A DIFFICULT QUESTION.
During the week, Ansel, Peter, and Samuel were busy reviewing and fixing in memory what they had already learned of the nature and laws of heat. They were not only interested in the new line of study, and desirous of pleasing Mr. Wilton, but they also felt that their scholarship was to be tested, and each one was ambitious of standing equal to the best.
Ansel, of course, was busy and ambitious. The lesson was coming somewhat upon his own ground, and he felt in no wise unwilling to show how well he had mastered the subject. He entered upon it with feelings a little different, however, from his anticipations. The explanation which Mr. Wilton had given of the purpose of the Creator in making such a world seemedto him very reasonable. He could make no objection to it. But that explanation had taken away at one sweep a whole store of objections to God’s goodness which he was waiting to bring out as soon as a good opportunity was presented. A world designed for the dwelling-place of sinners—sinners not already given over and doomed to final wrath, but to be recovered from sin and trained in virtue and holiness, or, if incorrigible, to be held in check and used as helps in the discipline of the righteous—he plainly saw must be as unlike a world fitted up for holy beings as a reform school is different from a home for kind and obedient children. Those arrangements which he had thought the most painful and objectionable might, after all, be the wisest and best. He did not see where to put in a reasonable objection to Mr. Wilton’s unexpected argument, yet he did not feel quite satisfied to confess to himself that he was so soon and so easily defeated.
In this state of mind, on Saturday morning he met Mr. Hume upon the street.
“Good-morning, Ansel,” said Mr. Hume.
“Good-morning,” returned Ansel.
“I hear,” said Mr. Hume, “that you havegiven up studying the Bible in your Bible class, and have begun the study of natural philosophy. Is that so?”
“Not quite true, Mr. Hume. We are to examine some department of the works of Nature, and see what indications appear of the Creator’s wisdom and goodness.”
“That is a little different from the report which came to me. But what did you learn last Sunday?”
“Mr. Wilton told us that in order to judge of the wisdom and goodness of God in any of the affairs of this world we must consider the object for which that arrangement was designed. He said that if a man examine a cotton-gin, supposing it to be a threshing-machine, he would be likely to pronounce it a foolish and worthless contrivance; and that the fine edge of a razor would be worse than useless upon the cutter of a breaking-up plough. He told us that the earth was not prepared as the dwelling-place of sinless beings, but as a place of discipline for the fallen human race, and that we ought not to look upon it as the choicest specimen of workmanship which the Creator could construct.”
“I have heard that Mr. Wilton believessomething of that kind. Ansel, have you studied geology?”
“I have read a little upon that subject and have heard some lectures.”
“Can you tell me, then, whether or not the natural laws which prevailed on the earth ages and ages ago, before the earth was fit for men to live upon it, are the same as those which have been in operation in these later ages, since men have inhabited it?”
“I suppose that the same laws have prevailed from the beginning of the geologic periods. I think that geology makes that very evident.”
“If that were not so,” said Mr. Hume, “the past history of the globe would be a riddle to us; it would be confusion worse confounded. In regard to those early ages we could not reason from cause to effect, for we should know nothing of the forces and principles then in existence. In geologic studies we judge the past from the present, and if that be not a trustworthy method of reasoning, all the conclusions of geologists are as worthless as dreams. Have you any reason to suppose, from what you have read on this subject, that a curse changed the character of the earth as a dwelling-place for man somesix thousand years ago? Is it true, as Milton says, that then
‘The sunHadfirsthis precept so to move, so shine,As might affect the earth with cold and heatScarce tolerable, and from the north callDecrepit winter—from the south to bringSolstitial summer’s heat’?
Did the Creator then
‘Bid his angels turn askanceThe poles of earth twice ten degrees and moreFrom the sun’s axle’?
Or was death then first introduced among the brute creation, as Milton fancies?—
‘But Discord first,Daughter of sin, among the irrationalDeath introduced through fierce antipathy;Beast now with beast ’gan war, and fowl with fowl,And fish with fish; to graze the herb all leaving,Devoured each other.’”
“Animals must have died,” said Ansel, “for their remains lie imbedded in rock which certainly existed before man lived on the earth.”
“I wish you would ask Mr. Wilton one question for me.”
“I am willing to ask him any proper question,and I suppose you would not wish me to ask any other.”
“I certainly would not. Will you ask him how it was possible for man not to sin and fall if God created the world for a sinful race myriads of ages before man was brought into existence? It would seem that if man had remained obedient he could not have lived pleasantly in a world prepared for sinners, and at the same time, by man’s obedience, all the Creator’s plans touching this world would have been dislocated and disappointed.”
