Since the gla.s.s or pipeclay will contaminate the quartz which has been fused on to it, it is necessary to discard the end pieces at the close of the operation. A string of fragments having been collected and stuck together, the next step is to fuse them down into a uniform rod.

This is easily done by holding the string in the blow-pipe flame and allowing it to fuse down. Twisting the fused part has a good effect in a.s.sisting the operation. It is desirable to use a large jet and as powerful a flame as can be obtained during this part of the operation.

The final result should be a rod, say two or three inches long and one-eighth of an inch thick, which will in most cases contain a large number of air bubbles. Since the presence of drawn-out bubbles cannot be advantageous, it is often desirable to get rid of them, and this can conveniently be done at the present stage. The process at best is rather tedious; it consists in drawing the quartz down very fine before an intense flame, in order to allow the bubbles to get close enough to the surface to burst. A considerable loss of material invariably occurs during the process; for whenever the thin rod separates into two bits the process of flame-drawing of threads goes on, and entails a certain waste; moreover, the quartz in fine filaments is probably partially volatilised.

Sooner or later, however, a sufficient length of bubble-free quartz can be obtained. It must not be supposed that it is always necessary to eliminate bubbles as perfectly as is contemplated in the foregoing description of the treatment, but for special purposes it may be essential to do so, and in any case the reader's attention is directed to a possible source of error.

It may be mentioned in connection with this matter that crystals of quartz may look perfectly white and clear, and yet contain impurity.

For instance, traces of sodium are generally present, and lithium was found in large spectroscopic quant.i.ty in five out of six samples of the purest crystals in my laboratory. The presence of lithium in rock crystal has also been detected by Tegetmeier (Vied. Ann, xli. p.

19, 1890).

After some practice in preparing rods and freeing them of bubbles the operator will notice a distinct difference in the fusibility of the samples of quartz he investigates, though all may appear equally pure to the unaided eye. It should be mentioned, however, that high infusibility cannot always be taken as a test of purity, for the most infusible, or rather most viscous, sample examined by the writer contained more lithium than some less viscous samples.

Fig. 65.

During the process of freeing the quartz from bubbles the lithium and sodium will be found to burn away, or at all events to disappear.

A rod of quartz, say three inches long, one-sixteenth of an inch in diameter, and free from bubbles for half an inch of its length, even when examined by a strong lens, is suitable for drawing into threads.

The rod is manipulated exactly in the manner described under gla.s.s-blowing, and is finally drawn down at the bubble free part into a needle, say 0.02 inch in diameter (No. 25 on the Birmingham wire gauge), and 2 inches long.

Fig. 66.

There is one peculiarity about fused quartz which renders its manipulation easier than that of gla.s.s--it is impossible to break fused quartz, however suddenly it be thrust into the blow-pipe flame.

A rod having a diameter of three-sixteenths of an inch--and perhaps much more--may be brought right up to the tip of the inner cone of the oxy-gas flame and held there-till one side fuses, the other being comparatively cool, without the slightest fear of precipitating a smash. In seven years' experience I have never seen a bit of once fused quartz broken by sudden heating; whether it might be done if sufficient precautions were taken I do not know.

The reason of the fortunate peculiarity of quartz in this respect is, I presume, to be found in the fact that quartz once it has been fused is really a very strong material indeed, and is also probably the least expansible substance known. From some experiments of the writer upon the subject, it may be concluded that at the most quartz which has been fused expands only about one-fifth as fast as flint-gla.s.s, at all events between 20 and 70 C.

-- 84. Drawing Quartz Threads.

The thick end of the rod of quartz is held in the fingers or occasionally in a clip. The end of the fine point is attached to a straw arrow by means of a little sealing-wax. The arrow is laid on the stock of a crossbow in the proper position for firing. See Figs. 67 and 68, which practically explain themselves.

The needle is heated by the blow-pipe till a minute length is in a state of uniform fusion; the arrow is then let fly, when it draws a thread out with it. The arrow is preferably allowed to strike a wooden target placed, say, 30 feet away from the bow, and a width of black glazed calico is laid under the line of fire to catch the thread or arrow if it falls short. The general arrangements will be obvious from the figure.

The bow is of pine in the case where very long thin threads are required, though for ordinary purposes I have found a bow of lance-wood succeed quite as well. The trigger of the bow consists of a simple pin pa.s.sing through the stock and fastened at its lower end to a string connected with a board which can be depressed by foot. In the figure an ordinary trigger is shown, but the pin does just as well.

Fig 67.

