POPULAR CHEMISTRY.

Continued from Part V.

THE DIAMOND.

Why is carbon known by the names of diamond and charcoal?

Because the two latter substances, although so different, and almost opposite, in physical characters, are, according to unexceptionable experiments, almost chemically the same.

That diamond is simple carbon, is shown by the following experiment. M. Morveau exposed a diamond to intense heat, shut up in a small cavity in a piece of tough iron. When he opened the cavity, he found the diamond entirely gone, and the iron around it converted into steel. This shows that it is pure carbon, which combines with iron to form steel, and not charcoal, which is generally an oxide of carbon. The peculiar hardness of steel is to be ascribed to its union with a portion of pure carbon, or diamond. It is no uncommon thing for jewellers to expose such diamonds as are foul, to a strong heat, imbedded in charcoal, to render them clear; but, in this process, great care is taken to have a sufficient quantity of charcoal, to exclude the atmospheric air: otherwise, the intense heat would produce combustion.-Parkes.

Why is charcoal more inflammable than the diamond? Because of the looseness of its texture, and the hydrogen it contains. The latter is indeed the only chemical difference perceptible between diamond and the purest charcoal: but Dr. Ure asks, "can a quantity of an element, (hydrogen) less, in some cases, than 1-50,000th part of the weight of the substance, occasion so great a difference in physical and chemical characters?" In the Philosophical Transactions for 1797, is related Mr. Smithson Tennant's process for proving the identity of these two substances: he says, "It will appear, from experiments, that the diamond consists entirely of charcoal, differing from the usual state of that substance only by its crystallized form,”and Dr. Ure thinks this opinion to be correct.

The identity of charcoal and diamond is further illustrated in the following experiment. Sir Humphry Davy exposed charcoal to intense ignition, in vacuo, and in condensed azote, by means of Mr. Children's magnificent battery, when it slowly volatilized, and gave out a little hydrogen. The remaining was always much harder than before, and in one case, so hard as to scratch glass, while its lustre was increased. This fine experiment may be regarded as a near approach to the production of diamond; and we believe that similar experiments of French chemists have been equally successful.

Why did Newton infer that the diamond was inflammable?

Because of the circumstance, that inflammable substances refract light in a greater ratio than that of their densities. We readily acquiesce in Mr. Parkes's note: "It is wonderful that Newton, who had no chemical means of examining the diamond, should have conceived the idea of its inflammable nature."

It is not evident to whom the combustibility of the diamond first occurred; but, in the year 1694, the

Florentine Academicians proved its destructibility by heat, by means of a burning lens. The products of its combustion were first examined by Lavoisier, in 1772, and subsequently, with more precision, by Guyton Morveau, in 1785. Mr. Tennant's experiments, just referred to, demonstrated the important fact, that when equal weights of diamond and pure charcoal were submitted to the action of red-hot nitre, the results, in both cases, were the same; and, in 1807, the combustion of the diamond in pure oxygen, was found by Messrs. Allen and Pepys, to be attended with precisely the same results as the combustion of pure charcoal. Hence, observes Brande, the inevitable inference, that charcoal and the diamond are similar substances in their chemical nature, differing only in mechanical texture.

The combustion of the diamond may be most conveniently and perfectly effected, by placing it upon a platinum capsule, in a jar of pure oxygen, inverted over mercury, and throwing upon it the focus of a burning lens. Sir Humphry Davy, when at Florence, in 1814, (Phil. Trans.) used for this purpose the same lens which was employed in the first trials on the action of solar heat on the diamond, instituted by Cosmo III, Grand Duke of Tuscany; he found, that when strongly ignited by the lens in a thin capsule of platinum, perforated with many orifices, so as to admit a free circulation of air, the diamond continued to burn in the oxygen, after being withdrawn from the focus, with so brilliant a light as to be visible in the brightest sunshine, and with very intense heat. The results of these experiments demonstrate, that diamond affords no other substance by its combustion than pure carbonic acid gas; and that the process is merely a solution of diamond in oxygen, without any change in the volume of the gas. It likewise appears, that, in the combustion of the different kinds of charcoal, water is produced; and that, from the diminution of the volume of the oxygen, there is every reason to believe, that the

water is formed by the combustion of the hydrogen existing in strongly ignited charcoal.

Why is pure carbon, or diamond, so scarce, while its compounds, in different states, are so abundantly dispersed?

Because, (observes an eminent chemist*) "the wonder consists only in the opposition between facts and our opinions; it disappears in proportion as we discover and appropriate the powers of nature to produce the same effects. To dispel the astonishment of those who might consider this a ground of distrust, I shall remind them that aluminous earth is likewise one of the commonest substances, though the adamantine spar, no less rare than the diamond, is nevertheless alumina; that iron exists every where, under every form, except in the state of purity; and that the existence of native iron is still doubtful." Since the preceding observations were written, native iron is stated to have been found in Canaan, in the United States of America.

Diamonds are usually found in an ochreous, yellow earth, under rocks of grit-stone; they are likewise found detached, in torrents, which have carried them from their beds. They have no brilliancy when dug out of the earth, but are covered with an earthy crust. The diamond was first discovered in Asia, in the provinces of Golconda and Visapour, in Bengal, and in the island of Borneo. About the year 1720, diamonds were first found in Brazil; and a minute account of their discovery will be found in a volume of travels in that country, by the late Mr. Mawe, one of the most distinguished mineralogists of his time.

The primitive form of the diamond is the regular octoëdron, each triangular facet of which is sometimes replaced by six secondary triangles, bounded by curved lines; so that the crystal becomes spheroidal, and presents forty-eight facets. Diamonds with twelve and twenty-four facets, are not uncommon,

* Dr. John Thomson, notes on Fourcroy.