446 RESPIRATION 



only in a medium containing oxygen, while others, for example, the 

 bacillus of tetanus and the bacillus of anthrax, flourish only when this 

 gas is absent. It need scarcely be emphasized that by far the greatest 

 number of organisms are aerobic, i.e., they take in oxygen and give 

 off carbon dioxid. 



Diffusion Pressure. In the same way as the air moves from an area 

 of high pressure to an area of low pressure, so do the individual gases 

 constituting a mixture, move from places of high to places of low 

 pressure. The driving force responsible for this movement of diffusion, 

 is furnished by the partial pressures of these gases. The atmospheric 

 air rests upon us with a certain pressure which differs somewhat with 

 the temperature, altitude, and other conditions. For this reason, it 

 is necessary to have a fixed standard which is called an atmosphere. 

 This pressure is capable of supporting a column of mercury 760 mm. 

 in height at latitude 45 and at sea-level, when the temperature of the 

 mercury is C. But as air is composed of several gases, the total 

 pressure of 760 mm. Hg is equal to the sum of the separate pressures 

 of its constituents. Inasmuch as the pressure exerted by each gas in 

 a mixture is known as the partial pressure of that gas, the pressure of 

 the air is really the product of the different partial pressures of its 

 constituents. 



Dry atmospheric air shows the following composition: 



Oxygen ....................................... 20 . 94 per cent. 



Nitrogen ...................................... 78 . 40 per cent. 



Argon, krypton, neon: .......................... 0.63 per cent. 



Carbon dioxid ................................. . 03 per cent. 



As the partial pressure exerted by a certain gas is proportional to the 

 quantity of this gas present in the mixture, it can readily be seen that 



21 

 the partial pressure of the oxygen equals in round numbers ^^ X 



79 



760 = 159.6 mm. Hg, and that of the nitrogen 77^ X 760 = 600.4 



IUU 



mm. Hg. Carbon dioxid exerts practically no pressure at all in per- 

 fectly fresh air. 



Diffusion. In the lowest forms the interchanges of the gases is 

 effected by simple diffusion. The medium, whether it be water or air, 

 contains a certain normal quantity of oxygen. It is held here under a, 

 definite partial pressure. Inside the organism, on the other hand, the 

 partial pressure of this gas is much less, because it is constantly used 

 up during the processes of oxidation. On this account, it is present 

 here in smaller amounts than in the medium. Obviously, therefore, 

 the molecules of oxygen must move in a steady stream from without to 

 within directly through the enveloping membrane. The latter, quite 

 naturally, offers a slight resistance to the diffusing particles, but the 

 difference in the partial pressures is so great that this movement as a 

 whole is not noticeably hindered. Quite similarly, the fact that carbon 



