produced in Gases by Eontgen Eay^ &c. 129 



gen produces nuclei which also require approximately the same expan- 

 sion, v. 2 li\ = 1-25, in order that condensation may take place on them. 

 If the expansion be made while the discharge is taking place no fog is 

 obtained with smaller expansions. The results are not so simple if the 

 expansion be made after the discharge has ceased, apparently on 

 .account of some secondary effect of the discharge causing the nuclei to 

 grow. No nuclei were produced unless a glow could be observed at 

 the point of the wire. 



Effect of an Electric Field. When air exposed to X-rays is enclosed 

 by two parallel plates, between which a sufficient difference of potential 

 is maintained, the fogs obtained on expansion are very much less dense 

 than in the absence of the electric field, and if the rays be turned off 

 before expansion all the nuclei are found to have been removed, whereas 

 without any electric field a fog is obtained even if the expansion be not 

 made till some seconds after the rays have been cut off'. This behaviour 

 of the nuclei proves them to be charged particles or "ions." The 

 nuclei produced by uranium rays behave in a similar manner ; those 

 produced by the action of ultra-violet light on moist air, or by the pre- 

 sence of metals, are entirely unaffected by an electric field. They are 

 therefore not ions but uncharged nuclei. (Since the nuclei produced 

 by the action of metals on air or by weak ultra-violet light require just 

 .as great a degree of supersaturation as those produced by X-rays to 

 cause water to condense on them, the difference in the behaviour of the 

 two classes of nuclei can scarcely be due to a difference in size.) The 

 nuclei which escape from a negatively electrified plate under the influ- 

 ence of ultra-violet light are of course charged. 



It follows from the experimental results described in this paper that 

 the passage of electricity through gases is effected by charged particles 

 which have an identical effect as condensation nuclei, whether the con- 

 duction is the result of exposure of the gas to X-rays or uranium rays, 

 or of the action of ultra-violet light on a negatively charged zinc plate, 

 or consists in the escape of electricity from a pointed platinum wire. 

 In all cases the degree of supersaturation required to make condensa- 

 tion take place on these particles is approximately fourfold.* 



The nuclei which are produced and grow (in air or oxygen) under 

 the action of ultra-violet light are uncharged, in their initial stages at 

 least ; they are therefore not electrified water drops. It is possible 

 that they are water drops containing in solution some substance, per- 

 haps H 9 0. 2 , produced within them by the action of the ultra-violet light 

 in quantities sufficient to counterbalance the effect of the curvature of 

 the surface upon the vapour pressure necessary for equilibrium. 

 * ' Phil. Trans.,' loc. cit. 



