measured by the [Greek: alpha] rays, is about 25 per cent. of the maximum. After the escape of an [Greek: alpha] 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 [Greek: alpha] 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 [Greek: alpha] particle as before. These projected [Greek: alpha] 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 [Greek: alpha] 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).
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Fig. 50.
Thus as a result of the disintegration of the thorium atom, a series of chemical substances is produced, each of which has distinctive chemical properties. Each of these products is radio-active, and loses its activity according to a definite law. Since thorium has an atomic weight of 237, and the weight of the [Greek: alpha] particle is about 2, it is evident that, if only one [Greek: alpha] particle is expelled at each change, the process of disintegration could pass through a number of successive stages and yet leave behind,
