Take any quantity of linseed oil and put it in a clean iron pot, and hang it over a slow fire, and when it attains to a good heat add litharge and white vitriol (sulphate of zinc) in very small quantities, until the whole is added, when it should be boiled slowly for two or three hours. Twelve parts of litharge to three of the sulphate of zinc, are employed, and two ounces of this mixture to one pint of oil, does very well. If these drying materials were added hastily to the oil, it would fume over on the fire. Care must be exercised to prevent such an accident. After the mixture is boiled for two or three hours, the oil is taken off, and suffered to cool, when a sediment falls to the bottom, and the clear is poured off as drying oil. The sediment when mixed with whiting or ground chalk, and dry sand, makes capital cement for in seams in the roofs of buildings, or any crack to render the same impervious to water. It becomes as hard as stone in the course of a tew weeks, and is especially adapted for the joints of stones in exposed situations. We have been enquired of by three or four correspondents lately fespecting the above drying oil ; we have only to add that if an ounce ot resin be added to every pint of oil when boiling, it will improve the quality of the oil in no small degree.