2OO MODERN METHODS OF TESTING MILK 



timed with watch under the eye. The tubes are then 

 placed in the centrifuge within 20 minutes and whirled 

 7}4 to 8 minutes at a speed of 2000 revolutions a 

 minute. This must be done with such precision that the 

 use of a metronome is recommended as important dur- 

 ing the whirling. The whirling done, the tubes are re- 

 moved and placed in an upright position in a rack and 

 then read after 10 minutes or more. The end surfaces 

 of the cylindrical mass of casein should be flat ; the re- 

 sult in per cent, of casein is read directly on the scale. 

 Fresh milk is desirable, but it is said that 7-day com- 

 posite samples may be used by taking i -ounce samples 

 of milk daily, in a brown or amber-colored bottle, ad- 

 ding on the first and third days i*^ to 2 grains of bi- 

 chromate (one-fourth ordinary tablet). This test de- 

 mands more than ordinary exactness of manipulation. 



