CHAPTER IX.

EFFECTS OF HEAT.

1.--EXPANSION.

Q. _What are the principal EFFECTS of HEAT?_

A. 1.--Expansion. 2.--Liquefaction. 3.--Vaporization. 4.--Evaporation; and 5.--Ignition.

Q. _Does HEAT EXPAND the AIR?_

A. Yes; if a bladder (partially filled with air) be tied up at the neck, and _laid before the fire_, the air will _swell_ till the bladder _bursts_.

Q. _Why will the AIR SWELL, if the bladder be laid before the fire?_

A. Because the heat of the fire _gets between the particles of air_, and drives them _further apart from each other_; which causes the bladder to expand.

Q. _Why do unslit CHESTNUTS CRACK with a loud noise, when ROASTED?_

A. Chestnuts contain a great deal of air, which is expanded by the heat of the fire; and, as the thick rind prevents the air from escaping, it violently _bursts through, slitting the rind_, and making a great noise.

Q. _What occasions the loud CRACK or report which we hear?_

A. 1st--The _sudden bursting of the rind_ makes a report, in the same way as a piece of _wood_ or _glass_ would do, if _snapped in two_: and

2ndly--The _escape of hot air_ from the chestnut makes a report also, in the same way as _gunpowder_, when it escapes from a _gun_.

Q. _Why does the sudden BURSTING of the rind, or SNAPPING of a piece of wood, make a REPORT?_

A. As the attraction of the parts is suddenly overcome, _a violent jerk_ is given to the air; this jerk produces _rapid undulations_ in the air, which (striking upon the ear) give the brain the sensation of _sound_.

Q. _Why does the ESCAPE OF AIR from the chestnut, or the EXPLOSION of GUNPOWDER, produce a REPORT?_

A. Because a quantity of air (suddenly let loose) _pushes against the air around_, in order to make _room for itself_; and as the _air of the chestnut_ slaps against the _air of the room_, a _report_ is made, (as when I _slap_ a book or table).

Q. _If a CHESTNUT be SLIT, it will NOT CRACK; why is this?_

A. Because the _heated air_ of the chestnut can _freely escape_ through the _slit in the rind_.

Q. _Why does an APPLE spit and SPURT about, when roasted?_

A. An apple contains a vast quantity of _air_, which (being expanded by the heat of the fire) _bursts through the peel_, carrying the juice of the apple along with it.

Q. _Does an APPLE contain MORE AIR, in proportion, than a CHESTNUT?_

A. Yes, much more. There is as much condensed air in a common apple, as would fill a space 48 _times as big as the apple itself_.

Q. _Where is all this quantity of AIR stowed in the APPLE?_

A. The _inside_ of an apple is _made up of little cells_ (like a _honey-comb_), each of which contains a portion of the air.

Q. _When an APPLE is ROASTED, why is one part made SOFT, while all the rest remains hard?_

A. When an apple is roasted, the air in the _cells next to the fire_ is expanded and flies out; the _cells are broken_, and their juices _mixed together_; so the apple _collapses_ (from loss of air and juice), and feels _soft_ in those parts.

Q. _What is meant by the "apple COLLAPSING?"_

A. The _plumpness_ gives way, and the apple becomes _flabby_ and _shrivelled_.

Q. _Why do SPARKS of fire start (with a crackling noise) from pieces of WOOD laid upon a FIRE?_

A. The _air_ in the wood (expanded by the heat), _forces its way through the pores of the log_; and carries along with it the _covering of the pore_, which resisted its passage.

Q. _What is meant by the "PORES of the WOOD?"_

A. Very small _holes in the wood_, through which the _sap_ circulates.

Q. _What are the SPARKS OF FIRE, which burst from the WOOD?_

A. Very small pieces of wood _red hot_, separated from the log by the _force of the air_, as it bursts from its confinement.

Q. _Why does DEAL make more snapping than any OTHER WOOD?_

A. The pores of deal are _very large_, and contain much _more air_ than wood of a _closer grain_.

Q. _Why does DRY WOOD make more SNAPPING than GREEN WOOD?_

A. In _green wood_ the pores are filled with _sap_, and therefore contain _very little air_; but in _dry_ wood the sap is _dried up_, and the pores are filled with _air_ instead.

Q. _Why does DRY wood BURN more easily than GREEN or wet wood?_

A. Because the pores of dry wood are _filled with air_, which supports combustion; but the pores of green or wet wood are filled with _vapour_, which extinguishes flame.

Q. _Why does VAPOUR EXTINGUISH FLAME?_

1st--Because the coat of water (which wraps the fuel round) prevents the _oxygen_ of the air from getting to the _fuel_, to form into _carbonic acid gas_: and

2ndly--Heat is perpetually carried off, by the formation of the sap or water _into steam_.

