ms is great, and proper
ventilation is of vital importance.

47. Ventilation. In estimating the quantity of air necessary to keep
a room well aired, we must take into account the number of lights
(electric lights do not count) to be used, and the number of people to
occupy the room. The average house should provide at the _minimum_ 600
cubic feet of space for each person, and in addition, arrangements for
allowing at least 300 cubic feet of fresh air per person to enter
every hour.

In houses which have not a ventilating system, the air should be kept
fresh by intelligent action in the opening of doors and windows; and
since relatively few houses are equipped with a satisfactory system,
the following suggestions relative to intelligent ventilation are
offered.

1. Avoid drafts in ventilation.

2. Ventilate on the sheltered side of the house. If the wind is
blowing from the north, open south windows.

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