When the glass is coated with the bromide of silver, the action per se, is very slow, and the discoloration ultimately produced far short of blackness; but when moistened with nitrate of silver, sp. gr. 1.1, it is still more rapid than with the iodide, turning quite black in the course of a very few seconds' exposure to sunshine. Plates of glass thus coated may be easily preserved for the use of the camera, and have the advantage of being ready at a moment's notice, requiring nothing but a wash over with the nitrate of silver, which may be delayed until the image is actually thrown on the plate and adjusted to the correct focus with all deliberation. The sensitive wash being then applied with a soft, flat camel's-hair brush, the box may be closed and the picture impressed; after which it only requires to be thrown into water and dried in the dark to be rendered comparatively insensible, and may be finally fixed with hyposulphite of soda, which must be applied hot, its solvent power on the bromide being even less than on the iodide.

Sir John Herschel suggested a trial of the fluoride of silver upon glass, which, he says, if proved to be decomposable by light, might possibly effect an etching on the glass by the corroding property of the hydrofluoric acid.

The metallic fluorides have been found to be decomposable, and a very sensitive process on paper, called the fluorotype, will be described in the chapter on Miscellaneous Processes. I am not aware that any experiments have been made directly upon glass, but it is certainly worthy of a careful trial.

Herschel has remarked that we can not allow the wash of nitrate to dry upon the coating of the chloride or iodide of silver. If, however, we dip a glass which has one film of chloride upon it into a solution of common salt, and then spread upon it some nitrate of silver, we may very materially thicken the coating, and thus produce more intense effects. Mr. Towson employed glass plates prepared in this manner with much success. The mode adopted by that gentleman was to have a box the exact size of the glass plate, in the bottom of which was a small hole; the glass was placed over the bottom, and the mixed solution, just strong enough to be milky, of the salt and silver poured in. As the fluid finds its way slowly around the edges of the glass, it filters out; the peculiar surface action of the solid glass plate, probably a modified form of cohesive force, separating the fine precipitate which is left behind on the surface of the plate. By this means the operation of coating the glass is much quickened. Another method by which films of any of the salts of sil- . ver can be produced upon glass plates, is the following modification of the patent processes of Drayton and of Thompson for silvering glass: Take a very clear plate of glass, and having put around it an edging of wax about half an inch in depth, pour into it a solution of nitrate of silver made alkaline by a few drops of ammonia, taking care that no oxide of silver is precipitated; mix with this a small quantity of spirits of wine, and then add a mixture of the oils of lavender and cassia, or, which is perhaps the best process, a solution of grape sugar.

In a short time the glass will be covered with a very beautiful metallic coating. The solution is now poured off, the edging of wax removed, and the silver is exposed to the action of diluted chlorine, or to the vapor of iodine or bromide, until it is converted into a compound of one of these elements, after which we may proceed as recommended by Sir John Herschel.

ALBUMEN.

In the Technologiste for 1848, M. Niepce de Saint-Victor published his mode of applying albumen to glass plates. M. Blanquart Everard followed, and successively albumen, gelatine, and serum were employed. Messrs. Ross and Thomson, of Edinburgh, have been eminently successful operators with albumen on glass plates, many of their pictures leaving little to be desired. The manipulatory details of the albumen process will be found in the technical division. of this work.

COLLODION.

The successful application of a solution of gun cotton in ether, to form the film for receiving the sensitive surface on glass, has been claimed respectively by Mr. Fry and Mr. Archer. There is some difficulty in fixing precisely this point, since there was no actual publication of the process until long after it was generally in use. Mr. Fry certainly introduced the use of gutta-percha in combination with collodion.

THE GENESIS OF THE DIAMOND."

By GARDNER F. WILLIAMS, Kimberley, South Africa.

