradually ground circular.
+
+The operation of making a circular disc of given diameter does not
+differ in any important particular from the similar operation in the
+case of brass or iron, and is in fact merely a matter of turning at a
+slow speed.
+
+(2 and 3) Roughing or bringing the surfaces of the glass roughly to
+the proper convex or concave shape.  This is accomplished by
+grinding, generally with sand in large works, or with emery in the
+laboratory, where the time saved is of more importance than the value
+of the emery.
+
+Discs of iron or brass are cast and turned so as to have a diameter
+slightly less than that of the glass to be ground, and are, say, half
+an inch thick.  These discs are turned convex or concave on one face
+according as they are to be employed in the production of concave or
+convex glass surfaces.  The proper degree of convexity or concavity
+may be approximated to by turning with ordinary turning tools, using a
+circular arc cut from zinc or glass (as will be described) as a
+"template" or pattern.  This also is a mere matter of turning.
+
+The first approximation to the desired convex or concave surface of
+the glass is attained (in the case of small lenses, say up to three
+inches diameter) by rotating the glass on the lathe as described above
+(for the purpose of giving it a circular edge) and holding the tool
+against the rotating glass, a plentiful supply of coarse emery and
+water, or sand and water, being supplied between the glass and metal
+surfaces.  The tool is held by hand against the surface of the
+revolving glass, and is constantly moved about, both round its own
+axis of figure and to and fro across the glass surface.  In this way
+the glass gradually gets convex or concave.
+
+The curvature is tested from time to time by a spherometer, and the
+tool is increased or decreased in curvature by turning it on a lathe
+so as to cause it to grind the glass more at the edges or in the
+middle according to the indications of the spherometer.
+
+This instrument, by the way--so important for lens makers--consists
+essentially of a kind of three-legged stool, with an additional leg
+placed at the centre of the circle circumscribing the other three.
+This central leg is in reality a fine screw with a very large head
+graduated on the edge, so that it is easy to compute the fractions of
+a turn given to the screw.  The instrument is first placed on a flat
+plate, and the central screw turned till its end just touches the
+plate, a state of affairs which is very sharply discernible by the
+slight rocking which it enables the instrument to undergo when pushed
+by the hand.  See the sketch.
+
+On a convex or concave surface the screw has to be screwed in or out,
+and from the amount of screwing necessary to bring all four points
+into equal contact, the curvature may be ascertained.
+
+Let a be the distance between the equidistant feet, and d the distance
+through which the screw is protruded or retracted from its zero
+position on a flat surface.  Then the radius of curvature rho is given
+by the formula 2rho = a2/3d +d.
+
+Fig.  43.
+
+The process of roughing is not always carried out exactly as
+described, and will be referred to again.
+
+(4) The glass being approximately of the proper radius of curvature on
+one side, it is reversed on the chuck and the same process gone
+through on the other side.  After this the glass is usually dismounted
+from the lathe and mounted by cement on a pedestal, which is merely a
+wooden stand with a heavy foot, so that the glass may be held
+conveniently for the workman.  Sometimes a pedestal about four feet
+high is fixed in the floor of the room, so that the workman engaged in
+grinding the lens may walk round and round it to secure uniformity.
+For ordinary purposes, however, a short pedestal may be placed on a
+table and rotated from time to time by hand, the operator sitting down
+to his work.
+
+Rough iron or brass tools do not succeed for fine grinding--i.e.
+grinding with fine emery, because particles of emery become embedded
+in the metal so tightly that they cannot be got out by any ordinary
+cleaning.  If we have been using emery passing say a sieve with 60
+threads to the inch, and then go on to some passing say 100 threads to
+the inch, a few of the coarser particles will adhere to the "tool",
+and go on cutting and scratching all the time grinding by means of the
+finer emery is in progress.
+
+To get over this it is usual to use a rather different kind of
+grinding tool.  A very good kind is made by cementing small squares of
+glass (say up to half an inch on the side), on to a disc of slate
+slightly smaller than the lens surface to be formed (Fig.  51).  The
+glass-slate tool is then "roughed" just like the lens surface, but, of
+course, if the lens has been roughed "convex" the tool must be roughed
+"concave".
+
+The "roughed" tool is then used to gradually improve the fineness of
+grinding of the glass.  For this purpose grinding by hand is resorted
+to, the tool and lens being supplied continually with finer and finer
+emery.  Fig.  52 gives an idea of the way in which the tool is moved
+across the glass surface.  Very little pressure is required.  The tool
+is carried in small circular sweeps round and round the lens, so that
+the centre of the tool describes a many-looped curve on the lens
+surface.  The tool must be allowed to rotate about its own axis;  and
+the lens and pedestal must also be rotated from time to time.
+
+Every few minutes the circular strokes are interrupted, and simple,
+straight, transverse strokes taken.  In no case (except to correct a,
+defect, as will be explained) should the tool overhang the lens
+surface by more than about one quarter the diameter of the latter.
+After grinding say for an hour with one size of emery fed in by means
+of a clean stick say every five minutes, the emery is washed off, and
+everything carefully cleaned.  The process is then repeated with finer
+emery, and so on.
+
+The different grades of emery are prepared by taking advantage of the
+fact that the smaller the particles the longer do they remain
+suspended in water.  Some emery mud from a "roughing" operation is
+stirred up with plenty of water and left a few seconds to settle, the
+liquor is then decanted to a second jug and left say for double the
+time, say ten seconds;  it is decanted again, and so on till four or
+five grades of emery have been accumulated, each jug containing finer
+emery than its predecessor in the process.
+
+It is not much use using emery which takes more than half an hour to
+settle in an ordinary bedroom jug.  What remains in the liquid to be
+decanted is mostly glass mud and not emery at all.  The process of
+fine grinding is continually checked by the spherometer, and the art
+consists in knowing how to move the grinding tool so as to make the
+lens surface more or less curved.  In general it may be said that if
+the tool is moved in small sweeps, and not allowed to overhang much,
+the Centre of the lens will be more abraded, while if bold free
+strokes are taken with much overhanging, the edges of the lens will be
+more ground away.
+
+By the exercise of patience and perseverance any one will succeed in
+gradually fine grinding the lens surface and keeping it to the
+spherometer, but the skill comes in doing this rapidly by varying the
+shape of the strokes before any appreciable alteration of curvature
+has come about.
+
+Polishing.
+
+The most simple way of polishing is to coat the grinding tool with
+paper, as will be described, and then to brush some rouge into the
+paper.  The polisher is moved over the work in much the same way as
+the fine grinding tool, until the glass is polished.  Many operators
+prefer to use a tool made by squeezing a disc of slate, armed with
+squares of warm pitch, against the lens surface (finely ground), and
+then covering these squares with rouge and water instead of emery and
+water as in the fine grinding process.
+
+The final process is called "figuring."  It will in  general be
+unnecessary with a small lens.  With large lenses or mirrors the final
+touches have to be given after the optical behaviour of the lens or
+mirror has been tested with the telescope itself, and this process is
+called "figuring." A book might easily be written on the optical
+indications of various imperfections in a mirror or lens.  Suffice it
+to say here that a sufficiently skilled person will be able to decide
+from an observation of the behaviour of a telescope whether a lens
+will be improved by altering the curvature of one or all of the
+surfaces.
+
+A very small alteration will make a large difference in the optical
+properties, so that in general "figuring" is done merely by using the
+rouge polishing tool as an abrading tool, and causing it to alter the
+curves in the manner already suggested for grinding.  There are other
+methods based on knocking squares out of the pitch-polisher so that
+some parts of the glass may be more abraded than others.
+
+The "figuring" and polishing may be done by hand just like the
+grinding.  There are machines, however, which can be made to execute
+the proper motions, and a polisher is set in such a machine, and the
+mechanical work done is by no means inconsiderable.  In fact for
+surfaces above six inches in diameter few people are strong enough to
+work a polisher by hand owing to the intense adhesion between it and
+the exactly fitting glass surface.
+
+Such is a general outline of the processes required to produce a lens
+or mirror.  These processes will now be dealt with in much greater
+detail, and a certain amount of repetition of the above will
+unfortunately be necessary: the reader is asked to pardon this.  It
+will also be advisable for the reader to begin by reading the whole
+account before he commences any particular operation.  The reason for
+this is that it has been desirable to keep to the main account as far
+as possible without inserting special instructions for subsidiary
+operations, however important they may be;  consequently it may not
+always be quite clear how the steps described are to be performed.  It
+will be found, however, that all necessary information is really
+given, though perhaps not always exactly in the place the reader might
+at first expect.
+
+Sec. 54.  All the discs that I have seen, come from the makers already
+roughly ground on the edges to a circular figure--but occasionally the
+figure is very rough indeed--and in some cases, especially if small
+lenses have to be made, it is convenient to begin by cutting the glass
+discs out of glass sheet, which also may be purchased of suit-able
+glass.  To do this, the simplest way is to begin by cutting squares
+and then cutting off the corners with the diamond, the approximate
+circular figure being obtained by grinding the edges on an ordinary
+grindstone.
+
+If the pieces are larger, time and material may be saved by using a
+diamond compass, i.e.  an ordinary drawing compass armed with a
+diamond to cut circles on the glass, and breaking the superfluous
+glass away by means of a pair of spectacle-maker's shanks (Fig.  44),
+or what does equally well, a pair of pliers with soft iron jaws.  With
+these instruments glass can be chipped gradually up to any line,
+whether diamond-cut or not, the jaws of the pincers being worked
+against the edge of the glass, so as to gradually crush it away.
+
+Fig.  44.
+
+Assuming that the glass has been bought or made roughly circular, it
+must be finished on the lathe.  For this purpose it is necessary to
+chuck it on an iron or hardwood chuck, as shown in Fig.  46.  For a
+lens below say an inch in diameter, the centering cement may be used;
+but for a lens of a diameter greater than this, sufficient adhesion is
+easily obtained with Regnault's mastic, and its low melting point
+gives it a decided advantage over the shellac composition.
+
+The glass may be heated gradually by placing it on the water bath, or
+actually in the water, and gradually bringing the water up to the
+boiling-point.  The glass, being taken out, is rapidly wiped, and
+rubbed with a bit of waste moistened, not wet, with a little
+turpentine:  its surface is then rubbed with a stick of mastic
+previously warmed so as to melt easily.  The surface of the chuck
+being also warm, and covered with a layer of melted cement, it is
+applied to the glass.  The lathe is turned slowly by hand, and the
+glass pushed gradually into the most central position;  it is then
+pressed tight against the chuck  by the back rest, a bit of wood being
+interposed for obvious reasons.
+
+When all is cold the turning may be proceeded with.  The quickest way
+is to use the method already described (i.e.  actual turning by a file
+tool);  but if the student prefers (time being no object), he may
+accomplish the reduction to a circular form very easily by grinding.
+
+Fig.  45.
+
+Fig.  46.
+
+For this purpose he will require to make the following arrangements
+(Fig.  45).  If the lathe has a slide rest, a piece of stout iron may
+be bent and cut so as to fit the tool rest, and project beneath the
+glass.  The iron must be fairly rigid, for if it springs appreciably
+beneath the pressure of the glass, it will not grind the latter really
+round.  The lathe may run rather faster than for turning cast iron of
+the same size.  Coarse emery, passing through a sieve of 80 threads to
+the inch (run), may be fed in between the glass and iron, and the
+latter screwed up till the disc just grinds slightly as it goes round.
+
+A beginner will generally (in this as in all cases of grinding
+processes) tend to feed too fast--no grinding process can be hurried.
+If a slide rest is not available, a hinged board, carrying a bit of
+iron, may (see Fig.  45) be arranged so as to turn about its hinge at
+the back of the lathe;  and it may be screwed up readily enough by
+passing a long set-screw through the front edge, so that the point of
+the screw bears upon the lathe bed.  I may add that emery behaves as
+if it were greasy, and it is difficult to wet it with clean water.
+This is easily got over by adding a little soap or alcohol to the
+water, or exercising a little patience.
+
+A good supply of emery and water should be kept between the disc and
+the iron;  a little putty may be arranged round the point of contact
+on the iron to form a temporary trough.  In any case the resulting
+emery mud should on no account be thrown away, but should be carefully
+kept for further use.  The process is complete when the glass is
+perfectly round and of the required diameter as tested by callipers.
+
+Sec. 55.  The next step is to rough out the lens, and this may easily be
+done by rotating it more slowly, i.e.  with a surface speed of ten
+feet per minute, and turning the glass with a hard file, as explained
+in Sec. 42.  If it is desired to employ the slide rest, it is quicker and
+better to use a diamond tool--an instrument quite readily made, and
+of great service for turning emery wheels and the like,--a thing, in
+fact, which no workshop should be without.  A bit of diamond bort, or
+even a clear though off-colour stone, may be employed.
+
+An ordinary lathe tool is prepared by drawing down the tool steel to a
+long cone, resembling the ordinary practice in preparing a boring
+tool.  The apex of the cone must be cut off till it is only slightly
+larger than the greatest transverse diameter of the diamond splinter.
+The latter may have almost any shape--a triangular point, one side of
+a three-sided prism is very convenient.  A hole is drilled in the
+steel (which must have been well softened), only just large enough to
+allow the diamond to enter--if the splinter is thicker in the middle
+than at either end, so much the better--the diamond is fastened in
+position by squeezing the soft steel walls tightly down upon it.
+Personally I prefer to use a tool holder, and in this case generally
+mount the diamond in a bit of brass rod of the proper diameter;  and
+instead of pinching in the sides of the cavity, I tin them, and set
+the diamond in position with a drop of soft solder.
+
+Fig.  47.
+
+In purchasing diamond bort, a good plan is to buy fragments that have
+been employed in diamond drilling, and have become too small to reset;
+in this case some idea as to the hardness of the bits may be obtained.
+Full details as to diamond tool-making are given in books on
+watch-making, and in Holtzapffell's great work on Mechanical
+Manipulation;  but the above notes are all that are really
+necessary--it is, in fact, a very simple matter. The only advantage of
+using a diamond tool for glass turning is that one does not need to be
+always taking it out of the rest to sharpen it, which generally happens
+with hard steel, especially if the work is turned a little too fast.
+
+I recommend, therefore, that the student should boldly go to work
+"free hand" with a hard file;  but if he prefer the more formal
+method, or distrust his skill (which he should not do), then let him
+use a diamond point, even if he has the trouble of making it.  When
+using a diamond it is not necessary to employ a lubricant, but there
+is some advantage in doing so.
+
+The surface of the lens can be roughly shaped by turning to a template
+or pattern made by cutting a circular arc (of the same radius as the
+required surface) out of a bit of sheet zinc.  Another very handy way
+of making templates of great accuracy is to use a beam compass
+(constructed from a light wooden bar) with a glazier's diamond instead
+of a pencil.  A bit of thin sheet glass is cut across with this
+compass to the proper curvature--which can be done with considerable
+accuracy and the two halves of the plate, after breaking along the
+cut, are ground together with a view to avoiding slight local
+irregularities, by means of a little fine emery and water laid between
+the edges.  In this process the glass is conveniently supported on a
+clean board or slate, and the bits are rubbed backwards and forwards
+against each other.
+
+Sec. 56.  It is not very easy for a beginner to turn a bit of
+anything--iron, wood, or glass--with great accuracy to fit a template,
+and consequently time may be saved by the following procedure, applied as
+soon as the figure of the template is roughly obtained.  A disc of
+lead or iron, of the same diameter as the glass, and of approximately
+the proper curvature, is prepared by turning, and is armed with a
+handle projecting coaxially from the back of the disc.  The glass
+revolving with moderate speed on the lathe, the lead tool, supplied
+with coarse emery and water, is held against it, care being taken to
+rotate the tool by the handle, and also to move it backwards and
+forwards across the disc, through a distance, say, up to half an inch;
+if it is allowed to overhang too much the edges of the glass disc will
+be overground.  By the use of such a tool the glass can readily be
+brought up to the template.
+
+The only thing that remains, so far as the description of this part of
+the process goes, is to give a note or two as to the best way of
+making the lead tools, and for this purpose the main narrative of
+processes must be interrupted.  The easiest way is to make a set of
+discs to begin with.  For this purpose take the mandrel out of the
+lathe, and place it nose downwards in the centre of an iron ring of
+proper diameter on a flat and level iron plate.
+
+The discs are made by pouring lead round the screw-nose of the
+mandrel.  This method, of course, leaves them with a hole in the
+centre;  but this can be stopped up by placing the hot disc (from
+which the mandrel has been unscrewed) on a hot plate, and pouring in a
+sufficiency of very hot lead; or, better still, the mandrel can be
+supported vertically at any desired distance above the plate while the
+casting is being poured.  Lead discs prepared in this way are easily
+turned so as to form very convenient chucks for brass work, and for
+use in the case now being treated, they are easily turned to a
+template, using woodturners' tools, which work better if oiled, and
+must be set to cut, not scrape.
+
+If the operator does not mind the trouble of cutting a screw, or if he
+has a jaw chuck, the lead may be replaced by iron with some advantage.
+
+The following is a neat way of making concave tools.  It is an
+application of the principle of having the cutting tool as long as the
+radius of curvature, and allowing it to move about the centre of
+curvature.  Place the disc of iron or lead on the lathe mandrel or in
+the chuck, and set the slide rest so that it is free to slide up or
+down the lathe bed.  Take a bar of tool steel and cut it a little
+longer than the radius of curvature required.  Forge and finish one
+end of the bar into a pointed turning tool of the ordinary kind.
+Measure the radius of curvature from the point of the tool along the
+bar, and bore a hole, whose centre is at this point, through the bar
+from the upper to the lower face.  I regard the upper face as the one
+whose horizontal plane contains the cutting point when the tool is in
+use.  Clamp a temporary back centre to the lathe bed, and let it carry
+a pin in the vertical plane through the lathe centres, and let this
+pin exactly fit the hole in the bar.
+
+Fig. 48.
+
+Place the "radius" tool in position for cutting, and let it be lightly
+held in the slide rest nearly at the cutting point, the centre of
+rotation of the pedestal (or its equivalent) passing through the
+central line of the bar.  Then adjust the temporary back rest, so that
+the tool will take a cut.  In the sketch the tool is shown swinging
+about the back centre instead of about a pin--there is little to
+choose between the methods unless economy of tool steel is an object.
+The tool must now be fed across the work.  The pedestal must of course
+be free to rotate, and the slide rest to slip up and down the bed.  In
+this way a better concave grinding tool can be made than would be made
+by a beginner by turning to a template--though an expert turner would
+probably carry out the latter operation so as to obtain an' accuracy
+of the same order, and would certainly do it in much less time than
+would be required in setting up the special arrangements here
+described.
+
+On the other hand, if several surfaces have to be prepared, as in the
+making of an achromatic lens, the quickest way would be by the use of
+the radius tool, bored of course to work at the several radii
+required.  I have tried both methods, and my choice would depend
+partly on the lathe at my disposal, and partly on the number of
+grinding tools that had to be prepared.
+
+Having obtained a concave tool of any given radius, it is easily
+copied--negatively, so as to make a convex tool in the following
+manner.  Adjust the concave tool already made on the back rest, so
+that if it rotated about the line of centres, it would rotate about
+its axis of figure.
+
+Arrangements for this can easily be made, but of course they will
+depend on the detailed structure of the lathe.  Use the slide rest as
+before, i.e.  let it grasp an ordinary turning tool lightly, the
+pedestal being fixed, but the rest free to slide up or down the lathe
+bed.  Push the back rest up till the butt of the turning tool (ground
+to a rounded point) rests against the concave grinding tool.  If the
+diameter of the convex tool required be very small compared with the
+radius of curvature of the surface (the most usual case), it is only
+necessary to feed the cutting tool across to "copy" the concave
+surface sufficiently nearly.
+
+Fig. 49.
+
+There seems no reason, however, why these methods should not be
+applied at once to the glass disc by means of a diamond point, and the
+rough grinding thus entirely avoided.  I am informed that this has
+been done by Sir Henry Bessemer, but that the method was found to
+present no great advantage in practice.  A reader with a taste for
+mechanical experimenting might try radius bar tools with small
+carborundum wheels rapidly driven instead of a diamond.
+
+Enough has now been said to enable any one to prepare rough convex or
+concave grinding tools of iron or lead, and of the same diameter as
+the glass to be ground.
+
+The general effect of the process of roughing the rotating lens
+surface is to alter the radius of curvature of both tool and glass;
+hence it is necessary to have for each grinding tool another to fit
+it, and enable it to be kept (by working the two together) at a
+constant figure.  After a little practice it will be found possible to
+bring the glass exactly up to the required curvature as tested by
+template or spherometer.  The art of the process consists in altering
+the shape of the grinding tool so as to take off the glass where
+required, as described in Sec. 53, and from this point of view lead has
+some advantages;  (opinions vary as to the relative advantages of lead
+and iron tools for this purpose, however).  The subsidiary grinding
+tool is not actually needed for this preliminary operation, but it has
+to be made some time with a view to further procedure, and
+occasionally is of service here.
+
+Sec. 57.  'The glass disc must be ground approximately to the proper
+curvature on each side before any fine grinding is commenced.  It is
+precisely for this purpose that the previous turning of the disc is
+recommended, for it is easy to unmount and recentre a round object,
+but not so easy if the object have an indefinite shape.  Using a
+cement which is plastic before it sets, the disc may be easily taken
+off the chuck and centred by a little handicraft, i.e.  by rotating
+the lathe slowly and pushing the disc into such a position that it
+rotates about its axis.  The grinding of the second surface is
+accomplished exactly as in the former case;  of course on reversing
+the glass the chuck has to be slightly turned up to fit the convex or
+concave surface.
+
+Sec. 58.  There is, however, one point of interest and importance--attention
+to which will save a good deal of useless labour afterwards.
+The glass must be ground in such a manner that the thickness at the
+edge is the same all round.  In other words, the axes of figure of the
+two surfaces must coincide.  This will be the case if the recentering
+has been accurately performed, and therefore no pains should be spared
+to see that it is exactly carried out.  Any simple form of vernier
+gauge (such as Brown and Sharpe's vernier callipers) will serve to
+allow of a sufficiently accurate measurement of the edge thickness of
+the lens.  If any difference of thickness is observed as the gauge
+moves round the edge, one or other of the surfaces must be reground.
+Of course the latitude of error which may be permitted depends so much
+on the final arrangements for a special finishing process called the
+"centering of the lens"--which will be described--that it is
+difficult to fix a limit, but perhaps one-thousandth of an inch may be
+mentioned as a suitable amount for a 2-inch disc.  For rough work, of
+course; more margin may be admitted.
+
+Sec. 59.  In a large shop I imagine that lenses of only two inches
+diameter would be ground in nests;  or, in other words, a number would
+be worked at a time, and centering, even of a rough kind, would be
+left to the last;  but this process will be treated hereafter.  At
+present I shall assume that only one lens will be made at a time.
+Consequently we now enter on the stage of fine grinding by hand.  A
+leaden pedestal, for the sake of stability, must be provided on which
+to mount the lens, so that the surface to be operated on may be nearly
+horizontal (Fig.  50).  Before this can be done, however, fresh
+grinding tools (two for each surface) must be properly prepared.
+After trying several plans I unhesitatingly recommend that all
+fine-grinding surfaces should be made of glass.  This is easily done
+by taking two discs of lead, or iron, or slate, cut to a one-tenth
+inch smaller radius of curvature (in the case of a convex tool, and
+the opposite in the other case) than the lens surface (Fig.  51, A).
+On these, square bits of sheet glass, one-tenth of an inch thick, are
+to be cemented, so as to leave channels of about one-eighth of an inch
+between each bit of glass (Fig.  52, B).  The "mastic"    cement
+formerly described may be employed for this purpose.
+
+Fig.  50.
+
+The bits of glass ought first to have  their edges dressed smooth on
+the grind-stone.  A convex and concave glass surface having been thus
+roughly prepared, they must be mounted in turn in the lathe, and
+brought to the proper curvature by grinding with the tools formerly
+employed and tested by the template or spherometer.  It is well to
+control this process by means of a spherometer, so that the desired
+radius may be approximately reached.  The two glass-grinding tools
+are then ground together by hand (see Sec. 53 and Sec. 61), the spherometer
+being employed from time to time to check the progress of the work.
+In general, if large circular sweeps are taken, greatly overhanging
+the side of the glass surface to be figured, both the upper and lower
+surfaces will be more ground at the edges, while in the opposite
+event the centre will be chiefly affected.
+
+Fig.  51.
+
+A spherometer capable of measuring a 2-inch surface may be procured,
+having a screw of, say, 50 threads to the inch, and a micrometer
+surface divided into 200 parts, each part easily capable of
+subdivision--into tenths or even twentieths.  To get the full
+advantage of the spherometer it must screw exceedingly freely (i.e.
+must be well oiled with clock oil), and must not be fingered except at
+the milled head.  If one of the legs is held by the fingers the
+expansion is sufficient to throw the instrument quite out of
+adjustment.  The glass-grinding tools being brought to the proper
+figure, the next process is to transfer the same to the lens, and this
+is done by similar means, the fellow tool being used to correct the
+one employed in grinding the lens surface.  Before the grade of emery
+is changed all three surfaces must agree, as nearly, at least, as the
+spherometer will show.
+
+In order to prevent confusion the following summary of the steps
+already taken may be given.  The discs of glass are first ground or
+turned so as to be truly circular.  Four "tools" are made for each
+surface--a rough pair of iron or lead, and a finishing pair of iron,
+lead, or slate faced by glass squares.  For a small lens the iron or
+lead backing may be used, for a large one the slate.  The rough tools
+are used to give an approximate figure both to the lens and to the
+finishing tools.
+
+The final adjustment is attained by grinding one of the glass-faced
+tools alternately upon the lens and upon the fellow glass-faced tool.
+The spherometer is accepted at all stages of the process as the final
+arbiter as to curvature.  Some hints on the form of strokes used in
+grinding will be given later on (see Sec. 61).  It suffices to state here
+that the object throughout is to secure uniformity by allowing both
+the work and the tool to rotate, and exercising no pressure by the
+fingers.  The tool backing may weigh from one to two pounds for a
+2-inch lens.
+
+Sec. 60.  The tools and lens being all of the same curvature, the state
+of the surface is gradually improved by grinding with finer and finer
+emery.  The best way of grading the emery is by washing it with clean
+water, and allowing the emery (at first stirred up with the water) to
+settle out.  The longer the time required for this part of the process
+the finer will be the emery deposited.  An ordinary bedroom jug is a
+very good utensil to employ during this process; a large glass jug is
+even better.  The following grades will be found sufficient, though I
+daresay every operative's practice differs a little on this point.
+
+1st grade: Flour emery, with the grit washed out, i.e. allowed to
+stand for 2" (sec.) before being poured off.
+
+2nd grade: Stand 5" (secs.), settle in 1' (min.)
+
+3rd grade: Stand 1', settle in 10'.
+
+4th grade: Stand 10', settle in 60'.
+
+It is generally advisable to repeat the washing process with each
+grade.  Thus, selecting grade 2 for illustration, the liquor for grade
+3 must be poured off without allowing any of the sediment to pass over
+with it.  If any sediment at all passes, one has no security against
+its containing perhaps the largest particle in the jug.  As soon as
+the liquor for No.  3 has been decanted, jug No.  2 is filled up again
+with clean water (filtered if necessary), and after standing 5" is
+decanted into jug No.  2b, the sediment is returned to jug No.  1, and
+the liquor, after standing 1', is transferred to jug No.  3.
+
+The greatest care is necessary at each step of the operation to
+prevent "sediment" passing over with liquor.  There is a little danger
+from the tendency which even comparatively large particles of emery
+have to float, in consequence of their refusing to get wet, and the
+emery worked up on the side of the jug is also a source of danger,
+therefore wipe the jug round inside before decanting.
+
+In order to get a uniform grade stop the currents of water in the jug,
+which may work up coarse particles, by holding a thin bit of wood in
+the rotating liquid for a moment, and then gently withdrawing it in
+its own plane.  These precautions are particularly necessary in the
+case of grades Nos.  2, 3, and 4, especially No.  4, for if a single
+coarse particle gets on the tool when the work has progressed up to
+this point it will probably necessitate a return to grinding by means
+of No.  2, and involve many hours' work.
+
+The surface of the lens will require to be ground continuously with
+each grade till it has the uniform state of roughness corresponding to
+the grade in question.  Two hours for each grade is about the usual
+time required in working such a lens as is here contemplated.
+
+The coarser grades of emery may be obtained by washing ordinary flour
+of emery, but the finer ones have to be got from emery which has been
+used in the previous processes.  It is not a good plan to wash the
+finer grades of emery out of the proceeds of very rough grinding say
+with anything coarser than flour of emery--as there is a danger of
+thereby contaminating the finer grades with comparatively coarse glass
+particles (owing to their lightness) and this may lead to scratching.
+If the finer grades are very light in colour, it may be inferred that
+a considerable portion of the dust is composed of glass, and this does
+no good.  Consequently time may be saved by stirring up the
+light-coloured mass with a little hydrofluoric acid in a platinum
+capsule;  this dissolves the finely divided glass almost
+instantaneously.  The emery and excess of hydrofluoric acid may then
+be thrown into a large beaker of clean water and washed several times.
+Fine emery thus treated has much the same dark chocolate colour as the
+coarser varieties.
+
+The operator should not wear a coat, and should have his arms bare
+while working with fine emery, for a workshop coat is sure to have
+gathered a good deal of dust, and increases the chances of coarse
+particles getting between the surfaces.
+
+Sec. 61.  Details of the Process of Fine Grinding.
+
+A lens of the size selected for description is mounted as before
+mentioned on a leaden pedestal, and the operator places the latter on
+a table of convenient height in a room as free from dust as possible.
+Everything should be as clean as a pin, and no splashes of emery mud
+should be allowed to lie about.  I have found it convenient to spread
+clean newspapers on the table and floor, and to wear clean linen
+clothes, which do not pick up dust.  I have an idea that in large
+workshops some simpler means of avoiding scratches must have been
+discovered, but I can only give the results of my own experience.  I
+never successfully avoided scratches till I adopted the precautions
+mentioned.
+
+Fig.  52.
+
+The left hand should be employed in rotating the pedestal either
+continuously (though slowly) or at intervals of, say, one minute.
+This point is rather important.  Some operators require two hands to
+work the grinding tool, and in any case this is the safer practice.
+Under these circumstances the pedestal may be rotated through
+one-eighth or tenth of a revolution every three minutes, or
+thereabouts.  The general motion given to the grinding tool should be
+a series of circular sweeps of about one-fourth the diameter of the
+glass disc, and gradually carried round an imaginary circle drawn on
+the surface of the lens and concentric with it (Fig.  52).
+
+The tool may overhang the lens by a quarter of the diameter of the
+latter as a maximum. The circuit may be completed in from twelve to
+thirty sweeps.  The grinding tool should be lightly held by the
+fingers and the necessary force applied parallel to the surface.  The
+tool itself must be slowly rotated about its axis of figure.  If the
+tool be lightly held, it will be found that it tends to rotate by
+itself.  I say "tends to rotate," for if the tool be touching evenly
+all over the surface it will rotate in a direction opposite to the
+direction of the circular sweep.  For instance, if the tool be carried
+round its looped path clockwise, it will tend to rotate about its own
+axis of figure counter-clockwise.  If it touch more in the middle,
+this rotation will be increased, while if it touches more along the
+edge, the rotation will be diminished, or even reversed in an extreme
+case.
+
+Every fifty sweeps or so the tool should be simply ground backwards
+and forwards along a diameter of the lens surface.  This grinding
+should consist of three or four journeys to and fro along, say, eight
+different diameters.  About one-quarter of the whole grinding should
+be accomplished by short straight strokes, during which the tool
+should only overhang about one-quarter of an inch.  The object of the
+straight strokes is to counteract the tendency to a gradual
+accumulation of the emery in the centre, which results from the
+circular grinding.
+
+A great deal of the art of the process consists in knowing how to work
+the tool to produce any given effect.  For instance, if the lens
+requires to be ground down near the centre, the epicycloidal strokes
+must be nearly central;  the tool must never overhang very much.  If,
+on the other hand, it is the edges which require attention, these must
+be dealt with by wider overhanging strokes.  The tool must be
+frequently tested on its fellow, and, indeed, ground upon it if any
+marked unevenness of action (such as that just described) is required
+for the lens.  A check by spherometer will be applied at intervals
+according to the judgment of the operator, but, in any case, the
+fellow tool and lens should be kept at very nearly the same figure.
+
+The emery should never be allowed to become anything like dry between
+the tool and the lens, for in some way (probably by capillary action
+increasing the pressure of the tool) this seems to lead to scratching
+and "rolling" of the emery.  The channels in the glass tool between
+the squares are of the greatest importance in enabling the emery to
+distribute itself.  Perhaps the best guide in enabling one to judge
+as to when it is time to wash off the emery and apply fresh is the
+"feel" of the tool; also when the mud gets light in colour we know
+that it is full of glass dust, and proportionately inoperative.
+
+New emery may be put on, say, every five minutes, but no absolute rule
+can be given, for much depends on the pressure of the tool upon the
+lens.  In the case considered a brass or lead, or even slate tool, of
+an inch, or even less, in thickness, will press quite heavily enough.
+In washing the lens and tool before new emery is introduced, a large
+enamelled iron bucket is very handy;  the whole of the tool should be
+immersed and scrubbed with a nail-brush.  The lens surface may be
+wiped with a bit of clean sponge, free from grit, or even a clean damp
+cloth.
+
+When the time comes to alter the grade of emery, a fresh lot of
+newspapers should be put down, and tools, lens, and pedestal well
+washed and brushed by the nail-brush.  The surfaces should be wiped
+dry by a fresh piece of rag, and examined for scratches and also for
+uniformity of appearance; a good opinion can be formed as to the fit
+of the surfaces by noting whether--and if so, to what degree--they
+differ in appearance from point to point when held so that the light
+falls on them obliquely.
+
+It is necessary to exercise the greatest care in the washing between
+the application of successive grades of emery, and this will be
+facilitated if the edges of the glass squares were dressed on a
+grindstone before they were mounted.  An additional precaution which
+may be of immense advantage is to allow the tool to dry between the
+application of successive grades of emery (of course, after it has
+been scrubbed), and then to brush it vigorously with a hat-brush.  It
+sometimes happens that particles of mud which have resisted the wet
+scrubbing with the nail-brush may be removed by this method.
+
+As my friend Mr. Cook informs me that his present practice differs
+slightly from the above, I will depart from the rule I laid down, and
+add a note on an alternative method.
+
+Consider a single lens surface.  This is roughed out as before by an
+iron tool, a rough fellow tool being made at the same time.  The
+squares of glass are cemented to the roughing tool, and this is ground
+to the spherometer by means of the counterpart tool.  The glass-coated
+tool is then applied to the lens surface and grinding with the first
+grade of emery commenced.  The curvature is checked by the
+spherometer.  Two auxiliary tools of, say, half the diameter of the
+lens, are prepared from slate, or glass backed with iron, and applied
+to grind down either the central part of the lens surface or tool
+surface, according to the indications of the spherometer.  Any changes
+that may occur during grinding are corrected by these tools.  The
+spherometer is accepted as the sole guide in obtaining the proper
+curvature.  A slate backing is preferred for tools of any diameter
+over, say, 2 inches.
+
+Sec. 62.  Polishing.
+
+After the surface has been ground with the last grade of emery, and
+commences to become translucent even when dry, the grinding may be
+considered to be accomplished, and the next step is the polishing.
+There are many ways of carrying out this process, and the relative
+suitability of these methods depends on a good many, so to speak,
+accidental circumstances.  For instance, if the intention is to finish
+the polishing at a sitting, the polishing tool may be faced with
+squares of archangel--not mineral or coal-tar--pitch and brought to
+shape simply by pressing while warm against the face of the lens.  A
+tool thus made is very convenient, accurate, and good, but it is
+difficult to keep it in shape for any length of time;  if left on the
+lens it is apt to stick, and if it overhangs ever so little will, of
+course, droop at the edges.
+
+On the whole, the following will be found a good and sufficient plan.
+The glass-grinding tool is converted into a polishing tool by pasting
+a bit of thin paper over its surface;  a bit of woven letter paper of
+medium thickness with a smooth but not glazed surface does very well.
+We have found that what is called Smith's "21 lbs.  Vellum Wove" is
+excellent.  This is steeped in water till quite pliable and almost
+free from size.  The glass tool is brushed over with a little thin
+arrowroot or starch paste, and the paper is laid upon it and squeezed
+down on the glass squares as well as possible;  if the paper is wet
+enough and of the proper quality it will expand sufficiently to
+envelop the tool without creases, unless the curvature is quite out of
+the common.
+
+This being accomplished, and the excess of water and paste removed,
+the face of the paper is (for security) washed with a little clean
+water and a bit of sponge, and, finally, the tool is slightly pressed
+on the lens so as to get the paper to take up the proper figure as
+nearly as possible.  After the polishing tool has been thus brought to
+the proper figure, it is lifted off and allowed to dry slowly.  When
+the paper is dry it may be trimmed round the edges so as not to
+project sensibly beyond the glass squares.  The next step is to brush
+the surface over very carefully with polishing rouge (prepared as is
+described at the end of this section) by means of a hat-brush.  When
+the surface of the paper is filled with rouge all excess must be
+removed by vigorous brushing.
