CHEMISTRY: F. W. CLARKE 
183 
From the specific gravity of thorite lead Soddy 7 has deduced the atomic 
weight of Pb = 207.64; and Honigschmid 8 from analyses of lead chloride 
prepared from Soddy's original material has found Pb = 207.77. This 
value, however, is probably too low for the true thorium lead, for the reason 
that thorite, with a preponderant proportion of thoria, also contains some 
uranium. The thorite lead, therefore, must contain both isotopes, but with 
the higher one in much the largest quantity. The thorianite lead studied 
by Richards and Lembert had a still lower atomic weight, namely Pb = 
206.82, which shows that this variety of the metal is not of uniform character. 
That the atomic weight of uranium lead is extremely variable has already 
been shown. In order to interpret this variability its sources must be studied 
both geologically and mineralogically. On the geologic side of the question 
the uranium ore can be divided in to three principal classes, which are sharply 
distinct. The definitely crystallized varieties of uraninite occur in coarse 
pegmatites, associated with feldspar, quartz, mica, beryl, and other minor 
accessories. The massive pitchblende is found in metalliferous veins, together 
with sulphide ores of copper, lead, iron, zinc, and so forth. As for carnotite, 
that is a secondary mineral, found commonly as an incrustation on sandstone, 
and often, also upon fossil wood. There may be other modes of occurrence, 
but these are the most distinctive. 
In chemical composition the uraninites, as shown by Hillebrand's 9 splendid 
series of twenty-one analyses, fall into well defined groups. All contain 
uranium oxides, ranging from 65 to 90%, the low figures, however, represent- 
ing altered material. The crystallized, pegmatitic uraninites are charac- 
terized by their content in thoria and other rare earths, from 6 or 7 up to as 
much as 11%. They also contain subordinate proportions of lead, and the 
largest amount of helium. In broggerite and cleveite, however, lead is in 
excess of thoria. The massive pitchblendes, on the other hand, contain no 
thoria, usually much lead and little or no helium. That from Black Hawk, 
Colorado, is exceptional. It is intimately associated with sulphide ores, 
but contains little lead, and zirconia instead of thoria. Carnotite, which is 
quite unlike uraninite, is essentially a vanadate of uranium and potassium, 
with very little lead and no helium. It is, however, an important source 
of radium. 
It is now possible to correlate, at least roughly, the composition of the 
several minerals with the determinations of the atomic weight of uranium lead, 
although for a perfect comparison we should have analyses of the actual ores 
from which the various samples of lead were obtained. On theoretical grounds 
it is supposed that the true atomic weight of uranium lead is not far from 
206, and only determinations which approach that value are those which 
represent crystallized uraninite, including the varieties broggerite and cleve- 
ite. These minerals all contain helium, so that there seems to be a relation 
between the formation of these two degradation products of uranium. The 
