Take a little of the sample in a test tube, add pure, strong nitric acid and heat by a Bunsen burner until a white, solid residue is obtained. Then add water, when a clear, colorless solution will be obtained. A white residue would indicate adulteration with barytes, a red residue or a yellow solution with oxide of iron. The presence of iron may be ascertained by adding a few drops of a solution of potassium ferrocyanide (yellow prussiate of potash) to the solution, when a blue precipitate will be obtained if there be the least trace of iron present.