38 THE PREPARATION OF MATERIALS. 



2.756. The more recent results all tend towards the highest value, found 

 by Krichmeyer 1 (2.756). This appeared to be the most probable, because 

 the usual errors cause low results ; but as this determination was made 

 with only a single crystal, and as the salt weighed in the present research 

 had been fused, it was thought best to investigate further. Accord- 

 ingly, a pure sample of potassic bromide was further purified by being 

 three times recrystallized with centrifugal draining, when it gave no flame 

 test for sodium. 2 This product was fused in platinum and coarsely 

 pulverized ; its specific gravity was determined in an Ostwald pycnometer, 

 modified for use with solids, using toluol as the fluid to be displaced. The 

 toluol had been redistilled and possessed a density 0.8608 at 25, referred 

 to water at 4 ; 8.2568 grams of potassic bromide displaced 2.6009 grams 

 of the toluol at 25, a result which indicates a density of 2.73. Time was 

 lacking for a repetition of the experiment, but this preliminary result was 

 enough to show that the fused salt is essentially in the same state as the 

 crystallized salt. The difference between the two values is too small to 

 affect the vacuum correction ; the average value 2.74 was used. 



THE PRECIPITATION AND WEIGHING OF ARGENTIC BROMIDE. 



In order to determine the weight of argentic bromide obtainable from 

 the bromine in potassic bromide, the latter salt was precipitated by a very 

 slight excess of argentic nitrate, both substances being dissolved in large 

 quantities of the purest water. The method differed but slightly from 

 that which had been used recently in so many other cases of the same 

 kind. The argentic nitrate was prepared from a weighed amount of 

 pure silver, but no attempt was made in this series to determine the exact 

 amount of silver required, as it was desired not to complicate the process 

 before weighing the argentic bromide. Care was taken to have an excess, 

 but only a slight excess. The silver was dissolved in nitric acid with all 

 the usual precautions, and the precipitation was carried out in orange-red 

 light in the dark-room devoted to accurate work of this kind. The potas- 

 sic bromide had been dissolved out of the boat by digestion in a large 

 platinum dish, and been transferred to a 2-liter glass-stoppered Jena 

 Erlenmeyer flask with careful rinsing, and every precaution to prevent 

 any gain or loss. After being well shaken, the flask containing the silver 

 bromide was allowed to stand until the mother liquor had become clear. 

 The solution was filtered through a platinum Gooch-Munroe crucible, 

 and the precipitate was washed by decantation, first with an extremely 

 dilute acid solution of silver nitrate, and finally with exceedingly dilute 



iKrichmeyer, Z. physikal. Chem., 21, 81 (1896). 

 2 See Krichmeyer s experience, loc. cit. 



