29 



In process of time Priestley also discovered that the gas produced in 

 the distillation of charcoal was widely different from that evolved in 

 the solution of metals ; and hence his subsequent assertion that he "had 

 never been able to find the full weight of the air decomposed in the 

 water produced by the decomposition," was more relevant to this ques- 

 tion than M. Arago is aware. Priestley knew that the experiment 

 which M. Arago has recalled into notice, was erroneous in all its parts ; 

 and as it could tend only to mislead, when he reprinted in 1790 the 

 paper which had contained it, he exercised a sound discrimination, and 

 whilst he retained the account of those experiments in which he ima- 

 gined that he had converted water into common air, as, though erro- 

 neous, containing something instructive, he omitted this as utterly 

 worthless. It remains for MM. Arago and Dumas to follovv his 

 example of candid retractation, and restore the experiment, with the 

 claims that have been founded upon it, to the grave from Avhence it 

 has been disinterred. 



Having now disposed of two of the points on which I promised to 

 satisfy M. Arago, namely, the worth of Priestley's experiments on the 

 composition and decomposition of water, and of the deduction from 

 them of the theory, doctrine, or hypothesis of Watt, (except, indeed, 

 so far as M. Arago may coincide with my friend Lord Brougham in 

 thinking Watt's introduction of the matter of heat an improvement on 

 the views of Cavendish)*, I now turn to the real history of this great 

 discovery, to which, since the meeting of the Association at Birming- 

 ham, I have been enabled to make an addition that cannot fail to ex- 

 cite the interest of all who pay attention to experimental science. 



It is one of the privileges of genius to give duration even to its 

 perishable remains. In the expectation that I should find the MSS. 

 of Cavendish still in existence, I applied to Lord Burlington for in- 

 formation, and found that they had passed into the hands of his grand- 

 father, Lord George Cavendish, and thence to the present Duke of 

 Devonshire, from whom I have obtained permission to use them for 

 the elucidation of the present question. 



These carefully preserved MSS. exhibit the footsteps of a mind 



* Had Watt remarked only that there was here another instance of the 

 general fact, that whenever a combination is formed of greater density than 

 the combining bodies, heat is generated, the remark would have been just and 

 obvious ; but in representing the phsenomena as influenced by chemical affini- 

 ties of heat, and stating that " dephloyisticated water has a more powerful at- 

 traction for pJdoyiston than it has for latent heat" (Phil. Trans., vol. Ixxiv. p. 

 334), he introduces a principle which chemistry has no means of investigating. 



