RAPE OIL, an important fatty oil, known also as "sweet oil," either expressed or extracted from the crushed seeds of cultivated varieties of the cruciferous genus Brassica, the parent form of the whole apparently being the wild navew, B. campestris. Under the general name "rape oil" is included the produce of several plants having distinct and fairly constant characters, and one of these oils—colza (q.v.)—is a very well-known commercial variety. In Germany, where the production of rape oil centres, two prin cipal oil-seeds—rape and Riibsen—are recognized. (See RAPE.) The oil yielded by these seeds is, in physical and chemical properties, practically the same, the range of fluctuations not being greater than would be found in the oil of any specific seed under similar varying conditions of production ; the winter varieties of all the seeds are more productive than the summer varieties. Newly pressed rape oil has a dark sherry colour with, at first, scarcely any perceptible smell ; but after resting a short time the oil deposits an abundant mucilaginous slime, and by taking up oxygen it acquires a peculiar disagreeable odour and an acrid taste. Refined by the ordinary processes (see OILS), the oil assumes a clear golden yellow colour. In specific gravity it ranges between 0.9112 and 0.9117 (raw), and from 0.9127 to o.9136 (refined) ; the solidifying point is from —4° to —6° Centi grade.
The principal uses of rape oil are for lubrication and lighting; but since the introduction of mineral oils for both these purposes the importance of rape has considerably decreased. It is but little employed in soap-making, as it saponifies with difficulty and yields only an indifferent product. In Germany it is very
considerably used as a salad oil under the name of Schmalzol, being for that purpose freed from its biting taste by being mixed with starch, heated till the starch is carbonized, and filtered after the oil has cooled. The offensive taste of rape oil may also be removed by treatment with a small proportion of sweet spirits of nitre (nitrous ether). In the East Indies rape oil and its equiva lents, known under various names, are the most important of oils for native use. They are largely consumed as food instead of ghi under the name of "metah" or sweet oil, but for all other pur poses the same substance is known as "kurwah" or bitter oil. Most natives prefer it for the preparation of their curries, and other hot dishes. Rape oil is the subject of extensive adultera tion, principally with the cheaper hemp oil, rosin oil and mineral oils. These sophistications can be most conveniently detected, first by taste and next by saponification, rosin oil and mineral oil remaining unsaponified, hemp oil giving a greenish soap, while rape oil yields a soap with a yellow tinge. Rape oil and the other cruciferous oils are characterized by the presence of the unsatu rated fatty acid erucic acid, the identification of which affords a means of detecting rape oil in admixture with other oils. Lead plaster (Emplastrum lithargyri) boiled in rape oil dissolves, and, sulphide of lead being formed, the oil becomes brown or black. Other lead compounds give the same black col oration from the formation of sulphide.