664 FRAGMENTS OF SCIENCE. 
copper wire, while into the circuit of the other a thin 
platinum wire is introduced. The platinum glows with a 
white heat, while the copper wire is not sensibly warmed. 
Now an ounce of zinc, like an ounce of coal, produces 
by its complete combustion in air a constant quantity of 
heat. The total heat developed by an ounce of zinc 
through its union with oxygen in the battery is also 
absolutely invariable. Let our two batteries, then, continue 
in action until an ounce of zinc in each of them is con- 
sumed. In the one case the heat generated is purely 
domestic, being liberated on the hearth where the fuel is 
burned, that is to say in the cells of the battery itself. In 
the other case, the heat is in part domestic and in part 
foreign in part within the battery and in part outside. 
One of the fundamental truths to be borne in mind is that 
the sum of the foreign and domestic of the external and 
internal heats is fixed and invariable. Hence, to have 
heat outside, you must draw upon the heat within. 
These remarks apply to the electric light. By the inter- 
mediation of the electric current the moderate warmth of 
the battery is not only carried away, but concentrated so 
as to produce, at any distance from its origin, a heat next 
in order to that of the sun. The current might therefore 
be defined as the swift carrier of heat. Loading itself here 
with invisible power, by a process of transmutation which 
outstrips the dreams of the alchemist, it can discharge its 
load, in the fraction of a second, as light and heat, at the 
opposite side of the world. 
Thus, the light and heat produced outside the battery 
are derived from the metallic fuel burned within the bat- 
tery; and, as zinc happens to be an expensive fuel, though 
we have possessed the electric light for more than seventy 
years, it has been too costly to come into general use. But 
wjthin these waljs, in the autumn of 1831, Faraday dis- 
covered ;i m-\v source of electricity, which we have now to 
investigate. On the table before me lies a coil of covered 
copper wire, with its ends disunited. I lift one side of the 
coil from the table, and in doing so exert the muscular 
effort necessary to overcome the simple weight of the coil. 
I unite its two ends and repeat the experiment. The 
effort now required, if accurately measured, would be 
found greater than before. In lifting the coil I cut the 
lines of the earth's magnetic force, such cutting, as proved 
