8o RESEARCHES UPON ATOMIC WEIGHTS. 



The U-tube used for collecting volatilized silver chloride was washed out 

 thoroughly with dilute ammonia, and the solution was made up to definite vol- 

 ume after nearly all the ammonia had been expelled by boiUng. The silver 

 content of the solution was then compared in the nephelometer with that of 

 standard solutions of silver, care being taken that the tubes were treated in as 

 nearly as possible the same way. 



The second method of analysis consisted in heating the silver arsenate in a 

 platinum boat and, after weighing, dissolving the arsenate in nitric acid and 

 precipitating the silver as chloride or bromide. The platinum boat was heated 

 in a hard-glass tube forming part of a bottling apparatus,^ and was thus trans- 

 ferred without exposure to moist air to the weighing-bottle in which it was 

 always weighed. The boat and bottle were weighed by substitution by com- 

 parison with a counterpoise similar both in shape and volume. 



After the silver arsenate had been weighed, the boat with its contents was 

 transferred to a flask, and the salt was dissolved in warm 5N nitric acid. The 

 weighing-bottle was rinsed with acid, and the rinsings were added to the main 

 solution; then the solution was carefully transferred to a large glass-stoppered 

 precipitating flask and diluted to a volume of about i liter. 



From the weight of silver arsenate the amount of either hydrochloric or 

 hydrobromic acid necessary to precipitate the silver was calculated. A 

 slight excess of one acid or the other was then diluted to about 600 c.c, and the 

 solution was slowly poured into the solution of silver arsenate in the precipitat- 

 ing flask. After a few moments' shaking the precipitate was allowed to stand 

 for several days, with occasional agitation. 



The precipitated silver chloride or silver bromide was next collected upon a 

 weighed Gooch crucible, after it had been washed by decantation about ten times 

 with dilute hydrochloric acid in the case of silver chloride, with water in the 

 case of silver bromide. After several hours' heating in an electric air bath at 

 150° C, and about 2 hours' heating at 200° C, the precipitate was cooled in a 

 desiccator and weighed. 



In order to determine the moisture retained by the precipitate it was trans- 

 ferred as completely as possible to a small porcelain crucible and weighed. Then 

 the salt was fused by heating the small covered crucible contained in a large 

 crucible and was again weighed. 



The asbestos mechanically detached from the Gooch crucible, together with 

 a small quantity of silver chloride or silver bromide which escaped the crucible, 

 was collected upon a small filter through which the filtrate and wash-waters 

 were passed, and the filter paper was ignited in a small weighed porcelain cru- 

 cible. Before being weighed the ash was treated with a drop of nitric and a 

 drop of either hydrochloric or hydrobromic acid and again heated. 



* See page 9. 



