274 H. K. SCHACHMAN AND R. C. WILLIAMS 



the solute molecules, th.e treatment that follows refers to variations of 

 concentrations of only these molecules. This distinction is accomplished 

 experimentally by subtracting from the amount of light scattered by the 

 solution the amount that is scattered by the pure solvent. 



By the application of thermodynamic reasoning it can be shown that for 

 a solution we can write: 



where 



J?g„ := !^, for e = 90° 



•* 



R, T, and N = gas constant, absolute temperature, and Avogadro's 



number, respectively 

 P = osmotic pressure 

 e = optical dielectric constant 



From this equation it is seen that the amount of scattering is dependent 

 upon the osmotic work required to produce the fluctuations in concentration. 

 TiP/'bc can be written as the differential form of the Van't Hoff equation for 

 nonideal systems: 



^c \M J 



where M is the molecular w^eight. 



The optical dielectric constant and the experimentally determinable index of 



refraction are related thus: 



where n^ is the refractive index of the solvent. 

 Upon making the indicated substitutions we have: 



2-^^'«^W^^)^A = l+2£c+... (3^ 



Grouping the left-hand constants under the term K: 



Kc 1 



Kc . 



If -5— is plotted against c a straight line will result, the slope of which is the 



coefficient of nonideality, B, and the intercept of which is the reciprocal of 

 the molecular weight. 



