Q. What is the difference between a mist and fog?
A. Mist is generally applied to vapours condensed on marshes, rivers, and lakes.
Fog is generally applied to vapours condensed on land, especially if those vapours are laden with smoke.
Q. What is the reason why condensed vapour sometimes forms into clouds, and sometimes into fog?
A. If the surface of the earth be hotter than the air, then the vapour of the earth (being chilled by the cold air) becomes fog: but if the air be hotter than the earth, the vapour rises through the air, and becomes cloud.
A. 1st—Because less vapour is formed on a frosty day; and
2ndly—The vapour is frozen upon the ground before it can rise from the earth, and becomes hoar-frost.
Q. Why are fogs more general in autumn than in spring?
A. In spring the earth is not so hot as it is in autumn. In autumn the earth is generally warmer than the air; and, therefore, the vapour (issuing from the earth) is condensed into fog by the chill air.
Q. Why are fogs more common in valleys than on hills?
A. 1st—Because valleys contain more moisture than hills: and
2ndly—They are not exposed to so much wind, (which dissipates the vapour).
Q. How does wind dissipate fogs?
A. Either by blowing them away; or else by dissolving them into vapour again.
Q. What is the cause of the ground hoar-frost, or frozen dew?
A. Very rapid radiation of heat from the earth; in consequence of which, the surface is so cooled down, that it freezes the dew condensed upon it.
Q. Why is hoar-frost seen only after a very clear night?
A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to freeze the vapour condensed upon its surface.
Q. Why does hoar-frost very often cover the ground and trees, when the water of rivers is not frozen?
A. Hoar-frost is not the effect of cold in the air, but the cold of the earth (produced by excessive radiation); in consequence of which, the dew (condensed upon it) is frozen.
Q. Why is the hoar-frost upon grass and vegetables much thicker than that upon lofty trees?
Q. Why is the air (resting on the surface of the earth) colder than that in the higher regions?
A. Because the earth radiates more heat than the leaves of lofty trees; and, therefore, condenses and freezes the vapour of the air more rapidly.
Q. Why are evergreens often frost-bitten, when lofty trees are not?
A. Evergreens do not rise far above the surface of the earth; and (as the air contiguous to the earth is much colder than that in the higher regions) therefore, the low evergreen is often frost-bitten, when the lofty tree is uninjured.
Q. Why are tomb-stones covered with hoar-frost, long after it has melted from every object around?
A. White is a very bad absorbent of solar heat; and, therefore, the white tomb-stone remains too cold to thaw the frost congealed upon its surface.
Q. Why is there little or no hoar-frost under shrubs and shadowy trees?
Q. What is the cause of that hoar-frost which arises from frozen fog?
A. The thick fog (which invested the earth during the night) is condensed by the cold frost of early morning, and congealed upon every object with which it comes in contact.
Q. What is meant by the convection of heat?
A. Heat communicated by being carried to another thing or place; as the hot water resting on the bottom of a kettle, carries heat to the water through which it passes. (see p. 246).
Q. Are liquids good conductors of heat?
A. No; liquids are bad conductors; and are, therefore, made hot by convection.
Q. Why are liquids bad conductors of heat?
A. Because heat converts a liquid into steam, and flies off with the vapour, instead of being conducted through the liquid.
Q. Explain how water is made hot?
A. The water nearest the fire is first heated, and (being heated) rises to the top; other cold water succeeds, is also heated, and rises in turn; and this interchange keeps going on, till all the water boils.
Q. Why is water in such continual ferment, when it is boiling?
Q. How do these two currents pass each other?
A. The hot ascending current passes close by the metal sides of the kettle; while the cold descending current passes down the centre.
Q. Why does boiling water bubble?
A. The bubbles are portions of steam (formed at the bottom of the vessel) which rise to the surface, and escape into the air.
Q. Why does a kettle run over, when the water boils?
A. As the heat insinuates itself between the particles of water, it drives them asunder; and (as the particles of water are driven apart from each other) the same vessel will no longer hold the expanded water, and some runs over.
Q. Why does a kettle sing, when it is about to boil?
Q. Why does water boil?
A. Boiling is the effect of a more violent escape of air from the heated water; when, therefore, the air is not permitted to escape, water will never boil.
