July, 1910.1 



Saps and Exudations. 



formed in primary growth has a different 

 composition from that produced in the 

 secondary tissues by the cambium. The 

 difference does not seem to be wholly 

 due to a higher percentage of resin in 

 the latex from the young organs, though 

 in some cases this may occur. A sticky 

 weak caoutchouc may show on analysis 

 a chemical composition almost identical 

 with that of a standard product. 



The explanation evidently lies deeper, 

 and may be bound up in the mysteries 

 surrounding " tackiness " in rubber, 

 which is receiving attention from Dr. 

 Spence. Plantation rubber occasionally 

 arrives on the market in a sticky state, 

 though despatched from the tropics in 

 good condition. This change and great 

 deterioration have been laid to the 

 charge of bacteria. Spence shows 

 reasons for thinking that the alteration 

 is not directly due to bacterial growth 

 but rather to a change in the physical 

 nature of the rubber particles them- 

 selves, which ordinary chemical analysis 

 fails to reveal. There may be some 

 physical relationship between the weak 

 adhesive caoutchouc from young plant 

 organs, the so-called "tacky" rubber 

 and that rendered permanently sticky 

 by heat. 



Although the globules in the latex of 

 rubber-yielding plants 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 

 first suggested that the globules might 

 hold a mobile liquid. Weber 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 iso- 

 prene (Cs Hs), a hydrocarbon of the 

 terpene series. The latex globules may 

 contain a low polymer. Polyprenes 

 with double and four times the mole- 

 cular weight of isoprene are known and 

 are liquids. 



Darkening op Rubber and Oxidising 

 Ferments. 

 The darkening of latex, as it issues 

 from the tree, is a fairly frequent occur- 

 rence. The oxygen of the air gainiug 

 access to the latex, oxidises, with the 

 help of special ferments known as oxi- 

 dases, 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 j oung 

 shoots sometimes darkens, but not 

 always, whereas the blackening of that 

 from the capsule is without exception 

 in the writer's experience. 



The rubber samples prepared during 

 the Ceylon experiments of 1898-9 from 

 Hevea latex by the acetic acid process 

 have permanently retained their pale 

 colour. Much, however, of the planta- 

 tion 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 atten- 

 tion to the matter, and considers that 

 oxidases are really responsible for this 

 deep brown appearance. This explan- 

 ation is not without difficulties, when 

 the fact that the latex does not darken 

 on exposure to the air is taken into 

 consideration. Perhaps the action is slow 

 in taking place, and only at times are all 

 the substances necessary to cause darken- 

 ing present in the latex. 



The reason why no colour change was 

 observed in the early experiments may 

 be partly accounted for by the fact that 

 the samples were, as a rule, prepared by 

 the hot acid process ; any oxidase present 

 would naturally be destroyed by the 

 heat. The cold process, on the other 

 hand, is the one generally practised on 

 the estates. 



Considering that pale rubber (unknown 

 in the market before the advent cf plant- 

 ation) finds much favour with the 

 manufacturer, being advantageous for 

 certain purposes, the knowledge of the 

 conditions regulating its production is 

 of considerable importance. Bamber 

 recommends the plunging of the rubber 

 clots, just after their formation, into hot 

 water, at a temperature sufficiently high 

 to kill the oxidases. Such rubber should 

 then remain permanently pale in 

 colour. 



Dr. Spence has proved the presence of 

 oxidases in Para rubber and also in 

 Hevea latex itself. He writes: "These 

 observations prove conclusively that 

 the darkening in colour of raw rubber 

 is due to an oxidase which is associated 