+
+Fig.  53.
+
+The tool being placed on the lens, two or three strokes similar to
+those used in grinding may be taken, and the tool is then lifted off
+and examined.  It will be found to be dotted with a few bright points,
+produced by the adhesion of glass at the places of contact.  These
+points are then to be removed in the following manner.  An old
+three-cornered file is ground on each side till the file marks
+disappear, and sharp edges are produced (Fig.  53).  This tool is used
+as an ink eraser, and it will be found to scrape the paper of the
+polishing tool very cleanly and well.
+
+The bright spots are the objects of attention, and they must be erased
+by the old file, and the polisher reapplied to the glass.  A few
+strokes will develop other points, more numerous than before, and
+these in turn must be erased.  The process is continued till the whole
+surface of the polishing tool is evenly covered with bright specks,
+and then the polishing may be proceeded with.  The specks should not
+be more than about one-eighth of an inch apart, or the polishing will
+be irregular.
+
+The operation of polishing is similar to that of grinding.  A
+reasonable time for polishing a glass surface is twenty hours;  if
+more time is required it is a sign that the fine grinding has not been
+carried far enough.  The progress of the operation may be best watched
+by looking at the surface--not through it.  For this purpose a good
+light is requisite.  When the lens is dismounted it may be examined by
+a beam of sunlight in a dark room, under which circumstances the
+faintest signs of grayness are easily discernible.
+
+It may be mentioned here that if the surface is in any way scratched
+the rouge will lodge in the scratches with great persistence, and an
+expert can generally tell from the appearance of scratches what kind
+of polishing powder has been employed.
+
+The persistence with which rouge clings to a rough surface of glass is
+rather remarkable.  Some glass polishers prefer to use putty powder as
+a polishing material, and it is sometimes said to act more quickly
+than rouge;  from my rather limited experience I have not found this
+to be the case, but it may have merits that I do not know of.  Is it
+possible that its recommendation lies in the fact that it does not
+render scratches so obtrusively obvious as rouge does?
+
+Rouge is generally made in two or more grades.  The softer grade is
+used for polishing silver, and is called jewellers' rouge.  The harder
+grade, suitable for glass polishing, is best obtained from practical
+opticians (not mere sellers of optical instruments).  I mean people
+like Messrs.  Cook of York.  Many years ago I prepared my own hard
+rouge by precipitating ferrous sulphate solution by aqueous ammonia,
+washing the precipitate, and heating it to a red heat.  The product
+was ground up with water, and washed to get rid of large particles.
+This answered every purpose, and I could not find that it was in any
+way inferior to hard rouge as purchased.  The same precipitate heated
+to a lower temperature is said to furnish a softer variety of rouge;
+at all events, it gives one more suitable for polishing speculum
+metal.  Lord Rosso used rouge heated to a dull redness for this
+purpose.
+
+Rouge, whether made or bought, should always be washed to get rid of
+grit.  I ought to add that not the least remarkable fact about the
+polishing is the extraordinarily small quantity of the polishing
+material requisite, which suggests that the process of polishing is
+not by any means the same as that of exceptionally fine grinding.  Is
+it possible that the chief proximate cause of the utility of rouge is
+to be sought in its curious property of adhering to a rough glass
+surface, causing it, so to speak, to drag the glass off in minute
+quantities, and redeposit it after a certain thickness has been
+attained on another part of the surface?
+
+Sec. 63.  Centering.
+
+When a lens is ground and polished it will almost always happen that
+the axis of revolution of its cylindrical edge is inclined to the axis
+of revolution of its curved surfaces.  Since in practice lenses have
+to be adjusted by their edges, it is generally necessary to adjust the
+edge to a cylinder about the axis of figure of the active surfaces.
+This is best done on a lathe with a hollow mandrel..  The lens is
+chucked on a chuck with a central aperture--generally by means of
+pitch or Regnault's mastic, or "centering" cement for small
+lenses--and a cross wire is fixed in the axis of revolution of the lathe,
+and is illuminated by a lamp.  This cross wire is observed by an
+eye-piece (with cross wires only in the case of a convex lens, or a
+telescope similarly furnished in the case of a concave lens),
+also placed in the axis of rotation of the lathe.
+
+Both cross wires are thus in the axis of revolution of the mandrel,
+and the distant one (B in the figure) is viewed through the lens and
+referred to the fixed cross wires at A.  In general, as the lathe is
+rotated by turning the mandrel the image of the illuminated cross
+wires will be observed to rotate also.  The lens is adjusted until the
+image remains steady on rotating the mandrel and it is to give time
+for this operation that a slow-setting cement is recommended.  When
+the image remains stationary we know that the optical centre of the
+lens is in the axis of revolution, and that this axis is normal to
+both lens surfaces, i.e.  is the principal axis of the lens, or axis
+of figure.
+
+Fig.  54.
+
+A much readier method, and one, in general, good enough for most
+purposes, is to put a candle on the end of the lathe-bed where the
+back centre generally is, and observe the images of the flame by
+reflection from both the lens surfaces.  This method is very handy
+with small lenses;  the mandrel is turned, and the lens adjusted by
+hand till the images are immovable.  In both cases, of course, the
+edge of the lens is turned or ground till it is truly circular, the
+position of the lens remaining undisturbed on the chuck.  If the edge
+gauge has been properly used in the earlier stages of figuring, it
+will be found that very little turning or grinding is requisite to
+produce a true centering.
+
+The particular defect due to want of centering in a lens may be
+observed by using it as the objective of a telescope, and observing a
+star slightly out of focus.  The interference fringes will not be
+concentric circles  unless the lens is properly  centred.  I ought to
+say that I have not looked into the theory of this, but have merely
+taken it as a generally admitted fact.  The diseases of lenses and the
+modes of treating them are dealt with in a book by Messrs.  Cook of
+York, entitled On the Adjustment and Testing of Telescopic Objectives.
+
+The final process of figuring will be dealt with later on (Sec. Sec. 66 and
+67), as it applies not only to lenses but to mirrors, prisms, etc.  If
+the instructions given have been carefully carried out on a 2-inch
+lens, it should perform fairly well, and possibly perfectly, without
+any further adjustment of the glass.
+
+Sec. 64.  Preparation of Small Lenses, where great Accuracy is not of the
+first Importance.
+
+Such lenses may generally be made out of bits of good plate or sheet
+glass, and are of constant use in the physical laboratory.  They may
+be purchased so cheaply, however, that only those who have the
+misfortune to work in out-of-the-way places need be driven to make
+them.
+
+Suitable glass having been obtained and the curves calculated from the
+index of refraction, as obtained by any of the ordinary methods
+applicable to plates (the microscope method, in general, is quite good
+enough), squares circumscribing the desired circles are cut out by the
+help of a diamond.  [Footnote: Glazebrook and Shaw's Practical
+Physics, p.  383 (4th ed.).]  The squares are roughly snipped by means
+of a pair of pliers or spectacle-maker's shanks.  The rough circles
+are then mounted on the end of a brass or iron rod of rather greater
+diameter than the finished lenses are to possess.  This mounting is
+best done by centering cement.
+
+The discs are then dressed circular on a grindstone, the rod serving
+both as a gauge and handle.  A sufficient number of these discs having
+been prepared, a pair of brass tools of the form shown in the sketch
+(Fig.  55), and of about the proper radius of curvature, are made.
+One of these tools is used as a support for the glass discs.
+
+Fig. 55.
+
+A compass being set to scribe circles of the same diameter as the
+glass discs, centre marks are made on the surface of the appropriate
+tool, circles are drawn on this, and facets are filed or milled (for
+which the spiral head of the milling machine is excellent).  In the
+case of concave supporting surfaces, i.e.  in making concave lenses, I
+apprehend filing would be difficult, and the facets would have to be
+made by a rose cutter or mill;  but if the discs are fairly round,
+then, in fact, no facets are required.
+
+The facets being ready, the glass discs are cemented to them by
+centering cement, which may be used quite generally for small lenses.
+When the cutting of facets has been omitted on a concave surface, the
+best cement is hard pitch.  The grinding tool is generally rather
+larger than the nest of lenses.  Coarse and fine grinding is
+accomplished wholly on the lathe--the tool being rotated at a fair
+speed (see infra), and the nest of lenses moved about by its handle so
+as to grind all parts equally.  It must, of course, be held anywhere
+except "dead on," for then the part round the axis would not get
+ground;  this inoperative portion of the rotating tool must therefore
+be allowed to distribute its incapable efforts evenly over the nest of
+lenses.
+
+Polishing is accomplished by means of the grinding tool, coated with
+paper and rouge as before; or the tool may be coated with very thin
+cloth and used with rouge as before--in this case the polishing goes
+on fastest when the surface of the cloth is distinctly damp.  In
+working by this method, each grade of emery need only be applied from
+five to ten minutes.  The glass does not appear to get scratched when
+the emery is changed, provided everything is well washed.  A good
+polish may be got in an hour.  The lathe is run as for turning brass
+of the same diameter as the tool.
+
+One side of the lenses being thus prepared, they are reversed, and the
+process gone through for the other side in a precisely similar manner.
+[Footnote: Unless the radius of curvature is very short and the lenses
+also convex, there is no necessity to recess the facets, provided hard
+pitch is used as the cement.  See note on hard pitch.]  To save
+trouble, it is usual, to make such lenses of equal curvature on both
+faces;  but of course this is a matter of taste.
+
+Fig.  56.
+
+For very common work, bits of good plate glass are employed, and the
+manufacturer's surface treated as flat (Fig.  56).  In this way
+plano-convex lenses are easily and cheaply made.  Finally the lenses
+have to be centred, an essential operation in this case.  This is
+easily done by the reflection method--the edge being turned off by
+the file and kerosene and the centering cement being used in making
+the preliminary adjustment on the chuck.  I presume a lens made in
+this way is worth about a shilling, so that laboratory manufacture is
+not very remunerative.  Fig.  56 shows the method of mounting small
+lenses for lathe grinding, when only one lens is required.  The tool
+is generally rotated in the lathe and the lens held against it.
+
+Sec. 65.  Preparing Small Mirrors for Galvanometers.
+
+To get good mirrors for galvanometers, I have found the best plan is
+to grind and polish a large number together, on a disc perhaps 8 or 10
+inches in diameter.  I was led to this after inspecting and rejecting
+four ounces of microscope cover slips, a most wearisome process.  That
+regular cover slips should be few and far between is not unlikely,
+seeing that they are made (by one eminent firm at least) simply by
+"pot" blowing a huge thin bulb, and then smashing it on the floor and
+selecting the fragments.  As in the case of large mirrors, it is of
+course only necessary to grind one side of the glass, theoretically at
+all events.  The objections to this course are:
+
+(1) A silver surface cannot, in my experience, be polished externally
+(on a minute object like a cover slip) to be anything like so bright
+as the silver surface next the glass;  and,
+
+(2) if one side only is ground, it will be found that the little
+mirror hopelessly loses its figure directly it is detached from the
+support on which it has been worked.  Consequently, I recommend that
+these small mirrors should be ground and polished on both
+sides--enough may be made at one operation to last for a very long time.
+
+A slate back is prepared of the same radius of curvature as it is
+desired to impart to the mirrors.  Bits of thin sheet glass are then
+ground circular as described in the last section and cemented to this
+surface by the smallest quantity of clean archangel pitch, allowed to
+cool slowly and even to rest for a day before the work is proceeded
+with.  The whole surface is then ground and polished as before.
+
+The mirrors are now reversed, when they ought to nearly fit the tool
+(assuming that flats are being made, and the fellow tool in all other
+cases), and are recemented by pitch to the appropriate backing ground,
+and polished.  If very excellent results are required, these processes
+may be preceded by a preliminary rough grinding of one surface, so
+that the little discs will "sit" exactly on the tool surface, and not
+run the risk of being strained by capillary forces in the pitch.  We
+have always found this necessary for really good results.
+
+On removing such mirrors from the backing, they generally, more or
+less lose their figure, becoming (in general fairly uniformly) more
+concave or convex.  About 5 per cent of the mirrors thus prepared will
+be found almost perfect if the work has been well done, and the rest
+will probably be very fair, unless the diameter is very large as
+compared with the thickness.  The best way of grinding and polishing
+such large surfaces (nests 10 inches in diameter) is on a grinding
+machine, such as will be described below.  The polishing is best done
+by means of paper, as before described.
+
+Having occasion to require hitherto unapproached lightness and optical
+accuracy in such mirrors, I got my assistant to try making them of
+fused quartz, slices being cut by a diamond wheel from a rod of that
+material.  Chips of natural quartz were also obtained from broken
+"pebble" spectacles, and these were worked at the same time.  The
+resulting mirrors were certainly superior to the best we could make
+from glass, but the labour of grinding was greater, and the labour of
+polishing less, than in the latter case.  The pebble fragments gave
+practically as good mirrors as the fused slices.  For the future it
+will be better always to make galvanometer mirrors from quartz
+crystals.  These may be easily sliced, as will be described in Sec. 74.
+The slices are dressed on a grindstone according to instructions
+already given for small lenses.
+
+The silvering of these mirrors is a point of great importance.  After
+trying nearly every formula published, we have settled down to the
+following.
+
+A solution of pure crystallised nitrate of silver in distilled water
+is made up to a strength of 125 grams of the salt per litre.  This
+forms the stock solution and is kept in a dark bottle.
+
+Let the volume of silvering liquor required in any operation be
+denoted by 4 v.  The liquor is prepared as follows:
+
+I.  Measure out a volume v of the stock solution of silver nitrate,
+and calculate the weight of salt which it contains;  let this be w.
+In another vessel dissolve pure Rochelle salt to the amount of 2.6 w,
+and make up the solution to the volume v.  These two solutions are to
+be mixed together at a temperature of 55 deg. C, the vessels with their
+contents being heated to this temperature on the water bath.  After
+mixing the liquids the temperature is to be kept approximately
+constant for five minutes, after which the liquor may be cooled.  The
+white precipitate which first forms will become gray or black and very
+dense as the liquid cools.  If it does not, the liquor must be
+reheated to 55 deg. C, and kept at that temperature for a few minutes and
+then again allowed to cool.  The solution is in good order when all
+the precipitate is dense and gray or black and the liquor clear.  The
+blacker and denser the precipitate the better is the solution.  The
+liquor is decanted and filtered from the precipitate and brought up to
+the volume 2 v by addition of some of the wash water.
+
+II.  Measure out a volume 0.118 v of the stock solution into a
+separate vessel, and add to it a 5 per cent solution of ammonium
+hydrate, with proper precautions, so that the precipitate at first
+formed is all but redissolved after vigorous shaking.  It is very
+important that this condition should be exactly attained.  Therefore
+add the latter part of the ammonia very carefully.  Make up the volume
+to 2 v.
+
+Mix the solutions I.  and II.  in a separate vessel and pour the
+mixture into the depositing vessel.  The surface to be silvered should
+face downwards, and lie just beneath the free surface of the liquid.
+Bubbles must of course be removed.
+
+The silver deposit obtained in this manner is exceedingly white and,
+bright on the surface next to the glass, but the back is mat and
+requires polishing.
+
+The detail of the process described above was worked out in my
+laboratory by Mr. A. Pollock, to whom my thanks are due.
+
+This process gives good deposits when the solutions are freshly
+prepared, but the ammonia solution will not keep; The surfaces to be
+silvered require to be absolutely clean.  The process is assisted by a
+summer temperature, say 70 deg. Fahr, and possibly by the action of
+light.  Six or seven hours at least are required for a good deposit; a
+good plan is to leave the mirrors in the bath all night.  On removal
+from the bath the mirrors require to be well washed, and allowed to
+dry thoroughly in sun heat for several hours before they are touched.
+
+Care should be taken not to pull the mirrors out of shape when they
+are mounted for the bath.  A single drop of varnish or paint (a mere
+speck) on the centre will suffice to hold them. The back of the
+deposit requires to be varnished or painted as a rule to preserve the
+silver.  All paints and varnishes thus applied tend to spoil the
+figure by expanding or contracting.  On the whole, I think boiled
+linseed oil and white or red lead--white or red paint in fact--is
+less deleterious than other things I have tried.  Shellac varnish is
+the worst.
+
+Of course, the best mirror can be easily spoiled by bad mounting.  I
+have tried a great number of methods and can recommend as fairly
+successful the following:-  A little pure white lead, i.e.  bought as
+pure as a chemical--not as a paint--is mixed with an equal quantity
+of red lead and made into a paste with a little linseed oil.  I say a
+paste, not putty.  A trace of this is then worked on to the back of
+the mirror at the centre as nearly as may be, and to this is attached
+the support.  The only objection to this is that nearly a week is
+required for the paste to set.  If people must use shellac let it be
+remembered that it will go on changing its shape for months after it
+has cooled (whether it has been dissolved in alcohol or not).
+
+Sec. 66.  Preparation of Large Mirrors or Lenses for Telescopes.
+
+So much has been written on this subject by astronomers, generally in
+the English Mechanic and in the Philosophical Transactions for 1840,
+that it might be thought nothing could be added.  I will only say here
+that the processes already described apply perfectly to this case; but
+of course I only refer to silver on glass mirrors.  For any size over
+6 inches in diameter, the process of grinding and polishing by hand,
+particularly the latter, will probably be found to involve too much
+labour, and a machine will be required.  A description of a
+modification of Mr. Nasmyth's machine--as made by my assistant, Mr.
+Cook--will be found below.
+
+There is no difficulty in constructing or working such a machine, and
+considered as an all round appliance, it possesses solid advantages
+over the simple double pulley and crank arrangement, which, however,
+from its simplicity deserves a note.  Two pulleys, A and B, of about
+18 inches diameter by 4 inches on the face, are arranged to rotate
+about vertical axes, and belted together.  The shaft of one of these
+pulleys is driven by a belt in any convenient manner.  Each pulley is
+provided on its upper surface with a crank of adjustable length
+carrying a vertical crank-pin.
+
+Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6" long, and
+these rods are pinned together at their farther extremities, and this
+pin carries the grinding or polishing tool, or rather engages loosely
+with the back of this tool which lies below the rod.  It is clear that
+if the pulleys are of commensurable diameters, and are rigidly
+connected--say by belting which neither stretches nor slips--the
+polishing tool will describe a closed curve.  If, however, the belt is
+arranged to slip slightly, or if the pulleys are of incommensurable
+diameters, the curve traced out by the grinding tool will be very
+complex, and in the case of the ratio of the diameters being
+incommensurable, will always remain open;  for polishing purposes the
+consummation to be wished.
+
+Mirror surfaces are ground spherical, the reduction to parabolic form
+being attained in the process of polishing.  A very interesting
+account of the practice of dealing with very large lenses will be
+found in Nature, May 1886, or the Journal of the Society of Arts, same
+date (I presume), by Sir Howard Grubb.  The author considers that the
+final adjustment of surfaces by "figuring"--of which more anon--is
+an art which cannot be learned by inspection, any more than a man
+could learn to paint by watching an artist.  This is, no doubt, the
+case to some extent;  still, a person wishing to learn how to figure a
+lens could not do better than take Sir Howard at his word, and spend a
+month at his works.  Meanwhile the following remarks must suffice; it
+is not likely that anybody to whom these notes will be of service
+would embark on such large work as is contemplated by Sir Howard
+Grubb.
+
+Fig. 57.
+
+Description of Polishing Machine.  Power is applied through belting to
+the speed cone A.  By means of a bevel pinion rotation is communicated
+to the wheel D, which is of solid metal and carries a T-slot, C.  A
+pedestal forming a crank-pin can be clamped so as to have any desired
+radius of motion by the screw E.  A train of wheels E F G H K
+(ordinary cast lathe change wheels) communicate any desired ratio of
+motion to the tool-holder, which simply consists of two pins
+projecting vertically downwards from the spokes of wheel K.
+
+These pins form a fork, and each prong engages in a corresponding hole
+in the back of the slate-grinding tool (not shown in figure).  The
+connection with the tool is purposely loose.  The wheel E, of course,
+cannot rotate about the crank-pin D.  Provision for changing the ratio
+of tool rotation is achieved by mounting the wheels composing the
+train on pins capable of sliding along a long slot in the bar
+supporting them. The farther end of this bar is caused to oscillate
+to and fro very slowly by means of an additional crank-pin S and
+crank-shaft, the projecting face of the bed-plate W being placed
+so as to allow V to slide about easily and smoothly.
+Motion is communicated to this part of the system by means of
+gears at 0 and P, and a belt working from P to Q.
+Thus the vertical shaft R is set in motion and
+communicates by gears with S.  A pulley placed on the axle of the
+wheel carrying the crank-pin S gives a slow rotation to the work which
+is mounted on the table M. A small but important feature is the tray L
+below the gear K.  This prevents dirt falling from the teeth of the
+wheel on to the work.  The motion of S is of course very much less
+than of B--say 100 times less.  The work can be conveniently adjusted
+as to height by means of the screw N.
+
+The machine must be on a steady foundation, and in a place as free
+from dust as possible.  Though it looks complicated it is quite
+straight-forward to build and to operate.
+
+It is explained in Lord Rayleigh's article on Optics in the
+Encyclopaedia Britannica that a very minute change in the form of the
+curvature of the surface of a lens will make a large difference in the
+spherical aberration.  This is to be expected, seeing that spherical
+aberration is a phenomenon of a differential sort, i.e.  a measure of
+the difference between the curvature actually attained, and the
+theoretical curvature at each point of the lens, for given positions
+of point and image.  Sir H.  Grubb gives an illustration of the
+minuteness of the abrasion required in passing from a curve of one
+sort to a curve of another, say from a spherical to a parabolic curve,
+consequently the process of figuring by the slow action of a polishing
+tool becomes quite intelligible.  In making a large mirror or lens all
+the processes hitherto described under grinding and polishing, etc,
+have to be gone through and in the manner described, and when all this
+is accomplished the final process of correcting to test commences.
+This process is called figuring.
+
+Sec. 67.  Of the actual operation of this process I have no personal
+knowledge, and the following brief notes are drawn from the article by
+Sir H.  Grubb, from my assistant's (Mr. Cook) experience, and from a
+small work On the Adjustment and Testing of Telescopic Objectives, by
+T.  Cook and Sons, Buckingham Works, York (printed by Ben Johnson and
+Co, Micklegate, York).  This work has excellent photographs of the
+interference rings of star images corresponding to various defects.
+It must be understood that the following is a mere sketch.  The art
+will probably hardly ever be required in laboratory practice, and
+those who wish to construct large telescopes should not be above
+looking up the references.
+
+The process is naturally divided for treatment into two parts.
+
+(1) The detection of errors, and the cause of these errors.
+
+(2) The application of a remedy.
+
+(1) A lens, being mounted with its final adjustments, is turned on to
+a star, which must not be too bright, and should be fairly overhead.
+The following appearances may be noted:-
+
+A.  In focus, the star appears as a small disc with one or two rings
+round it; inside and outside of the focus the rings increase in
+number, are round, concentric with the disc, and the bright and dark
+rings are apparently equally wide.  The appearance inside the focus
+exactly resembles that outside when allowance is made for chromatic
+effects.  Conclusion:  objective good, and correctly mounted.
+
+B.  The rings round the star in focus are not circular, nor is the
+star at the centre of the system. In bad cases the fringes are seen at
+one side only.  Effects exaggerated outside and inside the focus.
+Conclusion:  the lens is astigmatic, or the objective is not adjusted
+to be co-axial with the eyepiece.
+
+C.  When in focus the central disc is surrounded by an intermittent
+diffraction pattern, i.e.  for instance the system of rings may appear
+along, and near, three or more radii.  If these shift when the points
+of support of the lens are shifted, flexure may be suspected.
+
+D.  On observing inside and outside the focus, the rings are not
+equally bright and dark.  This may be due to uncorrected spherical
+aberration, particularly to a fault known as "zonal aberration," where
+different zones of the lens have different foci, but each zone has a
+definite focus.
+
+E.  Irregular diffraction fringes point to bad annealing of the glass.
+This may be checked by an examination of the lens in polarised light.
+
+F.  If the disc appear blurred and coloured, however the focus be
+adjusted, incomplete correction for chromatic aberration is inferred.
+If in addition the colouring is unsymmetrical (in an extreme case the
+star disc is drawn out to a coloured band), want of centering is to be
+inferred.  This will also show itself by the interference fringes
+having the characteristics described in C.
+
+(2) The following steps may be taken in applying a remedy:
+
+A.  The adjusting screws of the cell mounting the object glass may
+be worked until the best result is attained;  this requires great
+care and patience.  Any errors left over are to be attributed to
+other causes than the want of collinearity of the axes of object
+glass and eyepiece.
+
+B.  Astigmatism is detected by rotating the object glass or object
+glass cell.  If the oval fringes still persist and the longer axis
+follows the lens, astigmatism may be inferred.  Similarly, by rotating
+one lens on the other, astigmatism, or want of centering (quite a
+different thing) may be localised to the lens.
+
+C.  The presence of flexure may be confirmed by altering the position
+of the points of support with respect to the eyepiece, the lens
+maintaining its original position.  The addition of more points of
+support will in general reduce the ill effects.  How to get rid of
+them I do not know;  they are only serious with large lenses.
+
+D.  Spherical aberration may be located by using stops and zonal
+screens, and observing the effect on the image.  Sir H.  Grubb
+determines whether any point on the lens requires to be raised or
+lowered, by touching the glass at that point with a warm hand or
+cooling it by ether.  The effects so produced are the differential
+results of the change of figure and of refractive index.  By observing
+the effect of the heating or cooling of any part, the operator will
+know whether to raise or lower that part, provided that by a suitable
+preliminary experiment he has determined the relation between the
+effect produced by the change of figure, and that due to the
+temperature variation of the refractive index.  In general it is
+sufficient to consider the change of shape only and neglect the change
+in refractive power.
+
+E.  Marked astigmatism has never been noticed by me, but I should
+think that the whole lens surface would require to be repolished or
+perhaps reground in this case.
+
+F.  To decide in which surface faults exist is not easy.  By placing a
+film of oil between the two surfaces nearly in contact these may be
+easily examined.  Thus a mixture of nut and almond oil of the right
+proportion, to be found by trial, for the temperature, will have the
+same refractive index as the crown glass, and will consequently reduce
+any errors of figure in the interior crown surface, if properly
+applied between the surfaces.  Similarly the interior of the flint
+surface may have its imperfections greatly reduced in effect by using
+almond oil alone, or mixed with bisulphide of carbon.  The outer
+surfaces, I presume, must be examined by warming or cooling over
+suitable areas or zones.
+
+The defects being detected, a matter requiring a great deal of skill
+and experience according to Sir H.  Grubb, the next step is to remedy
+them; and the remedial measures as applied to the glass constitute the
+process of figuring.  There are two ways of correcting local defects,
+one by means of small paper or pitch covered tools, which according to
+Sir H.  Grubb is dangerous, and according to the experience of Mr.
+Cook, and I think of many French opticians, safe and advantageous.
+
+Pitch polishing tools are generally used for figuring.  They are made
+by covering a slate backing with squares of pitch.  The backing is
+floated with pitch say one-eighth of an inch thick.  This is then
+scored into squares by a hot iron rod.  The tool, while slightly warm,
+is laid upon the lens surface, previously slightly smeared with dilute
+glycerine, until the pitch takes the figure of the glass.  The
+polishing material is rouge and water.  Small tools are applied
+locally, and probably can only be so applied with advantage for grave
+defects.
+
+The other method is longer and probably safer.  It consists in
+furnishing the polishing tool with squares of pitch as before.  These
+being slightly warm, the lens is placed upon them so that they will
+flow to the exact figure also as before.  I presume that the lens is
+to be slightly smeared with glycerine, or some equivalent, to keep the
+pitch from sticking.  The squares are most thickly distributed where
+the abrasion is most required, i.e.  less pitch is melted out by the
+iron rod.  This may be supplemented by taking advantage of differences
+of hardness of pitch, making some squares out of harder, others out of
+softer pitch.  The aim is to produce a polishing tool which will
+polish unequally so as to remove the glass chiefly from predetermined
+parts of the lens surface.  The tool is worked over the surface of the
+lens by the polishing machine, and part of the art consists in
+adjusting the strokes to assist in the production of the local
+variations required.
+
+A source of difficulty and danger lies in the fact that the pitch
+squares are rarely of the same hardness, so that some abrade the glass
+more rapidly than others.  This is particularly likely to occur if the
+pitch has been overheated.  [Footnote: When pitch is heated till it
+evolves bubbles of gas its hardness increases with the duration of the
+process.]  The reader must be good enough to regard these remarks as
+of the barest possible kind, and not intended to convey more than a
+general idea of the nature of the process of figuring.
+
+Sec. 68.  A few remarks on cleaning lenses will fittingly close this part
+of the subject.  There is no need to go beyond the following
+instructions given by Mr. Brashear in Popular Astronomy, 1894, which
+are reproduced here verbatim.
+
+"The writer does not advise the use of either fine chamois skin,
+tissue paper, or an old soft silk handkerchief, nor any other such
+material to wipe the lenses, as is usually advised.  It is not,
+however, these wiping materials that do the mischief, but the dust
+particles on the lenses, many of them perhaps of a silicious nature,
+which are always harder than optical glass, and as these particles
+attach themselves to the wiping material they cut microscopic or
+greater scratches on the surfaces of the objective in the process of
+wiping.
+
+"I write this article with the hope of helping to solve this
+apparently difficult problem, but which in reality is a very simple
+one.
+
+"Let us commence by taking the object glass out of its cell.  Take out
+the screws that hold the ring in place, and lift out the ring.
+Placing the fingers of both hands so as to grasp the objective on
+opposite sides, reverse the cell, and with the thumbs gently press the
+objective squarely out of the cell on to a book, block of wood, or
+anything a little less in diameter than the objective, which has had a
+cushion of muslin or any soft substance laid upon it.  One person can
+thus handle any objective up to 12 inches in diameter.
+
+"Before separating the lenses it should be carefully noted how they
+were put together with relation to the cell, and to one another, and
+if they art not marked they should be marked on the edges
+conspicuously with a hard lead pencil, so that when separated they may
+be put together in the same way, and placed in the same relation to
+the cell.  With only ordinary precaution this should be an easy
+matter.
+
+"Setting the objective on edge the two lenses may be readily
+separated.
+
+"And now as to the cleaning of the lenses.  I have, on rare occasions,
+found the inner surfaces of an object glass covered with a curious
+film, not caused directly by moisture but by the apparent oxidation of
+the tin-foil used to keep the lenses apart.  "A year or more ago a
+7-inch objective made by Mr. Clark was brought to me to clean.  It had
+evidently been sadly neglected.  The inside of the lenses were covered
+with such a film as I have mentioned, and I feared the glass was
+ruined.  When taken apart it was found that the tin-foil had oxidised
+totally and had distributed itself all over the inner surfaces.  I
+feared the result, but was delighted to find that nitric acid and a
+tuft of absorbent cotton cut all the deposit off, leaving no stains
+after having passed through a subsequent washing with soap and water.
+
+"I mention this as others may have a similar case to deal with.
+
+"For the ordinary cleaning of an objective let a suitable sized
+vessel, always a wooden one, be thoroughly cleaned with soap and
+water, then half filled with clean water about the same temperature as
+the glass.  Slight differences of temperature are of no moment.  Great
+differences are dangerous in large objectives.
+
+"I usually put a teaspoonful of ammonia in half a pail of water, and
+it is well to let a piece of washed 'cheese cloth' lie in the pail, as
+then there is no danger if the lens slips away from the hand, and, by
+the way, every observatory, indeed every amateur owning a telescope,
+should have plenty of 'cheese cloth' handy.  It is cheap (about 3 cts.
+per yard) and is superior for wiping purposes to any 'old soft silk
+handkerchief,' chamois skin, etc.  Before using it have it thoroughly
+washed with soap and water, then rinsed in clean water, dried and laid
+away in a box or other place where it can be kept clean.  When you use
+a piece to clean an objective throw it away, it is so cheap you can
+afford to do so.
+
+"If the lenses are very dirty or 'dusty,' a tuft of cotton or a
+camel's-hair brush may be used to brush off the loose material before
+placing the lenses in the water, but no pressure other than the weight
+of the cotton or brush should be used.  The writer prefers to use the
+palms of the hand with plenty of good soap on them to rub the
+surfaces, although the cheese cloth and the soap answers nicely, and
+there seems to be absolutely no danger of scratching when using the
+hands or the cheese cloth when plenty of water is used;  indeed when I
+wish to wipe off the front surface of an objective in use, and the
+lens cannot well be taken out, I first dust off the gross particles
+and then use the cheese cloth with soap and water, and having gone
+over the surface gently with one piece of cloth, throw it away and
+take another, perhaps a third one, and then when the dirt is, as it
+were, all lifted up from the surface, a piece of dry cheese cloth will
+finish the work, leaving a clean brilliant surface, and no scratches
+of any kind.
+
+"In washing large objectives in water I generally use a 'tub' and
+stand the lenses on their edge.  When thoroughly washed they are taken
+out and laid on a bundle of cheese cloth and several pieces of the
+same used to dry them.
+
+"I think it best not to leave them to drain dry;  better take up all
+moisture with the cloth, and vigorous rubbing will do no harm if the
+surfaces have no abrading material on them. I have yet to injure a
+glass cleaned in this way.
+
+"This process may seem a rather long and tedious one, but it is not so
+in practice, and it pays.
+
+"In some places objectives must be frequently cleaned, not only
+because they become covered with an adherent dust, but because that
+dust produces so much diffused light in the field as to ruin some
+kinds of telescope work.  Mr. Hale of the Kenwood Observatory tells me
+he cannot do any good prominence photography unless his objective has
+a clean surface; indeed every observer of faint objects or delicate
+planetary markings knows full well the value of a dark field free from
+diffused light.  The object-glass maker uses his best efforts to
+produce the most perfect polish on his lenses, aside from the accuracy
+of the curves, both for high light value and freedom from diffused
+light in the field, and if the surfaces are allowed to become covered
+with dust, his good work counts for little.
+
+"If only the front surface needs cleaning, the method of cleaning
+with cheese cloth, soap and water, as described above, answers very
+well, but always throw away the first and, if necessary, the second
+cloth, then wipe dry with a third or fourth cloth;  but if the
+surfaces all need cleaning I know of no better method than that of
+taking the objective out of its cell, always using abundance of soap
+and water, and keep in a good humor."
+
+Sec. 69.  The Preparation of Flat Surfaces of Rock Salt.
+
+The preliminary grinding is accomplished as in the case of glass,
+except that it goes on vastly faster.  The polishing process is the
+only part of the operation which presents any difficulty.  The
+following is an extract from a paper on the subject, by Mr. J.  A.
+Brashear, Pittsburg, Pa, U.S.A, from the Proceedings of the American
+Association for the Advancement of Science, 1885.  Practically the
+same method was shown me by Mr. Cook some years earlier, so that I can
+endorse all that Mr. Brashear says, with the following exceptions.  We
+consider that for small salt surfaces the pitch is better scored into
+squares than provided with the holes recommended by Mr. Brashear.
+
+Mr. Brashear's instructions are as follows.  After alluding to the
+difficulty of drying the polished salt surface--which is of course
+wet--Mr. Brashear says:-
+
+"Happily I have no trouble in this respect now, and as my method is
+easily carried out by any physicist who desires to work with rock salt
+surfaces, it gives me pleasure to explain it.  For polishing a prism I
+make an ordinary pitch bed of about two and one-half or three times
+the area of the surface of the prism to be polished.  While the pitch
+is still warm I press upon it any approximately flat surface, such as
+a piece of ordinary plate glass.  The pitch bed is then cooled by a
+stream of water, and conical holes are then drilled in the pitch with
+an ordinary counter sink bit, say one-quarter of an inch in diameter,
+and at intervals of half an inch over the entire surface.  This is
+done to relieve the atmospheric pressure in the final work.  The upper
+surface of the pitch is now very slightly warmed and a true plane
+surface (usually a glass one, prepared by grinding and polishing three
+surfaces in the ordinary way, previously wetted) is pressed upon it
+until the pitch surface becomes an approximately true plane itself.
+Fortunately, moderately hard pitch retains its figure quite
+persistently through short periods and small changes of temperature,
+and it always pays to spend a little time in the preparation of the
+pitch bed.
+
+"The polisher being now ready, a very small quantity of rouge and
+water is taken upon a fine sponge and equally distributed over its
+surface.  The previously ground and fined salt surface (this work is
+done the same as in glass working) is now placed upon the polisher and
+motion instantly set up in diametral strokes.  I usually walk around
+the polisher while working a surface.  It is well to note that motion
+must be constant, for a moment's rest is fatal to good results, for
+the reason that the surface is quickly eaten away, and irregularly so,
+owing to the holes that are in the pitch bed.  Now comes the most
+important part of this method.  After a few minutes' work the moisture
+will begin to evaporate quite rapidly.  No new application of water is
+to be made, but a careful watch must be kept upon the pitch bed, and
+as the last vestige of moisture disappears the prism is to be slipped
+off the polisher in a perfectly horizontal direction, and if the work
+has been well done, a clean, bright, and dry surface is the result.
