6/4 RUBBER 



has the desired effect. The impression is held by some 

 planters and others that micro-organisms do not develop 

 on smoked rubber unless the latter has not been smoked 

 sufficiently; the idea is, however, quite erroneous, since 

 common mildews readily develop on smoked sheet if the 

 latter is kept subsequently in a damp place or wrapped in 

 certain paper, i.e., newspaper or common brown paper. 



Darkening due to Oxidation. Numerous experiments 

 by various investigators have proved beyond doubt that 

 the more or less rapid darkening which takes place in 

 Hevea latex, and more markedly in other latices, such as 

 that of Castilloa, on standing and in the rubber after 

 coagulation, both before and after washing, is due to 

 oxidation caused by an enzyme which may be classified 

 as an oxidase, and which is a normal constituent of the 

 latex. The darkening occurs to a marked extent on the 

 surface of the latex or coagulum, due to contact with 

 atmospheric oxygen. Some coagula after standing over- 

 night in the serum have quite a mauve-grey surface. 

 The oxidizable substances in the latex on which the 

 enzyme acts are probably phenolic in character and may 

 be in combination with the proteins. I have shown, by 

 the addition of certain phenols, such as ordinary phenol 

 (carbolic acid) and hydroquinone, to the fresh latex as it 

 exudes from the cuts on the tree, that excessive darken- 

 ing is caused by an increase of such substances in the 

 latex or in the sap from the cut tissues with which the 

 exuding latex comes into contact; the exact cause of such 

 increase at particular periods is unknown. 



The older remedies for inhibiting the oxidation in the 

 preparation of pale crepe were : (a) Immersion of the 

 freshly machined and washed rubber for a short period 

 in nearly boiling water; this destroys the enzyme and 

 prevents further darkening during the drying stage; 

 (b) coagulation of the latex with an excess of acid, pro- 

 ducing rapid coagulation, and immediate maceration and 

 washing of the coagulum; (c) steaming the latex; (of) in 

 the case of sheet, covering the coagulum in the coagu- 

 lating pans, shortly after coagulation is complete, with 

 boards and weights, so that the coagulum is forced 

 beneath the surface of the serum or residual liquid. 



