I9IS.] MARSHALL-BANKS— NEW NEPHELOMETER. 177 



and comparison again made. This process was repeated until a 

 standard was obtained which when precipitated under the same con- 

 ditions and compared in the instrument with the unknown gave the 

 same amount of opalescence. The postulate involved, that the 

 same quantities of material precipitated under identical conditions 

 give equal opalescences, is undoubtedly correct, but the method is 

 somewhat tedious in application, although good accuracy was ob- 

 tained in about three approximations. 



Wells in 1906'' published the results of numerous experiments 

 in which silver chloride was precipitated under different condi- 

 tions, showing the influence of electrolytes both on the maximum 

 opalescence developed and on the time required for this maximum 

 to be reached. He came to the natural conclusion that the amount 

 of light reflected varies not only with the quantity of material in 

 suspension but also with its state of subdivision. In this investiga- 

 tion he used the Richards instrument of 1904 except that for the 

 usual standard suspension he substituted fixed standards of ground 

 glass as reflecting surfaces. 



P. A. Kober* in 19 13 took up the problem of determining quan- 

 titatively by the use of the nephelometric method, proteins and 

 other substances occurring in biochemical investigations for which 

 the ordinary gravimetric methods are either very tedious or in- 

 adequate. He used an instrument on the principle of the Richards 

 nephelometer but adapted to the framework and optical parts of 

 the Duboscq colorimeter. In comparing the opalescences of sus- 

 pensions differing considerably in concentration, he observed that 

 the readings were not quite inversely proportional to the concen- 

 tration of matter in suspension, and from a large number of ex- 

 periments with suspensions of different substances he developed an 

 empirical formula expressing the relation between scale readings 

 and concentration. This formula holds very well for ratios up to 

 1 : 3. He has successfully applied his instrument and method to the 

 determination of a number of organic substances such as casein 

 in milk, uric acid, and other purines. The nephelometer in various 

 modifications has been used by W. R. Bloor to determine the fat 



3 Wells, Am. Chem. Jour., XXXV., 99, 1906. 



4 P. A. Kober, Jour. Biol. Chem., XIIL, 485, 1913. 



