RICHARDS. — ATOMIC WEIGHT OF STRONTIUM. 375 



barium and crystallizing the mixture, both impurities tended toward 

 the mother liquors. Hence simple crystallization affords a method of 

 eliminating the two most likely impurities. 



The other properties of strontic bromide do not pertain especially 

 to the present work. 



Preparation of Materials. 



Strontic Bromide. — Six different specimens of the salt were ana- 

 lyzed, in order to establish the presence or absence of accidental 

 impurities. 



In the first place, five hundred grams of the purest strontic nitrate 

 of commerce were dissolved in two litres of pure water, and four times 

 in succession a cubic centimeter of pure sulphuric acid, diluted with 

 much water, was added to the solution. Each time only a small amount 

 of precipitate appeared at once, the rest appearing slowly. After 

 waiting in each case three or four days, the clear liquid was decanted. 

 No barium could be found even in the first precipitate of strontic 

 sulphate ; but it is true that the spectroscope is not a very satisfactory 

 means for the detection of barium under these circumstances. The 

 acid solution of strontic nitrate, which had been thus almost if not quite 

 freed from a possible trace of barium, was evaporated to small bulk, 

 filtered from the precipitated strontic sulphate, and twice successively 

 brought to crystallization. Each mass of crystals was washed three 

 times with alcohol upon the filter pump, to free it from the mother 

 liquor, which might contain calcium or magnesium. After having 

 been converted into pure carbonate by precipitation with ammonic 

 carbonate and long continued washing the strontium was combined 

 with bromine. For this purpose hydrobromic acid remaining from 

 the barium work, obtained by repeated fractional distillation of the 

 common acid, was used. 



The strontic bromide was evaporated in a platinum dish. This was 

 slightly attacked, bromine having been set free by a little occluded 

 strontic nitrate in the carbonate. After evaporation to dryness the 

 bromide was fused at a bright red heat in platinum. The alkaline 

 solution of the fused cake was treated with hydric sulphide, filtered, 

 acidified with hydrobromic acid, warmed, filtered from the platinic sul- 

 phide, boiled to free it from sulphuretted hydrogen, again filtered, and 

 crystallized twice from water. The crystals were washed with alcohol, 

 and the strontic bromide thus obtained is numbered I. below ; it was 

 used for the three preliminary experiments, as well as for Analysis 13, 



