On the Specific Heat of Gases and Vapours. 289 



The effects of hydrostatic pressure have been exemplified in the 

 same volcanic mountain, not only in fractures of the mountain, 

 but also in majestic fire-fountains, in which the lavas were thrown 

 to heights of from 100 to 700 feet*. 



Another Part, on the Formation of the Continental Plateaux 

 and Oceanic Depression, will finish this memoir. 



[To be continued.] 



XXXIV. On the Determination of the Specific Heat of Gases and 

 Vapours at Constant Volumes. By R. C. Nichols, Esq.f 



THE specific heat of gases and vapours at constant pressure 

 has been directly determined by experiment ; but that at 

 constant volume has hitherto been deduced from the former only 

 by a somewhat complicated process of experiment and calcula- 

 tion. The relation between the specific heat at constant pressure 

 c and that at constant volume c u for atmospheric air, thus ob- 



c 

 tained by Clement and Desormes is — = 1*348, by Masson 1*419, 



c i . 

 and has been calculated from experiments on the velocity of 

 sound at Paris at 1*4122, giving for the specific heat of air at 

 constant volume the values *1763, *1675, and *1683, if the value 

 for constant pressure be taken (as determined by Regnault) at 

 •2377. 



The value of c l} however, may be directly derived from that of 

 c by a simple calculation. It is obvious that the equivalent in 

 mechanical work of a given elevation of temperature at constant 

 pressure must be equal to that at constant volume plus the work 

 of expansion at constant pressure. Now this last is />(V t — V), 

 if p be the pressure per square foot, V and Y l the volumes in 

 cubic feet before and after expansion. And the expansion of air 

 is "003665 for every degree Centigrade above zero, or *002036 

 for every degree Fahrenheit above 32°. For one degree above 

 the temperature t, therefore, 



•002036V 



v-v= 



and 



^(V 1 -V 1 ) = 



1 + '002036 (/- 32)' 



•002036joV 

 1 + -002036 (t— 32)' 



and also various articles in Silliman's American Journal. Mallet's memoir, 

 already often referred to, has excellent observations on this subject, as on 

 others connected with volcanic action. 



* Silliman's American Journal, S. 2. vol. xiv. pp. 219, 254 (1852). 



f Communicated by the Author. 



Phil Mag. S. 4. Vol. 46. No. 306. Oct. 1873. X 