Q. Why is heat applied to the bottom, and not to the top of a kettle?
A. Because the heated water always ascends to the surface, heating the water through which it passes: if, therefore, heat were applied to the top of a vessel, the water below the surface would never be heated.
Q. As the lower part of a grate is made red-hot by the fire above, why would not the water boil, if fire were applied to the top?
A. The iron of a grate is an excellent conductor; and, therefore, if one part be heated, the heat is conducted to every other part: but water is a very bad conductor, and will not diffuse heat in a similar way.
Q. How do you know that water is a bad conductor of heat?
Q. If you wish to cool liquids, where should the cold be applied?
A. To the top of the liquid; because the cold portion will always descend, and allow the warmer parts to come in contact with the cooling substance.
Q. Does boiling water get hotter by being kept on the fire?
A. No; not if the steam be suffered to escape.
Q. Why does not boiling water get hotter, if the steam be suffered to escape?
A. Because as fast as the water boils, it is converted into steam; and the steam carries away the additional heat, as fast as it is communicated.
Q. Is steam visible or invisible?
A. Steam is invisible; but when it comes in contact with the air (being condensed into small drops) it instantly becomes visible.
A. If you look at the spout of a boiling kettle, you will find that the steam (which issues from the spout) is always invisible for about half an inch; after which, it becomes visible.
Q. Why is the steam invisible for only half an inch, and not either all invisible or all visible?
A. The air is not able to condense the steam as it first issues from the spout, but when it spreads and comes in contact with a larger volume of air, the invisible steam is readily condensed into visible drops.
Q. Why is our breath visible in winter-time?
A. Because it is condensed by the cold air into small drops, which are visible to the eye.
Q. Why do steam-engines sometimes burst?
Q. What becomes of the steam, after it has been condensed?
A. It is dissolved by the air, and forms a part of its invisible vapour.
Q. Is air a good conductor?
A. No; air is a very bad conductor, and is heated (like water) by convection.
Q. How is a room warmed by a stove?
A. The air nearest the fire is made hot first; the cold air descends, is heated also, and rises in turn; and this goes on, till all the air of the room is warmed.
Q. Why are fires placed on the floor of a room, and not towards the ceiling?
A. As heated air always ascends, if the fire were not near the floor, the lower part of the air (which we want to be the warmest) would never be benefited by the fire at all.
Q. If you take a poker out of the fire, and hold the hot end downwards, why is the handle so intensely hot?
Q. How should a red-hot poker be carried so as not to burn our fingers?
A. With the hot end upwards; because then the air (heated by the poker) would not pass over our hand to scorch it.
Q. Why is a poker (resting on the fender) cold; but if it leans against the stove, intensely warm?
A. The poker is an excellent conductor; while, therefore, it rests against the hot stove, the heat of the stove is conducted into the poker; but when it rests on the fender, it does not come in contact with the hot stove.
Q. Why does it feel so cold, when it rests on the fender?
A. Not being so warm as our hand, it imbibes the heat from it with such rapidity, that our loss of heat is palpable, and produces the sensation of coldness.
Q. Why are flues (which are carried through a church or room) always blackened with black lead?
Q. Why do country people touch the thick end of an egg with their tongue, to know if it be stale or not?
A. The thick end of an egg always contains a little air (between the shell and the white); but, when the egg is stale, the white shrinks, and the air expands.
Q. How can the tongue tell from this, whether the egg be stale or fresh laid?
A. As air is a very bad conductor, if the egg be stale, it will feel much warmer to the tongue, than if it be new-laid.
Q. Why will the big end of an egg feel warmer to the tongue, because it contains more air?
A. As air is a bad conductor, it will draw off the heat of the tongue very slowly, and, therefore, appear warm; but when there is only a very little air in the egg (as the white is a pretty good conductor), the heat of the tongue will be more rapidly drawn off, and the egg appear colder.
Q. Why is the large end of an egg cracked, when put into a saucepan to boil?
A. To let the air out; if the large end were not cracked, the air (expanded by the heat) would enter the white of the egg, and give it an offensive taste.
Q. Of what is atmospheric air composed?
A. Principally of two gases, oxygen and nitrogen; mixed together in the following proportion: viz. 1 part of oxygen, to 4 parts of nitrogen.
