6' 1 2 CHROMIC ACID. 



double oxalate of chromium and soda contains 9 HO, according 

 to Mitscherlich. In the analogous oxalate of peroxide of iron 

 and soda, the proportion of water appeared to me to be IOHO. 



The mineral chrome iron, Fe O, Cr 2 O 3 , crystallizes in octohe- 

 drons, and corresponds with the magnetic oxide of iron, having 

 the peroxide of iron replaced by oxide of chromium. Its density 

 is 4.5 ; it is about as soft as felspar, and infusible. When ex- 

 posed to long continued calcination, in contact with carbonate 

 of potash, in a reveberatory furnace, the oxide of chromium of 

 this compound absorbs oxygen, and combines as chromic acid 

 with the potash, while the protoxide of iron becomes peroxide. 

 The addition of nitre increases the rapidity of oxidation, but is 

 not absolutely required in the process. A yellow alkaline solu- 

 tion of carbonate and chromate of potash is obtained by lixi- 

 viating the calcined matter, which is generally converted into 

 the red chromate or bichromate of potash, by the addition of 

 the proper quantity of sulphuric acid, the latter salt being more 

 easily purified by crystallization than the neutral chromate. 



Chromic acid, CrO 3 , 651.8 or 52.19. This acid is not libe- 

 rated from the chromates in a state of purity by any acid except 

 the fluosilicic ; it is also easily altered. Fluosilicic acid gas is 

 conducted into a warm solution of bichromate of potash, till the 

 potash is completely separated as the insoluble fluoride of silicon 

 and potassium, which may be ascertained by testing a few drops 

 of the solution with tartaric acid or chloride of platinum. The 

 solution is evaporated to dryness by a steam heat, and the 

 chromic acid redissolved by water ; it gives an opaque dull red 

 solution. Chromic acid may also be obtained anhydrous and in 

 acicular crystals, by distilling, in a platinum retort, a mixture of 

 4 parts of chromate of lead, 3 parts of finely pulverised fluor 

 spar, and 7 parts of the Nordhausen sulphuric acid ; sulphate of 

 lime is formed, and the superfluoride of chromium, the vapour 

 of which is received in a large platinum crucible, covered by 

 wet paper, and used as a condenser. The superfluoride is 

 decomposed by the aqueous vapour from the paper, being 

 resolved into hydrofluoric acid and beautiful orange-red acicular 

 crystals of chromic acid, which fill the crucible. Chromic acid 

 differs remarkably from sulphuric acid, in having but little affi- 

 nity for basic water, so that it may be obtained anhydrous by 

 evaporating its solution to dryness. Indeed, the chromate of 

 water is not known to exist, even in combination, both the 



