A REVISION OF THE ATOMIC WEIGHT OF LEAD. 67 



in the least and without the production of basic salts. In the third place, silver 

 chloride, which has been precipitated from a dilute solution of lead chloride by 

 means of silver nitrate, was shown experimentally not to contain an amount of 

 occluded lead salt large enough to be detected. 



PURIFICATION OF MATERIALS. 



Water, hydrochloric acid, and nitric acid were carefully purified by distilla- 

 tion as described in the preceding papers. In the preparation of pure silver 

 also the usual methods were employed, one precipitation as chloride, one as 

 metal by ammonium formate, and one by electrolysis being followed by the 

 final fusion in hydrogen on a boat of pure lime. 



LEAD CHLORIDE. 



Three samples of lead chloride from two entirely diflrerent sources were em- 

 ployed. Sample A was prepared from metallic lead. Commercial lead was dis- 

 solved in dilute nitric acid, and the solution, after filtration, was precipitated 

 with a slight excess of sulphuric acid. The lead sulphate was thoroughly washed, 

 suspended in water, and hydrogen sulphide was passed in until the sulphate was 

 almost completely converted into sulphide. Next the sulphide was washed with 

 water, dissolved in hot dilute nitric acid, and the solution was freed from sulphur 

 and unchanged sulphate by filtration. The lead nitrate thus obtained was 

 crystallized twice, dissolved in water, and precipitated in glass vessels with a 

 slight excess of hydrochloric acid. The chloride was washed several times with 

 cold water and then crystallized from hot water eight times, the last five crys- 

 tallizations being carried out wholly in platinum, with centrifugal drainage 

 after each crystallization. In crystallizing the lead chloride the whole sample 

 was not dissolved at one time, but the same mother-liquor was used for dissolv- 

 ing several portions of the original salt. Needless to say, the chloride was not 

 exposed to contact with the products of combustion of illuminating gas, lest lead 

 sulphate be formed. 



Sample B was prepared from commercial lead nitrate. This salt was dissolved 

 and crystallized from dilute nitric acid once in glass and six times in platinum 

 vessels, with centrifugal drainage. Hydrochloric acid was then distilled into a 

 large quartz dish, and the solution of the nitrate was slowly added with constant 

 stirring with a quartz rod. The chloride was freed from aqua regia as far as 

 possible by washing with cold water, and was once crystallized from aqueous 

 solution in quartz dishes to remove last traces of aqua regia. Finally the salt 

 was crystalhzed three times in platinum. 



It could reasonably be expected that both of these samples were of a high 

 degree of purity; nevertheless, upon heating the salt in an atmosphere of hy- 

 drochloric acid, the salt itself turned somewhat dark, and upon solution of the 

 fused salt in water a slight dark residue remained. Although in a few preliminary 



