378 THE HUMAN BODY 



760 mm. (30 inches) of mercury; that is, to 152 mm. (6 inches) of 

 mercury pressure. It is only the fraction of the total pressure 

 exerted by the oxygen itself which affects the quantity of oxygen 

 absorbed by water at any given temperature. So, too, of all the at- 

 mospheric pressure -f- is due to nitrogen, and all the oxygen might 

 be removed from the air without affecting the quantity of nitrogen 

 which would be absorbed from it by a given volume of water. The 

 atmospheric pressure would then be of 760 mm. of mercury, or 

 608 mm. (24 inches), but it would all be due to nitrogen gas and 

 be exactly equal to the fraction of the total pressure due to that 

 gas before the oxygen was removed from the air. When several 

 gases are mixed together the fraction of the total pressure exerted 

 by each one is known as the partial pressure of that gas; and it is 

 this partial pressure which determines the amount of each in- 

 dividual gas dissolved by a liquid. If a liquid exposed to the air 

 for some time had taken up all the oxygen and nitrogen it could at 

 the partial pressures of those gases in the air, and were then put in 

 an atmosphere in which the oxygen had all been replaced by nitro- 

 gen, it would now give off all its oxygen, since, although the total 

 gaseous pressure on it was the same, no part of it was any longer 

 due to oxygen; and at the same time it would take up one-fifth 

 more nitrogen, since the whole gaseous pressure on its surface was 

 now due to that gas, while before only four-fifths of the total was 

 exerted by it. If, on the contrary, the liquid were exposed to pure 

 hydrogen under a pressure of one atmosphere it would give off all 

 its previously dissolved oxygen and nitrogen, since none of the 

 pressure on its surface would now be due to those gases ; and would 

 take up as much hydrogen as corresponded to a pressure of that 

 gas equal to 760 mm. of mercury (30 inches). 



3. The amount of gas taken up by a liquid varies, other things 

 being equal, inversely as the temperature. 



4. A liquid may be such as to combine chemically with a gas. 

 Then the amount of the gas absorbed is independent of the partial 

 pressure of the gas on the surface of the liquid. The quantity ab- 

 sorbed will depend upon how much the liquid can combine with. 

 Or, a liquid may partly be composed of things which simply dis- 

 solve a gas and partly of things which chemically combine with it. 

 Then the amount of the gas taken up under a given partial pres- 

 sure will depend on two things; a certain portion, that merely dis- 



