relative to Black, Watt, and Cavendish. 129 



manufactured, to identify with the oxygen of the atmosphere*, 

 just as Mayow identified with it the gas from saltpetre. 



In giving to the gases which he discovered the title of 

 "factitious airs" Boyle did not confound them with common 

 air. The extracts which I have given sufficiently show that 

 he used the word air generically, in the sense which he assigns 

 to it in the following passage : — M If I were to allow acids to 

 be one principle, it should be only in some such metaphysical 

 sense as that wherein air is said to be one body, though it 

 consist of the associated effluviums of a multitude of corpus- 

 cles of very different natures that agree in very little, save in 

 their being minute enough to concur in the composition of a 

 fluid aggregate consisting of flying partsf." 



It would indeed be a great mistake in the history of science, 

 to suppose that the notion of the air being a simple element 

 prevailed among philosophers down to the days of Black. 

 From the time of that remarkable revolution in the scientific 

 mind of Europe which attended the revival of the mechanico- 

 corpuscular philosophy, when the phaenomena of nature were 

 accounted for no longer by forms and qualities, but by the 

 sizes and motions, the cohesions and disjunctions of the par- 

 ticles of bodies, the atmosphere came at once to be conceived 

 of as a miscellaneous aggregate of the molecules of a variety 

 of heavy substances thrown into an elastic state, or floating in 

 an active medium of a still finer and more divided consistence. 

 " Tout corps invisible et impalpable," says Descartes, " se 

 nomme air t a, savoir en sa plus ample signification J." " By 

 air," says Dr. Wallis, M 1 find Mr. Hobbs would sometimes 

 have us understand a pure aether, 'aerem ab omni terrae aquae- 

 que effluviis purum, qualis putatur esse aether,' to which I 

 suppose answers the materia subtilis of Descartes, and M. 

 Hugens's 'more subtile matter* than air: on the other hand, 

 M. Hugens here by air seems to understand that feculent mat- 

 ter arising from those the earth's and water's effluvia, which 



and the loss of weight came to ^rd part of a grain. May 30. — I endea- 

 voured to burn the same minium again, but such plenty of air was pro- 

 duced, that the glass broke into a hundred pieces." 



* " At the same time that I got the air above-mentioned from mercurius 

 calcinatus and the red precipitate, I had got the same kind from red lead 

 or minium. In this process that part of the minium on which the focus of 

 the lens had fallen turned yellow. The experiment with red lead con- 

 firmed me more in my suspicion, that the mere, calcinatus must have got 

 the property of yielding this kind of air from the atmosphere, the process 

 by which that preparation and this of red lead is made being similar." — 

 Priestley's Experiments on Air, vol. ii. p. 111. 



f Reflections on the Hypothesis of Alkali and Acidum, ch. iv. 1676. 



X CEuvres, torn. vii. p. 237. 



