48 SCIENCE PROGRESS 



inquiry, as they are highly active substances and have provided 

 the starting-point for many series of derivatives which would 

 otherwise have remained unknown. 



Since 1885 the number of researches into the characters and 

 reactions of the terpenes and their derivatives has greatly in- 

 creased but while many have supplied facts relative to individual 

 series, the most important systematic inquiries which have led 

 to the present advanced state of knowledge of these compounds 

 are those of O. Wallach, 1 A. von Baeyer, 2 G. Wagner, 3 and 

 W. H. Perkinjun. 4 



The principal steps in the process of inquiry, taken in 

 approximately chronological order, are as follows. In 1856 it 

 had been shown by Greville Williams that turpentine produces 

 cymene by loss of two atoms of hydrogen. 5 In 1872 it was 

 announced by A. Oppenheim that oils of turpentine and lemon 

 combine with bromine and that when heated with aniline their 

 dibromides lose two molecules of hydrogen bromide and yield 

 common cymene. 6 C. R. A. Wright also showed that the 

 cymenes obtained from different sources, including camphor 

 and the terpenes, are identical. 7 From these and other facts 

 the terpenes were assumed to be dihydrocymenes. Cymene 

 was then known to be methyl-/>rtr^-propyl-benzene ; it was only 

 after a long controversy, in which several chemists took part, 

 that it was ultimately shown by O. Widman 8 that ordinary 

 cymene contains /so-propyl and not normal propyl, as had been 

 previously supposed. 



But much difficulty was felt in accepting Oppenheim's 

 formula for the terpenes owing to their peculiar properties, 

 for this formula was proposed at a time long antecedent to 

 Baeyer's important papers on hydrobenzene derivatives. No 

 benzene derivative was at that time known to combine directly 

 with hydrogen chloride for example, and accordingly Flawitzky 

 and others were led to propose open-chain formulae for the 

 terpenes as more in harmony with their quasi-aliphatic 

 character. 



A clue was obtained in 1885, when, in extending the study 

 of the oximes, a class of compounds recently discovered by 



1 Annalen, 225, etc. 5 Proc. R. Soc. 10, 516. 



3 Berickte, 26, etc. 6 Berichte, 5, 628. 



3 Ibid. 27, etc. 7 Chem. Soc. Trans. [1873], 686. 



1 Chetn. Soc. Trans. 1904, etc. 8 Berichte [1891], 24, 439. 



