or Centrifugal Jheory of Elasticity. 419 



But we have reason to believe that there is not time, during 

 the passage of sound, for an appreciable transfer of heat from 

 atom to atom ; so that for each particle 



6?Q + f/Q' = 0j or, K = in equation (29). 

 To fulfil this condition, we must have 

 dT _ r — K dp 

 dV K^ ■ d^' 



Consequently, 



or by equation (31), 



// dV Kp\ 



(34) 



That is to say, the action of heat increases the velocity of sound 

 in a fluid, beyond what it would be if heat did not act, in the ratio 

 of the square root of the specific heat at constant pressure to the 

 square root of the specific heat at constant volume. 



This is Laplace's law of the propagation of sound ; which is 

 here shown to be applicable, not only to perfect gases, but to all 

 fluids whatsoever*. 



General Note, September 1855. 



All that is said in articles (9.) and (10.) of mixtures of atoms 

 of different substances in equilih'io of pressure and temperature, 

 is applicable also to mixtures of portions of the same substance 

 in two different conditions, e. g. the liquid and the vapourous ; 

 and thus from equation (25) it is found, that the latent heat of 

 evaporation of unity of weight of a given fluid is represented by 



hQ!={r-K).f-SW, 

 dT 



where p is the pressure of the vapour in contact with its Hquid 

 at the absolute temperature t, and 6'V is the increase of volume 

 undergone by unity of weight of the fluid in the act of evapora- 



J 



ting. If for T — K be put — , as before, this equation is trans- 



formed into that deduced by Messrs. Clausius and Thomson 

 from Carnot's principle. 



The recent experiments of Mr. Joule and Professor William 



* For experimental verifications of this law, see the Philosophical Ma- 

 gazine for June 185.'^, and the Transactions of th« lloyal Society of Edin- 

 burgh, vol. XX. pp. .088, 589. (1853.) 



