232 A MANUAL OF PHYSIOLOGY 



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 



P, frictionless piston ; L, liquid in 

 cylinder; G, gas beginning to es- 

 cape from liquid. P is exactly 

 counterpoised. In addition to the 

 manner described in the text, the ex- 

 periment may be supposed to be per- 

 formed thus. Let the weight, W, be 

 determined which, when the receiver 

 is completely exhausted, suffices just 

 to keep the piston in contact with the 

 liquid. The pressure of the gas is 

 then just counterbalanced by W; 

 and if S is the area of the cross- 

 section of the piston the pressure of 



the gas per unit of area is . Or if 



O 



the piston is hollow, and mercury is 

 poured into it so as just to keep it in 

 contact with the liquid, the height of 

 the column of mercury required is 

 also equal to the pressure or tension 

 of the gas. 



FIG. 82 IMAGINARY EXPERIMENT TO ILLUSTRATE 'TENSION' OF A GAS IN 



A LIQUID. 



later on how this principle has been applied to determine the partial 

 pressure of oxygen or carbon dioxide which just suffices to prevent 

 blood, or any other of the liquids of the body, from losing or gaining 

 these gases. This pressure is evidently equal to that exerted by the 

 gases of the liquid at its surface, which is sometimes called their 

 * tension ' ; for if it were greater, gas would, upon the whole, pass 

 into the blood ; and if it were less, gas would escape from the blood. 

 Thus, the tension of a gas in solution in a liquid is equal to the partial 

 pressure of that gas in an atmosphere to which the liquid is exposed^ 

 which is just sufficient to prevent gain or loss of the gas by the liquid 

 (p. 240). 



The following imaginary experiment may further illustrate the 

 meaning of the term ' tension ' of a gas in a liquid in this connection: 



Suppose a cylinder filled with a liquid containing a gas in solution, 

 and closed above by a piston moving air-tight and without friction, 

 in contact with the surface of the liquid (Fig. 82). Let the weight 



