WATER SUPPLY AND SEWAGE DISPOSAL 161 



in many ways, but it is not known to have any serious effect 

 on health. The chief annoyance is the curd that forms in 

 it when soap is used, and it is also wasteful of soap. One of 

 the commonest substances is lime, which is really dissolved 

 limestone. Lime is deposited in a teakettle when hard water 

 is boiled in it, and in this way the water is made soft. Some 

 other substances in hard water cause more serious annoy- 

 ance because they can only be removed by the addition of 



WESTERN EUROPE 



UNITED STATES 



EASTERN EUROPE 



Petrogmd Bi^^^^_n^^.^^__ ___ .^ 50 



FIG. 80. Death rate from typhoid fever 



The figures show the number of deaths per hundred thousand population, 



based upon data for 1912. The dark line indicates the proportionate number 



of deaths in each city 



chemicals. Borax, sal soda, ammonia, and lye are commonly 

 used for this purpose in order to save soap. 



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 sedi- 

 ments present in lake waters have been brought in by rivers 

 or stirred up by waves along the shore, there being little sedi- 

 ment in the water a few miles from land ; but in rivers they 

 remain in suspension for a longer time. Sediments make the 

 water less inviting, but may not in themselves be dangerous. 



