Yes, I am; it is much used in painting, both in oil and in water
colours; but it is not reckoned a permanent oil-colour.

MRS. B.

That defect arises, I believe, in general, from its being badly
prepared, which is the case when the iron is not so fully oxydated as to
form a red oxyd. For a solution of green oxyd of iron (in which the
metal is more slightly oxydated), makes only a pale green, or even a
white precipitate, with prussiat of potash; and this gradually changes
to blue by being exposed to the air, as I can immediately show you.

CAROLINE.

It already begins to assume a pale blue colour. But how does the air
produce this change?

MRS. B.

By oxydating the iron more perfectly. If we pour some nitrous acid on
it, the prussian blue colour will be immediately produced, as the acid
will yield its oxygen to the precipitate, and fully saturate it with
this principle, as you shall see.

CAROLINE.

It is very curious to see a colour change so instantaneously.

MRS. B.

Hence you perceive that prussian blue cannot be a permanent colour,
unless prepared with red oxyd of iron, since by exposure to the
atmosphere it gradually darkens, and in a short time is no longer in
harmony with the other colours of the painting.

CAROLINE.

But it can never become darker, by exposure to the atmosphere, than the
true prussian blue, in which the oxyd is perfectly saturated?

MRS. B.

Certainly not. But in painting, the artist not reckoning upon partial
alterations in his colours, gives his blue tints that particular shade
which harmonises with the rest of the picture. If, afterwards, those
tints become darker, the harmony of the colouring must necessarily be
destroyed.

CAROLINE.

Pray, of what nature is the paint called _carmine_?

MRS. B.

It is an animal colour prepared from _cochineal_, an insect, the
infusion of which produces a very beautiful red.

CAROLINE.

Whilst we are on the subject of colours, I should like to learn what
_ivory black_ is?

MRS. B.

It is a carbonaceous substance obtained by the combustion of ivory.
A more common species of black is obtained from the burning of bone.

CAROLINE.

But during the combustion of ivory or bone, the carbon, I should have
imagined, must be converted into carbonic acid gas, instead of this
black substance?

MRS. B.

In this, as in most combustions, a considerable part of the carbon is
simply volatilised by the heat, and again obtained concrete