s poured into this tube up to zero or 0, and allowed to stand
for twelve hours in summer and sixteen or eighteen in winter. At the end
of that time the cream will have risen to the top, and its per-centage
may be easily seen. In good milk the cream will generally extend 11 to
15 degrees down from 0. This instrument, although very useful, is not
reliable in every case, especially in detecting the adulteration of
milk.

I have already stated that the complete separation of the butter from
the other constituents of the milk is never accomplished in the dairy.
Now although the proportion of curd in the butter is very small--rarely
more than two per cent. and often not a fourth of one per cent.--yet it
is more than sufficient, under a certain condition, to cause the butter
to become speedily rancid. That condition is simply contact with the
air. If the curd, before it becomes dry and firm, is subjected to the
influence of the air, it rapidly passes into a state of fermentation,
which is very soon communicated to the fatty and saccharine constituents
of the butter (substances not spontaneously liable to sudden changes in
composition) and those peculiar compounds--such, for example, as butyric
and capric acids, are generated, which confer upon rancid butter its
characteristic and very disagreeable odor and flavor. The fermentation
of the curd is prevented by incorporating common salt with the butter,
and by preventing, so far as possible, the access of air to the
vessels in which the article is placed. If fresh butter be placed in
water--which apparently protects it from the influence of the air--it
will soon become rancid. The reason of this is, that water always
contains air, which differs in composition, though derived, from the
atmosphere, by being very rich in oxygen. Now, it is precisely this
oxygen which effects those undesirable changes in the casein, or curd,
to which I have so repeatedly referred; hence its presence in a
concentrated state in water causes that fluid to produce an injurious
effect on the butter placed in it. A saturated solution of salt contains
very little air, and, so long as the curd is immersed therein, it
undergoes no change. The salt, too, acts as a decided preservative; for
although it was long considered to be capable of preserving animal
matters, merely by virtue of its property of absorbing water from them
(the presence of water being a condition in the decomposition of organic
matter), it h