146.Which are the better conductors of heat, fluids or solids?
Generally speaking,solids, especially those of them that are dense in their substance.
147.Why are dense substances the best conductors of heat?
Because the heat more readily travels from particle to particle until it pervades the mass.
148.Why are fluids bad conductors of heat?
Because of the want ofdensityin their bodies; and because a portion of the imbibed heat always passes off from fluids byevaporation.
"He casteth forth his ice like morsels: who can stand before his word,"—Psalm cxlvii.
149.Why are woollen fabrics bad conductors of heat?
Because there is a considerable amount ofairoccupying the spaces of the texture.
150.Is air a good or a bad conductor?
Air is abad conductor, and it chiefly transmits heat, as water does, byconvection.
151.Is water a good or a bad conductor?
Water is an indifferent conductor, but it is abetter conductor than air.
152.Why, when we place our hands in water, which may be of the same temperature as the air, does the water feel some degrees colder?
Because water,being a better conductor than air, takes up the warmth of the handmore rapidly.
153.Why, when we take our hands out of water do they feel warmer?
Because the air does not abstract the heat of the hand so rapidly as the water did, and the change in the degree of rapidity with which the heat is abstractedproduces a sensation of increased warmth.
154.Why do we see blocks of ice wrapped in thick flannel in summer time?
Because the flannel, being a non-conductor, prevents theexternal heatfromdissolving the ice.
Flannel wrapped around awarmbodykeeps in its heat; and wrapped around acoldbody, prevents heat frompassing into it.
155.How do we know that air is not a good conductor of heat?
Because,in still air, heat would travel to a given point much more rapidly, and in greater intensity, through even an indifferentsolid conductor, than it would through theair.
156.How do we know that water is not a good conductor of heat?
Because in a deep vessel containingice, and with heat applied at the top, some portion of the water may be made to boilbefore the ice, which lies a little under the surface, is melted.
"As snow in summer, and as rain in harvest; so honour is not seemly for a fool."—Prov. xxvi.
157.Why would you apply the heat at the top, in this experiment?
Because in heating water itexpands and rises. The boiling of water is caused by the heated waterascending from the bottom, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly byconvection, but not byconduction.
158.Why are bottles of hot water, used as feet-warmers, wrapped in flannel?
Because the flannel,being a bad conductor, allows the heat topass only gentlyfrom the bottle, and preserves the warmth for amuch longer time.
159.Why are hot rolls sent out by the bakers, wrapped up in flannel?
Because the flannel,being a bad conductor, does notcarry off rapidly the heat of the rolls.
160.Why is it said that snow keeps the earth warm?
Because snow is abad conductor, and prevents the frosty air fromdepriving the earth of its warmth.
161.Why are snow huts which the Esquimaux build found to be warm?
Because snow,being a bad conductor, keeps inthe internal heat of the dwelling, and prevents thecold outer air from taking away its warmth.
162.Why is snow, being composed of congealed water (and water being a better conductor than air), so good a non-conductor?
Because in the process of congealation it is frozen into crystalline forms, which, being collected into a mass, form a woolly body, thusproving the truthfulness of the Bible simile, which says, God "giveth snow like wool."
"He giveth snow like wool: he scattereth the hoar frost like ashes."—Psalm cxlvii.
FIG. 1.—CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE.
FIG. 1.—CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE.
163.Why does it frequently feel warmer after a frost has set in?
Because, in the act of congealation a great deal of heat is given out, andtaken up by the air, and thusthe severity of the cold is in some degree moderated.
164.Why is it frequently colder when a thaw takes place?
Because, in the process of thawing, a certain amount of heat iswithdrawn from the air, and enters the thawed ice.
165.What benefit results from these provisions of Nature?
They moderate both theseverity of frosts, andthe rapidity of thaws, which, in changeable climates, would be seriously detrimental tolife, and tovegetation.
166.Why are furs and woollens worn in the winter?
Because, being non-conductors, they prevent the warmth of the body from beingtaken up by the cold air.
167.Why are the skins of animals usually covered with fur, hair, wool, or feathers?
Because their coverings, beingnon-conductors of heat, preserve the warmth of the bodies of the animals.
