216 RUBBER AND 



the constitution of the rubber molecule only represents 

 one of the most recent theories. The problem of the 

 structure of this molecule cannot yet be regarded as 

 finally settled. 



In some of the modern processes which have been 

 proposed for the manufacture of synthetic rubber on 

 a commercial scale, isoprene is prepared from fusel oil, 

 obtained from starch by special methods of fermenta- 

 tion ; and polymerisation is effected by the action of 

 metallic sodium. Various other substances have also 

 been synthesised, which, though differing in composition, 

 appear to possess a similar molecular structure to india- 

 rubber. The physical properties of some of these bodies 

 are closely similar to those of rubber, and their use has 

 been proposed commercially for similar purposes. So 

 far, none of these substances have been placed upon the 

 market in appreciable quantities, and the reports of the 

 cheap production of synthetic rubber have not carried 

 with them a fall in the value of plantation shares. 



Vulcanisation. 



The molecule of rubber is unsaturated, and is able 

 to enter into direct combination with different elements. 

 This fact is expressed in its constitutional formula by 

 the presence of double links between two pairs of 

 carbon atoms in the ring. Among the addition pro- 

 ducts of rubber, those with sulphur possess the greatest 

 technical importance. Rubber combined with sulphur 



