640 On Thunderstorm Electricity. 



surface, and a super-saturated layer will begin to form 

 above it. 



Although I have never observed the peculiar formation 

 described by Mr. Simpson as being required for thunder- 

 clouds according to the theory he is criticising, I see no 

 reason on that account to conclude that condensation on 

 negative ions may not take place in the atmosphere. If 

 the supersaturated layer be thin, the condensation on the 

 negative ions will not in itself be at all a conspicuous phe- 

 nomenon although its effects may be by no means unimportant. 

 It is possible, however, that more striking phenomena may 

 also be the result of condensation on the negative ions. In a 

 typical fully developed cumulo-nimbus cloud, while the lower 

 part of the cloud has the appearance of ordinary cumulus,, 

 the upper part has an entirely different appearance, of which 

 there are several different varieties. In some cases the 

 appearance is suggestive of rain or snow falling from some 

 height above the lower cumuliform portion of the cloud. It 

 is possible that this upper portion of the cumulo-nimbus (the 

 false cirrus) may be due to condensation on the negative ions. 

 Until more has been done by experiment and observation to 

 throw light on the processes going on in clouds, it hardly 

 seems profitable to consider in detail the possible cloud forms 

 that might be expected to result from condensation on 

 negative ions. 



There is one statement in Mr. Simpson's paper which, 

 while not seriously affecting his argument, appears to call for 

 remark. In discussing the effect of a lightning-flash he 

 says : — " Now Barus has shown that whenever ions are 

 formed in dust-free air there is produced at the same time 

 a large number of nuclei which exist for a long time after 

 the ions have disappeared. As water- vapour is very readily 

 deposited on the nuclei, fourfold supersaturation is prevented 

 in air in which they are present unless the supersaturation 

 is caused by exceedingly rapid rarefaction." It is quite true 

 that spark-discharges in air, as well as other ionizing pro- 

 cesses which are accompanied by chemical effects, do produce 

 large and persistent nuclei in addition to the ordinary ions ; 

 a lightning-flash will therefore almost certainly produce such 

 nuclei. But the ordinary ionizing radiations when of moderate 

 intensity do not produce, in uncontaminated air, nuclei more 

 efficient than the negative ions. 



I shall conclude with the following three statements ex- 

 pressing summarily my views on the questions raised by 

 Mr. Simpson. 



1. I have never regarded condensation on the ions as in 



