208 CHEMISTRY OF FARM PRACTICE 



Put the test bottles in the centrifuge and whirl for four 

 or five minutes at a speed of 600-1200 revolutions per min- 

 ute. Then add hot water to fill the test bottle to the neck 

 and whirl again for one minute; next add hot water to near 

 the top of the graduated portion and whirl one minute 

 more. 



The reading of the per cent of fat should be taken at 

 about 130 F. This is best accomplished by means of a 

 pair of dividers. The appearance of the test bottle after 

 completed test is shown in Fig. 70. 



The results obtained in this determination are due to the 

 actions of strong sulphuric acid and the use of centrifugal 

 force. The action of the acid is threefold: It destroys 

 the adhesive force exercised by the casein, albumin, sugar, 

 and salts present in the milk; the mixture of sulphuric acid 

 and milk generates heat, which causes the fat globules to 

 run together and makes their separation from the mass 

 comparatively easy; third, the weight of the sulphuric 

 acid increases the specific gravity of the non-fatty materials, 

 causing the lighter fat the more readily to rise to the sur- 

 face. The completion of the separation of the fat is accom- 

 plished by the use of centrifugal force when the bottle con- 

 taining the mixture is whirled in a suitable apparatus, which 

 may be run either by hand or by power. 



153. Determination of Specific Gravity. The normal 

 specific gravity of milk from a herd usually falls between 

 1.030 and 1.034. The specific gravity of milk is quickly 

 determined by taking the reading of a lactometer floating in 

 the milk. The lactometer is a specialized form of a hydrom- 

 eter. The Quevenne lactometer has a scale graduated 

 from to 15 to 40, corresponding to specific gravities from 

 1.015 to 1.040. The milk should be tested at a temperature 

 ranging from 55 F. to 60 F., adding a correction of 1 

 (equivalent to 00.001 specific gravity) to the reading for 

 each degree F. above 60 F. and subtracting 1 for each 

 degree F. below 60 F. 



