1821.] Causes of Calorific Capacity, Latent Heat, §c. 367 



and seem to bear no proportion to the masses of the water, 

 which, perhaps, may be five or ten times as great in one vessel as 

 in the other. Now if we conceive the evaporation to commence 

 in each fluid at the same temperature and from equal superficial 

 portions, it is plain, all other things being the same, that the first 

 increment of evaporation will be equal in each. And the same 

 will likewise be the case with the second and successive incre- 

 ments, because the diminution of temperature by the evaporation 

 being generally much less than that communicated by the sur- 

 rounding air, the loss is immediately supplied, and the two por- 

 tions of water kept at precisely, or very nearly, the same 

 temperature ; and, consequently, their evaporations equally 

 supported. 



The same will also hold good, as I have mentioned in the Cor. 

 to the preceding Prop, if the depths be ever so unequal, but too 

 great to have the temperatures sensibly affected by the evapora- 

 tions ; for if the temperatures be constantly equal, and the sur- 

 faces equal, the evaporations must be equal, whatever the depths 

 may be. 



By such experiments as these philosophers appear to have 

 been much deceived, and to have formed very erroneous ideas of 

 the laws and effects of evaporation uninfluenced by other circum- 

 stances. Such phenomena are undoubtedly a decided proof 

 that evaporation takes place at the surface, and not in the interior 

 of the body ; but to conclude from this, that the mass of the 

 fluid under all circumstances has nothing to do with evaporation 

 is quite a paralogism. Let us, for instance, imagine two unequal 

 portions of the same fluid, exposing at the same temperature, 

 equal parts of their superficies to equal and similar actions of the 

 atmosphere. Then, because the temperatures, exposed surfaces, 

 and atmospheric actions, are equal, the first increments of evapo- 

 ration must likewise be equal. To carry on the same idea, let 

 us, therefore, conceive that successive contemporaneous incre- 

 ments of evaporation for half a given portion of time continue 

 also to be equal. Then, since equal evaporations would produce 

 equal diminutions of temperature in equal masses of the fluid, in 

 unequal masses they would produce unequal diminutions ; and 

 the temperature of the less mass would be much more diminished 

 than the temperature of the greater mass. But at a less temper- 

 ature there is, cateris paribus, a less evaporation. Therefore, for 

 the other moiety of the time, the evaporation of the less portion 

 will be less than that of the other; so that the two parts of time 

 being considered together, the whole evaporation in a given time 

 will be less from the less mass than from the other. This, how- 

 ever, is only to be considered as true when the effects of evapo- 

 ration are not accelerated or retarded, or at least not effectually 

 counterbalanced, by any foreign interference. Even should that 

 foreign interference counteract, but not be sufficient to overcome 

 entirely, the effects of evaporation on the temperature, it will even 



