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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.
