Page:Popular Science Monthly Volume 85.djvu/453

Rh, the increased flow when the second incision is made near the first is because latex has flowed to the wound in order to repair it.

Though Hevea seems to be in general the best rubber-producing tree, there is a little doubt whether it should be everywhere introduced; for instance, in Africa, where another species is native. African labor is less intelligent than that in the Malay States and the African natives can not tap the trees so successfully. The native trees and vines are usually cut down.

Moreover, experiments should be made with plantations of from three to five thousand trees before a decisive judgment is given, for it is possible that in large plantations diseases might rise and spread which have not appeared in small plantations. In very large estates, protective belts of other trees either of the original forest or of another genus of planted rubber should be made use of to prevent spread of diseases.

While, as stated above, in the Amazon valley five pounds of latex containing approximately two pounds of caoutchouc is considered a large yield, on the plantations trees ten years old are expected to yield three or four pounds of rubber. During 1908 nine thousand trees in the Cicely estate, one of the older Malay companies, gave an average of six pounds per tree, though the trees were between five and ten years old. In the Perah State there were eight trees seventeen years old, of an average girth of 55 inches, which yielded 28 pounds of rubber each. From the economic point of view the yield per acre is of more importance than the yield per tree. Six-year-old trees will yield about a hundred pounds per acre, while ten-year-old trees will yield three or four times as much.

In the East, rubber is coagulated from the latex by acetic acid. Smaller quantities of other acids would serve the same purpose, but an excess prevents coagulation, while with acetic acid the quantity may vary within fairly wide limits. When coagulation is brought about by acetic acid either pains must be taken to dry the rubber very thoroughly or some antiseptic must be put in. The method of smoking carried out in the Amazon district provides both acetic acid and the antiseptic fumes of creosote. Coagulation could be brought about by simple drying, but in this case the rubber is apt to become moldy and putrid. The precise cause of coagulation by acid is not certain. It has been ascribed to the small amount of protein in the latex, but, on the other hand, it is claimed that if the protein is removed the rubber can still be coagulated. The rubber produced has a composition something like the following: 94 per cent, caoutchouc, 3 per cent, resin, 2.5 per cent, protein, and 0.5 per cent, each of moisture and ash. One should perhaps add that what is usually called protein may not really be that substance, but some other which contains nitrogen.

The competition between Amazon hard Para rubber, which is so far the standard, and plantation rubber is keen. The latter is the purer, but