[HENDERSON] DEPOSITS OF RADIUM, THORIUM AND ACTINIUM 165 
on the cathode in air at atmospheric pressure. The conclusion seems 
justified that with a sufficiently high potential gradient all the rest- 
atoms of actinium could be concentrated on the cathode. The fact 
that Lucian, using cylindrical vessels, came to the conclusion that a 
limiting value of about 95% was reached only serves to emphasize 
the unsuitability of this type of vessel. 
THEORETICAL DISCUSSION. 
So far this paper has dealt only with rest-atoms after they have 
reached the end of recoil, and no suggestion has been made, which 
would in any way explain their “initial” charged or uncharged con- 
dition. The word “initial” as used in this connection does not apply 
to the rest-atom at the instant the @ particle is separated from the 
emanation atom, but rather to the instant when it has reached the 
end of its recoil path, for it is only then that even the strongest exter- 
nal field can exert an appreciable effect. However, the mechanism 
responsible for its initial charged condition is of considerable theoret- 
ical interest and has been much discussed since it was first discovered 
that the active deposits were largely collected on the cathode in spite 
of the fact that the rest-atom is produced when an emanation atom 
loses a positively charged a particle. It is felt that the present in- 
vestigation throws much light on this much discussed subject. In 
the first place the experiments on thorium rest-atoms in air, ether, 
and a mixture of the two, show conclusively that the initial positive or 
neutral condition of the rest-atom does not depend on what happens on 
the break up of the emanation atom, but rather upon the nature of the 
atoms met during recoil. In the second place it has been shown that 
the fraction of the rest-atoms initially neutral in a mixture of air 
and ether depends upon the relative amount of ether present, not 
merely on the absolute amount present. This can only mean that 
both air and ether molecules exert on the recoiling rest-atom some 
definite influence. It is not enough to assume that ether alone is an 
active agent, while the air is merely passive. 
The following explanation of what takes place during recoil 
seems to satisfy both the above results. When a neutral rest-atom 
while recoiling meets a molecule of nitrogen or oxygen, for example, 
it not only ionizes the molecule of the gas, but itself as well, that is, 
an election escapes from it and it goes on its way positively charged. 
Thus the rest-atom is always positively charged after collision with 
certain kinds of gas molecules. On the other hand after collision 
with certain other kinds of molecules, such as those of ether the rest- 
atom always escapes uncharged whether on meeting the molecule 
it is charged or not. If it meets such a molecule when positively 
