It was found by Crookes that a screen covered with phosphorescent zinc sulphide was brightly lighted up when exposed to the radiations. This is due to the bombardment of the zinc sulphide by a type of ray called the alpha ray. Under a magnifying glass this light is seen to be made up of a number of scintillating points of light and is not continuous, each scintillation being of very short duration. By proper subdivision of the field under the lens, the number of scintillations can be counted with close accuracy.
A simple form of apparatus called the spinthariscope has been devised to show these scintillations. A zinc sulphide screen is fixed in one end of a small tube and a plate carrying a trace of radium is placed very close to it. The scintillations can be observed through an adjustable lens at the other end of the tube. Outer light should be cut off, as in a dark room. The screen then appears to be covered with brilliant flashes of light. Other phosphorescent substances, such as barium platino-cyanide, may be substituted for the zinc sulphide, but they do not answer so well.
Penetrating Power
By penetrating power is meant the power exhibited by the rays of passing through solids of different thicknesses and gases of various depths. This power varies with different radiations and with the nature of the solid or gas. For instance, a sheet of metallic foil may be used and the effect of aluminum will differ from that of gold and the different rays vary in penetrating power. In the case of gases air will differ from hydrogen, and it is noticed that certain rays disappear after penetrating a short distance, while others can penetrate further before being lost.
Magnetic Deflection
If the radiations are subjected to the action of a strong magnetic field, it is found that part of them are much deflected in the magnetic field and describe circular orbits, part are only slightly deflected and in the opposite direction from the first, and the remaining rays are entirely unaffected.
Fig. 5.—Showing Magnetic Deflection of α, β, and γ Rays.
Three Types of Rays
By the use of these methods of investigation it is learned that the radiations consist of three types of rays. These have been named the alpha, beta, and gamma rays, respectively. Some radio-active bodies emit all three types, some two, and some only one. The distinguishing characteristic of these types of rays may be summed up as follows: