When new milk is left at rest the cream will rise to the top and after 12 to 24 hours a cream-line can be seen in the bottle. This cream-line is sharper and more easily seen in raw milk than in pasteurized milk and its absence is not always a sure sign of lack of richness or purity of the milk. By cooling the milk thoroughly so that it will keep, almost all the cream will be at the top in forty-eight hours and can be skimmed off. The cream can be used for coffee or on cereals or fruits or puddings; the skim milk left will still hold ½% or more of butter-fat and can be used to drink or for cooking. =The Separator.=—On the farm or in the creamery the cream is no longer raised by gravity, that is, by letting the milk “set” either in shallow pans on the kitchen shelf or in deep cans in ice water, but the fresh, warm milk is run through the separator in a continuous stream. [Illustration: Early conception of the separator ] It was noticed that the rising of the cream due to the difference in specific gravity between the butter-fat and the milk-“serum” (the watery solution of the other constituents) might be greatly hastened by subjecting the milk to centrifugal force. This physical phenomenon was taken advantage of in the first conception of the separator where it was shown that if a pail of milk was whirled around like a stone in a sling the heavier milk-serum would be thrown towards the bottom of the separator pail with so much greater force than the lighter cream (butter-fat mixed with a small part of the serum) that the separation which would take 48 hours in the milk at rest, could be accomplished in a few minutes when exposed to centrifugal force. From this early crude attempt the continuous _bowl-separator_ was developed and still later a number of divisions in the bowl were designed which increased the capacity and efficiency of the machine wonderfully. The most successful separator was designed by Dr. Gustaf De Laval of Sweden and the machines bearing his name are used all over the world where butter is made. But there are many other excellent separators on the market. [Illustration: Dr. Gustaf De Laval of Sweden, inventor of the separator bearing his name ] In the machine the milk is separated instantaneously by centrifugal force and runs out through two tubes, one for cream and the other for skim milk. A small modern hand machine will take care of from 200 to 1,000 lbs. of milk an hour, and power machines are built to separate 6,000 lbs. or more. By regulating the cream-outlet a heavier or lighter cream can be produced. [Illustration: Hand separator ]