500 SCIENCE PROGRESS 



paper read before the Philosophical Society of Birmingham in 

 May 1892 : 



Specimens of isoprene were made from several terpenes in 

 the course of my work on those compounds, and some of them I 

 have preserved. I was surprised a few weeks ago at finding the 

 contents of the bottles containing isoprene from turpentine 

 entirely changed in appearance. In place of a limpid colourless 

 liquid, the bottles contained several large masses of a solid of a 

 yellowish colour. Upon examination this turned out to be india- 

 rubber. The change of isoprene by spontaneous polymerisation 

 has not, to my knowledge, been observed before. I can only 

 account for it by the hypothesis that a small quantity of acetic 

 or formic acid had been produced by the oxidising action of the 

 air, and that the presence of this compound had been the means 

 of transforming the rest. The liquid was acid to test-paper, and 

 yielded a small portion of unchanged isoprene. 



The artificial rubber, like natural rubber, appears to consist 

 of two substances, one of which is more soluble in benzene or in 

 carbon disulphide than the other. A solution of artificial rubber 

 in benzene leaves on evaporation a residue which agrees in all 

 characters with a similar preparation from Para rubber. The 

 artificial rubber unites with sulphur in the same way as ordinary 

 rubber, forming a tough elastic compound. 



Previous to this time Wallach * had recorded that, after 

 allowing isoprene to remain in a sealed tube for a long time in 

 the light, and subsequently adding alcohol to the liquid, a tough 

 indiarubber-like mass separated out, which, on standing in the 

 air for some time resinified. C. O. Weber 2 in 1 894 also confirmed 

 Tilden's observation with regard to the spontaneous polymerisa- 

 tion of isoprene. After allowing 300 grammes of isoprene to 

 stand in a bottle for nine months, he found that it was converted 

 into a treacly viscous mass, which on treatment with methyl 

 alcohol yielded 211 grammes of a spongy substance identical 

 with indiarubber. 



It thus appears that isoprene, besides being one of the de- 

 composition products of caoutchouc, is also a possible, if not 

 probable, source of caoutchouc in the plant organism. 



Constitution oj Isoprene 



Greville Williams showed that isoprene contained carbon and 

 hydrogen in the proportion C 5 H 8 , and that it readily combined 

 with bromine, a fact which indicated its unsaturated character. 



1 Ann. der Chem. 1887 (vol. 238), p. 88. 



2 Journ. Soc. Chem. Industry, 1894, p. 1 1. 



