ODOMETER. An instrument for measuring the distance travelled over by a post-chaise or other carriage ; it is attached to the wheel, and shows, by means of an index and dial-plate, the distance gone over.
OIL. The distinctive characters of oil are unctuosity and inflammability, insolubility in water, and fluidity at moderate temperature'. Oils are distin guished into fixed, or fat oils, which do not rise in distillation, at the temperature of boiling water ; and volatik, or essential oils, which do rise at that temperature with water, or under 320° by themselves. The latter having been treated of under the word ESSENTIAL, in the preceding part of this work, we shall here confine our attention to the former class chiefly.
Fixed oils are generally contained in the seeds and fruits of those vegetables of which they are the products, and are formed principally at the period of maturity. They are extracted by pressure, sometimes with, and sometimes without, the aid of heat. They are usually impregnated with the mucilaginous or extractive matter of the vegetable, whence they acquire colour, odour, and taste ; and if heat has been employed to favour their extraction, they acquire acrimonious qualities, and undergo a change in some of their chemical properties. The purest oils are those expressed from the fruit of the olive, or the seeds of the almond ; others less pure are extracted from linseed, hemp seed, and numerous other seeds of plants. Fixed oils are usually fluid, but of a thick consistence, and they congeal at moderate temperatures ; some are even naturally concrete. When fluid, they are transparent, colourless, or of a yellowish or greenish tinge, inodorous, and insipid ; they are lighter than water. The following table exhibits the specific gravities of the principal sorts, of commerce ; water being 1.000 :— Fixed oils are incapable of combining with water ; and are very sparingly soluble in alcohol, in the cold, with the exception of castor, which is abundantly dissolved by rectified alcohol, and of linseed oil, which is dissolved, thong', more sparingly; boiling oil dissolves it, and also the others in sensible quality.
Expressed oils cannot be volatilised by heat, without a change of their pro perties. At temperatures below 6000 of Falw. they remain fixed, if the heat has not been for a long time continued. At the temperature mentioned, they are converted into vapour, but the oil condensed therefrom is altered in its properties ; it has lost its mildness, and has become more limpid and volatile, a portion of carbon having been deposited. Transmitted through an ignited tube, oil is converted into carbonic acid, and carburetted hydrogen, with a small portion of acid liquor, and a residium of charcoal. Exposed to a warm atmo sphere, expressed oils gradually acquire a sharp taste and smell, and become thick. This change, termed rancidity, is owing to absorption of oxygen. Drying oils, as those expressed with the aid of heat are named, do not become rancid, but by absorbing oxygen, are partially converted into a resinous kind of matter. At the temperature of ignition, at which it is converted into vapour, oil burns in atmospheric air, a large quantity of light and heat being extricated by its combustion. When the access of the air to the vapour of the oil is insufficient, it burns with a black smoke, and a quantity of carbonaceous matter which has the combustion is deposited. Hence the utility of a slender wick, up the oil by capillary attraction, and when kindled, produces suffi cient heat to convert it into vapour. In a hollow cylindrical wick, like that in the Argand lamp, through which an internal circulation of air is established, the supply of air is more abundant, and the whole of the oil is consumed ; the illumination therefore is greater, though there is some diminution of it in con sequence of the light from the internal surface having to pass through the flame. Expressed oils combine with the alkalies, and form soap. which see.
Expressed oils dissolve phosphorus by the aid of heat, forming a which becomes luminous when exposed to the air. They combine with a number of the metallic oxides, and acquire thereby a drying property. Boiled with Wilde of lead, expressed cal forms a compound of firm consistency, constituting the "common plaster" of the apothecaries. Expressed oils form the basis of paints (see PAINTING), and are hence called oil-colours, (see also, OIL-COLOUR VALES, subjoined to this article.) Expressed oils, combined with resins and turpentine, form varnishes ; (see Vault's.) Combined with lamp-black, they form printing ink ; (see Ian.) For most of these uses, however, the drying-oils are employed. There are two distinct processes of obtaining Oil by pressure; one cold, the other warm ; the cold-drawn oil being preferable for one purpose, and the warm for another. In the former, the substances are submitted to pressure, without increasing their natural temperature; in the latter, heat is artificially applied, generally through the medium of steam or air. The application of heat to seeds and most oleaginous matters, causes a great quantity of the oil to flow out, without pressure; and heat softens them so much, that less mechanical force becomes necessary to expel the remainder. It is therefore an indispensable point of economy to make use of heat whenever the application of it does not deteriorate the quality of the oil ; for more oil is thus obtained with less labour. In the large manufactories, linseed and rape-seed are the chief vegetable substances from which oil is obtained in this country ; heat is usually employed before pressure, and the separate products of oil in the different stages of manu facture are preserved, as distinct qualities. The ordinary " mill ' for this pur pose consists usually of an extensive range of machinery, and is usually distinguished by the denomination of the Dutch Mill, as the industrious people of Holland were the inventors, or chief improvers of it. In these, the seeds are put into bags, and covered with envelopes, consisting of hair-cloth, and sheep skin sewn together; in this state, they are subjected to pressure by the force of wedges, that are continually being struck by perpendicular stamper,. These 'tampers are raised by cams fixed to a revolving axis, (worked by a steam engine, or other adequate power,) and fall from the height they are thus raised upon the wedge'. The oil thus expressed, runs off, and is conducted to a cistern ; and therefore the purest, first flows into, and fills the lower chamber of the filter, and is followed by the less pure portions, according to their respective specific gra vity; but as the pipe enters this chamber at the top, those impurities that are con siderably heavier than the oil will subside to the bottom, and are from time to time to be discharged through the cock. The rest of the oil rises through the perforated plate, is separated from the lighter impu rities by the charcoal or sand in the mid dle chamber, and then passes through the upper plate into the top chamber, whence it flows through the cock 1. The two perforated plates must rest on rings or projecting ledges, that the charcoal may be renewed and the lower plate may be taken out occasionally, and cleared of the dregs which otherwise would stop its holes.