RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OF URANIUM. 371 



observed, in this connection, that Zimmermann used the chlorides in his 

 vapor density determinations, and his analyses show good agreement. 

 This does not show conclusively, however, that his material was free 

 from small, but fairly constant quantities of higher or lower chlorides as 

 impurities. 



Ou the other hand, bromine forms with uranium only three distinct 

 compounds: the tribromide, UBrg ; uranous bromide, UBr4; and the 

 oxybromide, or uranyl compound, UO.iBro. The tribromide can be 

 produced only from the tetrabromide by the action of reducing agents. 

 Uranyl bromide, UOaBr.,, has been certainly formed only in solution, 

 resulting in hydrated crystals. It has never been definitely obtained in 

 an-hydrous form. Zimmermann made many attempts to form the penta- 

 bromide, corresponding to the pentachloride, by passing bromine at 

 high temperatures over sublimed uranous bromide. Every attempt 

 gave negative results, showing tliat at temperatures up to the subliming 

 point of uranous bromide higher bromides cannot exist. Since higher 

 bromides are non-existant under the conditions prevailing in the forma- 

 tion of the tetrabromide, the objections to the use of the tetrachloride 

 are not applicable in the case of uranous bromide. The investigations 

 of Zimmermann * have shown that the tetrabromide can be formed in 

 an apparently definite state. It seemed probable, therefore, from the 

 literature on the subject, that in uranous bromide we had a compound 

 well suited to the purposes of our investigation. 



The method of preparation followed at first was essentially that 

 described by Zimmermann.* In an apparatus constructed wholly of 

 glass, a mixture of dry nitrogen and bromine vapor was passed over a 

 mixture of the green oxide of uranium, UsOg, and pure carbon. The 

 air was first thoroughly swept out of the apparatus by a current of 

 nitrogen, and the oxide was heated to a high temperature. When the 

 bromine vapor was passed in, uranous bromide formed, and sublimed in 

 brilliant crystalline j^lates of a brownish color. After cooling in a 

 current of nitrogen, the sublimate was transferred to a weighing bottle. 

 At this point, however, unexpected difficulties arose, owing to the rapid 

 oxidation of the bromide. Uranous bromide is extremely deliquescent, 

 and forms with water and oxygen the oxybromide, with liberation of 

 hydrobromic acid. Consequently, when exposed to the moist air of the 

 laboratory even for the short time required for removing the sublimate 

 from the combustion tube, the bromide loses its brilliant lustre, and 



* Annalen der Chemie, 216,3. 



