Q. _Whence does the HEAT of our own BODY arise?_ A. The _carbon of the blood_ combines with the _oxygen of the air inhaled_, and produces _carbonic acid gas_; which produces heat in a way similar to burning fuel. Q. _Whence does the HEAT of a DUNGHILL arise?_ A. The _straw, &c. of the dunghill_ undergoes _fermentation_ as it decays: the fermentation produces _carbonic acid gas_, and heat is evolved by a species of combustion (as in the two former cases). Q. _What changes do VEGETABLES undergo from PUTREFACTION?_ A. The _hydrogen of the vegetables_ combines with the _oxygen of the air_, and forms _water_: again, the _carbon of the vegetables_ combines with _oxygen of the air_, and forms _carbonic acid gas_. Putrefaction, therefore, is only another species of combustion. Q. _What changes do ANIMAL bodies undergo from PUTREFACTION?_ A. The same as vegetables, with this addition--they give out _ammonia_, _sulphur_, and _phosphorus_ also; which causes the _offensive smell_ of putrefying animal bodies. Q. _Why is LIME heated by a KILN?_ A. All marl and chalk abound _in carbonic acid_; and (when heated by a fire) the carbonic acid _flies off in gas_, producing great heat. Q. _What is MORTAR?_ A. Lime mixed with sand and water. Q. _What is LIME?_ A. _Lime-stone burnt_ produces lime. Q. _Why is the lime-stone BURNT, in order to make it into LIME?_ 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_ 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?_ A. By _stirring the mud_ at the bottom of any stagnant pool, and collecting the gas (as it escapes upwards) in an inverted glass 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.