From vegetable substances, such as seeds, the fat is extracted either by pressure or by the use of solvents. For quantitive purposes, only solvents are employed. The dry, finely ground material is exhausted with anhydrous ether or petroleum spirit, in one of the convenient forms of apparatus already described (=33->43=). In very oily seeds great difficulty is experienced in securing a fine state of subdivision suited to complete extraction. In such cases it is advisable to conduct the process in two stages. In the first stage the material, in coarse powder, is exhausted as far as possible and the percentage of oil determined. The residue is then easily reduced to a fine powder, in an aliquot part of which the remaining oil is determined in the usual way. [Illustration: FIG. 79.—OIL PRESS.] In securing oils for physical and chemical examination both pressure and solution may be employed. The purest oils are secured by pressure at a low temperature. To obtain anything like a good extraction some sort of hydraulic pressure must be used. In this laboratory a press is employed in which the first pressure is secured by a screw and this is supplemented by hydraulic pressure in which glycerol is the transmitting liquid. The construction of the press is shown in the accompanying figure. The whole press is warmed to nearly 100°. The hot finely ground oily material, enclosed in a cloth bag, is placed in the perforated cylinder and compressed as firmly as possible by turning with the hands the wheel shown at the top of the figure. The final pressure is secured by the screw shown at the bottom of the figure whereby a piston is driven into a cylinder containing glycerol. The degree of pressure obtained is equal to 300 atmospheres. Even with the best laboratory hydraulic pressure not more than two-thirds of the total oil contents of oleaginous seeds can be secured and the process is totally inapplicable to securing the oil from tissues when it exists in quantities of less than ten per cent. To get practically all of the oil the best method is to extract with carefully distilled petroleum of low boiling point.