JOSEPH PRIESTLEY. 117 



belief was, that atmospheric air, freed from accidental 

 impurities, is a simple elementary substance, indestructi- 

 ble and unalterable, as water was supposed to be. "When 

 a combustible burned, or when an animal 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 sup- 

 porting life and combustion. Thus, air contained in a 

 vessel in which a lighted candle had gone out, or a liv- 

 ing animal had breathed until it could breathe no longer, 

 was called " phlogisticated." The same result was sup- 

 posed 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 experiment. Hence 

 it appeared that common air consists, to the extent of 

 four-fifths of its volume, of air which is already " phlo- 

 gisticated ;" while the other fifth is free from phlogiston, 

 or " dephlogisticated." On the other hand, Priestley 

 found that air " phlogisticated " by combustion or respira- 

 tion could be " dephlogisticated," or have the properties 

 of pure common air restored to it, by the action of 

 green plants in sunshine. The question, therefore, would 

 naturally arise as common air can be wholly phlogisti- 

 cated 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 com- 



