78 SCIENCE IX SHORT CHAPTERS. 



To commence the ebullition of To commence the dissociation of 



water under ordinary pressure, aqueous vapor under ordinary 



a temperature of 100 C., or pressures, a temperature of 



212 F., must be attained. 2800 C., or 5072 F., must be 



attained. 



To complete the ebullition of a To complete the dissociation of 



.given quantity of water, an a given quantity of aqueous 



amount of heat must be ap- vapor, an amount of heat must 



plied, sufficient to have raised be applied sufficient to have 



the water 537 C., or 968 F., raised the vapor 4532 C., or 



above its boiling-point, had it 8158 F., above its dissociation- 



not evaporated. point had it not decomposed. 



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

 vapor of water shall condense, the elements of water may coin- 

 it must give off sufficient heat bine, they must give off suffi- 

 to raise its own weight of water cient heat to raise their own 

 537 C., or 968 F. weight of aqueous vapor 4532 



C., or 8158 F. 



I have expressed these generalizations and analogies rather 

 more definitely than they have been hitherto stated, but those 

 who are acquainted with the researches of Devi lie, Cailletet, 

 Bansen, etc. 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 action of the above- 

 stated pressure and consequent evolution of heat in the lower 

 regions of the solar atmosphere upon the large proportion of 

 aqueous vapor 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 temperature 

 corresponding to the latent heat of dissociation. We may 

 assume that in the lower regions of the solar atmosphere the 

 free heat evolved by mechanical compression will be more than 

 sufficient to dissociate the whole of the aqueous vapor, and 

 thus the dissociated gases will be left at a higher temperature 

 than was necessary to effect their dissociation. Their condi- 

 tion will thus be analogous to that of superheated steam, they 

 will have to give off some heat before they can begin to com- 

 bine. } 



* Since the above was written, these analogies have been generally 

 accepted. 



f Since the publication of " The Fuel of the Sun," Mr. Norman Lock - 

 yer has adopted this view of solar dissociation, and has gone so far as to 

 suppose that it splits metals and other substances regarded by modern 

 chemists as simple elements into more elementary and simple con- 

 stituents. He assumes that the temperature of the solar atmosphere, 

 growing higher at increasing depths, becomes somewhere capable of 



