Q. _Why do SPARKS of fire start (with a crackling noise) from pieces of WOOD laid upon a FIRE?_ A. The _air_ in the wood (expanded by the heat), _forces its way through the pores of the log_; and carries along with it the _covering of the pore_, which resisted its passage. Q. _What is meant by the "PORES of the WOOD?"_ A. Very small _holes in the wood_, through which the _sap_ circulates. Q. _What are the SPARKS OF FIRE, which burst from the WOOD?_ A. Very small pieces of wood _red hot_, separated from the log by the _force of the air_, as it bursts from its confinement. Q. _Why does DEAL make more snapping than any OTHER WOOD?_ A. The pores of deal are _very large_, and contain much _more air_ than wood of a _closer grain_. Q. _Why does DRY WOOD make more SNAPPING than GREEN WOOD?_ A. In _green wood_ the pores are filled with _sap_, and therefore contain _very little air_; but in _dry_ wood the sap is _dried up_, and the pores are filled with _air_ instead. Q. _Why does DRY wood BURN more easily than GREEN or wet wood?_ A. Because the pores of dry wood are _filled with air_, which supports combustion; but the pores of green or wet wood are filled with _vapour_, which extinguishes flame. Q. _Why does VAPOUR EXTINGUISH FLAME?_ 1st--Because the coat of water (which wraps the fuel round) prevents the _oxygen_ of the air from getting to the _fuel_, to form into _carbonic acid gas_: and 2ndly--Heat is perpetually carried off, by the formation of the sap or water _into steam_. (Carbonic acid gas is a compound of carbon and oxygen. The solid part of the fuel is _carbon_, and one of the gases of the air is oxygen.) Q. _What has CARBONIC ACID GAS to do with COMBUSTION?_ A. Combustion is produced by the _chemical action_ which takes place, while the _carbon_ of fuel unites with the _oxygen_ of air, and forms "_carbonic acid gas_." (See p. 36.) Q. _Why do STONES SNAP and fly about, when heated in the FIRE?_ A. The air in the stones (expanded by the heat of the fire), _meets with great resistance_ from the close texture of the stone; and, therefore, _bursts forth with great violence_, tearing the stone to atoms, and forcing the fragments into the room. Q. _Must not AIR be very STRONG, to shatter into atoms a hard stone?_ A. Yes. All the dreadful effects of _gunpowder_ are merely the results of the _sudden expansion of air_. Q. _When bottled ALE and PORTER is set before a FIRE, why is the CORK FORCED OUT sometimes?_ A. If the bottle be _not quite full_, there will be _air_ between the liquor and the cork; this _air_ (expanded by the heat of the fire) _forces out the cork_. Q. _Why does ALE or PORTER FROTH more, after it has been set before the fire?_ A. The _froth_ of ale or porter _depends upon the pressure_ to which it is subjected; and as the air (between the liquor and the cork) is _expanded_ by the heat, _it presses against the liquor_, and increases the quantity of froth.