Iv] NATURE OF THE RADIATIONS 141 
in consequence, the number of ions produced per unit length of 
path will diminish far more rapidly than the law observed for 
higher velocities would lead us to expect. This offers an explana- 
tion of the great increase of absorption of the a rays by matter 
which is observed when the rays are nearly all absorbed. 
PART IV. 
THE y OR VERY PENETRATING Rays. 
96. In addition to the a and @ rays, the three active sub- 
stances, uranium, thorium, and radium, all give out a radiation of 
an extraordinarily penetrating character. These y rays are con- 
siderably more penetrating than the X rays produced in a “hard” 
vacuum tube. Their presence can readily be observed for an active 
substance like radium, but is difficult to detect for uranium and 
thorium unless a large quantity of active material is used. 
Villard?, using the photographic method, first drew attention 
to the fact that radium gave out these very penetrating rays, and 
found that they were non-deviable by a magnetic field. This result 
was confirmed by Becquerel’. 
Using a few milligrams of radium bromide, the y rays can 
readily be detected in a dark room by the luminosity they excite 
in the mineral willemite or a screen of platinocyanide of barium. 
The « and 8 rays are completely absorbed by placing a thickness 
of 1 centimetre of lead over the radium, and the rays which then 
pass through the lead consist entirely of y rays. The very great 
penetrating power of these rays 1s readily observed by noting the 
slight diminution of the luminosity of the screen when plates of 
metal several centimetres thick are placed between the radium and 
the screen. These rays also produce ionization in gases and are 
best investigated by the electrical method. The presence of the 
y rays from 30 mgs. of radium bromide can be observed in an 
electroscope after passing through 30 cms. of iron. 
1 C. R. 130, pp. 1110, 1178. 1900. 2 C. R. 130, p. 1154, 1900. 
