212 RUBBER AND 



particular sample. Proteins and other nitrogenous 

 bodies are also invariably present in varying amounts, 

 and there is always a certain amount of ash remaining 

 on ignition. After the various impurities have been 

 determined as accurately as possible, the weight of 

 these is deducted from the original total weight, and 

 the residue is regarded as representing the amount of 

 pure caoutchouc present. The approximate compo- 

 sition of biscuit rubber prepared from the latex of old 

 Hevea trees at Henaratgoda was as follows : 



Resin, etc 2 per cent. 



Proteins, etc i 



Ash 0-3 



Rubber 96 



Physical Properties. 



In the highest attainable state of chemical purity, 

 india-rubber is a practically colourless and odourless 

 substance, possessing a specific gravity of approximately 

 0*911. It is insoluble in water, but is capable of 

 absorbing about 25 per cent, of its own weight of 

 water on prolonged immersion. Rubber is a bad 

 conductor both of heat and of electricity. Some of 

 its most important technical uses are dependent upon 

 the last named property. In so-called rubber solvents 

 the rubber swells into a jelly which finally " runs " 

 when a sufficient amount of the solvent is added. In 

 this way about 15 parts of rubber are soluble in 100 

 parts of benzene, and smaller amounts in turpentine, 



