142. COLOUR-REACTIONS. 123 



Fruhde's Reagent 1 resembles sulphuric acid in its action ; but the 

 colours are purer and make their appearance more rapidly. Very 

 characteristic colours are produced with some oils by sulphuric 

 acid mixed with Jth of its volume of a 5 per cent, aqueous solution 

 of ferric chloride. 



The oils should be dissolved in chloroform, in which the colour- 

 ing matter is also soluble and thus seen to advantage. 



Oils of pennyroyal, parsley, coriander, fennel, anise, savin and 

 turpentine cause no colouration in the chloroform, even after the 

 lapse of some time ; with oils of ledum and peppermint it assumes 

 a red tinge ; with ledum-camphor, and oils of thyme, cajeput, 

 galangal, pepper, cubebs, copaiba, juniper, violet or bluish-violet ; 

 with serpyllum, sweet marjoram, rosemary, caraway, dill, nutmeg, 

 cloves, worm-seed, cinnamon, green or bluish-green; with oil of 

 bergamot, etc., olive-green. 



Fuming nitric acid (5 drops to 1 of oil) gives specially charac- 

 teristic colourations with oils of mace and nutmeg (blood-red), 

 cubebs (green), copaiba (bluish-violet), gaultheria (cherry-red), 

 cinnamon (carmine), myrrh (reddish-violet), pimento (blood-red), 

 .and pennyroyal (violet). 



Picric acid (0'05 gram to 5 to 6 drops of oil) is easily dissolved 

 by some oils in the cold (caraway, cardamoms, cloves, rosemary, 

 mentha crispa, sweet marjoram, anise, star-anise, dill, valerian, 

 cummin, gaultheria, cinnamon, sweet flag) ; by others only on 

 warming. Some of the solutions deposit crj^stals on standing 

 (turpentine, lemon, bergamot, sweet marjoram, mace, dill, galangal, 

 bitter orange, worm-seed, valerian, cedar-wood, lavender, cajeput, 

 nutmeg, thyme, laurel and sandal-wood) ; others gradually assume 

 characteristic colourations : thus oil of mentha crispa becomes 

 olive-green ; cloves, sweet marjoram, anise, star-anise, nutmeg, 

 cinnamon, cummin, amomum and thyme, orange; fennel and 

 myrrh, blood-red ; dill, cascarilla and galangal, brown ; worm-seed, 

 reddish-brown ; sweet- flag, deep brown ; peppermint, deep grass 

 green. 



Fliickiger 2 recommends acting upon a solution of the ethereal 

 oil in bisulphide of carbon (1 in 15) with sulphuric and nitric acids. 

 With oil of valerian and nitric acid (specific gravity, 1'2) he 

 observed a green colouration of the bisulphide, and red of the acid 



1 1 cc. cone, sulphuric acid with O'Ol gram molybdate of soda. 



2 Schweiz. Wochenschr. f. Pharm. 261, 1870. 



