CHAPTER XIII.. Communication of Heat.

COMMUNICATION OF HEAT.

1.--CONDUCTION.

Q. How is HEAT COMMUNICATED from one body to another?

A. 1. By Conduction. 2. By Absorption. 3. By Reflection. 4. By Radiation: and 5. By Convection.

Q. What is meant by CONDUCTION of heat?

A. Heat communicated from one body to another, by actual contact.

Q. Why does a PIECE of WOOD (blazing at ONE end) NOT FEEL HOT at the OTHER end?

A. Wood is a bad conductor of heat; and, therefore, heat does not traverse freely through it: hence, though one end of a stick be blazing-hot, the other end may be quite cold.

Q. Why do SOME THINGS feel so much COLDER than others?

A. Principally because they are better conductors; and, therefore, draw off the heat from our body (which touches them) so much faster.

Q. What are the BEST CONDUCTORS of HEAT?

A. Dense solid bodies, such as metal and stone.

Q. Which METALS are the most RAPID CONDUCTORS of HEAT?

A. Silver is the best conductor, then copper, then gold or tin, then iron, then zinc, and then lead.

Q. What are the WORST CONDUCTORS of HEAT?

A. All light and porous bodies, such as hair, fur, wool, charcoal, and so on.

Q. Why are COOKING VESSELS so often furnished with WOODEN HANDLES?

A. Wood is not a good conductor, like metal; and, therefore, many vessels (which are exposed to the heat of the fire) have wooden handles, lest they should burn our hands when we take hold of them.

Q. Why is the HANDLE OF A METAL TEA-POT made of WOOD?

A. As wood is a bad conductor, the heat of the boiling water is not so quickly conveyed to the wooden handle, nor so quickly poured into the hand by it, as when the handle is made of metal.

Q. Why would a METAL HANDLE BURN the HAND of the tea-maker?

A. As metal is an excellent conductor, the heat of the boiling water rushes quickly into the metal handle, and into the hand that touches it.

Q. How do you know that a METAL HANDLE would be HOTTER than a WOODEN one?

A. By touching the metal collar into which the wooden handle is fixed: though the wooden handle is quite cold, this metal collar is intensely hot.

Q. Why do persons use paper or WOOLLEN KETTLE-HOLDERS to take hold of a kettle with?

A. Paper and woollen are both very bad conductors of heat; and, therefore, the heat of the kettle does not readily pass through them to the hand.

Q. Does the heat of the boiling kettle NEVER get through the woollen or paper kettle-holder?

A. Yes; but though the kettle-holder became as hot as the kettle itself, it would never feel so hot.

Q. Why would not the kettle-holder FEEL so hot as the kettle, when it really is of the same temperature?

A. Because (being a very bad conductor) it disposes of its heat so slowly, that it is scarcely perceptible; but metal (being an excellent conductor) disposes of its heat so quickly, that the sudden influx is painful.

Q. Why then does HOT METAL feel so much MORE intensely WARM than HOT WOOL?

A. Because it gives out a much greater quantity of heat in the same space of time; and the influx of heat is, therefore, more perceptible.

Q. Why does MONEY in our pocket feel so HOT, when we stand BEFORE a FIRE?

A. Metal is an excellent conductor; and, therefore, becomes rapidly heated. For the same reason it becomes rapidly cold, when it comes in contact with a body colder than itself.

Q. Why does a PUMP-HANDLE feel intensely COLD in WINTER?

A. As metal is an excellent conductor, when the hot hand touches the cold pump-handle, the heat passes rapidly from the hand into the iron; and this rapid loss of heat produces a sensation of intense coldness.

Q. Is the iron HANDLE of the pump really COLDER than the wooden PUMP itself?

A. No; every inanimate substance (exposed to the same temperature) possesses the same degree of heat.

Q. Why then does the IRON HANDLE seem so MUCH COLDER than the WOODEN PUMP?

A. Merely because the iron is a better conductor; and, therefore, draws off the heat from our hand much more rapidly than wood does.

Q. Why does a STONE or marble HEARTH feel to the feet so much COLDER than a CARPET or hearth-rug?

A. Because stone and marble are good conductors, but woollen carpets and hearth-rugs are very bad conductors.

Q. Why does the STONE HEARTH make our FEET COLD?

A. As soon as the hearth-stone has absorbed a portion of heat from our foot, it instantly disposes of it, and calls for a fresh supply; till the hearth-stone has become of the same temperature as the foot placed upon it.

