When oil-boilers were compelled to adopt some expedient to give a reddish-brown colour to the oil, they added a small amount of litharge, the introduction of which actually spoils the oil, and makes it unsuitable for many purposes to which it is otherwise applicable. Of late years, pale boiled oils have been more largely manufactured for special purposes. It is obvious that, for decorative house painting, in which delicate tints are a leading feature, they may be advantageously employed. Notwithstanding that some of the brown oils, when mixed with white lead, do not entirely retain the brownish tint, but, to some extent, lose it upon drying, yet they never preserve the whiteness of white lead. It follows, therefore, that a pale colour in the oil, provided it is not the yellow colour of raw oil, is greatly to be preferred. Moreover, when paints are mixed with zinc white, no trace of lead should be contained in the oil, otherwise, one of the valuable properties of zinc white pigments is destroyed, namely, its power to retain its whiteness in the atmosphere of a town, because its colour is not affected by sulphuretted hydrogen. Very generally, zinc white and white lead paints are not mixed with drying oils, but with refined linseed or bleached oil. This, at any rate, is the practice on the Continent. That is to say, the pigments are mixed with an oil from which the impurities, and the natural yellow and red colouring matter, have been removed, so that the colour of the paint is white. If ordinary oil be used, the paint is more or less yellow. In order to render such paint quick drying, a certain amount of dryers, in a solid or liquid form, is added. These dryers almost invariably contain lead, so that zinc white paint is contaminated by lead in another way, which may not be suspected, or which is overlooked. Now as to the chemical action of dryers on oils. Raw oil contains water and mucilage; the former can be absorbed by anhydrous zinc salts and by dried alum, and solutions of the salts and the salts themselves are capable of precipitating mucilage from the oil; hence these substances cause the impurities to become insoluble, so that they are carried down as “foots.” Heat greatly facilitates this action, particularly by causing the oil to become more fluid; and by the action of the anhydrous salts water is withdrawn from the oil. On the “drying” or oxidation of the oil, they exert no chemical action whatever. Zinc linoleate and lead linoleate do not act as dryers when simply added to the oil. Though the former is soluble in hot oil it is insoluble in cold oil, and it therefore separates from the oil as it cools. The latter is very soluble in linseed oil, but only adds to its drying power when heated therewith. In conjunction with a high temperature, lead dissolves in oil at the expense of the glycerine, which is decomposed into acrolein, while lead linoleate is formed.