Most vegetable juices exposed to the air harden into firm substances, commonly called gum. Some of these dissolve, or at least soften, in water; these technically are known as "gums," and usually are so designated in commerce. Others are insoluble in water, but dissolve readily in alcohol, in naphtha, in turpentine, or in other essential oils; these are designated as "gum-resins." Still others yield oils or pitchy substances on distillation; these are known as "oleo-resins." There are many other dried vegetable juices, however, that in commerce are not classified among the gums and resins, and of these the most important is the substance commonly known as india-rubber.

Rubber and Rubber Products.—"Caoutchouc" is approximately the name given by Indians of the Amazon forests to a substance that had also been found in India. Some of it was brought to Europe from the Amazon region as early as 1736, and for nearly one hundred years no general purpose was discovered for which it could be used, except to erase lead-pencil marks—hence the name india-rubber, which has held ever since.

Common rubber is the prepared juice of a dozen or more shrubs and trees, all of which grow in tropical regions.[37] The belt of rubber-producing plants extends around the world and includes such well-known species as the fig, the manihot (or manioc), and the oleander; indeed, it is a condition of sap rather than a definite species of plant that produces rubber, and the latter is a manufactured rather than a natural product. The process of preparing the juice is practically the same in every part of the world.

The rubber-gatherer of the Amazon, who is practically a slave, wades into the swamp, makes several incisions in the bark of the tree, fashions a rough trough of clay under it, and waits till the sap fills the clay vessel. When the sap has been gathered he makes a fire of the nuts of the urucuri palm and places an inverted funnel over it to concentrate the smoke. He first dips the end of a wooden spindle into the juice and then holds it in the smoke until the juice coagulates; this process is repeated until there has formed a ball of rubber weighing from five to ten pounds. The smoke of the palm-nuts is a chemical agent that converts the juice into the crude rubber of commerce.

Crude gum, however, is lacking both in strength and elasticity. The process that makes it a finished product is known as vulcanization. The crude rubber, having been exported to the manufacturer in the United States or Europe, is shredded, washed, and cleansed, and partly fused with varying proportions of sulphur. For a very soft product, such as the inner surface of tires, only a small proportion is used; where the wear is considerable, a larger proportion is employed.[38] White clay is sometimes added to give body to the product; coloring matter is also sometimes added.

By far the greater part of the crude rubber comes from the Amazon forests. Brazil produces about one-half, but a considerable quantity is obtained in AcrÉ, the territory formed where the borders of Brazil, Bolivia, and Peru meet, and now ceded to Brazil. Nearly all this product, that of the CearÁ region excepted, is marketed at ParÁ and is known as ParÁ rubber. It is the best produced. The African product, mainly from the forests of the Kongo, and Madagascar, and nearly all the East Indian product is sent to Europe.

The world's product is about one hundred and thirty-three million pounds of crude rubber. Of this product the United States takes nearly one-half. The greater part is used in the manufacture of pneumatic tires, hose, and overshoes. A large part is used for making water-proof cloth,[39] and considerable is made into the small elastic bands for which there is a growing use.

Gutta-Percha.—Gutta-percha is obtained from the juices of several plants (chiefly Dichopsis gutta and Supota mÜlleri) both of which abound in the Malay peninsula and the East Indies. It is prepared in a manner somewhat similar to that employed in making crude rubber; it is also easily vulcanized by heating with sulphur. It is used to a limited extent in the manufacture of golf-balls, but mainly as the insulating cover of copper wires used in ocean telegraph cables. For this purpose it has no known substitute, and its essential merit is the fact that it is not altered by salt water. Nearly all the product is shipped from Singapore to England.

Pine-Tree Products.—The various members of the pine and cone-bearing trees yield valuable essential oils and oleo-resins that are very important in the arts and sciences. These, in nearly every instance, are prepared from the sap of the tree.

Oil of turpentine is known as an "essential oil," and in chemical structure and properties it does not differ from the various essential oils, such as lemon, orange, peppermint, etc. Commercial turpentine is generally made from the sap of the long-leafed pine of the Atlantic coast-plain.

The bark of the tree is cut near the foot, and the sap that oozes from the scar quickly hardens into a gum. The gum, generally known as "crude turpentine," is distilled and yields about one-fourth its weight of oil or "spirit" of turpentine. It is a staple article of manufacture in Europe, India, and the United States, and is used chiefly to dilute the oil paints and varnishes used in indoor work. The United States supplies about two-thirds of the world's product, a large part of which is shipped from Savannah and Brunswick, Ga., to Great Britain.[40]

Resin is the substance remaining after the crude turpentine has been distilled. It is used in the manufacture of varnish, sealing-wax, and soap. Finely powdered resin is also mixed with wood-pulp in the manufacture of wrapping-paper. It gives the latter a glazed surface and renders it almost water-proof. Most of the world's product of resin comes from the turpentine district of the United States, and about four-fifths of it is exported to Europe.

When resin is subjected to distillation at a still higher temperature, resin oil, a very heavy turpentine, is given off, and a viscous substance known as pitch remains. A considerable amount of this is still made in the United States, but the greater part comes from the pine-forests of Russia and Scandinavia. When pine-wood is distilled, tar is the chief product. In Russia tar is generally made by burning green logs covered with turf, over a pit. Creosote, or wood preservative, is made from tar. The various pine-tree products, creosote excepted, are commonly known as "naval stores," the tar being used in water-proofing the rigging of vessels, the pitch in calking the seams in between planks, in the decks and hulls.

Other Resins and Gums Used in the Arts.—Most of the gums and resins used in the arts and sciences are the hardened sap of plants—in some cases exuding by natural means from the bark, in others resulting from the puncture of the bark.

The lac of commerce is due to the puncture of the young branches of a tree, frequently a fig (Ficus religiosa) growing in the tropical forests of India. The hardened sap incrusts twigs forming stick-lac; when crushed, washed, and freed from the woody matter it is seed-lac; when melted and cooled in flakes it is shell-lac, the form best known in commerce. It is the chief ingredient in sealing-wax, and is extensively used as a varnish. It is also used in fireworks on account of its inflammability.

Dammar is the product of a tree growing in the East Indies; it is the basis of a very fine white varnish. Copal is a term applied to oleo-resins soluble in turpentine, and used almost universally as varnishes. They come from the tropical regions of South America, Africa, and from the East Indies. Kauri is the fossil gum of a cone-bearing tree dug from the ground in northern New Zealand. Amber is the fossil gum of extinct cone-bearing trees found mainly along the Baltic coast of Prussia. It is used chiefly for the mouth-pieces of tobacco-pipes and cigar-holders; the inferior product is made into varnish. It is sold wherever tobacco is used. Sandarach, found on the north African coast, is used principally in Europe, being employed as a varnish. The United States and Great Britain consume most of the foregoing products.

QUESTIONS FOR DISCUSSION

FOR STUDY AND REFERENCE