“I will ask him, sir,” said Ansel, “at the first good opportunity.”
This good opportunity occurred sooner than Ansel expected, for, before entering upon the proposed lesson the next Lord’s Day, Mr. Wilton said to the class:
“I wish in these lessons to advance carefully and safely, and, as far as possible, have everything well understood. For that reason I invite you to speak freely of any difficulties or objections which may suggest themselves to your own minds or which you may hear presented by others. At the close of the last lesson the views which I had presented to you seemed veryreasonable, but it is possible that, as you have thought upon the subject during the week, objections may have arisen in your minds. If so, I should be glad to hear them now.”
“There are many things,” said Peter, “of which I cannot see the use, even if we suppose that the earth was designed as the dwelling-place of sinners.”
“It would be very surprising indeed if you could unravel all the mysteries of creation in a week’s time. Wiser men than any of us have spent a lifetime in searching out the meaning of God’s works, and died still in the dark upon many points. We need not expect to unravel and understand all the deep, complex, and delicately-interwoven contrivances in a world so vast and curious as this. The world is a great mystery—mysterious as a whole, and mysterious in all its parts—upon any supposition. But the explanation which I gave of its design furnishes a sufficient reason for the great outline of creation. This gives a reason for the pains and miseries which dog man at every step. This gives a reason for the earth’s being left rugged and sluggish, bringing forth thorns and thistles, and requiring to be subdued by patient industry. Itshows a ground for the necessity of exhausting toil under a frowning sky and mid miasmatic airs—for the liability to diseases and accidents, and the hard necessity of death. These great elements of divine providence are not stripped of their halo of mystery, but with this explanation they are seen to form a harmonious whole for the accomplishment of a great and glorious purpose.”
Mr. Wilton paused. Then Ansel said, “Mr. Hume wished me to ask you a question.”
“Very well, I should be glad to hear it. I hope, indeed, that he sends his question from interest in the subject, and not with the design of perplexing us. I wish also that he were here to ask the question and hear the answer for himself. But what is the question?”
“He wished me to ask how it was possible for man not to sin and fall if God placed him in a world prepared for a race of sinners and unfitted for a sinless race. He said that in such a case, if man had remained obedient, the plans of God would have been disarranged.”
“What answer did you try to give him, Ansel?”
“I did not try to make any explanation. Itseemed to me a very great objection. I did not see how such a course was consistent with God’s righteousness.”
“And you are not the first person who has objected to this as a great inconsistency. I am afraid the discussion will take more time than we ought to spare, but now that the question has been asked and the objection presented, I must take time to answer it, even if it consume the whole half hour.
“In considering this subject, as well as many others, we need to remember that the existence of difficulties is no objection to a principle or a fact. Difficulties wholly inexplicable by man attend facts and principles which must be true. A fact may be incomprehensible, though undeniable. The great Doctor Johnson said, ‘There are insuperable objections against a plenum, and insuperable objections against a vacuum, yet one of these must be true.’ What did he mean by that, Samuel?”
“He meant, I suppose, that we could not explain the possibility that any space should be wholly empty of matter, and could no more explain the possibility that any space should be filled with matter, but that all space must be filled,or else there must be empty space. Whether we can explain the possibility or not, one of them must be true.”
“That is right. The same is true of many other facts besides a plenum and a vacuum. We cannot conceive of infinite space; we cannot conceive that space should not be infinite, but bounded. We cannot conceive of the creation of the world from nothing, and no more can we conceive of its eternal existence. The truth is that the mind of man cannot grasp such subjects so as to reason upon them correctly. No sooner do we attempt to reason about the infinite things of God than we run into absurdities and reach the most contradictory conclusions. And in this respect it makes no difference with what principle or proposition we start if it only contain some infinite element. Let me give you a simple illustration from geometry—an illustration which, very likely, is familiar to you: the larger a circle, the less is the curvature of the line which bounds it; that is, the more nearly does that line approach a straight line. An infinite circle must be bounded by a straight line, because with any degree of curvature the circle would be less than infinite. But astraight line cannot bound a circle. The attempt to reason about an infinite circle brings us at once to the most palpable absurdities and contradictions. Or take this illustration: the whole of a thing is greater than any of its parts. But divide a line of infinite length in the middle, and each part is infinite. We reach the conclusion either that the half is equal to the whole or that other wholly incomprehensible proposition, that one infinity is twice as great as another infinity. I have made these statements to show you that the existence of difficulties does not indicate, much less prove, that a fact is not real and true.
