CHAP, iv.] ELECTRICITY AND MAGNETISM. 237 



fested in air and other gases at a low pressure, in tubes 

 exhausted by an air pump. 



The discharge of a quantity of electricity in any of 

 the above ways is always accompanied by a transform- 

 ation of its energy into energy of some other kind, 

 sound, light, heat, chemical actions, and other pheno- 

 mena being produced. These effects must be treated in 

 detail. 



284. Mechanical Effects. Chief amongst the 

 mechanical effects of the disruptive spark discharge is 

 the shattering and piercing of glass and other insulators. 

 The dielectric strength of glass, though much greater 

 than that of air, is not infinitely great. A slab of glass 

 3 inches thick has been pierced by the discharge of a 

 powerful induction-coil. The so-called "toughened" 

 glass has a greater dielectric strength than ordinary 

 glass, and is more difficult to pierce. A sheet of glass 

 may be readily pierced by a spark from a large Leyden 

 jar or battery of jars, by taking the following precau- 

 tions : The glass to be pierced is laid upon a block of 

 glass or resin, through which a wire is led by a suitable 

 hole, one end of the wire being connected with the outer 

 coating of the jar, the other being cut off flush with the 

 surface. Upon the upper surface of the sheet of glass 

 that is to be pierced another wire is fixed upright, its 

 end being exactly opposite the lower wire, the other 

 extremity of this wire being armed with a metal knob to 

 receive the spark from the knob of the jar or discharger. 

 To ensure good insulation a few drops of paraffin oil, or 

 of olive oil, are placed upon the glass round the points 

 where the wires touch it. A piece of dry wood similarly 

 treated is split by a powerful spark. 



If a spark is led through a tightly corked glass tube 

 containing water, the tube will be shattered into small 

 pointed fragments by the sudden expansion of the 

 liquid. 



The mechanical action of the brush discharge at 



