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experiment). Hence, if in a circuit of copper and iron, one junction 

 be kept about 280, that is, at the neutral temperature, and the other 

 at any lower temperature, a thermo-electric current will set from 

 copper to iron through the hot, and from iron to copper through 

 the cold junction ; causing the evolution of heat at the latter, and 

 the raising of weights too if it be employed to work an electro-mag- 

 netic engine, but not causing the absorption of any heat at the hot 

 junction. Hence there must be an absorption of heat at some part 

 or parts of the circuit consisting solely of one metal or of the other, 

 to an amount equivalent to the heat evolved at the cold junction, to- 

 gether with the thermal value of any mechanical effects produced in 

 other parts of the circuit. The locality of this absorption can only 

 be where the temperatures of the single metals are non-uniform, 

 since the thermal effect of a current in any homogeneous uniformly 

 heated conductor is always an evolution of heat. Hence there must 

 be on the Whole an absorption of heat, caused by the current 

 in 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 

 electromotive force is applied. The aggregate reversible thermal 

 effect, thus demonstrated 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 the two. 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 

 temperature may be called a convection of heat ; and to avoid cir- 

 cumlocution, I shall express it, that the vitreous electricity carries heat 

 with it, or that the specific heat of vitreous electricity is positive, 



