214 Biographical Memoir of'Dr Priestley, 



ledge in this department of science, when Priestley took up the 

 subject, and treated it with great success. 



Happening to lodge at Leeds near a brewery, he liad the cu- 

 riosity to examine the fixed air which exhales from beer in fer- 

 mentation, and the deleterious power with which that air ope- 

 rates upon animals, as well as its effect upon the flame of candles. 

 His investigations having afforded him striking results, he sub- 

 mitted inflammable air to similar experiments. Wishing after- 

 wards to determine all the circumstances in which these two 

 gases manifest themselves, he soon remarked, that, in a great 

 number of combustions, especially in the calcinations of metals, 

 the air in which these operations are performed is altered in its 

 nature, without either fixed or inflammable air being produced. 

 Whence his discovery of a third kind of noxious air, which he 

 called pJihgisticated air, and which was afterwards named azotic 

 gas. 



He made use of small animals for trying the pernicious action 

 of these different gases, and found himself obliged to inflict tor- 

 tures on sensible beings. His character is well illustrated in the 

 joy which he experienced on the discovery of a fourth kind, 

 which freed him from the necessity of having recourse to these 

 cruel means. This was Jiitrmis gas, which possesses the pro- 

 perty of suddenly diminishing the volume of any other gas with 

 which it is mixed, nearly in the proportion in which that other 

 gas is respirable, and consequently the property also of measur- 

 ing, to a certain extent^ the degree of salubrity of different 

 airs. 



This discovery gave origin to that branch of natural philoso- 

 phy named Eudiometry, and was of primary importance. All 

 the natural sciences were interested in possessing such a mea- 

 sure, and medicine in particular might have been highly be 

 nefited by it, were it not so diflicult to introduce scientific pro- 

 cesses into the practice of even the most scientific arts. 



Combustion, fermentation, respiration, and putrefaction, pro- 

 duced sometimes fixed air, at other times inflammable air, and 

 sometimes phlogisticated air. There were therefore a multitude 

 of causes capable of vitiating the air ; and yet its purity not being 

 sensibly altered during the long period that these causes have 

 been in action, it was necessary that there should be in nature 

 some constant means of keeping up this purity. 



