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.