CHAPTER XIII.

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.