GENERAL EFFECT OF HEAT 



sun are very striking. The surface of many a mountain peak 

 is covered with cracked rock so insecure that a touch or step 

 will dislodge the fragments and start them down the mountain 

 slope. The lower levels of mountains (Fig. 6) are frequently 

 buried several feet under debris which has been formed in this 

 way from higher peaks, 

 and which has slowly ac- 

 cumulated at the lower 

 levels. Under the action 

 of heat and cold exposed 

 rock "weathers"; that 

 is, it cracks and crumbles 

 into fragments which can 

 be blown away by wind 

 and washed away by 

 water. Heat and cold 

 are powerful agents of 

 weathering. 



General effect of heat. 

 Change in temperature. 

 - Perhaps the best- 

 known effect of heat is 



the warmth it imparts to substances which come under its in- 

 fluence. In summer the sun's rays heat the cement walks until 

 they burn the feet ; in winter, the coals glowing in furnace and 

 stove heat the icy air, and make it warm. When an object 

 feels hot to the touch, we say it has a high temperature; when 

 it feels cold to the touch, that it has a low temperature. 

 Changes in temperature are caused by heat : when cold water, 

 that is, water with a low temperature, is placed over glowing 

 coal it becomes warm water, that is, water with a high tempera- 

 ture. The change in the temperature of the water is due to the 

 heat given out from the burning fuel. As the fire dies out, it 



FIG. 6. Debris formed from crumbling rock. 



