TRANSPORTATION OF HEAT.

To-day we come to that subject which we should have looked at a week ago, if that I hope not unprofitable discussion of the uses of trials and the ministry of pain had not prevented. We must now examine the arrangement for softening the rigors of winter and toning down the heat of summer. The general principle is that in summer the earth receives an excess of heat, while in winter the opposite is true. These extremes are mitigated by transferring heat from summer to winter. How is this accomplished? Any one who has thoughts upon this subject may answer.”

“I have some thoughts,” said Ansel, “but whether right or wrong, I cannot tell. I should think heat might be carried from summer to winter in the same way as from day to night.”“What are some of those means for transferring heat which seem to you to operate the same in the annual as in the daily changes of temperature?”

“One is the absorption and radiation of heat, and another is the evaporation of water and the condensation of vapor.”

“You are right,” said Mr. Wilton. “The effect of these operations in the equalization of the annual extremes of heat is in no wise different from their effect upon the temperature of day and night, but from summer to winter their effect is vaster and more impressive. During the summer, sea and land, and ‘all that in them is,’ are receiving heat and rising in temperature. The heat of summer penetrates and warms the earth nearly a hundred feet in depth. Into the sea heat penetrates still deeper. How vast the amount of heat required to warm the whole surface of the earth and sea to such depths! By withdrawing so much heat from active use the intensity of the summer temperature is softened. During the colder months the land and sea slowly radiate their heat. We can hardly over-estimate the effect of this alternate absorption and radiation of heat. So great is the effect of this stored up heat that the sea and the great lakes never freeze even in the coldest winter weather, except in the polar regions, and the temperature must fall far below freezing and continue for a long time below the freezing point before the earth begins to freeze. The great bodies of water, remaining always at a temperature above thirty-two degrees, are especially important in warming the wintry air. In the coldest weather they seem like steaming caldrons throwing up their warm vapor. It is the absorption and radiation of heat alone which prevent the temperature of the atmosphere from rising or falling suddenly to the highest or lowest point possible. The sun breaks forth in all its splendor at noonday in summer: what if the sun were to remain stationary, shining thus in his strength for days and months? Everything would be consumed with heat. But why do not the glowing rays of the sun raise the temperature at once to the highest possible point? Because the earth and sea and every object upon the earth absorb the heat, storing it up and holding it in reserve. On the other hand, when the sun sets and his heat is withdrawn, why does not the temperature fall suddenly to the lowest possible point? Because the heat held in store is slowly radiated and the change of temperature rendered gradual.

“In this work of absorbing and radiating heat every object, earth, air, and sea, does its appropriate share. But water stands chief, and performs the largest service. Its high specific heat enables it to hold in store the largest calorific treasure, and causes it to change its temperature more slowly.

“The formation and condensation of vapor also operate in the same manner as in the transitions of day and night. During the summer the higher average temperature makes it possible for a much larger amount of vapor to be formed than in winter. You remember that at eighty degrees vapor equal to thirteen inches of water can sustain itself, while at thirty-six degrees the elastic force of vapor is equal to the pressure of only two inches and two-fifths of water, and at four degrees to three-fifths of an inch. If the mean summer temperature at any place were eighty degrees, it would be possible for more than one foot of water to be held in the form of vapor. In the formation of this vapor heat would be consumed sufficient to boil more than five and a half feet of ice water. If the mean winter temperature at the same place be thirty-six degrees, more than three-fourths of this vapor must be condensed and give out its latent heat to warm the air. It is not to be supposed that the full amount of vapor which can support itself does commonly exist, but the difference between the average amount of vapor in summer and in winter must be very great. I suppose this difference often amounts to four or six inches of water. If we suppose it to be four inches, an amount of heat is transferred from summer to winter sufficient to boil twenty-two inches of ice water. In estimating the effect of this we must consider that this heat is not given out gradually and regularly for three months, but whenever there is a sudden fall of temperature vapor is condensed, latent heat becomes sensible, and the suddenness and intensity of the fall are diminished. We need also to bear in mind that every open body of water is sending up its clouds of vapor constantly. The open lakes, and especially the sea, are like a seething caldron; and thus immensely more vapor is condensed during the winter months than is brought over from summer to winter. Much of the vapor formed in winter is to be set to the account of summer, for it is the summer’s heat absorbed by the water, which maintains its temperature and enables it to throw up such clouds of vapor, even in midwinter. But this comes in more properly at another place, and we will leave it for the present.

“There is another transition experienced by water by which heat treasured up in summer is made available for softening the rigors of winter. Who will suggest it?”

“It is the freezing of water,” said Mr. Hume. “In the process of crystallization one hundred and forty degrees of latent heat become sensible.”

“And this,” continued Mr. Wilton, “is no inconsiderable matter. Every pound of water frozen upon the surface of our lakes and rivers, every pound of water frozen in the wet earth, every pound of water frozen as snow or sleet in the air, gives out as much heat as would boil an equal amount of water at seventy-two degrees. Have you never heard of setting tubs of water in cellars to keep vegetables from freezing?”

