10 M‘Cirettanp—The Energy of Secondary Radiation. 
in the first place, whether or not any easily-absorbed radiation is produced 
at the surface of a substance which is exposed to 6 rays. 
This was investigated in the following manner :— 
Two thick lead plates were insulated and placed parallel to each other, 
the upper having a hole at the centre, covered only with tinfoil, through which 
passed a stream of 8 rays from the radium. The whole was enclosed in an 
air-tight vessel which could be exhausted to any desired pressure. One plate 
was kept at a high potential by joining it to a battery of small cells; the other 
plate was connected to an electrometer, and the saturation-current between the 
plates was measured in the usual way. 
In some experiments the plate connected to the electrometer was provided 
with a guard-ring. 
The pressure was reduced step by step; and the saturation-current was 
measured at each step. 
Now the total ionisation between the plates is due to two possible causes: 
first, the penetrating rays direct from the radium, together with the penetrating 
secondary radiation emitted by the plates—meaning by penetrating radiation one 
that can readily pass through the space between the plates without suffering any 
marked diminution of intensity even at atmospheric pressure; secondly, there 
may be an easily-absorbed radiation produced at the surface of the plates which 
will be completely stopped in the first few millimetres of air when at atmospheric 
pressure. 
When the pressure is reduced, the ionisation due to the first cause will 
obviously be proportionately reduced; but the ionisation due to the second cause 
will not be diminished, as this radiation will merely penetrate a greater thickness 
of the rarefied air, and produce the same ionisation as before, until the pressure 
is so much reduced that the easily-absorbed radiation reaches the opposite plate 
before being completely stopped. If, therefore, we plot the saturation-current 
against the pressure, the curve should be a straight line if there is no easily- 
absorbed radiation present; if there is such a radiation, the curve should be a 
straight line at low pressures, but becoming less inclined to the axis of pressure 
at higher pressures. ‘The change of slope will obviously occur at different pressures 
for different distances between the plates. 
A number of experiments were made in this way with the plates at different 
distances apart in different experiments varying from 2 mm. to 130 mm., 
the pressures being varied in each experiment from atmospheric down to a 
few centimetres of mercury. In every case the saturation-current was strictly 
proportional to the pressure throughout the whole range of pressures, the_ 
curve of current against pressure being a straight line passing through the 
origin. 
