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.

Identification of the Rays

The alpha rays have been identified as similar to the so-called canal
rays. These were first observed in the study of the _X_ rays. When an
electrical discharge is passed through a vacuum tube with a cathode
having holes in it, luminous streams pass through the holes toward the
side away from the anode and the general direction of the stream. They
travel in straight lines and render certain substances phosphorescent.
These rays are slightly deflected by a magnetic field and in an
opposite direction from that taken by the cathode rays in their
deflection. The rays seem to be positive ions with masses never less
than that of the hydrogen atom. Their source is uncertain, but they
may be derived from the electrodes.

The beta rays are identical in type with the cathode rays and are
negative electrons.

The gamma rays are analogous to the _X_ rays and are of the order of
light. They are in general considerably more penetrating than _X_
rays. For example, the gamma rays sent out by 30 milligrams of radium
can be detected by an electroscope after passing through 30
centimeters of iron, a much greater thickness than can be penetrated
by the ordinary _X_ rays.

CHAPTER III

CHANGES IN RADIO-ACTIVE BODIES

Is Radio-activity a Permanent Property?

Is this power of emitting radiations a permanent property or is it
lost with the passage of time? The first investigations of the
activity of uranium and thorium showed no loss of intensity at the end
of several years, and radium also se