CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 285 



the reduction of chromates (for example, of ammonium or mercurio 

 chromate) and by the decomposition (splitting up) of the saline corn- 

 it separates out from its solution in combination with these oxides, forming, for example, 

 ZnO,Cr 2 O 5 . Viard obtained compounds of Cr 2 O 3 with the oxides of Mg, Zn, Cd, &c.) 

 On precipitating the violet solution of chrome alum with ammonia, a precipitate contain- 

 ing Cr 2 O 3 ,6H 2 O is obtained, whilst the precipitate from the boiling solution with causth 

 potash was a hydrate containing four equivalents of water. When fused with borax chromic 

 Baits give a green glass. The same coloration is communicated to ordinary glass by the 

 presence of traces of chromic oxide. A chrome glass containing a large amount of 

 chromic oxide may be ground up and used as a green pigment. Among the hydrates 

 of oxide of chromium Guignet's green forms one of the widely-used green pigments which 

 have been substituted for the poisonous arsenical copper pigments, such as Schweinfurt 

 green, which formerly was much used. Guignet's green has an extremely bright green 

 colour, and is distinguished for its gre,it stability, not only under the action of light but 

 also towards reagents ; thus it is not altered by alkaline solutions, and even nitric acid 

 does not act on it. This pigment remains unchanged up to a temperature of 250 ; it 

 contains Cr 2 O 3 ,2HO 2 , and generally a small amount of alkali. It is prepared by fusing 3 

 parts of boric acid with 1 part of potassium dichromate ; oxygen is disengaged, and a 

 green glass, containing a mixture of the boratesof chromium and potassium, is obtained. 

 When cool this glass is ground up and treated with water, which extracts the borio 

 acid and alkali and leaves the above-named chromic hydroxide behind. This hydroxide 

 only parts with its water at a red heat, leaving the anhydrous oxide. 



The chromic hydroxides lose their water by ignition, and in so doing become spon- 

 taneously incandescent, like the ordinary ferric hydroxide (Chapter XXII.). It is not 

 known, however, whether all the modifications of chromic oxide show this phenomenon. 

 The anhydrous chromic oxide, Cr<,O 5 , is exceedingly difficultly soluble in acids, if it 

 has passed through the above recalescence. But if it has parted with its water, or the 

 greater part of it, and not yet undergone this self-induced incandescence (has not lost a 

 portion of its energy), then it is soluble in acids. It is not reduced by hydrogen. It is 

 easily obtained in various crystalline forms by many methods. The chromates of mer- 

 cury and ammonium give a very convenient method for its preparation, because when 

 ignited they leave chromic oxide behind. In the first instance oxygen and mercury are 

 disengaged, and in the second case nitrogen and water : 2Hg 2 Cr0 4 = Cr 2 O 3 + 05 + 4Hg or 

 (NH 4 ) 2 Cr 2 O 7 = Cr 2 5 + 4H 2 + N 2 . The second reaction is very energetic, and the mass 

 of salt burns spontaneously if the temperature be sufficiently high. A mixture of potas- 

 sium sulphate and chromic oxide is formed by heating -potassium dichrbmate with an 

 equal weight of sulphur : K 2 Cr 2 O 7 + S = K 2 SO 4 + Cr 2 O 5 . The sulphate is easily extracted 

 by water, and there remains a bright green residue of the oxide, whose colour is more 

 brilliant the lower the temperature of the decomposition. The oxide thus obtained is 

 used-as a green pigment for china and enamel. The anhydrous chromic oxide obtained 

 from chromyl chloride, Cr0 2 Cl 2 , has a specific gravity of 5'21, and forms almost black 

 crystals, which give a green powder. They are hard enough to scratch glass, and have a 

 metallic lustre. The crystalline form of chromic oxide is identical with that of the oxide 

 of iron and alumina, with which it is isomorphous. 



7 bis The most important of the compounds corresponding with chromic oxide is chromic 

 chloride, Cr 2 Cl 6 , which is known in an anhydrous and in a hydrated form. It resembles 

 ferric and aluminic chlorides in many respects. There is a great difference between 

 the anhydrous and the hydrated chlorides ; the former is insoluble in water, the latter 

 easily dissolves, and on evaporation its solution forms a hygroscopic mass which is very 

 unstable and easily evolves hydrochloric acid when heated with water. The anhydrous 

 form is of a violet colour, and Wb'hler gives the following method for its preparation : an 

 intimate mixture is prepared of the anhydrous chromic oxide with carbon and organic 

 matter, and charged into a wide infusible glass or porcelain tube which is heated in a 

 combustion furnace ; one extremity of the tube communicates with an apparatus generat- 

 ing chlorine which is passed through several bottles containing sulphuric acid in order 



