PARA RUBBER. 127 



There is a slightly higher percentage of proteids and resins in 

 Para rubber from young trees, and the poor physical properties of 

 young plantation rubber may be ultimately associated with the 

 proportion of these constituents present in the samples. 



Ash. — This impurity is presenl in almosl negligible quantities — 

 0T8 to 0"5percent. Generally, Para rubber contains 0*3 per cent, 

 of ash, as against 0*2 per c< at. in oilier rubbers. Welter is respon- 

 sible for the statement "thai it may yet be possible to chemically 

 identify the brand of indiarubber from ash analyses." Lime is 

 said to predominate' hi Para rubber, magnesia in (Vara, and ferrous 

 iron in African rubbers. The presence of the ash impurities is 

 undesirable on account of their tendency to interfuse with the india- 

 rubber and the resinous constituents during the processes of 

 manufacture. 



The insoluble constituent present in rubber is a substance which 

 is freefrom stickiness, is remarkably tough, and has moderate dis- 

 tensibility. Its nature and importance is imperfectly understood. 



There is a quantity of oxygen present in indiarubber, but the 

 proportion of this is. according to Weber, reduced practically 

 to vanishing point in successive purifying processes. 



General Properties of [ndiarubber. 



Alkalies have not a pronounced action upon indiarubber at low 

 temperatures. Heinzerling states that on prolonged digestion with 

 ammonia the indiarubber passes into the state of an emulsion, 

 in appearance closely resembling indiarubber milk. 



The effect of chlorine, bromine, and iodine on indiarubber is 

 very complicated, and for a full knowledge of the various changes 

 which are induced by their action reference must lie made to Weber 

 (pp. 31-37 ). Acids exert a strong action on indiarubber articles 

 commonly used. Strong sulphuric acid oxidises rubber; strong 

 nitric acid attacks rubber vigorously, forming at first a yellow com- 

 pound which is subsequently decomposed. The effect of oxygen on 

 crude and vulcanized rubber is to cause deterioration, a compound 

 known as Spiller's resin being formed. Crude indiarubber, 

 particularly just after it Leaves the washing machine for the drying 

 room, is apt to sutler considerably from oxidation during the drying 

 process, and it seems possible that similar changes may occur after 

 coagulation and pressing in the tropics. 



Though indiarubber is insoluble in water, it rapidly swells when 

 immersed in it and absorbs a considerable amount of the liquid, the 

 actual amount capable of being absorbed increasing with a decrease 

 in the resin and oily substances. On this account the rubber from 

 young trees may perhaps be roughly detected by the water capacity 

 of the sample of rubber, allowing for normal variations. When 

 vulcanized the water absorption power of indiarubber is small. 

 Though indiarubber docs not readily react with many common 

 reagents, it does react in a surprising degree with sulphur in its 

 various forms, the process of combination being commonly spoken 



