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CHEMISTRY: F. W. CLARKE 
A series of analyses of lead bromide gave values practically identical with 
these. 
This evidence as to constancy of atomic weight is conclusive, but it has 
also been confirmed by three investigations by Richards and his colleagues 
in this country, and by Honigschmid in Vienna. These later determinations 
were made as checks upon determinations of the atomic weight of isotopic 
lead derived from uranium minerals. 
In 1914 Richards and Lembert 2 published their determinations of the 
atomic weight of isotopic lead. Their results may be summarized as follows : 
Source Atomic weight 
Lead from Ceylonese thorianite 206 . 82 
Lead from English pitchblende 206.86 
Lead from Colorado carnotite 206.59 
Lead from Bohemian pitchblende 206 . 57 
Lead from North Carolina uraninite 206.40 
Two years later another series of determinations by Richards and Wads- 
worth 3 appeared. The average results obtained were as follows: 
Source Atomic weight 
Australian carnotite 206 . 375 
Colorado carnotite 207 . 004 
Broggerite, Norway 206 . 122 
Cleveite, Norway , 206 . 085 
Still another series of six determinations by Richards and Hall 4 on lead 
from Australian carnotite gave a mean value for the atomic weight of Pb = 
206.415. 
In a preliminary study of lead from Bohemian pitchblende, Honigschmid 5 
found values for the atomic weight ranging from 206.719 to 206.749. In 
a later investigation by Honigschmid and Horovitz, 6 lead was extracted 
from three different minerals, namely, the purest Joachimsthal pitchblende, 
a crystallized uranium ore from Morogoro, German (?) East Africa, and 
broggerite from Norway. The average values for the atomic weight were 
as follows: 
Source Atomic weight 
Pitchblende 206.406 
The Morogoro ore 206 . 042 
Broggerite 206.067 
All of these determinations of atomic weight, including those of Baxter 
and Gover on normal lead, were made by the same method, the same care 
as to purity of materials, and the same refinements of technique. Even 
Honigschmid, now in Vienna, had worked on atomic weight determinations 
with Richards, and so was familiar with the best procedure. The results 
obtained are therefore strictly comparable. 
For the atomic weight of thorium lead the data as yet are scanty, and 
based entirely upon material derived from Ceylonese thorite and thorianite. 
