[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 perpendicularly downward from a nozzle made of lead pipe through an aperture 1/8 in. in diameter.  Under these circumstances the absorption of the steam is nearly perfect, and takes place without any splashing.  The temperature rises with the increasing dilution, and may be regulated by the less experienced by manipulating the cooling tank.  An actual boiling is not desired, because it protracts unnecessarily the operation by the less perfect condensation of the steam.  No separation of silver sulphate occurs during this operation (and, consequently, there is no clotting of the steam nozzle), the large amount of free acid, combined with the increase of temperature, compensating for the diminution of the solubility of the sulphate by the dilution.  The most important point in this procedure is to know when to stop the admission of steam.  To determine this, the operator takes a drop or two of the solution upon a cold iron plate by means of a glass rod and observes whether after cooling the sample congeals partly or wholly into a white mass of silver bisulphate, or whether the silver separates as a monosulphate in detached yellow crystals, leaving a mother liquor behind.  As soon as the latter point has been reached, steam is shut off and the solution is allowed to crystallize, cold water being admitted into the outer pan.  The operator may now be certain that the liquid will no longer congeal into a soft mass of silver bisulphate, which on contact with water will disintegrate into powder, obstinately retaining a large amount of free acid; but the silver will separate as a monosulphate in hard and large yellow crystals retaining no acid and preserving their physical characteristics when thrown into water.  After cooling to, say, 80 deg.  F., the silver sulphate will have coated the pan C about 1 in. thick.  There will also be found a deposit of copper sulphate when the mother acid, after having been used over and over again, has been sufficiently saturated therewith.  Lead sulphate separates in a cloud, which, however, will hardly settle at this stage.

The whole operation just described, which constitutes the most essential feature of the author’s improvement upon his old process described in Dr. Percy’s work, is a short one, as the acid requires by no means great dilution.  The steam has merely to furnish enough water to dilute the free acid present to, say, 62 deg.  B. Areometrical determination is, of course, not possible, on account of the dissolved sulphates.