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Aγ -ray is a projected entity, according to the theories of J. J. Thomson, of Bragg, and of J. Stark, and the effect of it will be localised in space in the sense that the effect of a bullet is. Any one of these theories requires the ionisation produced byγ -radiation in a gas at any instant to fluctuate both in space and time. In this respect the effect ofγ -rays would be similar to that ofα -particles; as is well known, the spinthariscope exhibits for the latter rays both fluctuations in time and discontinuity in space. Similarly, the effects predictable forβ -rays have been observed, but, when the experiments recorded below were begun, no such fluctuation had been observed forγ -rays. In 1908 one of us described how they might be sought for experimentally: "At the same distance from a source ofγ rays, two identical and adjacent ionisation chambers would be placed to catch theγ -rays. With one chamber at +v volts and the other at —v , the electrode of one receives positive ions, the other negative; connecting the electrodes, the electroscope would receive the small excess of ions of one sign. If theγ -radiation is a spherical wave surface, then the ionisations in the chambers will have a constant ratio. This is true .... even when the energy varies continuously over the wave front; the excess current to the electroscope is then to a first approximation constant. If, on the other hand, theγ -rays are particles emitted in random directions asα -rays are, then the number entering each chamber in the given time will fluctuate. In the extreme case, during a short time a fewγ -particles might enter one chamber, and none the other. In the more general case, when some large number of particles enter both chambers, the current to the electroscope will fluctuate.

The observation by means of a string electrometer of fluctuations in the ionisation produced by γ -rays