MINT. 215 



drops of the essential oil, ^yithout alcohol. Some specmiens of oil 

 of peppermint imported from Germany and England showed by 

 this test a mixture with Mentha arvensis. To make this test 

 valuable, however, samples of known purity should be first operated 

 upon, and note taken of the depth of coloration required for a pure 

 oil. Another test for the detection of pennyroyal, which also 

 indicates Mentha arvensis when in sufficient quantity, is the 

 following : — Take 1 drachm each of chloral hydrate and pure 

 sulphuric acid, adding 12 drops of alcohol. "When this solution is 

 mixed with a like quantity of pure oil of peppermint, a dark cherry 

 colour is quickly produced and maintained for a long time. Penny- 

 royal (or oil of peppermint heavily adulterated with this oil) gives 

 no such colour, being more of a yellowish cast, and changing to an 

 olive-green. With Mentha arvensis, a yellowish -browm colour is 

 produced which is maintained for ten or twelve hours, and thirty- 

 six hours later, has a slight tendency to assume a cherry colour or 

 one intermediate between the cherry and the browm. It was 

 noticed that when the true oil of peppermint was mixed in ec[ual 

 proportions with that of Mentha arvensis, a deficiency in the 

 intensity of the cherry colour was plainly observable." 



The colour reactions produced by acids on oil of peppermint 

 were noticed by Professor Pliickiger in 1871* ; he found that 50 to 

 70 drops of oil of peppermint shaken with one drop of nitric acid 

 sp. gr. 1-2, turn faintly yellowish, brownish, and after an hour or 

 two exhibit a most beautiful blue-violet, or greenish-blue colour 

 when examined in transparent light. When observed in reflected 

 light, the liquid is of a copper colour and not transparent. If this 

 is warmed, the green or blue coloration takes place speedily ; it 

 may also be immediately provoked by adding a greater amount of 

 nitric acid, say 1 drop to 19, or nine drops of the essential oil. The 

 colour is remarkable on account of its persistency, for it lasts a 

 week or two at least in cold. He adds that " unfortunately this 

 reaction cannot be applied as a true test, as an admixture of 5 per 

 cent, of oil of turpentine does not at all prevent peppermint oil 

 from assuming the blue or green colour. . . . Carvene, the 

 more volatile portion of carraway oil, also acquires a slight similar 

 fluorescence, but by no means comparable to the above-described 

 as regards purity and intensity of colour. Peppermint oil which 



* Pharm. Journ. [3], i., p. 6S2. 



