Fusel Oil, Or Amyl Alcohol, a liquid colorless when pure, of offensive smell and burning taste, obtained by continuing the distillation of the fermented infusions used for the preparation of ardent spirits after the alcoholic portion has been drawn off. In this condition, however, it is mixed with water, from which it should be separated by a second distillation, the water coming over first. As this brings with it a portion of oil, it is to be set aside for the latter to separate, and form a layer on the surface. Ardent spirits contain fusel oil, particularly if the distillation has been pushed far. It is detected by redistilling whiskey, especially that obtained from potatoes, a milky fluid coming over at the last, from which the oil separates by standing; or by redistillation, water first coming over, and then the oil at its boiling point of 269°. Thus obtained, it is usually of a pale yellow, of specific gravity 0.818; at 4° below zero it congeals in crystalline leaves. It inflames only when heated to 130°. It unites with alcohol in all proportions, but has little affinity for water. The resins, fats, camphor, sulphur, phosphorus, etc, are dissolved by it. Upon the animal system it acts as an irritant poison; its vapor produces nausea, headache, and giddiness.

Its composition is represented by the formula C5H12O; or, on the supposition of its being a hydrated oxide of amyle, its formula is C5H11HO. Fusel oil is used to some extent for burning in lamps, and for dissolving copal and other resins for varnishes, etc. Its presence is highly injurious to liquors, and when in sufficient quantity to be perceptible to the smell and taste indicates bad rectification or the use of damaged grain. It may be detected by agitating the liquor with water, and leaving it to stand for the oil to rise and appear at the surface. It is separated in rectifying by the introduction of some soft wood charcoal. Olive oil may also be added, and the mixture being well shaken the oils will afterward collect together at the surface, when they may be decanted and the spirits be again distilled.