“Mr. Hume thinks the fact that the earth existed in its present condition before men sinned an insuperable objection to the view that this world was prepared as a place for the discipline of a fallen race. But let us look at the other side, and see if equal objections do not exist. The Creator foresaw the fall of man; is there no objection to the supposition that, knowing that man would sin, God made no provision for it? On the one supposition he foresees the evil and makes no provision; on the other, he foresees it and provides forthe catastrophe. The former supposition certainly involves the greater difficulties.
“The objector may reply that the plan of God, by embracing the fall of man and including it as one of its essential elements, made that fall necessary. But why should not God embrace in his plan that great event, the fall of man, which he foresaw in the future? Would it have been wiser and better to leave out of account that most stupendous fact in the history of the human race? This same objection, which Mr. Hume and many others have brought forward, lies with equal force against the great central fact of the gospel, the death of Christ. God’s plan touching this world included the incarnation and death of his Son. Jesus, the ‘Lamb of God,’ is spoken of as ‘slain from the foundation of the world.’ Rev. xiii. 8. But the incarnation and death of Christ presuppose the apostasy of the human race. Did this plan touching Christ make the apostasy of man a necessity? If preparing a world—fallen, so to speak, beforehand—for a race which God foresaw would fall, be inconsistent with his righteousness, it must be equally inconsistent to prepare a Saviour beforehand for that same race.
“Again, the divine plan touching the death of his Son included his betrayal by Judas and his crucifixion by the Jews. If Judas had known that God had poised the salvation of man upon the pivot of his treachery, he would doubtless have argued as Mr. Hume and others are accustomed to do. But did God’s plan excuse his treason against his Lord? His own conscience, piercing and rending his soul with remorse, drove him to self-destruction, and Christ confirmed the sentence of his conscience and called him the ‘son of perdition.’ The fact that God weaves the foreseen crimes of men into his plans is no palliation of their guilt.
“Would it be wise and well to take no account of foreseen events? Jesus has gone to prepare mansions for those who will, as he foresees, believe in him: why not make provision for foreseen evils also? Our civil government, knowing the liability to crime among men—a liability which the experience of man has shown to be a practical certainty—makes provision for those crimes by maintaining a police, reform schools, prisons, and armies. The Governor of the universe, knowing the liability of man to sin and fall—a liability which by his foreknowledge wasto him a certainty—made provision for that foreseen apostasy. He made provision, both by the creation of a world suited to a sinful race kept under a probation of mercy, and by appointing a Redeemer, the ‘Lamb of God,’ slain, in the eternal purpose, before the foundation of the world. If Mr. Hume’s objection has force at all, it has force against every wise provision of God to meet the consequences of man’s foreseen wickedness. It is wise, forsooth, on man’s part, to foresee coming evil and prepare for it; but if God do this, men count it worse than folly: they declare it to be an endorsement of the evil! So foolishly do men reason about the high things of God! My answer to Mr. Hume, then, has four parts:
“1. The existence of unexplainable difficulties does not disprove the truth and reality of any fact or principle.
“2. The supposition that God made provision for the present apostasy of the human race is burdened with fewer and smaller difficulties than its denial.
“3. The word of God declares that he did make provision for the fall of man by the pre-appointment of a Redeemer.
“4. That style of reasoning which seeks to justify or palliate man’s first sin because God prepared this world for a fallen race would palliate and justify all wickedness, because the sins of men are woven into every figure of the web of divine providence. Not the treason of Judas alone, but the whole sum of man’s evil-doing, is embraced in the far-reaching plan of God. How this magnifies the wisdom of God! He binds together in one bundle his own righteousness and the sins of men, in a most intricate interlacing, yet without blending the two and without staining the glory of his holiness.
“I hope I have made this plain. Do you think, Ansel, that you can repeat the substance of this answer to Mr. Hume?”
“I will try, sir, if he asks.”
“You will all notice,” added Mr. Wilton, “that I have not denied that there is a deep mystery in this preparation for the sins of men not yet created, and that I have not attempted to explain this mystery. I have only tried to show that the admission of the view I have given you is more satisfactory to reason than its denial, and that the mysteries of this view are not unreasonable and self-contradictory, forthe greatest mysteries are often the most reasonable things in the world.
“My introduction has become much longer than I designed, but now let us turn our attention to the subject of the lesson.
“To aid us in understanding God’s wise arrangements in the management of heat, we need, first, to consider what heat is and to review the laws of its action. Without this, we could look on and wonder at God’s working in nature, but could not explain that which we saw.
“Ansel, will you state the theories which have been held touching the nature of heat?”
