310 Mr Vincent, On the Action of 



rubber bung through which tubes passed, which enabled the whole 

 to be employed as a cloud chamber. The tube having been 

 filled with saturated dust-free air, an aluminium spark was placed 

 at the ultra-violet light focus of the lens. A dense cloud was 

 formed throughout the chamber in a minute. The spark was 

 kept on for an hour but no change in the cloud could be observed 

 after the first 10 minutes. In this arrangement also the time 

 that any drop is in the light is limited. As soon as a drop has 

 fallen a centimetre or so in the tube the drops above it shield it 

 from the light. 



The light which produces these clouds is the light which 

 Lenard has investigated with his steam-jet. In fact the pro- 

 duction of clouds and the alteration in the appearance of the jet 

 are practically the same phenomenon. The light to produce 

 the clouds goes through quartz, but I could get no clouds when 

 the light was filtered through Iceland spar or through 5 cm. 

 of water. Iceland spar is opaque to light shorter than 2064 

 tenth metres, while quartz stops all light shorter than 1852 tenth 

 metres 1 , so that the effective light has a wave-length between 

 these limits. Lenard found the wave-length of the light pro- 

 ducing the steam-jet effect in air to be 1800 tenth-metres. 



The explanation of the formation of these clouds is still to 

 be found. The fact that their particles are charged some posi- 

 tively, some negatively and some not at all may be attributed 

 to the gas in which the cloud is formed being ionised by the 

 light which at the same time produces some chemical change 

 which originates the drops. The ions in the gas will get on the 

 drops and charge them. A similar thing happens with a cloud 

 produced by a large expansion on the ions produced by X-rays, if 

 the rays are kept on after the drops are formed 2 . 



But we need use no specific ionising agent and yet get 

 charged droplets. The ions always present in the air are sufficient 

 to charge some of the dust if this is present. On filling the cloud 

 chamber with ordinary unfiltered air and condensing water on 

 the dust by expansion a dense cloud is formed. The heavier 

 particles soon fall and a few lighter ones are left floating in 

 the chamber. These are seen as bright luminous points ; on 

 putting on a field some of these move up, others sink, while 

 a few remain stationary. Reversing the field changes the direction 

 of the motion. 



The effect of ultra-violet light on air unfreed from dust is 

 most marked. The cloud forms at once. A similar effect is 

 produced by letting ultra-violet light act on air in which the 



1 Gifford, B. A. Report, 1898. 



2 H. A. Wilson, Phil. Mag. April, 1903. 



