THE BABCOCK TEST 63 



2 to 5 minutes after the mixing appears complete and 

 then to agitate a second time with rotary motion just 

 before placing in the tester. 



The action of the sulphuric acid upon the water and 

 organic solids of the milk produces a marked degree 

 of heat, as soon as the acid and milk begin to mix. 

 The color of the solution becomes yellow at first and 

 then passes through varying darker shades of yellow 

 to violet, brown and finally dark-brown, if the acid is 

 of the right strength. (See p. 43). The coloration is due 

 to the action of the acid upon the milk-sugar and milk- 

 casein. Too strong acid produces a dense black color. 

 In samples of milk containing too much bechromate of 

 potash, the color becomes greenish black. 



Samples of milk that have been preserved for some ' 

 time with biochromate or formalin, especially when the 

 preservation is used in larger than usual amounts, re- 

 quire more time and agitation to redissolve the coag- 

 ulated casein than do ordinary samples, since these 

 preservatives harden the coagulated casein. (See p. 29). 



WHIRLING THE TEST-BOTTLES 



The test-bottles containing the mixture of milk and 

 acid, after being agitated a second time as stated above, 

 are placed in the centrifugal tester (p. 38), and 

 whirled. This is better done soon after the milk and 

 acid are mixed, but it may be delayed without harm 

 .for 24 hours, in which case, however, the bottles 

 should be placed in water at 160 to 180 F. for 15 

 or 20 minutes before whirling. 



An even number of bottles should be whirled at the 



