32 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
gravity, see page 228.) Determination of the specific 
gravity of the milk solids and of the per cent. of fat 
in the milk solids will assist in detecting milk which has 
been skimmed or skimmed and watered. (See page 246 
for methods.) The milk solids of normal market milk 
have a specific gravity of 1.81 to 1.36, and the per cent. 
of fat in the milk solids is 20 to 84. When milk is 
skimmed or skimmed and watered, the specific gravity 
of the milk solids is increased, while the per cent. of fat in 
the solids is decreased. 
The influence of disease on the specific gravity of 
individual milk is not constant, but the specific gravity 
is usually lowered. However, the specific gravity of 
individual cow’s milk cannot be made use of to discover 
diseased conditions, because milk from different cows 
shows such great variations under normal conditions. 
The specific gravity of milk is lower when it is drawn 
from the udder than it is several hours later. 
Refraction.—Rays of light passing through one me- 
dium into another of different optical density, as through 
air into milk, are broken or refracted at the point of con- 
tact of the two media. The degree of refraction, or the 
refractive power compared with that of air, is called 
the refractive index. Since the calcium caseinate and 
fat contained in milk prevent the light rays from passing 
through it, these substances must be removed before the 
refractive index can be determined. The refractive index 
of milk, so-called, is really the refractive index of the 
milk serum or whey, 7.e., the milk minus the calcium 
caseinate and fat. 
The refractive power of the milk serum depends upon 
the quantity of lactose or of lactose and salts present. 
Adding water to milk reduces the proportion of these 
