Part 51 (1/2)
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.