96 CAVENDISH. 



phlogiston, and that inflammable air is either pure 

 phlogiston, or else water united to phlogiston;" and 

 he then gives his reasons in favour of the second in- 

 ference, namely, that inflammable air is water united 

 to phlogiston ; but he repeatedly dwells on the prefer- 

 ence due to this inference over the conclusion that in- 

 flammable air is pure phlogiston.* This statement of 

 the theory is somewhat less distinct than Mr. Watt's, 

 who considered water to be dephlogisticated air united 

 to inflammable air or pure phlogiston, and both de- 

 prived of their latent heat. But he, as well as Mr. Caven- 

 dish, expresses himself with some hesitation, and even, 

 like him, in some passages entertains 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 Lavoisier, 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 experi- 

 ment which should ascertain the nature of this acid, 

 and in what manner it was formed. He passed the 

 electric spark through common air without any hydro- 

 gen 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 combina- 

 tion is formed of the two gases whereof the atmos- 

 phere is composed. He had not supposed that the 

 hydrogen had any share in forming the acid: his 

 theory being that water, and not acid, is the produce 

 of that gas's combustion. He naturally suspected the 

 acid to be the produce of some union between the azote 

 and the oxygen of the atmosphere. He left the pro- 

 cess in the hands of a committee of his scientific 



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

 f See Appendix to the Life of Watt. 