"He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."—Psalm cxlvii.
168.How is the greater warmth of animals provided for in the winter?
It is observed that, as winter approaches, there comes a short woolly or downy growth, which,adding to the non-conducting property of their coats, confines their animal warmth.
In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind ofblackdown next their bodies. Black is thewarmest colour, and the purpose here is tokeep in the heat, arising from the respiration of the animal.
169.How is warmth provided for in animals that have no such coats?
They are furnished with a layer offat, which lies underneath the skin. Fat consists chiefly ofcarbon, and is anon-conductor.
170.Why are summer breezes said to be cool?
Because, as they pass over the heated surface of the body, they bear away a part of its heat.
171.Why is a still summer air said to be sultry?
Because, being heated by the sun's rays,and being a bad conductor, it does not relieve the body bycarrying off its heat.
172.Why does fanning the face make it feel cooler?
Because, by inducing currents of air to pass over the face, a part of the excessive heat is taken upand carried away.
173.Why does perspiration cool the body?
Because it takes up a part of the heat, and, evaporating,carries it into the air.
174.Why does blowing upon hot tea cool it?
Because it directs currents of air over the surface of the tea, and these currents take up a part of the heatand bear it away.
175.Why does air in motion feel cooler than air that is still?
Because each wave of aircarries away a certain portion of heatand being followed by another portion of air,a further amount of heat is borne away.
"Though I walk in the valley of the shadow of death I will fear no evil, for thou art with me."—Psalm xxiii.
176.Is the atmosphere ever as hot as the human body?
Not in this country. On the hottest day it is 10 or 12 deg.cooler than the temperature of our bodies.
177.What is the highest degree of artificial heat which man has been known to bear?
A man may be surrounded with air raised to the temperature of 300 deg. (the boiling point being 212), and yet not have the heat of his body raised more than two or three degrees above its natural temperature of from 97 deg. to 100 deg.
178.Why may man endure this degree of heat for a short time without injury?
Because the skin, and the vessels of fat that lie underneath it, are bad conductors of heat.
And because perspiration passing from the skin and evaporating, wouldbear the heat awayas fast as it was received.
Because, also, the vital principle (life) exercises a mysterious influence in the preservation of living bodies from physical influences.
179.Is the air ever hot enough, in any part of the world, to destroy life?
Yes. The hot winds of the Arabian deserts, which are calledsimooms, scatter death and desolation in their track, withering trees and shrubs, and burying them under waves of hot sand. When camels see the approach of a simoom they rush to the nearest tree or bush, or to some projecting rock, where they place their heads in an opposite direction to that from which the wind blows, and endeavour to escape its terrible violence. The traveller throws himself on the ground on the lee side of the camel, and screens his head from the fiery blast within the folds of his robe. But frequently both man and beastfall a prey to the terrible simoom.
180.Why are these hot winds so terrible in their effects?
Because, being in motion, they search their way to every part ofthe body, and passing over itleave some portion of their heat behind, which is again followed byadditional heat from every fresh blast of wind.
"The fear of the Lord is the beginning of knowledge: but fools despise wisdom and instruction."—Proverbs i.
181.What is Radiation?
The radiation of heat is amotion of the particles, in a series of rays, diverging in every direction from a heated body.
182.What is this phenomena of Radiation understood to arise from?
From a strongly repulsive power, possessed by particles of heat, by which they are excited to recede from each other with great velocity.
183.What is the greatest source of Radiation?
The sun, which sends forth rays ofboth light and heatin all directions.
184.When does a body radiate heat?
When it is surrounded by a medium which isa bad conductor.
185.When we stand before a fire, does the heat reach us by conduction or by radiation?
By radiation.
186.What becomes of the heat that is radiated from one body to another?
It is eitherabsorbedby those bodies, or transmitted through them and passed to other bodies byconduction, or diffused byconvection, or returned byreflection.
187.How do we know that heat is diffused by radiation?
If we set a metal plate (or any other body, though metal is best for the experiment) before the fire,rays of heat will fall upon it. If we turn the plate at a slight angle, and place anotherobject in a line with it, we shall find that the plate willreflect the rays it has received by radiation, on to the object so placed; but if we place an objectbetween the fire and the plate, we shall find that the rays of heatwill be intercepted, and that the latter can no longerreflect heat.
