24 NOMOS. 



glass of the Ley den jar and the conducting powers of 

 the discharging rod should be in any way connected ; 

 but so it is. This is seen in many ways. Water, 

 for example, is a conductor ; ice is an insulator. If 

 a plate of ice be coated on both sides with tinfoil, 

 and one coating be connected with an electrical 

 machine and the other with the earth, it may be 

 charged like the Leyden jar, and the charge remains 

 so long as the ice continues to insulate the two 

 coatings ; but as the ice melts away the insulation 

 fails, and as it fails the electricities in the two 

 coatings unite, not by discharge, but by conduction, 

 for the water into which the ice melts is a con- 

 ductor. As the ice melts away, the distinctions 

 between conduction, discharge, and insulation seem 

 to melt away also. The non-conducting properties 

 of ice and the conducting properties of water are 

 also paralleled in other substances. Solid sulphuret 

 of silver is an insulator ; fused sulphuret of silver is 

 a conductor. Solid fluoride of lead is an insulator ; 

 fused fluoride of lead is a conductor. And so also 

 with several other substances. Insulation, more- 

 over, is never absolutely perfect ; and for this reason 

 a charged Leyden jar speedily becomes uncharged 

 when left to itself. How is this ? Is it that insu- 

 lation is only slow conduction ? Again, conduction, 

 even in the best conductors, is not instantaneous : 

 time is required. Is, therefore, the difference be- 

 tween conduction and insulation a mere difference of 

 time ? That conduction is not instantaneous is seen 

 in a beautiful experiment of Mr. Wheatstone's, in 

 which a Leyden battery is discharged through a 



