162 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Balances and "Weights. 



The preliminary determinations were made upon a long-armed 

 Becker balance with the help of very carefully standardized platinized 

 brass weights. The final determinations were made upon the admi- 

 rable Troemer balance procured for the research upon copper, and sub- 

 sequently used for those upou barium and strontium. The weights 

 used in these final determinations were the same as those used in the 

 researches just cited ; they were compared with one another at the 

 beginning and at the close of the research, with satisfactory results. 

 All weighings were made by substitution, the tare weights being ves- 

 sels as nearly as possible similar to those being weighed ; and all 

 were reduced to the vacuum standard by the usual formula. 



The Specfic Gravity of Zincic Bromide. 



In the course of recent investigations upon the atomic weights so 

 many of the usually accepted specific gravities of hygroscopic sub- 

 stances have been found to be seriously in error, that it seemed advis- 

 able to redetermine the constant which influences the reduction to 

 vacuum of the present results. 



Pure zincic bromide was dried for a long time at a temperature of 

 200°, and then fused and heated for a sliort time at 300°. The pyc- 

 nometer in which this drying was effected was then stoppered and 

 cooled in a desiccatoi-. Carefully dried toluol having a specific grav- 

 ity of 0.8646 at 20°, when referred to water at 4°, was used as the 

 liquid to be displaced. Toluol is convenient for the purpose, because 

 so few inorganic substances are soluble in it, and because its volatility 

 is not so great as to cause serious loss during the weighing, but is 

 great enough to allow of the rapid drying of the exterior of the appa- 

 ratus. The first sample of zincic bromide was made from very pure 

 hydrobromic acid and ordinary pure zinc, and was distilled in an 

 atmosphere of carbon dioxide ; the second was made from the purest 

 electrolytic zinc and the purest bromine. Boih samples gave a per- 

 fectly clear dilute solution in water after the expulsion of the toluol 

 on the steam bath after the experiment. The decanted toluol left 

 upon evaporation on the steam bath only a trace of residue, which 

 was insoluble in water. Water decanted from this residue gave no 

 trace of precipitation with argentic nitrate ; hence it is evident that 

 zincic bromide is insoluble in toluol. The liquid contained in glass 



