Put a hot weak pellucid infusion of galls int a glass, and throw into it a grain of the vitriol of iron, calcined to whiteness, and considerably heated; then, as it falls to the bottom, it will make a black cloud, which will uniformly diffuse itself through the transparent liquor, and gradually turn it black.

The same effect may also be produced by the addition of a little vitriol of iron calcined to a yellow colour, or by the col-cothar of vitriol calcined to redness.

The black liquor, produced as above, may be rendered pellucid again, by pouring the liquor hot into a glass rinsed with the pure acid of vitriol. And to make this transparent liquor black again, pour to it as much hot oil of tartar per deliquium as will saturate the acid, which has attracted the metallic matter.