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 accumulated 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 accumulated 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?