42 MILK 
other solids increase it, it is possible to take off 
a small amount of the former and add some 
water without disturbing the sp. gr., but, of 
course, the above analytical methods will detect 
this procedure. It is now admitted that, except 
in cases of wide departure from the usual limits, 
the adulteration of milk cannot be detected by 
the sp. gr. alone but the employment of a care- 
fully graduated lactometer is of use in routine 
milk inspection. 
Direct Detection of Added Water. Serum-refrac- 
tion.—Of late years several methods have been 
proposed for this purpose but most of them have 
no positive value and have not come into general 
use. The refractive index of the whey (milk- 
serum) offers a rapid and satifactory method for 
detecting watering. Several methods of pre- 
paring this whey have been proposed, but 
Lythgoe has found, as the result of extended 
experience, the following to be satisfactory. 
Dissolve 7.25 grams of crystallized copper sulfate 
in water and dilute to 1000 c.c. If this solution 
does not refract 36 on the scale of the immersion 
refractometer at 20°, add water or copper sulfate 
until the desired result is obtained. To 8 c.c. 
of the copper solution add 32 c.c. of milk. Shake 
well and pour upon a dry filter. When the filtrate 
begins to come through clear, change the receiver, 
pour the small quantity of cloudy filtrate upon 
