THE development of our knowledge of radio-activity has emphasised the primary importance of the α particles, which are projected in great numbers from most of the active substances. As Rutherford showed in 1903, the α particles are veritable atoms of matter which are ejected from radio-active matter at a speed of about 10,000 miles per second. The great number of α particles which are projected from radium is weil illustrated by the multitude of scintillations observed when the α particles from a trace of radium fall on a screen of zinc sulphide. We shall see later that 136 million α particles are expelled every second from one milligram of radium in radioactive equilibrium. From the point of view of modern theory, the appearance of an α particle is the sign of a violent atomic explosion in which a fragment of the atom—an α particle—is ejects at a high speed. In the majority of the known active substances, the expulsion of an α particle accompanies the transformation of one substance into another, and the decrease of atomic mass consequent upon the loss of an α particle at once offers a reasonable explanation of the appearance of an entirely new kind of matter in place of the old.