3. Oil, mixed with white lead, zinc white, antimony white, and arseniate of tin, acts differently, thus:--The white lead dries most rapidly, the zinc white next, but antimony white and arseniate of tin are incapable of acting as dryers, in fact, they retard the drying process. 4. Oil boiled alone for five hours, and boiled for a further period of three hours with manganese dioxide becomes a superior drying oil to one which has been boiled with manganese dioxide for eight hours. From a series of experiments, which were continued for two years, on twenty-five weighed quantities of raw linseed oil, Prof. Hartley draws the following deductions:-- 1. The chemical action of a manganese compound, when dissolved in linseed oil, is that of a carrier of oxygen from the atmosphere to the oil. Manganese oxide takes up oxygen from the air, and transfers it to the oil, and in so doing it suffers alternately the opposite processes of oxidation and reduction. 2. To obtain the best result, the amount of manganese present must not exceed a certain small proportion of the oil. 3. Oil to which turpentine has been added dries more rapidly than oil without such addition, because the oil being diluted and rendered thinner, it spreads over a larger surface, and is in contact, therefore, with a much larger quantity of oxygen. 4. Turpentine does not act as a dryer, that is, as a carrier of oxygen to linseed oil. 5. Different white pigments behave differently when drying, because the more powerfully basic the properties of the pigment, the more powerful is its action as a dryer. Lead oxide and white lead (basic lead carbonate) combine more easily with the acids of linseed oil than zinc oxide does. But zinc oxide dries better than antimony oxide, because it is a stronger base, while arseniate of tin has no basic properties, therefore does not act as a dryer. Different substances, that is to say, those without chemical action on oil, such as lamp-black, sulphate of baryta, and sulphate of lead, cannot act as “dryers.” Linseed oil is a glyceride of a peculiar acid, called linoleic acid. Whatever the exact constitution of linoleic acid may be, linseed oil for the most part is composed of trilinolein. Raw linseed oil contains the following constituents:-- 1. Glyceride of linoleic acid or trilinolein {C_{18}H_{31}O}O_{3}. { C_{3}H_{5} } 2. Water. 3. Mucilage, with the composition _n_(C_{6}H_{10}O_{5}). On boiling with dilute acids this yields a gum and a sugar. 4. An essential oil, present in minute proportions, and of unknown composition. 5. A mixture of colouring matters of intense tinctorial power, viz. blue and yellow chlorophylls and erythrophyll. The only useful and desirable substance is the trilinolein.