440 The Fuel of the Sun. [October, 



compound was dissociated by heat alone under the same 

 pressure. 



According to the recently adopted figures of M. Deville, 

 the temperature at which the vapour of water becomes dis- 

 sociated under ordinary atmospheric pressure is 2800 C, and 

 the quantity of heat which disappears as temperature in the 

 course of dissociation is 2153 calories, i.e., sufficient to raise 

 2153 times its own weight of liquid water i° C. ; but, as the 

 specific heat of aqueous vapour is to that of liquid water as 

 0*475 to 1, the latent heat expressed in the temperature it 

 would have given to aqueous vapour is == 4532 C, or 

 8158 F. 



In order to render the analogy between the ebullition and 

 dissociation of water more evident and intelligible, I will 

 state it as follows : — 



To commence the ebullition of water To commence the dissociation of 



under ordinary pressure, a tempera- aqueous vapour under ordinary 



ture of ioo° C, or 212 F., must be pressures, a temperature of 2800 



attained. C, or 5072 F., must be attained. 



To complete the ebullition of a given To complete the dissociation of a 



quantity of water, an amount of given quantity of aqueous vapour, 



heat must be applied, sufficient to an amount of heat must be applied 



have raised the water 537 C, or sufficient to have raised the vapour 



968° F., above its boiling-point, had 4532° C, or 8158 F., above its dis- 



it not evaporated. sociation-point had it not decom- 

 posed. 



In order that a given quantity of In order that a given quantity of the 



vapour of water shall condense, it elements of water may combine, 



must give off sufficient heat to raise they must give off sufficient heat 



its own weight of water 537 C, to raise their own weight of aque- 



or 968 F. ous vapour 4532 C, or 8158 F. 



I have expressed these generalisations and analogies 

 rather more definitely than they have been hitherto stated, 

 but those who are acquainted with the researches of 

 Deville, Cailletet, Bunsen, &c, will perceive that I am 

 justified in doing so. 



With the general laws of the dissociation of water thus 

 before us, we may follow out the necessary results of the 

 above-stated pressure and consequent evolution of heat in 

 the lower regions of the solar atmosphere upon the large 

 proportion of aqueous vapour which I have shown that it 

 should contain. 



It is evident that the first result will be separation of this 

 water into its elements, accompanied with a loss of tempe- 

 rature corresponding to the latent heat of dissociation. We 

 may assume that in the lower regions of the solar atmo- 

 sphere the free heat evolved by mechanical compression will 

 be more than sufficient to dissociate the whole of the 



