THE MOLECULAR WEIGHT OF LIQUIDS. 269 



the relative number of molecules along a side of the cube, 

 whilst the square of this number represents the relative 

 number of molecules distributed on a face of the cube. 

 For different liquids, therefore, the molecular volume raised 

 to the power two-thirds (Mz>f, represents the surface on 

 which the same number of molecules lies. This surface s is 

 sometimes called the molecular surface of the liquid, so that 

 instead of s in the above equation we may substitute (Mz/) s . 

 The critical temperature and the correction d being deter- 

 mined, we have all the requisite data for calculating the value 

 of k by the equation y (Mvf = k (r — d). 



After carefully investigating a series of liquids Ramsay 

 and Shields found that the mean value of the constant k 

 was 2 '12 1. All liquids, however, as Eotvb's also found, do 

 not give constants approaching the value 2*121. The chief 

 exceptions are water and the fatty acids and alcohols, which 

 have values less than 2*121. It is noteworthy that in the 

 case of the apparent exceptions, the values of k vary with 

 the temperature and more nearly approach the normal value 

 the higher the temperature. In other words the molecular 

 surface energy, the product of the surface tension y into the 

 molecular surface (Mz*) 1 , is no longer a linear function of the 

 temperature. 



Returning once more to the equation y (Mz*) 1 = k (r - d), 

 it follows that the quotient of the differences of the molecu- 

 lar surface energy between any two temperatures by the 

 difference in temperature, should remain constant, that is — 



y(Mvf - y (Mv'Y 



T — T 



y and v corresponding to the temperature r, and y and v 

 to the temperature /. 



In the case of a great many liquids this was found to be 

 the case. The variation in the constant from 2*121 lies 

 within the limits of experimental error in the case of 

 such compounds as the esters, the hydrocarbons, carbon 

 tetrachloride, chlorobenzene, ether, mercaptan, the mus- 

 tard oils, chloral, paraldehyde, benzoic aldehyde, nitro- 

 benzene, aniline, pyridine, quinoline, silicon tetrachloride, 

 phosphorus trichloride, phosphorus oxychloride, sulphur 



