vul] RADIO-ACTIVE EMANATIONS 203 
has been reached is to be expected, since the emanation, which 
takes several minutes to diffuse through the layer above it, has 
already lost a large proportion of its activity. 
With a thick layer of thorium oxide in a closed vessel, the 
current between the plates is largely due to the radiation from the 
emanation lying between the plates. The following tables illus- 
trate the way in which the current varies with the thickness of 
paper for both a thin and a thick layer. 








TABLE I. Thin Layer. TaBLE II. Thick Layer. 
Thickness of sheets of paper ‘0027. Thickness of paper *008 cm. 
| reall 
| No. of layers . No. of layers | 
of paper Current of paper | Current 
| 
| 0 lea 0 1 
1 37 1 “74 
2 “16 2 “74 
3 08 5 "72 
10 67 
20 55 



The initial current with the unscreened compound is taken as 
unity. In Table L., for a thin layer of thorium oxide, the current 
diminished rapidly with additional layers of thin paper. In this 
case the current is due almost entirely to the « rays. In Table II. 
the current falls to ‘74 for the first layer. In this case about 26°/, 
of the current is due to the a rays, which are practically absorbed 
by the layer 008 cm. in thickness. The slow decrease with 
additional layers shows that the emanation diffuses so rapidly 
through a few layers of paper that there is little loss of activity 
during the passage. The time taken to diffuse through 20 layers 
is however appreciable, and the current consequently has decreased. 
After passing through a layer of cardboard 1:6 mms. in thickness 
the current is reduced to about one-fifth of its original value. In 
closed vessels the proportion of the total current, due to the emana- 
tion, varies with the distance between the plates as well as with the 
thickness of the layer of active material. It also varies greatly 
with the compound examined. In the nitrate, which gives off only 
a small amount of emanation, the proportion is very much smaller 
than in the hydroxide which gives off a large amount of emanation. 
