THE ANALYSIS OF SILVER CHROMATE. I47 



The small quantity of asbestos, together with a trace of silver bromide 

 which escaped the crucible, was collected by passing the entire filtrate and wash- 

 ings through a small filter. The ash of this filter was treated with nitric and 

 with hydrochloric or hydrobromic acid, then it was reheated and the crucible 

 was weighed. After correction for the ash of the filter, the gain in weight of 

 the crucible was added to the weight of the main mass of silver halide. 



Another correction was necessary. The filtrate contained dissolved silver 

 salt, even though an excess of halogen acid was used in the precipitation. 

 The larger part of the dissolved halide is due to the marked solubility in solutions 

 of chromic salts, the amount dissolved increasing with increasing concentration 

 of the chromic salts. Berlin overlooked this correction which was afterwards 

 pointed out by Siewert. Meineke later determined experimentally the quantity 

 of dissolved material, and also proposed the method of separation which was 

 adopted in this work. The entire filtrate of 3 to 4 liters was evaporated to small 

 bulk, nearly neutralized with ammonia, and then the silver was precipitated 

 from a hot solution as sulphide. The precipitate was collected upon a filter 

 paper, which was ignited. The residue was converted to the nitrate by digestion 

 with dilute nitric acid, and the solution was then filtered into a graduated flask, 

 in v/hich it was diluted to known volume. By comparison in the nephelometer 

 of this solution with standard solutions of silver the quantity of silver in solu- 

 tion was determined. 



That all dissolved silver was recovered in this way was shown by adding an 

 excess of ammonia to the filtrate of the silver sulphide in one analysis, the hy- 

 drogen sulphide having been expelled, and after removal of the chromic hydrox- 

 ide by filtration, testing the acidified filtrate for silver. None could be detected. 



DETERMINATION OF MOISTURE IN DRIED SILVER CHROMATE. 



The proportion of moisture in the silver chromate was found by fusing 

 weighed quantities of the salt in a current of pure dry air and collecting the 

 water vapor produced in a weighed phosphorus pentoxide tube. During the 

 fusion of the salt oxgyen is evolved, but since the fusing point is low, there is on 

 danger of volatilization of either silver or chromium compounds. 



In order to avoid the necessity of removing the fused silver chromate from a 

 platinum boat, boats of copper foil which had been cleaned and ignited were 

 employed. 



It was desirable to determine not only whether the proportion of water could 

 be made constant at any one temperature, but also how much the proportion 

 of water is affected by variations in temperature. Experiments were therefore 

 carried out with silver chromate which had been dried for 2 hours at 200°, 225°, 

 and 300°, in dry air which had been purified as previously described. 



After the salt had been dried, a carefully weighed U-tube containing resub- 

 limed phosphorus pentoxide was attached to the end of the tube, This U -tube 



