GASES BY THE ACTION OF RONTGEN RAYS AND OTHER AGENTS. 
429 
become charged with nuclei, on which condensation takes place when supersaturation 
is brought about by expansion. He was unable to detect any such effect when moist 
air was exposed in this way to the action of the sun’s rays. 
In experiments such as Aitken’s, in which the sunlight has to traverse glass, any 
ultra-violet rays which may be present are cut off before reaching the gas under 
investigation. Now the light from an arc lamp, when deprived of its ultra-violet 
rays by passing through glass, was found to have no cloud-producing effect. An 
apparatus provided vdth a quartz window was therefore used. 
This had the form shown in fig. 9. The quartz plate was fixed, as in the other 
Fig 9. 
expansion experiments in which a quartz window was required, with the help of an 
indiarubber washer. There was a joint on the horizontal j^art of the tube, made by 
means of an indiarubber stopper, as shown in the figure ; the connection between the 
expansion apparatus and the gauge also was made by means of an indiarubber tube, 
instead of glass tubing with the joints made by the blow-pipe, as in other experi¬ 
ments. The cloud-vessel could thus be placed so that the quartz plate was directly 
facing the sun. 
The presence of indiarubber vapour is doubtless a disadvantage, but the experi¬ 
ments with ultra-violet light from other sources make it highly improbable that any 
complications are thereby introduced. 
On account of the heating effect of the sunlight, accurate measurements of the 
expansion were not possible. 
The experiments were made during the month of August, between the hours of 
10 A.M. and noon. The apparatus was placed at an open window, which was closed 
when it was desired to make an experiment with the ultra-violet rays intercepted by 
glass. Even wlien a quartz lens was used to concentrate the sunlight no nuclei were 
produced which could be caught with slight expansions. Even with a pressure fall 
of 137 millims. no drops were seen under these conditions. 
With pressure falls exceeding 15 centims., showers or fogs were obtained in which 
the drops were plainly more numerous when no glass was interposed than when a 
glass screen was used to cut off the ultra-violet rays. 
