OGIVE, in architecture, an arch, or branch of a Gothic vault ; which instead of being circular, passes diagonally from one angle to another, and forms a cross with the other arches.
OIL. The general character of oils are combustibility, insolubility in water, and fluidity. From the peculiar properties of different oils, they are naturally divided into two kinds ; fixed or fat oils, and vo latile or essential oils. The fixed, or fat oils, require a high temperature to raise them to the state of vapour, a tempera ture above that of boiling water ; but the volatile, or essential oils, are volatilized. at the temperature of boiling water, and even at a lower one. Both the volatile and fixed oils are obtained from plants, and sometimes from the same plant ; but always from different parts of it. While the seeds yield fixed oil, the volatile oil is extracted from the bark or wood. One of the most distinguishing characteristics of the fixed oils is, that they exist only in one part of the vegetable, in the seeds. No trace of fixed oil can be detected in the roots, the stem, leaves, or flowers of those plants, whose seeds afford it in great abundance, The olive may seem an exception to this. The oil which it yields is extracted, not from the seeds, but from its covering. Among plants too, fixed oils are only found existing in those whose seeds have a peculiar structure. The seeds of plants have sometimes one lobe, in which case they are called " monocotyledonous" plants ; and some times they have two, when they are de nominated " dicotyledonous." The formation of fixed oil in plants is exclusive ly limited to the latter class. There is no instance of fixed oils being found in the seeds of plants which have only one lobe. Those seeds which yield the fixed oils contain also a considerable portion of mucilage, so that when such seeds are bruised and mixed with water, they form what is called an emulsion, which is a white fluid, containing a quantity of the oil of the seed mixed with the mucilage. Fixed oils are extracted from the seeds of a great number of plants, Those which yield it in greatest abundance are, the olive, thence called olive oil ; the seeds of lint, and the kernels of almonds, called linseed, or almond oil. Fixed oils are also obtained from animals ; such as train oil, as it is called, which is extract ed from the fat or blubber of the whale. Fixed oil is obtained also in great abun dance from the liver of animals, and is found to exist in the eggs of fowls. These
different kinds of fixed oils, although they possess many common properties, yet in others they are very different. Many cf the vegetable oils hate no smell, and scarcely any perceptible taste. The animal oils, on the contrary, are generally extremely nauseous and offensive. These differences are supposed to be owing to the mixture of extraneous bodies, or to certain chemical changes which arise from the action of these bodies upon each other, or on the oil itself. As the fixed oils exist ready formed in the seeds of plants, they are generally obtained by " expression," and hence they have been called " expressed oils." This is done by reducing the seeds to a kind of pulp, or paste, which is enclosed in bags, and subjected by means of machinery, when it is obtained in the large way, to strong pressure, so that the oil flows out, and is easily collected. The oil which is obtained by this process, which has been called " cold drawn oil," be cause it is procured without the ap plication of heat, and merely by pressure, is the purest ; but the quantity which seeds in general yield is comparatively small, and some seeds which contain a considerable portion of oil scarcely afford any when treated in this way. It there fore becomes necessary for extracting the oil from seeds of the latter description, and to have it in greater abundance from all seeds, to employ heat to facilitate the separation of the oil from the mucilage, or other matters with which it is com bined. For this purpose heat is applied, either to the apparatus which is employ ed in pressing out the oil, or the bruised seeds are exposed to the vapour of water, and sometimes they are boiled in the wa ter itself; by which means those sub stances which are soluble in water are separated, and thus the oily part which adhered to these substances is disen gaged. The oils which are obtained in this manner are very impure. They are mixed with mucilage, and other parts of the substances from which they have been extracted. Many of these matters separate from the oils when they are left at rest. They are sometimes mechanical ly purified by filtration through coarse cloths, by which means the grosser parts are separated. Different oils too, it is slid, undergo different kinds of purihca.