rne discovered and partially
separated another radio-active element which he called actinium. It
gives rise to an intermediate product actinium-X, which yields an
emanation with the short half-life of 3.9 seconds. The emanation
deposits two successive disintegration products actinium-A and
actinium-B.

Evidence gradually accumulated that the amounts of actinium in
radio-active minerals were, roughly at any rate, proportional to the
amounts of uranium. This result pointed to a lineal connection between
them, and led Boltwood to undertake a direct attack on the problem.
Separating a quantity of actinium from a kilogramme of ore, Boltwood
observed a growth of 8.5 x (10 to the power -9) gramme of radium in
193 days, agreeing with that indicated by theory within the limits of
experimental error. ("American Journal of Science", December, 1906.)
We may therefore insert provisionally actinium and its series of
derivatives between uranium and radium in the radio-active pedigree.

Turning to the other end of the radium series we are led to ask what
becomes of radium-F when in turn it disintegrates? What is the final
non-active product of the series of changes we have traced from uranium
through actinium and radium?

One such product has been indicated above. The alpha-ray particles
appear to possess the mass of helium atoms, and the growth of helium has
been detected by its spectrum in a tube of radium emanation. Moreover,
helium is found occluded in most if not all radio-active minerals in
amount which approaches, but never exceeds, the quantity suggested by
theory. We may safely regard such helium as formed by the accumulation
of alpha-ray particles given out by successive radio-active changes.

In considering the nature of the residue left after the expulsion of the
five alpha-particles, and the consequent passage of radium to radium-F
we are faced by the fact that lead is a general constituent of uranium
minerals. Five alpha-particles, each of atomic weight 4, taken from the
atomic weight (about 225) of radium gives 205--a number agreeing fairly
well with the 207 of lead. Since lead is more permanent than uranium, it
must steadily accumulate, no radio-active equilibrium will be reached,
and the amount of lead will depend on the age of the mineral as well as
on the quantity of uranium present in it. In primary minerals from
the same locality, Boltwood has shown that the contents of lead are
proportional to the amounts of u