280 Dr. Wallace on the Equivalent of Bromine. 



series of experiments performed with bromide of potassium pur- 

 chased as pure, and submitted to four recrystallizationsj I ob- 

 tained the equivalent 78-4. Marignac prepared, in the first 

 place, bromate of potash, and after repeated crystallizations, 

 converted it into bromide of potassium by the action of heat. 

 His experiments, which arc entitled to the utmost confidence, 

 gave the number 79-94' to 7999, or in round numbers 80; and 

 this equivalent is now universally accepted. Notwithstanding, 

 however, the very high reputation of Marignac, the wide differ- 

 ence between his results and those of other authorities render it 

 expedient that his number should be confirmed by another series 

 of experiments. 



I have recently had occasion to employ bromide of ai'senic in 

 an investigation, in the course of which I observed that this 

 compound offers peculiar facilities for thorough purification. It 

 distils readily, but at a much higher temperature than the chlo- 

 ride, and, on slow cooling, forms beautiful large rhombic prisms, 

 from which the still liquid portion may be poured off. The ob- 

 servation of these properties determined me to employ the bro- 

 mide of arsenic for the estimation of the equivalent of bromine. 



I prepared, by the simple union of the two elements, about 

 three-quarters of a pound of the bromide, and in the first place 

 distilled off a large proportion in order to get rid of the traces of 

 chlorine by which bromine is always accompanied. The bromide 

 was then collected, rejecting the last portions, however; and the 

 distillate was slowly cooled until a part crystallized out, after 

 which the remainder was poured off and rejected. The distilla- 

 tion, excluding the first and last portions, was repeated, and like- 

 wise the crystallization, several times. The first quantity em- 

 ployed for experiment was distilled three times and crystalUzed 

 as often ; the second portion was distilled three times and cry- 

 stallized four times ; and the third was distilled four times and 

 crystallized five times. 



The method of procedure resembled that of Pelouze for arsenic, 

 and of Marignac for bromine; and to ensure accuracy, all the 

 reagents — silver, nitric acid, and water — were obtained of the 

 greatest possible purity. Three experiments only were made. 



In the first experiment 8-58 grms. of silver required 8-3246 

 of bromide of arsenic ; in the second, 4*573 of silver took 4*4368 

 of the bromide ; in the third, the quantities were respectively 

 5-257 and 5-098. Taking the equivalent of silver at 107-97, 

 and that of arsenic at 75*0, the results are — 



1 79-756 



II 79-754 



III 79-705 



Mean 79-738 or 79-74 



