RICHARDS AND MERIGOLD. ATOMIC WEIGHT OF URANIUM. 367 



analysis he determined the ratio between uranium oxide and carbon 

 dioxide. Thus he eliminated the error involved in weighing a crystal- 

 lized salt which would probably contain more or less included water. 

 The principal sources of error are the questionable use of combustion 

 analysis in atomic weight investigations, and the possibility of unoxidized 

 carbon remaining in the uranium oxide. His best results vary from 

 239.4 to 241.1. 



The work of these chemists, though a great improvement over the 

 attempts of Rammelsberg and the other early workers, leaves much to 

 be desired, and does little more than give an approximate idea of the 

 probable value of the atomic weight of uranium. 



Zimmermann's investigation of the ratio between the oxides UO.. and 

 U 3 8 was much more carefully carried out, and is the only work thus far 

 published that is worthy of serious consideration. Using carefully purified 

 material, and giving attention to detail, Zimmermann oxidized the lower 

 oxide by means of a stream of oxygen, and observed the gain in 

 weight. His results for the atomic weight varied from 239.49 to 

 239.76, an extreme difference of 0.27, or 0.11 per cent. The average 

 was about 239. G. The chief probable cause of error in this method is 

 the difficulty which is always experienced in forming a more voluminous 

 solid from a less voluminous one. Uranous oxide has a specific gravity 

 of 10.2, while the " Uranoso-uranic " oxide has a specific gravity of 

 only 7.3. The great increase of volume which occurs when the higher 

 oxide is formed must tend to protect particles of the lower oxide from 

 the action of the oxygen. Hence the gain in weight will be too small, 

 and the apparent atomic weight of the metal too large.* It is clear 

 that a very small deficiency in the weight of the higher oxide must 

 cause a great increase in the apparent atomic weight. 



Moreover, any incompleteness in the reduction by which the lower 

 oxide was prepared, or any retention or occlusion of gases within this 

 oxide, would also tend to raise the apparent atomic weight. Hence one 

 is inclined to believe, even without further evidence, that Zimmermann's 

 result for uranium must be too high. 



A new determination of the atomic weight of uranium has recently 

 been made by J. Aloy.f The method employed differs materially from 

 any previously used in uranium work. The values obtained are derived 



* Compare Richards and Baxter, These Proceedings 34, 351 (1898). Ztsch. 

 anorg. Chem. 21, 251 (1890). 



t Comptes Rendus, 132, 551 (1901). This work is discussed rather fully here, 

 since it is too recent to have been included in Clarke's book. 



