RUBBER PLANTING 217 



is said to be vulcanised. We have already referred in 

 Chapter I to the remarkable properties of resistance 

 and durability possessed by vulcanised rubber. 



Rubber is vulcanised by two entirely different 

 processes. In hot vulcanisation the rubber is directly 

 combined with sulphur under the influence of heat. In 

 cold vulcanisation, rubber is combined, without heating, 

 with chloride of sulphur dissolved in benzene or other 

 solvent. The molecule of rubber vulcanised by the 

 second method contains chlorine as well as sulphur. 



Vulcanisation with hot sulphur was discovered by 

 Nelson Goodyear in 1839. At the melting point of 

 sulphur there is little or no action. Combination begins 

 at about I2OC., and the temperatures used in practice 

 range from I25C. upwards. According to the amount 

 of sulphur employed, the time of action and the 

 temperature, the final product ranges in character from 

 soft elastic to hard vulcanite. 



Weber considered that the process of vulcanisation 

 was one of definite chemical combination. He believed 

 that a series of sulphides of rubber were formed ranging 

 from (C 10 H 16 )2oS 2 to the final product vulcanite, to which 

 he attributed the formula C 10 H 16 S 2 . This opinion, which 

 has been subjected to severe criticism, appears to be 

 confirmed by the results of the most recent researches. 

 The process is a reversible one, and the vulcanised 

 rubber apparently always contains a certain amount 

 of uncombined rubber and a certain amount of free 

 sulphur. As a result, goods which have only been 



