134 THE RUBBER TREE BOOK 



lates chemical action, from which discoloration is certain to 

 follow. 



Coagulation is usually effected by means of the addition 

 of acetic acid. The general amount of acetic acid added to 

 the latex is, in most cases, three to four drops of the glacial 

 acid to the gallon of latex. Most planters dilute the acetic 

 acid with water, for greater convenience of measurement, and 

 have marked glasses, which are filled by the coolies up to the 

 point marked out of the glass jars containing the diluted 

 mixture. As the latex is usually emptied from the buckets 

 into large Shanghai earthenware jars, it is easy to regulate the 

 quantity of acetic acid which should be added. Complete 

 coagulation should leave the remaining liquid clear. When it 

 becomes turbid or milky it is a sign that complete coagulation 

 has not been effected. 



It is now a part of the general routine of rubber manufacture 

 on many leading estates to add some sodium bisulphite to the 

 latex after it has been sieved and before it is skimmed. The 

 acetic acid is added thereafter. This sodium bisulphite not 

 only acts as an antiseptic, but also preserves the colour of the 

 rubber. Fermentations arise very rapidly in the latex, and 

 unless promptly checked by such means as this tend to dis- 

 coloration of the rubber sheets. This will be most readily 

 observed when latex is coagulated in field-sheds at a distance 

 from the factory. The rubber made from latex so coagulated 

 is always lower in shade. On some estates endeavour is made 

 to obviate this by more thoroughly expressing the water from 

 the coagulated latex by heavy hand machines, but even this 

 precaution does not prevent a lower colour. One half-pound of 

 sodium bisulphite mixed with one gallon of water is about the 

 usual strength of the solution employed. It should be very 

 thoroughly stirred in or the rubber will be streaky. This 

 quantity is sufficient for 20 to 25 gallons of latex. The sodium 

 bisulphite pays for itself, as very careful investigation goes to 

 show that by its addition the total weight of the rubber obtained 

 is increased by a weight in excess of that of the sodium bi- 

 sulphite itself. The reason of this is at present obscure. The 

 addition in weight is equivalent to i per cent, in ordinary 

 practice. Not only does sodium bisulphite check discolora- 

 tion, but it also to some extent improves or, rather, perhaps 



