CONDUCTION THROUGH GASES 133 



clouds, illuminated by a beam of light from an 

 electric lantern. The nuclei in this case were 

 the ions produced by a piece of radium contained 

 in the tube seen to the right of the glass cloud- 

 chamber. The cloud has settled down to the 

 lower part of the hemispherical chamber, and its 

 sharply-defined upper surface is clearly visible. 

 The expansion is effected by the movement of a 

 piston within the vertical brass cylinder, the lower 

 part of which is put suddenly into communication 

 with the exhausted vessel seen lying on the table. 



In 1893, Professor Thomson had shown that, 

 in causing condensation, negative electrification 

 was more effective than positive, and Wilson, in 

 1899, further examined this point. He found 

 that, while negative ions produced condensation 

 of a cloud when the volume of the gas was in- 

 creased in the ratio of i : 1.28, positive ions did 

 not cause an equal effect till the expansion reached 

 1.3 1. It is possible that this difference may have 

 an important meteorological significance. If, as 

 there is reason to suppose, the atmosphere some- 

 times contains a considerable number of gaseous 

 ions, an expansion or fall of temperature would 

 result in the formation of drops of water round 

 the negative ions sooner than round the positive 

 ions. The negative ions thus would be removed 

 first, and the air would be left with an excess of 

 positive electrification. It is not unlikely that 

 the origin of the commonly observed potential 

 of the atmosphere, positive relative to that of 

 the earth, is, partially at any rate, to be found in 

 this selective withdrawal of the negative ions. 



If the ionization be not too intense, it is possible 

 to remove completely the ions from air by means 



