behind an air-tight but
freely-moving piston, it will bombard the walls of the cylinder and the
piston; and if the united push of the molecules on the one side of the
latter is greater than the resistance on the other side opposing its
motion, the piston must move. Having thus partly got their liberty, the
molecules become less active, and do not rush about so vigorously. The
pressure on the piston decreases as it moves. But if the piston were
driven back to its original position against the force of the steam, the
molecular activity--that is, pressure--would be restored. We are here
assuming that no heat has passed through the cylinder or piston and been
radiated into the air; for any loss of heat means loss of energy, since
heat _is_ energy.

THE BOILER.

The combustion of fuel in a furnace causes the walls of the furnace to
become _hot_, which means that the molecules of the substance forming
the walls are thrown into violent agitation. If the walls are what are
called "good conductors" of heat, they will transmit the agitation
through them to any surrounding substance. In the case of the ordinary
house stove this is the air, which itself is agitated, or grows warm. A
steam-boiler has the furnace walls surrounded by water, and its function
is to transmit molecular movement (heat, or energy) through the furnace
plates to the water until the point is reached when steam generates. At
atmospheric pressure--that is, if not confined in any way--steam would
fill 1,610 times the space which its molecules occupied in their watery
formation. If we seal up the boiler so that no escape is possible for
the steam molecules, their motion becomes more and more rapid, and
_pressure_ is developed by their beating on the walls of the boiler.
There is theoretically no limit to which the pressure may be raised,
provided that sufficient fuel-combustion energy is transmitted to the
vaporizing water.

To raise steam in large quantities we must employ a fuel which develops
great heat in proportion to its weight, is readily procured, and cheap.
Coal fulfils all these conditions. Of the 800 million tons mined
annually throughout the world, 400 million tons are burnt in the
furnaces of steam-boilers.

A good boiler must be--(1) Strong enough to withstand much higher
pressures than that at which it is worked; (2) so designed as to burn
its fuel to the greatest advantage.

Even in the best-designed boilers a large part of the combustion h