394 PROCEEDINGS OF THE AMERICAN ACADEMY. 



during the analysis ; for every step of this procedure was verified by 

 confirmatory evidence. If a flaw existed, it must have been in the 

 purity of the original substance. Since the observed atomic weight is 

 lower than the former results, it is important to examine into only 

 those possible irregularities which could have had the effect of lowering 

 the apparent value. 



The probable impurities tending to lower the atomic weight are, first, 

 sodic bromide ; second, hydrobromic acid ; third, free bromine ; fourth, 

 uranic pentabromide ; and fifth, an unknown metal with a lesser equiva- 

 lent. The first impurity was found to be present, its amount was deter- 

 mined, and a suitable correction was applied. The second could not have 

 been formed during the sublimation of the uranous bromide, because com- 

 pounds of hydrogen were scrupulously excluded. If formed by the action 

 of water after the sublimation, the atomic weight would have appeared 

 too high — for moist uranous bromide emits hydrobromic acid instead of 

 absorbing it. The third impurity, free bromine, could hardly have been 

 imprisoned or absorbed by the sharply crystalline salt to any appreciable 

 extent, since the concentration of the bromine vapor in the issuing gases 

 was but small. 



The evidence in regard to the absence of pentabromide is fiiirly conclu- 

 sive, although somewhat indirect. All attempts by many investigators 

 to form this compound have failed, in spite of the recognized existence of 

 the corresponding chlorine compound. It seemed possible, however, 

 that while this compound is not formed at high temperatures, lower 

 temperatures might permit the addition of the extra bromine. Accord- 

 ingly the jireparations used in Analyses 7, 8, 10, and 11, were cooled 

 in a current of dilute bromine vapor, instead of in pure nitrogen. The 

 presence of a comparatively small amount of pentabromide would make 

 a very decided difference in the quantity of bromine found. Hence the 

 essential agreement of the average result of these analyses, 238.50, with 

 the average result of all the others, 238.52, is good evidence of the 

 absence of uranium pentabromide. 



"With regard to the fifth possible impurity nothing can be said except 

 to point out the many operations involved in the purifications. These 

 seem to point toward probable purity ; but it is nevertheless to be re- 

 gretted that lack of time prevented the analysis of many different fractions 

 of material, prepared in varying ways. 



The presence of oxybromide would of course cause low bromine anal- 

 yses, and too high an apparent atomic weight. Therefore this possible 

 cause of error need not be considered, even if the oxybromide had ever 



