94 McCiettanp— On the Emanation given off by Radium. 
closed vessel falls off with time in a geometrical progression, showing that the 
rate of decay of the ionising power is proportional to the ionising power at every 
instant—a result which readily admits of the interpretation that the radiation 
arises from the emanation particles undergoing some change, and that the 
number changing at any instant is proportional to the total number present. 
The ionising power J may, from experiment (Rutherford, Phil. Mag., April, 
1903), be represented by 
T= LE, 
where d is a constant, and ¢ the time measured from the instant when J = J). 
Since dl 
= ME 
we see that d is the fraction of the total emanation that undergoes change or emits 
radiation in one second. And we know (Rutherford, Phil. Mag., April, 1903) 
that J falls to half its value in about 4 days, so that \ is approximately equal to 
PISS MOTs 
If, therefore, we accept the theory that the emanation undergoes a further 
change, and that each particle acts as a centre of radiation and ionisation only 
when undergoing change—and this is the only theory that seems to fit in with 
experiment—we see that the number calculated above, giving the minimum 
ionisation that must be produced by each emanation particle in one second, 
assuming it to be charged, would have to be multiplied by the factor $ + 10°. 
Multiplying 12000 by 4° 10°, we get 6 x 10° as the minimum number of 
ions produced in one second by each emanation particle when its turn comes to 
disintegrate, assuming that it is charged. This number is not a possible one for 
several reasons. Rutherford (Phil. Mag., May, 1903) gives 10° as the probable 
number of ions produced by each a ray before it is absorbed by the gas. The 
ionisation is chiefly due to a rays, so that to produce the above ionisation each 
emanation particle would require to emit 
6 x 10° 
10° y) 
The mass of the a particle being of the same order as that of the hydrogen 
atom, and the emanation haying been produced by a disintegration of the radium 
atom, each emanation particle could not possibly emit more than about 200 a rays. 
We can, therefore, finally conclude that the emanation is not charged. 
This fact—that the emanation is uncharged—has an important bearing on our 
conception of the manner in which the radium atom breaks up. The radium 
atom certainly gives off positively charged particles—the a rays. The emanation 
particles cannot be what remains of the atom after the emission of one or more 
a rays, because, in that case, it would be negatively charged. The atom must 
have parted with an equal negative charge, either by the emission of negative 
particles, or in some other way. 
or 6 x 10* a rays. 