"The fear of the Lord is the beginning of wisdom: a good understanding have all they that do his commandments."—Psalm cxi.
188.Does the agitation of the air interfere with the direction of rays of heat?
It has been found that the agitation of the air doesnotaffect the direction of rays of heat.
189.Why, then, if a current of air passes through a space across which heat is radiating, does the air become warmer?
Because it takes upsome portion of the heat, but it does not alter the direction of the rays.
This is clearly illustrated by reference torays of lightwhich are seen under many circumstances. But they are never bent, moved, nor in any way affected by the wind.
190.Why will not a current of air disturb the rays of heat, just as it would a spider's web, or threads of silk?
Because heat is animponderableagent, that is, something which cannot be acted upon by the ordinary physical agencies. It hasno weight, presents nosubstantial body, and is, in these latter respects, similar tolight and electricity.
191.What other sources of radiation of heat are there besides the sun and the fire?
Theearth, and allminor bodies, are, in some degree,radiators of heat.
192.What substances are the best radiators?
Allroughanddarkcoloured substances and surfaces are thebest radiators of heat.
193.What substances are the worst radiators of heat?
Allsmooth,bright, andlight colouredsurfaces arebad radiators of heat.
Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence which thecoloursof bodies have upon thevelocity of radiation. He surroundedthe bulb of a thermometer successively with equal weights ofblack,red, andwhitewool, and placed it in a glass tube, which was heated to the temperature of 180 deg. by immersion in hot water. The tube was then cooled down to 50 deg. by immersion in cold water; theblackcooled in 21 minutes, theredin 26 minutes, and thewhitein 27 minutes.
"Say unto wisdom, Thou art my sister; and call understanding thy kinswoman."—Proverbs vii.
194.If you wished to keep water hot for a long time, should you put it into a bright metal jug, or into a dark earthenware one?
You should put it into abright metaljug, because,being a bad radiator,it would not part readily with the heat of the water.
195.Why would not the dark earthenware jug keep the water hot as long as the bright metal one?
Because the particles of earthenware being rough, and of dark colour,they radiate heat freely, and the water would thereby be quickly cooled.
196.But if (as stated in the Lessons upon Conduction) metal is a better conductor of heat than stone or earthenware, why does not the metal jug conduct away the heat of the water sooner than the earthenware jug?
It would do so,if it were in contact with another conductor; but, being surrounded by air,which is a bad conductor, the heat must pass offby radiation, and as bright metal surfaces are bad radiators, the metal jug would retain the heat of the waterlonger than the earthenware one.
197.Supposing a red-hot cannon ball to be suspended by a chain from the ceiling of a room, how would its heat escape?
Almost entirely byradiation. But if you were to rest upon the ball a cold bar of iron, a part of the heat would be drawn off byconduction. Warm air would rise from around the ball, and, moving upwards, would distribute some of the heat byconvection.And some of its rays, falling upon a mirror, or any other bright surface, might be diffused byreflection.
"I will teach you by the hand of God; that which is with the Almighty will I not conceal."—Job xxvii.
198.Do some substances absorb heat?
Yes; those substances which arethe best radiatorsare alsothe best absorbersof heat.
199.Why does scratching a bright metal surface increase its power of radiation?
Because every irregularity of the surface acts as a point of radiation, oran outletby which the heat escapes.
200.Why does a bright metal tea-pot produce better tea than a brown or black earthenware one?
Because bright metalradiates but little heat, therefore the water is kept hot much longer,and the strength of the tea is extracted by the heat.
201.But if the earthenware tea-pot were set by the fire, why would it then make the best tea?
Because the dark earthenware tea-pot is a goodabsorber of heat, and the heat it wouldabsorbfrom the fire would more than counterbalance the loss byradiation.
202.How would the bright metal tea-pot answer if set upon the hob by the fire?
The bright metal tea-pot would probablyabsorb less heatthan it would radiate. Therefore it would not answer so well,being set upon the hob, as the earthenware tea-pot.
