248 A MANUAL OF PHYSIOLOGY 



atmosphere. Nevertheless, the quantity of nitrogen absorbed by the 

 water will be exactly the same as was absorbed from the air. If 

 the box was completely exhausted, and then a quantity of oxygen, 

 equal to that in it at first, introduced before the water was exposed 

 to it, the pressure would be found to be only about one-fifth that of 

 the external atmosphere ; but the quantity of oxygen taken up by 

 the water would be exactly equal to that taken up in the first 

 experiment. 



Two well-known physical laws are illustrated by our supposed 

 experiments : ( 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 would exert if the others were absent. (2) The 

 quantity (mass) of a gas absorbed by a liquid which does not act chemi- 

 cally 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 is was absorbed, the same volume of gas is 

 taken up at every pressure. 



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 

 pressure. 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 there- 

 fore 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. Therefore, on the 

 average 5 nitrogen molecules will in a given time get entangled by 

 liquid molecules for every 4 which came within their sphere of attrac- 

 tion before. On the whole, then, the water will lose oxygen and gain 

 nitrogen, while the atmosphere of the air-tight box will gain oxygen 

 and lose nitrogen. 



If, now, the partial pressures of oxygen and nitrogen under which 

 the water had been originally saturated were unknown, it is evident 

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

 afterwards determining the changes produced in the composition of 

 that atmosphere by loss to, or gain from, the gases of the water, we 

 could find out something about the original partial pressures. If, 

 for example, the quantity of oxygen in the atmosphere of the chamber 

 was increased, we could conclude that the partial pressure of oxygen 

 under which the water had been saturated was greater than that in 

 the chamber at the beginning of the experiment. And if we found 

 that with a certain partial pressure of oxygen in the atmosphere of 

 the chamber there was neither gain nor loss of this gas, we might be 

 sure that the partial pressure (the temperature being supposed not 

 to vary) was the same when the water was saturated. We shall see 



