RUBBER PLANTING 171 



Quality. 



Light coloured rubber is valuable for certain 

 purposes ; for example transparent tubes for feeding 

 bottles are now made from pale plantation rubber. The 

 number of uses for which a very pale tint is necessary 

 is however comparatively small. One obvious advantage 

 of light coloured transparent rubber lies in the fact that 

 its purity is unmistakable, as compared with dark com- 

 mercial rubbers, and until recently a higher price was 

 commanded by the palest crepe rubbers. The quality 

 of raw rubber, however, depends mainly upon what is 

 known as nerve. This expression more or less sums up 

 the results of tests upon the breaking strain, extensibility 

 and resiliency, all of which should, generally speaking, 

 be as high as possible. The resiliency of the rubber is 

 measured by the pull exerted after a certain period of 

 extension, and by the permanent extension shown after 

 a certain period of stretching, followed by a certain 

 interval of rest. The most resilient rubber exerts a 

 large pull, and shows a small permanent extension. In 

 practice, the strength and resiliency of the rubber are 

 estimated by the brokers simply by handling and 

 stretching ; and although expert buyers arrive at a high 

 degree of skill and judgment, such methods cannot be 

 regarded as infallible. It is even whispered on estates 

 that thick crepe is preferred to thin on account of its 

 greater apparent strength. In this particular form of 

 rubber, strength is a feature particularly difficult to 



