When a drying oil which has been treated with metallic lead, or with litharge, is shaken up with a solution of zinc sulphate, all the lead is precipitated from the oil, and zinc passes into solution therein. By manganese sulphate or copper sulphate, the lead is removed by manganese or copper. Oil charged with lead dries in 24 hours when spread out in a thin layer on glass; it will dry completely in 5 or 6 hours if charged with manganese, in 30 or 36 hours with copper, zinc, or cobalt; and it requires more than 48 hours with nickel, iron, chromium, &c. Although solidification of a drying oil charged with manganese takes place in from 5 to 6 hours when spread in thin films, the solidification of thicker films requires a longer time. A temperature of 122° to 140° F. accelerates the oxidation of the drying oils, partly because the oil becomes more fluid, and partly because the oxygen is more active at a higher temperature. Hence, oil which has been mixed with an equal volume of turpentine, or a light hydrocarbon, such as benzene, dries more rapidly than oil without such admixture. When a boiled oil, prepared with manganese, is dissolved in an equal volume of benzene, and shaken up with air in a bottle, rapid absorption of oxygen occurs, especially about 120° F. If fresh air is repeatedly provided, the oxidation is sufficient to cause the liquid to become thick, and, on distilling off the sapient, a perfectly dry and elastic solid remains. An oil containing manganese is a very superior drying oil to one which has been prepared with lead. This fact, however, is to be noted, that though a large proportion of manganese in an oil may hasten its drying, yet it is disadvantageous, because it does not form so tough a film. This arises from the film becoming hard upon the surface, and so protecting the oil underneath from absorbing oxygen from the air. Though the oils containing large quantities of dryers dry, they afterwards lose weight, and become viscous under the same conditions. Pure linoleates of lead and of zinc are not dryers; but if heated until it has turned brown, or begun to blacken, a lead dryer becomes effective, although it contains less of the lead compound. In this case, some compound of lead is formed by absorption of oxygen, which either itself actually oxidises or causes the oxidation of ordinary linseed oil. Having treated of the materials used for producing boiled oil, and of their action upon the oil, let us now consider how the operation is brought about. _Process 1._--Oil is boiled at a high temperature, that is to say, it is heated until frothing and bubbles of gas escape, when litharge or a manganese compound is added. _Process 2._--Oil is boiled at a steam heat, with litharge or a manganese compound, in conjunction with a blast of air.