vur| RADIO-ACTIVE EMANATIONS 223 
The greater proportion of the conductivity is thus due to 
a rays, as in the case of the radio-active elements. The amount 
of absorption of these a rays by aluminium foil is about the same 
as that of the rays from the active bodies. No direct comparison 
can be made, for the a rays from the emanation show the charac- 
teristic property of increased rate of absorption with thickness 
of matter traversed. Before testing, the rays have been largely 
absorbed by the mica window, and the penetrating power has 
consequently decreased. 
No alteration in the radiation from the emanation was ob- 
served on placing an imsulated wire inside the emanation vessel, 
and charging it to a high positive or negative potential. When 
a stream of air through the vessel carried away the emanation as 
fast as 1t was produced, the intensity of the radiation fell to a small 
fraction of its former value. 
No evidence of any @ rays in the radiations was found in 
these experiments, although a very small effect would have been 
detected. After standing some hours, however, 8 rays began to 
appear. These were due to the excited activity deposited on the 
walls of the vessel from the emanation, and not directly to the 
emanation itself. 
The radium emanation, like that of thorium, only gives rise to 
arays. This was tested in the following way!?: 
A large amount of emanation was introduced into a cylinder 
made of sheet copper 005 cm. thick, which absorbed all the 
a rays but allowed the @ and y rays, if present, to pass through 
with but little loss. The external radiation from the cylinder 
was determined at intervals, commencing about two minutes after 
the introduction of the emanation. The amount observed at first 
was extremely small, but increased rapidly and practically reached 
a maximum in three or four hours. Thus the radium emanation 
only gives a rays, the @ rays appearing as the excited activity is 
produced on the walls of the vessel. On sweeping out the emana- 
tion by a current of air, there was no immediately appreciable 
decrease of the radiation. This is another proof that the emanation 
does not give out any 8 rays. Ina similar way it can be shown 
1 Rutherford and Soddy, Phil. Mag. April, 1903. 
