30 MILK ANALYSIS 



beaker or casserole will be a suitable container, the positive 

 terminal being placed within the negative. 



Four cells of a gravity battery will suffice for a single decom- 

 position, and will operate two, but more slowly. It is usual to 

 arrange the apparatus so that the operation may be continued 

 during the night. When the electricity is taken from the general 

 supply of the laboratory, it is usually necessary to interpose 

 resistance and to have some means of measuring the current- 

 flow. This is sometimes done with a gas evolution cell and 

 incandescent lamp, but an ammeter and adjustable rheostat 

 are better. 



Lactose may be determined by the polarimeter after removal 

 of the fat and proteids, which is best effected, as recommended 

 by Wiley, by a mercuric nitrate solution, prepared by dissolving 

 mercury in twice its weight of nitric acid of 1.42 sp. gr. and add- 

 ing to the solution five volumes of water. The A. O. A. C. 

 optical method is as follows : 



For polarimeters reading to 100 for 26.048 grams sucrose 

 (corresponding to 32.98 grams lactose), measure, in c.c., the 

 amount obtained by dividing double this (i. e., 65.96) by the 

 specific gravity, add 10 c.c. mercuric nitrate solution, make up 

 to 102.6 c.c., shake, filter through a dry filter and examine in a 

 200 mm. tube. Half the observed reading will be the per- 

 centage of lactose. For example, if the specific gravity of the 

 milk is 1.030, the amount taken will be 65.96 -r- 1.030=64 c.c. 



The allowance for volume of precipitate by making up to 

 102.6 c.c. is not accurate, except with closely- skimmed milks. 



The correction may be made more closely by calculating 

 the actual volume of the precipitate by multiplying the fat- 

 percentage by 1.075 (average specific volume of fat) and the 

 proteid-percentage by 0.8 (average specific volume of coagulated 

 proteids), deducting the sum of these products from 100 c.c. 

 and correcting the observed reading by proportion. For 

 ordinary milk, the volume of the proteids from 65.96 grams may 