“I have,” replied Peter. “I visited my grandfather two years ago, and his cellar sometimes froze. I asked him why he put tubs of water in his cellar, but he could not tell me, only he said that he knew that tubs of water in his cellar did keep his vegetables from being nipped with the frost.”

“Can you tell us, Peter, why tubs of water set in a cellar should have this effect?”

“I suppose that when the water begins to freeze it begins to give out its latent heat.”

“That is one part of the reason. The water is drawn from the well at perhaps fifty degrees; it must lose eighteen degrees of heat before it begins to freeze, and all the heat which the water loses the air of the cellar gains. And then, as you said, as soon as the water begins to freeze latent heat begins to become sensible. Every pound of water frozen sets free heat enough to raise a pound of water through one hundred and forty degrees. But why do not the vegetables begin to freeze as soon as the water?”

“I don’t know.”

“Water holding salt or other minerals in solution freezes at a lower temperature than pure water. For this reason the juices of vegetables and fruits and the sap of trees may be cooled below thirty-two degrees without freezing. On this account the water set in cellars tends to prevent vegetables from freezing; the water begins to freeze at thirty-two degrees, while potatoes and turnips may be cooled a little lower than thirty-two degrees without harm. In this manner the buds of trees are sometimes warmed and protected by the coating of ice which forms around them. The drops of water, falling through the sleety air, touch upon the twigs of trees and freeze upon them, an icy coat embracing them all around. In freezing, the water gives out one hundred and forty degrees of heat, a part of which goes to the air and a part to the twig.”

“This reminds me,” said Ansel, “of what the Irishman said on being told that snow contains heat, that ‘it would be a blessed thing for the poor if one could tell how many snowballs it would take to boil a tea-kettle.’”

“It might be difficult to use snowballs to boil the tea-kettle, but the heat given out in the formation of the snowflakes is doubtless employed quite as usefully for the poor as if used in preparing their tea. You have all noticed that before a snow-storm, or perhaps during the early part of the storm, the temperature generally becomes milder, and you have often heard the remark, ‘It is too cold to snow.’ Men have learned that the coming of a snow-storm is attended by a warming of the air. This popular impression is philosophical, yet few understand its philosophy. A foot of snow falls, equal to two or three inches of water. In the condensation of the vapor which formed this snow one thousand degrees of latent heat become sensible, and then in the congelation of the clouds into snowflakes one hundred and forty degrees of heat are evolved. This softening of the rigors of winter is, I think, as great a blessing to the poor as the heating of the tea-kettle. Let us make an estimate of the amount of heat set free in the production of one great snow-storm. Two feet of snow falls, equal, we will suppose, to five inches of water. In the condensation of the watery vapor one thousand degrees of heat are evolved, and in the congelation one hundred and forty degrees—an amount of heat which would boil three feet of cold spring water. In every square mile there are 27,878,400 square feet, and a square mile of water three feet in depth would contain 83,625,200 cubic feet. The production of such a snow-storm sets free for every square mile of surface heat which would boil more than 80,000,000 of cubic feet of spring water. Such a storm sometimes extends over a region of country a thousand miles square, that is, over a million of square miles. In the production of one such storm—a very heavy and extensive storm, I have supposed—heat is generated which would boil eighty millions of millions (80,000,000,000,000) of cubic feet of spring water—an amount altogether too vast for our comprehension. To accomplish this result by combustion would require more than 500,000,000 of tons of anthracite coal—an amount at least three times as great as the yearly product of all the coal-mines of the world. And this is but one heavy storm. The amount of rainfall in the United States may be thirty-six inches or forty or forty-five inches. Supposing the average rainfall of the whole earth to be twenty-four inches—an estimate very far below the truth—we have this result: There are, in round numbers, two hundred millions of square miles of surface, more than five and a half quadrillions (5,575,680,000,000,000) of square feet and more than eleven quadrillions of cubic feet of water. The condensation of this amount of vapor would boil more than sixty quadrillions of cubic feet of ice water. One pound of anthracite coal burned under the most favorable circumstances will boil sixty pounds of ice water. To boil sixty quadrillions of cubic feet of ice water would require sixty quadrillions of pounds of coal—thirty billions of tons—not less than twenty-five tons to every inhabitant of the globe. At this rate a very few years would exhaust the coal-fields of the world. Calculations like these are useful in showing upon how stupendous a scale the Creator carries on his operations. But we must remember that these works are carried on, not to amaze men, but to benefit them. The works go on silently and unseen, challenging no attention from fools, receiving no thought except from the patient student of Nature, and eliciting no thankful recognition save from a few reverent worshipers.

“But I have been led away from a point which I had in mind. While considering the effect of heat in expanding bodies, I reminded you that water presents a marked peculiarity, and promised to speak of it more fully. This is the place for us to look at this singular and beautiful peculiarity of water. What is the general principle touching the effect of heat upon bodies?”

