Page:Popular Science Monthly Volume 85.djvu/450

446 small pieces, steeped in warm water and run through washing rolls, after which it is dried. Rubber thus obtained is mainly a hydrocarbon of the empirical formula C5H8, that is, it contains sixty parts by weight of carbon to eight of hydrogen. There is a small amount of resin, a very little protein and somewhat less than one per cent, of inorganic matter which forms an ash when the rubber is burned.

Freshly coagulated rubber has a spongy or reticular structure, due to the way in which the particles come together. This shows even in dried rubber and the particles can still be seen in globular form after solution, but films made by the evaporation of the solvent from the solution have apparently lost the reticular character. Rubber on being heated becomes sticky and if cooled to near the freezing point of water (about 40° Fahr.) it becomes hard and loses its elasticity. Stretched rubber has the very peculiar property of contracting on being heated. This curious property was predicted from theoretical considerations and was later confirmed by experiment. A suitable way to carry out the experiment is to stretch a rubber tube to nearly double its length by means of a heavy weight and then to pass steam through the tube. A tube a couple of feet in length will under these circumstances contract several inches.

Pine rubber softens too readily with rise of temperature and hardens before the temperature has fallen much below normal; the range of temperature through which it retains its properties of toughness and elasticity is too limited, but by the addition of sulphur the range of temperature can be very much extended. Rubber can be made to take up sulphur in various ways, the process being called "vulcanization." One method is by heating with sulphur, another is by treatment in the cold with a mixture of chloride of sulphur and carbon bisulphide. The properties of vulcanized rubber vary with the amount of sulphur, soft rubbers contain 3-4 per cent., while hard rubber, or ebonite, contains 20-30 per cent. The sulphur seems to be combined in some form, at least partially with the rubber. No matter how much sulphur may be mixed with the rubber or what the temperature or length of time, the maximum of combined sulphur is about thirty-two per cent.

The main source of rubber supply, almost up to the present, has been the wild-growing trees and vines. In 1006 about 400 tons (approximately one per cent, of the whole) were obtained from plantations, by 1909-10 the amount had risen to about five per cent.: now plantation rubber has almost overtaken that derived from uncultivated plants. Java produced 73 tons in 1910 and 491 tons in 1911, while, during the first three months of 1912, the Malay States produced 3,810 tons, and during the corresponding three months of 1913 the amount was 5,625 tons, or over a half more. The rapid increase is due to the fact that each year more and more of the trees are reaching the productive age.