Specific Heat of Elastic Fluids. 479 



8. The quantity of heat absorbed by a gas when it produces 

 during its expansion motive work which is completely expended 

 in the interior of the calorimeter, or of which the greater part is 

 rendered available externally. 



9. Lastly, the densities of vapour of maximum density under 

 different pressures. 



The experiments which refer to these different questions are, 

 with the exception of the latter, already almost completed. But 

 as it will still require some considerable time to arrange them in 

 their proper order and discuss them with suitable care, I purpose 

 to bring before the Academy in succession the general results, 

 preparatory to publishing them in a complete form. 



I shall, in the present instance, limit myself to the considera- 

 tion of my researches upon the capacity for heat of elastic fluids. 



Capacity for Heat of Elastic Fluids. 



Two definitions of the specific heat of elastic fluids may be 

 given. According to the former, the specific heat is that quan- 

 tity of heat which must be communicated to a gas in order to 

 raise its temperature one degree, under conditions which admit 

 of its free expansion, so that it may preserve a constant tension ; 

 according to the second, it is the quantity of heat necessary to 

 elevate its temperature one degree under such conditions that its 

 volume remains the same while its tension increases. 



The former of these capacities has been termed the specific 

 heat of gases under constant pressure ; the second has been 

 termed the specific heat with constant volume. The former defi- 

 nition alone coincides with that which has been adopted for the 

 capacity for heat of solids and liquids ; it is therefore the only 

 one which has hitherto been found to admit of a direct experi- 

 mental determination. 



During the last century a great number of physicists have 

 been engaged with investigations relating to the specific heat of 

 elastic fluids. Crawford, Lavoisier and Laplace, Dalton, Clement 

 and Desormes, Delaroche and Berard, Haycraft, Gay-Lussac, 

 Dulong, Pe la Rive and Marcet, have successively published 

 researches upon this subject. Most of these physicists have en- 

 deavoured to demonstrate by experiment certain laws, to which 

 they had been led by a priori reasoning on the constitution of 

 elastic fluids. They applied themselves less to determine the 

 numerical values of the capacity of different gases for heat in 

 relation to that of liquid water, generally adopted as unity, than 

 to seek for simple relations, which they supposed ought to exist 

 between them. The conclusions at which they arrived are in 

 most instances very erroneous. 



The memoir of Delaroche and Berard, which was rewarded 



