Q. Where does the oxygen of the air come from, which is blown to the lighted tinder?
Q. What is the good of blowing oxygen gas to lighted tinder?
A. Oxygen gas supports combustion; and lighted tinder is quickened by the breath, in the same way as a dull fire is revived by a pair of bellows.
Q. Why do horses sometimes strike fire with their feet?
A. When iron horse-shoes strike against the flint-stones of the road, very small fragments (either of the shoe or stones) are knocked off red-hot, and look like sparks.
Q. What makes these fragments red-hot?
A. The percussion condenses the part struck, and squeezes out its latent heat.
Q. What is meant by friction?
Q. How do the Indians produce fire, by merely rubbing two pieces of dry wood together?
A. They take a piece of dry wood (sharpened to a point), which they rub quickly up and down a flat piece, till a groove is made; and the saw-dust (collected in this groove) soon catches fire.
Q. Why does the saw-dust of the wood catch fire by rubbing?
A. The latent heat of the wood is developed by friction; because the particles of the wood are squeezed closer together, and the heat pours out, as water from a sponge.
(The best woods for this purpose are box-wood against mulberry, or laurel against poplar or ivy.)
Q. Do not carriage wheels sometimes catch fire?
A. Yes; if the wheels be dry,—or fit too tightly,—or revolve very rapidly,—they often catch fire.
Q. Why do wheels catch fire in such cases?
Q. What is the use of greasing cart wheels?
A. The grease lessens the friction; and (by diminishing the friction) the latent heat is less disturbed.
Q. Why is the top of a mountain colder than the valley beneath, although it be two or three miles nearer to the sun?
A. 1st—Because the air on a mountain is less compressed, than the air in a valley.
2ndly—It is more rarefied: and
3rdly—It is less heated by reflection.
Q. Why is air colder on a mountain “because it is less compressed?”
A. As the air in a valley is more compressed (by the mass of air above) than that on the top of a mountain, therefore more heat runs out; just as more water runs from a sponge, the closer it is squeezed together.
Q. Why is a mountain-top colder than a valley, “because the air there is more rarefied?”
Q. Why is a mountain-top colder than a valley, “because the air there is less heated by reflection?”
A. Air is not heated by the sun, but by reflection from the surface of the earth; and as there is no earth round a mountain-top to reflect heat, therefore the air there is intensely cold.
Q. Why does rubbing our hands and faces make them feel warm?
A. Chiefly because the friction excites the latent heat of our hands and faces, and makes it sensible to our feeling.
Q. When a man has been almost drowned, why is suspended animation restored by rubbing?
A. The vital heat of the body (which had become latent by the action of the water) is again developed by friction: and, as soon as this animal heat can be excited, the vital powers of the body are restored.
Q. Why do two pieces of ice (rubbed together) melt?
Q. Are not forests sometimes set on fire by friction?
A. Yes; when two branches or trunks of trees (blown about by the wind) rub violently against each other, their latent heat is developed, and sets fire to the forest.
Q. What is meant by compression?
A. The act of bringing parts nearer together; as a sponge is compressed by being squeezed in the hand.
Q. Cannot heat be evolved from common air merely by compression?
Q. Why will the tinder catch fire?
A. Because the air is compressed; and its latent heat being squeezed out, sets fire to the tinder at the bottom of the tube.
Q. What are the principal effects of heat?
A. 1.—Expansion. 2.—Liquefaction. 3.—Vaporization. 4.—Evaporation; and 5.—Ignition.
Q. Does heat expand the air?
A. Yes; if a bladder (partially filled with air) be tied up at the neck, and laid before the fire, the air will swell till the bladder bursts.
Q. Why will the air swell, if the bladder be laid before the fire?
Q. Why do unslit chestnuts crack with a loud noise, when roasted?
A. Chestnuts contain a great deal of air, which is expanded by the heat of the fire; and, as the thick rind prevents the air from escaping, it violently bursts through, slitting the rind, and making a great noise.
Q. What occasions the loud crack or report which we hear?
A. 1st—The sudden bursting of the rind makes a report, in the same way as a piece of wood or glass would do, if snapped in two: and
2ndly—The escape of hot air from the chestnut makes a report also, in the same way as gunpowder, when it escapes from a gun.
