CONDENSATION NUCLEI. 201 



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

 tainiii<»" ions from :i zinc plate exposed to weak ultra-violet li<!:1it 

 lu'liaves on cxiJansion like aii- exposed to X i-ays, I'o^s l)ein<>' piotluceil 

 ill air initially saturated if the lower ex[)aiisi()ii limit rj i\=--\.)ii) Ik; 

 exceeded. The action of an electric (ield in removing- the nuclei is 

 the same in air ionized by Recquerel rays as in air ionized by X rays. 

 The ions produce<l by the dischar<»:e from a point are similar in their 

 action, but there is here a tendency, due probably to the products of 

 chemical combinations bi-ou^-ht about by the luminous discharge, for 

 the nuclei to izrow or for larger uncharged nuclei to be formed, so 

 that a much sum Her degree of supersaturation may be required to 

 produce a chuid. The ions produced by these various methods are 

 also identical in the velocity with which they move through air 

 under a given [xiteiitial gradient. The degree of supersaturation 

 reijuired to make water condense on the ions is independent of 

 the gas. 



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

 necting the maximum supersaturation with the charge of the drop, 

 we obtain the result ^^=0X10 — '" electrostatic units for a fourfold 

 supei-saturation. To obtain this number, we have, of course, ex- 

 tended to drops of almost molecular smallness, r=7'XlO — '' cm., an 

 equation which could only be used with confidence wdien the radius 

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

 renuu-kable that the value obtained approximates fairly closely to 

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

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

 iiowever, completely explained, for our fornnda would make effi- 

 ciency of the electrification in helping condensation independent of 

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

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

 ui)on them than do the positive. 



DIFFERENCE BETWEEN POSITIVE AND NEGATIVE IONS. 



To study this question we may use an expansion apparatus i)ro- 

 vided with a cloud chaml)er, in which the air under examination 

 is contained between tAvo hcrizontal plates kept at slightly difterent 

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

 plate is exposed to the action of X rays. A series of obsei-vations 

 are then made, in which the rays are cut off at a definite intei-val of 

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

 the downward-moving ions have had time to reach the lower plate 

 while only 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- 



