186 SCIENCE OF COMMON THINGS. 



Why glass breaks when placed in hot water. "Why a stove snaps. 



hot water, and not the outside / so tlie glass snaps in 

 consequence of this unequal expansion. 



1338 Why is not the outside of the glass expanded by the hot water as 

 well as the inside ? 



Because glass is a bad conductor of heat, and breaks 

 before the heat of the inner surface is conducted to the 

 outside. 



1339 Why does a glass snap because the inner surface is hotter than 

 the outer ? 



Because the inner surface is expanded, and not the 

 outer ; in consequence of which an opposing force is 

 created which breaks the glass. 



1330 Why does a cooper heat his hoops red hot when he puts them on 

 a tub? 



1. As iron expands by heat, the hoops will be larger 

 when they are red hot ; in consequence of which they 

 w T ill fit on the tub more easily ; and 



2. As iron contracts by cold, the hoops w r ill shrink 

 as they cool down, and girt the tub with a tighter 

 grasp. 



1331 Why does a wheelwright make the tire red hot which he fixes on 

 a wheel f 



1. That it may^ on more easily and 



2. That it may girt the wheel more tightly. 



1333 Why will the wheelwrights tire Jit the wheel more easily for beiny 

 made red hot f 



Because it will be expanded by the heat, and (being 

 larger) will go on the wheel more easily. 



1333 Why will the tire which has been put on hot girt the wheel more. 

 firmly ? 



Because it will shrink when it cools down, and 

 therefore girt the wheel with a tighter grasp. 



1334 Why does a stove make a crackling noise when afire is very hot? 



Because it expands from the heat ; and the parts of 

 the stove rulbing against each other, or driving against 

 the bricks, produce a crackling noise. 



1335 Why does a stove make a similar crackling noise when a large 

 fire is put out f 



