200 PROFESSOR E. RUTHERFORD ON THE 



the experiments. In the first place, it is assumed that the carriers which give rise to 

 excited activity are deposited on the surface of the body, to be made active imme- 

 diately after their formation. There is some evidence, however, that some of these 

 carriers exist for a considerable interval in the gas before their deposit on the body. 

 For example, it is found that if a body is introduced for a short interval, about 

 1 minute, into a vessel containing the radium emanation, which has remained undis- 

 turbed for several hours, the activity after the first rapid decay (see fig. 4, curve B) 

 is in much greater proportion than if an electric field had been acting for some time 

 previously. This result indicates that the carriers of B and C both collect in the gas 

 and are swept to the electrode when an electric field is applied. This effect may in part 

 be due to a slight volatility of the matter B at ordinary temperatures.* If the matter B 

 exists to some extent in the gas, the difference between the theoretical curves for 

 three successive changes would be explained ; for, in transferring the emanation to 

 another vessel, the matter B mixed with it would commence at once to change into C 

 and give rise to a part of the radiation observed. 



The equal division of the activity between the products A and C (see fig. 12) 

 supports the view that C is a product of A, for when radioactive equilibrium is 

 reached, the number of particles of A changing per second is equal to the number of 

 B or C changing per second. If each atom of A and C expels an a particle of the 

 same mass and with the same average velocity, the activity due to the matter A 

 should be equal to that due to the matter C ; and this, as we have seen, is the case. 

 Further investigations are in progress, which it is hoped will throw more light on 

 this difficult question. 



23. Very Slow Change in the Active Deposit from Radium. M. and Mme. CURIE! 

 have observed that bodies which have been exposed for a long interval in the presence 

 of the radium emanation do not lose all of their activity. The excited activity at 

 first decays according to the equations already considered, but a residual activity 

 always remains, of the order of 1/20,000 of the initial activity. This residual activity 

 seemed fairly permanent, for it did not decay during an interval of six months. 

 GIESEL observed that a platinum wire which has been exposed to the radium emanation 

 shows residual activity, and he states that the radiation consists entirely of a rays. 



I have examined this residual activity in the following way. The emanation from 

 30 milligs. of pure radium bromide was condensed in a small glass tube and the ends 

 of the tube sealed. After standing for a month the tube was opened, and left to 



h This result is supported by some recent experiments of Miss BROOKS (' Nature,' July 21, 1904). It 

 was shown that the matter B is volatile at ordinary temperatures, and a small part escapes from the active 

 body and is deposited in the neighbourhood. It was also observed that the volatility of the matter B was 

 far more marked during the first 10 minutes after removal, i.e., during the time the first change is in 

 progress. If A is the parent of B, the expulsion of a charged a particle must set B in motion, and in 

 consequence some of the atoms of B may acquire sufficient velocity to escape from the active body. 



t 'These presentee & la Facult^ des Sciences,' Paris, 1903, p. 116. 



