142 PROFESSOR W. THOMSON ON THE 



heat. Hence there must be on the whole absorption of heat caused by the current 

 passing from cold to hot in copper, and from hot to cold in iron. When a current 

 is forced through the circuit against the thermo-electric force, the same reasoning 

 establishes an evolution of heat to an amount equivalent to the sum of the heat 

 that would be then taken in at the cold junction, and the value in heat of the 

 energy spent by the agency (chemical, or of any other kind) by which the electro- 

 motive force is applied. The aggregate reversible thermal effect, thus demon- 

 strated to exist in the unequally-heated portions of the two metals, might be 

 produced in one of the metals alone, or (as appears more natural to suppose) it 

 may be the sum or difference of effects experienced by each. Adopting, as a matter 

 of form, the latter supposition, without excluding the former possibility, we may 

 assert that either there is absorption of heat by the current passing from hot to 

 cold in the copper, and evolution to a less extent, in the iron of the same circuit ; 

 or there is absorption of heat produced by the current from hot to cold in the 

 iron, and evolution of heat to a less amount in the copper ; or there must be 

 absorption of heat in each metal : with the reverse effect in each case, when the 

 current is reversed. The reversible effect in a single metal of non-uniform tem- 

 perature may be called a convection of heat ; and, to avoid circumlocution, I shall 

 express it, that the vitreous electricity carries heat with it, or that the specific 

 heat of vitreous electricity is positive, when this convection is in the nominal 

 ' direction of the current ;' and I shall apply the same expressions to ' resinous 

 electricity,' when the convection is against the nominal direction of the current. 

 It is established, then, that one or other of the following three hypotheses must 

 be true : — 



124. " Vitreous electricity carries heat with it in an unequally heated con- 

 ductor, whether of copper or iron ; but more in copper than in iron : 



" Or, resinous electricity carries heat with it in an unequally heated conductor, 

 whether of copper or iron ; but more in iron than in copper : 



" Or, vitreous electricity carries heat with it in an unequally heated conductor 

 of copper, and resinous electricity carries heat with it in an unequally heated 

 conductor of iron. 



125. " Immediately after communicating this theory to the Royal Society of 

 Edinburgh, I commenced trying to ascertain by experiment which of the three hy- 

 potheses is the truth, as Theory, with only thermo-electric data, could not decide 

 between them. I had a slight bias in favour of the first rather than the second, 

 in consequence of the positiveness which, after Franklin, we habitually attri- 

 bute to the vitreous electricity, and a very strong feeling of the improbability of 

 the third. With the able and persevering exertions of my assistant, Mr M 'Far- 

 lane, applied to the construction of various forms of apparatus, and to assist me 

 in conducting experiments, the research has been carried on with little intermis- 

 sion for more than two years. Mr Robert Davidson, Mr Charles A. Smith, and 



