XXV. AN ELEMENTARY SUBSTANCE IS CHEMICALLY SIMPLE.

I HAVE proved then that an elementary substance is not a simple substance, and is composed of two really distinct principles. But, though elementary substances are not simple in their essence, they are chemically simple as not being generated by two elementary substances of different natures. All chemists agree in this that substances obtained by union of substances differing in their nature, are not elementary substances. They are called chemical compounds and though at one time they were not called so, they were not said to be elementary. But the practical question, "Which of them are elementary?" is not a question for philosophers as such to answer. It belongs to the experimental physicists, who by little and little, and after centuries of study, have corrected many errors in purely experimental science.

The proof of this is indisputably evident.

In the corporeal universe there certainly are many substances that differ in their nature specifically, as the specific diversity of their operations plainly shows. These either do or do not result from the union of others. Non datur medium, because the opposition is contradictory. If they do not result from the union of substances, they are chemically simple. If they do, we must either admit an endless series of combinations and unions, which is against reason, or come at last to substances chemically simple. In fact the substances that result from the combination of other substances are like numbers, and those that do not like units. However great a number may be, it always consists of units, and proceeds from a unit as from its principle. Thus compound substances are such in relation to their components and therefore we must acknowledge the components, unless we are prepared to suppose the Begun without a beginning, and the Reasoned without a sufficient reason. But to declare which are simple substances and which are not, belongs to experimental science, and would be foreign to the purpose of a treatise, in which the rational principles only of St. Thomas's Physical system are explained.

Let it suffice for us to infer by reasoning that elementary substances must be of more than one species. I know very well that many scientists would have all elementary substances to be of one specific nature. Why they do so I know not. Perhaps it comes from a natural tendency to suppose in all substances one and the same matter, as the common principle that subjects them to the various transformations on which the unity, order and beauty of the universe depend. But since it is not easy to form a just conception of such matter, those who hardly give a glance at the inmost essence of things may be led to imagine it to be a set of atoms, each having the same nature. Hence the well-known hypotheses of the Epicureans, Cartesians and the modern followers of the Mechanic system. Some few there have been, who, while disowning these theories, and inclining to the Physical system, admitted the double principle of extension and activity, but stood out for the one species of elementary substance, fancying that, as all colours may be had from one light, so may all corporeal substances be constituted from one species of elements. But is that probable?

Most certainly the progress of science has not shown that the elements are of one species. They were at one time reduced by Physicists to a few species: but gradually, as empirical teaching advanced, the species increased, and now there are a great many. This cannot be said to show that the elements are of one species, but contrariwise would lead us infer from experiment what Cardinal Toledo said as a philosopher, viz., that the elements are of more than one species. And in fact, how can they be not of more than one species? The mixtum resulting from the elements has a nature different from theirs: but this could not be, if the elements themselves had the same nature. Therefore they differ in nature and consequently in species. If hydrogen and oxygen were the same, the quantity could indeed be increased by aggregation: but not the nature changed belonging thereto.

Anyone seeking to weaken the force of this argument by saying that these, even though of different species, are not chemically simple, because they may be composed of others equal in species, would greatly deceive himself. The proof is of such a sort that its conclusion is universal for chemically simple elements, whatsoever and wheresoever they may be -- elements to which we must come in the last resort, unless we like to loiter in the region of the absurd.

There is another objection which at first sight may seem to have some importance: i.e. that a specific diversity in the elements is not required, because an accidental diversity is sufficient. The answer is that it would be sufficient, if the diversity of the mixta compounds were accidental, but not otherwise. Now chemistry shows them to be substantial.

In conclusion, therefore, we say that elementary substances are of different species -- are atoms composed of materia prima and substantial form -- two principles really distinct -- and that these elementary substances are simple, being free from chemical composition. This we have defined and proved.

From the elementary we pass on to the mixed or composite.