THE BIOLOGICAL CONDITIONS IN THE SOIL 227 



to a fraction of an atmosphere, and finally so weak that gravity 

 easily overcomes them. Over such a wide range it is con- 

 venient for the investigator to make divisions, and he is quite 

 at liberty to do this so long as he realises that his divisions 

 are arbitrary and not real. The various coefficients referred 

 to in the preceding pages have their uses, but they become 

 dangerous when they are taken to imply a break or an abrupt 

 change in the condition of the water. There appears to be 

 neither break nor division in the state of the soil water. 



Air Supply. 



The figures given in Table LVIII. show that about 10 

 per cent, of the volume of a normally moist soil is occupied 

 by air, but this volume is perpetually varying inversely as the 

 amount of water varies. These changes alone would lead to 



Table LIX. — Composition of the Air of Soils, Per Cent, by Volume. 



(Atmospheric air contains 21 per cent, of oxygen and "03 per cent, of COj. 



15* 



