Page:Popular Science Monthly Volume 67.djvu/18

12 $$\gamma$$ rays of radium themselves carry a negative charge. The lead envelope in his experiment acquired a positive charge in consequence of the emission of a secondary radiation consisting of negatively charged particles, projected with great velocity from the surface of the lead. The electric charge acquired by the metal ring was due to the absorption of these secondary rays by it, and the diminution of this charge in a magnetic field was due to the ease with which these secondary rays are deflected. It is thus to be expected that the envelope surrounding the radium, whether made of lead or other metal, will always acquire a positive charge, provided the metal is not of sufficient thickness to absorb all the $$\gamma$$ rays in their passage through it.

No conclusive evidence has yet been brought forward to show that the $$\gamma$$ rays can be deflected either in a magnetic or an electric field. In this, as in other respects, the rays are very analogous to the Röntgen rays.

According to the theory of Stokes, J. J. Thomson and Weichert, Röntgen rays are transverse pulses set up in the ether by the sudden arrest of the motion of the cathode particles on striking an obstacle. The more sudden the stoppage the shorter is the pulse, and the rays, in consequence, have greater power of penetrating matter. In some recent experiments Barkla found that the secondary rays set up by the Röntgen rays, on striking an obstacle, vary in intensity with the orientation of the vacuum tube, showing that the Röntgen rays exhibit the property of one-sidedness or polarization. This is the only evidence so far obtained in direct support of the wave nature of the Röntgen rays.

If Röntgen rays are not set up when the cathode particles are stopped, conversely, it is to be expected that Röntgen rays will be set up when they are suddenly expelled. Now this effect is not observable in an X-ray tube, since the cathode particles acquire most of their velocity, not at the cathode itself, but in passing through the electric field between the cathode and anticathode. It isy however, to be expected theoretically that a type of Röntgen rays should be set up at the sudden expulsion of the $$\beta$$ particles from the radio atoms. The rays, too, should be of a very penetrating kind, since not only are the charged particles projected with a speed approaching that of light, but the change of motion must occur in a distance comparable with the diameter of an atom.

On this view the $$\gamma$$ rays are a very penetrating type of Röntgen rays, having their origin at the moment of the expulsion of the $$\beta$$ particle from the atom. If the $$\beta$$ particle is the parent of the $$\gamma$$ rays the intensity of the $$\beta$$ and $$\gamma$$ rays should, under all conditions, be proportional to one another. I have found this to be the case, for the $$\gamma$$ rays always accompany the $$\beta$$ rays and, in whatever way the $$\beta$$ ray