Chapter I (3)

My reason for describing the file method at such length is to be found in the fact that a Morse drill requires to be sharpened after drilling glass before it can be used in the ordinary way, and this is often a difficulty.

I ought to say that I have never succeeded in boring the barrel of a glass tap by either of these methods. [Footnote: I have been lately informed that it is usual to employ a splinter of diamond set in a steel wire holder both for tap boring and for drilling earthenware for riveting. The diamond must, of course, be set so as to give sufficient clearance for the wire holder.

For methods of using and setting diamond tools see § 55. It will suffice to say here that a steel wire is softened and filed at one end so as to form a fork; into this the diamond is set by squeezing with pliers. The diamond is arranged so as to present a point in the axis of the wire, and must not project on one side of the wire more than on the other. It is not always easy to get a fragment satisfying these conditions, and at the same time suitable for mounting. A drop of solder occasionally assists the process of setting the diamond.

In drilling, the diamond must be held against the work by a constant force, applied either by means of weight or a spring. I made many trials by this method, using a watchmaker's lathe and pressing up the work by a weight and string, which passed over a pulley. I used about 40 ounces, and drilled a hole 3/32 in diameter in flint glass at a speed of 900 revolutions per minute to a depth of one-eighth of an inch in eight minutes. I used soap and water as a lubricant, and the work was satisfactory.

Since this was set up, I have been informed by Mr. Hicks of Hatton Garden that it is necessary to anneal glass rod by heating it up to the softening point and allowing it to cool very slowly under red-hot sand or asbestos before boring. If this be done, no trouble will be experienced. The annealing must be perfect.]

§ 39. For boring large holes through thick glass sheets, or, indeed, through anything where it is necessary to make sure that no accident can happen, or where great precision of position and form of hole is required, I find a boring tube mounted as shown in the picture (Fig. 36) is of great service. Brass or iron tube borers do perfectly well, and the end of the spindle may be provided once for all with a small tube chuck, or the tubes may be separately mounted as shown. A fairly high speed is desirable, and may be obtained either by foot, or, if power is available, is readily got by connecting to the speed cone of a lathe, which is presumably permanently belted to the motor.

Fig. 36.

After trying tubes armed with diamond dust, as will be presently explained, I find that emery and thin oil or turpentine, if liberally supplied below the glass, will do very nearly as well. The tube should be allowed to rise from the work every few seconds, so as to allow of fresh emery and oil being carried into the circular grooves. This is done by lifting the hinged upper bearing, the drill being lifted by a spiral spring between the pulley and the lower bearing shown at B. The glass may be conveniently supported on a few sheets of paper if flat, or held firm in position by wooden clamps if of any other shape. In any case it should be firmly held down and should be well supported. Any desired pressure upon the drill is obtained by weighting the hinged board A.

§ 40. The following method was shown to me by Mr. Wimshurst, but I have not had occasion to employ it myself. It is suitable for boring large holes through such glass as the plates of Mr. Wimshurst's Influence machines are usually made of. A diamond is mounted as the "pencil" of a compass, and with this a circle is drawn on the glass in the desired position. The other leg of the compass of course rests on a suitable washer.

To the best of my recollection the further procedure was as follows. A piece of steel rod about one-eighth inch in diameter was ground off flat and mounted in a vice vertically, so as to cause its plane end to form a small horizontal anvil. The centre (approximately) of the diamond-cut circle of the glass was laid on this anvil so as to rest evenly upon it, and the upper surface (i.e. that containing the cut) was then struck smartly with a hammer, completely pulverising the glass above the anvil. The hole was gradually extended in a similar manner right up to the diamond cut, from which, of course, the glass broke away.

A similar method has long been known to glaziers, differing from the preceding in that a series of diamond cuts are run across the circle parallel to two mutually perpendicular diameters. A smart tap on the back of the scored disc will generally cause the fragments to tumble out. I have never tried this myself, but I have seen it done.

Large discs may easily be cut from sheet glass by drawing a circular diamond cut, and gradually breaking away the outer parts by the aid of additional cuts and a pair of pliers or "shanks" (see Fig. 44).

§ 41. Operations depending on Grinding: Ground-in Joints.

