te of solution--that is to say, is dissolved in milk in the same
way that we dissolve sugar in water. When milk becomes sour, either
naturally or by the addition of rennet, it can no longer hold casein in
solution, and the curd consequently separates. Casein is the substance
which forms the basis of cheese. The substance that remains after the
removal of the butter and cheese is called _serum_, or whey, and is
composed of a sweetish substance termed _sugar of milk_, and certain
saline bodies, termed the ash, dissolved in water.

The butter and the sugar of milk are employed in the animal economy in
the production of fat, and are what have been styled by physiologists
_heat-producers_ and _fat-formers_. The casein resembles the gluten of
wheat in composition; it belongs to the class of food substances termed
_flesh-formers_. The ash, or mineral part of the milk, is chiefly
employed in forming the bones of the young animals it is destined to
nourish.

The quality of milk is influenced by the quantity and quality of the
food given to the animal. The milk of cows fed on distillery wash,
turnip, and mangel tops, coarse herbage, and other kinds of inferior
food, is always of inferior quality. Hence it is of great importance
that dairy stock be kept in good old pastures in summer, and fed on
Swedish turnips, mangel-wurtzel, and oil-cake during winter. It is true
economy to supply dairy cows with abundance of nutritious food; and it
should be constantly borne in mind that the milk from two well-fed cows
will give more butter than can be obtained from the produce of three
badly-fed animals.

The butter is the constituent of milk which is most affected by the
nature and amount of the animal's food; and butter is precisely the
article which is of the greatest importance to the Irish dairy farmer,
as the quantity of cheese prepared in this country is inconsiderable.
When, therefore, it is found that a cow pastured on inferior land, or
badly fed in the byre, yields a large supply of milk of a high specific
quantity (which, however, is rarely the case), it must not be concluded
that the result is satisfactory; for if such milk be tested by the
lactometer it will certainly be found wanting in butter. The average
composition of English milk, according to Way, is:--

Water 87.02
Butter 3.23
Casein 4.48
Sugar of milk