vi HYGIENE OF RESPIRATION 63 



5. Factories. The manufacture of ammonia, hydro- 

 chloric acid, and other things by chemical works, and 

 the making of glue, candles, leather, and bricks pollute 

 the air of our cities with dangerous, nauseous products. 



6. Old Wells and Close Cellars. The carbon dioxid, 

 marsh gas, and organic impurities that form at the 

 bottom of old wells and in close, dark, and dirty cellars 

 are responsible for many diseases. 



7. Marshes and Stagnant Pools. The formation of 

 organic matter and marsh gas by the slow decay of 

 vegetables in damp, swampy places makes them sources 

 of contamination. Malarial fevers are due to germs 

 that abound in such localities. 



Many other causes might be named which tend to 

 make the air unfit to breathe by robbing it of oxygen, 

 by pouring into it useless and dangerous gases, and by 

 filling it with disease-producing germs (bacteria). It 

 would seem that with all these sources of pollution the 

 air would become utterly unfit for breathing. We shall 

 see why it does not. 



1. Burn a candle in a wide-mouthed bottle. When it goes out 

 add a little clear limewater and shake thoroughly. What do you 

 infer ? 



2. In the same way try the burning of paper, wood, etc. 



3. Test the gas that escapes from fermenting liquids. See 

 section 37. 



58. The Air Purified. There are several natural 

 means by which the air tends to purify itself. 



i. Diffusion. All of the gases of the atmosphere are 

 kept in constant motion and so tend to become equally 



