A. The intensity of fire-light depends upon the whiteness to which the carbon is reduced, by combustion. If the carbon be white hot, its combustion is perfect, and the light intense; if not, the light is obscured by smoke.
Q. Why will not cinders blaze, as well as fresh coals?
A. The flame of coals is made chiefly by hydrogen gas. As soon as this gas is consumed, the hot cinders produce only an invisible gas, called carbonic acid.
Q. Where does the hydrogen gas of a fire come from?
A. The fuel is decomposed (by combustion) into its simple elements, carbon and hydrogen gas. (see p. 33)
Q. Why does not a fire blaze on a frosty night, so long as it does upon another night?
A. The air (being very cold) rushes to the fire so rapidly, that the coals burn out faster, and the inflammable gas is sooner consumed.
Q. Why does a fire burn clearest on a frosty night?
A. Because the volatile gases are quickly consumed; and the solid carbon plentifully supplied with air, to make it burn bright and intensely.
Q. Why does a fire burn more intensely in winter than in summer time?
A. Because the air is colder in winter, than in summer-time.
Q. How does the coldness of the air increase the heat of a fire?
A. For two reasons: 1st—Because cold air being more condensed than hot air, contains a greater body: and
2ndly—Cold air rushes more quickly to the fire, and supplies more oxygen.
Q. Why does the sun, shining on a fire, make it dull, and often put it out?
A. 1st—When the sun shines, the air is rarefied; and, therefore, flows more slowly to the fire.
2ndly—As the air is rarefied, even that which reaches the fire, affords less nourishment.
Q. Why does the air flow to the fire more tardily for being rarefied?
A. The greater the contrast (between the external air, and that which has been heated by the fire) the more rapid will be the current of air towards that fire.
Q. Why does rarefied air afford less nourishment to fire, than cold air?
A. Because it is spread out, (like a piece of gold beaten into leaf); and as a square inch of gold leaf will not contain so much gold as a square inch of bullion—so, a square inch of rarefied air has less body, than a square inch of cold air.
Q. Why does a fire burn more fiercely in the open air?
A. 1st—Because the air out-of-doors is more dense, than the air in-doors: and
2ndly—Because air is more freely supplied to a fire out-of-doors.
Q. Why is the air out-of-doors more dense than that in-doors?
A. Because the circulation is more free; and as soon as any portion has been rarefied, it instantly escapes, and is supplied by colder currents.
Q. Why does not a fire burn so freely in a thaw, as in a frost?
A. During a thaw, the air is filled with vapour; and, both moves too slowly, and is too much diluted to nourish the fire.
Q. Why does a fire burn so fiercely in windy weather?
A. In windy weather the air is rapidly changed, and affords plentiful nourishment to the fire.
Q. Why do a pair of bellows get a fire up?
A. A pair of bellows, (like the wind), drives the air more rapidly to the fire; and the plentiful supply of oxygen soon makes the fire burn intensely.
Q. Why is a candle blown out by the breath, and not made more intense, like a fire?
A. As the flame of a candle is confined to a very small wick, it is severed from it by the breath; and (being unsupported) must go out.
Q. Why is a smouldering wick sometimes rekindled by blowing it?
A. The breath carries the air to it with great rapidity; and the oxygen of the air kindles the red hot wick, as it kindles charred wood.
Q. Why is not the red hot wick kindled by the air around it, without blowing it?
A. Because oxygen is not supplied with sufficient freedom, unless it be blown to the wick.
Q. When is this experiment most likely to succeed?
A. In frosty weather; because the air contains more oxygen then, being condensed by the cold.
Q. Why does a poker, laid across a dull fire, revive it?
A. For two reasons. 1st—Because the poker concentrates the heat, and therefore increases it: and
2ndly—Because the poker arrests the air which passes over the fire, and produces a draught.
Q. Why do several pieces of wood or coal burn better than one?
A. When there are two or three pieces of wood on a fire, the air (circulating round them) produces an eddy or draught, which draws up the fire.
Q. Why are stoves fixed on the floor of a room?
A. In order that the air, on the lower part of the room, may be heated by the fire.
Q. Would not the air of the lower part of a room be heated equally well, if the stoves were fixed higher up?
A. No; the heat of a fire has a very little effect upon the air below the level of the grate; and, therefore, every grate should be as near to the floor as possible.
Q. Why are our feet so cold when we sit close by a good fire?
A. As the fire consumes the air which passes over it, cold air rushes through the crevices of the doors and windows along the bottom of the room to supply the deficiency; and these currents of cold air, rushing constantly over our feet, deprive them of their warmth.
Q. If a piece of paper be laid flat on a clear fire, it will not blaze, but char. Why so?
A.
