[Illustration:  FIG 30.—­Lepidodendron.  Portion of Sandstone stem after removal of bark of a giant club-moss]

There is but one more form of carbon with which we have to deal in running through the series.  We have seen that coal is not the summum bonum of the series.  Other transformations take place after the stage of coal is reached, which, by the continued disentanglement of gases, finally bring about the plumbago stage.

What the action is which transforms plumbago or some other form of carbon into the condition of a diamond cannot be stated.  Diamond is the purest form of carbon found in nature.  It is a beautiful object, alike from the results of its powers of refraction, as also from the form into which its carbon has been crystallised.  How Nature, in her wonderful laboratory, has precipitated the diamond, with its wonderful powers of spectrum analysis, we cannot say with certainty.  Certain chemists have, at a great expense, produced crystals which, in every respect, stand the tests of true diamonds; but the process of their production at a great expense has in no way diminished the value of the natural product.

The process by which artificial diamonds have been produced is so interesting, and the subject may prove to be of so great importance, that a few remarks upon the process may not be unacceptable.

The experiments of the great French chemist, Dumas, and others, satisfactorily proved the fact, which has ever since been considered thoroughly established, that the diamond is nothing but carbon crystallised in nearly a pure state, and many chemists have since been engaged in the hitherto futile endeavour to turn ordinary carbon into the true diamond.

Despretz at one time considered that he had discovered the process, which consisted in his case of submitting a piece of charcoal to the action of an electric battery, having in his mind the similar process of electrolysis, by which water is divided up into the two gases, hydrogen and oxygen.  He obtained a microscopic deposit on the poles of the battery, which he pronounced to be diamond dust, but which, a long time after, was proved to be nothing but graphite in a crystallised state.  This was, however, certainly a step in the right direction.

The honour of first accomplishing the task fell to Mr Hannay, of Glasgow, who succeeded in producing very small but comparatively soft diamonds, by heating lampblack under great pressure, in company with one or two other ingredients.  The process was a costly one, and beyond being a great scientific feat, the discovery led to little result.

A young French chemist, M. Henri Moissau, has since come to the front, and the diamonds which he has produced have stood every test for the true diamond to which they could be subjected; above all, the density of the product is 3.5, i.e., that of the diamond, that of graphite reaching 2 only.