290 



PRINCIPLES OF CHEMISTRY 



be dissolved in dilute nitric acid, and grey crystalline chromium (sp. gr. 

 6-81) is left behind. Fre"my also prepared crystalline chromium by the 

 action of the valour of sodium on anhydrous chromic chloride in a 

 stream of hydrogen, using the apparatus shown in the accompanying 

 drawing, and placing the sodium and the chromic chloride in separate 

 porcelain boats. The tube containing these boats is only heated when 

 it is quite full of dry hydrogen. The crystals of metallic chromium 

 obtained in the tube are grey cubes having a considerable hardness and 

 withstanding the action of powerful acids, and even of aqua regia. 

 The chromium obtained by Wohler by the action of a galvanic current 

 is, on the contrary, acted on under these conditions. The reason of 

 this difference must be looked for in the presence of impurities, and in 

 the crystalline structure. But- in any case, among the properties of 



FlG 92. Apparatus for the preparation of metallic chromium by igniting chromic chloride 

 and sodium iu a stream of hydrogen. 



metallic chromium, the following may be considered established : it is 

 white in colour, with a specific gravity of about 6-7, is extremely hard 

 in a crystalline form, is not oxidised by air at the ordinary temperature, 

 and with carbon it forms alloys like cast iron and steel. 



The two analogues of chromium, molybdenum and tungsten (or wol- 

 fram), are of still rarer occurrence in nature, and form acid oxides, R0 3 , 

 which are stillless energetic than Cr0 3 . Tungsten occurs in the some- 

 what rare minerals, scheelite, CaWO 4 , and wolfram ; the latter being an 

 isomorphous mixture of the normal tungstates of iron and manganese, 

 (MnFe)W0 4 . Molybdenum is most frequently met with as molybdenite, 

 MoS 2 , which presents a certain resemblance to graphite in its physical 

 properties and softness. It also occurs, but much more rarely, as a 

 yellow lead ore, PbMo0 4 . In both these forms molybdenum occurs m 

 the primary rocks, in granites, gneiss, &c., and in iron and copper ores 



