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495one employed in these experiments. The number of ions produced by a single a-particle under the special conditions of the experiment is easily found from the curve given in fig. 3. The determination of the ionisation current in the bulb then gives at once the total number of a-particles. Care has to be taken to obtain saturation and to avoid ionisation by collision, which occurs when too large a voltage is applied. I wish to acknowledge the assistance which Mr. E. Marsden has given me in some of these observations.In conclusion, I desire to express my gratitude to Prof. Rutherford for his valuable suggestions and his kind interest in the experiments.On a Diffuse Reflection o f the a.-Particles. When /3-particles fall on a plate, a strong radiation emerges from the same side of the plate as that on which the /3-particles fall. This radiation is regarded by many observers as a secondary radiation, but more recent experi ments seem to show that it consists mainly of primary ^-particles, which have 'been scattered inside the material to such an extent that they emerge again at the same side of the plate* For a-particles a similar effect has not previously been observed, and is perhaps not to be expected on account of the relatively small scattering which a-particles suffer in penetrating matter.f On a Diffuse Reflection of theIn the following experiments, however, conclusive evidence was found of the existence of a diffuse reflection of the a-particles. A. small fraction of the a-particles falling upon a metal plate have their directions changed to such an extent that they emerge again at the side of incidence. To form an idea of the way in which this effect takes place, the following three points were investigated:-

But whatever value is attached to the numerical evidence by those competent to judge, the broad conclusion from the direction of the changes is unaffected. I should llke also to reply to one criticism which has been made. It is not necessary to the hypothesis that the atomic free frequency should be absolutely invariable throughout all chemical changes. If it were the case that a slight change (say 1 per cent.) in the atomic free frequency would account for all the observed changes of refractivity and dispersion, the criticism would have force. But any one who examines the figures in lhe table for (e. g.) hydrogen, nitrogen, and ammonia, will see that no slight change in a frequency can possibly account for the observed changes. For hydrogen no2X 10-~r=12409, for nitrogen 17095. For ammonia the nmnber which expresses its average value is 8135, an enormous drop. At the same time the refractivity has gone up only 3} per cent. But if the number of vibrators had remained constant while the average free frequency decreased, the increase of the refractivity must have been much greater than this. Hence, to account for the observed changes, one must also assume that the number of vibrators (? electrons) has fallen off in about the same proportion as the frequency. These two hypotheses seem much more improbable than that which is here put forward. LXI. The Laws of .Deflexion of a Particles through Large Angles*.

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