Platinum is detected when in the metallic state by its physical characters and insolubility in acids. In alloys it may be found by dissolving them in nitric acid or in aqua regia, evaporating with hydrochloric acid, and treating the filtrate with ammonic chloride and alcohol. A heavy yellow precipitate marks its presence. The assay of bullion, or of an alloy containing platinum, may be made as follows: Take 0.2 gram of the alloy and an equal weight of fine silver, cupel with sheet lead, and weigh. The loss in weight, after deducting that of the silver added, gives the weight of the base metals, copper, lead, &c. Flatten the button and part by boiling with strong sulphuric acid for several minutes. _When cold_, wash, anneal, and weigh. The weight is that of the platinum and gold. The silver may be got by difference. Re-cupel the metal thus got with 12 or 15 times its weight of silver, flatten and part the gold with nitric acid in the usual way (see under _Gold_), and the platinum will dissolve. The gold may contain an alloy of osmium and iridium; if so, it should be weighed and treated with aqua regia. The osmiridium will remain as an insoluble residue, which can be separated and weighed. Its weight deducted from that previously ascertained will give the weight of the gold. When the platinum only is required, the alloy must be dissolved by prolonged treatment with aqua regia, the solution evaporated to dryness, and the residue extracted with water. The solution thus obtained is treated with ammonic chloride in large excess and with some alcohol. A sparingly soluble[47] yellow ammonic platinum chloride is thrown down, mixed, perhaps, with the corresponding salts of other metals of the platinum group. Gold will be in solution. The solution is allowed to stand for some time, and then the precipitate is filtered off, washed with alcohol, dried, and transferred (wrapped in the filter paper) to a weighed crucible. It is ignited, gently at first, as there is danger of volatilising some of the platinum chloride, and afterwards intensely. With large quantities of platinum the ignition should be performed in an atmosphere of hydrogen. Cool and weigh as metallic platinum. IRIDIUM Occurs in nature alloyed with osmium as osmiridium or iridosmine, which is "rather abundant in the auriferous beach sands of Northern California" (Dana). It occurs in bright metallic scales, which do not alloy with lead, and are insoluble in aqua regia. Iridium also occurs in most platinum ores, and forms as much as two per cent. of some commercial platinum. In chemical properties it resembles platinum, but the ammonic irido-chloride has a dark red colour, and on ignition leaves metallic iridium, which does not dissolve in aqua regia diluted with four or five times its volume of water and heated to a temperature of 40° or 50° C.