506 THE CHEMISTRY OF ESSENTIAL OILS
reached about 10° below the point of solidification, this being at 6° to 8°
for anise and star-anise oils, the solidification is induced by gentle fric-
tion with the thermometer upon the wall of the tube holding the oil.
Should this simple procedure fail in its object, a minute crystal of solidified
oil or of anethol may be introduced into the fluid, whereupon the crystal-
lisation sets in at once, disengaging considerable heat. It may be ac-
celerated by constantly stirring with the thermometer, whose mercurial
column rapidly rises, soon reaching the maximum height, called the
solidification-point of the oil."
The solidification point taken in this way, or the melting-point, should
not be below 15°. It usually varies in pure samples between 15° and 18°,
according to the percentage of anethol. A quite satisfactory method of
determining the solidifying-point is to carefully supercool a small amount
in a test tube—say to 10°, add a crystal of anethol and stir with the ther-
mometer, which will now rapidly rise to the correct solidifying-point.
Small quantities of petroleum do not bring this point below the lowest
limit for a pure oil, but even 2 or 3 per cent, greatly interferes with the
solubility in alcohol. The pure oil is soluble in 3 volumes at most of
90 per cent, alcohol, whereas an oil adulterated with 5 per cent, of
petroleum will not dissolve in 10 volumes. On keeping for any con-
siderable time, the oil becomes partially oxidised, with the formation of
anisic aldehyde and anisic acid with a lowering of the melting-point and
raising of the specific gravity. A few years ago some samples of aniseed
oil appeared on the German and London markets, which possessed an
abnormally low melting-point. No adulterant could be detected in these,
although the melting-point varied from 8° to 11°. Two views have been
taken on this point. Umney has examined a sample of star-anise leaf
oil, which, according to Simon, is largely distilled in the Po-Se district.
He finds its specific gravity to be *9878, its rotation + 1°, and its solidi-
fication-point below that of the ordinary fruit oil. A larger percentage of
anisic aldehyde was also indicated by its behaviour on fractionation, and
towards the usual aldehydic reagent—fuchsine decolorised by sulphurous
acid. Possibly the oil in question was distilled from leaves and fruits.
The other view is that the flowers, or very early fruits, are distilled and
yield an oil with a much lower anethol content. It is believed that some
of the flowers of very young fruits are removed in order to allow the re-
mainder to develop to the best advantage, and instead of being thrown
away, these are distilled. Some oil from Tonkin, said to be oil of this
description, absolutely free from petroleum and apparently quite authentic,
gave the following results:—
Specific gravity. -9916
Rotation + 0° 23'
Solidifying-point 12°.Gross adulteration with up to 40 per cent, of petroleum oil was found
in a number of samples a few years ago, but the great alteration in the
physical characters of the oil render this easy of detection.
The following figures represent samples of this type :—
Specific gravity 0'950
Refractive index.1-5280
Congealing-point 8°
Melting-point 10°