Page:Popular Science Monthly Volume 67.djvu/16

The evidence that the $\alpha$ particles are atomic in size mainly rests on the deflection of the path of the rays in a strong magnetic and electric field. It has, however, been suggested by H. A. Wilson that the $\alpha$ particle may in reality be a 'positive' electron, whose magnitude is minute compared with that of the negative. The electric mass of an electron for slow speeds is equal to $2e^{2}/3a$. Since there is every reason to believe that the charge carried by the $\alpha$ particle and the electron are the same, in order that the mass of the positive electron should be about 2,000 times that of the negative, it would be necessary to suppose that the radius of the sphere over which the charge is distributed is only 1/2000 of that of the electron, i. e., about 10^—10 cms. The magnetic and electric deflection would be equally well explained on this view. This hypothesis, while interesting, is too far reaching in its consequences to be accepted before some definite experimental evidence is forthcoming to support it. The evidence at present obtained strongly supports the view that the $\alpha$ particles are in reality projected matter atomic in size. The probability that the a particle is an atom of helium is discussed later.

Becquerel showed that the $\alpha$ rays of polonium were deflected by a magnetic field to about the same extent as the $\alpha$ rays of radium. On account of the feeble activity of thorium and uranium, compared with radium and polonium, it has not been found possible to examine whether the rays emitted by them are deflectable. There is little doubt, however, that the particles of all the radio-elements are projected matter of the same kind (probably helium atoms). The $\alpha$ rays from the different radioactive products differ in their power of penetrating matter in the proportion of about three to one, being greatest for the a rays from the imparted or 'induced' activity of radium and thorium, and least for uranium. This difference is probably mainly due to a variation of the velocity of projection of the a particles in the various cases. The interpretation of results is rendered difficult by our ignorance of the mechanism of absorption of the a rays by matter. Further experiment^[1] on this point is very much required.

It is of importance to settle whether the a particles of radium and polonium have the same ratio $e/m$. Becquerel states that the amount of curvature of the $\alpha$ rays from polonium in a field of constant strength was the same as for the $\alpha$ rays from radium. This would show that the product of the mass and velocity is the same for the $\alpha$ particles from the two substances. The $\alpha$ rays of polonium, however, certainly have less penetrating power than those of radium, and

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1. Bragg and Kleeman (Phil. Mag., Dec, 1904) have recently attacked this question and have offered a very satisfactory explanation of the mechanism of the absorption of the $\alpha$ rays by matter.