RICHARDS AND MERIGOLD. ATOMIC WEIGHT OP URANIUM. 395 



been made in the absence of water. In the light of all these consid- 

 erations, there would seem to be no good reason to question the purity 

 of our bromide. 



On comparing the result of this investigation, 238.53, with that of 

 Zimmermann's, 239.59 (the only previous work worthy of serious consid- 

 eration), the difference of over a unit seems at first to be one of great 

 magnitude. The percentage difference (0.45%) is however smaller than 

 many a difference which often has been passed by unheeded in small atomic 

 weights, such as those of magnesium or aluminum. This point illustrates 

 the difficulty of obtaining results with high atomic weights which can 

 satisfy the cursory reader. 



Nevertheless, such a difference is far too great to pass unchallenged. 

 It seems highly probable that the greater part of it is due to the 

 previously discussed sources of inaccuracy in Zimmermann's method, — 

 especially to the difficulty of wholly re-oxidizing the lower oxide. 

 The failure to oxidize half a per cent of the uranous oxide, involving an 

 error in the weight of only 0.017 per cent of the total weight of the 

 substance, would account for the discrepancy. 



Heuce it seems not unlikely that the atomic weight of uranium is 

 really as low as 238.53. Nevertheless, the question cannot be looked 

 upon as conclusively settled. Certainty can be obtained only by the 

 application of a new method, radically different from the two just com- 

 pared. Our experience of nearly four years of varied work seems to 

 indicate that the search for such method will not be an easy one. The 

 many degrees of quantivalence of uranium and the unsuitable properties 

 of its compounds combine to render the problem one of unusual difficulty. 

 When face to face with a problem of this kind one cannot but admire 

 Stas's wisdom in selecting chiefly univalent elements with powerful 

 affinities in order to prove the constancy of the atomic weights. 



The result of our analyses of uranous bromide may be summed up iu 

 the following words: If oxygen is taken as 16.000, and bromine as 

 79.955, the atomic weight of uranium appears to be not far from 

 238.53. 



Cambridge, Mass., U. S. A. 1897-1901. 