+The surface is now tested by the well-known method of interference
+from a perfect glass test plate (see Fig.  178).
+
+"If an error of concavity presents itself the process of polishing is
+gone over again, using short diametral strokes.  If the error is one
+of convexity, the polishing strokes are to be made along the chords,
+extending over the edge of the polisher.  The one essential feature of
+this method is the fact that the surface is wiped dry in the final
+strokes, thus getting rid of the one great difficulty of pitch
+polishing, a method undoubtedly far superior to that of polishing on
+broadcloth.  If in the final strokes the surface is not quite cleaned
+I usually breathe upon the pitch bed, and thus by condensation place
+enough moisture upon it to give a few more strokes, finishing just the
+same as before.  In ten minutes I have polished prisms of rock salt in
+this manner that have not only shown the D line double, but Professor
+Langley has informed me that his assistant, Mr. Keeler (J.  E.), has
+seen the nickel line clearly between the D lines.  This speaks for the
+superiority of the surfaces over those polished on broadcloth.
+
+"In polishing prisms I prefer to work them on top of the polisher, as
+they can be easily held, but as it is difficult to hold lenses or
+planes in this way without injuring the surfaces, I usually support
+them in a block of soft wood, turned so as to touch only at their
+edges, and work the polisher over them. Though it takes considerable
+practice to succeed at first, the results are so good that it well
+repays the few hours' work it requires to master the few difficulties
+it presents."
+
+Fig.  58.
+
+Sec. 70.  Casting Specula for Mirrors.
+
+According to Sir H. Grubb (loc. cit.) the best alloy is made of four
+atoms of copper and one of tin;  this gives by weight,
+copper 252, tin 117.8.
+
+The copper is melted first in a plumbago crucible;  the tin is added
+gradually.  Of course, in the process of melting, even though a little
+fine charcoal be sprinkled over the copper, some loss of that metal
+will occur from oxidation.  It is convenient in practice, therefore to
+reserve a portion of the tin and test the contents of the crucible by
+lifting a little of the alloy out and examining it.
+
+The following indications may be noted:  When the copper is in excess
+the tint of the alloy is slightly red, and the structure, as shown at
+a fractured surface, is coarsely crystalline.  As the proper
+proportions are more nearly attained, the crystalline structure
+becomes finer, the colour whiter, and the crystals brighter.  The
+alloy is ready for use when the maximum brightness is attained and the
+grain is fine.
+
+If too much tin be added, the lustre diminishes.  The correct
+proportion is, therefore, attained when a further small addition of
+tin produces no apparent increase of brightness or fineness of grain.
+About three-quarters of the tin may be added at first, and the other
+quarter added with testing as described.  The alloy is allowed to cool
+until on skimming the surface the metal appears bright and remains so
+without losing its lustre by oxidation for a sensible time;  it will
+still be quite red-hot.
+
+Fig. 59.                                            Fig.   60.
+
+As the speculum alloy is too difficult to work with ordinary tools, it
+is best to cast the speculum of exactly the required shape and size.
+This is done by means of a ring of iron turned inside (and out) and on
+one edge.  This ring is laid on a plate of  figured iron, and before
+the metal is poured the plate (G)  (Figs 59 and 61) is heated to, say,
+300 deg. C.  In order to avoid the presence of oxide as far as possible,
+the following arrangements for pouring are made.  A portion of the
+lower surface of the ring is removed by radial filing until a notch
+equal to, say, one-twentieth of the whole circumference is produced.
+This is cut to an axial depth of, say, half an inch.
+
+A bar of iron is then dovetailed loosely into the notch (Fig.  60, B),
+so that it will rest on the iron plate, and half fill the notch.  The
+aperture thus left forms the port of ingress for the hot metal (see
+Fig.  61, M).  A bit of sheet iron is attached to the upper surface of
+the ring, and lies as a sort of flap, shaped like a deep shovel,
+against the outside of the ring overhanging the port (Figs.  59 and 61
+at F).  This flap does not quite reach the iron plate, and its sides
+are bent so as to be in contact with the ring.  A portion of a smaller
+ring is then applied in such a manner as to form a pouring lip or pool
+on the outside of the main ring at E, and the metal can only get into
+the main ring by passing under the edge of the flap and up through the
+port.  This forms an efficient skimming arrangement.  The process of
+casting is carried out by pouring steadily into the lip.
+
+To avoid air bubbles it is convenient to cause the metal to spread
+slowly over the chill, and Mr. Nasmyth's method of accomplishing this
+is shown in the figure (61).  The chill rests on three pins, A B C
+(Figs.  59 and 61).  Before pouring begins the chill is tilted up off
+C by means of the counterpoise D, which is insufficient to tilt it
+after the speculum is poured.  It is important that the chill should
+be horizontal at the close of the operation, in order that the
+speculum may be of even thickness throughout.  This is noted by means
+of levels placed on the ring (at K for instance).
+
+Fig.  61.
+
+This apparatus may appear unnecessarily complex, but it is worth while
+to set it up, for it makes the operation of casting a speculum fairly
+certain.  If the metal is at the right temperature it will form a
+uniformly liquid disc inside the ring.  The mass sets almost directly,
+and as soon as this occurs it is pushed to the edge of the plate and
+the metal in the lip broken off by a smart upward tap with a hammer.
+The dovetailed bit of iron is knocked downwards and falls off, and the
+ring may then be lifted clear of the casting.  The object of the
+dovetail will now be understood, for without it there is great risk of
+breaking into the speculum in knocking the "tail" off.
+
+A box of quite dry sawdust is prepared in readiness for the process of
+annealing before the speculum is cast.  The box must be a sound wooden
+or metal box, and must be approximately air-tight.  For a speculum a
+foot in diameter the box must measure at least 3 feet both ways in
+plan, and be 2 feet 6 inches deep.  Half the sawdust is in the box and
+is well pressed down so as to half fill it.  The other half must be
+conveniently ready to hand.  As soon as possible after casting, the
+speculum is thrown into the box, covered over with the sawdust, and
+the lid is put on.
+
+The object in having the box nearly air-tight is to avoid
+air-currents, which would increase the rate of cooling.  A speculum a
+foot in diameter may conveniently take about three days to anneal, and
+should be sensibly warm when the box is opened on the fourth day.  For
+larger sizes longer times will be required.  We will say that the
+sawdust thickness on each side must be proportional to the dimensions
+of the speculum, or may even increase faster with advantage if time is
+of no moment.
+
+The process of annealing may be considered successful if the disc does
+not fly to pieces in working;  it is to be worked on the chilled side.
+The object of giving the chill the approximate counterpart form will
+now appear; it saves some rough grinding, and causes the finished
+surface to be more homogeneous than it would be if the centre were
+sunk by grinding through the chilled surface.
+
+In 1889 I learned from Mr. Schneider, Professor Row-land's assistant
+at Baltimore, that in casting specula for concave gratings a good deal
+of trouble had been saved by carrying out the operation in an
+atmosphere consisting mostly of coal gas.  It was claimed that in this
+way the presence of specks of oxide was avoided.  I did not see the
+process in operation, but the results attained are known and admired
+by all experimenters.
+
+Sec. 71.  Grinding and polishing Specula.
+
+The rough grinding is accomplished by means of a lead tool and coarse
+emery; the size of grain may be such as will pass a sieve of 60
+threads to the inch.  The process of grinding is quite similar to that
+previously described, but it goes on comparatively quickly.  The rough
+grinding is checked by the spherometer, and is interrupted when that
+instrument gives accordant and correct measurements all over the
+surface.
+
+The fine grinding may be proceeded with by means of a glass-faced tool
+as before described, or the labour may be reduced in the following
+manner.  A slate tool, which must be free from green spots (a source
+of uneven hardness), is prepared, and this is brought nearly to the
+curvature of the roughly ground speculum, by turning or otherwise.  It
+is finished on the speculum itself with a little flour of emery.  The
+fine grinding is then carried on by means of slate dust and water, the
+slate tool being the grinder.  The tool is, of course, scored into
+squares on the surface.
+
+If the casting process has been carried out successfully, the rough
+grinding may take, say six hours, and the fine grinding say thirty
+hours for a disc a foot in diameter.  The greatest source of trouble
+is want of homogeneity in the casting, as evidenced by blowholes, etc.
+In general, the shortest way is to discard the disc and start afresh
+if there is any serious want of perfection in the continuity or
+homogeneity of the metal.
+
+Fig.  62.
+
+The finely ground surface must, of course, be apparently correct in so
+far as a spherometer (with 3 inches between the legs for a disc 1 foot
+in diameter) will show.  Polishing and figuring are carried out
+simultaneously.  Half an hour's polishing with a slate-backed pitch
+tool and rouge and water will enable an optical test to be made.  The
+most convenient test is that of Foucault, a simple appliance for the
+purpose being shown in the figure (62).  It essentially consists of a
+small lamp surrounded by an opaque chimney (A) through which a minute
+aperture (pin-hole) is made.  A small lens may be used, of very great
+curvature, or even a transparent marble to throw an image of the flame
+on the pin-hole.
+
+A screen (B) is placed close to the source, and is provided with a
+rocking or tilting motion (C) in its own plane.  The source and screen
+are partly independent, and each is provided with a fine adjustment
+which serves to place it in position near the centre of curvature.
+The screen is so close to the pin-hole in fact that both the source
+and a point on the edge of the screen may be said to be at the centre
+of curvature of the mirror.  The mirror is temporarily mounted so as
+to have its axis horizontal, in a cellar or other place of uniform
+temperature.
+
+The final focussing to the centre of curvature is made by the fine
+adjustment screws;  the image may be received on a bit of paper placed
+on the screen and overlapping the edge nearest the source.  The screws
+are worked till the image has its smallest dimension and is bisected
+by the edge of the screen.  The test consists in observing the
+appearance of the mirror surface while the screen is tilted to cut off
+the light, as seen by an eye placed at the edge of the screen, a
+peephole or eye lens being provided to facilitate placing the eye in a
+correct position.  The screen screws are worked so as to gradually cut
+off the light, and the observer notes the appearance of the mirror
+surface.  If the curves are perfect and spherical, the transition from
+complete illumination to darkness will be abrupt, and no part of the
+mirror will remain illuminated after the rest.
+
+For astronomical purposes a parabolic mirror is required.  In this
+case the disc may be partially screened by zonal screens, and the
+position of the image for different zones noted;  the correctness or
+otherwise of the curvature may then be ascertained by calculation.  A
+shorter way is to place the source just outside the focus, to be found
+by trial, and then, moving the extinction screen (now a separate
+appliance) to, say, five times the radius of curvature away, where the
+image should now appear, the suddenness of extinction may be
+investigated.  This, of course, involves a corresponding modification
+of the apparatus.
+
+Whether the tests indicate that a deepening of the Centre, i.e.
+increase of the curvature, or a flattening of the edges is required,
+at least two remedial processes are available.  The "chisel and
+mallet" method of altering the size of the pitch, squares of the
+polisher may be employed, or paper or small pitch tools may be used to
+deepen the centre.  The "chisel and mallet" method merely consists in
+removing pitch squares from a uniformly divided tool surface by means
+of the instruments mentioned.  This removal is effected at those
+points at which the abrasion requires to be reduced.
+
+When some practice is attained, I understand that it is usual to try
+for a parabolic form at once, as soon as the polishing commences.
+This is done by dividing the pitch surface by V-shaped grooves, the
+sides of the grooves being radii of the circular surface, so that the
+central parts of the mirror get most of the polishing action.  If
+paper tools are used they must not be allowed much overhang, or the
+edges of the mirror betray the effects of paper elasticity.  Most
+operators "sink" the middle, but the late Mr. Lassell, a most
+accomplished worker, always attained the parabolic form by reducing
+the curvature of the edges of a spherical mirror.
+
+Sec. 72.  Preparation of Flat Surfaces.
+
+As Sir H.  Grubb has pointed out, this operation only differs from
+those previously described in that an additional condition has to be
+satisfied.  This condition refers to the mean curvature, which must be
+exact (in the case of flats it is of course zero) to a degree which is
+quite unnecessary in the manufacture of mirrors or lenses.
+
+A little consideration will show that to get a surface flat the most
+straightforward method is to carry out the necessary and sufficient
+condition for three surfaces to fit each other impartially.  If they
+each fit each other, they must clearly all be flat.  To carry out the
+process of producing a flat surface, therefore, two tools are made,
+and the glass or speculum is ground first on one and then on the
+other, the tools being kept "in fit" by occasional mutual grinding.
+The grinding and polishing go on as usual.  If paper is employed, care
+must be taken that the polisher is about the same size as the object
+to be polished.
+
+There is a slight tendency to polish most at the edges; but if the
+sweeps are of the right shape and size, this may be corrected
+approximately.  The best surfaces which have come under my notice are
+those prepared as "test surfaces" by Mr. Brashear of Alleghany, Pa,
+U.S.A.  These I believe to be pitch polished.  A pitch bed is
+prepared, I presume, in a manner similar to that described for
+rocksalt surfaces; but the working of the glass is an immense art, and
+one which I believe--if one may judge by results--is only known to
+Mr. Brashear.
+
+In general, the effect of polishing will be to produce a convex or
+concave surface, quite good enough for most purposes, but distinctly
+faulty when tested by the interference fringes produced with the aid
+of the test plate.  The following information therefore--which I draw
+from Mr. 'Cook--will not enable a student to emulate Mr. Brashear,
+but will undoubtedly help him to get a very much better surface than
+he usually buys at a high price, as exhibited on a spectroscope prism.
+
+The only difference between this process and the one described for
+polishing lenses, lies in the fact that the rouge is put into the
+paper surface while the latter is wet with a dilute gum "mucilage."
+It is of course assumed that the object and the two tools have been
+finely ground and fit each other impartially.  The paper is rubbed
+over with rouge and weak gum water.  The tool, when dry, is applied to
+the flat ground surface (of the object), and is scraped with the
+three-cornered file chisel as formerly described.  This process must
+be very carefully carried out.  The paper must be of the quality
+mentioned, or may even be thinner and harder.  The cross strokes
+should be more employed than in the case of the curved surfaces.
+
+A good deal will depend on the method employed for supporting the
+work; it is in general better to support the tool, which may have a
+slate backing of any desired thickness, whereby the difficulty
+resulting from strains is reduced.  The work must be mounted in such a
+way as to minimise the effect of changes of temperature.  If a pitch
+bed is selected, Mr. Brashear's instructions for rock salt may be
+followed, with, of course, the obvious necessary modifications.  See
+also next section.
+
+Sec. 73.  Polishing Flat Surfaces on Glass or on Speculum Metal.
+
+The above process may be employed for speculum metal, or pitch may be
+used.  In the latter case a fresh tool must be prepared every hour or
+so, because the metal begins to strip and leave bits on the polisher;
+this causes a certain amount of scratching to take place.  As against
+this disadvantage, the process of polishing, in so far as the state of
+the surface is concerned, need not take an hour if the fine grinding
+has been well done.
+
+For the finest work changes of temperature, as in the case of glass,
+cause a good deal of trouble, and the operator must try to arrange his
+method of holding the object so as to give rise to the least possible
+communication of heat from the hand.
+
+The partial elasticity of paper, which is its defect as a polishing
+backing, is, I believe, partly counterbalanced by the difficulty of
+forming with pitch an exact counterpart tool without introducing a
+serious rise of temperature (i.e.  warming the pitch).  The rate of
+subsidence of the latter is very slow at temperatures where it is hard
+enough to work reliably as a polisher.
+
+A student interested in the matter of flat surfaces will do well to
+read an account of Lord Rayleigh's work on the subject, Nature, vol.
+xlviii, 1893, pp.  212, 526 (or B.  A.  Reports, 1893).  In the first
+of these communications Lord Rayleigh describes the method of using
+test plates, and shows how to obtain the interference fringes in the
+clearest manner.
+
+For the ordinary optician a dark room and a soda flame afford all
+requisite information;  and if a person succeeds in making three glass
+discs, say 6 inches in diameter, so flat that, when superposed in any
+manner, the interference fringes are parallel and equidistant, even to
+the roughest observation, he has nothing to learn from any book ever
+written on glass polishing.  Lord Rayleigh has also shown how to use
+the free clean surface of water as a natural test plate.
+
+Since the above was written the following details of his exact course
+of procedure have been sent to me by Mr. Brashear, and I hereby tender
+my thanks:-
+
+"It really takes years to know just what to do when you reach that
+point where another touch either gives you the most perfect results
+attainable, or ruins the work you have already done.  It has taken us
+a long time to find out how to make a flat surface, and when we were
+called upon to make the twenty-eight plane and parallel surfaces for
+the investigation of the value of the metre of the international
+standard, every one of which required an accuracy of one-twentieth of
+a wave length, we had a difficult task to perform. However, it was
+found that every surface had the desired accuracy, and some of them
+went far beyond it.
+
+It is an advantage in making flat surfaces to make more than one at a
+time; it is better to make at least three, and in fact we always grind
+and 'fine' three together.  In making speculum plates we get up ten or
+twelve at once on the lead lap.  These speculum plates we can test as
+we go on by means of our test plane until we get them nearly flat.  In
+polishing them we first make quite a hard polisher, forming it on a
+large test plane that is very nearly correct.  We then polish a while
+on one surface and test it, then on a second and test it, and after a
+while we accumulate plates that are slightly concave and slightly
+convex.  By working upon these alternately with the same polisher, we
+finally get our polisher into such shape that it approximates more and
+more to a flat surface, and with extreme care and slow procedure we
+finally attain the results desired.
+
+All our flats are polished on a machine which has but little virtue in
+itself, unmixed with brains.  Any machine giving a straight
+diametrical stroke will answer the purpose.  The glass should be
+mounted so as to be perfectly free to move in every direction--that
+is to say, perfectly unconstrained.  We mount all our flats on a piece
+of body Brussels carpet, so that every individual part of the woof
+acts as a yielding spring.  The flats are held in place by wooden
+clamps at the edges, which never touch, but allow the bits of glass or
+metal to move slowly around if they are circular; if they are
+rectangular we allow them to tumble about as they please within the
+frame holding them.
+
+For making speculum metal plates either plane or concave we use
+polishers so hard that they scratch the metal all over the surface
+with fine microscopic scratches.  We always work for figure, and when
+we get a hard polisher that is in proper shape, we can do ever so many
+surfaces with it if the environments of temperature are all right.  If
+we have fifty speculum flats to make, and we recently made three times
+that number, we get them all ready and of accurate surface with the
+hard polisher.  Then we prepare a very soft polisher, easily indented
+when cold with the thumb nail.  A drop of rouge and about three drops
+of water are put on the plate, and with the soft polisher about one
+minute suffices to clean up all the scratches and leave a beautiful
+black polish on the metal.  This final touch is given by hand; if we
+do not get the polish in a few minutes the surface is generally ruined
+for shape, and we have to resort to the hard polisher again.
+
+I assure you that nothing but patience and perseverance will master
+the difficulty that one has to encounter, but with these two elements
+'you are bound to get there.'"
+
+CHAPTER III
+
+MISCELLANEOUS PROCESSES
+
+Sec. 74.  Coating Glass with Aluminium and Soldering Aluminium.
+
+A process of coating glass with aluminium has been lately discovered,
+which, if I mistake not, may be of immense service in special cases
+where a strongly adherent deposit is required.  My attention was first
+attracted to the matter by an article in the Archives des Sciences
+physiques et naturelles de Geneve, 1894, by M. Margot.  It appears
+that clean aluminium used as a pencil will leave a mark on clean damp
+glass.  If, instead of a pencil, a small wheel of aluminium--say as
+big as a halfpenny and three times as thick--is rotated on the lathe,
+and a piece of glass pressed against it, the aluminium will form an
+adherent, though not very continuous coating on the glass.
+
+Working with a disc of the size described rotating about as fast as
+for brass-turning, I covered about two square inches of glass surface
+in about five minutes.  The deposit was of very uneven thickness, but
+was nearly all thick enough to be sensibly opaque.  By burnishing the
+brilliance is improved (I used an agate burnisher and oil), but a
+little of the aluminium is rubbed off.  The fact that the burnisher
+does not entirely remove it is a sign of the strength of the adherence
+which exists between the aluminium and the glass.  In making the
+experiment, care must be taken to have the glass quite clean--or at
+all events free from grease--in order to obtain the best results.
+
+M. Margot has contributed further information to the Archives des
+Sciences physiques et naturelles (February 1895).  He finds that
+adherence between aluminium and glass is promoted by dusting the glass
+with powders, such as rouge.  There is no doubt that a considerable
+improvement is effected in this way;  both rouge and alumina have in
+my hands greatly increased the facility with which the aluminium is
+deposited.  M. Margot finds that zinc and magnesium resemble aluminium
+in having properties of adherence to glass, and, what is more, carry
+this property into their alloys with tin.  Thus an alloy of zinc and
+tin in the proportions of about 92 per cent tin and 8 per cent zinc
+may be melted on absolutely clean glass, and will adhere strongly to
+it if well rubbed by an asbestos crayon.
+
+A happy inspiration was to try whether these alloys would, under
+similar circumstances, adhere to aluminium itself, and a trial showed
+that this was indeed the case, provided that both the aluminium and
+alloy are scrupulously clean and free from oxide.  In this way M.
+Margot has solved the problem of soldering aluminium. I have satisfied
+myself by trial of the perfect ease and absolute success of this
+method.  The alloy of zinc and tin in the proportions above mentioned
+is formed at the lowest possible temperature by melting the
+constituents together.  It is then poured so as to form thin sticks.
+
+The aluminium is carefully cleaned by rubbing with a cuttle bone, or
+fine sand, and strong warm potash.  It is then washed in water and
+dried with a clean cloth.  The aluminium is now held over a clean
+flame and heated till it will melt the solder which is rubbed against
+it.  The solder sticks at once, especially if rubbed with another bit
+of aluminium (an aluminium soldering bit) similarly coated.  To solder
+two bits of aluminium together it is only necessary to tin the bits by
+this process and then sweat them together.
+
+The same process applies perfectly to aluminium caused to adhere to
+glass by the previously mentioned process, and enables strong soldered
+contacts to be made to glass.  In one case, while I was testing the
+method, the adhesion was so strong that the solder on contracting
+while cooling actually chipped the surface clean off the glass.  In
+order to get over this I have endeavoured to soften the solder by
+mixing in a little of the fusible metal mercury amalgam;  and though
+this prevents the glass from being so much strained, it reduces the
+adherence of the solder.  It is a comfort to be able to solder
+aluminium after working for so many years by way of electroplating, or
+filing under solder.  An alternative method of soldering aluminium
+will be described when the electroplating of aluminium is discussed, Sec.
+138.
+
+Gilding Glass.  In looking over some volumes of the Journal fuer
+praktische Chemie, I came across a method of gilding glass due to
+Boettger (Journ.  f.  prakt.  Chem. 103, p.  414).  After many trials
+I believe I am in a position to give definite instructions as to the
+best way of carrying out this rather troublesome operation.  The films
+of gold obtained by the process are very thick, and the appearance of
+the gold exceedingly fine.  The difficulty lies in the exact
+apportionment of the reducing solution.  If too much of the reducing
+solution be added, the gold deposits in a fine mud, and no coating is
+obtained.  If, on the other hand, too little of the reducing solution
+be added, little or no gold is deposited.  The secret of success turns
+on exactly hitting the proper proportions.
+
+The reducing solution consists of a mixture of aldehyde and glucose,
+and the difficulty I have had in following Boettger's instructions
+arose from his specifying "commercial aldehyde" of a certain specific
+gravity which it was impossible to reproduce.  I did not wish to
+specify pure aldehyde, which is not very easily got or stored, and
+consequently I have had to determine a criterion as to when the
+proportion of reducing solution is properly adjusted.
+
+The aldehyde is best made as required.  I employed the ordinary
+process as described in Thorpe's Dictionary of Applied Chemistry, by
+distilling alcohol, water, sulphuric acid, and manganese dioxide
+together.  The crude product is mixed with a large quantity of calcium
+chloride (dry--not fused), and is rectified once.  The process is
+stopped when the specific gravity of the product reaches 0.832 at 60 deg.
+F.  The specific gravity of pure aldehyde is 0.79 nearly.
+
+The following is a modification of Boettger's formula:-
+
+Solution I
+
+1 gram of pure gold is converted into chloride--got acid free--i.e.
+to the state represented by AuCl3, and dissolved in 120 cc.  of water.
+
+This solution is the equivalent of one containing 6.5 grains of
+trichloride to the ounce of water.
+
+Solution II.
+
+  6     grams sodium hydrate.
+
+100     grams water.
+
+Solution III.
+
+ 0.2     grams glucose (bought as pure).
+
+12.6    cubic centimetres 95 per cent alcohol.
+
+12.6    cubic centimetres water.
+
+ 2.0     cubic centimetres aldehyde, sp. gr. 0.832.
+
+To gild glass these solutions are used in the following proportions by
+volume:-
+
+16     parts of     No.  I.
+
+ 4    parts of    No.  II.
+
+ 0.8    parts of    No.  III.
+
+The glass is first cleaned well with acid and washed with water:  it
+is then rinsed with Solution No.  III.  If it is desired to gild the
+inside of a glass vessel, Solution No.  III.  may be placed in the
+vessel first, and the walls of the vessel rinsed round carefully.
+Solutions I.  and II.  are mixed separately and then added
+to III.--after about two minutes the whole is well shaken up.
+
+If it be desired to gild a mirror of glass, the glass-plate is
+suspended face downwards in a dish of the mixed solutions--care being
+taken to rinse the glass with Solution III.  first.
+
+If the mixture darkens in from 7' to 10' in diffuse daylight and at
+60 deg.F.  it will gild well, and it generally pays to make a few trials
+in a test tube to arrive at this.  If too much reducing solution is
+present the liquid will get dark more rapidly, and vice versa.  The
+gilding will require several hours--as much as twelve hours may be
+needed.
+
+The reaction is one of great chemical interest, being one of that
+class of reactions which is greatly affected by capillarity.  Thus it
+occasionally happens that when the reducing solution is not in the
+right proportion, gold will be deposited at the surface of the liquid
+(so as to form a gilt ring on the inside of a test tube), the
+remainder of the gold going down as mud.  The gold deposited is at
+first transparent to transmitted light and is deeply blue.  I thought
+this might be due to a trace of copper or silver, but on carefully
+purifying the gold no change of colour was noted.  If the reducing
+solution is present in slightly greater proportions than that given in
+the formula, the gold comes down with a richer colour, and has a
+tendency to form a mat surface and to separate from the glass.  The
+gold which is deposited more slowly has a less rich colour but a
+brighter surface.  The operation should be interrupted when a
+sufficient deposit has been obtained, because it is found that the
+thicker the deposit, the more lightly is it held to the glass surface.
+
+Sec. 75.  The Use of the Diamond-cutting Wheel.
+
+A matter which is not very well known outside geological circles is
+the manipulation of the diamond-cutting wheel, and as this is often of
+great use in the physical laboratory, the following notes may not be
+out of place.  I first became acquainted with the art in connection
+with the necessity which arose for me to make galvanometer mirrors out
+of fused quartz, and it was then that I discovered with surprise how
+difficult it is to obtain information on the point.  I desire to
+express my indebtedness to my colleagues, Professor David and Mr.
+Smeeth, for the instruction they have given me.  In what follows I
+propose to describe their practice rather than my own, which has been
+of a makeshift description.  I will therefore select the process of
+cutting a slice of rock for microscopical investigation.
+
+Sec. 76.  Arming a Wheel.
+
+Fig. 63.
+
+A convenient wheel is made out of tin-plate, i.e.  mild steel sheet,
+about one-thirtieth of an inch thick and seven inches in diameter.
+This wheel must be quite flat and true, as well as round;  too much
+pains cannot be taken in securing these qualities.  After the wheel is
+mounted, it is better to turn it quite true by means of a
+watch-maker's "graver" or other suitable tool.  The general design of
+a rock-cutting machine will be clear from the illustration (Fig.  63).
+
+The wheel being set up correctly, the next step is to arm it with
+diamond dust.  For this purpose it is before all things necessary that
+real diamond dust should be obtained.  The best plan is to procure a
+bit of "bort" which has been used in a diamond drill, and whose
+properties have therefore been tested to some extent.  This is ground
+in a diamond mortar--or rather hammered in one--and passed through a
+sieve having at least 80 threads to the inch.  The dust may be
+conveniently kept in oil.
+
+To arm the wheel, a little dust and oil is taken on the finger, and
+laid on round the periphery of the wheel.  A bit of flint or agate is
+then held firmly against the edge of the wheel and the latter is
+rotated two or three times by hand.  The rotation must be quite
+slow--say one turn in half a minute--and the flint must be held firmly
+and steadily against the wheel.  Some operators prefer to hammer the
+diamond dust into the wheel with a lump of flint, or agate, but there
+is a risk of deforming the wheel in the process.  When a new wheel is
+set up, it may be necessary to repeat the above process once every
+half hour or so till the cutting is satisfactory, but when once a
+wheel is well armed it will work for a long time without further
+attention.
+
+Sec. 77.  Cutting a Section.
+
+A wheel 7 inches in diameter may be rotated about 500 times per
+minute, and will give good results at that speed.  The work, as will
+be seen from the diagram, is pressed against the edge of the wheel by
+a force, which in the case quoted was about the weight of eleven
+ounces.  This was distributed along a cutting arc of three-quarters of
+an inch.
+
+A convenient cutting lubricant is a solution of Castile soap in water,
+and this must be freely supplied;  if the wheel gets dry it is almost
+immediately spoiled owing to the diamond dust being scraped off.  In
+the figure the lubricant is supplied by a wick running into the
+reservoir.  I have used both clock oil and ordinary gas-engine oil as
+lubricants, with equally satisfactory results.  As to the speed of
+cutting, in the experiment quoted a bit of rather friable "gabbro,"
+measuring three-quarters of an inch on the face by five-eighths of an
+inch thick, was cut clean through in six minutes, or by 3000 turns of
+the wheel.  The travel of the edge was thus between 5000 and 6000
+feet, or say 9000 feet, nearly 2 miles, per inch cut.
+
+A good solid rock, like basalt, can be cut into slices of about 3/32
+inch thick.  A very loose rock is best boiled in Canada balsam, hard
+enough to set, before it is put against the wheel.
+
+Instead of a grinding machine a lathe may be employed.  The disc is,
+of course, mounted on the mandrel, and the work on the slide-rest.
+The latter must be disconnected from its feed screws, and a weight
+arranged over a pulley so as to keep the work pressed against the
+wheel by a constant force.
+
+It may, perhaps, occur to the reader to inquire whether any clearance
+in the cut is necessary.  The answer is that in all probability, and
+in spite of every care, the wheel will wobble enough to give
+clearance.  If it does not, a little diamond dust rubbed into the side
+of the wheel, as well as the edge, will do all that is required.  The
+edge also, after two or three armings, "burrs" a little, and thus
+provides a clearance naturally.  It is not unlikely that in the near
+future the electric furnace will furnish us with a number of products
+capable of replacing the diamond as abrading agents.  The cost of the
+small amount of diamond dust; required in a laboratory is so small,
+however, that it; is doubtful whether any appreciable economy will be,
+effected.
+
+Sec. 78.  Grinding Rock Sections, or Thin Slips of any Hard Material.
+
+A note on this is, perhaps, worth making, for the same reasons as were
+given for note, Sec. 75, which it naturally follows.  Just as
+trout-fishing; is described by Mr. Francis as the "art of fine and far
+off," [Footnote: In the Badminton Library, volume on Fishing.]
+section grinding may be called "the art of Canada balsam cooking," as
+follows.  A section of rock having been cut from the lump as just
+described, it becomes; necessary to grind it down for purposes of
+microscopical investigation.  For this purpose it is placed on a slip
+of glass, and cemented in position by Canada, balsam. Success in the
+operation of grinding the mounted section depends almost entirely on
+the way in which the mounting is done, and this in its turn depends on
+the condition to which the Canada has been brought.
+
+To illustrate the operations, I will describe a specific case, viz.
+that of grinding the section of "gabbro"' above described, for
+microscopical purposes.  One side of the section is probably
+sufficiently smooth and plane from the operation of the diamond wheel;
+if not, it must be ground by the finger on a slab of iron or gun-metal
+with emery and water, the emery passing a sieve of 80 threads to the
+inch.  The glass base on which the section is to be mounted for
+grinding is placed on a bit of iron or copper plate over a Bunsen
+burner, and three or four drops of natural Canada balsam are placed
+upon it.  The section is placed on the plate to heat at the same time.
+
+The temperature must not rise so high as to cause any visible change
+in the Canada balsam, except a slight formation of bubbles, which rise
+to the surface, and can be blown off.  The heating may require to be
+continued, say, up to twenty minutes.  The progress of the operation
+is tested by examining the balsam as to its viscous properties.
+
+An exceedingly simple and accurate way of testing is to dip a pair of
+ordinary forceps in the balsam, which may be stirred a little to
+secure uniformity.  The forceps are introduced with the jaws in
+contact, and, as soon as withdrawn, the jaws are allowed to spring
+apart, thus drawing out a balsam thread.  In a few moments the thread
+is cold, and if the forceps be compressed, this thread will bend.
+
+The Canada must be heated until it is just in such a state that on
+bringing the jaws together the thread breaks.  The forceps may open to
+about three-quarters of an inch.  If the Canada is more viscous, so
+that the thread does not break, the section when cemented by it will
+most probably slip on the slide.  On the other hand, if the balsam is
+more brittle, it will crumble away during the grinding.
+
+Assuming that the proper point has been reached, the section is
+mounted with the usual precautions to avoid air bubbles, i.e.  by
+dropping one edge on the balsam first.  When all is cold, the surface
+of the section may be ground on an iron plate with emery passing the
+80 sieve, till it is about 1/40 inch thick.  From this point it must
+be reduced on ground glass by flours of emery and water;  the rough
+particles of the former may be washed out for fine work.
+
+The process of grinding should not take more than half an hour if the
+section is properly cut, etc.  Beyond this point the allowable
+thickness must depend on the nature of the rock; a good general rule
+is to get the section just so thin that felspars show the yellow of
+the first order in a polarising, microscope.  The section is then
+finished with, say, two minutes emery or water of Ayr-stone dust.  It
+is better not to have the surface too smooth.
+
+To transfer the section, the hard Canada round the sides is scraped
+away, and the section itself covered with some fresh Canada from the
+bottle.  It is then warmed till it will slip off when a pin, or the
+invaluable dentist's chisel, is pressed against one side.  If the
+section be very delicate, the cover slip should be placed over it
+before it is moved to the proper slide.  The Canada used for mounting
+is not quite so hard as that employed in grinding, but it should be
+hard when cold, i.e. not sticky.
+
+The art of preparing Canada balsam appears to consist in heating it
+under such conditions as will ensure its being exposed in thin layers.
+I have wasted a good deal of time in trying to bake Canada in
+evaporating basins, with the invariable result that it was either over
+or under-baked, and got dark in colour during the process.
+
+On reviewing the process of rock section-cutting and mounting as just
+described, I cannot help thinking that, if properly systematised, it
+could be made much more rapid by the introduction of proper automatic
+grinding machinery.  It also seems not improbable that a proper
+overhaul of available gums and cements would be found to lead to a
+cementing material less troublesome than Canada balsam.
+
+Sec. 79.  Cutting Sections of Soft Substances.
+
+Though this art is fully treated of in books on practical biology, it
+is occasionally of use to the physicist, and the following note treats
+of that part of the subject which is not distinctly biological.
+
+Soft materials, of which thin sections may be required, generally
+require to be strengthened before they are cut.  For this purpose a
+variety of materials are available.  The one most generally used is
+hard paraffin.  The only point requiring attention is the embedding.
+The material must be dry.
+
+This is accomplished by soaking in absolute alcohol, i.e.  really
+absolute alcohol made by shaking up rectified spirit with potassium
+carbonate, previously dried, and then digesting for a day with large
+excess of quick-lime, making use of an inverted condenser and finally
+distilling off the alcohol without allowing it to come in contact with
+undried air.  After soaking for some time in absolute alcohol, the
+material may be transferred to oil of bergamot, or oil of cloves, or
+almost any essential oil.  After soaking in this long enough to allow
+the alcohol to diffuse out, the material may be lifted into a bath of
+melted paraffin (melting at, say, 51 deg. C.).  The process of soaking is
+in some cases made to go more rapidly by exhausting, and, if the
+material will stand it, by raising the temperature over 100 deg. C.  The
+soaking process may require minutes, hours, or days, according to the
+size and density of the material;  but a few hours are usually
+sufficient.
+
+When cold, the sections may be cut in any of the ordinary forms of
+microtome.
+
+Another way of embedding is to soak in collodion, and then precipitate
+the latter in the material and around it by plunging into nearly
+absolute alcohol.  The collodion yields a harder matrix than the
+paraffin.
