英文科學(xué)讀本 第五冊(cè)·Lesson 43 Resin and Turpentine

Lesson 43 Resin and Turpentine

If we examine a piece of common resin, we shall be struck with its resemblance, in many respects, to the gums, which we have already described. It has a semi-transparent, glassy appearance, and is somewhat sticky to the touch. On the other hand, further examination would show that it differs from them in being highly inflammable, and insoluble in water, while it is soluble in spirits of wine, and the essential or volatile oils. Resin is employed chiefly in the manufacture of soap, and also in the preparation of a common kind of wax, which is used for sealing the corks of bottles.

Oil of turpentine, or spirits of turpentine, is a clear, transparent, very limpid liquid. It has a peculiar and powerful odor, and is highly inflammable, burning with flame and smoke. If we let fall a drop on a sheet of paper, it will gradually pass away, leaving no stain. It is this last-named property which places it among the volatile oils.

Indeed, oil of turpentine is unquestionably the most valuable of all the volatile oils. Large quantities of it are used in making oil-colors for painting. Its thin, limpid nature helps the paint to flow freely from the brush, and when it is laid on, the oil of turpentine, owing to its volatility, flies off rapidly in vapor, leaving behind the substance of the color, with the thin coating of linseed-oil.

It is also largely used in the manufacture of varnish, and in making the common lacquer for covering iron goods.

It is a powerful solvent of fat and oily matter of all kinds. Hence we use it to take grease-stains out of our clothes, etc.

Now that we know something of the nature and properties of both these substances—the solid resin and the liquid oil of turpentine—we may go a step further, and say that the two come from the same source.

They are the product of certain trees belonging to the pine family. Several species of these trees elaborate a peculiar secretion—a thick, very sticky liquid, of a yellowish color, not unlike honey in appearance. It is obtained by making a number of cross-cut incisions in the trunk almost to the pith, hollowing out each one so as to form a sort of bowl or basin, into which the turpentine—a honey-like liquid—drains, and is collected in vessels placed round.

This liquid is known as crude, or common turpentine. The great pine-forests of North America supply immense quantities of this crude turpentine—in fact, they supply the world. No less than 30,000 tons are exported annually to England alone.

In the preparation of the secretion for use, it is heated with a certain quantity of water in a copper retort or still. When the water boils, the steam passes away into the condenser to be re-converted into water, and the vapor from the heated liquid distils over with it.

This vapor condenses too, and forms a clear, limpid fluid which, being lighter than water, floats on the surface. The new liquid, when collected, is the volatile oil or spirits of turpentine. In the retort itself a yellowish, solid substance is left behind. This is the resin.

The crude or common turpentine, therefore, consists of resin dissolved in the oil of turpentine.

One more thought about the volatile oils before we leave the subject. The list of these oils includes, amongst others, the oils of lemon, cloves, nutmeg, lavender, bergamot, rosemary, and peppermint, in addition to the oil or spirit of turpentine. They are clear, limpid liquids, much lighter than water; they leave no greasy stain on paper; they are obtained (like the oil of turpentine) by distillation; and they are characterised by their powerful odor and taste.

They were at one time thought to contain the subtle essence of the plant from which they were derived; this explains why they were called essential oils.