ranium, while, accepting this theory,
the age of minerals with a given content of uranium may be calculated
from the amount of lead they contain. The results vary from 400 to
2000 million years. ("American Journal of Science", October, 1905, and
February, 1907.)

We can now exhibit in tabular form the amazing pedigree of radio-active
change shown by this one family of elements. An immediate descent is
indicated by >, while one which may either be immediate or involve an
intermediate step is shown by.... No place is found in this pedigree
for thorium and its derivatives. They seem to form a separate and
independent radio-active family.

Atomic Weight Time of half Radio-Activity
decay

Uranium 238.5 alpha

Uranium-X ? 22 days beta, gamma
...
Actinium ? ? no rays

Actinium-X ? 10.2 days alpha (beta, gamma)

Actinium Emanation ? 3.9 seconds alpha

Actinium-A ? 35.7 minutes no rays

Actinium-B ? 2.15 minutes alpha, beta, gamma
...
Radium 225 about 2600 years alpha

Radium Emanation ? 3.8 days alpha

Radium-A ? 3 minutes alpha

Radium-B ? 21 minutes no rays

Radium-C ? 28 minutes alpha, beta, gamma

Radium-D ? about 40 years no rays

Radium-E ? 6 days beta (gamma)

Radium-F ? 143 days alpha
...
Lead 207 ? no rays

As soon as the transmutation theory of radio-activity was accepted,
it became natural to speculate about the intimate structure of the
radio-active atoms, and the mode in which they broke up with the
liberation of some of their store of internal energy. How could we
imagine an atomic structure which would persist unchanged for long
periods of time, and yet eventually spontaneously explode, as here an
atom and there an atom reached a condition of instability?

The atomic theory of corpuscles or electrons fortunately was ready to
be applied to this new problem. Of the resulting speculation