Ix] EXCITED RADIO-ACTIVITY 265 
Giesel' has also observed that a platinum wire which has been 
exposed in the presence of the radium emanation possesses residual 
activity, and he has shown that the radiation consists entirely of 
a rays. A further discussion of this residual activity and its 
possible connection with polonium is given later in section 188. 
174. Connection between decay curves for different 
times of exposure. The decay of excited activity, im cases 
where there is no occlusion of the emanation by the substance 
made active, is a function only of the time of exposure. The 
decay curves are all intimately connected with each other, and can 
be theoretically deduced provided the decay curve for a very short 
exposure is accurately known. 
It is supposed that the excited activity produced on a body is 
due to a deposit of radio-active particles. On an average a certain 
number of these particles will break up per second, giving rise 
to rays which ionize the gas. If a large number of particles is 
deposited, the rate of production of ions in the gas by the rays will 
be practically a continuous function of the time. The rate of pro- 
duction of ions, at any time, divided by the total number of radio- 
active particles deposited, will be called the average number of 
ions produced by each particle at that time. 
Suppose the radio-active particles which cause excited activity 
are deposited at a uniform rate of q per second. The number 
deposited in a short time dt= qdt. 
Let n,= the average number of ions produced in the gas per 
second by each particle, at the instant of removal. 
n=the average number of ions per particle per second 
after an interval t. 
Suppose n =n, f(t) where f(t) is a function of ¢ such that 
F@®=!1 when t=0, 
f(t)=90 when t=, 
J (t) may im some cases pass through a maximum value greater 
than unity. The variation of the rate of production of ions with 
1 Ber. deutsch. Chem. Ges. p. 2368, 1903. Chem. News, Aug. 7, 1903. 
