304 HUMAN PHYSIOLOGY CHAP. 



In many places where it' is difficult to obtain pure, 

 wholesome water, attempts have been made to purify 

 it by artificial means. For the past fifty years filters 

 have been used very extensively in Europe to purify the 

 water for cities. We are now introducing them in this 

 country. The safest and most effective way to purify 

 water for private use is to boil it for at least twenty 

 minutes. By this method all the disease-producing bac- 

 teria are killed, and the water becomes innocent. We 

 cannot be too careful as to the water we drink. 



214. The Air. The subject of pure, fresh air has 

 been discussed quite fully in former chapters. The 

 matter of wholesome, odorless air is of great importance 

 to our comfort and health. In low, damp, swampy 

 localities the air always contains dangerous bacteria. 

 Doubtless many diseases are due to damp, poorly ven- 

 tilated basements. Cellars are especially dangerous if 

 there are no cement floors to exclude the gases that 

 may come from the ground beneath. Dead animals, 

 waste matter, and excretions from the body must not be 

 allowed to pollute the air we breathe ; they should be 

 promptly disposed of in the country as well as in the 

 city. Of course not all bad odors are dangerous, 

 neither is all odorless air safe to inhale. 



In cities it is often quite impossible to get an abun- 

 dance of pure, odorless air. The many factories and 

 trades keep the air filled with odors of all kinds. While 

 most of these are harmless, they are not agreeable, and 

 our cities are doing a great deal to control all such 

 nuisances. The plumbing must be inspected frequently, 



