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 uranium, 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?