Page:Radio-activity.djvu/482

 *—which is also a measure of the rate of change of uranium—balances the rate of change of that product. In this respect the process would be exactly analogous to the production of the emanation by radium, with the difference that the radium changes much more slowly than the emanation. But since radium itself in its disintegration gives rise to at least five changes with the corresponding production of α rays, the activity due to the radium (measured by the α rays), when in a state of radio-active equilibrium with uranium, should be about five times that of the uranium that produces it; for it has been shown that only one change has so far been observed in uranium in which α rays are expelled. Taking into account the presence of actinium in pitchblende, the activity observed in the best pitchblende is about the same as would be expected if the radium were a disintegration product of uranium. If this hypothesis is correct, the amount of radium in any pitchblende should be proportional to the amount of uranium present, provided the radium is not removed from the mineral by percolating water.

This question has been experimentally attacked by Boltwood , McCoy and Strutt. McCoy measured the relative activities of different minerals in the form of powder by means of an electroscope, and determined the amount of uranium present by chemical analysis. His results indicated that the activity observed in the minerals was very approximately proportional to their content of uranium. Since actinium is present as well as uranium and its products, this would indicate that the amount of radium and actinium taken together is proportional to the amount of uranium. This problem has been attacked more directly by Boltwood and Strutt by measuring the relative amount of the radium emanation evolved by different minerals. By dissolving the mineral and then setting it aside in a closed vessel, the amount of emanation present reaches a maximum value after about a month's interval. The emanation is then introduced into a closed vessel containing a gold-leaf electroscope similar to that shown in Fig. 12. The rate of movement of the gold-leaf is proportional to the amount