Q. _Why are TOMB-STONES covered with HOAR-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 HOAR-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 HOAR-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 passes_. (_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 PASS each other?_ A. The _hot ascending current_ passes close by the _metal sides_ of the kettle; while the _cold descending current_ passes _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?_