There is no doubt that the process which takes place in the emission of radiation from the radioactive elements is a _transformation of the element_, an explosion of the atoms accompanied by the emission either of double-charged helium atoms or of electrons, and the forming of the atoms of a new element. The energy of the rays is an internal atomic energy, freed by these transformations. The element uranium, with the greatest of all known atomic weights (238), passes, by several intermediate steps, into radium with atomic weight 226; from radium there comes, after a series of steps, lead, or, in any case, an element which, in all its chemical properties, behaves like lead. We shall go no further into this subject, merely remarking that the transformations are quite independent of the chemical combinations into which the radioactive elements have entered, and of all external influences. When α-particles from radium are sent against a screen with a coating of especially prepared zinc sulphide, on this screen, in the dark, there can be seen a characteristic light phenomenon, the so-called scintillation, which consists of many flashes of light. Each individual flash means that an α-particle, a helium atom, has hit the screen. In this bombardment by atoms the individual atom-projectiles are made visible in a manner similar to that in which the individual raindrops which fall on the surface of a body of water are made visible by the wave rings which spread from the places where the drops meet the water. This flash of light was the first effect of the individual atom to be available for investigation and observation. The incredibility of anything so small as an atom producing a visible effect is lessened when, instead of paying attention merely to the small size or mass of the atom, its kinetic energy is considered; this energy is proportional to the square of the velocity, which is here of overwhelming magnitude. For the most rapid α-particles the velocity is 2·26 × 10⁹ cm. per second; their kinetic energy is then about ⁴/₃₀ of the kinetic energy of a weight of one milligram of a substance at a velocity of one centimetre per second. This energy may seem very small, but, at least, it is not a magnitude of “inconceivable minuteness,” and it is sufficient under the conditions given above to produce a visible light effect. We must here also consider the extreme sensitiveness of the eye. [Illustration: FIG. 19.—Photograph of paths described by α-particles (positive helium ions) emitted from a radioactive substance.] More practical methods of revealing the effects of the individual α-particles and of counting them are founded on their very strong ionization power. By strengthening the ionization power of α-particles, Rutherford and Geiger were able to make the air in a so-called ionization chamber so good a conductor that an individual α-particle caused a deflection in an electrical apparatus, an electrometre.