62 CHEMICAL STATICS 



and of iV/50 acetic acid solution (prepared by diluting the N /2b 

 acetic acid used above to exactly one-half with distilled water) 

 are determined simultaneously. By determining the refractive 

 indices of the fluid and of iV/50 acetic acid simultaneously, the 

 necessity for regulation of the temperature at which the readings 

 are made is obviated, for although the absolute values of the 

 refractive indices are affected by temperature, the differences 

 between them are independent of the temperature at which they 

 are determined (19). In carrying out a large number of esti- 

 mations, however, it is necessary to eliminate the possible error 

 due to progressive changes in the temperature of the dark-room by 

 redetermining the refractive index of the solvent (in this instance 

 7V^/50 acetic acid) at frequent intervals. 



The refractive index of a 1 per cent solution of NaCl is 0.00160 

 greater than that of distilled water. The refractivities of 1 per 

 cent solutions of other inorganic salts and of glucose and urea are 

 of very similar magnitude. To within a sufficient degree of 

 accuracy, therefore, the percentage of non-protein substances 

 in the serum may be estimated by dividing their refractivity 

 by the factor 0.00160. The result must be multiplied by 2, 

 because the serum has been diluted to one-half with iV/25 acetic 

 acid. 



For the determination of the refractive index, I employ a 

 Pulfrich refractometer, which reads the angle of total reflection 

 to within one minute. A sodium flame is used as the source of 

 light. The refractive index may be estimated from the angle 

 of total reflection with the aid of the table which is supplied with 

 the instrument. 



When it is desired to make a number of successive determina- 

 tions, the cup of the refractometer should be carefully dried with 

 absorbent cotton and lens paper before a new sample of fluid is 

 introduced. 



(6) Estimation of the albumins. 



Glass tubes 9-10 cm. long are prepared, having an inside diam- 

 eter of 5 mm. and one end closed. Tubes which have been em- 

 ployed in the estimation of the non-protein constituents, after 

 having been cleaned and dried, may be cut down to the proper 

 length and utilized for this purpose. Small corks or short pieces 

 of glass tubing sealed at one end, the sealed end being pressed 

 against the open end of the longer tubes and held in position by 



