78 PROGRESS OF SCIENCE IN THE CENTURY. 



possessed in the highest degree the faculty of obser- 

 vation " had also discovered oxygen, he was unable 

 to free himself from the bondage of phlogistic 

 theory. The same was true of many others, and it 

 is to Lavoisier (1743-1794) that we must give the 

 credit of destroying the old theory by replacing it 

 with a better. Here we have one of the many 

 instances which lead us to say with confidence that 

 to destroy effectively one must replace. It is true 

 that Lavoisier stood on the shoulders of other 

 workers, but his own experiments were not less in- 

 genious, and, more than any of his predecessors or 

 contemporaries, he reached the importance of precise 

 quantitative measurement. Thus he was led to state 

 about 1777 the fundamental conclusion that in the 

 process of combustion, the burning substance unites 

 with oxygen, whereby an acid is usually produced; 

 and that the increase in weight of the substance 

 burned is equal to the loss in weight of the air. His 

 researches also led him to the general proposition 

 that in all chemical reactions it is only the kind of 

 matter that is changed, the quantity remaining 

 constant ; and to the brilliant idea that " heat is the 

 energy which results from the imperceptible move- 

 ments of the molecules of a substance." 



The Conservation of Matter. One of the foun- 

 dation-stones of chemistry which every worker 

 builds upon with unquestioning confidence is the 

 conservation of matter. We can neither create nor 

 destroy the smallest particle; the elements which 

 enter into the composition of the soap-bubble film are 

 as lasting as those which form the granite rocks. 

 The state of the matter may wholly change from 

 solid to gaseous, or in other ways, the particular com- 

 binations of the elements may wholly change as they 



