FIXED OILS. There are two speci mens of oil in vegetables, agreeing iu the common properties of unctuosity and in flammability, but essentially different in many of their chemical qualities. The one capable of being volatilized without decomposition, is named volatile oil, the other is fixed oil.
The latter is generally contained in the seeds and fruits of vegetables, and varies in its properties, according to the plants from which it is obtained by pressure, and frequently called expressed oils. When the process is aided by heat, the action of which is to render the oil more fluid, the product is esteemed less pure. The purest fixed oils are those expressed from the fruit of the olive, or the seeds of the almond ; others, less pure, come from the flax-seed and hemp-seed. These oils are usually fluid, but of a somewhat thick consistence, and liable to congeal at very moderate colds ; palm-oil is even, na turally, concrete. When fluid, they are transparent, of a yellow or yellowish green color, and capable of being render ed quite transparent by the use of animal charcoal. They arc inodorous and in sipid, at least if they have been obtained with due care ; and free from the muci laginous and extractive matter of the plants from whence they come ; are lighter than water, with which they do not unite, and are very sparingly soluble in alcohol, with the exception of castor oil. At a temperature below 600° Fahr., they remain unchanged.
Near this temperature, however, they begin to boil, and to disengage an in flammable vapor ; but the oil thus con densed is altered in its properties ; it loses its mildness, becomes more limpid and volatile, a portion of carbon being likewise deposited. Transmitted through an ignited tube, fixed oil is convert and into carbonic acid and carbureted hydro gen, with a small portion of acid liquor, and a residuum of charcoal. In the open air, it burns with a clear white light, and formation of water and car bonic acid gas. Accordingly, the fixed oils are capable of being employed for the purposes of artificial illumination, as well in lamps as for the manufacture of gas. Fixed oils undergo considerable change by exposure to the air. The ran cidity which then takes place is occa sioned by the mucilaginous matters which they contain becoming acid.
From the operation of the same cause, they gradually lose their limpidity, and some of them, which are hence called drying-oils, become so dry, that they no longer feel unctuous to the touch, nor give a stain to paper. This property, for
which linseed-off is remarkable, may be communicated quickly, by heating the oil in an open vessel. The drying-oils are employed for making oil-paint, and, mixed with lamp-black; constitute print ers' ink.
During the process of drying, oxygen is absorbed in considerable quantity. This absorption of oxygen is, under certain circumstances, so abundant and rapid, and accompanied with such a free disengagement of caloric, that light, porous, combustible materials, such as lamp-black, hemp, or cotton-seed may be kindled by it. Many instances of spontaneous combustion have occurred from this cause. It appears that if hemp, flax, or linen cloth, steeped in linseed oil, lie in a heap, and be somewhat press ed together and confined, its temperature rises, a smoke issues from it, and, at length, sometimes within 24 or even 12 hours, it takes fire. The same thing happens with mixtures of oil and fine charcoal, and with lamp-black wrapped up in linen ; from whence it is conjec tured, that many extensive fires, which have broken out in cotton manufactories, and for which no cause could be assigned, must have arisen from this spontaneous inflammability of oils.
Fixed oils unite with the common me tallic oxides. Of these compounds, the most interesting is that with the oxide of lead. When linseed-oil is heated with a small quantity of litharge, a liquid re sults which is powerfully drying, and is employed as oil-varnish. Olive-oil, com bined with half its weight of forms the common diachyonykster. The fixed oils are readily attacked by alkalies. With ammonia, they form a soapy liquid, to which the name of volatile liniment is applied.
They are oxidated by a number of the acids. Sulphuric acid soon renders them black ; the oxygen of the acid attracting part of the hydrogen of the oil, and causing the deposition of charcoal ; and, if beat is applied, a large portion of sul phurous acid is disengaged, and even sulphur is evolved. Nitric acids renders them thick; if heat is applied, the action is more rapid, and a yellow color is com municated, the oil being rendered crete. Chlorine thickens oil, and renders it white. When boiled in sulphur, a compound is formed of a brown color, a very fetid smell, and acrid taste. It like wise, when heated, dissolves phosphorus, forming a liquid which becomes lumi nous, when exposed to the air. Olive oil consists of carbon 77.218, oxygen 9.427, and hydrogen 13'360.