A. Evergreens do not _rise far above the surface of the earth_; and (as the air _contiguous to the earth_ is much _colder than that in the higher regions_) therefore, the _low evergreen is often frost-bitten_, when the lofty tree is uninjured.

Q. _Why are TOMB-STONES covered with h.o.a.r-FROST, long after it has melted from every object around?_

A. _White is a very bad absorbent of solar heat_; and, therefore, the _white tomb-stone_ remains _too cold_ to thaw the frost congealed upon its surface.

Q. _Why is there little or NO h.o.a.r-FROST under SHRUBS and shadowy TREES?_

A. 1st--Because the leafy shrubs and trees _arrest the process of radiation_ from the earth: and

2ndly--Shrubs and trees _radiate a little heat_ towards the earth; and, therefore, the _ground beneath_ is never _cold enough to congeal the little dew_ which rests upon it.

Q. _What is the cause of that h.o.a.r-FROST which arises from FROZEN FOG?_

A. The thick fog (which invested the earth during the night) is condensed _by the cold frost_ of early morning, and _congealed upon every object_ with which it comes in contact.

CHAPTER XVII.

5.--CONVECTION.

Q. _What is meant by the CONVECTION of HEAT?_

A. Heat communicated _by being carried_ to another thing or place; as the hot water resting on the _bottom_ of a kettle, carries heat to the water _through which it pa.s.ses_. (_see p._ 246).

Q. _Are LIQUIDS good CONDUCTORS of heat?_

A. No; liquids are _bad conductors_; and are, therefore, made hot by _convection_.

Q. _Why are LIQUIDS BAD CONDUCTORS of heat?_

A. Because heat _converts a liquid into steam_, and flies off with the vapour, instead of being _conducted through the liquid_.

Q. _Explain how WATER is made HOT?_

A. The water _nearest the fire is first heated_, and (being heated) _rises to the top;_ other cold water succeeds, is _also_ heated, and rises in turn; and this interchange keeps going on, _till all the water boils_.

Q. _Why is WATER in such continual FERMENT, when it is BOILING?_

A. This commotion is mainly produced by the _ascending and descending currents_ of hot and cold water.

The escape of _air_ from the water contributes also to increase this agitation.

Q. _How do these two currents Pa.s.s each other?_

A. The _hot ascending current_ pa.s.ses close by the _metal sides_ of the kettle; while the _cold descending current_ pa.s.ses _down the centre_.

Q. _Why does BOILING WATER BUBBLE?_

A. The bubbles are _portions of steam_ (formed at the bottom of the vessel) which _rise to the surface_, and escape into the air.

Q. _Why does a KETTLE RUN OVER, when the water BOILS?_

A. As the heat insinuates itself between the particles of water, _it drives them asunder_; and (as the particles of water are _driven apart from each other_) the _same_ vessel will no longer hold the expanded water, and some runs over.

Q. _Why does a KETTLE SING, when it is ABOUT to BOIL?_

A. Water contains _a great deal of air_, which (being expanded by the heat of the fire) escapes by fits _through the spout of the kettle_; which sings in the same way as a trumpet does, when a person blows in it.

Q. _Why does WATER BOIL?_

A. Boiling is the effect of a _more violent escape of air_ from the heated water; when, therefore, the air is _not permitted to escape_, water will _never boil_.

Q. _Why is HEAT applied to the BOTTOM, and not to the top of a KETTLE?_

A. Because the heated water _always ascends to the surface_, heating the water through which it pa.s.ses: if, therefore, heat were applied to the _top of a vessel_, the water _below the surface_ would _never be heated_.

Q. _As the lower part of a GRATE is made RED-HOT by the fire ABOVE, why would not the WATER boil, if fire were applied to the TOP?_

A. The _iron_ of a grate is an _excellent conductor_; and, therefore, if _one_ part be heated, the heat is conducted to _every other part_: but _water_ is a _very bad conductor_, and will not diffuse heat in a similar way.

Q. _How do you know that WATER is a BAD CONDUCTOR of heat?_

A. When a blacksmith immerses his red-hot iron in a tank of water, the water which _surrounds_ the red-hot iron is made _boiling hot_, but the water _below_ the surface remains quite cold.

Q. _If you wish to COOL LIQUIDS, where should the cold be applied?_

A. To the _top of the liquid_; because the _cold_ portion will always _descend_, and allow the warmer parts to come in contact with the cooling substance.