Q. Why does the sudden bursting of the rind, or snapping of a piece of wood, make a report?
Q. Why does the escape of air from the chestnut, or the explosion of gunpowder, produce a report?
A. Because a quantity of air (suddenly let loose) pushes against the air around, in order to make room for itself; and as the air of the chestnut slaps against the air of the room, a report is made, (as when I slap a book or table).
Q. If a chestnut be slit, it will not crack; why is this?
A. Because the heated air of the chestnut can freely escape through the slit in the rind.
Q. Why does an apple spit and spurt about, when roasted?
A. An apple contains a vast quantity of air, which (being expanded by the heat of the fire) bursts through the peel, carrying the juice of the apple along with it.
Q. Does an apple contain more air, in proportion, than a chestnut?
Q. Where is all this quantity of air stowed in the apple?
A. The inside of an apple is made up of little cells (like a honey-comb), each of which contains a portion of the air.
Q. When an apple is roasted, why is one part made soft, while all the rest remains hard?
A. When an apple is roasted, the air in the cells next to the fire is expanded and flies out; the cells are broken, and their juices mixed together; so the apple collapses (from loss of air and juice), and feels soft in those parts.
Q. What is meant by the “apple collapsing?”
A. The plumpness gives way, and the apple becomes flabby and shrivelled.
Q. Why do sparks of fire start (with a crackling noise) from pieces of wood laid upon a fire?
A. The air in the wood (expanded by the heat), forces its way through the pores of the log; and carries along with it the covering of the pore, which resisted its passage.
Q. What is meant by the “pores of the wood?”
Q. What are the sparks of fire, which burst from the wood?
A. Very small pieces of wood red hot, separated from the log by the force of the air, as it bursts from its confinement.
Q. Why does deal make more snapping than any other wood?
A. The pores of deal are very large, and contain much more air than wood of a closer grain.
Q. Why does dry wood make more snapping than green wood?
A. In green wood the pores are filled with sap, and therefore contain very little air; but in dry wood the sap is dried up, and the pores are filled with air instead.
Q. Why does dry wood burn more easily than green or wet wood?
A. Because the pores of dry wood are filled with air, which supports combustion; but the pores of green or wet wood are filled with vapour, which extinguishes flame.
Q. Why does vapour extinguish flame?
1st—Because the coat of water (which wraps the fuel round) prevents the oxygen of the air from getting to the fuel, to form into carbonic acid gas: and
2ndly—Heat is perpetually carried off, by the formation of the sap or water into steam.
(Carbonic acid gas is a compound of carbon and oxygen. The solid part of the fuel is carbon, and one of the gases of the air is oxygen.)
Q. What has carbonic acid gas to do with combustion?
A. Combustion is produced by the chemical action which takes place, while the carbon of fuel unites with the oxygen of air, and forms “carbonic acid gas.” (See p. 36.)
Q. Why do stones snap and fly about, when heated in the fire?
A. The air in the stones (expanded by the heat of the fire), meets with great resistance from the close texture of the stone; and, therefore, bursts forth with great violence, tearing the stone to atoms, and forcing the fragments into the room.
Q. Must not air be very strong, to shatter into atoms a hard stone?
Q. When bottled ale and porter is set before a fire, why is the cork forced out sometimes?
A. If the bottle be not quite full, there will be air between the liquor and the cork; this air (expanded by the heat of the fire) forces out the cork.
Q. Why does ale or porter froth more, after it has been set before the fire?
A. The froth of ale or porter depends upon the pressure to which it is subjected; and as the air (between the liquor and the cork) is expanded by the heat, it presses against the liquor, and increases the quantity of froth.
Q. Why is the froth of ale and porter increased by pressure?
A. Because the liquor absorbs carbonic acid so long as it is under pressure; and the moment that the pressure is removed, the carbonic acid escapes in foam or froth.
Q. When a boy makes a balloon, and sets fire to the cotton or sponge (which has been steeped in spirits of wine), why is the balloon inflated, or blown out?
Q. Why does the balloon rise, after it has been inflated by the expanded air?
A. The same quantity of air is expanded to three or four times its original volume; and is made so much lighter than common air, that even when all the paper, wire, and cotton are added, it is still lighter bulk for bulk.
Q. What is meant by being lighter “bulk for bulk?”
A. If the balloon be 3 square feet in size, it is lighter (when inflated) than 3 square feet of common air, and therefore floats through it; as a cork (at the bottom of a tub of water) would rise to the surface.
Q. Why does smoke rush up a chimney?
A. The heat of the fire expands the air in the chimney; and (being thus made lighter than the air around), it rises up the chimney, and carries the smoke in its current.