Q. Do not the woollen CARPET and HEARTH-RUG, also, conduct heat from the human body?

A. Yes; (but being very bad conductors) they convey the heat away so slowly, that it is scarcely perceptible.

Q. Is the COLD HEARTH-STONE and WARM CARPET then of the SAME TEMPERATURE?

A. Yes; everything in the room is really of the same temperature; but some feel colder than others because they are better conductors.

Q. How LONG will the hearth-stone feel cold to the feet resting on it?

A. Till the feet and the hearth-stone are both of the same temperature; and then the sensation of cold in the hearth-stone will go off.

Q. Why would not the HEARTH-STONE feel COLD, when it is of the SAME temperature as our FEET?

A. Because the heat would no longer rush out of our feet into the hearth-stone, in order to produce an equilibrium.

Q. Why does the HEARTH-STONE (when the fire is lighted) feel so much HOTTER than the HEARTH-RUG?

A. The hearth-stone is an excellent conductor; and, therefore, parts with its heat more readily than the woollen hearth-rug; which (being a very bad conductor) parts with its heat reluctantly.

Q. Why does PARTING with HEAT RAPIDLY make the HEARTH-STONE feel WARM?

A. As the heat of the stone rushes quickly into our foot, it raises its temperature so suddenly, that we cannot help perceiving the increase of heat.

Q. Why does the non-conducting power of the HEARTH-RUG prevent its feeling so HOT as it really is?

A. Because it parts with its heat so slowly and gradually, that we scarcely perceive its transmission into our feet.

Q. When we plunge our HANDS into a basin of WATER, why does it produce a sensation of COLD?

A. Though the water (in which we wash) is really warmer than the air of our bed-room; yet because it is a better conductor, it feels colder.

Q. Why does the CONDUCTING power of water make it feel COLDER than the air, though in reality it is WARMER?

A. Because it abstracts heat from our hands so rapidly, that we feel its loss; but the air abstracts heat so very slowly, that its gradual loss is hardly perceptible.

Q. Is water a GOOD CONDUCTOR of heat?

A. No; no liquid is a good conductor of heat; but yet water is a much better conductor than air.

Q. Why is WATER a BETTER CONDUCTOR of heat than AIR?

A. Because it is less subtile; and the conducting power of any substance depends upon its solidity, or the closeness of its particles.

Q. How do you know that WATER is NOT a GOOD CONDUCTOR of heat?

A. Because water may be made to boil at its surface, without imparting sufficient heat to melt ice a quarter of an inch below the boiling surface.

Q. Why are NOT LIQUIDS GOOD CONDUCTORS of heat?

A. Because the heat (which should be transmitted) produces evaporation, and flies off in the vapour.

Q. Why does a POKER (resting on the fender) feel so much COLDER than the HEARTH-RUG, which is further off the fire?

A. The poker (being an excellent conductor) draws heat from the hand much more quickly than the rug, which is a bad conductor: and, therefore, (though both are equally warm) the poker seems to be much colder.

Q. Why are HOT BRICKS (wrapped in cloth) employed in cold weather to KEEP the FEET WARM?

A. Bricks are bad conductors of heat, and cloth or flannel still worse: therefore a hot brick (wrapped in flannel) will retain its heat a very long time.

Q. Why is a TIN PAN (filled with HOT WATER) employed as a FOOT WARMER?

A. Because polished tin (being a bad radiator of heat) keeps hot a very long time; and warms the feet resting upon it.

Q. What is meant by being a "bad RADIATOR of heat?"

A. To radiate heat is to throw off heat by rays, as the sun; a polished tin pan does not throw off the heat of boiling water from its surface, but keeps it in.

Q. Why is the TIN FOOT-WARMER covered with FLANNEL?

A. 1st--To prevent the perspiration of the foot from taking off the polish of the tin:

2ndly--Flannel is a very bad conductor; and, therefore, helps to keep the tin hot longer: and

3rdly--If the feet were not protected, the conducting surface of the tin would feel painfully hot.

Q. What harm would it be if the POLISH of the tin were injured by the perspiration of our feet?

A. Polished tin throws off its heat very slowly; but dull, scratched, painted, or dirty tin, throws off its heat very quickly: if, therefore, the tin foot-warmer were to lose its polish, it would get cold in a much shorter time.

Q. Why are FURNACES and stoves (where much HEAT is required) built of porous BRICK?

A. As bricks are bad conductors, they prevent the escape of heat: and are, therefore, employed where great heat is required.

