362 On the Specific Heat of Gases and Vapours. 



fi, and H ; so that if any four of these are given, the fifth may- 

 be deduced independently of any other determination. 



According to Regnault's observations, the density of air at 

 0° C. and a pressure of 760 millims. compared with mercury is 

 1 to 10517'3, by which estimation the value of //, for a cubic 

 foot will be -000038132. This gives c-c x = '069166, whence 

 c 2 = '16853 if c=*2377*. Assuming these values to be correct, 



-=1-4104. 

 c i 



The accuracy of the result c — c 1 = '069166 depends on that 



of the values of the coefficient of expansion, '003665 per degree 

 Centigrade, the density of air as above, and the mechanical equi- 

 valent of heat, 772 for one pound of water raised 1° Fahr. If 



in addition — be taken at 1*4122, c, = *1678, and c = '23696, 



an independent determination of the specific heat of air for con- 

 stant pressure, which is a little lower than that of Regnault. 



Q 



The value of — , as calculated from the velocity of sound, has 



been given by Regnault at 1*3945 ; but this does not accord so 

 well with his determination of the other constants as the value 

 usually assigned, 1*4122. 



The specific gravity of hydrogen being about *0692, the value 

 of c—c } for hydrogen is very nearly 1 ; and if E represent the 

 atomic weight of any other gas (hydrogen being 1), 



-..-J (nearly), 



a coincidence which is remarkable, though apparently accidental. 

 The observations of Regnault lead to the conclusion that the 

 specific heat c of all simple gases at temperatures and pressures 

 sufficiently remote from their points of condensation, varies in- 

 versely as their atomic weight ; and since c—c l varies in the 



same manner, it follows that — must have the same value for all 



these gases. This same conclusion was arrived at by Dulong 

 from experiments on the velocity of sound. 



Although the expression for the value of c—c ] is perfectly 

 general, it must be observed that it varies directly with the co- 

 efficient of expansion, a, which cannot be assumed to be the same 

 for vapours or gases near their points of condensation as for per- 



* The same result will be obtained from the equations given in the pre- 

 ceding part of this paper if the weight of 1 cubic foot of air be taken as 

 535'81 grs. at a pressure of 147346 lbs. and at 60°, instead of 535*68 grs. 

 at a pressure of 14706 lbs. 



