S62 RESPIRATION [CH. XXVI. 



The quantity of gas which a liquid will dissolve depends not only 

 on the solubility of the gas, but upon the pressure of the gas to which 

 the liquid is exposed. Thus, in the instance given above, if the 

 oxygen had been rarefied in the bottle until it only exerted a pressure 

 of one-fifth of an atmosphere, the water would have taken up not 

 0-04 c.c. of oxygen, but only one-fifth of that amount, 0-008 c.c. 

 To take another example, 1 c.c. of water shaken up with pure nitrogen 

 at 760 mm. pressure will dissolve 0-02 c.c. ; but suppose the pressure 

 to be reduced to four-fifths of the atmospheric pressure the water 

 will dissolve -02xf = -016 c.c. If we represent the coefficient of 

 solubility of a gas by K, and the pressure of the gas to which the 

 liquid is exposed by P', and the atmospheric pressure by P ; then the 

 quantity (Q) of the gas dissolved by 1 c.c. of the liquid may be 

 obtained by the following formula 



Dalton-Henry Law. 



What has been said above is as true of gases which are mixed 

 together as of pure gases. For instance, we have seen that a cubic 

 centimetre of water shaken up with oxygen at one-fifth of an atmos- 

 phere (153 mm. pressure) will absorb -04x-J =-008 c.c. ; or if shaken 

 with nitrogen at a pressure of four-fifths of an atmosphere, it will 

 dissolve -02 x = -016 c.c. If now a c.c. of water be shaken with air 

 (a mixture of one part of oxygen to four of nitrogen), it will have 

 absorbed -008 c.c. of oxygen and -016 c.c. of nitrogen. This fact has 

 been stated as the Dalton-Henry Law in the following words : 

 When two or more gases are mixed together, each of them produces 

 the same pressure as if they separately occupied the entire space and 

 the other gases were absent. The total pressure of the mixture is the 

 sum of the partial pressures of the individual gases in the mixture. 



The Tension of Gases in Fluids. 



In the cases which have been discussed up to this point, a con- 

 dition of equilibrium exists between the gas dissolved in the fluid 

 and the gas in the atmosphere to which the fluid is exposed, so that 

 as m.any molecules of the gas leave the surface of the fluid as enter 

 it. The gas dissolved in the fluid therefore exercises a pressure 

 which is the same as that of the gas in the atmosphere when 

 equilibrium exists. For the sake of convenience the word Tension 

 is applied to the pressure of the gas in the fluid. 



Definition of Tension. The tension of a gas dissolved in a fluid 

 is equal to the pressure of the same gas in an atmosphere with which 

 the gas in the fluid would be in equilibrium. Above, we have called 



