206 



like common air, excellent non-conductors,* and^ mixing 

 with the air of the room, give a preponderance of positive 

 influence to the testing insulated iiame (that is to say, 

 render the air potential positive at the place occupied by this 

 flame). 



Half an hour, or often much more, elapses after such 

 an operation, before the natural negatively electrified air 

 becomes again paramount in its influence on the testing 

 flame. 



That either positive or negative electricity may be carried, 

 even through narrow passages, by air, I have tested by 

 turning an electric machine, with a spirit lamp on its prime 

 conductor, for a short time in a room separated from the 

 lecture room by an oblique passage about two yards long, 

 and then stopping the machine and extinguishing the lamp, 

 so as to send a limited quantity of positive electricity into 

 the air of that room. When the lecture room window was 

 kept open, and the door leading to the adjoining room shut, 

 the testing spirit lamp showed the natural negative. When 

 the window was closed, and a small chink (an inch or less 

 wide) opened of the door, the indication quickly became 

 positive. If the door was then shut, and the window again 

 opened, the natural effect was slowly recovered. A current 

 of air, to feed the lecture room fire, was found entering by 

 either door or window when the other was shut. This 

 alternate positive and negative electric ventilation may be 



* I find that steam from a kettle boiling briskly on a common fire is an 

 excellent insidator. I allow it to blow for a quarter of an hour or more 

 against an insulated electrified conductor, without discovering that it has any 

 effect on the retention of the charge. The electricity of the steam itself, m 

 such circumstances, as is to be expected from Faraday's investigation, is not 

 considerable. Common au* loses nearly all its resisting power at some tem- 

 perature between that of boihng water and red hot iron, and conducts 

 continuously (not, as I believe, is generally supposed to be the case, by 

 disruption) as glass does, at some temperature below the boihng point, with 

 so groat case as to discharge any common insulated conductor almost com- 

 pletely in a few seconds. 



