Professor Leslie on Electrical Theories . ® 
or of tinfoil, be advanced nearer to the conductor, and paral- 
lel to it ; as the edge conies gradually opposite to the metal- 
lic point, the light will be observed to vanish, and not to le-ap- 
pear till the other edge has completely emerged. To what cause, 
then, shall we ascribe this effect? It cannot surely be asserted, 
that the passage of the electric fluid is obstructed by the tinfoil, 
since metals are allowed to be the best of all conductors. But 
the appearan e is precisely the same, whatever be the natuie of 
the substances interposed ; and the only common property which 
they can have in this position, is that of intercepting the cur- 
rent of air. 
If the tinfoil be suspended near the conductor, and the point 
held close behind it, the lucid spangle, though greatly diminish- 
ed, may, in certain cases, be still discernible ; for, in conse- 
quence of the strong current of air between the conductor and 
the tinfoil, the latter becomes a second conductor. This objec- 
tion may be obviated by varying the experiment Thus, let a 
round hole, near an inch in diameter, be made in the middle of 
a sheet of paper which is kept insulated at the distance of half 
a foot from the prime conductor, and let the metallic point be 
held about half an inch behind the middle of the hole ; it will 
there continue totally dark. If the aperture be enlarged, the 
effect will be the same, only the point must be removed some- 
what farther back ; and if the aperture he increased to several 
inches diameter, or if the paper be brought very near to the 
conductor, the point will resume its lucid spangle. This expe- 
riment is so conclusive, that it scarcely needs any comment. 
The resistance which air meets with in passing through a nar- 
row aperture is very considerable ; and therefore a small part only 
of the aerial current, which diverges in all directions from the 
prime conductor, can flow through the hole in the paper. This 
experiment evinces, that not only the transmission of air is ne- 
cessary to produce the luminous appearance, but that, in order 
that the effect be sensible, there must also be a quick succession 
of particles. Hence whatever tends to detain the air which has 
already flowed upon a body, or retards the reflux of the stream, 
must diminish the electrical communication. Thus, let a point- 
ed wire be stuck through the middle of a round card of two or 