(Carbonic acid gas is a compound of carbon and oxygen. The solid part of the fuel is _carbon_, and one of the gases of the air is oxygen.)

Q. _What has CARBONIC ACID GAS to do with COMBUSTION?_

A. Combustion is produced by the _chemical action_ which takes place, while the _carbon_ of fuel unites with the _oxygen_ of air, and forms "_carbonic acid gas_." (See p. 36.)

Q. _Why do STONES SNAP and fly about, when heated in the FIRE?_

A. The air in the stones (expanded by the heat of the fire), _meets with great resistance_ from the close texture of the stone; and, therefore, _bursts forth with great violence_, tearing the stone to atoms, and forcing the fragments into the room.

Q. _Must not AIR be very STRONG, to shatter into atoms a hard stone?_

A. Yes. All the dreadful effects of _gunpowder_ are merely the results of the _sudden expansion of air_.

Q. _When bottled ALE and PORTER is set before a FIRE, why is the CORK FORCED OUT sometimes?_

A. If the bottle be _not quite full_, there will be _air_ between the liquor and the cork; this _air_ (expanded by the heat of the fire) _forces out the cork_.

Q. _Why does ALE or PORTER FROTH more, after it has been set before the fire?_

A. The _froth_ of ale or porter _depends upon the pressure_ to which it is subjected; and as the air (between the liquor and the cork) is _expanded_ by the heat, _it presses against the liquor_, and increases the quantity of froth.

Q. _Why is the FROTH of ale and porter INCREASED by PRESSURE?_

A. Because the liquor absorbs _carbonic acid_ so long as it is under _pressure_; and the moment that the pressure is _removed_, the carbonic acid _escapes_ in foam or froth.

Q. _When a boy makes a BALLOON, and sets fire to the cotton or sponge (which has been steeped in spirits of wine), why is the balloon INFLATED, or blown out?_

A. The _air_ inside the balloon is _expanded by the flame_, till the whole balloon is _blown out_ without a crumple.

Q. _Why does the BALLOON RISE, after it has been inflated by the expanded air?_

A. The same quantity of air is expanded _to three or four times its original volume_; and is made so much _lighter than common air_, that even when all the paper, wire, and cotton are added, it is still lighter bulk for bulk.

Q. _What is meant by being lighter "bulk for bulk?"_

A. If the balloon be 3 square feet in size, it is _lighter_ (when inflated) than 3 square feet of _common air_, and therefore _floats through it_; as a cork (at the bottom of a tub of water) would rise to the surface.

Q. _Why does SMOKE RUSH UP a CHIMNEY?_

A. The heat of the fire _expands the air in the chimney_; and (being thus made _lighter_ than the air around), it _rises up the chimney_, and carries the smoke in its current.

Q. _Why has a LONG CHIMNEY a greater DRAUGHT than a short one?_

A. Because air rises faster and faster the _higher it ascends_ in a chimney flue; the same as a stone falls faster and faster the _nearer it approaches to the ground_.

Q. _Why will a LONG chimney SMOKE, unless the FIRE be pretty FIERCE?_

A. If the fire be not pretty fierce, its heat will not be sufficient to _rarefy all the air in the chimney_; and then the chimney will _smoke_.

Q. _Why will the chimney smoke, if the fire be not BIG enough to heat ALL the air in the CHIMNEY FLUE?_

A. Because the _cold air_ (condensed in the upper part of the flue), _will sink from its own weight_, and sweep the ascending smoke _back with it_ into the room.

Q. _What is the use of a COWL upon a chimney-pot?_

A. The cowl acts as a _screen against the wind_, to prevent it from blowing into the chimney.

Q. _What HARM would the WIND do, if it were to BLOW into a CHIMNEY?_

A. 1st--It would prevent the smoke from getting out: and

2ndly--The _cold air_ (introduced into the chimney by the wind) _would fall down the flue_, and drive the smoke with it _back into the room_.

Q. _Why does a SMOKE-JACK turn round in a chimney?_

A. The current of hot air up the chimney, striking against the _oblique vanes of the smoke-jack_, drives them round and round; in the same way as the sails of a _wind-mill_ are driven round by the _wind_.

Q. _Why are some things SOLID, others LIQUID, and others GASEOUS?_

A. As _heat_ enters any substance, _it drives its particles further asunder_; and a _solid_ (like _ice_) becomes a _liquid_; and a _liquid_ (like _water_) becomes a _gas_.

