§§ 29, 30. CONSTITUENTS OF ETHEBEAL OILS. 27 



heated with excess of hydrochloric acid as long as nitrous fumes 

 are evolved, and then tested for sulphuric acid by chloride of 

 barium. 



Warming a small quantity of an ethereal oil containing sulphur 

 with a solution of caustic potash of specific gravity 1 "3, and adding 

 nitro-prusside of sodium, after diluting with water, often suffices 

 to show the presence of sulphur by the sulphide of potassium 

 formed striking a bluish-violet colour with the nitro-prusside. 



Some ethereal oils contain nitrogcv, and many of these are re- 

 garded as nitriles. This element may be detected by heating a drop 

 of the oil with metallic sodium, dissolving the cooled mass in water, 

 adding a drop of solution of ferric and ferrous salt, and, after a few 

 minutes, acidifying with hydrochloric acid, when a precipitate of 

 Prussian blue makes its appearance if nitrogen is present. 



If the ethereal oil contains a sulpliocyanide (oil of mustard or 

 horse-radish), both the sulj)hur and nitrogen test must yield a 

 positive result. 



§ 30. Constituents. — Ethereal oils distilled from vegetable sub- 

 stances are generally mixtures that can be separated into their 

 constituents. If this is to be attempted we must, from the first, 

 admit that, in the present state of our knowledge, an exact 

 quantitative separation is not to be thought of. The principal 

 reason for this must be sought for in the ease with which ethereal 

 oils undergo decomposition, and the great disposition many of 

 them show to form polymers. In the majority of cases only one 

 method of separating the constituents of an oil is feasible, viz., 

 that oi fractional distillation, which must be repeated until products 

 of constant boiling-point have been olitained. But ifc is in this 

 very distillation that a change in the oil often takes place, either 

 by the formation of polymers of the original oil Avith higher 

 boiling-points, or by the production of hydrocarbons by the 

 liberation of the elements of water from constituents of the oil 

 containing oxygen. 



An important improvement in these operations might perha})s 

 be made in conducting the distillations under diminished pressure. 

 In order to make this modification available, the temperature 

 must first be ascertained at which the more commonly occurring 

 constituents of oils can be distilled. ]\Iany of the terpenes present 

 in ethereal oils may be distilled under the ordinary pressure at 

 155° to 157°; many of their poljaners at about 190°; others at 



