CAVENDISH. 431 



aeriform substances are each .sui generis, and the 

 same from whatever substances, by whatever pro- 

 cesses, they are obtained ; nor satisfied with the mere 

 fact that one of them is heavier, and the other much 

 lighter, than atmospheric air, he inquired into the 

 precise numerical relation of their specific gravities 

 with one another and with common air, and first 

 showed an example of weighing permanently elastic 

 fluids : unless, indeed, Torricelli may be said before 

 him to have shown the relative weight of a column of 

 air and a column of mercury : or the common pump 

 to have long ago compared in this respect air with 

 water. It is, however, sufficiently clear, that neither 

 of these experiments gave the relative measure of one 

 air with another : nor, indeed, could they be said to 

 compare common air with either mercury or water, 

 although they certainly showed the relative specific 

 gravities of the two bodies, taking air for the middle 

 term or common measure of their weights. 



The common accounts in chemical and in biogra- 

 phical works are materially incorrect respecting the 

 manner in which Mr. Cavendish was led to make his 

 great experiment upon the composition of water in 

 1781 and the following years. It is said, that while 

 making his experiments on air in 1765 and 1766, he 

 had observed for the first time, that moisture is pro- 

 duced by the combustion of inflammable air, and that 

 this led him, sixteen or seventeen years later, " to com- 

 plete the synthetical formula of water, and to find 

 that the moisture that he had before observed was 

 simple water."* Nothing can be more erroneous than 



* Penny Cyclopaedia, vol. vi. p. 39?. This and other similar 



