OXIDE OF CHROMIUM. 609 



with peroxide of iron and with alumina, and the higher or 

 chromic acid, Cr O 3 , is isomorphous with sulphuric acid. 



Oxide of chromium, Cr 2 O 3 ; 1003.6 or 80.42. This oxide 

 exists in chrome iron, but is not immediately derived from that 

 mineral. When chromate of mercury, the orange precipitate 

 obtained on mixing nitrate of mercury and chromate of potash, 

 is strongly ignited, oxide of chromium remains as a powder of a 

 good green colour. The oxide of chromium is also obtained, by 

 deoxidising the chromic acid of bichromate of potash in various 

 ways ; by ignition with sulphur, for instance, or by igniting toge- 

 ther 1 part of bichromate of potash with \\ parts of sal ammo- 

 niac, and 1 part of carbonate of potash, whereby chloride of 

 potassium and oxide of chromium are formed ; the chromic 

 acid losing half its oxygen which is converted into water by the 

 hydrogen of the ammonia. Another process, interesting from 

 affording the oxide in the state of crystals, is to pass the vapour 

 of chloro-chromic acid (Cr O 2 Cl) through a tube heated to 

 whiteness, when oxygen and chlorine gases are disengaged, and 

 oxide of chromium attaches itself to the surface of the tube. 

 The crystals have a metallic lustre, and are of so deep a green 

 as to appear black ; they have the same form as specular iron 

 ore, the density 5.21, and are as hard as corundum (Wohler). 

 The ignited oxide of chromium is not soluble in acids ; heated 

 with access of air, and in contact with an alkali, it absorbs oxy- 

 gen and becomes chromic acid. Fused with borax or other 

 vitreous substances, oxide of chromium communicates to them 

 a beautiful green colour ; it is the colouring matter of the eme- 

 rald, and is employed to produce a green colour upon earthen- 

 ware. Oxide of chromium (and not chromic acid) is also the 

 colouring matter of pink colour, applied to stoneware. This 

 substance is formed by igniting strongly a mixture of 1 00 parts 

 of peroxide of tin, 33 parts of chalk and not more than 1 part of 

 oxide of chromium.* 



To obtain the same oxide in a hydrated condition, a solution 

 of bichromate of potash is brought to the boiling point, and 

 hydrochloric acid and alcohol alternately added in small quan- 

 tities, till the solution passes from a red to a deep green colour, 



* Malaguti. An. de Chim. et de Phys. t. 61, p. 433. Mr. O. Sims finds 

 that peroxides of iron and manganese may be substituted for oxide of chromium 

 in pink colour, so that the coloration of that substance is of an extraordinary 

 character. 



R II 2 



