serve as fuel and to make and repair muscular tissue--this element is
regarded as an important ingredient of milk. The protein in milk is
called _casein_. The opaque whiteness of milk is largely due to the
presence of this substance. As long as milk remains sweet, the lime
salts it contains hold this casein in solution; but when it sours, the
salts are made soluble and the casein thickens, or coagulates. In
addition to casein, milk contains a small amount of protein in the form
of _albumin_. This substance, upon being heated, coagulates and causes
the formation of the skin that is always found on the top of milk that
has been heated. The skin thus formed contains everything that is found
in milk, because, as it forms, casein is dried with it and sugar and
fat, too, are caught and held there. It is the protein of milk and its
characteristic coagulation that are made use of in the making of cheese.
In cooking, the protein of milk is probably more affected than any of
the other substances, but the degree to which the digestion of milk is
thus affected is not definitely known, this being a much
disputed question.

9. FAT IN MILK.--The other substance in milk that serves as fuel, or to
produce energy, is fat. It occurs in the form of tiny particles, each
surrounded by a thin covering and suspended in the liquid. Such a
mixture, which is called an _emulsion_, is the most easily digested form
in which fat is found. The fat in milk varies more than the other food
substances, it being sometimes as low as 2 per cent, and again as high
as 6 per cent. However, the average of these two, or 4 per cent., is the
usual amount found in most milk.

As has been mentioned, the fat globules of milk rise to the top because
fat is lighter than water, so that when milk has been undisturbed for
some time the top, which is known as _cream_, will be found to contain