383 THE HUMAN BODY. 



phere it would give off all its previously dissolved oxygen, 

 and nitrogen, since none of the pressure on its surface would 

 now be due to those gases; and would take up as much 

 hydrogen as corresponded to a pressure of that gas equal to- 

 760 mm. of mercury (30 inches). 



3. A liquid may be such as to combine chemically with 

 a gas. Then the amount of the gas absorbed is indepen- 

 dent of the partial pressure of the gas on the surface of the 

 liquid. The quantity absorbed will depend upon how much 

 the liquid can combine with. Or, a liquid may partly be 

 composed of things which simply dissolve a gas and partly 

 of things which chemically combine with it. Then the- 

 amount of the gas taken up under a given partial pressure- 

 will depend on two things; a certain portion, that merely 

 dissolved, will vary with the pressure of the gas in question;, 

 but another portion, that chemically combined, will remain 

 the same under different pressures. The amount of this 

 second portion depends only on the amount of the sub- 

 stance in the liquid which can chemically combine with it. 

 and is totally independent of the partial pressure of th# 

 gas. 



4. Bodies are known which chemically combine with 

 certain gases when the partial pressure of these is consider- 

 able; but the compounds thus formed are broken up, and 

 the gas liberated, when its partial pressure on the surface 

 of the liquid falls below a certain limit. 



5. A membrane, moistened by a liquid in which a gas is 

 soluble, does not essentially alter the laws of absorption, by 

 a liquid on one side of it, of a gas present on its other side, 

 whether the absorption be due to mere solution or to 

 chemical combinations or to both. 



The Absorption of Oxygen by the Blood. Applying 

 the physical and chemical facts stated in the preceding 

 paragraph to the blood, we find that the blood contains (1) 

 plasma, ^hich simply dissolves ox} T gen, and (2) haemoglobin, 

 which combines with it under some partial pressures of 

 that gas, but gives it up under lower. 



Blood plasma or, what comes to the same thing, fresh 

 serum, exposed to the air, takes up no more oxygen than so 



