1871J The Fuel of the Sun. 441 



aqueous vapour, and thus the dissociated gases will be left 

 at a higher temperature than was necessary to effect their 

 dissociation. Their condition will thus be analogous to that 

 of superheated steam, they will have to give off some heat 

 before they can begin to combine. 



There will, however, be somewhere an elevation where 

 the heat evolved by the joint compression of the elementary 

 and combined gases will be just sufficient to dissociate the 

 latter, and here will be the meeting surface of the combined 

 and the uncombined constituents of water. There will be 

 a sphere containing uncombined oxygen and hydrogen sur- 

 rounded by an atmospheric envelope containing large quan- 

 tities of aqueous vapour, and the temperature at this 

 limiting surface will be just equal to that of the oxyhydrogen 

 flame under a corresponding pressure. 



What will occur under these conditions ? Will the 

 " detonating gases " behave as in the laboratory ? 

 Obviously not, as a glance at the third of the above 

 parallel propositions will show. The dissociated gases 

 cannot combine without giving off their 4532° of latent 

 heat as actual temperature. This can only be effected by 

 communication with matter which is cooler than itself. 



If a bubble of steam is surrounded by water maintained 

 at the boiling temperature, it will not condense at all, 

 because any effort of condensation would be accompanied 

 with an evolution of heat exactly sufficient to evaporate its 

 own result. If, however, the surrounding water is slowly 

 radiating, or otherwise losing its heat, the enclosed bubble 

 of steam will condense proportionately by giving off to 

 its envelope an amount of its latent heat just sufficient to 

 maintain the water at the boiling-point. 



For further illustration, let us conceive the case of a 

 certain quantity of the elements of water heated exactly to 

 the temperature of dissociation, and confined in a vessel 

 the sides of which are maintained externally at precisely 

 the same temperature as the gases within, so that no heat 

 can be added or taken away from them. No sensible 

 amount of combination could now take place, as the first 

 infinitesimal effort of combustion, or combination, would 

 set free just the amount of heat required to decompose its 

 own result. Let us now suppose a modification of these 

 conditions, viz., that the vessel containing the dissociated 

 gases, at the temperature of dissociation, shall be surrounded 

 with bodies cooler than itself, i.e., capable of receiving more 

 heat from it than they radiate towards it ; there would then 

 take place just so much combustion as would set free the 



