feme EleBrical Experiments. 55 
end of the tube, the outfide of the glafs, by the known 
property of charged electrics, muft be pofitive. The re- 
maining part of the tube being pofitive on its inner fur- 
face, muft be negative on the outfide. But as there is a 
vacuum within the tube, it may be afked, why is not the 
equilibrium between the negative electricity of the quick- - 
iilver and the pofitive electricity of the glafs inftantly 
balanced? 
When about two inches of each extremity of this tube 
are coated with tin-foil, that coating aflifts to make the 
electricities more confpicuous. 
With regard to the conftruCtion of fuch tubes (which I 
have made of feveral lengths from nine inches to two 
feet feven inches) I find that fome will aCt very well, 
while others will hardly acquire any electricity at all, 
even when they are made very hot. I am not as yet tho- 
roughly fatisfied with refpeCt to the caufe of this difference, 
but fufpeCt that the thicknefs of the glafs is more con- 
eerned in it than any thing elfe ; for I find that a tube 
whofe glafs is about one twentieth of an inch thick, 
anfwers better than either a thicker or a thinner one. 
November 13, 1776. 
