VIIt| RADIO-ACTIVE EMANATIONS 225 
independent of the pressure and nature of the gas. This was true 
in the case of a solution down to the pressure of the saturated 
vapour, and in the case of solid salts to very low pressures. When 
the pump was kept going at pressures of the order of ‘001 mm. of 
mercury, the amount of excited activity was much diminished. 
This was probably not due to any alteration of the rate of escape 
of the emanation, but to the removal of the emanation by the 
action of the pump as fast as it was formed. 
Since the amount of excited activity, when in a state of 
radio-active equilibrium, is a measure of the amount of emana- 
tion producing it, these results show that the amount of emanation 
present when the rate of production balances the rate of decay is 
independent of the pressure and nature of the gas. It was also 
found that the time taken to reach the point of radio-active equi- 
librium was independent of the size of the vessel or the amount 
of active matter present. These results show that the state of 
equilibrium cannot in any way be ascribed to the possession by the 
emanation of any appreciable vapour pressure ; for if such were the 
case, the time taken to reach the equilibrium value should depend 
on the size of the vessel and the amount of active matter present. 
The results are, however, in agreement with the view that the 
emanation is present in minute quantity in the tube, and that the 
equilibrium is governed purely by the radio-active constant 2, the 
constant of decay of activity of the emanation. This has been seen 
to be the same under all conditions of concentration, pressure and 
temperature, and, provided the rate of supply of the emanation 
from the active compound is not changed, the time-rate of increase 
of activity to the equilibrium value will always be the same, 
whatever the size of the vessel or the nature and pressure of the 
surrounding gas. 
Chemical Nature of the Emanations. 
149. Earlier experiments. We shall now consider some 
experiments on the physical and chemical properties of the emana- 
tions themselves, without reference to the material producing them, 
in order to see if they possess any properties which identify them 
with any known kind of matter. 
R. R.-A. 15 
