This is an art about which more can be learned in five minutes by watching it well practised than by pages of written description. My advice to any one about to commence the practice of the art would be to make friends with a glazier and see it done. What follows is therefore on the supposition that this advice has been followed.

After some experience of cutters made of especially hardened steel, I believe better work can generally be got out of a diamond, provided the cost is not an objection. It is economy to pay a good price for a good diamond. As is well known, the natural angle of the crystal makes the best point, and a person buying a diamond should examine the stone by the help of a lens, so as to see that this condition is fulfilled. The natural angle is generally, if not always, bounded by curved edges, which have a totally different appearance from the sharp edges of a "splinter."

When a purchase is to be made, it is as well for the student to take a bit of gla.s.s and a foot-rule with him, and to test the diamond before it is taken away. When a good diamond has been procured, begin by taking cuts on bits of clean window gla.s.s until the proper angle at which to hold the tool is ascertained. Never try to cut over a scratch, if you value your diamond, and never press hard on the gla.s.s; a good cut is accompanied by an unmistakable ringing sound quite different from the sound made when the diamond is only scratching.

Perhaps the most important advice that can be given is, Never lend the diamond to anybody--under any circ.u.mstances.

The free use of a diamond is an art which the physicist will do well to acquire, for quite a variety of apparatus may be made out of gla.s.s strips, and the accuracy with which the gla.s.s breaks along a good cut reduces such an operation as gla.s.s-box-making to a question of accurate drawing.

-- 48. Cementing.

One of the matters which is generally confused by too great a profusion of treatment is the art of cementing gla.s.s to other substances.

The following methods will be found to work, subject to two conditions:

(1) The gla.s.s must be clean;

(2) it must be hot enough to melt the cement.

For ordinary mending purposes when the gla.s.s does not require to be placed in water (especially if hot) nothing is better than that kind of glue which is generally called "diamond cement." This may be easily made by dissolving the best procurable isingla.s.s in a mixture of 20 per cent water and 80 per cent glacial acetic acid--the exact proportions are not of consequence.

First, the isingla.s.s is to be tightly packed into a bottle with a wide neck, then add the water, and let the isingla.s.s soak it up.

Afterwards pour in the acetic acid, and keep the mixture near 100C.

for an hour or two on the water bath--or rather in it. The total volume of acetic acid and water should not be more than about half of the volume of isingla.s.s when the latter is pressed into the bottle as tightly as possible.

The proper consistency of the cement may be ascertained by lifting a drop out of the bottle and allowing it to cool on a sheet of gla.s.s.

In ten minutes it ought not to be more than slightly sticky, and the ma.s.s in the bottle, after standing a few hours cold, should not be sticky at all, and should yield, jelly-like, to the pressure of the finger to only a slight degree. If the glue is too weak, more isingla.s.s may be added (without any preliminary soaking).

A person making the mixture for the first time almost always gets it too weak. It is difficult to give exact proportions by weight, as isingla.s.s and gelatine (which may replace it) differ greatly in quality. This cement is applied like glue, and will cement nearly anything as well as gla.s.s. Of course, as much cement as possible must be squeezed out of any joint where it is employed. The addition of gums, as recommended in some books, is unnecessary.

Ordinary glue will serve perfectly for cementing gla.s.s to wood.

"Chipped gla.s.s" ware is, I understand, made by painting clean gla.s.s with glue. As the glue dries and breaks by contraction, it chips off the surface of the gla.s.s. I have never seen this done. In nearly all cases where alcohol is not to be employed very strong joints may be made by sh.e.l.lac. Orange sh.e.l.lac is stronger than the "bleached"

variety.

A sine qua non is that the gla.s.s be hot enough to melt the sh.e.l.lac.

The best way is to heat the gla.s.s surfaces and rub on the sh.e.l.lac from a bit of flake; the gla.s.s should not be so hot as to discolour the sh.e.l.lac appreciably, or its valuable properties will be partly destroyed. Both gla.s.s surfaces being thus prepared, and the sh.e.l.lac being quite fluid on both, they may be brought together and clamped tightly together till cool. Sh.e.l.lac that has been overheated, or dissolved in alcohol, or bleached, is of little use as compared with the pale orange flaky product. Dark flakes have probably been overheated during the preliminary refining.

