The art of assaying gold and silver is founded upon the feeble affinity which these have for oxygen in comparison with copper, tin, and other cheap metals, and on the tendency which the latter metals have to oxidize rapidly in contact with lead at a high temperature, and sink with it into any porous, earthy vessel in a thin, glassy, vitrified mass. The precious metal having previously been accurately weighed and prepared, the first process is Cupellation. The muffle, with cupel properly arranged on the " muffle plate," is placed in the furnace, and the charcoal added, and lighted at the top by means of a few ignited pieces thrown on last. After the cupels have been exposed to a strong white heat for about half an hour, and have become white hot, the lead is put into them by means of tongs. As soon as this becomes bright red and "circulating," as it is called, the specimen for assay, wrapped in a small piece of paper or lead-foil, is added; the fire is now kept up strongly until the metal enters the lead and "circulates" well, when the heat, slightly diminished, is so regulated that the assay appears convex and more glowing than the cupel itself, whilst the "undulations" circulate in all directions, and the middle of the metal appears smooth,with a margin of litharge, which is freely absorbed by the cupel. When the metal becomes bright and shining, or, in the technical language, begins to "lighten," and prismatic hues suddenly flash across the globules, and undulate and cross each other, followed by the metal becoming very brilliant and clear, and at length bright and solid (called the brightening), the separation is ended, and the process complete. The cupels are then drawn to the mouth of the " muffle," and allowed to cool slowly. When quite cold, the resulting "button," if of SILVER, is removed by the "pliers" or "tongs" from the cupels, and after being flattened on a small anvil of polished steel, with a polished steel hammer, to detach adhering oxide of lead, and cleaned with a small, hard brush, is very accu redely weighed. The weight is that of pure silver, and the difference between the weight before cupellation and that of the pure metal represents the proportion of alloy in the sample examined. In the case of gold, the metal has next to undergo the operations of QUARTATION. The cupelled sample is fused with three times its weight of pure silver (called the "witness,") and in this state may be easily removed by parting. The alloy, alter quartation, is hammered or rolled out into a thin strip or leaf, carted into a spiral form, and boiled for a quarter of an hour with about 2½ to 8 ounces of nitric acid (specific gravity, 1.3); and the fluid being poured off, it is again boiled in a similar manner, with 1½ to 2 ounces more nitric acid (sp. gr., 1.2); after which the gold is carefully collected, washed in pure water, and dried. When the operation of parting is skilfully conducted, the acid not too strong, the metal preserves its spiral form; otherwise it falls into flakes or powder. The second boiling is termed the "reprise" The loss of weight by parting corresponds to the quantity of silver originally in the specimen.

For Alloys containing Platinum, which usually consist of copper, silver, platinum, and gold, the method of assaying is as follows: The alloy Is cupelled in the usual way, the loss of weight expresses the amount of copper, and the "button," made into a riband and treated with sulphuric acid, indicates by the portion dissolved that also of the silver present. By submitting the residuum to quartation, the platinum becomes soluble in nitric acid. The loss after digestion in this menstruum expresses the weight of that metal, and the weight of the portion now remaining is that of pure gold. Gold containing palladium may be assayed in the same manner.