RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OP URANIUM. 369 



was used in the work to be described in the following pages. It was 

 invariably found, however, that during the decomposition of the dried 

 nitrate, and the subsequent oxidation, the oxide first produced forms a 

 protecting crust, as it were. This prevents, or at least very materially 

 retards, the decomposition of the material within the interior, even when 

 the temperature is maintained for several hours at red heat. On the 

 outside, the material had the appearance of being completely converted 

 to oxide. On powdering the lumps, however, and again heating, there 

 was in every case a further evolution of nitric fumes. Moreover, nitrogen 

 itself is often retained by oxides prepared in this way.* It seems thus 

 extremely probable that the quantities of nitrogen measured by Aloy 

 were in every case too small. Obviously, until this point is definitely 

 settled, Aloy's results must be regarded with more or less suspicion. 



It has been pointed out that reduction is usually much more complete 

 than oxidation. t During the reduction of an oxide, there is formed, 

 perhaps, by the removal of a portion of the oxygen, a kind of skeleton 

 framework, giving to the remaining substance a porous structure which 

 enables the reducing gas to penetrate farther into the interior of the 

 mass, until reduction is complete. Owing to this action, it is probable 

 that when the green oxide of uranium is finally reduced by hydrogen, 

 all the nitrogen is expelled, and the final product is pure uranous oxide. 

 Consequently, the weight of uranous oxide used in the calculation is 

 probably nearly correct, the principal error being in the volume of 

 nitrogen . 



Aside from this special objection to the use of this method in its 

 application to uranium, there is the general objection to the use of such 

 a method where great accuracy is desired. The exact measurement of 

 small quantities of gas offers considerable opportunity for error, especially, 

 when, as in this case, the gas is first to be transferred from the collect- 

 ing to the measuring apparatus. When the volume or weight of a gas 

 is involved in an atomic weight investigation, it is customary to work 

 with as large volumes as possible, thus reducing to a minimum the 

 effect of the errors inevitably connected with the measurement of the 

 gas. The exact measurement of a volume no larger than 165 cubic 

 centimeters, even, — the largest volume measured by Aloy, — is a 

 matter of considerable experimental difficulty, while with the smaller 

 volumes, 15, 33, and 38 cubic centimeters, errors of at least 0.1% are 



* Richards and Rogers, These Proceedings, 28, 200 (1893) ; also Richards, 

 Ibid. 33, 399 (1898). 



t Richards and Baxter, loc. cit. 

 VOL. xxxvii. — 21 