+
+Whatever form of cutting machine is employed, the art of sharpening
+the knife is the only one requiring any particular notice.  The
+easiest way of obtaining a knife hard enough to sharpen, is to use a
+razor of good quality.  If it has to be ground, it is best to do this
+on a fine Turkey stone which is conveniently rested on two bits of
+rubber tubing, to avoid jarring the blade.  Many stones are slightly
+cracked, but on no account must the razor be dragged across a crack,
+or the edge will suffer.
+
+The necessary and sufficient condition is that the razor must be
+worked in little sweeps over the stone, and pressed against the latter
+by little more than its own weight, and the grinding must be regular.
+The edge may be inspected under a microscope, and it must be perfectly
+smooth and even before it will cut sections.  A finishing touch may be
+given on a leather strap, but it must be done skilfully, otherwise it
+is better omitted.
+
+The necessity for providing exceptionally keen and sharp edges arose
+in the manufacture of phonographs, where the knife used to turn up the
+wax cylinders must leave a perfectly smooth surface.  In 1889 this was
+being accomplished on an ivory lap fed with a trace of very fine
+diamond dust.
+
+I have had this method in mind as a possible solution of the
+difficulty of razor-grinding, but have not tried it.  I imagine one
+would use a soft steel or ivory slip rubbed over with fine diamond
+dust and oil by means of an agate.  The lap used in the phonograph
+works was rotated at a high speed.
+
+Sec. 80.  On the Production of Quartz Threads.
+
+[Footnote: Since this was written an article on the same subject by
+Mr. Boys appeared in the Electrician for 1896.  The instructions
+therein given are in accordance with what I had written, and I have
+made no alteration in the text.]
+
+In 1887 the important properties of fused quartz were discovered by
+Mr. Vernon Boys (Philosophical Magazine, June 1887, p.  489, "On the
+Production, Properties, and Some Suggested Uses of the Finest
+Threads").  A detailed study of the properties of quartz threads was
+made by Mr. Boys and communicated to the Society of Arts in 1889
+(Journal of the Society of Arts, 1889).  An independent study of the
+subject was made by the present writer in 1889 (Philosophical
+Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").
+
+There is also a paper in the Philosophical Magazine for 1894 (vol.
+xxxvii.  p.  463), by Mr. Boys, on "The Attachment of Quartz Fibres."
+This paper also appeared in the Journal of the Physical Society at
+about the same date, together with an interesting discussion of the
+matter.  In the American Journal, Electric Power, for 1894, there is a
+series of articles by Professor Nichols on "Galvanometers," in which a
+particular method of producing quartz threads is recommended.  The
+method was originally discovered by Mr. Boys, but he seems to have
+made no use of it.  A hunt through French and German literature on the
+subject has disclosed nothing of interest--nothing indeed which
+cannot be found in the papers mentioned.
+
+Sec. 81.  Quartz fibres have two great advantages over other forms of
+suspension when employed for any kind of torsion balance, from an
+ordinary more or less "astatic" galvanometer to the Cavendish
+apparatus.  In the first place the actual strength of the fibres under
+longitudinal stress is remarkably high, ranging from fifty to seventy
+tons weight per square inch of section, and even more than this in the
+case of very fine threads;  the second and more important point in
+favour of quartz depends on the wide limits within which cylindrical
+threads of this material obey the simplest possible law of torsion,
+i.e.  the law that for a given thread carrying a given weight at a
+given temperature and having one end clamped, the twist about the axis
+of figure produced by a turning moment applied at the free end is
+proportional simply to the moment of the twisting forces, and is
+independent of the previous history of the thread.
+
+It is to be noted, however, that the torsional resilience of quartz as
+tested by the above law is not so perfect but that our instrumental
+means allow us to detect its imperfections, and thus to satisfy
+ourselves that threads made of quartz are not things standing apart
+from all other materials, except in the sense that the limits within
+which they may be twisted without deviating in their behaviour from
+the law of strict proportionality by more than some unassigned small
+quantity, are phenomenally wide.
+
+A torsion balance--if we except the case of certain spiral
+springs--is almost always called upon for information as to the magnitude
+of very small forces, and for this purpose it is not essential merely
+that some law of twisting should be exactly obeyed, but also that the
+resistance to twisting of the suspension should be small.
+
+Now, regarded merely as a substance possessing elastic rigidity,
+quartz is markedly inferior to the majority of materials, for it is
+very stiff indeed;  its utility depends as much as anything upon its
+great strength, for this allows us to, use threads of exceeding
+fineness.  In addition to this it must be possible, and moreover
+readily possible, to obtain threads of uniform section over a
+sufficient length, or the rate of twist per unit length of the thread
+will vary in practice from point to point, so that the limits of
+allowable twist averaged over the whole thread may not be exceeded,
+and yet they may be greatly overpassed at particular points of the
+thread.
+
+It is interesting to note that in the case of quartz we not only have
+a means for readily producing very uniform cylindrical threads, but
+that the limits of allowable rate of twist are so wide that a small
+departure from uniformity of section produces much less inconvenience
+than in the case of any other known substance.
+
+Sec. 82.  There are three methods generally in use for drawing quartz
+fibres, all depending on the fact that quartz when fused is so viscous
+that it may be drawn into threads of great length, without these
+threads breaking up into drops, or indeed without their showing any
+sign of doing so.  The surface tension of the melted quartz must,
+however, be very considerable, as may be seen by examining the shape
+of a drop of the molten material, and this suffices to impress a
+rigidly cylindrical form upon the thread, the great viscosity
+apparently damping down all oscillation.
+
+The first method is the one originally employed by Mr. Boys.  A needle
+of quartz is melted somewhere in its length and is then drawn out
+rapidly by a light arrow, to which one end of the needle is attached,
+and which is projected from a kind of crossbow.
+
+A modification of this method, which the writer has found of service
+when very thick threads are required, is to replace the bow and arrow
+by a kind of catapult.
+
+The third method, which yields threads of almost unmanageable
+fineness, depends on the experimental fact that when a fine point of
+quartz is held in a high pressure oxygen gas blow-pipe flame, the
+friction of the flame gases suffices to overcome the tendency of the
+capillary forces to produce a spherical drop, and actually causes a
+fine thread to be projected outwards in the direction of the flame.
+
+Sec. 83.  A preliminary operation to any method is the production of a
+stick of fused quartz.  This is managed as follows.  A rock crystal or
+quartz pebble is selected and examined.  It must be perfectly white,
+transparent, and free from dirt.  Surface impurity can of course be
+got rid of by means of a grindstone.  The crystal is placed in a
+perfectly clean Stourbridge clay crucible, furnished with a cover, and
+heated to bright redness for about an hour in a clean fire or in a
+Fletcher's gas furnace.  The contents of the crucible are turned out
+when sufficiently cool on to a clean brick or bit of slate.  It will
+be found that the crystal is completely broken up and the fragments
+must be examined in case any of them have become contaminated by the
+crucible, but this will not have happened if the temperature did not
+rise beyond a bright red heat.
+
+The heap of fragments being found satisfactory, the next thing is to
+fuse some of the pieces together.  Unless the preliminary heating has
+been efficiently carried out this will prove an annoying task, because
+a rock crystal generally contains so much water that it splinters
+under the blow-pipe in a very persistent manner.  There are two ways
+of assembling the fragments.  One is to place two tiles or bricks on
+edge about the heap of quartz lying upon a third tile, so that the
+heap occupies the angular corner or nook formed by the tiles (Fig.
+64).
+
+The oxygas blow-pipe previously described is adjusted to give its
+hottest flame, the bags being weighted by at least two hundredweight,
+if of the size described (see Sec. 15).
+
+The tip of the inner cone of the blow-pipe is brought to bear directly
+upon one of the fragments, and if the operation is performed boldly it
+will be found that the surface of the fragment can be fused, and the
+fragment thus caused to hold together before the lower side gets hot
+enough to suffer any contamination from the tile or brick.  A second
+fragment may be treated in the same way, and then a third, and so on.
+
+Finally, the fragments may be fused together slightly at the corners,
+and a stick may thus be formed.  Of course a good deal of cracking and
+splitting of the fragments takes place in the process;  the best
+pieces to operate upon are those which are well cracked to begin with,
+and that in such a way that the little fragments are interlocked.
+
+An alternative method which has some advantages is to arm a pair of
+forceps with two stout platinum jaws, say an inch and a half long, and
+flattened a little at the ends.  The fragments are held in these
+platinum forceps and the blow-pipe applied as before.  This method
+works very well in adding to a rod which has already been partly
+formed, but the jaws require constant renewals.  The first fragment
+which is fused sufficiently to cohere may also be fused to a bit of
+tobacco pipe, or hard glass tube or rod, and the quartz stick
+gradually built up by fusing fresh pieces on to the one already in
+position.
+
+Fig. 64.
+
+Since the glass or pipeclay will contaminate the quartz which has been
+fused on to it, it is necessary to discard the end pieces at the close
+of the operation.  A string of fragments having been collected and
+stuck together, the next step is to fuse them down into a uniform rod.
+This is easily done by holding the string in the blow-pipe flame and
+allowing it to fuse down.  Twisting the fused part has a good effect
+in assisting the operation.  It is desirable to use a large jet and as
+powerful a flame as can be obtained during this part of the operation.
+
+The final result should be a rod, say two or three inches long and
+one-eighth of an inch thick, which will in most cases contain a large
+number of air bubbles.  Since the presence of drawn-out bubbles cannot
+be advantageous, it is often desirable to get rid of them, and this
+can conveniently be done at the present  stage.  The process at best
+is rather tedious;  it  consists in drawing the quartz down very fine
+before an intense flame, in order to allow the bubbles to get close
+enough to the surface to burst.  A considerable loss of material
+invariably occurs during the process;  for whenever the thin rod
+separates into two bits the process of flame-drawing of threads goes
+on, and entails a certain waste; moreover, the quartz in fine
+filaments is probably partially volatilised.
+
+Sooner or later, however, a sufficient length of bubble-free quartz
+can be obtained.  It must not be supposed that it is always necessary
+to eliminate bubbles as perfectly as is contemplated in the foregoing
+description of the treatment, but for special purposes it may be
+essential to do so, and in any case the reader's attention is directed
+to a possible source of error.
+
+It may be mentioned in connection with this matter that crystals of
+quartz may look perfectly white and clear, and yet contain impurity.
+For instance, traces of sodium are generally present, and lithium was
+found in large spectroscopic quantity in five out of six samples of
+the purest crystals in my laboratory.  The presence of lithium in rock
+crystal has also been detected by Tegetmeier (Vied.  Ann, xli.  p.
+19, 1890).
+
+After some practice in preparing rods and freeing them of bubbles the
+operator will notice a distinct difference in the fusibility of the
+samples of quartz he investigates, though all may appear equally pure
+to the unaided eye.  It should be mentioned, however, that high
+infusibility cannot always be taken as a test of purity, for the most
+infusible, or rather most viscous, sample examined by the writer
+contained more lithium than some less viscous samples.
+
+Fig.  65.
+
+During the process of freeing the quartz from bubbles the lithium and
+sodium will be found to burn away, or at all events to disappear.
+
+A rod of quartz, say three inches long, one-sixteenth of an inch in
+diameter, and free from bubbles for half an inch of its length, even
+when examined by a strong lens, is suitable for drawing into threads.
+The rod is manipulated exactly in the manner described under
+glass-blowing, and is finally drawn down at the bubble free part into
+a needle, say 0.02 inch in diameter (No.  25 on the Birmingham wire
+gauge), and 2 inches long.
+
+Fig.  66.
+
+There is one peculiarity about fused quartz which renders its
+manipulation easier than that of glass--it is impossible to break
+fused quartz, however suddenly it be thrust into the blow-pipe flame.
+A rod having a diameter of three-sixteenths of an inch--and perhaps
+much more--may be brought right up to the tip of the inner cone of
+the oxy-gas flame and held there-till one side fuses, the other being
+comparatively cool, without the slightest fear of precipitating a
+smash.  In seven years' experience I have never seen a bit of once
+fused quartz broken by sudden heating;  whether it might be done if
+sufficient precautions were taken I do not know.
+
+The reason of the fortunate peculiarity of quartz in this respect is,
+I presume, to be found in the fact that quartz once it has been fused
+is really a very strong material indeed, and is also probably the
+least expansible substance known.  From some experiments of the writer
+upon the subject, it may be concluded that at the most quartz which
+has been fused expands only about one-fifth as fast as flint-glass, at
+all events between 20 deg. and 70 deg. C.
+
+Sec. 84.  Drawing Quartz Threads.
+
+The thick end of the rod of quartz is held in the fingers or
+occasionally in a clip.  The end of the fine point is attached to a
+straw arrow by means of a little sealing-wax.  The arrow is laid on
+the stock of a crossbow in the proper position for firing.    See
+Figs.  67 and 68, which practically explain themselves.
+
+The needle is heated by the blow-pipe till a minute length is in a
+state of uniform fusion;  the arrow is then let fly, when it draws a
+thread out with it.  The arrow is preferably allowed to strike a
+wooden target placed, say, 30 feet away from the bow, and a width of
+black glazed calico is laid under the line of fire to catch the thread
+or arrow if it falls short.  The general arrangements will be obvious
+from the figure.
+
+The bow is of pine in the case where very long thin threads are
+required, though for ordinary purposes I have found a bow of
+lance-wood succeed quite as well.  The trigger of the bow consists of
+a simple pin passing through the stock and fastened at its lower end
+to a string connected with a board which can be depressed by foot.  In
+the figure an ordinary trigger is shown, but the pin does just as
+well.
+
+Fig 67.
+
+The arrow is made out of about 6 inches of straw, plugged up aft by a
+small plug of pine or willow fastened in with sealing-wax, and
+projecting backwards one-eighth of an inch.  This projection serves a
+double purpose:  it gives a point of attachment for the quartz needle,
+and on firing the bow it forms a resisting anvil on which the string
+of the bow impinges.  The head of the arrow is formed by a large
+needle stuck in with sealing-wax, and heavy enough to bring the centre
+of gravity of the arrow forward of one-third of its length, the
+condition of stability in flight.
+
+Fig.  68.
+
+It is not necessary to employ any feathering for these arrows;  though
+I have occasionally used feathers or mica to "wing the shaft" no
+advantage has resulted therefrom.
+
+To get fine threads a high velocity is essential.  This is obtained by
+considering (and acting upon) the principles involved.  The bow may be
+regarded as a doubly-tapering rod clamped at the middle.  After
+deflection it returns towards its equilibrium position at a rate
+depending in general terms on the elastic forces brought into play,
+directly, and on the effective moment of inertia of the rod, inversely
+(see Rayleigh, Sound, vol.  ii.  chap.  viii.)  If the mass of the
+arrow is negligible compared with the bow, the rate at which the arrow
+moves is practically determined by that attained by the end of the
+bow, which is a maximum in crossing its equilibrium position.
+
+The extent to which the arrow profits by this velocity depends on the
+way the bow is strung.  It will be greatest when the string is
+perpendicular to the bow when passing its equilibrium position;  or in
+other words, when the string is infinitely long.  Since the string has
+mass, however, it is not permissible to make it too long, or its
+weight begins to make itself felt, and a point is soon reached at
+which the geometrical gain in string velocity is compensated for by
+the total loss of velocity due to the inertia of the string.  In
+practice it is sufficient to use a string 10 per cent longer than the
+bow.
+
+It is well to use a light fiddle string, served with waxed silk at the
+trigger catch;  if this be omitted the gut gets worn through very
+quickly.  In order to decide how far it is permissible to bend the
+bow, the quickest way is to make a rough experiment on a bit of the
+same plank from which the bow is to be cut, and then to allow a small
+factor of safety.  In the figure the bow is of lance-wood and is more
+bent than would be suitable for pine.
+
+The bow itself is tapered from the middle outwards just like any other
+bow.  If thick threads are required, the above considerations are
+modified by the fact that quartz opposes a considerable resistance to
+drawing, and that consequently the arrow must not only have a high
+velocity, but a fair supply of energy as well;  in other words, it
+must be heavy.  A thin pine arrow instead of a straw generally does
+very well, but in this case the advantage of using pine for the bow
+vanishes;  and in fact lance-wood does better, owing to the greater
+displacement which it will stand without breaking.  This of course
+only means that a greater store of energy can be accumulated at one
+bending.
+
+I had occasion to investigate whether the unavoidable spin of an arrow
+about its axis produces any effect on the thread, and for this purpose
+made arrows with inertia bars thrust through the head, i.e.  an arrow
+with a bit of wire run through it, perpendicular to its
+length--forming a cross in fact--the arms of the cross being weighted
+at the extreme ends by shot.  This form of arrow has a considerable
+moment of inertia about its longer axis, and consequently rotates less
+than a mere straw, provided that the couples tending to produce rotation
+are not increased by the cross arm, or the velocity too much reduced.
+Shooting one of these arrows slowly, I could see that it did not
+rotate, and when fired at a high velocity, it generally arrived at the
+target (placed at varying distances front bow) with the arms nearly
+horizontal, thus showing that it probably did not rotate much.
+
+I did not succeed in this at the first trial, by any means.  The
+threads got in this way were no better than those made with a single
+straw, whence we may conclude very provisionally that the spin of the
+arrow has only a small effect, if  any, on the quality of the threads.
+
+Feathering the arrow, in my experience, tends, if anything to make it
+spin more;  for one thing, because it is practically impossible to lay
+the feathering on straight.
+
+After the arrow is shot, it remains to gather in the thread, and if
+the latter is at all thin, we have a rather troublesome job.  In a
+thread thirty or forty feet long, the most uniform part generally lies
+in the middle if the thread is thin, i.e.  of the order of a
+ten-thousandth of an inch in diameter.  If the thread is thick the
+most uniform part may be anywhere.  The part of the thread required is
+generally best isolated by passing a slip of paper under it at each
+end and cementing the thread to the paper by means of a little
+paraffin or soft wax, and then cutting off the outer portions.  One
+bit of paper may then be lifted off the calico, and the thread will
+carry the other bit.  In this way the thread may be taken to a
+blackened board, where it may be mounted for stock.
+
+By passing the two ends of the thread under a microscope, or rather by
+breaking bits off the two ends and examining them together, it is easy
+to form an Opinion as to uniformity.
+
+Mr. Boys has employed an optical method of examining threads, but the
+writer has invariably found a  high-power microscope more convenient
+and capable of giving more exact information as to the diameter of the
+threads.
+
+The beginner--or indeed the practised hand--need not expect to get a
+thread of the exact dimensions required at the first shot.  A little
+experience is necessary to enable one to judge of the right thickness
+of the needle for a thread of given diameter.  The threads are so
+easily shot, however, that a few trials take up very little time and
+generally afford quite sufficient experience to enable a thread of any
+required diameter to be prepared.
+
+It is no use attempting to heat an appreciable length of needle; if
+this be done the thread almost invariably has a thick part about the
+middle of its length..  It is sufficient to fuse at most about
+one-twentieth of an inch along the needle before firing off the bow.
+This can be done by means of the smaller oxygas blow-pipe jet
+described in the article on blow-pipes for glass-blowing, Sec. 14.  The
+flame must of course be turned down so as to be of a suitable size.  A
+sufficiently small flame may be got from almost any jet.
+
+If the needle be not equally heated all round, the thread tends to be
+curly;  indeed by means of the catapult, threads may be pulled which,
+when broken, tend to coil up like the balance-springs of watches, if
+only care be taken to have one side of the needle much hotter than the
+other.
+
+Sec. 85.  When examining bits of threads, say thicker than the
+two-thousandth of an inch, under the microscope it is convenient to
+use a film of glycerine stained with some kind of dye, in order to
+render the thread more sharply visible.  The thread is mounted beneath
+a cover slip, and a drop of the stained glycerine allowed to run in.
+Such a treatment gives the image of the thread a sharply defined edge
+3 and the contrast between the whiteness of the thread and the colour
+of the background allows measurements to be made with great ease.
+
+On the whole the easiest way of measuring the diameter of a thick
+thread is to use a measuring microscope, i.e.  one in which the lens
+system can be displaced along a plane bed by means of a finely cut
+micrometer screw.  The instruments made by the Cambridge Scientific
+Instrument Company do fairly well.  Direct measurements up to 0.0001
+inch are easily made by means of a microscope provided with a Zeiss
+"A" objective, and rather smaller differences of thickness can be made
+out by it.  For thin threads the method next to be described is more
+fitting, because higher powers can be more conveniently used.
+
+In this method an ordinary microscope is employed together with a
+scale micrometer, and either an eyepiece micrometer, or a camera and
+subsidiary scale.  The eyepiece micrometer is the more convenient.  If
+a camera be employed, i.e.  such an one as is supplied by Zeiss, it is
+astonishing how the accuracy of observation may be increased by
+attending carefully to the illumination of both the subsidiary scale
+and of the thread.  The two images should be as far as possible of
+equal brightness, and for this purpose it will be found requisite to
+employ small screens.
+
+The detail of making a measurement by means of the micrometer eyepiece
+is very simple.  The thread is arranged on the stage so as to point
+towards the observer, and the apparent diameter is read off on the
+eyepiece scale.  In order to calibrate the latter it is only necessary
+to replace the thread by the stage micrometer, and to observe the
+number of stage micrometer divisions occupying the space in the
+eyepiece micrometer formerly occupied by the thread.  It is essential
+that both thread and stage micrometer should occupy the same position
+in the field, for errors due to unequal distortion may otherwise
+become of importance.  For this reason it is best to utilise the
+centre of the field only.
+
+The same remark applies to measurements by means of the camera, where
+the image of the thread is projected against the reflected image of
+the subsidiary scale laid alongside the microscope.  In this case the
+value of the subsidiary scale divisions must be obtained from the
+divisions of the stage micrometer, coinciding as nearly as possible
+with the position occupied by the thread.  Before commencing a
+measurement the screens are moved about till both images appear
+equally bright.
+
+Threads up to about one twenty-thousandth of an inch in diameter may
+be sufficiently well measured by means of a Zeiss "4 centimetre
+apochromatic object-glass" and an eyepiece "No. 6" with sixteen
+centimetre tube length. [Footnote: The objective certainly had "4 cm."
+marked on it, but the focal length appeared to be about I.5 mm. only.]
+
+Sec. 86.  Drawing Threads by the Catapult.
+
+The bow-and-arrow method fails when threads of a greater diameter than
+about 0.0015 inch are required--at least if any reasonable uniformity
+be demanded, and no radical change in the bow and arrow be carried
+out.
+
+Thus in the writer's laboratory a thread of about this diameter,
+within 1/10000  of an inch-13 inches long and free from air
+bubbles--was required.  A fortnight's work by a most skilful operator
+only resulted in the production of two lengths satisfying the
+conditions.
+
+The greatest loss of time occurs in the examination of the thread by
+means of the microscope.
+
+Threads for galvanometer suspensions are conveniently from 0.0001 to
+0.0004 inch in diameter, and are much more easily made and got uniform
+than thicker threads, to the production of which the catapult method
+applies.
+
+A reference to the diagram will make the construction of the
+instrument quite clear.  The moving end of the quartz is attached to a
+small boxwood slider working on a tubular girder or between wires.
+The quartz is secured in position by clamps shown at A and B, and
+motion is imparted to the slider by a stretched piece of catapult
+elastic (C).  An easy means of regulating the pull of the elastic is
+to hold it back by a loop of string whose length can be varied by
+twisting it round a pin.
+
+Fig.  69.  [Footnote: For greater clearness of drawing, the tube
+carrying the slider is shown somewhat higher above the base than is
+convenient in practice;  and the slide itself is shown too thin in the
+direction of the hole through it.]
+
+Since it is not permissible to allow the slider to rebound at the end
+of its journey, some such arrangement of breaks as is shown must be
+adopted.  In the diagram the bottom of the slider runs on to a brass
+spring between the girder and the base of the appliance, and so gets
+jammed; the spiral spring acts merely as an additional guard.  The
+diagram does not show the lower spring very clearly;  it is a mere
+strip lying in the groove.
+
+A rod of quartz, with a needle at one end, is prepared as before and
+secured in the clamps.  During the operation of fastening down the
+clamps, there is some danger of breaking the needle, and consequently
+it is advisable to soften the latter before and while adjusting the
+second clamp.
+
+The process of drawing a thread by this method is exactly similar to
+the operation already described in connection with the arrow method.
+Though short thick threads form the product generally obtained from
+the catapult, it must not be supposed that thin threads cannot be
+obtained in this way.  If a short length of a very fine needle be
+heated, it will be found to yield threads quite fine enough for
+ordinary suspension purposes, but naturally not so uniform as those
+obtained from the 40-foot lengths obtainable by the bow-and-arrow
+method.
+
+It is easy to make spiral quartz springs resembling watch
+balance-springs by means of the catapult.  All that is necessary is to
+see that the quartz is rather unequally heated before the shot is
+fired.  In the future it is by no means impossible that such springs
+may have a real value, for the rigidity of quartz is known to increase
+as temperature rises.  Hence it is probable that the springs would
+become stiffer as temperature rises, even though they work chiefly by
+bending, and little or not at all by twisting.  As this is the kind of
+temperature variation required to compensate an uncompensated watch
+balance wheel, it may turn out to have some value.
+
+Sec. 87.  Drawing Threads by the Flame alone.
+
+A stick of quartz is drawn down to a fine point, and the tip of this
+point is held in the blow-pipe flame in the position shown in Fig.
+70.
+
+Fig.  70.
+
+The friction of the flame gases is found to be sufficient to carry
+forward the fused quartz and to draw it into threads in spite of the
+influence of the capillary forces.  If a sheet of paper be suspended
+at a distance of two or three feet in front of the blow-pipe flame, it
+will be found to be covered with fine threads tangled together into a
+cobwebby mass.  As this method is an exceedingly simple one of
+obtaining threads, I have endeavoured to reduce it to a systematic
+operation.
+
+A sheet of cardboard, about two feet square, is painted dead black and
+suspended horizontally, painted side downwards (Fig.  70, A), at a
+height of about two feet above the blow-pipe flame.  The latter is
+adjusted so as to point almost vertically upwards and towards the
+centre of the cardboard.  A few half-inch pins are thrust through the
+card from the upper surface and pushed home; about one dozen pins
+scattered over the surface will be sufficient.  Their object is to
+prevent the threads being carried away round the edge of the screen.
+
+The flame from the jet described so often is fed from gas bags
+weighted to about eighty pounds per square foot of  (one) surface,
+i.e.  "4-foot" bags require from three to four hundredweight to give
+an advantageous pressure. [Footnote: The resulting threads were really
+too fine for convenient manipulation, so that unless extremely fine
+threads are required it will be better to reduce the pressure of the
+gases considerably.]
+
+Two sticks of quartz are introduced and caused to meet just in front
+of the inner cone--the hottest part of the flame.  They are then
+drawn apart so as to form a fine neck, which softens and is bent in
+the direction of motion of the flame gases.  When fusion is complete
+the neck separates into two parts, and a thread is drawn from each of
+them. By alternately lightly touching the rods together, and drawing
+them apart, quite a mass of threads may be obtained in two or three
+minutes, when the process should be stopped.  If too many threads get
+entangled in the pins, one gives one's self the unnecessary trouble of
+separating them. On taking down the card it will be found that the
+threads have been caught by the pins;  but the card now being laid
+black side upwards, the former easily slip off the points.
+
+Threads at least a foot long, and perhaps vastly longer, may be
+obtained by this method, and are extraordinarily fine.  When I first
+read Professor Nichols' statement (Electric Power, 1894) as to the
+value of these fibres for galvanometer purposes, I was rather
+sceptical on the ground that the threads would tend to get annealed by
+being drawn gradually, instead of suddenly, from a place of intense
+heat to regions of lower temperature.
+
+Now annealing threads by a Bunsen makes them rotten.  The threads
+being immersed in the hot flame gases could only cool at the same rate
+as the gas, and it was not--and is not--clear to me that annealing
+of the threads can be avoided.  On the other hand, it may be possible
+that a thread cooled slowly from the first does not suffer in the same
+way as a cold thread would do when annealed in a Bunsen flame.
+
+Again the velocity of the gases is beyond doubt exceedingly high, so
+that the annealing, even supposing it to be deleterious, might not be
+carried very far.  Threads drawn by this method and measured "dry,"
+i.e.  by mounting them on a slide without the addition of any liquid,
+turned out to have a diameter of about 1/20000 of an inch.
+
+I do not think I could manage to mount such fine threads without very
+special trouble.  All the threads lying on the board, however, were
+found in reality to consist of three or four separate threads, and
+there is no reason why several threads should not be mounted in
+parallel, provided, of course, that they are equally stretched and
+touching each other.  Equality of tension in the mounting could be
+secured by making one attachment good, then cementing the other
+attachment to the other end of the threads, and "drawing" the two
+attachments slightly apart at the moment the cement commences to set.
+This method may turn out to be very valuable, for, so far as I can
+see, the carrying power would be increased without an increase of
+torsional stiffness of anything like so high an order as would be the
+case were one thread only employed.  On the other hand, the law of
+torsion could hardly be quite so simple, at all events, to the second
+order of approximations.
+
+Sec. 88.  Properties of Threads.
+
+A large number of experiments on the numerical values of the elastic
+constants of quartz threads have been made by Mr. Boys and his
+students, and by the writer.  As the methods employed were quite
+distinct and the results wholly independent, and yet in good agreement
+with each other, a rounded average may be accepted with considerable
+confidence.
+
+TENACITY OF QUARTZ FIBRES (BOYS).
+
+Diameter of Thread.
+
+Tenacity in Tons' Weight per Square Inch of Section.
+
+Tenacity in Dynes per Square Centimetre.
+
+Inches
+
+Centimetres
+
+0.00069
+
+0.00175
+
+51.7
+
+8 X 109
+
+0.00019
+
+0.00048
+
+74.5
+
+11.5 X 109
+
+Rounded mean of Boys' and Threlfall's results:
+
+Young's Modulus at 20 deg. C,
+
+5.6   X 1011 C.G.S.
+
+Modulus of Simple Rigidity at 20 deg. C,
+
+2.65 X 1011 C.G.S.
+
+Modulus of Incompressibility,
+
+1.4   X 1011 C.G.S.
+
+Modulus of Torsion,
+
+3.7   X 1011 C.G.S.
+
+Approximate coefficient of linear expansion of quartz per degree
+between 80 deg. C.  and 30 deg. C.  is 0.0000017 (Threlfall = loc. cit.).
+
+This must be regarded with some suspicion, as the data were not
+concordant.  There is no doubt, however, about the extreme
+inexpansibility of quartz.
+
+Temperature coefficient of modulus of torsional rigidity per degree
+centigrade, 22 deg. to 98 deg. C, 0.000133
+
+Ditto, absolute simple rigidity, 0.000128 (Threlfall).
+
+Limit of allowable rate of twist in round numbers is, one-third turn
+per centimetre, in a fibre 0.01 cm. diameter.
+
+The limiting rate is probably roughly inversely as the diameter.
+
+Attention must be called to the rapid increase in the torsional
+rigidity of these threads as the temperature rises.  A quartz spiral
+spring-balance will be appreciably stronger in hot weather.
+
+Sec. 89.  In the majority of instances in which quartz threads are
+applied in the laboratory, it is desirable to keep the coefficient of
+torsion as small as possible, and hence threads are used as fine as
+possible.
+
+It is convenient to remember that a thread 0.0014 cm. or 0.0007 inch
+in diameter breaks with a weight of about ten grammes, and may
+conveniently be employed to carry, say, five grammes.  With threads
+three times finer the breaking strength per unit area increases, say,
+50 per cent.  In ordinary practice--galvanometric work for
+instance--where it is desirable to use a thread as fine and short as
+possible to sustain a weight up to, say, half a gramme, it will be
+found that fibres five centimetres long or over give no trouble
+through defect of elastic properties.  A factor of safety of two is
+a fair allowance when loading threads.
+
+No difficulty will be experienced in mounting threads having a
+diameter of 0.0002 inch or over.  With finer threads it is necessary
+to employ very dark backgrounds (Mr. Boys uses the darkness of a
+slightly opened drawer), or the threads cannot be sufficiently well
+seen.
+
+In the case of instruments in which threads remain highly twisted for
+long periods of time, the above rule as to the safe limit of twist
+does not allow of a sufficient margin;  it is only applicable to
+galvanometric and similar purposes.
+
+The cause of the increase in tenacity as the diameter diminishes is at
+present unknown.  It is due neither to an effect of annealing
+(annealed threads are rotten), nor is it a skin effect, nor is it due
+to the cooling of the thread under higher capillary pressure.  It is,
+however, possible that it may be associated with some kind of
+permanent set taken by the fibres during the stage of passage from the
+liquid to the solid state.
+
+Sec. 90.  On the Attachment of Quartz Fibres.
+
+For many purposes it is sufficient to cement the fibres in position by
+means of ordinary yellow shellac, but where very great accuracy is
+aimed at, the shellac (being itself imperfectly elastic and exposed to
+shearing stress) imposes its imperfections on the whole system. This
+source of error can be got over by soldering the threads in position.
+Attempts were made by the writer in this direction, with fair success,
+in 1889, but as Mr. Boys has carried the art to a high degree of
+perfection, I will suppress the description of my own method and
+describe his in preference.  It has, of course, been frequently
+repeated in my laboratory.
+
+In many cases, however, if not in all, it may be replaced by Margot
+soldering, as already described, a note on the application of which to
+this purpose will follow.
+
+A thread of the proper diameter having been selected, it is cut to the
+right length.  With fine threads this is not always a perfectly easy
+matter.  The best way is for the operator to station himself facing a
+good light, not sunlight, which is too tiring to the eye, but bright
+diffused light.  The thread will be furnished with bits of paper stuck
+on with paraffin at both ends, as already described.
+
+A rough sketch of the apparatus--or, at all events, two lines showing
+the exact length which the free part of the thread must have--are
+marked on a smooth board, and this is supported with its plane
+vertical.  The thread is held against the board, and the upper piece
+of paper is stuck lightly to the board with a trace of soft wax, so
+that the lower edge of the paper is at any desired height above the
+upper mark.  This distance is measured, and forms the length of thread
+allowed to overlap the support.  A second bit of paper is attached
+below the lower mark, a margin for the attachment of the lower end
+being measured and left as before.  The thread will be most easily
+seen if the board is painted a dead black.
+
+If it is desired to attach the thread to its supports merely by
+shellac, this is practically all that needs to be done.  The supports
+should resemble large pins.  The upper support will be a brass wire in
+most cases, and will require to be filed away as shown in the sketch
+(Fig.  71).  It is then coated with shellac by heating and rubbing
+upon the shellac.  As previously noted, the shellac must not be
+overheated.
+
+The thread is cut off below the lower slip of paper, and the upper
+support being conveniently laid in a horizontal position on another
+dead-black surface, the thread is carried to it and laid as designed
+against the shellac, which is now cold.  When the thread is in place,
+a soldering iron is put against the brass wire, and the shellac
+gradually melted till it closes over the thread.
+
+Fig.  71.
+
+The iron is then withdrawn and the thread pulled away from the point
+for one-twentieth of an inch or less.  This ensures that the thread
+makes proper contact with the cement, and also that it is free from
+kinks; of course, it must leave the cement in the proper direction.  A
+similar process is next carried out with respect to the lower
+attachment, and the ends of the thread are neatly trimmed off.
+
+Both ends of the thread being secured, the next step is to transfer
+the upper support to a clip stand, the suspended parts being held by
+hand, so that the weight comes on the thread very gradually.  In this
+way it will be easily seen whether the thread is bent where it enters
+the shellac, and should this be the case, a hot iron must be brought
+up to the shellac and the error rectified.
+
+When both the support and the suspended parts are brought nearly to
+the required bearing, the hot iron is held for a moment close up to
+each attachment, the hand being held close below but not touching the
+suspended parts, and both attachments are allowed to straighten
+themselves out naturally.
+
+These details may appear tiresome, and so they are when written out at
+length, but the time occupied in carrying them out is very short, and
+quartz threads break easily, unless the pull upon them is accurately
+in the direction of their length at all points.
+
+In the event of its being decided to attach the thread by soldering,
+the process is rather more expensive in time, but not otherwise more
+troublesome.
+
+Fig.  72.                                     Fig.  73.
+
+The thread being cut as before to the proper length, little bits of
+aluminium foil are smeared all over with melted shellac and suspended
+from the thread replacing the paper slips before described.  It is
+important that no paraffin should be allowed to touch the thread
+anywhere near a point intended to be soldered.  The thread is hung up
+from a clip stand by one of the bits of foil, and the lower end is
+washed by dipping it into strong nitric acid for a moment and thence
+into water.  The object of smearing the foils all over with shellac is
+to prevent them being acted upon by the acid.  The threads are not
+very easily washed acid free, but the process may be assisted by means
+of a fine camel's-hair pencil.
+
+Some silvering solution made as described (Sec. 65) is put into a test
+tube;  the thread, after rinsing with distilled water, is lowered into
+the solution so far as is required, and is allowed to receive a
+coating of silver.  It has been observed that the coating of silver
+must not be too thick--not sufficiently thick to be opaque.  A watch
+may be kept on the process by immersing a minute strip of mica
+alongside the thread.
