MATERIALS COMPOSING THE LAND 75 



to be condensed and to fall as rain or snow. It takes a great 

 deal of heat to evaporate water and all this heat is given 

 off when it condenses. Water seeks the lowest place it can 

 find, giving out energy as it flows. In fact, the earth has 

 been likened by some writers to a water engine, since 

 water has played such an important part in its history. 



Another property of water which is of great importance 

 is its power to take up heat. This was shown in Experi- 

 ment 27. When it cools, it gives out the heat it took 

 up when its temperature was raised. It is for this reason 

 that hot water bags are used to keep people warm, and 

 that farmers sometimes in winter, when they fear that 

 their cellars will freeze, carry down tubs of water to keep 

 their cellars above the freezing point. This is why orange 

 groves are often irrigated just before there is danger of a 

 heavy frost. 



A pound of water in cooling one degree gives out about 

 as much heat as a pound of iron in cooling 9 degrees. 

 This capacity for holding heat makes bodies of water 

 warm up slowly in the summer and cool off slowly as 

 winter approaches. As they cool they give back to the 

 air the heat they have taken up. During the early part 

 of the summer the air above them is kept cool and in the 

 fall it is warmed. This property of water will be found 

 later to be of great importance. 



40. Materials Composing the Land. Experiment 39. Ob- 

 tain specimens of the igneous rocks, lava, obsidian, basalt, granite ; 

 of the sedimentary rocks, sandstone, fossiliferous limestone, conglom- 

 erate, peat ; of the metamorphic rocks, shale, schist, marble, anthra- 

 cite coal. Examine these carefully with the eye and with the lens, 

 noting whether they have a uniform composition or are made up of 

 different particles. Are the particles composing the rocks crystalline ? 

 Are they scattered irregularly or arranged in layers ? Test with a file 

 or knife-blade the hardness of the rock as a whole and of its differ- 

 ent constituents. Try a drop of hydrochloric acid on the different 



