Mr. W. J. M. Rankine on the Mechanical Theory of Heat. 483 



These are all the volatile substances of which I was able to pre- 

 pare considerable quantities in a state of purity. 



The specific heat which I have obtained for the vapour of 

 water in a great number of experiments is 0*475 ; this is barely 

 one-half of that found by Delaroche and Berard. It is remark- 

 able that the specific heat of the vapour of water should be nearly 

 equal to that of solid water, and only one-half that of liquid 

 water. 



It now remains for me to discuss the values which I have 

 found for the specific heat of compound elastic fluids, in relation 

 to the specific heat of the simple gases of which they are com- 

 posed, and to the amount of condensation which the latter have 

 experienced ; to compare the specific heat connected respectively 

 with the solid, liquid, and gaseous states in several substances 

 which are capable of assuming all these diff*erent states. I shall, 

 however, reserve this discussion for a later communication, in 

 which I shall give the latent heat of vaporization of the same 

 substances. , 



LXXVI. Mechanical Theory of Heat. — Velocity of Sound in 

 Gases. % W. J. Macquorn Rankine, C.E., F.R.S.E. ^^c* 



SINCE the date of my last communication on this subject, I 

 have had an opportunity of reading M. Regnault^s table of 

 the specific heats of a great number of gases under constant 

 pressure, published in the Comptes Rendus for the 18th of April. 



The results contained in this table, taken in conjunction with 

 other data obtained by the aid of the mechanical theory of heat, 

 furnish the means of calculating a priori the velocity of sound 

 in various gases. 



The velocities thus calculated being compared with the velo- 

 cities as determined directly by observation, afibrd a very satis- 

 factory experimental test of the accuracy of Laplace's theory of 

 the propagation of sound in gases, and of the fundamental prin- 

 ciples of the mechanical theory of heat. 



Let V represent the volume of unity of weight of a fluid under 

 the pressure P, at a given fixed temperature (such as that of 



melting ice) ; -Ar- the rate of variation of pressure with density 



V ^p 



for a sudden change of volume, and — j- the rate of variation at 



d.^ 



constant temperature ; Ky the specific heat of the fluid at con- 

 stant volume, and Kp at constant pressure; u the velocity of 

 sound in the fluid at the given temperature ; then it has been 

 * Communicated by the Author. 



