SCIENCE OP COMMON THINGS. 179 



Cause of wcuthcr-worn rocks. Cause of icicles. "What is ice f 



IIT* 7 Is the force created by the expansion of water in the act of freezing 

 very great? 



Yes ; as an illustration the following experiment 

 may be quoted : Cast-iron bomb-shells, thirteen inches 

 in diameter and two inches thick, were filled with wa- 

 ter, and their apertures or fuse-holes firmly plugged 

 with iron bolts. Thus prepared, upon exposure to the 

 severe cold of a Canadian winter, about 19 below 

 " zero," at the moment the water froze, the iron plugs 

 were violently thrust out, and the ice protruded, and 

 in some instances the shells burst asunder, thus demon- 

 strating the enormous interior pressure to which they 

 were subjected by water assuming the solid state. 



117*8 What is the principal cause of the rounded and weather-worn 

 aspect of some rocks, especially the limestone and sandstone rocks ? 



The expansion of freezing water : water is absorbed 

 into their fissures and pores by capillary attraction, 

 and when it freezes during winter, it expands and de- 

 taches successive fragments, so that the original sharp 

 and abrupt outline is gradually rounded and softened 

 down. 



117*9 Why, in the winter, do we let the water run to prevent its freezing 

 in the service pipe ? 



Because the motion of the water prevents the crys- 

 tals of ice from forming or attaching themselves to the 

 sides of the pipe. 



1180 Can a lens be made of ice capable of concentrating the rays of the 

 sun with sufficient intensity to inflame substances ? 



Yes ; a burning-lens can be formed of transparent 

 ice, of power sufficient to produce effects nearly equal 

 to those of the glass lens. 



1181 What is " ground ice," or " anchor ice ?" 



Ice formed at the bottom of streams or rivers. 



1183 Upon what does the formation of icicles depend ? 



Upon the successive congelation of drops or slender 

 streams of water. 



1183 What is ice t 



Frozen water. "When the temperature of water un- 