For many purposes a cement is required capable of resisting carbon bisulphide. This is easily made by adding a little treacle (say 20 per cent) to ordinary glue. Since the mixture of glue and treacle does not keep, i.e. it cannot be satisfactorily melted up again after once it has set, no more should be made up than will be wanted at the time. If the glue be thick, gla.s.s boxes for carbon disulphide may be easily put together, even though the edges of the gla.s.s strips are not quite smooth, for, unlike most cements, this mixture remains tough, and is fairly strong in itself.

I have found by experiment that most fixed and, to a less degree, essential oils have little or no solvent action on sh.e.l.lac, and I suspect that the same remark applies to the treacle-glue mixture, but I have not tried. Turpenes act on sh.e.l.lac slightly, but mineral oils apparently not at all. The tests on which these statements are based were continued for about two years, during which time kerosene and mineral oils had no observable effect on sh.e.l.lac--fastened galvanometer mirrors.

-- 49. Fusing Electrodes into Gla.s.s.

This art has greatly improved since the introduction of the incandescent lamp; however, up to the present, platinum seems to remain the only substance capable of giving a certainly air-tight result. I have not tried the aluminium-alumina method.

Many years ago it was the fashion to surround the platinum wire with a drop of white enamel gla.s.s in order to cause better adhesion between it and the ordinary gla.s.s. [Footnote: Hittorf and Geissler (Pogg.

Ann. 1864, -- 35; English translation, Phys. Soc. London, p. 138) found that it was impossible to make air-tight joints between platinum and hard potash gla.s.s, but that soft lead gla.s.s could be used with success as a cement.] However, in the case of flint gla.s.s, if one may judge from incandescent lamps, this is not essential--a fact which entirely coincides with my own experience.

On the other hand, when sealing electrodes into German gla.s.s I have often used a drop of enamel with perfect results, though this is not always done in Germany. In all cases, however, in which electrodes have to be sealed in--especially when they are liable to heat--I recommend flint gla.s.s, and in this have the support of Mr. Rain (The Incandescent Lamp and its Manufacture, p. 131). The exact details for the preparation of eudiometer tubes are given by Faraday (Chemical Manipulation, -- 1200).

In view of what has preceded, however, I will content myself with the following notes. Make the hole through which the wire is to protrude only slightly larger than the wire itself, and be sure that the latter is clean. Allow the gla.s.s to cool sufficiently not to stick to the wire when the latter is pushed in. Be sure that, on heating, the gla.s.s does not get reduced, and that it flows up to the wire all round; pull and push the wire a little with a pair of pincers, to ensure this.

It is not a bad plan to get the gla.s.s exceedingly fluid round the wire--even if the lump has to be blown out a little afterwards--as it cools. The seal should finally be well annealed in asbestos, but first by gradually moving it into the hot air in front of the flame.

It was observed by Professor J. J. Thomson and the author some years ago (Proc. Roy. Soc. 40. 331. 1886) that when very violent discharges are taken through lightly sealed-in electrodes in lead-gla.s.s tubes--say from a large battery of Leyden jars--gas appears to be carried into the tube over and above that naturally given off by the platinum, and this without there being any apparent want of perfection in the seal. This observation has since been confirmed by others. Consequently in experiments on violent discharges in vacuo where certainty is required as to the exclusion of air, the seals should be protected by a guard tube or cap containing mercury; this must, of course, be put in hot and clean, on hot and clean gla.s.s, and in special cases should be boiled in situ.

A well-known German physicist (Warburg, I think) recommends putting the seals under water, but I cannot think that this is a good plan, for if air can get in, why not water? which has its surface tension in its favour. The same reasoning prevents my recommending a layer of sulphuric acid above the mercury-a method used for securing air-tightness in "mercury joints" by Mr. Gimingham, Proc. R. S.

1874.

Further protection may be attained for many purposes by coating the platinum wire with a sheath of gla.s.s, say half an inch long, fused to the platinum wire to a depth of one-twentieth of an inch all round.

In some cases the electrodes must be expected to get very hot, for instance, when it is desired to platinise mirrors by the device of Professor Wright of Yale. In this and similar cases I have met with great success by using "barometer" tubes of about one-twelfth of an inch bore, and with walls, say, one-tenth of an inch thick.

