BAXTER. — ATOMIC WEIGHT OF CHROMIUM. 411 



Another correction was necessary. The filtrate contained dissolved 

 silver salt, even though an excess of halogen acid was used in the pre- 

 cipitation. 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 dis- 

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

 adopted in this work. The entire filtrate of three to four liters was 

 evaporated to small bulk, nearly neutralized with ammonia, and then 

 the silver was precipitated from a hot solution as sulphide. The pre- 

 cipitate was collected upon a filter paper, which was ignited. The resi- 

 due was converted to the nitrate by digestion with dilute nitric acid, 

 and the solution was then filtered into a graduated flask, in which it 

 was diluted to known volume. By comparison in the nephelometer of 

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

 solution was determined. In using the nephelometer all necessary pre- 

 cautions, as pointed out by Richards, 28 were taken. 



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 hydrogen sulphide having been expelled, and after re- 

 moval of the chromic hydroxide by filtration, testing the acidified filtrate 

 for silver. None could be detected. 



The Determination of Moisture in 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 pen- 

 toxide tube. During the fusion of the salt oxygen is evolved, but 

 since the fusing point is low, there is no 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 twp hours at 200°, 225°, and 300°, in dry air which 

 had been purified as previously described. 



28 Am. Chem. Jour., 35, 510 (1906). 



