CHEMISTRY: F. W. CLARKE 
181 
NOTES ON ISOTOPIC LEAD 
By Frank Wigglesworth Clarke 
United States Geological Survey, Washington 
Read before the Academy, April 23, 1918 
One of the most remarkable discoveries in the field of radioactivity, has 
been the fact that the elements of highest atomic weight, uranium and 
thorium, are unstable, and undergo slow transformations into other sub- 
stances; especially into helium and lead. The lead thus produced is iden- 
tical with normal lead in its spectrum and its distinctively chemical 
properties, but different in its atomic weight; and this difference, which is 
thoroughly established, is of peculiar significance. The purest lead fron^ 
uranium minerals has an atomic weight fully a unit lower than that of or- 
dinary lead, while that from thorium minerals is nearly a unit higher. These 
are the extreme differences, so far as the present evidence goes; but the ac- 
tual determinations of the atomic weights of these isotopes of lead show 
wide variations due to differences between the minerals from which the lead 
was obtained. Furthermore, these isotopes differ from ordinary lead in 
specific gravity; one being lighter and the other heavier than ordinary lead, 
these differences being proportional to the variations in atomic weight. Con- 
sequently the three kinds of lead have the same atomic volume, and occupy 
the same place in the periodic classification of the chemical elements. 
Ordinary or normal lead differs from isotopic lead in one important re- 
spect, namely, its atomic weight is constant, and the actual determinations 
vary only within the limits of experimental uncertainty. This constancy was 
established by Baxter and Grover, 1 who studied lead from a number of dis- 
tinct sources. Their material was derived from four mineral species; galena, 
cerussite, vanadinite, and wulfenite, and also from commercial lead nitrate. 
Furthermore, the minerals examined came from seven widely separated 
localities; two from Germany, and one each from Australia, Missouri, Idaho, 
Washington, and Arizona. The lead in each case was carefully purified, 
and converted into chloride, with which the determinations of atomic weight 
were made. The method of determination was the standard method long 
in use at Harvard, and based upon large experience and the most thorough 
technique. The values found for the atomic weight are shown in the fol- 
lowing table: 
Source Atomic weight of lead 
Commercial nitrate 207 . 22 
Cerussite, New South Wales 207 . 22 
Cerussite, Eif el Mountains, Germany 207.20 
Galena, Joplin, Missouri 207.22 
Cerussite, Wallace, Idaho 207 . 2 1 
Galena, Nassau, Germany 207.21 
Vanadinite and wulfenite, Arizona 207.21 
Galena, Metalline Falls, Washington 207 . 2 1 
