5.--CONVECTION.
Q. _What is meant by the CONVECTION of HEAT?_
A. Heat communicated _by being carried_ to another thing or place; as
the hot water resting on the _bottom_ of a kettle, carries heat to the
water _through which it passes_. (_see p._ 246).
Q. _Are LIQUIDS good CONDUCTORS of heat?_
A. No; liquids are _bad conductors_; and are, therefore, made hot by
_convection_.
Q. _Why are LIQUIDS BAD CONDUCTORS of heat?_
A. Because heat _converts a liquid into steam_, and flies off with the
vapour, instead of being _conducted through the liquid_.
Q. _Explain how WATER is made HOT?_
A. The water _nearest the fire is first heated_, and (being heated)
_rises to the top;_ other cold water succeeds, is _also_ heated, and
rises in turn; and this interchange keeps going on, _till all the water
boils_.
Q. _Why is WATER in such continual FERMENT, when it is BOILING?_
A. This commotion is mainly produced by the _ascending and descending
currents_ of hot and cold water.
The escape of _air_ from the water contributes also to increase this
agitation.
Q. _How do these two currents PASS each other?_
A. The _hot ascending current_ passes close by the _metal sides_ of the
kettle; while the _cold descending current_ passes _down the centre_.
Q. _Why does BOILING WATER BUBBLE?_
A. The bubbles are _portions of steam_ (formed at the bottom of the
vessel) which _rise to the surface_, and escape into the air.
Q. _Why does a KETTLE RUN OVER, when the water BOILS?_
A. As the heat insinuates itself between the particles of water, _it
drives them asunder_; and (as the particles of water are _driven apart
from each other_) the _same_ vessel will no longer hold the expanded
water, and some runs over.
Q. _Why does a KETTLE SING, when it is ABOUT to BOIL?_
A. Water contains _a great deal of air_, which (being expanded by the
heat of the fire) escapes by fits _through the spout of the kettle_;
which sings in the same way as a trumpet does, when a person blows in
it.
Q. _Why does WATER BOIL?_
A. Boiling is the effect of a _more violent escape of air_ from the
heated water; when, therefore, the air is _not permitted to escape_,
water will _never boil_.
Q. _Why is HEAT applied to the BOTTOM, and not to the top of a KETTLE?_
A. Because the heated water _always ascends to the surface_, heating the
water through which it passes: if, therefore, heat were applied to the
_top of a vessel_, the water _below the surface_ would _never be
heated_.
Q. _As the lower part of a GRATE is made RED-HOT by the fire ABOVE, why
would not the WATER boil, if fire were applied to the TOP?_
A. The _iron_ of a grate is an _excellent conductor_; and, therefore, if
_one_ part be heated, the heat is conducted to _every other part_: but
_water_ is a _very bad conductor_, and will not diffuse heat in a
similar way.
Q. _How do you know that WATER is a BAD CONDUCTOR of heat?_
A. When a blacksmith immerses his red-hot iron in a tank of water, the
water which _surrounds_ the red-hot iron is made _boiling hot_, but the
water _below_ the surface remains quite cold.
Q. _If you wish to COOL LIQUIDS, where should the cold be applied?_
A. To the _top of the liquid_; because the _cold_ portion will always
_descend_, and allow the warmer parts to come in contact with the
cooling substance.
Q. _Does BOILING water get hotter by being KEPT on the FIRE?_
A. No; not if the steam be suffered to escape.
Q. _Why does not boiling water get HOTTER, if the steam be suffered to
ESCAPE?_
A. Because _as fast as the water boils_, it is converted into _steam_;
and the steam _carries away_ the additional heat, as fast as it is
communicated.
Q. _Is STEAM visible or INVISIBLE?_
A. Steam is _invisible_; but when it comes in contact with the air
(being _condensed into small drops_) it instantly becomes visible.
Q. _How do you know that STEAM is INVISIBLE?_
A. If you look at the spout of a boiling kettle, you will find that the
steam (which issues from the spout) is always invisible _for about half
an inch_; after which, _it becomes visible_.
Q. _Why is the steam INVISIBLE for only HALF AN INCH, and not either all
INVISIBLE or all VISIBLE?_
A. The air is not able to condense the steam as it first issues from the
spout, but when it _spreads_ and comes in contact with a larger volume
of air, the _invisible steam_ is readily condensed into _visible drops_.
Q. _Why is our BREATH VISIBLE in winter-time?_
A. Because _it is condensed by the cold air_ into small drops, which are
visible to the eye.
Q. _Why do STEAM-ENGINES sometimes BURST?_
A. Steam is very _elastic_; and this elasticity increases in a greater
proportion than the heat which produces it; unless, therefore, some
_vent_ be freely allowed, the steam heaves and swells, till it bursts
the vessel which confined it.
Q. _What BECOMES of the steam, after it has been condensed?_
A. It is _dissolved by the air_, and forms a part of its invisible
vapour.
Q. _Is AIR a good CONDUCTOR?_
A. No; _air is a very bad conductor_, and is heated (like water) _by
convection_.
Q. _How is a ROOM WARMED by a STOVE?_
A. The air _nearest the fire_ is made hot _first_; _the cold air
descends_, is heated also, and rises in turn; and this goes on, _till
all the air of the room is warmed_.
Q. _Why are FIRES placed on the FLOOR of a room, and not towards the
CEILING?_
A. As heated air always _ascends_, if the fire were not _near the
floor_, the lower part of the air (which we want to be the warmest)
would never be benefited by the fire at all.
Q. _If you take a POKER out of the fire, and hold the HOT END DOWNWARDS,
why is the HANDLE so intensely HOT?_
A. Because the hot end of the poker _heats the air around it_, and this
hot air (in its ascent) _scorches the poker_, and the _hand which holds
it_.
Q. _How should a RED-HOT POKER be carried so as not to BURN our
fingers?_
A. With the hot end _upwards_; because then the air (heated by the
poker) _would not pass over our hand_ to scorch it.
Q. _Why is a POKER (resting on the FENDER) COLD; but if it leans against
the STOVE, intensely warm?_
A. The poker is an _excellent conductor_; while, therefore, it rests
against the hot stove, the heat of the stove is _conducted into the
poker_; but when it _rests on the fender_, it does not come in _contact
with the hot stove_.
Q. _Why does it feel so COLD, when it rests on the FENDER?_
A. Not being so warm as our hand, it _imbibes the heat from it_ with
such _rapidity_, that our loss of heat is _palpable_, and produces the
sensation of coldness.
Q. _Why are FLUES (which are carried through a church or room) always
BLACKENED with BLACK LEAD?_
A. In order that the heat of the flue _may be more readily diffused_
throughout the room. Black lead radiates heat more freely than any other
known substance.
Q. _Why do country people touch the thick end of an EGG with their
TONGUE, to know if it be STALE or not?_
A. The thick end of an egg always contains _a little air_ (between the
shell and the white); but, when the egg is stale, _the white shrinks_,
and the air expands.
Q. _How can the TONGUE tell from this, whether the egg be STALE or FRESH
laid?_
A. As air is a _very bad conductor_, if the egg be _stale_, it will feel
much _warmer to the tongue_, than if it be new-laid.
Q. _Why will the big end of an egg feel WARMER to the tongue, because it
contains more AIR?_
A. As air is a _bad conductor_, it will draw off the heat of the tongue
_very slowly_, and, therefore, _appear warm_; but when there is only a
_very little air in the egg_ (as the _white_ is a pretty good
conductor), the heat of the tongue will be _more rapidly_ drawn off, and
the egg _appear colder_.
Q. _Why is the large END of an EGG CRACKED, when put into a saucepan to
boil?_
A. _To let the air out_; if the large end were _not cracked_, the air
(expanded by the heat) _would enter the white of the egg_, and give it
an _offensive taste_.
PART II.
AIR.
CHAPTER XVIII.
Q. _Of what is atmospheric AIR composed?_
A. Principally of two gases, _oxygen_ and _nitrogen_; mixed together in
the following proportion: viz. 1 part of oxygen, to 4 parts of nitrogen.
Q. _What are the uses of the OXYGEN of the air?_
A. It is the _oxygen_ of the air which _supports combustion_, and
_sustains life_.
Q. _What is meant when it is said, that the OXYGEN of the air "SUPPORTS
COMBUSTION?"_
A. It means this; that it is the _oxygen of the air_ which makes _fuel
burn_.
Q. _How does the OXYGEN of the air make FUEL BURN?_
A. The fuel being decomposed (by heat) into _hydrogen_ and _carbon_; the
_carbon combines with the oxygen of the air_, and produces _combustion_.
Q. _What does the combination of carbon and oxygen produce?_
A. The _carbon of the fuel_ combining with the _oxygen of the air_ makes
CARBONIC ACID GAS. (_see pp. 36, 37_).
Q. _What becomes of the HYDROGEN of the FUEL?_
A. Hydrogen (being very inflammable) _burns with a blaze_, and is the
cause of the _flame_ which is produced by combustion. (_see p. 34_).
Q. _What becomes of the NITROGEN of the air, amidst all these changes
and combinations?_
A. The _nitrogen of the air escapes_, and is _absorbed by the leaves_ of
grass, trees, and various other vegetables.
Q. _What is meant when it is said, that OXYGEN "SUSTAINS LIFE?"_
A. It means this: if a person _could not inhale oxygen_, he would _die_.
Q. _What GOOD does this inspiration of OXYGEN do?_
A. 1st--It gives _vitality to the blood_: and
2ndly--It is the _cause of animal heat_.
Q. _How is FOOD converted into BLOOD?_
A. After it is swallowed, it is dissolved in the stomach into a _grey
pulp_; it then passes into the intestines, and is converted by the
"bile" _into a milky substance_ (called _chyle_).
Q. _What BECOMES of the milky substance, called CHYLE?_
A. It is absorbed by the vessels called "_lacteals_," and poured into
the veins _on the left side of the neck_.
Q. _What becomes of the chyle AFTER it is POURED into the VEINS?_
A. It then _mingles with the blood_, and is itself _converted into
blood_.
Q. _How does the OXYGEN we inhale MINGLE with the BLOOD?_
A. The oxygen of the air mingles with the blood _in the lungs_, and
converts it into a _bright red colour_.
Q. _What colour is the blood BEFORE it is oxydized in the lungs?_
A. _A dark purple._ The oxygen turns it to _a bright red_.
Q. _Why are PERSONS so PALE who live in CLOSE ROOMS and CITIES?_
A. The blood derives its redness from the _oxygen_ of the air inhaled;
but, as the air in close rooms and cities _is not fresh_, it is
_deficient in oxygen_, and cannot turn the blood to a beautiful bright
red.
Q. _Why are PERSONS who live in the OPEN AIR and in the country, of a
RUDDY complexion?_
A. As the blood derives its bright red colour from the _oxygen_ of the
air inhaled, therefore, country-people (who inhale _fresh air_) are more
ruddy than citizens.
Q. _Why is not the air in CITIES so FRESH as that in the COUNTRY?_
A. Because it is impregnated with the _breath of its numerous
inhabitants_, the _odour of its sewers_, the _smoke of its fires_, and
many other impurities.
Q. _How does the COMBINATION of OXYGEN with the BLOOD produce animal
HEAT?_
A. The principal element of the blood is _carbon_, which (combining with
the oxygen of the air inhaled) produces _carbonic acid gas_, (in the
same way as burning fuel.) (_see pp._ 33, 36).
Q. _What becomes of the NITROGEN of the air, after the oxygen enters the
blood?_
A. The nitrogen is _exhaled_, and taken up by the leaves of trees and
other vegetables. (_see p._ 35).
Q. _Why does the vitiated air (after the oxygen has been absorbed) COME
OUT of the MOUTH, and not sink into the stomach?_
A. The vitiated air (being _heated by the heat of the body_) _ascends
naturally_, and passes by the _heavier fresh air_ (which we inhale)
without obstruction or injury.
Q. _If (both in combustion and in respiration) the OXYGEN of the air is
CONSUMED, and the NITROGEN REJECTED--Why are not the PROPORTIONS of the
AIR DESTROYED?_
A. Because the _upper surface of vegetable leaves_ (during the day)
_gives out oxygen_ and _absorbs nitrogen_, and thus the proper balance
is perpetually restored.
Q. _Show how God has made ANIMAL and VEGETABLE life DEPENDENT on each
other?_
A. _Animals_ require _oxygen_ to keep them alive, and _draw it from the
air_ by inspiration; the upper surface of _leaves_ (all day long) _gives
out oxygen_, and thus supplies the air with the _very gas_ required by
man and other animals.
Q. _Do not animals EXHALE the VERY GAS needed by VEGETABLES?_
A. Yes; animals reject the _nitrogen of the air_ (as not suited to the
use of animal life), but _vegetables absorb it_, as it is the food they
live on; and thus the vegetable world restores the equilibrium of the
air, disturbed by man and other animals.
Q. _Is AIR a good CONDUCTOR?_
A. No; air is a very _bad conductor_.
Q. _How is AIR HEATED?_
A. By "convective currents."
Q. _What are meant by "CONVECTIVE CURRENTS?"_
A. When a portion of air is heated, _it rises upward in a current_,
carrying the heat with it: other _colder air succeeds_, and (being
_heated_ in a similar way) _ascends also_; and these are called
convective currents.
("Convective currents;" so called from the Latin words, cum-vectus
(_carried with_) because the _heat_ is "carried with" the current.)
Q. _Is AIR HEATED by the RAYS of the SUN?_
A. No; air is _not heated_ (in any sensible degree) _by the action of
the sun's rays_ passing through it.
Q. _Why then is the AIR HOTTER on a SUNNY DAY, than on a CLOUDY one?_
A. On a fine day, the sun _heats the surface of the earth_, and the air
(resting on the earth) _is heated by contact_; as soon as it is heated
_it ascends_, and _other_ air succeeding is _heated in a similar way_,
till all is heated by convection.
Q. _If AIR be a BAD CONDUCTOR, why does hot IRON get COLD, by being
EXPOSED to the AIR?_
A. A piece of hot iron exposed to the air, is made cold--1st--By
"convection;" and
2ndly--By "radiation."
Q. _How is hot iron (exposed to the air) made cold by CONVECTION?_
A. The air around the iron (being intensely heated by contact) rapidly
ascends, _carrying some of its heat with it_: other air succeeds,
_absorbs more heat_, ascends, and gives place to that which is _colder_;
till the hot iron _is cooled completely down_.
Q. _How is hot iron cooled by RADIATION?_
A. While the heat of the iron is being carried off by "convection," it
is _throwing off heat_ (on all sides) _by radiation_.
Q. _What is meant by RADIATION?_
A. Heat emitted (in all directions) from any surface, by _innumerable
rays_.
Q. _Why is BROTH COOLED by being left exposed to the AIR?_
A. Hot broth throws off _some_ heat by _radiation_; but it is _mainly_
cooled down _by convection_.
Q. _How is hot BROTH cooled down by CONVECTION?_
A. The air _resting on the hot broth_ (being heated) _ascends_; _colder_
air succeeding _absorbs more heat_, and _ascends also_; and this process
is repeated, till the broth is _made cool_.
Q. _Why is hot TEA and BROTH COOLED faster, for being STIRRED about?_
A. 1st--The agitation assists the liquor in _bringing its hottest
particles to the surface_:
2ndly--The action of stirring _agitates the air_, and brings it
_quicker_ to the broth or tea: and
3rdly--As the hottest particles are more rapidly brought into contact
with the air, therefore _convection is more rapid_.
Q. _Why is HOT TEA, &c. cooled more rapidly by BLOWING it?_
A. Because the heated air is _blown more rapidly away_; in consequence
of which, _cold air more rapidly succeeds_ to _absorb heat_ from the
surface of the tea or broth.
