PUTREFACTION AND OXIDATION 343 



or vegetable tissues into simpler inorganic bodies, the chief 

 products being water, carbonic acid, and ammonia, together 

 with smaller quantities of other products, some of them of 

 particularly evil odour and poisonous properties. 



The bacteria of putrefaction are for the most part 

 anaerobes, and are therefore found at some little depth in 

 the soil. In order that the soil should be kept in proper 

 condition for producing crops, it is essential that not only 

 these bacteria shall be present (in the lower layers), but 

 that in the upper layers there shall exist oxidation bacteria 

 that shall change the carbonaceous matter into carbonic 

 acid and the ammonia into nitrates. Plants cannot absorb 

 ammonia direct ; their nitrogen must be in the form of 

 nitrates before it is capable of assimilation. Hence we see 

 the necessity of maintaining due porosity in the soil, and 

 of not unduly loading it with more organic matter than the 

 organisms can successfully deal with. The failure of this 

 process of oxidation is well seen in a sewage farm where 

 sewage matter, either liquid or in the form of sludge-cake, 

 has been applied too freely to the land. Instead of large 

 crops being obtained, the reverse is found to be the case, 

 and the land is said to be ' sewage-sick.' 



The experiments of James Buchanan Young on the 

 soil of graveyards show that they are very rich in micro- 

 organisms, particularly those of a liquefying type, the 

 Proteus vulgaris being present in great numbers. Their 

 action is so effective that he found no notable quantities of 

 organic carbon and nitrogen in the upper layers, while 

 that in the lower layers was not so very much in excess of 

 that found in virgin soil. 



The oxidation bacteria also play a very important part in 

 the purification of water artificially. 



The filter-beds at the waterworks are constructed of a 

 layer of large stones with unjointed pipes placed at intervals 