Q. Why has a long chimney a greater draught than a short one?
Q. Why will a long chimney smoke, unless the fire be pretty fierce?
A. If the fire be not pretty fierce, its heat will not be sufficient to rarefy all the air in the chimney; and then the chimney will smoke.
Q. Why will the chimney smoke, if the fire be not big enough to heat all the air in the chimney flue?
A. Because the cold air (condensed in the upper part of the flue), will sink from its own weight, and sweep the ascending smoke back with it into the room.
Q. What is the use of a cowl upon a chimney-pot?
A. The cowl acts as a screen against the wind, to prevent it from blowing into the chimney.
Q. What harm would the wind do, if it were to blow into a chimney?
Q. Why does a smoke-jack turn round in a chimney?
A. The current of hot air up the chimney, striking against the oblique vanes of the smoke-jack, drives them round and round; in the same way as the sails of a wind-mill are driven round by the wind.
Q. Why are some things solid, others liquid, and others gaseous?
A. As heat enters any substance, it drives its particles further asunder; and a solid (like ice) becomes a liquid; and a liquid (like water) becomes a gas.
Q. Why does water simmer before it boils?
A. The particles of water near the bottom of the kettle (being formed into steam sooner than the rest) shoot upwards; but are condensed again (as they rise) by the colder water, and produce what is called “simmering.”
Q. What is meant by simmering?
Q. Why does a kettle sing when the water simmers?
A. Because the air (entangled in the water) escapes by fits and starts through the spout of the kettle; which makes a noise like a wind instrument, when it is blown into.
Q. Why does not a kettle sing, when the water boils?
A. As all the water is boiling hot, the steam meets with no impediment, but freely escapes in a continuous stream.
Q. When does a kettle sing most?
A. When it is set on a hob to boil.
Q. Why does a kettle sing more when it is set on the side of a fire, than when it is set in the midst of the fire?
Q. Why does a kettle sing, when the boiling water begins to cool again?
A. Because the upper surface cools first; and the steam (still rising from the lower parts of the kettle) is again entangled, and escapes fitfully.
Q. Why does boiling water swell?
A. Water (like air) expands by heat. The heat of the fire drives the particles of water further apart from each other; and (as they are not packed so closely together) they take up more room; or (in other words) the water swells.
Q. What is meant when it is said, “that heat drives the particles of water further apart from each other.”
A. Water is composed of little globules, like very small grains of sand; the heat drives these particles away from each other; and (as they then require more room) the water swells.
Q. Why does boiling water bubble?
Q. Why does a kettle sometimes boil over?
A. Liquids expand very much by heat; if, therefore, a kettle be filled with cold water, some of it must run over as soon as it is expanded by heat.
Q. But I have seen a kettle boil over, although it has not been filled full of water; how do you account for that?
A. If a fire be very fierce, the air is expelled so rapidly, that the bubbles are very numerous; and (towering one above the other) reach the top of the kettle, and fall over.
Q. Why is a pot, which is full to overflowing (while the water is boiling hot), nothing like full, when it has been taken off the fire for a short time?
A. When the water was swelled by boiling heat, it filled the pot even to overflowing; but as soon as the water is condensed by cold, it contracts again, and occupies a much less space.
Q. Why does the water of a kettle run out of the spout when it boils?
Q. How can the pressure of steam on the surface of the water, force the water through the kettle-spout?
A. In the same manner as the pressure of air on the mercury of a barometer, forces the quicksilver up the glass tube.
Q. What causes the rattling noise so often made by the lid of a saucepan or boiler?
A. The steam (seeking to escape) forces up the lid of the boiler, and the weight of the lid causes it to fall back again: this being done frequently, produces a rattling noise.
Q. If the steam could not lift up the lid of the boiler, how would it escape?
A. If the lid fitted so tightly, that the steam could not raise it up, the boiler would burst into fragments, and the consequences might be fatal.
Q. When steam pours out from the spout of a kettle, the stream begins apparently half an inch off the spout; why does it not begin close to the spout?
Q. Why is not all the stream invisible, as well as that half-inch?
A. As the steam comes in contact with the colder air, the invisible particles (being condensed), roll one into another, and look like a thick mist.
