44 ELEMENTARY SCIENCE 



well as the water vapor in the air, are perfectly trans- 

 parent, and are evenly distributed. Just as the substances 

 that make up a solution tend to become equally distributed, 

 so gases that come together tend to become equally dis- 

 tributed. The process by which one gas spreads itself 

 evenly through another is called the process of diffusion. 



Water, as we find it in nature, is never perfectly pure. 

 That is, it always contains particles of other substances 

 along with the particles of water. However, these natural 

 impurities do not discolor the water, and may not make it 

 less fit to drink. In fact, the most agreeable drinking water 

 does contain impurities, while perfectly pure water tastes 

 flat and insipid. Any one who drinks distilled water, 

 having been in the habit of drinking well-water, finds it 

 disagreeable at first. Springs that are famous for the 

 health-giving value of their water owe this to the presence 

 in solution of certain minerals or gases, or both. 



Hard and Soft Water. The difference between hard 

 and soft water is that the former contains more of dissolved 

 matter than the latter. There are, of course, all sorts of 

 degrees of "hardness" and "softness" of water, depending 

 upon the varying amounts of matter dissolved in them. 



Spring-water is usually regarded as the purest for drink- 

 ing purposes, because it has been thoroughly filtered by 

 soaking through the soil. This process removes particles 

 which are suspended in the water (as particles of mud), 

 and so renders the water quite clear, but it has no effect 

 on the material which is dissolved in the water. In fact, 

 this process of soaking through the soil is the very process 

 by which water becomes hard. It dissolves certain soil ma- 

 terials as it goes along and carries them with it in solution. 



