10 



RESPIRATORY MECHANISMS 



phere and in the following discussion concentrations will 

 generally be given in mm pressure of mercury. 



In the surface layers of the soil the increase in pressure is 

 insignificant, but oxidative processes both inorganic and or- 

 ganic are at work by which O2 is used up and sometimes C0 2 

 produced. Normally, however, the exchange with the free 

 atmosphere taking place by diffusion and by barometric pres- 

 sure variations is so considerable that the air differs only 

 slightly from the atmospheric. Ege (1916) found in a forest 

 30 cm below the surface 0.2% C0 2 and 20.6% 2 , correspond- 

 ing to pressures of 1.5 and 152 mm respectively. In a field 

 the O2 pressure was 153, in an anthill 145 and in a decaying 

 beech trunk 133 mm. Portier and Duval (1929) found C0 2 

 pressures up to 14 mm in anthills in summer when the tem- 

 perature was also several degrees higher than in the earth in 

 the neighborhood. Fairly heavy rain will sometimes clog the 

 pores of the surface so as to seriously reduce the exchange, and 

 in one case Ege observed 30 cm below the surface in a field 

 after rain the compositions and pressures given in Table 4. 



Table 4 



In such circumstances some of the animals living in the earth 

 are apt to suffer and the earthworms may react by coming 

 to the surface. 



At greater depths, where only man will penetrate, the 

 peculiar conditions first studied by Haldane sometimes give 

 rise to serious accidents. Inorganic oxidative processes which 

 do not give rise to any production of C0 2 take place regularly, 



