Respiration 



175 



with the changes of fermentation and decomposition to 

 which all organized matter is liable, all tend to pollute 

 the atmosphere. 



The breathing of dust and smoke in the air of towns, 

 which is to a greater or less extent laden with the germs of 

 disease, is an ever-present and most potent menace to public 

 and personal health. It is one of the main causes of the 

 excess of mortality in towns and cities over that of country 

 districts. 



. This is best shown in the overcrowded streets and houses 

 of great cities, which are deprived of the purifying influence 

 of sun and air. The fatal effect of living in vitiated air is 

 especially marked in the mortality among infants and chil- 

 dren living in the squalid and overcrowded sections of our 

 great cities. 



Experiment 80. Simple apparatus to illustrate the movements of 

 the lungs in the chest. T is a bottle from which the bottom has been 

 removed ; Z>, a flexible and elastic mem- 

 brane tied on the bottle and capable 

 of being pulled out by the string -5", so 

 as to increase the capacity of the bottle. 

 L is a thin elastic bag representing the 

 lungs. It communicates with the exter- 

 nal air by a glass tube fitted air-tight 

 through a cork in the neck of the bottle. 

 When D is drawn down the pressure of 

 the external air causes L to expand. 

 When the string is let go L contracts 

 again by virtue of its elasticity. FIG. 91. 



- 264. The Necessity for a Proper Supply of Pure Air ; the 

 Need of the Removal of Impure Air. As we have learned, 

 the expired air contains, besides carbon dioxide, not only 

 a certain amount of organic matter in the form of vapor, 

 but minute dust particles including many micro-organisms. 



