"It really takes years to know just what to do when you reach that point where another touch either gives you the most perfect results attainable, or ruins the work you have already done. It has taken us a long time to find out how to make a flat surface, and when we were called upon to make the twenty-eight plane and parallel surfaces for the investigation of the value of the metre of the international standard, every one of which required an accuracy of one-twentieth of a wave length, we had a difficult task to perform. However, it was found that every surface had the desired accuracy, and some of them went far beyond it.

It is an advantage in making flat surfaces to make more than one at a time; it is better to make at least three, and in fact we always grind and 'fine' three together. In making speculum plates we get up ten or twelve at once on the lead lap. These speculum plates we can test as we go on by means of our test plane until we get them nearly flat. In polishing them we first make quite a hard polisher, forming it on a large test plane that is very nearly correct. We then polish a while on one surface and test it, then on a second and test it, and after a while we acc.u.mulate plates that are slightly concave and slightly convex. By working upon these alternately with the same polisher, we finally get our polisher into such shape that it approximates more and more to a flat surface, and with extreme care and slow procedure we finally attain the results desired.

All our flats are polished on a machine which has but little virtue in itself, unmixed with brains. Any machine giving a straight diametrical stroke will answer the purpose. The gla.s.s should be mounted so as to be perfectly free to move in every direction--that is to say, perfectly unconstrained. We mount all our flats on a piece of body Brussels carpet, so that every individual part of the woof acts as a yielding spring. The flats are held in place by wooden clamps at the edges, which never touch, but allow the bits of gla.s.s or metal to move slowly around if they are circular; if they are rectangular we allow them to tumble about as they please within the frame holding them.

For making speculum metal plates either plane or concave we use polishers so hard that they scratch the metal all over the surface with fine microscopic scratches. We always work for figure, and when we get a hard polisher that is in proper shape, we can do ever so many surfaces with it if the environments of temperature are all right. If we have fifty speculum flats to make, and we recently made three times that number, we get them all ready and of accurate surface with the hard polisher. Then we prepare a very soft polisher, easily indented when cold with the thumb nail. A drop of rouge and about three drops of water are put on the plate, and with the soft polisher about one minute suffices to clean up all the scratches and leave a beautiful black polish on the metal. This final touch is given by hand; if we do not get the polish in a few minutes the surface is generally ruined for shape, and we have to resort to the hard polisher again.

I a.s.sure you that nothing but patience and perseverance will master the difficulty that one has to encounter, but with these two elements 'you are bound to get there.'"

CHAPTER III

MISCELLANEOUS PROCESSES

-- 74. Coating Gla.s.s with Aluminium and Soldering Aluminium.

A process of coating gla.s.s with aluminium has been lately discovered, which, if I mistake not, may be of immense service in special cases where a strongly adherent deposit is required. My attention was first attracted to the matter by an article in the Archives des Sciences physiques et naturelles de Geneve, 1894, by M. Margot. It appears that clean aluminium used as a pencil will leave a mark on clean damp gla.s.s. If, instead of a pencil, a small wheel of aluminium--say as big as a halfpenny and three times as thick--is rotated on the lathe, and a piece of gla.s.s pressed against it, the aluminium will form an adherent, though not very continuous coating on the gla.s.s.

Working with a disc of the size described rotating about as fast as for bra.s.s-turning, I covered about two square inches of gla.s.s surface in about five minutes. The deposit was of very uneven thickness, but was nearly all thick enough to be sensibly opaque. By burnishing the brilliance is improved (I used an agate burnisher and oil), but a little of the aluminium is rubbed off. The fact that the burnisher does not entirely remove it is a sign of the strength of the adherence which exists between the aluminium and the gla.s.s. In making the experiment, care must be taken to have the gla.s.s quite clean--or at all events free from grease--in order to obtain the best results.

