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_.