36 BUTTER-MAKING. 
upon the specific gravity of the serum; hence, by adding water 
to milk, the resistant force is decreased to such an extent as to 
get a more rapid and more efficient separation of the fat. The 
water dilution separators are based upon this principle. In 
normal milk, the amount of fat left in the skimmed milk by 
natural creaming is about .4%. The fat which is left in this 
skimmed milk is largely composed of very small globules. 
This is due to the fact that the resistant force of these small 
globules is equal to cr greater than the buoyant force acting 
upon them. 
Fie. 5.—Standardized milk. Showing the amount of cream on milk con- 
taining the designated per cent of butter-fat. (From Bul. 92, Ill.) 
This completeness of natural skimming is to a certain extent 
based upon the mathematical law which is stated as follows: 
‘‘The surfaces of two spheres are to each other as the squares of 
their diameters, and their cubical contents are to each other 
as the cubes of their diameters.” The larger the globules are, 
the greater the surface is, and the greater the resisting force to 
which they are subjected. From the law stated it can be seen 
that as the size of the globule increases, the cubical content 
increases more rapidly than the surface. If a fat-globule were 
split up into smaller ones, there would be more surface exposed 
