78 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC. 



sample. If the argentic sulphate was kept fused 10 or 15 seconds too long 

 after the sulphur trioxide current was stopped, a yellowish-brown product 

 was obtained. This indicates that the salt contained either metallic silver 

 or argentic sulphide. Richards, Wells, and Forbes 1 have shown that a 

 slight discoloration of a fused silver salt is a very delicate test for im- 

 purity. The loss of weight due to this discoloration was very slight, as 

 will be shown on page 79. 



The entire tube was later heated in a current of pure air at a tempera- 

 ture above the boiling-point of sulphuric acid for about 5 minutes, in order 

 to drive out any possible accidental trace of acid in the tube. While still 

 warm it was placed in a desiccator, and later weighed. The tube was kept 

 horizontal until the end of the experiment, to avoid any chance of mechan- 

 ical loss. 



The device of spreading the salt in a thin layer around the tube by rotat- 

 ing the tube during the cooling had four very important advantages : 

 First, the chance of breaking the very fragile quartz tube during the solid- 

 ification and cooling was greatly reduced by the more even distribution of 

 the strain ; secondly, the salt was agitated and spread out in a thin layer 

 while still fused in a current of pure air, thus facilitating the escape of any 

 possible trace of dissolved sulphur trioxide ; thirdly, the cooling was made 

 more uniform and rapid, so that the argentic sulphate had no time to 

 decompose before it had solidified; and fourthly, a much larger surface 

 was exposed to the later action of the hydrochloric acid. 



The possibility just mentioned that the colorless argentic sulphate might 

 contain an excess of sulphur trioxide needed careful consideration. Un- 

 fortunately a direct test for acid seemed to be impracticable, owing to the 

 slight solubility of silver sulphate, hence light upon the question was 

 sought in several distinct ways. 



Weber 2 has found that in order to prepare a compound Ag 2 S,O 7 argen- 

 tic sulphate must be heated with sulphur trioxide under pressure in a 

 sealed tube. This, together with the fact that argentic sulphate decom- 

 poses so easily when fused, indicates that it does not have a great tendency 

 to retain sulphuric oxide, and that the vapor tension of the trioxide in the 

 disulphate is far above that maintained in the present experiments. In 

 order to obtain quantitative evidence, the pure white sulphate in six of the 

 final experiments, after being weighed, was fused again for about 10 

 to 15 seconds in order to decompose it slightly. This gave a slight but 

 unmistakable dark color to the salt. The resulting losses of weight are 

 tabulated on page 79. 



iRichards and Wells, Carnegie Inst. Pub. 28, 31 (1905) ; Richards and Forbes, the 

 present publication, p. 55. 



2 Weber, Ben, 17, 2503 (1884). 



