884 ESSENTIAL OILS. 



by mixing them with other substances. Some of these essences 

 are colourless, most of them yellow, red or brown, others green, 

 and a small number are blue. They are not unctuous to the 

 touch like the fixed oils, but feel harsh. They produce an oily 

 stain upon paper which disappears on drying. Their density 

 varies from 0.759, the density of oil of coriander, to 1.096, the 

 density of oil of sassafras ; but they are generally lighter than 

 water. Although volatile at the ordinary temperature, their 

 boiling point is usually not under 320. The volatile oils are 

 nearly all decomposed in part when distilled alone. They burn 

 in air with a bright but smoky flame. When exposed to cold 

 they freeze, but generally separate into a solid and fluid por- 

 tion, indicating that they are mixtures of two oils differing in 

 fluidity ; the concrete portion is termed the stearopten, and the 

 liquid portion the elaopten of the oil.* 



The essences exposed to air deepen in colour and absorb 

 oxygen. It has been observed that the odour of oils is closely 

 related with this chemical change. Those which oxidate most 

 rapidly have the strongest smell, and the characteristic odour of 

 no oil can be perceived immediately after its distillation in an 

 atmosphere of carbonic acid gas. The oil becomes in time thick, 

 loses its odour, and is transformed into a resin which in the end 

 becomes hard. A small quantity of carbonic acid is formed in 

 this transformation, but no water ; it is greatly promoted by 

 light. An oil which has commenced to undergo this change 

 consists of a portion of oil unchanged, holding a resin in solu- 

 tion, from which the former may be separated by distillation 

 with water. The substance turpentine is in this condition, and 

 gives oil of turpentine, when distilled, with common resin as the 

 fixed residue. Essences should therefore be preserved in well- 

 stopt bottles. They are strongly acted upon and frequently 

 inflamed by concentrated nitric acid. Some of them produce a 

 sort of explosion when mixed with dry iodine. 



Essential oils are very slightly soluble in water, but suffi- 

 ciently so to communicate their odour and taste to that liquid. 

 Water which has been distilled from an odoriferous plant is a 

 saturated solution of the oil ; the distilled waters used in me- 



* These terms were first applied to the solid and fluid portions of fixed oils ; they 

 are derived from (map suet and t\aiov oil. 