M. Margot has contributed further information to the Archives des Sciences physiques et naturelles (February 1895). He finds that adherence between aluminium and gla.s.s is promoted by dusting the gla.s.s with powders, such as rouge. There is no doubt that a considerable improvement is effected in this way; both rouge and alumina have in my hands greatly increased the facility with which the aluminium is deposited. M. Margot finds that zinc and magnesium resemble aluminium in having properties of adherence to gla.s.s, and, what is more, carry this property into their alloys with tin. Thus an alloy of zinc and tin in the proportions of about 92 per cent tin and 8 per cent zinc may be melted on absolutely clean gla.s.s, and will adhere strongly to it if well rubbed by an asbestos crayon.

A happy inspiration was to try whether these alloys would, under similar circ.u.mstances, adhere to aluminium itself, and a trial showed that this was indeed the case, provided that both the aluminium and alloy are scrupulously clean and free from oxide. In this way M.

Margot has solved the problem of soldering aluminium. I have satisfied myself by trial of the perfect ease and absolute success of this method. The alloy of zinc and tin in the proportions above mentioned is formed at the lowest possible temperature by melting the const.i.tuents together. It is then poured so as to form thin sticks.

The aluminium is carefully cleaned by rubbing with a cuttle bone, or fine sand, and strong warm potash. It is then washed in water and dried with a clean cloth. The aluminium is now held over a clean flame and heated till it will melt the solder which is rubbed against it. The solder sticks at once, especially if rubbed with another bit of aluminium (an aluminium soldering bit) similarly coated. To solder two bits of aluminium together it is only necessary to tin the bits by this process and then sweat them together.

The same process applies perfectly to aluminium caused to adhere to gla.s.s by the previously mentioned process, and enables strong soldered contacts to be made to gla.s.s. In one case, while I was testing the method, the adhesion was so strong that the solder on contracting while cooling actually chipped the surface clean off the gla.s.s. In order to get over this I have endeavoured to soften the solder by mixing in a little of the fusible metal mercury amalgam; and though this prevents the gla.s.s from being so much strained, it reduces the adherence of the solder. It is a comfort to be able to solder aluminium after working for so many years by way of electroplating, or filing under solder. An alternative method of soldering aluminium will be described when the electroplating of aluminium is discussed, -- 138.

Gilding Gla.s.s. In looking over some volumes of the Journal fuer praktische Chemie, I came across a method of gilding gla.s.s due to Boettger (Journ. f. prakt. Chem. 103, p. 414). After many trials I believe I am in a position to give definite instructions as to the best way of carrying out this rather troublesome operation. The films of gold obtained by the process are very thick, and the appearance of the gold exceedingly fine. The difficulty lies in the exact apportionment of the reducing solution. If too much of the reducing solution be added, the gold deposits in a fine mud, and no coating is obtained. If, on the other hand, too little of the reducing solution be added, little or no gold is deposited. The secret of success turns on exactly hitting the proper proportions.

The reducing solution consists of a mixture of aldehyde and glucose, and the difficulty I have had in following Boettger's instructions arose from his specifying "commercial aldehyde" of a certain specific gravity which it was impossible to reproduce. I did not wish to specify pure aldehyde, which is not very easily got or stored, and consequently I have had to determine a criterion as to when the proportion of reducing solution is properly adjusted.

The aldehyde is best made as required. I employed the ordinary process as described in Thorpe's Dictionary of Applied Chemistry, by distilling alcohol, water, sulphuric acid, and manganese dioxide together. The crude product is mixed with a large quant.i.ty of calcium chloride (dry--not fused), and is rectified once. The process is stopped when the specific gravity of the product reaches 0.832 at 60 F. The specific gravity of pure aldehyde is 0.79 nearly.

The following is a modification of Boettger's formula:-

Solution I

1 gram of pure gold is converted into chloride--got acid free--i.e.

to the state represented by AuCl3, and dissolved in 120 cc. of water.

This solution is the equivalent of one containing 6.5 grains of trichloride to the ounce of water.

Solution II.

6 grams sodium hydrate.

100 grams water.

Solution III.

0.2 grams glucose (bought as pure).

12.6 cubic centimetres 95 per cent alcohol.

12.6 cubic centimetres water.

2.0 cubic centimetres aldehyde, sp. gr. 0.832.

To gild gla.s.s these solutions are used in the following proportions by volume:-

16 parts of No. I.

4 parts of No. II.

0.8 parts of No. III.

