72 THE PREPARATION OF RUBBER 



In the Amazon vaUey the natives coagulate the latex 

 of the Para tree by exposing it in thin layers to the smoke 

 given ofif by bm-ning palm-nuts. The coagulation of the 

 latex is probably effected partly by the heat and partly 

 by the action of the acetic acid present in the smoke (see 

 below). 



The natives of some parts of Africa obtain a similar 

 result by spreading Landolphia latex over their arms and 

 chest, when it quickly coagulates, forming a film of rubber 

 which is then stripped off. 



(6) Action of Reagents. — The coagulation of latex can 

 be brought about by a large number of chemical 

 agents, and it will only be possible to notice here the 

 methods which are in common use for the preparation 

 of rubber. 



Acids. — Many latices coagulate almost immediately 

 on the addition of small quantities of mineral or organic 

 acids. The spontaneous coagulation which frequently 

 occurs when latex is allowed to stand is often due to the 

 acidity which develops in the latex. Organic acids are 

 generally preferable to mineral acids for the preparation 

 of rubber, and acetic, citric, and tannic acids are chiefly 

 used for the purpose. The latex obtained from cultivated 

 Para trees in the East is coagulated almost entirely by 

 the addition of small quantities of acetic acid ; formic 

 acid and hydrofluoric acid (purub) are also used. In 

 Africa many Landolphia latices are coagulated by means 

 of the acid juices of fruits or by acid infusions made from 

 species of Costus, Hibiscus, and Tamarindus. The latex 

 of Funtumia elastica is not affected by the addition of 

 acetic or citric acid, but it may be quickly and conveniently 

 coagulated by means of a hot solution of tannic acid. A 

 hot infusion of the leaves of BauJiinia reticulata, which 

 contain tannic acid (tannin), is employed in parts of 

 West Africa as a coagulant for Funtumia latex. 



Salts. — Many salts, such as sodium chloride (common 

 salt), alum, cream of tartar, mercuric chloride (corrosive 

 sublimate), sodium sulphate, and magnesium sulphate, 

 effect the coagulation of latex. A solution of salt is used 

 by the natives in Africa to coagulate the latex of species 

 of Landolphia ; alum is commonly adopted in South 

 America for tiie preparation of the rubber of Hancornia 

 speciosa ; cream of tartar has been used forj^ preparing 



