Or Gum Elastic, is a product of the milky juices of several tropical and sub-tropical plants found in the West Indies, Central and South America, West Africa, and India; there is evidence that its properties were partially known to the Spaniards in the West Indies early in the 17th century; but its first introduction to this country was about 1770, when it was employed by artists for erasing black-lead pencil marks, hence its familiar name; it is collected by making incisions in the tree trunk and gathering the slowly exuding juice, which is first solidified by drying, then purified by boiling and washing; it is flexible and elastic, insoluble in water, and impenetrable to gases and fluids, and these qualities give it great commercial importance; the use of pure rubber has been greatly superseded by that of "vulcanised" rubber; mixed with from 1/40 to ½ of its weight of sulphur and combined by heat, the rubber acquires greater elasticity, is not hardened by cold or rendered viscid by heat, and is insoluble in many of the solvents of pure rubber; its usefulness is thus largely increased and greatly extended of late; the demand for rubber is in excess of the supply, but no substitute has been found effective; in recent years care has been bestowed on its economical collection and on its scientific culture.