285 



The following laws, wliich have already been inferred from experi- 

 ment by Dulong, are then deduced from the theory : 



The specific heat of unity of volume, at constant volume, varies 

 for different perfect gases inversely as the fraction by which the 

 ratio of the two specific heats exceeds unity. 



Equal volumes of all substances in the state of perfect gas, at 

 the same pressure, and at equal and constant temperatures, being 

 compressed by the same amount, disengage equal quantities of heat. 



The data now obtained being employed to calculate the value of 

 heat in terms of the force of gravity, it is found that the real specific 

 heat of atmospheric air is equivalent to a fall of 238"66 feet per 

 centigrade degree, and the apparent specific heat of liquid water at 

 the temperature of melting ice (being what is commonly termed a 

 thermal unit) to a fall of 1252 feet per centigrade degree, or 695'6 

 feet per degree of Fahrenheit. 



The author next investigates the apparent specific heat of vapour 

 at saturation. This quantity, according to his theory, is altogether 

 different from the variation of the total heat of evaporation, with 

 which, according to the theory of Carnot, it is identical. It is in 

 general negative ; so that if vapour at saturation is allowed to ex- 

 pand, being cut off from external sources of heat, a portion of it must 

 be liquefied in order to supply the heat necessary for the expansion 

 of the rest, in addition to the heat set free by the fall of tempera- 

 ture. 



The third section treats of the latent and total heat of evapora- 

 tion, especially for water. 



It is in the first place proved, from the principle of vis viva, that 

 the latent heats of evaporation and liquefaction, at a given tempera- 

 ture, are equal, with contrary signs. 



The total heat of evaporation is defined to be the sum of the 

 latent heat of evaporation, and of the heat required to raise the 

 liquid to the temperature at which it is evaporated, from some arbi- 

 trary fixed temperature — (generally that of melting ice). 



The law of variation of the total heat of evaporation with tempera- 

 ture is then deduced from the principle of the conservation of vis 

 viva, which, as applied to this subject, takes the following form ; — 



Let a portion of fiuid in the liquid state be viixcd from a cer- 

 tain temperature to « higher temperature ; ht it be evaporated at 

 the higher tiniperalnre ; let the vapour then be allowed to expand, 