Q. What are the uses of the oxygen of the air?
A. It is the oxygen of the air which supports combustion, and sustains life.
Q. What is meant when it is said, that the oxygen of the air “supports combustion?”
A. It means this; that it is the oxygen of the air which makes fuel burn.
A. The fuel being decomposed (by heat) into hydrogen and carbon; the carbon combines with the oxygen of the air, and produces combustion.
Q. What does the combination of carbon and oxygen produce?
Q. What becomes of the hydrogen of the fuel?
A. Hydrogen (being very inflammable) burns with a blaze, and is the cause of the flame which is produced by combustion. (see p.34).
Q. What becomes of the nitrogen of the air, amidst all these changes and combinations?
A. The nitrogen of the air escapes, and is absorbed by the leaves of grass, trees, and various other vegetables.
Q. What is meant when it is said, that oxygen “sustains life”?
A. It means this: if a person could not inhale oxygen, he would die.
A. 1st—It gives vitality to the blood: and
2ndly—It is the cause of animal heat.
Q. How is food converted into blood?
A. After it is swallowed, it is dissolved in the stomach into a grey pulp; it then passes into the intestines, and is converted by the “bile” into a milky substance (called chyle).
Q. What becomes of the milky substance, called chyle?
A. It is absorbed by the vessels called “lacteals,” and poured into the veins on the left side of the neck.
Q. What becomes of the chyle after it is poured into the veins?
A. It then mingles with the blood, and is itself converted into blood.
Q. How does the oxygen we inhale mingle with the blood?
A. The oxygen of the air mingles with the blood in the lungs, and converts it into a bright red colour.
Q. What colour is the blood before it is oxydized in the lungs?
Q. Why are persons so pale who live in close rooms and cities?
A. The blood derives its redness from the oxygen of the air inhaled; but, as the air in close rooms and cities is not fresh, it is deficient in oxygen, and cannot turn the blood to a beautiful bright red.
Q. Why are persons who live in the open air and in the country, of a ruddy complexion?
A. As the blood derives its bright red colour from the oxygen of the air inhaled, therefore, country-people (who inhale fresh air) are more ruddy than citizens.
Q. Why is not the air in cities so fresh as that in the country?
A. Because it is impregnated with the breath of its numerous inhabitants, the odour of its sewers, the smoke of its fires, and many other impurities.
Q. How does the combination of oxygen with the blood produce animal heat?
Q. What becomes of the nitrogen of the air, after the oxygen enters the blood?
A. The nitrogen is exhaled, and taken up by the leaves of trees and other vegetables. (see p.35).
Q. Why does the vitiated air (after the oxygen has been absorbed) come out of the mouth, and not sink into the stomach?
A. The vitiated air (being heated by the heat of the body) ascends naturally, and passes by the heavier fresh air (which we inhale) without obstruction or injury.
Q. If (both in combustion and in respiration) the oxygen of the air is consumed, and the nitrogen rejected—Why are not the proportions of the air destroyed?
A. Because the upper surface of vegetable leaves (during the day) gives out oxygen and absorbs nitrogen, and thus the proper balance is perpetually restored.
A. Animals require oxygen to keep them alive, and draw it from the air by inspiration; the upper surface of leaves (all day long) gives out oxygen, and thus supplies the air with the very gas required by man and other animals.
Q. Do not animals exhale the very gas needed by vegetables?
A. Yes; animals reject the nitrogen of the air (as not suited to the use of animal life), but vegetables absorb it, as it is the food they live on; and thus the vegetable world restores the equilibrium of the air, disturbed by man and other animals.
Q. Is air a good conductor?
A. No; air is a very bad conductor.
Q. How is air heated?
A. By “convective currents.”
Q. What are meant by “convective currents?”
A. When a portion of air is heated, it rises upward in a current, carrying the heat with it: other colder air succeeds, and (being heated in a similar way) ascends also; and these are called convective currents.
(“Convective currents;” so called from the Latin words, cum-vectus (carried with) because the heat is “carried with” the current.)
Q. Is air heated by the rays of the sun?
A. No; air is not heated (in any sensible degree) by the action of the sun’s rays passing through it.
Q. Why then is the air hotter on a sunny day, than on a cloudy one?
A. On a fine day, the sun heats the surface of the earth, and the air (resting on the earth) is heated by contact; as soon as it is heated it ascends, and other air succeeding is heated in a similar way, till all is heated by convection.
