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 

 be 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. Dissolve mercury 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. Mix 33.2 grams of po- 

 tassium iodid, 13.5 grams of mercuric 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 deli- 

 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.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 4) to obtain the per cent of lactose in the 

 sample. 



