March 1907. 131 Saps and Exudations. 



it is the result or the polymerisation of a liquid which is held in suspension in the 

 latex aud on polymerisation changes into the solid colloid which we know as 

 caoutchouc Weber, by experiments conducted in South America with fresh latex, 

 arrived at a similar conclusion, which later workers have confirmed. Although the 

 nature of the process is not yet completely elucidated, there is little room for doubt 

 that the coagulation is due to the polymerisation of a liquid and possibly of aliquid 

 hydrocarbon contained in the latex. For the chemist the important question 

 remains as to the nature of this liquid from which caoutchouc is formed. 



The chemical nature of caoutchouc is a subject which has attracted the 

 attention of the distinguished chemists from the middle eighteenth century, among 

 them being Faraday, Liebig, and Dal ton. Faraday was the first to examine the 

 constituents of the latex of Hevea brasiliensis. It is only in recent years that our 

 knowledge of the constitution of the organic compounds, and especially of the 

 terpene group has rendered it possible to make any great advance. It is interesting 

 to record that Greville Williams, in 18(50, made most important contributions to this 

 subject. He identified a new hydrocarbon, isoprene, as a decomposition product of 

 caoutchouc, and recognised its polymeric relation to caoutchouc. 



The results obtained from the analytical side, and especially the formation 

 of di-pentene and isoprene by pyrogenic decomposition of caoutchouc, had pointed 

 to the fact that caoutchouc was essentially a terpenoid polymer of the formula 

 C H Harries finds, however, that the oxonide of caoutchouc, when distilled with 



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steam, breaks up into laevulinic aldehyde, laevulinic acid, and hydrogen peroxide, 

 and he concludes from this that caoutchouc is a polymer of a 1 : 5 dimethyl cyclo 

 octadien. Whilst Harries' s work has brought us much nearer the goal, and has led 

 to the discovery of a new method of investigation through the oxonides, which is 

 obviously of wide application, it cannot yet be said that the constitution of 

 caoutchouc has been settled or its relation to the parent substance of the latex 

 definitely established. It has still to be shown how a closed-chain hydrocarbon such 

 as Harries's octadien can under go polymerisation forming the colloid caoutchouc. 



There are strong arguments for the view that the constitution of the parent 

 substance present in the latex is nearly related to that of isoprene. This remarkable 

 hydrocarbon of the formula C H , first obtained by Greville Wiiliams from the dry 



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distillation ot rubber, is a saturated olefinic hydrocarbon which is found among the 

 products, resulting from heating caoutchouc. It readily polymerises, forming 

 dipentene. Bouchardt noticed that this hydrocarbon obtained from the pyrogenic 

 decomposition of caoutchouc furnished a substance identical with rubber when acted 

 on by hydrochloric acid and under other conditions. To Wallach and also to 

 Tilden is due the further important observation that when isoprene prepared from 

 oil of turpentine is kept for sometime, i t gradually passes into a substance having 

 all the characteristic properties of caoutchouc. 



I have very briefly drawn attention to the present position of our knowledge 

 of the chemistry of caoutchouc in illustration of the interest which attaches to the 

 examination of vegetable products, and also because of the immense importance 

 of the problem from the practical and commercial standpoint. Chemistry in thLs 

 case holds the premier position in reference to this subject, and to a large extent 

 may be said to hold the key to the future of the rubber industry in all its phases 

 The discovery of better methods of coagulation, preparation, and purification 

 will be affected through chemical investigation, as will also the determination of 

 the manner ot utilising of various other plants which furnish rubberdike latices. 

 That the physical properties of raw rubber, on which its technical value depends, 

 are to be correlated with the chemical composition of the material there can be no 

 doubt. The chemical analysis of raw rubber, as at present conducted, is, however. 



