446 THE POPULAR SCIENCE MONTHLY 



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 C 5 H S , that is, it contains sixty parts by weight of car- 

 bon 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 solu- 

 tion 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 ex- 

 periment 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 tem- 

 perature through which it retains its properties of toughness and elas- 

 ticity is too limited, but by the addition of sulphur the range of tem- 

 perature 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 prop- 

 erties of vulcanized rubber vary witli 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 com- 

 bined 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 (approxi- 

 mately 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 

 fust three months of 1912, the Malay States produced 3.S10 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. 



