278 THE CIRCULATION OF NITROGEN [PART III 



for several days in some water in a warm place it will be seen that 

 the liquid soon becomes turbid, and if we examine it microscopically 

 we will find that innumerable bacteria, cocci, and spirilla are 

 present. But infusoria will also usually be present and one can 

 generally increase the numbers of the latter organisms by adding 

 a little sugar to the putrefying liquid. There is a succession 

 of organisms in a mass of decomposing sewage : first bacteria, then 

 flagellates, infusoria, turbellarian worms, nematodes, oligochaete 

 worms, and dipterous larvae. Much of the organic matter of the 

 sewage passes into the air in the form of the bodies of flies which 

 have fed on the other organisms mentioned. 



Modern sanitary science has availed itself of the power of 

 decomposing organic matter possessed by bacteria and other 

 micro-organisms. In the "septic" or "biological " methods of treat- 

 ing the sewage of great human communities, the offensive matter 

 is broken down by bacterial action. In these installations the 

 crude sewage, after being " screened " so as to remove the coarser 

 solid matters, is led into the septic tanks. There are two types 

 of the latter. In one air is excluded from the putrefying sewage, 

 and the bacteria which flourish in the tanks are anaerobes, that is, 

 species which can live in the absence of oxygen. In the other 

 type oxygen is freely admitted. In crude sewage we have a liquid 

 which already contains enormous numbers of bacteria (from 6 to 

 12 millions per c.c. in London sewage) and which also contains food 

 matters for the bacteria in the shape of dissolved and suspended 

 organic matters, broken down proteids, fats, carbohydrates, cellulose, 

 &c. These food substances are utilised by the micro-organisms, 

 both bacteria and infusoria, and the latter multiply and live upon 

 the organic matter, which therefore rapidly disappears. Even after 

 remaining in the septic tanks for 24 hours there is already a con- 

 siderable diminution in the proportion of putrescible matters 

 present in the sewage. 



Nitrification. Thus a number of substances, unstable and 

 capable of further decomposition, are continually being produced 

 as the waste substances of the bodies of living organisms. These 

 waste substances, or excretory products, reach the drains of human 

 communities, or enter the subsoil water, and so pass into lakes or 



