M. Stas on the Determination of Atomic Weights, 195 



mide of silver. By this means bromide of silver becomes 

 yellow, as also when bromide of potassium is poured upon it. 

 By cold water it again becomes white with a very feeble tinge 

 of yellow. White bromide of silver rapidly becomes violet in 

 the light, yellow more slowly ; fused bromide of silver is quite 

 permanent in the light. In the presence of traces of free 

 bromine, white bromide of silver is also permanent in the light. 

 The bromide of silver obtained, like the iodide of silver, was 

 weighed in a glass bulb. It can be fused without loss of weight, 

 provided no gases or vapours enter the apparatus. 



For the complete synthesis of bromide of silver, pure bromine 

 had to be weighed. The latter was prepared by distilling bromide 

 of potassium and bromate of potassium, or bromide of barium 

 and bromate of barium, with sulphuric acid. The bromide of 

 potassium of commerce was freed from admixed iodine by add- 

 ing to a quarter of the solution bromine-water until the brown 

 colour at first formed passed into a very bright orange-yellow 

 colour. The other three-fourths of the bromide-of-potassium 

 solution were then added, and all the iodine removed by shaking 

 with CS 2 . The solution, which had been freed from CS 2 by boiling, 

 was mixed with not quite six equivalents of hydrate of potash 

 for one of KBr, and chlorine passed in. By evaporating the 

 liquid, at first only half the bromate of potassium formed was 

 utilized, the mother-liquors served for the preparation of bro- 

 mate of barium. The bromate of potassium was recrystallized 

 three times, and suitably lixiviated each time. The pure salt 

 ought neither to render turbid nor colour a boiling and perfectly 

 neutral solution of sulphate of silver. By heating in porcelain 

 vessels (platinum vessels are attacked by it), the bromate is con- 

 verted into bromide of potassium. Towards the end there is a 

 little explosion and separation of bromine. Five equivalents of 

 this bromide of potassium are mixed with one equivalent of the 

 unchanged bromate of potassium, and distilled with sulphuric 

 acid. The bromine which separates in the cold is collected in a 

 retort containing a solution of bromide and bromate of potas- 

 sium, from which it is distilled off in the water-bath. To free 

 it from every trace of chlorine, it is dissolved in concentrated 

 solution of bromide of calcium, and precipitated therefrom by 

 water. Bromide of calcium was prepared by dissolving purified 

 bromine in caustic lime mixed with ammonia and then concen- 

 trating the solution. The precipitated bromine is repeatedly 

 dried over bromide of calcium and distilled off anhydrous phos- 

 phoric acid. The latter was sublimed in a current of air. The 

 bromine is finally agitated with finely powdered caustic baryta, 

 and distilled after being poured off. 



The mother-liquors from the preparation of bromate of potas- 

 sium were diluted with five times as much water, heated to 80°, 



02 



