We may cut this triangle of residual products where we please, and by careful analysis determine accurately the chemical composition of a section at this point, and we may please ourselves with the illusion, as many chemists have done, that the definite proportions found represent the formula of a specific compound; but an adjacent section above or below would show a different composition, and so in the entire triangle we should find an infinite series of formulae, or rather no constant formulae at all. We should also find that the slice, taken at any point while lying in the laboratory or undergoing chemical treatment, would change in composition, and become a different substance. In the same way we can snatch a brand from the fire at any stage of its decomposition, or analyze a decaying tree trunk during any month of its existence, and thus manufacture as many chemical formulae as we like, and give them specific names; but it is evident that this is child's play, not science. The truth is, the slowly decomposing tissue of the plants of past ages has given us a series of phases which we have grouped under distinct names, and we have called one group peat, one lignite, another coal, another anthracite, and another graphite. We have spaced off the scale, and called all within certain lines by a common name; but this does not give us a common composition for all the material within these lines. Hence we see that any effort to define or describe coal, lignite, or anthracite accurately must be a failure, because neither has a fixed composition, neither is a distinct substance, but simply a conventional group of substances which form part of an infinite and indivisible series. But this sliding scale of solid compounds, which we designate by the names given above, is not the only product of the natural and spontaneous distillation of plant tissue. Part of the original organic mass remains, though constantly wasting, to represent it; another part escapes, either completely oxidized as carbonic acid and water, or in a volatile or liquid form, still retaining its organic character, and destined to future oxidation, known as carbureted hydrogen, olefiant gas, petroleum, etc. Hence, in the decomposition of vegetable tissue, two classes of resultant compounds are formed, one residual and the other evolved; and the genesis and relation of the carbon minerals may be accurately shown by the following diagram: