142 On the Combinations of Chromium. 



not be retained by the acid, and therefore is precipitated 

 with the oxide of chromium ; another portion of the oxide of 

 iron, is thrown down by the potash disengaged from the 

 chromate. Iron does not dissolve in chromic acid, even 

 when its affinity is aided by attaching it to a slip of platina ; 

 — heated with the acid, it decomposes it. 



Chromate of Copper. — It is uncrystalizable. When moist, 

 its color is chesnut brown, when dried at 212° F. it is black. 

 It is soluble in ammonia, and affords a rich, dark green solu- 

 tion ; by evaporating the ammonia, it separates unaltered. 

 It may be formed, by adding chromate of potash, to sulphate 

 or acetate of copper. 



Chromate of Lead. — Its crystals are acicular prisms, often 

 grouped so as to form radiated masses ; the color of the 

 crystals is pure orange yellow, and they present a rich silky 

 lustre. When in the form of scales, this salt is of a dull scar- 

 let color ; when moist the color is a bright scarlet ; and 

 when in powder its color when dry, is scarlet. It is con- 

 verted into the bichromate by nitric acid, and subsequently 

 dissolved ; pure alkalies dissolve it, and leave it unchanged, 

 when neutralized by acids. Its crystals are obtained, by dis- 

 solving oxide of lead in a solution of pure soda ; adding a 

 solution of chromate of potash, and placing the mixed solu- 

 tion, — contained in a conical or hemispherical vessel, — in a 

 jar, at the bottom of which, carbonic acid is slowly elimina- 

 ted from chips of marble, by sulphuric acid. As the soda 

 absorbs the acid, beautiful groups of radiated crystals ap- 

 pear.* It is readily obtained, by fusing nitre on bichro- 

 mate of lead, at a low temperature, dissolving the nitre and 

 washing the powder. In this way it might be prepared by 

 our manufacturing chemists, and introduced into use as a 

 pigment. 



Bichromate of Lead. — In the form of powder, its color at 

 ordinary temperatures, is yellow and orange yellow ; it is so* 

 luble in nitric acid ; decomposed by muriatic acid, and also 



* Mr. Faraday, first published the fact, that chromate of lead might be crys- 

 talized, or rather, he first observed crystals, which had been deposited from an 

 alkaline solution, and it is due to this accurate observer, for me to state, that 

 his observation led me to make experiments on this salt. 



