88 WATER AND OTHER BEVERAGES 



common salt. Such water is not irritating when in con- 

 tact with the delicate tissues of the body. 



Organic contents. Besides these ordinary contents, 

 surface waters frequently contain minute living plants 

 and animals. Many of these organisms are entirely 

 harmless, since they cannot grow in the bodies of ani- 

 mals. Some of them, however, when introduced into 

 the body thrive and multiply as parasites. The most 

 harmful of these, because of their power to cause disease, 

 are certain kinds of microbes which have been introduced 

 into the water from animals or man. When taken into 

 the body, they multiply at so rapid a rate and develop 

 such powerful poisons that they may even overwhelm 

 and kill the animal which acts as their host. Some of 

 the most dangerous of these are the microbes of typhoid, 

 cholera, dysentery and scarlet fever. They may be in- 

 troduced into the system not only by drinking the 

 water contaminated by them but also by eating raw shell- 

 fish which have lived in the contaminated water, by 

 drinking milk from receptacles washed in it, or by eat- 

 ing raw vegetables watered or washed with it. 



Water supply. To secure pure drinking water has 

 become one of the great problems of civilized life, espe- 

 cially in the larger towns and cities. The main sources 

 of supply are surface water, spring, well and lake water. 



Surface water. Surface water, as found in rivers 

 and brooks, is always liable to ^contamination and should 

 be avoided for drinking purposes, even in sparsely set- 

 tled regions. For example, in a country district in the 

 hilly part of Pennsylvania there was an epidemic of 

 typhoid fever in the spring. It was finally traced to 

 the contamination of the water supply by the excretions 

 of a case of typhoid. These had been thrown out upon 

 the snow on a hillside, and when the snow melted in 



