164 PLANT PRODUCTS 



such trees, creeper, shrubs, etc. The laticiferous system, 

 which is distinct from the sap-bearing cell system, generally 

 lies between the outer bark and cambium. By cutting 

 through the bark into the latex cells the latex is obtained. 

 This operation is referred to as tapping. In wild rubber 

 V-shaped cuts are generally made, but in plantation rubber 

 the trees are tapped by one central channel. The latex is 

 collected in a cup which is fastened to the tree below the 

 channel. In wild rubber the sticky latex is smoked over a 

 fire from very smoky materials, which produce much 

 creosote, tarry matter, acetic acid, etc. Only small quantities 

 are treated at a time, and gradually a substantial piece 

 of rubber, thirty or forty pounds weight, is produced. 

 Plantation latex is generally coagulated by the addition 

 of a small quantity of acetic acid, the smoking process being 

 carried out later whilst drying. Recently some efforts have 

 been made to produce on the estates themselves a crude 

 pyro-ligneous acid obtained by the distillation of waste wood 

 in a small form of retort (see p. 131), as apparently the 

 single application of crude pyro-ligneous acid is better than 

 successive applications of acetic acid and smoke. The 

 plantation rubber, being produced under at least some 

 partial scientific treatment, is much superior to the wild 

 rubber. The crude material often includes much resin 

 and other vegetable matter. The impure varieties require 

 to be cleaned in a special machine. Rubber, when stretched, 

 does not return to its original condition, but remains stretched 

 for some time. It does not, however, alter in volume. 

 Rubber appears to be as incompressible as water. The fact 

 that rubber does not return to its original length when 

 stretched is commonly alluded to as hysteresis. The 

 freshly cut surfaces of rubber readily adhere to one another. 

 As rubber is, strictly speaking, an organic gel, it absorbs 

 water freely, and may increase to an extent of twenty five 

 per cent, in its weight, and fifteen per cent, in its volume. 

 Many organic liquors, like petroleum, coal tar, etc., are 

 absorbed by rubber, and some of these make good typical 

 colloidal solutions. 



