ly becomes much
concentrated, and it requires high heat and careful management to
finish the solution of the bullion. Bars containing more than about 10
per cent. of copper cannot be dissolved at all, owing to the
separation of copper sulphate insoluble in the small amount of free
acid finally remaining. The advantage gained by dissolving bullion
with abundance of free acid in the improved process is so evident that
it merely requires to be pointed out. For bullion containing 20 per
cent. of copper the author employs six parts of acid to one of
bullion; for baser metal still more acid, and so on, never losing more
than the stochiometrical percentage of acid and recovering the
remainder. In this description he, however, confines himself to the
treatment of ordinary silver ore with less than 10 per cent. of
copper.

In the diagram A A represent two refining pots, 4 ft. in diameter and
3 ft. in depth, each capable of dissolving at one operation as much as
400 pounds of bullion. The acid is stored in the cast iron reservoir,
B, which is placed on a level sufficiently high to charge into A by
gravitation, and is composed of fresh concentrated acid mixed with the
somewhat dilute acid regained from a previous operation. After the
bullion is fully dissolved all the acid still available is run from B
into A A. The temperature and strength are thereby reduced, the fuming
ceases, any still undissolved copper sulphate dissolves, and the gold
settles. In assuming that the settling of the gold takes place in A
itself, the author follows the practice of the United States mints. In
private refineries, where refining is carried on continuously, the
settling may take place in an intermediate vessel, and A A be at once
recharged. Owing to the large amount of free acid present, the
temperature must fall considerably before the separation of silver
sulphate commences, and sufficient time may be allowed for settling if
the intermediate vessel be judiciously arranged.

[Illustration]

_Separating the Silver Sulphate._--The clarified solution is siphoned
off the gold from A A into C, which is an open cast iron pan, say 8
ft. by 4 ft. and 1 ft. deep. It is supported by means of a flange in
another larger pan--not shown in the diagram--into which water may be
admitted for cooling. Steam is blown into the acid solution, still
very hot, as soon as C is filled. The steam is introduced about 1 in.
below the surface of the liquid, blowing perpend