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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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