CHARACTERS OF MILK. 43 



that a membrane immediately forms around every drop of fat 

 that is brought in contact with a solution of albumen ; and I 

 have found that fat shaken with a caseous substance (crystallin) 

 in a state of solution, causes a partial coagulation by the for- 

 mation of such membranes or capsules. I have shown that 

 when woman's milk is evaporated, and the residue reduced to a 

 fine powder, and extracted with ether (which takes up the butter), 

 there are left the capsules of the fat-vesicles, which, when mixed 

 with water, and placed on the object-stage, may be observed 

 with the microscope. 



Milk is materially affected by a large number of substances, 

 especially by those that precipitate its casein. The addition of 

 any of these substances causes it to coagulate, that is to say, 

 the casein becomes insoluble and incloses the butter, and thus 

 produces the separation of a whey-like fluid from the caseous mass. 

 A precipitation of this nature is brought about by alcohol which, 

 at the same time, takes up a very small quantity of fat : when 

 milk is shaken with ether, no precipitation of casein ensues, 

 but the milk becomes rather clearer and the ether is found to 

 contain fat, but only a small portion of all that is contained in 

 the milk. When milk is left to itself for a considerable time, 

 it coagulates, in consequence of the conversion of a portion of 

 its sugar into lactic acid : this change often takes place very 

 rapidly in cow's milk, and generally more quickly than in 

 woman's milk. If the milk is allowed to remain still longer 

 exposed to an ordinary temperature, the surface becomes covered 

 with peculiar forms of mould, and, under certain conditions 

 which are not accurately known, particular species of infusoria 

 are developed. These infusoria are the cause of a blue or yellow 

 colouring matter, which is especially distributed over the sur- 

 face, a phenomenon that has long been observed, and which 

 has recently been carefully investigated by Fuchs. 



Rennet likewise precipitates the casein apparently by a cata- 

 lytic action on the sugar of milk, by which it is converted into 

 lactic acid ; hence the precipitation is hindered by the addition 

 of an alkali, and, as Herberger has observed, does not occur in 

 milk which abounds in alkaline salts. 



The solid constituents of the milk vary from about 9 to 35g ; 

 the specific gravity usually lies between 1028 and 1042. 



