162 ELEMENTS OF GENERAL SCIENCE 



The third kind of material bacteria is in reality a sort 

 of sediment, but bacteria are of such importance that they 

 will be discussed separately. Indeed, the principal importance 

 of other sediments lies in the fact that bacteria are likely to 

 be associated with them. The mud along the shore usually 

 has a great deal of refuse mixed with it. There are great 

 numbers of bacteria living in this mud, and if it is stirred up 

 by the waves and mixed with water, the bacteria are likely 

 to be distributed as widely as the mud is. Water which is 

 muddy is often dangerous and always open to suspicion. 



177. Sources of bacteria in lakes and rivers. Natural 

 waters are never wholly free from bacteria. Every rain 

 washes them into the streams. The most important source 

 of bacteria in many of our streams is the sewage of the 

 cities on their banks. The bacteria from this source are 

 particularly objectionable because the sewage includes human 

 wastes and therefore commonly contains the germs of such 

 infectious diseases as are present in the community. As the 

 water flows down the river many of the bacteria settle to the 

 bottom along with other sediments or are killed by exposure 

 to air and sunlight. Excepting near the source, very few 

 rivers in their natural condition are suitable for domestic use. 



The larger lakes are fed by many rivers and therefore 

 receive large numbers of bacteria from them. Contamina- 

 tion is increased by refuse dumped alongshore and by the 

 sewage of adjoining cities. The principal sources of contami- 

 nation of lake waters are located near shore. The waters of 

 the center of the lakes are polluted only as bacteria and 

 other sediments are carried out into the lake by currents. 

 The movements of lake water are so slow that bacteria and 

 other sediments usually settle to the bottom before they 

 have traveled many miles from land. For this reason the 

 water in the central part of large lakes is usually of excel- 

 lent quality. As cities increase in size, however, the zone of 

 pollution alongshore extends farther into the lake. 



