20 Prof. Rutherford and Miss Brooks : Comparison of 



since the rate of decay of the excited radiation as a whole 

 more than compensates for the increase of radiation due to 

 radioactive particles deposited in the last few hours of exposure. 

 The general shape of the decay-curves for the two speci- 

 mens of radium points to the conclusion that two kinds of 

 radiation are emitted by the excited body which decay at 

 different rates. The relative amount of excited radiation 

 due to the two kinds is different for the two samples of 

 radium. The amount of that radiation which decays more 

 rapidly is greater for specimen (c) than for specimen (e). 



If, in addition, we suppose, as in the case of thoria, that 

 the excited radiation which has the slower normal rate of 

 decay does not reach its maximum for some time after 

 deposit of the radioactive matter, the general shape of the 

 decay-curves can be satisfactorily explained. 



The observed curve of decay is in that case due to the 

 addition of the effect of two types of radiations, one of which 

 decays regularly with the time, while the other increases at 

 first, passes through a maximum, and then decays. Such a 

 combination would completely explain the peculiarities ex- 

 hibited by the experimental curves of the excited activity 

 from radium (c) and (e). 



It will be observed that the shape of the decay-curves for 

 radium depends upon the time of exposure to the emanation. 

 The initial decrease of excited activity with time is greater 

 for a body exposed a short interval (see fig. 8) than for a 

 body exposed several hours (see fig. 7, curve I.) . 



From general theoretical considerations the rafe of rise 

 of excited activity with time of exposure can be deduced 

 from the rate of decay and vice versa In a recent paper * it 

 has been shown that excited radioactivity is due to the con- 

 veyance of radioactive matter of some kind on positively- 

 charged carriers, which travel through air in an electric field 

 with about the same velocity as the positive ions produced in 

 air by Rontgen and Becquerel rays. 



Suppose n is the average number of ions produced per 

 second by a single radioactive carrier at the instant of deposit. 

 Let n = number after a lapse of time t. 



Suppose n — n f{t) where f{t) is a function of t, 



such that f(t) = l when t = 0, 



f(t) = when t= qo ; 



f(t) may in some cases pass through a maximum value 

 greater than unity. 



* Phys. Zeit. No. 10, 1902. 



