ANALYTIC PROCESSES 9 
the inclosure of the air, and to allow sufficient 
time for the escape of any bubbles that may be 
present. Sp. gr. is understood to be taken at 15.5°; 
samples should be brought near to this. If at 
a few degrees above or below, it will suffice to 
make the determination at once and obtain the 
correct figure by reference to the annexed table. 
The sp. gr. of normal milk ranges between 
1.027 and 1.035. The figure alone does not 
indicate the character of the sample, but taken 
in conjunction with the figure for fat or for 
total solids, it is of value as a check on the 
results furnished by other determinations. 
The simplest method of determining sp. gr. is 
by the lactometer, a delicate and accurately gradu- 
ated hydrometer. The instrument must be im- 
mersed carefully so as not to wet the stem above 
the point at which it will rest. Its accuracy 
should be verified by immersion in distilled 
water at 15.5° and milks of known sp. gr. 
More accurate determinations may be made 
with a balance. A special form, the Westphal 
balance, is adapted to the determination of 
sp. gr. only, the weights being so arranged 
that a simple enumeration of them gives the 
gravity directly. The cheap forms of this in- 
strument are not satisfactory, but some made 
by German houses are excellent. The ordinary 
