360 PHYSICAL PROPERTIES 



Reiss measured the change in the refractive indices of dilute 

 salt solutions which results from the introduction of varying 

 amounts of the " pseudoglobulins " of blood serum. His pseudo- 

 globulins were prepared by fractional coagulation with ammonium 

 sulphate and purified by prolonged dialysis. Fraction I (pseudo- 

 globulin I) was coagulated at 32 to 36 per cent saturation with 

 ammonium sulphate, fraction II (pseudoglobulin II) was coagu- 

 lated at 36 to- 39 per cent saturation. The change in the refractive 

 index of the solvent due to the introduction of these proteins was 

 found to be directly proportional to the quantity of protein dis- 

 solved in it; the change due to the introduction of 1 per cent of 

 the "pseudoglobulin I " being 0.00224 and that due to the introduc- 

 tion of 1 per cent of the " pseudoglobulin II" 0.00230. The 

 difference between these determinations is not sufficient to con- 

 stitute a basis for distinction between the two globulin-fractions, 

 since it is not greater than that which might have arisen through 

 experimental error. Reiss also measured the influence of other 

 constituents of serum (especially crystallized and amorphous 

 serum albumin) upon the refractive indices of dilute saline solu- 

 tions. 



I have amplified and confirmed these results of Reiss, employing 

 for this purpose a variety of proteins (casein, paranuclein, ovo- 

 mucoid, ovovitellin, serum globulin, gliadin, etc.) and not only 

 aqueous solvents but alcohol-water mixtures, acetone-water mix- 

 tures, etc. A large number of determinations upon casein in 

 aqueous solutions, between the concentrations which it is feasible 

 to employ* show that the change in the refractive indices of 

 aqueous solvents due to the introduction of this protein is very 

 accurately proportional to its concentration (43) (56). Accord- 

 ingly the refractive index of a solution of this protein is given by: 



n HI = a X c 



where n is the refractive index of the solution, n\ that of the solvent, 

 c the percentage of casein in the solution and a a constant which 

 is characteristic of the protein (for example, casein) which is 



* If the concentration of protein be too small (less than about 0.5 per cent) 

 the experimental error of the determination becomes a significant proportion 

 of the observed difference in the refractive index of the solvent, due to the 

 introduction of the protein. If the concentration of protein be too large then 

 the solutions are too opaque to enable an exact reading to be obtained. 