Q. Why are FURNACE DOORS, &c. frequently COVERED with a paste of CLAY and SAND?

A. Because this paste is a very bad conductor of heat; and, therefore, prevents the escape of heat from the furnace.

Q. If a stove be placed in the MIDDLE of a room, should it be made of bricks or IRON?

A. A stove in the middle of a room should be made of iron; because iron is an excellent conductor, and rapidly communicates its heat to the air around.

Q. Why does the Bible say, that God "giveth SNOW like WOOL?"

A. As snow is a very bad conductor of heat, it protects vegetables and seeds from the frost and cold.

Q. How does the non-conducting power of SNOW PROTECT VEGETABLES from the FROST and cold?

A. As snow is a bad conductor, it prevents the heat of the earth from being drawn off by the cold air which rests upon it.

Q. Why are WOOLLENS and FURS used in COLD weather for CLOTHING?

A. Because they are very bad conductors of heat; and, therefore, prevent the warmth of the body from being drawn off by the cold air.

Q. Do not woollens and furs actually IMPART heat to the body?

A. No; they merely prevent the heat of the body from escaping.

Q. Where would the heat ESCAPE to, if the body were NOT wrapped in wool or fur?

A. The heat of the body would fly off into the air; for the cold air (coming into contact with our body) would gradually draw away its heat, till it was as cold as the air itself.

Q. What then is the PRINCIPAL USE of CLOTHING in winter-time?

A. To keep the body air-tight; and prevent the external air (or wind) from coming into contact with it, to absorb its heat.

Q. Why are BEASTS COVERED with FUR, HAIR, or WOOL?

A. Because fur, hair, and wool are very slow conductors of heat; and (as dumb animals cannot be clad like human beings) God has given them a robe of hair or wool, to keep them warm.

Q. Why are BIRDS covered with DOWN or FEATHERS?

A. Because down and feathers are very bad conductors of heat; and (as birds cannot be clad like human beings) God has given them a robe of feathers to keep them warm.

Q. Why are WOOL, FUR, HAIR, or FEATHERS such SLOW CONDUCTORS of heat?

A. Because a great quantity of air lurks entangled between their fibres; and air is a very bad conductor of heat.

Q. If AIR be a BAD CONDUCTOR of heat, why should we not feel as warm WITHOUT clothing, as when we are wrapped in wool and fur?

A. Because the air (which is cooler than our body) is never at rest; and, therefore, fresh particles (perpetually passing over our body) keep drawing off the heat little by little.

Q. Why does the ceaseless CHANGE of air tend to DECREASE the WARMTH of a naked body?

A. Thus:--the air which cases the body absorbs as much heat from it as it can, while it remains in contact; it is then blown away, and makes room for a fresh coat of air, which does the same.

Q. Does the AIR (which encases a naked body) become by contact as WARM as the BODY itself?

A. It would do so, if it remained motionless; but as it remains only a very short time, it absorbs as much heat as it can in the time, and passes on.

Q. Why do we feel COLDER in WINDY WEATHER, than in a CALM day?

A. Because (in windy weather) the particles of air pass over us more rapidly; and every fresh particle takes from us some portion of heat.

Q. Show the wisdom of God in making the AIR a BAD CONDUCTOR.

A. If air were a good conductor (like iron and stone) the heat would be drawn so rapidly from our body, that we must be chilled to death. Similar evils would be felt also by all the animal and vegetable world.

Q. Does not the bad conducting power of air enable persons to judge whether an EGG be NEW or STALE?

A. Yes; touch your tongue against the shell at the larger end; if it feels warm to the tongue, the egg is stale; if not, it is new-laid.

Q. Why will the SHELL of a STALE EGG feel WARM to the tongue?

A. Between the shell and the "white of the egg" there is a small quantity of air, which expands in a stale egg, from the shrinking of the white.

Q. Why does the expansion of air (at the end of an egg) make it feel WARM to the tongue?

A. As air is a very bad conductor, the more air an egg contains, the less heat will be drawn from the tongue when it touches the shell.

Q. Why do ladies FAN themselves in summer, to make their FACES COOL?

A. The fan puts the air in motion, and makes it pass more rapidly over their face; and (as the temperature of the air is always lower than that of the human face) each puff of air carries off some portion of heat from the face.

Q. Does FANNING the air make the AIR itself COOLER?

A. No; fanning makes the air hotter and hotter.

Q. Why does FANNING the air increase its HEAT?

A. By causing the air continually to absorb heat from the human body which it passes over.

Q. If fanning makes the AIR HOTTER, how can it make a PERSON feel COOLER?

A. Fanning makes the air hotter, but the face cooler; because it keeps taking the heat out of the face, and giving it to the air.

