Solubility of the Casein. 23 
called skins, as was formerly thought, but main- 
tain their form by reason of the surface tension of 
the liquid fat, and also to some extent because of 
a layer of more or less condensed casein that im- 
mediately surrounds them. The permanency of the 
emulsion is further increased by the viscous nature 
of the milk serum, due to the presence of sugar 
and other solids in solution. 
The albuminoids.—The casein is the chief albu- 
minoid of milk, although there is always present a 
small amount of albumin, and, according to some 
authorities, of fibrin. Casein is of prime impor- 
tance in the manufacture of cheese, and it is the 
chief constituent that goes to form tissue when milk 
is used as a food. 
There is a good deal of doubt as to the form 
in which the casein exists. Formerly it was sup- 
posed that the casein is in solution. This idea 
was brought about by the fact that it is impossible 
to filter the casein from the milk, even though it 
. 1s passed through several thicknesses of fine paper. 
Later, though, it was found that when milk is 
passed through a fine porcelain filter a certain part 
is removed, and it was then supposed that there 
were two forms of casein, one of which was in 
solution and the other in suspension in very fine 
particles of a colloidal or gummy character. Still 
later investigations have shown that in all proba- 
bility a large part of the casein is in this extremely 
fine colloidal state. When milk is subjected to 
the action of weak organic or mineral acids, to 
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