In applying this scientific fact to the purpose of recovering gold from the concentrates, the latter are placed in vats with a weak solution of the cyanide in water. The time during which they are allowed to remain depends upon the size of the gold particles. If they be comparatively large, it stands to reason that it must be longer than if they be small, for they will take longer to dissolve. After the proper time, which is found by experiment, the liquid is drawn off, and in some cases the concentrates are given a second dose to ensure that the gold shall be thoroughly removed and none left undissolved. If the material being operated upon be very fine, as it often is, forming what the mining people call "slimes," then mechanical stirrers have to be used in the vats to keep the stuff moving, as otherwise the cyanide would not get to all the particles and some would not be acted upon. The liquid, having been the appropriate time in the vat, is drawn off, placed in wooden tanks or boxes, and fine shreds of zinc are added to it. Discs of sheet zinc are put into a lathe and a fine shaving taken off them, and it is these fine shavings which are used. Now zinc, as we know from the fact that it is the essential part in electric batteries, has very pronounced electrical properties, and it is believed that these come into play here. At all events the gold becomes deposited upon the zinc, while the zinc itself is to a certain extent eaten away by the solution. The result is (_a_) a solution weaker than it was before, (_b_) the remains of the shavings, and (_c_), at the bottom of the box in which this process takes place, _a dark mud_. That black mud, on being heated, produces the bright metallic gold, and the object of the whole operation is achieved. The solution is then led to another tank, brought up to its proper strength again and is ready to be used once more, while the remains of the shavings are used for the next batch of material to be treated. In some cases the crushed ore straight from the crushing mill is cyanided, in others it is simply the remains left over from the previous amalgamating process which is thus treated. All depends upon the nature of the material in question. There are other chemical methods besides the cyaniding, but it is the chief. It has been found specially useful with the Johannesburg ores, and to it the South African goldfields owe a great deal of their success.