[Footnote: "Barometer" tube is merely very thick-walled gla.s.s tubing, and makes particularly bad barometers, which are sold as weather gla.s.ses.]

This tube is drawn down to a long point--say an inch long by one-eighth of an inch external diameter, and the wire is fused in for a length, say, of three-quarters of an inch, but only in the narrow drawn--down part of the tube. At different times I have tried four such seals, and though the electrodes were red hot for hours, I have never had an accident--of course they were well annealed.

Fig. 37.

For directions as to the making of high vacuum tubes, see the section dealing with that matter.

-- 50. As economy of platinum is often of importance, the following little art will save money and trouble. Platinum is easily caused to join most firmly to copper--with which, I presume, it alloys--by the following method. Hold the platinum wire against the copper wire, end to end, at the tip of the reducing flame of a typical blowpipe--or anywhere--preferably in the "reducing" part of the oxygas flame; in a moment the metals will fuse together at the point of contact, when they may be withdrawn.

Such a joint is very strong and wholly satisfactory, much better than a soldered joint. If the work is not carried out successfully so that a considerable drop of copper-platinum alloy acc.u.mulates, cut it off and start again. The essence of success is speed, so that the copper does not get "burned." If any considerable quant.i.ty of alloy is formed it dissolves the copper, and weakens it, so that we have first the platinum wire, then a bead of alloy, and then a copper wire fused into the bead, but so thin just outside the latter that the joint has no mechanical strength.

-- 51. The Art of making Air-light Joints.

Lamp-manufacturers and others have long since learned that when gla.s.s is in question not only are fused joints made as easily as others, but that they afford the only reliable form of joint. An experimenter who uses flint gla.s.s, has a little experience, an oxygas blow-pipe and a blowing apparatus, will prefer to make his joints in this way, simply from the ease with which it may be done. When it comes to making a tight joint between gla.s.s and other substances the problem is by no means so easy. Thus Mr. Griffiths (Phil. Trans. 1893, p. 380) failed to make air-tight joints by cementing gla.s.s into steel tubes, using hard sh.e.l.lac, and the tubes fitting closely. These joints were satisfactory at first, but did not last; the length of the joint is not stated. The difficulty was finally got over by soldering very narrow platinum tubes into the steel, and fusing the former into the gla.s.s.

Mr. Griffiths has since used an alloy with success as a cement, but I cannot discover what it is made from. Many years ago Professor Hittorf prepared good high vacuum tubes by plugging the ends of gla.s.s tubes with sealing wax merely, though in all cases the s.p.a.ces to be filled with wax were long and narrow (Hittorf, Pogg. Ann. 1869, -- 5, English translation, Phys. Soc. p. 113). Again, Regnault habitually used bra.s.s ferules, and cemented gla.s.s into them by means of his mastic, which can still be procured at a low rate from his instrument-makers (Golan, Paris). Lenard also, in his investigations on Cathode Rays (Wied. Ann, vol. li. p. 224), made use of sealing wax covered with marine glue.

Surely in face of these facts we must admit that cement joints can be made with fair success. I do not know the composition of M.

Regnault's mastic, but Faraday (Manipulations, -- 1123) gives the following receipt for a cement for joining ferules to retorts, etc:

Resin 5 parts.

Beeswax 1 part.

Red ochre or Venetian red, finely powdered and sifted 1 part.

I believe this to be substantially the same as Regnault's mastic, though I have never a.n.a.lysed the latter.

For chemical work the possibility of evolution of gas from such a cement must be taken into account, and I should certainly not trust it for this reason in vacuum tube work, where the purity of the confined gas could come in question. Otherwise it is an excellent cement, and does not in my experience tend to crack away from gla.s.s to the same extent as paraffin or pure sh.e.l.lac.

This cracking away from gla.s.s, by the way, is probably an effect of difference in rate of expansion between the gla.s.s and cement which probably always exists, and, if the cement be not sufficiently viscous, must, beyond certain temperature limits, either produce cracks or cause separation. Professor Wright of Yale has used a hard mineral pitch as a cement in vacuum work with success.

My attention has been directed to a fusible metal cement containing mercury, and made according to the following receipt, given by Mr. S.

G. Rawson, Journal of the Society of Chemical Industry, vol. ix.