To-day we review the modes in which heat passes or is conveyed from place to place. It is evident that if heat were confined to the very place or point where it is generated, it could subserve none of those uses to which it is now applied1 in the economy of Nature or in the works and arts of man. But heat passes from place to place with great facility, and by one method, with the speed of light, it tends to diffuse2 itself evenly through all; it seeks an equilibrium3. The modes of its diffusion4, or conveyance5, are three in number. Ansel may name them.”
“Heat passes from place to place and from body to body by ‘conduction,’ by ‘radiation,’ and by ‘convection.’”
“What is meant, Ansel, by the ‘conduction’ of heat?”
[Pg 101]“The passing of heat from atom to atom and from particle to particle through a body is called conduction.”
“That is right. I will call upon Peter to give some illustrations of the conduction of heat.”
“The examples are so many,” Peter answered, “that I hardly know what to mention first. If I hold a pin in the flame of a lamp, the part of the pin that touches the flame is first heated, but soon the heat runs along the whole length of the pin and burns my fingers. The parts of a stove which touch the fire are first heated, and from them the heat spreads through the whole stove. A pine-wood shaving, kindled6 at one end, is heated by conduction, but the heat passes through it very little faster than the flame follows. Heat escapes from our bodies by being slowly conducted through our clothing. There is no end to the examples of conduction which one might give.”
“We must not think of the conduction of heat,” said Mr. Wilton, “as if it were a fluid slowly absorbed by a porous7 body, as water poured upon the ground soaks into it, or as water percolates8 through a lump of sugar and moistens the whole of it. We must [Pg 102]remember that the transfer of heat is not a transfer of any substance, but a transfer of motion. One atom is set in motion, and strikes against another atom and sets that in motion, and thus motion is communicated from atom to atom and from molecule9 to molecule through the whole mass of matter till every atom is agitated10 with the heat vibrations11. Do all bodies conduct heat with equal rapidity?”
“No, sir,” replied Ansel; “there is the greatest possible difference. Some substances are called good conductors, because heat permeates12 them so readily and rapidly; others conduct heat very slowly, and are called poor conductors or bad conductors.”
“That is right. Every child soon learns by experience to make a practical distinction of this kind. He very soon understands that he can hold a stick of wood without burning his hand, even though it be blazing at the other end, but that when a piece of iron is red hot at one end he must not take hold of it at the other. The child very soon learns to know the different feeling of a cotton night-gown from one of flannel13, and the difference in apparent warmth between a linen14 pillow-case and a woolen15 blanket.[Pg 103] After a room has been heated for a considerable time the various objects in it all become of the same temperature, and the same is true in a cold room; but how great the difference in the sensations produced by touching16 the oil-cloth and a woolen carpet in a cold room! Good conductors of heat, if hot, feel very hot; or if cold, feel very cold; while poor conductors make a much less decided17 impression. Why is this, Samuel?”
“The good conductors receive heat or part with it very readily. If the good conductor be hotter than our bodies, it imparts its heat rapidly to our hand, and because we receive heat rapidly from it, it feels to us very hot. Or if it be colder than our bodies, it takes heat from our hands very rapidly, and gives the impression of being very cold. Poor conductors impart heat to the skin or take it away more slowly, and hence feel as if their temperature were more nearly like that of the body.”
“The conducting qualities of bodies,” said Mr. Wilton, “seem to depend chiefly upon their structure or the arrangement of their atoms. Bodies which are compact and solid in their structure convey heat more rapidly than those[Pg 104] which are loose and porous. Hence solids are better conductors than fluids, and fluids are better conductors than gases, and among solids the metals are better conductors than organized bodies, like wood or flesh, and better than the loose and porous minerals. In bodies of loose, porous, or fibrous texture18, the continuity of the conductory substance is constantly broken. The particles in a mass of sawdust touch only at a few points, leaving frequent spaces. In woolen and cotton fabrics19 the points of junction20 of the fibres are very few, comparatively. For this reason the motion is not readily communicated from atom to atom.
