124 EFFECTS OF HEAT. 



as soon as the fire is removed; and (as 

 it shrinks into a smaller space) the parts 

 rub against each other again, and the 

 bricks are again disturbed; and this 

 produces a cracking noise. 



Q. Why does the PLASTER round a STOVE 

 CRACK and fall away ? 



A. When the fire is lighted, the iron- 

 work (which expands more than the 

 hrick-work and plaster) pushes away the 

 bric/cs and plaster : but when the fire is 

 put out, the metal shrinks again, and 

 leaves the "setting 11 behind. 



Q. Why does the PLASTER FALL AWAY ? 



A. As a chink is left (between the 

 "setting" and the stove), the plaster will 

 frequently fall away Jrom its own weight. 



Q. What OTHER cause contributes to BRING 



the PLASTER DOWN ? 



A. As the heat of the fire varies, 

 the size of the iron stove varies also ; and 

 this swelling and perpetually contracting, 

 keeps up such a constant disturbance 

 about the plaster, that it cracks and Jails 

 off, leaving the fire-place very unsightly. 



Q. Why does the MERCURY of a THERMOMETER 

 RISE in hot u-eather ? 



