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 pa.s.sage 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 quant.i.ty 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.