“I will do it as well as I can. The ancient philosophers supposed fire to be one of the four elements of which all bodies were composed. The three other elements were earth, air, and water. These four elements were mingled in various proportions. Of these, fire was esteemed the purest and most ethereal; this constantly tended upward to the empyrean, the highest heaven, where the element of fire and light was supposed to exist unmingled and pure. In the seventeenth century, Beccher and Stahl, two German chemists, brought forward what is known as thephlogistic hypothesis.They supposed that every combustible body held in composition a pure, ethereal substance which they calledphlogiston, a Greek word which signifiesburned, and that in combustion this phlogiston escaped. Flame was supposed to be this escaping phlogiston. These were the notions held about fire and combustion, but they are hardly worthy to be called theories of heat. The discovery of oxygen by Dr. Priestley of England, in 1774, and the introduction of the balance by Lavoisier of France, joined with the ever-enlarging circle of facts to be explained, rendered the phlogistic hypothesis untenable, and it was thrown aside.
“Until a few years since thecalorictheory was generally received. According to this theory, heat is asubstance, a subtle ether, diffused through all bodies and surrounding their atoms. This ether has been supposed to have a strong attraction for the atoms of every other substance, while between its own atoms a strong repulsion exists. In solid bodies each atom of matter, or in compound bodies each cluster of atoms, has been supposed to be surrounded by a little atmosphere, so to speak, of caloric, which prevented the atoms from coming into absolute contact.According to this theory, heat expands bodies by increasing and deepening these minute atmospheres, thus pressing the atoms farther from each other.”
“You need not explain this theory farther,” said Mr. Wilton; “we have hardly time to go into the history of theories. Tell us the latest received theory.”
“The theory now commonly believed is called themechanicalordynamictheory. According to this theory, the essence of heat ismotion. A hot body is one whose atoms are in a state of rapid and intense motion or vibration; and the sensation of heat on touching a hot body arises from the impact, or rapid blows, of the agitated atoms, communicating the same atomic vibration to the flesh and nerves of the hand.”
“Very well stated, Ansel. This is the theory now more commonly received. The caloric theory, like the crude notions of the old Greek philosophers about fire, and like the phlogistic hypothesis, has been rejected because it failed to explain the phenomena of heat. Whether the dynamic theory is destined to share the same fate remains to be seen. It seems, however, to have a better foundation than its predecessors.The dynamic theory, though recently made popular, is by no means a recent conception. It was advocated by such men as Bacon, Newton, Rumford, Davy, Locke, and others. Locke, the distinguished intellectual philosopher who lived in the latter half of the seventeenth century (born 1632, died 1704), said, ‘Heat is a very brisk agitation of the insensible parts of an object, which produces in us that sensation from which we denominate the object hot, so that what in our sensations is heat in the object is nothing but motion.’ Benjamin Thompson, an American gentleman who went to Europe in the time of our revolution, and for his scientific fame was made Count Rumford, and became the founder of the Royal Institution of England, declared that he could form no conception of the nature of heat generated by friction unless it were motion.
“A beautiful generalization has been made to show how well this idea of heat harmonizes with the entire plan of the universe. In the whole boundless universe each system of worlds, like our solar system, may be regarded as a molecule, or complex atom. These cosmical molecules, or complex atoms of the universe, are in motionthrough unmeasured space. In these systems of worlds the planets, with their satellites, are the molecules, and they are in motion—indeed, they commonly have several motions. Our earth, for example, rotates upon its axis once each day; it revolves in its orbit around the sun once each year, and the axis of the earth has a slow wabbling motion which produces the precession of the equinoxes, requiring 25,868 years for a complete revolution. The earth also is made up of parts, and all these are in ceaseless motion. As said the old Greek philosopher, ‘All things flow’—that is, everything is in a state of change. Solomon has well described this perpetual movement and change: ‘One generation passeth away, and another generation cometh. The sun also ariseth, and the sun goeth down, and hasteth to his place whence he arose. The wind goeth toward the south, and turneth about unto the north. It whirleth about continually, and the wind returneth according to his circuits. All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither do they return again. All things are full of labor; man cannot utter it. The eye is not satisfied with seeing, nor the ear filledwith hearing. The thing that hath been, it is that which shall be; and that which is done is that which shall be done, and there is no new thing under the sun.’ Eccles. i. 4-9. It is certainly in harmony with this universal movement that the atoms of matter, though they seem so closely packed, should in their inconceivable smallness through inconceivably minute spaces vibrate, or rotate, or revolve through an orbit, never at rest. Intensity of heat we may think of as intensity of this atomic motion—a wider swing, so to speak, in their vibration or revolution. This, of course, requires a wider separation of the atoms and a consequent expansion of bodies. A feebler atomic motion permits the atoms to approach each other. In this manner we explain the enlargement of bodies by heat and their contraction by decrease of temperature. ‘The ideas of the best-informed philosophers are as yet uncertain regarding the exact nature of the motion of heat, but the great point at present is to regard it as a motion of some kind, leaving its more precise character to be dealt with in future investigation.’ This is the most we can do at present.”
