104 ATMOSPHERIC ELECTRICITY. 



separated to produce an electric field of such magnitude as we know 

 must exist in a thundercloud. 



It is to be remarked, however, that the laws of contact electricity 

 must be applicable to gases as well as to solids, and that if water 

 becomes positive when rubbing against air, and negative when rubbing 

 against ice, there must be a strong contact difference between ice and 

 air. In other words, it does not matter whether there is direct friction 

 between ice and water, or whether the air forms an intermediate body. 

 We may imagine air rising through a cloud containing drops of water 

 negatively electrified, and then passing through an ice cloud having 

 its negative electricity increased, thus leaving the ice and water par- 

 ticles at a difference of potential which may, by a fusion of the drops, 

 increase sufficiently to produce a lightning discharge. This seems to 

 me the most plausible theory which, in the present state of our knowl- 

 edge, can be formed. As regards the permanent negative charge of 

 the earth's surface, the time has not yet arrived for forming a definite 

 opinion. Although we know that the earth, once electrified, would 

 gradually lose its charge into the atmosphere, yet we can express no 

 opinion as to the rate at which the loss is going on. That loss may 

 be exceedingly slow, and consequently equilibrium might be attained 

 by a very small preponderance of negative electricity brought back to 

 its surface through some cause or other. Rain, as has already been 

 mentioned, is more frequently electrified negatively than positively in 

 our own climate, and though we do not know how far this holds in the 

 tropical belt, it is at any rate possible that the surface of the earth may 

 in this way alone make up for the loss. "We may also reasonably think 

 that Lenard's observation on salt water may account for the permanent 

 charge. Every wave that breaks into spray under the action of a 

 strong wind would leave the water negatively electrified, the air carry- 

 ing away the positive charge. It would be of great interest to possess 

 observations on atmospheric electricity on board ship while waves are 

 breaking in the neighborhood. So far we have only Exner's observa- 

 tions to guide us, who found, while observing at Lavinia, in Ceylon, 

 that the spray from breaking waves affected the indications of the 

 electrometer, proving its positive electrification (Wiener Akad. Sit- 

 zungsberichte, Yob XCVIII). 



But although the loss of electricity from the earth's surface may be 

 very slow, it is equally possible that it is considerable. We shall not 

 be able to treat this question satisfactorily until we have some clearer 

 notion of the causes of the aurora. We know that the aurora implies 

 electric currents, and the circuit of these currents may lie completely 

 within the earth's atmosphere, and have nothing to do with the observed 

 fall of potential near the ground. It is also possible that the body of 

 the earth forms part of the electric circuit, and if that is the case, 

 there must be across different parts of the surface an outward and 

 inward flow of positive electricity. Such a discharge could not fail to 



