602 SCIENCE PROGRESS 



that oxidases are really responsible for this deep brown 

 appearance. This explanation 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 darkening 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 of plantation) finds much favour with the 

 manufacturers, 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 

 with the protein or the so-called insoluble constituent of the 

 rubber." x 



Why, it may be asked, does not Hevea latex itself darken 

 on exposure to the air, and why, further, do some samples 

 of rubber prepared from it darken and others remain light 

 in colour ? 



Procedure on Estates 



The "biscuit" was the earliest form of plantation rubber 

 to appear on the market. The diluted latex, after careful 

 straining to free it from all foreign particles, and after the 

 addition of the necessary amount of acetic acid, is poured into 

 flat circular dishes. Coagulation gradually sets in, and after 

 twenty-four hours the caoutchouc from each vessel is capable 

 of being removed in the form of a spongy clot. This, on being 

 passed through a mangle, is compressed into a thin circular 



1 Spence, Biochemical Journal, 1908, 3, 179. 



