V.] JOSEPH PRIESTLEY. 109 



breathed in air, it was supposed that a substance, 

 "phlogiston," the matter of heat and light, passed 

 from the burning or breathing body into it, and 

 destroyed its powers of supporting life and combus- 

 tion. Thus, air contained in a vessel in which a 

 lighted candle had gone out, or a living animal had 

 breathed until it could breathe no longer, was called 

 "phlogisticated." The same result was supposed to 

 be brought about by the addition of what Priestley 

 called " nitrous gas " to common air. 



In the course of his researches, Priestley found 

 that the quantity of common air which can thus 

 become " phlogisticated," amounts to about one-fifth 

 the volume of the whole quantity submitted to experi- 

 ment. Hence it appeared that common air consists, 

 to the extent of four-fifths of its volume, of air which 

 is already "phlogisticated;" while the other fifth is 

 free from phlogiston, or " dephlogisticated." On the 

 other hand, Priestley found that air " phlogisticated " 

 by combustion or respiration could be " dephlogis- 

 ticated," or have the properties of pure common air 

 restored to it, by the action of green plants in sun- 

 shine. The question, therefore, would naturally arise 

 as common air can be wholly phlogisticated by 

 combustion, and converted into a substance which 

 will no longer support combustion, is it possible to 

 get air that shall be less phlogisticated than common 

 air, and consequently support combustion better than 

 common air does ? 



Now, Priestley says that, in 1774, the possibility 

 of obtaining air less phlogisticated than common air 



