438 CAVENDISH. 



of the process called the destructive distillation. The 

 substances found might have been the product, and not 

 merely the educt of the process. It is known that if coal 

 or oleaginous bodies be distilled in close vessels there are 

 obtained gases and water and acids which never existed 

 in the matters subjected to the action of the fire. The 

 component parts of these matters enter into new com- 

 binations with one another under the action of heat, just 

 as a tallow candle or an oil lamp gives lamp-black and 

 water in burning, though no water, but only hydrogen, 

 nor of course any lamp-black, exists in the tallow and the 

 oil. So, in Lavoisier's experiment, the water might 

 contain only oxygen and hydrogen, and the action of 

 the hot iron might have separated them from each other, 

 But it was also quite possible that the iron gave out 

 hydrogen, and that the hot water was partly kept in 

 solution by this gas, partly combined with the iron, for 

 on that supposition the combined weight of the calcined 

 iron and the hydrogen gas would be exactly equal 

 to the united weight of the water evaporated, and of 

 the iron before calcination. The previous discovery of 

 Watt and Cavendish is liable to no such ambiguity ; 

 and it has the merit of also removing all ambiguity 

 from the experiment of Lavoisier, which it manifestly 

 suggested. 



These great discoveries placed Cavendish in the 

 highest rank of philosophers. No one doubted of 

 nitrous acid ; that he was the undisputed discoverer of 

 the composition of water, before Mr. Watt's claim, is 

 .equally certain ; nor, even now, is it necessary for 

 the defenders of Watt's priority to deny that Cavendish 

 made the great step without any previous knowledge 



