THE INTERNAL PRESSURES OF UQUIDS 



BY ALBERT p. MATHEWS 



The fundamental significance of the constant "a" of van 

 der Waals' equation makes its exact determination important. 



Various methods have been proposed for the determination 

 of this constant, but none of them are entirely satisfactory. 

 In a recent paper 1 attention was drawn to a method by which 

 it could be determined from the surface tension by the use of 

 Thomas Young's formula, T = rK/3, combined with the 

 law of Ramsay and Shields, and the values thus computed 

 were shown to be closely similar for most substances to the 

 values computed by van der Waals' method from the critical 

 temperature and pressure, but in some cases they deviated 

 considerably from his values. I found, also, that the values 

 of the constant "a" obtained by this method were simple 

 functions of the products of the molecular weight and the 

 number of valences in the molecule and that they could be 

 computed from these values. Inasmuch as the method 

 used in computing "a" from the surface tension involved 

 the value of the density at absolute zero, which was computed 

 from Cailletet and Mathias' law of the rectilinear diameter, 

 and involved, therefore, some uncertainty and was certainly 

 too high, it was desirable to find a method of computing "a" 

 directly from the surface-tension measurements. 



The desire of finding such a method was stimulated by the 

 present great uncertainty of the value of the internal pressures 

 of liquids. Traube 2 has within the past few years computed 

 the internal pressure for many liquids, but the results he has 

 obtained are, in my opinion, unreliable because his method of 

 computation involves the use of the value "&" the real molec- 

 ular volume or co-volume, a very doubtful value. The values 

 he finds for the internal pressure at zero degrees are, also, 

 widely different from those computed by the use of ' V found 



1 Mathews: Jour. Phys. Chem., 17, 154 (1913). 



2 Traube: Zeit. phys. Chem., 68, 291 (1909). 



