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 or greater than the buoyant force acting 

 upon them. 



Fig. 5. — Standardized milk. Showing the amount of cream on milk con- 

 taining the designated per cent of butter-fat. (From Bui. 92, III.) 



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 



