64 Royal Society : — • 



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 recommended, as the tube becomes 

 constantly stopped up by the formation of the solid tribromide. 



The analysis of the tribromide was made by dissolving the com- 

 pound in water, and precipitating the bromine with excess of nitrate 

 of silver, the vanadium being estimated as V 2 5 , either in the filtrate 

 from the bromide of silver or in a separate portion. The bromine 

 in the above determinations, obtained by precipitation as silver-salt, 

 was invariably found to be too high, whilst the vanadium nearly 

 agreed with the theoretical percentage. This is due to the fact 

 pointed out by Stas, in his.' Recherches,' p. 156, that bromide of 

 silver, when boiled, encloses mechanically a portion of the precipi- 

 tant, which then cannot be washed out. The loss of weight ob- 

 tained 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 17*61 18-44 



Mean of 3 determinations. 

 Bromine Br 3 = 240-0 82-39 80'86 



291-3 100-00 99-30 



2. Vanadium Oxy tribromide, 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 (V 2 3 ) heated 

 to redness. Moisture prevents the formation of the oxytribromide ; 

 and it not only undergoes sudden decomposition when heated to 1 80°, 

 but also slowly decomposes at the ordinary atmospheric temperatures. 

 The boiling-point of the tribromide can, however, be brought below 

 its temperature of decomposition by distillation in vacuo, and the 

 liquid can then be freed completely from bromine by passing a 

 current of dry air through the liquid. Under a pressure of 100 mil- 

 lims. the oxytribromide boils from 130° to 135°, and may be 

 distilled almost without decomposition. Vanadium oxytribromide 

 dissolves in water, yielding a yellow-coloured solution, in which both 

 vanadium and bromine were determined, after reduction with sul- 

 phurous acid : — 



Calculated. Mean of several analyses. 



V = 51-3 1G-G9 16-75 



Br. =240-0 78-10 79-20 



O.". = 16-0 , 5-21 



307-3 100-00 



The specific gravity of the oxytribromide at 0° is 2-967. 



