A REVISION OF THE ATOMIC WEIGHT OF LEAD. 



THE ANALYSIS OF LEAD CHLORIDE. 



INTRODUCTION. 



Although lead is one of the most common elements, its atomic weight has re- 

 ceived comparatively little attention, the value at present accepted being based 

 almost wholly upon the work of Stas.^ Of the earlier determinations of this 

 constant those of Dobereiner ^ and Longchamps ^ can hardly be considered as 

 possessing other than historic interest. The first results which can lay claim to 

 accuracy are those of Berzelius,^ who obtained values ranging from 206.7 to 

 207.3 by reduction of litharge in a current of hydrogen. Berzelius also synthe- 

 sized the sulphate from metallic lead with the result 207.0.* Shortly after, 

 Turner ^ criticized the first method employed by Berzelius and attributed the 

 irregularity of his results to the action of lead oxide on the silicious matter of 

 the tube at the temperature employed in the reduction. By the conversion of 

 both the metal and the oxide into sulphate Turner in a painstaking research 

 deduced the values 207.0 and 207.6 respectively, and by converting the nitrate 

 into sulphate, 204.2. Marignac ' converted metallic lead into the chloride by 

 heating in a stream of chlorine and obtained the result 207.42. Both Marig- 

 nac * 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 chloride, 206.85. Dumas subse- 

 quently 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 



* Earlier work on the atomic weight of lead has been carefully summarized by Clarke. 

 Smithsonian Miscellaneous Collections, Constants of Nature, "A Recalculation of the Atomic 

 Weights," 1910. The earlier results have been recalculated on the basis of the following atomic 

 weight?: O = 16.00; N = 14.01; S = 32.07; CI = 35-46; Ag = 107.88. 



* Schweig. Jour., 17, 241 (1816). ' Ann. Chim. Phys., 34, 105 (1827). 



* Fogg. Ann., 19, 314 (183c). * Lehrbuch, 5th ed., 3, 1187 (1845). 

 ^ Phil. Trans., 527 (1833). ' Lieb. Ann., 59, 289 (1846). 



^ Jour. Prakt. Chcm., 74, 2j8 (1858). ^ Lieb. Ann., 113, 35 (i860). 



^° CEuvres Completes, i, 383. 



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