THE GASES OF THE PJ.OOD 



*\\ 



Two well-loiown physical laws are illustrated by our supposed ex- 

 periments: (i) In a mixture of gases which do not act chemically on each 

 other the pressure exerted, by each gas (called the partial pressure of the 

 gas) is the same as it ivould exert if the others were absent. (2) The quan- 

 tity (mass) of a gas absorbed by a liquid which does not act chemically upon 

 it is proportional to the partial pressure of the gas. It also depends upon 

 the nature of the gas and of the liquid, and on the temperature, increase 

 of temperature in general diminishing the quantity of gas absorbed. 

 It is to be noted that when the volume of the absorbed gas is measured 

 at a pressure equal to the partial pressure under which ic was absorbed, 

 the same volume of gas is taken up at every pressure. 



The volume of a gas (reduced to o C. and 760 mm. pressure) physi- 

 cally absorbed or dissolved in i c.c. of a liquid exposed to the gas at 

 760 mm. pressure is called the absorption coefficient of the gas in that 

 liquid. The following table from Bohr shows the absorption coefficients 

 of the three gases of physiological interest oxygen, nitrogen, and 

 carbon dioxide in water, blood-plasma, whole blood and blood-corpuscles 

 at the body temperature (38 C.) : 



Suppose, now, that a vessel of water, saturated with oxygen and 

 nitrogen for the partial pressures under which these gases exist in the 

 air, is placed in a box filled with pure nitrogen at full atmospheric pres- 

 sure. As we have seen, there is a constant interchange going on between 

 a liquid which contains gas in solution and the atmosphere to which it 

 is exposed. Oxygen and nitrogen molecules will therefore continue to 

 leave the water; but if the box is large, few oxygen molecules will find 

 their way back to the water, and ultimately little oxygen will remain 

 in it. In other words, the quantity of oxygen absorbed by the water 

 will become again proportional to the partial pressure of oxygen, which 

 is not now much above zero. On the other hand, molecules of nitrogen 

 will at first enter the water in larger number than they escape from it, 

 for the pressure of the nitrogen is now that of the external atmosphere, 

 of which its partial pressure was formerly only four-fifths. In unit 

 volume of the gas above the water there will be 5 molecules of nitrogen 

 for every 4 molecules in the same volume of atmospheric air. There- 

 fore, on the average 5 nitrogen molecules will in a given time get en- 

 tangled by liquid molecules for every 4 which came within their sphere 

 of attraction before. On the whole, then, the water will lose oxygen 

 and gain nitrogen, while the atmosphere of the airtight box will gain 

 oxygen and lose nitrogen. 



In the case of water, in which oxygen and nitrogen are absorbed 

 solely in solution, the partial pressures of these gases under which the 

 water was originally saturated could, of course, be easily" calculated 

 from the amount dissolved and the ccefficient of absorption. But 

 supposing that these partial pressures were unknown, it is evident that 

 by exposing it to an atmosphere of known composition, and afterwards 

 determining the changes produced m the composition of that atrnp' 



