Prof. Thomson on the Dynamical Theory of Heat. 221 



appear, experimental evidence in the special pheenomena of 

 thermo-electricity is quite necessary to prove it. Not only are 

 the conditions prescribed in the second law of the dynamical 

 theory not completely fulfilled, but the part of the agency which 

 does fulfil them is in all known circumstances of thermo-electric 

 currents excessively small in proportion to agency insepai'ably 

 accompanying it and essentially violating those conditions. 

 Thus, if the current be of the full strength which the thermal 

 electromotor alone can sustain against the resistance in its cir- 

 cuit, the whole mechanical energy of the thermo-electric action 

 is at once spent in generating heat in the conductor, — an essen- 

 tially irreversible process. The whole thermal agency imme- 

 diately concerned in the current, even in this case when the cur- 

 rent is at the strongest, is (from all we know of the magnitude 

 of the thermo-electric force and absorptions and evolutions of 

 heat) probably very small in comparison with the transference of 

 heat from hot to cold by ordinary conduction through the metal 

 of the circuit. It might be imagined, that by choosing, for the 

 circuit, materials which are good conductors of electricity and 

 bad conductors of heat, we might diminish indefinitely the effect 

 of conduction in comparison with the thermal effects of the cur- 

 rent; but unfortunately we have no such substance as a non- 

 conductor of heat. The metals which are the worst conductors 

 of heat are nearly in the same proportion the worst conductors 

 of electricity ; and all other substances appear to be compara- 

 tively very much worse conductors of electricity than of heat ; 

 stones, glass, dry wood, and so on, being, as compared with metals, 

 nearly perfect non-conductors of electricity, and yet possessing 

 very considerable conducting powers for heat. It is true we 

 may, as has been shown above, diminish without limit the waste 

 of energy by frictional generation of heat in the circuit, by using 

 an engine to do work and react against the thermal electromotive 

 force ; but, as we have also seen, this can only be done by keep- 

 ing the strength of the current very small compared with what 

 it would be if allowed to waste all the energy of the electromotive 

 force on the frictional generation of heat, and it therefore requires 

 a very slow use of the thermo-electric action. At the same time 

 it does not in any degree restrain the dissipation of energy by 

 conduction, which is always going on, and which will therefore 

 bear an even much greater proportion to the thermal agency 

 electrically spent than in the case in which the latter was sup- 

 posed to be unrestrained by the operation of the engine. By 

 far the greater part of the heat taken in at all, then, in any 

 thermo-electric arrangement is essentially dissipated, and there 

 would be no violation of the great natural law expressed in Car- 

 not's principle if the small part of the whole action, which is 



