"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 ill.u.s.tration of the convection of heat," said Mr. Wilton, "is seen in the common method of heating water. The heat is applied at the bottom of the vessel 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 ill.u.s.tration is seen in winds and ocean currents. Warm winds carry heat enough to warm a continent, and the mighty 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, pa.s.s 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, 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 pa.s.ses through certain substances. It pa.s.ses through gases and through some liquids and some solids. The best of gla.s.s, though it is so solid, interposes very little hindrance to the pa.s.sage of light. Heat in like manner radiates through certain solids. Luminous heat is radiated through gla.s.s. Rock-salt transmits dark heat also. A plate of alum permits light to pa.s.s, 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 pa.s.ses. I have seen boys make burning-gla.s.ses of ice. The heat pa.s.ses through them and burns that upon which it is concentrated, while the ice itself through which the heat pa.s.ses 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 inverse ratio to each other; the better the 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 _concealed_. 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 otherwise than in raising the temperature. You will understand this best by an ill.u.s.tration.

"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 trans.m.u.ted 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 cohesive attraction of the molecules of water and changing the liquid to a gas. About one thousand degrees of heat is thus expended, but 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 molecular attractions is trans.m.u.ted 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 trans.m.u.ted to force or force to heat.

"These trans.m.u.tations 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 vapor, and in raising that vapor to the higher regions of the atmosphere. This heat-force, or, as we might call it, 'sunpower,' expended upon the earth, amounts to thousands of millions of horse-power daily.

[Ill.u.s.tration: TRANs.m.u.tATION of HEAT.

Page 113.]

"Examples of the trans.m.u.tation of force into heat abound 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. Savages who have no better methods of kindling fire rub dry wood together till the sticks ignite. The force expended in overcoming the friction is changed to heat. In the combustion of coal beneath the steam boiler 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 trans.m.u.ted to force, and drives the engine and the machinery attached. In our study of G.o.d'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 trans.m.u.tation 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 anvil be struck with a hammer, or if a bar of iron or copper be moved back and forth 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 pa.s.sing through an imperfect conductor, produces heat sufficient 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 trans.m.u.ted the one into another.

"By this principle of the trans.m.u.tation 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 ill.u.s.tration 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 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 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 sufficiently uniform to establish this supposition.

"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 G.o.d'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 G.o.d 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 principles, is one of G.o.d's works. Already have we been looking at the Creator's handiwork. Already have we been trying to trace out the thoughts of G.o.d as they are written in the "Bible of Nature." The thoughts of G.o.d 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.

CHAPTER VI.

MANAGEMENT AND SOURCES OF HEAT.

While the lessons which have been reported were going on, the religious interest in the church was deepening. Mr. Wilton did not cease to make his sermons instructive, but, in addition to the instruction, he made them more and more pungent and persuasive. He aimed to gather up the impressions and convictions already wrought in the minds of his hearers and combine them for united and immediate effect. He believed that this was to be a reaping-time.

Mr. Hume was becoming interested, not because he had been at church, for he had not been there, but the Holy Spirit of G.o.d was working upon his heart. He was becoming uneasy in his unbelief. For some reason, he knew not why, his opinions were becoming more and more unsettled. He did not like to go to the house of G.o.d; his self-will and pride of consistency rebelled against the thought of hearing and believing the gospel; but he was restless and discontented away from the place of worship. His a.s.sociations with his infidel comrades grew distasteful. His Sundays were days of distress: with his attention relieved from business cares, thoughts of G.o.d and eternity pressed upon him, and he could not escape them. At length he determined to go and hear Mr. Wilton again: perhaps he should hear something which he could so positively reject as to set his mind at rest. He went, accordingly, the next Lord's Day, and heard a very impressive sermon.

The text for the forenoon was Ps. lxvi. 5: "Come and see the works of G.o.d: he is terrible in his doing toward the children of men." The sermon gave first a brief and rapid review of some striking displays of G.o.d's displeasure at the sins of men: that ancient world of men whose "thoughts were only evil continually" he overwhelmed with the flood; he burned with fire from heaven Sodom and Gomorrah, Zeboim and Admah, those lascivious and festering cities of the plain; he sent his torturing and consuming plagues upon the Egyptians, and sunk the army of Pharoah like a stone in the deep waters of the Red Sea: "they sank as lead in the mighty waters;"

he caused the earth to open and receive Korah and his adherents, and bade his angel in "one night" to touch with death the thousands of Sennacherib's army. This record of divine wrath against evil-doers has startled the consciences of wicked men, and will continue to startle them so long as the unG.o.dly live upon the earth. It is easy for unbelievers to call the word of G.o.d a record of fabulous wonders, but that record lives and will live, and its words a.s.sert their divinity by touching and burning the consciences of men as if they were tongues of fire.

