RESPIRATION 191 



that of the food which he eats, and it is at least quite as 

 important that this air should be pure as that the food 

 consumed should be wholesome. 



Every expiration of each pair of lungs in a closed room 

 reduces the quantity of oxygen in the room and increases 

 the carbonic acid gas and other impurities. Now, experi- 

 ence and experiment have proved that the relative propor- 

 tions of these gases in the air inhaled cannot be greatly 

 changed without injuriously affecting animal organisms. 

 The presence of 1 per cent of carbonic acid gas is harm- 

 ful, though 1 per cent may be endured for a time ; but it 

 is the impurities always present with the gas, other than 

 the carbonic acid gas itself, together with the increase of 

 moisture and heat and unpleasant odors , that produce the 

 bad effects. When the amount of carbon dioxide becomes 

 10 per cent death is only a matter of time. 



A person may be suffocated to death in an ill-ventilated 

 room from lack of oxygen, from an excess of carbon 

 dioxide, or from the two causes combined, and he is also 

 exposed to other dangers whose effects are not manifest 

 at once. If several persons are present, germs of disease 

 are liable to be floating in the air or clinging to walls or 

 floor, and may easily be drawn into the lungs along with 

 other dust. The diseases most often communicated in 

 this way are consumption and pneumonia, each of which 

 is believed to be caused by a specific bacterium. 



272. Methods of Warming and Ventilation. In some mod- 

 ern buildings, provision for ventilation is made in connec- 

 tion with the heating apparatus. Hot air furnaces provide 

 for a constant flow of warm air into a room, with the removal 

 of that already present. The danger is lest the air brought 

 in should be taken not from pure outdoor sources but from 

 cellars, or from rooms where it has been already vitiated, 



