RESPIRATION 231 



spheric gases in certain proportions in round numbers, about T J^ of 

 its volume of oxygen and -^ of its volume of nitrogen (measured at 

 760 mm. mercury and o C.). 



Now, let a similar vessel of gas-free water be placed in a large air- 

 tight box filled with air at atmospheric pressure, and let the oxygen 

 be all absorbed before the water is exposed to the atmosphere of the 

 box. The latter now consists practically only of the nitrogen of the 

 air, and its pressure will be only about four-fifths that of the external 

 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) /;/ a mixture of gases which do not act chemically 

 on each other the pressure exerted by each gas (called the partial pres- 

 sure 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 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 it 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 now not 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 niirogen. 



