XXVIII. WHAT IS MEANT BY SUBSTANTIAL TRANSFORMATION.

SOME people seem to think that substantial transformation, as understood according to the Physical system, means annihilation of the elements, or at least of their substantial forms, and the production of a new form to be the specific principle of the chemical compound's activity. If so, we should not have a chemical composition, but annihilation and creation. Mixtio, says Aristotle, est alteratorum unio.^{1} It therefore consists in this: When, e.g. oxygen and hydrogen combine, there remains, 1. the matter (materia prima) of both; 2. in each the principle of activity undergoes a change; 3. by virtue of this change every atomic part of the compound molecule has in itself the nature of the mixtum; so that every atomic part of the water has the nature of water, and the qualities of the previous elements (oxygen and hydrogen) are hidden. All this implies a substantial transformation of the elements. That, in virtue of this, the mixtum is a whole, of a new and homogeneous nature in every part, is taught by Aristotle. He says "Is a mixtum, nothing else than minute particles. varying in their nature, but so confused that, although retaining their former nature, they seem a homogeneous whole? No: for in the mixtum every part must have the nature of the whole, as each part of water has the nature of water." This principle of his is quite in accordance with the division of elements into atoms and the disposal of the mixtum into molecules of a certain dimension and a certain figure.

People are so afraid of this in our times, that a few words about it may not be amiss. We said before that every substantial form requires determinate matter, and must naturally have also, according to its various states, a determinate figure. Oportet ut determinatae formae determinata figura debeatur.^{2} In that which has life the substantial form requires a determinate organism: and thus the soul, which is the substantial form of the human body, could not be adapted to the organism of an eagle, nor the substantial form of a stag to that of a fish, nor the substantial form of a peach tree to that of a camellia or of a tulip. Now this doctrine must be equally true of the inorganic: so that, in virtue of a substantial form, an atom of gold has not under the same temperature the same figure as an atom of phosphorus, nor has a molecule of a mixed body the same figure as that of another mixed body. True it is that there are bodies, both elementary and mixed, whose atoms or molecules, in a solid state, show equal figures, and therefore may he called isomorphous: but that is not in contradiction with the doctrine. Rather does it lead thereto by a sort of analogy; for in the different species the likeness between the external form or figures of bodies becomes greater as the species descend from Man, the noblest of the animates, towards the lowest species of corporeal substances. Anyone may see that what we have said about the atoms of the simple bodies and the molecules of the mixed is in no way against those beautiful theories about crystals, which are so deservedly honoured in physics.

{1} De Gener., i.

{2} Comm. in Sent., ii. Dist. xix. 1, 1.