MINT. 211 



the cone is applied to the skin, which presently assumes a bright 

 scarlet appearance, the irritating effect produced by these substances 

 being quite distinct from the cooling sensation imparted by 

 menthol. 



The action of menthol depends on its rapid and complete 

 evaporation from the skin, but when a cone adulterated with greasy 

 matter is used, the grease retards, if it does not actually prevent, 

 this evaporation, and thus causes the menthol to act as an irritant. 

 Pure menthol cones slowly evaporate on exposure to the air ; in 

 cones made partly of fatty matters this tendency is greatly checked. 

 When any powder has been used to adulterate the menthol, it is 

 seen as a coating on the outside of the cone after the latter has 

 been exposed for a few^ days, the non volatile powder having been 

 left behind after the menthol had evaporated. 



It is clear therefore, that only the purest menthol is suitable for 

 the preparation of cones, and that no admixture of any kind ought 

 to be employed in their manufacture. 



The following are regarded as reliable tests for the purity of 

 menthol* : — 



1. It should be completely and readily soluble in twice its 

 weight of chloroform. On the addition of a small quantity of 

 iodine to this solution, a rich indigo-blue colour (not dark-green) 

 ought to be imparted, and this colour should be completely 

 discharged on shaking the mixture with solution of caustic potash 

 or soda. 



2. The melting-point ought to be 42^-2 0. (108° F.) and the 

 boiling point not lower than 212° C. (413 °'6 F.) and there should 

 be no residue on evaporation. 



3. Menthol shaken with an oxidising mixture, such as sulphuric 

 acid and bichromate of potash, ought to be entirely converted 

 after prolonged heating into a dark green flocculous substance 

 (menthene). 



4. Strong sulphuric acid ought not in the cold to blacken menthol 

 to any considerable extent. 



5. When menthol is heated with a small quantity of anhydrous- 

 zinc chloride, the mixture ought to give off the odour of menthene- 



* Read before the Harwich Pharmaceutical and Chemical Association, 

 10th February, 1885, by G. C. Waldie, New York. 



