174. When concentrations which are to be chlorinated for gold, contain also copper, the roasting is not pushed to the decomposition of copper sulphate. The latter is converted into chloride by an addition of salt, a short time before discharging the ore from the furnace. The leach is then green, and, after precipitating and settling the gold, it is drawn off, either through a filter or not, as desired, and led into tanks containing-iron, where it is allowed to stand until the tanks are again required. The iron goes into solution, and the copper is precipitated in the form known as "cement copper," which is collected periodically, washed on a filter, with hot acidulated water, dried and sold. As it may contain gold it should be assayed for that metal.