Useful Receipts. Preparations of the Purple Powder of Cassius.

Scientific American 4, 13.10.1849

Dissolve 300 grains of gold in five times their weight of aqua regia, prepared from four parts of hydrochloric acid, and one part of nitric acid; evaporate the solution almost to dryness; this evaporation is requisite to get rid of the acid. The chloride of gold being redissolved in water, and filtered, the solution is to be diluted till it measures 26 ounces. Fragments of granulated tin are then to be put into it, which becomes turbid and brown in a few minutes; its tint gradually becomes deeper, and, at the end of a quarter of an hour, it assumes a fine purple colour; the precipitate is deposited, and it remains only to collect it on a filter.

It sometimes happens, and especially when large quanties are operated on, that the precipitate does not separate, but remains in the liquid, to which it gives a deep purple colour; in this case, it is merely requisite to heat the liquid slightly, and to add a little common salt, the product then immediately separates.

When the liquid holding the purple powder in suspension is decanted, to separate the excess of metallic tin which remains at the bottom of the metallic vessel, in the state of a black powder, are poured off with it; it is proper to allow the liquor to settle for some time, and afterwards to decant it. This operation should be repeated three or four times.

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