GASES BY THE ACTION OF EONTGEN BAYS AND OTHEE AGENTS. 
427 
hydrogen was obtained, as in previous experiments, by heating a tube containing 
palladium saturated with the gas. 
In hydrogen prepared in this way nuclei were produced under the influence of the 
ultra-violet rays, but never in very large numbers, and always requiring great 
supersaturation to make water condense upon them, however long the exposure. On 
replacing the hydrogen by air, without making any other alteration in the apparatus, 
and again exposing to the rays, a fog was obtained without expansion in less than 1 
minute. 
Hydrogen exposed to Ultra-violet Rays of Arc. 
Gauge reading (in millinis.) 
(approx. = pressure fall). 
Eesult of expansion. 
137 
0 
147 
Sliglit shower 
167 
Dense shower 
167 (mica interposed) 
0 
In all the above observations the time of exposure was thirty seconds. 
Fresh hydrogen prepared. 
Gauge reading (in millims.) 
(approx. = pressure fall). 
Eesult of expansion. 
147 
142 (exposed for 60 seconds) 
150 
Very few drops 
0 
Dense shower 
The expansion required to make condensation take place upon these nuclei is, it 
will be seen, approximately the same as is required in the case of the nuclei produced 
by X-rays or Uranium rays. 
With CO., exposed to ultra-violet rays fogs were obtained with slight expansion 
and even without expansion, but stronger radiation was found to be necessary than in 
the case of air or oxygen. For these experiments an apparatus like that used for 
hydrogen was used. The 00-2 was prepared by heating potassium bicarbonate. 
The nuclei produced by the action of ultra-violet light on moist air, oxygen or 
carbonic acid, are thus seen to be capable of growing under the action of the rays till 
they actually become large enough to scatter ordinary light. At least in the case of 
air these visible fogs may persist for hours, although the air be not saturated with 
water vapour. Later experiments make it very improbable that the growth of these 
nuclei is due to each one becoming charged with electricity under the action of the 
ultra-violet light. The most obvious way of accounting for the growth of the nuclei 
3 I 2 