Chemically the diamond is composed of the element carbon in its pure crystallized state. The diamond crystallizes in the isometric system and the most common forms are the octahedron and dodecahedron, while the (24-sided) tetrahexahedron is not uncommon. Cube diamonds with beveled edges, representing the combination ∞0 and 02 are occasionally found in the Bultfontein and Wesselton mines at Kimberley, South Africa. The diamonds from various mines have distinctive forms of crystallization, or variations of the same forms, so characteristic that those familiar with South African diamond mines and their products can determine. positively from which mine any given parcel of diamonds has been obtained. It is not always possible to determine the source of each individual diamond, for similar stones are occasionally found in different mines; but these are exceptions to the rule. There is a difference in the luster, shape, or crystalline form of the diamonds from the various mines that gives each mine some distinctive characteristic. In one mine nearly all the crystals are sharp-edged octahedrons, while in another dodecahedrons with rounded faces predominate. One might give no end of peculiarities of the diamonds from the various mines, but it will suffice for the purposes of this paper to state the fact that such distinctive characteristics do occur. From this observation it may be concluded that the diamonds in the mines of the Kimberley district, which occupies a small area (see fig. 1), did not have a common origin.

The diamond is the most impenetrable of all known substances and will scratch any other stone or the hardest steel. During his lecture at Kimberley, Sir Wm. Crookes squeezed a diamond between

Reprinted, with the author's revision and additions, from Transactions American Institute of Mining Engineers, 1905. Read at Lake Superior meeting, September, 1904.

A lecture delivered before the British Association at Kimberley, September 5, 1905.

two blocks of steel until the blocks touched without injury to the stone in the slightest degree. The pressure is said to have been

FIG. 1.-Diamond mines in the Kimberley District, South Africa.

170 tons to the square inch. It is a very strong reflector of light and refracts incident rays more than any other substance except crocoite."

a Table of indices of refraction in Dufrenoy's Traité de Mineralogie.

While crocoite is the only mineral that exceeds the dispersive power of diamond to dissolve white light into rainbow tints, in its powers of reflection, refraction, and dispersion taken together the diamond is unmatched." It is highly phosphorescent, and even the blackest diamond is transparent to the X ray. The diamond glows under the influence of the B rays from radium. Diamonds when subjected to the action of radium for several months assume a green color, but cut stones when treated in this manner seem to lose part of their brilliancy. It is insoluble in all acids, and can easily be burned and converted into carbon dioxide. It volatilizes at a temperature of about 3,600° C. and passes from the solid to the gaseous state without liquefying.

Sir William Crookes went through the process of producing diàmonds before the eyes of his audience, but was only able to show them the result of this experiment by reproducing a lantern slide of microscopical diamonds which he had made in the same way previously, for it takes a fortnight to separate them from the iron and other substances in which they are embedded. The scientific principle upon which this experiment rests, according to Sir William Crookes, is that molten iron absorbs carbon, and as iron increases in volume as it passes from the liquid to the solid state, if the outer crust of the iron is suddenly cooled and the center remains in a liquid state, the enormous pressure caused by its expanding while cooling affords the two factors necessary for the crystallization of a diamond-heat and pressure.

Authorities differ somewhat as to the exact moment when molten iron expands on cooling, but it is the generally accepted theory that expansion takes place at the moment of solidification. It is also a well-known fact that shrinkage or contraction takes place as the solidified metal cools. It is therefore possible to obtain enormous pressure in the molten center of a casting by the contraction of the . outer shell which has been rapidly cooled and the expansion of the inner mass just as it begins to solidify.'

It is noteworthy that the diamond is a nonconductor of electricity, while graphite and amorphous carbon, substances so closely allied to it in chemical composition, are good electrical conductors. By the application of friction the diamond can be positively electrified, but it very soon loses its electricity. The diamond is easily cleaved in planes parallel to the octahedral faces. Pieces may be easily broken from the facets of a cut stone by striking it with a hard substance.

Feuchtwanger's Treatise on Gems.

American Society Mechanical Engineers, Vol. XVIII, pp. 419 and 431; Vol. XVII, pp. 126 and 1015.