ox THE PRESENT POSITION OF THE CHEMISTRY OF RUBBER. 233 



The Fresent Position of the Chemistry of Rubber. 

 By S. S. Pickles, M.Sc. 



[Ordered by the General Committee to be printed in extenso."] 



Indiarubber, or caoutchouc, is a hydrocarbon which is present in the 

 milky juices of various trees and shrubs, belonging chiefly to the natural 

 orders Euphorbiacese, Moracese, Artocarpacete, and Apocynacese. The 

 milky juice, technically known as latex, and which exudes from certain 

 plants when wounded, is quite distinct from the so-called sap of plants. 

 It is contained chiefly in the middle layer of the bark, in a network of 

 minute tubes known as laticiferous vessels. These vessels run for the 

 most part longitudinally in the other plant tissues, fornting usually a 

 closed and connected system. 



The latex possesses to some extent the properties of a vegetable 

 emulsion, the caoutchouc being suspended in it in the form of minute 

 transparent globules averaging about x^^s's inch in diameter (Adriani). 

 On coagulation by means of heat, or treatment with various chemicals, 

 the coagulum consists largely of the indiarubber hydrocarbon, mixed, 

 however, with certain quantities of resinous and albuminoid substances. 

 The proportion of resinous matter varies in different varieties of latex, 

 and depends upon the botanical source, habitat, and age of the plant, 

 and also upon the portion of the plant from which the latex was 

 taken. 



The botanical function of the latex forms a very interesting and 

 important study, and much has been written regarding this portion of the 

 subject ; this, however, falls outside the province of the present report, 

 which is intended to be confined as closely as possible to the pure chemistry 

 of the indiarubber hydrocarbon. 



When pure, indiarubber is an almost colourless, elastic substance, and 

 when obtained in thin sheets is quite transparent ; the specific gravity 

 varies slightly in diflferent samples between '91 and '93. 



It belongs to that class of substances known as colloids, and this fact 

 makes the investigation from a chemical standpoint a matter of consider- 

 able difiiculty. Even dilute solutions are gelatinous in nature, and it is 

 almost impossible to prepare definite and characteristic derivatives direct 

 from the hydrocarbon. Its analysis shows it to have the composition 

 expressed by the empirical formula C.^Hg, though its molecular weight is 

 probably many multiples of this number ; it is an unsaturated hydrocarbon, 

 having one unsaturated bond or double linking for every complex CjHg. 

 On heating it is decomposed, giving a mixture of hydrocarbons, most of 

 which have the same empirical composition as the parent rubber. The 

 earlier work on the chemistry of indiarubber was almost entirely confined 

 to the examination of the products obtained by destructive distillation, 

 and it is proposed to devote the first part of the report to an historical 

 survey of these investigations. 



The first mention of indiarubber on record was made about four 

 hundred years ago by Herrera,' who, in his account of the second voyage 

 of Columbus to America (1493-1496), observed that the inhabitants of 



' JEncycloj)adm Britannica, vol. xii. 1881, p. 835. 



