relative to Black, Watt, and Cavendish. 499 



that of a similar mixture of nitrous and common air. After 

 this I had no doubt but that the air from mere, calcinatus was 

 fit for respiration, and that it had all the other genuine pro- 

 perties of common air. But I did not take notice of what I 

 might have observed if I had not been so fully possessed by 

 the notion of there being no air better than common air, that 

 the redness was really deeper, and the diminution something 

 greater, than common air would have admitted. / now con- 

 cluded that all the constituent parts of the air were equally and 

 in their proper proportion imbibed in the preparation of this 

 substance, and also in the process of making red lead*" — a 

 conclusion identical with the ideas of Rey in 1630. 



The next step in Priestley's inquiry was the employment 

 of Mayow's mice, which convinced him that this air was 

 longer respirable than common air; but his ideas of it were 

 less accurate than Mayow's, for instead of considering it, with 

 him, a constituent part of nitric acid, he thought it a compound 

 of nitric acid and earth', and in December 1777, "no doubt 

 remained on his mind that atmospheric air, or the thing that 

 we breathe, consists of the nitrous [nitric] acid and earth, 

 with so much phlogiston as is necessary to its elasticity, and 

 likewise so much more as is necessary to bring it from its 

 state of perfect purity to the mean condition in which we 

 find it." 



You now see the error into which you have fallen when 

 you represent Priestley as discovering before Lavoisier that 

 " this was a gas wholly different from all other gases formerly 

 known," and may perhaps suspect that you are not justified 

 in condemning as " an unworthy and lamentable proceeding " 

 on Lavoisier's part, " the intruding himself into the history of 

 this discovery, knowing that Priestley was the sole discoverer." 

 A property of this gas, which under Priestley's observation 

 had led to nothing, in the hands of Lavoisier gave rise to one 

 of the most important investigations in the annals of chemistry ; 

 he, it appears from your own admission, had ascertained the 

 relations of this elementary substance to various bases and to 

 the atmosphere, between August 1774 and March 1775, at 

 which date the foregoing extracts show the "sole author of the 

 discovery" to have "had no doubt that it had all the genuine 

 properties of common air" Whoever may be called the 

 discoverer of oxygen, whether Hook and Mayow, who first 

 inferred its existence in nitre and in air, — or Boyle, who first 

 disengaged the elastic gas from minium, — or Hales, who col- 

 lected it from the same material, — or Nieuwentyt, who attri- 



* Experiments and Observations on different kinds of Air, vol. ii. p. 1 13, 

 ed. 1790. 



