BAXTER AND COFFIN. — ANALYSIS OF SILVER ARSENATE. 185 



chloric acid gas or dry air. The other end of the quartz tube slipped 

 into one of the arms of a large U-tube filled with glass pearls, which 

 served to condense any silver chloride vapor which might escape from 

 the quartz tube. The other arm of the U-tube was connected with the 

 flue of a hood, the suction thus caused being sufficient to prevent the 

 escape of gaseous arsenic compounds from the apparatus. The quartz 

 tube was protected from dust by a covering of sheet mica. 



The usual method of procedure was as follows: The quartz tube 

 containing the silver arsenate being in place, a current of hydrochloric 

 acid gas was passed through the tube, and the tube was slowly re- 

 volved with pincers tipped with platinum wire in order that the salt 

 might be thoroughly exposed to the action of the acid. Neglect to do 

 this at the commencement of the reaction always resulted in the caking 

 of the salt in the tube, thereby rendering the action of the acid less rapid. 

 The hydrochloric acid was dried by passing through three towers con- 

 taining beads moistened with concentrated sulphuric acid. The ap- 

 paratus for generating and purifying the acid was constructed wholly 

 of glass. 



In the earlier experiments the salt was gently heated from the com- 

 mencement of the reaction. To all outward appearance it was entirely 

 converted into silver chloride in a few hours. Upon fusion, however, 

 it presented a very cloudy appearance, owing to the presence of con- 

 siderable arsenic, which could not be completely removed even by 

 keeping the silver chloride fused in the current of hydrochloric acid 

 for as long as eight hours. This is the cause of the larger quantities of 

 arsenic found in the chloride obtained in the earlier analyses. Fur- 

 thermore, the longer period of heating at a temperature above the fus- 

 ing point of silver chloride accounts for the larger amounts of volatil- 

 ized silver chloride found in these experiments. 



As experience was gained it was found best to expose the salt first 

 in the cold for about eight hours to the action of the hydrochloric acid 

 gas, next to heat the salt gently below its fusing point for from ten to 

 fifteen hours, and finally to keep it barely fused for from five to ten 

 hours longer. When the reaction was apparently at an end, the cur- 

 rent of hydrochloric acid gas was stopped, and dry air was passed 

 through the tube for about fifteen minutes in order to eliminate hydro- 

 chloric acid. The silver chloride was then allowed to solidify in a 

 uniform thin layer around the inside of the quartz tube by slowly re- 

 volving the tube during solidification. The platinum wire used in 

 weighing the tube was slipped on, the tube was transferred to its desic- 

 cator, and 'after standing several hours beside the balance it was 

 weighed. 



