(^230) THE CHEMISTEY OF ESSENTIAL OILS.
Specific Gravity
Rotation ....
Solubility in 80 % Alcohol \
Acid Value ....
Ester ,, ....
Menthol Acetate
Ester Value after Acetylation
Total Menthol
Free „ ...
Menthone ....
Ordinary Peppermint,
Pallescens.
0-9191
- 10° 54'
1vol.
afterwards
clouding
0-8
40-5
14-3 °/ 0
160-8
50-9 °/ 0
39-6 °/ 0
7'3
°
/
0-9184
- 8° 2'
—
0-8
31-7
11-2 °/ 0
169-1
53-9 %
45-1 °/ 0
—
|
Red Peppermint,
Rubescens.0-9170- 16° 38'
1vol.
afterwards
clouding
1-0
18-9
6-7 °/ 0
180-2
58-0 °/ 0
52-8 °/ 0
16-8 o
0-9136
- 13° 44'
1-2
17-5
6-2 °/ 0
185-6
60-0°
55-1 °/ 0
The limit values for French peppermint oils are, however, wider
than those indicated for these two particular varieties. Most—if not
all—genuine oils will have characters which fall within the fol-
lowing :—
Specific gravity 0910 to 0930
Optical rotation... • - 5° ,, - 35°
Refractive index 1-4610 „ 1-4690
Menthol as esters 6 to 20 per cent.
Total menthol 48 „ 70 „
Many French oils are not soluble in 3 volumes of 70 per cent,
alcohol, but will generally dissolve clearly in 4 volumes.
In rare cases where the plants have been attacked by certain insect
pests, oils have been obtained which are dextro-rotatory up to + 7°.
French oil of peppermint contains,
1
in addition to menthol and its.
esters, isovaleric aldehyde, isoamyl alcohol, Z-pinene, A
8
- ^-menthene and
cineol.
ENGLISH PEPPERMINT OIL.Two varieties of the plant are grown in England, known as " black "
and " white" mint. Of these the former is the coarser plant, rarely
flowering in England, and yielding more oil of somewhat less delicate
aroma than the white mint. These appear to be forms of Mentha
piperita, var. officinalis, but are certainly not identical with the French
forms (vide supra) named by A. and E. G. Camus rubescens and
pallescens, although these botanists believed them to be so. The herbs
are distilled in the green state, although sometimes left for a day or two
in heaps, after being cut. In England most of the stills used hold from
4 to 8 cwt. of herbs, and have a false bottom which is covered to the
depth of 2 ft. with water, and direct heat is then applied. The distilla-
tion is carried on for about five hours. The yield varies according to the
season from about •£• to 1 per cent.
According to the Journal of the Board of Agriculture? from 500 to
1000 acres are under peppermint cultivation in England. In this country
black mint yields twice as much oil as does white mint, and as a result
the cultivation of the latter has been supplanted to a considerable extent.
Peppermint grows best at a height not exceeding 200 ft. The principal
1
Roure-Bertrand Fils, Report, April, 1909, 33.
a 15 (1908), No. 5.