atter and the substance to be dyed, by which
means it causes them to combine and adhere together.

CAROLINE.

And what are the substances that perform the office of thus reconciling
the two adverse parties?

MRS. B.

The most common mordant is sulphat of alumine, or alum. Oxyds of tin and
iron, in the state of compound salts, are likewise used for that
purpose.

_Tannin_ is another vegetable ingredient of great importance in the
arts. It is obtained chiefly from the bark of trees; but it is found
also in nut-galls, and in some other vegetables.

EMILY.

Is that the substance commonly called _tan_, which is used in
hot-houses?

MRS. B.

Tan is the prepared bark in which the peculiar substance, tannin, is
contained. But the use of tan in hot-houses is of much less importance
than in the operation of _tanning_, by which skin is converted into
leather.

EMILY.

Pray, how is this operation performed?

MRS. B.

Various methods are employed for this purpose, which all consist in
exposing skin to the action of tannin, or of substances containing this
principle, in sufficient quantities, and disposed to yield it to the
skin. The most usual way is to infuse coarsely powdered oak bark in
water, and to keep the skin immersed in this infusion for a certain
length of time. During this process, which is slow and gradual, the skin
is found to have increased in weight, and to have acquired a
considerable tenacity and impermeability to water. This effect may be
much accelerated by using strong saturations of the tanning principle
(which can be extracted from bark), instead of employing the bark
itself. But this quick mode of preparation does not appear to make
equally good leather.

Tannin is contained in a great variety of astringent vegetable
substances, as galls, the rose-tree, and wine; but it is nowhere so
plentiful as in bark. All these substances yield it to water, from which
it may be precipitated by a solution of isinglass, or glue, with which
it strongly unites and forms an insoluble compound. Hence its valuable
property of combining with skin (which consists chiefly of glue), and of
enabling it to resist the action of water.

EMILY.

Might we not see that effect by pouring a little melted isinglass into a
glass of wine, which you say contains tannin?

MRS. B.

Yes. I have prepared a solution of isinglass for that very purpose. --Do
you observe the thick muddy precipitate? --That is the tannin combin