68 MODERN METHODS OF TESTING MILK 



these points instead of concave. While there is such 

 an apparent error, the excessive reading thus caused 

 is only enough to make up for the loss of fat which 

 can not be separated from the rest of the liquid by 

 centrifugal force and brought into the fat-column. The 

 amount of fat thus left in the mixture of milk-serum 

 and acid is ordinarily about .2 per cent, and this is 

 about the amount of excess obtained by the approved 

 method of reading the upper limit of the fat-column. 



(4) The fat appearing in the neck of the test-bot- 

 tle at the end of a successful test is of a clear, yellow 

 color, and the line of division between its lower limit 

 and the acid solution beneath it is sharply distinct. 

 However, the fat is apt to be light-colored in the case 

 of milk from cows far along in lactation. 



(5) The fat-column may show certain defects, if 

 the conditions of the test have not been properly car- 

 ried out, among which are (a) black particles below 

 or above or in the fat-column, or a darkened appear- 

 ance of the whole column of fat; (by white particles 

 below or above or in the fat-column, or a cloudy ap- 

 pearance of the whole column; and (c) bubbles on 

 the surface. 



(a) Black particles in the neck of the test-bottle at 

 the end of the test, or a darkened appearance of the 

 fat itself, are due to one or more of the following 

 causes: (i) Too strong acid (above 1.83 specific grav- 

 ity), (2) too much acid (more than 18 cc.), (3) too 

 high temperature of the milk or acid (over 75 F.), 

 (4) allowing milk and acid to stand in test-bottle too 

 long before mixing, (5) allowing the acid to drop 



