166 SCIENCE AND THE HUMAN MIND 



neglect the duty of publishing to the world the results 

 of experiments made solely to satisfy his own curiosity. 

 This was not the case with his chief chemical discovery, 

 the compound nature of water. But when, in 1781, 

 Cavendish thus dethroned water from its old and 

 proud position as one of the elements, he still described 

 its constituents as phlogiston and dephlogisticated air. 



But, with this gradual accumulation of knowledge, 



the accepted theory was becoming more and more in- 



Lavoisierand ^equate. New ideas were better able 



the Conserva- to describe the phenomena, and, in due 



tion of Matter. tim6) the new ideag appeared> In ^g^ 



Lavoisier repeated Cavendish's experiment, and 

 grasped the fact that there was no need to invent 

 a body with properties fundamentally unlike those of 

 other material substances. Lavoisier regarded the 

 constituents of water as ordinary gases possessing 

 mass and weight, and named them hydrogen (the 

 water-forming element) and oxygen (the acid-forming 

 element). The conception of phlogiston became 

 unnecessary, and with it vanished the last of the 

 essentially light bodies with negative weights. Thus 

 the principles which, for a century and a half, had 

 revivified the science of mechanics were carried over 

 into chemistry. 



In Lavoisier's hands, too, another principle which 

 had slowly been emerging from obscurity became 

 clear : the principle of the persistence of matter, or 

 conservation of mass. Throughout any series of 

 chemical changes, the matter involved may alter its 

 state of chemical combination, and change its form 

 from solid to liquid or seemingly disappear as a gas. 



