48. What Becomes of the Carbon Dioxide. When we reflect that carbon
dioxide is constantly being supplied to the atmosphere and that it is
injurious to health, the question naturally arises as to how the air
remains free enough of the gas to support life. This is largely
because carbon dioxide is an essential food of plants. Through their
leaves plants absorb it from the atmosphere, and by a wonderful
process break it up into its component parts, oxygen and carbon. They
reject the oxygen, which passes back to the air, but they retain the
carbon, which becomes a part of the plant structure. Plants thus serve
to keep the atmosphere free from an excess of carbon dioxide and, in
addition, furnish oxygen to the atmosphere.

[Illustration: FIG. 24.--Making carbon dioxide from marble and
hydrochloric acid.]

49. How to Obtain Carbon Dioxide. There are several ways in which
carbon dioxide can be produced commercially, but for laboratory use
the simplest is to mix in a test tube powdered marble, or chalk, and
hydrochloric acid, and to collect the effervescing gas as shown in
Figure 24. The substance which remains in the test tube after the gas
has passed off is a solution of a salt and water. From a mixture of
hydrochloric acid (HCl) and marble are obtained a salt, water, and
carbon dioxide, the desired gas.

50. A Commercial Use of Carbon Dioxide. If a lighted splinter is
thrust into a test tube containing carbon dioxide, it is promptly
extinguished, because carbon dioxide cannot support combustion; if a
stream of carbon dioxide and water falls upon a fire, it acts like a
blanket, covering the flames and extinguishing them. The value of a