To a diluted colorless solution of chloride of gold, add a few drops of a solution of any salt of tin, or stir the solution of gold with a slip of metallic tin; in either case, the production of a beautiful purple or port wine color will be the immediate result. If the mixture is allowed to settle, it becomes colorless; a purple powder (which is an oxide of gold combined with a little tin) being precipitated. This powder is employed in the painting of china, and is called the purple precipitate of Cassius.
Let fall a drop of a solution of nitrate of silver into a glassful of water, and add to it a grain of common salt. Mutual decomposition of the salts will take place, and chloride of silver (in the form of a white powder) will be precipitated. This precipitate is soluble in ammonia, and blackens on exposure to light.
Add a few drops of a solution of nitrate of copper to a test glass of water; the mixture will be colorless; pour into it a little liquid ammonia. The mixture will then assume a fine deep blue color.
Another.--Ferrocyanide of potassium gives a dense brown precipitate with the salts of copper. This is very delicate.
Infusion of galls gives a bluish black, and ferrocyanide of potassium a blue precipitate.
Sulphydrate of ammonia (made by passing a current of sulphuretted hydrogen gas through solution of ammonia until no more is absorbed) gives a flesh-colored precipitate.
Protochloride of tin gives a grayish precipitate. A piece of gold introduced into a solution containing mercury and touched with a piece of iron has the mercury deposited upon it.
Sulphydrate of ammonia gives a black precipitate; chromate of potassa and iodide of potassium, yellow.
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