AMOUNT OF REgm tN RUBBER 



51 



It will be seen from these figures that the Para tree 

 will furnish rubber containing 96" 5 per cent, of caoutchouc 

 and only 1"9 per cent, of resin, whilst at the other end 

 of the scale the products derived from the latex of certain 

 species of Euphorbia may contain only 6* 7 per cent, of 

 caoutchouc and 92- 8 per cent, of resin. Between these 

 extremes every variation is possible, and the analysis 

 of the specimen of Ficus Vogelii rubber quoted above 

 illustrates a case in which caoutchouc and resin are in 

 nearly equal proportions. The presence of a large amount 

 of resin adversely affects the physical properties of the 

 rubber, rendering it soft, weak, and inclined to be sticky, 

 whilst the products in which resin predominates do not 

 usually exhibit the characteristic properties of rubber, 

 but are hard, inelastic, and more or less brittle. 



The amount of resm in latex varies not only in the 

 different rubber-yieldmg plants, but also to a greater 

 or less degree in the same species according to the age 

 of the tree. In general it may be stated that the latex 

 from the stem of a very young tree will be more resinous 

 than that from an older tree, i.e. the amount of resin 

 in the latex tends to diminish with the age of the tree. 

 This variation in the composition of the latex, and con- 

 sequently of the rubber, due to the age of the tree, is much 

 more marked m the case of certain species than in others. 

 Thus Weber obtained the following results in some ex- 

 periments with Castilloa trees in Central America : 



