As the varnish dries, the alcohol evaporates, and the dry varnish or resin which remains, not being soluble in water, will not be acted on by it. There is a class of compound resins called _gum-resins_, which are precisely what their name denotes, that is to say, resins combined with mucilage. Myrrh and assafœtida are of this description. CAROLINE. Is it possible that a substance of so disagreeable a smell as assafœtida can be formed from a volatile oil? MRS. B. The odour of volatile oils is by no means always grateful. Onions and garlic derive their smell from volatile oils, as well as roses and lavender. 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.