x88i.] The Source of Electric Energy. 379 
be homogeneous throughout its interior, heat is regularly 
conducted through it. But if the two surfaces be hetero- 
geneous, or if the one substance be heterogeneous internally, 
a marked difference results. Part of the heat is converted 
into electricity. Instead of the energy passing from mole- 
cule to molecule in the gradual and continuous heat method, 
part of it passes in the rapid and instantaneous eleCtric 
method. 
Very slight changes yield this result, and these changes 
are probably all changes in tension. If a wire be twisted or 
stretched, the heat that flows over this tense part becomes 
partly converted into electricity. Of if it be homogeneous, 
and one part of it be intensely heated, electricity likewise 
appears. Yet in this latter case there is also a change in 
tension, for the heat causes expansion of the wire, and con- 
sequently increased atmospheric pressure, which must affeCt 
the tension of the molecules. In the contact of diverse sub- 
stances, or of similar substances in different states, there 
undoubtedly exist differences of molecular tension, and, 
therefore, of vibratory pitch. 
If, instead of heat being conducted from one substance to 
another, two substances be rubbed together, similar results 
appear. If they are homogeneous it partly or entirely 
becomes electricity. And if the energy emitted be that pro- 
duced by chemical aCtion between diverse substances, a like 
result appears. Electricity is produced. In short, in every 
case in which two homogeneous surfaces receive excess 
motive energy, this energy flows from one to the other as 
heat. In every case where two heterogeneous surfaces receive 
excess motive energy, this energy, partly or completely, flows 
from one to the other as electricity. But all this electricity 
is eventually converted into heat, or into some condition in 
correlation with heat. Its existence as electricity is usually 
but temporary. 
And the necessary condition for the temporary conversion 
of heat energy, or of chemical or mechanical energy, into 
electricity, appears to be diversity of tension in the molecules 
of contiguous surfaces. The same diversity exists between 
the molecules of heated solids and those of the gas or ether 
in contact with them. There is thus an analogy of condition 
between the production of light and of electricity. Whether 
this is significant of a resemblance in character remains to 
be proved. 
Why does this difference in the relations of the two sur- 
faces produce such a difference in results ? To this 
important question it may be possible to give a probable 
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