191 1.] BOLTWOOD— RADIOACTIVITY. 339 



change following radium is a striking one for the product in this case 

 is gaseous. It is known as the radium emanation and has the inert 

 chemical character of the rarer gases of the atmosphere, helium, 

 neon, argon, krypton and xenon. \\'hen the atoms of radium emana- 

 tion undergo transformation, the succeeding product known as ra- 

 dium A is formed. This is a solid and is deposited in the form of a 

 thin coating of active matter on the walls of a vessel containing the 

 emanation. This acquirement of activity by the surface of objects 

 in contact with the emanation was observed some time before a 

 satisfactory explanation of the phenomenon was suggested. It was 

 therefore known as "imparted" or "induced" activity. It is now 

 called the active deposit. Radium A, which has a half value period 

 of three minutes and emits alpha rays, is followed by radium B, 

 which emits beta rays and is half transformed in about twenty-six 

 minutes, and this in turn is succeeded by radium C with a half value 

 period of about nineteen minutes. The transformation of radium 

 C is accompanied by the expulsion of alpha, beta and gamma rays. 

 An interesting product known as radium D then ensues, its transfor- 

 mation being characterized by the absence of any detectible radiation 

 whatever. A product of this sort is known as a rayless change and 

 other examples to such a change occur in both the thorium and 

 actinium series. On account of the similarity of its chemical prop- 

 erties to those of ordinary lead, radium D is known as radio-lead. 

 It undergoes transformation more slowly than the immediately pre- 

 ceding products and has a half value period of about sixteen years. 

 It is followed by radium E, a beta ray change, with a period of five 

 days, and this is succeeded by radium F, otherwise known as polo- 

 nium. Polonium emits alpha rays only and is half transformed in 

 one hundred and forty-three days. 



In addition to the ionium-radium series, uranium is also the pro- 

 genitor of another group of radio-elements of which actinium is the 

 first and most stable member. The rate of change of actinium has 

 not yet been determined, but is comparatively slow and is probably 

 of the same order as that of the radium. Actinium offers another 

 example of the rayless changes which have already been referred 

 to, and no radiations have been observed to accompany its trans- 

 formation. 



