RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OF URANIUM. 383 



phuric acid was added, and the mixture was slowly i"un into the solution 

 of uraiious bromide. The green color of the uranous salt soon changes to 

 the yellow color characteristic of uranyl com[)ounds. On adding the 

 first few cubic centimeters of the dilute hydrogen dioxide solution, a 

 greenish white precipitate came down. Addition of more of the acid 

 dioxide solution redissolved it, and the resulting solution was perfectly 

 clear. This peculiar hydrolytic action is due to the acid, and not to tlie 

 hydric dioxide, lor the same reaction occurs if dilute sulphuric acid alone 

 is added to the solution. * 



The explanation of this interesting phenomenon, which is just the 

 opposite of what might have been expected, is, undoubtedly, that the 

 bromide is already hydrolyzed to a great extent by merely dissolving in 

 water. The hydrate is probably in solution in the colloidal state. Evi- 

 dence of this is found in the fact that if the clear aqueous solution of 

 uranous bromide is allowed to stand exposed to the air, a hydrate gradu- 

 ally separates, giving to the solution a cloudy, murky appearance. After 

 two or three days this precipitate disappears, giving place to a clear 

 yellow solution of oxybromide and hydrobromic acid. The addition of 

 sulphuric acid coagulates the colloid before it can all be converted into 

 uranyl salt. 



In order to be sure that no bromine or hydrobromic acid is lost by 

 this method of oxidation, the following experiment was made. 0.5 gram 

 of bromide was dissolved in 250 c.c. of water, 50 c.c. of dilute sulphuric 

 acid (1 :10) was added, and the hydrogen dioxide solution was run in. 

 This was done in a closed flask, similar in construction to a gas washing 

 bottle. A current of air was drawn through the bottle and then through 

 starch solution containing potassium iodide to see if bromine is liberated. 

 Not the slightest trace of blue color appeared in the starch solution, even 

 after adding a large excess of hydrogen peroxide and allowing it to stand 

 over night. A test for hydrobromic acid was sought in a similar way, 

 by drawing the air through a solution of silver nitrate, again with nega- 

 tive results, as was to have been expected. These experiments show 

 conclusively that uranous bromide can be oxidized completely by hydro- 

 gen dioxide, without loss of bromine. 



Silver nitrate, in moderately concentrated solutions, is not acted upon 

 by a three per cent solution of hydrogen peroxide. Consequently a con- 

 siderable excess of the latter reagent could do no harm. Nevertheless 

 care was taken never to add more than the calculated amount of hydro- 

 gen dioxide. Moreover, the solution of hydrogen dioxide used contained 

 only one per cent of this reagent, and this was diluted ten times before 



