THE GASES OF THE BLOOD 



247 



sphere 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 or 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 atmo- 

 sphere of the chamber there was neither gain 

 nor loss of this gas, we might be sure that the 

 partial pressure (the temperature being sup- 

 posed not to vary) was .the same when the 

 water was saturated. We shall see later on 

 how this principle has been applied to deter- 

 mine 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 when they are not 

 merely dissolved, but also combined in the 

 form of dissociable compounds. 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 ex- 

 posed, which is just sufficient to prevent gain or 

 loss of the gas by the liquid (p. 256). 



The following imaginary experiment may 

 further illustrate the meaning of the term ' ten- 

 sion ' of a gas in a liquid in this connection. 



Suppose a cylinder filled with a liquid con- 

 taining a gas in solution, and closed above by 

 a piston moving airtight and without friction, 

 in contact with the surface of the liquid (Fig. 

 117). Let the weight of the piston be balanced 

 by a counterpoise. The pressure at the sur- 

 face 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 

 gradually 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 become 

 free being very small in proportion to that 

 still in solution. 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 upwards. 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 



Fig. 117. Imaginary Ex- 

 periment to illustrate 

 * Tension ' of a Gas in a 

 Liquid. P, frictionless 

 piston; L, liquid in cy- 

 linder; G, gas beginning 

 to escape from liquid. 

 P is exactly counter- 

 poised. In addition to 

 the manner described in 

 the text, the experiment 

 may be supposed to be 

 performed thus: Let the 

 weight, W, be deter- 

 mined 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 counter- 

 balanced by W; and if 

 S is the area of the cross- 

 section of the piston, the 

 pressure of the gas per 



W 

 unit of area is -g . Or, if 



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. 



