ON LABORATORY ARTS
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
MACMILLAN AND CO, LIMITED
NEW YORK: THE MACMILLAN COMPANY
All rights reserved
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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. 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).
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.
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.
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.
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.
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.]
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).
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.
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.
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).
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.
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.
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.
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.
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.
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.