The arrow is made out of about 6 inches of straw, plugged up aft by a small plug of pine or willow fastened in with sealing-wax, and projecting backwards one-eighth of an inch. This projection serves a double purpose: it gives a point of attachment for the quartz needle, and on firing the bow it forms a resisting anvil on which the string of the bow impinges. The head of the arrow is formed by a large needle stuck in with sealing-wax, and heavy enough to bring the centre of gravity of the arrow forward of one-third of its length, the condition of stability in flight.

Fig. 68.

It is not necessary to employ any feathering for these arrows; though I have occasionally used feathers or mica to "wing the shaft" no advantage has resulted therefrom.

To get fine threads a high velocity is essential. This is obtained by considering (and acting upon) the principles involved. The bow may be regarded as a doubly-tapering rod clamped at the middle. After deflection it returns towards its equilibrium position at a rate depending in general terms on the elastic forces brought into play, directly, and on the effective moment of inertia of the rod, inversely (see Rayleigh, Sound, vol. ii. chap. viii.) If the ma.s.s of the arrow is negligible compared with the bow, the rate at which the arrow moves is practically determined by that attained by the end of the bow, which is a maximum in crossing its equilibrium position.

The extent to which the arrow profits by this velocity depends on the way the bow is strung. It will be greatest when the string is perpendicular to the bow when pa.s.sing its equilibrium position; or in other words, when the string is infinitely long. Since the string has ma.s.s, however, it is not permissible to make it too long, or its weight begins to make itself felt, and a point is soon reached at which the geometrical gain in string velocity is compensated for by the total loss of velocity due to the inertia of the string. In practice it is sufficient to use a string 10 per cent longer than the bow.

It is well to use a light fiddle string, served with waxed silk at the trigger catch; if this be omitted the gut gets worn through very quickly. In order to decide how far it is permissible to bend the bow, the quickest way is to make a rough experiment on a bit of the same plank from which the bow is to be cut, and then to allow a small factor of safety. In the figure the bow is of lance-wood and is more bent than would be suitable for pine.

The bow itself is tapered from the middle outwards just like any other bow. If thick threads are required, the above considerations are modified by the fact that quartz opposes a considerable resistance to drawing, and that consequently the arrow must not only have a high velocity, but a fair supply of energy as well; in other words, it must be heavy. A thin pine arrow instead of a straw generally does very well, but in this case the advantage of using pine for the bow vanishes; and in fact lance-wood does better, owing to the greater displacement which it will stand without breaking. This of course only means that a greater store of energy can be acc.u.mulated at one bending.

I had occasion to investigate whether the unavoidable spin of an arrow about its axis produces any effect on the thread, and for this purpose made arrows with inertia bars thrust through the head, i.e. an arrow with a bit of wire run through it, perpendicular to its length--forming a cross in fact--the arms of the cross being weighted at the extreme ends by shot. This form of arrow has a considerable moment of inertia about its longer axis, and consequently rotates less than a mere straw, provided that the couples tending to produce rotation are not increased by the cross arm, or the velocity too much reduced.

Shooting one of these arrows slowly, I could see that it did not rotate, and when fired at a high velocity, it generally arrived at the target (placed at varying distances front bow) with the arms nearly horizontal, thus showing that it probably did not rotate much.

I did not succeed in this at the first trial, by any means. The threads got in this way were no better than those made with a single straw, whence we may conclude very provisionally that the spin of the arrow has only a small effect, if any, on the quality of the threads.

Feathering the arrow, in my experience, tends, if anything to make it spin more; for one thing, because it is practically impossible to lay the feathering on straight.

After the arrow is shot, it remains to gather in the thread, and if the latter is at all thin, we have a rather troublesome job. In a thread thirty or forty feet long, the most uniform part generally lies in the middle if the thread is thin, i.e. of the order of a ten-thousandth of an inch in diameter. If the thread is thick the most uniform part may be anywhere. The part of the thread required is generally best isolated by pa.s.sing a slip of paper under it at each end and cementing the thread to the paper by means of a little paraffin or soft wax, and then cutting off the outer portions. One bit of paper may then be lifted off the calico, and the thread will carry the other bit. In this way the thread may be taken to a blackened board, where it may be mounted for stock.

By pa.s.sing the two ends of the thread under a microscope, or rather by breaking bits off the two ends and examining them together, it is easy to form an Opinion as to uniformity.

Mr. Boys has employed an optical method of examining threads, but the writer has invariably found a high-power microscope more convenient and capable of giving more exact information as to the diameter of the threads.

The beginner--or indeed the practised hand--need not expect to get a thread of the exact dimensions required at the first shot. A little experience is necessary to enable one to judge of the right thickness of the needle for a thread of given diameter. The threads are so easily shot, however, that a few trials take up very little time and generally afford quite sufficient experience to enable a thread of any required diameter to be prepared.

