and mercury on the other, form, it is true, notable exceptions; nevertheless, the bulk of the facts sustain this law, and indicate that the genesis of those elements which we know only as solids or liquids, and which we have supposed to have taken place during the fiery ordeal through which the solar system has had to pass, is rather a process of integration than of subdivision, since they have much larger molecules than the gases that exist in the nebulæ, and out of which we have supposed them to be formed.
We have seen that matter, in its cosmical history, as enacted in the development of a planetary system, assumes a great variety of forms, and resolves itself into numerous specifically distinct molecular aggregates. The different substances which we know on our planet are the result of the cohesion into homogeneous masses of these different aggregates, all the constituent units of any one of these masses consisting of the same species of molecular aggregate. We saw reason to suppose that, at an early period in the development of the solar system (and we may infer the same for all systems), the number of distinct substances was small, and that these substances were gaseous at very low temperatures. The two abundant gases, nitrogen and hydrogen, exist in the irresolvable gaseous nebulæ, and these, doubtless, went far to constitute the original substance of our infant system. These gases, though differing greatly from each other in their atomic weights, nevertheless have small molecules compared with those of most substances now found in the earth, and which are, for the most part, either solid or liquid at life-supporting temperature. There has, therefore, been upon the whole increase of mass among the molecules of substances later developed.
When we rise to the point of view which removes all distinction between elements and compounds, except the subjective one that in the former we do not know and cannot prove their composition, while in the latter we can do this in so far as to resolve them into the former, we can make the further generalization that along with this increase of mass there has gone decrease of stability in such molecules. We are of course unable to predicate this, except inferentially, of the elements which we can not decompose, although these, doubtless, vary greatly in their relative stability, and, as before remarked, some substances which had been supposed to be elementary have already been reduced to simpler forms, and others may still be so reduced. Moreover, those which have thus yielded possess large molecules (counting that of the compound), and this should serve as an index to future attempts of a like nature. There is, for example, little hope of resolving hydrogen or carbon into simpler elements, but the reverse of the alchemist's dream may yet be realized, and gold reduced, if not to baser, at least to simpler materials.
All the known chemical compounds must be supposed to have been