106 Ol.iervat'ions on Mr. Donovan's Refiections, <^c. 



eqj/al dhirlhut'ion is gratuitously supposed, since glass is but 

 an imperfect non-conductor when not covered with some resinous 

 varnish. 



38. Mr. Donovan says (p. 345) : "As to the fact whether 

 glass is actually impermeable, many experiments have been 

 made; but they all appear to be of doubtful force, and may be 

 explained in some manner, without supposing that electricity 

 passes through." 



39. I have made in this respect some decisive experiments, 

 proving that when a glass rod transmits the electric fiziid, it is 

 only by its surface. I used for that purpose three glass rods of 

 the same glass and the same diameter — one remaining naked — 

 another covered all over with sealing-wax — 'the third covered 

 with sealing-wax, with only an interruption in the middle of its 

 length. These rods were supported horizontally on insulating 

 pillars, which left their extremities accessible to the knob of a 

 Leyden vial. I used three very sensible electrometers, each 

 having a long brass conductor in order to connect them with 

 the rods. 



Exp. 1. Having placed the naked rod on the pillars, with 

 an electrometer at each end and one in the middle, and applied 

 the Leyden vial to one extremity, th# electrometer near it first 

 diverged, then that in the middle, and seon that of the other 

 extremity, showing that some time was required for the propa- 

 gation of the electric fluid even on the naked surface of glass. 



Exp. 2. Having used the rod entirely covered with sealing- 

 wax, a small motion was produced in tiie next electrometer, 

 by the influence of the vial, but none in the remote electro- 

 meters. 



Exp. 3. The rod with an interruption of the sealing-wax in 

 the middle of its length was to be the test. U glass were permeahle, 

 the electrometer applied to that part ought to have been put in 

 niotion ; but if it moves only along the surface of the glass, be- 

 ing stopped by the sealing-wax, the electrometer applied to the 

 naked part in the middle cannot be put in motion. The result 

 of the experiment was, that, whatever time the Leyden vial re- 

 mained applied to that extremity, no motion was produced in 

 tiiat electrometer, any more than in that of the other extremity. 

 It is therefore demonstrated that glass is absolutely impermeahle 

 to the electric, fluid. 



40. A very specious objection of Mr. Donovan against Frank- 

 lin's doctrine is thus expressed, p. 349 : '* Franklin supposes 

 that no electricity can be received on one surface, unless the 

 other can part with an equal quantity. In the case of excitation 

 of the common cylinder, the inner surfacG having no communi- 

 cation 