The process will be perfectly understood by reference to a simple case. Suppose it is desired to grind the end of a tube into the neck of a bottle. If a stoppered bottle is available, the stopper must be taken out and measured as to its diameter at the top and bottom. Select a bit of tube as nearly as possible of the same diameter as the stopper at its thickest part. Draw down the glass in the blow-pipe flame rather by allowing it to sink than by pulling it out. After a few trials no difficulty will be experienced in making its taper nearly equal to that of the stopper, though there will in all probability be several ridges and inequalities. When this stage is reached anneal the work carefully and see that the glass is not too thin. Afterwards use emery and water, and grind the stopper into the bottle.

There are six special directions to be note

(1 )Turn the stopper through at least one revolution in each direction.

(2) Lift it out often so as to give the fresh emery a chance of getting into the joint.

(3) Rotate the bottle as well as the stopper in case there is any irregularity in the force brought to bear, which might cause one side of the neck to be more ground than another, or would cause the tube to set rather to one side or the other.

(4) Use emery passing a 50 sieve, i.e. a sieve with fifty threads to the inch run (see § 144) to begin with, and when the stopper nearly fits, wash this thoroughly away, and finish with flour emery, previously washed to get rid of particles of excessive size; the process of washing will be fully discussed in the chapter on glass-grinding, which see.

(5) Any degree of fineness of surface may be obtained by using graded emery, as will be explained, but, in general, it is unnecessary to attempt a finer surface than can be got with washed flour emery. A superficial and imperfect polish may be given by grinding for a short time with powdered pumice stone.

(6) If the proper taper is not attained by blowing, or if ridges are left on the tapered part, the process may be both hastened and improved by giving the taper a preliminary filing with a three-cornered file and kerosene, just as one would proceed with iron or brass. A little filing will often save a good deal of grinding and make a better job.

If a bottle without a tapered neck is to be employed, it is as well to do the preliminary grinding by means of a cone turned up from a bit of cast iron. This is put in the lathe and pushed into the mouth of the bottle, the latter being supported by the hands. Use about the same surface speed as would be employed for turning cast iron. In this case the emery is better used with kerosene.

If a cylindrical bit of cast iron about an inch in diameter is turned down conically nearly to a point, it will save a good deal of trouble in making separate cones. If it gets ground into rings, and it becomes necessary to turn it up, use a diamond tool until the skin is thoroughly removed; the embedded emery merely grinds the edge off any ordinary steel tool.

For diamond tools see § 55.

§ 42. Use of the Lathe in Glass-working.

If it is necessary to remove a good deal of glass, time may be saved by actually turning the glass in a lathe. According to the direction given above for grinding a tube into the neck of a bottle, very little glass need be removed if the drawing down is well done, so that for this purpose turning is often unnecessary.

If the taper of the stopper be small and it is permissible to use a thick tube, or if a solid stopper only has to be provided, or an old stopper quickly altered to a new form, turning is very useful. The glass may be "chucked" in any suitable manner, and run at a speed not exceeding 10 feet per minute. Prepare a three-cornered file by mercury-hardening and by grinding the end flat so as to form a cutting angle of about 80°, and use a moderate amount of kerosene lubrication, i.e. enough to keep the glass damp, but even this is not essential. Use the file as an ordinary brass turning tool, and press much more lightly than for metal turning. The glass will be found to scrape off quite pleasantly.

By chucking glass tubes on wooden mandrells the ends may be nicely turned in this manner ready for accurate closing by glass plates.

The process of grinding also is made much more rapid--at all events in the earlier stages--by chucking either the stopper or the bottle and holding the other member in the fingers, or in a wooden vice held in the hands. The finishing touches are best given by hand.

I ought to say that I think a good deal of glass-grinding, as practised in laboratories, might be advantageously replaced by glass turning or filing and certainly will be by any one who will give these methods a trial.

If one tube is to be ground into another, as in grinding a retort into a receiver, the latter must be drawn down from a larger piece, few beginners being able to widen a tube by the method explained with sufficient ease and certainty. The other operations are similar to the operations above described.

§ 43. Funnels often require to be ground to an angle of 60°. For this purpose it is well to keep a cast-iron cone, tapering from nothing up to four inches in diameter. This may be mounted on a lathe, and will be found of great use for grinding out the inside of funnels. Care must be taken to work the funnel backwards and forwards, or it will tend to grind so as to form rings, which interfere with filtering. A rough polish may be given on the lines explained in the next section.

§ 44. A rough polish may be easily given to a surface which has been finished by washed flour emery, in the following manner. Turn up a disc of soft wood on the lathe, and run it at the highest wood-turning speed. Rub into the periphery a paste of sifted powdered pumice stone and water.

