66 RESEARCHES UPON ATOMIC WEIGHTS. 



careful consideration of his work discloses minor defects, many of which he 

 recognizes himself. The metallic lead used in the syntheses was finally fused 

 under potassium cyanide. Whether or not this treatment introduced impuri- 

 ties into the metal is uncertain. Stas himself suspected the presence of alkali 

 metals. Since the nitrate could not be dried above 150° without decom- 

 position, it undoubtedly 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 moisture, 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 can not 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 only serve 

 to emphasize the need of a redetermination of the value in question, and it was 

 with this object in view that the work embodied in this paper was undertaken. 



The search for a suitable method for determining the atomic weight of lead 

 failed to reveal any more promising line of attack than those already employed 

 for the purpose. With an element of so high an atomic weight as lead, in any 

 method involving the change of one of its compounds into another, errors which 

 may be insignificant with elements of small atomic weight are magnified in the 

 calculations to undesirable proportions. Furthermore, during the following 

 investigation, reduction of the chloride and oxide in hydrogen was investigated 

 far enough to show that complete reduction of either compound was extremely 

 difficult, if not impossible, without loss of material from the containing vessel by 

 sublimation, aside from the fact that all available material for containing ves- 

 sels is acted upon by either the fused salt or the reduced metal. The elimination 

 of moisture from lead nitrate or lead sulphate without decomposition of the 

 salts seemed likely to prove a stumbling-block in the use of these substances. 

 Finally, in spite of the slight solubility of lead chloride, the determination of 

 the chlorine in this salt by precipitation with silver nitrate was chosen as pre- 

 senting fewest difficulties. In the first place, the determination of a halogen can 

 be effected with great accuracy. In the second place, the elimination of mois- 

 ture from lead chloride is an easy matter, since the salt may be fused in a plat- 

 inum vessel in a current of hydrochloric-acid gas without attacking the platinum 



* Ann. Chim. Pkys. (3), 9, 254 (1843). 



