314 REPORT— 1880. 



tlie spectrum transmitted by the solution, form a strongly marked and 

 highly characteristic feature of very many substances. 



In consequence of the satisfactory results which they had obtained, 

 Messrs. Hartley and Huntington thought it worth while to make a 

 special examination of essential oils.' In addition to the scientific interest 

 which these bodies possess, many have considerable commercial value, 

 and consequently are subject to adulteration. 



It is now well known that essential oils consist for the most part of 

 isomeric hydrocarbons, which may be divided into three polymeric 

 groups, having the composition represented by the formulae — CigHig — 



^15ll24 — C20H32. 



To the first class belong the hydrocarbons derived from turpentine, 

 orange, nutmeg, myrtle, and others ; the second group includes the 

 hydrocarbons from rosewood, cubebs, calumus, cascarilla, patchouli, and 

 cloves. The third group is represented by colophene. Though of 

 unknown constitution, these bodies exhibit a close relationship to benzene 

 derivatives. 



The report on this investigation contains twenty-five diagrams having 

 reference to about fifty specimens which had been examined. In the 

 examination of the following bodies no absorption- bands were discovered, 

 but the absorption of the extreme ultra-violet rays was found to be 

 greater the higher the number of carbon atoms in the molecule : 

 australene, terebene,^ terebenthene, hesperidene, cajputene, dihydrate, 

 the oils of lign aloes, Indian geranium, santal wood, cedrat, birch bark, 

 juniper, rosemary, rosewood, lavender, vitivert, turpentine, cubebs, 

 patchouli, citronella, elder, melaleuca ericifolia, and cedar wood, the 

 hydrocarbons from cedrat, nutmeg, carraway, and menthole, otto of rose, 

 and otto of citron. 



The presence of cymene in small quantity was indicated by absorption- 

 bands in the case of the hydrocarbons from thyme, lemon, and nutmeg, 

 the blue oil from patchouli, and in one specimen of carraway hydrocarbon. 



The following bodies cause powerful absorption-bands, and are, there 

 can be but small doubt, composed largely of some benzene derivative : 

 oils of bay, thyme, peppermint, bergamot, cloves, aniseed, and cassia, 

 carvole, myristicol, and otto of pimento. It is generally admitted that 

 the oils of bay, pimento, and cloves contain eugenol, CgHg . OH . OCH3 . 

 C3H5 ; oil of aniseed, anethol, CgH4 . OCH3 . C3H5 ; oil of thyme, thymol, 

 CgHg . OH3 . O3H7. Some other oils, such as bergamot and oil of pepper- 

 mint, as likewise the bodies menthole, carvole, and myristicol, have an 

 unknown constitution. The three latter substances are said to be iso- 

 meric' A special interest is attached to their examination, since the 

 character of the spectra they transmit appears to show that the nucleus 

 of menthole is a terpene ; while the benzene ring is the inner basis of 

 carvole and myristicol. An examination of the absorption-spectrum of 

 myristicol throws further light on the nature of this substance. On 

 reference to the diagram, we find that the absorption-band is not well- 

 defined, and that a comparatively small amount of dilution has sufficed to 

 eliminate it. Now these are the characteristics of an absorption-spectrum 

 due to a mixture of two substances, one of which only is capable of causing 

 an intermediate absorption. 



' Proc. Roy. Soc, 1880. 



" Since shown to be chiefly camphene (Jourti. Cliem. Soc. vol. sxxv. p. 758). 



^ Jour. Cfwm. Soc, Gladstone, vol. xxv. p. 1. 



