i88 PHYSICAL SCIENCE 



activity of these bodies consists in the emission of 

 a rays, the disintegration of each atom resulting 

 in the projection of one or more a particles with a 

 velocity about one-twelfth that of light, while the 

 residues break down into new and simpler atoms, 

 which are themselves in a state of instability, and 

 are known to us as thorium- J^ and uranium-JY. 



The further transformation of these bodies is 

 very rapid, their activity disappearing in a time to 

 be measured in days. In radium we possess an 

 analogous substance, also an intermediate product 

 in a state of instability, the life of which is enor- 

 mously longer. The primary substance, standing 

 to radium as thorium stands to thorium-A^, was 

 discovered by Boltwood and named ionium. It is 

 itself derived from uranium through uranium-X 

 and two other intermediate products. 



In compounds of radium and thorium, we get 

 the emanations as a step in the process of atomic 

 dissociation. These bodies also are unstable, 

 that is, radio-active. They emit new a rays, and 

 produce the radio-active deposit which generally 

 appears on the walls of the containing vessel. 

 This again breaks down, with the usual accom- 

 paniment of a radiation. The decay of the active 

 deposit on a rod, exposed for a very short time 

 to the radium emanation, is shown in Fig. 34. 

 The curve is a complicated one, and may profit- 

 ably be compared with the simple curves giving 

 the rate of decay of the activity of uranium-J\^, 

 the curve of Fig. 33 on page 184, and with the 

 curve of decay of the radium emanation. Fig. 32 

 on page 178. Rutherford has shown, however, 

 that the complex curve of the decay of the excited 

 activity of radium can be made up by the con- 



