The white colour used in its preparation being white lead, it cannot be depended upon to keep its colour entirely. Some pieces originally white, which the writer has put by during the last twenty years, are of a dun grey or chamois colour, although they have been exposed only to ordinary atmospheric influences. If the canvas were prepared with some white substance other than white lead, as, for instance, sulphate of barium and zinc oxide, or silica, or zinc sulphide in an oleoresinous medium, or drying oil prepared without alkali or acid, we might expect such a canvas to retain its colour and not be darkened when exposed to foul air.,