208 proceedings of the american academy. 



The Ratio of Silver to Silver Bromide. 



The ratio of silver to silver bromide was determined as follows : 

 Weighed quantities of silver were dissolved in the purest redistilled 

 nitric acid diluted with an equal volume of water, in a flask provided 

 with a column of bulbs to catch possible spatterings. However, during 

 the solution of the silver the temperature was kept so low that almost 

 no gas was evolved, and hence there was very little danger from this 

 source. Next the acid solution of the silver was diluted with an equal 

 volume of water, and was heated until free from nitrous acid and oxides 

 of nitrogen. After still further dilution, the solution was added slowly 

 with constant agitation to a dilute solution of an excess of ammonium 

 bromide in a glass-stoppered precipitating flask, and the whole was 

 violently shaken for some time to promote coagulation. By adding the 

 silver solution to the bromide, occlusion of silver nitrate was almost 

 wholly precluded. In some experiments the solutions were as dilute 

 as twentieth normal, in others as concentrated as fourth normal. The 

 final results seemed to be independent of the concentration of the 

 solutions. At the end of about twenty-four hours the flask with its 

 contents was again shaken, and then it was allowed to stand until 

 the supernatant liquid was perfectly clear. The precipitate of silver 

 bromide was collected upon a weighed Gooch crucible, after thorough 

 washing by decantation with water, and was dried in an electric oven, 

 first for several hours at 130°, finally for about fourteen hours at 180°. 

 Then it was cooled and weighed. 



The operations of precipitation and filtration were performed in a 

 large cupboard lighted with red light, and if the flask was taken out of 

 this cupboard it was enveloped in several thicknesses of black cloth. 



Even after the prolonged drying, traces of moisture were retained 

 by the salt, and could be expelled only by fusion. This was done by 

 transferring the bulk of the silver bromide, freed as completely as pos- 

 sible from asbestos, to a small porcelain crucible which was weighed 

 with its cover. The silver bromide was then fused by heating the 

 small crucible, contained in a large crucible to prevent direct contact 

 with the flame of the burner. A temperature much above the fusing 

 point of silver bromide was avoided so that volatilization of the salt 

 could not take place. This treatment must have eliminated occluded 

 ammonium salts as well as water. Finally, in order to convert any 

 occluded silver nitrate, metallic silver, or silver sub-bromide into silver 

 bromide, the salt was again fused in a current of dry air containing 

 bromine vapor. This treatment seldom produced any measurable 

 effect either upon the weight or the appearance of the salt, which was 



