6 io Albert P. Mathews 



Y, of Stefan, or 3 / 20 as computed by Lord Rayleigh, we have 

 the complete formula 



(5) (i /V, - i / V.) 1 3 -- TV: /3 N I/3 (Tc/ (T, T)) v> . 

 Changing to density in place of volume on the left hand side 



we have 



(6) a - 3 rv; /3 N 1/3 M(T c / (T c T)) 2/3 / (^ ^) ; 



or if Ramsay and Shields' surface-tension figures are used 



(7) a = 3 ^ /3 N 1/3 M(Tc/ (T c T))-*V (^ _ </) . 



The result is given in dynes for gram mol quantities. M is 

 the molecular weight; d 1 and d v , liquid and vapor density, 

 respectively; N, the number of molecules in a gram mol, is 

 6.21 X io 23 ; T is the surface tension in dynes; Vj, the volume / 

 of a gram mol at temperature, T. 



From the foregoing it appears that 1 / s of the internal latent 

 heat of vaporization is due to the cooling caused by the in- 

 crease of the surface brought about by the transfer, of molecules 

 from the region of uniform density into the surface, and their 

 passage through the surface. 



In Table 2, I have given the calculation of "a" by formula 

 (7) for a number of substances using Ramsay and Shields', 

 or Ramsay and Acton's or Renard and Guye's surface-tension 

 determinations and the densities from S. Young's 1 recent 

 work. 



In Table 2 the constancy of "a" is shown to be good for 

 normal substances, except near the critical temperature where 

 it generally falls off somewhat. This may be due to the 

 inaccuracy of the surface tension close to the critical tempera- 

 ture, but it is more probably due to the fact that the thickness 

 of the surface layer in molecular diameters is not properly 

 represented by the expression (T C /(T C T))' 76 and possibly 

 to the influence of the angle of contact. 



There is in general a tendency of the value of "a" to rise 

 except near the critical point, and this tendency is most pro- 

 nounced in an associating substance such as ethyl alcohol. 



The agreement is admirabj^clear to the critical temperature 



1 Young, S: Proc. Roy. Dublin Soc., 12, 374 (1910). 



