k.

MRS. B.

Because you know it is customary, in order to save time and labour, to
make butter from cream alone. In this case, therefore, the butter-milk
is deprived of the creamed milk, which contains both the curd and whey.
Besides, in consequence of the milk remaining exposed to the atmosphere
during the separation of the cream, the latter becomes more or less
acid, as well as the butter-milk which it yields in churning.

EMILY.

Why should not the butter be equally acidified by oxygenation?

MRS. B.

Animal oil is not so easily acidified as the other ingredients of milk.
Butter, therefore, though usually made of sour cream, is not sour
itself, because the oily part of the cream had not been acidified.
Butter, however, is susceptible of becoming acid by an excess of oxygen;
it is then said to be rancid, and produces the sebacic acid, the same as
that which is obtained from fat.

EMILY.

If that be the case, might not rancid butter be sweetened by mixing with
it some substance that would take the acid from it?

MRS. B.

This idea has been suggested by Sir H. Davy, who supposes, that if
rancid butter were well washed in an alkaline solution, the alkali would
separate the acid from the butter.

CAROLINE.

You said just now that creamed milk consisted of curd and whey. Pray how
are these separated?

MRS. B.

They may be separated by standing for a certain length of time exposed
to the atmosphere; but this decomposition may be almost instantaneously
effected by the chemical agency of a variety of substances. Alkalies,
rennet*, and indeed almost all animal substances, decompose milk by
combining with the curds.

Acids and spirituous liquors, on the other hand, produce a decomposition
by combining with the whey. In order, therefore, to obtain the whey
pure, rennet, or alkaline substances, must be used to attract the curds
from it.

But if it be wished to obtain the curds pure, the whey must be separated
by acids, wine, or other spirituous liquors.

[Footnote *: Rennet is the name given to a watery infusion of the
coats of the stomach of a sucking calf. Its remarkable efficacy in
promoting coagulation is supposed to depend on the gastric juice
with which it is impregnated.]

EMILY.

This is a very useful piece of information; for I find white-wine whey,
which I sometimes take when I have a cold, extremely heating; now, if
the whey were separated by means of an alkali instead of wi