Obfervaiions in Ele&ricity . 91 
of thick plate glafs, and that of a cylinder of -ivory three- 
quarters of an inch diameter, preffed by a weight of 
about fix or eight ounces Troy ; the glafs is always lhi- 
vered into very fmall fragments, and part of it is fome- 
times reduced to an impalpable powder. But ythly, If the 
plate of glafs be covered with a Hip of writing-paper, 
painted with lamp-black and oil, or with a flip of oiled 
filk (fuch as is frequently ufed for garments) the charge 
pafles over thefe fubftances without leaving the fmalleft 
trace on either of them, though the glafs under them be 
broken by the blow of the explofion. 
From thefe experiments, and the obfervations above 
recited, I think the following corollaries may be deduced, 
1 ft, That a charge of .electricity, or a ftroke of lightning, 
which is the fame thing, paffes, in many cafes, upon the 
furface of bodies, in a much larger proportion than 
through the interior fubftance of them, as appears by the 
malls of fhips, coated with lamp-black, &c,. 'T andhy the 
experiment above recited, with the cylinder of ivory and 
the glafs See. ; for in this experiment, the charge being re- 
filled by the ivory (which however is fometimes fplit by 
the explofion) forces a paffage between that and the 
glafs, and being there confined by heavy weights, exert? 
(c) See a curious inFuuice of this kind in M. adanson’s Voyage to 
Senegal, ^p. 239. 
its 
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