CHemicaL TESTS AND ANALYSES 347 
‘To the filtrate of the casein determination add 0.3 c.c. of 10 per 
cent acetic acid, boil until the albumin is completely precipitated and 
proceed as directed in previous paragraph. 
In the place of the above methods the per cent of albumin may 
he determined by subtracting the per cent of casein from the per 
cent of total nitrogen. 
Milk Sugar (Lactose). 
Optical Method. 
Preparation of Reagents. 
“(a) Acid mercuric nitrate.—[issolve mereury in double its 
weight of nitric acid, specific gravity 1.42, and dilute with an equal 
volume of water. One cubic centimeter of this reagent is sufficient 
for the quantities of milk mentioned below. Larger quantities may 
be used without affecting the results of polarization. 
(b) Mercuric iodid with acetic acid.—\ix 33.2 vrams of po- 
tassium iodid, 13.5 grams of mereuric chlorid, 20 c.c. of glacial 
acetic acid and 640 c.c. of water.” 
Determine the specific gravity of the milk by means of a deh- 
cate hydrometer, or, if preferred, a pycnometer. The quantity of 
sample to be taken for the determination varies with the specific 
gravity and is to be measured at the same temperature at which the 
specific gravity is taken. The volume to be measured is indicated 
in the following table, which is based upon twice the normal weight 
of lactose (32.9 grams per 100 metric ¢.c.) for the Ventzke sugar 
scale. 
Place the quantity of milk indicated in the table in a flask 
graduated at 102.6 c.c., add 1 c¢.c. of the acid mercuric nitrate solu- 
tion or 30 c.c. of the mercuric iodid solution (an excess of these re- 
agents does no harm), fill to the mark, shake, filter through a dry 
filter and polarize. It is not necessary to heat before polarizing. If 
a 200 m.m tube is used, divide the polariscope reading by 2( or, if a 
400 m.m. tube is used, by +) to obtain the per cent of lactose in the 
sample. 
