9 8 THE POPULAR SCIENCE MONTHLY. 



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 combustion, of respiration, and 

 of the composition of water, to the end of his days fought against the 

 inevitable corollaries from his own labors. His last scientific work, 

 published in 1800, bears the title, "The Doctrine of Phlogiston es- 

 tablished, and that of the Composition of Water refuted." 



When Priestley commenced his studies, the current belief was, 

 that atmospheric air, freed from accidental impurities, is a simple ele- 

 mentary substance, indestructible and unalterable, as water was sup- 

 posed 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. 



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 experi- 

 ment. Hence it appeared that common air consists, to the extent of 

 four-fifths of this volume, 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 com- 

 bustion or respiration couid be " dephlogisticated," or have the prop- 

 erties of pure common air restored to it, by the action of green plants 

 in sunshine. The question, therefore, would naturally arise as com- 

 mon air can be wholly phlogisticated by combustion, and converted 

 into a substance which will no longer support combustion, is it pos- 

 sible to get air that shall be less phlogisticated than common air, and 

 consequently, support combustion better than common air does ? 



Now, Priestley says that, in 1774, the possibility 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 which was then called mer- 

 curius calcinatus per se, and which is commonly known as red precipi- 

 tate : 



" I presently found that, by means of this lens, air was expelled from it very 

 readily. Having got about three or four times as much as the bulk of my ma- 

 terials, I admitted water to it, and found that it was not imbibed by it. But, 

 what surprised me more than I can well express was, that a candle burned in this 



1 " Experiments and Observations on Different Kinds of Air," vol. ii., p. 31. 



