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"T 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 materials."
"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 soufflée, 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 frozen."
"I should like," said Emile, "to have had a taste of that cheese, so cold under its hot crust of cooked eggs!"
"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?" queried Marie.
"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 example.
"If on going to bed at night we cover a bed of live coals with ashes, we shall find the coals still alive the next 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."