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_ acc.u.mulated 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_ acc.u.mulated 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 gla.s.s 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 gla.s.s or horn.

Q. _How does this fine GAUZE WIRE prevent an EXPLOSION in the coal mine?_

A. 1st--Because flame will _never pa.s.s 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 Pa.s.s 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.