A Guide To The Scientific Knowledge Of Things Familiar Part 10

Q. _Why are CINDERS lighter than COALS?_

A. Because their vapour, gases, and volatile parts, have been driven off by _previous combustion_.

Q. _Why will not STONES do for fuel, as well as COALS?_

A. Because they contain no _hydrogen_ (or inflammable gas) like coals.

Q. _Why will not WET KINDLING light a fire?_

A. 1st--Because the moisture of the wet kindling prevents the _oxygen of the air from getting to the fuel_ to form it into carbonic acid gas: and

2ndly--The heat of the fire is perpetually _drawn off_, by the conversion of _water_ into _steam_.

Q. _Why does DRY wood burn BETTER than GREEN?_

A. 1st--Because no heat is _carried away_, by the conversion of _water into steam_: and

2ndly--The pores of dry wood _are filled with air_, which supply the fire with oxygen.

Q. _Why do TWO pieces of WOOD burn BETTER than ONE?_

A. 1st--Because they help to entangle the _heat of the pa.s.sing smoke_, and _throw it on the fuel_: and

2ndly--They help to _entangle the air_ that pa.s.ses over the fire, and create a kind of _eddy_ or draught.

Q. _Why does SALT CRACKLE when thrown into a FIRE?_

A. Salt contains _water_; and the _cracking_ of the salt is owing to the sudden _conversion of the water into steam_.

Q. _Why will not wood or paper burn, if they are steeped in a solution of POTASH, phosphate of LIME, or AMMONIA (hartshorn)?_

A. Because any "al'kali" (such as potash) will _arrest the hydrogen_ (as it escapes from the fuel), and prevent its _combination_ with the _oxygen of air_.

Q. _What is an al'kali?_

A. The con'verse of an _acid_; as _bitter_ is the con'verse of _sweet_, or _insipid_ the con'verse of _pungent_.

Q. _Why does a JET of FLAME sometimes burst into the room THROUGH THE BARS OF A STOVE?_

A. The iron bars conduct heat to the _interior of some lump of coal:_ and its volatile gas (bursting through the weakest part) is kindled by the glowing coals over which it pa.s.ses.

Q. _Why is this JET sometimes of a GREENISH YELLOW colour?_

A. When a lump of coals lies _over the hot bars_, or the coals below it are not _red hot_, the gas which bursts from the lump _escapes unburnt_, and is of a greenish colour.

Q. _Why does the gas escape UNBURNT?_

A. Because neither the _bars_ nor _coals_ (over which it pa.s.ses) are _red-hot_.

Q. _Why does a BLUISH FLAME sometimes flicker on the surface of hot cinders?_

A. Gas from the hot coals _at the bottom of the grate_ mixing with the _carbon of the coals above_, produces an inflammable gas (called carbonic oxide), which burns with a blue flame.

Q. _Why is the FLAME of a good fire YELLOW?_

A. Because both the hydrogen and carbon of the fuel are in a state of _perfect combustion_. It is the _white heat of the carbon_, which gives the pale yellow tinge to the flaming hydrogen.

Q. _What is LIGHT?_

A. Rapid _undulations_ of a fluid called _ether_, striking on the eye.

Q. _How does COMBUSTION make these undulations of LIGHT?_

A. The atoms of matter (set in motion by heat) _striking against_ this ether, produce _undulations_ in it; as a _stone_ thrown into a stream, would produce undulations in the _water_.

Q. _How can UNDULATIONS of ether produce LIGHT?_

A. As _sound_ is produced by _undulations of air_ striking on the _ear_; so _light_ is produced by undulations of _ether_ striking on the _eye_.

Q. _What is ETHER?_

A. A very subtile fluid, which pervades and surrounds _every thing we see_.

Q. _Mention a simple experiment to prove that LIGHT is produced by rapid MOTION._

A. When a fiddle-string is _jerked_ suddenly, its rapid vibration produces a grey _light_; and when a carriage wheel revolves very quickly, it sends forth a similar light.

Q. _Does HEAT ALWAYS produce LIGHT?_

A. No: the heat of a stack of hay, or reeking dunghill, though very _great_, is not sufficient to produce _light_.