600 PHILOSOPHICAL TRANSACTIONS. [aNNO 178g. 



ceeding from the edge of the cushion over its surface, and thence half round the 

 cylinder. The cylinder was then excited by applying an amalgamed leather in 

 . the usual manner. The electricity was received by a conductor, and passed off 

 in sparks to Lane's electrometer. By the frequency of these sparks, or Vjy the 

 number of turns required to cause spontaneous explosion of a jar, the strength 

 of the excitation was ascertained. — 2. The cushion was withdrawn about 1 inch 

 from the cylinder, and the excitation performed by the silk only. A stream of 

 fire was seen between the cushion and the silk; and much fewer sparks passed 

 between the balls of the electrometer. — 3. A roll of dry silk was interposed, to 

 prevent the stream from passing between the cushion and the silk. Verv few 

 sparks then appeared at the electrometer. — 4. A metallic rod, not insulated, was 

 then interposed, instead of the roll of silk, so as not to touch any part of the 

 apparatus. A dense stream of electricity appeared between the rod and the silk, 

 and the conductor gave many sparks. — 5. The knob of ajar being substituted 

 in the place of the metallic rod, it became charged negatively. — 6. The silk 

 alone, with a piece of tin foil applied behind it, afforded much electricity, though 

 less than when the cushion was applied with a light pressure. The hand, being 

 applied to the silk as a cushion, produced a degree of excitation seldom equalled 

 by any other cushion. — 7' The edge of the hand answered as well as the palm.— 

 8. When the excitation by a cushion was weak, a line of light appeared at the 

 anterior part of the cushion, and the silk was strongly disposed to receive elec- 

 tricity from any uninsulated conductor. These appearances did not obtain when 

 the excitation was by any means made very strong. — Q. A thick silk, or 2 or 

 more folds of silk, excited worse than a single very thin flap. He used the silk 

 called Persian. — 10. When the silk was separated from the cylinder, sparks 

 passed between them ; the silk was found to be in a weak negative, and the cy- 

 linder in a positive state. 



The foregoing experiments show, that the office of the silk is not merely to 

 prevent the return 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 seems 

 also 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 compensation 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. 



11. When the excitation was very strong in a cylinder newly mounted, flashes 

 of light were seen to fly across its inside, from the receiving surface to the sur- 



