the heat that goes
through an iron rod will beat the heat that goes through a glass rod,
or the other way round:

[Illustration: FIG. 56. The metal balls are fastened to the iron and
glass rods with drops of wax.]

EXPERIMENT 41. Take a solid glass rod and a solid iron rod,
each about a quarter inch in diameter and about 6 inches long.
With sealing wax or candle grease stick three ball bearings or
pieces of lead, all the same size, to each rod, about an inch
apart, beginning 2 inches from the end. Hold the rods side by
side with their ends in a flame, and watch the balls fall off
as the heat comes along through the rods. The heat that first
melts off the balls beats.

[Illustration: FIG. 57. Does the heat travel faster through the iron
or through the glass?]

What really happens down among the molecules when the heat travels
along the rods is that the molecules near the flame are made to move
more quickly; they joggle their neighbors and make them move faster;
these joggle the ones next to them, and so on down the line. Heat
that travels through things in this way is called _conducted_ heat.
Anything like iron, that lets the heat travel through it quickly, is
called a _good conductor_ of heat. Anything like glass, that allows
the heat to travel through it only with difficulty, is called a _poor
conductor_ of heat, or an _insulator_ of heat.

A silver spoon used for stirring anything that is cooking gets so
hot all the way up the handle that you can hardly hold it, while the
handle of a wooden spoon never gets hot. Pancake turners usually have
wooden handles. Metals are good conductors of heat; wood is a poor
conductor.

An even more obvious example of the conducting of heat is seen in a
stove lid; your fire is under it, yet the top gets so hot that you can
cook on it.

When anything feels hot to the touch, it is because heat is being
conducted to and through your skin to the sensitive little nerve ends
just inside. But when anything feels cold, it is because heat is being
conducted away from your skin into the cold object.

AIR CARRIES HEAT BY CONVECTION. One of the poorest conductors of heat
is air; that is, one particle of air can hardly give any of its heat
to the next particle. But particles of air move around very easily and
carry their heat with them; and they can give the heat they carry with
them to any solid thing they bump into. So when air can move around,
the part