Page:Radio-activity.djvu/440

 CHAPTER XII.

RATE OF EMISSION OF ENERGY.

243. It was early recognised that a considerable amount of energy is emitted by the radio-active bodies in the form of their characteristic radiations. Most of the early estimates of the amount of this energy were based on the number and energy of the expelled particles, and were much too small. It has been pointed out (section 114) that the greater part of the energy emitted from the radio-active bodies in the form of ionizing radiations is due to the α rays, and that the β rays in comparison supply only a very small fraction.

Rutherford and McClung made an estimate of the energy of the rays, emitted by a thin layer of active matter, by determining the total number of ions produced by the complete absorption of the α rays. The energy required to produce an ion was determined experimentally by observations of the heating effect of X rays, and of the total number of ions produced when the rays were completely absorbed in air. The energy required to produce an ion in air was found to be 1·90 × 10^{-10} ergs. This, as will be shown in Appendix A, is probably an over-estimate, but was of the right order of magnitude. From this it was calculated that one gram of uranium oxide spread over a plate in the form of a thin powdered layer emitted energy into the air at the rate of 0·032 gram calories per year. This is a very small emission of energy, but in the case of an intensely radio-active substance like radium, whose activity is about two million times that of uranium, the corresponding emission of energy is 69000 gram calories per year. This is obviously an under-*