174 PHILOSOPHICAL TRANSACTIONS. » [aNNO 1787. 



Other, 2 slips of leaf gold are fastened with paste, gum-water, or varnish. These 

 slips, suspended by the peg, and that in the tube fast to the centre of the cap, 

 hang in the middle of the glass, about 3 inches long, and a quarter of an inch 

 broad. In one side of the cap there is a small tube, to place wires in. It is 

 evident, that without the glass the leaf gold would be so agitated by the least 

 motion of the air, that it would be useless ; and if the electricity should be 

 communicated to the surface of the glass, it would interfere with the repulsion 

 of the leaf gold ; therefore two long pieces of tin-foil are fastened with varnish 

 on opposite sides of the internal surface of the glass, where the leaf gold may 

 be expected to strike, and in connexion with the foot. The upper end of the 

 glass is covered and lined with sealing-wax as low as the outermost rim, to 

 make its insulation more perfect. 



The following experiments will show the sensibility of this instrument. 



1st. Powdered chalk was put into a pair of bellows, and blown on the cap, 

 which electrified it positively when the cap was about the distance of 6 inches 

 from the nozzle of the bellows ; but the same stream of powdered chalk electrifi- 

 ed it negatively at the distance of 3 feet. In this experiment there is a change of 

 electricity from positive to negative, by the dispersion or wider diffusion of the 

 powder in the air. It is also changed by placing a bunch of fine wire, silk, or 

 feathers, in the nozzle of the bellows, and is wholly negative when blown from 

 a pair of bellows without their iron pipe, so as to come out in a larger stream ; 

 this last experiment did not answer in dry weather so well as in wet. The posi- 

 tive electricity of the chalk, thus blown, is communicated because part of the 

 powder sticks to the cap ; but the negative is not communicated, the leaf gold 

 collapsing as soon as the cloud of chalk is dispersed. 



2dly, A piece of chalk drawn over a brush, or powdered chalk put into the 

 brush, and projected on the cap, electrifies it negatively ; but its electricity is 

 not communicated. 



3dly, Powdered chalk blown with the mouth or bellows from a metal plate 

 placed on the cap, electrifies it permanently positive. Or if the chalk is blown 

 from the plate, either insulated or not, so that the powder may pass over the 

 cap, if not too far off, it is also positive. Or if a brush is placed on the cap, 

 and a piece of chalk drawn over it, when the hand is withdrawn the leaf gold 

 gradually opens with positive electricity as the cloud of chalk disperses. 



4thly, Powdered chalk falling, from one plate to another, placed on the 

 instrument, electrifies it negatively. 



Other methods of producing electricity with chalk and other powders have 

 been tried ; as projecting chalk from a goose wing, chalking the edges of books 

 and clapping the book suddenly together, also sifting the powder on the cap ; 

 all which electrified it negatively : but the instrument being placed in a dusty 