“The crystalline arrangement of atoms has an influence upon conduction of heat. Heat is conducted more rapidly in a direction parallel with the axis21 of crystallization than across that axis. Wood conducts heat more rapidly in the direction of the grain. This arrangement seems to be well adapted for keeping trees warm in winter. Their roots reach down into the earth, which remains22 warm in the coldest weather. This heat of the earth travels along the fibres up through the tree, while the heat conducted across the fibres escapes much more slowly into the open air.[Pg 105] The bark also, being a very bad conductor, hinders the escape of heat. Of metals, silver is the best conductor. I will give you a brief table which will show the great difference in the conducting qualities of some of the metals. Counting the conducting qualities of silver as 100, the table is: ‘Silver, 100; Gold, 53; Copper23, 74; Iron, 12; Platinum24, 8; German Silver, 6; Bismuth, 2.’—Youmans.
“What is the second method by which heat passes from place to place?”
“It is radiated,” replied Ansel.
“And what is radiation?”
“It is motion in straight lines or rays diverging25 from a centre. From a hot body heat is passing off in straight lines in every direction. As a lamp radiates light, so does a hot body radiate heat.”
“Radiant heat,” said Mr. Wilton, “moves with the same velocity26 as light, that is, one hundred and ninety-two thousand miles per second. It also follows the same general principles as light in all its motions. It is absorbed, reflected, or transmitted in the same manner as light. And this is true of either luminous27 heat—that is, heat radiated from a[Pg 106] body which is red hot—or obscure, or dark heat.
“As there are good and poor conductors, so there are good and bad radiators28 of heat. The radiation of heat depends upon three conditions:
“1. Upon the temperature of the body. The higher the temperature, the more rapid and energetic is its radiation.
“2. Upon the surface of the radiating body. A dull, rough surface radiates heat more rapidly than a surface bright and polished.
“3. Upon the substance of the radiating surface. With surfaces equally smooth and bright, some substances radiate heat much better than others. A surface of varnish29 radiates heat much more powerfully than a surface of gold or silver.
“Ansel, you may, if you can, explain the radiation of heat.”
“I can give no other explanation than that radiation is conduction through that subtle ether which is supposed to pervade30 all space.”
“Very well; perhaps that is as good an explanation as can be given. But it seems rather like the propagation of an impulse than the spreading of atomic vibrations in every [Pg 107]direction. The motion is propagated in straight lines. If it be conduction, it must be carried on by different vibrations from those of ponderable substances. Heat, light, and electricity are supposed to be all propagated through the same theoretical ether. Sir Isaac Newton estimated the density31 of the ether as seventy thousand times less than the density of our atmosphere, and its elasticity33 in proportion to its density as four hundred and ninety millions times greater. But the very existence of this universally-diffused ether is a supposition made to account for the phenomena34 of light, heat, and electricity; and, of course, all its qualities must be theoretical also. Radiation is believed to be the propagation of a motion or impulse through an inconceivably rare and elastic32 ether.
“Peter, what is the third method by which heat passes from place to place?”
“Convection,” was his reply.
“What is meant by convection of heat?”
“The conveyance of heat by carrying a heated body. If I remove a hot iron or a kettle of hot water, I must of course carry the heat which it contains.”
“A very good illustration of the convection[Pg 108] of heat,” said Mr. Wilton, “is seen in the common method of heating water. The heat is applied at the bottom of the vessel35 containing the water; as fast as the water at the bottom next the fire is heated, it rises and carries the heat to the top; cold water comes to take its place, and this in turn is heated and rises and carries heat to the top. This process is carried on till all the water comes to the same temperature. Thus water is heated by convection of heat.
“A grander illustration is seen in winds and ocean currents. Warm winds carry heat enough to warm a continent, and the mighty36 ocean currents are still more efficient in transferring heat from one part of the earth to another.
“Another point we need to understand. When radiant heat falls upon a body, what becomes of it?”
“It is disposed of,” answered Samuel, “in one of three ways: it may be reflected according to the same principles by which light is reflected; or it may be transmitted, that is, pass through the body; or it may be absorbed, that is, stop in it.”
“Very well stated, Samuel. In regard to reflection I need to say very little. You know how light is reflected from a polished surface,[Pg 109] such as a lamp reflector: heat is reflected in the same manner. One fact you must bear in mind touching reflected heat: it does not heat the reflecting body.
“There is no need of telling you that light passes through certain substances. It passes through gases and through some liquids and some solids. The best of glass, though it is so solid, interposes very little hindrance37 to the passage of light. Heat in like manner radiates through certain solids. Luminous heat is radiated through glass. Rock-salt transmits dark heat also. A plate of alum permits light to pass, but stops both luminous heat and dark heat. Remember that transmitted heat, as was said of reflected heat, does not heat the body through which it passes. I have seen boys make burning-glasses of ice. The heat passes through them and burns that upon which it is concentrated, while the ice itself through which the heat passes is not melted.
