29* HEAT. 



Janiin and Richard carried out a method based upon this result, in which 

 a definite quantity of heat H was communicated to the gas contained in 

 a large vessel by means of a wire heated by an electric current. 



In one case the gas was allowed to expand at constant pressure, and 

 dv was measured, and in the other the volume was kept constant and dp 

 was observed. No absolute measures could be made owing to the loss of 

 heat from the wire by radiation, which was unknown. But it was the 

 same in both parts of the experiment, and so the quantity H remaining 

 in the gas was the same. 



They obtained as values of 7 



Air ..... . 1-41 



Hydrogen ..... 1-41 



Carbon dioxide . . . . 1'29 



Joly has succeeded in making direct measurements of the specific 

 heat at constant volume by the steam calorimeter, as already described 

 in chap. vi. His result for air at and 760 mm. is 



O v = 0-17154. 

 If we take Wiedemann's value for that at constant pressure, C p = 0*2389, 



C 

 we get 7 = ^=1-393. 



With Swann's value taken as 0-2414 at 0. we get 7= 1-407. 



The Determination of the Absolute Temperature at C. from the Value 

 of y. If in the equation for the difference of specific heats 



K p -K,, = a 2 ve,0 



pr 



we substitute for K, its value 2 



7 



we get K D = J o?veeO 



y-l 



Rankine, taking the value of y given by the velocity of sound, used 

 this equation to determine K p and C p , but if we use the value of K p 

 found by direct experiment, it may also be regarded as an equation 

 giving 6, the absolute temperature at 0., the length of the degrees of 

 the absolute and air scales so nearly coinciding at 0, that a may be 

 taken as the same for each. 



Putting the equation in the form 



y a.*ve 



if we take K p = -2389 x 4-2 x 10 T 



a = -00367 

 v = 773-4 

 e = 1-0136 x 10 6 (Paris atmosphere), 



and use the value of y given by the velocity of sound as 1 -406 we get 



(9=274-4 

 But if we use the value of 7 obtained by Joly, viz. 1-393, then we get 



= 268-1 



