CONDENSATION NUCLEI.^ 



I'.y (". T. 11. Wilson. V. II. S., 

 Felluw of Sitliirii Siisxc.r ('oUcijc, ('iiiiihri(h/c. I'ni/ldnd. 



If we take <lie ordinary air of a room and inclose it in a jjflass vessel 

 containini!; some water and j)rovided Avith some means of increasing 

 or diminishing the volume at will, we are ablc^ to observe the follow- 

 ing ])hen()niena : If tire air has been allowed to stand sufficiently long 

 to become saturated with water Aapor, any increase of volume, even 

 if very slight, causes the formation of a fog throughout the volume 

 of the moist air. This is easily made visible by concenti-ating a 

 powerful l>eam of light on the contents of the vessel; or, by placing 

 a small source of light behind the vessel, brilliant-colon^d rings or 

 coronas may l)e seen surrounding the source. If the air be made to 

 conti'act again to its original volume, a second expansion like the first 

 will again give a similar fog, but Avhen this ])i'ocess has been sevcM'al 

 times rei)eated the fogs become thinnei", the dro])s l)eing fewer and 

 larger; we get at length a tine rain on ex})ansion rather than a fog, 

 the dro])s falling to the bottom of the vessel within a fev,- seconds 

 instead of renuiining in sus])ension for many minutes like the first- 

 formed fog })articles. When this stage has been reached, the next 

 and all succeeding exj^fansions procbice no drops at all, no condensation 

 resulting elsewhere than on tlie walls of the vessel. If ordinary iiir 

 be now admitted into the vessel, dro])s will again l)e seen on expan- 

 sion, unless the air introduced has entered through a tightly pressed 

 plug of cotton wool, or has been otherwise filtered, in which case no 

 drops are seen. 



The phenomena are readily explained if we sup))()S(> that Avater can 

 not under ortUnary circumstances condense in the foi-m of drops 

 unless suitable nuclei ai-e present to serve as starting points for the 

 drops. These nuclei are present in very varying numlx'rs in ordinary 

 atmospheric air, from which they may be removed by filtering, or 



a A paper presented at the International Electrical Coiit^ress of St. Louis, 1!)(i4. 

 Reprinttd from author's revised copy. 



195 