Q. _If a shutter be closed in the day-time, the stream of light
(piercing through the crevice) seems in CONSTANT AGITATION. WHY is
this?_
A. The air (in the sun-beam piercing through the shutter-crevice) is
_more heated_, than _that in its neighbourhood_; the convective current,
therefore, is _distinctly seen_, where little motes and particles of
dust are _thrown into agitation_ by the _violence_ of the current.
Q. _Why is the GALLERY of a CHURCH or theatre HOTTER than the AISLE or
pit?_
A. The hot air ascends from the _bottom_ to the _top of the room_, and
cold air (from the doors and windows) flies to the _bottom_ to supply
its place.
Q. _Why does a CROWDED ROOM produce HEAD-ACHE?_
A. Because we breathe air _vitiated by the crowd_.
Q. _How does a CROWD VITIATE the AIR of a ROOM?_
A. Whenever we breathe, the elements of the air are _separated_ in the
lungs, _some of the oxygen is absorbed by the blood_, and some of it is
converted into _carbonic acid gas_, and exhaled with the nitrogen.
Q. _Is ALL the NITROGEN REJECTED by the lungs?_
A. Yes; all the nitrogen of the air is always exhaled.
Q. _What is CARBONIC ACID GAS?_
A. As carbon has a very great affinity for oxygen, therefore, whenever
they are exposed to heat, they _combine_, and form carbonic acid gas
(or what is vulgarly called fixed air).
Q. _Is CARBONIC ACID GAS wholesome?_
A. No; it is quite _fatal to animal life_; and whenever it is inhaled,
it acts like a narcotic poison, (producing drowsiness which ends in
death).
Q. _Why is a CROWDED ROOM UNWHOLESOME?_
A. Because the oxygen of the air is either _absorbed by the lungs_, or
substituted for _carbonic acid gas_, which is a noxious poison.
Q. _Mention the historical circumstances, so well known in connection
with the "BLACK HOLE of CALCUTTA."_
A. In the reign of George II, the Raja (or Prince) of Bengal[12] marched
suddenly to Calcutta to drive the English from the country; as the
attack was unexpected, the English were obliged to submit, and 146
persons were taken prisoners.
[12] The Sur Raja, at Dowlat; a young man of violent passions, who had
but just succeeded to the throne. A. D. 1756.
Q. _What became of these prisoners?_
A. They were driven into a place about 18 feet square, and 15 or 16 feet
in height, with only two small grated windows. 123 of the prisoners died
in one night; and (of the 23 who survived) the larger portion died of
putrid fevers, after they were liberated in the morning.
Q. _Why were 123 persons SUFFOCATED in a few hours, from confinement in
this close hot PRISON-hole?_
A. Because the _oxygen of the air_ was soon consumed by so many lungs,
and its place supplied by _carbonic acid_ exhaled by the hot breath.
Q. _Why do persons in a crowded CHURCH feel DROWSY?_
A. 1st--Because the crowded congregation _inhale a large portion of the
oxygen of the air_, which alone can sustain vitality and healthy action:
and
2ndly--The air of the church is impregnated with carbonic acid gas,
which (being a strong narcotic) produces drowsiness in those who inhale
it.
Q. _Why did the captives in the BLACK HOLE die SLEEPING?_
A. 1st--Because the _absence of oxygen_ quickly affects the vital
functions, depresses the nervous energies, and produces a lassitude
which ends in death: and
2ndly--The _carbonic acid gas_ inhaled by the captives (being a narcotic
poison) would also produce _drowsiness and death_.
Q. _Why do PERSONS, who are so much in the OPEN AIR, enjoy the best
HEALTH?_
A. Because the air they inhale is _much more pure_.
Q. _Why is COUNTRY AIR more PURE than the air in CITIES?_
A. 1st--Because there are fewer inhabitants to vitiate the air:
2ndly--There are more trees to restore the equilibrium of the vitiated
air: and
3rdly--The free circulation of air keeps it pure and wholesome (in the
same way as running streams are pure and wholesome, while stagnant
waters are the contrary).
Q. _Why does the SCANTINESS of a country POPULATION render the COUNTRY
AIR more PURE?_
A. Because the fewer the inhabitants, _the less carbonic acid will be
exhaled_; and thus country people will inhale _pure oxygen_, instead of
air _impregnated with the narcotic poison_, called carbonic acid gas.
Q. _Why do TREES and FLOWERS help to make country AIR WHOLESOME?_
A. Because trees and flowers _absorb the carbonic acid_ generated by the
lungs of animals, putrid substances, and other noxious exhalations.
Q. _Why is the AIR of CITIES LESS wholesome than COUNTRY air?_
A. 1st--Because there are _more inhabitants_ to vitiate the air:
2ndly--The _sewers_, _drains_, _bins_, and _filth of a city_, very
greatly vitiate the air:
3rdly--The streets and alleys prevent a free circulation: and
4thly--Besides all this, there are fewer trees to absorb the excess of
carbonic acid gas, and _restore the equilibrium_.
Q. _Why are PERSONS who live in CLOSE ROOMS and crowded CITIES,
generally SICKLY?_
A. Because the air they breathe is not pure, but is both _defective in
oxygen_, and impregnated with _carbonic acid gas_.
Q. _Where does the CARBONIC ACID of close ROOMS and CITIES COME from?_
A. From the lungs of the inhabitants, the sewers, drains, and so on:
besides, trees and gardens are not numerous enough _to absorb the
noxious gas_ as fast as it is generated.
Q. _What BECOMES of the CARBONIC ACID of crowded cities?_
A. Some of it is _absorbed by vegetables_, and the rest is _blown away
by the wind_, and diffused through the whole volume of the air.
Q. _Does not this constant diffusion of carbonic acid affect the PURITY
of the WHOLE AIR?_
A. No; because after it is thus diffused, _it is carried to various
lands_, and _absorbed_ in its passage by the _vegetable world_.
Q. _Why do persons who ascend in balloons feel intense pain in their
eyes and ears?_
A. Because the air of the upper regions is _more rarefied than the air
on the earth_; and the air inside their bodies (seeking to become of the
same rarity) _bursts through their eyes and ears_, producing an intense
pain.
Q. _Why is it often PAINFUL, and difficult to BREATHE, on a MOUNTAIN
top?_
A. Because the pressure of air on the mountain top is _not so great as
on the plain_; and the air inside our bodies (seeking to become of the
same rarity) _bursts through the pores of the body_, and produces great
pain.
Q. _Why do we feel OPPRESSED just PREVIOUS to a STORM?_
A. Because the air is _greatly rarefied by heat and vapour_; and the air
inside us (seeking to become of the same rarity) produces an oppressive
and suffocating feeling.
Q. _Why do DIVERS suffer great pain in their eyes and ears under water?_
A. Because the air at the bottom of the sea _is more dense_ than the air
_on the surface_; and while the air inside the diver's body is settling
into the same density, he feels oppressed with pain, especially in the
ears.
Q. _Why is this PAIN felt especially about the EARS of a DIVER?_
A. The ear is fitted with a small membrane called _the drum_ (or
tympanum), through which the dense air bursts, and the rupture very
often _produces incurable deafness_.
Q. _Why do our CORNS ache just previous to RAIN?_
A. Previous to rain, the density of air is greatly lowered (as every one
knows from the fall of the barometer); in consequence of an unequal
pressure, _our feet swell_; but the hard corn, _not being elastic_, is
painfully stretched and pressed.
(Some of this pain is due to electricity.)
Q. _Why do CELLARS feel WARM in WINTER?_
A. As the external air has not free access into cellars, they remain at
a _pretty even temperature_, which (in winter time) is about 10 degrees
_warmer_ than the external air.
Q. _Why do CELLARS feel COLD in SUMMER time?_
A. As the external air has not free access into cellars, they remain at
a _pretty even temperature_, which (in summer time) is about 10 degrees
_colder_ than the external air.
Q. _Why does lightning strike the OAK-tree more frequently than any
OTHER tree?_
A. 1st--Because the _grain of the oak, being closer_ than that of any
other tree, renders it a better conductor: and
2ndly--The _sap_ of the oak contains a _large quantity of iron_ in
solution, which is a most admirable conductor of lightning.
Q. _Why does AIR rust IRON?_
A. The _oxygen of the air_ combines with the _surface of the iron_, and
produces _oxide of iron_, which is generally called rust.
This rust is a species of combustion.
Q. _Why does hot iron SCALE and PEEL off, when struck with a HAMMER?_
A. The _oxygen of the air_ very readily unites with _the surface of the
hot iron_, and forms a metallic oxide (or rust) which scales off when
struck with a hammer.
Q. _Does iron RUST in DRY air?_
A. No; iron undergoes no change in dry air.
Q. _Why do STOVES and FIRE-IRONS become RUSTY, in rooms which are not
OCCUPIED?_
A. Because the air is damp; and moist air _oxidizes_ (or rusts) iron and
steel.
Q. _In what part of the year is it most difficult to keep STOVES and
FIRE-IRONS BRIGHT?_
A. In _autumn and winter_; because the capacity of the air for holding
water _being on the decrease_, its vapour is deposited on every-thing
with which it comes in contact.
Q. _Why does GREASING iron prevent its becoming RUSTY?_
A. Because _grease_ prevents the humidity of air from coming in contact
with the _surface of the iron_.
Q. _Why do not STOVES rust so frequently as POKERS and TONGS?_
A. Because stoves are generally _covered with plumbago_, or black lead.
Q. _What is plumbago, or black lead?_
A. A mixture of charcoal and iron filings.
A most excellent varnish to prevent rust is made of 1 pint of fat oil
varnish, mixed with 5 pints of highly rectified spirits of turpentine,
rubbed on the iron or steel with a piece of sponge. This varnish may be
applied to bright stoves and even mathematical instruments, without
injuring their delicate polish.
Q. _Why does ornamental STEEL (of a purple or LILAC colour) rust more
readily than polished WHITE steel?_
A. Because the lilac tinge is produced by _partial oxidation_; and the
process which forms rust has, therefore, _already_ commenced.
Q. _How can lilac STEEL be kept FREE from RUST?_
A. By keeping it in a very _dry place_; for then no additional oxygen
will come in contact with it, to increase its amount of rust.
Q. _Do any OTHER metals (besides iron) combine rapidly with oxygen?_
A. Yes; copper, lead, mercury, and even silver to some extent.
Q. _Why does COPPER TARNISH?_
A. The tarnish of copper is caused by its _oxidation_; that is, the
oxygen of the air combines with the surface of the copper, and instead
of _rusting it_, covers it with a _dark tarnish_.
Q. _Why does LEAD lose its BRIGHTNESS, and become DULL and of a DARKER
hue, by being exposed to the air?_
A. The vapour of the air combines with the lead, and _oxidizes its
surface_; but instead of becoming _rusty_, the surface becomes _dull_,
and of a _darker colour_.
Q. _Why is it difficult to keep SILVER BRIGHT?_
A. Because the vapour of the air oxidizes its surface, and _tarnishes_
it.
Q. _Why do silver TEA-POTS and SPOONS tarnish more quickly than silver
ore or bullion?_
A. Because alloy (of some _baser_ metal) is used to make it more _hard
and lasting_; and this _alloy_ oxidizes more quickly than silver itself.
Q. _Why does GERMAN silver turn a dingy yellow in a few hours?_
A. German silver has a great affinity for oxygen, and shows its
oxidation by a _sickly yellow tarnish_, instead of rust.
Q. _If quicksilver (or mercury) is tarnished like copper and lead,--Why
does it preserve its BRILLIANCY in BAROMETERS and THERMOMETERS?_
A. Because _air_ is excluded from it, and no moisture comes in contact
with it to _oxidize_ (or _tarnish_ it).
Q. _Is GOLD affected by the atmosphere?_
A. Not readily: gold will never combine with oxygen of itself, (or
without aid).
Q. _Which of the METALS is capable of resisting oxidation altogether?_
A. Plat'inum; in consequence of which, the graduated arcs of delicate
instruments for observation are made of plat'inum instead of any _other_
metal.
Q. _Why is PLAT'INUM used for the graduated arcs of delicate
mathematical instruments, instead of any other metal?_
A. Because it will never oxidize; but retain its _bright surface_ in all
weathers free from both _rust and tarnish_.
Q. _Before plat'inum was discovered, which of the metals was employed
for the same purpose?_
A. Gold.
Platinum, (a white metal), so called from "plata," the Spanish word for
_silver_. It was first introduced into England by Mr. Wood, (A. D. 1749)
from South America.
Q. _For what other SCIENTIFIC purposes is PLAT'INUM now used?_
A. For crucibles in which _acids_ are employed, and for galvanic
batteries.
Q. _Why are CRUCIBLES (in which acids are employed) made of PLAT'INUM?_
A. Because the acid would act upon _other metals_, or upon _glass_, and
prevent the experimenter's success.
Q. _Which of the METALS have the GREATEST affinity to OXYGEN?_
A. Those called _potassium_ and _sodium_.
Potassium and sodium derive their names from potash and soda. Potassa is
the oxide of potassium; and soda is the oxide of sodium.
Q. _How is the affinity of potassium and sodium for oxygen shewn?_
A. They _decompose water_ the moment they are brought into contact with
it.
Q. _What effect has POTASSIUM on WATER?_
A. It _catches fire_ the moment it is thrown into water, and burns with
a vivid flame, which is still further increased by the combustion of
_hydrogen_ separated from the water.
(N.B. Water is composed of oxygen and hydrogen; and potassium separates
the two gases.)
Q. _What effect has SODIUM on WATER?_
A. It does _not take fire_ as potassium does, but undergoes very rapid
_oxidation_.
Q. _Is the FURR of KETTLES an oxide?_
A. No; the furr (or deposit of boiling water) is a precipitate of
_lime and mineral salt_, separated from the water by the process of
boiling.
Q. _Is not this FURR of boiling water often DANGEROUS?_
A. Yes; especially in _tubular boilers_, such as those employed in
railways.
Q. _Why is this FURR especially TROUBLESOME in RAILWAY engines?_
A. Because it is a _bad conductor of heat_; in consequence of which, it
hinders the evaporating effect of the fire, and prevents the economy of
fuel.
Q. _Why is this FURR especially DANGEROUS in RAILWAY engines?_
A. Because when it is deposited in the boilers, they are likely to
become _over-heated_; and then _explosion_ will take place from the
sudden generation of highly elastic steam.
Q. _Why cannot RAILWAY engines be fed with BRACKISH WATER?_
A. Because brackish water contains _mineral salt_, which makes a much
larger deposit of furr, than that which contains _only vegetable
matter_.
CHAPTER XIX.
CARBONIC ACID GAS.
Q. _What is CHOKE DAMP?_
A. _Carbonic acid gas_ accumulated at the bottom of wells and pits,
which renders them noxious, and often fatal.
Q. _Why is not this carbonic acid TAKEN UP by the AIR, and DIFFUSED, as
it is in cities?_
A. Because (being _heavier than common air_) it cannot _rise from the
well or pit_; and no wind can get to it to blow it away.
Q. _Is CARBONIC ACID WHOLESOME?_
A. No; it is _fatal to animal life_, when inhaled through the mouth;
acting on the stomach, _as a narcotic poison_ (i. e. a poison which
produces death from drowsiness).
Q. _How can any one KNOW, if a place be infested with CARBONIC ACID
GAS?_
A. If a pit or well contain carbonic acid, _a candle_ (let down into it)
_will be instantly extinguished_. The rule, therefore, is this--Where a
_candle will burn, a man can live_; but _what will extinguish a candle_,
will _also destroy life_.
Q. _Why does a MINER lower a CANDLE into a mine, before he descends?_
A. Because the _candle will be extinguished_, if the mine contains
carbonic acid gas: but if the candle is _not extinguished_, the mine is
_safe_, and the man may fearlessly descend.
