462 TURPENTINE OIL. 



D. ETHEREAL OILS. 



The name ethereal or volatile oils has been applied to 

 all those compounds, which pass over with the vapor 

 on heating certain plants or parts of plants with water, 

 and form the odorous constituent of these plants. 

 Most of them are mixtures of compounds containing 

 oxygen and hydrocarbons. The oxygenized bodies are 

 of very various character, and belong to entirely dif- 

 ferent chemical groups. They are in some cases acids 

 (valeric acid in oil of valerian, pelargonic acid in the 

 oil of Pelargonium roseum) ; in some, aldehydes (cumi- 

 nol in oil of cumin, cinnamic aldehyde in oil of cinna- 

 mon) ; in others, ethers (methyl salicylate in gaultheria 

 oil) ; in others still, phenols (thymol in oils of thy- 

 mian and monarda), etc. They have already been 

 described, as far as they are well known, in connection 

 with these compounds, to which they bear a close 

 chemical relation. The hydrocarbons called terpenes, 

 contained in the various ethereal oils, have nearly all 

 the same composition in percentages. Their formula 

 is a multiple of the simple formula, C 5 H 8 . 



By far the greater number boils without decomposi- 

 tion at 160-170, and these have the molecular for- 

 mula, C 10 H 16 . A smaller number boils at 250-260, 

 and has the molecular formula, C 15 H 24 ; and a still 

 smaller number, which boils above 300, has the for- 

 mula, C 20 H 32 . 



The hydrocarbons of the formula C 10 H 16 show the 

 greatest similarity in their chemical and physical pro- 

 perties, and with many the observed difference between 

 them is confined to the smell and the action upon 

 polarized light. Most of them are imperfectly investi- 

 gated, and a more careful investigation will probably 

 show a thorough chemical identity of many of them. 



The best known is 



Turpentine-oil. 

 C 10 H 16 . 



Occurrence. In all parts "of all coniferous trees. 

 When fir, pine, larch trees, etc., are accidentally 



