200 CONDENSATION NUCLEI. 



which give rise to the rainlike eondensation and which are at any 

 moment present in quite small numbers are, as we shall see, ions con- 

 tinually beino {produced in the gas. They can be removed by an elec- 

 tric field. The cloudlike ccmdensation occurring with large expan- 

 sions is entirely unaffected by an electric field; it is independent also 

 of the nature of the gas. If we calculate how large a drop of Avater 

 would require to be in order that it should just be able to grow in 

 vapor of eightfold supersaturation, we obtain the very small valne 

 t).4 by 10 — ^ cm. for the radius of such drops. Thus drops not large in 

 comparison with molecular dimensions might be expected to grow 

 into visible drops in an atmosphere supersaturated to this extent. 



TIIK IONS AS CONDENSATION NITCLKI. 



. If we expose the cloud chauii)er of an expansion aj)j)aratus to the 

 action of Roentgen rays, the air having been previously fi-ced from 

 dust, just th(^ same expansion is required as in the absence of the 

 rays to produce drops, but now we get comparatixely dens* fogs 

 in place of the rainlike condensation. The cloudlike condensation 

 obtained with ex])ansions exceeding (he second limit is not sensibly 

 affected. Thus, when X rays ])ass through moist air they produce 

 nuclei of exactly the same efficiency in |)i-om<)tiiig condensation as 

 those wdiich are always l)eing produced in small numi)ers and to 

 which the rainlike condensation is (Uh\ The conducting power 

 imparted to air by the action of X rays being explaine(l as due to 

 the setting free of ions in the gas, it was natui'al (o identify the 

 nuclei with the ions. 



This view was verified by studying the action of an electric field 

 on the nuclei produced by X rays, l^etween two jjarallel plates, 

 which formed the top and bottom of the cloud chambei- of an 

 expansion appaiatus, a difference of potential of sonu^ hundred volts 

 could be ai)plie(l. With the electric field acting, the number of drops 

 produced on expansion in air exposed to the rays was exceedingly 

 small in comparison with the nuuibei- seen in the absence of the field. 

 The nuclei carry a charge of electricity, and are driven by the electric 

 field against the ])lates innnediately after being set free. The direct 

 [u-oof that the few nuclei, which are always present and give rise 

 to the rainlike condensation, are also ions has been more difficult to 

 carry out. Attempts made with small apparatus led to negative 

 results, the nunibei- of drops being inconveniently small whether the 

 field was apj^iied or not. Recent experiments on a large scale, how- 

 ever, showed in a striking way the removal of these nuclei by the 

 electric field. The subject has been further cleared up by the inde- 

 pendent proof l)y purely electrical measurements that the air in a 

 closed vessel is continually being ionized. 



