THE CHEMICAL HISTORY OF THE TERPENES 47 



For the purposes of this article it will be convenient to 

 restrict the name " terpene " to the unsaturated hydrocarbons 

 having the empirical formula C 10 H 16 , of which oil of turpentine 

 and essence of lemon may be regarded as typical examples. 

 All, with one exception, are liquids having characteristic odours, 

 lighter than water and soluble in that liquid only to an extent 

 sufficient to communicate their peculiar taste and smell. Nearly 

 all the natural terpenes are optically active, some rotating the 

 ray to the right, others to the left. 



A few of the derivatives of turpentine were known long 

 before any means were available for determining their com- 

 position. Thus terpin hydrate, in the form of crystals deposited 

 from old specimens of turpentine oil, was described nearly 

 two hundred years ago ; moreover, the hydrochloride, which 

 was at first confused with camphor, was obtained by Kind in 

 1803 * by passing hydrogen chloride gas into turpentine oil. 

 This was before chlorine had been recognised as an element 

 and when the nature of muriatic acid was unknown. 



The first definite knowledge of the composition of terpenes 

 was provided by Dumas and by Blanchet and Sell, who in 1833 2 

 analysed, by Liebig's then new process, both oil of turpentine 

 and essence of lemon and found that, in spite of a difference of 

 some 20 in boiling point, these hydrocarbons had the same 

 composition. 



Twenty years later Berthelot studied the two commercial 

 varieties of turpentine — French and American — and the action 

 of heat upon them 3 but then and for many years after it was 

 supposed that there was an indefinitely large number of hydro- 

 carbons of the same composition, C 10 Hi G . 



The first classification attempted was based on the production, 

 properties and interactions of the nitrosochlorides which are 

 formed by the union of the terpene with nitrosyl chloride, 

 NOC1. 4 The three classes then recognised were, however, 

 found by further research to be insufficient and as explained 

 later the number of hydrocarbons of distinct constitution is 

 very much greater than was then supposed. The nitroso- 

 chlorides, however, have played a very important part in the 



1 Tromsdorff's J, d. Pharrn. xi. pt. ii. 132. 



2 Anna/en, 6, 245 and 262. 



3 Annates de Chitn. Phys. [3], 39, 5. 



4 Tilden, Chem. Soc. T?-ans. 1875, 1877, 187s. 