The gla.s.s is first cleaned well with acid and washed with water: it is then rinsed with Solution No. III. If it is desired to gild the inside of a gla.s.s vessel, Solution No. III. may be placed in the vessel first, and the walls of the vessel rinsed round carefully.

Solutions I. and II. are mixed separately and then added to III.--after about two minutes the whole is well shaken up.

If it be desired to gild a mirror of gla.s.s, the gla.s.s-plate is suspended face downwards in a dish of the mixed solutions--care being taken to rinse the gla.s.s with Solution III. first.

If the mixture darkens in from 7' to 10' in diffuse daylight and at 60F. it will gild well, and it generally pays to make a few trials in a test tube to arrive at this. If too much reducing solution is present the liquid will get dark more rapidly, and vice versa. The gilding will require several hours--as much as twelve hours may be needed.

The reaction is one of great chemical interest, being one of that cla.s.s of reactions which is greatly affected by capillarity. Thus it occasionally happens that when the reducing solution is not in the right proportion, gold will be deposited at the surface of the liquid (so as to form a gilt ring on the inside of a test tube), the remainder of the gold going down as mud. The gold deposited is at first transparent to transmitted light and is deeply blue. I thought this might be due to a trace of copper or silver, but on carefully purifying the gold no change of colour was noted. If the reducing solution is present in slightly greater proportions than that given in the formula, the gold comes down with a richer colour, and has a tendency to form a mat surface and to separate from the gla.s.s. The gold which is deposited more slowly has a less rich colour but a brighter surface. The operation should be interrupted when a sufficient deposit has been obtained, because it is found that the thicker the deposit, the more lightly is it held to the gla.s.s surface.

-- 75. The Use of the Diamond-cutting Wheel.

A matter which is not very well known outside geological circles is the manipulation of the diamond-cutting wheel, and as this is often of great use in the physical laboratory, the following notes may not be out of place. I first became acquainted with the art in connection with the necessity which arose for me to make galvanometer mirrors out of fused quartz, and it was then that I discovered with surprise how difficult it is to obtain information on the point. I desire to express my indebtedness to my colleagues, Professor David and Mr.

Smeeth, for the instruction they have given me. In what follows I propose to describe their practice rather than my own, which has been of a makeshift description. I will therefore select the process of cutting a slice of rock for microscopical investigation.

-- 76. Arming a Wheel.

Fig. 63.

A convenient wheel is made out of tin-plate, i.e. mild steel sheet, about one-thirtieth of an inch thick and seven inches in diameter.

This wheel must be quite flat and true, as well as round; too much pains cannot be taken in securing these qualities. After the wheel is mounted, it is better to turn it quite true by means of a watch-maker's "graver" or other suitable tool. The general design of a rock-cutting machine will be clear from the ill.u.s.tration (Fig. 63).

The wheel being set up correctly, the next step is to arm it with diamond dust. For this purpose it is before all things necessary that real diamond dust should be obtained. The best plan is to procure a bit of "bort" which has been used in a diamond drill, and whose properties have therefore been tested to some extent. This is ground in a diamond mortar--or rather hammered in one--and pa.s.sed through a sieve having at least 80 threads to the inch. The dust may be conveniently kept in oil.

To arm the wheel, a little dust and oil is taken on the finger, and laid on round the periphery of the wheel. A bit of flint or agate is then held firmly against the edge of the wheel and the latter is rotated two or three times by hand. The rotation must be quite slow--say one turn in half a minute--and the flint must be held firmly and steadily against the wheel. Some operators prefer to hammer the diamond dust into the wheel with a lump of flint, or agate, but there is a risk of deforming the wheel in the process. When a new wheel is set up, it may be necessary to repeat the above process once every half hour or so till the cutting is satisfactory, but when once a wheel is well armed it will work for a long time without further attention.

-- 77. Cutting a Section.

A wheel 7 inches in diameter may be rotated about 500 times per minute, and will give good results at that speed. The work, as will be seen from the diagram, is pressed against the edge of the wheel by a force, which in the case quoted was about the weight of eleven ounces. This was distributed along a cutting arc of three-quarters of an inch.