TI1E FUEL OF THE SUK. 7 



tion." Having myself been unable to find any satisfactory 

 English account of this subject at a time when it had already 

 been well treated by French and German authors, in the form 

 of published lectures and cyclopaedia articles, I assume that 

 others may have encountered a similar difficulty, and therefore 

 Iwell rather more fully upon this part of my present sum- 

 .nary. 



It appears that all chemical compounds may be decomposed 

 by heat, and that, at a given pressure, there is a definite and 

 special temperature at which the decomposition of each com- 

 pound is effected. For the absolute and final establishment of 

 the universality of this law further investigations are necessary, 

 actual investigations having established it as far as they have 

 gone, but these have not been exhaustive. 



There appears to be a remarkable analogy between disso- 

 ciation and evaporation. When a liquid is vaporized, a cer- 

 tain amount of heat is " rendered latent," and this quantity 

 varies with the liquid and with the pressure, but is definite and 

 invariable for each liquid at a given pressure. In like manner, 

 when a compound is dissociated, a certain amount of heat is 

 " rendered latent" or conveited into dissociating force, and 

 this varies with each compound and with the pressure, but is 

 definite and invariable for each compound at a given pressure. 

 Further, when condensation occurs, an amount of heat is evolv- 

 ed, as temperature, exactly equal to that which was rendered 

 latent in the evaporation of the same substance under the same 

 pressure ; and, in like manner, when chemical recombination 

 of dissociated elements occurs, an amount of heat is evolved, 

 as temperature, exactly equal to that which disappeared when 

 the 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 dissociated 

 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 1 C. ; but, as the 

 specific heat of aqueous vapor is to that of liquid water as 0*475 

 to 1, the latent heat expressed in the temperature 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 : 



