DETERMINATION OF LACTOSE. 55 



this the amount of copper found represents (0.204^ 

 o.204o)xo.78+o. 1 50=0.0006x0.78+0. 1 50=0. 1 505 grams 

 ot lactose. In case the precipitate is weighed as cuprous 

 oxide (Cu^O) or a cupric oxide (CuO), the following ad- 

 ditional data is required to complete the calculation : One 

 gram of cuprous oxide represents 0.8882 gr. of metalHc 

 copper and i gram of cupric oxide represents 0.7989 

 grams of copper.* 



C. ESTIMATION BY MEANS OF CIRCUL,AR POLARI- 

 ZATION. 



In a flask of about 150 c. c. capacity are placed 50 c. c. 

 of milk; 25 c. c. of neutral lead acetate are added, thor- 

 oughly^mixed by shaking, and heated to boiling. In or- 

 der to prevent the loss of water by evaporation, the flask 

 is connected by means of a cork to a long glass tube 

 which serves as a condenser. After heating for a short 

 time, the contents of the flask are cooled to room tem- 

 perature and filtered through a dry filter. The filtrate 

 should be perfectly clear, otherwise it must be filtered 

 again. In some cases it may be necessary to filter several 

 times. With the clear filtrate fill an observation tube 

 200 mm. long, taking care to force out all the air bub- 

 bles and determine the angle of rotation in a suitable 

 polarization apparatus. The specific rotatory power of 

 lactose at 20 degrees is -j- 52.53 circular degrees, for .so- 

 dium light. If the angle be ( x ), and if the ttibe used be 

 200 mm. (2 X 100) long, then the number of grammes of 

 lactose per 100 c. c. of milk will be calculated by the 

 formula 



100 3 1 , . T ,^-.,7 



( cr ^ • . - . - =( oc ) • 1.42/7 



^ ' 52.53 2 2^^ 



*Compare these methods .with the methods adopted by the 

 Association of Official Agricultural Chemists. See Bulletm No. 

 46 U. S. Dept. of Agr., Division of Chemistry, page 40-41,— 

 Translators. 



