DECOMPOSITION OF WATER BY HEAT. 325 



from observing the obstinacy with which water retained air, 

 was led to believe that water was convertible into nitrogen 

 (phlogisticated air). I have repeated several of Priestley's 

 experiments under much more stringent circumstances, and 

 have never been able to free water from air, or so to boil water 

 that for every ebullition of vapour a minute bubble of perma- 

 nent gas was not left, which appeared to have been an in- 

 dispensable nucleus to the vapour. 



The difficulty of boiling water increases, as M. Donny has 

 proved, in proportion to its freedom from air, and at last the 

 bursts of vapour become so enormous that the vessels em- 

 ployed are generally broken. There appears to me a point 

 beyond which this resistance does not extend ; but even at 

 this point a minute bubble of air is left for each burst of va- 

 pour, though they are so few and distant that the aggregate 

 amount of gas is very trifling. I have produced from water 

 which had been previously carefully deprived of air by the ordi- 

 nary methods three-fourths of its own volume of permanent gas, 

 which proved to be nitrogen ; but as the water in this experi- 

 ment was boiled under a long column of oil, it is probable that 

 if any oxygen were present it might have been absorbed by the 

 oil ; I have, however, always found the proportion of oxygen 

 to decrease as the boiling was continued. It may be worth 

 noticing, as having had some influence on my mind, that many 

 months ago, when considering the experiments of Henry and 

 Donny on the cohesion of water, I mentioned to Mr. Gassiot, 

 and also to Mr. Bingham, my assistant (to whose assiduity I 

 am much indebted), that I was inclined to think if water could 

 be absolutely deprived of air it would be decomposed by heat, 

 a result which I have now attained by a totally different series 

 of inductions. It is a circumstance worthy of remark, that I 

 find the greater part of the air to be expelled at a compara- 

 tively low temperature, and when the water has come in con- 

 tact with the platinum, while the decomposition all takes 

 place when the platinum is surrounded by an atmosphere of 

 steam, if steam it may be called, for the state of this atmo- 

 sphere at the first immersion of the platinum is at present very 

 mysterious. 



I think I may now safely regard it as proved that plati- 



