A. When the fire is lighted, _the iron-work_ (which expands more than the brick-work and plaster) _pushes away the bricks and plaster_: but when the fire is put out, the metal _shrinks_ again, and _leaves the "setting" 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 _from 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 falls off_, leaving the fire-place very unsightly. Q. _Why does the MERCURY of a THERMOMETER RISE in hot weather?_ A. Heat _expands the metal_; and as the metal is _increased in bulk_, it occupies a _larger space_, (or, in other words, rises higher in the tube.) Q. _Why is a GLASS BROKEN, when HOT WATER is poured into it?_ A. Because the _inside of the glass_ is expanded by the hot water, and _not the outside_; so the glass _snaps_ for want of _flexibility_. Q. _Why is not the OUTSIDE of the GLASS expanded by the hot water, as well as the INSIDE?_ A. Glass is a _non-conductor of heat_; and, therefore, _breaks_ before the heat of the _inner surface_ is _conducted to the outside_. Q. _Why does a GLASS snap, because the INNER surface is HOTTER than the OUTER?_ A. _Glass is expanded by heat_; and as the inner surface expands, _it stretches the outer surface till it snaps_. Q. _Why is a CHINA CUP broken, if HOT WATER be poured over it, or into it?_ A. China is a _non-conductor_; and, as the _inner surface expands by the heat_, before the _outer one_, _it forms an arch_, and pulls the parts of the cup asunder. Q. _Why does the BOTTOM COME OFF, if a GLASS BEAKER be set on a warm HOB?_ A. Glass is a _non-conductor_; and, as the _bottom of the glass_ (from the warmth of the hot stove) _expands, before the sides are heated_, the two parts _separate_ the one from the other. CHAPTER XI. 2.--LIQUEFACTION. 3.--VAPORIZATION. Q. _What is meant by LIQUEFACTION?_ A. The _state of being melted_; as ice is melted by the heat of the sun. Q. _Why is ICE MELTED by the HEAT of the SUN?_ A. The _heat of the sun_ (entering the solid ice) _forces its particles asunder_, till their attraction of cohesion is sufficiently overcome, to _convert the solid ice into liquid_. (See p. 112.) Q. _Why are METALS MELTED by the heat of FIRE?_ A. The _heat of the fire_ (entering the solid metal) _forces its particles asunder_, till their attraction of cohesion is sufficiently overcome, to _convert the solid metal to a liquid_. Q. _Why is WATER converted to STEAM by the heat of FIRE?_ A. The _heat of the fire_ (entering the water) _divides its globules into very minute bubbles_, which (being made lighter than air) fly off from the surface _in the form of steam_.