Q. What becomes of the steam? for it soon vanishes.
A. After it is condensed into mist, it is dissolved by the air, and dispersed abroad as invisible vapour.
Q. And what becomes of the invisible vapour?
A. Being lighter than air, it ascends to the upper regions, where (being again condensed) it contributes to form clouds.
Q. Why does a metal spoon, left in a saucepan, retard the process of boiling?
Q. Why will a pot (filled with water) never boil, when immersed in another vessel full of water also?
A. Because water can never be heated above the boiling point: all the heat absorbed by the water after it boils, is employed in converting the water into steam.
Q. How does the conversion of water into steam prevent the inner pot from boiling?
A. The moment the water in the larger pot is boiling hot (or 212°), steam is formed, and carries off some of its heat; therefore, 212 degs. of heat can never pass through it, to raise the inner vessel to the same heat.
Q. Why do sugar, salt, &c. retard the process of boiling?
A. Because they have a tendency to fix water by chemical attraction; and therefore retard its conversion into steam.
Q. If you want water to boil, without coming in contact with the saucepan, what plan must you adopt?
Q. Why would the inner vessel boil, if the outer vessel contained strong brine?
A. Though water boils at 212 degs. of heat, yet brine will not boil till raised to 218 or 220 degs. Therefore, 212 degs. of heat may easily pass through brine to raise the vessel immersed in it to boiling heat, before any of it is carried off by steam.
Q. Why will brine impart to another vessel more than 212°, and water not so much?
A. Because both liquids will impart heat till they boil, and then they can impart heat no longer.
Q. Why can they impart no extra heat after they boil?
A. Because all extra heat is spent in making steam. Hence water will not boil a vessel of water immersed in it, because it cannot impart to it 212 degs. of heat: but brine will, because it can impart more than 212 degs. of heat, without being converted itself into steam.
| Ether boils at | 104 degs. |
| Alcohol boils at | 173-1/2 degs. |
| Water boils at | 212 degs. |
| Water with one-fifth salt at | 219 degs. |
| Syrup boils at | 221 degs. |
| Oil of turpentine at | 304 degs. |
| Sulphuric acid at | 472 degs. |
| Linseed oil at | 640 degs. |
| &c. &c. |
Any liquid which boils at a lower degree can be made to boil if immersed in a liquid which boils at a higher degree. Thus a cup of ether can be made to boil in a saucepan of water. A cup of water in a saucepan of brine or syrup. But a cup of water will not boil if immersed in ether; nor a cup of syrup in water.
Q. Why are clouds higher on a fine day?
A. 1st—Because the air (expanded by heat) drives them higher up: and
2ndly—The clouds themselves are lighter, and therefore more buoyant.
Q. Why are the clouds lighter on a fine day?
A. Because their mists are either absorbed by the dry air, or vapourized by the hot sun.
Q. Why is a cup put topsy-turvy into a fruit-pie?
A. Its principal use is to hold the crust up, and prevent it from sinking, when the cooked fruit gives away under it.
Q. Does not the cup prevent the fruit of the pie from boiling over?
A. No, by no means; it would rather tend to make it boil over, than otherwise.
Q. Why would the cup tend rather to make the fruit boil over?
Q. If the juice is driven out of the cup, why is the cup always full of juice, when the pie is cut up?
A. Immediately the pie is drawn, the air in the cup begins to condense again, and occupy a smaller space; in consequence of which, there is no longer enough air to fill the cup, and so juice rushes in to fill up the deficiency.
Q. Why does juice rush into the cup, because the cup is not full of air?
A. As the external air presses upon the surface of the juice, it rushes into the cup unobstructed; as mercury rises through the tube of a barometer through similar pressure.
Q. Does heat expand every thing else besides air and water?
A. Yes; every thing (that man is acquainted with) is expanded by heat.
Q. Why does a cooper make his hoops red-hot, when he puts them on a tub?
A. 1st—As iron expands by heat, the hoops will be larger when they are red-hot; and will, therefore, fit more easily on the tub: and
2ndly—As iron contracts by cold, the hoops will shrink as they cool down, and girt the tub with a tighter grasp.
Q. Why does a wheelwright make his hoops red-hot, which he fixes on the nave of a wheel?
Q. Why will the wheelwright’s hoop fit the nave more easily, because they are made red-hot?
A. As iron expands by heat, the hoops will be larger when they are hot; and (being larger) will go on the nave more easily.
Q. Why will the hoops, which have been put on hot, girt the nave more firmly?
A. As iron contracts by cold, the hoops will shrink as they cool down; and, therefore, girt the nave with a tighter grasp.