234 POPULAR LECTURES AND ADDRESSES. 



shut the door and open the window in your 

 observing room. Then, whatever electric oper- 

 ations you may have been performing, after a 

 short time you find indications of negative electri- 

 fication of the air. During all that time, let us 

 suppose that an electric machine has been turned 

 in the neighbouring room, and a spirit-lamp 

 burning on its prime conductor. Keep turning 

 the electric machine in the neighbouring room, 

 with the spirit-lamp as before. Make no other 

 difference but this shut the window and open 

 the door. I am supposing that there is a fire in 

 your experimenting room. When the window 

 was open and the door closed, the fire drew its 

 air from the window, and you got the air direct 

 from without. Now shut the window and open 

 the door into the next room, and gradually the 

 electric manifestation changes. And here some- 

 body may suggest that it is changed because of 

 the opening of .the door, and the inductive effect 

 from the passage. But I anticipate that criticism 

 by saying that my observation has told me that 

 the change takes place gradually. For a time 