Q. Why is BROTH COOLED by BLOWING it?

A. The breath causes a rapid change of air to pass over the broth; and (as the air is not so hot as the broth) it keeps absorbing heat, and thus makes the broth cooler and cooler.

Q. Would not the air absorb heat from the broth just as well WITHOUT BLOWING?

A. No; air is a very bad conductor; unless, therefore, the change be rapid, the air nearest the surface of the broth would soon become as hot as the broth itself.

Q. But would not the hot air PART with its heat instantly to the CIRCUMJACENT air?

A. No; not instantly. Air is so bad a conductor, that it parts with its heat very slowly: unless, therefore, the air be kept in continual motion, it would cool the broth very slowly indeed.

Q. Why does WIND generally feel COOL?

A. Wind is only air in motion; and the more quickly the air passes over our body, the more rapidly it absorbs the heat therefrom.

Q. Why does AIR ABSORB heat more QUICKLY by being set in MOTION?

A. Because every fresh gust of air absorbs a fresh portion of heat; and the more rapid the succession of gusts, the greater will be the quantity of air absorbed.

Q. If the AIR were HOTTER than our body, would the WIND feel COOL?

A. No; if the air were hotter than our body, it would feel insufferably hot.

Q. Why would the AIR feel INTENSELY HOT, if it were WARMER than our BLOOD?

A. Because then the wind would add to the heat of our body, instead of diminishing it.

Q. Is the AIR EVER as HOT as the human BODY?

A. Not in this country: in the hottest summer's day, the air is always 10 or 12 degrees cooler than the human body.

Q. Is the EARTH a GOOD CONDUCTOR of heat?

A. No; the power of conducting heat depends upon the continuity of matter; if the particles of which a thing is composed are not continuous, they have very little power to conduct heat.

Q. Why is the earth (BELOW the SURFACE) WARMER in WINTER than the surface itself?

A. Because the earth is a bad conductor of heat; and, therefore (although the ground be frozen) the frost never penetrates above an inch or two below the surface.

Q. Why is the earth (BELOW the SURFACE) COOLER in SUMMER than the surface itself?

A. Because the earth is a bad conductor of heat; and, therefore, (although the surface be scorched with the burning sun) the intense heat cannot penetrate to the roots of the plants and trees.

Q. Shew the WISDOM of GOD in making the EARTH a BAD CONDUCTOR.

A. If the heat and cold could penetrate the earth (as freely as the heat of a fire penetrates iron), the springs would be dried up in summer and frozen in winter, and all vegetation would perish.

Q. Why is WATER from a SPRING so COOL in SUMMER?

A. As the earth is a bad conductor, the burning rays of the sun can penetrate only a few inches below the surface; in consequence of which, the springs of water are not affected.

Q. Why is it COOL under a SHADY tree in a hot summer's day?

A. 1st--Because the overhanging foliage screens off the rays of the sun:

2ndly--As the rays of the sun are warded off, the air (beneath the tree) is not heated by the reflection of the earth: and

3rdly--The leaves of trees, being non-conductors, allow no heat to penetrate through them.

Q. Why do the LAPLANDERS wear SKINS, with the FUR INWARDS?

A. The dry skin prevents the wind from penetrating to their body; and as the fur contains a quantity of air between its hairs (which soon becomes heated by the body) the Laplander is clad in a case of hot air, impervious to the cold and wind.

Q. Why does a LINEN SHIRT feel COLDER than a COTTON ONE?

A. Linen is a much better conductor than cotton; and, therefore, (as soon as it touches the body) it more rapidly draws away the heat, and produces a sensation of cold.

Q. Why is the FACE COOLED by wiping the temples with a fine CAMBRIC HANDKERCHIEF?

A. The fine fibres of the cambric have a strong capillary attraction for moisture; and are excellent conductors of heat: thus the moisture and heat are both abstracted from the face, and a sensation of coolness is produced.

"Capillary attraction," i. e. the attraction of a thread or hair. The wick of a candle is wet with grease, because the melted tallow runs up the cotton from capillary attraction.

Q. Why would not a COTTON handkerchief do as well?

A. The coarse fibres of cotton have much less capillary attraction, and are nothing like such good conductors as linen: and, therefore, wiping the face with a cotton handkerchief, increases the sensation of warmth.