PUBLIC HYGIENE, OR GENERAL SANITATION 363 



live for a single hour without air, and he is unable to 

 select what air he will use, for he must breathe that which 

 immediately surrounds him. Several things are needful 

 that the air of a dwelling or other building may be fit to 

 breathe. 



(1) Drainage of the Ground on which a House stands. 

 More or less of the air in a building comes from the ground 

 beneath and around it. A wet soil favors the multiplica- 

 tion of the bacteria always present in the ground, and 

 among them are often germs of specific disease. Water 

 is found at varying distances everywhere beneath the 

 earth's surface, and, in order that a spot may be fit for 

 building upon, what is called the ground water should be 

 not less than fifteen feet below the surface (some authori- 

 ties say thirty feet), and the level should not greatly vary 

 from time to time. That a house may be healthful, the 

 soil beneath it must be thoroughly drained by pipes laid 

 deep enough in the ground ; and public authorities should 

 have power to see that this is done. 



(2) G-round Air should so far as possible be excluded from 

 a House. The air in a house is usually warmer than that 

 outside, and so there is a tendency to suck up the air in 

 the porous ground below. Even if the soil is dry, there 

 are often gases from decaying vegetation or other sources, 

 which are more or less injurious to life, mixed with the 

 earth. These should be excluded from the house by cov- 

 ering cellar walls and floor with an impervious coating. 

 Care must also be taken that nothing is left to decay in 

 the cellar or any other part of a building. 



(3) Sunshine is necessary to Health and to the purity of 

 the air in a house. No room is fit for human occupancy 

 if it does not at some time receive the direct rays of the 

 sun. Many dangerous germs are killed outright by direct 



