b
oxygen at the common temperature of the atmosphere. Hence arises a
variety of changes in oils which modify their properties and uses in the
arts.

If oil simply absorbs, and combines with oxygen, it thickens and changes
to a kind of wax. This change is observed to take place on the external
parts of certain vegetables, even during their life. But it happens in
many instances that the oil does not retain all the oxygen which it
attracts, but that part of it combines with, or burns, the hydrogen of
the oil, thus forming a quantity of water, which gradually goes off by
evaporation. In this case the alteration of the oil consists not only in
the addition of a certain quantity of oxygen, but in the diminution of
the hydrogen. These oils are distinguished by the name of _drying oils_.
Linseed, poppy, and nut-oils, are of this description.

EMILY.

I am well acquainted with drying oils, as I continually use them in
painting. But I do not understand why the acquisition of oxygen on one
hand, and a loss of hydrogen on the other, should render them drying?

MRS. B.

This, I conceive, may arise from two reasons; either from the oxygen
which is added being less favourable to the state of fluidity than the
hydrogen, which is subtracted; or from this additional quantity of
oxygen giving rise to new combinations, in consequence of which the most
fluid parts of the oil are liberated and volatilised.

For the purpose of painting, the drying quality of oil is further
increased by adding a quantity of oxyd of lead to it, by which means it
is more rapidly oxygenated.

The rancidity of oil is likewise owing to their oxygenation. In this
case a new order of attraction takes place, from which a peculiar acid
is formed, called the _sebacic acid_.

CAROLINE.

Since the nature and composition of oil is so well known, pray could not
oil be actually _made_, by combining its principles?

MRS. B.

That is by no means a necessary consequence; for there are innumerable
varieties of compound bodies which we can decompose, although we are
unable to reunite their ingredients. This, however, is not the case with
oil, as it has very lately been discovered, that it is possible to form
oil, by a peculiar process, from the action of oxygenated muriatic acid
gas on hydro-carbonate.

We now pass to the _volatile_ or _essential oils_. These form the basis
of all the vegetable perfumes, and are contained, more or less, in every
part of the plan