290 APPLIED BIOLOGY 



sewage by distributing it in concrete tanks or reservoirs, 

 arranged so as to allow decomposing bacteria to grow rapidly. 

 The result is that sewage is changed so that it will be harm- 

 less. Even pathogenic (disease-producing) bacteria are thus 

 killed. Many cities now purify their sewage by this method 

 before discharging it into rivers. For private sewer systems 

 in villages and in rural districts, the best method consists 

 in allowing sewage to run into porous drain-tiles laid a short 

 distance below the surface in loose soil. In the soil around 

 the tiles the decomposing bacteria flourish and convert the 

 sewage into harmless substances which may be used by roots 

 of plants. The old-fasjhioned cesspools are being super- 

 seded by the modern bacterial methods, for the cesspools do 

 not favor rapid growth of bacteria, and moreover they are 

 often deep enough to allow escape of dangerous sewage into 

 subterranean water courses. 



Summarizing the useful aspects of bacteria, it is evident 

 that their usefulness Jar exceeds their harmfulness. A few 

 individual organisms may die from bacterial diseases, but 

 the continuance of life on this planet depends directly upon 

 the decomposing action of bacteria, which allows a cycle of 

 organic matter (116). And quite apart from this all- 

 important aspect of our relation to bacteria, their value in 

 the other ways we have noted above far outweighs their 

 harmfulness as producers of disease. Moreover, in the not 

 distant future, when civilized people will carefully apply the 

 already well-known biological laws (482^90), so that 

 bacterial diseases will be kept under control, the usefulness 

 of bacteria will attract more popular attention than at 

 present, when the average citizen thinks of " germs ;> only as 

 producers of disease. 



Most of the useful aspects of bacteria are well treated in 

 Lipmann's " Bacteria in Relation to Country Life," and 

 more briefly in Conn's " Bacteria, Yeasts, and Molds in the 

 Home." 



