In a preceding chapter on the chemical properties of the vegetable substances many of their characteristics have been described. In this place we need only describe the physical properties of the essential oils, and with some of them to lay stress on those peculiarities by which they are specially differentiated. This knowledge is of the greatest importance to the manufacturer of perfumery because no single individual is in a position to prepare all aromatic substances himself, but must rely on commerce for some of them; and in no group of chemicals is adulteration as frequent and as difficult of demonstration as among the aromatics. These adulterations are carried so far that many essential oils occurring in commerce under certain names often have nothing in common with the substance for which they are sold but the name. OIL OF CASSIE. The oil of Acacia farnesiana is greenish-yellow and viscid; the density and boiling-point, which are of the greatest importance with reference to the genuineness of an essential oil, are not yet accurately known. Moreover, this oil never occurs in commerce as such, but its odor is present in perfumes, fixed oils, and pomades. OIL OF ANISE should be colorless or faintly yellow; a dark yellow color indicates old and inferior quality. The characteristics of this oil are the odor, its aromatic sweet taste, and especially the property of solidifying at a comparatively high temperature, 10-15° C. (50-59° F.), which is due to the separation of a stearopten, anethol. Oil of anise is frequently adulterated with or replaced by oil of star-anise. The easy solidification of the oil of anise is not always proof of its good quality, for the oil from anise chaff, which congeals at a still higher temperature, is sometimes mixed with it, and this has a less fine odor than that distilled from the seed. One part by weight of oil of anise is soluble in an equal weight of alcohol of 94%. OIL OF BERGAMOT has a pale yellow color which becomes greenish when the oil is kept in copper vessels, and a strong agreeable odor. This oil requires the greatest care in its preservation, as it abstracts oxygen from the air with extreme rapidity, when it changes its superior odor so that it can hardly be distinguished from oil of turpentine. OIL OF BITTER ALMOND (OLEUM AMYGDALE AMARÆ), when pure, is a colorless, refractive liquid which is heavier than water. The vessels in which this product is preserved must be stoppered air-tight, for in the air the oil very quickly changes into a white, odorless mass of crystals consisting of benzoic acid.