t 46 1 ] 
P. S. Having formerly obferved, that the fridtion be- 
tween Mercury and Glafs in vacuo , would not 
only produce the light of eledlricity, as in the 
luminous barometer, or within an evacuated 
glafs ball, but would alfo eledtrify the glafs on 
the outfide ; I im merged a piece of dry Glafs 
in a balm of Mercury, and found, that by 
taking it out, the Mercury was electrified minus, 
and the Glafs eledtrified plus , to a confiderable 
degree. I found alfo, that Amber, Sealing- 
wax, and Illand Cryltal, when taken out of 
Mercury, were all eledtrified pofitively (3). 
How does it then appear, that the eledlricity, 
which was obferved upon rubbing the laft 
mentioned fubftance, after it was taken out of 
Mercury furrounded by Ice, was owing to cold, 
and not to the fridlion between it and the Mer- 
cury, in taking it out ? Illand Cryltal, when 
warm, is a non-condudtor ; and all non-con- 
dudtors may be excited with proper rubbers. 
(3) A fmall quantity of an amalgama , or mixture, of Mer- 
cury and Tin, with a very little Chalk or Whiting, being 
rubbed on the cufhion of a globe, or on the oiled filk-rubber of 
a tube, will excite the globe or tube to a great degree, with 
very little fridlion ; efpecially if the rubbers be made more damp,, 
tr dry, as occafion may require. 
LXXII. An 
