152 INTRODUCTION TO GENERAL SCIENCE 



References : 



1. 1103:296-299. The Effect of the Ocean on Climate. 



2. 1304:277. Influence of Water, 

 a. 1302:244-249. The Oceans. 



6. 1303 : 96-98. The Forms and Exploration of the Ocean. 



c. 1305:109-121. The Sea. 



d. 1309 : 192-199. Ocean Currents ; their Causes and Effects. 



e. 1310 : 492-495. Composition of Sea Water. 

 /. 1311:279-290. The Ocean. 



g. 1312:169-196. The Ocean. 



112. PURIFICATION OF WATER 



Water is purified by nature, and also by man, who employs 

 the same methods. In fact, it should be clearly comprehended 

 that man can make use only of natural phenomena in any of 

 his work. All he can do is to control these phenomena within 

 certain limits. 



Water evaporates from the surface of the ocean, or other 

 bodies of water, and from the land, to form water vapor, which 

 condenses again to water. Only pure water evaporates. 

 This is nature's method of distillation. Man distills water 

 in a still. See Section 27, Distillation of Liquids. 



Some rocks are very porous and allow water to pass through, 

 but prevent solid matter from passing. Man uses a filter 

 composed of some porous material, such as cotton, charcoal, 

 sand, or unglazed clay. The latter is often in the form of a 

 cylinder, the filtered water passing into the cylinder and 

 leaving the dirt on the outside, whence it can be easily re- 

 moved. This is the Pasteur filter. 



Some chemicals may be held in solution and cannot be 

 removed by a filter. See Section 115, Solution and its 

 Effects. Chemical means may then be employed instead of 