203.Why should dish covers be plain in form, and have bright surfaces?
Because, being bright and smooth, they will not allow heat to escapeby radiation.
204.Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?
Because athincoating of soot acts as agood absorber of heat, and overcomes thenon-absorbingquality of thebright surface.
"And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."
205.But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?
Because, although soot is agood absorberof heat, it is avery bad conductor; an accumulation of it, therefore, would cause a waste of fuel, byretarding the effects of heat.
206.Why should the lids and fronts of kettles and saucepans be kept bright?
Because bright metalwill not radiate heat; therefore, the heat which is taken up readily through theabsorbingandconductingpower of the bottom of the vessel, is kept in and economised by thenon-radiatingproperty of the bright top and front.
207.Does cold radiate as well as heat?
It was once thought thatcold radiatedas well asheat. But a mass of ice can only be said to radiate cold,by its radiating heat in less abundance than that which is emitted from other bodies surrounding it. It is, therefore,incorrectto speak of theradiation of cold.
208.Why, if you hold a piece of looking-glass at an angle towards the sum, will light fall upon an object opposite to the looking-glass?
Because the rays of the sun arereflectedby the looking-glass.
209.Why, when we stand before a mirror, do we see our features therein?
Because the rays of light that fall upon us arereflectedupon the bright surface of the mirror.
210.Why, if a plate of bright metal were held sideways before a fire, would heat fall upon an object opposite to the plate?
Because rays of heat may bereflectedin the same manner as the rays of light.
"But the wise answered saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves."—Matt. xxv.
211.Why would not the same effect arise if the plate were of a black or dark substance?
Because black and dark substances are notgood reflectors of heat.
212.What are the best reflectors of heat?
Smooth, light-coloured, and highly polished surfaces, especially those ofmetal.
213.Why does meat become cooked more thoroughly and quickly when a tin screen is placed before the fire?
Because the bright tin reflects the rays of heat back again to the meat.
214.Why is reflected heat less intense than the primary heat?
Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact.
215.Can heat be reflected in any great degree of intensity?
Yes; to such a degree that inflammable matters may be ignited by it. If a cannon ball be made red hot, and then be placed in an iron stand between two bright reflectors, inflammable materials, placed in a proper position to catch the reflected rays,will ignite from the heat.
There is a curious and an exceptional fact with reference toreflectedheat, for which we confess that we are unable to give "The Reason Why." It is found that snow, which lies near the trunks of trees or the base of upright stones, melts before that which is at a distance from them, though the sun may shine equally upon both. If a blackened card is placed upon ice or snow under the sun's rays, the frozen body underneath it will be thawed before that which surrounds it. But if wereflectthe sun's rays from a metal surface, the result isdirectly contrary—the exposed snow is the first to melt, leaving the card standing as upon a pyramid. Snowmeltsunder heat which isreflectedfrom the trees or stones while it withstands the effect of thedirect solar rays. In passing through a cemetery this winter (1857), when the snow lay deep, wewere struck with the circumstance that the snow in front of the head-stones facing the sun was completely dissolved, and, in nearly every instance, the space on which the snow had melted assumed a coffin-like shape. This forced itself so much upon our attention that we remained some time to endeavour to analyse the phenomena; and it was not until we remembered the curious effect ofreflected heatthat we could account for it. It is obvious that the rays falling from the upper part of the head-stone on to thefootof the grave would be less powerful than those that radiated from thecentreof the stone to the centre of the grave. Hence it was that the heat dissolved at the foot of the grave only a narrow piece of snow, which widened towards the centre, and narrowed again as it approached the foot of the head-stone, where the lines of radiation would naturally decrease. Such a phenomena would prove sufficient to raise superstition in untutored minds.
"The light of the righteous rejoiceth, but the lamp of the wicked shall be put out."—Proverbs xiii.
216.Are good reflectors of heat also good absorbers?
No; for reflectors at oncesend backthe heat which they receive, while absorbersretain it. It is obvious, therefore, thatreflectorscannot be goodabsorbers.
217.How do fire-screens contribute to keep rooms cool?
Because they turn away from the persons in the room rays of heat which would otherwise make the warmth excessive.