“If a body have a good radiating surface, that is, if its surface be dull and rough, the heat which falls upon it will be mostly absorbed. The reflecting and absorbing qualities hold an inverse38 ratio to each other; the better the [Pg 110]reflecting qualities, the worse the absorbing, and the worse the reflecting, the better the absorbing. Heat which is absorbed by a body commonly raises its temperature, and remains in the body till it is slowly radiated or is conducted away by the air or other bodies which come in contact with it.
“What is that heat called, Ansel, which is absorbed by a body with no rise of temperature?”
“It is called latent heat.”
“That is the old and common expression, but what is meant by latent heat?”
“The word latent signifies lying hidden or concealed39. Latent heat, as you suggested in your first question, is that heat which a body receives without showing it by a change of temperature.”
“That name ‘latent heat,’” said Mr. Wilton, “expresses the opinion of those who invented it; they supposed that heat was in some manner hidden in certain bodies. We must not suppose, however, that this latent heat continues to exist in bodies as heat; latent heat is that heat which is converted into force or some other motion than the atomic heat vibrations, and is employed[Pg 111] otherwise than in raising the temperature. You will understand this best by an illustration.
“Take one hundred pounds of ice at the temperature of thirty-two degrees, that is, as warm as is possible without melting. That one hundred pounds of ice will absorb heat which would raise one hundred pounds of ice water through one hundred and forty degrees, and by receiving that heat it is melted, but the water produced has the temperature of thirty-two degrees. It has received one hundred and forty degrees of heat, but its temperature is not raised a single degree. This one hundred and forty degrees of heat has been transmuted40 into force and employed in overcoming the crystalline attraction of the atoms of water.
“Let that ice water at thirty-two degrees of temperature receive one hundred and eighty degrees of heat, and the water rises to two hundred and twelve degrees, the temperature of boiling. But whatever additional heat is absorbed brings no increase of temperature, but transforms the water into steam. It is employed in overcoming the cohesive41 attraction of the molecules42 of water and changing the liquid to a gas. About one thousand degrees of heat is thus expended43, but[Pg 112] the steam which is produced has only the temperature of two hundred and twelve degrees. If the process be reversed, the steam gives up, as it is said, the one thousand degrees of heat in returning to the condition of water and the one hundred and forty degrees in resuming the crystalline structure of ice. The heat which was employed as force in overcoming the atomic and molecular44 attractions is transmuted again to heat, and shows itself in raising the temperature. And that which is true of water is true of any other substance in changing its form from a solid to a liquid or from a liquid to a gas, or the opposite. In an amount different for each kind of matter, in all these changes of condition, heat is transmuted to force or force to heat.
“These transmutations are going on ceaselessly in the operations of Nature, and without understanding them we cannot appreciate the wonderful operations of heat in the world. The heat of the sun beams upon the ocean; the greater part of that heat is expended as force in overcoming the molecular attraction of water, thus converting it to vapor45, and in raising that vapor to the higher regions of the atmosphere. This heat-force, or, as we might call it, [Pg 113]‘sunpower,’ expended upon the earth, amounts to thousands of millions of horse-power daily.
Transmutation of Heat.
Page 113.
“Examples of the transmutation of force into heat abound46 everywhere. A boy strikes his heel upon the stone pavement; from the point of contact between the stone and the steel points in his boot heel sparks of fire fly out. Force is changed to heat so intense that particles of steel are set on fire. Savages47 who have no better methods of kindling48 fire rub dry wood together till the sticks ignite. The force expended in overcoming the friction49 is changed to heat. In the combustion50 of coal beneath the steam boiler51 we see both processes going on. The atoms of carbon dash against the atoms of oxygen, and the force of the collision generates the heat of the combustion. This heat, born thus of force, is again transmuted to force, and drives the engine and the machinery52 attached. In our study of God’s management of heat we shall constantly meet with these changes. You will need, therefore, to study carefully this subject of latent heat.
