The manner of our attack upon the problems of radioactivity, including the action of the Röntgen rays, depends materially on whether we suppose that an atom can or can not be made to yield energy from an internal store. In the former case we have nothing to guide us to an estimate of how much energy may be expected to be put into circulation in this way. If, for example, a
β
-ray in passing through an atom prompts the atom to emit new secondary radiation with energy drawn from a source usually beyond the reach of transformation into physical or chemical or other known forms, or if the atom sometimes absorbs energy from that of the
β
-ray motion and locks it up, then the quantities of energy thus added to or subtracted from the amounts we may hope to measure must be no more than subjects of experiment. We shall have to be content with registering them without accounting for them. But if we take the second of the two alternatives the problem is immensely simplified, and we may work for a more complete solution. An
α
-ray or
β
-ray begins its career with so much energy. While we cannot, of course, explain this initial liberation of energy, we may try to account completely for its subsequent expenditure in various ways, since we have no unknown or unexpected items to take into account on either side of the balance-sheet. Exactly the same statement can be made in respect to each X- or
γ
-ray, since each such ray, as has been shown in previous papers, can be considered by itself, being independent of all its companions in what we call a “beam of X- or of
γ
-rays.” Energy considerations lead us directly to the supposition that the X- and
γ
-rays are corpuscular in nature in so far as each ray is a separate identity moving through space unaltered in form and energy- content, just as an unhindered projectile would do. No X- or
γ
-ray spends energy in its passage through matter; the only way in which the existence of such rays is made manifest is through their replacement by swiftly moving electrons which ionise the gas through which they pass. The single X-ray disappears as such, and in its place is a cathode ray, an electron moving with energy inherited from the X-ray. Ionisation by X- or
γ
-rays is an indirect process.
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