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eminent experimentalists already. Now, in the theory of heat, the 

 research of the specific heats of the gases is one not far removed from 

 such primary or central position, being no mere question of detail, 

 but intimately connected with the inquiry into the nature of heat 

 itself; it is also one which has been agitated by eminent men, and 

 results have been obtained by some, and disputed by others, of which 

 it is interesting, in a high degree, to examine the correctness or in- 

 validity. For a new examination of this kind, conducted by new 

 methods of experiment, the present award has been made. Of the 

 nature and grounds of this award, I now proceed briefly to speak ; 

 and first, it may be proper that I should remind the Academy of the 

 meaning of this phrase specific heats, and of the phenomena which 

 suggest the name and the conception. 



When any two equal volumes of water at any two unequal tempera- 

 tures are mixed together, the mixture acquires, in general, a tempera- 

 ture which is either exactly or at least very nearly intermediate between 

 the two original temperatures, being as many degrees of the thermo- 

 meter below the one, as it is higher than the other. Bat if a pint 

 of mercury at 60° and a pint of water at 80° be brought in contact 

 and acquire thereby a common temperature, it is found that this last 

 is not so low as 70° ; and that thus, this passage of heat, from the 

 warmer water to the colder mercury, has cooled the former less than 

 it has warmed the latter, as indicated by the degrees of a thermo- 

 meter. Phenomena of this kind suggest the conception, that only a 

 part of the heat contained at any one time, in any particular body, 

 aflfects the senses or the thermometer ; and that the remainder of the 

 heat is insensible, latent, or hidden: so that water, for example, 

 absorbs or hides more heat than the same bulk of mercury at any 

 temperature common to both, and that for any given increase of 

 that temperature (measured by the thermometer) the former absorbs 

 or renders latent more than the latter, while, on the contrary, in 

 cooHng through any given number of degrees, it sets a greater quan- 

 tity free. Many other phenomena are made intelligible by such 

 a conception, and even more immediately suggest it. Thus, if we put 

 a pound of freshly frozen ice in contact with a pound of water, which 

 is warmer than it by about 140° of Fahrenheit's thermometer, the 

 result will be two pounds of water, not at an intermediate, but at the 



