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then as many of our terms are used now, to cover up ignorance. 

 Priestley furthermore recognized that all of these components 



guished flame and life alike. What is it then, that makes burning 



He sought the answer to this question in nature herself. The 

 method was that of experiment. In order to ascertain the effect 

 of these different "airs," he placed in them small animals. He 



putrefaction all have a similar effect on the surrounding air. He 

 became especially interested in trying to find out why the air 



mals do not suffocate, though a multitude of generations of living 

 beings have worked for millions of years to vitiate the air by ab- 

 sorbing immense quantities of " dephlogisticated " air (oxygen), 

 and returning oceans of " fixed air" (C0 2 ), and though the fixed 

 air is continually supplied from flames" volcanoes, and other 

 sources. The theory of Saluce, referred to above, was based 

 upon the fact that cold prevents fermentation and putrefaction, 

 while heat promotes them. Priestley resolved to test that theoiy 

 by means of experiment. To that end he burned candles in an 

 enclosed space, .or let animals remain there until the air would 

 no longer support combustion or respiration. This air was then 

 exposed to the cold of a hard frost, but even then flames went 

 out, and animals expired when placed in it. Thus the theory of 

 Saluce was disproved, as well as another current theory that heat 

 vitiated the air, for animals lived at ease in air that had been 

 passed through hot tubes. What could the true explanation be ? 

 Again the question was put direct to nature. "It becomes," 

 said Priestley, "a great object of philosophical inquiry, to ascer- 

 tain what change is made in the constitution of the air by flames, 

 and to discover what provision there is in nature for remedying 

 the injury which the atmosphere receives by this means." 



Priestley found that animals could not live in air in which a 

 candle had burned out ; he also demonstrated the converse, show- 

 ing that a flame would not burn in air vitiated by the respiration 

 of a mouse. We can hardly overestimate the importance of this 



