154 ELEMENTARY PHYSIOLOGY LEsa 



as to the behaviour of gases towards each other and 

 towards liquids with which they may be in contact is 

 essential as a preliminary. 



When a gas is enclosed in a vessel, it exerts a 

 pressure on its walls which is measured by the height 

 of the column of mercury which it can support in a 

 vertical tube connected with the vessel. If two gases 

 are mixed in the vessel, each gas exerts its own 

 pressure just as if the other gas were not present ; the 

 total pressure of the two gases is therefore eqrial to the 

 swn of their separate pressures. The pressure due to 

 each gas in the mixture is called the partial pressure 

 of that gas, and is proportional to the qnantitij of the gas. 

 Hence if the toUil pressure of the mixture is measured 

 and its composition is determined by analysis the partial 

 pressure of each gas is at once known. Take for instance, 

 ordinary air when the barometer stands at 760 mm. (30 

 inches of mercury). The partial pressure of the oxj'gen is 

 1% ^ 7^0 ~ 159 6 mm. (6 '3 inches of mercury), and 

 that of the nitrogen is ^^q x 700 = 600-4 mm. (23-7 

 inches of mercury). 



When a gas is in contact with a liquid some of the 

 gas is absorbed by the liquid, the amount being dependent 

 on the pressure of the gas. If <«"o gases of eijual solubility 

 are in contact with the some liquid tliey will be absorbed in 

 quantities proportional to their respective partial pressures 

 in the space over the liquid, and when the absorption is 

 complete the partial pressures of the gases in the liquid 

 are the same as the partial pressures of the gases in the 

 space. If the partial pressure of one of the gases be made 

 less in the space over the liquid, then some of tliat gas will 

 make its exit from the licjuid ; and if its partial pressure be, 

 on the other hand, increased, then more of that gas will 

 enter the liquid. Thus we see that changes in the partial 

 pressures of the gases in contact with the liquid determine 



