42 Elementary Biology. 



quence of the metabolic changes taking place in the organ- 

 ism. Thus air expired from the lungs contains only 16 per cent, 

 instead of 2 1 per cent, of oxygen, and 4^ per cent, of carbonic 

 acid instead of yj^- per cent. The amount of nitrogen remains 

 unaltered. Expired air also contains traces of poisonous 

 and foetid organic matter, and a large amount of water- 

 vapour. Such air is termed ' foul ' air. If the atmosphere 

 of such a room is to remain perfectly wholesome, about 20 

 cubic feet of fresh air per minute should be supplied for each 

 individual in it. 



Small as is the proportion of carbonic acid found in 

 fresh air (only 4 volumes in 10,000), yet that proportion is 

 amply sufficient for the supply of the carbon required by 

 the vegetal economy ; whilst if the proportion be increased 

 beyond that amount, danger would ensue to animal life. 

 The atmospheric conditions, therefore, necessary for the 

 proper maintenance of animal and vegetal life are manifestly 

 that the air should be pure and dry, with a proportion of 

 carbonic acid present not over '04 per cent. 



We have hitherto considered the atmospheric conditions 

 necessary for the maintenance of life in terrestrial plants and 

 animals; we have now to glance at the conditions necessary 

 for the maintenance of aquatic life. 



No life of any kind could be possible under water were 

 it not that all gases are soluble in water. The solubility 

 of different gases, however, depends on first, the tempe- 

 rature and pressure of the gas, and, secondly, the degree 

 of inherent tendency of the gas to dissolve. The last, which 

 is called the coefficient of absorption of the gas, varies very 

 greatly. For example, at a temperature of o C., and under 

 a barometric pressure of 760 mm. one volume of water 

 will absorb, of 



Nitrogen ..... '02035 v l s - 



Oxygen -04114 ,, 



Carbonic acid .... 1 7967 ,, 



Ammonia ... - 1148-8 ,, 



