90 GENERAL SCIENCE 



Water can hold gases as well as solids in solution. Both marine 

 and land plants get the needed supply of oxygen for their tissues from 

 the water. Fish are provided with special organs known as gills to 

 get from the water that passes in and over them the oxygen they 

 require. When water containing any gas is freed from pressure, more 

 or less of the dissolved gas will escape. This is seen in "soda water," 

 and in all carbonated mineral waters. 



VAPORIZATION AND CONDENSATION 



As elevation above the earth's surface increases it is found 

 that the pressure exerted by the atmosphere decreases. In 

 the study of Physics it is also found that when the pressure 

 upon any gas is decreased the gas not only expands but its 

 temperature falls. This result follows from the molecules 

 being further apart. As atmospheric moisture rises in the 

 form of invisible vapor of water it expands and cools, and the 

 formation of clouds with or without rain or snow may follow. 

 The air may cool, also, by reason of radiation of its heat out 

 into space independent of any expansion. 



Clouds and fogs, consist of minute droplets of water as a 

 liquid. These cloud masses of water particles are heavier 

 than air, and are all the time settling toward the earth's 

 surface. Where the under surfaces of the cloud mass come 

 in contact with warmer air, or air that is unsaturated, the 

 liquid particles change back into the vapor form. But 

 while the cloud is thus wasting below, it may be forming 

 anew elsewhere, especially at the upper surfaces. Thus 

 the form of the cloud is more or less constant. 



The amount of rainfall in some of the regions of earth is a 

 negligible quantity, while the amazing total of 500 inches or 

 over forty feet a year is recorded for the region northeast 

 of the Bay of Bengal. Where the annual rainfall is under 

 twenty inches, the raising of crops without irrigation is 

 hazardous. The distribution of the rainfall through ^the 

 growing season is a factor quite as important as the matter 