Q. _Why are PERSONS sometimes KILLED, by leaning over BEER VATS?_
A. Vats (where beer has been made) contain a _large quantity of carbonic
acid gas_, produced by the "vinous fermentation" of the beer; and when a
man incautiously _leans over a beer vat_, and inhales the carbonic acid,
he is immediately _killed_ thereby.
Q. _Why are PERSONS often KILLED, who enter BEER VATS to clean them?_
A. Carbonic acid (being _heavier than atmospheric air_) often rests upon
the _bottom of a vat_: when, therefore, a person enters the vat, and
_stoops to clean the bottom_, he inhales the pernicious gas, which
_kills_ him.
Q. _Why are the JUNGLES of Jarva and Hindostan so FATAL to life?_
A. Because vast quantities of _carbonic acid_ are thrown off by decaying
_vegetables_; and (as the wind cannot penetrate the thick brushwood to
blow it away) _it settles there_, and destroys animal life.
Q. _Why are PERSONS sometimes KILLED by having a CHARCOAL FIRE in their
bed-rooms?_
A. When charcoal is burned, the _carbon of the charcoal_ unites with the
_oxygen of the air_, and forms _carbonic acid gas_, which is a narcotic
poison.
Q. _Why does the carbonic acid gas of a CHARCOAL FIRE RISE and DISPERSE
itself about the room; whereas the carbonic acid gas of a BEER VAT
SETTLES near the FLOOR?_
A. The carbonic acid gas of a charcoal fire _is heated_ by the
_combustion of the fuel_, and rises; but the carbonic acid gas of a beer
vat is _not_ heated, and, therefore, rests on the _bottom of the vat_.
Q. _Why do persons throw LIME into BINS to PREVENT their offensive
SMELL, in summer time?_
A. Bins contain large quantities of _carbonic acid gas_, which readily
_combines with lime_, and produces "_carbonate of lime_," which is
entirely free from all offensive odour.
Q. _Why do persons throw LIME into SEWERS in summer time?_
A. Sewers (like bins) contain large quantities of _carbonic acid_, which
readily _combines with lime_, and produces _carbonate of lime_; and thus
the offensive gas of the sewer is neutralized.
Q. _Can carbonic acid be removed in any way BESIDES by LIME?_
A. Yes; _water_ thrown into a pit will disperse the carbonic acid.
Q. _What effect has WATER on CARBONIC ACID GAS?_
A. Water (under _pressure_) _absorbs_ carbonic acid gas; and _parts_
with it (when the _pressure is removed_) in the form of EFFERVESCENCE.
Q. _Why does AERATED WATER effervesce, when the CORK is removed?_
A. While the _cork was fastened down_, the water _absorbed_ the carbonic
acid; but the moment _the pressure is removed_ (by taking out the cork)
the gas is given out with _effervescence_.
Q. _Why does SODA WATER effervesce?_
A. Soda water contains 8 times its own bulk of carbonic acid gas, which
makes its escape in _effervescence_, the moment that the _cork is
removed_.
Q. _Why does GINGER POP fly about in froth, when the string of the cork
is cut?_
A. All vinous fermentation produces carbonic acid gas. While the _cork
is fast_, the water of the liquor _absorbs_ the carbonic acid; but the
moment that the _pressure is removed_, the gas is given off in
_effervescence_.
Q. _Why does BOTTLED ALE froth, more than DRAUGHT ale?_
A. Because the _pressure_ is greater in a _bottle_ than in a tub which
is perpetually tapped: and effervescence is always produced _in
proportion to the pressure_.
Q. _Why does bottled ALE and PORTER become "LIVELY" and FROTHY by being
SET before the FIRE?_
A. The heat of the fire _expands the air_ (between the liquid and the
cork), and as this air expands, _it presses the liquid down_, which
causes effervescence.
Q. _What produces the FROTH of BOTTLED PORTER?_
A. The _carbonic acid gas_, produced by its _vinous fermentation_; which
is _absorbed by the liquor_ so long as the bottle is _well corked_, but
is _given off in froth_ as soon as the pressure of the cork is
_removed_.
Q. _What gives the pleasant ACID taste to soda water, ginger beer,
champagne, and cider?_
A. The presence of _carbonic acid_, generated by fermentation, and
liberated by effervescence when the pressure of the cork is removed.
Q. _Why does fresh SPRING WATER SPARKLE, when poured from one vessel to
another?_
A. Because fresh spring and pump water contain _carbonic acid_; and it
is the presence of this gas _which makes the water sparkle_.
Q. _What is the FERMENTATION of BEER and WINE?_
A. The production of carbonic acid gas and al'cohol.
Q. _How is CARBONIC ACID GAS produced by FERMENTATION?_
A. Malt and fruit _both contain sugar_; and sugar consists of carbon,
oxygen, and hydrogen. In fermentation, a part of the _carbon and oxygen_
of the sugar escape, _in the form of carbonic acid gas_.
Carbonic acid gas is a compound of carbon and oxygen, in the following
proportions:--3 lbs. of carbon and 8 lbs. of oxygen will form 11 lbs. of
carbonic acid gas. Now, 100 lbs. of white sugar contains 43 lbs. of
carbon; 50 lbs. of oxygen; and 7 lbs. of hydrogen.
Q. _How is AL'COHOL produced by FERMENTATION?_
A. The _hydrogen of the sugar_ combines with the _residue of the oxygen
and carbon_ to form "AL'COHOL."
Q. _What is AL'COHOL?_
A. Al'cohol is the _spirit_ of wine or beer, obtained by _fermentation_.
(100 gallons of alcohol consist of 38 gallons of oxygen; 43-1/2 of
carbon; 15 of hydrogen; and 3-1/2 of nitrogen.)
Q. _Why is BARLEY MALTED?_
A. Because _germination_ is produced by the artificial heat; and in
germination the _starch of the grain_ is converted into _sugar_.
Q. _How is barley malted?_
A. The barley is _moistened with water_, and _heaped up_; by which
means, great _heat_ is produced, which makes the _barley sprout_.
Q. _Why is not the BARLEY suffered to GROW, as well as SPROUT?_
A. Plants in the _germ_ contain _more sugar_ than in any _other state_;
as soon as the germ _puts forth shoots_, the _sugar_ of the plant is
_consumed_, to _support the shoot_.
Q. _How is BARLEY PREVENTED from SHOOTING, in the process of MALTING?_
A. The barley is _put into a kiln_ as soon as it sprouts; and the _heat_
of the kiln checks or _destroys the young shoot_.
Q. _Why is YEAST put into BEER to make it WORK?_
A. Yeast supplies the beer with _nitrogen_, which is one of the
ingredients of alcohol.
Alcohol consists of _oxygen_, _carbon_, and _hydrogen_, (obtained from
the sugar of malt), and _nitrogen_, (obtained from yeast).
Q. _Why is it NOT needful to put YEAST into WINE?_
A. Because fruit contains _carbon_, _hydrogen_, _oxygen_, and
_nitrogen_, in the form of "gluten;" and, therefore, ferments
_spontaneously_.
(Gluten is explained fully in the Appendix.--Turn to the word in the
Index.)
Q. _Does not MALT contain carbon, hydrogen, oxygen, and nitrogen, as
well as FRUIT?_
A. No; the sugar of malt contains _carbon_, _hydrogen_, and _oxygen_,
but _no nitrogen_; in consequence of which, _yeast_ (which contains
_nitrogen_) is _added to the wort_.
Q. _Why do NOT GRAPES ferment while they hang on the VINE?_
A. 1st--Because the skin lets out the _water of the pulp_, which causes
the grapes to shrivel and dry up: and
2ndly--The skin _prevents_ the admission of _oxygen into the pulp_, from
the air without.
Q. _What is the FROTH or SCUM of fermented LIQUORS?_
A. _Carbonic acid gas_, which (being heavier than common air) _settles
on the top of the liquor_, in the form of scum.
Q. _Why does a small piece of raw MEAT, or a few RAISINS improve FLAT
BEER?_
A. 1st--Because they supply it with _nitrogen_ to form it into al'cohol.
2ndly--As the raw meat, &c. _putrifies_, it gives off _carbonic acid
gas_ into the beer, which gives it "life."
Q. _Why is BEER FLAT, if the cask be open too long?_
A. Because _too much of the carbonic acid gas_ (produced by
fermentation) is suffered to _escape_.
Q. _How is the CARBONIC ACID GAS of BEER generated?_
A. The saccharine (or sugar) of the malt is converted by _fermentation_
into carbonic acid gas and alcohol.
Q. _Why does BEER turn FLAT, if the VENT PEG be left OUT of the tub?_
A. Because the _carbonic acid gas escapes_ through the vent hole.
Q. _Why will NOT beer RUN OUT of the tub, till the VENT PEG is taken
out?_
A. When the tap is turned, _air rushes through the tap_ into the bottom
of the tub, and _holds the liquor in_.
The _upward_ pressure of air is illustrated by the
following simple experiment:--Fill a wine-glass with water; cover the
top of the glass with a piece of writing paper; turn the glass topsy
turvy, and the water will not run out. The paper is used merely to give
the air a medium sufficiently dense to act against.
Q. _Why does the BEER RUN FREELY, immediately the VENT PEG is taken
out?_
A. As soon as the vent peg is taken out, air rushes _through the vent
hole_ at the _top of the tub_,--presses the liquor _down_, and _forces
it through the tap_.
Q. _Why does liquor flow reluctantly out of a BOTTLE held upside down?_
A. Because the _upward pressure of the air_ prevents the liquor from
flowing out.
Q. _Why should a bottle be held OBLIQUELY, in order to be emptied of its
liquor?_
A. Because _air_ will then _flow into the bottle_, and help the liquor
out by _balancing the upward pressure_.
Q. _Why does wine (poured from a bottle QUICKLY) SPIRT about without
going into the decanter?_
A. The liquor fills the _top of the decanter_ (like a _cork_), and
leaves _no room_ for the air inside _to escape_; therefore, the decanter
(being _full of air_) refuses to admit the _wine_.
Q. _Why is BEER made STALE, by being exposed to the AIR?_
A. Because air _absorbs its carbonic acid_, which gave it "life."
Q. _Why is PORTER made STALE, by being exposed to the AIR?_
A. Because air _absorbs its carbonic acid_, which gave it "life."
Q. _Why does the EFFERVESCENCE of soda water and ginger beer so soon go
off?_
A. Because air _absorbs the carbonic acid_, which produced the
effervescence.
Q. _Why is BOILED WATER FLAT and insipid?_
A. Because the whole of the _carbonic acid is expelled_ by boiling, and
_absorbed by the air_.
Q. _Why does WATER become FLAT and insipid, after it has been DRAWN some
time?_
A. Because air _absorbs its carbonic acid_; and when its carbonic acid
is absorbed, the water is flat and insipid.
Q. _Why should spring WATER (used for WASHING) be exposed to the AIR?_
A. Spring water _contains carbonic acid_; but (by being exposed to the
_air_) this carbonic acid is _absorbed_, and the water becomes _more
soft_.
Q. _Why does YEAST make BREAD LIGHT?_
A. Flour contains a small portion of _saccharine matter_ (or sugar); and
the yeast (mixing with this) produces _fermentation_, as it does in
brewing.
Q. _How does FERMENTATION make the DOUGH RISE?_
A. During fermentation, _carbonic acid gas is evolved_; but the sticky
texture of the dough will not allow it to _escape_, so it _forces up
little bladders_ all over the dough.
Q. _Why is DOUGH placed BEFORE the FIRE?_
A. 1st--Because the heat of the fire _increases the fermentation_: and
2ndly--It _expands the gas_ which is confined in the little bladders; in
consequence of which, the bladders are _blown up larger_, and the dough
becomes lighter and more porous.
Q. _Why is BREAD HEAVY, if the dough be removed from the fire?_
A. Because the dough _gets cold_, and then the air in the bladders
_condenses_,--the paste falls,--and the bread is close and heavy.
Q. _Whence does the HEAT of FIRE arise?_
A. The _carbon of fuel_ (when heated) _combines with the oxygen of the
air_, and produces _carbonic acid gas_: again, the _hydrogen of the
fuel_ combining with _other portions of oxygen_, condenses into _water_;
by which chemical actions _heat is evolved_.
Q. _Whence does the HEAT of our own BODY arise?_
A. The _carbon of the blood_ combines with the _oxygen of the air
inhaled_, and produces _carbonic acid gas_; which produces heat in a way
similar to burning fuel.
Q. _Whence does the HEAT of a DUNGHILL arise?_
A. The _straw, &c. of the dunghill_ undergoes _fermentation_ as it
decays: the fermentation produces _carbonic acid gas_, and heat is
evolved by a species of combustion (as in the two former cases).
Q. _What changes do VEGETABLES undergo from PUTREFACTION?_
A. The _hydrogen of the vegetables_ combines with the _oxygen of the
air_, and forms _water_: again, the _carbon of the vegetables_ combines
with _oxygen of the air_, and forms _carbonic acid gas_. Putrefaction,
therefore, is only another species of combustion.
Q. _What changes do ANIMAL bodies undergo from PUTREFACTION?_
A. The same as vegetables, with this addition--they give out _ammonia_,
_sulphur_, and _phosphorus_ also; which causes the _offensive smell_ of
putrefying animal bodies.
Q. _Why is LIME heated by a KILN?_
A. All marl and chalk abound _in carbonic acid_; and (when heated by a
fire) the carbonic acid _flies off in gas_, producing great heat.
Q. _What is MORTAR?_
A. Lime mixed with sand and water.
Q. _What is LIME?_
A. _Lime-stone burnt_ produces lime.
Q. _Why is the lime-stone BURNT, in order to make it into LIME?_
A. The fire _expels the carbonic acid_, and converts the hard lime-stone
into a _loose powder_.
Q. _Why does MORTAR become HARD, after a few days?_
A. Because the lime _re-imbibes_ the carbonic acid of the air, which was
_expelled by fire_; and the loose _powder_ again becomes as hard as the
original _lime-stone_.
Q. _Why is MORTAR adhesive?_
A. When the carbonic acid is expelled, the hard lime-stone is _converted
into a loose powder_, which (being mixed with sand and water) becomes a
_soft and sticky plaster_; but, as soon as it is placed between bricks,
it _imbibes carbonic acid again_, and hardens into _lime-stone_.
CHAPTER XX.
CARBURETTED HYDROGEN GAS.
Q. _What is CHOKE-DAMP?_
A. _Carbonic acid gas_ accumulated at the bottom of wells and pits. It
is called CHOKE damp, because it _chokes_ (or suffocates) _every animal
that inhales it_. (_see p._ 264).
Q. _What is marsh-gas or FIRE-DAMP?_
A. _Carburetted hydrogen gas_ accumulated on marshes, in stagnant
waters, and coal pits; it is frequently called "inflammable air."
Q. _What is CARBURETTED HYDROGEN GAS?_
A. _Carbon_ combined with _hydrogen_.
Q. _How may CARBURETTED HYDROGEN GAS be PROCURED on marshes?_
A. By _stirring the mud_ at the bottom of any stagnant pool, and
collecting the gas (as it escapes upwards) in an inverted glass vessel.
Q. _What is COAL GAS?_
A. _Carburetted hydrogen_ extracted from coals, by the heat of _fire_.
Q. _Why is carburetted hydrogen gas called FIRE-DAMP, or inflammable
air?_
A. Because it very readily _catches fire and explodes_, when a light is
introduced to it.
Q. _Why is carburetted hydrogen gas frequently called MARSH GAS?_
A. Because it is generated in _meadows and marshes_ from putrefying
vegetable substances. (_See ignis fatuus_, _p._ 285).
Q. _What gas is evolved by the WICK of a burning CANDLE?_
A. _Carburetted hydrogen gas_: that is, the _carbon and hydrogen_ of the
tallow _combine into a gas_ from the heat of the flame; and this gas is
_carburetted hydrogen_, or inflammable air.
