64 MODERN METHODS OF TESTING MILK 



same time and they should be placed about the disc 

 in pairs opposite to each other, so that the equilibrium 

 of the tester will not be disturbed. When all the sam- 

 ples to be tested are placed in the tester, the cover 

 is placed on the jacket and the machine turned for 4 

 or 5 minutes at proper speed, 600 to 1,200 revolutions 

 per minute, according to the diameter of the centri- 

 fugal disc. (p. 41). 



The whirling brings the fat to the top of the mix- 

 ture in the test-bottle. The whirling of the bottles 

 should never be done without having the cover on the 

 jacket, for two reasons: (i) The cover prevents the 

 cooling of the fat in the test-bottles, and (2) the 

 operator is protected from injury in case a bottle should 

 break and scatter its contents while being whirled. 



In the case of hand-testers, it may be necessary to 

 put hot water in the jacket in cold weather in order 

 to keep the bottles warm enough. 



ADDING HOT WATER TO THE TEST BOTTLES 



When the bottles have been whirled 4 or 5 minutes, 

 moderately hot water is added to each bottle until the 

 contents come to the lower end of the neck. The 

 water may be added with a pipette or by means of 

 any convenient arrangement. The cover of the ma- 

 chine is replaced and the bottles are whirled at full 

 speed for one minute. Hot water is again added to 

 the bottles until the fat, which is lighter than the rest 

 of the liquid, rises in the neck to the 8 or 9 per cent, 

 mark. One must be careful never to raise the fat 

 above the 10 per cent. mark. The whirling is then re- 

 peated for one minute at full speed. 



