CAVENDISH. 435 



dwells on the preference due to this inference over 

 the conclusion that inflammable air is pure phlogiston.* 

 This statement of the theory is somewhat less distinct 

 than Mr. Watt's, who considered water to be dephlo- 

 gisticated air united to inflammable air or pure phlo- 

 giston, and both deprived of their latent heat. But he, 

 as well as Mr. Cavendish, expresses himself with some 

 hesitation, and even, like him, in some passages enter- 

 tains the idea of water as united in a small proportion 

 with inflammable air. The theory, though nearly 

 completed by those great chemists, was perhaps first 

 stated with perfect certainty and distinctness by La- 

 voisier, f 



In the combustion of hydrogen gas with common 

 air, and even with impure oxygen gas, Cavendish had 

 observed that the water was slightly tinged with 

 acid, though not always when pure oxygen gas 

 was used for the operation. He therefore devised 

 an experiment which should ascertain the nature 

 of this acid, and in what manner it was formed. He 

 passed the electric spark through common air with- 

 out any hydrogen gas being present ; the air was 

 in a receiver over mercury, and the operation was of 

 long continuance, on account of the slowness with 

 which the combination is formed of th^ two gases 

 whereof the atmosphere is composed. He had not 

 supposed that the hydrogen had any share m forming 

 the acid : his theory being that water, and not acid, is 

 the produce of that gas's combustion. He naturally 



* Philosophical Transactions, 1784, p. 137, 140. 

 t See Appendix to the Life of Watt. 



2 F 2 



