Mr. W. J. M. Rankine on the Mechanical Action of Heat. 119 



Section III. — Of the Latent and Total Heat of Evaporation, 

 especialhj for Water. 



(17.) The latent heat of evaporation of a given substance at a 

 given temperature, is the amount of heat which disappears in 

 transforming unity of weight of the substance from the Hquid 

 state, to that of vapour of the maximum density for the given 

 temperature, being consumed in producing an increase of volume, 

 and an unknown change of molecular arrangement. 



It is obvious that if the vapour thus produced is reconverted 

 into the liquid state at the same temperature, the heat given out 

 during the liquefaction must be equal to that consumed during 

 the evaporation ; for as the sum of the expansive and compress- 

 ive powers, and of those dependent on molecular arrangement 

 during the whole process, is equal to zero, so must the sum of 

 the quantities of heat absorbed and evolved. 



The heat of liquefaction, at a given temperature, is therefore 

 equal to that of evaporation, with the sign reversed. 



(18.) If to the latent heat of evaporation at a given tempera- 

 ture is added the quantity of heat necessary to I'aise unity of 

 weight of the liquid from a certain fixed temperature (usually 

 that of melting ice) to the temperature at which the evaporation 

 takes place, the result is called the total heat of evaporation from 

 the fixed temperature chosen. 



According to the theory of Carnot, this quantity is called the 

 constituent heat of vapour ; and it is conceived, that if liquid at 

 the temperature of melting ice be raised to any temperature and 

 evaporated, and finally brought in the state of vapour to a certain 

 given temperature, the whole heat expended will be equal to the 

 constituent heat corresponding to that given temperature, and 

 will be the same, whatsoever may have been the intermediate 

 changes of volume, or the temperature of actual evaporation. 



According to the mechanical theory of heat, on the other hand, 

 the quantity of heat expended must vaiy with the intermediate 

 circumstances ; for otherwise no power could be gained by the 

 alternate evaporation and liquefaction of a fluid at diflerent 

 temperatures. 



(19.) The law of the latent and total heat of evaporation is 

 immediately deducible from the principle of the constancy of the 

 total vis viva in the two forms of heat and expansive power, when 

 the body has returned to its primitive density and temperature, 

 as already laid down in article 7. 



That principle, when applied to evaporation and liquefaction, 

 may be stated as follows : — 



Let a portion of fluid in the liquid state be raised from a cer- 

 tain temperature to a higher temperature : let it be evaporated 