Q. _Why do COAL-MINES so frequently EXPLODE?_
A. Because the _carburetted hydrogen gas_ (which is generated in these
mines by the coals) explodes, when a light is incautiously introduced.
Q. _How can miners SEE in the coal-pits, if they may NEVER introduce a
LIGHT?_
A. Sir Humphrey Davy invented a lantern for the use of miners, called
"the Safety Lamp," which may be used without danger.
Q. _Who was SIR HUMPHREY DAVY?_
A. A very clever chemist, born in Cornwall. (1778--1829).
Q. _What kind of thing is the SAFETY LAMP?_
A. It is a kind of lantern _covered with a fine gauze wire_, instead of
glass or horn.
Q. _How does this fine GAUZE WIRE prevent an EXPLOSION in the coal
mine?_
A. 1st--Because flame will _never pass through fine gauze wire_: and
2ndly--Though the wire get _red-hot_, it will not ignite the gas; for
carburetted hydrogen gas can be ignited only by _flame_.
(N. B. The interstices of the gauze wire must not exceed the 7th of an
inch in diameter.)
Q. _Why will not FLAME PASS THROUGH very fine wire-GAUZE?_
A. Because the metal wire is a very _rapid conductor of heat_; and when
the flame of burning gas in the lamp reaches the wire gauze, the heat
(which is needful to produce flame) is _conducted away by the wire_, and
the flame is _extinguished_.
Q. _Does the gas of the COAL-PIT get THROUGH the wire gauze INTO the
LANTERN?_
A. Yes; but the inflammable gas ignites and burns _inside the lamp_: as
soon, however, as this is the case, _the miner is in danger_, and should
withdraw.
Q. _Why is the miner in DANGER, if the gas ignites and burns in the
INSIDE of the safety-lamp?_
A. Because the heat of the burning gas will soon _destroy the wire
gauze_, and then the flame (being free) will set fire to the mine.
CHAPTER XXI.
PHOSPHURETTED HYDROGEN GAS.
Q. _From what does the very OFFENSIVE EFFLUVIA of CHURCH-YARDS arise?_
A. From a gas called PHOSPHURETTED HYDROGEN; which is _phosphorus_
combined with _hydrogen gas_.
Q. _What is PHOSPHORUS?_
A. A pale amber-coloured substance, resembling wax in appearance. The
word is derived from two Greek words, which mean "_to produce or carry
light_." ([Greek: phôs-pherein]).
Q. _How is PHOSPHORUS OBTAINED?_
A. By heating bones to a white heat; by which means the animal matter
and charcoal are _consumed_, and what is left is called "_phosphate of
lime_."
Q. _How is PHOSPHATE OF LIME converted into PHOSPHORUS?_
A. It is reduced to _powder_, and mixed with _sulphuric acid_; which
(being heated and filtered) is converted into _phosphorus_.
Q. _Of what are LUCIFER MATCHES made?_
A. Of phosphorus; and above 250 thousand lbs. of phosphorus are used
every year in London alone, merely for the manufacture of lucifer
matches.
Q. _Why does a PUTREFYING dead BODY SMELL so offensively?_
A. From the _phosphuretted hydrogen gas_, which always arises from
putrefying animal substances.
The escape of _ammonia_ and _sulphur_ contributes also to this offensive
effluvia.
Q. _What is the cause of the IGNIS FATUUS, Jack o'Lantern, or Will o'the
Wisp?_
A. This luminous appearance (which haunts meadows, bogs, and marshes)
arises from the _gas of putrefying animal and vegetable substances_;
especially decaying fish.
Q. _What gases arise from these PUTREFYING substances?_
A. _Phosphuretted hydrogen gas_ from putrefying _animal_ substances: and
_Carburetted hydrogen_, (or inflammable gas) from fermenting _vegetable_
matters.
Some persons erroneously think that the AURORA BOREALIS, or Northern
Lights, may be attributed to the same gases, burning in the upper
regions of the air.
Q. _How are these gases IGNITED on bogs and meadows?_
A. By the electricity of the air, the rays of the sun, some accidental
spark, the lamp of some traveller, or in some similar way.
And sometimes from the spontaneous combustion of some dung-heaps, &c. in
the locality.
Q. _Why does an ignis fatuus or Will o'the Wisp FLY from us when we RUN
to MEET it?_
A. When we run _towards_ an ignis fatuus, we produce a current of air,
which drives the light gas _forwards_.
Q. _Why does an ignis fatuus run AFTER us, when we FLEE from it in
fright?_
A. When we run _away_ from the ignis fatuus, we produce a current in the
way we run, which _attracts_ the light inflammable gas in the _same
course_.
Q. _Is not a kind of Jack o'Lantern sometimes produced by an INSECT?_
A. Yes; a swarm of luminous insects sometimes passes over a meadow, and
produces an appearance exactly like that of the ignis fatuus.
Q. _May this meteoric appearance be attributed to any OTHER cause,
besides those mentioned?_
A. Yes; if many horses, sheep, pigs, or oxen, are pastured on a meadow,
the _animal vapour_ arising from them (strongly electrified by the air)
_will ignite_, and produce a luminous appearance.
Q. _May not many GHOST stories have risen from some ignis fatuus lurking
about church-yards?_
A. Perhaps _all_ the ghost stories (which deserve any credit at all)
have arisen from the ignited gas of church-yards lurking about the
tombs, to which _fear_ has added its own creations.
CHAPTER XXII.
WIND.
Q. _What is WIND?_
A. Wind is _air in motion_.
Q. _What PUTS the air in motion, so as to produce WIND?_
A. The principal causes are the _variations of heat and cold_, produced
by the succession of _day and night_, and the _four seasons_.
Q. _What effect has HEAT upon the air?_
A. Heat _rarefies_ the air, and causes it to _expand_.
Q. _How do you KNOW that heat causes the air to EXPAND?_
A. If a bladder _half full of air_ (tied tight round the neck), were
laid before a _fire_, the heat of the fire would expand the air so
much, that the bladder would soon be _entirely inflated_; (in this case,
the air in the bladder is expanded to _twice its original bulk_, by the
heat of the fire).
Q. _What EFFECT is produced upon air by RAREFACTION?_
A. It causes the air to _ascend through colder strata_, as a cork (put
at the bottom of a basin of water) would ascend through the water.
Q. _How do you KNOW that rarefied air ASCENDS?_
A. When a boy sets fire to the cotton of his balloon, the flame _heats
the air_ inside the balloon; and the air becomes _so light_, that it
ascends, and _carries the balloon with it_.
Q. _What effect is produced upon AIR by COLD?_
A. Air is _condensed by cold_, or squeezed into a smaller compass; in
consequence of which, _it becomes heavier_, and descends towards the
ground.
Q. _How do you KNOW that air is CONDENSED by COLD?_
A. After the bladder is _fully inflated_, (by lying before the fire), if
it be taken _away from the fire_, the bladder will _collapse_, and show
that it is not half full.
Q. _What is meant by the bladder "COLLAPSING?"_
A. The skin will become _wrinkled, shrivelled, and flabby_, because
there is not sufficient air inside to _fill it out_.
Q. _How do you KNOW that CONDENSED air will DESCEND?_
A. As soon as the cotton of the balloon _is burnt out_, the air inside
becomes _cold again_, and the balloon _falls to the earth_.
Q. _Does the SUN HEAT the AIR as it does the EARTH?_
A. No; the air is _not heated by the rays of the sun_, because air (like
water) is a very _bad conductor_.
Q. _How is the AIR HEATED?_
A. By _convection_, thus:--The _sun_ heats the _earth_, and the _earth_
heats the _air resting upon it_; the air thus heated _rises_, and is
succeeded by _other air_, which is heated in a similar way, till _all
is warmed_ by "convective currents."
Q. _What is meant by "CONVECTIVE CURRENTS of air?"_
A. Streams of air heated by the earth, which _rise upwards_ and _carry
heat with them_, are called "convective currents" of hot air.
Q. _Is the air in a ROOM in perpetual motion, as the air ABROAD is?_
A. Yes; there are always _two currents of air_ in the room we occupy,
one of _hot_ air flowing _out_ of the room, and another of _colder_ air
flowing _into_ the room.
Q. _How do you KNOW, that there are these TWO currents of air in every
occupied ROOM?_
A. If I hold a lighted candle near the crevice _at the top of the door_,
the flame will be blown _outward_ (towards the _hall_); but if I hold
the candle _at the bottom of the door_, the flame will be blown
_inwards_ (into the _room_).
Q. _Why would the flame be blown OUTWARDS (towards the HALL), if the
candle were held at the TOP of the door?_
A. Because as the air of the room is warmed by the fire, &c., _it
ascends_; and (floating about the upper part of the room) some of it
escapes _through the crevice_ at the _top of the door_, and thus
produces a current of air _outwards_ (into the _hall_).
Q. _Why would the flame be blown INWARDS (into the ROOM), if the candle
were held at the BOTTOM of the door?_
A. Because after the warm air of the room has ascended to the ceiling,
or made its escape into the hall, &c., a partial _vacuum_ is made at the
_bottom of the room_; and cold air (from the hall) _rushes under the
door_ to supply the void.
Q. _What is meant by a "partial VACUUM being made, at the BOTTOM of the
ROOM?"_
A. A vacuum means a place _from which the air has been taken_: and a
"_partial_ vacuum" means, a place from which a _part of its air_ has
been taken away. Thus when the air on the floor _ascends to the
ceiling_, a partial vacuum is made _on the floor_.
Q. _And how is the VACUUM filled UP again?_
A. It is filled up by _colder air_, which rushes (under the _door_, and
through the _window_ crevices) into the room.
Q. _Give me an ILLUSTRATION._
A. If I dip a pail into a pond and fill it with water, a hole (or
vacuum) is made in the pond _as big as the pail_; but the moment I _draw
the pail out_, the hole is _filled up_ by the water around.
Q. _Show how this illustration APPLIES._
A. The heated air which ascends from the bottom of a room, is as much
_taken away_, as the water in the pail; and (as the void was instantly
supplied by _other water in the pond_) so the _void of air is supplied_
by a current from _without_.
Q. _What is the CAUSE of WIND?_
A. The _sun_ heats the _earth_, and the _earth_ heats the _air resting
upon it_; as the warm air ascends, the void is filled up by a _rush of
cold air_ to the place, and this _rush of air_ we call WIND.
Q. _Does the WIND ALWAYS blow?_
A. Yes; there is always _some_ motion in the air; but the _violence_ of
the motion is perpetually varying.
Q. _Why is there always SOME motion in the air?_
A. As the earth is _always turning round_, the vertical rays of the sun
are always _varying_.
Q. _What do you mean by "the VERTICAL RAYS of the SUN?"_
A. The rays made at _noon-day_: when the sun is in a _direct line_ above
any place, his rays are said to be "vertical" to that place.
Q. _How are the VERTICAL rays of the sun always VARYING?_
A. Suppose the brass meridian of a globe to represent the vertical rays
of the sun; as you turn the globe round, _different parts_ of it will
pass under the brass rim, in constant _succession_.
Q. _And is it NOON-DAY to the place over which the SUN is VERTICAL?_
A. Yes; as each place passes _under the brass meridian_, it is
_noon-day_ to _one_ half, and _mid-night_ to the _other_.
Q. _Show how this ROTATION of the earth affects the AIR._
A. If we suppose the brass meridian to be the vertical sun, the whole
column of air _beneath_ will be heated by the _noon-day rays_; that part
which the sun has _left_, will become gradually _colder and colder_; and
that part to which the sun is _approaching_, will grow constantly
_warmer and warmer_.
Q. _Then there are THREE qualities of air about this spot?_
A. Yes; the air over the place which _has passed the meridian_ is
_cooling_: the air under _the vertical sun_ is the _hottest_; and the
air which is over the place _about to pass under the meridian_, is
_increasing in heat_.
Q. _How does this VARIETY in the HEAT of AIR produce WIND?_
A. The air always seeks to _preserve an equilibrium_; so the _cold air_
rushes to the _void_, made by the _upward current of the warmer air_.
Q. _Why does not the wind ALWAYS BLOW ONE way, following the direction
of the SUN?_
A. Because the direction of the wind is subject to perpetual
interruptions from _hills and valleys_, _deserts and seas_.
Q. _How can HILLS and MOUNTAINS ALTER the course of the WIND?_
A. Suppose a wind, blowing from the north, comes to a mountain, as it
cannot pass _through it_, it must either rush _back again_, or _fly off
at one side_ (as a _marble_ when it strikes against a _wall_).
Q. _Do MOUNTAINS affect the wind in any OTHER way?_
A. Yes; many mountains are _capped with snow_, and the _warm air_ is
_condensed_ as it comes in contact with them; but as soon as the
_temperature of the wind_ is changed, its _direction_ may be changed
also.
Suppose A B C to be _three columns of air_. A, the column of air which
is _cooling down_; B, the column to which the _sun is vertical_; and C,
the column which _is to be heated next_. In this case the _cold_ air of
A will rush towards B C, because the air of B and C is _hotter_ than A.
But suppose now C to be a _snow-capped mountain_. As the hot air of B
reaches C, it is _chilled_; and (being now _colder_ than the air
_behind_) it rushes _back again_ towards A, instead of following the
sun.
Q. _How can the OCEAN affect the direction of the WIND?_
A. When the ocean rolls beneath the _vertical sun_, the water is _not
made so hot_ as the _land_; and (as another _change of temperature_ is
produced) another obstacle is offered to the _uniform direction of the
wind_.
Q. _Why is not the WATER of the sea made so HOT by the vertical sun, as
the surface of the LAND?_
A. 1st--Because the _evaporation_ of the sea is greater than that of the
land:
2ndly--The waters are _never still_: and
3rdly--The rays of the sun strike _into_ the water, and are _not
reflected from its surface_, as they are by _land_.
Q. _Why does the EVAPORATION of the sea prevent its surface from being
HEATED by the vertical sun?_
A. As water _absorbs_ heat by being _converted into vapour_; the surface
of the sea is continually _losing heat by evaporation_.
Q. _How does the MOTION of the sea prevent its surface from being HEATED
by the vertical sun?_
A. As one portion is heated _it rolls away_, and is succeeded by
_another_; and this constant motion prevents _one_ part of the sea from
being heated _more than another_.
Q. _How is the WIND affected by the SEA?_
A. When air from the hot earth _reaches the sea_, it is often
_condensed_, and either rushes _back again_, or else its violence is
very greatly _abated_.
Q. _Do CLOUDS affect the WIND?_
A. Yes. As passing clouds screen the direct heat of the sun from the
earth, they diminish the _rarefication of the air also_: and this is
_another_ cause why neither the strength nor direction of the wind is
_uniform_.
Q. _Would the winds blow regularly from east to west, if these
OBSTRUCTIONS were REMOVED?_
A. Without doubt they would. If the whole earth were covered with
_water_, the winds would always _follow the sun_, and blow from east to
west. Their irregularity is owing to the interspersion of _sea and
land_, and the irregularities of the earth's surface.
Q. _Do winds NEVER blow REGULARLY?_
A. Yes; in those parts of the world, where these obstructions do not
exist; as on the Atlantic and Pacific Ocean, the winds are pretty
uniform.
Q. _What are the winds, which blow over the ATLANTIC and PACIFIC Ocean,
called?_
A. They are called "Trade Winds."
Q. _Why are they called TRADE WINDS?_
A. Because (as they blow uniformly in one direction) they are very
convenient to those who _carry on trade_ by means of these oceans.
Q. _In what DIRECTION do the TRADE WINDS blow?_
A. That in the _northern_ hemisphere blows from the _north-east_: that
in the _southern_ hemisphere from the _south-east_.
Q. _Why do they not blow from the FULL NORTH and SOUTH?_
A. Because the _polar current_, combining with the _equatorial current_,
give the wind a _new direction_.
