THE ABSORPTION OF GASES BY LIQUIDS. 381 



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 \ of the total pressure is due to 

 oxygen and % 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 there- 

 fore reduced to ^ 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 

 f is due to nitrogen, and all the oxygen might be removed 

 from the air without affecting the quantity of nitrogen 

 which would be absorbed from it by a given volume of 

 water. The atmospheric pressure would then be 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 to- 

 gether 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 oxygen had all been replaced by nitrogen, it would now 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 % 

 more nitrogen, since the whole gaseous pressure on its sur- 

 face was now due to that gas while before only f of the 

 total was exerted by it. If, on the contrary, the liquid were 

 exposed to pure hydrogen under a pressure of one atmos- 



