VIIt] RADIO-ACTIVE EMANATIONS 233 
and -043 for the negative ions. The radium emanation thus 
diffuses more rapidly than the ions produced by its radiation in 
the gas, and behaves as if its mass were smaller than that of 
the ions produced in air, but considerably greater than that of 
the air molecules with which it is mixed. 
It is not possible to regard the emanation as a temporarily 
modified condition of the gas originally in contact with the active 
body. Under such conditions a much larger value of K would be 
expected. The evidence derived from the experiments on diffusion 
strongly supports the view that the emanation is a gas of heavy 
molecular weight. 
Diffusion of the Thorium Emanation. 
154. On account of the rapid decay of the activity of the 
thorium emanation, it 1s not possible to determine the value of K 
its coefficient of diffusion into air by the methods employed for the 
radium emanation. The value of K has been determined by the 
writer in the followmg way. A plate C, 
Fig. 43, covered with thorium hydroxide, was 
placed horizontally near the base of a long 
vertical brass cylinder P. The emanation 
released from the thorium compound diffuses 
upwards in the cylinder. 
Let p be the partial pressure of the emana- 
tion at a distance « from the source C. This 
will be approximately uniform over the cross 
section of the cylinder. From the general 
principles of diffusion we get the equation 
dp dp 
a da? dt’ 
The emanation is continuously breaking 
up and expelling « particles. The emanation-residue gains a posi- 
tive charge, and, in an electric field, is at once removed from the 
gas to the negative electrode. 
Since the activity of the emanation at any time is always 
proportional to the number of particles which have not broken up, 
and since the activity decays with the time according to an 


Fig. 43. 
