Technically pure rubber is by no means pure in the chemical sense. The chief impurities present are various resinous and allied bodies, which can be removed more or less completely by prolonged extraction with boiling acetone. The amount of the resins and oily bodies extracted in this way varies from i per cent, upwards, according to the origin of the 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 composition of biscuit rubber prepared from the latex of old Hevea trees at Henaratgoda was as follows:

Resin, etc

2 per cent

Proteins, etc