54 Testing Milk and Its Products. 



the same standard measure used. The part of the scale 

 from to 10 being calibrated first, then from 10 to 20 

 and 20 to 30 per cent, in the same way. 



63. Pipette and acid cylinder. The pipette and 

 the acid cylinder used in the Babcock test may be cali- 

 brated by any of the methods already given. Sufficiently 

 accurate results are obtained by weighing the quantity 

 of water which each of these pieces of apparatus will 

 hold, viz., 17.6 grams and 17.5 grams, respectively. The 

 necessity of previous thorough cleaning of the glassware 

 is evident from what has been said in the preceding. 

 The pipette and the acid measure may be weighed empty 

 and then again when filled to the mark with pure water, 

 or the measureful of water may be emptied into a small 

 weighed vessel, and this weighed a second time. In 

 either case the weight of the water contained in the 

 pipette or acid measure is obtained by difference. 1 



Calibrations of the acid cylinder are generally not called 

 for, except as a laboratory exercise, since small variations 

 in the amount of acid measured out do not affect the 

 accuracy of the test. 



2. CENTRIFUGAL MACHINES. 



64. The capacity of the testing machine to be selected 

 should be governed by the number of tests which are 

 likely to be made at one time. For factory purposes a 

 twenty-four to thirty bottle tester is large enough, and 

 to be preferred for a larger tester, even if toward a hun- 



1 1 cubic centimeter of distilled water weighs I gram, when weighed in 

 a vacuum at the temperature of the maximum density of water (4 C); for 

 the purposes of calibration of glassware used in the Babcock test, suf- 

 ficiently accurate results are, however, obtained by weighing the water in 

 the air and at a low room-temperature (60 F.). 



