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 dioxide and water falls upon a fire, it acts like a blanket, covering the flames and extinguishing them.  The value of a fire extinguisher depends upon the amount of carbon dioxide and water which it can furnish.  A fire extinguisher is a metal case containing a solution of bicarbonate of soda, and a glass vessel full of strong sulphuric acid.  As long as the extinguisher is in an upright position, these substances are kept separate, but when the extinguisher is inverted, the acid escapes from the bottle, and mixes with the soda solution.  The mingling liquids interact and liberate carbon dioxide.  A part of the gas thus liberated dissolves in the water of the soda solution and escapes from the tube with the outflowing liquid, while a portion remains undissolved and escapes as a stream of gas.  The fire extinguisher is therefore the source of a liquid containing the fire-extinguishing substance and further the source of a stream of carbon dioxide gas.