RESPIRATION 



249 



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. 256). 



The following imaginary ex- 

 periment 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 solu- 

 tion, and closed above by a piston 

 moving air-tight and without 

 friction, in contact with the 

 surface of the liquid (Fig. 106). 

 Let the weight of the piston be 

 balanced by a counterpoise. 

 The pressure at the surface of 

 the liquid is evidently that of 

 the atmosphere. Now, let the 

 whole be put into the receiver of 

 an air-pump, and the air gradu- 

 ally exhausted. Let exhaustion 

 proceed until gas begins to 

 escape from the liquid and lies 

 in a thin layer between its 

 surface and the piston, the 



quantity of gas which has be- 

 come free being very small in 

 proportion to that still in solu- 

 tion. At this point the piston 

 is acted upon by two forces 

 which balance each other, the 

 pressure of the air in the 

 receiver acting downwards, and 

 the pressure of the gas escaping 

 from the liquid acting ut>- 



FIG. 106. IMAGINARY EXPERIMENT TO 

 ILLUSTRATE ' TENSION ' OF A GAS IN 

 A LIQUID. 



P, frictionless piston ; L, liquid in 

 cylinder ; G, gas beginning to escape 

 from liquid. P is exactly counterpoised. 

 In addition to the manner described in 

 the text, the experiment may be sup- 

 posed to be performed thus : Let the 

 weight, W, be determined which, when 

 the receiver is completely exhausted, 

 suffices just to keep the piston in con- 

 tact 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 



W 



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. 



wards. If the pressure in the 

 receiver is now slightly increased, the gas is again absorbed. The 

 pressure at which this just happens, and against which the piston is 

 still supported by the impacts of gaseous molecules flying out of the 

 liquid, while no pressure is as yet exerted directly between the liquid 

 and the piston, is obviously equal to the pressure or tension of the gas 

 in the liquid. 



