15 6 HANDBOOK OF PHYSIOLOGY. 



over mercury. By repeating the experiment several times the whole of the 

 gases of the specimen of blood is obtained, and may be estimated. 



A. The Oxygen of the Blood. It has been found that a very 

 small proportion of the oxygen which can be obtained, by the aid of the 

 mercurial pump from the blood, exists in a 

 state of simple solution in the plasma. If 

 the gas were in simple solution, the amount 

 of oxygen in any given quantity of blood, 

 exposed to any given atmosphere, ought to 

 vary with the amount of oxygen contained in 

 the atmosphere. Since, speaking generally, 

 the amount of aiiy gas absorbed by a liquid 

 such as plasma would depend upon the pro- 

 portion of the gas in the atmosphere to 

 which the liquid is exposed if the propor- 

 tion is great, the absorption will be great; if 

 small, the absorption will be similarly small. 

 The absorption continues until the propor- 

 tions of the gas in the liquid and in the at- 

 mosphere are equal. Other things will, of 

 course, influence the absorption, such as the 

 nature of the gas employed, the nature of 

 the liquid and the temperature, but cceteris 

 paribus, the amount of a gas which a liquid 

 absorbs depends upon the proportion the 

 so-called partial pressure of the gas in 

 the atmosphere to which the liquid is sub- 

 jected. And conversely, if a liquid contain- 

 ing a gas in solution be exposed to an atmo- 

 sphere containing none of the gas, the gas 

 will be given up to the atmosphere until the 

 amount in the liquid and in the atmosphere becomes equal. This con- 

 dition is called a condition of equal tensions. 



The condition may be understood by a simple illustration. A large amount 

 of carbonic acid gas is dissolved in a bottle of water by exposing the liquid to 

 extreme pressure of the gas, and a cork is placed in the bottle and wired down: 

 The gas exists in the water in 'a condition of tension, and therefore exhibits 

 a tendency to escape into the atmosphere, in order to relieve the tension ; this 

 produces the violent expulsion of the cork when the wire is removed, and if 

 the aerated water is placed in a glass the gas will continue to be evolved until 

 it has almost entirely passed into the atmosphere, and the tension of the gas 

 in the water approximates to that of the atmosphere, in which, it should be 

 remembered, the carbon dioxide is, naturally, in very small amount, viz., 

 .04 per cent. 



Fig. 134. Ludwig's Mercurial 

 Pump. 