It is no use attempting to heat an appreciable length of needle; if this be done the thread almost invariably has a thick part about the middle of its length.. It is sufficient to fuse at most about one-twentieth of an inch along the needle before firing off the bow.

This can be done by means of the smaller oxygas blow-pipe jet described in the article on blow-pipes for gla.s.s-blowing, -- 14. The flame must of course be turned down so as to be of a suitable size. A sufficiently small flame may be got from almost any jet.

If the needle be not equally heated all round, the thread tends to be curly; indeed by means of the catapult, threads may be pulled which, when broken, tend to coil up like the balance-springs of watches, if only care be taken to have one side of the needle much hotter than the other.

-- 85. When examining bits of threads, say thicker than the two-thousandth of an inch, under the microscope it is convenient to use a film of glycerine stained with some kind of dye, in order to render the thread more sharply visible. The thread is mounted beneath a cover slip, and a drop of the stained glycerine allowed to run in.

Such a treatment gives the image of the thread a sharply defined edge 3 and the contrast between the whiteness of the thread and the colour of the background allows measurements to be made with great ease.

On the whole the easiest way of measuring the diameter of a thick thread is to use a measuring microscope, i.e. one in which the lens system can be displaced along a plane bed by means of a finely cut micrometer screw. The instruments made by the Cambridge Scientific Instrument Company do fairly well. Direct measurements up to 0.0001 inch are easily made by means of a microscope provided with a Zeiss "A" objective, and rather smaller differences of thickness can be made out by it. For thin threads the method next to be described is more fitting, because higher powers can be more conveniently used.

In this method an ordinary microscope is employed together with a scale micrometer, and either an eyepiece micrometer, or a camera and subsidiary scale. The eyepiece micrometer is the more convenient. If a camera be employed, i.e. such an one as is supplied by Zeiss, it is astonishing how the accuracy of observation may be increased by attending carefully to the illumination of both the subsidiary scale and of the thread. The two images should be as far as possible of equal brightness, and for this purpose it will be found requisite to employ small screens.

The detail of making a measurement by means of the micrometer eyepiece is very simple. The thread is arranged on the stage so as to point towards the observer, and the apparent diameter is read off on the eyepiece scale. In order to calibrate the latter it is only necessary to replace the thread by the stage micrometer, and to observe the number of stage micrometer divisions occupying the s.p.a.ce in the eyepiece micrometer formerly occupied by the thread. It is essential that both thread and stage micrometer should occupy the same position in the field, for errors due to unequal distortion may otherwise become of importance. For this reason it is best to utilise the centre of the field only.

The same remark applies to measurements by means of the camera, where the image of the thread is projected against the reflected image of the subsidiary scale laid alongside the microscope. In this case the value of the subsidiary scale divisions must be obtained from the divisions of the stage micrometer, coinciding as nearly as possible with the position occupied by the thread. Before commencing a measurement the screens are moved about till both images appear equally bright.

Threads up to about one twenty-thousandth of an inch in diameter may be sufficiently well measured by means of a Zeiss "4 centimetre apochromatic object-gla.s.s" and an eyepiece "No. 6" with sixteen centimetre tube length. [Footnote: The objective certainly had "4 cm."

marked on it, but the focal length appeared to be about I.5 mm. only.]

-- 86. Drawing Threads by the Catapult.

The bow-and-arrow method fails when threads of a greater diameter than about 0.0015 inch are required--at least if any reasonable uniformity be demanded, and no radical change in the bow and arrow be carried out.

Thus in the writer's laboratory a thread of about this diameter, within 1/10000 of an inch-13 inches long and free from air bubbles--was required. A fortnight's work by a most skilful operator only resulted in the production of two lengths satisfying the conditions.

The greatest loss of time occurs in the examination of the thread by means of the microscope.

Threads for galvanometer suspensions are conveniently from 0.0001 to 0.0004 inch in diameter, and are much more easily made and got uniform than thicker threads, to the production of which the catapult method applies.

A reference to the diagram will make the construction of the instrument quite clear. The moving end of the quartz is attached to a small boxwood slider working on a tubular girder or between wires.

The quartz is secured in position by clamps shown at A and B, and motion is imparted to the slider by a stretched piece of catapult elastic (C). An easy means of regulating the pull of the elastic is to hold it back by a loop of string whose length can be varied by twisting it round a pin.

Fig. 69. [Footnote: For greater clearness of drawing, the tube carrying the slider is shown somewhat higher above the base than is convenient in practice; and the slide itself is shown too thin in the direction of the hole through it.]