matter is composed of two substances--one, a solid, termed
_margarin_; the other fluid, and styled by chemists _elaine_. The solid
fat is identical in composition with the solid fat of the human body.
The elaine is peculiar to milk, but it differs very slightly from
_olein_, or fluid fat. The relative proportions of the fluid and solid
fats vary with the seasons. According to Braconnot, the solid fat forms
in summer 40 per cent. of the butter, but in winter the proportion rises
to 65. This decrease in the proportion of the liquid fat in winter is
the cause of the greater hardness of the butter in that season, which is
often incorrectly attributed solely to the cold.

The cheesy and acid matters contained in butter are by no means
essential; on the contrary, if it were quite free from them, it might
be retained with little or no salt for a very long period without
becoming rancid. The cheesy matter contains nitrogen; and nearly all
the substances into which this element enters as a constituent are
remarkably prone to decomposition. Yeast, and ferments of every
kind--gunpowder, fulminating silver, chloride of nitrogen--and almost
every explosive compound, contain this element. The cheesy matter is
a very nitrogenous body, and in presence of air and moisture not only
rapidly decomposes, or decays, itself, but induces by mere contact a
like state of decomposition in other substances--such, for instance, as
fat, sugar, and starch, which naturally have no tendency to change their
state. Bearing the foregoing facts in mind, it is obvious that the chief
precautions to be observed in the manufacture of butter are:--Firstly,
to separate to as great an extent as practicable the casein from the
butter; and, secondly, as in practice a small portion of the curd
remains in the butter, to prevent it from undergoing any change--at
least for a prolonged period. How these desiderata may best be
accomplished I shall now proceed to point out.

_The Butter Manufacture._--The theory of the process of churning is very
simple. By violently agitating the milk or cream the little vesicles, or
bags containing the butter, are broken, and, the fatty matter adhering,
_lumps of butter_ are formed. The operation of churning also introduces
atmospheric air into the milk, which, aided by the high temperature to
which the fluid is raised, converts a portion of the _sweet_ sugar of
milk into the _sour_ lactic acid. By the alteration produced in this way