516 ELECTRICITY. 



7. Mechanical effects of electricity. 



Whenever a large charge of the electric fluid passes 

 through or among bodies of very little conducting pow- 

 er, it seems to exert no little mechanical force in prelbra- 

 ting, rupturing, or dispersing them. These experiments 

 may be performed in a variety of ways. 



Perforating paper. Let a card of ordinary thickness, 

 be laid upon the table, with a piece of tin foilunder it, 

 one end of which is connected with the outside of the 

 Leyden jar. Place one knob of the discharger, on the 

 top of the card, and bring the other into contact with 

 the knob or inside of the jar. In this way the spark, if 

 it passes at all. will pass through the paper, and on ex- 

 amining it, a small perforation will be found to be made, 



and if the charge is great several perforations. 

 There is something very singular in the appearance of 



these perforations. The paper is protruded on both sides, 



forming a sort of double bur, one looking down to- 

 wards the tinfoil, and the other up towards the knob of 

 the discharger. In former times when the controversy 

 between the supporters of the two theories, those of 

 Franklin and Du Faye, was going on, this experi- 

 ment was considered by many as very decisive proof of 

 the opposite motion of two fluids. It seems, however, 

 not probable that the protrusion of the paper is owing to 

 the mechanical impetus of the two fluids, but to the 

 very violent and sudden rarifaction of the air contain- 

 ed in the substance of the paper, which by the explo- 

 sive force thus given it, forces the parts of the paper out 

 each way. 



This experiment may be varied by using not a card, 

 but several sheets of paper, a quire, or twenty or thirty 

 leaves of a book. The effect will be precisely analogous. 

 The separate leaves will be protruded from the middle 

 sheet outwards, each way. 



Perforation of glass. If instead of a sheet of paper 

 a thin plate of gla.ss be used, it will be shivered to frag- 

 ments at the spot where the electricity passes, perhaps, 

 however, without falling to pieces. In one small s|K>t 

 the glass will be completely pulverized, and from that 

 spot cracks will radiate to a greater or less distance ac- 



