RAIN AND ITS ORIGIN IN THUNDERSTORMS. 411 



ELSTEE and GEITEL'S* observations, recorded in ' Terrestrial Magnetism and 

 Atmospheric Electricity,' give the same result. In the article referred to no tables 

 are given, but 17 figures show the results of the observations. In four of these 

 figures (figs. 2, 7, 10, and 17) we find " Platzregen" recorded, and in each case high 

 positive charges accompany the heavy rain. 



The only exceptions to this general rule are to be found in ELSTER and GEITEL'S! 

 first paper on the electricity of rain, published in the ' Wiener Berichte ' for 1890, in 

 which we find that the " Platzregen " is more often accompanied by negative than 

 positive electricity. In view of all the other observations, which show without 

 exception the positive charge of " Platzregen," it is difficult to explain this 

 discrepancy ; biit, accepting it in full, it may still be said that the evidence in favour 

 of the view that the excessive rain within a thunderstorm is positively charged is 

 overwhelming. 



Advancing a step further, it will be necessary to consider what happens to the 

 negative electricity which is separated when a drop breaks up. It is very probable 

 that this charge is given to the air in the form of free negative ions, and it appears 

 certain that these will, on formation, be carried upwards with the full velocity of 

 the ascending air ; they will then quickly leave behind the drops of water which 

 retain the positive electricity. 



But the negative charge cannot exist long as free ions, for the latter will be rapidly 

 absorbed by the cloud particles with which the air is filled. In this way the cloud 

 particles may become exceedingly highly electrified. Now within a highly electrified 

 cloud there must be rapid combination of the water drops, and from it considerable 

 rain will fall : this rain will be negatively charged, and, under suitable conditions, 

 both the charge on the rain and the rate of rainfall could be large. But it is 

 important to notice that the negatively charged rain has an entirely different origin 

 from that of the positively charged rain, and therefore the character of the rainfall 

 might be expected to be different in the two cases. It has already been shown that 

 the positively charged rain is likely to occur in heavy downpours in consequence of 

 its intimate connection with the ascending currents, but as the negatively charged 

 rain is formed in the large cloud masses, which are more or less uniformly charged 

 and extend over and around the ascending currents, the negatively charged rain is 

 likely to have a much more uniform rate of fall, and also to occur in the intervals 

 between the bursts of the positively charged rain. 



The observations bear out these considerations in a remarkable way. In storm 

 after storm it was found that negatively charged rain fell in the lulls after a heavy 

 downpour of positively charged rain. Negatively charged rain never occurred in 

 heavy downpour, but was very often associated with steady rain from a lightly clouded 

 sky. Also negative electricity was measured but rarely, and in large quantities 



* ELSTER and GEITEL, 'Terr. Mag. and Atm. Elect.,' vol. 4, pp. 13-32, 1899. 

 t ELSTER and GEITEL, ' Wien. Ber.,' vol. 99, p. 421, 1890. 



3 G 2 



