1.930 + 200 65.0 4.3 1 54 1.810 + 176 69.6 4.6 1 70 1.650 + 250 75.2 5.6 1 45 1.520 + 500 79.4 4.2 1 9.8 1.520 + 119 85.1 5.5 1 Mean 29 Mean 5.1 So long, then, as we are considering the ionization of neutral atoms through the absorption of an ether wave of any kind, the evidence [Pg 137] at present available indicates that the act always consists in the detachment from the atom of one single negative electron, the energy with which this electron is ejected from the atom depending, as we [Pg 138] shall see in chap. X, in a very definite and simple way upon the frequency of the ether wave which ejects it. IV. IONIZATION BY -RAYS When the ionization is due to the passage of -rays through matter, the evidence of the oil-drop experiments as well as that of C. T. R. Wilson’s experiments (see chap. IX) on the photographing of the tracks of the -rays is that here, too, the act consists in the detachment of one single electron from a single atom. This experimental result is easy to understand in the case of the -rays, when it is remembered that Wilson’s photographs prove directly the fact, long known from other types of evidence, that a -ray, in general, ionizes but a very minute fraction of the number of atoms through which it shoots before its energy is expended. If, then, its chance, in shooting through an atom, of coming near enough to one of the electronic constituents of that atom to knock it out is only one in one thousand, or one in one million, then its chance of getting near enough to two electronic constituents of the same atom to knock them both out is likely to be negligibly small. The argument here rests, however, on the assumption that the electrons within the atom are independent of one another, which is not necessarily the case, so that the matter must be decided after all solely by experiment.