The free air can be ionized in various ways; this ionization can be detected because the air becomes more or less conducting. In fact, electric forces will drive the positive and negative ions through the air in opposite directions, thus giving rise to an electric current. If the ionization process is not steadily continued, the air gradually loses its conductivity, since the positive and negative ions recombine into neutral atoms or molecules. Ionization can be produced by flames, since the air rising from a flame contains ions. A strong ionization can also be brought about by X-rays and by ultra-violet rays. In the higher strata of the atmosphere the ultra-violet rays of the sun exercise an ionizing influence. Most of all, however, the air is ionized by rays from the so-called _radioactive_ substances which in very small quantities are distributed about the world. The characteristic radiation from these substances was discovered in the last decade of the nineteenth century by the French physicist, Becquerel, and afterwards studied by M. and Mme. Curie. From the radioactive uranium mineral, pitchblende, the latter separated the many times more strongly radioactive element radium. The proper nature of the rays was later explained, particularly through the investigations of the English physicists, Rutherford, Soddy and Ramsay. These rays, which can produce heat effects, photographic effects and ionization, are of three quite different classes, and accordingly are known as α-rays, β-rays, and γ-rays. The last named, like the X-rays, are ultra-violet rays, but they have often even shorter wave-lengths and a much greater power of penetration than the usual X-rays. The β-rays are electrons which are ejected with much greater velocity than the cathode rays; in some cases their velocity goes up to 99·8 per cent. that of light. The α-rays are positive atomic ions, which move with a velocity varying according to the emitting radioactive element from ¹/₂₀ to almost ¹/₁₀ that of light. It has further been proved that the α-particles are atoms of the element helium, which has the atomic weight 4, and that they possess two positive charges, _i.e._, they must take up two electrons to produce a neutral helium atom.