“What is the evidence,” asked Samuel, “that the dynamic theory of heat is true?”
“The evidence that any theory is true is its ability to explain the facts or phenomena with which it has to do. If it explains all the facts and contradicts no known principles, it is regarded as true, or at least no objection can be made to it. Let me illustrate. Astronomers had long inquired what force or law controlled the movements of the heavenly bodies. At length Newton answered, A force of attraction between bodies which decreases in proportion as the square of the distance between them increases. This explanation has been found sufficient to explain all the known facts in the working of the heavenly bodies. Upon the basis of this theory astronomers calculate the positions of planets and comets for years and centuries to come.
“This theory led to the discovery of the planet Neptune, the last discovered of the primary planets. For thirty years irregularities in the motion of Uranus had been noticed. These variations were so slight that if another planet had revolved in the proper orbit of Uranus they would have seemed to the naked eye, throughout their course, one and the same star.This slight irregularity of motion was so nicely measured that the place of the unseen planet which caused it was almost exactly calculated from the estimated force and direction of its attraction. This theory of a universal attraction of gravitation so well explains all the facts in the case, and has become so universally received, that we are liable to forget that, after all, it is nothing but a theory.
“Our idea of the structure of the solar system was at first only a theory. The astronomer does not see the planets revolving in regular circles through the heavens and moving around the sun. He only sees the shining points moving back and forth upon the concave vault, doubling and crossing their tracks apparently in the greatest disorder. How shall their motions be explained? Astronomers have found that the motions of planets revolving around a central sun, when seen from one of the planets, must present just these apparent irregularities. This explanation is so full and complete that it is now counted not a theory, but an established fact. The same may be said of the shape of the earth.
“The dynamic theory of heat explains thephenomena of heat better than any other explanation that has been proposed. It explains the radiation of heat from the sun or from any other hot body: vibrations or impulses are propagated through that ether which is supposed to fill all space. It explains the conduction of heat through solid bodies in the same manner. It explains the expansion of bodies: the atomic motion forces the atoms of bodies farther apart. It explains the production of heat by friction or collision, which no other theory is able to do: the shock of the collision generates this atomic vibration. It explains the production of heat by combustion: the atoms of oxygen and carbon or hydrogen dash against each other and generate heat by the collision. This theory explains the transmutation of motion, or living force, and electricity, into heat, and the transmutation of heat into electric or mechanical force. These points will come up again, and I now only refer to them in answering Samuel’s question. The dynamic theory explains the phenomena of heat and its relations to force, light, and electricity exceedingly well, and for this reason men look upon it with favor and count it as probably true. If in the progress of scientific investigation itshall be found to explain all the new facts discovered and meet well all the demands made upon it, it will at length be received as an admitted principle in physical science. Thewavetheory of light and thevibratorytheory of sound may be looked upon as thus established.
“At our next lesson we shall take a rapid review of the effects and laws of heat.”
HEAT A GIFT OF GOD.
The class is again promptly in place and ready for work.
“As I announced a week ago,” said Mr. Wilton, “we will to-day take a rapid review of the effects and laws of heat. Will you tell us, Peter, the first and chief of these effects?”
“Yes, sir: combustion.”
“What is combustion?”
“Commonly the rapid union of oxygen with some combustible substance, attended with the evolution of heat.”
“Was your answer correct, then?”
“No, sir,” said Peter, blushing; “I spoke before I thought.”
“Will you correct your answer?”
“The first and chief effect of heat is expansion.”
“That is right. Our sensation of heat is ofcourse only asensation—merely thefeelingwhich results from the effects of heat upon our nerves—but the chief physical effect of heat is the expansion of bodies. The chemical qualities of bodies are not changed: they are not made either heavier or lighter. A sufficiently high temperature renders bodies luminous, and then we call them red hot or white hot. Solid bodies begin to be luminous at a temperature of about one thousand degrees. But the one invariable effect of heat, with two or three apparent exceptions, is expansion. You may mention, Samuel, some familiar illustrations of the effect of heat in expanding bodies.”