+
+The silvered thread is rinsed with distilled water and allowed to dry.
+
+Meanwhile the other end of the thread may be silvered.  When both ends
+are silvered the process of coppering by electro deposit is commenced.
+A test tube is partially filled with a ten per cent solution of
+sulphate of copper, and several copper wires are dipped into it to
+form an anode.  The thread is lowered carefully into the solution so
+as not to introduce air bubbles, and the silvered part is allowed to
+project far enough above the surface of the solution to come in
+contact with a fine copper wire.  The circuit is closed through a
+Leclanche cell and a resistance box.
+
+It is as well to begin with a fair resistance, say 100 ohms out in the
+box, and the progress of the deposit is watched by means of a
+low-power microscope set up in front of the thread.  If the copper
+appears to come down in a granular form, the resistance is too small
+and must be increased;  if no headway appears to be made, the
+resistance must be diminished.
+
+As soon as a fair coat of copper has come down, i.e.  when the
+diameter of the thread is about doubled, the process is interrupted.
+The thread is withdrawn, washed, dipped in a solution of chloride of
+zinc, and carefully tinned by dragging it over a small clean drop of
+solder on a soldering bit.
+
+During this part of the process the shellac is apt to get melted if
+the iron is held too close, so that it is advisable to begin by making
+the thread somewhat over long.  The end of the thread must only be
+trimmed off at the conclusion of the operation, i.e.  after the thread
+is soldered up.  The thread is attached to the previously tinned
+supports much in the same way as has been described under the head of
+shellac attachments.  It does not very much matter whether both ends
+are coppered before one is soldered up or not.  At the conclusion of
+the whole process the superfluous copper and silver are dissolved off
+by a little hot strong nitric acid applied on a glass hair pencil.
+This is best done by holding the thread horizontally with the
+assistance of clip stands.
+
+If the thread is too delicate to bear brushing, the nitric acid may be
+applied by pouring out a big drop into a bit of platinum foil and
+holding this below the thread so as to touch it lightly.  The
+dissolving of the copper and silver is, of course, followed by copious
+washing with hot water.  This process is more laborious than might be
+imagined, but it may be shortened by heating the platinum foil
+supporting the water (Fig.  74).
+
+Fig.  74
+
+The washing part of the process is, in the opinion of the writer, the
+most difficult part of the whole business, and it requires to be very
+thorough, or the thread will end by drawing out of the solder.  In
+many cases it is better to try to do without any application of nitric
+acid at all, but, of course, this involves silvering and coppering to
+exact distances from the ends of the thread--at all events, in
+apparatus where the effective length of the thread is narrowly
+prescribed.
+
+It is important not to leave the active parts of the thread
+appreciably silvered, for the sake of avoiding zero changes due to the
+imperfect elasticity of the silver.  In this soldering process
+ordinary tinman's solder may be employed;  it must be applied very
+free from dust or oxide.
+
+Sec. 91.  Other Modes of soldering Quartz.
+
+Thick rods of quartz may be treated for attachment by solder in the
+same way as glass was treated by Professor Kundt to get a foundation
+for his electrolytically deposited prisms. [Footnote: See Appendix at
+end of book.]
+
+The application of a drop of a strong solution of platinum
+tetrachloride to the rod will, on drying, give rise to a film of the
+dry salt, and this may be reduced in the luminous gas flame.  During
+the process, however, the quartz is apt to get rotten, especially if
+the temperature has been anything approaching a full red heat.  The
+resulting platinum deposit adheres very strongly to the quartz, and
+may be soldered to as before.  This method has been employed by the
+writer with success since 1887, and may even be extended to thick
+threads.
+
+It was also found that fusible metal either stuck to or contracted
+upon clean quartz so as to make a firm joint.  In the light of M.
+Margot's researches (already described), it occurred to me that
+perhaps my experience was only a special case of the phenomena of
+adhesion investigated with so much success by M. Margot.  I therefore
+tried whether the alloy of tin and zinc used for soldering aluminium
+would stick to quartz, and instantly found that this was indeed the
+case.
+
+Adhesion between the alloy and perfectly clean quartz takes place
+almost without rubbing.  A rod of quartz thus "tinned" can be soldered
+up to anything to which solder will stick, at once.  On applying the
+method to thick quartz threads, success was instantaneous (the threads
+were some preserved for ordinary galvanometer suspensions);  but when
+the method was applied to very fine threads, great difficulty in
+tinning the threads was experienced.  The operation is best performed
+by having the alloy on the end of an aluminium soldering bit, and
+taking care that it is perfectly free from oxide before the thread is
+drawn across it.  There was no difficulty in soldering a thread
+"tinned"    in this manner to a copper wire with tinman's solder, and
+the joint appeared perfect, the thread breaking finally at about an
+inch away from the joint.
+
+I allow Mr. Boys' method to stand as I have written it, simply because
+I have not had time as yet to make thorough tests of the durability of
+"Margot" joints on the finest threads;  but I have practically no
+doubt as to its perfect applicability, provided always that the solder
+can be got clean enough when melted on the bit.  Very fine threads
+will require to be stretched before tinning, in order to enable them
+to break through the capillary barrier of the surface of the melted
+solder.
+
+Sec. 92.  Soldering.
+
+It is almost unfair to the arts of the glass-blower or optician to
+describe them side by side with the humble trade of soldering.
+Nevertheless, no accomplishment of a mechanical kind is so serviceable
+to the physicist as handiness with the soldering bit;  and, as a rule,
+there is no other exercise in which the average student shows such
+lamentable incapacity.  The following remarks on the subject are
+therefore addressed to persons presumably quite ignorant of the way in
+which soldering is carried out, and do not profess to be more than of
+the most elementary character.
+
+For laboratory purposes three kinds of solder are in general
+sufficient.  One is the ordinary tinman's solder composed of lead and
+tin.  The second is "spelter," or soft fusible brass, and the third is
+an alloy of silver and brass called silver solder.
+
+Tinman's solder is used for most purposes where high temperatures are
+not required, or where the apparatus is intended to be temporary.  The
+"spelter," which is really only finely granulated fusible brass, is
+used for brazing iron joints.  The silver solder is convenient for
+most purposes where permanency is required, and is especially suited
+to the joining of small objects.
+
+Sec. 93.  Soft tinman's solder is made by melting together two parts of
+grain tin and one of soft lead--the exact proportions are not of
+consequence--but, on the other hand, the purer the constituents the
+better the solder.  Within certain limits, the greater the proportion
+of tin the cleaner and more fusible is the solder.  It is usually
+worth while to prepare the solder in the laboratory, for in this way a
+uniform and dependable product is assured.  Good soft lead is melted
+in an iron ladle and skimmed;  the temperature is allowed to rise very
+little above the melting-point.  The tin is then added little by
+little, the alloy stirred vigorously and skimmed, and sticks of solder
+conveniently cast by sweeping the ladle over a clean iron plate, so as
+to pour out a thin stream of solder.  If the solder be properly made
+it will have a mat and bright mottled surface, and will "crackle" when
+held up to the ear and bent.
+
+Perhaps the chief precaution necessary in making solder is to exclude
+zinc.  The presence of a very small percentage of this metal entirely
+spoils the solder for tinman's work by preventing its "running" or
+flowing smoothly under the soldering bit.
+
+Fig.  75.
+
+Fig.   76.
+
+Fig.  77.
+
+Sec. 94.  Preparing a Soldering Bit.
+
+The wedge-shaped edge of one of the forms of bit shown in the sketch
+is filed to shape and the bit heated in a fire or on a gas heater.  A
+bit of rough sandstone, or even a clean soft brick, or a bit of tin
+plate having some sand sprinkled over it, is placed in a convenient
+position and sprinkled with resin.
+
+As soon as the bit is hot enough to melt solder it is withdrawn and a
+few drops of solder melted on to the brick or its equivalent.  The
+iron or bit is then rubbed to and fro over the solder and resin till
+the former adheres to and tins the copper head.  It will be found
+advisable to tin every side of the point of the bit and to carry the
+tinning back at least half an inch from the edge.
+
+If the solder obstinately refuses to adhere, the cause is to be sought
+in the oxidation of the copper, or of the solder, or both--in either
+case the result of too high a temperature or too prolonged heating.
+The simple remedy is to get the iron hot, and then to dress it with an
+old file, so as to expose a bright surface, which is instantly passed
+over the resin as a means of preserving it from oxidation.  If the
+process above described be now carried out, it will be found that the
+difficulty disappears.
+
+Before using the iron, wipe off any soot or coke or burned resin by
+means of an old rag.  An iron tinned in this way is much to be
+preferred to one tinned by means of chloride of zinc.
+
+A shorter and more usual method is carried out as follows:  The
+solution of chloride of zinc is prepared by adding bits of zinc to
+some commercial hydrochloric acid diluted with a little (say 25 per
+cent) of water.  The acid may conveniently be placed in a small glazed
+white jar (a jam pot does excellently), and this should only be filled
+to about one-quarter of its capacity.  An excess of zinc may be added.
+
+It may be fancy, but I prefer a soldering solution made in this way to
+a solution of chloride of zinc bought as a chemical product.  The jar
+is generally mounted on a heavy leaden base, so as to avoid any danger
+of its getting knocked over, for nothing is so nasty or bad for tools
+as a bench on which this noxious liquid has been upset (Fig.  78).
+
+Fig. 78.
+
+To tin a soldering bit, a little of the fluid is dipped out of the jar
+on to a bit of tin plate bent up at the edges--a few drops is
+sufficient--and the iron is heated and rubbed about in the liquid
+with a drop of solder.  If the iron is anything like clean it will tin
+at once and exhibit a very bright surface, but quite dirty copper may
+be tinned by dipping it for a moment in the liquid in the pot and then
+working it about over the solder.  An iron so tinned remains covered
+with chloride of zinc, and this must be carefully wiped off if it is
+intended to use the iron with a resin or tallow flux in lead
+soldering.
+
+One disadvantage of this process is that the copper bit soon gets
+eaten into holes and requires to be dressed up afresh.  On the other
+hand, an iron so tinned always presents a nice clean solder surface
+until the next time it is heated, when it generally becomes very dirty
+and requires to be carefully wiped before using.
+
+In my experience also an iron so tinned is more easily spoiled as to
+the state of its surface, "detinned," in fact, by overheating than
+when the tinning is carried out by resin and friction.  When this
+happens, the shortest way out of the difficulty is the application of
+the old file so as to obtain a perfectly fresh surface.  No one who
+knows his business ever uses an iron that is not perfectly clean and
+well tinned.
+
+The iron may be cleaned from time to time by heating it red hot and
+quenching it in water to get rid of the oxide, which scales off in the
+process.
+
+Sec. 95.  Soft Soldering.
+
+In the laboratory the chief application of the process is to copper
+soldering during the construction of electrical apparatus and to zinc
+soldering for general purposes.
+
+In ninety-nine cases out of every hundred where difficulties occur
+their origin is to be traced to dirt.  There seems to be some
+inexplicable kink in the human mind which renders it callous to
+repeated proofs of the necessity for cleaning surfaces which it is
+intended to solder.  The slightest trace of albuminous or gelatinous
+matter or shellac will prevent solder adhering to most metals and the
+same remark applies in a measure to the presence of oxides, although
+these may be removed by chloride of zinc or prevented from forming by
+resin or tallow.  A touch with an ordinarily dirty hand--I refer to a
+solderer's hand--will often soil work sufficiently to make the
+adherence of solder difficult.
+
+The fluxes most generally employed are tallow for lead, resin or
+Venice turpentine for copper, chloride of zinc for anything except
+lead, which never requires it.  The latter flux has the property (also
+possessed by borax at a red heat) of dissolving any traces of oxide
+which may be formed, as well as acting as a protecting layer to the
+metal.
+
+We may now turn to the consideration of a simple case of soldering,
+say the joining of two copper wires.  The wires are first cleaned
+either by dipping in a bath of sulphuric and nitric acids--a thing no
+laboratory should be without--or by any suitable mechanical means.
+The cleaned wires are then twisted together--there is a regulation
+way of doing this, but it presents no advantage in laboratory
+practice--and the joint is sprinkled over with resin, or painted
+with a solution of resin in alcohol.
+
+The iron, being heated and floated with solder, is held against the
+joint, the latter being supported on a brick, and the solder is
+allowed to "sweat" into the joint.  Enough solder must be present to
+penetrate right through the joint.  Nothing is gained by rubbing
+violently with the iron.  If the copper is clean it will tin, and if
+it is dirty it won't, and there the matter ends.
+
+Beginners generally use too small or too cold a bit, and produce a
+ragged, dirty joint in consequence.  If the saving of time be an
+object, the joint may be twisted together on ordinarily dirty oxidised
+wires and heated to, say, 200 deg. C.  It is then painted with chloride of
+zinc and soldered with the bit.
+
+There is a difference of opinion as to the relative merits of chloride
+of zinc and of resin as a flux in soldering copper.  Thus the standing
+German practice is, or was, to employ the former flux in every case
+for soldering electric light wires, while in England the custom  used
+to be to specify that soldering should be done by resin, and this
+custom may still prevail;  it lingers in Australia at all events.
+
+However, it is agreed on all hands that when chloride of zinc is used
+it must be carefully washed off.  I have known of an electrical
+engineer insisting on his workmen "licking" joints with their tongues
+to ensure the total removal of chloride of zinc; it has a horrible
+taste; and I have occasionally pursued the same plan myself when the
+soldering of fine wires was in question.
+
+In any case, it is very certain that chloride of zinc left in a joint
+will ruin it sooner or later by loosening the contact between copper
+and solder.
+
+Very often it is requisite to solder together two extensive flat
+surfaces--for instance, in "chucking" certain kinds of brass work.
+The surfaces to be soldered must be carefully tinned, most
+conveniently by the help of the blow-pipe and chloride of zinc.  After
+tinning, the surfaces are laid together and heated so as to "sweat"
+them together; the phrase, though inelegant, is expressive.
+
+96.  Soldering Tin Plate.
+
+If the plate be new and clean, a little resin or its solution in
+alcohol is all that is necessary as a flux.  If the tin plate is rusty
+the rust must be removed and the clean iron, or rather mild steel,
+surface exposed.  The use of chloride of zinc is practically essential
+in this case.  Tin plate is often spotted with rust long before it
+becomes rusty as a whole, when, of course, it may be regarded as worn
+out, and such rust spots are most conveniently removed by means of the
+plumber's shave-hook.  The shave-hook is merely a peculiarly shaped
+hard steel scraping knife on a handle (Fig.  79).
+
+Fig.  79
+
+With tin plate the soldering of long joints is often necessary.  The
+plate must be temporarily held in position either by binding with iron
+wire, fastening by clamps, or holding by an assistant.  The flux is
+applied and the iron run slowly along the joint.  Enough solder is
+used to completely float the tip of the iron.  By arranging the joint
+so that it slopes downward slightly, and commencing at the upper end,
+the solder may be caused to flow after the iron, and will leave a
+joint with the minimum permissible amount of solder in it.  By
+regulating the slope, heat of iron, etc, any desired quantity of
+solder may be run into the joint.
+
+Sec. 97.  Soldering Zinc.
+
+Zinc alloys with soft solder very easily, and by so doing entirely
+spoils it, making, it "crumbly," dirty, and preventing it running.
+Consequently, in soldering up zinc great care must be taken to prevent
+the solder becoming appreciably contaminated by the zinc.  To this end
+the zinc surfaces are cleaned by means of a little hydrochloric acid,
+which is painted on instead of chloride of zinc.  Plenty of solder is
+melted on to the work, and is drawn along over the joint by a single
+slow motion of the soldering bit.  The iron must be just hot enough to
+make the solder flow freely, and it must never be rubbed violently on
+the zinc or allowed to linger in one spot;  the result of the latter
+action will be to melt a hole through the zinc, owing to the tendency
+of this metal to form an easily fusible alloy with the solder.
+
+The art of soldering zinc is a very useful one in the laboratory.  The
+majority of physicists appear to overlook the advantages of zinc
+considered as a material for apparatus construction.  It is light,
+fairly strong, cheap, easily fusible, and yet hard and elastic when
+cold.  It may be worked as easily as lead at a temperature of, say,
+150 deg. to 200 deg. C, and slightly below the melting-point (423 deg. C.) it is
+brittle and may & powdered.  The property of softening at a moderate
+temperature is invaluable as a means of flattening zinc plate or
+shaping it in any way.  During the work it may be held by means of an
+old cloth.  Zinc sheet which has been heated between iron plates and
+flattened by pressure retains its flatness very fairly well after
+cooling.
+
+Sec. 98.  Soldering other Metals.
+
+Iron.
+
+The iron must be filed clean and then brushed with chloride of zinc
+solution.  Some people add a little sat ammoniac to the chloride of
+zinc, but the improvement thus made is practically inappreciable.  If
+the iron is clean it tins quite easily, and the process of soldering
+it is perfectly easy and requires no special comment.
+
+Brass.
+
+The same method as described for iron succeeds perfectly.  The brass,
+if not exceedingly dirty, may be cleaned by heating to the temperature
+at which solder melts (below 200 deg. C.), and painting it over with
+chloride of zinc, or dipping it in the liquor.  If now the brass be
+heated again in the blow-pipe flame, it will be found to tin perfectly
+well when rubbed over with solder.
+
+German Silver, Platinoid, Silver, and Platinum are treated like iron.
+With regard to silver and platinum the same precautions as recommended
+in the case of zinc must be observed, for both these metals form
+fusible alloys with solder.
+
+Gold when pure requires no flux.  Standard gold, which contains copper,
+solders better with a little chloride of zinc.
+
+Lead must be pared absolutely clean and then soldered quickly with a
+hot iron, using tallow as a flux.  Since solder if over hot will
+adhere to lead almost anywhere, plumbers are in the habit of specially
+soiling those parts to which it is not intended that solder shall
+adhere. The "soiling" paint consists of very thin glue, called size,
+mixed with lampblack; on an emergency a raw potato may be cut in half,
+and the work to be soiled may be rubbed over with the cut surface of
+the potato.
+
+Hard Carbon or gas coke may be soldered after coating with copper by
+an electrolytic process, as will be described.
+
+Sec. 99.  Brazing.
+
+Soldering at a red heat by means of spelter is called brazing.
+Spelter is soft brass, and is generally made from zinc one part,
+copper one part; an alloy easily granulated at a red heat;  it is
+purchased in the granular form.
+
+The art of brazing is applied to metals which will withstand a red
+heat, and the joints so soldered have the strength of brass.
+
+The pieces to be jointed by this method must be carefully cleaned and
+held in their proper relative positions by means of iron wire.  It is
+generally necessary to soften iron wire as purchased by heating it red
+hot and allowing it to cool in the air;  if this is not done the wire
+is usually too hard to be employed satisfactorily for binding.
+
+Very thin wire--i.e. above No. 20 on the Birmingham wire gauge--does
+not do, for it gets burned through, and perhaps allows the work
+to fall apart at a critical moment.
+
+The work being securely fastened, the next step is to cover the
+cleaned parts with flux in order to prevent oxidation.  For this
+purpose "glass borax" is employed. "Glass" borax is simply ordinary
+borax which has been fused for the purpose of getting rid of water of
+crystallisation.  The glass borax is reduced to powder in an iron
+mortar, for it is very hard, and is then made up into a cream with a
+little water.  This cream is painted on to the parts of the work which
+are destined to receive the solder.
+
+The next step is to prepare the spelter, and this is easily done by
+mixing it with the cream, taking care to stir thoroughly with a
+flattened iron wire till each particle of spelter is perfectly covered
+with the borax.  The mixture should not be too wet to behave as a
+granular mass, and may then be lifted on to the work by means of the
+iron spatula.
+
+Care must be taken to place the spelter on those parts only which are
+intended to receive it, and when this is done, the joint may be
+lightly powdered over with the dry borax, and will then be ready for
+heating.
+
+If the object is of considerable size it is most conveniently heated
+on the forge;  if small the blowpipe is more convenient.  In the
+latter case, place the work on a firebrick, and arrange two other
+bricks on edge about it, so that it lies more or less in a corner.  A
+few bits of coke may also be placed on and about the work to increase
+the temperature by their combustion, and to concentrate the flame and
+prevent radiation.  The temperature is gradually raised to a bright
+red heat, when the spelter will be observed to fuse or "run," as it is
+technically said to do.
+
+If the cleaning and distribution of flux has been successful, the
+spelter will "run" along the joint very freely, and the work should be
+tapped gently to make sure that the spelter has really run into the
+joint.  The heating may be interrupted when the spelter is observed to
+have melted into a continuous mass.  As soon as the work has fallen
+below a red heat it may be plunged into water, a process which has the
+effect of cracking off the glass-like layer of borax.
+
+There is, however, some risk of causing the work to buckle by this
+violent treatment, which must of course be modified so as to suit the
+circumstances of the case.  If the joint is in such a position that
+the borax cannot be filed off, a very convenient instrument for its
+removal by scraping is the watchmaker's graver, a square rod of hard
+steel ground to a bevelled point (Fig.  80).
+
+Fig.  80.
+
+Several precautions require to be mentioned.  In the first place,
+spelter is merely rather soft brass, and consequently it often cannot
+be fused without endangering the rest of the work.  A good protection
+is a layer of fireclay laid upon the more delicate parts, such for
+instance as any screwed part.
+
+Gun-metal and tap-metal do not lend themselves to brazing so readily
+as iron or yellow brass, and are usually more conveniently treated by
+means of silver solder.
+
+Spelter tends to run very freely when it melts, and if the brass
+surface in the neighbourhood of the joint is at all clean, may run
+where it is not wanted.  Of course some control may be exercised by
+"soiling" with fireclay or using an oxidising flame;  but the erratic
+behaviour of spelter in this respect is the greatest drawback to its
+use in apparatus construction.  The secret of success in brazing lies
+in properly cleaning up the work to begin with, and in disposing the
+borax so as to prevent subsequent oxidation.
+
+Sec. 100.  Silver Soldering.
+
+This process resembles that last described, but instead of spelter an
+alloy of silver, copper, and zinc is employed.  The solder, as
+prepared by jewellers to meet special cases, varies a good deal in
+composition, but for the laboratory the usual proportions are:
+
+For soft silver solder
+
+Fine silver         2 parts
+Brass wire        1 part
+
+For hard silver solder
+
+Sterling silver     3 parts
+Brass wire      1 part
+
+The latter is, perhaps, generally the more convenient.
+
+Silver solders may, of course, be purchased at watchmakers' supply
+shops, and as thus obtained, are generally in thin sheet.  This is
+snipped fine with a pair of shears preparatory to use.
+
+As odds and ends of silver (from old anodes and silver residues)
+generally accumulate in the laboratory, it is often more convenient to
+make the solder one's self.  In this case it must be remembered in
+making hard solder by the second receipt that standard silver contains
+about one-twelfth of its weight of copper--exactly 18 parts copper to
+220 silver.
+
+The silver is first melted in a plumbago crucible in a small furnace
+together with a little borax; if any copper is required this is then
+added, and finally the brass is introduced.  When fusion is complete,
+the contents of the crucible are poured into any suitable mould.
+
+The quickest and most convenient way of preparing the alloy for use is
+to convert it into filings with the assistance of a coarse file, or by
+milling it, if a milling machine is available.
+
+Equal volumes of filings and powdered glass borax are made into a thin
+paste with water, and applied in an exactly similar manner to that
+described under the head of "brazing."  In fact all the processes
+there described may be applied equally to the case under discussion,
+the substitution of silver for spelter being the only variation.
+
+The silver solder is more manageable than spelter, and does not tend
+to run wild over the work:  a property which makes it much more
+convenient both for delicate joints and in cases where it is desired
+to restrict the solder to a single point or line.  Small objects are
+almost invariably soldered with silver solder, and are held by forceps
+or on charcoal in the pointed flame of an ordinary blow-pipe.
+
+Sec. 101.  On the Construction of Electrical Apparatus: Insulators.
+
+It is not intended to deal in any way with the design of special
+examples of electrical apparatus, but merely to describe a rather
+miscellaneous set of materials and processes constantly required in
+its construction.
+
+It is not known whether there is such a thing as a perfect insulator,
+even if we presuppose ideal circumstances.  Materials as they exist
+must be regarded merely as of high specific resistance, that is if we
+allow ourselves to use such a term in connection with substances,
+conduction through which is neither independent of electromotive force
+per unit length, nor of previous history.
+
+Even the best of these substances generally get coated with a layer of
+moisture when exposed to the air, and this as a rule conducts fairly
+well.  Very pure crystalline sulphur and fused quartz suffer from this
+defect less than any other substances with which the writer is
+acquainted, but even with them the surface conductivity soon grows to
+such an extent as totally to mask the internal conduction.
+
+It is proposed to give a brief account of the properties of some
+insulating substances and their application in electrical
+construction, and at the same time to indicate the appliances and
+methods requisite for working them.
+
+With regard to the specific resistances which will be quoted, the
+numbers must not be taken to mean too much, partly for the reason
+already given.  It is also in general doubtful whether sufficient care
+has been taken to distinguish the body from the surface conductivity,
+and consequently numerical estimates are to be regarded with
+suspicion.  The question of "sampling" also arises, for it must be
+remembered that a change in composition amounting to, say, 1/10000 per
+cent may be accompanied  by a million-fold change in specific
+resistance.
+
+Sec. 102.  Sulphur.
+
+This element exists in several allotropic forms, which have very
+different electric properties.  After melting at about 125 deg. C, and
+annealing at 110 deg. for several hours, the soluble crystalline
+modification is formed.  After keeping for some days--especially if
+exposed to light--the crystals lose their optical properties, but
+remain of the same melting-point, and are perfectly soluble in carbon
+bisulphide.  The change is accompanied by a change in colour, or
+rather in brightness, as the transparency changes.
+
+The "specific resistance" of sulphur in this condition is above 1028
+C.G.S.E.M. units, or 1013 megohms per cubic centimetre for an electric
+intensity of say 12,000 volts per centimetre.  This is at ordinary
+temperatures.  At 75 deg. C.  the specific resistance falls to about 1025
+under similar conditions as to voltage.
+
+In all cases the conductivity appears to increase with the electric
+intensity, or at all events with an increase in voltage, the thickness
+of the layer of sulphur remaining the same.
+
+The specific inductive capacity is 3.162 at ordinary temperatures, and
+increases very slightly with rise of temperature. [Footnote: March
+1897.--It is now the opinion of the writer that though the specific
+inductive capacity of a given sample of a solid element is perfectly
+definite, yet it is very difficult to obtain two samples having
+exactly the same value for this constant, even in the case of a
+material so well defined as sulphur.]
+
+The total residual charge, after ten minutes' charging with an
+intensity of 12,000 volts per centimetre, is not more than 4 parts in
+10,000 of the original charge.  In making this measurement the
+discharge occupied a fraction of a second.  The electric strength for
+a homogeneous plate of crystalline sulphur is not less than 33,000
+volts per centimetre, and probably a good deal more.  If the sulphur
+is contaminated with up to 3 per cent of the amorphous variety, as is
+the case if it is cooled fairly quickly from a temperature of 170 deg. C.
+or over, the specific resistance falls to from 10^25 to 10^26 at
+ordinary temperatures;  and the specific inductive capacity increases
+up to 3.75, according to the amount of insoluble sulphur present.
+
+The residual charge under circumstances similar to those described
+above, but with an intensity of about 4000 volts per centimetre is,
+say, 2 per cent of the initial charge.  So far as the writer is aware
+sulphur is the only solid non-conductor which can be easily obtained
+in a condition of approximate purity and in samples sufficiently
+exactly comparable with one another;  it is the only one, therefore,
+that repays any detail of description.
+
+Very pure sulphur can be bought by the ton if necessary from the
+United Alkali Company of Newcastle-on-Tyne.  It is recovered from
+sulphur waste by the Chance process, which consists in converting the
+sulphur into hydrogen sulphide, and burning the latter with
+insufficient air for complete combustion.  The sulphur is thrown out
+of combination, and forms a crystalline mass on the walls and floor of
+the chamber.
+
+The sulphur which comes into the market consists of this mass broken
+up into convenient fragments.  In order to purify it sufficiently for
+use as an insulator, the sulphur may be melted at a temperature of
+120 deg. to 140 deg. C, and filtered through a plug of glass wool in a zinc
+funnel; as thus prepared it is an excellent insulator.  To obtain the
+results mentioned in the table it is, however, necessary to conduct a
+further purification (chiefly from water) by distillation in a glass
+retort.
+
+The sulphur thus obtained may be cast of any desired form in zinc
+moulds, the castings and moulds being immediately removed to an
+annealing oven at a temperature of from 100 deg. to 110 deg. C, where they
+are left for several hours.  If the sulphur is kept melted for some
+time at 125 deg. C.  the annealing is not so important.
+
+The castings may be removed from the mould by slightly heating the
+latter, but many breakages result.  Insulators made on this plan are
+much less affected by the condensation of moisture from the air than
+anything except fused quartz.  They are, however, very weak
+mechanically, and apt to crack by exposure to such changes of
+temperature as go on from day to day.  It is clear, however, that in
+spite of this their magnificent electrical properties fit them for
+many important uses.
+
+If the sulphur be cooled rapidly from 170 deg. C.  or over, a mixture of
+the crystalline and amorphous varieties of sulphur is obtained.  This
+mixture is very much stronger and tougher than the purely crystalline
+substance, and may be worked with ordinary hardwood tools into fairly
+permanent plates, rods, etc.  Sheets of pure thick filter paper may
+also be dipped into sulphur at 170 deg. C, at which temperature air and
+moisture are mostly expelled, and such sheets show a very considerable
+insulating power.  The sulphur does not penetrate the paper, which
+therefore merely forms a nucleus.
+
+Cakes of the crystalline or mixed varieties may be made by grinding up
+some purified sulphur, moistening it with redistilled carbon
+bisulphide, or toluene, or even benzene (C6H6), and pressing it in a
+suitable mould under the hydraulic press.  The plates thus formed are
+porous, but are splendid insulators, especially if made from the
+crystalline variety of sulphur, and they appear to keep their shape
+very well, and do not crack with ordinary temperature changes.
+
+The metals which resist the action of sulphur best are gold and
+aluminium;  while platinum and zinc are practically unacted upon at
+temperatures below a red heat--in the former case,--and below the
+boiling-point of sulphur in the latter.
+
+A very convenient test of the purity of sulphur is the colour assumed
+by it when suddenly cooled from the temperature at which it is
+viscous.  Quite pure sulphur remains of a pale lemon yellow under this
+treatment, but the slightest trace of impurity, such as arises from
+dust containing organic matter, stains the sulphur, and renders it
+darker in colour.
+
+Sec. 103.  Fused Quartz.
+
+This is on the whole the most reliable and most perfect insulator for
+general purposes.  No exact numerical data have been obtained, but the
+resistivity must certainly be of the same order as that of pure
+sulphur at its best.  The influence of the moisture of the air also
+reaches its minimum in the case of quartz, as was originally observed
+by Boys.
+
+As yet, however, the material can only be obtained in the form of rods
+or threads.  For most purposes rods of about one-eighth of an inch in
+diameter are the most convenient.  These rods may be used as
+insulating supports, and succeed perfectly even if they interpose less
+than an inch of their length to electrical conduction.  The sketch
+(Figs.  81 and 81A) shows (to a scale of about one-quarter full size)
+a complete outfit for elementary electrostatic experiments, such as
+has been in use in the writer's laboratory for five years.  With these
+appliances all the fundamental experiments may be performed, and the
+apparatus is always ready at a moment's notice.
+
+Fig.  81.
+
+Though quartz does not condense moisture or gas to form a conducting
+layer of anything like the same conductivity as in the case of glass
+or ebonite, still it is well to heat it if the best results are to be
+obtained.  For this purpose a small pointed blow-pipe flame may be
+used, and the rods may be got red-hot without the slightest danger of
+breaking them. They then remain perfectly good and satisfactory for
+several hours at least, even when exposed to damp and dusty air.
+
+The rods are conveniently held in position by small brass ferrules,
+into which they are fastened by a little plaster of Paris. Sealing-wax
+must be avoided, on account of the inconvenience it causes when the
+heating of the rods is being carried out.
+
+One useful application of fused quartz is to the insulation of
+galvanometer coils (Fig. 82), another to the manufacture of highly
+insulating keys (Fig. 83); while as an insulating suspension it has
+all the virtues.  If it is desired to render the threads conducting
+they may be lightly silvered, and will be found to conduct well enough
+for electrometer work before the silver coating is thick enough to
+sensibly impair their elastic properties.
+
+Fig.  81A.
+
+Fig.  82 is a full-size working drawing of a particular form of
+mounting for galvanometer coils.  The objects sought to be attained
+are:
+(1) high insulation of the coils,
+
+(2) easy adjustment of the coils to the suspended system.
+
+The first object is attained as follows.  The ebonite ring A is bored
+with four radial holes, through which are slipped from the inside the
+fused quartz bolt-headed pins B.  The coil already soaked in hard
+paraffin is placed concentrically in the ring A by means of a special
+temporary centering stand.  The space between the coil and the ring is
+filled up with hard paraffin, and this holds the quartz pins in
+position.  The system of ebonite ring, coil, and pins is then fastened
+into the gun-metal coil carrier, which is cut away entirely, except
+near the edges, where it carries the pin brackets C.  These brackets
+can swivel about the lower fastening at E before the latter is
+tightened up.
+
+The coil is now adjusted in the adjusting stand to be concentric with
+the axis of symmetry of the coil carrier, and the supporting pins are
+slipped into slot holes cut in the brackets, the brackets being
+swivelled as much as necessary to allow of this.  When the pins are
+all inserted the brackets are screwed up by the screws at E.  The pins
+are then cemented firmly to the brackets by a little plaster of Paris.
+The coil carrier can now be adjusted to the galvanometer frame by
+means of screws at D, which pass through wide holes in the carrier and
+bold the latter in position by their heads.  In the sectional plan the
+parts of the galvanometer frame are shown shaded.  The front of the
+frame at F F is of glass, and the back of the frame is also made of
+glass, though this is not shown in the section.
+
+A represents an ebonite ring into which the wire coil is cemented by
+means of paraffin.  B B B B are quartz pins, with heads inside the
+ebonite ring.  C C C are slotted brackets adjustable to the pins and
+capable of rotation by releasing the screws E E.  D D are the screws
+holding the coil carriage to the galvanometer framework.  These screws
+pass through large holes in the carriage so as to allow of some
+adjustment.
+
+Fig.  82.
+
+Fig.  83.
+
+Sec. 104.  Glass.
+
+When glass is properly chosen and perfectly dry it has insulating
+properties possibly equal to those possessed by quartz or crystalline
+sulphur.  For many purposes, however, its usefulness is seriously
+reduced by the persistence with which it exhibits the phenomena of
+residual charge, and the difficulty that is experienced in keeping it
+dry.
+
+The insulating power of white flint glass is much in excess of that of
+soft soda glass, which is a poor insulator, and of ordinary green
+bottle glass.  The jars of Lord Kelvin's electrometers, which insulate
+very well, are made of white flint glass manufactured in Glasgow, but
+it is found that occasionally a particular jar has to be rejected on
+account of its refusing to insulate, and this, if I understand aright,
+even when it exhibits no visible defects.
+
+A large number of varieties of glass were tested by Dr. Hopkinson at
+Messrs.  Chance Bros.  Works, in 1875 and 1876 (Phil.  Trans, 1877),
+and in 1887 (Proc.  Roy.  Soc.  xli.  453), chiefly with a view to the
+elucidation of the laws regulating the residual charge;  and
+incidentally some extraordinarily high insulations were noted among
+the flint glasses.  The glass which gave the smallest residual charge
+was an "opal" glass;  and flint glasses were found to insulate 105
+times as well as soda lime glasses.  The plates of Wimshurst machines
+are made of ordinary sheet window glass, but as the insulating
+property of this material appears to vary, it is generally necessary
+to clean, dry, and test a sheet before using it.  With regard to hard
+Bohemian glass, this is stated by Koeller (Wien Bericht) to insulate
+ten times as well as the ordinary Thuringian soft soda glass.
+
+On the whole the most satisfactory laboratory practice is to employ
+good white flint glass.  The only point requiring attention is the
+preparation of the glass by cleaning and drying.  Of course all grease
+or visible dirt must be removed as described in an earlier chapter (Sec.
+13), but this is only a beginning.  The glass after being treated as
+described and got into such a state as to its surface that clean water
+no longer tends to dry off unequally, must be subjected to a further
+scrub with bibulous paper and a clear solution of oleate of soda.  The
+glass is then to be well rinsed with distilled water and allowed to
+drain on a sheet of filter paper.
+
+A very common cause of failure lies in the contamination of the glass
+with grease from the operator's fingers.  Before setting out to clean
+the glass the student will do well to wash his hands with soap and
+water, then with dilute ammonia and finally with distilled water.
