PARA-RUBBER CULTIVATION 601 



are often spoken of as composed of caoutchouc, it does not 

 necessarily follow that they are so. There is some evidence 

 for the view that the globules contain a liquid rather than a 

 solid and, that in the formation of rubber from the latex, this 

 liquid polymerises to true caoutchouc. Preyer 1 first suggested 

 that the globules might hold a mobile liquid. Weber 2 has 

 shown that they are soluble in ether, whereas the caoutchouc 

 prepared from them is not. 



Caoutchouc is generally regarded chemically as a high 

 polymer of isoprene (C 5 H 8 ), a hydrocarbon of the terpene series. 

 The latex globules may contain a low polymer. Polyprenes 

 with double and four times the molecular weight of isoprene 

 are known and are liquids. 



Darkening of Rubber and Oxidising Ferments 



The darkening of latex, as it issues from the tree, is a fairly 

 frequent occurrence. The oxygen of the air gaining access to 

 the latex, oxidises, with the help of special ferments known 

 as oxidases, certain substances occurring in solution, and from 

 them are produced dark-coloured bodies. Castilloa latex 

 markedly exhibits this peculiarity. If this latex dries naturally, 

 a dark brown, almost black, rubber is produced. As the deeply 

 coloured substance is in solution, the creaming or centrifugal 

 method permits the preparation of an almost colourless rubber. 



The latex of Hevea, such as comes from the trunk or 

 branches of the tree, does not darken on exposure to the air. 

 That, however, from the green wall of the unripe capsule 

 (fruit) changes rapidly from a white to a black colour. The 

 latex caused to exude from young shoots sometimes darkens, 

 but not always, whereas the blackening of that from the capsule 

 is without exception in the writer's experience. 3 



The rubber samples prepared during the Ceylon experi- 

 ments of 1898-9 from Hevea latex by the acetic acid process 

 have permanently retained their pale colour. Much, however, 

 of the plantation rubber made in this way appears on the 

 market quite dark in colour. A gradation of tints from pale 

 amber to black can be observed in the sale rooms. Kelway 

 Bamber has turned his attention to the matter, and considers 



1 Preyer, Beihefte zum Tropenpflanzer, 1900, 1, 29. 



2 Weber, India-Rubber Journal, 1903, vol. xxvi. 373. 



3 Parkin, Annals of Botany, 1900, 14, 199. 



