RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OF URANIUM. 389 



crystals of electrolytic silver were then dried over potash and fused 

 in vacuo on a boat of pure lime. The buttons of silver thus obtaiiit-d 

 were treated with nitric acid to remove the surface, dried, and kept over 

 potash. A second sample was obtained by fusing in vacuo electrolytic 

 silver which had been prepared from the silver bromide obtained in Dr. 

 Baxter's work upon cobalt, which was known to be very pure. 



Hydric dioxide was purified as follows : To a solution of the ordinary 

 commercial peroxide prepared for medicinal use, was added a solution of 

 baric hydroxide, which had been purified by recrystallization. The pre- 

 cipitated baric dioxide was washed until a nitric acid solution of the 

 same showed no trace of halogen. It was then added to pure dilute sul- 

 phuric acid, and the resulting solution of hydric dioxide was filtered and 

 distilled in a partial vacuum. The solution thus obtained showed no trace 

 of halogen, and left no visible residue on evaporation in platinum. 



Ammonium sulphide was made from pure ammonia, which had been 

 redistilled in platinum, and pure sulphuretted hydrogen. It left no visible 

 residue on evaporation in platinum. 



Hydrochloric and nitric acids were redistilled in a platinum still, and 

 throut^hout the work platinum vessels were used wherever possible. 



Water was twice redistilled, once over alkaline potassic permanganate, 

 and again over acid potassic sulphate from a Jena glass flask, a block-tia 

 condenser and Jena glass receiver being used. 



The Results of the Analyses of Ueanous Bromide. 



The method of analysis has been already fully described. 



The analyses recorded in the first series were made by adding an 

 excess of silver nitrate to the solution of uranyl bromide. From the 

 ratio of the observed weights of urauous bromide to argentic bromide, 

 the molecular weight of uranous bromide was calculated, that of argentic 

 bromide being assumed to be 187.885. From the results obtained from 

 this preliminary series the weight of silver necessary to precipitate the 

 bromine in one gram of uranous bromide was calculated. In the subse- 

 quent determinations the exact weight of silver required was weighed 

 out, as nearly as possible, and dissolved in pure nitric acid with suitable 

 precautions to avoid loss. The exact end point was reached by standard 

 hundredth normal solutions of argentic nitrate and hydrobromic acid, by 

 means of the nephelometer.* After determining the end point a slight 

 excess of argentic nitrate was always added, and the weight of the total 



* Richards, These Proceedings, 30, 385 (1894). Z. anorg. Chem., 8, 269 (1895). 



