344 FKAGMENTS OF SCIENCE. 



bustion 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 recombine, the heat 

 absorbed in the decomposition 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 pos- 

 sess. 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 battery is therefore rich enough to pay for 

 that decomposition, 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 bis- 

 muth and antimony soldered alternately together. The 

 electric current is here evoked by warming the soldered 

 junctions of one face of the pile. Like the Voltaic cur- 

 rent, the thermo-electric current can heat wires, pro- 



