AIR 



6 7 



tion nearly balance each other, so that the carbon dioxid 

 in the air remains fairly constant. While the percentage 

 amount in the air is small, the total amount is quite 

 large ; it is estimated that over each acre of the earth's 

 surface, there are about 30 tons of carbon dioxid at the 

 disposal of plant bodies. Carbon dioxid itself is not 

 such a poisonous gas, but it is usually associated in 



pig. 36. Ventilating board in window for obtaining fresh air. 



respired air with noxious and poisonous products thrown 

 off by the lungs. Hence the amount in a room is taken 

 as the index of the completeness of ventilation. When- 

 ever the carbon dioxid in a room exceeds o.i per 

 cent., the poisonous products associated with it are 



