90 ROYAL SOCIETY OF CANADA 
method of experiment. In other words, the emanation must be mixed 
with some gas, and conducted through tubes containing absorbing, or 
non-absorbing, material into a testing vessel, where it ionizes the air, 
and the ionization currents can be measured by means of an electrometer. 
The apparatus used for the experiments is represented by the accom- 
panying diagram, fig. 2. 
An air-pump supplies a current of air through a tube of thorium 
hydroxide, where the emanation mixes with the air and thence passes 
through tubes, containing absorbing or non-absorbing material, as the 
case may be, into a testing vessel of the ordinary cylindrical type. An 
accurately calibrated manometer in the circuit measures the velocity 
of the air current. he testing vessel is connected to one pole of a 
battery of E.M.F. sufficient for complete saturation, the other pole of 
the battery being earthed. The central rod of the testing vessel is con- 
nected with a suitable electrometer, and a condenser of adjustable cap- 
acity is placed in parallel with it. It should be noticed thet, with this 
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arrangement, absorption of the emanation can only take place after the 
charcoal has absorbed its full amount of the atmospheric gases. 
In the first place, it is necessary to consider the way in which the 
ionization current in the testing vessel will vary with the velocity of 
flow of the current of air. This can be calculated from theoretical 
considerations. 
The thorium hydroxide, in radio-active equilibrium, gives off free 
atoms: of emanation at a constant rate, and these are all removed by the 
air current. In consequence, there will be the same number of emana- 
tion atoms leaving the thorium tube per second for any speed of the 
air current. . . 
Case [.—With no absorbing material between the thorium hydroxide 
and the testing vessel. 
The ionization current in the testing vessel is proportional to the 
number of emanation atoms breaking up per second in the testing vessel, 
and this number is A times the number of emanation atoms present, À 
being the coefficient of decay of the emanation. 
