THE CHEMISTRY OF RESPIRATION. 407 



would now give off all its oxygen, since, although the total 

 gaseous pressure on it was the saihe, no part of it was any 

 longer due to oxygen; and at the same time it would take 

 up one fifth more nitrogen, since the whole gaseous pressure 

 on its surface was now due to that gas, while before only four 

 fifths of the total was exerted by it. If, on the contrary, the 

 liquid were exposed to pure hydrogen under a pressure of one 

 atmosphere 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 independent 

 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 com- 

 posed 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 de- 

 pend on two things; a certain portion, that merely dissolved, 

 will vary with the pressure of the gas in question; but an- 

 other portion, that chemically combined, will remain the 

 same under different pressures. The amount of this second 

 portion depends only on the amount of the substance in the 

 liquid which can chemically combine with it, and is totally 

 independent of the partial pressure of the gas. 



4. Bodies are known which chemically combine with 

 certain gases when the partial pressure of these is consider- 

 able, forming compounds which break up, or dissociate, 

 liberating the gas, when its partial pressure falls below a 

 certain limit. Oxygen forms such a compound with heemo- 

 globin. 



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 chem- 

 ical 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, which simply dissolves oxygen, and (2) JicK r m.oglobin > 



