202 RADIO-ACTIVE EMANATIONS [CH. 
the radio-active particles are not destroyed by the electric field. 
The current through the gas at any particular instant, after 
stoppage of the flow of air, was found to be the same whether the 
electromotive force had been acting the whole time or had been 
just applied for the time of the test. 
The emanation itself is unaffected by a strong electric field and 
so cannot be charged. By testing the activity of the emanation 
after passing through long concentric cylinders, charged to a high 
potential, it was found that the emanation certainly did not move 
with a velocity greater than ‘00001 cm. per second, for a gradient 
of 1 volt per cm., and there was no evidence to show that 1t moved 
at all. 
The rate at which the emanation is produced is independent 
of the gas surrounding the active matter. If in the apparatus of 
Fig. 37, air is replaced by hydrogen, oxygen, or carbonic acid, 
similar results are obtained, though the current observed in the 
testing vessel varies for the different gases on account of the 
unequal absorption by them of the radiation from the emanation. 
If a thorium compound, enclosed in paper to absorb the a 
radiation, is placed in a closed vessel, the saturation current due to 
the emanation is found to vary directly as the pressure. Since 
the rate of lonization is proportional to the pressure for a constant 
source of radiation, this experiment shows that the rate of emission 
of the emanation is independent of the pressure of the gas. The 
effect of pressure on the rate of production of the emanation is 
discussed in more detail later in section 148. 
133. Effect of thickness of layer. The amount of emana- 
tion emitted by a given area of thorium compound depends on 
the thickness of the layer. With a very thin layer, the current 
between two parallel plates, placed in a closed vessel as in Fig. 16, 
is due very largely to the a rays. Since the a radiation is very 
readily absorbed, the current due to 1t practically reaches a maximum 
when the surface of the plate is completely covered by a thin layer 
of the active material. On the other hand the current produced 
by the emanation increases until the layer is several millimetres m 
thickness, and then is not much altered by adding fresh active 
matter. This falling off of the current after a certain thickness 
