SOILS 389 



greater. There are annual and seasonal fluctuations of 

 the carbonic acid in the soil, and larger amounts are found 

 in the deeper than in the surface layers. As water passes 

 through the soil it takes up carbonic acid, and as noticed at 

 p. 3, this is one of the causes of the wearing away of 

 certain rocks by the solvent action of the acid. 



The oxygen in the soil air diminishes as the deeper layers 

 are reached, until finally it ceases to exist. This fact has 

 an important practical bearing on the disposal of the dead 

 by burial, especially animals which have died from in- 

 fectious diseases. The anthrax bacillus, for example, 

 cannot sporulate excepting in the presence of free oxygen , 

 so that deep burial is a safe method of disposing of these 

 cases. 



The nitrogen in the soil air is much the same as that in 

 the atmospheric air. If plants could utilize the nitrogen of 

 the air agriculture would be greatly simplified ; with a few 

 exceptions they cannot. The exceptions are certain 

 leguminous crops in which nodules form on the roots ; these 

 nodules contain organisms which are capable of utilizing 

 free nitrogen as plant food. To these organisms the name 

 Bacillus radicicola is given ; it is found that though in 

 appearance they cannot be distinguished from each other, 

 yet, stating the case broadly, each leguminous plant has its 

 own organism, so that, for example, the organisms of the 

 pea are of no use for the clover. 



The movement of the soil air is a process constantly 

 occurring more or less freely, depending upon the porosity 

 and dryness of the soil. The actual motive power is 

 furnished by the rise and fall of the ground water, as the 

 water rises the air is forced out, as it sinks the air is drawn 

 in. It is during the rise of the ground water that soil 

 emanations are forced out, and where the surface pollution 

 is great, as in the neighbourhood of inhabited buildings, 

 these emanations must be a means of rendering the air 

 impure. 



This is the reason for the necessity of paving in and 

 around buildings, and placing them on an impervious 



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