PARA RUBBER. Ill 



Tannic acid. — This is, according to Weber, the most powerful of 

 the acids which can be used for this process, and he asserts that on 

 a laboratory scale it is excellent for use with the latex of Para rubber. 

 If rubber coagulated by tannic acid, while still wet, be placed in an 

 incubator at temperatures from 100° P. upwards, it rapidly passes 

 into a state of putrescid fermentation, but such a change does not 

 occur if the rubber is thoroughly dry. 



Mercuric choloridc. — Corrosive sublimate effectively coagulates 

 the proteid while the latex is cold, and also acts as an antiseptic. 

 It is very poisonous, and if used a small quantity of mercury is un- 

 avoidably left in the rubber. 



Amount of Acid to be used. 



The quantity of acetic or formic acid required depends upon the 

 proportion and condition of the albumen in the latex. According 

 to Weber the latex of Para rubber in its native habitat contains 

 only about 1*5 per cent, of albumen, and one-third of an ounce of 

 anhydrous formic or half an ounce of glacial acetic acid per gallon of 

 the latex is quite sufficient to produce a rapid and complete coagula- 

 tion. The behaviour of the latex from Para rubber trees with acids is 

 due to the fact that the milk is, when fresh, usually slightly alkaline or 

 neutral, and the proteid substances are insoluble in a feebly acid solu- 

 tion but soluble in alkaline or strongly acid solutions. It has been 

 asserted that the proteid matter is insoluble in a neutral solution, but 

 on several occasions the fresh latex from the Henaratgoda trees 

 remained liquid, though the reaction with litmus paper did not 

 indicate acidity or alkalinity. Only a small quantity of acid is 

 required to neutralize or acidify the latex, and therefore lead to 

 the precipitation of the proteids. It is a mistake to add excess 

 of acetic acid, as the proteids Mould be partly re-dissolved and, 

 therefore, still remain in solution. 



The amount of pure acetic acid necessary for complete coagulation 

 depends upon the quantity of proteids to be precipitated, and the 

 latex in Ceylon, according to the analyses already quoted, contains 

 from 2*3 to 28 per cent, of these substances. If ordinary latex is 

 allowed to stand for some time, the proteid matter decomposes and 

 acidity sufficient to lead to coagulation is developed. Diluting the 

 latex will not reduce the total quantity of acid required. Every 

 100 volumes of pure Ceylon latex require approximately one volume 

 of pure acetic acid. Many planters add one or tw r o drops of acetic- 

 acid to about half a gallon of the diluted latex. If the acetic acid 

 is added until the mixture becomes neutral after stirring — i.e., will 

 neither turn litmus paper red nor blue, or until it is feebly acid — very 

 little harm will be done. The addition of excess of acid may bring 

 about a re -solution of the proteids and coagulation be there by 

 delayed. It is very rare that the latex on a large scale is heated 

 before treatment with acetic acid. 



The completeness of the precipitation is judged by the clearness 

 or turbidity of the liquid in which the rubber floats. When the 

 separation of caoutchouc is complete, the mother liquor is quite clear. 