Any fairly smooth ground glass surface may be more or less polished by holding it for a moment against the revolving disc. Exact means of polishing will be described later on. Meanwhile this simple method will be found both quick and convenient, and is often quite sufficient where transparency, rather than figure, is required. I daresay a fine polish may be got on the same lines, using putty powder or washed rouge (not jewellers' rouge, which is too soft, but glass-polishers' rouge) to follow the pumice powder, but I have not required to try this.

§ 45. It is sometimes required to give to ground glass surfaces a temporary transparency. This is to be done by using a film of oil of the same refractive index as the glass. Cornu has employed a varnish consisting of a mixture of turpentine and oil of cloves, but the yellow-brown colour of the latter is often a disadvantage. It will be found that a mixture of nut oil and oil of bitter almonds, or of bromo-napthalene and acetone, can be made of only a faint yellow colour; and by exact adjustment of the proportions will have the same refractive index for any ray as crown glass (ordinary window glass).

Procure a sample of the glass and smash it up to small fragments in an iron mortar. Sift out the fine dust and the larger pieces; bits about as large as small beads--say one-sixteenth inch every way--do very well. Boil the sifted glass with strong commercial hydrochloric acid to remove iron, wash with distilled water and a few drops of alcohol, dry on blotting paper in the sun or otherwise. Put the dry glass into a bottle or beaker, and begin by adding almond oil (or bromo-napthalene), then add nut oil (or acetone) till the glass practically disappears when examined by sodium light, or light of any other wave-length, as may be required.

The adjustment of the mixture is a matter of great delicacy, one drop too much of either constituent, in, say, 50 cubic centimetres, makes all the difference. The final adjustment is best accomplished by having two mixtures of the oils, one just too rich in almond, the other in nut oil; by adding one or other of these, the required mixture is soon obtained.

It is to be noted

(1) That adjustment is only perfect for light of one wave-length.

(2) That adjustment is only perfect at one temperature.

On examining a bottle of rather larger fragments of glass immersed in an adjusted mixture by ordinary daylight, a peculiarly beautiful play of colours is seen.

Of course, if it is only desired to make ground glass fairly transparent, these precautions are unnecessary, but it seemed better to dispose of the matter once for all in this connection.

M. Cornu's object was to make a varnish which would prevent reflection from the back of a photographic plate on to the film. I have had occasion to require to do the same when using a scale made by cutting lines through a film of black varnish on a slip of glass. This succeeded perfectly by making the varnish out of Canada balsam stained with a black aniline dye.

Mr. Russell, Government Astronomer of New South Wales, finds that the "halation" of star photographs can be prevented by pouring over the back of the plate a film of collodion suitably stained.

§ 46. Making Ground Glass.

This is easily done by rubbing the surface of polished glass with a bit of cast iron and washed "flour of emery." Of course, if the fineness of grain of the surface is of importance, appropriate sizes of emery must be employed. The iron may be replaced by a bit of glass cut with transverse grooves to allow the emery to distribute itself, or even by a bit of glass without such grooves, provided it does not measure more than one or two inches each way. If great speed is an object rather than the fineness of the surface, use a bit of lead and coarse emery, say any that will pass a sieve with fifty threads to the inch.

It may perhaps be mentioned here that it is a pity to throw away emery which has been used between glass and glass. In the chapter dealing with fine optical work the use of emery of various grades of fineness will be treated, and the finer grades can only be obtained (to my knowledge) from emery which has been crushed in the process of glass or metal grinding, especially the former. A large jam-pot covered with a cardboard lid does well as a receptacle of washings.

§ 47. Glass-cutting.

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

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

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

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

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

§ 48. Cementing.

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

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

(1) The glass must be clean;

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

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

First, the isinglass is to be tightly packed into a bottle with a wide neck, then add the water, and let the isinglass soak it up. Afterwards pour in the acetic acid, and keep the mixture near 100°C. for an hour or two on the water bath--or rather in it. The total volume of acetic acid and water should not be more than about half of the volume of isinglass when the latter is pressed into the bottle as tightly as possible.

The proper consistency of the cement may be ascertained by lifting a drop out of the bottle and allowing it to cool on a sheet of glass. In ten minutes it ought not to be more than slightly sticky, and the mass in the bottle, after standing a few hours cold, should not be sticky at all, and should yield, jelly-like, to the pressure of the finger to only a slight degree. If the glue is too weak, more isinglass may be added (without any preliminary soaking).

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

Ordinary glue will serve perfectly for cementing glass to wood.

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

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

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

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

§ 49. Fusing Electrodes into Glass.

