448 RELATIONS OF OXYGEN IN BLOOD. [BOOK n. 



atmosphere above ; the greater the pressure of the oxygen, the 

 larger the amount which will be absorbed. If the pressure of 

 the whole atmosphere remain the same, at 760 mm. of mercury 

 for instance (the ordinary atmospheric pressure), the pressure 

 of the oxygen may be increased or diminished by increasing or 

 diminishing the proportion of oxygen in the atmosphere. So 

 that with an atmosphere remaining at any given pressure the 

 quantity of oxygen absorbed will depend on the quantity 

 present in that atmosphere. If on the other hand water, 

 already containing a good deal of oxygen dissolved 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 estab- 

 lished 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 independent 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 difference, as far as the absorption of oxygen by the water 

 was concerned, whether the remaining two-thirds of the atmos- 

 phere 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 atmos- 

 phere 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 absorption 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, 



