CONDENSATION NUCLEI. 201 



Air ionized by any of the various types of Beequerel rays or con- 

 taiiiiiio- ions from a zi-nc plate exposed to weak ulti-a-violet light 

 heiiaves on ex])ansion like air exposed to X rays, fo<>s being produced 

 in air initially saturated if the lower exi)ansion limit /"., 'rj=l.'ir) be 

 exceeded. The action of an eh-ctric field in remo\'inii' the nuclei is 

 the same in air ionized by P)ec(|uerel rays as in air ionized by X rays. 

 The ions produced by the discharge from a point are similar in their 

 action, but there is liere a tendency, due probably to tiie pi-ochicts of 

 chemical combinations l)rought about by the luminous discharge, for 

 the nuclei to grow or for larger uncharged nuclei to be formed, ^o 

 that a UHU-h smaUer degree of supersaturation may be re(iuired to 

 produce a cloud. The ions produced by these various metliods are 

 also identical in the velocity with which they move through air 

 under a given potential gradient. The degree of sujjersaturation 

 recjuired to make water condense on the ions is independent of 

 the gas. 



If we make use of the equation which has been given above, con- 

 necting the maxinnun supersaturation with the charge of the dro]), 

 we ol)tain the result (=()X10 — ^" electrostatic units for a fourfold 

 supersaturation. To obtain this number, we have, of coui'se, ex- 

 tended to drops of almost mok'cular smallness, y'=7XlO — "* cm., an 

 equation which could only l)e used with confidence when the radius 

 was at least a thousand times as great. It is therefore somewhat 

 remarkable that the value obtained a})]n'oximates fairly closely to 

 the values foiuid by J. J. Thomson and by H. A. Wilson for the 

 ionic charge. 'Jlie action of the ions as condensation nuclei is not, 

 however, completely explained, for our formula would make effi- 

 ciency of the electrification in heli)ing condensation independent of 

 the sign of the cliarge. Now, the negative ions are found to require 

 a less degree of supersaturation to make water form visible dro})s 

 upon them than do the positive. 



DIFFERENCE BETW'EEN POSITIVE AND NEGATIVE IONS. 



To study this question we may use an expansion apparatus pro- 

 vided wnth a cloud chamber, in which the air undei- examination 

 is contained between two horizontal plates kept at slightly diti'erent 

 potentials. A thin stratum of the air immediately over the lower 

 plate is exposed to the action of X rays. A sei-ies of ol)servations 

 are then made, in which the rays are cut off at a definite interval of 

 time before the expansion is made, the interval being such that all 

 the downward-moving ions have had time to reach the lower ])late 

 while onl}^ a small proportion of the upward-moving ones have 

 reached the much more distant upper plate l)efore the expansion 

 takes place. Thus at the moment of expansion we will have prac- 



