The view that the atoms of the radio-elements are undergoing spontaneous disintegration was put forward by Rutherford and Soddy as a result of evidence of this character. The discovery of the _material_ nature of the α rays added strong confirmation to the hypothesis; for it has been pointed out (section 95) that the expulsion of α particles must be the result of a disintegration of the atoms of the radio-element. Taking the case of thorium as an example, the processes occurring in the atom may be pictured in the following way. It must be supposed that the thorium atoms are not permanently stable systems, but, on an average, a constant small proportion of them—about one atom in every 10¹⁶ will suffice—break up per second. The disintegration consists in the expulsion from the atom of one or more α particles with great velocity. For simplicity, it will be supposed that each atom expels _one_ α particle. It has been shown that the α particle of radium has a mass about twice that of the hydrogen atom. From the similarity of the α rays from thorium and radium, it is probable that the α particle of thorium does not differ much in mass from that of radium, and may be equal to it. The α particles expelled from the thorium atoms as they break up constitute what is known as the “non-separable activity” of thorium. This activity, measured by the α rays, is about 25 per cent. of the maximum. After the escape of an α particle, the part of the atom left behind, which has a mass slightly less than that of the thorium atom, tends to rearrange its components to form a temporarily stable system. It is to be expected that it will differ in chemical properties from the thorium atom from which it was derived. The atom of the substance Th X is, on this view, the thorium atom minus one α particle. The atoms of Th X are far more unstable than the atoms of thorium, and one after the other they break up, each atom expelling one α particle as before. These projected α particles give rise to the _radiation_ from the Th X. Since the activity of Th X falls to half its original value in about four days, on an average half of the atoms of Th X break up in four days, the number breaking up per second being always proportional to the number present. After an atom of Th X has expelled an α particle, the mass of the system is again reduced, and its chemical properties are changed. It will be shown (section 154) that the Th X produces the thorium emanation, which exists as a radio-active gas, and that this in turn is transformed into matter which is deposited on solid bodies and gives rise to the phenomena of excited activity. The first few successive changes occurring in thorium are shown diagrammatically below (Fig. 50). [Illustration: Fig. 50.]