nly the opposite end of
the poker has actually been in the fire. Heat from the fire passed
into the poker, traveled along it, and warmed it. When heat flows in
this way from a warm part of a body to a colder part, the process is
called _conduction_. A flatiron is heated by conduction, the heat from
the warm stove passing into the cold flatiron and gradually heating
it.

In convection, air and water circulate freely, carrying heat with
them; in conduction, heat flows from a warm region toward a cold
region, but there is no apparent motion of any kind.

Heat travels more readily through some substances than through others.
All metals conduct heat well; irons placed on the fire become heated
throughout and cannot be grasped with the bare hand; iron utensils are
frequently made with wooden handles, because wood is a poor conductor
and does not allow heat from the iron to pass through it to the hand.
For the same reason a burning match may be held without discomfort
until the flame almost reaches the hand.

Stoves and radiators are made of metal, because metals conduct heat
readily, and as fast as heat is generated within the stove by the
burning of fuel, or introduced into the radiator by the hot water, the
heat is conducted through the metal and escapes into the room.

Hot-water pipes and steam pipes are usually wrapped with a
non-conducting substance, or insulator, such as asbestos, in order
that the heat may not escape, but shall be retained within the pipes
until it reaches the radiators within the rooms.

The invention of the "Fireless Cooker" depended in part upon the
principle of non-conduction. Two vessels, one inside the other, are
separated by sawdust, asbestos, or other poor conducting material
(Fig. 18). Foods are heated in the usual way to the boiling point or
to a high temperature, and are then placed in the inner vessel. The
heat of the food cannot escape through the non-conducting material
which surrounds it, and hence remains in the food and slowly cooks it.

[Illustration: FIG. 18.--A fireless cooker.]

A very interesting experiment for the testing of the efficacy of
non-conductors may be easily performed. Place hot water in a metal
vessel, and note by means of a thermometer the _rapidity_ with which
the water cools; then place water of the same temperature in a second
metal vessel similar to the first, but surrounded by asbestos or other
non-conducting material, and note the _slowness_ with which