A. A solid substance, generally of a black colour; well known under the forms of charcoal, lamp-black, coke, black-lead, &c. Q. _What is HYDROGEN GAS?_ A. The principal 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?_