for Equal Volumes under Constant Pressure. 211 



whence, on the one hand, 



7'- 7 = 0-06792 nearly; 

 on the other hand, 



y— 7=00691. 



From these values of y'— 7 there follows further the value of 

 the mechanical equivalent of the unit of heat, namely : — employ- 

 ing the first number, 



A 1 . 000364166 . 10334-5 * . OQ . „ . .. 



A= 1-2932.006792 =428 ' 47 kll ^ l ' ammetres > 



employing the second number, 



A =425 *33 f kilogrammetres. 



The values of - and 7'— 7 are smaller, and that of A is greater, 



than those hitherto given ; that is to say, the difference is in the 

 direction which might be expected a priori when it is considered 

 that the values hitherto deduced from observation have reference 

 to gases which, although they approach nearly to the condition 

 of perfect gases that is presupposed in the case of the values 

 here deduced, do not quite attain to it. 



In any case the value ( - = 1 "41 ) that has been found for the 



permanent, but not quite strictly speaking perfect, gases is at 

 least not so well established, even for the ideally perfect gaseous 



state, as to overthrow the value (- =1*4 J deduced from the con- 

 siderations above set forth. The approximate agreement of the 

 values here compared, as well as the direction in which they 

 differ, accordingly justify us in adhering to the values of the two 

 specific heats expressed by the equations (6), (7), (8), and (9) ; 

 and at the same time an additional confirmation is afforded to 

 the views from which they are derived, respecting the molecular 

 and atomic constitution of gases. 



* 000364166 = coefficient of expansion of air. 



10334*5 = pressure on 1 square metre in kilogrammes. 



1'2932 = weight of 1 cubic metre of air in kilogrammes. 



006792 = heat of expansion, 

 t Ad. Dronke, Pogg. Ann. vol. cxix. p. 399 (1863). 