Q. _What is the CAUSE of the EQUATORIAL current?_
A. The rotation of the earth upon its axis.
Q. _What is the cause of the POLAR CURRENT?_
A. As the heat in the _torrid zone_ is always _greatest_, and at the
_poles_ the _least_, therefore a constant current of air rushes _from
the poles_ towards the _equator_.
Q. _How does the COMBINATION of these two currents give a new direction
to them both?_
A. When these currents of air meet at the equator, they _clash
together_, and fly off in a new direction.
Q. _Do trade winds blow from the north-east and south-east ALL the YEAR
ROUND?_
A. Yes, _in the open sea_; that is, in the Atlantic and Pacific Oceans
for about 30 degs. each side of the equator.
Q. _Do the TRADE WINDS blow uniformly from north-east and south-east in
the INDIAN OCEAN?_
A. No; nor yet in those parts of the _Atlantic_ and _Pacific_ which
_verge on the land_.
Q. _Why do not the trade winds blow uniformly from north-east and
south-east in the INDIAN OCEAN?_
A. Because when Arabia, Persia, India, and China, are exposed to the
enormous heat of their summer sun, the _air is so rarefied_, that the
colder air from the south pole rushes _towards these nations_, and not
to the _equator_; in consequence of which, a SOUTH-WEST wind is produced
for _six months of the year_.
Q. _How does it blow for the OTHER 6 months?_
A. When the sun has left the _northern_ side of the equator for the
_southern_, then the _southern part of the torrid zone_ is most heated;
and the cold air from the north (rushing towards the southern tropic) is
diverted to the NORTH-EAST, where it continues for the _other_ six
months of the year.
Q. _What are the six-month trade winds called?_
A. They are called MONSOONS; and blow from the _north-east_ from
September to April, and from the _south-west_ for the _other_ six months
of the year.
Q. _Have WE any regular winds in ENGLAND?_
A. No; our island (having a _continent on one side_, and a _sea on the
other_) has a most _variable_ climate.
Q. _Have the winds in England NO general direction throughout the year?_
A. We generally find that _easterly_ winds prevail during the _spring_
of the year, and _westerly_ winds are most common in the _summer_ and
_autumn_.
S-West winds are most frequent in July and August. N-East winds in
January, March, April, May, June; and most seldom in July, September,
and December.
Q. _When are the WINDS in ENGLAND generally the HIGHEST?_
A. The winds in December and January are generally the highest. Those in
February and November the next; and those in August and September the
least boisterous.
Q. _Why are the winds of Europe generally HIGHEST in DECEMBER and
JANUARY?_
A. Because the sun is _furthest south_ in those months; and (as the heat
in these northern regions rapidly _decreases_) the _contrast between our
temperature_ and that of the _torrid zone_ is greater in December and
January, than in any _other_ two months throughout the year.
Q. _Why does this CONTRAST of heat increase the VIOLENCE of the WINDS?_
A. As the air always seeks to _preserve an equilibrium_, therefore the
_greater the contrast_, the more violent will be the rush of air to
_equalize_ the two volumes.
Q. _Why are the winds in Europe generally the most PLACID during the
months of SEPTEMBER and AUGUST?_
A. August and September are our _warmest months_, when we approach
nearer to the heat of the torrid zone than in any _other two months_;
therefore, the air (to and from the equator) _moves with less velocity_
in our northern hemisphere.
Q. _Show the GOODNESS and WISDOM of GOD in the constant tendency of air
to equilibrium._
A. If the cool air of the polar regions did not rush into the torrid
zone, _it would become so hot_, that no human being could endure it. If
(on the other hand) the hot air from the torrid zone did not modify the
polar regions, they would soon become _insufferably cold_.
Q. _Why are EAST WINDS in England generally DRY?_
A. Because, as they come over the _vast continents_ of Asia and Europe,
they absorb _very little water_.
Q. _Why does their imbibing so little water make them DRY winds?_
A. Being thirsty when they reach our island, they readily imbibe
moisture from the air and clouds; and, therefore, _bring dry weather_.
Q. _Why is the NORTH WIND in England generally COLD?_
A. The north wind comes from the _polar regions_, over mountains of
snow, and seas of ice; in consequence of which, it is very _cold_.
Q. _Why are NORTH WINDS in England generally DRY and biting?_
A. As they come from regions _colder than our own_, they are _warmed by
the heat of our island_; and (as their temperature is raised) _they
absorb moisture_ from every thing they touch; in consequence of which,
they are both _dry and parching_.
Q. _Why is the SOUTH WIND generally WARM in England?_
A. The south wind comes over the hot sandy deserts of Africa, and is
heated by the land it traverses.
Q. _Why does the SOUTH WIND often bring us RAIN?_
A. The south wind (being much _heated_ by the hot sands of Africa)
_imbibes water very plentifully_, as it passes over the Mediterranean
Sea and British Channel.
Q. _Why does the SATURATION of the south wind cause RAIN?_
A. As soon as it reaches our cold climate, _it is condensed_, and its
vapour is squeezed out (as water from a wet sponge).
Q. _Why are WEST WINDS in England generally RAINY?_
A. The west winds come over the _Atlantic Ocean_, and are laden with
_vapour_: if, therefore, they meet with the least _chill_, some of the
vapour is squeezed out.
Q. _Why is a fine CLEAR DAY sometimes OVERCAST in a few minutes?_
A. Because some _sudden change of temperature_ has condensed the vapour
of the air _into clouds_.
Q. _Why are CLOUDS sometimes DISSIPATED quite as suddenly?_
A. Because some _dry wind_ (blowing over the clouds) has _imbibed their
moisture_, and carried it off in invisible vapour.
Q. _Why does a SOUTH-WEST wind bring us RAIN?_
A. As it comes from the _torrid zone_, and _crosses the ocean_, the hot
wind is _laden with vapour_; and as some of the heat escapes (as soon
as it reaches our northern island) the _vapour is condensed_, and
precipitated as rain.
Q. _Why does a NORTH-EAST wind RARELY bring RAIN?_
A. As it comes from a climate _colder than our own_, its capacity for
imbibing vapour is _increased_ when it reaches our island; in
consequence of which, it _dries the air_, dispels the clouds, and
promotes evaporation.
Q. _Why does WIND sometimes bring RAIN, and sometimes FINE weather?_
A. If the wind be _colder than the clouds_, it will condense their
vapour into _rain_: if the wind be _warmer than the clouds_, it will
_dissolve_ them, and cause them to disappear.
Q. _Why are MARCH winds DRY?_
A. Because they generally blow from the east or north-east; and,
therefore, _sweep over the continent of Europe_.
Q. _What is the USE of MARCH winds?_
A. They _dry the soil_ (which is saturated with the floods of February),
_break up the heavy clods_, and fit the land for the _seeds_ which are
committed to it.
Q. _Why does "MARCH COME IN like a LION?"_
A. Because it comes in with _blustering east winds_, which are essential
to dry the soil, which would otherwise _rot the seed_ committed to it.
Q. _Why does "MARCH GO OUT like a LAMB?"_
A. Because the water (evaporated by the high winds) falls again in
_showers_ to fertilize the earth, and _breaks the violence of the
winds_.
Q. _Why is it said that "A bushel of MARCH DUST is worth the king's
ransom?"_
A. Because it indicates that there has been a continuance of _dry
weather_; and unless _March be dry_, the seed will rot in the wet soil.
Q. _Why is it said "A DRY cold MARCH never BEGS BREAD?"_
A. Because the _dry cold winds_ of March prepare the soil for _seeds_,
which germinate, and produce fruit in the autumn.
Q. _Why is it said that "A WET MARCH makes a SAD autumn?"_
A. Because, if _March be wet_, so much of the seed _rots in the ground_,
that the autumn crops are spoiled.
Q. _Why is it said that "MARCH FLOWERS make NO summer BOWERS?"_
A. Because, if the _spring be very mild_, vegetation gets too forward,
and is _pinched by the nightly frosts_, so as to produce neither fruits
nor flowers.
Q. _Why is it said "A LATE SPRING makes a FRUITFUL YEAR?"_
A. Because if the vegetation of spring be _backward_, the frosty nights
will _do no harm_; for the fruits and flowers will not put forth their
tender shoots, till the nights become _too warm to injure them_.
Q. _Why is it said that "APRIL SHOWERS bring MAY FLOWERS?"_
A. Before seeds can germinate, _three_ things are essential:--Darkness,
Heat, and Moisture. April showers supply the principal nourishment on
which seeds depend for existence.
Q. _Does RAIN-water possess any fertilizing properties BESIDES that of
mere MOISTURE?_
A. Yes; rain-water contains "AMMONIA," to which much of its fertilizing
power may be attributed.
(Ammonia is a compound of nitrogen and hydrogen. Common hartshorn is
only ammonia and water.)
Q. _Why has God made NOVEMBER a very RAINY month?_
A. Because the rain hastens the _putrefaction of the fallen leaves_, and
this makes the earth fertile.
Q. _Why is there MORE rain FROM SEPTEMBER to MARCH than from March to
September?_
A. From September to March, the temperature of the air is _constantly
decreasing_; on which account, its _capacity for holding vapour_ is on
the _decrease_, and the vapour is precipitated as rain.
Q. _Why is there LESS rain FROM MARCH to SEPTEMBER, than from September
to March?_
A. From March to September, the temperature of the air is _constantly
increasing_; on which account, _its capacity for holding vapour_ is on
the _increase_, and very little is precipitated as rain.
Q. _Why is the RISING SUN in summer accompanied with a BREEZE?_
A. Because the heat of the rising sun _stops the radiation of heat_
from the earth, and _warms its surface_.
Q. _How does this WARMTH produce a BREEZE?_
A. The air (resting on the earth's surface) is _warmed by contact_,
ascends upwards, and _colder air rushes in_ to fill up the void, which
is the cause of the _morning_ breeze.
Q. _Why is there often an EVENING BREEZE during the summer months?_
A. The earth _radiates heat at sun-set_, and the air is cooled down
quickly by contact: this condensation causes a _motion in the air_,
which is the evening breeze.
Q. _Why are TROPICAL ISLANDS always subject to a SEA-breeze every
MORNING (i. e. a breeze blowing from the sea to the land)?_
A. The solar rays are unable to heat the surface of the _sea_ as they do
the _earth_; therefore, the _air resting on the earth_ is more _heated_
than the _air resting on the sea_; and the colder sea air blows _inland_
to restore the equilibrium.
Q. _Why is the LAND BREEZE UNHEALTHY?_
A. Because it is frequently loaded with exhalations from _putrefying
animal_ and _vegetable_ substances.
Q. _Why is the SEA BREEZE fresh and HEALTHY?_
A. Because it passes over the fresh sea, and is _not_ laden with noxious
exhalations.
It is _healthy_, therefore, to walk on the sea-beach before ten o'clock
in the morning; but _unhealthy_ after sun-set.
Q. _Why is there generally a fresh breeze from the SEA (in English
watering places) during the summer and autumn MORNINGS?_
A. As the _land_ is _more heated by the sun_ than the _sea_; therefore,
air resting on the _land_ is hotter than air resting on the _sea_; in
consequence of which, cooler sea air glides _inland_, to restore the
equilibrium.
Q. _Why does the SEA BREEZE feel COOL?_
A. As the sun cannot make the surface of the _sea_ so hot as the surface
of the _land_; therefore, the air which blows from the sea, feels
_cooler than the air of the land_.
Q. _Why are TROPICAL ISLANDS subject to a LAND BREEZE every EVENING
(i. e. a breeze blowing from the land towards the sea)?_
A. The _surface of land_ cools down _faster_ (after sun-set) than the
surface of the _sea_: in consequence of which, the air of the cold land
_is condensed, sinks down_, and spreads itself into the warmer _sea
air_, causing the LAND BREEZE.
Q. _Why is the LAND BREEZE COOL?_
A. As the surface of the land is cooled at sun-set _quicker than the
surface of the sea_; therefore, the seaman feels the air from the land
to be chill.
Q. _Why is the TEMPERATURE of ISLANDS more EQUABLE than that of
CONTINENTS?_
A. Because the _water_ around the island _absorbs_ the extreme heat of
summer, and _gives out_ heat to mitigate the extreme cold of winter.
Q. _Why does the sea round an island GIVE OUT heat in winter?_
A. Unless the _sea be frozen_ (which is rarely the case), it is _warmer_
than the frozen land; and, therefore, the warmth of the sea air (mixing
with the cold land air) helps to mitigate the intense cold.
Q. _Why are there WAVES in the sea?_
A. The wind (acting on the surface of the sea) _piles up ridges of
water_, which leave behind an _indentation_: as the water on all sides
rushes to _fill up this indentation_, the disturbance spreads on all
sides, and billow rolls after billow.
Q. _Why does WIND in England generally feel COLD?_
A. Because a _constantly changing surface_ comes in contact with our
body, to draw off its heat.
Q. _Why is a ROOM (even without a fire) generally WARMER than the OPEN
AIR?_
A. As the air in a room is _not subject to much change_, it soon becomes
of the same temperature as our skin, and no longer feels cold.
Q. _Why do we generally feel COLDER out-of-doors?_
A. Because the air (which surrounds us) _is always changing_; and as
fast as _one_ portion of air has become warmer by contact with our body,
_another colder portion_ surrounds us to absorb more heat.
Q. _Why are HOT FOODS made COOL by BLOWING them?_
A. Blowing causes the air (which covers the hot food) _to change more
rapidly_; in consequence of which, the hot air is _quickly blown away_,
and gives place to fresh _cold air_.
Q. _Why do ladies FAN THEMSELVES in hot weather?_
A. By the action of the fan, _fresh particles of air_ are perpetually
brought in contact with the face, and every fresh particle of air
_absorbs some heat_ from the skin.
Q. _Does the fan COOL the AIR?_
A. No; it makes the _air hotter_, by imparting to it the heat _out of
our face_: but it cools the _face_ blown upon, by transferring its heat
to the _air_.
Q. _Is the AIR in SUMMER time ever so hot as our bodies?_
A. No, not in England. In the hottest day in summer, the air of England
is 15 or 20 degrees cooler than the human body.
Q. _How fast does wind travel?_
A. A gentle breeze goes at about the rate of 5 miles an hour. A high
wind from 20 to 60. A hurricane from 80 to 100 miles an hour.
Q. _How is the VELOCITY of WINDS ascertained?_
A. By observing the velocity of the clouds, and by an instrument for the
purpose.
This instrument is called an ANEMOMETER.
Q. _How is the VELOCITY of the CLOUDS ascertained?_
A. By observing the speed of their shadow along the ground; which is
found in a high wind to vary from 20 to 60 miles an hour.
Q. _Why is there always a strong DRAUGHT through the KEYHOLE of a door?_
A. As the air of the room we occupy is _warmer than the air in the
hall_, therefore the cold hall air _rushes through the keyhole_ into the
room, and causes a draught.
Q. _Why is there always a strong DRAUGHT UNDER the DOOR, and through the
crevice on each side?_
A. The cold air _rushes from the hall_ under the door, &c. into the
room, to supply the _void_ caused in the room (by the escape of warm
air up the chimney, &c.)
Q. _Why is there always a DRAUGHT through the WINDOW crevices?_
A. The external air (being colder than the air of the room we occupy)
rushes through the window crevices _to supply the deficiency_, caused by
the escape of air up the chimney, &c.
Q. _Why is there more DRAUGHT if you open the LOWER SASH of a window,
than if you open the UPPER sash?_
A. If the _lower_ sash be open, the _cold external air_ will rush more
freely _into the room_; but if the _upper_ sash be open the _heated air
of the room_ will _rush out_; and (of course) there will be less
draught.
Q. _By which means is the ROOM better VENTILATED, by opening the lower
or the upper sash?_
A. A room is better _ventilated_ by opening the _upper sash_; because
the hot vitiated air (which always ascends towards the ceiling) _can
better escape_.
