subjected to Radium Rays. 
345 
radium rays. The electroscope itself, was shielded by thick lead 
blocks, S, S from the direct action of the radium rays. The 
plates A and P were placed about 05 cms. apart, and were pro- 
tected by an outer earthed vessel V. P was supported on a rod 
passing out through an ebonite stopper C, and was charged to 
a high potential by a battery of storage cells. 
The ionization between the two plates was very intense, and it 
was necessary to connect a condenser of considerable capacity to 
the gold-leaf system in order to reduce the rate of motion of the 
leaf to a measurable amount. 
In this apparatus a copper plate was exposed, after careful 
polishing, to the a-rays from the radium bromide, continuously 
from March 22nd to May 3rd, a period of six weeks, readings being 
made of the ionization between the plates at frequent intervals 
during the period. The ionization is due to the combined effects 
of primary and secondary radiation. On account of the short- 
ness of the range of the a-particle it was not found practicable to 
separate the two effects. However, as the primary radiation is 
constant, this is not a matter of importance, as any change in 
the total ionization must be due to a change in the secondary 
rays. 
The observed ionizations were corrected for variations in 
temperature and pressure, but as described in the case of the 
/3-rays (supra), the readings varied somewhat from time to time. 
These variations, amounting only to a few per cent., were quite 
irregular, and there was not the slightest indication of any 
alteration in the amount of secondary radiation from the copper 
plate. Thus calling the ionization on March 22nd 100, its value 
on April 16th was 100 - 2, and on May 3rd 100 8, or a difference of 
less than one per cent. As it is somewhat doubtful whether the 
a-rays produce any radiation which is capable of ionizing a gas, 
and as the ionization produced by the a-rays themselves is very 
large, it was feared that the effect due to the latter might be 
sufficiently large to mask any small alteration in the secondary 
radiation. In order to avoid this, the lower plate P was covered 
with 1/10 mm. of aluminium during the actual measurements of the 
secondary radiation, in order to cut off the a-rays. The radiation 
measured was, therefore, that due to /3 and y rays only. The 
aluminium was removed immediately after taking the reading, 
and the exposure to a-rays continued. 
Measurements were, however, also made of the ionization 
between the plates, without screening off the a-rays by alumi- 
nium. The results obtained, however, were very similar, and I was 
unable to detect any sign of fatigue. 
We must, therefore, conclude that the prolonged exposure of 
a substance to either a, /3 or y rays from radium does not produce 
