295. 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 oils. All oils become thickened by age, and more rapidly so by contact of air and combination with its oxygen; in which case if the oil be fat or unctious oil, such as olive oil and all animal oils, stearine, or tallow, is produced and separated from the elain, olein, or fluid oil; if it be a drying oil, such as linseed and painter's oil, caoutchouc or gluten, is in like manner produced; and if it be a volatile or essential oil, such as that of turpentine, solid resin is formed therein: a third and acid substance is formed in oils when they become rancid, called margarine, which is inimical to drying. Wax is produced by the action of oxygen on a compound fat and essential oil; way is therefore a substance between resin and stearine or tallow. 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 pureness and permanence, as the fat oils are for burning in lamps.