Chemistry of Essential Oils

240 THE CHEMISTKY OF ESSENTIAL OILS'

OIL OF MENTHA SILVESTRIS.
The dry herb, Mentha silvestris, yields about 1 per cent, of essential'
oil, having the following characters:—

Specific gravity 0-968 to 0971
Optical rotation about + 30°
Refractive index 1'4954
Acid value ......... 2-4
Ester,, 20-9
„ ,, (after acetylation) 170 to 175
The oil contains very little menthol, but a considerable amount of
pulegone, with a small amount of a phenol (carvacrol ?).
An oil examined by Tschirch was stated by him to have the extra-
ordinary optical rotation - 132° 52' and refractive index 146856. The
authenticity of this specimen must be regarded with suspicion.

OLLS OF MONARDA.
The best known oil of the Monarda species is that known as America
horse-mint oil, which is obtained to the extent of about 1 per cent, from
the herb Monarda punctata. The oil has a specific gravity from 0 930
to 0940, and is either optically inactive or slightly dextro-rotatory.
The plant is of considerable importance, as it may before long be
available as a regular source of thymol, which has hitherto been manu-
factured principally from ajowan seeds. Experiments have been carried
out by the United States Department of Agriculture,
1
with this end in
view.
In 1910 seeds obtained from Volusia County were sown, and the first
year's growth yielded 020 per cent, of oil, and the second 024 per cent.,,
the oils containing 64 per cent, of phenols. The plants were observed
to vary, the variations falling into two classes:—

1. Darker plants with more serrate leaves and a pronounced red
   colour in the stems and branches.


2. Plants with larger leaves of a light green colour and little or no
   red colour in the stems and branches. The lighter type also produced
   more leaves. The average yield of oil from the two types (thirty-four
   tests being made) was 040 per cent, of oil from the dark plants, contain-
   ing 66 per cent, of phenols, and 0 42 per cent, from the light type, con-
   taining 69 per cent, of phenols.
   By continuing the cultivation of the light green variety, the yield of
   oil was increased to 044 per cent, in 1914, containing 72 per cent, of
   phenols; and in 1915 the yield of phenols was 74 per cent., although
   the percentage of oil was only 0 42 per cent.
   The oil has the following characters :—
   Specific gravity 0-923 to 0-933
   Optical rotation + 1° „ - 2°
   Refractive index at 20° 1-5020 „ 1-5040
   The oil contains about 70 to 80 per cent, of phenols, of which a small
   portion consists of carvacrol and the remainder of thymol, the rest of the
   oil being chiefly cymene, which boils at 175° to 176° C., whilst thymol
   boils at 232° C.
   Fractional distillation in a copper retort is therefore resorted to, and

(^1) Bulletin, No. 372; and P. and E.O.R. (1916), 331.