THE ANALYSIS OF MILK 31 



violent shaking. The solution is poured on a dry 

 filter, and a polarimeter tube filled with the clear 

 filtrate. 



As an example : the milk has a sp. gr. of 1.032, the 



degrees of gravity are 32.0, and (a) is -^ 1.60 c.c. ; 



the fat is 3.60 and (b) is ^- = 2.00 c.c. ; if an instrument 

 1 .8 



graduated in angular degrees and a 200 mm. tube are 

 used (c) is 5.43. The water added is 1.60-1-2.00+5.43= 

 9.03 c.c. 



The reading is made by placing the tube in the 

 instrument (Fig. 21), focusing the eye-piece on the 

 half -shadow plate when the analyser is' so turned that 

 one side is darker than the other, and adjusting the 

 analyser till both sides are equal in intensity ; the scale 

 is then read by means of the vernier provided. Several 

 readings should be made, the adjustment to equality 

 being made from either side alternately, and the mean 

 of the readings taken as the correct reading. A blank 

 estimation, i.e. one with a tube filled with distilled 

 water, should always be made, and the reading if to 

 the right subtracted from, or if to the left added to, the 

 reading of the sample. The corrected reading of the 

 scale gives the percentage by weight of anhydrous milk 

 sugar. 



Mercuric nitrate does not remove the proteins quite 

 completely ; for greater accuracy add to the filtrate 

 ^o of its volume each of phospho-tungstic acid 

 solution and of dilute (i : i) sulphuric acid solution, 

 filter and polarise. Multiply the reading by i.i. The 

 error with whole milk is, however, very small and hardly 

 exceeds the experimental error of reading. 



Gravimetric Estimation. About ten grammes of milk 

 are placed in a 100 c.c. flask with 60 to 70 c.c. water, 

 5 c.c. of Fehling's copper sulphate solution added (see 

 Appendix), and the Solution neutralised with caustic 

 soda ; the liquid is made up to 100 c.c., and the contents 

 of the flask, after mixing, filtered through a dry filter ; 



