Q. _Why is a mountain-top COLDER than a valley, "because the AIR there is LESS HEATED by REFLECTION?"_

A. Air is _not_ heated by the _sun_, but by _reflection from the surface of the earth_; and as there is _no earth_ round a _mountain-top_ to reflect heat, therefore the air there is intensely cold.

Q. _Why does RUBBING our HANDS and FACES make them feel WARM?_

A. Chiefly because the friction _excites the latent heat_ of our hands and faces, and makes it sensible to our feeling.

Q. _When a man has been almost DROWNED, why is suspended animation RESTORED by RUBBING?_

A. The vital heat of the body (which had become _latent_ by the action of the water) is _again developed by friction_: and, as soon as this animal heat can be excited, the vital powers of the body are restored.

Q. _Why do two pieces of ICE (rubbed together) MELT?_

A. Ice contains 140 _degrees of latent heat_, and (when two pieces are _rubbed together)_ their _particles are compressed_, and this _latent heat_ rolls out and _melts the ice_.

Q. _Are not FORESTS sometimes SET on FIRE by friction?_

A. Yes; when two branches or trunks of trees (blown about by the wind) _rub violently against each other_, their _latent heat is developed_, and sets fire to the forest.

Q. _What is meant by COMPRESSION?_

A. The act of _bringing parts nearer together_; as a sponge is _compressed_ by being _squeezed in the hand_.

Q. _Cannot HEAT be evolved from common air merely by COMPRESSION?_

A. Yes; if a piece of _German tinder_ be placed at the _bottom of a gla.s.s tube_, and the air in the tube _compressed by a piston_,[11] the tinder will catch fire.

[11] In a common syringe or squirt, the _handle_ part which _contains the sucker_ (and is forced up and down), is called "The Piston."

Q. _Why will the tinder catch fire?_

A. Because the _air is compressed_; and its _latent heat being squeezed out_, sets fire to the tinder at the bottom of the tube.

CHAPTER IX.

EFFECTS OF HEAT.

1.--EXPANSION.

Q. _What are the princ.i.p.al EFFECTS of HEAT?_

A. 1.--Expansion. 2.--Liquefaction. 3.--Vaporization. 4.--Evaporation; and 5.--Ignition.

Q. _Does HEAT EXPAND the AIR?_

A. Yes; if a bladder (partially filled with air) be tied up at the neck, and _laid before the fire_, the air will _swell_ till the bladder _bursts_.

Q. _Why will the AIR SWELL, if the bladder be laid before the fire?_

A. Because the heat of the fire _gets between the particles of air_, and drives them _further apart from each other_; which causes the bladder to expand.

Q. _Why do unslit CHESTNUTS CRACK with a loud noise, when ROASTED?_

A. Chestnuts contain a great deal of air, which is expanded by the heat of the fire; and, as the thick rind prevents the air from escaping, it violently _bursts through, slitting the rind_, and making a great noise.

Q. _What occasions the loud CRACK or report which we hear?_

A. 1st--The _sudden bursting of the rind_ makes a report, in the same way as a piece of _wood_ or _gla.s.s_ would do, if _snapped in two_: and

2ndly--The _escape of hot air_ from the chestnut makes a report also, in the same way as _gunpowder_, when it escapes from a _gun_.

Q. _Why does the sudden BURSTING of the rind, or SNAPPING of a piece of wood, make a REPORT?_

A. As the attraction of the parts is suddenly overcome, _a violent jerk_ is given to the air; this jerk produces _rapid undulations_ in the air, which (striking upon the ear) give the brain the sensation of _sound_.

Q. _Why does the ESCAPE OF AIR from the chestnut, or the EXPLOSION of GUNPOWDER, produce a REPORT?_

A. Because a quant.i.ty of air (suddenly let loose) _pushes against the air around_, in order to make _room for itself_; and as the _air of the chestnut_ slaps against the _air of the room_, a _report_ is made, (as when I _slap_ a book or table).

Q. _If a CHESTNUT be SLIT, it will NOT CRACK; why is this?_

A. Because the _heated air_ of the chestnut can _freely escape_ through the _slit in the rind_.

Q. _Why does an APPLE spit and SPURT about, when roasted?_

A. An apple contains a vast quant.i.ty of _air_, which (being expanded by the heat of the fire) _bursts through the peel_, carrying the juice of the apple along with it.

Q. _Does an APPLE contain MORE AIR, in proportion, than a CHESTNUT?_