RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OF URANIUM. 873 



a complexity of operations incompatible with the degree of accuracy 

 requisite iu an atomic weight investigation. 



On account of these formidable difficulties in the preparation and 

 analysis of pure uranous bromide, it was thought best to search for some 

 compound which offered fewer obstacles. It will be seen that this 

 search was vain, although it required many mouths. 



In view of the great tendency of uranous bromide to oxidize, under 

 ordinary conditions, the use of uranyl bromide seemed to offer the 

 simplest solution of the problem. Anhydrous uranyl bromide has never 

 been prepared in a pure state. In the prepai'ation of uranous bromide, 

 if the nitrogen used contains a little oxygen, or if traces of moisture are 

 present, there is formed, in addition to the uranous bromide, a yellow 

 powder, very different in appearance from the brown color of finely 

 divided uranous bromide. This powder has been assumed by various 

 investigators to be the oxybromide. Owing to the fact that it is always 

 mixed with uranous bromide, an analysis has never been obtained. 



There seemed to be, however, some basis for belief that under suitable 

 conditions of temperature, moisture, and oxygen supply, it might be 

 possible to obtain anhydrous uranyl bromide entirely free from the 

 uranous compound. With this end in view, the green oxide, without any 

 admixture of carbon, was heated in a stream of bromine, also iu a 

 current of hydrobromic acid. In each case there was apparently no action 

 whatever other than a partial and gradual reduction to the black oxide. 

 This slight reducing action is probably not due to the gases used, in the 

 sense of being peculiar to them, for Zimmermann has shown that this 

 reduction takes place whenever the green oxide is heated in a current of 

 inactive gas such as nitrogen or carbon dioxide. * 



Both moist and dry gases were used. Mixtures of these gases and air 

 were also tried, at different temperatures. The green oxide was then 

 reduced by hydrogen to uranous oxide, UO2, and this was then treated 

 with various combinations of dry and moist bromine vapor, hydrobromic 

 acid, and air, at various temperatures. Again the results were negative. 

 Under these conditions the bromine did not combine to the slightest 

 extent with the uranium. Since combination fails to take place, even in 

 the presence of considerable quantities of oxygen, there is naturally 

 some cause to doubt that the light colored powder above mentioned is 

 really an oxybromide. Possibly it is, after all, uranous bromide in a 

 different state of aggregation. 



* Log. cit. See also Richards, These Proceedings, 33, 423 (1898). 