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

Many years ago it was the fashion to surround the platinum wire with a drop of white enamel glass in order to cause better adhesion between it and the ordinary glass. [Footnote: Hittorf and Geissler (Pogg. Ann. 1864, § 35; English translation, Phys. Soc. London, p. 138) found that it was impossible to make air-tight joints between platinum and hard potash glass, but that soft lead glass could be used with success as a cement.] However, in the case of flint glass, if one may judge from incandescent lamps, this is not essential--a fact which entirely coincides with my own experience.

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

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

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

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

A well-known German physicist (Warburg, I think) recommends putting the seals under water, but I cannot think that this is a good plan, for if air can get in, why not water? which has its surface tension in its favour. The same reasoning prevents my recommending a layer of sulphuric acid above the mercury-a method used for securing air-tightness in "mercury joints" by Mr. Gimingham, Proc. R. S. 1874.

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

In some cases the electrodes must be expected to get very hot, for instance, when it is desired to platinise mirrors by the device of Professor Wright of Yale. In this and similar cases I have met with great success by using "barometer" tubes of about one-twelfth of an inch bore, and with walls, say, one-tenth of an inch thick. [Footnote: "Barometer" tube is merely very thick-walled glass tubing, and makes particularly bad barometers, which are sold as weather glasses.]

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

Fig. 37.

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

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

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

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

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

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

Surely in face of these facts we must admit that cement joints can be made with fair success. I do not know the composition of M. Regnault's mastic, but Faraday (Manipulations, § 1123) gives the following receipt for a cement for joining ferules to retorts, etc:

Resin 5 parts.

Beeswax 1 part.

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

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

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

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

My attention has been directed to a fusible metal cement containing mercury, and made according to the following receipt, given by Mr. S. G. Rawson, Journal of the Society of Chemical Industry, vol. ix. (1890), P. 150:-

Bismuth 40 per cent

Lead 25 per cent

Tin 10 per cent

Cadmium 10 per cent

Mercury 15 per cent

This is practically one form of Rose's fusible metal with 15 per cent mercury added. It takes nearly an hour to set completely, and the apparatus must be clean and warm before it is applied.

As the result of several trials by myself and friends, I am afraid I must dissent from the claim of the author that such a cement will make a really air-tight joint between glass tubes. Indeed, the appearance of the surface as viewed through the glass is not such as to give any confidence, no matter what care may have been exercised in performing all the operations and cleaning the glass; besides which the cement is rigid when cold, and the expansion difficulty comes in.

On the other hand, if extreme air-tightness is not an object, the cement is strong and easily applied, and has many uses. I have an idea that if the joints were covered with a layer of soft wax, the result would be satisfactory in so far as air-tightness is concerned.

This anticipation has since been verified.

In many cases one can resort to the device already mentioned of enclosing a rubber or tape-wrapped joint between two tubes in a bath of mercury, but in this case the glass must be clean and hot and the mercury also warm, dry, and pure when the joint is put together, otherwise an appreciable air film is left against the glass, and this may creep into the joint.

Perhaps the easiest way of making such a joint is to use an outer tube of thin clean glass, and bore a narrow hole into it from one side to admit the mercury; if the mercury is to be heated in vacuo, it is better to seal on a side joint. It is always better, if possible, to boil the mercury in situ, which involves making the wrapping of asbestos, but, after all, we come back to the position I began by taking up, viz. that the easiest and most reliable method is by fusion of the glass--all the rest are unsuitable for work of real precision.

I should be ungrateful, however, were I not to devote a few lines to the great convenience and merit of so-called "centering cement." This substance has two or three very valuable properties. It is very tough and strong in itself, and it remains plastic on cooling for some time before it really sets. If for any reason a small tube has to be cemented into a larger one, which is a good deal larger, so that an appreciable mass of cement is necessary, and particularly if the joint requires to have great mechanical strength, this cement is invaluable. I have even used a plug of it instead of a cork for making the joint between a gas delivery tube and a calcium chloride tower. (Why are these affairs made with such abominable tubulures?)

The joint in question has never allowed the tube to sag though it projects horizontally to a distance of 6 inches, and has had to withstand nearly two years of Sydney temperature. The cement consists of a mixture of shellac and 10 per cent of oil of cassia.

The shellac is first melted in an iron ladle, and the oil of cassia quickly added and stirred in, to an extent of about 10 per cent, but the exact proportions are not of importance. Great care must be taken not to overheat the shellac.