Volatile Oils, procured by distillation from turpentine, and other vegetal substances, are almost destitute of the strength of the expressed oils, having hardly more cementing power in painting than water alone, and are principally useful as solvents, and media of resinous and other substances introduced into vehicles and varnishes. In drying they partly evaporate, and partly by combination with oxygen form resins, and become fixed. They are not, however, liable to change colour like expressed oils of a drying nature; and, owing to their extreme fluidness, are useful diluents of the latter: they have also a bleaching quality, whereby they, in some degree, correct the tendency of drying and expressed oils to discolour-ment. Of essential oils, the most volatile, and nearest in this respect to alcohol is the oil of sassafras, but that most used in painting is the OIL OF TURPENTINE; the rectified oil, improperly called spirit of turpentine, etc. is preferable only on account of its being thinner, and more free from resin. By the action of oxygen upon it water is either generated or set free, and the oil becomes thickened, but is again rendered limpid by a boiling heat upon water, in which the oxygen and resin are separated from it. When coloured by heat or otherwise, oil of turpentine may be bleached by agitating some lime powder in it, which will carry down the colour.