360 PLANTS AND MAN 



was applied by the British chemist Priestley, in 1770, after he 

 had found it useful in rubbing out, or erasing, lead pencil marks. 

 Although rubber articles, and clothing waterproofed with rubber 

 solutions, were used somewhat, it was the accidental discovery 

 of rubber vulcanization by the American, Goodyear, that led 

 to the widespread usefulness of rubber today. Until the stabiliza- 

 tion of rubber by vulcanization, or heating in the presence of 

 sulphur, was developed, rubber was stiff and brittle when cold, 

 soft and sticky when warmed. Vulcanized rubber is not as subject 

 to changes with variations in temperature or moisture. Today 

 widely different types of rubber articles are manufactured, their 

 hardness or softness predetermined by the amount of sulfur 

 added and the degree of heat to which the compound is sub- 

 jected. The United States is the world's leading rubber user, 

 importing about 70% of the annual rubber production to supply 

 our rubber industry. The manufacture of automobile tires and 

 tubes, the last phase of the rubber industry to develop, uses 

 almost nine-tenths of the crude rubber imported by the United 

 States. Other uses include the manufacture of rubber boots and 

 shoes, druggists sundries such as hot water bottles, rubber gloves, 

 etc., electrical goods, hard rubber goods, and other uses too 

 numerous to mention. 



The Brazilian Rubber Tree 



Of the multitude of rubber-yielding plant species common to 

 the hot, damp forests of South America and Africa, one — the 

 Brazilian or Para rubber tree — stands out far beyond all 

 others in importance. It is a member of the Spurge Family, 

 which also includes Ceara and Intisy rubber plants; but the 

 Para rubber tree produces about 95% of the world's rubber 

 supply today. In addition to being the chief wild source in its 

 South American jungle home, it is the most common tree used 

 for planting in the extensive rubber plantations of the British 

 and Dutch East Indies. In South America the tree attains heights 

 of over one hundred feet with diameters of three feet, living to 

 an age of two hundred years. The small, inconspicuous flowers 

 are in marked contrast to the giant, three-parted leaves (fig. 237). 



