120 THE CHEMISTRY OF THE FAEM. 



16 inches deep, and surrounded by ice. The cream rises 

 quickly, and can all be obtained in twelve to twenty-four 

 hours from the time of setting. Cream thus prepared is 

 perfectly sweet, and free from curd, the low temperature at 

 which the milk has been kept having reduced chemical 

 change to a minimum. It occasionally happens that milk 

 will not yield its cream at low temperatures ; this is 

 sometimes the case with the milk of cows several 

 months after calving, and especially, when receiving a 

 winter diet. 



A third plan of separating cream is by subjecting the 

 milk to extremely rapid horizontal revolution in a centri- 

 fugal machine ; under these circumstances the fat globules 

 rise into the centre of the revolving mass. In Laval's 

 machine the new milk enters in a continuous stream, and 

 is immediately separated into cream and skim-milk, the 

 former leaving the apparatus by a pipe at the top, the 

 latter by another pipe from the side. The cream thus 

 obtained is, of course, perfectly sweet. 



Cream varies considerably in composition. Good cream, 

 not scalded on the Devonshire plan, may contain 55 to 65 

 per cent, of water, and 25 to 40 per cent, of fat. Casein 

 and the other constituents of milk are present in small 

 quantity. In sweet cream the casein may be about one- 

 tenth of the fat ; in cream which has soured during setting 

 the casein forms a much larger proportion. 



Skim-Milk. — Milk thoroughly skimmed in the ordinary 

 way will contain about 08 per cent, of fat ; more than this 

 quantity is frequently present. When ice has been used, 

 the percentage of fat left in the milk will be 0*3 to 0*6 ; 

 and when the centrifugal machine has been employed. 



