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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
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