Radioactivity of the Atmosphere. 143 
part of the leather over the wire every five feet, a large quan- 
tity of excited activity could be transferred from the copper 
wire to the leather. This radioactive leather was then tested 
n the following apparatus:—Two parallel zinc plates were 
placed horizontally over one another, and insulated. The 
upper plate was connected to one pair of quadrants of the 
electrometer, the lower plate being connected to one pole of 
the battery, the other pole of which was earthed. The radio- 
active leather was placed on the lower plate, and the rate of 
movement of the needle taken as a measure of the quantity of | 
radioactivity present. About 100 volts P.D. between the 
plates was sufficient for saturation. The apparatus was 
inclosed in a metal box connected to earth, which acted as a 
guard-ring. 
It was found that with thin close-grained leather the 
excited activity decayed to half value in about 45 minutes, 
whilst with a thicker and more absorbent leather it took 48 
to 52 minutes. 
A piece of felt, moistened with ammonia, was also rubbed 
over the wire, and tested in the same way. Only a small 
quantity of excited activity could be observed in this, but it 
was found to decay much more slowly, falling only to half 
value in 60 minutes. Another thick spongy piece of felt 
gave a rate of decay even slower. If, however, the felt was 
reduced to ashes and the residue tested, much more radio- 
activity was observed than the felt itself showed. The reason 
for this seems to be that the ammonia dissolves off the matter 
which causes the radioactivity and carries it into the interior 
of the felt. Some of the radiation is thus absorbed in the 
felt before it can reach the surface. 
The rate of decay of the activity of the ashes from the felt is 
about the same as for the copper wire. In Pl. XIV. fig. 2, 
curve I. shows the decay of the activity of the unburnt felt, and 
curve II. that of the ashes from the felt. The difference in 
the rate of decay in the two cases is probably due to the fact 
that the penetrating part of the radiation which passes 
through the felt has a rate of decay different from that of the 
whole radiation. 
A piece of cotton wool, moistened with ammonia and 
rubbed over an active copper wire, then reduced to ashes and 
- tested, gave a very large amount of radioactivity. The rate 
of decay is the same as for the excited activity on copper. 
This is a good method of obtaining a large quantity of excited 
activity in a concentrated form. It produces more ionization 
thav uranium bulk for bulk. 
When a piece of copper wire which has been made radio- 