+
+In the case of an electrometer jar which has become conducting but is
+not perceptibly dirty, rubbing with a little oleate of soda and a silk
+ribbon, followed, of course, by copious washing, does very well.  If
+there is any tin-foil on the jar, great care must be taken not to
+allow the glass surface to become contaminated by the shellac varnish
+or gum used to stick the tin-foil in position.
+
+Finally, the glass should be dried by radiant heat and raised to a
+temperature of 100 deg. C.  at least, and kept at it for at least half an
+hour.  Before drying it is of course advisable to allow the water to
+drain away as far as possible, and if the water is only the ordinary
+distilled water of the laboratory, the glass is preferably wiped with
+a clean bit of filter paper; any hairs which may be left upon the
+glass will brush off easily when the glass is dry.
+
+In order to obtain satisfactory results the glass must be placed in
+dry air before it has appreciably cooled.  This is easily done in the
+case of electrometer jars, and so long as the air remains perfectly
+dry through the action of sulphuric acid or phosphorus pentoxide, the
+jar will insulate.  The slightest whiff of ordinarily damp air will,
+however, enormously reduce the insulating power of the glass, so that
+unvarnished glass surfaces must be kept for apparatus which is
+practically air-tight.
+
+For outside or imperfectly protected uses the glass does better when
+varnished.  It is a fact, however, that varnished glass is rarely if
+ever so good as unvarnished glass at its best.  Too much care cannot
+be taken over the preparation of the varnish; French polish, or
+carelessly made shellac varnish, is likely to do more harm than good.
+
+The best orange shellac must be dissolved in good cold alcohol by
+shaking the materials together in a bottle.  The alcohol is made
+sufficiently pure by starting with rectified spirit and digesting it
+in a tin flask over quick-lime for several days, a reversed condenser
+being attached.  A large excess of lime must be employed, and this
+leads to a considerable loss of alcohol, a misfortune which must be
+put up with.
+
+After, say, thirty hours' digestion, the alcohol may be distilled off
+and employed to act on the shellac.  In making varnish, time and
+trouble are saved by making a good deal at one operation--a
+Winchester full is a reasonable quantity.  The bottle may be filled
+three-quarters full of the shellac flakes and then filled up with
+alcohol; this gives a solution of a convenient strength.
+
+The solution, however, is by no means perfect, for the shellac
+contains insoluble matter, and this must be got rid off.`' One way of
+doing this is to filter the solution through the thick filtering paper
+made by Schleicher and Schuell for the purpose, but the filtering is a
+slow process, and hence requires to be conducted by a filter paper
+held in a clip (not a funnel) under a bell jar to avoid evaporation.
+
+Another and generally more convenient way in the laboratory is to
+allow the muddy varnish to settle--a process requiring at least a
+month--and to decant the clear solution off into another bottle,
+where it is kept for use.  The muddy residue  works up with the next
+lot of shellac and alcohol, which may be added at once for future use.
+
+The glass to be varnished is warmed to a temperature of, say, 50 deg. C,
+and the varnish put on with a lacquering brush; a thin uniform coat is
+required.  The glass is left to dry long enough for the shellac to get
+nearly hard and to allow most of the alcohol to evaporate.  It is then
+heated before a fire, or even over a Bunsen, till the shellac softens
+and begins to yield its fragrant characteristic smell.
+
+If the coating is too heavy, or if the heating is commenced before the
+shellac is sufficiently dry, the latter will draw up into "tears,"
+which are unsightly and difficult to dry properly.  On no account must
+the shellac be allowed to get overheated.  If the varnish is not quite
+hard when cold it may be assumed to be doing more harm than good.
+
+In varnishing glass tubes for insulating purposes it must be
+remembered that the inside of the tube is seldom closed perfectly as
+against the external air, and consequently it also requires to be
+varnished.  This is best done by pouring in a little varnish
+considerably more dilute than that described, and allowing it to drain
+away as far as possible, after seeing that it has flooded every part
+of the tube.
+
+During this part of the process the upper end of the tube must be
+closed, or evaporation will go on so fast that moisture will be
+deposited from the air upon the varnished surface.  Afterwards the
+tube may be gently warmed and a current of air allowed to pass, so as
+to prevent alcohol distilling from one part of the tube to another.
+The tube is finally heated to the softening point of shellac, and if
+possible closed as far as is practicable at once.
+
+Sec. 105.  Ebonite or Hard Rubber.
+
+This exceedingly useful substance can be bought of a perfectly useless
+quality.  Much of the ebonite formerly used to cover induction coils
+for instance, deteriorates so rapidly when exposed to the air that it
+requires to have its surface renewed every few weeks.
+
+The very best quality of ebonite obtainable should be solely employed
+in constructing electric works.  It is possible to purchase good
+ebonite from the Silvertown Rubber Company (and probably from other
+firms), but the price is necessarily high, about four shillings per
+pound or over.
+
+At ordinary temperatures ebonite is hard and brittle and breaks with a
+well-marked conchoidal fracture.  At the temperature of boiling water
+the ebonite becomes somewhat softened, so that it is readily bent into
+any desired shape;  on cooling it resumes its original hardness.
+
+This property of softening at the temperature of boiling water is a
+very valuable one.  The ebonite to be bent or flattened is merely
+boiled for half an hour or so in water, taken out, brought to the
+required shape as quickly as possible, and left to cool clamped in
+position.
+
+The sheet ebonite as it comes from the makers is generally far from
+flat.  It is often necessary to flatten a sheet of ebonite, and of
+course this is the more easily accomplished the smaller the sheet.
+Consequently a bit of ebonite of about the required size is first cut
+from the stock sheet by a hack-saw such as is generally used for
+metals.  This piece is then boiled and pressed between two planed iron
+plates previously warmed to near 100 deg. C.
+
+With pieces of ebonite such as are used for the tops of resistance
+boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is
+experienced.  The sheets when cold are found to retain the flatness
+which has been forced upon them perfectly well.  It is otherwise with
+large thin sheets such as are used for Holtz machines.  I have
+succeeded fairly, but only fairly, by pressing them in a "gluing
+press," consisting of heavy planed iron slabs previously heated to
+100 deg. C.
+
+I do not know exactly how best to flatten very thin and large sheets.
+It is easy to make large tubes out of sheet ebonite by taking
+advantage of the softening which ebonite undergoes in boiling water.
+A wooden mandrel is prepared of the proper size and shape.  The
+ebonite is softened and bent round it; this may require two or three
+operations, for the ebonite gets stiff very quickly after it is taken
+out of the water.  Finally the tube is bound round the mandrel with
+sufficient force to bring it to the proper shape and boiled in water,
+mandrel and all.  The bath and its contents are allowed to cool
+together, so that the ebonite cools uniformly.
+
+Tubes made in this way are of course subject to the drawback of having
+an unwelded seam, but they do well enough to wind wire upon if very
+great accuracy of form is not required.  If very accurate spools are
+needed the mandrel is better made of iron or slate and the spool is
+turned up afterwards.  The seam may be strapped inside or at the ends
+by bits of ebonite acting as bridges, and the seam itself may be
+caulked with melted paraffin or anthracene.
+
+Working Ebonite.
+
+Ebonite is best worked as if it were brass, with ordinary
+brass-turning or planing tools.  These tools should be as hard as
+possible, for the edges are apt to suffer severely, and blunt tools
+leave a very undesirable woolly surface on the ebonite.  In turning or
+shaping ebonite sheets it is as well to begin by taking the skin off
+one side first, and then reversing the sheet, finishing the second
+side, and then returning to the first.  This is on account of the fact
+that ebonite sometimes springs a little out of shape when the skin is
+removed.
+
+Turned work in ebonite, if well done, requires no sand-papering, but
+may be sufficiently polished by a handful of its own shavings and a
+little vaseline.  The advantage of using a polished ebonite surface is
+that such a surface deteriorates more slowly under the influence of
+light and air than a surface left rough from the tool.  If very highly
+polished surfaces are required, the ebonite after tooling is worked
+with fine pumice dust and water, applied on felt, or where possible by
+means of a felt buff on the lathe, and finally polished with rouge and
+water, applied on felt or cloth.
+
+Ebonite works particularly well under a spiral milling cutter, and
+sufficiently well under an ordinary rounded planing tool, with cutting
+angle the same as for brass, and hardened to the lightest straw
+colour.
+
+It is not possible, on the other hand, to use the carpenter's plane
+with success, for the angle of the tool is too acute and causes the
+ebonite to chip.
+
+In boring ebonite the drill should be withdrawn from the hole pretty
+often and well lubricated, for if the borings jam, as they are apt to
+do, the heat developed is very great and the temper of the drill gets
+spoiled.  Ebonite will spoil a drill by heating as quickly as anything
+known;  on the other hand, it may be drilled very fast if proper
+precaution is taken.
+
+It is advisable to expose ebonite to the light as little as possible,
+especially if the surface is unpolished, for under the combined action
+of light and air the sulphur at the surface of the ebonite rapidly
+oxidises, and the ebonite becomes covered with a thin but highly
+conducting layer of sulphurous or sulphuric acid or their compounds.
+When this happens the ebonite may be improved by scrubbing with hot
+water, or washing freely with alcohol rubbed on with cotton waste in
+the case of apparatus that cannot be dismounted.
+
+A complete cure, however, can only be effected by scraping off the
+outer layer of ebonite so as to expose a fresh surface.  For this
+purpose a bit of sheet glass broken so as to leave a curved edge is
+very useful, and the ebonite is then scraped like a cricket bat.  In
+designing apparatus for laboratory use it is as well to bear in mind
+that sooner or later the ebonite parts will require to be taken down
+and scraped up.  Rods or tubes are, of course, most quickly treated on
+the lathe with rough glass cloth, and may be finished with fine
+sandpaper, then pumice dust and water, applied on felt.  After
+cleaning the pumice off by means of water and a rag, the final touch
+may be given by means of vaseline, applied on cloth or on ebonite
+shavings.
+
+Sec. 106.  Mica.
+
+A great variety of minerals go under this name.  Speaking generally,
+the Russian micas coming into commerce are potash micas, and mica
+purchased in England may be taken to be potash mica, especially if it
+is in large sheets.
+
+At ordinary temperatures "mica" of the kind found  in commerce is an
+excellent insulator.  Schultze (Wied. Ann. vol. xxxvi. p. 655) comes
+to the conclusion that both at high and at low temperatures mica (of
+all kinds?) is a better insulator than white "mirror glass," the
+composition of which is not stated.  The experiments of the author
+referred to were apparently left unfinished, and altogether too much
+stress must not be laid on the results obtained, one of which was that
+mica conducts electrolytically to some extent at high temperatures.
+
+Bouty (Journal de Physique, 1890 [9], 288) and J.  Curie (These de
+Doctorat, Paris, 1888) agree in making the final conductivity of the
+mica used in Carpentier's condensers exceedingly small--at all events
+at ordinary temperatures.  Bearing in mind that for such substances
+the term specific resistance has no very definite meaning, M. Bouty
+considers it is not less than 3.19 x 1028 E.M. units at ordinary
+temperatures.  M. Bouty gives a note or illustration of what such
+numbers mean--a precaution not superfluous in cases where magnitudes
+are denoted logarithmically.  Referring to the value quoted, viz.
+3.19 x 1028, M. Bouty says, "Ce serait la resistance d'une colonne de
+mercure de 1mmq de section et de longueur telle que la lumiere se
+propageant dans le vide, mettrait plus de 3000 ans A se transmettre
+d'une extremite a I'autre de la colonne."
+
+M. Bouty returns to the study of mica (muscovite) in the Journal de
+Physique for 1892, p.  5, and there deals with the specific inductive
+capacity, which for a very small period of charge he finds has the
+value 8--an enormous value for such a good insulator, and one that it
+would be desirable to verify by some totally distinct method.  This
+remark is enforced by the fact that M. Klemencic finds the number 6
+for the same constant.  The temperature coefficient of this constant
+was too small for M. Bouty to determine.  The electric intensity was
+of the order of 100 volts per centimetre, and the experiments seem to
+indicate that the specific inductive capacity would be only slightly
+less if referred to a period of charge indefinitely short.
+
+I have found that the residual charge in a mica condenser, made
+according to Carpentier's method (to be described below), is about 1
+per cent of the original charge under the following circumstances.
+
+Voltage 300 volts on a plate 0.2 mm. thick, duration of charge ten
+minutes, temperature about 20 deg. C.  To get this result the mica must be
+most carefully dried.  This and other facts indicate that the
+so-called residual charge on ordinary condensers is, to a very large
+extent, due to the creeping of the charge from the armatures over the
+more or less conducting varnished surfaces of the mica, and its slow
+return on discharge.
+
+This source of residual charge was carefully guarded against by
+Rowland and Nichols (Phil. Mag.  1881) in their work on quartz, and is
+referred to by M. Bouty, who adduces some experiments to show that his
+own results are not vitiated by it.  On the other hand, M. Bouty shows
+that a small rise in temperature enormously affects the state of a
+mica surface, and that the surface gets changed in such a way as to
+become very fairly conducting at 300 deg. C.  Also anybody can easily try
+for himself whether exposing a mica condenser plate which has been
+examined in presence of phosphorus pentoxide to ordinary air for five
+minutes will not enormously increase the residual charge, as has
+always been the case in the writer's experience, and if so, it is open
+to him to suggest some cause other than surface creeping as an
+explanation.
+
+M. Bouty, using less perfectly dried mica, did not get so good a
+result as to smallness of residual charge as the one above quoted.
+
+The chief use of mica for laboratory purposes depends on the ease with
+which it can be split, and also upon the fact that it may be
+considerably crumpled and bent without breaking.  It therefore makes
+an excellent dielectric in so far as convenience of construction is
+concerned in the preparation of condensers, and lends itself freely to
+the construction of insulating washers or separators of any kind.  Its
+success as a fair insulator at moderate temperatures has led to its
+use in resistance thermometers, where it appears to have given
+satisfaction up to, at all events, 400 deg. C.
+
+It is worth a note that according to Werner Siemens, who had immense
+experience (Wied. Ann. vol. clix.), soapstone is the only reliable
+insulator at a red heat, but, no doubt, a good deal depends on the
+particular specimen investigated.
+
+Sec. 107.  Use of Mica in Condensers.
+
+If good results are desired it is essential to select the mica very
+carefully.  Pieces appreciably stained,--particularly if the stain is
+not uniformly distributed,--cracked pieces, and pieces tending to
+flake off in patches should be rejected.  The best samples of mica
+that have come under the writer's observation are those sheets sold
+for the purpose of giving to silver photographic prints that hideous
+glazed surface which some years ago was so popular.
+
+Sheets of mica about 0.1 to 0.2 mm. thick form good serviceable
+condenser plates, and will certainly stand a pressure of 300 volts,
+and most likely a good deal more.  The general practice in England
+seems to have been to build up condensers of alternate sheets of
+varnished or paraffined-mica and tin-foil.
+
+This practice is open to several objections.  In the first place, the
+capacity of a condenser made in this way varies with the pressure
+binding the plates together.  In the second place, the amount of mica
+and tin-foil required is often excessive in consequence of the
+imperfect contact of these substances.  Again, the inevitable air film
+between the mica and tin-foil renders condensers so made unsuitable
+for use with alternating currents, owing to the heating set up through
+air discharges, and which is generally, though often (if not always)
+wrongly, attributed to dielectric hysteresis.
+
+These imperfections are to a great extent got over by M. Carpentier's
+method of construction, which is, however, rather more costly both in
+material and labour.  On the other hand, wonderful capacities are
+obtained with quite small amounts of mica.  M. Bouty mentions a
+condenser of one microfarad capacity weighing 1500 grms.  and
+contained in a square box measuring 12 centimetres on the side, and
+about 3 centimetres thick.
+
+The relation between the capacity and surface of doubly-coated plates
+is in electro-static units:
+
+Capacity = (sp. ind. capacity X area of one surface)/(4pi X thickness)
+
+This may be reduced to electro-magnetic units by dividing by 9x10^20,
+and to microfarads by further multiplying by 10^15.
+
+M. Carpentier begins, of course, by having his mica scrupulously clean
+and well selected.  It is then silvered by one of the silvering
+processes (Sec. 65) on both sides, for which purpose the sheets may be
+suspended in a paraffined wood rack, so as to lie horizontally in the
+silvering solution, a space of about half an inch being allowed
+between the sheets.  The silvering being finished, the sheets are
+dipped along two parallel edges in 75 per cent nitric acid.  With
+regard to the third and fourth edges of the sheet, the silver is
+removed on one side only, using a spun glass brush;  if we agree to
+call the two surfaces of the mica A and B respectively, and the two
+edges in question C and D, then the silver is removed from the A side
+along edge C, and from the B side along edge D.  The silvered part is
+shown shaded in Fig.  84.  By this arrangement the silver and mica
+plates may be built up together so as to form the same mutual
+arrangement of contacts as in an ordinary mica tin-foil condenser.
+
+Fig. 84.
+
+It need hardly be said that the sheets require very complete washing
+after treatment with nitric acid, followed by a varnishing of the
+edges as already described in the case of glass, and baking at a
+temperature of 140 deg. C.  in air free from flame gases, till the shellac
+begins to emit its characteristic odour and is absolutely hard when
+cold.
+
+The plates are then built up so as to connect the sheets which require
+to be connected, and to insulate the other set.  General contact is,
+if necessary, secured by means of a little silver leaf looped across
+from plate to plate--a part of the construction which requires
+particular attention and clean hands, for it is by no means so easy to
+make an unimpeachable contact as might at first appear.
+
+The condenser, having been built up, may be clamped solid and placed
+in its case;  the capacity will not depend appreciably on the
+tightness of the clamp screws--a great feature of the construction.
+Such a condenser will not give its best results unless absolutely dry.
+I have kept one very conveniently in a vacuum desiccator over
+phosphorus pentoxide, but if of any size, the condenser deserves a box
+to itself, and this must be air-tight and provided with a drying
+reagent, so arranged that it can be removed through a manhole of some
+sort.
+
+Contact to the brass-work on the lid may be made by pressing spring
+contacts tightly down upon the ends of the silver foils and carrying
+the connections through the lid.  This also serves to secure the
+condenser in position.
+
+Sec. 108.  Micanite.
+
+This substance, though probably comparing somewhat unfavourably with
+the insulators already enumerated, and being subject to the
+uncertainties of manufacture, has during the last few years achieved a
+considerable success in American electrical engineering construction.
+It is composed of scrap mica and shellac varnish worked under pressure
+to the desired shape, and may be obtained in sheets, plates, and rods,
+or in any of the forms for which a die happens to have been
+constructed.
+
+Of course, in special cases it would be worth while to prepare a die,
+and then the attainable forms would be limited by moulding
+considerations only.  The writer's experience is very limited in this
+matter, but Dr. Kennelly, with whom he communicated on the subject,
+was good enough to reply in favour of micanite for engineering work.
+
+Sec. 109.  Celluloid.
+
+This material is composed of nitrocellulose and camphor.
+
+It has fair insulating properties, and may be obtained in a variety of
+forms, but has now been generally abandoned for electrical work on
+account of its inflammability.
+
+Sec. 110.  Paper.
+
+Pure white filter paper, perfectly dry, is probably a very fair
+insulator;  the misfortune is that in practice it cannot be kept dry.
+Under the most favourable circumstances its specific resistance may
+approach 1024 E.M. units.  It must therefore be considered rather as a
+partial conductor than as an insulator.  The only case of the use of
+dry paper as an insulator in machine construction which has come under
+the writer's notice is in building up the commutators of the small
+motors which used to drive the Edison phonographs.
+
+Its advantages in this connection are to be traced to the fact that a
+commutator so built up is durable and keeps a clean surface.  Of
+course, the use of paper as an insulator for telephone wires is well
+known, but its success in this direction depends less upon its
+insulating properties than upon the fact that it can be arranged in
+such a way as to allow of the wires being partially air insulated, an
+arrangement tending to reduce the electrostatic capacity of the wire
+system.
+
+At one time it was the custom of instrument makers to employ ordinary
+printed paper in the shape of leaves torn from books or the folios of
+old ledgers to form the dielectric of the condensers used in
+connection with the contact breakers of induction coils.  This
+practice has nothing but economy to recommend it, for cases often
+occur in which the paper, by gradual absorption of moisture from the
+air, comes to insulate so badly that it practically short circuits the
+spark gap, and so stops the action of the coil.  Three separate cases
+have come within the writer's experience.
+
+Some measurements of the resistance of paper have been made by F.
+Uppenborn (Centralblatt fuer Electrotechnik, Vol.  xi.  p.  215,
+1889).  There is an abstract of the paper also in Wiedemann's
+Beiblaetter (1889, vol.  xiii.  P.  711).  Uppenborn examined the
+samples of paper under normal conditions as to moisture and obtained
+the following results:-
+
+Description of Paper
+
+I
+
+Pressure Intensity
+
+II.
+
+Specific Resistance corresponding to pressures as in Column I. Ohms.
+
+III
+
+Pressure Intensity.
+
+IV.
+
+Specific Resistance corresponding to Column III. Ohms.
+
+Common cardboard 2.3 mm. thick
+
+0.05 kilo. per 6000 sq. mm.
+
+4.85 x 1015
+
+20 kg. per 6000 sq. mm.
+
+4.7 x 1014
+
+Gray paper, 0.26 mm. thick
+
+0.05 kilo. per 5000 sq. mm.
+
+3.1 x 10^15
+
+20 kg. per 5000 sq. mm.
+
+8 x 1014
+
+Yellow parchment paper-09 mm. thick
+
+0.05 kilo. per 5300 sq. mm.
+
+3.05 x 1016
+
+20 kg. per 5300 sq. mm.
+
+8 x 1016
+
+Linen tracing cloth
+
+0.05 kilo. per 6000 sq. mm.
+
+1.35 x 1016
+
+20 kg. per 33,000 sq. mm.
+
+1.86 x 10^15
+
+Sec. 111.  Paraffined Paper.
+
+Like wood and other semiconductors, paper can be vastly improved as an
+insulator by saturating it with melted paraffin.  To get the best
+results a pure paper free from size must be employed--gray Swedish
+filter paper does well.  This is dried at a temperature above 100 deg. C.
+for, say, half an hour, and the sheets are then floated on the top of
+paraffin, kept melted at 140 deg. C.  or thereabout in a baking dish.  As
+soon as the paper is placed upon the melted paraffin the latter begins
+to soak through, in virtue of capillary action, and drives before it
+the air and moisture, causing a strongly marked effervescence.
+
+After about one minute the paper may be thrust below the paraffin to
+soak.  When a sufficient number of papers have accumulated, and when
+no more gas comes off, the tray may be placed in a vacuum box (Fig.
+85), and the pressure reduced by the filter pump.  As the removal of
+the air takes time, provision must be made for keeping the bath hot.
+
+A vacuum may be maintained for about an hour, and air then readmitted.
+Repeated exhaustions and readmissions of air, which appear to improve
+wood, do not give anything like such a good result with paper.  In
+using a vacuum box provision must be made in the shape of a cool
+bottle between the air pump and the box.  If this precaution be
+omitted, and if any paraffin splashes on to the hot surface of the
+box, it volatilises with decomposition and the products go to stop up
+the pump.  Paraffin with a melting-point of 50 deg. C. or upwards does
+well.
+
+The bath should be allowed to cool to about 60 deg. C.  before the papers
+are removed, so that enough paraffin may be carried out to thoroughly
+coat the paper and prevent the entrance of air.
+
+Fig.  85.
+
+Fig.  85 is a section of a vacuum vessel which has been found very
+convenient.  It measures about two feet in diameter at the top.  It is
+round, because it is much easier to turn one circular surface than to
+plane up four surfaces, which has to be done if the box is square.
+Both the rim of the vessel and the approximating part of the cover
+require to be truly turned and smoothly finished.  A very good packing
+is made of solid indiarubber core about half an inch thick.  This is
+carefully spliced--cemented by means of a solution of rubber in
+naphtha, and the splice sewed by thick thread.  The lid ought to have
+a rim fitting inside the vessel, for this keeps the rubber packing in
+place;  the rim has been accidentally omitted in Fig.  85.  The bolts
+should not be more than five inches apart, and should lie at least
+half an inch in diameter, and the rim and lid should be each half an
+inch thick.
+
+Condensers may now be built up of sheets of this prepared paper
+interleaved with tin-foil in the ordinary way.  If good results are
+required, the condenser when finished is compressed between wooden or
+glass end-pieces by means of suitable clamps.  It can then be put in a
+box of melted paraffin, heated up to 140 deg. C, and exhausted by means
+of the water pump for several hours.
+
+In this process the air rushes out from between the paper and foils
+with such vehemence that the paraffin is generally thrown entirely out
+of the box.  To guard against this the box must be provided with a
+loosely fitting and temporary lid, pierced with several holes.
+
+The real test as to when exhaustion is complete would be the cessation
+of any yield of air or water.  Since it is not generally convenient to
+make the vacuum box so air-tight that there are absolutely no leaks at
+all, and as the paraffin itself is, I think, inclined to "crack"
+slightly at the temperature of 140 deg. C, this test or criterion cannot
+be conveniently applied.
+
+Two exhaustions, each of about two hours' duration, have, however, in
+my experience succeeded very well, provided, of course, that the
+dielectric is prepared as suggested.  At the end of the exhaustion
+process the clamping screws are tightened as far as possible, the
+condenser remaining in its bath until the paraffin is pasty.
+
+Condensers made in this way resist the application of alternating
+currents perfectly, as the following tests will show.  The dielectric
+consisted of about equal parts of hard paraffin and vaseline.  A
+condenser of about 0.123 microfarads capacity and an insulation
+resistance of 2000 megohms, [Footnote: As tested by a small voltage.]
+having a tin-foil area of 4.23 square metres (about), and double
+papers each about 0.2 mm. thick, designed to run at 2000 volts with a
+frequency of 63 complete periods, was tested at this frequency.
+
+The condenser was thoroughly packed all round in cotton-wool to a
+thickness of 6 inches, and its temperature was indicated more or less
+by a thermometer plunged through a hole in the lid of the containing
+box and of the condenser box, and resting on the upper surface of one
+set of tin-foil electrodes, from which the soft paraffin mixture had
+been purposely scraped away.  The following were the results of a four
+hours' run at a voltage 50 per cent higher than that for which the
+condenser was designed--i.e.  3000 volts.
+
+Times. Voltage Temperature   Temperature  Difference
+              in Condenser.   in Air.
+
+Hrs.  Min.
+
+2    10  2750    22.8 deg. C.      23.0 deg. C.      + 0.2 deg.
+
+3    10  2700    23.0 deg. C.      23.3 deg. C.      + 0.3 deg.
+
+3    18  3200    23.1 deg. C.      23.0 deg. C.      -0.1 deg.
+
+4    10  3200    23.3 deg. C.      23.7 deg. C.      + 0.4 deg.
+
+5    10  3100    23.6 deg. C.      23.4 deg. C.      -0.2 deg.
+
+6    10  3000    23.8 deg. C.      23.35 deg. C.     -0.45 deg.
+
+An idea of the order of the amount of waste may be formed from the
+following additional experiment.
+
+A condenser similar to the one described was filled with oil of a low
+insulating power.  It was tested calorimetrically, and also by the
+three voltmeter method, which, however, proved to be too insensitive.
+The temperature rise in the non-conducting box in air was about 0.3 deg.
+C.  per hour, and the loss of power was found to be less than 0.1 per
+cent.  In the present case the actual rise was only 1 deg. in four hours,
+and the integral give and take between the condenser and the air is
+practically nothing; consequently we may consider with safety that the
+rate of rise is certainly less than 1 degree per three hours.  The
+voltage and frequency were about the same in both experiments,
+consequently the energy passed is about proportional to the capacity
+used in the two experiments.
+
+From this it follows that since the specific heat of both condensers
+was the same (nearly), the loss in the present case is a good deal
+less than one-tenth per cent.  The residual charge is also much less
+than when the condenser is simply built up of paper paraffined in an
+unsystematic manner, and from which the air and water have been
+imperfectly extracted, as by baking the condenser first, and then
+immersing it in paraffin or oil.
+
+It is usual to consider that the phenomena of residual charge and
+heating in condensers, to which alternating voltages are applied, are
+closely allied.  This is true, but the alliance is not one between
+cause and effect--at all events, with regard to the greater part of
+the heating.  The imperfect exclusion of air and moisture,
+particularly the latter, certainly increases the residual charge by
+allowing surface creeping to occur;  but it also acts directly in
+producing heating, both by lowering the insulation of the condenser
+and by allowing of air discharges between the condenser plates.
+
+Of these causes of heating, the discharges in air or water vapour are
+probably the more important.  Long ago a theory of residual charge was
+given by Maxwell, based on the consideration of a laminated
+dielectric, the inductivity and resistance of which varied from layer
+to layer.  It was shown that such an arrangement, and hence generally
+any want of homogeneity in a direction inclined to the lines of force
+leading to a change of value of the product of specific resistance and
+specific inductive capacity, would account for residual charge.
+
+This possible explanation has been generally accepted as the actual
+explanation, and many cases of residual charge attributed to want of
+homogeneity, which are certainly to be explained in a simpler manner.
+For instance, the residual charge in a silvered mica plate condenser,
+carefully dried, can be increased at least tenfold by an exposure of a
+few minutes to ordinarily damp air.  The same result occurs with
+condensers of paraffined or sulphured paper;  and these are the
+residual changes generally observed.  The greater part must be due to
+creeping.
+
+Sec. 112.  Paraffin.
+
+This substance has long enjoyed great popularity in the physical
+laboratory.  Its specific resistance is given by Ayrton and Perry as
+more than 1025, but it is probably much higher in selected samples.
+The most serviceable kind of paraffin is the hardest obtainable,
+melting at a temperature of not less than 52 deg. C.  It is a good plan to
+remelt the commercial paraffin and keep it at a temperature of, say,
+120 deg. C.  for an hour, stirring it carefully with a glass rod so that
+it does not get overheated;  this helps to get rid of traces of water
+vapour.
+
+Hard paraffin, when melted, has an enormous rate of expansion with
+temperature, so great, indeed, that care must be taken not to overfill
+the vessels in which it is to be heated.  Castings can only be
+prepared by cooling the mould slowly from the bottom, keeping the rest
+of the mould warm, and adding-paraffin from time to time to make up
+for the contraction.  The cooling is gradually allowed to spread up to
+the free surface.
+
+The chief use of paraffin in the laboratory is in the construction of
+complicated connection boards, which are easily made by drilling holes
+in a slab of paraffin, half filling them with mercury, and using them
+as mercury cups.
+
+Since paraffin is a great collector of dust, it should be screened by
+paper, otherwise the blocks require to be scraped at frequent
+intervals, which, of course, electrifies them considerably.  This
+electrification is often difficult to remove without injuring the
+insulating power of the paraffin.  A light touch with a clean Bunsen
+flame is the readiest method, and does not appear to reduce the
+insulation so much as might be expected.  The safest way, however, is
+to leave the key covered by a clean cloth, which, however, must not
+touch the surface, for a sufficient time to allow of the charges
+getting away.
+
+The paraffin often becomes electrified itself by the friction of the
+key contacts, so that in electrometer work it is often convenient to
+form the cups by lining them with a little roll of copper foil twisted
+up at the bottom. In this case the connecting wires should, of course,
+be copper.  Small steel staples are convenient for fastening the
+collecting wires upon the paraffin;  or, in the case where these wires
+have to be often removed and changed about, drawing-pins are very
+handy.
+
+With mercury cups simply bored in paraffin great trouble will often be
+experienced in electrometer work, owing to a potential difference
+appearing between the cups--at all events when the contacts are
+inserted and however carefully this be done.  A few drops of very pure
+alcohol poured in above the mercury often cures this defect.  The
+surface of paraffin is by no means exempt from the defect of losing
+its insulating power when exposed to damp air, but it is not so
+sensitive as glass, nor does the insulating power fall so far.
+
+Two useful appliances are figured.
+
+Fig.  86.                           Fig.  87.
+
+One, in which paraffin appears as a cement, is an insulating stand
+made out of a bit of glass or ebonite tube cemented into an Erlenmeyer
+flask, having its neck protected from dust when out of use by a rubber
+washer, the other a "petticoat" insulator made by cementing a flint
+glass bottle into a glass dish with paraffin.  In course of time the
+paraffin will be found to have separated from the glass.  When this
+occurs the apparatus may be melted together again by placing it on
+the water bath for a few minutes.
+
+Sec. 113.  Vaseline, Vaseline Oil, and Kerosene Oil.
+
+These, when dry, insulate almost, but not quite as well as solid
+paraffin.  H. Koeller (Wien Berichte, 98, ii.  201, 1889; Beibl. Wied.
+Ann. 1890, p. 186), working with very small voltages, places the
+final(?) specific resistance of commercial petroleum, ether, and
+vaseline oil at about 2 X 1027 C.G.S.  This is ten times higher than
+the value assigned to commercial benzene (C6H6), and a hundred times
+higher than the value for commercial toluene.
+
+All these numbers mean little or nothing, but the petroleum and
+vaseline oil were the best fluid insulators examined by Koeller.  By
+mixing vaseline with paraffin a soft wax may be made of any desired
+degree of softness, and by dissolving vaseline in kerosene an
+insulating liquid of any degree of viscidity may be obtained.
+
+Hard paraffin may be softened somewhat by the addition of kerosene,
+and an apparently homogeneous mass cast from the mixture.  It will be
+found, however, that in course of time the kerosene oozes out, unless
+present in very small quantity.  Koeller has found (loc. cit.) that
+some samples of vaseline oil conducted "vollstaendig gut," but I have
+not come across such samples.  Vaseline oil, however, is sold at a
+price much above its value for insulating purposes.
+
+Kerosene oil is best obtained dry by drawing it directly from a new
+tin and exposing it to air as little as possible.  Of course, it may
+be dried by chemical means and distillation, but this is usually (or
+always) unnecessary.
+
+Fig 88.
+
+There is some danger of kerosene containing minute traces of sulphuric
+acid, and it and other oils may be conveniently tested for insulation
+in the following manner.  The quartz electroscope is taken, and the
+insulating rod heated in the blow-pipe.  The electroscope will now
+insulate well enough to show no appreciable collapse of the leaves in
+one or two hours' time.  Upon the plate of the electroscope is put a
+platinum or copper cylinder, and this is filled with kerosene (say) up
+to a fixed mark.
+
+The electroscope is placed on a surface plate, or, at all events, on a
+sheet of plate glass, and a "scribing block" is placed along side it
+and the scriber adjusted to dip into the kerosene to any required
+depth.  This is done by twisting a bit of wire round the scribing
+point and allowing it to project downwards.  The point itself serves
+to give an idea of the height to which the vessel may be filled.  The
+liquid is adjusted till its surface is in contact with the end of the
+scribing point, and the wire then projects into the liquid and forms
+an electrode of constant area of surface.  The scribing block is put
+to earth.  A charge is given to the electroscope, and the time
+required for a given degree of collapse of the leaves noted.
+
+The kerosene is then removed and its place taken by vaseline or
+paraffin, known to insulate well as a standard for comparison.  The
+experiment is then repeated, and the time noted for the same degree of
+collapse.  This test, though of course rough, is generally quite
+sufficient for workshop purposes, and is easily applied.  Moreover, it
+does not require correction for electrometer leakage, as generally
+happens when more elaborate appliances are used.
+
+The actual resistance of insulating oils depends so much on the
+electrical intensity, on the duration of that intensity, and on the
+previous history of the oil as to the direction of the voltage to
+which it has been subjected--to say nothing of the effects of traces
+of moisture--that quantitative experiments are of no value unless
+they are extremely elaborate.  I shall therefore only append the
+following numbers due to Bouty, Ann.  de Chemie et de Physique (6),
+vol. xxvii. p. 62, 1892, in which the effect of the conductivity on
+the determination of the specific inductive capacity was properly
+allowed for:-
+
+Carbon
+
+Bisulphide.
+
+Turpentine.
+
+Benzene (C6H6) at 20 deg. C.
+
+Benzene at
+
+60 deg. C.
+
+Specific inductive capacity
+
+2.715
+
+2.314
+
+2.21
+
+2.22
+
+Specific resistance in ohms per cubic centimetre
+
+1.5 x 1013,
+
+1.75 x 1012
+
+1.56 x 1011
+
+7.9 x 1011
+
+[Footnote: Professor J.  J.  Thomson, and Newall (Phil.  Proc.  1886)
+consider that carbon bisulphide showed traces of a "residual charge"
+effect;  hence, until this point is cleared up, we must regard Bouty's
+value as corresponding only to a very short, but not indefinitely
+short, period of charge.  On this point the paper must be consulted.
+
+March 1897--The writer has investigated this point by an independent
+method, but found no traces of "residual charge."]
+
+Information as to the specific inductive capacity of a large number of
+oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a
+paper by Quincke in Wiedemann's Annalen, 1883.
+
+Sec. 114.  Imperfect Conductors.
+
+Under this heading may be grouped such things as wood, slate, marble,
+etc--in fact, materials generally used for switchboard insulation.
