EXPANSION. 53 



project beyond the building. The nut with which it 

 is provided is then screwed up on the projecting rod, 

 until it touches the outside of the wall. The lamps 

 being then removed, the rod cools, and, by its con- 

 traction, draws up the walls with it. 



115. FRACTURE OF GLASS VESSELS. 

 fracture of Glass expands less than iron by heat, yet 

 glass vessels sufficiently, when expansion is unequal on 



by heat? '' H 



opposite surfaces, to occasion its fracture. 

 Thus if hot water be poured on a thick glass plate, it 

 cracks. The first effect is to expand the upper surface, 

 while the under one is but slightly affected. The ob- 

 vious tendency of this unequal expansion, is to warp 

 the plate, and curve it inward toward the under side. 

 But, as the glass cannot bend, it breaks. 



116. HOW TO CUT GLASS BY HOT WIRE. 



How can heat 



be used to cut In consequence of the same unequal expan- 



sion, a crack once commenced in glass may 

 be made to follow the heated end of a rod of iron or pipe- 

 stem drawn over its surface. Broken vessels of glass 

 may be thus cut into useful shapes. A glass vial may 

 be cut evenly in two, by encircling it with a ring of 

 iron heated to redness, and afterward plunging it into 

 cold water. The glass beneath the ring becomes ex- 

 panded through and through, and the subsequent im- 

 mersion in water, causes a sudden contraction in the 

 exterior, and consequent fracture, on the principle above 

 stated. 



117. WOOD AND MARBLE EXPAND LIT- 



TLE.^-Wood and marble expand but little 



used for pen- "by heat, and are therefore sometimes used 

 dulum rods ? ' 



for pendulum rods, where careful provision 



