BAXTER AND WILSON. — THE ATOMIC WEIGHT OF LEAD. 367 



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 al- 

 ready 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 investi- 

 gation, reduction of the chloride and oxide in hydrogen was investi- 

 gated 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 vessels 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 determin- 

 ation of the chlorine in this salt by precipitation with silver nitrate 

 was chosen as presenting fewest difficulties. In the first place, the 

 determination of a halogen can be effected with great accuracy. In the 

 second place, the elimination of moisture irom lead chloride is an easy 

 matter, since the salt may be fused in a platinum vessel in a current 

 of hydrochloric acid gas without attacking the platinum 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, does not contain an amount of 

 occluded lead salt large enough to be detected. 



Purification of Materials. 



Water. — All of the water used in either the purification or the 

 analyses was distilled twice, once from an alkaline permanganate solu- 

 tion and once from very dilute sulphuric acid. Block tin condens- 

 ers were used in both distillations, and rubber and cork connections 

 were avoided. Generally receivers of Jena glass were employed, but 

 in certain cases the water was collected in platinum or quartz vessels. 



Hydrochloric acid. — Commercial C. P. hydrochloric acid was diluted 

 with an equal volume of water and distilled with a quartz condenser, 

 only the middle fraction being collected. 



Nitric acid. — Nitric acid was distilled with a platinum condenser, 



