SCIENCK AND MAN. 603 



of ways we can distribute the items of a never- varying sum, 

 but even the subtle agency of the electric current places no 

 creative power in our hands. 



Instead of generating external heat, we may cause the 

 current to effect chemical decomposition at a distance 

 from the battery. Let it, for example, decompose water 

 into oxygen and hydrogen. The heat generated in the 

 battery under these circumstances by the combustion of a 

 given weight of zinc falls short of what is produced when 

 there is no decomposition. How far short? The question 

 admits of a perfectly exact answer. When the oxygen and 

 hydrogen recornbine, the heat absorbed in the decomposi- 

 tion is accurately restored, and it is exactly equal in amount 

 to that missing in the battery. We may, if we like, bottle 

 up the gases, carry in this form the heat of the battery to 

 the polar regions, and liberate it there. The battery, in 

 fact, is a hearth on which fuel is consumed; but the heat 

 of the combustion, instead of being confined in the usual 

 manner to the hearth itself, may be first liberated at the 

 other side of the world. 



And here we are able to solve an enigma which long 

 perplexed scientific men, and which could not be solved 

 until the bearing of the mechanical theory of heat upon 

 the phenomena of the voltaic battery was understood. 

 The puzzle was, that a single cell could not decompose 

 water. The reason is now plain enough. The solution 

 of an equivalent of zinc in a single cell develops not 

 much more than half the amount of heat required to 

 decompose an equivalent of water, and the single cell 

 cannot cede an amount of force which it does not possess. 

 But by forming a battery of two cells instead of one, 

 we develop an amount of heat slightly in excess of that 

 needed for the decomposition of the water. The two- 

 celled buttery is therefore rich enough to pay for that de- 

 composition, and to maintain the excess referred to within 

 its own cells. 



Similar reflections apply to the thermo-electric pile, an 

 instrument usually composed of small bars of bismuth and 

 antimony soldered alternately together. The electric cur- 

 rent is here evoked by warming the soldered junctions of 

 one face of the pile. Like the voltaic current, the thermo- 

 electric current can hear, wires, produce decomposition, 

 magnetize iron, and deflect a magnetic needle at any dis- 



