406 THE HUMAN BODY. 



same volume of oxygen as before, but this volume of the- 

 compressed gas will contain exactly four times as much 

 oxygen as did the same volume of the gas under the original 

 pressure; and if, now, the pressure be again diminished the 

 oxygen will be given off exactly in proportion as its pressure 

 on the surface of the water decreases. Finally, when a com- 

 plete vacuum is formed above the surface of the water, it will 

 be found that the latter has given off all its dissolved oxygen. 

 This law, that the quantity of a gas dissolved by a liquid 

 varies directly as the pressure of that gas on the surface of 

 the liquid is known as Dalton's law. 



2. The amount of a gas dissolved by a liquid depends, not 

 on the total pressure exerted by all the gases pressing on its 

 surface, but on the fraction of the total pressure which is 

 exerted by the particular gas in question. For example, the 

 average atmospheric pressure is equal to that of a column 

 of mercury 760 mm. (30 inches) high. But 100 volumes of 

 air contain approximately 80 volumes of nitrogen and 20 of 

 oxygen : therefore -J of the total pressure is due to oxygen 

 and f to nitrogen : and the amount of oxygen absorbed by 

 water is just the same as if all the nitrogen were removed 

 from the air and its total pressure therefore reduced to -J- of 

 760 mm. (30 inches) of mercury; that is, to 152 mm. (6 inches) 

 of mercury pressure. It is only the fraction of the total 

 pressure exerted by the oxygen itself which affects the 

 quantity absorbed by water at any given temperature. So, 

 too, of all the atmospheric pressure ^ is due to nitrogen, and 

 all the oxygen might be removed from the air without affect- 

 ing the quantity of nitrogen which would be absorbed from 

 it by a given volume of water. The atmospheric pressure 

 would then be f of 760 mm. of mercury, or 608 mm. (24 

 inches), but it would all be due to nitrogen gas and be 

 exactly equal to the fraction of the total pressure due to that 

 gas before the oxygen was removed from the air. When 

 several gases are mixed together the fraction of the total pres- 

 sure 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 



