ABSTRACT
We find this principle stated in many different ways. In the first place, let us put aside the formula “Nothing is created, nothing is lost,” which is still sometimes employed and which evidently is much too broad, for it would apply just as well to the conservation of velocity and to that of energy; this is not surprising, since the formula, we have seen, is only one of the expressions of the principle of causality. We must at least say: Matter is neither created nor lost. But even this formula lacks precision. Here is an ingot of silver : its colour, its lustre, its hardness, its malleability, its conductivity for electricity and heat, in a word, all the physical properties which I know it to have, are certainly an integral part of my conception of this matter. Does one mean to affirm that all of this is indestructible? Assuredly not, since it suffices to dissolve the metal in nitric acid for these properties to vanish. The term matter is therefore taken here in a narrower sense than the one commonly assigned to it, and the statement of the principle must be completed by a definition of this term. In order to follow the evolution of matter through modifications, analogous to that undergone by the ingot of silver, the chemist uses the balance. It was with the aid of the balance that Lavoisier accomplished his “chemical revolution,” as Berthelot calls it. It seems, then, that the term matter must be defined as that which has weight. Let us remark, however, that this weight, the indestructibility of which we apparently affirmed, we see without surprise change with the place where the experiment is performed ; it will not be strictly the same at the Pole and near the Equator, and we suppose that the same ingot of silver, weighed, of course, on a spring balance, would have an entirely different weight on the surface of the moon. These variations, due to the variations of the constant of gravitation in these different places, we shall eliminate in dividing the weight by this constant. We thus come to the concept of mass, related to that of weight by the equation w = mg, and our principle finally crystallizes into that of conservation of mass.
