PHYSIOLOGICAL CHEMISTRY 



ter where the cups move and the prisms are stationary. The changes 

 required to convert the older type of instrument are more complicated 

 and scarcely to be advised unless the instrument is to have fairly 

 continuous use as a nephelometer. If the change is desired the nephe- 

 lometer tubes are to be supported in the same way as above, but the 

 jackets must be carried on special brackets which are made to replace 

 the brackets carrying the plungers. The nephelometer tubes must be 

 stationary, the jackets being the movable parts. 



Kober 1 has devised a combined colorimeter and nephelometer less 

 expensive than the Duboscq apparatus and which may be obtained 

 in this country. 2 A cut of this nephelometer-colorimeter is given in 

 Fig. 96, page 297. 



Nephelometric Calculations. The amounts of precipitate in solu- 

 tions examined nephelometrically is not exactly inversely proportional 

 to the readings of the scale. When the concentration of the unknown 

 and of the standard are within 10 per cent of each other (or within 

 about 20 per cent if the readings are made at depths as great as 50- 

 60 mm.) accurate results may however be obtained directly. If the 

 variations are greater than this a correction is necessary. Kober 3 

 has proposed an equation to supply this correction and thus make 

 possible very accurate work under conditions of moderate variations 

 of concentration. The equation is as follows: 



5 (i x)sk 



or 



2 y 



where y = height of unknown solution, on the left side of the instru- 

 ment, when standard solution is kept on the right side at a definite 

 height, 5 = height of standard solution on the left side and % = the 

 ratio of the concentrations of the two solutions. 



T 



k = where K = a constant, obtained by substitution of standardi- 



s . . 



zation values of s, y, and x. The instrument should be checked up 

 for each series of analyses by reading the standard against itself and 

 determining the potential height of the standard solution by reading 

 the scale on the left side when the solution on the right side is kept at a 

 definite height, and the two are matched. 



1 Kober: Jour. Ind. and Eng. Chem., 7, 843, 1915; Jour. Biol. Chem., 29, 155, 1917. 

 * The instrument is manufactured by the Klett Manufacturing Co., 202 E. 46th St., 

 New York City. 



8 Kober: J. Am. Chem. Soc., 37, 2379, 1915; Jour. Biol. Chem., 13, 485, 1913. 



