§§ 33, 34, 35. ALDEHYDES, VOL. ACIDS, ETC. 21> 



composition, have, when prepared from different plants, somewhat 

 dissimilar properties (odour, behaviour to polarized light, etc.) — 

 some ethereal oils contain other substances which may belong to 

 tolerably distant groups. Aldehydes, ethereal salts, alcohols, 

 acids, etc., have been found in various oils. 



§ 33. Aldehydes. — If an aldehyde is to be looked for in an 

 ethereal oil, it must first be ascertained whether that oil precipitates 

 metallic silver from an ammoniacal solution of the nitrate.^ If 

 this is the case, it must be shaken with a concentrated solution 

 of acid sulphite of soda. The majority of aldehydes are dissolved 

 by acid sulphite of soda, and may be separated from other consti- 

 tuents which do not enter into such combination by removing the 

 aqueous liqui 1. The aldehyde may then be liberated from com- 

 bination by neutralizing with caustic soda or decomposing with 

 dilute sulphuric acid, and, when thus separated, should be tested 

 as to its physical properties, odour, etc. It should also be ascer- 

 tained if it produces a crystalline jDrecipitate in ethereal solution 

 of ammonia. Finally, an ultimate analysis may be made. 



Of the aldehydes to which particular attention should be 

 directed, I may mention those of pelargonic, capric and methyl- 

 capric acid, of angelic, cinnamic, salicylic, and benzoic acid 

 (§§ 25, 26). Many, perhaps all, vegetable substances contain- 

 ing chlorophyll, when distilled in the fresh state, appear to 

 yield a substance with the characters of an aldehyde. ^ 



§ 34. Volatile Acids. — Acids may be removed from the ethereal 

 oil by shaking with dilute solution of potash or soda, and may be 

 liberated, after evaporation of the solution, by the addition of 

 dilute sulphuric acid. (Cf. §§ 25, 139.) Besides the volatile 

 acids already mentioned, the possible presence of hydrocyanic 

 acid, which is partially converted into formic acid by shaking 

 with soda, is not to be forgotten. It may best be looked for in 

 the aqueous part of the distillate (§ 25), and recognised by the 

 well-known silver precipitate and sulphocyanide and Prussian- 

 blue tests. 



§ 35. Ethereal Salts. — If an essential oil is to be examined for 

 ethereal salts that may be mixed with it, it should be remembered 

 that such salts may be decomposed by heating in autoclaves with 



1 See Tollens, Ber. d. d. chem. Ges. xv. 1635 ; Salkowski, ibid, 1739 (1882). 

 " See Ber. d. d. chem. Ges. xiv. 2144, 2508, for an account of this most 

 interesting observation. 



