366 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and 207.6 respectively, and by converting the nitrate into sulphate, 

 204.2. Marignac "^ converted metallic lead into the chloride by heat- 

 ing in a stream of chlorine and obtained the result 207.42. Both 

 Marignac ^ and Dumas ^ analyzed lead chloride. Marignac, who dried 

 the salt at 200°, by titration against silver found the atomic weight of 

 lead to be 206.81, and from the ratio of lead chloride to silver chlo- 

 ride, 206.85. Dumas subsequently showed that lead chloride, even 

 when dried at 250°, retains moisture and is somewhat basic, and in 

 one analysis in which corrections are applied for these errors, found a 

 somewhat higher value, 207.07, as was to be expected. Chloride 

 analyses by early investigators are, however, to be universally dis- 

 trusted, owing to neglect of the very considerable solubility of silver 

 chloride, thus producing too low results. 



Stas's work upon the syntheses of lead nitrate and sulphate from the 

 metal is undoubtedly the most accurate contribution upon the subject, 

 although a careful consideration of his work discloses minor defects, 

 many of which he recognized himself. The metallic lead used in the 

 syntheses was finally fused under potassium cyanide. Whether or not 

 this treatment introduced impurities into the metal is uncertain. 

 Stas himself suspected the presence of alkalies in the metal. Since 

 the nitrate could not be dried above 150° without decomposition, it un- 

 doubtedly contained moisture, and Stas calls attention to this point. 

 The sulphate was made by treatment of lead nitrate, resulting from 

 the nitrate syntheses, with sulphuric acid. The sulphate was dried 

 finally at dull redness, and was probably free, or nearly free, from mois- 

 ture, although it may have contained traces of lead oxide resulting from 

 occluded nitrate, as well as sulphuric acid. Most of these probable errors 

 tend to lower the observed atomic weight, so that Stas's value from the 

 series of nitrate syntheses, 206.81, and that from the sulphate series, 

 206.92, are to be regarded as minimum values. The reader of Stas's 

 own account of his work upon lead cannot fail to be impressed with 

 the fact that he was somewhat dissatisfied with the outcome of his 

 research. Mention should also be made of the work of Anderson 

 and Svanberg ^^ on the conversion of lead nitrate into oxide, although 

 the method was primarily employed in an endeavor to fix the atomic 

 weight of nitrogen. Their results yield the value 207.37. 



The discrepancies between the. results of these various experiments 



' Lieb. Ann., 59, 289 (1846). 



8 Jour. Prakt. Chem., 74, 218 (1858). 



9 Lieb. Ann., 113, 35 (IBGO). 

 " CEuvres Completes, 1, 383. 



" Ann. Chim. Phys. (3), 9, 254 (1543). 



