52 PRESIDENTS ADDRESS—SECTION B. 
the effect of pressure on air and other gases, and to enunciate 
the law wnmich governs these phenomena. We owe to him our 
conception of the mysterious affinity of the particles of one 
kind of matter for those of another, which is one of the funda- 
mental conceptions of the science. 
He was the founder of our systematic method of analytical 
chemistry, and the tests he employed to detect some of the com- 
moner chemical substances are in use in our laboratories to this 
day. He also made many discoveries in technical applications. 
From Boyle’s time we are able to trace with accuracy the 
growth of a well defined science of chemistry. It is to the accept- 
tance of Boyle’s ideas, and to his discoveries of a few of the funda- 
mental laws governing matter, that we owe the existence of che- 
mistry as a science. From this time on we are able to follow the 
successive steps of its progress, and to connect each successive 
discovery with those that have preceded it. 
PHLOGISTON. 
It is true that Boyle’s work did not bear fruit immediately, 
not, indeed, till quite 100 years after his time. During this 
period a theory concerning the constitution of matter was ac- 
cepted which effectually prevented further advance. This was 
Stahl’s theory of phlogiston, according to which a certain ele- 
ment or principle was held to exist more or less in all combus- 
tible substances. 
Bodies that do not burn contain no phlogiston, those that 
burn readily contain much phlogiston. When a substance 
burns it parts with its phlogiston. 
Phlogiston was, in fact, the principle of inflammability. It 
also came to be regarded as the principle of lightness, since many 
substances were found to increase by weight on burning. 
This theory was finally overthrown by the discovery of oxygen 
by Priestley in 1774, and of the compound nature of water 
by Cavendish ten years later, and these two discoveries may be 
regarded as the two most important landmarks on the experi- 
mental side of the science. 
LAVOISIER. 
But it is to their contemporary, the great French chemist, 
Lavoisier, that the credit is due of seeing the full interpreta- 
tion of these discoveries, of affording the proper explanation of 
the phenomena of oxidation, combustion, and reduction, and of 
dealing the final death blow to the phlogistic theory. 
At Lavoisier’s time, as we have seen, the few scattered facts 
known with regard to chemical phenomena stood in no apparent 
relation to each other. Except in one or two instances, no ex- 
