HEALTH AND DISEASE 419 



In addition to the carbon dioxide, water and heat are given off as 

 well as a very small amount of organic material of a poisonous 

 nature. It is the presence of this material that gives rise to the 

 odor noticeable in a close room. But other organic material is 

 found in air. Dust from the street contains bacteria of all 

 kinds, some of which may be disease-producing. Thus may be 

 spread bacteria from the respiratory tracts of people who have 

 colds, pneumonia, diphtheria, or tuberculosis. Much dust is dried 



Two cultures (A) were exposed to the air of a dirty street in the crowded part of 

 Manhattan. (B) was exposed to the air of a well-cleaned and watered street in 

 the uptown residence portion. Which culture has the most colonies of bacteria ? 

 How do you account for this ? 



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 sufHcient quantities to poison people. Pure 

 air is, as can be seen, almost an impossibility in a great city 



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 mjrth of " night air " being injurious has long since been 

 exploded, and thousands of people of delicate health, especially 



