206 CONDENSATION NUCLEI. 



evajioration of the nucleus. From recombination of these ions per- 

 sistent uncharged nuclei will result. The facts are found to be in com- 

 plete accord with these considerations. 



If we produce a cloud in dust-free air by an expansion exceeding 

 1.25 after exposure to X ra^i^s, or exceeding 1.38 in the absence of 

 ionizing agents, the drops formed, if made to evaporate by com- 

 |)ression, aj^pear to leave behind nuclei requiring only slight ex- 

 ])ansion to make water condense on them. J. J. Thomson has pointed 

 out that there may be a certain size for which even uncharged drops 

 of })ure water may be stable in an unsaturated atmosphere. For, 

 according to the experiments of Keinold and Kiicker, the surface 

 tension of thin films has a minimum for a certain thickness. There 

 may. therefore, be a certain size (somewhat smaller than that corre- 

 sponding to minimum surface tension) foi- which the potential energ}' 

 of a drop due to surface tension has a minimum value. Such a drop 

 would be in equilibrium in vapor saturated with respect to a flat 

 surface. 



Bloch. following Langevin, works out, in the paper already re- 

 ferred to, the theory of condensation of water vapor on ions. He 

 shows that we might expect drops of about 10 yuyu in diameter to be 

 stal)le, on account of the variation in surface tension in that region, 

 but we should not expect to meet Avith drops of Avhich the diameter 

 was com])rised between that limit and a very low value, the equilib- 

 riuui of such particles being unstable. The behavior of other sub- 

 stances than Avater would probably be similar. In this way Bloch 

 explains the fact that we do not meet w^ith ions of mobility inter- 

 mediate betAveen about 1 centimeter and one three-hundredths centi- 

 meter per second for a field of 1 volt per centimeter. 



There are then three principal classes of nuclei: (1) The ions 

 proper, requiring a foui'fold or sixfold supersaturation to cause water 

 to condense on them, and having a mobility exceeding 1 centimeter 

 per second in a field of 1 volt per centimeter; (2) loaded ions requir- 

 ing little or no sui)ersaturation to make water condense on them, and 

 liaving a mobility generally less than a thousandth part of that of the 

 ions proper; (3) uncharged nuclei, resembling the second class in 

 requiring little or no supersaturation in order that visible drops may 

 form upon them. 



[The accompanying illustrations — most of them from an article on 

 " Nucleation during cold weather," by Dr. Carl Barns, Avhich ap- 

 peared in the Physical Review for April, 1903 — are here appended to 

 shoAv in a grapliic manner the number of nuclei in atmospheric 

 air. Doctor Barus, \A'ho prepared these charts, has for several years 

 been conducting investigati(ms on this subject, under the auspices of 

 the Smithsonian Institution, which has published several of his 

 memoirs. — Editor.] 



