368 CHEMICAL DISCOVERY AND INVENTION 



is liable to putrefy, and in its decomposition to affect the quality 

 of the rubber. By the smoking process the rubber is preserved 

 and the dark colour commonly exhibited by the loaves or blocks 

 is accounted for. 



COMPOSITION AND CONSTITUTION OF RUBBER 



Rubber has long been known to consist essentially of a mixture 

 of two or more hydrocarbons having the ultimate composition 

 expressed by the formula C 10 H 16 . But rubber always contains 

 larger or smaller amounts of substances containing oxygen, 

 which for want of more knowledge are commonly called resins. 

 Part of these resins probably result from the absorption of 

 atmospheric oxygen by the rubber hydrocarbons. A small 

 quantity of nitrogenous matter is also present in natural rubber, 

 and is attributable in part to the retention of albuminous matter 

 from the latex. 



When pure rubber is heated it splits up completely into com- 

 pounds having the same percentage composition. Of these the 

 most volatile is a liquid called isoprene, the formula of which is 

 C 5 H 8 . It boils at about 37 under atmospheric pressure, and has 

 been the subject of much experiment in connection with the 

 chemical synthesis of rubber. This will be referred to on a later 

 page. 



Beside isoprene rubber also yields a large proportion of 

 dipentene C 10 H 16 (boiling point 175) which belongs to the series 

 of terpenes (see Essential Oils), together with hydrocarbons of 

 the same composition but higher molecular weight. The mole- 

 cule of rubber is undoubtedly very large and complex, how large 

 it is impossible as yet to say with certainty, but it has been 

 suggested that the molecule consists of at least eight groups of 

 the composition C 5 H 8 , that is that the molecular formula is 

 C 40 H 64 . The difficult solubility of rubber and its colloidal 

 character would be consistent with a still more complex formula. 



Whatever the constitution of rubber may be it has one definite 

 chemical characteristic ; it is unsaturated. On this depends its 

 power of entering into direct chemical combination with such 

 elements as chlorine, bromine, and sulphur as well as with 

 certain oxides of nitrogen. Its capacity for combination with 

 sulphur and with sulphur chloride is the explanation of the 

 important process known as " vulcanisation," upon which 

 depend so many applications of rubber to practical purposes. 



