Certain aromatic substances that occur in large amounts in some parts of plants, are best obtained by pressure. The rinds of certain fruits contain an essential oil in considerable quantities inclosed in receptacles easily distinguished under the microscope. When these vegetable substances are subjected to strong pressure, the oil receptacles burst and the essential oil escapes. The force is usually applied through a screw press with a stout iron spindle; the vegetable substances being inclosed in strong linen or horse-hair cloths, placed between iron plates, and subjected to a gradually increasing pressure. Comparative experiments have shown us that even with the most powerful presses a considerable, amount of oil is lost owing to the fact that a large number of oil receptacles remain intact. For this reason, when oil is to be extracted by pressure, a hydraulic press is preferable, as it develops greater power than any other press. In the hydraulic presses used for this purpose the piston fits exactly into a hollow iron cylinder with sieve-like openings in its circumference. The vegetable substances are filled into this cylinder; when the pressure is applied, the fluids escape through the perforations, and the residue forms a compact woody cake which is then free from oil. Besides the essential oil, watery fluid is expressed, the whole appearing as a milky liquid, owing to the admixture of vegetable fibres, mucilage, etc. It is collected in a tall glass cylinder which is set in a place free from any vibration. After remaining at rest for several hours the liquid separates into two layers, the lower being watery and mixed with mucilage, that floating on top being almost pure oil. The latter is separated, and finally purified by filtration through a double paper cone in a funnel covered with a glass plate. [Illustration: FIG. 4.] It is best to separate the water and oil in a regular separatory funnel, or in a simple apparatus illustrated in Fig. 4. It is made by cutting the bottom from a tall flask, and fitting into the neck by means of a cork a glass tube having a diameter of one-fourth to one-half inch. A rubber tube with stop-cock is fastened to the glass tube. By careful opening of the stop-cock, the watery fluid can be drained off to the last drop. To the perfumer this method is of little importance, since it is applicable only to a few substances which, moreover, give cheap odors. Still, the possession of a hydraulic press is advisable to every manufacturer who works on a large scale, as it is useful also in the preparation of several fixed oils frequently employed in perfumery, for instance, oils of almonds, nuts, etc. [Illustration: FIG. 5.] [Illustration: FIG. 6.]