1 8 BACTERIA. 



forms of nitrogen, phosphorus, carbon, and hydrogen, 

 during which they become finally oxidized or mineralized to 

 nitric acid (HNO 3 ), phosphoric acid (H 3 PO 4 ), carbonic acid, 

 CO 2 , and water, H 2 O. In nature this process goes on in 

 the superficial layers of the earth or in the presence of the 

 atmosphere. That it takes place much more readily near the 

 surface of the ground and in porous earth can easily be 

 understood if what takes place in the oxidation that goes 

 on in spongy platinum is borne in mind. In the earth we 

 have a spongy material, the upper surface of which is well 

 supplied with air, and usually also with moisture ; there is 

 also a certain amount of organic matter present on which 

 micro-organisms feed, and any additional organic matter 

 brought to this spongy mass is rapidly seized upon by the 

 micro-organisms, is oxidized, and thus prepared for the 

 nutrition of the plants that are growing in this soil. So 

 necessary is this whole process of oxidation by micro- 

 organisms that Duclaux insists that soil rendered sterile (as 

 regards micro-organisms), and supplied only with sterilized 

 water and air, is incapable of supplying sufficient nutrient 

 material to plants to enable them to flourish even moder- 

 ately well. All the organisms found in these superficial 

 layers under ordinary conditions are serobes ; in the deeper 

 layers of the soil are micro-organisms that give rise to the 

 second kind of decompositions. These bacteria, which are 

 anaerobic (that is, they can flourish without being supplied 

 with free oxygen) in character, have a special power of taking 

 up the oxygen that is contained or combined in the products 

 which have filtered down from the surface where the decom- 

 positions by direct oxidation are going on. Living away from 

 the atmosphere and being unable to obtain or to utilize 

 free oxygen, these organisms have developed the faculty of 

 being able to wrest oxygen, by force, as it were, from the 

 oxygen-containing bodies that come down to them from 

 nearer the surface, sometimes however using part only of 

 the combined oxygen and setting free another part by 

 which further oxidation may go on ; in doing this they 

 carry the process of decomposition a stage further, and 

 after the altered organic materials have been attacked by 

 these anaerobic organisms they appear to contain little that 

 will provide nutrition for micro-organisms of any kind ; so 

 that, after we come to a certain depth, bacteria are not 



