92 BACTERIA less. 



(NHg), sulphuretted hydrogen (HgS), and ammonium 

 sulphide ( (NH4)oS), the evolution of which produces the 

 characteristic odour of putrefaction. 



The filial stage in putrefaction is the formation of nitrates 

 and nitrites. The process is a double one, both stages 

 being due to special forms of Bacteria. In the first place, 

 by the agency of the nitrous ferment, ammonia is converted 

 into nitrous acid — 



NH3 + 30 = H._,0 + HNOo 

 Ammonia. Oxygen. Water. Nitrous Acid. 



The nitric ferment then comes into action, converting the 

 nitrous into nitric acid — 



NHO2 + O = HNO3 



Nitrous Acid. Oxygen. Nitric Acid. 



This process is one of vast importance, since by its agency 

 the soil is constantly receiving fresh supplies of nitric acid 

 which is one of the most important substances used as 

 food by plants. 



Besides holophytes and saprophytes there are included 

 among Bacteria many parasites, that is, species which feed 

 not on decomposing but on living organisms. Many of the 

 most deadly infectious diseases, such as tuberculosis, diph- 

 theria, typhoid fever, and cholera, are due to the presence 

 in the tissues or fluids of the body of particular species of 

 microbes, which feed upon the parts affected and give rise 

 to the morbid symptoms characteristic of the disease. 



Some Bacteria, like the majority of the organisms pre- 

 viously studied, require free oxygen for their existence, but 

 others, like Saccharomyces during active fermentation (see 

 p. 80), are quite independent of free oxygen and must there- 

 fore be able to take the oxygen, without which their metabolic 



