x1] AND OF ORDINARY MATERIALS 369 
wire, exposed in the open air, decays according to an exponential 
law with the time, falling to half value im 45 minutes. On the 
other hand, the activity of freshly fallen rain and snow falls to 
half value in about 30 minutes. Now the activity of a wire, made 
active by exposure to the radium emanation, is at first irregular, 
but about an hour after removal it decays according to an expo- 
nential law with the time, falling to half value in 28 minutes. 
The agreement between the rates of decay of the activity of the 
emanation in the air and the excited activity produced on rain and 
snow, with the similar effects produced by radium, strongly sup- 
ports the view that the radium emanation is present in the soil 
and atmosphere. Allan* has also obtained evidence to show that 
the rate of decay of the excited activity produced on a negatively 
charged wire is the resultant of the rates of decay of several types 
of matter which have different rates of decay. For example, the 
activity transferred from the active wire to a piece of leather 
moistened with ammonia, fell to half value in 38 minutes, while on 
a piece of absorbent felt treated similarly the activity fell to half 
value in 60 minutes. Thus it seems probable that different types 
of active matter are collected by the negatively charged wire, 
which are soluble in ammonia in different degrees. An accurate 
determination of the rate of decay of the excited activity from 
actinium would be of interest, in order to see if the activity derived 
from the air may be due in part to the presence of the actinium 
emanation. 
Considering the results as a whole, there is evidence that other 
radio-active substances besides radium and thorium are present in 
the earth. There can be little doubt, however, that part of the 
radio-activity of the atmosphere is due to the radium emanation, 
which is continually diffusmg into the atmosphere from the pores 
of the earth. Since radio-activity has been observed in the 
atmosphere at all pomts at which observations have, so far, been 
made, there can be little doubt that radio-active matter is dis- 
tributed in minute quantities throughout the soil of the earth. 
The volatile emanations escape into the atmosphere by diffusion, 
or are carried to the surface in spring water or by the escape of 
underground gases, and cause the radio-active phenomena observed 
1 Phys. Rev. 16, p. 306, 1903. 
R. B.-A. 
