CHAPTER IV. 
NATURE OF THE RADIATIONS. 
PART Tf. 
COMPARISON OF THE RADIATIONS. 
65. 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, Rontgen 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. 
