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 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. 
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 Thérner. In this, ro cc. of milk are diluted 
with 20 c.c. of water, a few drops of a dilute 
4 
