is is a colorless gas usually having a
disagreeable odor due to impurities. It is now made in large quantities
from calcium carbide (CaC_{2}). This substance is formed when coal and
lime are heated together in an electric furnace. When treated with water
the carbide is decomposed, yielding acetylene:

CaC_{2} + 2H_{2}O = C_{2}H_{2} + Ca(OH)_{2}.

Under ordinary conditions the gas burns with a very smoky flame; in
burners constructed so as to secure a large amount of oxygen it burns
with a very brilliant white light, and hence is used as an illuminant.

~Laboratory preparation.~ The gas can be prepared readily in a generator
such as is shown in Fig. 60. The inner tube contains fragments of
calcium carbide, while the outer one is filled with water. As long as
the stopcock is closed the water cannot rise in the inner tube. When the
stopcock is open the water rises, and, coming into contact with the
carbide in the inner tube, generates acetylene. This escapes through the
stopcock, and after the air has been expelled may be lighted as it
issues from the burner.

[Illustration: Fig. 60]

Carbon forms two oxides, namely, carbon dioxide (CO_{2}) and carbon
monoxide (CO).

~Carbon dioxide~ (CO_{2}). Carbon dioxide is present in the air to the
extent of about 3 parts in 10,000, and this apparently small amount is
of fundamental importance in nature. In some localities it escapes from
the earth in great quantities, and many spring waters carry large
amounts of it in solution. When these highly charged spring waters reach
the surface of the earth, and the pressure on them is removed, the
carbon dioxide escapes with effervescence. It is a product of the
oxidation of all organic matter, and is therefore formed in fires as
well as in the process of decay. It is thrown off from the lungs of all
animals in respiration, and is a product of many fermentation processes
such as vinegar making and brewing. Combined with metallic oxides it
forms vast deposits of carbonates in nature.

~Preparation.~ In the laboratory carbon dioxide is always prepared by the
action of an acid upon a carbonate, usually calcium carbonate, the
apparatus shown in Fig. 39 serving the purpose very well. This reaction
might be expected to produce carbonic acid, thus:

CaCO_{3} + 2HCl = CaCl_{2} + H_{2}CO_{3}.

Carbonic acid is very unstable, however, and decomposes into its
anhydride, CO_{2}, and water, thus:

H_{2}CO_{3} = H_{2}O + CO_{2}.