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 
