Part 62 (1/2)
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?_