170 ELECTRICAL PROPERTIES OF SILK. 
of the silk is not merely to prevent the retwm of 
electricity from the cylinder to the cushion, but that 
it is the chief agent in the excitation, while the 
cushion serves only to supply the electricity, and 
perhaps increase the pressure at the entering part. 
There likewise seems to be little reason to doubt, 
but that the disposition of the electricity to escape 
from the surface of the cylinder, is not prevented 
by the interposition of the silk, but by a compen- 
sation after the manner of a charge ; the silk being 
then as strongly negative as the cylinder is positive ; 
and lastly, that the line of light between the silk 
and cushion in weak excitations, does not consist 
of returning electricity, but of electricity which 
passes to the cylinder, in consequence of its not 
having been sufficiently supplied during its contact 
with the rubbing surface. 
