ant and serviceable, both from their
elasticity, and their being water-proof.

The substance which comes next in our enumeration of the immediate
ingredients of vegetables, is _extractive matter_. This is a term,
which, in a general sense, may be applied to any substance extracted
from vegetables; but it is more particularly understood to relate to the
extractive _colouring matter_ of plants. A great variety of colours are
prepared from the vegetable kingdom, both for the purposes of painting
and of dying; all the colours called _lakes_ are of this description;
but they are less durable than mineral colours, for, by long exposure to
the atmosphere, they either darken or turn yellow.

EMILY.

I know that in painting, the lakes are reckoned far less durable colours
than the ochres; but what is the reason of it?

MRS. B.

The change which takes place in vegetable colours is owing chiefly to
the oxygen of the atmosphere slowly burning their hydrogen, and leaving,
in some measure, the blackness of the carbon exposed. Such change cannot
take place in ochre, which is altogether a mineral substance.

Vegetable colours have a stronger affinity for animal than for vegetable
substances, and this is supposed to be owing to a small quantity of
nitrogen which they contain. Thus, silk and worsted will take a much
finer vegetable dye than linen and cotton.

CAROLINE.

Dying, then, is quite a chemical process?

MRS. B.

Undoubtedly. The condition required to form a good dye is, that the
colouring matter should be precipitated, or fixed, on the substance to
be dyed, and should form a compound not soluble in the liquids to which
it will probably be exposed. Thus, for instance, printed or dyed linens
or cottons must be able to resist the action of soap and water, to which
they must necessarily be subject in washing; and woollens and silks
should withstand the action of grease and acids, to which they may
accidentally be exposed.

CAROLINE.

But if linen and cotton have not a sufficient affinity for colouring
matter, how are they made to resist the action of washing, which they
always do when they are well printed?

MRS. B.

When the substance to be dyed has either no affinity for the colouring
matter, or not sufficient power to retain it, the combination is
effected, or strengthened, by the intervention of a third substance,
called a _mordant_, or basis. The mordant must have a strong affinity
both for the colouring m