Olive oil is refined, for the use of watchmakers, in the following way. Into a bottle or phial containing the oil, a slip of sheet-lead is immersed, and the bottle is placed at a window, where it may receive the rays of the sun. The oil by degrees gets covered with a curdy mass, which after some time settles to the bottom, while itself becomes limpid and colourless. As soon as the lead ceases to separate any more of that white substance, the oil is decanted off into another phial for use.
Passing over the processes of manufacture of many kinds of oil of less importance, we proceed to take linseed-oil, or the oil extracted from the seeds Of the flax-plant, as an illustration of the manufacture of oil from seeds. " Linseed, rape-seed, poppy-seeds, and other oleiferous seeds were," Dr. Ure observes, "formerly treated for the extraction of their oil, by pounding in hard wooden mortars with pestles shed with iron,zet in motion by cams driven by a shaft turned with horse or water-power ; then the triturated seed was put into woollen bags, which were wrapped up in hair-cloths, and squeezed between upright wedges in press-boxes, by the impulsion of vertical rams driven also by a cam mechanism." In the best mills on the old construction, he adds, the cakes of crushed seeds obtained by this first wedge-pressure were ground anew upon the bed of an edge-mill, and subjected to a second pressure, with the aid of heat. " These mortars and presses," ho states, " constitute what are called Dutch mills : they are still in very general use both in this country and on the Continent, and are by many persons supposed to be preferable to the hydraulic presses." Owiug to the extreme hardness and smoothness of the seeds of flax and hemp, and to the circumstance that the fragments' of their shells, however broken, form minute concavities which will retain the oil unless a greater pressure be applied than could be given by an ordinary serew-press, the presses employed for extracting oil from such seeds differ materially from those used in crushing olives and other com paratively soft oleagioous substances. Hence it is that the wedge press and Brainah's hydraulic press have been introduced for the purpose. Of these two powerful machines some manufacturers prefer the former, believing that the same degree of pressure is more efficient when imparted by means of sudden impulses or blows upon the end of a wedge, than when it is applied gradually and steadily as in the hydraulic press. In the wedge-press, of which there are many varieties, the crushed seeds are put into bags of hair-cloth or some similar material, and these bags are then placed between plates of iron united together like the covers of a book, or between boards or blocks of wood, within a very strong and massive framework. The small end of a wedge is then introduced in such a way between the plates or boards that, when it is driven down by the blows of a ram or pestle, it may compress the bags with enormous force. The driving of the wedges is continued until they are so tight that the pestle rebounds from them throe times, when they are judged to be sufficiently driven.
The use of the hydraulic press instead of this apparatus needs no minute explanation. Some of these act horizontally, the bags being, as in the wedge-press, placed vertically, and separated from one another by cast-iron plates ; but in others the bags are piled upon one another in cast-iron cases, and placed in a vertical press. The seed is put into bags of flannel or of horsehair. Among other advantages it is stated that the hydraulic or hydrostatic press requires less space than a stamping-mill which could do the same work, and that the hairs and bags are found to last longer with it than with the old machine.
A mode much adopted in this country, of obtaining linseed oil, we shall now describe. As the hardness and smoothness of the seed gives it a tendency to slide away unbroken under the rolling action of the millstones by which they are ground, it is desirable, before taking it to the grinding-mill, to bruise or crush the seed by causing it to fall from a hopper between two iron crushing-rollers, placed side by side, and capable of being pressed against each other with any determinate degree of force; but the use of such rollers is by no moans universal, the seeds being, in many cues, submitted to the grinding-mill without any such preparation. This mill, which is sometimes called an edga ms1/, consists of a pair of stones, technically called running-4mm, or -sinners, usually made of granite, resembling grindstones in shape, and from five to seven feet or upwards in diameter, so mounted as to roll sound in a circular path of small diameter upon a solid horizontal bed if stone or iron laid beneath them. These stones, which roll round he bed from thirty to thirty-six times in a minute, are sometimes moped with iron, though many prefer the rough surface of the granite, ;which may be re-dressed with a hammer as often as is needful. They ;rind the seed partly by their weight, which often amounts to three ;ens each, and partly by the peculiar rubbing motion which arises `rem the circumstance that the outer edge of the stone has to perform t larger circuit than the inner, although the two must of course revolve round the axle at one and the same speed. The action is therefore similar to that of a cone when forced to roll onward in a straight path. The two running-stones are mounted on the same horizontal axis, but at rather different distances from the central vertical shaft or iris round which they roll, so that they do not follow one another in precisely the same path on the bed of this mill. The bed is surrounded by a rim which prevents the seeds from being scattered; and the revolving framework in which the running-stones are mounted carries also two rakes or sweeps, which collect and lay the seeds in a ridge along the circular path of the runners. By this means the seeds are reduced, by the partial expression of the oil, to a pasty mass, from which a limited quantity of very fine cold-drawn oil may be obtained by the simple action of the press.