218.Why are white and light articles of clothing cool?
Because theyreflectthe rays of heat.
White, as acolour, is also a badabsorberandconductor.
219.Why is the air often found excessively hot in chalk districts?
Because the soilreflectsupon objects near to it the heat of the solar rays.
220.How does the heat of the sun's rays ultimately become diffused?
It is firstabsorbedby the earth. Generally speaking, the earthabsorbsheat by day, andradiatesit by night. In this way an equilibrium of temperature is maintained, which we should not otherwise have the advantage of.
221.Does not the air derive its heat directly from the sun's rays?
Only partially. It is estimated that the air absorbs onlyone-thirdof the caloric of the sun's rays—that is to say, that a ray ofsolar heat, entering our atmosphere at its most attenuated limit (a height supposed to be aboutfifty miles), would, in passing through the atmosphere to the earth, part with only one-third of its calorific element.
"As for the earth, out of it cometh bread; and under it is turned up as it were fire."—Job xxviii.
222.What becomes of the remaining two-thirds of the solar heat?
They areabsorbedchiefly by theearth, the great medium of calorificabsorption; but some portions are taken up byliving things, both animal and vegetable. When therays of heatstrike upon the earth's surface, they are passed from particle to particle into the interior of the earth's crust. Other portions are distributed through the air and water byconvection, and a third portion is thrown back into space byradiation. These latter phenomena will be duly explained as we proceed.
223.How do we know that heat is absorbed, and conducted into the internal earth?
It is found that there is a given depth beneath the surface of the globe at which an equal temperature prevails. The depth increases as we travel south or north from the equator, and corresponds with the shape of the earth's surface,sinking under the valleys, and rising under the hills.
224.Why may we not understand that this internal heat of the earth arises, as has been supposed by many philosophers, from internal combustion?
Because recent investigations have thrown considerable and satisfactory light upon the subject. It has been ascertained that the internal temperature of the earthincreasesto a certain depth,one degree in every fifty feet. But that below that depth the temperaturebegins to decline, and continues to do so with every increase of depth.
225.Do plants absorb heat?
Yes. They bothabsorbandradiateheat, under varying circumstances. The majestic tree, the meek flower, the unpretending grass, all perform a part in the grand alchemy of nature.
"Consider the lilies of the field, how they grow; they toil not, neither do they spin."
When we gaze upon a rose it is not its beauty alone that should impress us: every moment of that flower's life is devoted to the fulfilment of its part in the grand scheme of the universe. It decomposes the rays of solar light, and sends the red rays only to our eyes. It absorbs or radiates heat, according to the temperature of the ærial mantle that wraps alike the flower and the man. It distills the gaseous vapours, and restores to man the vital air on which he lives. It takes into its own substance, and incorporates with its own frame, the carbon and the hydrogen of which man has no immediate need. It drinks the dew-drop or the rain-drop, and gives forth its sweet odour as a thanksgiving. And when it dies, it preaches eloquently to beauty, pointing to the end that is to come!
226.How do we know that plants operate upon the solar and atmospheric heat?
A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.
227.What is the chief cause of variation in the temperature of flowers?
It is generally supposed that their temperature is affected by theircolours.
228.Why is it supposed that the colour of a flower influences its temperature?
Because it is found by experiment that thecoloursof bodies bear an important relation to their properties respectingheat, and hold some analogy to the relation ofcolourstolight.
If when the ground is covered with snow, pieces of woollen cloth, of equal size and thickness, and differing only in colour, are laid upon the surface of the snow, near to each other, it will be found that the relation ofcolourto temperature will be as follows:—In a few hours theblackcloth will have dissolved so much of the snow beneath it, as to sink deep below the surface; thebluewill have proved nearly as warm as the black; thebrownwill have dissolved less of the snow; theredless than the brown; and the white theleast, or none at all. Similar experiments may be tried with reference to thecondensation of dew, &c. And it will be uniformly found that thecolourof a body materially affects its powers ofabsorptionand ofradiation.
"And yet I say unto you, that even Solomon, in all his glory, was not arrayed like one of these."—Matt. vi.
229.Why do we know that these effects are not the result of light?
