The α-rays are the ones that are active in the very beautiful experiments with the spinthariscope of Crookes. In this apparatus a very small fragment of a salt of radium (a fraction of a milligram) is held by a metallic thread a short distance (0.5 mm.) from a phosphorescent screen of zinc sulphide. On examining, in the dark, the surface of the screen which is toward the radium by means of a magnifying glass, luminous points are seen sprinkled over the screen, reminding one of a starry sky. These luminous points appear and disappear continuously. From the ballistic theory it might be imagined that each luminous point that appeared and disappeared resulted from the shock of a projectile. This is the first instance of a phenomenon which enables one to distinguish the individual action of an atom.
γ-Rays.—The rays are exactly like the Roentgen rays. They seem to comprise but a small part of the total radiation. The γ-rays have a very extraordinary power of penetration, and they diffuse very little in passing through most substances.
Diffusion of the Radium Rays.—Suppose we have a pencil of Becquerel rays issuing from radium and limited by a screen of lead. If the pencil meets a thin screen the α-rays are absorbed, the β-rays are diffused in all directions, and the γ-rays go part way through the screen as a well-defined pencil with sharp edges. The γ-rays can also pass through a prism of thick glass as a well-defined straight pencil. The question has been raised whether the β-rays are always completely diffused when they penetrate a solid screen. The experiments of Becquerel show that a pencil of β-rays can propagate itself in a well defined way in paraffin. Becquerel made use of the action of the β-rays on the photographic plate to study the path of the rays when dispersed by a magnetic field. It can be seen from the prints that the most penetrating rays pass through 7 or 8 mm. of paraffin without marked diffusion, while the least penetrating rays are completely diffused after passing through 2 mm. The magnetic field deflects the β-rays in paraffin as in air.
Conductivity of Dielectric Liquids under the Influence of the Radium Rays.—Dielectric liquids become poor conductors under the influence of the radium rays. This can be shown with petroleum, ether, vaseline oil, benzene, amylene, carbon disulphide, and liquid air.
Radiation of Other Radioactive Substances.—Polonium emits only very slightly penetrating rays, which seem to be identical with the α-rays of radium. They possess almost the same power of penetration and are deflected in the same way by the magnetic field. Finally, the experiment of the spinthariscope can be performed with the α-rays of polonium. Hence, polonium is a source of the α-rays unmixed with other kinds of rays, which is of value in certain studies. But the source exhausts itself, and some years after it has been separated from the minerals containing it the polonium loses its activity.
Thorium, uranium, and actinium seem to emit the α- and β-rays; the deviability of the β-rays has been shown.
The Electric Charge of the Radium Rays.—According to the ballistic theory, the α-rays should carry positive electric charges and the β-rays negative electric charges. Mme. Curie and I have shown that, in conformity to the theory, the β-rays of radium charge positively the bodies that absorb them. To show this, a plate of lead is connected with an electrometer. The plate of lead is entirely covered with a layer of paraffin, which is in turn enveloped in fine aluminium foil connected with the earth. The radium, placed in a small dish, sends its rays upon the plate of lead, which is thus protected. The α-rays are stopped by the exterior layer of aluminium. Part of the β-rays pass through the aluminium and the paraffin and are absorbed by the lead, which becomes charged negatively. The paraffin is necessary to sufficiently insulate the plate of lead, which could be charged if it were surrounded by air, for it is made a conductor by Becquerel rays.
We have also shown that a salt of radium is charged positively when it is enveloped in an insulating layer, and that it emits β-rays from the exterior, while the α-rays cannot escape.
A sealed glass bulb containing a salt of radium becomes spontaneously charged like a Leyden jar. If after a sufficient time a mark is made on the wall of the flask with a glass-cutter, a spark is discharged, which pierces the glass at the point where it was made thin by the cutter. At the same time the experimenter feels a slight shock in his fingers from the passage of the discharge.
Phosphorescence of Substances under the Action of the Becquerel Rays. Light Emitted by Salts of Radium. Coloration of Substances by the Action of the Rays.—The radiation from radium causes phosphorescence in a great many substances: the salts of the alkalies and alkaline earths, uranyl-potassium sulphate, organic substances, cotton, paper, cinchonine sulphate, skin, glass, quartz, etc. The most sensitive substances are barium platinocyanide, willemite (silicate of zinc), sulphide of zinc, and diamond. With the penetrating β-rays willemite and the platinocyanide are the most sensitive, while with the α-rays it is better to use phosphorescent sulphide of zinc.