The scoria[1] of lead is also made use of; the best kind being that which approaches nearest to a yellow colour, without any vestiges of lead, or which has the appearance of sulphur without any terreous particles. It is broken into small pieces and washed in a mortar, until the mortar assumes a yellow colour; after which, it is poured off into a clean vessel, the process being repeated until it deposits a sediment, which is a substance of the greatest utility. It possesses the same properties as lead, but of a more active nature. How truly wonderful is the knowledge which we gain by experiment, when even the very dregs and foul residues of substances have in so many ways been tested by mankind!