ix] Doctrines of Respiration. 239 



" I hardly know any experiment that is more adapted to 

 " amaze and surprise than this is, which exhibits a quantity of 

 " air, which, as it were, devours a quantity of another kind of 

 " air, half as large as itself, and yet is so far from gaining any 

 " addition to its bulk, that it is considerably diminished by it." 



He found this nitrous air could be conveniently used as 

 avtest of the fitness of air for breathing. Either of these lines 

 of inquiry might have led him to the discovery which he 

 afterwards made. But they did not. His mind was too full of 

 phlogiston, and under the idea that common air consisted of 

 acid gas, and phlogiston, he pursued long inquiries into other 

 acid gases than the nitrous air, into marine acid air, vitriolic 

 acid air, and even vegetable or acetous acid air. 



These inquiries did not lead far ; but another independent 

 inquiry suddenly brought him, accidentally as it were, upon his 

 great discovery. 



He obtained after some difficulty an adequate burning-glass 

 such as would enable him to raise to the requisite heat bodies 

 enclosed in a glass vessel, the gases developed in which he could 

 study with success. By the help of this burning-glass he, 

 following up Hales' views, " tried to find out what kind of air a 

 " great variety of substances natural and artificial would yield." 



While engaged on this inquiry, which was quite independent 

 of his earlier researches, he found that mercuric oxide, mercurius 

 calcinatus per se, yielded under the action of the sun's rays a 

 quantity of gas which was not inflammable, and which so far 

 from quenching flame was exceedingly favourable for com- 

 bustion. These are his words : 



" With this apparatus, after a variety of other experiments, 

 " an account of which will be found in its proper place, on the 

 " 1st August, 1774, I endeavoured to extract air from mercurius 

 " calcinatus per se ; and 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 materials, I 

 " admitted water to it, and found that it was not imbibed by it. 

 "But what surprised me more than I can yet well express, was 

 " that a candle burned in this air with a remarkably vigorous 

 " flame, very much like that of the enlarged flame with which a 