“Dr. Joule, of Manchester, England, has discovered the ratio between heat and force, that is, the amount of force which by transmutation[Pg 114] produces any given amount of heat. The force of a one-pound weight which has fallen one foot is taken as the unit of force, and the amount of heat which is required to raise one pound of water one degree is taken as the unit of heat. By many and various careful experiments, Dr. Joule demonstrated that 772 units of force are the equivalent of one unit of heat. A pound weight falling 772 feet, or 772 pounds falling one foot, and then arrested, produces heat sufficient to raise one pound of water one degree. The result is the same whatever the method by which the force is expended. If water be agitated or shaken, if sticks of wood or iron plates be rubbed together, if an anvil53 be struck with a hammer, or if a bar of iron or copper be moved back and forth54 between the poles of an electromagnet, the force expended is changed to heat. You must remember, however, that force becomes heat only so far as the force is actually expended, or used up so that it no longer exists as force.
“These conclusions are supported by other beautiful experiments. ‘An electric current which, by resistance in passing through an imperfect conductor, produces heat sufficient[Pg 115] to raise one pound of water one degree, sets free an amount of hydrogen which, when burned, raises exactly one pound of water one degree. Again, the same amount of electricity will produce an attractive magnetic force by which a weight of 772 pounds may be raised one foot high.’—Youmans. We conclude from experiments like these that heat, mechanical force, and electricity are interchangeable forces; they may be transmuted the one into another.
“By this principle of the transmutation of heat and mechanical force we explain the production of heat by compression and the loss of heat by expansion. Samuel, you may state the fact upon this point.”
“If any substance be suddenly compressed,” answered Samuel, “heat appears; if it be expanded, cold is produced. Since gases expand or yield to pressure so readily, they furnish the best illustration of this principle.”
“The suddenness of the compression or expansion,” said Mr. Wilton, “is a matter of no consequence. The effect is the same whether the operation be sudden or slow, but if the compression or expansion be slow, the heat or cold generated is less apparent; the heat is [Pg 116]dissipated as fast as produced and the colder gas is warmed by the vessel which contains it. Ansel, how shall we explain this?”
“I cannot explain it, sir.”
“The explanation is very simple,” said Mr. Wilton. “Mechanical force is employed in the compression of the gas; the force is expended and used up upon the gas, and appears again in the form of atomic heat motion. In the expansion of gases the operation is just the reverse; the atomic heat motion is expended in producing expansion, and hence disappears as heat. The general principle is that no force can be expended in two ways at the same time.
“One other point we must notice to-day, that is, specific heat. What is understood, Ansel, by this term, specific heat?”
“The relative amount of heat which different substances require to raise their temperature through any given number of degrees.”
“That is right. I think that you all must have noticed that it requires much more heat to raise the temperature of some bodies than others. What an amount of heat is required to raise the temperature of water! That heat which will raise one pound of water one degree[Pg 117] will cause an equal increase of temperature in five pounds of sulphur, or four pounds of air, or nine pounds of iron, or eleven pounds of copper, or thirty pounds of mercury, lead, or gold. This is what is meant by saying that one substance has a greater capacity for heat than another. The specific heat of water is greater than that of any other known substance except hydrogen gas. This fact, taken in connection with its great specific latent heat and its poor conducting qualities, renders it exceedingly important in regulating climate and moderating extremes of temperature; of this you will be reminded very often as our lessons go on.
“No law or principle determining the specific heat of the various elements and explaining the different capacities for heat has as yet been discovered. It has been suggested that specific heat depends upon the number of atoms, that it holds an inverse ratio to their combining numbers, or, what is the same thing, a direct ratio to the number of atoms. This would harmonize well with the dynamic theory of heat, but the harmony between the specific heat of substances and the number of atoms is not sufficiently55 uniform to establish this supposition.
[Pg 118]“This completes our review of first principles. I hope that this not very entertaining review of your academic studies has not wearied you of the very word heat and worn out your interest in examining God’s management of heat before making a beginning.”
“I think,” said Samuel, “that we are not in the habit of becoming disgusted with our studies.”
“You may expect,” continued Mr. Wilton, “if the past has been interesting to you, that the lessons to come will prove more interesting still. Next week we shall consider the abundant provision which the Creator has made for warming the earth.”
And let me say to you, patient reader, that if I had known that you were as familiar with the laws and principles of heat as Ansel, Peter, and Samuel seem to have been, this and the preceeding chapter would not have been written. However dull this review may have seemed to you, it was needful, perhaps, for others, that they might understand the wonderful works of God which we shall now proceed to examine. And, reader, do not forget that heat itself, that subtle motion and mighty force, with all its laws and[Pg 119] principles, is one of God’s works. Already have we been looking at the Creator’s handiwork. Already have we been trying to trace out the thoughts of God as they are written in the “Bible of Nature.” The thoughts of God are great and wonderful. It has been useful and interesting to read thus far in this book written with the finger of the Creator of worlds and of man, even if we turn not another page.
