498 



EESPIRATION 



determining the pressure or tension of this gas in the liquid. Thus, 

 it will be seen that a gas can be extracted from a liquid by simply 

 bringing it into relation with an atmosphere in which its partial pres- 

 sure is slight. The procedure usually followed is to subject the liquid 



FIG. 255. GAS PUMP FOR EXTRACTING THE GASES OF BLOOD. (Grehant.) 

 M and F, the mercury receivers; P, the windlass for raising and lowering M ; m, a 

 three-way stopcock protected by a seal of mercury or water; C, a cup with mercury over 

 which the receiving eudiometer is placed to collect the gases; B, the bulb in which, after a 

 vacuum is made, the blood is introduced by the graduated syringe, S. By means of the 

 stopcock m the vacuum in F, caused by the fall of the mercury, can be placed in commu- 

 nication with B. After the gases have diffused over into F, M is raised, and when the 

 stopcock m is properly turned these gases are driven out through C into the receiving 

 tube. The operation is repeated until no more gas is given off from B. (Howett.) 



in which the gas is dissolved, to the vacuum of an air-pump or to bring 

 it into relation with some other gas. 



The gases of the blood, however, present certain peculiarities be- 

 cause they are not entirely in pure physical solution, but enter loose 

 chemical combinations; in fact, a part of the carbon dioxid forms a 

 stable compound, the dissociation of which necessitates the use of 



