CH. x.] CONDUCTION 107 



rays : the space near the cathode represents the length 

 between the breech and the muzzle of the gun, and 

 the rest of the path is analogous to the trajectory 

 of a bullet. When the projectile strikes an atom the 

 shock may cause it to pass another on, and so 

 continue the convection. 



The fire-bucket method must be the method of 

 conduction in solids, where the atoms are not sus- 

 ceptible of locomotion and can only pass electrons 

 on from hand to hand ; oscillating a little in one 

 direction to receive them, and in another direction 

 to deliver them up, and so getting thrown gradually 

 into the state of vibration which we call heat. 

 But it may be observed that this need for motion, 

 in order to pass electrons on, becomes less and less 

 according as the body is less subject to the irregular 

 molecular disturbance we call heat. It may be the 

 expansion and molecular separation, or it may be 

 the irregular jostling and disturbance, that impede 

 easy conduction ; but certainly conduction improves 

 as temperature falls, and transmission becomes quite 

 easy at very low temperatures. The conduction of 

 heterogeneous alloys is a less simple matter, being 

 probably mixed up with back E.M.F. developed at 

 innumerable junctions, otherwise it would be in- 

 structive to examine the effect of low temperature 

 on the conductivity of a metal which did not 

 contract with cold. 



The extra conductivity of hot electrolytes is a 

 totally different phenomenon : it is not true conduc- 

 tion, but convective locomotion of ions, in their 

 case. The same effect of temperature which lessens 

 their viscosity increases their conductivity. 



Insulators are bodies where conduction can only be 

 accomplished with violence. Metals are bodies in 



