ATMOSPHERIC ELECTRICITY. 103 



ference of potential. The electrification of the drops is, therefore, not 

 wanted, and a change in the distribution takes place. The natural 

 supposition would be that this equilibrium would be restored very 

 quickly through the surface of the water, but a certain time seems to 

 be required for this. Meanwhile, the strong current of air which in 

 Lenard's experiments is brought down with the water drops carry some 

 of the electricity away, the water remaining positive. More recent 

 experiments of Lord Kelvin's, with air bubbling through water, point 

 similarly to contact forces between gases and liquids, and in these 

 experiments also it appears that a considerable time is required to 

 establish electric equilibrium between a gas and a solid. Lenard finds 

 very important differences caused by small impurities in the water, the 

 water acting much more strongly when it is pure. If it contains as 

 much salt as is contained in the sea, the effect is reversed, and the air 

 becomes positively electrified. The explanation which is given above 

 is practically that of Lenard, whose observations have been confirmed 

 and further extended by Prof. J. J. Thomson. These experiments, no 

 doubt, account for the behavior of air in the neighborhood of water- 

 falls, and they probably also explain the negative electrification of air 

 in the neighborhood of districts in which rain is falling. The strong 

 positive electrification of mist may also be due to the same cause. 



There seems to be no doubt that the formation of a cloud is often 

 accompanied by electrical effects. A few years ago, descending from 

 the Dent Blanche, I found myself, after sunset, at a height of about 

 12,000 feet. A current of air was apparently blowing up the valley 

 which stretches from Evolena toward Ferpecle, and I could observe a 

 cloud condensing below me at a height a little below the snow line. 

 As night came on and we continued our descent over the glacier and 

 down the valley, a series of electric discharges were noticed between 

 the cloud, which was lying in a deep-cut valley, the sides of the moun- 

 tain, and the blue sky overhead. Here the moist air was evidently 

 streaming through the cloud, depositing its moisture in the form of 

 drops, and it seemed the most natural explanation at the time that the 

 air left the cloud in an electrified state. 



But while by means of experiments we have been able to produce 

 some of the phenomena of atmospheric electricity, we have other 

 important effects which can not be accounted for in so simple a way. 

 The electric discharges during a thunderstorm give evidence of electric 

 fields which could hardly be explained by contact electricity between 

 drops of water and air alone. The fact that thunderstorms are nearly 

 always connected with the formation of hail, and Faraday's experi- 

 ments showing that water rubbing against ice becomes negatively elec- 

 trified, is made use of in the theories of Sohnke and Luvini. It is quite 

 likely that there is some truth in these theories. Their weak point lies 

 in the difficulty of seeing how particles of ice and water can be first 

 sufficiently mixed to allow of friction, and then become sufficiently 



