DISCOVERY AND NATURE OF RADIOACTIVITY II 



great violence, expelling in many cases a small portion of the disrupted 

 atom at a high speed. The residue of the atom forms a new atomic 

 system of less atomic weight, and possessing physical and chemical 

 properties which markedly distinguish it from the parent atom. The 

 atoms composing the new substance formed by the disintegration of the 

 parent matter are also unstable, and break up in turn. The process 

 of disintegration of the atom, once started, proceeds through a num- 

 ber of distinct stages. These new products formed by the succes- 

 sive disintegrations of the parent matter are in most cases present in 

 such extremely minute quantity that they cannot be investigated by 

 ordinary chemical methods. . . . For any simple substance, the aver- 

 age number of atoms breaking up per second is proportional at any 

 time to the number present. In consequence the amount of radio- 

 active matter decreases in a geometrical progression with time." 

 Rutherford'"^ illustrates these changes by the following diagram* 

 (figure i). The time periods given indicate how long is required 



oo 



RAOIUNl EMAN. FtAD.A RAO.B RADC RAD.D RAD.E RAD F RAD.G 



iOOQifr3. zjSiaAji Smins,. zSmini- iSmina. •'K)j<ri. Sdaj^& 4SAaAfa /4oa<U)-s. 



f^aetio-LeO'i' RaiiO'Tcfluriurry.Thtoniujn J 



-v— ' 



Active Deposit Rapid Change Active Deposit Slow Change 



Fig. I. Theory of Atomic Disintegration. (After Rutherford.) 



forthe given product to become half transformed. Thus, it requires 

 2,000 years f for a given quantit}^ of radium to become half trans- 

 formed into the next following product. 



Uranium is now generally regarded as the ancestor of radium, 

 but there are several intermediate disintegration products of uranium, 

 and of these tonmni, discovered by Boltwood,^^ may be the immediate 



is static or potential. The internal energy of the hypothetical atom at which others 

 are working is kinetic. 



" The disintegration of radium in the former case is comparable to the explosion 

 of an unstable chemical compound, like gun-cotton. In the latter case it must be rep- 

 resented by something more akin to the flying to pieces of a single rapidly spinning 

 unit, such as a fly wheel." 



*Duane** states that radium 5, formerly held by Rutherford "^ to be non-radio- 

 active, emits as much negative electricity as does radium C. 



t Results obtained by Boltwood'^ on the growth of radium in preparations of 

 ionium separated from uranium minerals indicate that the half-value period for radium 

 is 2,000 years. Rutherford'-^" gives 1,760 years. 



