636 Mr. C. T. R. Wilson on 



formed are likely to be negatively charged ; that the drops, 

 formed in ionized air by expansions slightly exceeding that 

 required to cause condensation on negative ions, are actually 

 negatively charged has been proved by H. A. Wilson (Phil. 

 Mag. April 1903). Since, however, each drop will only 

 carry the very small ionic charge, the electrical effect will be 

 small if only a few large drops are formed ; if a large number 

 of negative ions serve as nuclei of condensation, the drops 

 will be small, and will only fall slowly relatively to the air ; 

 the resulting electric field cannot exceed that which drives 

 positive ions downwards as fast as the negatively charged 

 drops fall under the action of gravity. The field initially 

 produced may, however, be strong enough to induce coales- 

 cence of drops which come in contact (Lord Rayleigh, Roy. 

 Soc. Proc. vol. xxviii. p. 406), and we may thus get drops 

 carrying many times the charge of one ion, and large enough 

 to fall rapidly. Strong fields may then result. 



Again, we should expect (Nature, vol. lxii. p. 149) drops 

 falling through ionized air to become negatively charged as 

 a result of the difference in the mobility of the positive and 

 negative ions. This effect has in fact been experimentally 

 demonstrated by Schmauss {Ann. d. Phi/sik, vol. ix. p. 224). 



If collisions resulting in splashing occur between raindrops 

 (and they are likely to be frequent in the up-rush of air in 

 thunderstorms) positively charged rain may be formed. For, 

 as Lenard has shown, when splashing of pure water occurs, 

 as for example in waterfalls, the air in the neighbourhood 

 acquires a negative, the water a positive charge. 



Apart from the Lenai'd effect, the splashing resulting from 

 the collision of drops in an electric field may have large 

 effects, either in intensifying or diminishing the electric 

 fields already existing, the action being like that of an electro- 

 static influence- machine. The result would be to increase 

 the intensity of the field if the splashes were thrown out from 

 the lower portion of the combined drop. If, for example, 

 the field were such as to produce positive electrification on 

 the lower surface of a neutral drop, a droplet leaving the 

 lower surface would be positively charged, and being carried 

 upwards by the air relatively to the large drop, would add to 

 the intensity of the primary field/' 



The difficulty mentioned in the first paragraph of the 

 passage quoted is in itself sufficient to make untenable the 

 theory of thunderstorms which Mr. Simpson criticises. 

 Taking the example given by him and assuming a cloud 

 layer of continually increasing thickness to be formed, we 

 can readily see that unless larger drops are produced by 



