IROK ORES. 243 



eists in making a chromate of lime from the chrumic iron. 

 It is as follows : 1. Pulverize very finely chalk and chromic 

 iron, and mix the sifted material well by means of a revolv- 

 ing barrel. 2. Calcine for nine or ten hours at a bright red 

 heat in a reverberatory furnace, when, if complete, the whole 

 has a yellowish-green color, and dissolves entirely in muri- 

 atic acid. 3. The porous mass after being crushed under a 

 mill, to be mixed with hot water and kept agitated, adding 

 a little sulphuric acid till it slightly reddens blue litmus 

 paper. 4. Triturated chalk should then be added, and tha 

 oxyd of iron is thus removed. 5. After being left quiet for 

 a while, the clear supernatant liquid is to be drawn off: it 

 contains bichromate, with a little sulphate of lime. The 

 chromate of potash may then be made from it by adding car- 

 bonate of potash ; the chromate of lead, by adding acetate 

 of lead ; chromate of zinc, by adding chlorid of zinc. 



The bichromate of potash has a fine red color, and is much 

 used by calico printers. It is made from the chromate by 

 adding nitric or acetic acid to its solution, (enough to give it 

 a sour taste,) and setting it aside to crystallize. The green 

 oxyd of chromium gives the fine green color to glass of 

 borax in blowpipe experiments with chromic iron ; and it 

 is used to produce this tint in porcelain and enamel painting* 

 It is the coloring ingredient of the emerald, and the emerald- 

 colored chrysoberyl of the Urals ; and occurs in some varie- 

 ties of diallage and serpentine. It has been found native. 

 Chromic acid is said to be the coloring matter of the red 

 sapphire or ruby. With oxyd of tin, it affords a pink color, 

 which is used in porcelain painting. 



COLUMBITE. 



In rectangular prisms, more or less modified 

 Also massive. Disseminated in the gangue 

 Cleavage parallel to the lateral faces of the 

 prism, somewhat distinct. 



Color iron-black, brownish-black ; often 

 with a characteristic iridescence on a surface 

 of fracture ; streak dark brown, slightly red- 

 dish. Luster submetallic, shining. Opaque. 



Describe another mode of treating chromic iron 1 What is the color- 

 ing ingredient of the emerald ? what of the red sapphire? What are 

 the color, luster and form of columbite ] 