Q. _By which means is a HOT ROOM more quickly COOLED--By opening the
upper or the lower sash?_
A. A hot room is _cooled more quickly_ by opening the _lower sash_;
because the cold air can enter more freely by an _under_ current, than
by one _higher up_.
Q. _Why does WIND DRY damp LINEN?_
A. Because dry wind (like a dry sponge) imbibes the particles of vapour
from the surface of the linen, as fast as they are formed.
Q. _Which is the HOTTEST PLACE in a church, chapel, or theatre?_
A. The gallery.
Q. _Why is the GALLERY of all public places HOTTER than the lower parts
of the building?_
A. Because the heated air of the room _ascends_, and all the _cold air_
(which can enter through the doors and windows) _keeps to the floor_,
till it has become heated.
Q. _Why do PLANTS often grow out of WALLS and TOWERS?_
A. Because sometimes the _wind_ blows the seed there with the dust; and
sometimes _birds_ (flying over) drop the seed which they had formerly
eaten.
CHAPTER XXIII.
BAROMETER.
Q. _What is a BAROMETER?_
A. A weather-glass, or instrument to show the changes of the weather, by
marking the _variations in the weight of air_.
Q. _What is a THERMOMETER?_
A. An instrument to show how _hot or cold_ anything is.
Q. _What is the DIFFERENCE between a THERMOMETER and a BAROMETER?_
A. In a THERMOMETER the mercury is _sealed up from the air_:
In a BAROMETER the mercury is left _exposed_ (or open) to the air.
Q. _If the mercury of the thermometer be SEALED UP from the air, how can
the air AFFECT it?_
A. The heat of the air passing _through the glass tube_ into the
mercury, causes it to _expand more or less_, and rise in the tube
accordingly.
Q. _Why is the TUBE of a BAROMETER left OPEN?_
A. That the air may _press upon it_ freely; and as this pressure is
_more_ or _less_, the mercury _rises or falls_ in the tube.
Q. _How can weather be affected by the WEIGHT of the air?_
A. When air is warm or moist, it is _lighter_ than usual:
When it is cold or dry, it is _heavier_: and as a barometer marks
whether the air be _light_ or _heavy_, it indicates these _changes_.
Q. _How can you tell (by looking at a BAROMETER) what KIND of WEATHER it
will be?_
A. Because the mercury in the tube _rises and falls_, as the air becomes
lighter or heavier: and we can generally tell by the _weight_ of the
air, what kind of weather to expect.
Q. _Does the WEIGHT of the air VARY MUCH?_
A. Yes; the atmosphere in England varies as much as _one-tenth part_
more or less.
Q. _What is the chief USE of a BAROMETER?_
A. To warn _sailors_ how to _regulate their ships_, before squalls come
on.
Q. _How can a BAROMETER warn SAILORS to regulate their SHIPS?_
A. As the barometer will tell when _wind_, _rain_, or _storm_ is at
hand, the sailor can make his ship trim before it overtakes him.
Q. _Are there any RULES which can be depended on?_
A. Yes; there are _ten special rules_ to direct us how to know the
changes of weather, by marking the mercury of a barometer.
Q. _What is the 1ST SPECIAL RULE in regard to the barometer?_
A. The barometer is _highest of all_ during a _long frost_; and it
generally rises with a _north-east wind_.
Q. _Why is the barometer HIGHEST of all during a long FROST?_
A. Because long frost _condenses the air very greatly_; and the more air
is _condensed_, the greater is its _pressure_ on the mercury of the
barometer.
Q. _Why does the barometer generally RISE with a NORTH-EAST wind?_
A. Because NORTH-EAST winds make the air both _cold and dry_: the air,
therefore, is both _condensed_, and _without vapour_.
Q. _What is the 2ND SPECIAL RULE in regard to the barometer?_
A. The barometer is _lowest of all_ during a _thaw which follows a long
frost_: it generally falls with SOUTH and WESTERN winds.
Q. _Why does the barometer fall LOWEST of all at the BREAKING UP of a
long FROST?_
A. 1st--Because the air (which had been much _dried_ by the frost)
_absorbs the moisture_ of the fresh warm current of wind from the south
or south-west: and
2ndly--The air (which had been much _condensed_ by the frost) is
suddenly _expanded_ by the warm wind which is introduced.
Q. _Why does the barometer fall very low with SOUTH and WEST winds?_
A. Because SOUTH and WEST winds come heavily _laden with vapour_; and
_vaporized_ air is lighter than _dry air_.
Q. _What effect has WIND on the mercury?_
A. _All_ winds make the barometer _drop_, except EASTERN winds: those
winds which blow from the SOUTH, and SOUTH-WEST make it _drop the
lowest_.
Q. _Why do WINDS generally make the mercury of a barometer DROP?_
A. Wind is caused by a _partial vacuum_ in some parts of the globe; and
as the air _rushes in_ to supply this deficiency, _its general pressure
is lessened_, and the barometer falls.
Q. _What is the 3RD SPECIAL RULE in regard to the barometer?_
A. While the barometer stands above 30°, the air must be very _dry_ or
very _cold_, or perhaps _both_, and _no rain_ may be expected.
Q. _Why will there be NO RAIN if the AIR be very DRY?_
A. If the air be very _dry_ it will _absorb moisture_, and not part with
what it has _in rain_.
Q. _Why will there be NO RAIN if the AIR be very COLD?_
A. If the air be very _cold_ it is _so much condensed_, that it has
already parted with as much moisture as it can spare.
Q. _What is the 4TH SPECIAL RULE in regard to the barometer?_
A. When the barometer stands _very low_ indeed, there is never _much_
rain, although a _fine day_ will seldom occur at such times.
Q. _What kind of WEATHER will it be when the barometer is UNUSUALLY
LOW?_
A. There will be _short heavy showers_, with sudden _squalls of wind_
from the _west_.
Q. _Why will there be VERY LITTLE RAIN if the barometer be UNUSUALLY
LOW?_
A. Because the air must be very _warm_, or very _moist_, or perhaps
_both_.
Q. _Why will there be little or no rain, if the_ AIR _be very_ WARM?
A. If the air be very warm it will have a tendency to _imbibe more
moisture_, and not to part with what it has.
Q. _Why will there be little or no rain if the air be MOIST, and the
barometer remains very LOW?_
A. If the air be ever so moist, rain will never fall till _cold air_ has
been introduced to _condense the vapour_; and the moment that the _cold_
air is introduced, the barometer will _rise_.
Q. _What is the 5TH SPECIAL RULE in regard to the barometer?_
A. In summer-time (after a long continuance of fair weather) the
barometer will _fall gradually_ for 2 or 3 days before _rain_ comes; but
if the fall of the mercury be very _sudden_, a _thunder-storm_ is at
hand.
Q. _What is the 6TH SPECIAL RULE in regard to the barometer?_
A. When the sky is cloudless, and seems to promise fair weather, if the
barometer be _low_, the face of the sky will soon be suddenly
_overcast_.
Q. _What is the 7TH SPECIAL RULE in regard to the barometer?_
A. Dark dense clouds will pass over _without rain_, when the barometer
is _high_; but if the barometer be low, it will often rain _without any
gathering of clouds_.
Q. _What is the 8TH SPECIAL RULE in regard to the barometer?_
A. The _higher_ the barometer, the greater is the probability of _fair
weather_.
Q. _Why is the barometer HIGH in FINE weather?_
A. Because the air contains but _very little vapour_. The _drier_ the
air, the _higher_ does the mercury of the barometer rise.
Q. _What is the 9TH SPECIAL RULE in regard to the barometer?_
A. When the mercury is in a _rising_ state, _fine_ weather is at hand;
but when the mercury is in a _sinking_ state, _foul_ weather is near.
Q. _Why does the mercury RISE at the approach of FINE weather?_
A. Because the air is becoming more _dry_, and therefore its _pressure_
is greater.
Q. _Why does the mercury SINK at the approach of FOUL weather?_
A. Because the air is _laden with vapour_, or _disturbed by wind_.
Q. _Why does VAPOUR in the air make the mercury SINK?_
A. Because vaporized air is _lighter than dry air_, and therefore its
_pressure is less_ on the mercury of the barometer.
Q. _What is the 10TH SPECIAL RULE in regard to the barometer?_
A. If (in frosty weather) it _begins to snow_, the barometer generally
rises to 32°, where it remains as long as the snow continues to fall;
if, after this, the weather _clear up_, you may expect _very severe
cold_.
Q. _How can you know if the MERCURY of the barometer be RISING?_
A. If it be _convex_ (i. e. higher in the _middle_ than at the _sides_;)
it is in a _rising state_.
Q. _How can you tell if the MERCURY of the barometer be about to FALL?_
A. If it be _concave_ (i. e. _hollow_ in the _middle_) it is in a
_falling state_.
Q. _Why is the mercury CONVEX when it is RISING?_
A. The sides of the mercury _rub against the glass tube_, and are
_delayed_ by it, so that the _middle_ part _rises faster_ than the
_sides_.
Q. _Why is the mercury CONCAVE when it is FALLING?_
A. The sides of the mercury _rub against the glass tube_, and are
_delayed_ by it, so that the _middle_ part _sinks faster_ than the
_sides_.
Q. _What effect does a THUNDER-STORM produce on the weather?_
A. Thunder is generally _preceded by hot_ weather, and _followed by
cold_ and showery weather.
Q. _What effect does a SUDDEN CHANGE produce on the weather?_
A. A great and sudden change (either from hot to cold, or from cold to
hot) is generally followed _by rain within 24 hours_.
Q. _Why is a sudden CHANGE from HOT to COLD followed by RAIN?_
A. The cold _condenses the air_ and its vapour; which, being condensed
and squeezed out, _falls in rain_.
Q. _Why is a sudden CHANGE from COLD to HOT followed by RAIN?_
A. Because the air is _quickly saturated with moisture_; and as soon as
_night_ comes on, the temperature is _lowered again_, and some of the
abundant moisture falls in rain.
Q. _Why is the air quickly SATURATED with MOISTURE, when HEAT succeeds
rapidly from COLD?_
A. Because the evaporation (which was checked by the cold) is _carried
on very rapidly_, in consequence of the _diminished pressure_ of the
air.
(N. B. The _less the pressure_ of the air, the more _rapidly it
evaporates_ moisture.)
Q. _When does the barometer VARY MOST?_
A. In winter time.
Q. _Why does the barometer vary MORE in WINTER than in SUMMER time?_
A. Because the _difference of temperature_ between the torrid and
temperate zones is _so great_, that the state of the air is perpetually
_disturbed_ by their mixing together.
Q. _When does the barometer VARY LEAST?_
A. In summer time.
Q. _Why does the barometer vary LESS in SUMMER than in WINTER time?_
A. Because the temperature of our island is _so nearly equal_ to that of
the torrid zone, that its state is _not much disturbed_ by interchange
of currents.
Q. _What effect has WIND on the barometer?_
A. NORTH and EAST winds make the mercury _rise_; all _other_ winds make
it _sink_; but SOUTH and WEST winds make it _sink lower_ than any other
winds.
Q. _Have HEAT and COLD any effect on the barometer?_
A. No, not of _themselves_; but because _cold_ weather is generally
either _dry_, or _rough with north-east winds_, therefore the mercury
_rises_ in cold weather; and because warm weather is often _moist_ or
_fanned by south-west winds_, therefore, the mercury sinks.
Q. _Why is the mercury of a barometer LOWER in the TORRID than in the
FRIGID zones?_
A. Because the warm air of the torrid zone contains much more _vapour_
than the condensed air of the frigid zone; and the _moister_ the air,
the _less is its pressure_.
Q. _In what MONTHS is the barometer HIGHEST?_
A. In May and August; next to these, in June, March, September, and
April.
Q. _In what MONTHS is the barometer LOWEST?_
A. In November and February; then in October, July, December, and
January.
Q. _What are the DRIEST months?_
A. March and June; then May and August; then April and November.
Q. _What are the WETTEST months?_
A. October and February; then July and September; then January and
December.
Q. _Why is there LESS wet from MARCH to AUGUST, than there is from
August to March?_
A. Because the _heat is constantly increasing_; and the capacity of the
air to absorb and retain moisture increases likewise.
Q. _Why is there MORE wet from AUGUST to MARCH, than there is from March
to August?_
A. Because the _heat is constantly decreasing_, and the capacity of the
air to retain moisture decreases also; so that (although it often rains)
yet the air is always on the point of saturation.
Q. _Why does the mercury of a barometer RISE in a FROST?_
A. Because frost _condenses the air_; and condensed air is heavier than
_rarefied_ air.
Q. _Why does the mercury of a barometer FALL in a THAW?_
A. Because the air is both _warmer_ (or more rarefied), and also filled
with _vapour_.
Q. _What does a SUDDEN rise or fall of the barometer indicate?_
A. If the _rise_ be sudden, fine weather will not continue long:
If the _fall_ be sudden, foul weather will not continue long.
Q. _What sort of weather may we expect if the barometer be very
FLUCTUATING?_
A. If the mercury fluctuates much, the weather will be very _changeable
and unsettled_.
The FALL of the barometer.
In very _hot_ weather, the fall of the mercury denotes _thunder_.
Except in very hot weather, the sudden falling of the barometer denotes
high wind.
In _frosty_ weather, the fall of the barometer denotes _thaw_.
If _wet_ weather happens _soon_ after the fall of the barometer, expect
but _little_ of it.
In _wet_ weather if the barometer falls, expect much wet.
In _fair_ weather, if the barometer falls much and _remains_ low, expect
much wet in a few days, and probably _wind_.
N. B. The barometer sinks lowest of all for wind and rain together, next
to that for wind (except it be an east or north-east wind).
* * * * *
The RISE of the barometer.
In _winter_ the rise of the barometer presages _frost_.
In _frosty_ weather, the rise of the barometer presages _snow_.
If _fair_ weather happens _soon_ after the rise of the barometer, expect
but _little_ of it.
In _wet_ weather, if the mercury rises high and _remains_ so, expect
continued _fine_ weather in a day or two.
In wet weather, if the mercury rises suddenly very high, fine weather
will not last long.
N. B. The barometer rises highest of all for north and east winds; for
all _other_ winds it sinks.
* * * * *
If the barometer be UNSETTLED.
If the motion of the mercury be _unsettled_, expect unsettled weather.
If it stand at "MUCH RAIN" and rise to "CHANGEABLE," expects _fair
weather of short continuance_.
If it stand at "FAIR" and fall to "CHANGEABLE," expect _foul_ weather.
N. B. Its motion _upwards_ indicates the approach of fine weather: its
motion _downwards_ indicates the approach of foul weather.
CHAPTER XXIV.
SNOW. HAIL. RAIN.
Q. _What is SNOW?_
A. The condensed vapour of the air _frozen_, and precipitated to the
earth.
Q. _What is the CAUSE of SNOW?_
A. When the air is nearly saturated with vapour, and condensed by a
current of air _below freezing point_, some of the vapour is squeezed
out, and frozen into snow.
A few years ago, some fishermen (who wintered at Nova-Zembla), after
they had been shut up in a hut for several days, _opened the window_,
and the cold external air rushing in, instantly condensed the air of the
hut, and the vapour (which was squeezed out) fell on the floor _in a
shower of snow_.
Q. _Why does SNOW fall in WINTER time?_
A. Because the sun's rays are too _oblique_ to heat the surface of the
earth; and (as the _earth has no heat_ to radiate into the air) the air
is very cold.
Q. _What is SLEET?_
A. When flakes of snow (in their descent) pass through a bed of air
_above freezing point_, they melt; and fall to the earth as half-melted
snow or sleet.
Q. _What is the USE of SNOW?_
A. To keep the _earth warm_, and to _nourish_ it.
Q. _How can snow keep the EARTH WARM?_
A. Because it is a very _bad conductor_; in consequence of which, the
earth which is covered with snow, very rarely descends _below freezing
point_, even when the air is 15 or 20 degrees colder.
