BROWN IRON ORE. 185 



solution filtered from any impurities, and evaporated 

 to crystallization. (See Molybdate of Lead.) 



The molybdic acid may also be precipitated from 

 the solution of an alkaline molybdate neutralized with 

 nitric acid, by basic nitrate of suboxide of mercury. 

 The yellow precipitate is allowed to subside, filtered 

 off, washed with a dilute solution of the mercury-salt, 

 dried and ignited. Molybdic acid may also be quan- 

 titatively determined, by this method. The precipitate 

 is collected upon a filter (previously dried at 100 and 

 weighed), dried at 100, and a weighed portion of it 

 gently ignited in a bulb-tube (Fig. 19), through which 

 a stream of hydrogen is passed, when dark brown 

 binoxide of molybdenum, MoO 2 , is left. 



100. BROWN IRON-ORE, CONTAINING VANADIUM. 



In order to extract the vanadium, the quantity of 

 which does not amount even to 1 per cent., the finely- 

 powered ore is intimately mixed with ^ its weight of 

 nitre, and exposed, for an hour, in a crucible, to a 

 gentle ignition. When cool, the mass is powdered and 

 boiled with water. 



The filtered solution has a yellow color, and con- 

 tains the vanadates, chromates (molybdates ?), arsenates, 

 phosphates, nitrites and silicates of potassa and alu- 

 mina. 



It is gradually mixed, and well stirred, with nitric 

 acid, taking care that it may still remain slightly alka- 

 line and that no nitrous acid is liberated, which 

 would reduce the vanadic and chromic acids. The 

 precipitate of alumina and silica thus separated is 

 filtered off. 



The solution is then mixed with an excess of solu- 

 tion of chloride of barium, as long as any precipitate 



16* 



