Page:Popular Science Monthly Volume 50.djvu/707

Rh grades are dark and mixed with impurities. The former are tenacious, the latter often very friable. Macassar and Sarawak, the finest grades of gutta, are light-brown verging on yellow, while Balata is rose-brown.

On being refined and drawn thin, gutta-percha is translucent; drawn very thin, it is transparent; but placed on a white surface, it is rose or gray. At ordinary temperatures it is supple, flexible, and very tenacious and extensible, so that it may be drawn to three times its length, when it retains almost all of the extension. If a dent be made with the finger nail, a trace will remain. It softens above 50° C. and becomes adhesive above 100° C. Two pieces may be permanently joined by applying a hot iron to the surfaces and using light pressure. It is a bad conductor of heat and electricity, but may be electrified by friction; so it is sometimes employed for the disks of electric machines. Exposed to the air, it undergoes, at length, a great change, losing its fibrous structure and becoming granular and friable; more quickly in hot countries. This is a result of the action of oxygen under the influence of light. It is insoluble in water, softens in boiling water and steam; resists alkalies, hydrofluoric acid (being used for a receptacle for this acid), and ordinary dilute acids, but, when strong, they attack it.

The property of gutta-percha of greatest value to mankind is its dielectric or nonconducting property. This is not lessened by atmospheric conditions, nor is it destroyed by plunging under water or burying in the ground or subjection to other deteriorating influences. Hence its fitness for cables, telephone wires, etc. Its power of insulation decreases as the temperature increases. Sea water is a medium in which gutta-percha undergoes no alteration, and the enormous pressure at great depths exerts a favorable action on it by closing up accidental splits. Wrappings are put upon electric cables to protect the insulation from abrasion and the attacks of marine animals, as well as to strengthen the cable. Gutta insulation is preferred for telegraphy, telephony, bell service, etc.; rubber insulation for lighting and power, as intense currents are liable to lead to accidents by fusion of gutta insulation. What the world owes to gutta-percha may be most easily illustrated by more than one hundred and thirty-nine thousand miles of ocean cables, not to speak of the myriads of wires on land, under it, and in buildings for every conceivable purpose.

As soon as this valuable substance became known, industrial enterprise at once sought to make use of it. From the year 1844 the new product received numerous applications and gave rise to many patents. It was used for making stoppers, glues, and wires; then shoes, surgical instruments, and clothing. The most fortunate application was as coating for telegraph cables (patents to