GASES BY THE ACTION OF EONTGEN EAYS AND OTHEE AGENTS. 
415 
Distance of Spark from Quartz Plate =42 ceritims. 
p = gauge reading 
(approx. = pressm-e fall). 
Duration of 
exposime. 
Interval after 
exposure. 
Eesult of expansion. 
(38) 148 millims. 
60 secs. 
0 
0 
(39) 152 ,-, 
60 „ 
0 
Slight shower 
Distance of Spark from Quartz Plate =21 centims. 
p = gauge reading 
(approx. = pressure fall). 
Duration of 
exposure. 
Interval after 
exposiire. 
Eesult of expansion. 
(40) 
128 millims. 
60 secs. 
0 
Very few drops 
(41) 
140 „ 
5 „ 
0 
0 
(42) 
140 „ 
60 „ 
0 
Few drops 
(43) 
155 „ 
3 „ 
0 
Shower 
The nuclei produced by the action of ultra-violet light differ in many ways from 
those produced by X-rays or Uranium rays. 
The expansion required to make water condense upon them depends on the strengtli 
of the radiation, and when this is strong, only a very slight expansion is necessary, as is 
shown in Experiments (l) and (29) in the above table. The smallest expansion of 
which the apparatus admitted was, in fact, sufficient to form a fog with strong- 
radiation, and indeed, in later experiments, fogs were obtained without any expansion. 
Under the action of the weakest rays used, however (Ex|)ts. 30-39), the expansion 
required to obtain condensation is as great {p — about 150 millims.) as that required 
to catch the nuclei produced by X-rays. 
The expansion required to make condensation take place on these nuclei depends 
on the time during which the apparatus has been exposed to the action of the rays 
before the expansion, being less the longer the exposure. The number which can be 
caught by a given expansion also increases with the time of exposure. (See Expts. 
16-21, 23-29.) The nuclei thus appear to grow under the action of the ultra-violet 
rays. The increase in the size of the nuclei, or in the numljer exceeding a given 
size, does not, however, continue indefinitely with increasing time of exposure, but 
after a time a steady state is reached, the result of a given expansion liecoming 
independent of the time of exposure, if this be long enough. (Expts. 3-7, 10-12.) 
The time for which the nuclei persist depends on the size to whicli they have 
attained. When the radiation is so weak that the nuclei are only caught if an 
expansion be made as great as would be required for X-ray nuclei, by far the larger 
number have disappeared in 30 seconds after cutting off tlie radiation. (Expts. 
