GASES BY THE ACTION OF RONTGEN RAYS AND OTHER AGENTS. 
445 
upper plate consisting of a sheet of aluminium, forming the roof of the cloud-vessel, 
the lower plate being formed by the upper surface of the water wliich fills the lower 
part of this. The ahiminimn plate was fixed l^y means of sealing wax. The thickness 
of the layer of air between the plates was 1'6 centim. By means of a battery of 
secondary cells, any difference of potential up to 240 volts could Ije maintained 
between the plates. The positive terminal (')f the battery was connected to the 
aluminium. An aperture at tlie side, closed by a quartz plate, fixed with sealing- 
wax, enabled a horizontal l:)eam of ultra-violet light from a zinc spark to enter the 
apparatus. The light did not impinge on tiie aluminium plate. The air could be 
exposed either to the ultra-violet light or to the Tlontgen rays from a focus-tube 
placed above the aluminium plate. 
Fig. 13. 
The first experiments were made with idtra-violet light weak enough to give no 
condensation with pressure fall less than about 15 centims. When a somewliat 
greater expansion was used { ‘P = 172 millims.), an equally dense fog was obtained, 
wliether the difference of potential between the plates was 240 volts or zero. The 
ultra-violet light in both cases was applied for 30 seconds, and the expansion was 
made before cutting ofi:' the light. In other experiments the expansion was not made 
till 3 seconds after cutting off the ultra-violet rays ; in tliese experiments the expan¬ 
sion was somewhat greater than before, p* being equal to 183 millims. The fogs 
obtained, when a difference of potential of 240 volts was maintained between the 
aluminium and the water during the exposure and till after the expansion had been 
made, were again indistinguishable in appearance from those obtained in the absence 
of any difference of potential. 
On exposing the air to Tlontgen rays, instead of idtra-violet light, the expansion 
being the same as before (gauge reading — 183), very dense fogs were obtained in the 
absence of electromotive force, while, when a difierence of potential of 240 volts was 
maintained between the metal and water surfaces, only a very slight fog appeared on 
expansion. An expansion of the same amount, made 3 seconds after the rays were 
cut off, gave a fog in the absence of any difference of potential, whereas, when the 
potential diiference amounted to 240 volts, no drops at all were produced, even when 
the expansion was brought about 2 seconds after cutting off the rays. In fact, with 
