• On the Internal Pressure of a Liquid. 105 



Recently the internal pressure tt of a number of normal 

 liquids has been calculated by H. Davies (Phil. Mag. [6] 

 xxiv. p. 415, 1912). The calculations of tt carried out in 

 the present paper are of a similar nature to those of Davies, 

 in which the relation 



RT 



7T= 



V-Vt 



has been employed at 0° C, vi being taken to be 0*27 v e , the 

 values for v c being mainly those recently compiled by Young 

 (Proc. Roy. Dub. Soc.xii., 1910). It is clear that the results 

 obtained represent minimal values of tt °, since one cannot 

 assume vi to be absolutely constant, and any possible change 

 is an increase with temperature. The values are, however, 

 of importance as fixing the inferior limit for tt for a con- 

 siderable number of liquids. (It will be observed that the 

 values obtained are approximately one half of the values 

 given by the latent heat of vaporization of unit volume of 

 the liquid.) (Table I.) 



In a paper in the Phil. Mag. [5J xxxii. p. 113 (1891) 

 E. Obach has pointed out that there is an approximate direct 

 proportionality between the latent heat of vaporization of 

 a liquid and its dielectric constant. The data, especially 

 those on the dielectric constant K, were and are meagre, 

 and further the quantities were not compared at the same 

 temperature. Comparison of substances of similar chemical 

 constitution leads, however, to a fairly constant proportionality 

 factor. Now, many years previously, Dupre had put forward 

 the view that tbe latent heat of vaporization of unit volume 

 of the liquid was a direct measure of the internal pressure tt, 

 and although this statement is not exact it is certainly safe 

 to say that a large latent heat accompanies large cohesion. 

 Hence one would expect that there should be a rough pro- 

 portionality between the dielectric capacity of a liquid and 

 its internal pressure. This has been stated to be the case by 

 Walden (Zeitsch. phys. Chem. Ixvi. p. 407, 1909). Walden 

 inferred the relationship after having observed that solvents 

 with large cohesion possess likewise a marked dissociating 

 power upon dissolved substances, which property in turn 

 has been connected with the dielectric constant of the 

 solvent by the Nernst-Thomson rule. 



