Page:Popular Science Monthly Volume 69.djvu/49

These products appear to be elements, and this idea that some elements may have existences of but short duration, from a few seconds to many years, is a decidedly novel one. It has been suggested that this may account for some of the vacant spaces in our periodic table of the elements, particularly in the neighborhood of thorium, radium and uranium. Perhaps these spaces never will be occupied except by transients. Indeed it is not impossible that all our elements are mere transients, mere conditions of things, all undergoing change. But there is no immediate danger of their all vanishing away in the form of rays and emanations. Rutherford has calculated that radium will be half transformed in about 1,300 years, that uranium will be half transformed in $6\times 10^{8}$ years, and thorium in about $2.4\times 10^{9}$ years. We may safely say the other elements are decaying much more slowly, so we may continue to direct our anxieties towards the probable duration of our coal beds and deposits of iron ore as matters of more present concern.

The objection may be raised that perhaps radium should not be classed as an element, but rather should be considered as an unstable compound in the act of breaking down into its elements. But the answer to this objection is at hand. The evolution of energy accompanying these changes is far in excess of that obtainable from any known chemical process, so far in excess that it is certain we are dealing with a source of energy hitherto unknown to us, with a wholly new class of phenomena. The following quotation from Whetham^[1] will convey an adequate conception of the magnitude of the forces at work here:

It is possible to determine the mass and the velocity of the projected particles, and. therefore, to calculate their kinetic energy. From the principles of the molecular theory, we know that the number of atoms in a gram of a solid material is about $10^{20}$. Four or five successive stages in the disintegration of radium have been recognized, and, on the assumption that each of these involves the emission of only one particle, the total energy of radiation which one gram of radium could furnish if entirely disintegrated seems to be enough to raise the temperature of 10 s grams, or about 100 tons, of water through one degree centigrade. This is an underestimate; it is possible that it should be increased ten or a hundred times. As a mean value, we may say that, in mechanical units, the energy available for radiation in one ounce of radium is sufficient to raise a weight of something like ten thousand tons one mile high.

Again,

the energy liberated by a given amount of radioactive change. . . is at least 500,000 times, and may be 10,000,000 times, greater than that involved in the most energetic chemical action known.

The theory that the source of most of the sun's energy is a decay of elements analogous to radium, to disintegration of atoms, is acknowledged to account better than any previous theory for the great quantity

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1. 'The Recent Development of Physical Science,' W. C. D. Whetham.