There is still another form under which volatile oils present themselves, which is that of balsams. These consist of resinous juices combined with a peculiar acid, called the benzoic acid. Balsams appear to have been originally volatile oils, the oxygenation of which has converted one part into a resin, and the other part into an acid, which, combined together, form a balsam; such are the balsams of Peru, Tolu, &c.
We shall now take leave of the oils and their various modifications, and proceed to the next vegetable substance, which is caoutchouc. This is a white milky glutinous fluid, which acquires consistence, and blackens in drying, in which state it forms the substance with which you are so well acquainted, under the name of gum-elastic.
CAROLINE.
I am surprised to hear that gum-elastic was ever white, or ever fluid! And from what vegetable is it procured?
MRS. B.
It is obtained from two or three different species of trees, in the East-Indies, and South-America, by making incisions in the stem. The juice is collected as it trickles from these incisions, and moulds of clay, in the form of little bottles of gum-elastic, are dipped into it. A layer of this juice adheres to the clay and dries on it; and several layers are successively added by repeating this till the bottle is of sufficient thickness. It is then beaten to break down the clay, which is easily shaken out. The natives of the countries where this substance is produced sometimes make shoes and boots of it by a similar process, and they are said to be extremely pleasant 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.