ons have been sent
on one wire both ways at the same time, and no set of signals has in
any way interfered with the completeness and audibility of the rest.
Sixteen sets of waltzes were being performed at one and the same time
by the particles of one wire without confusion. Because the air is
transmitting the notes of an organ from the loft to the opposite end of
the church, it is not incapable of bringing the sound of a voice in an
opposite direction to the organist from the other end of the church.

The extreme mobility of steel is seen when the red-hot metal is plunged
into water. Instantly every particle takes a new position, making it a
hundredfold more hard than before it was heated. But these particles
of transferred steel are still mobile. A man's razor does not cut
smoothly. It is dull, or has a ragged edge that is more inclined to
draw tears than cut hairs. He draws the razor over the tender palm of
his hand a few times, rearranges the particles of the edge and builds
them out into a sharper form. Then the razor returns to the lip with
the dainty touch of a kiss instead of a saw. Or the tearful man dips
the razor in hot water and the particles run out to make a wider blade
and, of course, a thinner, sharper edge. Drop the tire of a wagon
wheel into a circular fire. As the heat increases each particle says
to its neighbor, "Please stand a little further off; this more than
July heat is uncomfortable." So the close friends stand a little
further apart, lengthening the tire an inch or two. Then, being taken
out of the fire and put on the wheel and cooled, the particles snuggle
up together again, holding the wheel with a grip of cold iron. Mobile
and loose, with plenty of room to play, as the particles have, neither
wire nor tire loses its tensile strength. They hold together, whether
arms are locked around each other's waist, or hand clasps hand in
farther reach. What change has come to iron when it has been made red
or white hot? Its particles have simply been mobilized. It differs
from cold iron as an army in barracks and forts differs from an army
mobilized. Nothing has been added but movement. There is no caloric
substance. Heat is a mode of motion. The particles of iron have been
made to vibrate among themselves. When the rapidity of movement
reaches four hundred and sixty millions of millions of vibrations per
second it so affects the eye that we say it is red-hot. When other
systems