A convenient cutting lubricant is a solution of Castile soap in water, and this must be freely supplied; if the wheel gets dry it is almost immediately spoiled owing to the diamond dust being sc.r.a.ped off. In the figure the lubricant is supplied by a wick running into the reservoir. I have used both clock oil and ordinary gas-engine oil as lubricants, with equally satisfactory results. As to the speed of cutting, in the experiment quoted a bit of rather friable "gabbro,"

measuring three-quarters of an inch on the face by five-eighths of an inch thick, was cut clean through in six minutes, or by 3000 turns of the wheel. The travel of the edge was thus between 5000 and 6000 feet, or say 9000 feet, nearly 2 miles, per inch cut.

A good solid rock, like basalt, can be cut into slices of about 3/32 inch thick. A very loose rock is best boiled in Canada balsam, hard enough to set, before it is put against the wheel.

Instead of a grinding machine a lathe may be employed. The disc is, of course, mounted on the mandrel, and the work on the slide-rest.

The latter must be disconnected from its feed screws, and a weight arranged over a pulley so as to keep the work pressed against the wheel by a constant force.

It may, perhaps, occur to the reader to inquire whether any clearance in the cut is necessary. The answer is that in all probability, and in spite of every care, the wheel will wobble enough to give clearance. If it does not, a little diamond dust rubbed into the side of the wheel, as well as the edge, will do all that is required. The edge also, after two or three armings, "burrs" a little, and thus provides a clearance naturally. It is not unlikely that in the near future the electric furnace will furnish us with a number of products capable of replacing the diamond as abrading agents. The cost of the small amount of diamond dust; required in a laboratory is so small, however, that it; is doubtful whether any appreciable economy will be, effected.

-- 78. Grinding Rock Sections, or Thin Slips of any Hard Material.

A note on this is, perhaps, worth making, for the same reasons as were given for note, -- 75, which it naturally follows. Just as trout-fishing; is described by Mr. Francis as the "art of fine and far off," [Footnote: In the Badminton Library, volume on Fishing.]

section grinding may be called "the art of Canada balsam cooking," as follows. A section of rock having been cut from the lump as just described, it becomes; necessary to grind it down for purposes of microscopical investigation. For this purpose it is placed on a slip of gla.s.s, and cemented in position by Canada, balsam. Success in the operation of grinding the mounted section depends almost entirely on the way in which the mounting is done, and this in its turn depends on the condition to which the Canada has been brought.

To ill.u.s.trate the operations, I will describe a specific case, viz.

that of grinding the section of "gabbro"' above described, for microscopical purposes. One side of the section is probably sufficiently smooth and plane from the operation of the diamond wheel; if not, it must be ground by the finger on a slab of iron or gun-metal with emery and water, the emery pa.s.sing a sieve of 80 threads to the inch. The gla.s.s base on which the section is to be mounted for grinding is placed on a bit of iron or copper plate over a Bunsen burner, and three or four drops of natural Canada balsam are placed upon it. The section is placed on the plate to heat at the same time.

The temperature must not rise so high as to cause any visible change in the Canada balsam, except a slight formation of bubbles, which rise to the surface, and can be blown off. The heating may require to be continued, say, up to twenty minutes. The progress of the operation is tested by examining the balsam as to its viscous properties.

An exceedingly simple and accurate way of testing is to dip a pair of ordinary forceps in the balsam, which may be stirred a little to secure uniformity. The forceps are introduced with the jaws in contact, and, as soon as withdrawn, the jaws are allowed to spring apart, thus drawing out a balsam thread. In a few moments the thread is cold, and if the forceps be compressed, this thread will bend.

The Canada must be heated until it is just in such a state that on bringing the jaws together the thread breaks. The forceps may open to about three-quarters of an inch. If the Canada is more viscous, so that the thread does not break, the section when cemented by it will most probably slip on the slide. On the other hand, if the balsam is more brittle, it will crumble away during the grinding.

a.s.suming that the proper point has been reached, the section is mounted with the usual precautions to avoid air bubbles, i.e. by dropping one edge on the balsam first. When all is cold, the surface of the section may be ground on an iron plate with emery pa.s.sing the 80 sieve, till it is about 1/40 inch thick. From this point it must be reduced on ground gla.s.s by flours of emery and water; the rough particles of the former may be washed out for fine work.

The process of grinding should not take more than half an hour if the section is properly cut, etc. Beyond this point the allowable thickness must depend on the nature of the rock; a good general rule is to get the section just so thin that felspars show the yellow of the first order in a polarising, microscope. The section is then finished with, say, two minutes emery or water of Ayr-stone dust. It is better not to have the surface too smooth.

To transfer the section, the hard Canada round the sides is sc.r.a.ped away, and the section itself covered with some fresh Canada from the bottle. It is then warmed till it will slip off when a pin, or the invaluable dentist's chisel, is pressed against one side. If the section be very delicate, the cover slip should be placed over it before it is moved to the proper slide. The Canada used for mounting is not quite so hard as that employed in grinding, but it should be hard when cold, i.e. not sticky.

