White Lead is used in very large quantities by painters, not only as white paint, but to mix with and qualify every shade of color, and to give body to them. It is prepared by different processes, and several patents have been taken out for improvements in its manufacture; but the most usual, and, perhaps, the best process, is as follows:--A layer of spent tan (from the tanner’s pits) is spread out three or four feet thick, and in it a number of earthen pots are arranged in rows, each partly filled with a mixture of vinegar, water, and treacle, or some other acid fermenting liquid, each of these pots is covered with a piece of lead made in the form of a grating (fig. 1), and over all these a flooring of boards, and then again a layer of tan, pots covered with lead, and boards, two or three times repeated; this is called a white lead bed, and is left for several weeks; the tan ferments and gets warm, causing the acid vapours to rise and corrode the lead, at the same time giving off carbonic acid. At the end of about three months the leaden covers are found to have become completely changed into a white shining substance (carbonate and oxide of lead), this is washed from impurities and ground in a mill, under water, to a fine powder. It is the white lead of commerce; but what is familiarly called white lead, is this substance ground up with linseed oil into a thick paste so as to be ready for the use of painters. White lead is a very poisonous substance, and produces the disease called painters’ colic, when taken into the system in minute quantities and for a long time, so that all who have much to do with this dangerous substance, as house-painters and artists, should be extremely careful that their hands are well washed frequently, and especially before going to meals. Cisterns of lead, used for containing water, very soon become coated inside with a thin film of sulphate of lead, this prevents the water from acting further on the lead, and the water from such cisterns is never found to be poisonous; but, if distilled water were used, it would act rapidly on the lead, corroding it, and causing a deposit in the water of white lead, which would render such water dangerous in the extreme. PRUSSIAN BLUE.