38 Testing Milk and Its Products. 



sudden changes of temperature without breaking, and 

 should be sufficiently heavy to withstand the maximum 

 centrifugal force to which they are likely to be sub- 

 jected in making tests. This force may, on the average, 

 be not far from 30.65 Ibs. (see 66), which is the pres- 

 sure exerted in whirling the bottles filled with milk and 

 acid in a centrifugal machine of 18 inches diameter at 

 a speed of 800 revolutions per minute. 



Special forms of test bottles used in testing cream 

 and skim milk are described under the heads of cream 

 and skim-milk testing (89, 90, 91, 99). 



When 17.6 cc., or 18 grams of milk (48), are meas- 

 ured into the Babcock test bottle, the scale on the neck 

 of the bottles will show directly the per cent, of fat 

 found in the milk. The scale is graduated from to 

 10 per cent. 10 per cent, of 18 grams is 1.8 grams. As 

 the specific gravity of pure butter fat (i. e., its weight 

 compared with that of an equal volume of pure water) 

 at the temperature at which the readings are made 

 (about 140 P.), is 0.9, then 1.8 grams of fat will oc- 

 cupy a volume of J^-=2 cubic centimeters. The space 

 between the and 10 per cent, marks on the necks of 

 the test bottles must therefore hold exactly 2 cubic cen- 

 timeters. The scale is divided into 10 equal parts, each 

 part representing one per cent., and each of these is 

 again sub-divided into five equal parts. Each one of the 

 latter divisions therefore represents two-tenths of one per 

 cent, of fat when 17.6 cc. of milk is measured out. The 

 small divisions are sufficiently far apart in most Bab- 

 cock test bottles to make possible the estimation of one- 

 tenth, or even five-hundredths, of one per cent, of fat 

 in the samples tested. 



