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 this law of Lavoisier or of the conservation of weight, the verification of one of the great laws of nature which we extend to every kind of matter, ponderable or not. It is the law of the conservation of matter, or again, of the indestructibility of matter—"Nothing is lost, nothing is created, all is transformation." This is exactly what Tait held, this impossibility of creating or destroying matter which at the same time is a proof of its objective existence. This indestructibility of ponderable matter is at the same time the fundamental basis of chemistry. Chemical analysis could not exist if the chemist were not sure that the contents of his vessel at the end of his operations ought to be quantitatively, that is to say by weight, the same as at the beginning, and during the whole course of the experiment.

§ 2..

''The Idea of Energy Derived from the Kinetic Theory.''—The notion of energy is not less clear than the notion of matter, it is only more novel to our minds. We are led to it by the mechanical conception which now dominates the whole of physics, ''the kinetic conception'', according to which in the sensible universe there are no phenomena but those of motion. Heat, sound, light, with all their manifestations so complex and so varied, may, according to this theory, be explained by motion. But then, if outside the brain and the mind which has consciousness and which perceives, Nature really offers us only motion, it follows that all phenomena are essentially homo-*