RUBBER: BOTANY, CULTIVATION AND CHEM ISTRY. During his second visit to South America, Columbus was astonished to see the native Indians amusing themselves with a black, heavy ball made from a vegetable gum. Later explorers were equally impressed by these balls, and an historian of the time remarked that they rebounded so much that they appeared alive. Three centuries elapsed before the material was brought into commercial use in Europe, and it was then marketed not for - its elastic properties but to rub out lead pencil marks—hence the name indiarubber or rubber. Since then the principal constitu ent (caoutchouc) of this material has been found as a vegetable product in many parts of the world, sometimes mixed with as little as one twentieth and sometimes with eight times its own weight of other substances. Some of these mixtures are strong and elastic, others are weak and brittle, but they are all classified as rubber. In addition, the term has been broadened by common use to include a wide range of vulcanised products derived from rubber by heating with sulphur, generally after mixing with large propor tions of powders and plastic solids.
Although raw rubber is a coherent, elastic solid, it is obtained from a milky liquid known as latex, which occurs in special tubes in the roots, stem, branches, leaves and fruit of a wide variety of trees growing for the most part in the tropics. Rubber latex con sists of a watery solution (serum), in which float small globules of rubber visible under a microscope. Although neither the rub ber nor the serum is definitely opaque in thin layers, the many reflecting surfaces presented by these globules cause the latex to have the appearance of cows' milk, but whereas cows' milk con tains only about 12% solid matter, rubber milk contains 3o-4o%. When rubber milk is suitably treated the globules unite (coagu late) and float in the serum as a soft, doughy mass (coagulum), which can be easily rolled to a sheet or other convenient form. On drying the coagulum loses its doughy character and becomes the firm, elastic solid known as raw or crude rubber.
Certain tropical African plants yield rubber of good quality, but the methods of preparation employed by the natives are crude and tend to degrade the product. The most important are Funtu mia elastica, a tall and stately tree, and various species of Lan dolphia, which are big woody climbers.
A tree which is well known because it is cultivated in Europe as an ornamental plant under the name of the indiarubber tree is Ficus elastica, indigenous to Assam and Burma, where it grows to a considerable size. It yields moderately resinous rubber.
Guayule rubber from Parthenium argentatum—a silvery- leafed shrub found in north Mexico and is also moderately res inous.
The tropical American rubbers include Ceara rubber from Manihot Glaziovii, a tree of moderate size growing in Brazil, and Caucho rubber from Castilloa elastica, a large tree found in Cen tral America and portions of Brazil. In both cases the rubber is of fairly good quality. The purest and best, however, is un doubtedly the Para rubber obtained chiefly from Hevea brasili ensis and to a lesser extent from Hevea Benthamiana. Both these trees are found in the Amazon valley, the former around the southern and the latter around the northern tributaries. Hevea Benthamiana is not so widely distributed as Hevea brasiliensis, but both yield rubber which is classed as "hard fine para"—the highest grade on the market.