“The blacksmith heats the wagon-tire in order that it may easily slip over the wheel. If a kettle be filled with cold water, by heating it the water is expanded and runs over. I have noticed that the spaces between the ends of the successive iron rails upon the railroad are larger in winter than in summer, showing that the rails are shorter in winter than in summer. While skating during the cold winter evenings upon the mill-pond, I have seen cracks in the thick ice start and run across the mill-pond with a roar almost like thunder. The ice was contracted bythe cold till it could no longer fill the whole space between the banks, and being frozen fast to the banks, it was torn asunder. The mercury in the tube of a thermometer is constantly expanding or contracting by every change of temperature.”
“Yes, those are all good illustrations, and we might go on to mention others equally good by the score. In cold countries, during the intense cold of winter, the surface of the earth cracks by shrinkage, just as you have seen the ice upon the mill-pond torn in two. The Britannia iron tubular bridge over the St. Lawrence at Montreal rises and falls two and one-half inches on account of greater expansion of the upper surface when exposed to the heat of the sun, while a loaded freight train causes a depression of but one-fourth of an inch. A few years since, in order to make some philosophical experiments connected with the rotation of the earth upon its axis, a ball was suspended by a wire in the interior of Bunker Hill monument. By this means it was accidentally discovered that the heat of the sun, expanding the sides of the monument exposed to its rays, caused the whole monument to sway back and forth daily.”
Here Ansel raised his hand.
“What is it, Ansel?”
“I was going to mention the belief of geologists that the mountain ranges were thrown up by the contracting of the earth’s crust on account of cooling.”
“That is an illustration of contraction by loss of heat on an enormous scale. The materials which form our globe may have existed in the beginning in a nebulous or gaseous state. There is certainly very good reason for believing that the earth was once in a fluid state, the whole of its substance molten by intense heat. It is certain that the interior is now hot, and portions of it molten. It is by very many believed that the whole interior is molten. The crust of the earth may have been formed by cooling. If after an outer crust had been formed, and its temperature had fallen so low as to become nearly stationary, the interior mass continued to cool, the molten mass would tend to sink away from the crust and the crust would sink in upon it by wrinkling. Thus mountains may have been formed. Along the line of fracture the easiest vents would be formed for volcanoes. But this carries us somewhat aside from oursubject, and as the expansion of bodies by heat has been sufficiently illustrated, we will leave it. Will some one now state the manner in which the dynamic theory of heat explains this expansion?”
Samuel answered: “I think you have already given us the explanation.”
“I have briefly referred to it, but you may give it again.”
“The atomic motion which is supposed to constitute what we call heat, whatever that motion be, whether a vibration or rotation or revolution, requires that the atoms of bodies shall not be packed in absolute contact, and the more intense the agitation or the wider the swing of the vibration or revolution, the greater must be their separation. Hence heat expands bodies by thrusting their atoms farther apart.”
“That will do,” said Mr. Wilton. “Let us look now at some of the secondary effects of heat. You may mention some of them, Ansel.”
“Heat relaxes or overpowers the cohesive attraction of bodies.”
“What is cohesive attraction?”
“It is that force which binds together theatoms of matter in simple substances, that is, bodies like iron or copper or silver, composed of but one kind of substance, or in compound bodies it is the force which unites the compound molecules of matter.”
“Give us now some illustrations of the effect of heat in overcoming cohesive attraction.”
“The blacksmith heats his iron in order to overcome its cohesive attraction and render it soft, that he may easily hammer it. The founder heats his metal till its cohesion is so far destroyed that it becomes fluid and can be poured into the mould. Heat relaxes the cohesive force of ice and changes it to water, and by farther heating its cohesion is entirely overcome and the water is changed to a gas.”
“We use heat also in cooking our food,” spoke up Peter: “is it not because heat destroys the cohesive attraction, and thus softens it?”
“If that were the only effect of heat upon food,” said Mr. Wilton, “we should be obliged to eat our food hot, for as soon as it cooled the cohesion would return and the food would be raw again. The operation of heat in cooking is various, and part of the effect is commonlyto be ascribed to the water in which the food is cooked or to that which is contained in it. By the combined agency of heat and water starch swells to twenty or thirty times its original bulk and the minute starch grains burst open. In cooking potatoes the starch of the potato absorbs a portion of the water that is in it, and thus renders it dry and mealy. The action of heat and water upon rice, wheat, and other grains is similar to their operation upon starch. In the baking of bread the starch is converted into gum. In boiling flesh the effect is partly due to the solvent powers of water: the juices of the flesh are extracted, the gelatin is dissolved, the fat is liquefied, and the cells in which the fatty matter is held more or less burst, the albumen is solidified, and by long boiling the texture and fibre of the flesh are destroyed. The albumen of an egg, that is, the white, coagulates by heat. But in most of these processes the action of heat cannot be separated from that of water.
