I JOSEPH PRIESTLEY 17 



assisting in their rational explanation, his influence 

 to the end of his life was warmly exerted in favour 

 of error. From first to last, he was a stiff adherent 

 of the phlogiston doctrine which was prevalent 

 when his studies commenced ; and, by a curious 

 irony of fate, the man who by the discovery of 

 what he called " dephlogisticated air" furnished 

 the essential datum for the true theory of com- 

 bustion, of respiration, and of the composition of 

 water, to the end of his days fought against the 

 inevitable corollaries from his own labours. His 

 last scientific work, published in 1800, bears the 

 title, " The Doctrine of Phlogiston established, and 

 that of the Composition of Water refuted." 



When Priestley commenced his studies, the cur- 

 rent belief was, that atmospheric air, freed from 

 accidental impurities, is a simple elementary sub- 

 stance, indestructible 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 supporting 

 life and combustion. 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. 



VOL. Ill C 



