Theory of Thunderstorm Electricity. 623 



the air which enters any given cross-section at the base of 

 the cloud will rise vertically and leave the region under 

 discussion through a similar area vertically over that through 

 which it entered. 



We will now consider the electrical condition of the air at 

 the different stages of its ascent. Before entering the cloud 

 the air may be supposed to contain the mean number of ions 

 per cubic metre found in air under normal conditions. The 

 air enters the cloud with this amount of free electricity, and 

 commences its upward journey through the cloud-particles. 

 We have already agreed that a cloud acts as a filter for dust- 

 particles, and it will be quite as efficient a filter for ions ; in 

 fact, direct observations have shown that within a cloud (foo-) 

 practically no free ions can be measured. Thus the air which 

 enters the cloud with a large number of free ions will pass 

 out at the top with none, and will commence to travel throuoh 

 the supersaturated stratum with neither dust nor ions. On 

 account of what is generally called natural ionization new 

 ions will at once be formed, and as now there are no water- 

 particles to catch them they will continue to exist as free 

 ions. But the rate of formation is not rapid. Measurements 

 have been made of the rate at which new ions are beino- 

 formed in the atmosphere near the earth, and numbers 

 varying between 10 and 50 ions per c.c. per second found *. 



In the case under consideration it is not likely that the 

 ions would be formed at anything like the rate that thev are 

 formed near the earth, if for no other reason than that the 

 radioactive particles which are supposed to be the cause of a 

 great deal of the natural ionization of the air will also have 

 been filtered out in the cloud. Grerdienf has estimated that 

 the rate at which ions form in the region which we are con- 

 sidering is about 10 of each kind per c.c. per second. As, 

 however, I do not wish to err on the side of underestimation, 

 I will assume that the ions are generated at the rate of 2o 

 per c.c. per second. 



Now consider an area of 1 sq. cm. at any position on tne 

 upper surface of the lower cloud ; the air which passes out 

 of this area will rise vertically and enter the condensation 

 layer through another area of 1 sq. cm. Let us assume that 

 every ion which forms in a cubic cm. of air while passino- 

 from the lower cloud to the condensation layer above con- 

 tinues to exist as a free ion ; i. e. we will entirely neo-Iect 

 the recombination of ions : and let us further assume that 



* Schuster, Proc. Mean. Lit. and Phil. Soc. vol. xlviii. part ii p 1 

 (1904). ' r ' 



t Gerdien, I, c. p. 662. 



