Oils are distinguished into Fat oils, Drying oils, and Volatile oils; the two first are also called fixed and expressed oils, as the latter are essential oik. All oils become thickened by age, and more rapidly so by contact of air and combination with its oxigen; in which case, if the oil be fat or unctuous oil, stearine, or tallow, is produced and separated from the elain, olein, or fluid oil; if it be a drying oil, caoutchouc, or gluten, is in like manner produced; and if it be a volatile or essential oil, solid resin is formed therein. All these substances may be regarded as oxides of elain, into which oils are wholly convertible; and, finally, by the action of time, air, and heat, they approach an elementary state, suffer incipient combustion, develope hydrogen, and become ultimately carbonized and darkened: in all which states oils are deteriorated for working freely and for painting with purcncss and permanence, as the fat oils are for burning in lamps.

All oils are soluble or miscible in water by the medium of alkalis, absorbent earths, or other metallic oxides, and are, therefore, capable of chemical union with pigments; they are partially soluble also in alcohol, and absorb or take up by agitation small portions of both alcohol and water, which they resign upon being heated.