In testing a hard rocky gravel for gold, first finely powder a sample of the gravel, moisten it in a tall cylinder with water, and pass chlorine gas through it, whereby soluble chloride of gold is formed. After treating with chlorine the gravel should be washed with hot water, the solution collected in a dish, boiled to expel the chlorine, and then heated with solution of ferrous sulphate. If gold is present it will separate as a line brown powder. Another method is to take, say,1/4 lb. of the powdered gravel, mix it with litharge (oxide of lead) and flour or cream of tartar, and heat it in a crucible in a furnace. The litharge is reduced by the flour or cream of tartar forming metallic lead, which melts and, as it passes through the gravel, takes the gold with it to the bottom of the crucible. After heating, the crucible is broken open and the button of metallic lead is removed. It is first roasted in a dish in a muffle furnace to get rid of the greater portion of the lead as oxide; the oxidation is then finished on a bone-ash cupel, which absorbs the oxide of lead formed, leaving, at the end of the operation, a button of metallic gold, providing that metal was present in the gravel.