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Saps and Exudations. 



Hydrochloric Acid has at all times the same action as weak sulphur acid. 

 The pale yellow colour cannot be distinguished from that given by sulphuric acid. 

 Concentrated acid acts in the same way as a weak solution. 



Nitric Acid acts like hydrochloric acid, except that the colour of both black 

 water and the precipitate formed are deep orange. Concentrated nitric acid acts 

 in the same way on black water, but oxidises the latex forming a brittle orange 

 substance which becomes powdery when dry. 



Lime Juice acts like hydrochloric acid on the black water, but when in the 

 right proportion and under suitable conditions, it coagulates the latex. 



Sodium Hydroxide makes a very white latex, but turns the black water a 

 deep brownish red. Ammonia turns pure latex to a yellowish colour, which on the 

 surface when exposed to the air becomes green. Latex to which water has been 

 added turns greenish, and the black water becomes a very deep black on addition 

 of ammonia. Ammonia is a good substance to use in keeping latex from coagu- 

 lating. Latex may be kept pure for a long time without coagulating if ammonia is 

 in it. The actions are the same in concentrated and very weak ammonia. A 

 solution of ammonia, "001 per cent, will show these actions nearly as strongly as 

 concentrated ammonia. 



Sodium Carbonate acts practically in the same way as sodium hydroxide, 

 except that on one occasion it effected coagulation. Sodium Chloride has no 

 action on black water, but gives half-coagulation with latex. Calcium Chloride 

 causes no colour change in black water, but forms a dark-coloured precipitate. It 

 half -coagulates latex. Copper Sulphate, Zinc Chloride, and such salts act like weak 

 acid. Black water boils down to a black solid substance which might be found of 

 some commercial value, and could in that case be sold as a by-product. Sugar 

 has a slow action on black water which is not noticeable for two or three 

 days, but after that begins to take away the black colour and leave a pale 

 liquid. Black water treated with acid or other chemical substance and boiled 

 down, turns black just before dryness and forms apparently the same black 

 substance as is formed by pure black water, 



arguments regarding latex. 



What value latex is to the tree is still an open question. I think that it is 

 simply a protection against insects and evaporation whenever the tree is wounded. 

 Anything striking against the outside bark, if it hits hard enough, will bruise the 

 inner bark so that the latex flows. An examination of the place a day or two 

 later will show a thin coat of rubber entirely covering the bruise. Tropical 

 trees do not have the thick outer cork bark of nothern trees. Anything striking 

 them is liable to bruise the inner bark. This sheet of rubber would protect the 

 bruise from too much evaporation and from insect attacks. Leaf-cutter ants do not 

 attack the leaves of Castilloa and cattle do not seem to be fond of them, but I 

 believe this is not due to the latex but to the thick coat of epidermal hairs, a thing 

 which few tropical trees seem to possess. It is noticeable that ants do attack Hevea 

 which has not a hairy leaf. 



The study of the stucture of the latex shows that it has two distinct parts ; 

 watery solutions and solid substance in minute globules. The watery contain 

 no rubber. They contain the substance which forms the residue of the black 

 water, though this substance is apparently changed by oxidation before becoming 

 black water. They may aiso contain sugars and proteids, as these substances are 

 evidently there, but it is more than likely that these substauces are not in the 

 original latex, but come from some other bark tissue than the " milk tubes." 



17 



