PRESIDENTIAL ADDRESS, S.T.E. 1874. 235 



after the door is opened and the window closed, 

 the electrification of the air in your experimental 

 room continues negative, but it gradually becomes 

 zero, and a little later becomes positive. It 

 remains positive as long as you keep turning the 

 electric machine in the other room and the door 

 is open. If you stop turning the electric machine, 

 then, after a considerable time, the manifestation 

 changes once more to negative ; or if you shut 

 the door and open the window the manifestation 

 changes more rapidly to negative. It is, then, 

 proved beyond all doubt that the electricity which 

 comes in at the windows of an ordinary room in 

 town is ordinarily negative in fair weather. It is 

 not always negative, however. I have found it 

 positive on some days. In broken weather, rainy 

 weather, and so on, it is sometimes positive and 

 sometimes negative. 



Now, hitherto there is no proof of positive 

 electricity in the air at all in fine weather ; but 

 we have grounds for inferring that probably there 

 is positive electricity in the upper regions of the 

 air. To answer that question the direct manner 



