420 PROCEEDINGS OP THE AMERICAN ACADEMY. 



stable, and the only one which satisfies both the above conditions is 

 silver iodide. Accordingly synthesis of silver iodide from a weighed 

 amount of silver was selected for the first method of investigation. 



Purification of Materials. — The problem of purification of materials 

 was a comparatively simple one. Silver was prepared as follows : To 

 a solution of silver nitrate which had been made strongly acid with 

 nitric acid, there was added a large excess of concentrated hydrochloric 

 acid. After the precipitate had been washed many times with pure water, 

 it was reduced to metallic silver by means of a solution of sodium hydrox- 

 ide and invert sugar. The silver was washed well with water aud fused 

 with a blowpipe on charcoal. Surface impurities were removed by 

 scrubbing the buttons with sand, and boiling them, first with dilute 

 hydrochloric acid, and then with ammonia. Next, the whole of the 

 process just described was repeated, but with reagents which had been 

 purified with the greatest care. Water and acids were freshly distilled, 

 the sodium hydroxide was electrolyzed in a platinum dish until free from 

 iron, and the final fusion was carried out in a porcelain crucible lined 

 with the purest lime. This lining was made by the ignition of a mixture 

 of reprecipitated calcic carbonate and recrystallized calcic nitrate. After 

 the buttons had been cleansed as before, the silver was deposited 

 electrolytically by making one button the cathode, the other buttons in 

 succession the anode, in a cell containing a strong solution of silver 

 nitrate made by dissolving one of the buttons in pure nitric acid. Con- 

 nection was made with the battery by means of a platinum wire, which, 

 however, did not dip into the solution. The crystals of silver were 

 washed, first with water, then with ammonia, and again several times with 

 water. Finally they were dried and fused in a current of hydrogen 

 on a boat of pure lime contained in a porcelain tube. The hydrogen 

 was generated electrolytically from a cell composed of zinc amalgam, 

 hydrochloric acid, and platinum, and was purified from hydrochloric 

 acid and dried by passing over beads moistened with caustic potash 

 solution and then over solid potassic hydroxide. The silver was cut 

 into fragments weighing from three to five grams each by means of 

 a clean chisel and anvil, and the iron adhering to the surface was 

 removed by warming the pieces repeatedly with fresh portions of 

 dilute nitric acid, until iron could no longer be detected in the acid. 

 The fragments were washed thoroughly with the purest water, then 

 with ammonia, and again with water as before. They were allowed to 

 dry in the air, and finally the last traces of moisture were eliminated 

 by heating in a vacuum, siuce silver which is heated to a high tempera- 



