260 Dr. Adolf Liehmann [Feb. 27, 



sical researches by Wallacb ; tbey are cyclic compounds, and have the 

 general formula CioHje' 



Pinene, camphene, limonene, hipentine, phellandrene, sylvestrene, 

 terpinene, are the terpenes more frequently occurring in essential oils. 

 Other hydrocarbons found in oils are sesquiterpenes, C15H24, and 

 cadinene and caryaphillene are its chief members. Alcohols occur 

 frequently either combined as acid esters, phenolic esters, or in the 

 free state. 



Of the greatest interest and importance are the diolefinic alcohols 

 geraniol and linalool CgoHigO, and the olefinic alcohol citronellol. 

 Geraniol is the chief ingredient of attar of roses, and linalool is 

 also present, but in smaller quantity. 



Several modes of preparing these alcohols in pure state have been 

 devised, and they can now be obtained in the market as chemical 

 individuals. 



Whilst these alcohols are open chain compounds, essential oils 

 contain a number of cyclic alcohols which are true hydroxy deriva- 

 tives of terpenes proper. Amongst the more important, terpineol, 

 borneol and menthol may be mentioned. 



Open chain aldehydes are substances of strong odour, not always 

 very pleasant. Octyl and nonyl aldehydes are constituents of the 

 German attar of roses and of oil of lemon. Citral, however, is 

 more important, and occurs in nearly all essential oils, which are 

 distinguished by the characteristic lemon odour of this substance. 



It is contained, to the extent of from 70 to 80 per cent., in lemon 

 grass oil, and its chief importance lies in the fact that it forms the 

 raw material for the preparation of ionone or artificial violet. 



Citronellal frequently occurs along with citral, and has similar 

 properties. 



A number of aromatic aldehydes, benzaldehyde, salicylic alde- 

 hyde, cumaric aldehyde, anis aldehyde, vanilline, heliotropin and 

 cinnamic aldehyde belonging to the benzene series, occur in nature, 

 but most of them are now prepared synthetically. 



Of aliphatic ketones only methyl heptenone, distinguished by the 

 fruity odour of amylacetate, and methyl nonyl ketone are of any im- 

 portance. Cyclic ketones are, however, important constituents of 

 some essential oils, for example, fenchone, tujone, pulegone, all dis- 

 tinguished by a characteristic smell. But the most valuable in the 

 industry of perfumes is irone, the active principle obtained from the 

 roots of violets, and ionone or artificial violet. 



Acids as such do not assist the perfuming qualities of essential 

 oils. They are either odourless or have an objectionable smell ; but 

 ft number of esters have been found in natural perfumes and possess 

 highly valuable properties ; amongst them, methyl salicylate, methyl 

 anthranilate, etc., and a number of esters of fatty acids. 



Lactones are also represented amongst perfumes ; sedanolide with 

 a strong odour of celery is present in this plant, and cumarine with 



