METHODS OF OBTAINING CHROMIC ACID.

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Tannin precipitates it in flakes of a fawn colour. Heated by theblow-pipe on charcoal, it boils up, and leaves an infusible greenmatter. Fused with phosphoric acid, or with borax, a vitreousglobule is obtained of an emerald-green colour. Sulphuric acid,while cold, produces no effect upon it; but, when heated, itmakes it assume a bluish green colour. With a solution of ni-trate of mercury, it gives a precipitate of a dark cinnabar red.With a solution of nitrate of silver, it gives a precipitate, which,the moment it is formed appears of a beautiful carmine red, butbecomes purple by exposure to light. It unites with the alcaliesand forms crystallizable salts of a beautiful orange colour. Italso unites with the earths and metallic oxids, and forms com-pounds distinguished by the splendour of their colours.

METHODS OF OBTAINING CHROMIC ACID.

To obtain chromic acid boil one part of the red lead ore of Sibe-ria, reduced to an impalpable powder, with two parts of carbonateof potash and ten or twelve of water, for at least an hour. Thensuffer the mixture to subside, decant and filter the super-natantfluid. The clear solution must then be evaporated and sufferedto crystallize. When no more crystals can be obtained in thismanner, the whole quantity of salt is to be dissolved in a suffi-cient quantity of distilled vyater, and pure nitric acid added to thesaturation of the alcali. The solution now affords by evaporationcrystals of a ruby colour, which are chromic acid.

In order to disengage chromic acid from chromate of iron themethod described page 22 may be made use of.

On account of the scarcity of the minerals which contain thisacid, the properties and methods of obtaining chromates are stillunknown.