152 BACTERIA 



storage, and the bodies of animals which have been 

 engulfed in bogs are often found perfectly preserved 

 after many years. 



After the first putrefactive decompositions other 

 kinds of bacteria reduce the products of putrefaction 

 to simpler substances and so on until ammonium 

 (NH 4 ) compounds are produced, together with carbon 

 dioxide and water. This process is noticed in the 

 strong smell of ammonia (NH 3 ) arising from a heap of 

 stable manure at a certain stage of decomposition. 

 The debris of plants fallen leaves, twigs, etc. undergo 

 similar decompositions, but since a smaller proportion 

 of their substance is formed of proteins and there is 

 a much larger proportion of carbohydrates (cellulose, 

 etc.), the result is more carbon dioxide and water and 

 less ammonia than in the case of animal substances. 

 Also the early stages of decomposition of plant remains 

 are largely carried out by fungi, though bacteria also 

 play a part. The decaying plant debris, together 

 with any decaying animal bodies or other animal sub- 

 stances present, form the humus of the soil. Humus is 

 always in process of reduction to the simple chemical 

 substances named above, together with various mineral 

 salts, and is as constantly replenished by fresh organic 

 debris. 



When the organic substances have been brought 

 into the state of simple salts containing nitrogen, 

 other kinds of bacteria take up the work, some (Nitro- 

 somonas) converting the ammonium salts into nitrites, 

 i.e. salts formed from nitrous acid (HNO 2 ) and bases 

 present in the soil, others again (Nitrobacter) carrying 

 out a further oxidation and producing nitrates, formed 

 from bases and nitric acid (HNO 3 ). This process of 

 the oxidation of ammonium compounds to nitrates, 



