THE FUEL OF TEE SUN. 13 



compound was dissociated by heat alone under the same 

 pressure. 



According to the recently adopted figures of M. Deville, 

 the temperature at which the vapor 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 calorics, i.e., sufficient 

 to raise 2153 times its own weight of liquid water 1 C.; 

 but, as the specific heat of aqueous vapor is to that of liquid 

 water as 0'47o to 1, that latent heat expressed in the tem- 

 perature it would have given to aqueous vapor 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 T 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 corn- 

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

 to raise 'its own weight of wa- dent heat to raise their own 

 ter 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 Deville, 

 Cailletet, Bunsen, etc., will perceive that I am justified in 

 doing so.* 



* Since the above was written these analogies have been generally 

 accepted. 



