156 ELEMENTS OF GENERAL SCIENCE 



while others are of very little consequence. For our own 

 purpose it will be useful to put them all into three classes: 

 dissolved substances, sediments, and bacteria. 



One of the commonest substances is lime, which is really 

 dissolved limestone. This is the substance which is deposited 

 in a teakettle if hard water is boiled in it. The presence of 

 lime in water is an annoyance in many ways, but the sub- 

 stance is not known to have any serious effect upon health. 

 The same may be said of other substances which are commonly 

 present in solution in water. 



Sediments, or solid particles, of various sorts are present in 

 all natural waters. We usually speak of them as mud, clay, 

 sand, etc. If the water is allowed to stand for some time, the 

 sediment tends to settle to the bottom. The sediments present 

 in lake waters are such as have been brought in by rivers or 

 stirred up by waves along the shore. Very naturally there is 

 little sediment in water a few miles from land, while in rivers 

 these sediments remain in suspension for a longer time. Such 

 sediments as we have been describing make the water less 

 inviting, but they are not in themselves dangerous. 



The third kind of material present hi water bacteria 

 is in reality a sort of sediment, but the bacteria are of such 

 great importance that it seems best to discuss them separately. 

 Indeed, the principal importance of other sediments lies in the 

 fact that bacteria are likely to be associated with them. For 

 instance, the mud along the shore usually has a great deal of 

 refuse mixed with it, for the shore is likely to be made a 

 general dumping ground. Thus there are great numbers of 

 bacteria living in the mud. If waves stir up this mud and 

 it mingles with the water, the bacteria are likely to be dis- 

 tributed as widely as the mud is. Therefore water which is 

 muddy is often dangerous and always open to suspicion. 



174. Sources of bacteria in lakes and rivers. Natural waters 

 are never wholly free from bacteria. Every rain washes them 

 into the streams, and they are further increased by the common 



