534 SMITH'S INTERMEDIATE CHEMISTRY 



When the hydroxide is heated, chromic oxide Cr 2 03 remains as a 

 green powder. 



Other Elements of the Chromium Family. Reference 

 to the periodic system of the elements (p. 278) shows three ele- 

 ments molybdenum, tungsten and uranium which should 

 resemble chromium in their properties and derivatives. Molyb- 

 denum and tungsten, indeed, give acid anhydrides MoOs and 

 W0 3 , and the salts derived therefrom correspond to the chromates 

 and dichromates. The metals are used, like chromium, in making 

 special steel alloys (p. 493). Tungsten has a higher melting- 

 point (3540) than any other metal and, on this account, and 

 because it is less volatile than carbon, is now used for filaments 

 in electric lamps. A carbon filament also requires 3.25 watts per 

 candle power while a tungsten filament uses only 1.25 watts per 

 1 c.p. The powdered metal obtained by reduction can be pressed 

 into wire form and then rolled while strongly heated by an elec- 

 tric current until a compact wire is obtained. 



Uranium, besides giving uranates and diuranates, also ex- 

 hibits base-forming properties. Pitchblende, which contains 

 the oxide UsOg along with smaller amounts of many other elements, 

 is found mainly in Joachimsthal (Bohemia) and in Cornwall. 

 Carnotite, a uranate and vanadate of potassium, K 2 0,2U0 3 , 

 V 2 5 ,3H 2 O, occurs in Colorado. Pitchblende is roasted with 

 lime, the calcium uranate CaUO4 thus formed is decomposed with 

 sulphuric acid, giving uranyl sulphate U0 2 S0 4 . When excess of 

 sodium carbonate is added to the solution of the latter, the foreign 

 metals are precipitated and sodium diuranate Na^U-jOT^HjjO, 

 which is also thrown down, dissolves in the excess as Na 2 U0 4 . 



After filtration, the diuranate of sodium is reprecipitated by 

 neutralizing with sulphuric acid and boiling. This salt is used in 

 making uranium glass, which shows a yellowish-green fluorescence. 



The most striking property exhibited by uranium and its 

 compounds, radioactivity, will be dealt with in the final chapter. 



