37 



This oxygen having been obtained from nitrate of mercury, he sup- 

 posed that the nitric acid might have been derived from the gas, and 

 therefore repeated the experiment with oxygen disengaged from red 

 precipitate by oil of vitriol ; but he still found nitric acid. 



Such were the experiments made in 1781, " concerning the recon- 

 version of air* into water by decomposing it in conjunction with 

 inflammable air", which Priestley f and Cavendish | mention as having 

 been communicated to the former, and repeated in consequence by 

 him in April 1783. 



In the following year, 1782, Cavendish made further experiments 

 on the analysis of air, and the tests of oxygen, and in October resumed 

 the investigation of the cause of the production of nitric acid in the 

 combustion of that gas with hydrogen : he now employed the oxygen 

 evolved by plants, under the action of solar light, with the same result ; 

 but varying the proportions of the gases (MSS. pp. 203-5.) dis- 

 covered that an excess of oxygen conduced to the production of the 

 nitric acid: he had probably before conjectured the cause of the phae- 

 nomenon,and in his next experiment, in January 1783, (MSS. p. 211,) 

 he added a little nitrogen to the excess of oxygen, and found the quan- 

 tity of acid still further increased ; but when he mixed nitrogen with 

 oxygen (MSS. p. 217,) in the proportions of common air, and exploded 

 either this mixture, or common air, with a quantity of hydrogen in- 

 sufficient to consume all the oxygen in it, the excess of the latter no 

 longer determined the formation of acid, but, as in his first experi- 

 ments, pure water was the result. Hence he came to the following con- 

 clusions — that when hydrogen is burnt with oxygen, slightly contami- 

 nated with nitrogen, and in excess, the excess of oxygen forms with 

 the nitrogen nitric acid§ ; but that when it is burnt with oxygen mixed, 

 as in common air, with a large proportion of nitrogen, the heat of the 



* That Priestley by air, here means oxygen, which was often so called 

 x«T i^axnv, appears from his manner of repeating the experiment, and from 

 Watt's defective acknowledgement, (the only notice he takes of any of Caven- 

 dish's experiments,) " I believe that Mr. Cavendish was the first who disco- 

 vered that the combustion of dephlogisticated and inflammable air produced 

 moisture, on the sides of the glass in which they were fired." Cavendish does not 

 (as M. Arago says) insinuate, but states distinctly, and without contradiction, 

 that he mentioned to Priestley " all his experiments on the explosion of inflam- 

 mable with common and dephlogisticated airs, except those which related to the 

 cause of the acid found in the water." Phil. Trans, vol. Ixxiv, part 1. p. 134. 



t Phil. Trans. 1783, p. 426. J Ibid. 1785, p. 134. 



§ Experiments on Air, Phil. Trans, vol. Ixxiv. part 1. p. 139. 

 c 2 