Q. _Why does WATER SIMMER before it boils?_

A. The particles of water _near the bottom of the kettle_ (being formed into _steam_ sooner than the rest) _shoot upwards_; but are _condensed_ again (as they rise) _by the colder water_, and produce what is called "simmering."

Q. _What is meant by SIMMERING?_

A. A gentle tremor or _undulation_ on the surface of the water. When water _simmers_, the bubbles _collapse beneath the surface_, and the steam is condensed to _water again_: but when water _boils_, the bubbles _rise to the surface_, and _steam is thrown off_.

Q. _Why does a KETTLE SING when the water simmers?_

A. Because the _air_ (entangled in the water) escapes by _fits and starts_ through the _spout of the kettle_; which makes a noise like a wind instrument, when it is blown into.

Q. _Why does NOT a kettle SING, when the water BOILS?_

A. As _all_ the water is _boiling hot_, the steam meets with no _impediment_, but freely escapes in a continuous stream.

Q. _When does a kettle sing most?_

A. When it is set on a _hob_ to boil.

Q. _Why does a kettle SING MORE when it is set on the SIDE of a fire, than when it is set in the MIDST of the fire?_

A. When the kettle is set on the _hob_ to boil, the heat is applied very _partially: one side is hotter than the other_, and therefore the steam is more _entangled_.

Q. _Why does a KETTLE sing, when the boiling water begins to COOL again?_

A. Because the _upper_ surface cools _first_; and the steam (still rising from the lower parts of the kettle) is _again entangled_, and escapes fitfully.

Q. _Why does BOILING WATER SWELL?_

A. Water (like air) _expands by heat_. The heat of the fire drives the particles of water _further apart from each other_; and (as they are not _packed so closely together_) they take up _more room_; or (in other words) the water _swells_.

Q. _What is meant when it is said, "that HEAT drives the PARTICLES of water further APART from each other."_

A. Water is composed of little globules, like very small grains of sand; the heat _drives_ these particles _away from each other_; and (as they then require more _room_) the water _swells_.

Q. _Why does BOILING WATER BUBBLE?_

A. Water contains _air_; and (as the water is heated) _the air is driven out_, and raises a _bubble_ in that part of the water which resists its escape.

Q. _Why does a KETTLE sometimes BOIL OVER?_

A. Liquids _expand very much by heat_; if, therefore, a kettle be _filled with cold water_, some of it must _run over_ as soon as it is _expanded by heat_.

Q. _But I have seen a KETTLE BOIL OVER, although it has not been filled FULL of WATER; how do you account for THAT?_

A. If a fire be _very fierce_, the air is expelled so _rapidly_, that the _bubbles are very numerous_; and (towering one above the other) _reach the top of the kettle, and fall over_.

Q. _Why is a pot, which is full to OVERFLOWING (while the water is boiling HOT), NOTHING LIKE FULL, when it has been taken off the fire for a short time?_

A. When the water was _swelled by boiling heat_, it filled the pot even to overflowing; but as soon as the water is _condensed by cold_, it _contracts_ again, and occupies a much less space.

Q. _Why does the water of a KETTLE run out of the SPOUT when it BOILS?_

A. Because the steam cannot escape _so fast as it is formed_, and (being _confined in the kettle_) _presses on the water with great power_, and forces it out of the spout.

Q. _How can the PRESSURE of STEAM on the SURFACE of the water, FORCE the water through the KETTLE-SPOUT?_

A. In the same manner as _the pressure of air_ on the _mercury of a barometer_, forces the _quicksilver up the glass tube_.

Q. _What causes the RATTLING NOISE so often made by the LID of a saucepan or boiler?_

A. The steam (seeking to escape) _forces up the lid_ of the boiler, and the _weight_ of the lid causes it to _fall back again_: this being done _frequently_, produces a rattling noise.

Q. _If the steam COULD NOT LIFT UP THE LID of the boiler, how would it escape?_

A. If the lid fitted so tightly, that the steam could not raise it up, the boiler would _burst into fragments_, and the consequences might be fatal.

Q. _When steam pours out from the spout of a kettle, the STREAM begins apparently HALF AN INCH off the SPOUT; why does it not begin CLOSE to the spout?_

A. Steam is really _invisible_; and the half-inch (between the spout and the "_stream of mist_") is the _real steam_, before it has been condensed by air.

Q. _Why is not ALL the stream INVISIBLE, as well as that half-inch?_

A. As the steam _comes in contact with the colder air_, the invisible particles (being _condensed_), roll one into another, and look like a thick mist.

