58 CHEMICAL STATICS 



the protein-content of fluids by their optical rotation. This 

 particular physical property of the proteins was, however, a 

 most unfortunate choice, since it is one of the few physical prop- 

 erties which is independent of the absolute mass or volume of 

 the molecules displaying it, and the specific rotatory power of 

 proteins is extremely low in comparison with that of a large 

 proportion of optically active substances of much smaller molec- 

 ular dimensions. 



In recent years two methods of estimating protein, based upon 

 the measurement of physical qualities of their solutions or sus- 

 pensions, have been suggested and applied to specific problems 

 in very considerable detail. These are the nephelometric method, 

 based upon measurements of opacity, and the refractometric 

 method. Both methods are capable of attaining a high degree 

 of accuracy and may be rendered to a great extent independent 

 of contamination by non-protein substances. It therefore ap- 

 pears probable that they may eventually prove applicable 

 to the estimation of a variety of proteins under conditions which 

 would preclude, or render exceedingly difficult, reduction of the 

 estimation to a determination of nitrogen. 



2. The Nephelometric Method of Estimating Proteins. — 

 This method, which we owe to Kober and his collaborators (6) 

 (7) (8) (9) (10), depends upon the measurement of the relative 

 opacities of suspensions of coagulated proteins, the measurement 

 of relative opacity being carried out in a modified form of color- 

 imeter. A full description of this instrument will be found in a 

 recent article by Kober and Graves (9). 



The success of the nephelometric method depends upon the 

 discovery of specific coagulating agents which will bring the 

 protein into stable suspension in masses of approximately uni- 

 form, or at least reproducible, magnitude, of sufficiently large 

 diameter to scatter incident light-rays and insufficiently large to 

 result in instability of the suspension. The discovery of the most 

 suitable coagulating agent for each protein and under each set 

 of modifying circumstances constitutes, indeed, the chief diffi- 

 culty of the method, but, once the correct coagulant is found 

 and the circumstances attending its employment have been 

 standardized, the method is susceptible of a very high degree 

 of accuracy, far surpassing indeed that attainable through the 

 estimation of nitrogen. 



