It is a matter of some difficulty to judge the purity of essential oils, not only because of their complex nature, but owing to the very great effect upon their properties produced by growing the plants in different soils and under varying climatic conditions, and still more to the highly scientific methods of adulteration adopted by unscrupulous vendors. The following figures will be found, however, to include all normal oils.
Anise Stell, or Star Anise, from the fruit of Illicium verum, obtained from China. Specific gravity at 15° C., 0.980-0.990; optical rotation, faintly dextro- or lævo-rotatory, +0° 30' to -2°; refractive index at 20° C., 1.553-1.555; solidifying point, 14°-17° C.; solubility in 90 per cent. alcohol, 1 in 3 or 4.
The chief constituents of the oil are anethol, methyl chavicol, d-pinene, l-phellandrene, and in older oils, the oxidation products of anethol, viz. anisic aldehyde and anisic acid. Since anethol is the most valuable constituent, and the solidifying point of the oil is roughly proportional to its anethol content, oils with a higher solidifying point are the best.
Aspic oil, from the flowers of Lavandula spica, obtained from France and Spain, and extensively employed in perfuming household and cheap toilet soaps; also frequently found as an adulterant in lavender oil. Specific gravity at 15° C., 0.904-0.913; optical rotation, French, dextro-rotatory up to +4°, rarely up to +7°, Spanish, frequently slightly lævo-rotatory to -2°, or dextro-rotatory up to +7°; esters, calculated as linalyl acetate, 2 to 6 per cent.; most oils are soluble in 65 per cent. alcohol 1 in 4, in no case should more than 2.5 volumes of 70 per cent. alcohol be required for solution.
The chief constituents of the oil are: linalol, cineol, borneol, terpineol, geraniol, pinene, camphene and camphor.
Bay oil, distilled from the leaves of Pimenta acris, and obtained from St. Thomas and other West Indian Islands. It is used to some extent as a perfume for shaving soaps, but chiefly in the Bay Rhum toilet preparation. Specific gravity at 15° C., 0.965-0.980; optical rotation, slightly lævo-rotatory up to -3°; phenols, estimated by absorption with 5 per cent. caustic potash solution, from 45 to 60 per cent.; the oil is generally insoluble in 90 per cent. alcohol, though when freshly distilled it dissolves in its own volume of alcohol of this strength.
The oil contains eugenol, myrcene, chavicol, methyl eugenol, methyl chavicol, phellandrene, and citral.
Bergamot oil, obtained by expression from the fresh peel of the fruit of Citrus Bergamia, and used very largely for the perfuming of toilet soaps. Specific gravity at 15° C., 0.880-0.886; optical rotation, +10° to +20°; esters, calculated as linalyl acetate, 35-40 per cent., and occasionally as high as 42-43 per cent.; frequently soluble in 1.5 parts of 80 per cent. alcohol, or failing that, should dissolve in one volume of 82.5 or 85 per cent. alcohol. When evaporated on the water-bath the oil should not leave more than 5-6 per cent. residue.
Among the constituents of this oil are: linalyl acetate, limonene, dipentene, linalol, and bergaptene.
Bitter Almond Oil.—The volatile oil obtained from the fruit of Amygdalus communis. Specific gravity at 15° C., 1.045-1.06; optically inactive; refractive index at 20° C., 1.544-1.545; boiling point, 176-177° C.; soluble in 1 or 1.5 volumes of 70 per cent. alcohol.