ESSENTIAL OILS. 209 



The terpenes proper are a large class ; they are mobile 

 liquids boiling between 156 and 176. One member, cam- 

 phene, is a solid obtained artificially from camphor and pinene. 

 Most terpenes are optically active, possess characteristic 

 though very varied odours, are insoluble in water, but dissolve 

 readily in ether or alcohol. 



By strong sulphuric acid they are converted into resinous 

 polymers. Strong nitric acid oxidises them with inflamma- 

 tion. Many of them absorb oxygen from the air, forming 

 resinous products. They all greedily absorb chlorine, bromine, 

 or iodine. 



Varieties of terpenes form the main portion of the oils of 

 turpentine, orange, lemon, bergamot, dill, fir needles, cummin, 

 elemi, fennel, eucalyptus, &c. 



2. (i.) Oxygenated Essential Oils. Representatives of the 

 groups alcohols, aldehydes, ketones, phenols, and esters are 

 found in this class. They are usually present in the oil in 

 admixture either with one another or with a terpene. Many 

 members of this group are known. We may mention a few 

 typical ones : 



Alcohols. Borneol, C 10 H 17 OH ; menthol, C 10 H 19 OH. The 

 latter occurs in oil of peppermint (Mentha piperita). It is a 

 crystalline substance melting at 42 and boiling at 213. Its 

 acetate is also present in peppermint oil. 



Esters. Acetates and other salts of amyl and other radicals 

 are sometimes found. Also linalyl acetate, C 10 H 17 .C2H 3 2 , in 

 oils of lavender and bergamot. 



Aldehydes. Oils of cinnamon and cassia contain cinnamic 

 aldehyde, C (i H 5 .CH:CH.CHO ; oil of bitter almonds, benzoic 

 aldehyde, C (; H 5 .CHO. 



Ketones. Camphor, C 10 H 1(; 0, is probably a ketone. Its 

 constitution has given rise to much discussion. According to 

 Bredt," it has the following formula : 



* Ann. 289, 15; J.t'.S. 1894, abst. i. 141. 



