Page:Popular Science Monthly Volume 71.djvu/534

528 common constituents, the $$\alpha$$ and $$\beta$$ particles. It has been found that after a certain critical velocity has been reached, the $$\alpha$$ particles all at once cease to produce any ionization, phosphorescence or photographic action. If a substance emits particles with a velocity less than this critical velocity, we should have no method at present available for detecting them. As to the enormous amount of ionization produced by radium, one can partly grasp it when one considers that an a particle produces about 80,000 ions and one grm. of radium emits about $$6\ 10^{10}$$ a particles per second. The $$\gamma$$ rays also differ in penetrating power. The $$\gamma$$ rays from radium and thorium are very much stronger than those from uranium and actinium.

By actual experiment in the laboratory it is possible to watch the gradual formation of helium and actinium. The rate of formation of helium from radium is known roughly, so that if in any rock the helium formed from radium has not been allowed to escape, a quite accurate estimation of the age of the rock can be made. Radium has also been found by Boltwood and Rutherford to grow in actinium solutions. But by investigating elements which appear together in the rocks it is possible to learn much more. In fact it was from the occurrence of helium in radioactive minerals that the brilliant prediction of the production of helium was made. Boltwood, Strutt and McCoy have shown that the amount of radium present in radioactive minerals always bears a constant ratio to the amount of uranium present. For every gram of uranium there is present $$3.8\ 10^{-7}$$ grms. of radium. From this coexistence in a constant ratio, one is justified in assuming that radium is a product of uranium. If this is true it is easy to explain the existence of radium in rocks that contain uranium. Otherwise, on account of the short period of disintegration of radium, it would be difficult to account for its distribution through the rocks. It has also been found that minerals of the same age contain uranium and lead in the same ratio, so that it seems quite certain that lead is a disintegrated product resulting from radium. Recent experiments by Rutherford seem to indicate, however, that actinium is not a direct product of uranium, as radium is considered to be. The existence together of various other elements has been used as an argument for their relationship. At present, however, the evidences are very meager and often conflicting. This field of experiment is one that promises very important results, however.

Considerable work has recently been done by the Hon. E. J. Strutt upon the amount of radium contained in various kinds of rocks widely