1870.] 



Prof. Roscoc on Vanadium. 



317 



mine even when kept sealed up in glass tubes ; it is very deliquescent, and 

 on heating in the air rapidly loses all its bromine and takes up oxygen, 

 with formation of vanadic acid. On being thrown into water, the tribro- 

 mide readily dissolves, forming a brown liquid (in this respect resembling 

 the trichloride), which, on addition of a few drops of hydrochloric acid, 

 turns of a bright green colour, showing the presence of a solution of an 

 hypovanadic salt. No free bromine or hydrobromic acid is given off on 

 dissolving the tribromide in water. That a more volatile higher bromide 

 was not formed in this reaction was shown, inasmuch as, on distilling the 

 excess of liquid which had collected in the receiver, it was found to consist 

 of free bromine, containing mere traces of the tribromide mechanically 

 carried over. The tribromide is likewise formed when bromine is passed 

 over a red-hot mixture of vanadium trioxide and pure charcoal, as in the 

 preparation of the tetrachloride ; but this method is not one to be recom- 

 mended, as the tube becomes constantly stopped up by the formation of 

 the solid tribromide. 



The analysis of the tribromide was made by dissolving the compound in 

 water, and precipitating the bromine with excess of nitrate of silver, the 

 vanadium being estimated as V 2 0 ; , either in the nitrate from the bromide 

 of silver or in a separate portion. The bromine in the above determina- 

 tions, obtained by precipitation as silver-salt, was invariably found to be 

 too high, whilst the vanadium nearly agreed with the theoretical per- 

 centage. This is due to the fact pointed out^by Stas, in his ( Recherches/ 

 p. 156, that bromide of silver, when boiled, encloses mechanically a por- 

 tion of the precipitant, which then cannot be washed out. The loss of 

 weight obtained by reducing the bromide to metallic silver in a current of 

 hydrogen, taken as bromine, gave more nearly agreeing numbers : — 



Calculated. Mean of 6 determinations. 



Vanadium V = 51*3 1 7*61 18*44 



Mean of 3 determinations. 

 Bromine Br 3 = 240*0 82*39 80*86 



291*3 100*00 99*30 



2. Vanadium Oxytribromide, or Vanadyl Tribromide, VOBr 3 , molec. wt. 

 = 307'3. — The oxytribromide is a dark-red transparent liquid, evolving 

 white fumes on contact with the air, obtained by passing pure and dry 

 bromine over vanadium trioxide (Y 2 0 3 ) heated to redness. Moisture pre- 

 vents the formation of the oxytribromide ; and it not only undergoes sudden 

 decomposition when heated to 1 80°, but also slowly decomposes at the or- 

 dinary atmospheric temperatures. The boiling-point of the tribromide 

 can, however, be brought below its temperature of decomposition by dis- 

 tillation in vacuo, and the liquid can then be freed completely from bro- 

 mine by passing a current of dry air through the liquid. Under a pressure 

 of 100 millims. the oxytribromide boils from 130° to 135°, and may be 