"But to the thoughtful man," said Mr. Wilton, "there is a manifestation of G.o.d's displeasure at sin even more impressive than these miraculous judgments. The Creator has built his wrath against sin into the very fabric of the universe; he has written it upon the very atoms and elements of matter and of mind, and graved it upon the 'nature of things.' The forces of Nature are all instinct with holy wrath against unG.o.dliness.

Evil doing works out evil consequences by the regular course of nature.

Babylon, Nineveh, and Tyre were great and prosperous, and as mighty in wickedness as in commerce and war. In the height of their prosperity G.o.d denounced upon them disaster and desolation, and by the natural processes of evil their decay and destruction came upon them. No miracle broke the harmony of their mighty march to decay and the silence of death. Great nations have perished, but not till they became corrupt. Rome fell, but luxury first gendered luxuriant vices, and vices enervated her hardihood and undermined the defences of her courage. No righteous nation ever perished. No nation ever fell into decay till ripe in sin and ready for moral putrefaction. But against wicked and corrupt nations wars and desolations are determined, and the end thereof is with a flood. The very forces of Nature seem allied in firm compact with the laws of G.o.d, ready with resistless hand to avenge their transgression and to visit evil upon evil-doers. This steady march of all the forces of the world in bringing decay and wretchedness upon sinners is more impressive than any single desultory example of avenging wrath.

"But perhaps an unbeliever replies, 'Not so; there is a natural law of development, decay, and death, apart from sin. Trees grow up, become old, and die. Men pa.s.s from childhood up to manhood, and from manhood down to second childhood, and return to the dust whence they came. By a like principle, nations pa.s.s through similar changes of development, decay, and desolation. But in all this there is no manifestation of divine favor or disfavor.'

"This is narrow and false reasoning. If a single great city had become corrupt while all the world beside remained righteous, and G.o.d had denounced his displeasure upon it and had executed his wrath by sudden and tremendous judgment, that one city standing out in single and solitary unG.o.dliness and desolation, who would deny, who could deny, that the fate of that unhappy city was a manifestation of divine displeasure? If a second example were made of a second unG.o.dly city, would the expression of divine wrath be weakened? Nay; every man would say that it is made stronger. What if a third example be made of a third city? What if every wicked city is made an example? What if G.o.d embody his displeasure at evil-doing in the structure of the world, and give to the very atoms of matter and the elements of mind such natures that by the working of their own proper forces, without a miracle, they shall bring pain and evil, decay and death, upon the unG.o.dly? What is this but writing his wrath against sin upon the earth and sky, upon matter and the consciences of men, declaring by this that till the heavens and the earth and the spirits of men be no more he will never withdraw his indignation? This is what G.o.d has done. The wicked man sets in motion the machinery which works out his own everlasting undoing. His own hand sows the seeds of death, and as those seeds germinate they strike their roots into his corruptions and draw their nourishment from his evil life. Thus do sinners go on 'treasuring up wrath against the day of wrath and revelation of the righteous judgments of G.o.d.'

"But remember that G.o.d has not left the world in these later ages without the testimony of wrathful judgments which ought to startle and alarm the consciences of the wicked like the fires of Sodom. Let me give you what I suppose to be a true record of the fate which befell a band of bold blasphemers. In that uprising of infidelity which took place near the close of the last century there was formed at Newburg, N. Y., through the influence of a man known as 'Blind Palmer,' an a.s.sociation of infidels under the name of the Druidical Society. The object of the society was to uproot and destroy revealed religion. In pursuit of this object they descended to the most blasphemous mockery. At one of their meetings they burned the Bible, baptized a cat, partook of the bread and wine as appointed for the ordinance of the Lord's Supper, and gave the elements to a dog. Then the wrath of G.o.d broke out upon them. 'On the evening of that very day he who had administered the mock sacrament was attacked with a violent inflammatory disease; his inflamed eyeb.a.l.l.s were protruded from their sockets; his tongue was swollen, and he died before morning in great bodily and mental agony. Dr. H----, another of the same party, was found dead in his bed the next morning. D---- D----, a printer who was present, three days after fell in a fit, and died immediately. In a few days three others were drowned. Within five years from the time the Druidical Society was organized all the thirty-six original members--actors in the blasphemous ceremonies spoken of--died in some strange or unnatural manner. Two were starved to death, seven were drowned, eight were shot, five committed suicide, seven died on the gallows, one was frozen to death, and three died, the record says, _accidentally_.' Be sure of this: G.o.d has not left the world nor forgotten his judgments against his enemies, neither is he tied up and hampered by the laws of Nature. 'G.o.d is angry with the wicked every day. If he turn not, he will whet his sword: he hath bent his bow and made it ready. He hath also prepared for him the instruments of death.'