英文科學(xué)讀本 第五冊·Lesson 38 Vegetable Secretions—Camphor and Gums

Lesson 38 Vegetable Secretions—Camphor and Gums

Among the useful vegetable secretions, camphor undoubtedly holds an important position. Originally the juice of the plant which produces it, this substance, like others we have already met with, assumes the solid form during its preparation for use.

Camphor, as we see it, is a white, hard, tough, crystalline substance, with a very powerful aromatic odor, and a bitter, unpleasant taste. It is so volatile that a piece left exposed to the air would in time disappear altogether by evaporation. Very slight heat is required to change it from the solid form, but it can then scarcely be said to become a liquid, because it passes off at once as vapor.

It dissolves very slowly in water, so slowly that we might almost describe it as insoluble, were it not that it imparts some of its odor and taste to the water. It dissolves rapidly in alcohol, forming camphorated spirits or spirits of camphor.

Insects of all kinds dislike the strong, pungent smell of camphor. Little saucers of this substance are always kept, therefore, in cabinets of natural history specimens, to keep away insects. It is a good protection against moths, to put some camphor in the drawers where furs, blankets, and woollen clothes of all kinds are kept.

Camphor is used as a medicine, but it is a fallacy to think that it has the power of warding off infection. It is even a bad thing to wear camphor about the person, as it is weakening and lowering, both to the muscular and the nervous system.

A few drops of camphorated spirits taken on a lump of sugar, as soon as a chill is felt, will often save a person from catching cold. Camphor-ball, or camphor-ice, as it is sometimes called, is invaluable for chapped hands and faces, and also for burns and chilblains.

Camphor is obtained from a plant belonging to the laurel family. The camphor laurel grows chiefly in China and Japan, and there it is as large as an English oak. It has lately been introduced into several of the warmer countries of the world.

It is obtained in a curious way. The valuable secretion is stored in every part of the tree—root, stem, branches, and leaves. When, therefore, the tree is fully matured, it is uprooted, and every part of it is chopped up into small pieces.

These pieces are then put into iron retorts, with wooden lids or coverings perforated with holes, and above the lids are placed large dome-shaped hoods made of earthenware. The hood is filled with loose twigs, hay, or straw, and after all the crevices have been stopped, the retort is placed over a slow fire.

The heat causes the volatile camphor to rise in vapor, and as soon as the vapor touches the cool, rough surface of the hay and twigs, it condenses on them in flaky crystals, and is afterwards scraped off.

It is now a dirty brown color, and is known as crude camphor. It is purified by a second distilling process, somewhat similar to the first, and when it is again collected, it is the hard, white, crystalline substance which is familiar to us all.

Gums are another class of vegetable secretions, possessing their own special characteristics distinct from all others. Gum-like exudations may frequently be seen oozing through the cracks in the bark of trees, especially of many of our fruit trees. The juice hardens on exposure to the air, and then has the appearance of little, rounded, semi-transparent, glistening, solid drops. It is perfectly odorless, and has an insipid taste, but it dissolves in the mouth, and becomes very glutinous and sticky. This is the nature of all gums.

The most valuable of these secretions is Gum Arabic, which is obtained from a tree of the acacia tribe, that grows chiefly in Arabia, hence the name. It also grows in the East Indies, Abyssinia, and Egypt. It is an exudation from the stem, and hardens on exposure to the air.

It is very soluble in water, forming, when fully dissolved, a thick, glutinous solution, with strongly adhesive properties. It is on account of its adhesive and stiffening properties that this gum is so extensively used in the arts and manufactures. It makes a valuable cement for attaching labels to glass and other objects. It is largely used also in calico-printing, and in the manufacture of crape and other fabrics.

Gum Senegal is a similar though inferior secretion, and is much used in calico-printing.

The so-called gum, used for postage stamps, envelopes, and labels, as you already know, is not actually a gum, but is made from baked starch. Its proper name is dextrine or starch gum.