Radioactivity of the Atmosphere. 147 
for the light substances and for aluminium the absorption is 
nearly proportional to density, but for the heavier metals 
there isa wide divergence. 
-zLhese resulis are similar in character to those of Rutherford 
and Miss Brooks*, who examined the @ radiation from 

: r : 
uranium, and found that the ratio dense: 725 the same for 
Ss 
such materials as mica, ebonite, glass, and aluminium, but 
differed greatly for the substances of high density. 
Absorption in Gases. 
A series of experiments were also made on the absorption 
of the excited activity by air, coal-gas, carbonic-acid gas, and 
hydrogen. For this purpose a special apparatus was con- 
structed, the general arrangement of which is shown in fig. 8 
(Pl. XIV.), and is similar in principle to that used by Ruther- 
ford in his experiments on uranium radiation +. It consists 
of a cylindrical brass vessel, closed at the top by an air-tight 
cover, and at the bottom by a mercury trap. It is divided 
on the inside into two chambers by means of a horizontal 
partition, which has a circular hole cut in it, covered with a 
sheet of aluminium ‘00038 cm. thick. The partition was 
insulated from the sides of the cylinder, and connected to 
one pole of the battery, the other pole being earthed. Im- 
mediately below the partition was a circular table, which 
could be moved up and down by means of a screw passing 
through the bottom of the cylinder. At the top of the upper 
chamber was suspended an insulated disk, connected to the 
electrometer. 
The radioactive leather was placed on top of the table 
immediately underneath the aluminium foil. The radiation 
given off by this leather penetrated through a certain layer of 
air or any gas with which the cylinder might be filled, and 
thence through the aluminium foil into the upper chamber, 
where it could ionize the gasand produce a movement of the 
needle of the electrometer. The brass cylinder was earthed 
and acted as a guard-ring, preventing any leak along the 
sides. The radiation in passing through the layer of gas 
before reaching the upper chamber, would be absorbed to 
an extent depending on the thickness of layer traversed. 
This thickness could be regulated by means of the screw. 
The volume of gas in the upper chamber remaining constant 
the ionization produced there would always be a measure of 
the strength of the radiation unabsorbed after passing through 
* Phil. Mag. July 1902. r f~ Phil. Mag. Jan. 1899, 
