382 PROCEEDINGS OF THE AMERICAN ACADEMY. 



During the weighing in the final analyses, the bromide of uranium 

 was still surrounded by an atmosphere of pure dry nitrogen in the 

 tightly stoppered weighing bottle. Since this bottle had been full of 

 dry air when it was first weighed, a small correction had to be applied 

 on this account. The difference in weight between 6.70 cubic centi- 

 meters (the interior volume of the weighing bottle) of air and the same 

 volume of nitrogen at 20° C. is 0.000265 gram. Of this nitrogen a 



gram of uranous bromide displaced -rw, = 0.206 cubic centimeters, or 



0.24 milligram, while the brass weights used in weighing the bromide 

 displaced 0.145 milligram of air. Hence in vacuum a gram of uranous 

 bromide would weigh 0.265 + 0.24 — 0.145 = 0.36 milligram more 

 than the observed weight, while two grams would weigh 0.265 + 

 2(0.24 — 0.145) = 0.46 more than the observed weight. All the weights 

 given in the tables are corrected in this way to the vacuum standard. 



Methods of Analysis. 



By the use of these devices we were able to prepare and weigh pure 

 uranous bromide in a definite state. There still remained, however, the 

 problem of devising a suitable method of analysis. As previously men- 

 tioned, all uranous compounds reduce silver nitrate, making impossible 

 the usual method of procedure in halogen determinations. 



The method of precipitating the uranium and determining bromine in 

 the filtrate involves too much danger of loss of material in the multiplic- 

 ity of operations. The most satisfactory solution of the problem seemed 

 to be to oxidize the compound to the uranyl salt, provided this could be 

 done without loss of bromine. Nitric acid is of course effective as an 

 oxidizing agent, but the oxidation is accompanied by loss of bromine. 

 After much experimenting, hydrogen dioxide was found to be the most 

 suitable oxidizer. From neutral solutions of uranium compounds, hydro- 

 gen dioxide precipitates a hydrated peroxide of uranium. If the solution 

 is slightly acid, this precipitation is prevented and the uranous compound 

 completely oxidized to the uranyl state. The weighed sample of uranous 

 bromide was dissolved in considerable water — at least 400 cubic centi- 

 meters of water to each gram of bromide. The bottle containing the 

 bromide was opened by means of a suitable glass fork, either below the 

 water or just above the surface, so that it could be instantly submerged, 

 and thus avoid loss of hydrobromic acid by the action of moist air. The 

 calculated volume of a standard solution of pure hydrogen dioxide was 

 then diluted to about 100 c.c, one cubic centimeter of pure dilute sul- 



