66 LIFE AND DEATH. 



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 funda- 

 mental 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. Energy. 



TJie Idea of Energy Derived front 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- 



^ It must be added that the absolute rigour of this law has 

 been called in question in recent researches. It would only 

 have an approximate value. 



