August 9, 1906] 



NA TURE 



365 



rnerce which commands the highest price, and the forests 

 <if Africa, where climbing plants, generally of the Lan- 

 tlolphia class, also furnish rubber. The increased demand 

 for caoutchouc has led to the extensive planting of the 

 Para rubber tree, especially in Ceylon and in the Federated 

 Malay States. Systematic cultivation and improved 

 methods of preparation are responsible for the fact that 

 (he product of the cultivated tree, which begins to furnish 

 satisfactory rubber when six or seven years old, is now 

 commanding a higher price than the product of the wild 

 tree in Brazil. It is estimated that within the next seven 

 years the exports of cultivated india-rubber from Ceylon 

 and the Federated Malay Stales will reach between ten 

 and fifteen million pounds annually, and that after fifteen 

 years they may exceed the exports of the so-called wild 

 rubber from Brazil. 



The services which chemistry can render to the elucida- 

 tion of the problems of rubber production and utilisation 

 are very numerous. Methods of treatment depending on 

 a knowledge of the other constituents of the latex have 

 led to the production of rubber in a purer condition. Much 

 still remains to be elucidated by chemical means as to th(\ 

 nature of the remarkable coagulation of the latex. As is 

 well known, the latex is a watery fluid resembling milk 

 in appearance which contains the rubber, or, as I think 

 more probable, the immediate precursor of rubber, together 

 with proteids and other minor constituents. The con- 

 stituent furnishing rubber is in suspension, and rises like 

 cream when the latex is at rest. On the addition of an 

 acid, or sometimes of alkali, or even on mere exposure, 

 coagulation takes place and the rubber separates as a 

 solid, the other constituents for the most part remaining 

 dissolved in the aqueous liquid or " serum." The first 

 view taken of the nature of the coagulation process was 

 that, like the coagulation of milk by acids, it is dependent 

 upon a process of proteid coagulation, the separated proteids 

 carrying down the rubber during precipitation. 



This explanation cannot, however, be considered com- 

 plete by the chemist, and there are peculiarities connected 

 with the coagulation of the latex which are opposed to the 

 view that it is wholly explained by the coagulation of the 

 associated proteids. The experimental investigation of the 

 question on the chemical side is beset with many difficul- 

 ties, which are increased if access cannot be had to fresh 

 latex. A number of experiments were made at the Imperial 

 Institute with latex forwarded from India. The difficulties 

 contended with in preventing coagulation during transit 

 were great, but in the case of the latex derived from 

 certain plants these were to some extent surmounted, and 

 the results obtained, especially with reference to the 

 behaviour of certain solvents towards the latex, led to the 

 conclusion that " coagulation " can take place after re- 

 moval of the proteids, and that in all probability it is the 

 result of the polymerisation of a liquid which is held in 

 suspension in the latex and on polvmerisation 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 a liquid hydrocarbon contained in 

 the latex. For the chemist the important question re- 

 mains 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 distinguished chemists from 

 the middle of the eighteenth century, among them being 

 Faraday, Liebig, and Dalton. Faraday was the first to 

 examine the constituents of the latex of Hevea hrasiliensis. 

 It is only in recent years that our knowledge of the con- 

 stitution of 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 i860, made most important contributions to this sub- 

 ject. He identified a new hydrocarbon, isoprene, as a 

 decomposition product of caoutchouc, and recognised its 

 poh'meric relation to caoutchouc. 



The results obtained from the analytical side, and 

 especially the formation of di-pentene and isoprene by 



NO. 19 I 9, VOL. 74] 



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 ozonide of caoutchouc, when distilled with steam, 

 breaks up into laevulinic aldehyde, laevulinic acid, and 

 hydrogen peroxide, and he concludes from this that caout- 

 chouc is a polvmer 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 ozonides, which is obviously^ of 

 wide application, it cannot yet be said that the constitu- 

 tion 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 undergo polymerisation forming 

 the colloid caoutchouc. 



There are strong arguments for the view that the con- 

 stitution of the parent substance present in the latex is 

 nearly related to that of isoprene. This remarkable hydro- 

 carbon of the formula C,,H,, first obtained by Greville 

 Williams from the dry distillation of rubber, is an un- 

 saturated olefinic hydrocarbon which is found among the 

 products, resulting from heating caoutchouc. It readily 

 polymerises, forming di-pentene. Bouchardat 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 con- 

 ditions. To Wallach and also to Tilden is due the further 

 important observation that when isoprene prepared from 

 oil of turpentine is kept for some time, it gradually passes 

 into a substance having all the characteristic properties of 

 caoutchouc. 



I have very briefly drawn attention to the present posi- 

 tion of our knowledge of the chemistry of caoutchouc in 

 illustration of the interest which attaches to the examin- 

 ation of vegetable products, and also because of the 

 immense importance of the problem from the practical 

 and commercial standpoint. Chemistry in this 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 purifica- 

 tion will be effected through chemical investigation, as 

 will also the determination of the manner of utilising the 

 various other plants which furnish rubber-like 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, not always to be taken by itself 

 as a trustworthy criterion of quality, and more refined 

 processes of analysis are now needed. .Although the finest 

 caoutchouc for technical purposes is only yielded by some 

 half-dozen plants, under the names of which these varieties 

 of caoutchouc pass, there can scarcely be a doubt that the 

 elastic substance 'in each case possesses a very similar, if 

 not identical, chemical structure. Nearly all the latices 

 and similar fluids furnished by plants contain rnore or 

 less caoutchouc. Even opium, which is the dried juice of 

 the capsule of the poppy, contains caoutchouc, whilst the 

 opium yielded by certain Indian species contains a notable 

 proportion. Chemistry must determine the means by 

 which caoutchouc can best be separated from these 

 relatively poor latices. In view of the increasing^ produc- 

 tion of the nearly pure caoutchouc which is furnished by 

 Hevea hrasiliensis, Funtumia elasiica, Castilloa elastica. 

 Ficus elastica, and a few other plants which occur or can 

 be cultivated in several of our tropical Possessions, the 

 question is not a pressing one at the moment. 



Moreover, it cannot be doubted that chemical science 

 will sooner or later be able to take a definite step towards 

 the production of rubber by artificial means. 



The production of caoutchouc by chemical means has, 

 indeed, virtuallv been accomplished in its formation from 

 isoprene. The exact nature of this change has still to be 

 determined. When this has been done it will only remain 

 to cheapen the cost of production to make the manufacture 

 of synthetic rubber a purely practical problem. I should 

 be the last to discourage the great extension of rubber 

 planting which is now "taking place. It is warranted by 



