The Babcock Test. 59 



on this point. He should measure out a dozen tests of 

 the same sample of milk, and whirl half the number at 

 the speed required for machines of the diameter of his 

 tester. Whirl the other half at a somewhat higher 

 speed. If the averages of the two sets of determinations 

 are the same, within the probable error of the test (say, 

 less than one-tenth of one per cent.) the first whirling 

 was sufficient, as it is believed will generally be the case. 

 If the second set of determinations come higher than the 

 first set, the first whirling was too slow, and a new series 

 of tests of the same sample of milk should be made to 

 ascertain that the second whirling was ample. 



This method will test not only the speed required with 

 the particular machine at hand, but will also serve to 

 indicate the correctness of the calibration of the bottles. 

 A large number of tests of the same sample of milk 

 made as directed (pouring the milk once or twice previ- 

 ous to taking out a pipetteful for each test) should not 

 vary more than two-tenths of one per cent, at the outside, 

 and in the hands of a skilled operator will generally 

 come within one -tenth of one per cent. If greater dis- 

 crepancies occur, the test bottles giving too high or too 

 low results should be further examined, and calibrated 

 according to the directions already given (53 et seq.}. 



69. Hand testers. When only a few tests are made at 

 a time, and at irregular intervals, as in case of dairymen 

 who test single cows in their herds, a small hand tester 

 answers every purpose. These may be had in sizes from 

 two to twelve bottles. In selecting a particular make of 

 tester the dairyman has the choice of a large number of 

 different kinds of machines. It is a source of regret that 