Q. If air be a bad conductor, why does hot iron get cold, by being exposed to the air?
A. A piece of hot iron exposed to the air, is made cold—1st—By “convection;” and
2ndly—By “radiation.”
Q. How is hot iron (exposed to the air) made cold by convection?
Q. How is hot iron cooled by radiation?
A. While the heat of the iron is being carried off by “convection,” it is throwing off heat (on all sides) by radiation.
Q. What is meant by radiation?
A. Heat emitted (in all directions) from any surface, by innumerable rays.
Q. Why is broth cooled by being left exposed to the air?
A. Hot broth throws off some heat by radiation; but it is mainly cooled down by convection.
Q. How is hot broth cooled down by convection?
A. The air resting on the hot broth (being heated) ascends; colder air succeeding absorbs more heat, and ascends also; and this process is repeated, till the broth is made cool.
A. 1st—The agitation assists the liquor in bringing its hottest particles to the surface:
2ndly—The action of stirring agitates the air, and brings it quicker to the broth or tea: and
3rdly—As the hottest particles are more rapidly brought into contact with the air, therefore convection is more rapid.
Q. Why is hot tea, &c. cooled more rapidly by blowing it?
A. Because the heated air is blown more rapidly away; in consequence of which, cold air more rapidly succeeds to absorb heat from the surface of the tea or broth.
Q. If a shutter be closed in the day-time, the stream of light (piercing through the crevice) seems in constant agitation. Why is this?
Q. Why is the gallery of a church or theatre hotter than the aisle or pit?
A. The hot air ascends from the bottom to the top of the room, and cold air (from the doors and windows) flies to the bottom to supply its place.
Q. Why does a crowded room produce head-ache?
A. Because we breathe air vitiated by the crowd.
Q. How does a crowd vitiate the air of a room?
A. Whenever we breathe, the elements of the air are separated in the lungs, some of the oxygen is absorbed by the blood, and some of it is converted into carbonic acid gas, and exhaled with the nitrogen.
Q. Is all the nitrogen rejected by the lungs?
A. Yes; all the nitrogen of the air is always exhaled.
Q. What is carbonic acid gas?
Q. Is carbonic acid gas wholesome?
A. No; it is quite fatal to animal life; and whenever it is inhaled, it acts like a narcotic poison, (producing drowsiness which ends in death).
Q. Why is a crowded room unwholesome?
A. Because the oxygen of the air is either absorbed by the lungs, or substituted for carbonic acid gas, which is a noxious poison.
Q. Mention the historical circumstances, so well known in connection with the “Black Hole of Calcutta.”
A. In the reign of George II, the Raja (or Prince) of Bengal[12] marched suddenly to Calcutta to drive the English from the country; as the attack was unexpected, the English were obliged to submit, and 146 persons were taken prisoners.
[12] The Sur Raja, at Dowlat; a young man of violent passions, who had but just succeeded to the throne. A. D. 1756.
Q. What became of these prisoners?
A. They were driven into a place about 18 feet square, and 15 or 16 feet in height, with only two small grated windows. 123 of the prisoners died in one night; and (of the 23 who survived) the larger portion died of putrid fevers, after they were liberated in the morning.
Q. Why were 123 persons suffocated in a few hours, from confinement in this close hot prison-hole?
A. Because the oxygen of the air was soon consumed by so many lungs, and its place supplied by carbonic acid exhaled by the hot breath.
Q. Why do persons in a crowded church feel drowsy?
A. 1st—Because the crowded congregation inhale a large portion of the oxygen of the air, which alone can sustain vitality and healthy action: and
2ndly—The air of the church is impregnated with carbonic acid gas, which (being a strong narcotic) produces drowsiness in those who inhale it.
A. 1st—Because the absence of oxygen quickly affects the vital functions, depresses the nervous energies, and produces a lassitude which ends in death: and
2ndly—The carbonic acid gas inhaled by the captives (being a narcotic poison) would also produce drowsiness and death.
Q. Why do persons, who are so much in the open air, enjoy the best health?
A. Because the air they inhale is much more pure.
Q. Why is country air more pure than the air in cities?
A. 1st—Because there are fewer inhabitants to vitiate the air:
2ndly—There are more trees to restore the equilibrium of the vitiated air: and
3rdly—The free circulation of air keeps it pure and wholesome (in the same way as running streams are pure and wholesome, while stagnant waters are the contrary).