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 *metres distance, only an occasional one is seen. The experiment is extremely beautiful, and brings vividly before the observer the idea that the radium is shooting out a stream of projectiles, the impact of each of which on the screen is marked by a flash of light.

The scintillating points of light on the screen are the result of the impact of the [Greek: alpha] particles on its surface. If the radium is covered with a layer of foil of sufficient thickness to absorb all the [Greek: alpha] rays the scintillations cease. There is still a phosphorescence to be observed on the screen due to the [Greek: beta] and [Greek: gamma] rays, but this luminosity is not marked by scintillations to any appreciable extent. Sir William Crookes showed that the number of scintillations was about the same in vacuo as in air at atmospheric pressure. If the screen was kept at a constant temperature, but the radium cooled down to the temperature of liquid air, no appreciable difference in the number of scintillations was observed. If, however, the screen was gradually cooled to the temperature of liquid air, the scintillations diminished in number and finally ceased altogether. This is due to the fact that the screen loses to a large extent its power of phosphorescence at such a low temperature.

Not only are scintillations produced by radium, actinium, and polonium, but also by the emanations and other radio-active products which emit [Greek: alpha] rays. In addition, F. H. Glew has found that they can be observed from the metal uranium, thorium compounds and various varieties of pitchblende. In order to show the scintillations produced by pitchblende, a flat surface was ground, and a transparent screen, whose lower surface was coated with zinc sulphide, placed upon it. Glew has designed a modified and very simple form of spinthariscope. A transparent screen, coated on one side with a thin layer of zinc sulphide, is placed in contact with the active material, and the scintillations observed by a lens in the usual way.

Since there is no absorption in the air, the luminosity is a maximum. The relative transparency of different substances placed between the active material and the screen may, in this way, be directly studied.

The production of scintillations appears to be a general property of the [Greek: alpha] rays from all radio-active substances. The