relative to Black, Watt, and Cavendish. 121 



gases approached too nearly to common air in that respect to 

 enable the experimenters to establish a distinction. An at- 

 tempt too had been made by Greenwood, a Professor of 

 Mathematics at Cambridge in New England, to ascertain the 

 specific gravity of the deleterious air in a well, which was 

 doubtless chiefly carbonic gas ; but the method employed by 

 him was not sufficiently delicate to show a difference of den- 

 sity. Such was the state of knowledge, or rather ignorance, 

 on this subject previous to the experiments of Cavendish. 

 We have not the least reason to believe that any one had 

 observed the different weights of the different kinds of air. 

 Dr. Mayow* indeed about a century before had supposed 

 his " nitro-igneous aura" to the combinations of which he 

 ascribed the phaenomena of acidification, combustion and vi- 

 tality, to be heavier than the residual air from which it is 

 separated in those processes ; and this opinion, which proved 

 to be correct, he entertained so distinctly, as to represent the 

 specific lightness of the vitiated air, after it had served its 

 purpose of sustaining life, as a provision of nature for freeing 

 us from a noxious atmosphere. But he had no better ground 

 for entertaining such an opinion than his observation of the 

 movements of animals which he had confined in a close ves- 

 sel, and which appeared in his experiments to seek for a less 

 suffocating air in the lower part of the receiver, whilst they 

 avoided the upper. 



Such loose surmises as these detract nothing from the great 

 experimental discovery of Cavendish, the importance of which 

 cannot be better expressed than in the words of an eminent 

 chemist and chemical historian f: " It can scarcely be said that 

 pneumatic chemistry was properly begun till Mr. Cavendish 

 published his valuable paper on Carbonic Acid and Hydrogen 

 Gas, in the year 1766." On the fruits of this discovery, in 

 the hands of its author and of all succeeding chemists, and its 

 consequences to the study of gaseous substances and their 

 combinations, I need not dwell. It is enough to remark, that 

 the ascertainment of this physical difference in the gases was 

 the first conclusive proof of a 'plurality of elastic fluids. 



Another point of no small consequence to pneumatic che- 

 mistry was first made out in this paper. From the earliest 

 discovery of factitious airs, it had been observed that a consi- 

 derable portion of several of these disappeared after they had 

 been generated, though there had been no change of tempe- 

 rature or pressure. The usual statement of this phenomenon 



* De parte aeria igneaque Spir. Nitri. 



f Dr. J. Thomson's Biographical Account of Priestley, Ann. Phil., vol. i. 

 p. 91. 



Phil. Mag. S. 3. Vol. 28. No. 185. Feb. 1846. K 



