_Expression._--This is only practicable when the substances are especially rich in oil and of sufficient softness, as in the case with the peel of the orange, citron, lemon, etc. In such instances the material is simply placed in a linen cloth and subjected to a strong pressure until it ceases to yield oil. The press may be of any size according to the quantity to be expressed. For small quantities it generally consists of an iron vessel, having a small opening at the bottom so that the oil may flow out. The material is placed upon a perforated bottom inside of the vessel and covered with a well-fitting iron plate, that can be pressed down by means of a screw. Though the material is fairly exhausted by such a press, for large operations it is advisable to make use of a hydraulic press, which is constructed and managed in exactly the same manner as those used for the expression of fixed oils. By expression a turbid milky fluid is obtained, which consists of the volatile oil and aqueous substances. The latter are a solution of various extractive substances and salts in water. This fluid, as it runs from the press, is received in tall, narrow, glass vessels and brought into a cool place for clarification. This frequently requires several days, three distinct layers being generally distinguished. On the bottom is a mucous layer consisting of cell substances carried along by the liquid bodies. Over this is a clear fluid consisting of a solution of extractive substances, vegetable albumen, and salts, and upon this floats the volatile oil, being specifically the lightest body, which, by its greater refractive power, can be clearly distinguished from the aqueous fluid. The oil is separated by bringing all that has been expressed into a bottle provided near the bottom with a lateral neck closed by a cock. After separating the oil from the aqueous fluid, the latter is allowed to escape by opening the cock. The oil obtained in this manner is still impure, and requires further treatment to remove small vegetable fibres, invisible to the naked eye, which float in them, and cause them to be somewhat opaque and slightly opalescent. By their subsequent decomposition they would also give the oil a disagreeable odor. There are two methods of obtaining the oil entirely clear, viz., filtration and distillation. Filtration is the cheaper process, but requires special precautions to exclude the air as much as possible to prevent the oil from undergoing injurious changes. By arranging the filtering apparatus so that the oil always comes in contact with only the same quantity of air, the injurious action of the oxygen is reduced to a minimum. It is self-evident that the apparatus should not be placed in the sun, but in a semi-dark, cool place. [Illustration: FIG. 1.]