1895.] on Atmospheric Electricity. 473 



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 carries some of the electricity away, the water remaining posi- 

 tive. 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 im- 

 purities 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 ex- 

 planation 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 

 behaviour of air in the neighbourhood of waterfalls, and they pro- 

 bably also explain the negative electrification of air in the neighbour- 

 hood 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 towards Ferpecle, and I could observe 

 a cloud condensing below me at a height a little less than 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 mountain, 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 cannot 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 

 experiments showing that water rubbing against ice becomes nega- 

 tively electrified, 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 separated to produce an electric field of such 

 magnitude as we know must exist in a thunder cloud. 



