HARMFUL CONSTITUENTS OF LATEX. 71 



which the latex and its constituents may vary under different condi- 

 tions of growth. Parkin is probably in error in the idea that the latex 

 with which he experimented belonged to Cast ilia ma/rk7iamiana. The 

 tree which was introduced by Cross from Panama to Ceylon is more 

 likely to be the same as that with which Weber experimented in 

 Colombia. 



DISCOLORATION OF CASTILLA LATEX. 



Incidental to his principal discovery Dr. Weber reports several 

 observations of much interest, not alone in their practical significance, 

 but also as illustrations of the mistakes which can be made in a subject 

 so difficult of investigation as rubber. Thus it is found that the milk 

 of Castilla contains not a trace of tannic acid, the presence of which 

 has often been inferred, presumably because ferric chlorid produces 

 the same color reaction with latex as with tannic acid, turning it dark 

 green. This reaction Dr. AYeber finds to be due to the presence of a 

 glucoside, which also gives the latex its intensely bitter taste. The 

 addition of tannic acid precipitates the albumens of the latex, so that 

 the presence of albumens is itself deemed a sufficient evidence of the 

 absence of tannic acid in latex of any kind. 



The rapid color change of the milk of Castilla on exposure to the air 

 is found to be due to an enzym or oxydizing ferment (oxydase), which 

 is probably destroyed by the boiling water, as suggested by Parkin, to 

 whose work Dr. Weber does not refer, although in this part of the 

 subject it had covered the same ground. 



Parkin reported as follows: 



Several latices, which are pure white when they first issue from a wound on the 

 plant, rapidly darken on exposure to the air. This is due to the presence of an 

 oxydizing ferment, or oxydase, which with the aid of the oxygen of the air acts on 

 some constituent of the latex, changing it to a deep brown coloring matter. 



The latex of Castilloa is a good example. It rapidly darkens on exposure and 

 dries to an almost black rubber. By creaming the caoutchouc particles can be sepa- 

 rated from the dark beer-like liquid and made into a sheet of nearly colorless rubber. 

 By quickly heating the collected latex the darkening is arrested, owing to the 

 destruction of the enzyme. 



The latex of Hevea collected from the tree trunk does not darken at all on exposure 

 to the air, and, provided that molds and putrefactive organisms are kept away, 

 rubber prepared from it remains indefinitely of a light color. On the other hand, 

 the latex from the wall of the unripe capsule (fruit) changes on exposure from milk- 

 white to 1 ilack. The darkening is wholly prevented if the latex is quickly subjected 

 to heat. No doubt there is an oxydase present in the latex of the capsule." 



The expression ** coagulation of rubber"' appears objectionable to 

 Dr. Weber because he finds that it is the albuminous substances of 

 the latex which coagulate and not the rubber itself, but this objection 

 seems rather overtechnical, since, even in Dr. Weber's method, the 

 rubber is collected and compacted, and for this process a name is still 



a Parkin, Annals of Botany, 14:199-200, 1900. 



