BAXTER AND WILSON. — THE ATOMIC WEIGHT OF LEAD. 371 



by gentle heating. The solution was then further diluted until 

 not stronger than one per cent, and added slowly, with constant 

 agitation, to the solution of lead chloride contained in the precipi- 

 tating flask. The precipitation and handling of the silver chloride 

 were conducted in a room lighted with ruby light. The flask was 

 shaken for some time and allowed to stand for a few days, with 

 occasional agitation, until the supernatant liquid had become clear. 

 Thirty cubic centimeter portions of the solution were then removed 

 and tested with hundredth normal silver nitrate and sodium chloride, 

 in a nephelometer, 18 for excess of either chloride or silver, and, if 

 necessary, standard silver nitrate or sodium chloride was added, and 

 the process of shaking and testing repeated until the amounts of silver 

 and chloride were equivalent. The test solutions were always returned 

 to the flask, since they contained appreciable amounts of silver chlo- 

 ride, and the weight of silver chloride subsequently obtained was cor- 

 rected for the quantity thus introduced. Furthermore, if an excess of 

 silver was found, a negative correction of an equivalent quantity of 

 silver chloride was necessary. 



After the exact end point had been obtained, about two tenths of a 

 gram of silver nitrate in excess was added in order to precipitate the 

 dissolved silver chloride, and the flask was thoroughly shaken, and 

 allowed to stand again until the solution was perfectly clear. The 

 silver chloride was washed, first several times with a very dilute silver 

 nitrate solution containing four hundredths of a gram per litre, and 

 then eight times with pure water. It was next transferred to a Gooch 

 crucible and dried for several hours in an electric oven, the tempera- 

 ture being gradually raised to ISO , and was cooled in a desiccator and 

 weighed. In every case the moisture retained by the precipitate was 

 determined by fusion in a small porcelain crucible. The silver chlo- 

 ride, dissolved in the filtrate and washing, was determined by comparison 

 with standard solutions in the nephelometer in the usual manner. 

 Care was taken to treat both tubes in exactly the same manner, and 

 final readings were taken only when the ratio had become constant. 

 Before proceeding to the nephelometer tests, however, the filtrate and 

 washings were passed through a very small filter in order to collect a 

 small quantity of asbestos shreds mechanically detached from the Gooch 

 crucible. The filter was ignited and weighed, the ash being treated 

 with a drop of nitric and hydrochloric acid in order to convert any 

 reduced silver into chloride. In order to find out whether lead or 

 silver nitrates were appreciably adsorbed by the filter paper, a solution 



18 Richards and Wells, Am. Ch. J., 31, 235 (1904) ; 35, 510 (1906). 



