CONDENSATION NHCLEI. 205 



T\\v nuclei I'oimd in IVi'slilv |)r('])ar((l leases, and studied esj)eeially 

 hy Townsend. resemble in many ways those result in*:; i'rom the oxida- 

 tion of phosphorus. Like thejn, they form clouds without supersatu- 

 ration. and they carry a char<i-e of electricity. In some cases, at least, 

 as was shown hy TownstMid's ex[)eriinents. the cliarii'e on each nucleus 

 IS the ionic char<>v. Bloch has studied the mobility of these ions, 

 and in agreement with Townsend has found it to be of tlie same order 

 as that of the phosphorus ions. 



By the splashing of water or a(|ueous solutions, or tlu^ bubbling of 

 air through water or solutions, nuclei are produced reipiiring only a 

 slight expansion in order that ^v^ater may condense uj)on them. These 

 miclei have lately been studied by Barus. He finds that the nuclei 

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

 arising from distilled water. It is most natural to regard the.se 

 nuclei, as does Bai'us," as small drops which ha\'e e\a[)orated till the 

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

 stance on the vapor pi-essure counterbalances that of the surface ten- 

 sion. The splashing or bubbling process also impai'ts temporary 

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

 tilled water the conduction is i)ractically unipolar and i]iw to the pres- 

 ence of negative ions Inning a mobility equal to that of tlie ions pro- 

 duced by X rays. AA'ith salt water positive ions of very small 

 mobility are produced in addition. 



In the |)roducts of coml)ustion from Hames we find again ions of 

 snuill mobility, and no ai)})reciable degree of supersaturation is re- 

 (|uired to produce a cloud. 



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

 have, first, ions like those j)roduced i)y X rays antl similar agents, 

 which have a d(>finite velocity in an electric field of given strength 

 and recjuire a dehnite 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 molulity,, about one- 

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

 power of condensing water to form visible drops without supersat- 

 uration. Ions with intermediate properties are i-arely, if ever, met 

 with. Bloch points out that we should expect an important difi'er- 

 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 op])ositely charged ions (which we 

 nuiy regard as minute chaigcd (h'o])s) combine, we shoidd expect the 

 resultant uncharged nucleus to evaporate at once. ( )n the other 

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

 the charge alone, and neutralization of the charire will not result in 



Barus, Tlie Structure of the Nucleus. 



