On Laboratory Arts Part 1

On Laboratory Arts.

by Richard Threlfall.

PREFACE

EXPERIMENTAL work in physical science rests ultimately upon the mechanical arts. It is true that in a well-appointed laboratory, where apparatus is collected together in greater or less profusion, the appeal is often very indirect, and to a student carrying out a set experiment with apparatus provided to his hand, the temptation to ignore the mechanical basis of his work is often irresistible.

It often happens that young physicists are to be found whose mathematical attainments are adequate, whose observational powers are perfectly trained, and whose general capacity is unquestioned, but who are quite unable to design or construct the simplest apparatus with due regard to the facility with which it ought to be constructed.

That ultimate knowledge of materials and of processes which by long experience becomes intuitive in the mind of a great inventor of course cannot be acquired from books or from any set course of instruction.

There are, however, many steps between absolute ignorance and consummate knowledge of the mechanical arts, and it is the object of the following pages to a.s.sist 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 a.s.sistance, 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 t.i.tle 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 gla.s.s-blowing, in spite of there being an excellent and handy book by Mr. Shenstone (The Methods of Gla.s.s-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 a.s.sistant, 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 ill.u.s.trations thanks are due to Mrs. Threlfall and Mr. James Cook. With regard to matters which have come to the writer's knowledge by his being specifically instructed in them from time to time, due acknowledgment is, it is hoped, made in the text.

With regard to the question as to what matters might be included and what omitted, the general rule has been to include information which the author has obtained with difficulty, and to leave on one side that which he has more easily attained. All the "unities" have been consistently outraged by a deliberate use of the English and metric systems side by side. So long as all the materials for mechanical processes have to be purchased to specifications in inches and feet, it is impossible to use the centimetre consistently without introducing inconvenience. However, everybody ought to, and probably does, use either system with equal facility.

No attempt has been made at showing how work can be done without tools. Though, no doubt, a great deal can be done with inferior appliances where great economy of money and none of time is an object, the writer has long felt very strongly that English physical laboratory practice has gone too far in the direction of starving the workshop, and he does not wish, even indirectly, 'to give any countenance to such a mistaken policy. Physical research is too difficult in itself, and students' time is too valuable, for it to be remunerative to work with insufficient appliances.

In conclusion, the writer would ask his readers to regard the book to some extent as tentative, and as a means to the procuring and organising of information bearing upon laboratory arts. Any information which can be given will be always thankfully received, and the author hereby requests any reader who may happen to learn something of value from the book to communicate any special information he may possess, so that it may be of use to others should another edition ever be called for.

CHAPTER I

HINTS ON THE MANIPULATION OF GLa.s.s AND ON GLa.s.s-BLOWING FOR LABORATORY PURPOSES

-- 1. THE art of gla.s.s-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 gla.s.s-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 a.s.sumed by the reader to have come within my personal experience.

-- 2. Returning to gla.s.s-blowing, we may note that two forms of gla.s.s-blowing are known in the arts, "Pot" blowing and "Table"

blowing. In the former case large quant.i.ties of fluid "metal"

(technical term for melted gla.s.s) are a.s.sumed 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 gla.s.s-working.

I do not think that a man can become an accomplished gla.s.s-blower from book instructions merely--at all events, not without much unnecessary labour,--but he can learn to do a number of simple things which will make an enormous difference to him both as regards the progress of his work and the state of his pocket.

-- 3. The first thing is to select the gla.s.s. 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 gla.s.s most generally employed in laboratories.

-- 4. Soft Soda Gla.s.s, obtained for the most part from factories in Thuringia, and generally used in a.s.sembling chemical apparatus.--This gla.s.s 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 gla.s.s often devitrifies with age, and is then useless for blowpipe work, though it may be bent sufficiently for a.s.sembling chemical apparatus. Devitrified gla.s.s looks frosty, or, in the earlier stages, appears to be covered by cobwebs, and is easily picked out and rejected.

-- 5. It might be imagined that the devitrification would disappear when the gla.s.s is heated to the fusing point; and so it does to a great extent, but for many operations one only requires to soften the gla.s.s, and the devitrification often persists up to this temperature.

My experience is that denitrified gla.s.s is also more likely to crack in the flame than good new gla.s.s, though the difference in this respect is not very strongly marked with narrow tubes.

-- 6. Flint Gla.s.s.

Magnificent flint gla.s.s is made both in England and France. The English experimenter will probably prefer to use English gla.s.s, 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 gla.s.s made by Messrs. Powell of Whitefriars, but I daresay equally good gla.s.s may be obtained elsewhere.

For general purposes flint gla.s.s 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 gla.s.s is less liable to crack and break at places of unequal thickness than if made of soda gla.s.s. 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 gla.s.s 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 fas.h.i.+oned in soda gla.s.s. Indeed, if it is necessary to join two bits of soda gla.s.s of different kinds, it is better to separate them by a short length of flint gla.s.s; they are more likely to remain joined to it than to each other. A particular variety of flint gla.s.s, known as white enamel, is particularly suitable for this purpose, and, indeed, may be used practically as a cement.

-- 7, It is, however, when the necessity of altering or repairing apparatus complicated by joints arises that the advantage of flint gla.s.s is most apparent. A crack anywhere near to a side, or inserted joint, can scarcely ever be repaired in the case of soda gla.s.s apparatus, even when the gla.s.s is quite thin and the dimensions small.

It should also be mentioned that flint gla.s.s has a much more brilliant appearance than soda gla.s.s. Of course, there is a considerable difference between different kinds of flint gla.s.s 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 gla.s.s. The kind of flint gla.s.s made by Messrs. Powell becomes distinctly soft soon after it is hot enough to be appreciably luminous in a darkened room, and at a white heat is very fluid. This fluidity, though of advantage to the practised worker, is likely to give a beginner some trouble.

-- 8. As against the advantages enumerated, there are some drawbacks.

The one which will first strike the student is the tendency of the gla.s.s 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 gla.s.s it is quite possible to neglect to fuse the gla.s.s 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 gla.s.s, or is at least exceedingly unusual. An accident of this kind is particularly likely to happen if the gla.s.s be at all reduced. Hence, if a joint does not crack when cold, the presumption is, in the case of soda gla.s.s, that the joint is perfectly made, and will not allow of any leak; but this is not the case with flint gla.s.s, for which reason all joints between flint gla.s.s tubes require the most minute examination before they are pa.s.sed. If there are any air bubbles in the gla.s.s, especial care must be exercised.

-- 9. Hard or Bohemian, Gla.s.s.

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 gla.s.s. Fused joints may be made between hard gla.s.s and flint gla.s.s without using enamel, and though they often break in the course of time, still there is no reason against their employment, provided the work be done properly, and they are not required to last too long.

-- 10. On the Choice of Sizes of Gla.s.s 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 gla.s.s 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. Gla.s.s 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 gla.s.s 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 gla.s.s to order. It is a good plan to stock the largest and smallest size of each material as well as the most useful working sizes.

Fig. 1.