384 



BIOLOGY AND HUMAN LIFE 



In many workrooms it is necessary for the individual worker 

 to wear an air filter over his mouth and nose. This respirator 

 consists of a canvas cup holding a wet sponge or cloth (see Fig. 

 169) through which the breathed air must pass, leaving the dust 

 behind. The gas masks worn by soldiers during the World 



Fig. 169. Respirator 



In many industrial processes it is impracticable to remove the dust by mechanical 



means. The respirator is worn by the worker to filter the dust out of the air which 



he breathes. The sectional view shows the valve, v, and the sponge, s, through which 



the air is filtered. (From photograph lent by the American Museum of Safety) 



War, by firemen, and sometimes by miners usually carry, in 

 place of a mechanical filter, a chemical preparation to absorb or 

 to counteract the gas from which they protect the wearer. 



287. Fatigue. We can feel restored by sleep or rest without 

 taking additional food. Experiments show that the human 

 body or any other animal organism behaves, when fatigued, as 

 though some special substance in the cells prevented their full ac- 

 tivity. Fatigue has therefore been considered a poisoning, which 



