340 BOLTWOOD— RADIOACTIVITY. [April 22, 



Six products subsequent to actinium have thus far been identi- 

 fied in this series. The first is radioactiniuni, an element having a 

 half-value period of 19^ days and emitting both alpha and beta par- 

 ticles. The subsequent product is known as actinium X. The half 

 value period of actinium X is about ten days and its atoms disin- 

 tegrate with the expulsion of alpha particles. The next step in the 

 series of transformation is the gaseous product known as the actin- 

 ium emanation. This, like the radium emanation, is chemically inert 

 and incapable of entering into combination with other elements. 

 Actinium emanation is a very short lived substance and has a half 

 value period of only 3.9 seconds. It is transformed successively 

 into three other products, which are solids, known respectively as 

 actinium A, B and C, and together constitute the so-called active 

 deposit from the actinium emanation. Actinium A has a half value 

 period of 36 minutes, actinium V> of 3.1 minutes and actinium C of 

 5.1 minutes. The first emits beta rays, the second alpha rays, and 

 the third beta and gamma rays. 



As already stated, actinium and its products are genetically con- 

 nected with uranium and are. in some manner as yet obscure, derived 

 from it. The evidence in support of this conclusion is quite con- 

 vincing. All uranium minerals contain definite quantities of actin- 

 ium and in the older minerals the relative proportions of uranium 

 and actinium present arc so constant as to permit of no other expla- 

 nation. But the actual genealogical history of actinium is still 

 obscure and we are not yet in a position to clearly trace the line of 

 descent. Whether actinium is formed directly from uranium by a 

 special kind of transformation which involves only a small propor- 

 tion of the total number of the atoms changing, or whether its pro- 

 duction occurs at a later stage in the uranium-radium series, is at 

 present an open question and the discovery of the true relations is 

 one of the most interesting problems now awaiting solution. 



The thorium series presents another group of radio-elements 

 comprising ten successive members. I shall not stop to enumerate 

 these in detail, but their princi])al phy>ical and chemical charactei- 

 istics have already been determined. Thorium itself, the parent sub- 

 stance, has a verv slow rate of change, which is probably not more 

 than a fifth that of uranium. 



