190 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The crucible with its contents was dried for at least 12 hours in an 

 electric air-bath at 160°, cooled in a desiccator and weighed. The 

 crucible had originally been dried in exactly the same way. In order 

 to find out how much moisture had been retained by the dried silver 

 chloride, the greater part of the salt was transferred to a porcelain 

 crucible which was weighed. Then the crucible was heated to the 

 fusing point of the silver chloride and reweighed. The loss in weight 

 is assumed to represent residual moisture. On an average 0.004 

 percent of moisture was found, a proportion which is in accord with 

 earlier experience in the Harvard laboratories. 



The solubility of silver chloride in the filtrate and silver nitrate wash 

 waters, which both contained 0.05 gm. of silver nitrate per liter, was 

 computed from the solubility products as found by Kohlrausch 28 at 

 20° and 25°, 1 X 10' 10 and 1.7 X 10" 10 respectively. At 20° the solu- 

 bility in 0.0003 normal silver nitrate solution is 0.05 mg., at 25° 

 0.08 mg. per liter. The former correction was used during the colder 

 months when the laboratory was maintained at about 20°, the latter 

 correction in four analyses which were completed in summer. The 

 total correction in most cases fell between 0.2 and 0.3 mg. The 

 silver chloride dissolved in the aqueous rinsings as well as that ob- 

 tained by rinsing the precipitating flask with ammonia was determined 

 by comparison with standard solutions of chloride in a nephelometer, 

 the usual precautions being taken to secure uniformity of precipita- 

 tion. As in earlier researches it was found desirable to dissolve in 

 ammonia the cloud of silver chloride in both the standard solution and 

 that being analyzed, and then to reprecipitate with nitric acid. 



Corrections were of course applied for any chloride introduced in 

 order to compensate for excess of silver, and also for the standard 

 chloride solution added to the portions which were tested in the com- 

 parison, for these portions were always returned to the precipitating 

 flask. The latter quantity amounts to 1 .328 mg. for each test which 

 was made in the comparison. Because of the comparatively large size 

 of this correction, the standard silver and chloride solutions were made 

 up, preserved, and measured with great care. 



Weighings were carried out on a No. 10 Troemner balance, sensitive 

 at least to 0.02 mg. In order to avoid difficulties from changes in 

 atmospheric humidity and density, the receptacles containing the salts 

 were always weighed by substitution for counterpoises as nearly as 

 possible like the objects both in material, volume, and external surface. 

 The balance case was provided with a small amount of radio-active 



28 Zeit. physik. Chem., 64, 167 (1908). 



