THE ATMOSPHERIC GASES 



191 



ide. Such acceleration throws light upon 

 the supposed more rapid rate of the growth 

 of plants in earlier geological epochs, when, 

 presumably, the carbon dioxide content of 

 the atmosphere was greater than it is at 

 present. The volume of atmospheric carbon 

 dioxide represents a balance between the 

 amount fixed in photosynthesis and as 

 chemical carbonates or bicarbonates, on the 

 one hand, and the amount released by res- 

 piration, decay, and by geological or in- 

 dustrial processes, on the other. 



The respiratory nerve centers of man and 

 other vertebrates and of insects are sensitive 

 to variations in the concentration of carbon 

 dioxide. Addition of this gas to inspired 

 air produces an increase in the volume of 

 respiration in man that corresponds directly, 

 in lower ranges, to the amount of carbon 

 dioxide introduced. Likewise, a decrease in 

 carbon dioxide concentration, such as may 

 be brought about by repeated deep breath- 

 ing while otherwise at rest, retards later 

 respiration, until the normal internal atmos- 

 phere of some 5 to 6 per cent is reestab- 

 lished (Dill, 1938). 



DISSOLVED ATMOSPHERIC GASES 



Atmospheric gases dissolve in water in 

 accordance with certain well-established 

 principles of which the following are im- 

 portant: 



1. Given time and physical contact, a 

 gas soluble in water dissolves in it until 

 equilibrium is established. 



2. The solubility of a gas in water in- 

 creases with lowering of the temperature of 

 the water and decreases with increasing salt 

 content. 



3. Bohr's invasion coefficient approxi- 

 mates the rate at which gas enters at a 

 water-gas interface. This coefficient may be 

 calculated for a given temperature from 

 the following relations: 



(Volume of gas entering surface in one minute") 



X 760 



(Gas pressure in air) — (Gas pressure in water) 

 X (area of interface) 



The relation between small gas bubbles in 

 water can be stated in terms of Bohr's 

 coefficient: at 37° C, when water flows past 

 a small bubble, the invasion coefficient 

 equals 0.07. The value is smaller for large 

 bubbles, and Bohr's formula is approxi- 

 mated only when both air and water at the 



interface are steadily and rapidly renewed 

 (Harvey, 1928). 



4. The rate of solution is greater (a) for 

 dry gas than for one holding water vapor; 

 (b) the greater the partial pressure of the 

 gas in the atmosphere or the greater under- 

 saturation of that gas in water (these fac- 

 tors are combined in the statement that the 

 rate of solution is greater, the steeper the 

 concentration gradient between air and 

 water); (c) the greater the exposed sur- 

 face; and (d) the greater the agitation of 

 the water by waves or otherwise. 



Oxygen diflFuses slowly through the sur- 

 face of placid water. At 10° C, it would 

 require about a million years for Lake Con- 

 stance, Switzerland, to be saturated to its 

 greatest depth of 250 meters if the water 

 remained quiet and the oxygen entered by 

 diffusion alone. Conversely, water that has 

 much surface agitation, whether by waves, 

 by waterfalls or rapids, or by any other 

 agency, tends strongly to become supersat- 

 urated with atmospheric gases. 



5. The concentration of a saturated solu- 

 tion of a gas is proportional to the pressure 

 at which the gas is supplied (Henry's 

 "law") . 



6. The pressure exerted by each com- 

 ponent of a gaseous mixture is proportional 

 to its partial pressure in the mixture; the 

 total pressure of the gaseous mixture is 

 the sum of the partial pressures of its com- 

 ponents (Dalton's "law"). Each gas dis- 

 solves irrespective of the solution of other 

 gases. 



7. Solubilities differ for different gases. 

 With disHlled water at 0° C. and with 760 

 mm. pressure for each designated gas, each 

 liter of water contains, at equilibrium, 49.24 

 cc. of oxygen, 23 cc. of nitrogen, and 1715 

 CO. of carbon dioxide. That is, for equal 

 pressures and with other conditions similar, 

 oxygen is something over t^vice as soluble 

 as nitrogen, and carbon dioxide is approxi- 

 mately 35 times as soluble as oxygen. 



As shown in Table 14, the atmospheric 

 gases meet the surface of water with 

 widely different partial pressures; hence, in 

 place of the 21 volume per cent of oxygen 

 found in the atmosphere, the air dissolved 

 in water is almost 35 per cent oxygen, and 

 the percentage of nitrogen is correspond- 

 infjly reduced. Further, as a result of the 

 differential effect of salinity on the solubilitv' 

 of these two gases, the oxygen-nitrogen 

 ratio in a given volume of sea water is 