Q. _Why is SNOW a BAD CONDUCTOR of heat and cold?_
A. Because _air_ is confined and entangled between the crystals, and
_air_ is a very _bad conductor_; when, therefore, the earth is covered
with snow, it cannot throw off its heat by radiation.
Q. _Tell me the words of the PSALMIST_ (cxlvii. 16.) _respecting snow,
and explain what he means._
A. The Psalmist says--"The Lord giveth snow like wool:" and he means not
only that snow is as _white as wool_, but that it is also as _warm as
wool_.
Q. _Why is WOOL WARM?_
A. Because _air_ is entangled between the fibres of the wool, and air is
a _bad conductor_.
Q. _Why is SNOW WARM?_
A. Because _air_ is entangled between the crystals of the snow, and air
is a _bad conductor_.
Q. _Why does SNOW NOURISH the earth?_
A. Because it supplies it with _moisture_ for a considerable time; which
penetrates slowly into the soil, and insinuates itself through every
clod, ridge, and furrow.
Q. _Why is there NO SNOW in SUMMER time?_
A. No snow reaches the general surface of the earth in summer time,
because the _heat of the earth_ melts it in its descent.
Q. _Why are some MOUNTAINS ALWAYS COVERED with SNOW?_
A. 1st--Because the _air is more rarefied_; and rarefied air _abstracts
heat_ which it holds in a _latent state_:
2ndly--As the mountain top is _not surrounded by earth_ to radiate heat
into the air; therefore, the snow is _not melted_ in its descent, but
falls on the mountain, and lies there.
Q. _Why is SNOW WHITE?_
A. Snow is formed of an infinite number of very minute crystals and
prisms, which reflect all the colours of the rays of light; and these
colours _uniting_ before they meet the eye, cause snow to appear white.
Q. _What is HAIL?_
A. Rain, which has passed in its descent _through some cold bed of air_,
and has been frozen into drops of ice.
Q. _Why is ONE bed of air COLDER than another?_
A. This is frequently caused by _electricity_ in the air, _unequally
distributed_.
Q. _Why is HAIL frequently accompanied with THUNDER and LIGHTNING?_
A. 1st--Because the _congelation of water into hail_ disturbs the
electricity of the air: and
2ndly--The _friction_ (produced by the fall of hail) excites it still
more.
Q. _Why does HAIL fall generally in SUMMER and AUTUMN?_
A. 1st--Because the _air is more highly electrified_ in summer and
autumn: and
2ndly--The vapours (being rarefied) ascend to the more elevated regions,
where the _cold is greater_ than it is nearer the earth.
Q. _What TWO things are essential to cause HAIL?_
A. Two _strata of clouds_ having _opposite electricities_, and _two
currents of wind_. The _lower cloud_ (being negative) is the one
_precipitated_.
Q. _What is RAIN?_
A. The vapour of the clouds or air _condensed_, and precipitated to the
earth.
Q. _Why is the vapour of the air or clouds PRECIPITATED?_
A. When the air is _saturated with vapour_, if a cold current
_condenses_ it, it is no longer _able to hold all its vapour_ in
solution, and some of it is squeezed out, and falls as rain.
Q. _Why does RAIN fall in DROPS?_
A. The vapoury particles in their descent _attract each other_; and
those which are sufficiently near, _unite_ and form into a drop.
Q. _Why does not the COLD of NIGHT ALWAYS cause rain?_
A. When the air is not _near saturation_ (although condensed by the
chill of evening), it will still be able to hold its vapour in solution.
Q. _Why does a PASSING CLOUD often drop RAIN?_
A. Because the cloud (travelling about on the wind) comes into contact
with _something that chills it_; and its vapour being squeezed out,
_falls to the earth as rain_.
Q. _Why are RAIN-DROPS sometimes much LARGER than at OTHER times?_
A. When the rain-cloud is floating _near the earth_, the drops are
large, because such a cloud is _much more dense_ than one which is more
elevated.
The size of the rain-drop is increased according to the _rapidity_ with
which the vapours are condensed.
Q. _Does not WIND sometimes INCREASE the SIZE of rain-drops?_
A. Yes; by blowing two or more drops into one.
Q. _Why do CLOUDS FALL in RAINY weather?_
A. 1st--Because the _clouds are heavy_ with abundant vapour: and
2ndly--As the density of the air is _diminished_, it is less able to
buoy the clouds up.
Q. _How do you KNOW that the DENSITY of the air is DIMINISHED in RAINY
weather?_
A. Because the _mercury of a barometer falls_.
Q. _Why is RAIN-water more FERTILIZING than PUMP-water?_
A. Because it contains a compound of hydrogen and nitrogen (called
_ammonia_), which is a very excellent food for young plants.
Q. _Why is NOVEMBER made by God to be a RAINY MONTH?_
A. Because rain _hastens the putrefaction of the fallen leaves_ by
causing fermentation.
Q. _Why does RAIN PURIFY the AIR?_
A. 1st--Because it _beats down the noxious exhalations_ collected in the
air, and _dissolves_ them:
2ndly--It mixes the air of the _upper_ regions with that of the _lower
regions_: and
3rdly--It _washes the earth_, and sets in motion the stagnant _sewers
and ditches_.
Q. _Why are MOUNTAINOUS countries more RAINY than flat ones?_
A. The air (striking against the side of the mountains) is _carried up
the inclined plane_, and brought in contact with the _cold air of the
higher regions_, by which it is _condensed_, and its _vapour squeezed
out_.
Q. _Why does a SPONGE SWELL when it is WETTED?_
A. Because the water _penetrates the pores_ of the sponge, and drives
the particles of the sponge _further from each other_; in consequence of
which, the _bulk_ of the sponge is greatly _increased_.
Q. _Why do FIDDLE-strings SNAP in WET weather?_
A. Because the moisture of the air (penetrating the string) _causes it
to swell_; and (as the cord _thickens_) its _tension is increased_, and
the string snaps.
Q. _Why does PAPER PUCKER when it is WETTED?_
A. Because the moisture (penetrating the paper) _drives its particles
further apart_; and (as the moisture is absorbed _unequally_ by the
paper) some parts are more enlarged than others; in consequence of
which, the paper _blisters_ or _puckers_.
Q. _Why do the weather toys called CAPU'CHINS lift the cowl over the
figures in wet weather, and remove it in dry?_
A. The cowl of the capu'chin is _fastened to a piece of cat-gut_. When
the weather is _wet_, the moisture _swells the cat-gut_ and it is
_shortened_, by which means the _cowl is pulled up_; but in _dry_
weather, the _string is loosened_, and the cowl falls down.
Q. _In another weather toy, the MAN comes out in WET weather, and the
LADY in FINE:--Why is this?_
A. The two figures are attached to a piece of _cat-gut_ in such a
manner, that when the _cat-gut is shortened by moisture_, it pulls the
_man out_; but when it is _loose_, the woman _falls out by her own
weight_.
Q. _Why are WET STOCKINGS DIFFICULT to PULL ON?_
A. The moisture (by penetrating the threads of the stockings) causes
them to _shrink in size_.
Q. _What is the MOST RAINY spot in ENGLAND?_
A. Keswick (in Cumberland); and then Kendal (a market town in
Westmoreland).
(In Keswick, about 63 inches of rain fall in a year. In Kendal, 58;
Manchester, 38; Liverpool, 34; Dublin and Cambridge, 25; Lincoln, 24;
London, 21; and in Paris, only 18.)
Q. _In which PART of the DAY does the MOST RAIN fall?_
A. More rain falls by _night_ than by day; because the cold night
_condenses the air_, and diminishes its capacity for holding vapour in
solution.
Q. _Does more rain fall in SUMMER or in WINTER time?_
A. There are _more rainy days_ from September to March; but _heavier_
rains between March and September.
Q. _Why are there MORE RAINY DAYS from September to March, than from
March to September?_
A. Because the temperature of the air is _constantly decreasing_, and
its capacity for vapour decreases also; in consequence of which, it is
perpetually obliged to _part with some of its vapour_ in rain.
Q. _In what PART of the WORLD does RAIN fall MOST ABUNDANTLY?_
A. Near the _equator_; and the quantity of rain _decreases_ as we
approach the _poles_.
Q. _Why does MORE RAIN fall at the EQUATOR than at the POLES?_
A. Because the _contrast_ between the _night and day_ is very great. The
hot air _absorbs moisture very abundantly_ during the day; and when the
cold night _condenses_ the air, it is unable to _retain the moisture
imbibed_, and some of it falls in rain.
CHAPTER XXV.
WATER.
Q. _What is WATER?_
A. Water is composed of _two gases_, oxygen and hydrogen.
(In 9 lbs. of water, 8 are oxygen, and 1 is hydrogen.)
Q. _Why is WATER FLUID?_
A. Because its particles are kept separate by _latent heat_; but when a
certain quantity of this latent heat is driven out, _water becomes
solid_, and is called ice.
Q. _How can WATER be converted into a GAS?_
A. By increasing its _latent heat_, the particles, of water are again
_subdivided into invisible steam_.
Q. _Why is PUMP water called HARD water?_
A. Because it is laden with foreign matters, and will not readily
_dissolve substances_ immersed in it.
Q. _What makes PUMP-water HARD?_
A. Because when it filters through the earth, it becomes impregnated
with _sulphate of lime_, and many other impurities from the _earths and
minerals_ with which it comes in contact.
Q. _Why is it difficult to WASH our HANDS clean with HARD water?_
A. Because the _soda of the soap_ combines with the _sulphuric acid_ of
the hard water, and the _oil of the soap_ with the _lime_, and float in
flakes on the top of the water.
N.B. Sulphate of lime consists of sulphuric acid and lime.
Q. _Why is it difficult to wash in SALT WATER?_
A. Because salt water contains _muriatic acid_; and the _soda of soap_
combines with the _muriatic acid of the salt water_, and produces a
cloudiness.
Q. _Why does a BLACK HAT turn RED at the SEA SIDE?_
A. The _muriatic acid of the sea-water_ disturbs the _gallic acid of the
black dye_, and turns it _red_.
Q. _Of what is SOAP made?_
A. Of kelp (or the ashes of sea-weed dried and burnt in a pit) mixed
with oil or fat.
YELLOW SOAP is made of whale-oil, soda, and resin. SOFT SOAP is made of
oil and potash. HARD SOAP of oil and soda.
Q. _Why does WATER CLEAN dirty LINEN?_
A. Because the _oxygen_ of the water attaches itself to the _stains of
the linen_, and _dissolves_ them; as oxalic acid dissolves ink spots.
Q. _Why does SOAP greatly INCREASE the cleansing power of water?_
A. 1st--Because soap _increases the oxygen_ of the water: and
2ndly--It _neutralizes the grease_ of the things washed.
Q. _Why is RAIN WATER SOFT?_
A. Because it has _not come in contact with earths and minerals_.
Q. _Why is it MORE EASY to WASH with SOFT water than with HARD?_
A. Because it unites freely with the soap, _dissolving_ it instead of
_decomposing it_, as hard water does.
Q. _Why do WOOD ASHES make HARD water SOFT?_
A. 1st--Because the _carbonic acid of the wood ashes_ combines with the
_sulphate of lime in the hard water_, and converts it into _chalk_: and
2ndly--The _sulphuric acid of the water_ combines with the _potash of
the wood ashes_, and prevents it from neutralizing the oily matter of
the soap.
Q. _Why has RAIN water such an UNPLEASANT SMELL, when it is collected in
a rain water tub or tank?_
A. Because it is impregnated with _decomposed organic matter_, washed
from roofs, trees, or the casks in which it is collected.
Q. _Why does WATER MELT SUGAR?_
A. Because very minute particles of water _insinuate themselves into
the pores_ of the sugar, and force the crystals _apart from each other_.
Q. _Why does WATER MELT SALT?_
A. Because very minute particles of water insinuate themselves into the
_pores of the salt_, and force the crystals _apart from each other_.
Q. _Why does melted SUGAR or SALT give a FLAVOUR to the WATER?_
A. Because the sugar or salt (being disunited into very minute pieces)
_floats about the water_, and mixes with _every part_.
Q. _Why does HOT water melt sugar and salt QUICKER than COLD water?_
A. 1st--Because the _heat_ of the water entering the pores of the sugar
or salt, _opens a passage for the water_: and
2ndly--The _particles of hot water_ being _smaller_ than those of cold,
can _more readily penetrate_ the pores of salt or sugar.
Q. _Why is SEA-water SALT?_
A. 1st--Because it contains _mines of salt_ at the bottom of its bed:
2ndly--It is impregnated with _bituminous matter_, which is brackish:
and
3rdly--It contains many _putrid substances_, which increase its
brackishness.
Q. _Why is NOT RAIN-water SALT, although most of it is evaporated from
the SEA?_
A. Because _salt will not evaporate_; and, therefore, when sea-water is
turned to vapour, its _salt is left behind_.
Q. _Why does STAGNANT water PUTREFY?_
A. Because leaves, plants, insects, &c. are decomposed in it.
Q. _Why is STAGNANT water full of WORMS, EELS, &c.?_
A. Because numberless insects _lay their eggs_ in the leaves and plants
which float on the surface; these eggs are soon hatched, and produce
swarms of worms, eels, and insects.
Q. _Why are FLOWING waters FREE from these IMPURITIES?_
A. 1st--Because the motion of running water _prevents its fermentation:_
2ndly--It _dissolves the putrid substances_ which happen to fall into
it: and
3rdly--It casts _on the bank_ (by its current) such substances as it
cannot _dissolve_.
Q. _Why does RUNNING water OSCILLATE and WHIRL in its current?_
A. 1st--Because it _impinges against its banks_, and is perpetually
diverted from its forward motion: and
2ndly--Because the _centre_ of a river _flows faster_ than its _sides_.
Q. _Why do the SIDES of a river flow more TARDILY than its CENTRE?_
A. Because they _rub against the banks_, and are delayed in their
current thereby.
Q. _Why does SOAPY water BUBBLE?_
A. Because the soap _makes the water tenacious_, and prevents the
bubbles from _bursting_ as soon as they are formed.
Q. _Why will not water bubble WITHOUT SOAP?_
A. Because it is not tenacious enough _to hold together the bubbles_
that are formed.
Q. _When SOAP BUBBLES are blown from a pipe, why do they ASCEND?_
A. Because they are _filled with warm breath_, which is lighter than
air.
CHAPTER XXVI.
ICE.
Q. _What is ICE?_
A. FROZEN WATER. When the air is reduced to 32 degrees of heat, water
will no longer remain in a _fluid state_.
Q. _Why is SOLID ICE LIGHTER than WATER?_
A. Because water _expands by freezing_; and as the _bulk is increased_,
the _gravity_ must be _less_.
Nine cubic inches of water become ten when frozen.
Q. _Why do EWERS BREAK in a FROSTY NIGHT?_
A. Because the water in them _freezes_; and as the _water is expanded by
frost_, it bursts the ewers to make room for its increased volume.
Q. _Why does it not expand UPWARDS (like boiling water), and RUN OVER?_
A. Because the _surface_ is first frozen, and the frozen surface acts as
a _plug_, which is more difficult to burst than the earthen ewer
itself.
Q. _Why do TILES, STONES, and ROCKS often SPLIT in winter?_
A. Because the moisture (which they imbibed) _freezes_, and by its
expansion _splits the solid mass_.
Q. _In winter time, FOOT-MARKS and WHEEL-RUTS are often covered with an
icy NET-WORK, through the interstices of which the soil is clearly
seen,--WHY does the water freeze in NET-WORK?_
A. The water in these hollows froze first at the _sides_ of the
foot-prints: other crystals gradually shot across the water, and would
have _covered the whole surface_, had not the earth _absorbed_ the water
before it had time to freeze.