“But there is another effect of heat very important both in nature and in the arts. What is that?”
“The quickening of chemical affinity,” answered Samuel.
“That is right: heat is necessary for the operation of chemical affinity. Perhaps this is only a weakening of the cohesive force, thus allowing the chemical attractions to assert their strength. But the fact is that, while in many cases the chemical affinities act with great energy at ordinary temperatures, in other cases they slumber, however closely the substances are brought into contact, till their temperature is raised. Samuel, you may mention some illustrations of this principle.”
“A few months ago I visited Hazard’s powder mills, in Enfield, Connecticut, and there learned how gunpowder is made. The charcoal, the sulphur, and the nitre are first finely pulverized, then ground together for hours till thoroughly mixed, and afterward pressed together. This mass is then broken into grains and the grains polished. But though these elements are brought into so close contact, yet they do not combine and explode till heat is applied. The same is true of the combustion of wood and coal. The carbon and the hydrogen of the fuel are constantly surrounded with the oxygen of the air, but they do not take fire and burn, that is, they do not combine with theoxygen, till they are raised to a red heat, or perhaps even to a higher temperature. If a stove filled with burning coal be cooled down to a low temperature by applying ice, the combustion will cease, the fire will go out. Our teacher at the academy on one occasion heated a steel watch-spring red hot and plunged it into a jar of oxygen, and the steel spring began quickly to burn with great fury.”
“You have given us good illustrations, Samuel, and that which is true of carbon and hydrogen and oxygen is true of substances in general. The effect of heat in producing chemical changes is very important everywhere. It is seen not only in the chemist’s laboratory and in the artisan’s shop, but also in the laboratory of Nature. Plant a grain of corn in midwinter: why does it not germinate and grow? Nothing is needed but the requisite heat to quicken the chemical affinities into action. Earth and air furnish the needed material for the growth of forest trees in winter as well as in summer, but the cold holds in check the chemical forces and prevents the requisite chemical combinations. No sooner does the sun quicken that atomic vibration or revolution which we call heat thanvegetable growth begins. Heat is necessary for those chemical changes by which food is digested in the stomach and the processes of nutrition carried on in every part of the body. If a man finish his dinner with ice cream or ice water, the process of digestion is delayed till the contents of the stomach recover their proper temperature. This is one chief reason why warm, comfortable clothing is so very important, especially for children. All the vital processes are chemical processes: they are carried on through chemical affinities. Unless the body be kept at a suitable temperature, these processes are feeble and imperfect, nutrition and vital combustion are hindered, and diseases are engendered.
“These, then, are the chief effects of heat. It expands bodies, weakens cohesive attraction, and quickens the chemical affinities into activity.”
Ansel again raised his hand.
“What do you wish?”
“Will you please tell us, Mr. Wilton, how this weakening of cohesive attraction is explained upon the dynamic theory of heat?”
“I will do so with pleasure. The increased atomic motion in the heated body throws the atoms farther apart, as we have already learned,and by this increase of distance their attraction is diminished. If the earth were twice its present distance from the sun, their attraction for each other would be four times less than it now is; if its distance were three times as great, their attraction for each other would be nine times less. The attraction of gravitation diminishes in proportion as the square of the distance through which it must act increases. Perhaps cohesive attraction diminishes according to the same law, though the spaces are so small that this cannot be demonstrated, but it is certainly weakened by the expansion of bodies through the agency of heat.”
Here Peter raised his hand.
“What will you say, Peter?”
“Do not men heat and burn bricks, not to soften them, but to harden them?”
“That is true,” said Mr. Wilton; “but in this there is a process of drying as well as of heating, and the hardening is due chiefly to the complete drying by the intense heat. Too great heat will melt bricks while in the process of burning. I once heard a brick-burner say that he could melt the brick around the arches in his kiln in half an hour, if he pleased to put in fueland let the fire burn. Indeed, almost every known solid substance has been fused by heat. Whether carbon has ever been melted is an unsettled question.”
“I would like to inquire,” said Samuel, “why water will not burn. Is it because it evaporates before it reaches a sufficiently high temperature?”
“This is a little aside from our subject, but the incombustibility of water is a provision of the Creator so very important that we will stop to notice it. I think, however, that by a little thought you yourself can answer the question. Tell me again what combustion is.”
“Combustion is commonly the combining of oxygen with some other substance called a combustible. The rusting of iron and the decay of organic bodies are forms of slow combustion.”
“Now tell us the composition of water.”
“Water is composed of oxygen and hydrogen—eight parts of oxygen to one of hydrogen, by weight, or two parts of hydrogen to one of oxygen, by measure.”
