of the Active Deposit of Actinium. 745 
Conclusion. 
It may be useful here to draw attention to some o£ the 
differences that have been observed in the transmission in 
electric lields of the active deposits of the different radioactive 
substances . 
In dry and dust-free air at atmospheric pressure the active 
deposit of Thorium is almost completely confined to the 
negative pole ; on reduction of the gaseous pressure to a few 
mms. a marked diminution in the amount transmitted to the 
negative pole is observed, some of the active deposit then 
being found on the positive boundary of the system. 
In the case of Radium the transmission of its active deposit 
to the negative pole is not so complete as with thorium ; at 
atmospheric pressure about 95 per cent, of the active deposit 
is directed to the negative pole, the remaining 5 per cent, 
going to the positive pole. With reduction of the gaseous 
pressure similar effects are observed to those mentioned for 
thorium, the activity of the negative boundary decreasing, 
that of the positive increasing. 
With Actinium the active deposit may at atmospheric 
pressure be almost completely directed to the negative pole, 
but by suitable variation of the experimental conditions, the 
distribution may be rendered much less complete, the amount 
obtained on the negative pole being reduced to as little as 
twice that on the positive. 
At low pressures, there is the same possibility of the 
phenomena observed depending on experimental conditions. 
From the results of several independent observers it seems 
clear that the electrical charge exhibited by the active deposit 
particles of the radioactive substances is mainly determined 
by the numerous collisions between these particles and the 
ions or molecules of the gas with which they are mixed. 
Whether the active deposits are electrically charged at the 
moment of formation from the emanations, still remains an 
open question. Although experiments with the active deposit 
from Radium show that at very low pressures (about *01 mm.) 
almost as much is directed to a positive as to a negative pole, 
this does not preclude the possibility of the active deposit 
particles being really positively charged, for at this pressure 
the velocity of the particles is so high that no ordinary electric 
field would be capable of directing them to the negative 
boundary of the system. 
