200 CONDENSATION NUCLEI. 



which give rise to the rainlike condensation and ^^hich are at any 

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

 tinually being produced in the gas. They can be reiiioved by an elec- 

 tric field. The cloud like condensation 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 water 

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

 vapor of eightfold supersaturation, we obtain the very small value 

 (>.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. 



THE IONS AS CONDENSATION NUCLEI, 



If we expose the cloud chamber of an expansion apparatus to the 

 action of Roentgen rays, the air having been previously freed from 

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

 rays to produce drops, but now we get comparatively dens? fogs 

 in place of the rainlike condensation. The cloudlike condensation 

 obtained with expansions exceeding the second limit is not sensibly 

 affected. Thus, when X rays pass through moist air they produce 

 nuclei of exactly the same efficiency in promoting condensation as 

 those which are always being produced in small numbers and to 

 which the rainlike condensation is due. The conducting power 

 imparted to air by the action of X rays being explained as due to 

 the setting free of ions in the gas, it Avas natural to 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. Between two parallel plates, 

 which formed the top and bottom of the cloud chamber of an 

 expansion ap])aratus, a difference of potential of some hundred volts 

 could be applied. 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 number seen in the absence of the field. 

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

 field against the plates innnediately after being set free. The direct 

 proof 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 number of drops being inconveniently small whether the 

 field was ai)plied or not. Recent experiments on a large scale, how- 

 ever, showed in a striking way the removal ol' these nuclei hy the 

 electric field. The sul)ject has ))een furtlier cleared up by the inde- 

 pendent proof by i)urely electrical measurements that the air in a 

 closed vessel is continually being ionized. 



