CHAP. IL] RESPIRATION. 587 



depend on the quantity present in that atmosphere. If on the 

 other hand water, already containing a good deal of oxygen dis- 

 solved in it, be exposed to an atmosphere containing little or no 

 oxygen, the oxygen will escape from the water into the atmosphere. 

 The oxygen, in fact, which is dissolved in the water, like the 

 oxygen in the atmosphere above, stands at a certain pressure, the 

 amount of pressure depending on the quantity dissolved ; and 

 when water containing oxygen dissolved in it is exposed to any 

 atmosphere, the result, that is, whether the oxygen escapes from 

 the water into the atmosphere, or passes from the atmosphere into 

 the water, depends on whether the pressure of the oxygen in the 

 water is greater or less than the pressure of the oxygen in the 

 atmosphere. Hence when water is exposed to oxygen, the oxygen 

 either escapes or is absorbed until equilibrium is established 

 between the pressure of the oxygen in the atmosphere above and 

 the pressure of the oxygen in the water below. This result is, as 

 far as mere absorption and escape are concerned, quite inde- 

 pendent of what other gases are present in the water or in the 

 atmosphere. Suppose a half-litre of water was lying at the 

 bottom of a two-litre flask, and that the atmosphere in the flask 

 above the water was one- third oxygen; it would make no dif- 

 ference, as far as the absorption of oxygen by the water was 

 concerned, whether the remaining two-thirds of the atmosphere 

 was carbonic acid, or nitrogen, or hydrogen, or whether the space 

 above the water was a vacuum filled to one- third with pure 

 oxygen. Hence it is said that the absorption of any gas depends 

 on the partial pressure of that gas in the atmosphere to which 

 the liquid is exposed. This is true not only of oxygen and 

 water, but of all gases and liquids which do not enter into 

 chemical combination with each other. Different liquids will of 

 course absorb different gases with differing readiness ; but, with 

 the same gas and the same liquid, the amount absorbed will 

 depend directly on the partial pressure of the gas in the overlying 

 space. It should be added that the process is much influenced 

 by temperature. Hence, to state the matter generally, the ab- 

 sorption of any gas by any liquid will depend on the nature of 

 the gas, the nature of the liquid, the pressure of the gas, and the 

 temperature at which both stand. 



Now it might be supposed, and indeed was once supposed, 

 that the oxygen in the blood was simply dissolved by the blood. 

 If this were so, then the amount of oxygen present in any 

 given quantity of blood exposed to any given atmosphere, ought 

 to rise and fall steadily and regularly as the partial pressure 

 of oxygen in that atmosphere is increased or diminished; the 

 absorption (or 'escape) of oxygen ought to follow what is 

 known as the Henry-Dalton law of pressures. But this is 

 found not to be the case. If we expose blood containing little or 

 no oxygen to a succession of atmospheres containing increasing 



382 



