In testing carbonic acid in a state of combination, the gas must first
be liberated by acting upon the substance with a stronger acid, and
then applying the lime-water test. The presence of large quantities of
carbonic acid in a gaseous mixture can be readily detected by plunging
into the vessel a lighted taper, which will be immediately extinguished.
This ought always to be adopted in a brewery, where many fatal accidents
have happened through workmen going down into empty fermenting vats and
wells without first taking this precaution.

The presence of carbon in this colorless gas can be demonstrated by
causing some of it to pass over a piece of the metal potassium placed
in a hard glass tube, and heated to dull redness; the potassium then
eagerly combines with the oxygen, forming oxide of potassium, and the
carbon is liberated and can be separated in the form of a black powder
by washing the tube out with water.

_Carbon Monoxide, or Carbonic Oxide. Symbol CO._--This is formed when
carbon is burnt with an insufficient supply of oxygen, or when carbonic
acid gas is passed over some carbon heated to redness. This gas is
continually being formed in our furnaces and fire-places; at the lower
part of the furnace, where the air enters, the carbon is converted into
carbonic acid, which in its turn has to pass through some red-hot coals,
so that before reaching the surface it is again converted into carbonic
oxide; over the surface of the fire this carbonic oxide meets with a
fresh supply of oxygen, and is then again converted into carbonic acid.
The peculiar blue lambent flame often observed on the surface of our
open fire-places is due to the combustion of carbonic oxide, which has
been formed in the way we have just described. Carbonic oxide is a
colorless, tasteless gas, which differs from carbonic acid by being
combustible, and by not having any action on lime water.--_Brewers'