18 JOSEPH PRIESTLEY i 



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 " iihlogisticated," 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 vol- 

 ume, 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 " dephlogisticated/' or have 

 the properties of pure common air restored to it, by 

 the action of green plants in sunshine. The ques- 

 tion, 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? 



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

 bility of obtaining air less phlogisticated than 

 common air had not occurred to him.* But in 

 pursuing his experiments on tlie evolution of air 

 from various bodies by means of heat, it happened 

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



* ErpprimPTifs and Observations on Different Kinds of 

 Air, vol. ii. p. 31. 



