23 



The proofs wliicli I have already alleged, that Cavendish owed 

 nothing either to the experiments of which Priestley sent an account to 

 the Royal Society, on the 21st of April 1783, or to the conclusions 

 which Watt drew from them, were these — 



1. The experiments which Cavendish made in the summer of 1781 

 not only necessarily involved the notion (which is the claim set up for 

 Watt), but substantially established the fact (which is the claim set up 

 for Priestley) of the composition of water. 



2. The experiment which Priestley made in April 1783, /or the pro- 

 fessed purpose of verifying the fact of the co7iversion of air into water, 

 communicated to him hy Cavendish, added nothing to the proofs which 

 Cavendish had already obtained of it nearly two years before. 



3. Whilst the views of Cavendish are shown by the internal evi- 

 dence of the experiments themselves, and the train of reasoning which 

 they imply, to have been from the first precise and philosophical, those 

 of Priestley and Watt were always, as regards the former, and till after 

 the publication of Cavendish's and Lavoisier's papers, as regards the 

 latter, vague and wavering to a degree scarcely comprehensible to 

 those who have not studied the ideas prevalent at that period of chemi- 

 cal history. 



These three positions I hope now to establish in a manner which will 

 leave M. Arago nothing more to desire. 



The opinion of Watt has been called a theory, a doctrine, and even 

 a hypothesis; and a northern critic*, who views this question of indi- 

 vidual justice as one of national honour, allows the claim of Cavendish 

 to the proof of the fact, but reserves for Scotland the credit of the 

 Jtypothesis. So far, certainly, as it involved the theories of heat and 

 phlogiston, it ivas a hypothesis ; but so far as it related to the conver- 

 sion of inflammable and dephlogisticated airs into water, it was simply 

 an opinion that Priestley had succeeded in proving the point which he, 

 after Cavendish, had made an experiment expressly to ascertain. 



Whatever it be called, however, whether a statement of the result 

 of Priestley's experiment, or a hypothesis, or a doctrine, or a theory, 

 it was no sooner conceived than placed by Watt on the shelf, and 

 left there from April to November : the experiment of Priestley also 

 remained in abeyance. Priestley has given reason enough for his not 

 prosecuting so important an inquiry further, by informing us that it 

 belonged to Cavendish : Watt has also assigned a reason for his sus- 

 pense ; and T have shown in my address, that that reason proves him 



* Edinburgh Review, No. 142. 



