Footnote 47: Expose two sterilized dishes containing culture media; one in a room being swept with a damp broom, and the other in a room which is being swept in the usual manner. Note the formation of colonies of bacteria in each dish. In which dish does the more abundant growth take place? Many of the bacteria present in the air are active in causing diseases of the respiratory tract, such as diphtheria, membranous croup, and tuberculosis. Other diseases, as colds, bronchitis (inflammation of the bronchial tubes), and pneumonia (inflammation of the tiny air sacs of the lungs), are also caused by bacteria. [Illustration: Plate culture exposed for five minutes in a school hall where pupils were passing to recitations. Each spot is a colony of bacteria or mold.] Dust, with its load of bacteria, will settle on any horizontal surface in a room not used for three or four hours. Dusting and sweeping should always be done with a damp cloth or broom, otherwise the bacteria are simply stirred up and sent into the air again. The proper watering of streets before they are swept is also an important factor in health. Much dust is composed largely of dried excreta of animals. Soft-coal smoke does its share to add to the impurities of the air, while sewer gas and illuminating gas are frequently found in sufficient quantities to poison people. Pure air is, as can be seen, almost an impossibility in a great city. [Illustration: A sleeping porch, an ideal way to get fresh air at night.] How to get Fresh Air.--As we know, green plants give off in the sunlight considerable more oxygen than they use, and they use up carbon dioxide. The air in the country is naturally purer than in the city, as smoke and bacteria are not so prevalent there, and the plants in abundance give off oxygen. In the city the night air is purer than day air, because the factories have stopped work, the dust has settled, and fewer people are on the streets. The old myth of "night air" being injurious has long since been exploded, and thousands of people of delicate health, especially those who have weak throat or lungs, are regaining health by sleeping out of doors or with the windows wide open. The only essential in sleeping out of doors or in a room with a low temperature is that the body be kept warm and the head be protected from strong drafts by a nightcap or hood. Proper ventilation at _all_ times is one of the greatest factors in good health.