Because they would occur, in just the same order, in the absence of light.
230.Why are dark coloured dresses usually worn in winter, and light in summer?
Because blackabsorbsheat, and therefore becomes warm; whilelight coloursdonotabsorb heat in the same degree, and therefore they remain cool.
231.Why do iron articles, even when near fire, usually feel cool?
Because they are bad absorbers, and do not take up heat freely, unless they arein contactwith a hot body.
232.How is heat diffused through the atmosphere?
Byconvection. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course.
233.How do the waters of the ocean become heated?
Chiefly byconvection. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat byconvectionfrom the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude.
234.What is the cause of winds?
Currents of air, and winds, are the result ofconvection. The air, heated by the high temperature of the tropics,ascends, while the colder air of the temperate and the frigid zonesblows towards the equatorto supply its place.
"Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."—Psalm xxix.
235.What is the cause of sea breezes?
Sea breezes are also the result ofconvection. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surfaceflies away towards the ocean. As the sun goes down, the earth cools again, and the airflies backto find its equilibrium.
Many countries by the sea are subjected to these periodical breezes, known as either "land" or "sea breezes," according to their direction. About eight o'clock in the morning an ærial current begins to flow from the sea towards the land, and continues until about three o'clock in the day; then the current takes a reverse direction, flowing from the land to the sea. This it continues to do throughout the night, until the time of sunrise, when a temporary calm ensues.
236.Why does a soap bubble ascend in the air?
Because, being filled withwarmair, it islighterthan the surrounding medium, and therefore ascends.
237.Why does the bubble fall after it has been in the air some time?
Because the air contained in it has become cool, and, as it contains carbonic acid gas, it isheavierthan the air.
238.What became of the warmth at first contained in the bubble?
It has beendistributed in the airthrough which the bubble passed.
239.What does this simple illustration of the distribution of warmth explain?
It explains the law ofconvection, orheat distribution, over the surface of the globe.
240.Why does air ascend the chimney?
Because, being heated, it becomeslighterthan the surrounding medium, and therefore flies upwards, through the outlet provided for it.
241.Why does air fly from the doors and windows towards the fire-place?
Because, as the warm air flies away, cold air rushes in to occupy its place.
"How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."—Proverbs xvi.
242.What does this example of the motion of the air in our rooms explain?
It explains the movement of volumes of air byconvection, and illustrates the origin ofbreezesandwinds.
243.What is the chief effect of this law of convection?
Under its influence air and water are the greatequalisers of solar heat, rendering the earth agreeable to living things, and suited to the laws of their existence.
Owing, also, to this law ofconvection, the constituents of the air are equalised. The breath of life, supplied by the purer oxygen of the "sunny south," is diffused in salubrious gales over the wintry climes of the north. And the waters, evaporated from the bosom of the central Atlantic Ocean and the Pacific, are borne across vast continents, and poured down in fertilising showers upon distant lands.
To the educated mind, nothing is too simple to merit attention. To the ignorant, few things are sufficiently attractive to excite curiosity. Knowledge enables us to estimate the varied phenomena that are hourly arising around us, and to see, even in the most trifling effects, illustrations of those great causes and consequences that govern with mighty power the material world. Man, sitting by his fire-side, is enabled to witness the operation of some of nature's grandest laws:lightandheatare around him;conduction,radiation,reflection,absorption, andconvectionof heat are all going on before him; little winds are sweeping by his footstool, and warm currents, with miniature clouds folded in their arms, are passing upward before his view. Chemical changes are going on; the solid rock of coal disappears, flying away as an invisible gas. The little "hills are melted," and hard stones have been converted into "fervent heat." Although some of these changes are imperceptible to theeye, they are manifest to the educatedmind; and the pleasures of philosophical observation are as sweet as a poet's dreams.
"Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house."—Matt. v.
244.Why will a piece of paper, held three or four inches over the flame of a candle, become scorched?
Because the hot air and gas produced by the burning of the candleascendsrapidly.
245.Why will a piece of paper held about an inch below the flame of a candle scarcely become warmed?
Because the heatascends; and only a little of it falls upon the paper, and that byradiation.