366 PHYSICAL PROPERTIES 



Calculating the theoretical values of a, on the assumption that 

 Gladstone's law of mixtures holds good for solutions of protein in 

 any solvent we at once find that although a varies with the nature 

 of the solvent in the sense demanded by Gladstone's law, yet the 

 law does not hold good, for different solvents, with any approach 

 to quantitative exactitude. Thus the calculated value of a for 

 solutions of casein in 50 per cent alcohol is 0.00095, assuming the 

 density of casein dissolved in 50 per cent alcohol to be the same 

 as that of casein dissolved in water, while if we assume the 

 density of casein dissolved in 50 per cent alcohol to be that of un- 

 dissolved casein (Cf. Chap. XIII, section 11) the calculated value 

 of a becomes 0.00121; the experimental value, however, is 0.00149; 

 the calculated value of a for insoluble serum-globulin in 50 per cent 

 alcohol is 0.00169, the experimental value is 0.00119. Hence Glad- 

 stone's law of mixtures holds good for solutions of protein in a 

 specified solvent, but no longer holds good if we vary the nature 



N 1 

 of the solvent. Now it is a characteristic of the quantity -= 



a 



that although it is independent of temperature and of concentra- 

 tion, it varies when the state of aggregation varies (35). We may 

 conclude, therefore, as we have had occasion to indicate before, 

 that the physical condition (number of associated water molecules, 

 degree of dispersion, etc.) of proteins in alcohol-water mixtures and 

 other non-aqueous solvents is not the same as it is in water.* 



Comparing the above cited determinations by Robertson and 

 Greaves, of the refractive indices of gliadin in various solvents, 

 with Mathewson's determination of the rotatory power of gliadin 

 in various solvents, it is evident that, as might have been expected, 

 there is no correspondence between the effects of different solvents 

 upon these two physical properties of dissolved protein. 



It will be observed that the value of a for solutions of gliadin in 

 75 per cent phenol is negative. This is due to the fact that the 

 refractive index of 75 per cent phenol is nearly equal to that of 

 gliadin itself, and its density is greater so that a mixture of the two 

 substances has a refractive index which is less than that of the 75 

 per cent phenol employed as solvent. 



The influence of temperature upon the refractive indices of 

 protein solutions has been determined by myself and by Herlitzka 

 (13). As stated above, I find the effect of temperature upon the 



* Cf . Chaps. I and XIII. 



