CHAPTER X.

EXPANSION FROM HEAT.

_(Continued.)_

Q. _Does heat expand every thing ELSE BESIDES air and water?_

A. Yes; _every_ thing (that man is acquainted with) is expanded by heat.

Q. _Why does a COOPER make his HOOPS RED-HOT, when he puts them on a tub?_

A. 1st--As _iron expands by heat_, the hoops will be _larger_ when they are red-hot; and will, therefore, _fit more easily on the tub_: and

2ndly--As _iron contracts by cold_, the hoops will _shrink_ as they cool down, and _girt the tub with a tighter grasp_.

Q. _Why does a WHEELWRIGHT make his hoops RED-HOT, which he fixes on the NAVE of a WHEEL?_

A. 1st--That they may _fit on more easily_: and

2ndly--That they may _girt the nave more tightly_.

Q. _Why will the wheelwright's HOOP FIT the nave MORE EASILY, because they are made RED-HOT?_

A. As _iron expands by heat_, the hoops will be _larger_ when they are hot; and (being larger) will go on the nave more _easily_.

Q. _Why will the HOOPS, which have been PUT ON HOT, girt the nave more FIRMLY?_

A. As _iron contracts by cold_, the hoops will _shrink as they cool down_; and, therefore, _girt the nave with a tighter grasp_.

Q. _Why does a farrier put the HORSE-SHOE on HOT?_

A. That it may _stick the closer_, when it has contracted by cold.

Q. _Why does a STOVE make a CRACKING NOISE, when a fire is very hot?_

A. The iron stove _expands by heat_, and (as it swells) the parts rub both _against each other_, and _against the bricks around_, driving them further off; and this produces a _cracking_ noise.

Q. _Why does a STOVE make a similar CRACKING NOISE, when a large FIRE is TAKEN DOWN?_

A. The iron stove _contracts again_, 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 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.