Mr, W. J. M. Rankine on the Mechanical Theory of Heat, 485 



corresponding to total privation of heat, whose value is very small, 

 and according to the latest determination, is about 2°'l Centi- 

 grade. 



Of the five gases which I have selected as having their pro- 

 perties sufficiently well- ascertained to enable them to be used for 

 the purposes of the comparison in question, four, viz. atmo- 

 spheric air, oxygen, hydrogen, and carbonic oxide, approach 

 sufficiently near to the condition of a perfect gas to enable the 

 following approximations to be employed, 



and consequently 



Kp-Kv = -^^^.PV, . .... . (4) 



where \ is a small constant, being about 1°*6 Centigrade ; but 

 for carbonic acid the deviation from the perfectly gaseous con- 

 dition is sufficient to render it necessary to use a more compli- 

 cated method of calculation, taking into account the cohesion of 

 its particles. 



The data and results of these computations are given in the 

 annexed table ; from which it appears, that, in the case of atmo- 

 spheric air and carbonic oxide, the agreement between the calcu- 

 lated and observed velocities of sound is sensibly perfect ; that 

 for oxygen and hydrogen, the differences are so small as to be 

 nearly inappreciable ; and that for carbonic acid, the discrepancy 

 of about one-fortieth part is not greater than might have been 

 anticipated, from the number of additional causes of error which 

 affect the experimental data for the less perfect gases. 



The ratio of the two specific heats of air agrees to a minute 

 fraction with that which had been previously deduced from the 

 velocity of sound. 



Glasgow, May 6, 1853. 



