verage specimen of cow's milk, according to Letheby,
is:--

Nitrogenous matter.......................................4.1
Fat......................................................3.9
Sugar of milk............................................5.2
Mineral matter...........................................0.8
Water...................................................86.0

If a drop of milk be examined with a microscope, it will be seen as a
clear liquid, holding in suspension a large number of minute globules,
which give the milk its opacity or white color. These microscopic
globules are composed of fatty matter, each surrounded by an envelope of
casein, the principal nitrogenous element found in milk. They are
lighter than the surrounding liquid, and when the milk remains at rest,
they gradually rise to the top and form cream. Casein, unlike albumen,
is not coagulated by heat; hence when milk is cooked, it undergoes no
noticeable change, save the coagulation of the very small amount of
albumen it contains, which, as it solidifies, rises to the top, carrying
with it a small portion of the sugar and saline matter and some of the
fat globules, forming a skin-like scum upon the surface. Casein,
although not coagulable by heat, is coagulated by the introduction into
the milk of acids or extract of rennet. The curd of cheese is coagulated
casein. When milk is allowed to stand for some time exposed to warmth
and air, a spontaneous coagulation occurs, caused by fermentative
changes in the sugar of milk, by which it is converted into lactic acid
through the action of germs.

Milk is sometimes adulterated by water, the removal of more or less of
the cream, or the addition of some foreign substance to increase its
density.

The quality of milk is more or less influenced by the food upon which
the animal is fed. Watery milk may be produced by feeding a cow upon
sloppy food.

The milk of diseased animals should never be used for food. There is no
way by which such milk can invariably be detected, but Prof. Vaughan, of
Michigan University, notes the following kinds of milk to be avoided:

1. Milk which becomes sour and curdles within a few hours after it has
been drawn, and before any cream forms on its surface. This is known in
some sections as 'curdly' milk, and it comes from cows with certain
inflammatory affections of the udder, or digestive diseases, or those
which have been overdriven or worried.

2. "Bitte