A Guide to the Scientific Knowledge of Things Familiar. Ebenezer Cobham Brewer
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. The carbon of a clear fire, being sufficiently hot to unite with the oxygen of the air, produces carbonic acid gas, which soon envelops the paper laid flat upon the cinders: but carbonic acid gas will not blaze.
Q. If you blow the paper, it will blaze immediately. Why so?
A. By blowing, or opening the door suddenly, the carbonic acid is dissipated, and the paper is instantly fanned into flame.
Q. Why does water extinguish a fire?
1st—Because the water forms a coating over the fuel, and keeps it from the air:
2ndly—The conversion of water into steam, draws off the heat of the burning fuel.
Q. Why does a little water make a fire fiercer, while a larger quantity of water puts it out?
A. Water is composed of oxygen and hydrogen; when, therefore, the fire can decompose the water into its simple elements, it serves for fuel to the flame.
Q. How can water serve for fuel to fire?
A. The hydrogen of the water will burn with a flame; and the oxygen of the water will increase the intensity of that flame.
Q. If a house be on fire, is too little water worse than no water at all?
A. Certainly. Unless the water be supplied so plentifully as to quench the fire, it will increase the intensity, like fuel.
Q. When will water extinguish fire?
A. When the supply is so rapid and abundant, that the fire cannot convert it into steam.
Q. Does not a very little water slacken the heat of fire?
A. Yes, till it is converted into steam; but then it increases the intensity of fire, and acts like fuel.
Q. Why does the wick of a candle (when the flame has been blown out) catch fire so readily?
A. As the wick is already very hot, a little extra heat will throw it into flame.
Q. Why does the extra heat revive the flame?
A. Because it again liberates the hydrogen of the tallow, and ignites it.
Q. Cannot wood be made to blaze without actual contact with fire?
A. Yes; if a piece of wood be held near the fire for a little time it will blaze, even though it does not touch the fire.