18 JOSEPH PRIESTLEY I 



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 " phlogisticated " ; while the 

 other fifth is free from phlogiston, or " dephlogis- 

 ticated." On the other hand, Priestley found that 

 air " phlogisticated " by combustion or respiration 

 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 phlogisticated by combustion, and con- 

 verted 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 com- 

 mon air does ? 



Now, Priestley says that, in 1774, the possi- 

 bility of obtaining air less phlogisticated than 

 common air had not occurred to him. 1 But in 

 pursuing his experiments on the evolution of air 

 from various bodies by means of heat, it happened 

 that, on the 1st of August 1774, he threw the heat 

 of the sun, by means of a large burning glass 

 which he had recently obtained, upon a substance 



1 Experiments and Observations on Different Kinds of Air \ 

 vol. ii. p. 31. 



