THIS is a most important and arduous question, owing to its intrinsic difficulty and the discrepancy of opinion about it. So long as chemistry remains within the limits of its own natural boundaries, collecting facts and registering phenomena, the learned cannot really be at variance with each other, though there may be more or less exactness in explaining and more or less faithful accounts of things: but when, passing these limits, it takes to deciding philosophically about the nature and essence of things, then it is that discrepancies arise. This happens, in some cases, through deficient knowledge of philosophy and a want of sound logic, while in others it proceeds from the modern fashion of following experience only, and confusing the senses with reason. We must therefore be very cautious, ready to receive all facts that are clearly proved, to admit the principles that are absolutely connected with those facts, and then enquire whether such are the principles accepted in the philosophy of St. Thomas.
Chemistry is the science that treats of the substantial changes in bodies. A substantial change means that which happens when a substance is changed, not by mere variation of temperature, nor by the fleeting influence of extrinsic agents, but by a change in its inmost being, so that it changes its nature, and, instead of being what it was, becomes another substance. Such mutation takes place in corporeal substances only, because they alone have a composite essence: and since our minds have no immediate intuition of substances, but know them through their operations, the diversity or contrariety of these operations will guide us to discern whether or no their substantial being is changed. This does not require a change in all the operations. It is enough to know that the specific operations have changed: for since the genus remains in the substantial mutations of corporeal things, the operations or passions belonging to the genus itself must remain. Hence a substance may change into another and yet have the same gravity, the same weight, the same solidity or liquidness, &c. It is very difficult sometimes to be sure of a substantial change, but we often can be sure of it. Knowing, for instance, that substances without life are different from living substances, we cannot doubt that inorganic minerals, when changed into plants or into sentient things, are substantially changed. This may, sometimes at least, be extended beyond the living, without fear of falling into error.
The proper and principal object, therefore, of chemistry is to be found in those substantial changes, obtained by combination of elements, which all modern scientists call chemical combination. The accidental changes of substances, called alterations, are but a secondary object accidentally treated of in chemistry. Herein there seems to be no essential discrepancy between the doctrine of St. Thomas and the true principles of modern science, together with the facts that chemists have shown to be certain. But the modern doctrine is in disagreement with that of many old physicists, and also of some modern ones, who put forward, without any sound reason, principles founded on their own fancy, or facts that do not exist. We must therefore discuss elementary substances according to our own doctrines distinctly.
<< ======= >>