474 Professor Arthur Schuster [Feb. 22, 



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 rub- 

 bing 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 particles at a difference of potential which 

 may, by a fusion of the drops, increase sufficiently to produce a 

 lightning discharge. This seems to the writer the most plausible 

 theory which, in the present state of our knowledge, 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 carrying 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 neighbourhood. So far we have 

 only Exner's observations 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.* 



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 influence the phenomena we have discussed, 

 and it seems probable that we should have some evidence derived 



* ' Wiener Akad. Sitzungsberichte,' vol. xcviii. 



