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VI. Internal Pressure and Latent Heat of Liquids. By 

 Wm. C. McC. Lewis, M.A., D.Sc, Chemical Laboratory, 

 University College, London *. 



IN the Phil. Mag. (xxiii. p. 955, 1912) Mr. Sukhodski has 

 criticised an expression given by the writer for the 

 internal pressure of a liquid. The expression referred to is 



K-Z=t|? (1) 



where K is the internal pressure and I was taken to mean 

 the latent heat of vaporization of 1 c.c. of the liquid (Dnpre). 

 I was aware that the further assumption, viz. 



1BK_1B/ m 



KdT -/ 3T' K } 



could not hold over a wide range of temperature, being* 

 evidently incorrect at the critical temperature, at which 

 point / is zero while K is still positive (and probably fairly 

 large) as Mr. Sukhodski points out. 



It mav be mentioned that if I is taken to be the latent 

 heat of expansion of the homogeneous phase (entirely liquid 

 for example), that is, the heat which has to be added to the 

 system while it expands isothermally by 1 c.c., such a 

 quantity will not become zero at the critical temperature, 

 but will have a positive value. By attaching this meaning 

 to /, the physical significance of equation (1) would be of 

 course considerably modified. Equation (2) might, under 

 these circumstances, be fairly correct, but as there is no 

 direct means of testing this, it may be dropped. 



Whether we regard I as referring to the latent heat of 

 vaporization of 1 c.c. or to the latent heat of expansion of 

 the liquid itself, equation (1) would still be of little use for 

 experimental purposes, owing to the presence of the term 



^ rp Recently, however, it has become possible to obtain a 

 ol • 



fairly accurate estimate of this quantity by a new method." 



In the Phil. Mag. (April and September 1912) Mr. IL 



Davies has put forward several very interesting applications 



of the Cailletet-Mathias law of the rectilinear diameter. 



One of these is an expression for the temperature coefficient 



of the internal pressure of a liquid which is found to be 



simply identical with the coefficient of expansion, viz. 



1 dK 



K a* ~" *' 



* Communicated by the Author. 



