CHAPTER IV.
NATURE OF THE RADIATIONS.
PART I.
Comparison of the Radiations.
71. The Three Types of Radiation. All the radio-active
substances possess in common the power of acting on a photographic
plate and of ionizing the gas in their immediate neighbourhood.
The intensity of the radiations may be compared by means of their
photographic or electrical action; and, in the case of the strongly
radio-active substances, by the power they possess of lighting up
a phosphorescent screen. Such comparisons, however, do not throw
any light on the question whether the radiations are of the same
or of different kinds, for it is well known that such different types
of radiations as the short waves of ultra-violet light, Röntgen and
cathode rays, all possess the property of producing ions throughout
the volume of a gas, lighting up a fluorescent screen, and acting
on a photographic plate. Neither can the ordinary optical methods
be employed to examine the radiations under consideration, as
they show no trace of regular reflection, refraction, or polarization.
Two general methods can be used to distinguish the types of the radiations given out by the same body, and also to compare the radiations from the different active substances. These methods are as follows:
(1) By observing whether the rays are appreciably deflected
in a magnetic field.
(2) By comparing the relative absorption of the rays by solids and gases.
Examined in these ways, it has been found that there are three
different types of radiation emitted from radio-active bodies, which