Q. _In winter time these FOOT-MARKS and WHEEL-RUTS are sometimes covered
with a perfect SHEET of ice, and not an icy net-work,--Why is THIS?_
A. The _air being colder_ and the _earth harder_ (than in the former
case), the _entire surface_ of the foot-print is frozen over, before the
earth can _draw the water in_.
Q. _Why is not the ice SOLID in these ruts?--WHY is there only a very
thin FILM or NET-WORK of ice?_
A. Because the earth _absorbs the water_, and leaves the icy _film
behind_.
Q. _Does not water expand by HEAT as well as COLD?_
A. Yes; it expands as soon as it is more than 42 degrees _till it
boils_, and then it flies off in steam.
[Illustration: Here A B measures the bulk of a portion of water at 42
degrees.
It goes on increasing in bulk to C D, when it boils. It also goes on
increasing in bulk to E F, when it freezes.]
Q. _Why do WATER-PIPES frequently BURST in FROSTY weather?_
A. Because the water in them _freezes_; and as the water _expands by
frost_, it bursts the pipes to make room for its increased volume.
Q. _When does WATER begin to EXPAND from cold?_
A. Water (which is wisely ordained by God to be an _exception_ to a
very general rule) _contracts_ till it is reduced to 42 degrees, and
then it _expands till it freezes_.
(Water freezes at 32°.)
Q. _Why does water expand when it freezes?_
A. Because it is converted into _solid crystals_, which _do not fit
close_, like the particles of water.
Q. _Why is the water at the BOTTOM of a river NEVER FROZEN?_
A. Because when water is colder than 42 degrees, it instantly _ascends
to the surface_; and (if it freezes) _floats there_ till it is melted.
(When a river is frozen, the water below the surface is never less than
42°.)
Q. _Show the WISDOM of GOD in this wonderful exception to a general
law._
A. If ice were _heavier than water_, it would _sink_; and a river would
soon become a solid _block of ice_, which could never be dissolved.
Q. _Why does not the cold ICE on the SURFACE of a river CHILL the water
BENEATH, and make it freeze?_
A. 1st--Water is a _very bad conductor_, and is heated or chilled by
CONVECTION only:
2ndly--If the ice on the surface were to communicate its _coldness_ to
the water beneath, the _water beneath_ must communicate its _heat to the
ice_, and the ice would instantly _melt_: and
3rdly--The ice on the surface acts as a _shield_ to _prevent the cold
air from penetrating the river_ to freeze it below the mere crust.
Q. _Why does WATER FREEZE at the SURFACE first?_
A. Because the surface is in _contact with the air_, and the air
_carries away its heat_.
Q. _Why does the coat of ice grow THICKER and THICKER, if the frost
CONTINUES?_
A. Because the _heat of the water_ (immediately below the frozen
surface) _passes through the pores of the ice_ into the _cold air_.
Q. _Why then are not WHOLE RIVERS FROZEN (layer by layer) till they
become solid ice?_
A. Because water is _so slow a conductor_, that our frosts never
continue _long enough_ to convert a whole river into a solid mass of
ice.
Q. _Why does not RUNNING water freeze so fast as STILL water?_
A. 1st--Because the motion of the current _dissolves the crystals_ as
fast as they are formed; and
2ndly--The heat of the _under_ surface is more freely distributed to the
_upper_ surface by the _rolling water_.
Q. _When RUNNING water is FROZEN, why is the ICE generally very ROUGH?_
A. Because little flakes of ice are first formed and _carried down the
stream_, till they meet some _obstacle_ to _stop_ them; _other_ flakes
of ice (_impinging against them_) are arrested in like manner; and the
_edges_ of the different flakes _overlapping_ each other, _make the
surface rough_.
Q. _Why do SOME parts of a RIVER FREEZE LESS than OTHERS?_
A. Because _springs_ issue from the bottom, and (as they bubble upwards)
_thaw the ice_, or make it thin.
Q. _When persons FALL into a RIVER in winter time, why does the WATER
feel remarkably WARM?_
A. Because the _frosty air_ is at least 10 or 12 degrees _colder_ than
the water.
(The water below the surface is at least 42°; but the air 32°, or even
less.)
Q. _Why is SHALLOW water FROZEN QUICKER than DEEP water?_
A. Because (as the _whole volume_ of water must be cooled to 42 degrees
before the _surface can be frozen_) it will take a longer time to cool
down a _deep_ bed of water than a _shallow_ one.
Q. _Why is SEA-WATER RARELY FROZEN?_
A. 1st--Because the _mass of water is so great_ that it requires a very
long time to cool the whole volume down to 42 degrees:
2ndly--The _ebb and flow_ of the sea interfere with the cooling
influence of the air: and
3rdly--_Salt_ never freezes till the surface is cooled down at least 25
degrees _below the freezing point_.
Q. _Why do some LAKES RARELY if ever FREEZE?_
A. 1st--Because they are _very deep_:
2ndly--Because their water is supplied by _springs_, which bubble from
the bottom.
Q. _Why does the DEPTH of the water RETARD its FREEZING?_
A. As the _whole volume of water_ must be reduced to 42 degrees before
the _surface will freeze_, the _deeper_ the water, the _longer_ it will
be before the whole volume is thus reduced.
Q. _Why do SPRINGS at the bottom of a lake PREVENT its FREEZING?_
A. Because they keep continually sending forth _fresh water_, which
prevents the lake from being reduced to the necessary degree of
coldness.
Q. _Why is it COLDER in a THAW than in a FROST?_
A. When frozen water is _thawed_, it absorbs _heat from the air and
objects around_ to melt its ice, in consequence of which the cold is
greatly increased.
Q. _Why is it WARMER in a FROST than in a THAW?_
A. When water freezes it _gives out its latent heat_, in order that it
may be converted into _solid ice_; and as much _heat is liberated_ from
the water into the air, we feel warmer.
Q. _Why does SALT DISSOLVE ICE?_
A. Water freezes at 32°, but salt and water will not freeze _till the
air is 25° colder_: if, therefore, salt be added to frozen water it
becomes _liquid_, unless the thermometer stands below 7°, (which it
never does in our island).
Q. _Will any thing do INSTEAD of SALT?_
A. Yes; any _acid_, such as sulphuric, nitric, &c.
Q. _Why are SALT and SNOW mixed together, colder than SNOW?_
A. When _salt_ is mixed with snow, it _dissolves the crystals_ into a
fluid; and whenever a solid is converted to a liquid, _heat is
absorbed_, and the cold made more intense.
Q. _Why does FROST make the EARTH CRACK?_
A. During the warm weather the earth _absorbed abundance of moisture_,
which the winter _freezes_: and (as water _expands_ by frost) the
expanding water _thrusts the particles of earth apart from each other_,
and leaves a chink or crack behind.
Q. _Show the WISDOM of GOD in this arrangement._
A. These _cracks_ in the earth let in the air, the dew and rain, and
many gases favourable to vegetation.
Q. _Why does the EARTH CRUMBLE in SPRING?_
A. In spring the _ice_ of the clods _dissolves_, and the particles of
earth (which had been held apart by the expanded ice) are left
_unsupported_, and tumble into minute parts (because their _cement is
dissolved_).
Q. _Why does MORTAR CRUMBLE away in FROST?_
A. If the mortar was not _dried in the warm weather_, its moisture
_freezes_, _expands_, and thrusts the particles of the mortar away from
each other; but (as soon as the frost goes) the _water condenses_ and
leaves the mortar full of cracks and chinks.
Q. _Why does STUCCO PEEL from a WALL in FROSTY weather?_
A. If the stucco was not _dried in the warm weather_, its moisture
_freezes_, _expands_, and thrusts its particles away from the wall; but
as soon as the water condenses again by the thaw, the stucco (being
unsupported) _falls by its own weight_.
Q. _Why cannot BRICKLAYERS and PLASTERERS work in FROSTY weather?_
A. Because the bricks and plaster would _start from their position_ as
soon as the _frost_ came and expanded the mortar.
Q. _Why do BRICKLAYERS COVER their work with STRAW in spring and
autumn?_
A. Because straw is a non-conductor, and prevents the mortar of their
new work from _freezing_ during the cold nights of spring and autumn.
Q. _Why are WATER-PIPES often covered with STALL-LITTER in winter time?_
A. Because straw (being a non-conductor) prevents the _water of the
pipes from freezing_, and the _pipes from bursting_.
Q. _Why are delicate TREES covered with STRAW in WINTER?_
A. Because straw (being a non-conductor) prevents the _sap of the tree_
from being frozen.
Q. _Can WATER be FROZEN in any way BESIDES by frosty weather?_
A. Yes; in very many ways. For example--a bottle of water wrapped in
_cotton_, and frequently _wetted with ether_, will soon freeze.
Q. _Why would WATER FREEZE if the bottle were kept constantly wetted
with ETHER?_
A. Because _evaporation_ would carry off the heat of the water, and
reduce it to _freezing point_.
Q. _Why does ETHER freeze under the RECEIVER of an AIR-pump, when the
air is exhausted?_
A. Because _evaporation_ is very greatly increased by the _diminution of
atmospheric pressure_; and the ether freezes by evaporation.
FREEZING MIXTURES.
1. If nitre be dissolved in water, the heat of the liquid will be
reduced 16 degrees.
2. If 5 oz. of nitre, and 5 of sal-ammoniac (both finely powdered) be
dissolved in 19 oz. of water, the heat of the liquid will be reduced 40
degrees.
3. If 3 lbs. of snow be added to 1 lb. of salt, the mixture will fall to
0° (or 32 degrees below freezing point).
The two following are the coldest mixtures yet known:--
1. Mix 3 lbs. of muriate of lime with 1 lb. of snow.
2. Mix 5 lbs. of diluted sulphuric acid with 4 lbs. of snow.
Q. _Why is it more easy to SWIM in the SEA than in a RIVER?_
A. Because the _specific gravity_ of salt water is _greater than that of
fresh_, and therefore it _buoys_ up the swimmer better.
Q. _How do cooks ascertain if their BRINE be SALT ENOUGH for pickling?_
A. They put an _egg into their brine_. If the egg _sinks_ the brine is
_not strong enough_, if the egg _floats_ it _is_.
Q. _Why will the EGG SINK if the brine be NOT STRONG enough for
pickling?_
A. As an egg is _heavier than water_, it will _sink_ if immersed
therein; but if as much _salt_ be added as the water can dissolve, the
egg will _float_.
Q. _Why will the EGG FLOAT in strong BRINE?_
A. Because the specific gravity of _salt and water_ is greater than that
of water _only_.
Q. _Why do persons SINK in water when they are UNSKILFUL SWIMMERS?_
A. 1st--Because (in their floundering about) they _take in water_ at
their nose and mouth, which makes them _heavier_:
2ndly--FEAR _contracts the body_; and as the body is compressed by fear
into a smaller compass, it becomes _heavier_: and
3rdly--The water and fear _take away the breath_; and when the breath is
taken from the body, its _bulk is reduced_, and it becomes _heavier_.
Q. _Why can QUADRUPEDS swim MORE EASILY than MAN?_
A. 1st--Because the _trunk_ of a quadruped is _lighter than water_, and
this is the greatest part of them:
2ndly--The _position_ of a beast in water is a _natural_ one.
Q. _Why is it MORE DIFFICULT for a MAN to swim than for a BEAST?_
A. Because the _head and limbs_ of a man (like those of a beast) are
_heavier_ than water, and these compose more than _half his body_:
2ndly--The _position_ of a man in water is _unnatural_ to him.
Q. _Why can FAT men SWIM more EASILY than SPARE men?_
A. _Fat is lighter than water_; and the _fatter_ a man is, the more
_buoyant_ will he be.
Q. _How are FISHES able to ASCEND to the SURFACE of water?_
A. Fishes have an _air-bladder_ near their abdomen: when this bladder is
_filled with air_, the fish increases in size; and (being lighter)
ascends through the water to its surface.
Q. _How are fishes able to DIVE in a minute to the BOTTOM of a stream?_
A. They _expel the air_ from their air-bladder; in consequence of which,
their _size is diminished_, and they sink instantly.
CHAPTER XXVII.
LIGHT.
Q. _What is LIGHT?_
A. Rapid undulations of a fluid called ether, striking on the optic
nerve of the eye. (_See p. 46._)
The _heat_ of fire or of the sun sets the atoms of _matter_ in motion;
and these atoms, striking against the fluid _ether_, cause it to
undulate.
Q. _How FAST does LIGHT TRAVEL?_
A. Light travels so fast, that it would go eight times round the earth,
while a person counts "ONE."
Q. _Does ALL light travel equally fast?_
A. Yes; the light of the sun, or the light of a candle, or the light
from houses, trees, and fields.
Q. _Where does the LIGHT of HOUSES, TREES, and FIELDS come from?_
A. The light of the _sun_ (or of some lamp or candle) is _reflected from
their surfaces_.
Q. _Why are SOME surfaces BRILLIANT like glass and steel, and OTHERS
DULL like lead?_
A. Those surfaces which _reflect the most light_, are the most
_brilliant_; and those which _absorb_ light are _dull_.
Q. _What is meant by REFLECTING LIGHT?_
A. Throwing the rays of light _back again_, from the surface on which
they light.
Q. _What is meant by ABSORBING LIGHT?_
A. Letting the rays of light _sink below the surface_ which they touch,
so as not to be seen.
Q. _Why can a THOUSAND persons SEE the SAME OBJECT at the same time?_
A. Because it throws off from its surface _an infinite number of rays in
all directions_; and one person sees _one_ portion of these rays, and
another person _another_.
Q. _Why is the EYE PAINED by a SUDDEN light?_
A. Because the pupil of the eye is _burdened with rays_, before it has
had time to contract.
Q. _Why does it give us PAIN, if a CANDLE be brought suddenly towards
our BED at night time?_
A. In the dark _the pupils of the eyes dilate_ very much, in order to
_admit more rays_. When a candle is brought before them, the enlarged
pupil is _overladen with rays_, and feels pained.
Q. _Why CAN we BEAR the candle-light after a few moments?_
A, Because the pupil _contracts again_ almost instantly, and adjusts
itself to the quantity of light which falls upon it.
Q. _Why can we SEE NOTHING, when we leave a WELL-LIGHTED room, and go
into the DARK ROAD or street?_
A. Because the pupil (which _contracted_ in the bright room) does not
_dilate instantaneously_; and the contracted pupil is not able to
_collect rays enough_ (from the dark road or street) to enable us to see
before us.
Q. _Why do we SEE BETTER, when we get USED to the dark?_
A. Because the pupil _dilates_ again, and is able to gather together
more rays; in consequence of which, we see more distinctly.
Q. _If we look at the SUN for a few moments, why do all OTHER things
appear DARK?_
A. Because the pupil of the eye (which was _very much contracted_ by
looking at the sun) is _too small_ to collect sufficient rays from
_other objects_, to enable us to distinguish their colours. (_See_
"accidental colours.")
Q. _If we watch a bright FIRE for a few moments, why does the ROOM seem
DARK?_
A. Because the pupil of the eye (which was very much _contracted_ by
looking at the fire) is _too small_ to collect sufficient rays from the
objects around, to enable us to distinguish their colours.
Q. _Why can we see the PROPER COLOUR of every object again, after a few
minutes?_
A. Because the pupil _dilates_ again, and accommodates itself to the
light around.
Q. _Why can TIGERS, CATS, and OWLS see in the DARK?_
A. Because they have the power of _enlarging the pupil of their eyes_,
so as to collect several scattered rays of light; in consequence of
which, they can _see distinctly_ when it is not light enough for us to
see _any thing at all_.
Q. _Why do CATS and OWLS SLEEP almost all DAY?_
A. As the pupil of their eyes is _very broad_, daylight _fatigues_ them;
so they close their eyes for relief.