SCIENCE AND MAN. 603 
of ways we can distribute the itemsof 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 recombine, 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 battery 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 heat wires, produce decomposition, 
magnetize iron, and deflect a magnetic needle at any dis.- 
