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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 whichiseither exactly or atleast 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. Phenomenaof this kind suggest the conception, that only a 
part of the heat contained at any one time, in any particular body, 
affects 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 Jatent more than the latter, while, on the contrary, in 
cooling 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 
