44 



ELECTRICITY. 



the opposite sides of the card, but at the 

 same time half an inch distant from 

 each other; when the discharge is 

 made between the wires, and along 

 the surface of the card, the latter is 

 found to be perforated, but always at 

 the point where the wire communicating 

 with the negative side of the battery had 

 touched it. The same perforation takes 

 place at this point, even when a hole 

 has been previously made at the point, 

 where it is touched by the positive wire. 



The course of the electric fluid may 

 be traced with more precision, by hav- 

 ing both sides of the card coloured, 

 previously to the experiment, with ver- 

 milion, for it will then leave on the 

 card a well defined black line extending 

 from the point of the positive wire to 

 the perforation ; and a diffused black 

 mark on the opposite side of the card, 

 around the perforation, and next to the 

 negative wire. 



(160.) When the electrical discharge 

 is made to pass in a perpendicular di- 

 rection through the thickness of a card, 

 which may be effected by placing it 

 against the outer coating of a Leyden 

 jar, and setting the lower ball of the 

 discharging rod against the other side 

 of the card, so that its thickness may be 

 interposed between it and the tin-foil, 

 and making the explosion in the usual 

 way, as represented in Jig. 33, ( 134,) 

 the card will be perforated. At the edge 

 of the perforation, on each side of the 

 card, there will be a small bur or pro- 

 trusion, which is always larger on the 

 side next to the jar, than on that next to 

 the discharging rod ; the former being 

 the negative, and the latter the positive 

 side. By passing the shock through a 

 quire of paper, instead of a single card, 

 the progress of this effect at different 

 depths from the surface may be accu- 

 rately analysed. Mr. Symmer, who de- 

 vised this experiment, observed that the 

 ragged edges were for the most part 

 directed outwards from the body of the 

 quire. Upon examining the leaves se- 

 parately, however, he found that the 

 edges of the holes were bent regularly 

 two different ways, and more remarka- 

 bly so about the middle of the quire ; 

 one edge of each hole being throughout 

 its course forced one way, and the other 

 edge in the contrary direction, as if the 

 hole had been made in the paper by 

 drawing two threads through it in oppo- 

 site directions. 



(161.) r i he following variation of the 

 experiment illustrates the nature of the 



mechanical impressions made by elec- 

 tricity. Let a sheet of tin-foil be placed 

 in the middle of a quire of paper ; ori 

 making the discharge through it, the 

 tin-foil is found to have received two 

 indentations in opposite directions, and 

 the leaves of paper are rent in such a man- 

 ner, that on both sides of the tin-foil the 

 burs point towards the outsides of the 

 quire ; but the indentations upon the 

 tin -foil, and the burs on the paper, are in 

 opposite directions. If another quire of 

 paper be taken, and two sheets of tin- 

 foil be placed within it, so that they are 

 separated by the two middle leaves of 

 the quire, the result will be that all the 

 leaves will be perforated, excepting the 

 two within the tin-foil, and in these two 

 leaves there will be two impressions or 

 indentations in opposite directions. 



(162.) The mechanical effects we have 

 just described have been often adduced, 

 not only as proofs of the materiality of 

 the electric fluid, but also as positive 

 indications of the direction of its motions, 

 according as either the one or the other 

 of the two theories of electricity is 

 adopted. But this is a subject which we 

 reserve for future discussion. 



(163.) The fracture of glass by the 

 electrical explosion has already been 

 adverted to, ($ 129 ;) but there are still 

 a few circumstances attending it which 

 deserve to be noticed. The edges of the 

 fractured portion appear well defined on 

 the positive side ; while on the negative 

 side they are splintered, as might be 

 expected from the passage of a material 

 agent from the former to the latter. It 

 is remarkable also, that a perforation 

 may be made in glass by a very mode- 

 rate discharge, when the glass is in con- 

 tact with oil or sealing-wax. Thus if 

 a small phial, or glass tube, closed at 

 one end, be filled with olive oil, and a 

 pointed wire, bent at right angles, and 

 passing through a cork fitted to the 

 mouth of the phial or tube, be intro- 

 duced into it, so that the point may touch 

 any part of its inside beneath the sur- 

 face of the oil ; on suspending the ves- 

 sel by its wire to the prime conductor of 

 an electrical machine, and applying to 

 the outside, either the knuckle, or a 

 brass ball, exactly opposite to the point 

 of the wire within, so that a spark may 

 pass between them, it will be found to 

 have made a small perforation through 

 the glass ; by bringing the wire in con- 

 tact with different parts of the glass, a 

 great number of holes may thus be 

 made in it. The effect of the oil appears 



