382 EXPJSRIMENTS WITB PLANTS 



In the first, the sewage is left for six to twelve 

 hours in a shallow open basin (contact bed) , the 

 bottom of which is covered with furnace clinkers or 

 coke (these substances help to purify the liquid) : it is 

 then conducted into a similar bed for six to twelve 

 hours, at the end of which it is so purified that it may 

 be allowed to flow^into a neighboring stream. 



In the second, a closed underground chamber 

 (septic tank) with a vent-pipe for gases is employed: 

 the sewage is allowed to flow slowly through it in a 

 constant stream; on emerging from this it is greatly 

 purified. 



This helps us to understand the self-purification of 

 rivers and streams. For example, the sewage of 

 Chicago is now emptied into the Illinois river, which, 

 after flowing some three hundred miles, empties into 

 the Mississippi a few miles above the point from which 

 St. Louis takes its water supply. This at first seems 

 to be an alarming condition of things, but on exami- 

 nation it has been found that this water has no more 

 bacteria than neighboring rivers which are not recipi- 

 ents of sewage. The purification probably depends 

 largely on the fact that the bacteria use up the food 

 supply (i. e., the sewage matter) very rapidly and 

 then perish, and, further, on the fact that they slowly 

 sink to the bottom, are devoured by other organisms, 

 and are killed by sunlight and aeration. 



In order to study the efEeet of aeration, take an 



