relative to Black, Watt, and Cavendish. 113 



supposed facts; and you have in consequence entirely mis- 

 stated the nature of Cavendish's experiments. Where, allow 

 me to ask, do you find in his paper, or his notes, any such 

 matter as this? "He then weighed accurately the air of both 

 kinds, which he exposed to the stream of electricity ; and he 

 afterwards weighed the liquid formed by the combustion : he 

 found that the two weights corresponded with great accuracy " 

 (Life of Cavendish, p. 433): and again, " Water equal to the 

 weight of the two gases taken together remained as the produce 

 of the combustion." Cavendish made no such experiments ; as 

 you will find whenever you take the trouble to read either the 

 documents themselves*, or my account of themf. I have 

 already stated that this method of determining the composition 

 of water, which is attended with great practical difficulties, was 

 tried indeed at a later time by the French philosophers with 

 such accuracy as it admits of, but that Cavendish, with his usual 

 sagacity, had taken an easier and more certain road : having 

 mastered beyond any of his cotemporaries the analysis of gases, 

 and possessed himself of their specific properties, he was enabled 

 to substitute the method of volume for that of weight ; he 

 found that about two volumes of hydrogen and one of oxygen, 

 when burnt together, entirely disappeared without loss of 

 weight, and that pure 'water was the result. To draw from 

 these premises the obvious conclusion, there was no need to 

 weigh or compare the weight of the airs, and the water that 

 lined the glass after combustion ; and he did not compare it. 

 Lavoisier followed in his steps : and should you ever read his 

 papers, you will find that he too in the first instance contented 

 himself with deducing the equality of the weights as a corollary 

 from experiments of the same kind as those of Cavendish. 



Had you happened to consult the second edition of the En- 

 cyclopaedia Britannica as well as the fast, you would have found 

 it purged both of these, and some other of Robison's historical 

 mistakes. You would have found all that you have referred 

 to omitted ; and in the article ' Chemistry,' compiled under the 

 revision of friends and connections of Watt, the following ac- 

 count substituted in its place. "In the year 1781, Mr. Caven- 

 dish proved that water is not a simple element, but that it is 

 composed of pure or vital air, and inflammable air." " In 

 the mean time the French chemists were not idle ; the cele- 

 brated Lavoisier, in conjunction with some of his philosophical 

 friends, confirmed by the most decisive experiments the truth 

 of Mr. Cavendish's discovery of the composition of' water, which 



* Phil. Trans, vol. lxxiv. Experiments on Air, by H. Cavendish, Esq. 

 Report of the British Association for 1839, autograph notes of experiments. 

 f Report of the British Association for 1839, pp. 35, 36. 