“How is water formed from these two gases? Are they mixed together as oxygen and nitrogenare mingled in the air, or are they chemically united?”
“They are chemically united: they are burned together. When hydrogen burns, the product is water.”
“Water is then aproductofcombustion. Can you not now tell why water is incombustible?”
“I think I now see the reason. The oxygen, being itself the supporter of combustion, will not burn, and the hydrogen has been already once burned in the formation of water.”
“And that which is true of water is true, in a greater or less degree, of other products of combustion. The burning of charcoal produces carbonic acid, and carbonic acid will not burn because it is the production of combustion. A candle is extinguished by it as quickly as by water. By a recent invention carbonic acid is used to extinguish conflagrations. The carbon has once united with oxygen, and a second combination with an additional amount, or, as a chemist would say, with another equivalent, of oxygen is much more difficult.”
“I think,” said Samuel, “I now understand why water will not burn, but will you pleasealso to tell us why water puts out fire better than almost anything else?”
“In order to extinguish fire one of two things must be done: either the supply of oxygen must be cut off or the combustible must be cooled down to a temperature below the burning point, when the combustion will cease of itself. When we shut the draught of an air-tight stove, we check the combustion by shutting off the full supply of oxygen. If we could wholly prevent the access of oxygen to the fuel, the fire would at once be extinguished. If oxygen should then be admitted again before the fuel had cooled down below the burning point, combustion would at once begin again. A blazing brand is extinguished by being thrust into ashes, because it is shut away from oxygen. In the same way we extinguish the flame of a candle with a tin extinguisher. On the other hand, fires often go out because the necessary temperature is not maintained. Water puts out fire in both these ways, but especially by the second. Water poured in torrents from a fire engine upon a fire forms a film of water, and the burning material shuts out the oxygen. But the water acts chiefly by lowering the temperature. Noother known substance except hydrogen gas requires so much heat to raise it through a given number of degrees of temperature as water. As much heat is required to heat one pound of water as thirty pounds of mercury. Hence, water poured upon burning timber cools it to so low a temperature that it ceases to burn.
“In addition to this, we may notice that wood saturated with water cannot be heated above the boiling point of water till the water is evaporated. As fast as the wood and the water rise or tend to rise above two hundred and twelve degrees, the water changes into steam and carries away the additional heat. The consumption of heat in the formation of vapor we must look at more carefully in a future lesson. We will suppose that a house is in flames. A fire engine throws a stream of cold water into the midst of the conflagration. The cold water, dashing against the burning wood, cools the heated surface; it is absorbed into the pores of the wood and hinders its rapid heating; a portion of the water, being changed into steam, carries off the heat; the steam, mingling with the flame, lowers the temperature of the burning gas, and in proportion as steam fills the surrounding space oxygenis driven away. A burning coal mine in England was once extinguished by forcing steam into it, thus driving out the air which supported the combustion and cooling down the burning coal.
“The advantages which men receive from these agencies of heat are so manifest that we cannot help noticing them. I do not refer to the comfort of a pleasant temperature, nor the impossibility of living in a temperature extremely low, but to all those processes by which man subdues nature, provides for himself food, clothing, and dwelling-places, and builds up civilization. Heat is that force which enables man to accomplish his ends. Heat brings the iron from the native ore, and heat renders it malleable and plastic to be shaped for man’s uses. Heat quickens the chemical affinities and renders the arts of civilized life a possibility. Heat brings together oxygen and carbon in ten thousand furnaces, and the heat engendered by the combustion, changed to force, drives the ponderous or nimble machinery which carries on the work of the world. Heat quickens the chemical affinities and causes the wheat to grow; heat prepares the wheat for man’s food;and by the aid of heat that food is changed in man’s body, nutrition goes on, the body is built up, waste matter is removed, and all the vital processes are supported. Without these agencies of heat—softening and subduing stubborn matter on the one side, and quickening its forces on the other—man could not exist.
“Let me remind you that these agencies of heat are of God’s devising. If the operations of heat are beneficent to man, it is because God wished to bless his creatures. I am not much given to moralizing, but when I see how completely these simple effects of heat meet man’s wants, I cannot help remembering and admiring the wisdom of the great Designer. It isGodand not blind, unconscious Nature that is working.”
“This reminds me,” said Samuel, “of the tradition in Greek mythology that Prometheus stole fire from Jupiter and brought it down to man in a reed as a precious treasure. It seems to me like a gift from heaven.”
“This mythological tradition has, however, one falsehood: there was no need that men should steal fire from the gods; God freely gave it. Heat is indeed a gift from heaven.”