CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 277 



of some serpentines. The commonest ore, and the chief source of the 

 chromium compounds, is the chrome iron ore or chromite, which occurs 

 in the Urals l and Asia Minor, California, Australia, and other 

 localities. This is magnetic iron ore, FeO,Fe 2 O 3 , in which the ferric 

 oxide is replaced by chromic oxide, its composition being FeO,Cr 2 O 3 . 

 Chrome iron ore crystallises in octahedra of sp. gr. 4*4 , it has a feeble 

 metallic lustre, is of a greyish -black colour, and gives a brown powder. 

 It is very feebly acted on by acids, but when fused with potassium 

 acid sulphate it gives a soluble mass, which contains a chromic salt, 

 besides potassium sulphate and ferrous sulphate. Iri practice the 

 treatment of chrome iron ore is mainly carried on for the preparation 

 of chromates, and not of chromic salts, and therefore we will trace the 

 history of the element by beginning with chromic acid, and especially 

 with the working up of the chrome iron ore into potassium dichromate, 

 K 2 Cr 2 O 7 , as the most common salt of this acid. It must be remarked 

 that chromic anhydride, CrO 3 , is only obtained in an anhydrous state, 

 and is distinguished for its capacity for easily giving anhydro-salts 

 with the alkalis, containing one, two, and even three equivalents of the 

 anhydride to one equivalent of base. Thus among the potassium salts 

 there is known the normal or yellow chromate, K 2 CrO 4 , which corre- 

 sponds to, and is perfectly isomorphous with, potassium sulphate, easily 

 forms isomorphous mixtures with it, and is not therefore suitable for a 

 process in which it is necessary to separate the salt from a mixture 

 containing sulphates. As in the presence of a certain excess of acid, 

 thedichromate, K 2 Cr 2 7 = 2K 2 CrO 4 + 2HX 2KX H 2 O, is easily 

 formed from K 2 CrO 4 , the object of the manufacturer is 'to produce 

 such a dichromate, the more so as it contains a larger proportion of the 

 elements of chromic acid than the normal salt. Finely-ground chrome 

 iron ore, when heated with an alkali, absorbs" oxygen almost as easily 

 (Chapter III., Note 7) as a mixture of the oxides of manganese, with 

 an alkali. This absorption is due to the presence of chromic oxide, 

 which is oxidised into the anhydride, and then combines with the 

 alkali Cr 2 O 3 -f- O 3 = 2Cr0 3 . As the oxidation and formation of the 

 chromate proceeds, the mass turns yellow. The iron is also oxidised, 

 but does not give ferric acid:, because the capacity of the chromium for 

 oxidation is incomparably greater than that of the iron. 



A mixture of lime (sometimes with potash) and chrome iron ore 

 is heated in a reverberatory furnace, with free access of air and at a 



The working of the Ural chrome iron ore into chromium compounds ha8 been 

 firmly established in Russia, thanks to the endeavours of P. K. Ushakoff, who con- 

 structed large works for this purpose on the river Kama, near Elabougi, where as much 

 as 2,000 tons of ore are treated yearly, owing to which the importation of chromium pre- 

 parations into Russia has ceased, 



