of the Active Deposit of Actinium. 
743 
eatio of the activities on the two plates should remain the 
same. This is, however, far from being the case, and more- 
over the positive plate shows signs of increased activity as 
its distance from the layer of actinium is increased. It 
appears then that in their passage through the containing gas, 
the positively charged active deposit particles may become 
neutralized, and in fact may even become reversed in sign. 
To account for the increase in the ratio observed at the low 
pressure, it must be remembered that at the moment of 
formation the active deposit particles have a very high 
velocity. Rutherford* has suggested that a certain number 
of collisions may therefore be necessary to reduce their speed 
so that the electric field may be capable of directing them to 
the electrodes; hence as the distance of the plates from the 
actinium is increased at the low pressure, their total activity 
should increase, as is found to be the case. 
It might be expected that if the distance of the plates from 
the actinium were made sufficiently large, even at the lower 
pressure, similar effects to those obtained at the high pressure 
would be observed. With the small cylinder described this was 
not possible ; a larger one 40 cms. long and 7*5 cms. dia- 
meter was used, and experiments at an intermediate pressure 
(1*2 cms.) were conducted in an exactly similar way to that 
already described. 
The numerical data are collected in Table III. and i repro- 
duced graphically in fig. 5. 
Table III. 
Distance from 
Actinium. 
1 Eatio of Activities. 
Negative 
Positive 
1-0 cm 
26 
..! 62 
J 76 
82 
63 
37 
25 
1 
35 
4-7 
90 
110 
18-0 
257 
30-5 
* • Radioactivity,' p. 319. 
3 D 2 
