618 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



imbibition ; so the effect of caustic potash is to enable the casein to pass 

 into solution, and so render the oil accessible to ether. The absence of 

 any solid envelope of the oil-globule is proved by the absence of any 

 such formation after mechanically breaking up the oil-globules, while the 

 proof of the fact of non-solution of casein in the milk is found in the 

 fact that when milk is filtered through porous earthenware scarcely any 

 casein passes through, while the other albuminoids which are really in 

 solution do so pass. Casein, therefore, in milk acts like the gum around 

 the oil-globules in an artificial mucilage emulsion. 



On account of the lighter specific gravity, when milk stands the oil- 

 globules rise to the top and, accompanied by varying amounts of the 

 other constituents of the milk, constitute the cream ; the largest rise 

 first, while the smaller ones remain suspended in the body of the milk. 

 The ascent of the cream is the more rapid the smaller the distance the 

 fat-globules have to traverse. Hence, cream is formed more rapidly in 

 broad, shallow vessels than in tall, narrow ones. Unless the temperature 

 of the milk is constant, currents are set up in the milk from the differ- 

 ences of temperature, and the formation of cream is delayed until the 

 entire volume of milk and the external medium are of the same tempera- 

 ture. Hence, shallow metal pans, by leading to a rapid equalization of 

 temperatures, are usually employed for the separation of the cream. As 

 the milk has a greater volume when warm than when cold, the cream will 

 contain less serum when the molecules of the fluid are further separated 

 from each other. So cream formed from warm milk has a higher per- 

 centage of fat in a small volume than cream formed from cold milk, and 

 the higher the temperature at which the formation of cream takes place 

 the smaller the amount of fat left in the skimmed milk. 



If the cream has been removed from milk by means of the centri- 

 fugal machine the separation is much more complete (often only O.L 

 per cent, of fat remaining in the milk), while the milk remains sweet 

 instead of becoming sour, as ordinarily occurs in the usual method of 

 separating the cream. Skimmed milk so obtained is, therefore, a more 

 valuable food, since it still contains all the sugar and is less apt to pro- 

 duce disturbances of digestion. 



The addition of a small amount of water facilitates, the addition of 

 salt interferes with, the separation of the cream. 



The following table represents the composition of cream : 



Water, . . . . . *,.,% . 61.67 



Fat, . . . . . .... . . 33.43 



Casein, . " . . . 2.62 



Sugar, ... 1 56 



Salts, . 0.72 



By the process of churning, which is only effective after the milk 

 has become slightly acid, only about two thirds of the fat are removed ; 



