Electricity by the Action of Neat and Light. 453 



and 1 have not yet been able to make the experiment with 

 the quadrant electrometer. 



These experiments may perhaps furnish an explanation of 

 the negative charge o£ the earth. It appears to be due to 

 the direct action of sunshine. 



It has already been assumed that the negative charge of 

 the earth should produce by influence a positive charge in 

 the upper layers of the atmosphere. It has also been de- 

 finitely shown, that the normal electrical charge of the atmo- 

 sphere has a maximum in summer and a minimum in winter. 

 But the difference between summer and winter is generally 

 assumed to be due to the difference in the humidity of the 

 air in summer and winter ; and this assumption is confirmed 

 by recent observations carried out in different climates, and 

 observations on the daily change of atmospheric electricity 

 show also that it usually varies inversely as the temperature 

 and humidity of the air. The excitation of electricity by 

 friction is one of the oldest electrical phenomena known, but 

 the question as to which bodies are negatively and which 

 positively electrified has not yet been completely solved. 

 It is known that it depends partly on the nature of the 

 material and partly on the quality of the surface of a body, 

 and hence it is difficult to say how a body will behave in this 

 respect. The bodies which are electrified by friction have 

 often been arranged in so-called potential series, in which 

 each body should be negatively electrified by rubbing with 

 bodies standing before it in the series, and positively elec- 

 trified by rubbing with bodies coming after it. These series, 

 though agreeing with each other in their general arrange- 

 ment, differ greatly in detail. Hence it is evident that other 

 factors, such as the quality of the surface and the tempe- 

 rature of the body, have an influence on the phenomena. 



It was suggested to me by Prof. Wiener that the electri- 

 fication observed might perhaps be due to the friction of 

 air-currents, possibly carrying dust, produced by the heating- 

 effect of the sunshine ; and he further suggested that the 

 experiment should be tried with a glass plate placed over 

 the paraffin. On making the experiment, I found that the 

 electrification when exposed under a glass plate was consider- 

 ably smaller than before ; but afterwards, on separating the 

 glass and paraffin, I found that the glass plate was itself 

 positively charged, and that the negative charge on the paraffin 

 was as high as when exposed without the glass. The diminu- 

 tion in the effect shown by the electrometer was therefore due 

 to the condenser action of the two opposite charges. I also 

 made an experiment to find whether currents of air and dust 



