Part 79. Oils, whether fixed or volatile, are very generally present throughout the vegetable kingdom; and, as we have already seen, they are often of much economic importance as food or flavoring, and in medicine. They are of scarcely less value in the industrial arts, immense quantities of different vegetable oils being consumed in the manufacture of paints, printing-ink, varnishes, soaps, and perfumery, and as lubricants and illuminants.

As vehicles for pigments fixed oils are selected which not only will hold the particles of coloring matter in perfect suspension, and so make it easy to spread them evenly over a surface, but which also will harden promptly when thus spread into a film exposed to the air. Oils which harden in this way are called drying oils although the change which takes place depends not upon the evaporation of a volatile solvent, as in the drying of certain varnishes, but upon the absorption of oxygen which changes the oil into a varnish-like substance. Linseed-oil, which is obtained by pressure from the seeds of flax (Fig. 217), is the one most widely used by painters. Its "drying" qualities are much improved by boiling. For use in printing-ink the oil is boiled until it is very thick. Other drying oils which are somewhat superior to linseed-oil are poppy-oil, from the seeds of the opium poppy (Fig. 172), and nut-oil, from the kernels of the English walnut (Fig. 27). These being comparatively expensive are reserved for fine painting.

Linseed-oil is invaluable also as a solvent for copal and other resins, with which it unites at a high temperature to form the highest class of varnishes. Entirely by itself it is used extensively to give an attractive "oil finish" to woodwork. In certain varnishes the volatile oil or spirits of turpentine, known commonly to the trade as "turps," is the solvent used, and is likewise indispensable to painters as a means of thinning their colors.

Any of the fixed oils combined with an alkali makes soap. When potash (or lye from wood ashes) is used soft soap is formed; hard soap being made with soda. Chemically the fixed oils are mixtures, in various proportions, of compounds called glycerides. A glyceride is so called because it consists of glycerin (the familiar sweetish substance soluble in water) combined with an acid. Linoleic, oleic, and palmatic acids are among the most important in vegetable oils. The glyceride of linoleic acid, called linolein, forms 80% of linseed-oil, and gives to this and to other drying oils their peculiar power of hardening by oxidation. Olein, the glyceride of oleic acid, is the main constituent of olive-oil. It is liquid at ordinary temperatures and becomes rancid by oxidation. Palmatic acid forms a glyceride, palmatin, which is not liquid at ordinary temperatures. It is the main solid constituent of coconut and other palm-oils. When any fixed oil is mixed with an alkali, the glycerides present are decomposed each into its peculiar acid and glycerin, and the acids unite with the alkali to form soap, leaving the glycerin free.

Inferior grades of linseed oil and other cheap oils are used for soft-soap. Oil from the olive (Fig. 113) is used extensively for castile, and other fine toilet soaps. Other hard soaps of various grades are made from "cocoa-butter " (see section 39), and oils from coconut (Fig. 36), cotton-seed (Fig. 215), peanut (Fig. 32), and almond (Fig. 31).

To give an agreeable odor to soap a large variety of volatile oils are introduced during the process of preparing the product for market. The oils of wintergreen (Fig. 147), marjoram (Fig. 137), coriander (Fig. 143), thyme (Fig. 134), caraway (Fig. 140), and many others are thus used to a greater or less extent.

These same volatile oils enter also into the manufacture of perfumery, and for this purpose many other volatile oils are more or less in demand, as, for example, the oils of nutmeg (Fig. 129), allspice (Fig. 123), sassafras (Fig. 160), peppermint (Fig. 146), spearmint (Fig. 135), orange-peel and orange-flowers (Fig. 106), and the oil distilled from the wood of red cedar (Fig. 261). It is to the fragrant oil obtained from the bark of white birch (Fig. 254) that the characteristic odor of Russia leather is due.

None but fixed oils can serve as lubricants; and of these, only the non-drying ones are suitable. The vegetable lubricants most extensively employed are (1) olive-oil, used for this purpose mostly in southern European countries where a sufficiently good quality may be obtained at a low price, (2) rape-oil from the seed of a variety of turnip grown widely in northern Europe and India, and (3) cotton-seed oil used largely in this country.

As illuminants vegetable oils have not to-day the importance they had before the introduction of petroleum lamp-oil and paraffin candles. Nevertheless, large quantities of vegetable illuminants are still consumed, especially in regions where mineral or animal oils are comparatively expensive. Almost all the fixed oils in common use for other purposes have served for burning, but the non-drying oils are preferable. Olive, peanut, and rape oils, which are all rich in olein, are among the best. Palmatin, as we have seen, is an important constituent of coconut-oil. This substance separated from the more fluid parts of the coconut-oil and other palm-oils affords an excellent material for candles.