GASES BY THE ACTION OF RONTGEN RAYS AND OTHER AGENTS. 
41U 
strike the glass. To ensure that there should be iio eflect throughout the length of 
the tube due to rays grazing the walls, a tinfoil diaphragm with an aperture of 
5 millims. was inserted in front of the quartz plate. The fogs still remained uniform 
from end to end. A want of uniformity in the density of the fog at once shows itself 
in the curvature of the upper surface of the fog, due to the more rapid settling where 
the di'ops are fewer and larger. In these experiments the time of exposure before 
exjDansion was from 3 to 5 seconds. 
It is easy to understand the great effect produced by a slight dimness of the quartz 
plate, for scattering of the ultra-violet rays by the small droplets on the plate is likely 
to take place to a much greater degree than that of the luminous rays. The rays 
Avhich escape scattering or deflection at the quartz may not be strong enough of them¬ 
selves to produce, in the time for which the exposure lasts, nuclei large enough to he 
caught with the degree of expansion used, while together with the scatteind i-ays they 
may be more than strong enough close to the quartz for this purpose. Now the 
scattered light (the quartz plate being small compared with the length of the tube) 
Avill fall off approximately inversely as the square of the distance from the quartz. It 
is thus readily understood why the fog extended only a short distance from the quartz 
when this Avas coAmred AAuth a deposit of dew. 
That the uniformity of the fog from end to end of the tube, Avhen the contents are 
actually exposed to equally intense radiation throughout, is not due to diffusion of 
the nuclei before expansion, or mixing of the air in the tube in consequence of the 
expansion, is certain. For, in the experiments in Avhich there Avas a deposit of deAv on 
the quartz, no fog Avas produced at the far end, even Avith an exposure of 60 seconds ; 
Avhile a very dense fog was obtained near the quartz Avith an equal expansion, Avith an 
exposure of only 10 seconds. Similar results Avere obtained in experiments in AAdiich 
the Cadmium points were displaced to one side, so that only a small portion of tlie 
tube near the quartz Avas exposed to tlie rays. The fog obtained on expansion only 
extended a short distance beyond the part of the tube reached by the rays. 
The experiments Avith this apparatus make the superficial origin of the nuclei 
very improbable; for, if they arose only Avhere the rays fell on a surface, the fogs 
Avould have been confined to the ends of the tube. To account otherAAUse for 
the fact that Avhenever a fog Avas produced (with the light passing axially) it Avas 
uniform from end to end of the tube, AAm AAmuld haA'e to supjAose that on account of 
undetected scattering of the ultra-violet rays at the ends, the AAmlls throughout the 
A\Iiole length of the tube received approximately uniform ultra-violet illumination. 
Perhaps the most striking proof that the nuclei produced by ultra-violet light are 
formed throughout the Amluine of the moist air, and not at the surface of the vessel 
containing it, is furnished by experiments Avith very strong radiation. As already 
stated in a preliminary note on the subject (‘ Camb. Phil. Soc. Proc.,’ vol. 9, p. 392), 
under the influence of Amry strong ultra-Auolet light fogs are produced AAdthout any 
expansion, eAmn in unsaturated air. The nuclei Avhich, A\hen they are only ex})osed 
3 H 2 
