is known about their nature. Since, however, the energy carried by them is very insignificant as compared with that in the alpha and beta rays, we can leave them entirely out of account in most of the computations which we make upon the energy of radiations of radio-active substances. It is now conjectured that the gamma rays are ethereal pulses like the X-rays.
The Nature of the Alpha Rays.
It was at first conjectured that possibly the alpha rays might be X-rays, since, like them, they are not deflected by a magnet, and since, also like them, they are very effective in rendering a gas electrically conducting. But only last year Professor Rutherford contrived a very ingenious experiment by which he showed conclusively that the alpha rays are deflected very slightly by a magnet if the magnet is sufficiently powerful. He also succeeded in showing that they are deflected by a very strong electrical field. But in both of these cases the direction of the deflection is opposite to that obtained under the same conditions with beta rays. These results of Professor Rutherford's are of the utmost importance, and they have been recently confirmed both by Becquerel in Paris, and by a German physicist by the name of Des Coudres. The only possible interpretation which can be put upon them is that the alpha rays also consist of particles of matter shot off from the radio-active substances, but that, while the beta ray particles carry charges of negative electricity, the alpha ray particles carry charges of positive electricity.
Further, when from the amounts of the deflections produced by the magnet and by the electric charge, the size and velocity of the alpha particles are calculated, the results are again most interesting. For these particles are found to have a mass not one one-thousandth that of the hydrogen atom, like the cathode rays, but approximately twice as great as that of the hydrogen atom, or about the size of the atom of helium. (The atomic weight of helium is 4.) They are therefore about 2,000 times as heavy as the cathode ray particles. This explains why they do not pass through ordinary matter as readily as do the smaller beta particles. But despite this comparatively great mass, their velocity is found to be as much as 20,000 miles per second, more than a tenth that of the smaller particles. It will be seen, therefore, that the energy of the blows which they strike against the bodies upon which they fall is much greater than that of the beta particles. This explains why they knock the gas to pieces, or dissociate it and thus render it conducting, so much more energetically than do the beta particles.
The Crookes Spinthariscope.
We have attempted to follow, thus far, the evidence upon which we base the conclusion that the radiations from radioactive substances con-
