142 GENERAL SCIENCE 



has been in process for many years, an opening, called a 

 cave, is left. There is in Kentucky a large limestone 

 cave, which is called the Mammoth Cave. Such caves 

 are also found in Virginia. Sandstone caves also are 

 formed in the same manner as limestone caves. 



Erosion is a process by which the surface of the earth 

 is worn away by the water; the rock and soil are dis- 

 solved and some of the solid material also is carried away 

 by the physical force of the water. The Grand Canyon, 

 made by the Colorado River, is an example of erosion. 

 The small ravines and gullies on hillsides where the 

 timber has been removed are the results of erosion. 

 Farmers need to guard against erosion in order to prevent 

 the soil from being carried away. They usually protect 

 the soil by keeping plants or grasses growing in their fields. 



95. Physical Properties of Water. The boiling point 

 of water at sea level is 100 C. Since the boiling 

 point varies with the pressure of the air or other gases 

 on the water, on the top of a mountain water will boil 

 at a temperature below 100 C. Water freezes at o 

 C. in the open air. Water is at its maximum density 

 at 4 C., that is, a cubic centimeter of water weighs 

 more at 4 C. than at any other temperature. If water 

 at a temperature of 4 C. is heated, it will expand. If 

 water at 4 C. is cooled, it will expand slowly until its 

 temperature is o C. At o C. water freezes and increases 

 in volume about one-ninth. Nine cubic feet of water, 

 when frozen, will become 10 cubic feet of ice. Which 

 weighs more, a cubic foot of ice at o C., or a cubic foot 

 of water at o C? To melt one gram of ice 80 calories 

 of heat are required. How much heat will be given out 

 when one gram of water at,o C. freezes? To change one 

 gram of water at boiling point into steam requires 536 



