ations 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.

[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