EXPRES′SION

In the useful arts , the mechanical operation by which a fluid contained in the pores or cells of a solid is pre ed out or expelled. Many of the fluid substances employed in pharmacy and chemistry are obtained by expre ion. Thus, the unctuous vegetable oils, as those of almonds, linseed, ., are procured by submitting these substances to powerful pre ure between iron plates, which are either made warm, or the bruised seeds are previously exposed in bags to the steam of boiling water. The juices of fresh vegetables are also obtained by expre ion. The substances are first bruised in a marble mortar, or, on the large scale, in a mill, and immediately submitted to the pre , to prevent them pa ing into a state of fermentation, which would injure the quality of the product. Fruits which contain highly flavoured or fragrant seeds, or which have rinds containing e ential oil, are generally deprived of them before being sent to the pre . The subacid fruits are also allowed to lay together for some days before pre ing them, as the quantity and quality of the product is thereby increased. The fluid matter absorbed by the ingredients employed in the preparation of tinctures, infusions, decoctions, extracts, ., is generally obtained by powerful pre ure. Expre ion is also frequently had recourse to for the purpose of obtaining solids in a state of purity, as in the expulsion of olein from stearin, water from bicarbonate of soda, . On the small scale, the common screw-pre , or one of like construction, is usually employed; but the power thus obtained is insufficient to expel the whole of a fluid diffused through the pores of a solid. Hence has arisen the use of the hydraulic pre , which is now almost alone employed on the large scale. In all these cases the substances are placed in bags made of haircloth, or coarse canvas, previously to their being submitted to pre ure. For tinctures and like pharmaceuticals, a small screw-pre ( TINCTURE PRESS ) made of ‘galvanised’ or tinned iron, and varying in capacity from 1 quart to several gallons, is employed.