A Guide To The Scientific Knowledge Of Things Familiar Part 17

A. Princ.i.p.ally of carbon and hydrogen gas. The _solid_ part is carbon, the _volatile_ part is gas.

Q. _What is CARBON?_

A. A solid substance, generally of a black colour; well known under the forms of charcoal, lamp-black, c.o.ke, black-lead, &c.

Q. _What is HYDROGEN GAS?_

A. The princ.i.p.al ingredient of water. It is well known in the form of common _coal gas_: it burns so readily that it used to be called "inflammable air."[10]

[10] To make hydrogen gas, see p. 34.

Q. _Why does a CANDLE BURN when lighted?_

A. The heat of the lighted wick _decomposes the tallow_ into its elementary parts of carbon and hydrogen; and the _hydrogen of the tallow_, combining with the _oxygen of the air_, produces _flame_.

Q. _Why is the FLAME of a candle HOT?_

A. 1st--Because the flame liberates _latent heat_ from the air and tallow: and

2ndly--It throws into _rapid motion_ the _atoms of matter_.

Q. _How is LATENT HEAT liberated by the flame of a CANDLE?_

A. When the _hydrogen_ of the tallow and _oxygen_ of the air _combine_, they _condense into water_; and much of their latent heat is _squeezed out_.

Q. _How are the ATOMS of MATTER disturbed by the flame of a candle?_

A. 1st--When the _hydrogen_ of the tallow and _oxygen_ of the air _condense into water_, a _vacuum_ is made; and the air is disturbed, as a _pond_ would be, if a pail of _water_ were taken out.

2ndly--When the _carbon_ of tallow and _oxygen_ of the air expand into _carbonic acid gas_, the air is _again_ disturbed; in a similar way as by the explosion of _gunpowder_.

Q. _Why does the flame of a CANDLE produce LIGHT?_

A. The chemical changes made by combustion, excite _undulations of ether_, which (striking the eye) produce light. (see p. 46.)

Q. _Why is the FLAME of a CANDLE YELLOW?_

A. Only the _outer_ coat of the flame is yellow; the _lower_ part of the flame is _violet_; and the _inside_ of the flame is _hollow_.

Q. _Why is the outside of the flame YELLOW?_

A. Because the _carbon of the tallow_ (being in a state of _perfect combustion_) is made white-hot.

Q. _Why is the BOTTOM part PURPLE of the flame of a candle?_

A. The _bottom part_ of the flame is _overladen with hydrogen_, raised from the tallow by the burning wick; and this _half-burnt gas_ gives a _purple_ tinge to the flame.

Q. _Why is the INSIDE of the flame of a candle HOLLOW?_

A. Because it is _filled with vapour_, raised from the candle by the _heat of the wick_.

Q. _Describe the different parts of the FLAME of a common CANDLE._

A. The flame consists of _three cones_. The innermost cone is hollow; the intermediate cone of a dingy purple hue; and the outside cone is yellow.

Q. _Why is the intermediate cone of a flame PURPLE, as well as the BOTTOM of the flame._

A. Because the gases are not in a state of _perfect combustion_; but contain an _excess of hydrogen_, which gives this cone a purple tinge.

Q. _Why is not the MIDDLE cone in a state of perfect combustion, as well as the OUTER cone?_

A. Because the outer cone _prevents the oxygen of the air_ from getting freely to the _middle of the cone_; and without the free access of oxygen gas, there is no such thing as complete combustion.

Q. _Why does the FLAME of a candle point UPWARDS?_

A. The flame _heats the surrounding air_, which (being hot) _rapidly ascends_, and drives the flame upwards at the same time.

Q. _Why is the FLAME of a candle POINTED at the top, like a cone?_

A. The _upper_ part of a flame is more _volatile_ than the lower parts; and as it affords _less resistance to the air_, is reduced to a mere point.

Q. _Why is the UPPER part of a flame more VOLATILE than the lower parts?_

A. The _lower_ parts of the flame are laden with unconsumed gas and watery vapour; which present considerable resistance to the air.

Q. _Why is the FLAME of a candle BLOWN OUT by a puff of breath?_

A. As the flame of a candle is attached to a _very small wick_, a puff of breath _severs the flame from the wick_; and it goes out for want of support.

Q. _Why does the FLAME of a candle make a GLa.s.s DAMP, which is held over it?_