120 Generating Economic Cycles 



fonnation. Nuclei are essential, but they may be sup- 

 plied, and are supplied, not only by dust particles but 

 by the gaseous ions that result from the action of the 

 many kinds of rays which have already been described. 

 Indeed the objective existence of the gaseous ions was 

 disputed by many physicists until their reality was 

 placed beyond doubt by experiments in which pre- 

 cisely their capacity to act as nuclei for condensation 

 of water vapor was employed to make their presence 

 visible. C. T. R. Wilson, who devised the experiments, 

 succeeded in obtaining actual photographs of rain- 

 droplets formed about the ions along the tracks of the 

 various types of rays discharging in vapor-saturated air. 

 Wilson's experunents on the ionization of gases and 

 the condensation of water vapor were reported in 1911 

 and 1912. It was in the line of progress from his early 

 conclusions to turn his attention to the problem of the 

 relation of rainfall to electrical variations in the at- 

 mosphere. Accordingly we find this statement in the 

 Annual Report of the University of Cambridge for 1919, 

 in the section referring to the work of C. T. R. Wilson 

 at the Solar Physics Observatory: ^ 



''The results obtained in these investigations have 

 suggested a theory which both accounts for many 

 of the more important phenomena of thunderstonns, 

 and relates them to those of fine weather atmos- 

 pheric electricity and terrestrial magnetism." - 



^Quoted by Bauer: "Some of the Chief Problems in Terrestrial 

 Magnetism and Electricity," Proceedings in the National Academy of 

 Sciences, 1920, vol. 6, pp. 575-576. 



2 There is an ample account of Wilson's researches upon this topic 

 in a paper, "Investigations on Lightning Discharges and on the Electric 