+An examination of the insulating power of these substances has
+recently been made by B.  O.  Peirce (Electrical Review, 11th January
+1895) with quite sufficient accuracy, having in view the impossibility
+of being certain beforehand as to the character of any particular
+sample.  The tests were made by means of holes drilled in slabs of the
+material to be examined.  These holes were three-eighths of an inch in
+diameter, and from five-eighths to three-quarters of an inch deep, and
+one inch apart, centre to centre.  A voltage of about 15 volts was
+employed.  The following general results were arrived at:-
+
+(1) Heating in a paraffin bath always increases the resistance of
+wood, though only slightly if the wood be hard and dense.
+
+(2) Frequent exhaustion and readmission of air above the surface of
+the paraffin always has a good effect in increasing the resistance of
+wood.
+
+(3) When wood is once dry, impregnating it with paraffin tends to keep
+it dry.
+
+(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot
+be entirely waterproofed in this way.
+
+(5) The resistance of wood with stream lines along the grain is twenty
+to fifty per cent lower than when the stream lines cross the grain.
+
+(6) The "contact" resistance between slabs of wood pressed together is
+always very high.
+
+The following table will sufficiently illustrate the results obtained.
+The stone was dried in the sun for three weeks in the summer (United
+States), and the wood is described as having been well seasoned:-
+
+CURRENT WITH THE GRAIN
+
+Lowest Resistance  Highest Resistance Lowest Specific Highest Specific
+between two Cups   between two Cups   Resistance in   Resistance in
+in Megohms.        in Megohms.        Megohms.        Megohms.
+
+
+
+
+Ash.
+
+    550               920                 380             700
+
+Cherry
+
+   1100              4000                2800            6000
+
+Mahogany
+
+    430               730                 310             610
+
+Oak
+
+    220               420                1050            2200
+
+Pine.
+
+    330               630                  360            1470
+
+Hard pine.
+
+     10                48                   17            1050
+
+Black walnut
+
+   1100              3000                  320            2100
+
+Red fibre
+
+      2                 4                    3              60
+
+Slate
+
+                                           184             280
+
+Soapstone.
+
+                                           330             500
+
+White marble
+
+                                          2000            8800
+
+
+Sec. 115.  As to working the materials very little need be said.
+
+Fibre is worked like wood, but has the disadvantage of rapidly taking
+the edge off the tools.  In turning it, therefore, brass-turning
+tools, though leaving not quite such a perfect finish as wood-turning
+tools, last much longer, and really do well enough.  Fibre will not
+bear heating much above 100 deg.C--at all events in paraffin.  At 140 deg.
+C.  it becomes perfectly brittle.  Its chief merit lies in its great
+strength.  So far as insulation is concerned, Mr. Peirce's experiments
+show that it is far below most kinds of wood.
+
+Slate. This is a vastly more useful substance than it is generally
+credited with being.  It is very easily worked at a slow speed, either
+on the shaping machine or on the lathe, with tools adjusted for
+cutting brass, and it keeps its figure, which is more than can be said
+for most materials.  It forms a splendid base for instruments,
+especially when ground with a little emery by iron or glass grinders,
+fined with its own dust, and French polished in the ordinary way.
+Spools for coils of considerable radial dimension may be most
+conveniently made of slate instead of wood or brass, both because it
+keeps its shape, and because it insulates sufficiently well to stop
+eddy currents--at all events, sufficiently for ordinary practice.  An
+appreciable advantage is that slate may be purchased at a reasonable
+rate in large slabs of any desired thickness.  It is generally cut in
+the laboratory by means of an old cross-cut saw, but it does not do
+much damage to a hard hack saw such as is used for iron or brass.
+
+Marble. According to Holtzapffell, marble may be easily turned by
+means of simple pointed tools of good steel tempered to a straw
+colour.  The cutting point is ground on both edges like a wood-turning
+tool, and held up to the work "at an angle of twenty or thirty
+degrees" (?with the horizontal).  The marble is cut wet to save the
+tool.  As soon as the point gets, by grinding, to be about one-eighth
+of an inch broad it must either be drawn down or reground;  a flat
+tool will not turn marble at all.
+
+A convenient saw for marble is easily made on the principle of the
+frame saw.  A bit of hoop iron forms a convenient blade, and is
+sharpened by being hammered into notches along one edge, using the
+sharp end of a hammer head.  The saw is liberally supplied with sand
+and water--or emery and water, where economy of time is an object.
+The sawing of marble is thus really a grinding process, but it goes on
+rapidly.  Marble is ground very easily with sand and water;  it is
+fined with emery and polished with putty powder, which, I understand,
+is best used with water on cloth or felt.  As grinding processes have
+already been fully described, there is no need to go into them here.
+I have no personal knowledge of polishing marble.
+
+Sec. 116.  Conductors.
+
+The properties of conductors, more particularly of metals, have been
+so frequently examined, that the literature of the subject is
+appallingly heavy.  In what follows I have endeavoured to keep clear
+of what might properly appear in a treatise on electricity on the one
+hand, and in a wiring table on the other.  The most important work on
+the subject of the experimental resistance properties of metals has
+been done by Matthieson, Phil. Trans. 1860 and 1862, and British
+Association Reports (1864); Callender, Phil. Trans. vol. clxxiii;
+Callender and Griffiths, Phil. Trans. vol.  clxxxii;   The Committee
+of the British Association on Electrical Standards from 1862 to
+Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi.  (1893);
+
+Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p.
+838;  Feussner and St. Lindeck, Zeitsch. fuer Inst. 'Kunde, ix. 1889,
+p. 233, and B. A. Reports, 1892, p.  139.  Of these, Matthieson, and
+Dewar and Fleming treat of resistance generally, the latter
+particularly at low temperatures.
+
+[Footnote: The following is a list of Dr. Matthieson's chief papers on
+the subject of the electrical resistance of metals and alloys:  Phil.
+Mag. xvi. 1858, pp. 219-223;  Phil. Trans. 1858, pp. 383-388 Phil.
+Trans. 1860, pp. 161-176;  Phil. Trans. 1862, pp. 1-27 Phil. Mag. xxi.
+(1861), pp. 107-115;  Phil. Mag. xxiii. (1862), pp. 171-179;
+Electrician, iv. 1863, pp. 285-296;  British Association Reports,
+1863, p. 351.]
+
+Matthieson, and Matthieson and Hockin, Klemencic, Feussner, and St.
+Lindeck deal with the choice of metals for resistance standards.
+Callender's, and Callender and Griffiths' work is devoted to the study
+of platinum for thermometric purposes.
+
+The bibliography referring to special points will be given later.  The
+simplest way of exhibiting the relative resistances of metals is by
+means of a diagram published by Dewar and Fleming (loc. cit.), which
+is reproduced on a suitable scale on the opposite page.  For very
+accurate work, in which corrections for the volumes occupied by the
+metals at different temperatures are of importance, the reader is
+referred to the discussion in the original paper, which will be found
+most pleasant reading.  From this diagram both the specific resistance
+and the temperature coefficient may be deduced with sufficient
+accuracy for workshop purposes.  In interpreting the diagram the
+following notes will be of assistance.  The diagram is drawn to a
+scale of so-called "platinum temperatures"--that is to say, let R0,
+R100, Rt be the resistances of pure platinum at 0 deg., 100 deg., and t deg. C.
+respectively, then the platinum temperature pt is defined as
+
+pt = 100 X (Rt-R0)/(R100-R0)
+
+This amounts to making the temperature scale such that the temperature
+at any point is proportional to the resistance of platinum at that
+point.  Consequently on a resistance temperature diagram the straight
+line showing the relation between platinum resistance and platinum
+temperature will "run out" at the platinum absolute zero, which
+coincides more or less with the thermodynamic absolute zero, and also
+with the "perfect gas" absolute zero.  Platinum temperatures may be
+taken for workshop purposes over ordinary ranges as almost coinciding
+with air thermometer temperatures.  The metals used by Professors
+Dewar and Fleming were, with some exceptions, not absolutely pure, but
+in general represent the best that can be got by the most refined
+process of practical metallurgy.  We may note further that the
+specific resistance is only correct for a temperature of about 15 deg. C,
+since no correction for the expansion or contraction of material has
+been applied.
+
+The following notes on alloys suitable for resistance coils will
+probably be found sufficient.
+
+Sec. 117.  Platinoid.
+
+This substance, discovered by Martino and described by Bottomley
+(Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and
+1 per cent to 2 per cent of tungsten, but I have not been able to
+obtain an analysis of its exact composition.  It appears to be
+difficult to get the tungsten to alloy, and it has to be added to part
+of the copper as phosphide of tungsten, in considerably greater
+quantity than is finally required.  The nickel is added to part of the
+copper and the phosphide of tungsten, then the zinc, and then the rest
+of the copper.  The alloy requires to be remelted several times, and a
+good deal of tungsten is lost by oxidation.
+
+The alloy is of a fine white colour, and is very little affected by
+air--in fact, it is to some extent untarnishable.  The specific
+resistance will be seen to be about one and a half times greater than
+that of German silver, and the temperature coefficient is about 0.021
+per cent per degree C.  (i.e.  about nineteen times less than copper,
+and half that of German silver).  To all intents and purposes it may
+be regarded as German silver with 1 per cent to 2 per cent of
+tungsten.  It does not appear to have been particularly examined for
+secular changes of resistance.
+
+118.  German Silver.  This material has been exhaustively examined of
+late years by Klemencic and by Feussner and St. Lindeck.  Everybody
+agrees that German silver, as ordinarily used for resistances, and
+composed of copper four parts, zinc two parts, nickel one part, is
+very ill-fitted for the purpose of making resistance standards.  This
+is due
+(1) to its experiencing a considerable increase in resistance on
+winding.  Feussner and St.  Lindeck found an increase of 1 per cent
+when German silver was wound on a core of ten wire diameters.
+
+(2) To the fact that the change goes on, though with gradually
+decreasing rate, for months or years;
+
+(3) to the fact that the resistance is permanently changed (increased)
+by heating to 40 deg. C.  or over.  By  "artificially ageing" coils of
+German silver by heating to 150 deg. C, say for five or six hours, its
+permanency is greatly improved, and it becomes fit for ordinary
+resistance coils where changes of, say, 1/5000 do not matter.
+
+It is a remarkable property of all nickel alloys containing zinc that
+their specific resistance is permanently increased by heating, whereas
+alloys which do not contain zinc suffer a change in the opposite
+direction.  The manufacturers of German silver appear to take very
+little care as to the uniformity of the product put on the market;
+some so-called German silver is distinctly yellow, while other samples
+are bright and white.
+
+It is noted by Price (Measurements of Electrical Resistance, p. 24)
+that German silver wire is apt to exhibit great differences of
+resistance within quite short lengths.  This has been my own
+experience as well, and is a great drawback to the use of German
+silver in the laboratory, for it makes it useless to measure off
+definite lengths of wire with a view to obtaining an approximate
+resistance.  In England German silver coils are generally soaked in
+melted hard paraffin.  In Germany, at all events at the Charlottenburg
+Institute, according to St.  Lindeck--coils are shellac-varnished and
+baked.  In any case it appears to be essential to thoroughly protect
+the metal against atmospheric influence.
+
+Sec. 119.  Platinum Silver.
+
+In the opinion of Matthieson and of Klemencic the 10 per cent silver,
+90 per cent platinum alloy is the one most suitable for resistance
+standards.  At all events, it has stood the test of time, for, with
+the following exceptions, all the British Association coils
+constructed of it from 1867 to the present day have continued to agree
+well together.  The exceptions were three one-ohm coils, which
+permanently increased between 1888 and 1890, probably through some
+straining when immersed in ice.  One coil changed by 0.0006 in 1
+between the years 1867 and 1891.  According to Klemencic, absolute
+permanency is not to be expected even from this alloy.
+
+Its recommendation as a standard depends on its chemical inertness,
+its small temperature coefficient (0.00027 per degree), and its small
+thermo-voltage against copper, as the following table (taken from
+Klemencic) will show:-
+
+Thermo-voltages in Micro-volts per degree against Copper
+          over the Range 0 deg. to 17 deg. C.
+
+Platinum iridium           7.14 micro-volts per degree C.
+
+Platinum silver            6.62 micro-volts per degree C.
+
+Nickelin                  28.5  micro-volts per degree C.
+
+German silver             10.43 micro-volts per degree C.
+
+Manganin (St.  Lindeck)    1.5  micro-volts per degree C.
+
+Mechanically, the platinum silver is weak, and is greatly affected as
+to its resistance by mechanical strains--in fact, Klemencic considers
+it the worst substance he examined from this point of view--a
+conclusion rather borne out by Mr. Glazebrook's experience with the
+British Association standards already referred to (B. A. Reports, 1891
+and 1892).
+
+Taking everything into account, it will probably be well to construct
+standards either with oil insulation only, or to bake the coils in
+shellac before testing, instead of soaking in paraffin.  Fig. 89
+illustrates a form of an oil immersed standard which is in use in my
+laboratory, and through which a considerable current may be passed.
+The oil is stirred by means of a screw propeller.
+
+Fig.  89.
+
+Fig.  89 represents a standard resistance for making Clerk cell
+comparisons by the silver voltameter method.  The framework on which
+the coils are wound consists of a base and top of slate.  The pillars
+are of flint glass tube surrounding brass bolts, and cemented to the
+latter by raw shellac.  Grooves are cut in the glass sleeves to hold
+the wires well apart.  These grooves were cut by means of a file
+working with kerosene lubrication.  A screw stirrer is provided, and
+the whole apparatus is immersed in kerosene in the glass box of a
+storage cell.  The apparatus is aged to begin with by heating to a
+temperature a good deal higher than any temperature it is expected to
+reach in actual work.  After this the rigidity of the frame is
+intended to prevent any further straining of the wire.  The apparatus
+as figured is not intended to be cooled to 0 deg. C, so that it is put in
+as large a box as possible to gain the advantage of having a large
+volume of liquid.  The top and bottom slates measure seven inches by
+seven inches, and the distance between them is seven inches.  The
+inner coil is wound on first, and the loop which constitutes the end
+of the winding is brought up to a suitable position for adjustment.
+The insulation of the heavy copper connectors is by means of ebonite.
+
+Sec. 120.  Platinum Iridium.
+
+Platinum 90 per cent, iridium 10 per cent.  This material was prepared
+in some quantity at the cost of the French Government, and distributed
+for test about 1886.  Klemencic got some of it as representing
+Austria, and found it behaved very like the platinum silver alloy just
+discussed.  The temperature coefficient is, however, higher than for
+platinum silver (0.00126 as against 0.00027).  The mechanical
+properties of the alloy are, however, much better than those of the
+silver alloy; and in view of the experience with B.  A.  standards
+above quoted, it remains an open question whether, on the whole, it
+would not be the better material for standards, in spite of its
+higher price.  Improvements in absolute measurements of resistance,
+however, may render primary standards superfluous.
+
+Sec. 121.  Manganin.
+
+Discovered by Weston--at all events as to its application to
+resistance coils.  A glance at the diagram will exhibit its unique
+properties, on account of which it has been adopted by the
+Physikalisch Technischen Reichsanstalt for resistance standards.  The
+composition of the alloy is copper 84 per cent, manganese 12 per cent,
+nickel 4 per cent, and it is described as of a steel-gray colour.
+Unfortunately it is apt to oxidise in the air, or rather the manganese
+it contains does so, so that it wants a very perfect protection
+against the atmosphere.
+
+Like German silver, manganin changes in resistance on winding, and
+coils made of it require to be artificially aged by heating to 150 deg.
+for five hours before final adjustment.  The annealing cannot be
+carried out in air, owing to the tendency to oxidation.  The method
+adopted by St.  Lindeck (at all events up to 1892) is to treat the
+coil with thick alcoholic shellac varnish till the insulation is
+thoroughly saturated, and then to bake the coil as described.  The
+baking not only anneals the wire, but reduces the shellac to a hard
+and highly insulating mass.
+
+Whether stresses of sufficient magnitude to produce serious mechanical
+effects can be set up by unequal expansion of wire and shellac during
+heating and cooling is not yet known, but so far as tested (and it
+must be presumed that the Reichsanstalt tests are thorough) no
+difficulty seems to have been met with.  In course of time, however,
+probably the best shellac coating will crack, and then adieu to the
+permanency of the coil!  This might, of course, be obviated by keeping
+the coil in kerosene, which has no action on shellac, but which
+decomposes somewhat itself.
+
+The method of treatment above described suffices to render coils of
+manganin constant for at least a year (in 1892 the tests had only been
+made for this time) within a few thousands per cent.  Manganin can be
+obtained in sheets, and from this material standards of 10-2, 10-3,
+and 10-4 ohms are made by soldering strips between stout copper bars,
+and these are adjusted by gradually increasing their resistance by
+boring small holes through them. The solder employed is said to be
+"silver."
+
+Mr. Griffiths (Phil.  Trans.  vol.  clxxxiv.  [1893], A, p.  390) has
+had some experience with manganin carrying comparatively heavy
+currents, under which circumstances its resistance when immersed in
+water was found to rise in spite of the varnish which coated it.
+Other experiments in which the manganin wire was immersed in paraffin
+oil did not exhibit this effect, though stronger currents were passed.
+
+On the whole, manganin appears to be the best material for coil boxes
+and "secondary" resistance standards.  Whether it is fit to rank with
+the platinum alloys as regards permanency must be treated as an open
+question.
+
+Sec. 122.  Other Alloys.
+
+The following tables, taken from the work of Feussner and St.
+Lindeck, Zeitschrift fuer Instrumenten Kunde, 1889, vol.  ix.  p.
+233, together with the following notes, will suffice.
+
+Sec. 123.  Nickelin.
+
+This is only German silver with a little less zinc, a little more
+nickel, and traces of cobalt and manganese.  It behaves like German
+silver, but is an improvement on the latter in that all the faults of
+German silver appear upon a reduced scale in nickelin.
+
+Sec. 124.  Patent Nickel.
+
+Practically a copper nickel alloy, used to some extent by Siemens
+and Halske.  It stands pretty well in the same relation to nickelin as
+the latter does to German silver.  After annealing as for manganin it
+can be made into serviceable standards which do not change more than a
+few thousandths per cent.  I have not come across a statement of its
+thermo-voltage against copper.
+
+Sec. 125.  Constantin.
+
+Another nickel copper alloy containing 50 per cent of each
+constituent.  It appears to be a serviceable substance, having a
+temperature coefficient of 0.003 per cent per degree only, but an
+exceedingly high thermo-voltage, viz.  40 micro-volts per degree
+against copper.
+
+         1        2     3       4        5         6     7       8
+       German  Nickelin made    Rheo-    Patent Nickel   Manga-  Nickel
+       Silver  by Obermaier     tane                     nese    Manga-
+               Dia-     Dia-             Dia-   Dia-     Copper  nese
+               meter    meter            meter  meter            Copper
+               1.0mm    0.1mm            0.6mm  1.0mm
+
+
+Copper 60.16    61.63    54.57  53.28     74.41  74.71    70      73
+
+Zinc   25.37    19.67    20.44  16.89     0.23   0.52     ...     ...
+
+Tin    ...      ...      ...    ...       trace  ...      ...
+
+Nickel 14.03    18.46    24.48  25.31     25.10  24.14    ...     3
+
+Iron   0.30     0.24     0.64   4.46      0.42   0.70     ...     ...
+
+Cobalt trace    0.19     ...    ...       trace  trace    ...     ...
+
+Mang-  trace    0.18    0.27    0.37      0.13   0.17     30      24
+anese.
+
+     99.86     100.37  100.40  100.31    100.24 100.24  ...      ...
+
+Specific
+resistance
+     30.0     33.2     44.8    52.5      34.2    32.8    100.6   47.7
+
+Temperature
+coefficient
+     0.00036 0.00030 0.00033 0.00041 0.00019  0.00021  0.00004  0.00003
+
+The specific resistance is in microhms, i.e. 10-6 ohms per cubic
+centimetre, and the temperature coefficient in degrees centigrade.
+
+
+126.  Nickel Manganese Copper.
+
+I can find no other reference with regard to this alloy mentioned by
+Lindeck.  Nicholls, however (Silliman's Journal [3], 39, 171, 1890),
+gives some particulars of alloys of copper and ferromanganese.  The
+following table is taken from Wiedemann's Beiblatter (abstract of
+Nicholl's paper, 1890, p.  811).  All these alloys appear to require
+annealing at a red heat before their resistances are anything like
+constant.
+
+Let x be percentage of copper, then 100--x is percentage of
+"ferromanganese."
+
+Values of x.  100   99.26  91 .88  86.98  80.4  70.65
+
+Specific
+resistance
+with respect
+to copper
+(?  pure)       1  1.19    11.28   20.4   27.5  45.1
+
+Temperature
+coefficient
+per degree
+x 10^6(hard) 3202  2167    138     16      22   -24
+
+Ditto (soft) ...   ...     184     80      66    21
+
+If nickel is added, alloys of much the same character are obtained,
+some with negative temperature coefficients--for instance, one
+containing 52.51 per cent copper, 31.27 per cent ferromanganese, and
+16.22 nickel.
+
+A detailed account of several alloys will be found in a paper by
+Griffiths (Phil.  Trans.  1894, p.  390), but as the constants were
+determined to a higher order of accuracy than the composition of the
+material--or, at all events, to a higher degree  of accuracy than
+that to which the materials can be reproduced--there is no advantage
+in quoting them here.
+
+CHAPTER IV
+
+ELECTROPLATING AND ALLIED ARTS
+
+Sec. 127.  Electroplating.
+
+This is an art which is usually deemed worthy of a treatise to itself,
+but for ordinary laboratory purposes it is a very simple matter--so
+simple, indeed, that the multiplicity of receipts as given in
+treatises are rather a source of embarrassment than otherwise.
+
+The fundamental principles of the art are:-
+
+(1) Dirty work cannot be electroplated.
+
+(2) Electroplated surfaces may be rougher, but will not be smoother
+than the original unplated surface.
+
+(3) The art of electroplating being in advance of the science, it is
+necessary to be careful as to carrying out instructions in detail.
+This particularly applies to the conditions which determine whether a
+metallic deposit shall come down in a reguline or in a crystalline
+manner.
+
+Sec. 128.  The Dipping Bath.
+
+An acid dipping bath is one of the most useful adjuncts to the
+laboratory, not only for cleansing metals for electroplating, but for
+cleaning up apparatus made out of bits of brass tube and sheet, and
+particularly for quickly cleaning binding screws, etc, where it is
+necessary to ensure good electrical contact.
+
+The cheapest and most satisfactory way in the end is to make up two or
+three rather large baths to begin with.  The glass boxes of storage
+batteries do very nicely for the purpose, and being generally ground
+pretty flat at the top, they may be covered by sheets of patent plate
+glass, and thus preserved from the action of the air.
+
+First Bath. A 30 or 40 per cent solution of commercial caustic soda.
+Objects may be cleansed from grease in this bath by heating them as
+hot as is consistent with individual circumstances, and plunging them
+into it.
+
+It is a considerable advantage to begin by removing grease from
+articles subsequently to be dipped in an acid bath, both because it
+saves time and acid, and because more uniform results are obtainable
+when this is done than when it is omitted.  It is a great advantage to
+have the caustic soda solution hot.  This is always done in factories
+where nickel-plating is carried on, but it is inconvenient in the
+laboratory.  The articles after dipping in the alkali are swilled with
+water, and may even be scrubbed with a brush, so as to remove greasy
+matters that have been softened but not entirely removed.
+
+Acid Bath. A convenient bath for laboratory purposes is made by
+mixing two volumes of strong commercial nitric acid with one of strong
+sulphuric acid in a cell measuring, say, 12 X 10 X 15 inches.
+
+Copper or brass articles are dipped in this bath for a few seconds,
+then rinsed with water, then dipped again for a second or two, or
+until they appear equally white all over, and then withdrawn as
+rapidly as possible and plunged into a large quantity of clean water.
+Care must be taken to transfer the articles from the bath to the water
+as quickly as possible, for if time be allowed for gas to be evolved,
+the surfaces become mat instead of bright.
+
+In order to save acid it is advisable to make up a third bath, using
+those odds and ends of acids which gradually accumulate in the
+laboratory.  Sulphuric acid from the balance cases, for instance,
+mixed with its own volume of commercial nitric acid, does very well.
+
+The objects to be dipped receive a preliminary cleansing by a dip in
+this bath, the strong bath being reserved for the final dip.  Sheet
+brass and drawn tube, as it comes from the makers, possesses a really
+fine surface, though this is generally obscured by grease and oxide.
+Work executed in these materials, cleaned in alkali, and dipped in
+really strong acid, will be found to present a much better appearance
+than work which has been filed, unless the latter be afterwards
+elaborately polished.
+
+On no account must paraffin be allowed to get into any of the baths.
+When the final bath gets weak it must be relegated to a subordinate
+position and a new bath set up.  A weak acid bath leaves an ugly
+mottled surface on brass work.
+
+Sec. 129.  A metallic surface which it is intended to electroplate must,
+as has been mentioned, be scrupulously clean.  If the metal is not too
+valuable or delicate, cleaning by dipping is easy and effectual.  The
+following notes will be found to apply to special cases which often
+occur.
+
+(1) Silver Surfaces intended to be gilt. These are first washed
+clean with soap and hot water, and polished with whitening.  They are
+then dipped for a moment in a boiling solution of potassium cyanide.
+A 20 per cent solution of common commercial cyanide does well, but the
+exact strength is quite immaterial.  The cyanide is washed away in a
+large volume of soft water, and the articles are kept under water till
+they are scratch-brushed.
+
+Mat surfaces are readily produced on standard silver by dipping in hot
+strong sulphuric acid.  The appearance of new silver coins, which is
+familiar to everybody, is obtained by this process.
+
+(2) Finely turned and finished Brass Work. If it is intended to
+nickel-plate such work, and if it is desirable to obtain brightly
+polished nickel surfaces, the work must be perfectly polished to begin
+with.  Full details as to polishing may be found in workshop books or
+treatises on watch-making.  It will suffice here to say that the brass
+work is first smoothed by the application of successive grades of
+emery and oil, or by very fine "dead" smooth files covered with chalk.
+Polishing is carried out by means of rotten stone and oil applied on
+leather.
+
+In polishing turned work care must be taken to move the file, emery,
+or rotten stone to and fro over the work with great regularity, or the
+surface will end by looking scratchy and irregular.  The first process
+of cleaning is, of course, to remove grease, and this is accomplished
+best by dipping in a bath of strong hot caustic soda solution, and
+less perfectly by heating the work and dipping it in the cold caustic
+soda bath.
+
+During this process a certain amount of chemical action often occurs
+leading to the brass surface exhibiting some discoloration.  The best
+way of remedying this is to dip the brass into a hot bath of cyanide
+of potassium solution.  If it is inconvenient to employ hot baths or
+to heat the brass work, good results may be obtained by rubbing the
+articles over with a large rough cork plentifully lubricated with a
+strong solution of an alkali.
+
+If the surfaces are very soiled or dirty, a paste of alkali and fine
+slaked lime may be applied on a cork rubber, and this in my experience
+has always been most effective and satisfactory in every way, except
+that it is difficult to get into crevices.  If the alkali stains the
+work, a little cyanide of potassium may be rubbed over the surface in
+a similar manner.
+
+Brass work treated by either of these methods is to be washed in clean
+water till the alkali is entirely removed, and may then be
+nickel-plated without any preliminary scratch-brushing.  The treatment
+in hot baths of alkali and cyanide is the method generally employed in
+American factories as a preliminary to the nickelling of small brass
+work for sewing machines, etc.
+
+(3) Copper either for use as the kathode in electrolysis calibration
+experiments or otherwise is most conveniently prepared by dipping in
+the acid bath, rinsing quickly in cold water, scratch-brushing under
+cold water, and transferring at once to the plating bath.  In the case
+where the copper plates require to be weighed they are dipped into
+very hot distilled water after scratch-brushing, and then dried at
+once by means of a clean glass cloth.
+
+(4) Aluminium (which, however, does not readily lend itself to plating
+operations [Footnote: This difficulty has now been overcome.  See
+note, section 138.] ) is best treated by alkali rubbed on with a cork,
+or by a hot alkaline carbonate where rubbing is inexpedient.  The
+clean aluminium is scratch-brushed under water, and at once
+transferred to the plating bath.
+
+(5) Iron for Nickel-plating. According to Dr. Gore
+(Electra-metallurgy, p.  319) the best bath for cleaning iron is made
+as follows: "One gallon of water and one pound of sulphuric acid are
+mixed with one or two ounces of zinc (which of course dissolves); to
+this is added half a pound of nitric acid."  The writer has been
+accustomed to clean iron by mechanical means, to deprive it of grease
+by caustic alkali, and to finish it off by, means of a hard scratch
+brush.  This process has always worked satisfactorily.
+
+(6) Articles soldered with soft solder containing lead and tin do not
+readily lend themselves to electrolytic processes, the solder
+generally becoming black and refusing to be coated with the
+electro-deposit.  Moreover, if soldered articles are boiled for any
+length of time in caustic alkali during the preliminary cleansing,
+enough tin will dissolve to form a solution of stannate of potash or
+soda--strong enough to deposit tin on brass or copper.  A method of
+coppering soldered articles will be described later on.
+
+Sec. 130.  Scratch-brushing.
+
+This process is generally indispensable, and to its omission is to be
+traced most laboratory failures in electroplating.  Scratch-brushes
+may be bought at those interesting shops where "watchmakers' supplies"
+are sold.  It will be well, therefore, to purchase a selection of
+scratch brushes, for they are made to suit particular kinds of work.
+They are all made of brass wire, and vary both in hardness and in the
+fineness of the wire.  The simplest kind of scratch brush consists
+merely of a bundle of wires bound up tightly by another wire, and
+somewhat "frizzed" out at the ends (Fig. 90).  A more useful kind is
+made just like a rotating brush, and has to be mounted on a lathe
+(Fig.  91).
+
+Fig.  90.                                  Fig. 91.
+
+The scratch brush is generally, if not always, applied wet;  the
+lubricant generally recommended is stale beer, but this may be
+replaced by water containing a small quantity of glue, or any other
+form of gelatine in solution--a mere trace (say .1 per cent) is quite
+sufficient.  Very fair results may be got by using either pure or
+soapy water.  The rotating brushes require to be mounted on a lathe,
+and may be run at the same speed as would be employed for turning
+wooden objects of the same dimensions.
+
+Since the brush has to be kept wet by allowing water or its equivalent
+to drip upon it, it is usual to make a tin trough over which the brush
+can revolve, and to further protect this by a tin hood to keep the
+liquid from being thrown all over the room.  In many works the brush
+is arranged to lie partly in the liquid, and this does very well if
+the hood is effective.
+
+There is a superstition that electro-deposits stick better to
+scratch-brushed surfaces than to surfaces which have not been so
+treated, and consequently it is usual to scratch-brush surfaces before
+electro-deposit.  However this may be, there is no doubt that
+adherence and solidity are promoted by frequent scratch-brushing
+during the process of depositing metal, especially when the latter
+tends to come down in a spongy manner.
+
+Gilt surfaces--if the gilding is at all heavy--are generally dull
+yellow, or even brown, when they come from the bath, and require the
+scratch brush to cause the gold to brighten, an office which it
+performs in a quite striking manner.  The same remark applies to
+silvered surfaces, which generally leave the bath a dead white--at
+all events if the deposit is thick, and if ordinary solutions are
+employed.  In either case the touch of the scratch brush is magical.
+
+Sec. 131. Burnishing.
+
+Burnishers of steel, agate, or bloodstone can be bought at the shops
+where scratch brushes are sold, and are used to produce the same
+brightening effect as can be got by scratch-brushing.  The same
+solutions are employed, but rather stronger, and the burnisher is
+swept over the surface so as to compress the deposited metal.
+Burnishing is rather an art, but when well done gives a harder and
+more brilliant (because smoother) surface than the scratch brush.  On
+the whole, steel burnishers are the most convenient if in constant
+use.
+
+If the burnishing tools have to lie about, steel is apt to rust,
+unless carefully protected by being plunged in quicklime or thickly
+smeared with vaseline, and the least speck of rust is fatal to a
+burnisher.  In any case the steel requires to be occasionally
+repolished by rouge and water on a bit of cloth or felt.  The process
+of burnishing is necessarily somewhat slow and tedious, and as a rule
+is not worth troubling about except in cases where great permanence is
+required.
+
+The burnisher is moved over the work somewhat like a pencil with
+considerable pressure, and care is taken to make the strokes as
+uniform in direction as possible;  otherwise the surface looks
+non-uniform, and has to be further polished by tripoli, whitening,
+etc, before it is presentable.
+
+Sec. 132.  Silver-plating.
+
+The most convenient solution for general purposes is an 8 to 10 per
+cent solution of the double cyanide of silver and potassium together
+with 1 or 2 per cent of "free" potassium cyanide.  Great latitude is
+permissible in the strength of solution and density of current.  As
+commercial cyanide of potassium generally contains an unknown
+percentage of other salts, which, however, do not interfere with its
+value for the purpose of silver-plating, the simplest procedure is as
+follows.
+
+For every 100 c.c.  of plating solution about 7 grms.  of dry
+crystallised silver nitrate are required.  The equivalent amount of
+potassium cyanide (if dry and pure) is 5.2 grms, but commercial
+cyanide may contain from 50 per cent upwards to 96 per cent in the
+best fused cyanide made from ferrocyanide only.  An approximate idea
+of the cyanide content can be obtained from the dealers when the salt
+is purchased, and this is all that is required.
+
+A quantity slightly in excess of the computed amount of cyanide is
+dissolved in distilled water, and this is cautiously added to the
+solution of the silver nitrate till precipitation is just complete.
+The supernatant liquors are then drained away, and the precipitate
+dissolved by adding a sufficiency of the remaining cyanide; this
+process is assisted by warming and stirring.
+
+An allowance of about one-tenth of the whole cyanide employed may be
+added to form "free" cyanide, and the solution made up to the strength
+named.  It is advisable to begin with the cyanide in a moderately
+strong solution, for the sake of ease in dissolving the precipitate.
+
+This solution will deposit silver upon articles of copper or brass
+immersed in it even without the battery, but the coat will be thin.
+The solution is used cold, with a current density of about 10 to 20
+amperes per square foot.  The articles to be silvered are
+scratch-brushed, washed, and electroplated, till they begin to look
+undesirably rough.  They are then taken out of the bath, rebrushed,
+and the process continued till a sufficiency of silver is deposited.
+Four grammes weight of silver (nearly) is deposited per ampere hour.
+It is best to use a fine silver anode, so that the solution, does not
+get contaminated by copper.
+
+In most factories it is usual to "quicken" the objects to be silvered
+before placing them in the electrolysis vats, because the deposit is
+said to adhere better in consequence of this treatment.  I have never
+found it any improvement for laboratory purposes, but it is easy to
+do.  A dilute (say 2 per cent) solution of cyanide of mercury is
+required containing a little free cyanide.  The objects to be
+"quickened" are scratch-brushed and dipped into the cyanide of mercury
+solution till they are uniformly white;  it is generally agreed that
+the less the mercury deposited the better, so long as a perfect
+coating is obtained.  The objects are rinsed after quickening, and put
+in the depositing bath at once.
+
+The mat surface of silver obtained by electrolysis of the cyanide is
+very beautiful--one of the most beautiful things in nature--shining
+with incomparable crystalline whiteness.  So delicate is it, however,
+for so great is the surface it exposes, that it is generally rapidly
+deteriorated by exposure to the air.  It may be protected to some
+extent by lacquering with pale lacquer, but it loses some of its
+brilliancy and purity in the process.  The deposit is generally
+scratch-brushed or burnished down to a regular reflecting surface.
+
+Sec. 133.  Cold Silvering.
+
+A thin but brilliant coat of silver may be readily applied to small
+articles of brass or copper in the following way.  A saturated
+solution of sodium sulphite (neutral) is prepared, and into this a 10
+per cent solution of nitrate of silver is poured so long as the
+precipitate formed is redissolved.  A good deal of silver may be got
+into solution in this way.  Articles to be silvered need only to be
+cleaned, brushed, and dipped in this solution till a coat of the
+required thickness is obtained.
+
+I must admit, however, that the coating thus laid on does not appear
+to be so permanent as one deposited by simple immersion from the
+cyanide solution, even though it is thicker.  The cyanide plating
+solution will itself give a good coat of silver if it is used boiling,
+and if a little potassium cyanide be added.
+
+For purposes of instrument construction, however, a thin coat of
+silver is seldom to be recommended, on account of its liability to
+tarnish and its rapid destruction when any attempt is made to repolish
+it.  For these reasons, nickel or gold plating is much to be
+preferred.
+
+Sec. 134.  Gilding.
+
+This art deserves to be much more widely practised than is usual in
+laboratories.  Regarded as a means of preserving brass, copper, or
+steel, it is not appreciably more "time robbing" than lacquering, and
+gives infinitely better results.  Moreover, it is not much more
+expensive.  Strange as it may seem, the costliness of gilding seldom
+lies in the value of the gold deposited;  the chief cost is in the
+chemicals employed to clean the work, and in interest on the not
+inconsiderable outlay on the solution and anode.
