106 Chemical Philosophy . [Chap. II. 



vanced different theories in succession about the 

 nature of this principle ; but as they were formed 

 rather on conjecture and analogy than direct proof, 

 they obtained but few advocates. At last M. 

 I^voisier, in 1778, by a number of ingenious and 

 accurate experiments proved that several com- 

 bustible substances, when united with oxygen, 

 form acids ; that a great number of acids contain 

 oxygen ; and that, when this principle is separated 

 from them, they lose their acid properties. He 

 concluded, therefore, that oxygen is the acidifying 

 principle, as the wond imports, and that acids are 

 nothing more than combustible substances com- 

 bined Vv'ith oxygen, and differing from one another 

 according to the nature of the combustible basis. 

 This doctrine has been since confirmed by experi- 

 ment, and is now generally received among che*- 

 mists*. 



For our knowledge of the composition of atmo- 

 spheric air, we are indebted to the chemical philo- 

 sophers of this period. The first step in this in- 

 quiry was taken by that unwearied experimenter. 

 Dr. Priestley, in 1774, by the discovery of oxygen 

 gas. This gas, according to the prevailing theory 

 of tl>c time, he considered as air totally deprived 

 of phlogiston. Azotic gas, on the other hand, 

 was air saturated with phlogiston. Hence he con- 

 sidered common air as oxygen gas combined with 

 an indefinite portion of phlogiston, and varying in 

 purity according to that portion, being always pure 

 in an inverse proportion to the quantity of phlo- 

 giston it contained. 



* TJionjson's Chemistri/. 