The art of preparing Canada balsam appears to consist in heating it under such conditions as will ensure its being exposed in thin layers.

I have wasted a good deal of time in trying to bake Canada in evaporating basins, with the invariable result that it was either over or under-baked, and got dark in colour during the process.

On reviewing the process of rock section-cutting and mounting as just described, I cannot help thinking that, if properly systematised, it could be made much more rapid by the introduction of proper automatic grinding machinery. It also seems not improbable that a proper overhaul of available gums and cements would be found to lead to a cementing material less troublesome than Canada balsam.

-- 79. Cutting Sections of Soft Substances.

Though this art is fully treated of in books on practical biology, it is occasionally of use to the physicist, and the following note treats of that part of the subject which is not distinctly biological.

Soft materials, of which thin sections may be required, generally require to be strengthened before they are cut. For this purpose a variety of materials are available. The one most generally used is hard paraffin. The only point requiring attention is the embedding.

The material must be dry.

This is accomplished by soaking in absolute alcohol, i.e. really absolute alcohol made by shaking up rectified spirit with pota.s.sium carbonate, previously dried, and then digesting for a day with large excess of quick-lime, making use of an inverted condenser and finally distilling off the alcohol without allowing it to come in contact with undried air. After soaking for some time in absolute alcohol, the material may be transferred to oil of bergamot, or oil of cloves, or almost any essential oil. After soaking in this long enough to allow the alcohol to diffuse out, the material may be lifted into a bath of melted paraffin (melting at, say, 51 C.). The process of soaking is in some cases made to go more rapidly by exhausting, and, if the material will stand it, by raising the temperature over 100 C. The soaking process may require minutes, hours, or days, according to the size and density of the material; but a few hours are usually sufficient.

When cold, the sections may be cut in any of the ordinary forms of microtome.

Another way of embedding is to soak in collodion, and then precipitate the latter in the material and around it by plunging into nearly absolute alcohol. The collodion yields a harder matrix than the paraffin.

Whatever form of cutting machine is employed, the art of sharpening the knife is the only one requiring any particular notice. The easiest way of obtaining a knife hard enough to sharpen, is to use a razor of good quality. If it has to be ground, it is best to do this on a fine Turkey stone which is conveniently rested on two bits of rubber tubing, to avoid jarring the blade. Many stones are slightly cracked, but on no account must the razor be dragged across a crack, or the edge will suffer.

The necessary and sufficient condition is that the razor must be worked in little sweeps over the stone, and pressed against the latter by little more than its own weight, and the grinding must be regular.

The edge may be inspected under a microscope, and it must be perfectly smooth and even before it will cut sections. A finishing touch may be given on a leather strap, but it must be done skilfully, otherwise it is better omitted.

The necessity for providing exceptionally keen and sharp edges arose in the manufacture of phonographs, where the knife used to turn up the wax cylinders must leave a perfectly smooth surface. In 1889 this was being accomplished on an ivory lap fed with a trace of very fine diamond dust.

I have had this method in mind as a possible solution of the difficulty of razor-grinding, but have not tried it. I imagine one would use a soft steel or ivory slip rubbed over with fine diamond dust and oil by means of an agate. The lap used in the phonograph works was rotated at a high speed.

-- 80. On the Production of Quartz Threads.

[Footnote: Since this was written an article on the same subject by Mr. Boys appeared in the Electrician for 1896. The instructions therein given are in accordance with what I had written, and I have made no alteration in the text.]

In 1887 the important properties of fused quartz were discovered by Mr. Vernon Boys (Philosophical Magazine, June 1887, p. 489, "On the Production, Properties, and Some Suggested Uses of the Finest Threads"). A detailed study of the properties of quartz threads was made by Mr. Boys and communicated to the Society of Arts in 1889 (Journal of the Society of Arts, 1889). An independent study of the subject was made by the present writer in 1889 (Philosophical Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").

There is also a paper in the Philosophical Magazine for 1894 (vol.

x.x.xvii. p. 463), by Mr. Boys, on "The Attachment of Quartz Fibres."

This paper also appeared in the Journal of the Physical Society at about the same date, together with an interesting discussion of the matter. In the American Journal, Electric Power, for 1894, there is a series of articles by Professor Nichols on "Galvanometers," in which a particular method of producing quartz threads is recommended. The method was originally discovered by Mr. Boys, but he seems to have made no use of it. A hunt through French and German literature on the subject has disclosed nothing of interest--nothing indeed which cannot be found in the papers mentioned.

-- 81. Quartz fibres have two great advantages over other forms of suspension when employed for any kind of torsion balance, from an ordinary more or less "astatic" galvanometer to the Cavendish apparatus. In the first place the actual strength of the fibres under longitudinal stress is remarkably high, ranging from fifty to seventy tons weight per square inch of section, and even more than this in the case of very fine threads; the second and more important point in favour of quartz depends on the wide limits within which cylindrical threads of this material obey the simplest possible law of torsion, i.e. the law that for a given thread carrying a given weight at a given temperature and having one end clamped, the twist about the axis of figure produced by a turning moment applied at the free end is proportional simply to the moment of the twisting forces, and is independent of the previous history of the thread.

