Lion by different manufacturers, but these processes are kept secret. After they have remained at rest for some time, they are filtered and agitated with water, by which the parts that are soluble in this fluid are separated from the oil. Some times they are gently heated, for a short er or longer time, according to the na ture of the substances with which the oil is contaminated. Acids diluted with wa ter are employed to separate the muci lage ; lime and the alkalies are also used to combine with an acid which holds this mucilage in solution, and thus to favour its precipitation. Alum, chalk, clay, and ashes, are also employed in the purifica tion of oils.
Fixed oils are generally liquid, but of a thick, viscid consistence, and in general they are lighter than water. The speci fic gravity varies from 0.91, which is that of olive oil, to 0.94, that of linseed oil. The boiling point of the fixed oils is not under the temperature of 600°. When exposed to cold they congeal, and even crystalize. There is, however, a con siderable variety in this respect among fixed oils : some become solid at the tem perature of a few degrees above the freezing point of water ; while others, on the contrary, require a degree of cold .5° ; and some remain fluid when ex posed to the greatest cold. Those oils, it has been observed, which most readily become solid, such as olive oil, are least subject to change ; while those which congeal with difficulty have a greater tendency to spoil and become rancid. When fixed oil is exposed to heat it does not evaporate, till it is raised to the tem perature of boiling, or 600° ; but when it is thus raised in vapour its properties are changed. It is decomposed by the sepa ration of some of its principles. The part that is volatilized has a greater proportion of hydrogen ; charcoal is deposited, and water and sebacic acid are formed, while carbonated hydrogen gas is disengaged. When oil is exposed to the open air, and a burning body is brought in contact with it, it readily takes fire, and burns rapidly, with a yellowish white flame. It is on this conversion of oil, into vapour, and the inflammation of this vapour, that the ap plication of oil in lamps and candles de pends. The oil is gradually and in small quantities brought in contact with the burning part of the wick ; it is converted' into vapour, which is immediately in-, flamed, and continues to burn till new portions are supplied, to undergo the same change, and thus keep up a constant and uniform light and heat. According to the analysis of olive oil by Lavoisier, it is composed of hydrogen and carbon, 'Liz.
Carbon 78.92 Hydrogen 21.08 100.00 The fixed oils are insoluble in water. When it is necessary to combine them with this liquid, it is by means of mucila ginous substances, in which case the mix ture is known under the name of emul sion; or with alkaline substances, when it is distinguished by the name of soap.
Some of these oils become thick, opaque, white, granulated, and are analogous in appearance to tallow. Oils subject to this change are called fat oils ; such, for in stance, is olive oil, almond oil, and rape seed oil. This change is more or less rapid in different circumstances. If a thin layer of oil be spread on the surface of the water, and exposed to the air, it takes place in a few days, and this effect is owing to the absorption of oxygen, which combines with the oils. But other oils, when they are exposed to the air, dry altogether, yet have the property of retaining their transparency. Oils which have this peculiar property are called ing oils. The oil of poppies, hemp-seed oil, and particularly linseed oil, are pos sessed of this property. The nature of the change which takes place in these drying oils is supposed to depend on the absorption of oxygen ; and this oxygen combining with the hydrogen of the oil forms water. This opinion is supported by the practice which is followed to in crease the drying property of linseed oil. It is usually boiled with litharge, before it is employed by painters. The litharge in this case is partly reduced to the me tallic state, by being deprived of its oxy gen, which is supposed to combine with the oil. Phosphorus combines with oils, with the assistance of heat. A small por tion of the phosphorus is dissolved, which communicates a luminous property to the oils, so that when they are spread upon any surface they shine in the dark. Hence some twenty years ago a person exhibit ed in London, as the everlasting lamp of the ancients, a vessel containing phos phorus immersed in oil.
The various purposes to which fixed oils are applied, are too well known to require particular enumeration. They are employed in domestic economy, either as articles of food, and for this purpose are used alone, or in combination with other substances ; or they are employed for giving light, by being burnt in lamps. They are used in medicine, either on ac count of the properties which peculiar oils possess, or on account of the proper ties they communicate to other substances with which they are combined. In this state the use of oils is well known in the form of unguents, plasters, and liniments. In the arts, fixed oils are of the most ex tensive utility. They are employed in the fabrication of soaps, for mixing colours in painting, for some kinds of varnish, and for defending substances from the ac tion of air and moisture.