MILK AND MILK PRODUCTS . 31 



be taken at 1.68 c.c. Supposing the sample to contain 4.0 per 

 cent, of fat and the polarimetric reading to be 10, the calcula- 

 tion would be thus: 



65.96 X 0.04 = 2.63 Amount of fat in milk taken 

 2.63 X 1-075 = 2 -^ 2 c - c - Volume of fat in precipitate 



1.68 c c. Est. vol. of proteids in precipitate 



4.50 c.c. Total volume of precipitate 

 100 4.50 = 95.5 c.c. Actual volume of liquid. 

 100 : 95.5 : : 10 : 9.55 9.55 -f- 2 = 4.75, per cent, lactose 



The employment of a factor for correcting for the volume 

 of precipitate may be avoided by Scheibler's method of 

 "double dilution," in which two solutions of different volume 

 are compared. The following is a summary of the method 

 given by Wiley & Ewell : For polarimeters adapted to a normal 

 weight of 26.048 sucrose, 65.82 grams of milk are placed in 

 a 100 c.c. flask, 10 c.c. of the acid mercuric nitrate added, the 

 flask filled to the mark, the contents well mixed, filtered, and 

 a reading taken. A similar quantity of the milk is placed 

 in a 200 c.c. flask and treated in the same way. The true 

 reading is obtained by dividing the product of the two readings 

 by their difference. If the observations are made in a 200 mm. 

 tube the percentage is half the true reading. 



The instrument should be accurate, and great care taken in 

 the work, or the results will be less satisfactory than by the 

 method first described, in which an allowance is made for the 

 volume of the precipitate. 



Birotation. When freshly dissolved in cold water, lac- 

 tose shows a higher rotation than that given above. By stand- 

 ing, or immediately on boiling, the rotatory power falls to the 

 point mentioned. In preparing solutions from the solid, there- 

 fore, care must be taken to bring them to the boiling- 

 point previous to making up to a definite volume. This precau- 

 tion is unnecessary when operating on milk. 



