A REVISION OF THE ATOMIC WEIGHT OF LEAD. 69 



When the lead chloride was dissolved, a dilute solution of a very nearly equiv- 

 alent amount of pure silver was added, and, after standing, the amounts of 

 chloride and silver were carefully adjusted until exactly equivalent by means 

 of the nephelometer. When the exact end-point had been found, about 0.2 

 gm. of silver nitrate in excess was added to precipitate dissolved silver chloride, 

 and the precipitate was collected and determined upon a Gooch crucible. Dis- 

 solved silver chloride in the filtrate and wash-waters was estimated by com- 

 parison with standard solutions in the nephelometer. Asbestos displaced from 

 the Gooch crucible was collected, and the moisture retained by the dried pre- 

 cipitate was found by loss of weight during fusion. 



In order to find out whether lead or silver nitrates were appreciably adsorbed 

 by the filter paper, a solution containing lead nitrate, silver nitrate, and nitric 

 acid of the concentration of these filtrates, was passed through several small 

 filter papers, which were then very carefully washed. In four cases, after in- 

 cineration of the papers, there was found, — o.ooooi, + 0.00002, -j- 0.00003, 

 -\- 0.00001 gm. of residue, exclusive of ash. This correction is so small that it 

 is neglected in the calculations. In all the analyses the platinum boat behaved 

 admirably, the loss in weight never amounting to more than a few hundredths 

 of a milligram. 



The balance used was a short-arm Troemner, easily sensitive to 0.02 mg. 

 The gold-plated brass weights were carefully standardized to hundredths of a 

 milligram. All the weighings were made by substitution with tare vessels as 

 nearly like those to be weighed as possible. 



Vacuum corrections. The values of the density of lead chloride as given by 

 various observers range from 5.78 to 5.805,^ the mean of the more accurate de- 

 terminations being 5.80. This gives rise to a vacuum correction of -\- 0.000062 

 for each apparent gram of lead chloride, the density of the weights being as- 

 sumed to be 8.3. The other vacuum corrections applied were for silver chloride, 

 -f- 0.000071, and for silver, —0.000031. 



All analyses which were carried to a successful completion are recorded in the 

 tables on page 70. 



RESULTS AND DISCUSSION. 



The close agreement of the averages of the two series is strong evidence that 

 no constant error, such as occlusion, affects the results. In all, 19.55663 gm. 

 of silver produced 25.98401 gm. of silver chloride, whence the ratio of silver 

 chloride to silver is 132.865, a value in close agreement with the result 132.867 

 obtained by Richards and Wells. Furthermore, the different samples, A, B, 

 and C, all give essentially identical results. 



It appears, then, that if the atomic weight of silver is taken as 107.880, the 

 atomic weight of lead is 207.09, nearly 0.2 unit higher than the value now in 



* Landolt-Bomstein-Meyerhoffer, Tabellen. 



