12 



RESPIRATORY MECHANISMS 



may be present. From the atmosphere 2 and N 2 are ab- 

 sorbed simultaneously and independently, each according to 

 its own pressure, and therefore in 100 volumes of water 



- X 3.5 = 0.72 volume of 2 and -=tr 



/60 



1.7 = 1.35 vol- 



760. 



umes of N 2 . 



Gases are somewhat less soluble in salt solutions, and Table 

 5 gives the quantities of 2 in equilibrium with the atmos- 

 phere at different temperatures and salinities; 20 %o CI corre- 

 sponds to ocean water. 



Table 5 



Oxygen in fresh and salt water saturated with atmospheric air 



ml/liter 



Compared with air the quantities of oxygen available in 

 water are very small and for water in equilibrium with the 

 atmosphere they vary with temperature. It is often desirable 

 to express the concentration of a dissolved gas by its pressure 

 in the atmosphere with which it would be in equilibrium. 

 For this purpose we shall use the word "tension" to dis- 

 tinguish it from the actual gas pressure with which it would 

 be in equilibrium. Tensions are given in mm pressure of 

 mercury. 



The oxygen in natural waters is mainly derived from the 

 atmosphere and is taken up at the surface where the tension 

 reaches 155 mm. It is transported downwards by diffusion 

 (the movement of individual molecules) and by convection 

 along with the water in which it is dissolved. Over distances 

 above a few mm diffusion is a slow process, and it has been 