It is to be noted, however, that the torsional resilience of quartz as tested by the above law is not so perfect but that our instrumental means allow us to detect its imperfections, and thus to satisfy ourselves that threads made of quartz are not things standing apart from all other materials, except in the sense that the limits within which they may be twisted without deviating in their behaviour from the law of strict proportionality by more than some una.s.signed small quant.i.ty, are phenomenally wide.

A torsion balance--if we except the case of certain spiral springs--is almost always called upon for information as to the magnitude of very small forces, and for this purpose it is not essential merely that some law of twisting should be exactly obeyed, but also that the resistance to twisting of the suspension should be small.

Now, regarded merely as a substance possessing elastic rigidity, quartz is markedly inferior to the majority of materials, for it is very stiff indeed; its utility depends as much as anything upon its great strength, for this allows us to, use threads of exceeding fineness. In addition to this it must be possible, and moreover readily possible, to obtain threads of uniform section over a sufficient length, or the rate of twist per unit length of the thread will vary in practice from point to point, so that the limits of allowable twist averaged over the whole thread may not be exceeded, and yet they may be greatly overpa.s.sed at particular points of the thread.

It is interesting to note that in the case of quartz we not only have a means for readily producing very uniform cylindrical threads, but that the limits of allowable rate of twist are so wide that a small departure from uniformity of section produces much less inconvenience than in the case of any other known substance.

-- 82. There are three methods generally in use for drawing quartz fibres, all depending on the fact that quartz when fused is so viscous that it may be drawn into threads of great length, without these threads breaking up into drops, or indeed without their showing any sign of doing so. The surface tension of the melted quartz must, however, be very considerable, as may be seen by examining the shape of a drop of the molten material, and this suffices to impress a rigidly cylindrical form upon the thread, the great viscosity apparently damping down all oscillation.

The first method is the one originally employed by Mr. Boys. A needle of quartz is melted somewhere in its length and is then drawn out rapidly by a light arrow, to which one end of the needle is attached, and which is projected from a kind of crossbow.

A modification of this method, which the writer has found of service when very thick threads are required, is to replace the bow and arrow by a kind of catapult.

The third method, which yields threads of almost unmanageable fineness, depends on the experimental fact that when a fine point of quartz is held in a high pressure oxygen gas blow-pipe flame, the friction of the flame gases suffices to overcome the tendency of the capillary forces to produce a spherical drop, and actually causes a fine thread to be projected outwards in the direction of the flame.

-- 83. A preliminary operation to any method is the production of a stick of fused quartz. This is managed as follows. A rock crystal or quartz pebble is selected and examined. It must be perfectly white, transparent, and free from dirt. Surface impurity can of course be got rid of by means of a grindstone. The crystal is placed in a perfectly clean Stourbridge clay crucible, furnished with a cover, and heated to bright redness for about an hour in a clean fire or in a Fletcher's gas furnace. The contents of the crucible are turned out when sufficiently cool on to a clean brick or bit of slate. It will be found that the crystal is completely broken up and the fragments must be examined in case any of them have become contaminated by the crucible, but this will not have happened if the temperature did not rise beyond a bright red heat.

The heap of fragments being found satisfactory, the next thing is to fuse some of the pieces together. Unless the preliminary heating has been efficiently carried out this will prove an annoying task, because a rock crystal generally contains so much water that it splinters under the blow-pipe in a very persistent manner. There are two ways of a.s.sembling the fragments. One is to place two tiles or bricks on edge about the heap of quartz lying upon a third tile, so that the heap occupies the angular corner or nook formed by the tiles (Fig.

64).

The oxygas blow-pipe previously described is adjusted to give its hottest flame, the bags being weighted by at least two hundredweight, if of the size described (see -- 15).

The tip of the inner cone of the blow-pipe is brought to bear directly upon one of the fragments, and if the operation is performed boldly it will be found that the surface of the fragment can be fused, and the fragment thus caused to hold together before the lower side gets hot enough to suffer any contamination from the tile or brick. A second fragment may be treated in the same way, and then a third, and so on.

Finally, the fragments may be fused together slightly at the corners, and a stick may thus be formed. Of course a good deal of cracking and splitting of the fragments takes place in the process; the best pieces to operate upon are those which are well cracked to begin with, and that in such a way that the little fragments are interlocked.

An alternative method which has some advantages is to arm a pair of forceps with two stout platinum jaws, say an inch and a half long, and flattened a little at the ends. The fragments are held in these platinum forceps and the blow-pipe applied as before. This method works very well in adding to a rod which has already been partly formed, but the jaws require constant renewals. The first fragment which is fused sufficiently to cohere may also be fused to a bit of tobacco pipe, or hard gla.s.s tube or rod, and the quartz stick gradually built up by fusing fresh pieces on to the one already in position.

Fig. 64.