THE PRECIPITATION AND WEIGHING OF ARGENTIC BROMIDE. 39 



nitric acid to prevent colloidal solution. With this latter faintly acid 

 liquid the precipitate was transferred to the crucible. The silver bromide 

 was dried at least twelve hours in the filtering crucible at 130 in an elec- 

 tric oven, 1 and weighed. It was then fused in a porcelain crucible, in a 

 furnace where it was effectively protected from flame gases, in order to 

 determine the trace of moisture always retained. The wash-waters and 

 the solution obtained on neutralizing the ammoniacal rinsings of the flask 

 gave no indication of the presence of bromine on careful testing in the 

 nephelometer. 



In using the Gooch-Munroe crucible, all the precautions pointed out 

 in the preceding paper were carefully heeded. A perforated plate was 

 always placed on top of the friable sponge to prevent rupture. After each 

 determination any bromide clinging to it was leached out with potassic 

 cyanide, which was then washed away with nitric acid, and finally very 

 thoroughly with water. The solvent action of the cyanide occasionally 

 loosened portions of the spongy film, but there was no difficulty in repair- 

 ing the injury. Upon drying in the electric oven, the crucible was ready 

 for another analysis. 



In addition to the loss of weight on fusion, two other very small and 

 somewhat uncertain corrections were applied to the weight of the argentic 

 bromide, but because they were of about the same magnitude and of 

 opposite sign, their effect was practically negligible. These were a cor- 

 rection for platinum corroded from the boat during the fusion of potassic 

 bromide, and a correction for argentic bromide dissolved in the water used 

 for washing. Because these corrections have not been considered in most 

 work of this kind, a word about them may not be amiss. 



In the series under discussion the average loss of the boat in each deter- 

 mination was less than 0.00009 gram. All of this trace of platinum may 

 have been present as invisible dust in the solution, and thus may have been 

 weighed with the argentic bromide, or all of it may have been in a soluble 

 form and may have remained in solution. Because of this uncertainty, 

 a compromise was made, and half of the loss of weight of the boat (in the 

 mean 0.00004 gram) was subtracted from each weight of argentic bro- 

 mide. As the average total weight of argentic bromide was over 4 grams, 

 this compromise could not have introduced an error as great as 1 part in 

 100,000 in either direction. 



According to Stas, 2 argentic bromide is wholly insoluble in water, but 

 recent experiments show that in the flocculent form it is unquestionably 

 soluble to a slight extent. 3 As in the case of the chloride, its solubility is 



Richards, Am. Chem. Journ., 22, 45 (1899). 

 2 Stas, Oeuvres, I, 89. 



3 B6ttger, Z. fur physik. Chemie, 46, 602 (1903) ; Kohlrausch und Rose, Zeit. 

 phys. Chem., 12, 234 (1893) ; also, Richards, Proc. Am. Acad., 30, 385 (1894). 



