A. As a _c.h.i.n.k_ 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 s.p.a.ce_, (or, in other words, rises higher in the tube.)

Q. _Why is a GLa.s.s BROKEN, when HOT WATER is poured into it?_

A. Because the _inside of the gla.s.s_ is expanded by the hot water, and _not the outside_; so the gla.s.s _snaps_ for want of _flexibility_.

Q. _Why is not the OUTSIDE of the GLa.s.s expanded by the hot water, as well as the INSIDE?_

A. Gla.s.s 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 GLa.s.s snap, because the INNER surface is HOTTER than the OUTER?_

A. _Gla.s.s 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 GLa.s.s BEAKER be set on a warm HOB?_

A. Gla.s.s is a _non-conductor_; and, as the _bottom of the gla.s.s_ (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_.

Q. _Why does not WOOD MELT, like metal?_

A. Because the heat of the fire _decomposes_ the wood into _gas, smoke, and ashes_; and the different parts _separate from each other_.

Q. _What is meant by VAPORIZATION?_

A. The _conversion of liquid into vapour_; as water is converted into vapour by the heat of the sun.

Q. _What are CLOUDS?_

A. Moisture _evaporated from the earth_, and collected in the upper regions of the air.

Q. _What is the difference between a FOG and a CLOUD?_

A. Clouds and fogs differ only in one respect. _Clouds are elevated above our heads_: but _fogs come in contact with the surface of the earth_.

Q. _If CLOUDS are WATER, why do they FLOAT on the air?_

A. 1st--The vapour of clouds is composed of _very minute bubbles_ (called ves'cicles), which float like _soap bubbles_: and

2ndly--Warm air (between the bubbles) _keeps them apart_, and makes the ma.s.s _lighter_; and the currents of air (which constantly ascend from the warm earth) _buoy them up_.

Q. _Why does VAPOUR sometimes form into CLOUDS, and sometimes rest upon the earth as MIST or FOG?_

A. When the _surface of the earth_ is _warmer than the air_, the vapour of the earth (being condensed by the chill air) becomes _mist or fog_.

But when the _air_ is _warmer than the earth_, the vapour _rises through the air_, and becomes cloud.