+
+The easiest metal to gild is silver, and it is not unusual to give
+base metals a thin coating of silver or copper, or both, one after the
+other, before gilding, in order to secure uniformity.  To illustrate
+the virtue of a thin layer of gold, I will mention the following
+experiment.  About three years ago I learned for the first time that
+to "clean" the silver used in a small household required at least an
+hour's labour per diem. I further ascertained that most of this time
+is spent on the polishing part of the process.
+
+As this seemed a waste of labour, I decided to try the effect of
+gilding.  In order to give the proposal a fair trial I gilt the
+following articles:  half a dozen table spoons and forks, a dozen
+dessert forks and spoons, and a dozen tea spoons.  These were all
+common electroplated ware.  They were weighed before and after
+gilding, and it was with difficulty that the increase of weight was
+detected, even though a fine bullion balance was employed.  On
+calculating back to money, it appeared that the value of the gold
+deposited was about threepence.  Assuming that an equal weight of
+silver had been accidentally dissolved by the free cyanide during the
+plating--which is unlikely--the total amount of gold deposited would
+be worth, say, sixpence.
+
+After three years' continuous use the gilding is still perfect, except
+at the points on which the spoons and forks rest, where it is
+certainly rather shabby.  Meanwhile the "gold" plate only requires to
+be washed with hot water and soap to keep it in perfect order, a much
+more cleanly and expeditious process than that of silver cleaning.
+
+Sec. 135.  Preparing Surfaces for Gilding.
+
+Ordinary brass work--rough or smooth--may for purposes of
+preservation be dipped, scratch-brushed, and gilt at once.  Seven
+years ago the writer gilt the inside of the head of a copper water
+still, and simply scratch-brushed it; it is to-day in as good order as
+when it was first done.  If it is intended to gild work from the
+first, with the view of making an exceptionally fine job of it,
+"gilding metal," i.e.  brass containing one to one and a quarter
+ounces of zinc to the pound of copper may be specified.  From its
+costliness, however, this is only desirable for small work.
+
+Iron and steel are generally given a preliminary coating of copper,
+but this may be dispensed with though with no advantage--by using a
+particular process of gilding.
+
+Base metals, zinc, pewter, lead, etc, are first coppered in a cyanide
+of copper solution, as will be described under the head of
+Copper-plating.  If it is intended to gild soldered articles,
+the preliminary coating of copper is essential.
+
+The most convenient vessel for holding a gilding solution is
+undoubtedly one formed of enamelled iron.  Particularly useful are the
+buckets and "billies" (i.e.  cylindrical cans) made of this material.
+These vessels may be heated without any fear of a smash, and do not
+appear to be appreciably affected by gilding solutions--at all events
+during several days or weeks.  The avoidance of all risk of breakage
+when twenty or thirty pounds' worth of solution is in question is a
+matter of importance.
+
+Under no circumstances is it desirable to use anything but the purest
+gold and best fused cyanide (called "gold" cyanide) in the preparation
+of the solutions.  The appearance of a pure gold deposit is far richer
+than of one containing silver, and its resistance to the atmosphere is
+perfect;  moreover, in chemico-physical processes one has the
+satisfaction of knowing what one is dealing with.
+
+Sec. 136.  Gilding Solutions.
+
+The strength of solution necessary for gilding brass, copper, and
+silver is not very material.  About one to two pounds of "gold"
+potassium cyanide (? 96 per cent KCN) per gallon does very well.  The
+gold is best introduced by electrolysing from a large to a small gold
+electrode.  One purchases a plate of pure gold either from the mint or
+from reliable metallurgists (say Messrs.  Johnson and Matthey of
+London), and from this electrodes are cut.
+
+The relative areas of the electrodes do not really much matter.  I
+have used an anode of four times the area of the cathode.  The
+solution is preferably heated to a temperature of about 50 deg. C, and a
+strong current is sent through it, say twenty amperes to the square
+foot of anode.  The electrodes must be suspended below the surface of
+the solution by means of platinum wires.  If the gold plates are only
+partly immersed, they dissolve much more rapidly where they cut the
+surface, possibly on account of the effect of convection currents,
+though so far as the writer is aware no proper explanation has yet
+been given.
+
+After a time gold begins to be deposited on the cathode in a powdery
+form, for which reason it is a good plan to begin by wrapping the
+latter in filter paper.  The process has gone on for a sufficient time
+when a clean bit of platinum foil immersed in the place of the cathode
+becomes properly gilt at a current density of about ten amperes per
+square foot.
+
+The powdery gold deposited on the cathode while preparing the solution
+can be scraped off and melted for further use, or the whole cathode
+may now be used as an anode.  The platinum foil testing cathode may
+also be "stripped" by making it an anode, and is for this reason
+preferable to German silver or copper, which would contaminate the
+solution while the "stripping" process was in progress.
+
+For general purposes a current density of say ten to fifteen amperes
+per square foot may be used, but this may be considerably varied, so
+long as the upper limit is not greatly overpassed.  During
+gold-plating there is a considerable advantage in keeping the
+electrodes moving or the solution stirred.
+
+After immersing the cleaned and scratch-brushed articles, depositing
+may go on for about three minutes, after which they are removed from
+the bath and examined, in order to detect any want of uniformity in
+the deposit.
+
+The articles should be entirely immersed;  if this is not done,
+irregularity is apt to appear at the surface.  Platinum wires employed
+as suspenders, and coated along with the articles to be gilt, may also
+be cleaned without loss by making them anodes.  If, on examination,
+all is found to be going on well, reimmerse the cathodes, and continue
+plating till they appear of a dull yellowish brown (this will occur in
+about four minutes), then remove them, rinse and scratch-brush them,
+and replace them in the bath.
+
+When a second coat appears to be getting rather brown than yellowish
+brown, i.e.  of the colour of wet wash-leather, the removal, followed
+by scratch-brushing, may be repeated, and for nearly all laboratory
+purposes, the articles are now fully gilt.
+
+The coating of gold deposited from a hot cyanide solution is spongy in
+the extreme, and if the maximum wear-resisting effect is to be
+obtained, it is advisable to burnish the gold rather than to rely upon
+the scratch brush alone.
+
+If the area of the cathode exceeds that of the anode the solution is
+said to grow weaker, and vice versa.  This may be remedied in the
+former case by an obvious readjustment;  the latter introduces no
+difficulty so far as I know except when plating iron or steel.
+
+The student need not be troubled at the poor appearance of the deposit
+before it is scratch-brushed.  Heavy gold deposits are almost always
+dull, not to say dirty, in appearance till the burnisher or scratch
+brush is applied.  On the other hand, the deposit ought not to get
+anything like black in colour.
+
+The following indications of defects may be noted--they are taken from
+Gore.  I have never been really troubled with them.
+
+The deposit is blackish.  This is caused by too strong a current in
+too weak a bath.  This may be remedied to some extent by stirring or
+keeping the cathode in motion.  The obvious remedy is to add a little
+cyanide of gold.
+
+The gold anode gets incrusted.  This is a sign that the bath is
+deficient in potassium cyanide.  The gold anode gets black and gives
+off gas.  The solution is deficient in cyanide, and too large a
+current is being passed.
+
+If a bright surface is desired direct from the bath, some caustic
+potash (say 2 per cent) may, according to Gore, be added, or the
+articles may be plated only slightly by using a weak current and
+taking them out directly they show signs of getting dull.  By a weak
+current I mean one of about five amperes per square foot.
+
+The deposit is said to be denser if the solution be heated as
+directed;  but the bath will gild, though not quite so freely when
+cold.
+
+To gild iron or steel directly, dilute the bath as above recommended
+some five or six times, add about 1 per cent of potassium cyanide, and
+gild with a very weak current (say two or three amperes per square
+foot) in the cold.  Frequent scratch-brushing will be found requisite
+to secure proper adherence.
+
+It is generally recommended to gild brass or German silver in
+solutions which are rather weak, but in the small practice which
+occurs in the laboratory a solution prepared as suggested does
+perfectly for everything except iron or steel.  The scratch-brushing
+should be done over a large photographic developing dish to avoid loss
+of gold.  It is a good plan to rinse the articles after leaving the
+bath in a limited quantity of distilled water, which is afterwards
+placed in a "residue" bottle, and then to scratch-brush them by hand
+over the dish to catch fine gold.  When any loose dust is removed the
+articles may be scratched in the lathe without appreciable further
+loss.
+
+Silver-gilt articles tend to get discoloured by use, but this
+discoloration can be removed by soap and water.  After long use a gold
+cyanide bath tends to alter greatly in composition, In general, the
+bath tends to grow weaker, from the fact that there is a strong
+temptation to gild as many articles at once as possible.
+
+It is therefore a good plan to keep a rough profit and loss account of
+the gold in order to find the quantity in solution.  Fifty dwts.  per
+gallon (or 78 grms. per 4.5 litres) is recommended.  A gallon of
+solution of this strength is worth about eleven pounds sterling in
+gold and cyanide, and a serviceable anode will be worth about 10
+pounds.  (Fine gold is worth nominally four pounds four shillings and
+eleven pence ha'penny per oz.) Gold may be easily obtained containing
+less impurity than one part in ten thousand.
+
+Sec. 137.  Plating with Copper.
+
+Copper may be deposited from almost any of its salts in reguline form,
+the sulphate and nitrate being most usually employed.  In the
+laboratory a nearly saturated solution of sulphate of copper with 1 or
+2 per cent of sulphuric acid will answer most purposes.  A current
+density of, at most, fifteen amperes per square foot may be used,
+either for obtaining solid deposits for constructional purposes or for
+calibrating current measuring instruments by electrolysis.  A copper
+anode is of course employed.
+
+When coppering with a view to obtaining thick deposits it is a good
+plan to place the electrodes several inches apart, and, if possible,
+to keep the liquid stirred, as there is a considerable tendency on the
+part of copper deposits to grow out into mossy masses wherever the
+current density exceeds the limit mentioned.  As the masses grow
+towards the anode the defect naturally tends to increase of itself,
+hence the necessity for care.  The phenomenon is particularly marked
+at the edges and corners of the cathode.
+
+If the deposit becomes markedly irregular, the best plan is to stop
+the process and file the face of the deposit down to approximate
+smoothness.  In coppering it is of the utmost importance that the
+cathode be clean and free from grease; it must never be touched (by
+the finger, for instance) from the time it is scratch-brushed till it
+is immersed in the plating bath.  Any grease or oxidation tends to
+prevent the copper deposit adhering properly.
+
+A copper deposit oxidises very easily when exposed to the air.
+Consequently if the surface be required free from oxide, as, for
+instance, when it is to be silvered or gilt, it must be quickly washed
+when withdrawn from the coppering bath, scratch-brushed, and
+transferred immediately to the silvering or gilding bath.
+
+If the surface is to be dried, as in electrolysis calibrations, it
+must be rinsed quickly with boiling water and pressed between sheets
+of filter paper.  Another method which has been recommended is to
+rinse the copper in--water slightly acidulated with sulphuric acid
+(which prevents oxidation), then in distilled water, and to dry by
+blotting paper and in front of a fire, taking care not to make the
+plate too hot.  The wash water is sufficiently acidulated by the
+addition of two or three drops of acid per litre.  So far as I know,
+the method of washing in acidulated water was first proposed by Mr. T.
+Gray.
+
+Sec. 138.  Coppering Aluminium.
+
+A good adherent deposit of copper on aluminium used to be considered a
+desideratum in the days when it afforded the only means of soldering
+the latter.  Many receipts have been published from time to time, and
+I have tried, I think, most of them.  On no occasion, however, till
+this year (1896), have I succeeded in obtaining a deposit which would
+not strip after it was tinned and soldered, though it is not difficult
+to get apparently adherent deposits so long as they are not operated
+upon by the soldering iron.  The best of the many solutions which have
+been proposed in years gone by is very dilute cupric nitrate with
+about 5 per cent of free nitric acid.
+
+The problem of electroplating aluminium which I have indicated as
+awaiting a solution has at last found one.  In the Archives des
+Sciences physiques et naturelles de Geneve for December 1895 (vol.
+xxxiv.  p.  563) there is a paper by M. Margot on the subject, which
+discloses a perfectly successful method of plating aluminium with
+copper.  The paper itself deals in an interesting way with the theory
+of the matter--however, the result is as follows.
+
+(1) The aluminium articles are boiled for a few minutes in a strong
+solution of ordinary washing soda.  The aluminium surface is thus
+corroded somewhat, and rendered favourable to the deposit of an
+adherent film of copper.  After removal from the soda solution the
+aluminium is well washed and brushed in running water.
+
+(2) The articles are dipped for thirty seconds or so in a hot 5 per
+cent solution of pure hydrochloric acid.
+
+(3) After dipping in the hydrochloric acid, the work is instantly
+plunged into clean water for about one second, so as to remove nearly,
+but not quite, all of the aluminium chloride.
+
+(4) The work is transferred to a cold dilute (say 5 per cent) solution
+of cupric sulphate slightly acidulated with sulphuric acid.  The
+degree of acidulation does not appear to be very important, but about
+one-tenth per cent of strong acid does well.
+
+If the preliminary processes have been properly carried out the
+aluminium will become coated with copper, and the process is
+accompanied by the disengagement of gas.  It appears to be a rule that
+if gas is not given off, the film of copper deposited is non-adherent.
+The work must be left in the copper sulphate solution till it has
+received a uniform coating of copper.
+
+(5) When this is the case the work is removed--well washed so as to
+get rid of the rest of the aluminium chloride, and then electroplated
+by the battery in the ordinary copper sulphate bath.
+
+If the operation (4) does not appear to give a uniform coat, or if gas
+is not evolved from every part of the aluminium surface, I find that
+operations (2) and (3) may be repeated without danger, provided that
+the dip in the hydrochloric acid is shortened to two or three
+seconds.
+
+The copper layer obtained by Margot's method is perfectly
+adherent--even when used as a base for ordinary solder--though in
+this case it can be stripped if sufficient force is applied.
+
+Since the solder recommended by M. Margot for aluminium contains zinc,
+it does not run well when used to unite aluminium to copper, brass,
+iron, etc.  In this case, therefore, I have found the most
+advantageous method of soldering to be by way of a preliminary
+copper-plating.
+
+The success of M. Margot's method depends in my experience on
+obtaining just the proper amount of aluminium chloride in contact with
+the aluminium when the latter is immersed in the copper sulphate
+solution.
+
+Sec. 139.  The process of copper-plating from sulphate or nitrate may,
+according to Mr. Swan (Journal of the Royal Institution, 1892, p.
+630), be considerably accelerated by the addition of a trace of
+gelatine to the solution.  As success appears to depend upon hitting
+the exact percentage amount of the gelatine, which must in any case be
+but a fraction of one per cent, and as Mr. Swan refrains from stating
+what the amount is, I am unable to give more precise instructions.  A
+few experiments made on the subject failed, doubtless through the
+gelatine content not having been rightly adjusted.  Mr. Swan claims to
+be able to get a hard deposit of copper with a current density of 1000
+amperes per square foot, but seems to recommend about one-tenth of
+that amount for general use.
+
+The solution employed is a mixture of nitrate of copper and ammonium
+chloride--proportions not stated.  Electrolytic copper, as generally
+prepared, is very pure, but this is a mere accident depending on the
+impurities which, as a rule, have to be got rid of.  Electrolysis
+seems to have no effect in purifying from arsenic, for instance.
+
+Roughly speaking, about 11 grms.  of copper are deposited per ampere
+hour from cupric salt solutions.  When the current density is too high
+the anode suffers by oxidation, and this introduces a large and very
+variable resistance into the circuit.
+
+Sec. 140.  Alkaline Coppering Solution
+
+Coppering Base Metals. It is often desirable to coat lead, zinc,
+pewter, iron, etc, with a firm and uniform layer of copper
+preparatory to gilding or silvering.  If copper or brass articles are
+soldered with soft solder it is found that the solder does not become
+silvered or gilt along with the rest of the material, but remains
+uncoated and of an ugly dark colour.  This defect is got over by
+giving a preliminary coating of copper.
+
+This is done in an alkaline solution, generally containing cyanogen
+and ammonia.  The following method has succeeded remarkably well with
+me.  The receipt was taken originally from Gore's Electro-metallurgy,
+p.  208.  A solution is made of 50 grms.  of potassium cyanide
+(ordinary commercial, say, 75 per cent) and 30 grms.  of sodium
+bisulphite in I.5 litres of water.  Thirty-five grammes of cupric
+acetate are dissolved in a litre of water, and 20 cubic centimetres of
+the strongest liquid ammonia are added.  The precipitate formed must
+be more or less dissolved to a strong blue solution.  The cyanide and
+bisulphite solution is then added with warming till the blue colour is
+destroyed.  This usually requires the exact amount of cyanide and
+bisulphite mentioned, but I have not found it essential to entirely
+destroy the colour.
+
+The solution contains cuprocyanide of sodium and ammonium (?), which
+is not very soluble, and this salt tends to be deposited in granular
+crystalline masses on standing.  However, at a temperature of 50 deg. C.
+the above receipt gives an excellent coppering liquid, which will coat
+zinc with a fine reguline deposit.  Brass or copper partly smeared
+with solder will receive a deposit of copper on the latter as well as
+on the former, and, moreover, a deposit which appears to be perfectly
+uniform.
+
+In using the bath the anode tends, as a rule, to become incrusted, and
+this rapidly increases the resistance of the cell, so that the current
+falls off quickly.  The articles should be scratch-brushed and plated
+for about two minutes with a current density of about ten amperes per
+square foot.
+
+As soon as the deposit begins to look red the articles are to be
+removed and rebrushed, after which the process may be continued.
+About five minutes' plating will give a copper deposit quite thick
+enough after scratch-brushing to allow of a very even gilding or
+silvering.
+
+Aluminium appears to be fairly coated, but, as usual, the copper
+strips after soldering.  Iron receives an excellent and adherent coat.
+
+I do not think that the formation of a crust upon the anode can be
+entirely prevented.  According to Gore, its formation is due to the
+solution being too poor in copper, but I have added a solution of the
+acetate of copper and ammonium till the colour was bright blue without
+in any way reducing the incrustation.  If the solutions become
+violently blue it is perhaps as well to add a little more cyanide and
+bisulphite, but I have not found such an addition necessary.  The
+process is one of the easiest and most satisfactory in
+electro-metallurgy.
+
+Sec. 141.  Nickel-plating.
+
+An examination of several American samples of nickel-plated goods has
+disclosed that the coating of nickel is, as a rule, exceedingly thin.
+This is what one would expect from laboratory repetition of the
+processes employed.
+
+Commercial practice in the matter of the composition of nickelling
+solutions appears to vary a good deal.  Thin coatings of nickel may be
+readily given in a solution of the double sulphate of nickel and
+ammonia, which does rather better if slightly alkaline.  Deposits from
+this solution, however, become gray if of any thickness, and,
+moreover, are-apt to flake off the work.  The following solution has
+given very good results with me.  It is mentioned, together with
+others, in the Electrical Review, 7th June 1895.
+
+The ingredients are:-
+
+Nickel sulphate            5 parts
+
+Ammonia sufficient to neutralise the nickel salt.
+
+Ammonium tartrate        3.75 parts
+
+Tannin                0.025 parts
+
+Water                    100 parts
+
+The nickel sulphate and ammonia are dissolved in half the water, the
+ammonium tartrate in the other half with the tannin.  The solutions
+are mixed and filtered at about 40 deg. C.  This solution works well at
+ordinary temperatures, or slightly warm, with a current density of ten
+amperes per square foot.  In an experiment made for the purpose I
+found that plating may go on for an hour in this solution before the
+deposit begins to show signs of flaking off.  The deposit is of a fine
+white colour.
+
+The resistance of the bath is rather high and rather variable,
+consequently it is as well to have a current indicator in circuit, and
+it may well happen that five or six volts will be found requisite to
+get the current up to the value stated.  For nickelling small objects
+of brass, such as binding screws, etc, it is very necessary to be
+careful as to the state of polish and uniformity of their surfaces
+before placing them in the plating bath.  A polished surface will
+appear when coated as a polished surface, and a mat surface as a mat
+surface;  moreover, any local irregularity, such as a speck of a
+foreign metal, will give rise to an ugly spot in the nickelling bath.
+For this reason it is often advisable to commence with a coat of
+copper laid on in an alkaline solution and scratch-brushed to absolute
+uniformity.
+
+An examination of the work will, however, disclose whether such a
+course is desirable or not;  it is not done in American practice, at
+all events for small brass objects.  These are cleaned in alkali and
+in boiling cyanide, which does not render a polished surface mat, as
+weak acid is apt to do, and are then coated with a current density of
+about ten amperes per square foot.
+
+In spite of what is to be found in books as to the ease with which
+nickel deposits may be polished, I find that the mat surface obtained
+by plating on an imperfectly polished cathode of iron is by no means
+easily polished either by fine emery, tripoli, or rouge.
+Consequently, as in the case of brass, if a polished surface is
+desired, it must be first prepared on the unplated cathode.  In this
+case, even if the deposit appears dull, but not gray, it may be easily
+polished by tripoli and water, using a cork as the polisher.
+Scratch-brushing with brass wire, however, though possibly not with
+German silver wire, brightens the deposit, but discolours it.  When
+the deposit becomes gray I have not succeeded in polishing it
+satisfactorily.
+
+Soldered brass or iron may be satisfactorily coated with nickel by
+giving it a preliminary coating of copper in the cyanide bath.  On the
+whole, I recommend in general that iron be first coated with copper in
+the alkaline bath, scratch-brushed, and then nickel-plated, and this
+whether the iron appears to be uniform or not.  Much smoother,
+thicker, and stronger coats of nickel are obtained upon the
+copper-plated surface than on the iron one, and the coating does not
+become discoloured (?  by iron rust) in the same way that a coating on
+bare iron does.  The copper surface may be plated for at least an hour
+at a density of ten amperes per square foot without scaling.
+
+Scales or circles divided on brass may be greatly improved in
+durability by nickel--plating.  For this purpose the brass must be
+highly polished and divided before it is nickelled.
+
+The plating should be continued for a few minutes only, when a very
+bright but thin coat of nickel will be deposited;  it then only
+remains to wash and dry the work, and this must be done at once.  If
+the nickel is deposited before the scale or circle is engraved, very
+fine and legible divisions are obtained, but there is a risk that
+flakes of nickel may become detached here and there in the process of
+engraving.
+
+142.  Miscellaneous Notes on Electroplating.
+
+Occasionally it is desirable to make a metallic mould or other object
+of complex shape.  The quickest way to do this is to carve the object
+out of hard paraffin, and then copy it by electrotyping.  Electrotype
+moulds can be made in many ways.  The easiest way perhaps is to take a
+casting in plaster of Paris, or by means of pressure in warm
+gutta-percha.
+
+In cases where the mould will not draw, recourse must be had to the
+devices of iron-founders, i.e.  the plaster cast must be made in
+suitable pieces, and these afterwards fitted together.  This process
+can occasionally be replaced by another in which the moulding material
+is a mixture of treacle and glue.  The glue is soaked in cold water
+till it is completely soft.  The superfluous water thrown away,
+one-fourth part by volume of thick treacle is added, and the mixture
+is melted on the water bath;  during which process stirring has to be
+resorted to, to produce a uniform mixture.
+
+This liquid forms the moulding mixture, and it is allowed to flow
+round the object to be copied, contained in a suitable box, whose
+sides have been slightly oiled.  The object to be copied should also
+be oiled.  After some hours, when the glue mixture has set, it will be
+found to be highly elastic, so that it may be pulled away from the
+mould, and afterwards resume very nearly its original form.
+
+One drawback to the use of these moulds lies in the fact that the
+gelatine will rarely stand the plating solution without undergoing
+change, but this may be partially obviated by dipping it for a few
+seconds in a 10 per cent solution of bichromate of potash, exposing it
+to the sunlight for a few minutes, and then rinsing it.
+
+In order to render the surface conducting, it is washed over with a
+solution of a gold or silver salt, and the latter reduced in situ to
+metal by a suitable reagent.  A solution of phosphorus is the most
+usual one (see Gore, Electro-metallurgy, p.  216).  Such a mould may
+be copper-plated in the sulphate bath, connection being made by wires
+suitably thrust into the material.
+
+Plaster of Paris moulds require to be dried and waxed by standing on a
+hot plate in melted wax before they are immersed in the plating bath.
+In this case the surface is best made conducting either by silvering
+it by the silvering process used for mirrors, or by brushing it over
+with good black lead rendered more conducting by moistening with an
+ethereal solution of chloride of gold and then drying in the sun.
+
+The brushing requires a stiff camel's-hair pencil of large size cut so
+that the hairs project to a distance of about a quarter of an inch
+from the holder.  The brushing must continue till the surface is
+bright, and is often a lengthy process.
+
+The same process of blackleading may be employed to get a coat of
+deposited metal which will strip easily from the cathode.
+
+In all cases where extensive deposits of copper are required, the
+growth takes place too rapidly at the corners.  Consequently it is
+often desirable to localise the action of the deposit.  A "stopping"
+of ordinary copal varnish seems to be the usual thing, but a thin coat
+of wax or paraffin or photographic (black) varnish does practically as
+well.
+
+I do not propose to deal with the subject of electrotyping to any
+extent, for if practised as an art, a good many little precautions are
+required, as the student may read in Gore's Electro-metallurgy.  The
+above instructions will be found sufficient for the occasional use of
+the process in the construction of apparatus, etc.  There is no
+advantage in attempting to hurry the process, a current density of
+about ten amperes per square foot being quite suitable and
+sufficiently low to ensure a solid deposit.
+
+Sec. 143.  Blacking Brass Surfaces.
+
+A really uniform dead-black surface is difficult to produce on brass
+by chemical means.  A paste of nitrate of copper and nitrate of silver
+heated on the brass is said to give a dead-black surface, but I have
+not succeeded in making it act uniformly.  For optical purposes the
+best plan is to use a paint made up of  "drop" black, ground very fine
+with a little shellac varnish, and diluted for use with alcohol.  No
+more varnish than is necessary to cause the black to hold together
+should be employed.
+
+In general, if the paint be ground to the consistency of very thick
+cream with ordinary shellac varnish it will be found to work well when
+reduced by alcohol to a free painting consistency.
+
+A very fine gray and black finish, with a rather metallic lustre, may
+be easily given to brass work.  For this purpose a dilute solution of
+platinum tetrachloride (not stronger than 1 per cent) is prepared by
+dissolving the salt in distilled water.  The polished brass work is
+cleaned by rubbing with a cork and strong potash till all grease has
+disappeared, as shown by water standing uniformly on the metal and
+draining away without gathering into drops.
+
+After copious washing the work is wholly immersed in a considerable
+volume of the platinum tetrachloride solution at the ordinary
+temperature.  After about a quarter of an hour the brass may be taken
+out and washed.  The surface will be found to be nicely and uniformly
+coated if the above instructions have been carried out, but any
+finger-marks or otherwise dirty places will cause irregularity of
+deposit.  If the process has been successful it will be found that the
+deposit adheres perfectly, hardly any of it being removed by vigorous
+rubbing with a cloth.  If the deposit is allowed to thicken--either
+by leaving the articles in the solution too long or heating the
+solution, or having it too strong--it will merely rub off and leave
+an irregular surface.
+
+This process succeeds well with yellow brass and Muntz metal, either
+cast or rolled, but it does not give quite such uniform (though still
+good) results with gun-metal, on which, however, the deposit is darker
+and deader in appearance.
+
+A book might be written (several have been written) on the art of
+metal colouring, but though doubtless a beautiful and delicate art, it
+is of little service in the laboratory.  For further information the
+reader may consult a work by Hiorns.
+
+Sec. 144.  Sieves.
+
+Properly graded sieves with meshes of a reliable size are often of
+great use.  They should be made out of proper "bolting" cloth, a
+beautiful material made for flour-millers.  Messrs. Henry Simon and
+Company of Manchester have kindly furnished me with the following
+table of materials used in flour-milling.
+
+Sieves made of these materials will be found to work much more quickly
+and satisfactorily than those made from ordinary muslin or wire gauze.
+
+Relative Bolting Value of Silk, Wire, and Grit Gauze
+
+Threads per inch   Trade  No.   Trade  No.  Trade No. of
+Approximate.        of Silk.     of Wire.    Grit Gauze.
+
+18                  0000            18            16
+
+22                   000            20            20
+
+28                    00            26            26
+
+38                     0            32            34
+
+48                     1            40            44
+
+52                     2            45            50
+
+56                     3            50            54
+
+60                     4            56            58
+
+64                     5            60            60
+
+72                     6            64            66
+
+80                     7            70            70
+
+84                     8            80            80
+
+94                     9
+
+106                   10
+
+114                   11
+
+124                   12
+
+130                   13
+
+139                   14
+
+148                   15
+
+156                   16
+
+163                   17
+
+167                   18
+
+170                   19
+
+173                   20
+
+
+Sec. 145.  Pottery making in the Laboratory.
+
+When large pieces of earthenware of any special design are required,
+recourse must be had to a pottery.  Small vessels, plates, parts of
+machines, etc, can often be made in the laboratory in less time than
+it would take to explain to the potter what is required.  For this
+purpose any good pipeclay may be employed.  I have used a white
+pipe-clay dug up in the laboratory garden with complete success.
+
+The clay should be kneaded with water and squeezed through a cloth to
+separate grit.  It is then mixed with its own volume or thereabouts of
+powdered porcelain evaporating basins, broken basins being kept for
+this purpose.  The smoothness of the resulting earthenware will depend
+on the fineness to which the porcelain fragments have been reduced.  I
+have found that fragments passing a sieve of sixty threads to the inch
+run, do very well, though the resulting earthenware is decidedly
+rough.
+
+The porcelain and clay being thoroughly incorporated by kneading, the
+articles are moulded, it being borne in mind that they will contract
+somewhat on firing. [Footnote: The contraction depends on the
+temperature attained as well as on the time.  An allowance of one part
+in twelve will be suitable in the case considered.]  The clay should
+be as stiff as is convenient to work, and after moulding must be
+allowed to get thoroughly dry by standing in an airy place;  the
+drying must not be forced, especially at first, or the clay will
+crack.
+
+Small articles are readily fired in a Fletcher's crucible furnace
+supplied with a gas blow-pipe;  the furnace is heated gradually to
+begin with.  When a dull red heat is attained, the full power of the
+blast may be turned on, and the furnace kept at its maximum
+temperature for three or four hours at least, though on an emergency
+shorter periods may be made to do.
+
+The articles are supported on a bed of white sand;  after firing, the
+crucible furnace must be allowed to cool slowly.  It must be
+remembered that the furnace walls will get hot externally after the
+first few hours, consequently the furnace must be supported on bricks,
+to protect the bench.
+
+The pottery when cold may be dressed on a grindstone if necessary.
+This amateur pottery will be found of service in making small fittings
+for switch-boards, commutators, and in electrical work generally.
+
+Pottery made as described is very hard and strong, the hardness and
+strength depending in a great degree  on the proportion of powdered
+porcelain added to the clay, as well, of course, as on the quality of
+both of these materials.
+
+It is a good plan to knead a considerable quantity of the mixture,
+which may then be placed in a well-covered jar, and kept damp by the
+addition of a little water.
+
+Pottery thus made does not require to be glazed, but, of course, a
+glaze can be obtained by any of the methods described in works on
+pottery manufacture.  The following glaze has been recommended to me
+by a very competent potter:-
+
+Litharge
+
+7 parts by weight
+
+Ground flint
+
+2 parts by weight
+
+Cornish stone or felspar
+
+1 parts by weight
+
+These ingredients are to be ground up till they will pass the finest
+sieve--say 180 threads to the inch.  They are then mixed with water
+till they form a paste of the consistency of cream. They must, of
+course, be mixed together perfectly.  The ware to be glazed is dipped
+into the cream after the first firing; it is then dried as before and
+refired.  The glaze will melt at a bright red heat, but it will crack
+if not fired harder;  the harder it is fired the less likely is it to
+crack.
+
+If colouring matters are added they must be ground in a mill free from
+iron till they are so fine that a thick blanket filter will not filter
+them when suspended in water.  This remark applies particularly to
+oxide of cobalt.
+
+APPENDIX
+
+PLATINISING GLASS
+
+IN the Philosophical Magazine for July 1888 (vol.  xxvi.  p.  1) there
+is a paper by Professor Kundt translated from the Sitzungsberichte of
+the Prussian Academy.  This paper deals with the indices of refraction
+of metals.  Thin prisms were obtained by depositing metals
+electrolytically on glass surfaces coated with platinum. The
+preparation of these surfaces is troublesome.  Kundt recounts that no
+less than two thousand trials were made before success was attained.
+A detailed account of the preparation of these surfaces is not given
+by Kundt, but one is promised--a promise unfortunately unfulfilled so
+far as I am able to discover.  A hunt through the literature led to
+the discovery of the following references:  Central Zeitung fuer Optik
+und Mechanik, p.  142 (1888); Dingler's Polytechnik Journal, Vol.
+cxcv.  p.  464; Comptes Rendus, vol.  lxx.  (1870).
+
+The original communication is a paper by Jouglet in the Comptes
+Rendus, of which the other references are abstracts.  The account in
+Dingier is a literal translation of the original paper, and the note
+in the Central Zeitung is abbreviated sufficiently to be of no value.
+The details are briefly as follows:-
+
+One hundred grams of platinum are dissolved in aqua regia and the
+solution is dried on the sand bath, without, however, producing
+decomposition.  Though the instructions are not definite, I presume
+that the formation of PtCl4 is contemplated.
+
+The dried salt is added little by little to rectified oil of lavender,
+placed on a glass paint-grinding plate, and the salt and oil are
+ground together with a muller.  Care is required to prevent any
+appreciable rise of temperature which would decompose the compound
+aimed at, and it is for this reason that the salt is to be added
+gradually.  Of course the absorption of water from the air must be
+prevented from taking place as far as possible.  Finally, the compound
+is diluted by adding oil of lavender up to a total weight of 1400
+grams (of oil).
+
+The liquid is poured into a porcelain dish and left absolutely at rest
+for eight days.  It is then decanted and filtered, left six days at
+rest, and again decanted (if necessary).  The liquid should have a
+specific gravity of 5 deg. on the acid hydrometer.  (If by this the Baume
+scale is intended, the corresponding specific gravity would be 1.037.)
+A second liquid is prepared by grinding up 25 grams of litharge with
+25 grams of borate of lead and 8 to 10 grams of oil of lavender.  The
+grinding must be thoroughly carried out.
+
+This liquid is to be added to the one first described, and the whole
+well mixed.  The resulting fluid constitutes the platinising liquid,
+and is applied as follows:-
+
+A sheet of clean glass is held vertically, and the liquid is painted
+over it, carrying the brush from the lower to the upper edge.  The
+layer of oil dries slowly, and when it is dry the painting is again
+proceeded with, moving the brush this time from right to left; and
+similarly the process is repeated twice, the brush being carried from
+top to bottom and left to right.  This is with the object of securing
+great uniformity in the coating.  Nothing is said as to the manner in
+which the glass is to be dried.
+
+The dried glass is finally heated to a temperature of dull redness in
+a muffle furnace.  The resinous layer burns away without running or
+bubbling, and leaves a dull metallic surface.  As the temperature
+rises this suddenly brightens, and we obtain the desired surface
+(which probably consists of an alloy of lead and platinum).  It is
+bright only on the surface away from the glass.
+
+I have not had an opportunity of trying this process since I
+discovered the detailed account given by Jouglet;  but many
+modifications have been tried in the laboratory of the Sydney
+University by Mr. Pollock, starting from the imperfect note in the
+Central Zeitung, which led to no real success.
+
+It was found that it is perfectly easy to obtain brilliant films of
+platinum by the following process, provided that the presence of a few
+pin-holes does not matter.
+
+The platinum salt employed is what is bought under the name of
+platinic chloride;  it is, however, probably a mixture of this salt
+and hydro-chloro-platinic acid, and has all the appearance of having
+been obtained by evaporating a solution of platinum in aqua regia to
+dryness on the water bath.  A solution of this salt in distilled water
+is prepared;  the strength does not seem to matter very much, but
+perhaps one of salt to ninety-nine water may be regarded as a standard
+proportion.  To this solution is added a few drops of ordinary gum
+water (i.e.  a solution of dextrin).  The exact quantity does not
+matter, but perhaps about 2 per cent may be mentioned as giving good
+results.
+
+The glass is painted over with this solution and dried slowly on the
+water bath.  When the glass is dry, and covered with a uniform hard
+film of gum and platinum salt free from bubble holes, it is heated to
+redness in a muffle furnace.  The necessary and sufficient temperature
+is reached as soon as the glass is just sensibly red-hot.
+
+The mirrors obtained in this way are very brilliant on the free
+platinum surface.  If the gum be omitted, the platinum will have a mat
+surface;  and if too much gum be used, the platinum will get spotty by
+bubbles bursting.  There does not appear to be any advantage in using
+lead.
+
+It is quite essential that the film be dry and hard before the glass
+is fired.
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall
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