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. [Illustration: FIG. 5.--SHOWING MAGNETIC DEFLECTION OF [alpha], [beta], AND [gamma] 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: Alpha Rays The alpha rays have a positive electrical charge and a comparatively low penetrating power. They are slightly deflected in strong magnetic and electric fields. They have a great ionizing power and a velocity about one-fifteenth that of light. Beta Rays The beta rays are negatively charged and have a greater penetrating power than the alpha rays. They show a strong deflection in magnetic and electric fields, have less ionizing power than the alpha rays, and a velocity of the same order as light. Gamma Rays The gamma rays are very penetrating and are not deflected in the magnetic or electric fields. They have the least ionizing power and a very great velocity. The penetrating power of each type is complex and varies with the source, so the statements given are but generalizations. The alpha rays are projected particles which lose energy in penetrating matter. As to the power of ionizing gases, if that for the [alpha] rays is taken as 10,000, then the [beta] rays would be approximately 100 and the [gamma] rays 1. Measurement of Radiations The rays are examined and measured in several ways: 1. By their action on the sensitive photographic plates. The use of this method is laborious, consumes time, and for comparative measurements of intensity is uncertain as to effect. 2. By electrical methods, using electroscopes, quadrant electrometers, etc. These are the methods most used. 3. By exposure to magnetic and electric fields, noting extent and direction of deflection. 4. By their relative absorption by solids and gases. 5. By the scintillations on a zinc sulphide screen.