Iv] NATURE OF THE RADIATIONS 101 
direction of the rays, the ionization in the testing vessel due to 
the rays steadily decreases as the strength of the field increases, 
and in a strong field it is reduced to a very small fraction of its 
original value. In this case the rays are bent so that none of 
them enter the testing vessel. 
Examined in this way it has been found that the @ rays of 
uranium, thorium, and radium consist entirely of rays readily 
deflected by a magnetic field. The rays from polonium consist 
entirely of a rays, the deviation of which can be detected only in 
very intense magnetic fields. 
When the screen covering the active material is removed, in 
a strong magnetic field, the ionization in the vessel is mainly due 
to the a rays. On account of the slight deviation of the @ rays 
under ordinary experimental conditions, a still greater increase of 
the magnetic field does not appreciably alter the current due to 
them in the testing vessel. 
The action of a magnetic field on a very active substance like 
radium is easily shown by the electrical method, as the ionization 
current due to the deviable rays is large. With substances of 
small activity like uranium and thorium, the ionization current 
due to the deviable rays is very small, and a sensitive electrometer 
or an electroscope is required to determine the variation, in a 
magnetic field, of the very small current involved. This is 
especially the case for thorium oxide, which gives out only about 
1/5 of the amount of deviable rays that the same weight of uranium 
oxide gives. 
73. Experiments with a fluorescent screen. The 8 
rays from a few milligrams of pure radium bromide produce 
intense fluorescence in barium platino-cyanide and other substances 
which can be made luminous under the influence of the cathode 
rays. Using a centigram of radium bromide, the luminosity on 
a screen, placed upon it, is bright enough to be observed in 
daylight. With the aid of such a screen in a dark room many 
of the properties of the 8 rays may be simply illustrated and their 
complex nature clearly shown. A small quantity of radium is 
placed in the bottom of a short, narrow, lead tube open at one end. 
This is placed between the pole pieces of an electro-magnet, and 
