These particles move with enormous speeds, which depend upon the nature of the radio-active substance from which they have been shot out. The fastest of all, those emitted by Thorium Cʹ, move with a speed of 12,800 miles a second; even the slowest, those from Uranium 1, have a speed of 8800 miles a second, which is about 30,000 times the ordinary molecular velocity in air. Particles moving with these speeds knock all ordinary molecules out of their way; this explains the great penetrating power of the α-rays. β-PARTICLES. By examining their motion under magnetic forces, the β-rays were found to consist of negatively charged electrons, exactly similar to those which revolve orbitally in all atoms. As an α-particle carries a positive charge equal in amount to that of two electrons, an atom which has ejected an α-particle is left with a deficiency of positive charge, or what comes to the same thing, with a negative charge, equal to that of two electrons. Consequently it is natural, and indeed almost inevitable, that the ejections of α-particles should alternate with an ejection of negatively charged electrons, so that the balance of positive and negative electricity in the atom may be maintained. The β-particles move with even greater speeds than the α-particles, many approaching to within a few per cent. of the velocity of light (186,000 miles a second). [Illustration: PLATE XIII _C. T. R. Wilson_ The tracks of α- and β-particles] One of the most beautiful devices known to physical science, the invention of Professor C. T. R. Wilson, makes it possible to study the motions of the α- and β-particles as they thread their way through a gas, colliding with its molecules on their way. A chamber through which the particles are made to travel is filled with water-vapour in such a condition that the passage of an electrically charged particle leaves behind it a trail of condensations which can be photographed. As an example, Plate XIII shews a photograph taken by Professor Wilson himself, in which the trails of both α- and β-particles appear on the same plate. As the α-particles weigh about 7400 times as much as the β-particles, they naturally create more disturbance in the gas, and so leave broader and more pronounced tracks; also they pursue a comparatively straight course while the lighter β-particles are deflected from their courses by many of the molecules they meet. The plate shews four α-particle tracks and one (much fainter) β-ray track. The knobby-looking projections which may be seen on one of the α-ray tracks are of interest; they represent the short paths of electrons knocked out of atoms by the passage of the α-particle[9]. [9] These were called δ-rays by Bumstead. γ-RAYS. The γ-rays are not material particles at all; they prove to be merely radiation of a very special kind, which we shall now discuss. RADIATION