479 
1908-9.] A Simple Eadioscope and a Radiometer. 
larger, or, at least, not better nuclei to any perceptible extent. We can 
understand why this should be so : the life of an ion being so short, there 
is no time for any cumulative action of the rays. An interesting point, 
however, was observed. If after forming a dense, cloudy, or a foggy con- 
densation we leave it alone to settle for a short time, and if while there 
are still a good number of particles present we compress the air and cause 
the particles to evaporate, on again expanding the air slowly it will be 
found that many of the cloud particles have left a dust - like nucleus, 
as a slow and slight expansion makes them active and a shower results. 
But, on the other hand, if we compress and evaporate the particles immedi- 
ately after they are formed, it will be found that hardly any of the dust- 
like nuclei are present, only an odd drop appearing on slow expansion. 
Under the first conditions the cloud particles had time to differentiate in 
size, some growing larger, and it is probable that these larger cloud particles 
do not thoroughly evaporate but leave a nucleus large enough to be active 
with but slight supersaturation. 
To give an idea of the sensitiveness of this apparatus to radioactivity, 
it may be mentioned that any of the following substances, when held near 
the tube, give considerable increase in the condensation : pitchblende, 
radioactive mud, uranium, and, of course, any feeble radioactive salts or 
very minute quantities of them. For instance, the radium on a Crookes 
Spintariscope gives quite a dense shower. To get more decided effects with 
weak radioactive substances, an aluminium window in the tube B has 
been occasionally used. 
A number of gases and vapours were mixed with the air in the test 
chamber to see if any of them would improve its action. Amongst the 
substances tried were sulphurous acid, peroxide of hydrogen, sulphuric, 
nitric, and hydrochloric acids, and chlorine gas. None of these seemed to 
improve the action of the instrument as a detector of radioactivity. The 
most interesting of these tests were made with chlorine mixed with more 
or less air which acted differently from the others. With chlorine, after 
the tube had been freshly filled, there was always a great difficulty in 
getting the air cleared of the dust-like nuclei, and the action of the radio- 
activity was to manufacture great numbers of these dust-like nuclei, as it 
took a great number of slow expansions to clear out the nuclei after a test 
had been made with some radioactive substance. If while there was 
chlorine in the test chamber it was exposed to the light of burning 
magnesium, it gave dense condensation if expanded while the light was 
still burning, but all effect was gone if allowed to stand half a minute 
before expanding. On the other hand, an incandescent gas flame seems to 
