O give you a thorough understanding of the part
played by woven fabrics in our clothing and coverings I
must now call your attention to certain attributes of
heat. The little I can tell you on this subject will be
illustrated in many cases by important practical
applications. So be very attentive.
"A firebrand can with impunity be taken up in the
fingers by one end if the fire is confined to the
other; there is no risk of our being burnt, even though
the fingers be very close to the ignited part."
"I have often noticed that," said Jules, "when poking a
corner of the fire in winter, and when to save time I
have used my fingers instead of the tongs."
"But one could not, without getting burnt, grasp the
apparently cold end of a piece of iron, even a pretty
long one, if red at the other end. Neither could one
take hold of the handle of a flat-iron warming on the
heater. The hand must be protected by a thick holder.
"These two familiar examples, charcoal and iron, show
us that heat is not conducted with the same readiness
by all substances. It easily makes its way through
iron, which becomes very warm a long distance from the
part directly heated; but it is with
 difficulty that it permeates charcoal, which remains
cold a short distance from the ignited section.
"In this respect all substances are divided into two
classes: those that are easily permeated by heat or
that conduct it well, and those that are difficult for
heat to permeate or that conduct it poorly. The first
are called good conductors, such as iron; the second,
poor conductors, such as charcoal.
"Among the good conductors are all the
metals—iron, copper, silver, gold, and so on.
Non-metallic substances, such as wood, charcoal, brick,
glass, and the various kinds of stone, are, on the
contrary, poor conductors. Conductivity is still poorer
in powdery substances, such as ashes, earth, sawdust,
and snow; also in fibrous substances, such as cotton,
wool, silk, and hence the fabrics woven from these
"Then," said Marie, "the holder with which we grasp the
handle of a flat-iron keeps the hand from being burnt
because it prevents the heat from going farther. It is
a poor heat-conductor."
"Yes, it checks the heat of the iron and prevents its
reaching the hand. In like manner the various iron
implements that have to be thrust into the fire at one
end and heated are furnished at the other end with a
wooden handle by which they can be grasped without risk
of burning the hand.
"Of all substances air is the poorest heat-conductor,
as proved by the following curious experiment. A
scientist name Rumford, to whom we are indebted for
some noteworthy researches on the subject of heat, had
some frozen cheese placed in the middle of a dish. Over
this cheese was poured a
 light froth made of beaten eggs, and then the whole was
put into a very hot oven to cook the eggs quickly. Thus
was obtained a sort of omelette soufflé, piping
hot, and in the middle of it was the frozen cheese,
which had lost none of its coldness. How could the
cheese remain frozen in the oven? The explanation is to
be found in the poor conductivity of air. It was air,
imprisoned in the froth of the eggs, that sheltered the
cheese from the heat of the oven, obstructed the heat
in its passage, and prevented its going farther. The
heat not reaching it, the cheese in the center remained
"I should like," said Emile, "to have had a taste of
that cheese, so cold under its hot crust of cooked
"Now," resumed Uncle Paul, "I come to the practical
application of the peculiarities I have just pointed
out to you. A substance that conducts heat poorly may
serve two purposes which at first seem contradictory
and yet at bottom are alike in every respect. It can be
used to protect both from the cold and from the heat,
to keep an object warm as well as to keep it cool. To
cool off is to lose heat; to get warm is to gain it.
Accordingly, the point is to check, in the first case,
the inner heat that might escape, and, in the second,
the outer heat that might enter. Both requirements are
met by the same means, the interposition of an obstacle
impervious to heat in either direction; that is to say,
a covering that is a very poor heat-conductor."
Then what keeps out the cold keeps out the heat too?"
 "Precisely, singular though it may seem to you. The
same covering that prevents loss of heat in any given
body also prevents its receiving what might come from
outside. A few examples will prove this to you. Let us
first recall that chief among poor conductors are
powdery and fibrous substances. This property they owe
mainly to the air retained between one particle and
another, one fiber and another, just as water is
retained in the innumerable little cavities of a
sponge. These substances are used as a protection from
cold and heat alike. Ashes will furnish me my first
"If on going to bed at night we cover a bed of live
coals with ashes, we shall find the coals still alive
in the morning. The ashes, by keeping out the air,
check combustion; but they do still more: at the same
time that they check combustion they serve to retain in
the coals nearly all their original heat, so that the
next morning they are as glowing-hot as the night
before. This result is due to the obstacle that ashes
as a powdery substance oppose to the escape of heat.
Under this powdery covering the embers remain alive
because they cannot send out their heat, a poorly
conducting body standing in the way.
"These same ashes that prevent the cooling of an object
can also prevent its acquiring heat. I will tell you
later how the little girls in my village used to go and
borrow fire from a neighbor, returning with a live coal
on a bed of ashes in the hollow of their hand. This
shallow layer of ashes prevented their burning
themselves, its poor conductivity arresting the heat of
the glowing ember.
 "The two examples are convincing: you see the same
substance, ashes, acting as protector, first from the
cold and then from the heat, keeping the live coals
from cooling off and the hand from getting burnt."