WATER 65 



another, we change its conditions (see p. 7). Thus under at- 

 mospheric pressure water is converted to ice by reducing the 

 temperature below 0, and to steam by raising it above 100. 

 Under a pressure of 100 atmospheres, however, water freezes 

 at 1, and boils only at 330. With still higher pressures, the 

 change in the freezing-point is much more marked. Thus under 

 2000 atmospheres pressure water does not freeze above 20. 

 If the pressure is increased further, different crystalline forms of 

 ice make their appearance, and the freezing-point rises again 

 until under 20,000 atmospheres pressure water is found to freeze 

 at +78! 



Water as a Solvent: Natural Waters. Water has a re- 

 markable power of dissolving many other substances, and is said, 

 therefore, to have great power as a solvent. Rain is the purest 

 natural water. As it is formed by condensation of water vapor, 

 and has been in contact with the atmosphere only, it contains 

 only oxygen and other gases dissolved from the air, together 

 with a little dust. Sea water contains the greatest amount of 

 dissolved material, namely about 3.5 per cent. River and, espe- 

 cially, well waters contain materials in solution which have been 

 dissolved from the soil and the rocks. These materials act chem- 

 ically upon soap so that well waters are more or less hard, while 

 rain water is soft. We have already learned (p. 40) that natural 

 waters may also contain bacteria, which give rise to putrefaction 

 and disease. Many river waters contain large amounts of clay 

 and other insoluble substances suspended in them. The sus- 

 pended matter can be seen, as it renders the water turbid, but the 

 bacteria are invisible, so organic matter and bacteria may be pres- 

 ent in waters which look perfectly clear. 



Purification of Water. The suspended impurities, includ- 

 ing the bacteria, may be removed by filtration. City waters are 

 often filtered through extensive beds of gravel, but this treatment 



