ABSTRACT
The development of our knowledge of radio-activity has emphasised the primary importance of the a particles, which are projected in great numbers from most of the active substances. The general method employed was to allow the a particles to be fired through a small opening into a detecting vessel containing gas at low pressure exposed to an electric field not far from the sparking value. By adjustment of the electric field, it was found possible to obtain so large a magnification that the entrance of a single a particle was marked by a large excursion of the electrometer needle. In this way the expulsion of a particles was detected from uranium, thorium, radium, and actinium. Some very important consequences follow from the proof that the a particle is a helium atom. The number of a particles expelled from radium can be directly counted, and the corresponding volume of helium determined.
