ON THE CO.XbTlTUlION JiTC. Ol' ESSENTIAL OILS. 215 



and hydrobromic acid : a small quantity of nou-volatilc resinous matter is 

 formed; otherwise the yield of cymeuo approaches the theoretical (]uantit}'. 



Precisely the same result takes place on adding two equivalents of bromine 

 to the lowest-boiling fraction of nutmeg hydrocarbons (boiling at 1G3°-1G4°, 

 and containing 10 to 12 per cent, of cymene ready formed), with these dif- 

 ferences — that the yield of cymene is much less in this case, half the terpeno 

 present being converted into non-volatilc black resinous substances, and, 

 secondly, that much more heat is generated by the union of a given quantity 

 of bromine with the nutmcg-terpeue than is with the same amount of hes- 

 pei-idene. The higher the boiling-point of the original torpcne, the more 

 readily does its dibromide break up into cymene and hydrobromic acid : thus 

 hcspcridene dibromide gives not far from the theoretical yield ; nutmeg- ter- 

 pene dibromide about 50 per cent, only ; whilst turpentine dibromide is but 

 little affected by heat alone (Oppenheim), although it does yield some cymene 

 by this treatment (Grevillo Williams ; Barbier), — the boiling-points of the 

 three terpencs being respectively 178°, 163°-104°, and 160°. 



The same difference between hesperidene and the nutmcg-terpene is notice- 

 able when equal quantities of the two are shaken up with their own bulks of 

 strong sulphuric acid : the terpeues are polymerized, much heat being evolved, 

 this evolution being much greater in the case of the nutmeg-tcrpene. Attempts 

 to estimate quantitatively the difference in heat-development did not lead to 

 any trustworthy results, beyond indicating the bare fact that there is a great 

 difference. 



Taking into consideration these circumstances, together with the researches 

 of Fabre and Silbermann on the heats of combustion of acids of the acetic 

 series and compound others isomeric with them, and on the hydrocarbons of 

 the oletine family, it appears extremely probable that the higher the boiling- 

 point of any member of a series of isomeridcs, the greater is the " affinity " 

 between its constituent elements (t. e. the greater is the work performed in 

 their union), and consequently the less is what may be termed the intrinsic 

 chemical energij of the compound ((. e. the less work can be obtained by the 

 conversion of a given weight of the compound into other constant products) ; 

 or in other words, the heat of combustion of an isomeride of Jiighcr bollimj- 

 point is less than that of one of lower hoillng -point. It has not yet beeu found 

 practicable to test this point in the case of the isomeric terpenes, first, on 

 account of the difficulty of obtaining perfect combustion, and other experi- 

 mental errors, and, secondly, on account of the difficulty in getting terpenes 

 free from cymene to operate on. It is, however, hoped that some satisfac- 

 tory evidence on this head may be obtained whenever the experiments on 

 various oils &c. have disclosed the existence of a terpene which, like hesperi- 

 dene, appears to be one single homogeneous body of formula Cj^Hj^, ; in the 

 mean time the author cordially invites aU chemists who are interested in this 

 point, so vitaUy connected with the subject of isomerism, to submit it to the 

 tost of experiment in any cases that may seem to them promising. 



In order to make sure that the cymenes thus obtained from hesperidene and 

 nutmeg-terpene are identical with the ordinary cymene from cummin-oil, a 

 carefal examination was made of specimens of cymene derived from every 

 available source. Fittig, Kobrich, and Jilke have shown that the cymene ob- 

 tained tVom camphor by the action of zinc chloride is mixed with a largo 

 number of other substances ; this circumstance appears to have misled Kekide 

 and others into the belief that there are two distinct isomeridcs, a couclusiou 

 entirely negatived by the experiments described below. 



The cymenes from the dibromides obtained as above were purified by frac- 



