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, petrol, chloroform, carbon bisulphide and other solvents. The rubber is more easily taken up by the solvent after it has been mechanically kneaded in the rolling or masticating machines, than in the untreated condition. Weber considered that all these solutions should more properly be regarded as solutions of the so-called solvent in the india-rubber.
The mechanical strength of rubber which has once been dissolved is greatly inferior to that of the crude rubber. For this reason cut sheet rubber is superior for many purposes to sheet which has been "spread" from a solution. This fact renders it doubtful whether synthetic rubber can ever be made equal in physical properties to natural rubber. The mechanical kneading to which raw rubber is subjected in the course of manufacture has also a marked effect upon its physical properties.
The value of rubber for many purposes depends largely upon its chemical indifference, i.e. upon its want of power to enter into combination with many other chemical substances. Raw rubber undergoes slow oxidation on exposure to the air, and it enters rapidly into combination with ozone. Dilute acids and alkalies have little or no effect upon it, but it is rapidly destroyed by the action of strong sulphuric or nitric acid, or by exposure to the action of chlorine, bromine or iodine. The only compounds of rubber which have been at all closely studied are those with sulphur, with the halogens and with ozone.
On cooling raw rubber to the freezing point of water the substance becomes hard and brittle, but recovers its ordinary properties when restored to the normal temperature. At warm summer temperatures the elasticity of the rubber is increased, but on further raising the temperature the rubber becomes softer and more extensible. If the heating is not carried too far, the normal properties of the rubber are recovered on cooling. Heated still further the rubber becomes soft and sticky, becoming actually liquid at some point above 200°C. If cooled again from such a temperature the normal elasticity of the rubber is no longer recovered. At still higher temperatures the rubber undergoes dry distillation with destruction of the molecule