RICHARDS. — ATOMIC WEIGHT OP STRONTIUM. 373 



Properties of Strontic Bromide. 



The properties of the bromide of stiontium resemble very closely 

 those of the corresponding salt of barium. As is well known, how- 

 ever, the strontium salt usually crystallizes with six instead of with 

 two molecules of water. The crystals, unlike those of the barium 

 salt, are noticeably hygroscopic in ordinary air, so that they can- 

 not be weighed with great accuracy ; they melt easily in their own 

 water of crystallization at about 100°. This latter fact renders more 

 difficult the quantitative drying of the salts ; indeed, in the few cases 

 where the water of crystallization was determined, it was necessary to 

 allow the crystals slowly to lose their water in a desiccator before 

 ignition. Thus, it was found in the following experiment that five 

 molecules of water were given off, the sixth having very little, if 

 any, tension at ordinary temperatures. 



Grams. 



Initial weight of strontic bromide .... 1.3305 



Constant weight after three weeks over H2SO4 .9926 



Heated to 200° for three hours 9246 



T n ■ u.- 1 • ( Found . . = 25.41 



Loss of weight in dry air ■< 



^ ^ \ Calc. for 5 HjO = 25.33 



Additional loss on ignition \ ' ' 



(Calc. . . . = 5.06 



A week's standing in the air of the laboratory sufficed to supply 

 again all the water which had been lost. These results point without 

 doubt to the existence of a definite substance having the formula 

 SrBi'g . HoO, which is hygroscopic in the air and corresponds to the 

 compound BaBr.^ . HoO, obtained in a similar way.* The existence 

 of this substance has already been inferred by Lescoeur f from obser- 

 vations of the vapor tension of the crystal water. Anhydrous strontic 

 bromide is perhaps even more hygroscopic than the corresponding salt 

 of barium. 



Strontic bromide melts to a transparent liquid at 630° (Carnelly), 

 losing bromine in noticeable quantities if exposed to the air for some 

 time at this temperature. Fused in a current of dry hydrobromic 

 acid the salt soon recovers this lost bromine, and upon subsequent 

 solution in water shows itself to be wholly neutral both to phenol 

 phthalein and methyl orange. It will be seen that this fact is of the 



* These Proceedings, XXVIII. 12, foot-note. 



t Ann. de Chira. et de Phys., [6.], XIX. 553 (1890). 



