of radium as actually determined by the
ordinary methods. Uranium X_{1}, then, would have an atomic weight
of 234 and that of ionium would be 230. The other intermediate
elements, whose formation is due to the loss of beta particles only,
show no decrease in atomic weight.

Lead the End Product

From uranium to lead there is a loss of 8 alpha particles, or 32 units
in atomic weight. This would give for the final product an atomic
weight of 206. The atomic weight of lead is 207.17. It is not at all
certain that the final product of this series is ordinary lead. The
facts are such that they would lead one to think that it is not. It is
known only that the end product would probably be some element closely
resembling lead chemically and hence difficult or impossible to
separate from it. Several accurate determinations of lead coming from
uranium minerals, which always carry this element and in an
approximately definite ratio to the amount of uranium present, show
atomic weights of 206.40; 206.36; and 206.54. Even the most rigid
methods of purification fail to change these results. The lead in
these minerals might therefore be considered as coming in the main
from the disintegration of the uranium atom and, though chemically
resembling lead, as being in reality a different element with
different atomic weight.

Furthermore, in the thorium series 6 alpha particles are lost before
reaching the end product, which again is perhaps the chemical analogue
of lead. The atomic weight here should be 232 less 24, or 208.
Determinations of the atomic weight of lead from thorite, a thorium
mineral nearly free from uranium, gave 208.4.

The end product of the actinium series is also an element resembling
lead, but both the beginning and ending of this series are still in
obscurity.

Changes of Position in the Periodic System

The loss of 4 units in the atomic weight of an element on the
expulsion of an alpha particle is accompanied by a change of chemical
properties which removes the new element two groups toward the
positive side in the Periodic System.

Thus ionium is so closely related to thorium and so resembles it
chemically that it is properly classed along with thorium as a
quadrivalent element in the fourth group. Ionium expels an alpha
particle and becomes radium, which is a bivalent element resembling
barium belonging to the second group. Radium then expels an alpha
particle and becomes the gas, radium emanation,