122 GENERAL SCIENCE 



through the soil. In doing so the water takes into solu- 

 tion various salts. Sometimes these salts have certain 

 medicinal properties, and the water containing them is 

 sold for medicinal purposes. There now exist a number 

 of health resorts which base their claims on the peculiar 

 properties of the water of their springs. 



Purification of Water. In many cities the water is so 

 impure that some means must be taken to free it from 

 impurities. Some cities have established immense filtra- 

 tion plants to remove the solid matter from the water 

 supply. Columbus, Ohio, has such a plant which has 

 been in successful operation for a number of years, with 

 excellent results from both the standpoint of health and 

 of suitability for commercial uses. In this plant much 

 of the lime in solution is removed by the use of chemicals. 

 Ordinarily a city filtration plant consists simply of large 

 areas of sand, gravel, and sometimes charcoal through 

 which the water is allowed to percolate. After one filter 

 has been used for a while the water is turned into another 

 filter to permit the first to be purified by the action of the 

 air and sun. Such filters remove most organic matter, 

 but of course they do riot remove the soluble substances. 



A filter of sand and charcoal may be used for remov- 

 ing the solid particles from water, and such filters are 

 quite common both on a small and large scale. However, 

 such filters soon become clogged with small particles of 

 clay and other foreign substances and must be renewed 

 if their efficiency is to be maintained. 



To separate all the solid matter in the form of solutions 

 from water, distillation must be resorted to. This is 

 merely a process of driving the water off in steam and 

 catching and condensing the steam again. For home use 

 the water may be rendered safe by boiling, since all 