Q. _What BECOMES OF the STEAM? for it soon vanishes._

A. After it is condensed into mist, it is _dissolved by the air_, and dispersed abroad as _invisible vapour_.

Q. _And what BECOMES of the INVISIBLE VAPOUR?_

A. Being _lighter than air_, it _ascends_ to the upper regions, where (being again _condensed_) it contributes to form _clouds_.

Q. _Why does a METAL SPOON, left in a saucepan, RETARD the process of BOILING?_

A. The metal spoon (being an excellent _conductor_) _carries off the heat from the water_; and (as heat is carried off by the spoon) the water takes a longer time to boil.

Q. _Why will a POT (filled with water) NEVER BOIL, when immersed in ANOTHER vessel full of water also?_

A. Because water can _never be heated above the boiling point_: all the heat absorbed by the water after it _boils_, is employed in _converting the water into steam_.

Q. _How does the conversion of water into steam prevent the INNER POT from BOILING?_

A. The moment the water in the larger pot is _boiling hot_ (or 212°), _steam is formed_, and _carries off some of its heat_; therefore, 212 _degs._ of heat can never _pass through it_, to raise the _inner_ vessel to the _same heat_.

Q. _Why do SUGAR, SALT, &c. RETARD the process of BOILING?_

A. Because they have a tendency to _fix_ water by chemical attraction; and therefore retard its _conversion into steam_.

Q. _If you want water to boil, without COMING IN CONTACT with the SAUCEPAN, what plan must you adopt?_

A. _Immerse the pot_ (containing the water you want to boil) in a saucepan containing _strong brine_, or sugar.

Q. _Why would the INNER vessel boil, if the OUTER vessel contained strong BRINE?_

A. Though _water_ boils at 212 _degs._ of heat, yet _brine_ will not boil till raised to 218 or 220 _degs._ Therefore, 212 _degs._ of heat may easily pass through brine _to raise the vessel immersed in it to boiling heat_, before any of it is _carried off by steam_.

Q. _Why will brine impart to another vessel MORE than 212°, and water NOT SO MUCH?_

A. Because both liquids will _impart heat_ till they _boil_, and then _they can impart heat no longer_.

Q. _Why can they impart no extra heat after they boil?_

A. Because all _extra_ heat is spent _in making steam_. Hence water will _not_ boil a vessel of water immersed in it, because it cannot impart to it 212 _degs._ of heat: but _brine_ will, because it can impart _more than_ 212 _degs. of heat_, without being converted itself into steam.

Ether boils at 104 _degs._ Alcohol - - 173-1/2 " Water - - 212 " Water with one-fifth salt 219 " Syrup boils at 221 " Oil of turpentine, 304 " Sulphuric acid 472 " Linseed oil - 640 " &c. &c.

Any liquid which boils at a _lower_ degree can be made to boil if immersed in a liquid which boils at a higher degree. Thus a _cup of ether_ can be made to boil in a saucepan of _water_. A _cup of water_ in a saucepan of _brine or syrup_. But a _cup of water_ will _not_ boil if immersed in _ether_; nor a _cup of syrup_ in _water_.

Q. _Why are CLOUDS HIGHER on a FINE DAY?_

A. 1st--Because the _air_ (expanded by heat) _drives them higher up_: and

2ndly--The _clouds themselves are lighter_, and therefore _more buoyant_.

Q. _Why are the CLOUDS LIGHTER on a FINE DAY?_

A. Because their mists are either _absorbed by the dry air_, or _vapourized_ by the hot sun.

Q. _Why is a CUP PUT topsy-turvy into a FRUIT-PIE?_

A. Its principal use is to _hold the crust up_, and _prevent it from sinking_, when the cooked fruit gives away under it.

Q. _Does not the cup PREVENT the FRUIT of the pie from BOILING OVER?_

A. No, by no means; it would rather tend to _make it boil over_, than otherwise.

Q. _Why would the cup tend rather to MAKE the FRUIT BOIL OVER?_

A. As soon as the pie is put into the oven, the _air_ in the cup will _begin to expand_, and drive every particle of juice from under it; the pie dish, therefore, will have a cup-full _less room_ to hold its fruit, than if the cup were _taken out_.

Q. _If the juice is driven OUT of the cup, why is the CUP always FULL of JUICE, when the pie is cut up?_

A. Immediately the pie is drawn, the _air_ in the cup begins to _condense again_, and _occupy a smaller space_; in consequence of which, there is no longer _enough air to fill the cup_, and so _juice_ rushes in _to fill up the deficiency_.

Q. _Why does JUICE rush into the cup, because the cup is NOT FULL of AIR?_

A. As the external air _presses upon the surface of the juice_, it rushes into the cup _unobstructed_; as mercury rises through the tube of a barometer through similar pressure.