Preparing the Silver Sulphate.—­The bullion, containing, essentially, silver, copper and gold, is dissolved by boiling with sulphuric acid in cast iron pots.  The difference between the new process and the usual practice consists in the use of a much larger quantity of acid.  Thus, in refining ordinary silver “dore,” four parts of acid are used to one part of bullion.  Of this acid one part is chemically and mechanically consumed in the dissolving process, and the remaining three parts are fully recovered and at once ready for reutilization, as will be described hereafter.  In the usual process—­understanding thereby, here and in the following, the process practiced at the United States mints, for instance—­two parts of acid are employed for one of bullion; all of this is lost, partly through the dissolving and partly in being afterward mixed with water, previous to the precipitation of the silver by copper.  Economy in acid being therefore imperative, the silver solution finally 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.