EFFECT OF CULTURE AND CLIMATE ON PEPPERMINT OIL. 7 



From these physical properties it may be concluded that in several 

 instances there are material differences in the composition of the oils, 

 since the physical properties are greatly influenced by the con- 

 stituents of the oils. 



The color of the oils from the dry herb is uniformly darker than 

 that of the oils from the fresh herb and the odor is distinctly less 

 agreeable. The specific gravity is noticeably higher than in the oils 

 from the fresh herb, but the variations in refraction, rotation, and 

 solubility are less striking. Conspicuous among the oils from the dry 

 herb is that from the flowering plants in 1909. This oil was insoluble 

 in any proportion of either 80 or 90 per cent alcohol. 



Table IV also shows that the percentage of free acids, calculated 

 as acetic acid, in the various oils from the fresh plant varies con- 

 siderably, probably as a result of the breaking down of the ester of 

 the oil into free acid and menthol or of the inhibition of the esteri- 

 fication of the acid and menthol with the formation of menthyl 

 acetate. 



On comparing the percentage of ester or menthyl acetate in the 

 oils from the fresh herb during the three stages of growth, it will be 

 observed that the proportion of this constituent increases as the 

 plant develops. The oil obtained at the fruiting stage is in every 

 case richer in esters than the oil from the flowering or budding plants. 

 The content of menthyl acetate of the oils varies in the budding plant 

 during the several seasons from 6.72 to 16.62 per cent, in the flower- 

 ing plant from 7.07 to 14.5 per cent, and in the fruiting plant from 

 12.37 to 20.86 per cent. 



The free menthol content of the oils does not show the same 

 tendency to increase as is noted in the case of the ester content. A 

 certain relationship, however, seems to exist between the ester con- 

 tent and that of free menthol, since in many cases when the ester 

 content is high the free menthol content is low, and vice versa. The 

 oil distilled from the fruiting plant in 1910, in which year the per- 

 centage of esters was the highest, has the lowest percentage of free 

 menthol, while the oil from the flowering plant in 1909, which con- 

 tained a low percentage of esters, possessed the highest percentage 

 of free menthol. This, however, is not true in every instance. 

 That a relationship of this kind should exist between the ester content 

 and the free menthol content is but natural, as the higher the per- 

 centage of menthol in combination in ester form the lower is the 

 percentage of free menthol. 



The percentage of total menthol represents the combined content 

 of free menthol and the menthol held in combination in the form of 

 the ester. Therefore, the oils with the highest content of total 

 menthol are those which contain the highest content of free menthol 

 and also combined menthol, or ester. This is strikingly shown in 



