GASES BY THE ACTION OF EONTGEN RAYS AND OTHER AGENTS. 
409 
II. Oxygen exposed to X-rays. 
B = barometer reading = 767 millims. ; t = temperature = 21‘5° C. ; tt = maximum 
vapour pressure at C. = 19 millims. ; m = pressure required to sink 
plunger = 0. 
Focus bulb 3 centims. above aluminium window. 
Duration of exposure 
before expansion. 
Gauge reading 
(in millims.) = p. 
B — TT + 
Result of expansion. 
1 min. 
146 
1-243 
No drops 
1 
2 
152 
1-256 
Fog 
I „ 
149 
1-249 
Shower 
1 „ 
147 
1-245 
No drops 
10 secs. 
159 
1-271 
Dense fog- 
Least value V 2 IV 1 , with wbicb condensation was observed = t'249. 
The number of drops produced even with expansions exceeding any of those given 
in the table is exceedingly small in the absence of the rays. 
The minimum expansion required, in order that condensation in the form of drops 
may take place, is, it will be noticed, clearly defined ; the increase in the expansion 
corresponding to a cliange in the result from entire absence of drops to dense fog- 
being very small. It is also independent of the strength of the radiation, as is seen 
from the identity of the results here given with those previously obtained with weak 
rays (‘Phil. Trans.,’ loc. cit.). The increase of the density of the fogs with increasing- 
expansion continues till v^jv^ is about 1'31 ; beyond that point, as far as can he 
judged from the appearance of the fogs, the increase in the nuniher of the di-ops is 
slight, till the second limit t’a/'Ci = ^ is readied, beyond which the region of dense 
fogs, due to great supersaturation alone, is entered. Thus expansions exceeding 
v-y/'Vi = f‘31 appear to be sufficient to catch nearly all the nuclei produced by 
the rays. 
Prolonged exposure to the rays does not cause the nuclei to grow larger (or become 
otherwise more effective in helping the condensation) than tlie limit corresponding to 
the expansion ih/vi = 1'25. The oliservations given in Table II. show this, an 
exposure of 10 seconds producing- nuclei enough to give a dense fog with Vo/v^ = f-271, 
while even after 1 minute n<Dt one nucleus has grown sufficiently to lie caught by an 
expansion v-z/oi = L245. 
The nuclei introduced by the X-rays, as has already been pointed out (‘ Phil. 
Trans.,’ loc. cit.), rapidly diminish in number after the radiation has been cut offi, but 
several seconds are i-equired for their complete disappearance ; thus an expansion made 
5 seconds after switching off the current from the induction coil will give a shower 
very much denser than would have resulted had there been no exposure of the gas 
VOL. cxcii.—A. 3 G 
