BAXTER. REVISION OF THE ATOMIC WEIGHT OP IRON. 247 



As has been stated before, ferrous bromide prepared in tliis way 

 does not contain a weighable amount of ferric salt, and is a perfectly 

 stable substance in dry air. The pure salt varies in color from light 

 yellow to dark brown, according to the thickness of the crystals. 



In several cases the platinum boat which contained the salt during the 

 drying became covered with a black film, which disappeared when the 

 boat was ignited, without producing any change in weight. Since a 

 similar difficulty had never been met before when a platinum boat was 

 heated in hydrobromic acid, the phenomenon was at first ascribed to the 

 presence of either ferrous or ferric salt; but the eventual conversion of 

 the platinum into a brittle condition seemed to indicate some other cause 

 of the difficulty. The presence of phosphorus compounds in the mixed 

 gases was suspected, and when tests were made with amnionic molybdate, 

 both in the salt and in hydrobromic acid which had been generated in 

 the apparatus and collected in water, traces of phosphoric acid were 

 found in both cases. The source of the phosphoric acid was easily 

 discovered. 



Whenever bromine is passed for a sufficiently long time into a solu- 

 tion of hydrobromic acid containing red phosphorus, a yellow crystal- 

 line deposit is formed upon the walls of the flask above the solution. 

 This yellow substance resembles the pentabromide of phosphorus in 

 appearance and gives off much hydrobromic acid when treated with 

 water, hence it must have been the pentabromide. Since at ordinary 

 temperatures the pentabromide is not volatile while the tribromide is 

 volatile, probably the formation of pentabromide is due to volatilization 

 of the tribromide from the solution and consequent conversion into pen- 

 tabromide by bromine which escapes reduction in bubbling through the 

 flask. Obviously the formation of either substance in appreciable quan- 

 tity is impossible so long as the solution in the flask is very aqueous. 

 The aqueous tension of the solution is never high, however, since the 

 concentration of the hydrobromic acid is at a maximum, and the aqueous 

 tension is diminished still farther as the concentration of phosphorous and 

 phosphoric acids in the solution increases. Eventually, then, the aqueous 

 tension of the solution should be such as to make possible the existence 

 of the bromides of phosphorus. The attacking of the boat was evidently 

 due to the presence of compounds of phosphorus in the mixed gases. 



In order to remove the traces of bromine which escaped reduction in 

 the first tlask, containing hydrobromic acid and phosphorus, the gases, 

 after passing through this flask, were always conducted through a 

 short U-tube containing these same substances. The concentration 



