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CHEMISTRY: F. W. CLARKE 
minerals also contain thorium, which would tend to raise the atomic weight 
and so complicate any discussion of the figures. The most brilliantly crys- 
tallized uraninite, that from Branchville, Connecticut, contains 85% of 
U0 3 + U0 2 , with about 7% of Th0 2 , 4.35% of PbO, and a maximum, 0.4% 
of helium. The atomic weight of lead from that source, unfortunately, has 
not been determined; and it is doubtful whether material enough for accurate 
investigation could be obtained. 
The other determinations of the atomic weight of uranium lead give values 
much above 206, and even approaching 207. This is especially true of the 
lead from pitchblende, which contains no thorium and little if any helium. 
Its association with sulphide ores, however, leads to the suspicion that it may 
contain ordinary lead, perhaps in the form of occluded or dissolved galena. 
The atomic weight of the lead derived from it would, therefore, be that of 
a mixture, and not of the isotope alone. The carnotite lead would also 
seem to be a mixture, but of what kind is not clear. 
The atomic weight of isotopic lead now seems to be a complex of at least 
three quantities, namely, the atomic weights of normal lead, uranium lead, 
and thorium lead, in varying proportions. Since the atomic weights of the 
two isotopes differ from that of normal lead in opposite directions it is difficult 
to determine in any particular case the relative proportions of the three 
modifications of the element. It has been suggested that normal lead is a 
balanced mixture of its isotopes; but the constancy of the atomic weight of 
the ordinary metal seems to negative that supposition. In order to fulfill 
this condition it would be necessary that the isotopes should always commingle 
in equal or at least definite proportions; which is extremely improbable. 
The apparent variations in the atomic weight of lead, as shown in the older 
determinations, are due to varying methods, imperfect technique, different 
values for the atomic weights of the other elements with which that of lead 
is compared, and experimental errors. The modern determinations, which I 
have already cited, are the only ones that are strictly comparable. 
The suggestion that the lead contained in uranium ores is partly normal 
lead is not new. It has been advanced by other writers, 10 but the variable 
atomic weight of uranium lead gives the supposition a decided emphasis. 
It now acquires new importance because of its bearing upon certain attempts 
to use the ratio between uranium and lead in uranium minerals as a datum 
for computing the age of the earth. For this purpose the ratio has been 
employed by Boltwood, 11 who calculated it from almost all the trustworthy 
analyses of uraninite and its nearly allied species, and from it deduced their 
ages. These ages differ exceedingly. For a crystallized uraninite from 
Connecticut he found the age to be 410,000,000 years, and for Ceylonese 
thorianite 2,200,000,000 years. These calculations, and others, like them, 
involve two assumptions; first, that the rate of change from uranium to lead 
is accurately known, and secondly that all the lead was of radioactive origin. 
