Part 76. Elastic Gums Or Rubber. Continued

The discovery of vulcanization revolutionized the rubber industry. Not only were the old uses greatly extended but new uses for rubber have so multiplied that caoutchouc now ranks among the most important products of the vegetable kingdom. The elasticity of soft vulcanized rubber makes it invaluable in various articles of dress, for many surgical purposes, for elastic bands, solid or pneumatic tires, for various parts of machines, and for rubber balls, toys, and innumerable other articles of minor use. Its imperviousness to water and air, combined with its flexibility, render it of greatest service for waterproof garments or coverings, submarine diving-dresses, flexible tubes or hose, water-bottles, air-cushions, life-preservers, portable boats, etc. Hard rubber takes a high polish and is very resistant to the action of acids and other corrosive fluids. Therefore it makes the best possible material for photographer's developing trays, certain parts of fountains pens, telephones, surgical instruments, etc., while it is a most excellent and inexpensive substitute for horn or shell in such articles as combs and handles. Both vulcanite and the softer vulcanized rubber are extensively used for insulation in electric work. Pure rubber on account of its remarkable adhesiveness is an indispensable part of the best surgeon's plaster, and of the rubber tape used in repairing bicycle tires and in electric wiring. The curious erasing power of rubber, whether pure or vulcanized, is possessed by no other substance to anything like the same degree; hence one of its earliest uses still remains one of the commonest and most important.

Caoutchouc as a raw material bears, as we have seen, somewhat the same relation to the milky juice of plants that cheese bears to the milk of animals. That is to say, it separates from the fluid part as curd from whey, and becomes solid by drying. Chemically, however, caoutchouc is quite different from the proteid of which cheese mainly consists. Pure caoutchouc is a hydrocarbon; in other words, it contains only hydrogen and carbon in its composition. Commonly associated with it are various substances, regarded as impurities, among which are certain resins. These resins are believed to be derived from the caoutchouc through oxidation since they vary considerably in amount and differ chemically from the hydrocarbon merely by containing oxygen. Such compounds are appropriately called oxidized hydrocarbons, and are distinguished from carbohydrates by the fact that the oxygen and hydrogen they contain are not in the proportion of H₂0.

The distinctive characteristic of rubber is its extreme elasticity. A curious result of this is the heat developed when a piece of it is stretched. Thus a sudden warmth is perceptible when a rubber band is quickly stretched in contact with the lip. On account of this property means have to be taken in the manufacture of rubber to prevent overheating when large masses are vigorously worked.

Gutta-percha differs from india-rubber in being very firm and comparatively inelastic at ordinary temperatures, though at about 50°C. it becomes highly elastic and plastic. It resembles caoutchouc in flexibility, toughness, poor conductivity of heat and electricity, imperviousness to moisture, insolubility in dilute acids and in alcohol, and solubility in oil of turpentine, chloroform, naphtha, carbon bisulphid, etc. Unlike caoutchouc, however, gutta-percha is unaffected by fixed oils.

In chemical composition gutta-percha consists like caoutchouc, of a hydrocarbon similarly associated with resinous substances presumably derived from it by oxidation. Unless well purified soon after being collected the change into resin may go so far as to make the whole mass worthless.

Gutta-percha is obtained from several different species of trees all closely related to the taban-tree (Fig. 272) which was the original source. All are confined to the region of Sumatra and Borneo. Owing to the foolish practice of felling the trees to obtain the milky juice, what was for many years the main source of supply is now destroyed. More conservative methods of tapping, similar to those already described for caoutchouc-milk, give a continuous yield for many years. It has been also found that gutta-percha of the finest quality may be extracted from the leaves by using solvents. In separating the solid from the liquid part of the milk obtained by tapping no special means are necessary. A hard "curd" soon forms. After removal of the worst impurities (sometimes facilitated by boiling) the raw material is pressed into cakes or lumps and is then ready for export.

Fig. 272.-Taban-tree (Palaquium Gutta, Sapodilla Family, Sapotacece). A, flowering twig. B, young fruit. C, flower. D, ripe fruit. E, F, seed. (Burck.)-Tree 13 m. tall; leaves rusty-hairy beneath; flowers white; fruit fleshy. Native home, Malaysia.

The general use of gutta-percha dates only from about the middle of the 19th century. It was first brought prominently into notice by Dr. W. Montgomerie, an English surgeon stationed at Singapore. He found the natives using this extraordinary material for ax handles, sword hilts, and the like. This suggested to him important uses for it in surgery and various industrial arts; and, thanks to him, manufacturers soon came to realize that it was better adapted for certain purposes than any other substance.

Among the more important or familiar applications of gutta-percha may be mentioned its use as waterproof material in boot-soles, and as cement for leather, etc., its use for piping, for speaking tubes, various surgical appliances, golf balls, and molded ornaments. Its most important use is as insulating material for electric wires, especially cables. The great Atlantic cables and other submarine or subterranean electric lines, upon which modern civilization so much depends, owe their successful operation largely to the gutta-percha used to cover the wires and so protect them and at the same time prevent serious leakage of electricity.