THE FUEL OF THE SUN. 15 



tonating 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 boiliug temperature, it will not condense at all, be- 

 cause 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 main- 

 tain the water at the boiling-point. 



For further illustration, let us conceive the case of a cer- 

 tain 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 can 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 tem- 

 perature 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 amount 

 of heat required to maintain the temperature of the vessel 

 at the dissociation-point ; or, in other words, combustion 

 would go on to the extent of setting free just so much heat 

 as the gaseous mass was capable of radiating, or otherwise 

 transmitting to surrounding bodies ; and this amount of 

 combustion would continue till all the gases had combined. 



We have only to give this hypothetical vessel a spherical 

 form and an internal diameter of 853,380 miles to con- 

 struct its enveloping sides of a thick shell of aqueous vapor, 

 etc., and then, by placing in the midst of the contained dis- 



