MILK TESTING 21 



for every revolution of the crank wheel. Suppose 

 the test bottles make 12 revolutions while the crank 

 makes one. In order that the bottles may make 900 

 revolutions per minute, the crank must be turned 

 75 times in a minute (900 -t- 12 = 75). In case the 

 machine is less than 14 inches in diameter a greater 

 number of revolutions is necessary. 



21. Centrifugal Action. — As previously stated, the 

 fats are lighter than the milk serum, and when the 

 milk is whirled in the test bottles, there is a separation 

 of milk fats (lighter particles) from the milk serum 

 (heavier portion). The serum goes to the outside 

 of the circle of revolution, while the fats mass in the 

 center, where they are finally collected in the gradu- 

 ated stem of the test bottle. The sulphuric acid, 

 which is one and eight tenths heavier than water, 

 also aids in the separation both by increasing the 

 specific gravity of the milk serum and by chemical 

 action upon the albumin and casein. The cream 

 separator works on this same principle. In the case 

 of the separator, provision is made for the escape of 

 the fat into a tube as it collects at the center. Cen- 

 trifugal action is well illustrated by whirlpools, where 



