RESPIRATION. THE GASEOUS INTERCHANGES 419 



air. When several gases are mixed together the fraction of the 

 total pressure exerted by each one is known as the partial pressure 

 of that gas; and it is this partial pressure which determines the 

 amount of each individual gas dissolved by a liquid. If a liquid 

 exposed to the air for some time had taken up all the oxygen and 

 nitrogen it could at the partial pressures of those gases in the air, 

 and were then put in an atmosphere in which the oxygen had all 

 been replaced by nitrogen, it would now give off all its oxygen, 

 since, although the total gaseous pressure on it was the same, 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. The amount of gas taken up by a liquid varies, other things 

 being equal, inversely as the temperature. 



4. 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 ab- 

 sorbed will depend upon how much the liquid can combine with. 

 Or, a liquid may be composed partly of things which simply dis- 

 solve a gas and partly of things which combine with it chemically. 

 Then the amount of the gas taken up under a given partial pres- 

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

 solved, will vary with the pressure of the gas in question; but 

 another portion, that chemically combined, will remain the same 

 under different pressures. 



. 5. Bodies are known which combine chemically with certain 

 gases when the partial pressure of these is considerable, 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 hemoglobin. 



6. 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 



