Iv] NATURE OF THE RADIATIONS 129 
removed from the action of the radium rays for several days, 
showed the scintillations again when an electrified rod was brought 
near it. 
Although the scintillations from a particle of pure radium 
bromide are very numerous, they are not too numerous to be 
counted. Close to the radium, the luminosity is very bright, but, 
by using a high power microscope, the luminosity can be shown 
to consist of scintillations. This use of the microscope would offer 
a very convenient means of actually counting the number of the 
particles projected from the surface of the radium, if each particle 
gave rise to a flash of light. It is not likely, however, that this 
would be the case. The number of scintillations from a given mass 
of radium will depend upon its fineness of division, but on account 
of the ease with which the projected particles are absorbed, only 
a small portion of the total number projected from the mass of 
radium will escape from its surface. 
89. Absorption of the « rays by matter. The a rays from 
the different radio-active substances can be distinguished from 
each other by the relative amounts of their absorption by gases 
or by thin screens of solid substances. When examined under 
the same conditions, the a rays from the active substances can be 
arranged in a definite order with reference to the amount of 
absorption in a given thickness of matter. 
In order to test the amount of absorption of the a rays for 
different thicknesses of matter, an apparatus similar to that shown 
in Fig. 16, p. 82 was employed. A thin layer of the active 
material was spread uniformly over an area of about 30 sq. cms., 
and the saturation current observed between two plates 3°5 cms. 
apart. With a thin layer? of active material, the ionization between 
the plates is almost entirely due to the a rays. The ionization 
due to the 8 and y¥ rays is generally less than 1°/, of the total. 
The following table shows the variation of the saturation current 
between the plates due to the @ rays from radium and polonium, 
1 Rutherford and Miss Brooks. Phil. Mag. July 1902. 
2 In order to obtain a very thin layer, the compound to be tested is ground to a 
fine powder and then sifted through a fine gauge uniformly over the area, so that 
the plate is only partially covered. 
R. B.-A. 9 
