ACCESS TO OXYGEN 11 



but generally at a slow rate. The air is renewed mainly by 

 the variations in barometric pressure, and usually this is 

 sufficient to keep it respirable. When, however, water-bear- 

 ing strata are cut off from the surface by an impermeable 

 layer the air in such strata may be completely deprived of 

 oxygen, and when a well is carried down into such strata it 

 may be perfectly safe to work in it so long as the barometer is 

 rising and air flows in, but extremely dangerous when the 

 occluded air flows out by a falling barometer. A certain 

 number of accidents, usually ascribed to "poisonous" gases, 

 but simply due to lack of oxygen, are brought about in this 

 way (Haldane, 1922). 



Many animals live inside plants. In the few cases ex- 

 amined the air in plant tissues has been found to be perfectly 

 respirable. In the roots and rhizomes of aquatic plants the 

 oxygen pressure is, however, often greatly reduced. Figures 

 as low as 15 mm O2 in the summer and even down to 4 mm 

 in winter were observed by Ege. On the other hand the 

 assimilation of green submerged plants liberates free oxygen, 

 and the bubbles which collect from them under ice may 

 contain up to 45% 2 . Such air is utilized by several 

 animals. 



The access to oxygen in natural waters. Gases are soluble in 

 water. When air-free water is in contact with an atmosphere 

 consisting of one single gas the water will absorb that gas 

 until equilibrium is established. The absorption coefficient 

 is defined as the quantity of a gas, measured dry in volume 

 units at one atm. pressure and 0°C, taken up by one volume 

 of water from an atmosphere of that gas at normal pressure 

 (760 mm). I prefer to express the absorption coefficient in 

 per cent of the water volume. The absorption coefficient 

 varies with temperature, and at a temperature of 15°C. we 

 have for oxygen in pure water an absorption coefficient of 

 3.5%, for nitrogen 1.7%, and for C0 2 about 100%. 



The quantity of any gas absorbed is proportional to the 

 pressure of that gas and independent of any other gas that 