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applied
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| adj.应用的；v.应用，适用 | |
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diffuse
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| v.扩散；传播；adj.冗长的；四散的，弥漫的 | |
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equilibrium
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| n.平衡，均衡，相称，均势，平静 | |
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diffusion
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| n.流布；普及；散漫 | |
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conveyance
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| n.(不动产等的)转让，让与；转让证书；传送；运送；表达；（正）运输工具 | |
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kindled
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| （使某物）燃烧，着火( kindle的过去式和过去分词 )； 激起（感情等）； 发亮，放光 | |
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porous
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| adj.可渗透的，多孔的 | |
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percolates
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| v.滤( percolate的第三人称单数 )；渗透；（思想等）渗透；渗入 | |
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molecule
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| n.分子，克分子 | |
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agitated
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| adj.被鼓动的，不安的 | |
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vibrations
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| n.摆动( vibration的名词复数 )；震动；感受；（偏离平衡位置的）一次性往复振动 | |
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permeates
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| 弥漫( permeate的第三人称单数 )； 遍布； 渗入； 渗透 | |
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flannel
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| n.法兰绒；法兰绒衣服 | |
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linen
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| n.亚麻布，亚麻线，亚麻制品；adj.亚麻布制的，亚麻的 | |
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woolen
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| adj.羊毛(制)的；毛纺的 | |
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touching
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| adj.动人的，使人感伤的 | |
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decided
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| adj.决定了的，坚决的；明显的，明确的 | |
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texture
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| n.(织物)质地；(材料)构造；结构；肌理 | |
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fabrics
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| 织物( fabric的名词复数 )； 布； 构造； （建筑物的）结构（如墙、地面、屋顶）：质地 | |
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junction
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| n.连接，接合；交叉点，接合处，枢纽站 | |
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axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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remains
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| n.剩余物，残留物；遗体，遗迹 | |
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copper
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| n.铜；铜币；铜器；adj.铜(制)的；(紫)铜色的 | |
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platinum
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| n.白金 | |
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diverging
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| 分开( diverge的现在分词 )； 偏离； 分歧； 分道扬镳 | |
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velocity
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| n.速度，速率 | |
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luminous
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| adj.发光的，发亮的；光明的；明白易懂的；有启发的 | |
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radiators
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| n.（暖气设备的）散热器( radiator的名词复数 )；汽车引擎的冷却器，散热器 | |
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varnish
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| n.清漆；v.上清漆；粉饰 | |
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pervade
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| v.弥漫，遍及，充满，渗透，漫延 | |
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density
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| n.密集，密度，浓度 | |
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elastic
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| n.橡皮圈，松紧带；adj.有弹性的；灵活的 | |
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elasticity
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| n.弹性，伸缩力 | |
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phenomena
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| n.现象 | |
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vessel
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| n.船舶；容器，器皿；管，导管，血管 | |
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mighty
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| adj.强有力的；巨大的 | |
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hindrance
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| n.妨碍，障碍 | |
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inverse
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| adj.相反的，倒转的，反转的；n.相反之物；v.倒转 | |
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concealed
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| a.隐藏的，隐蔽的 | |
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transmuted
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| v.使变形，使变质，把…变成…( transmute的过去式和过去分词 ) | |
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cohesive
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| adj.有粘着力的；有结合力的；凝聚性的 | |
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molecules
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| 分子( molecule的名词复数 ) | |
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expended
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| v.花费( expend的过去式和过去分词 )；使用（钱等）做某事；用光；耗尽 | |
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molecular
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| adj.分子的；克分子的 | |
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vapor
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| n.蒸汽，雾气 | |
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abound
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| vi.大量存在；(in，with)充满，富于 | |
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savages
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| 未开化的人，野蛮人( savage的名词复数 ) | |
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kindling
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| n. 点火, 可燃物 动词kindle的现在分词形式 | |
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friction
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| n.摩擦，摩擦力 | |
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combustion
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| n.燃烧；氧化；骚动 | |
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boiler
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| n.锅炉；煮器(壶，锅等) | |
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machinery
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| n.(总称)机械，机器；机构 | |
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53
anvil
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| n.铁钻 | |
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54
forth
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| adv.向前；向外，往外 | |
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55
sufficiently
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| adv.足够地，充分地 | |
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