TON DENS ATION NU('LKI. 205 



Tlu' nuclei round in frcslily i)r('i)ariHl g-ascs, iiiid studied especially 

 l»v Tow UMMid. reseuihle in many ways those resultinjjj from tlie oxida- 

 tion of phosphorus. Like theui. they forui clouds without supcrsatu- 

 ration. and they carry a char<^e of electricity. Ill some cases, at least, 

 as was shown by Townseiid's experiments, the charije on each nucleus 

 IS the ionic charii-e. liloch has studied the mobility of these ions, 

 and in aiireement with Townsend has found it to be of the same order 

 as that of the ph<)S])liorus ions. 



P)y the splashin<2: of water or aciueous solutions, or the bubbling of 

 air through water or solutions, nuclei are [)roduced requiring only a 

 slight expansion in order that water may condense upon them. These 

 nuclei have lately been studied by P)arus. lie Hnds that the nuclei 

 produced from salt solutions are much more i)ersistent than those 

 :. rising from distilled water. It is most natural to regard these 

 iniclei, as does Barus," as small drops which have evaporated till the 

 strength of the solution is such that the eliect of the dissolved sub- 

 stance on the vapor pressure counterbalances that of the surface ten- 

 sion. The splashing or bubbling process also imparts temporary 

 conducting power to the gas. According to Kaehler, with pure dis- 

 tilled water the conduction is practically unipolar and due to the pres- 

 ence of negative ions having a mobility equal to that of the ions i)ro- 

 diiced by X rays. With salt water positive ions of very small 

 mobility are produced in addition. 



In the products of combustion from flames we find again ions of 

 small mobility, and no appreciable degree of supersaturation is re- 

 quired to produce a cloud. 



As Bloch points out, there are a])))arently two classes of ions. We 

 have. Hrst, ions like those produced i)y X rays and similar agents, 

 whicli have a definite velocity in an electric field of given strength 

 ;iiid recjuire a definite degree of supersaturation — fourfold or sixfold, 

 according to the sign of the charge — in order that water may con- 

 dense upon them. 



The second class consists of ions of variable mobility.^ about one- 

 thousandth part of that of the ions of the hrst class, and they have the 

 power of condensing water to form visible drops without su})ersat- 

 uration. Ions with intermediate })r()perties are rarely, if ever, met 

 witii. Bloch points out that we should expect an important differ- 

 ence between the two classes with respect to the result of recombination 

 of positive and negative ions. In the first class the nucleus owes its 

 existence to the charge: if two oj^iiositely charged ions (which we 

 ma}^ regard as minute chargiMl drops) combine, we should expect tlie 

 resultant uncharged- nucleus to evaporate at once. On the other 

 hand, the persistence of the ions of the second class can not be due to 

 the charm" alone, and neutralization of the charge will not result in 



o Bar us, The Structure of tlie Nucleus. 



