ANALYTIC PROCESSES 39 



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 ?oo 

 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. 



Multirotation. — When freshly dissolved in cold 

 water, lactose shows a higher rotation than that 

 given above. By standing, or immediately on 

 boiling, the rotary power falls to the point 

 mentioned. In preparing solutions from the 

 solid, therefore, care must be taken to bring them 

 to the boiling-point previous to making up to a 

 definite volume. This precaution is unnecessary 

 when operating on milk. 



Acidity. — Milk being often amphoteric to lit- 

 mus, that indicator cannot be employed in 

 estimating acidity. Phenolphthalein is usually 

 employed. Several methods differing in details 

 have been proposed. Probably the best is that 

 of Thorner. In this, 10 c.c. of milk are diluted 

 with 20 c.c. of water, a few drops of a dilute 

 4 