“Heat expands bodies and cold contracts them,” answered Ansel.

“Water both illustrates this rule and presents some very interesting apparent exceptions. It contracts by cold like other bodies till it reaches the temperature of thirty-nine and a half degrees; it then begins to expand, and expands regularly till it falls to thirty-two degrees; at that point it freezes, and in freezing it expands at once about one-ninth of its bulk. If the cooling process be continued, the ice produced contracts like any other solid. This peculiarity of the interrupted and unequal expansion of water is of the utmost importance in the affairs of our world. Consider the result if the water were to contract by cold as do other bodies down to the freezing point and below it. Water is cooled from the top by contact with the cold air. As the upper film of water cooled it would sink and a new stratum be brought to the surface; that in turn would be cooled and sink, and thus the cooling process would go on with the utmost rapidity till the whole body of water should be reduced to the freezing temperature. Then congelation would begin, and the first particles of ice formed would sink to the bottom, and as fast as the water became frozen at the top the ice would sink. In this manner a solid body of ice would be formed at the bottom of our lakes and rivers, while the surface would remain unfrozen in contact with the cold air till the whole body of water became a compact mass of ice. Great lakes turned to solid ice would not be thawed during the whole of the summer, for the water warmed from the top would not sink, but would form a warm stratum of water upon the surface, while, below, the solid ice would lie hardly feeling the summer heat. Nay, more; in the higher latitudes it would seem as if the very ocean must be turned to solid ice, never to be melted till the end of time. By the singular expansion of water below thirty-nine and half degrees and its great expansion in congelation, these disastrous consequences are prevented. Our lakes are cooled even in winter only to thirty-nine and a half degrees; below this temperature the colder water is lighter and remains upon the surface; ice floats upon the surface. The top becomes ice, but the great mass of the water remains at thirty-nine degrees, and the inhabitants of the waters live on unharmed. Spring comes, and the ice, being upon the surface, is soon melted, and the unbound waves begin again to ripple forth their unconscious joy.”

“Do you look upon this irregular expansion and contraction of water,” asked Mr. Hume, “as a real exception to the rule that heat expands bodies?”

“Not at all. In freezing, a new force comes in and asserts itself—the force of crystallization; or, more exactly, as the force of heat diminishes the force of crystallization becomes predominant, and throws the atoms into new positions and new relationships. To this new arrangement of atoms is due the expansion in freezing. Ice contracts and expands by cold and heat the same as any other solid. The attraction of crystallization begins, doubtless, to throw the atoms into their new and crystalline arrangement at the temperature of thirty-nine and a half degrees.

“We must remember that the heat which is set free in the condensation of vapor and in the freezing of water is absorbed in the formation of vapor and the melting of water. As much heat is taken from summer as is conferred upon winter. The summer is cooled as much as winter is warmed. The formation of vapor is a cooling process. Water is prevented from rising above the boiling point by the formation of vapor. Perspiration cools us by the evaporation to which it gives rise from the whole surface of our bodies. And the higher the temperature, the more rapid the evaporation, and the more vigorous the cooling process.

“We might look at other appliances for transferring heat from summer to winter, but they belong in principle to another department. We have now looked at some of the means for transferring heat in time. The heat is treasured up at the heated noonday, to be brought out for use during the cool hours of night; it is garnered from the excessive heats of summer to supply the deficiencies of winter. It is laid up in store to-day to be expended at any future time when needed. The transfer is a transfer not in space, but in time. We must hereafter examine those arrangements by which heat is transported through space. Some of these arrangements exert an influence upon day and night and upon summer and winter, and thus throw further light upon the subjects already discussed. Already more than once topics have been suggested and their full consideration put off till some more fitting time. In our next lesson we must begin the examination of these new principles. We have before spoken of the vicissitudes of days and seasons and years. We shall now have to do with the vicissitudes of zones and lands and seas, of deserts and mountain ranges. The elements become vaster, the stage is broader, and the movements more sublime.

“I am glad that you are so well interested in these great and beautiful works of God’s wisdom and power, but I hope that you do not forget that the crucified Christ is pre-eminently the power of God and the wisdom of God. These natural works are but the husk of which salvation from sin by Christ is the kernel. These outward things are wonderful and beautiful for the setting, but the gem, the royal precious stone, the Koh-i-noor, the ‘mountain of light,’ for which the setting was made, is the true knowledge of the true God and of his Son Jesus Christ. During the past few weeks you have heard others asking, ‘What shall we do to be saved?’ I should be greatly guilty if I allowed you to think earth, air, and sea, with all their silent and solemn movements, more important than our spiritual attitude toward God the Father and Christ the Saviour. Are you, Samuel, in your interest in studying Nature, forgetting Christ and the souls of men?”

“I hope not, and I think not. During the three years since my baptism I have never felt so much my obligation to Christ as now. I never felt before so deep a desire that my friends should repent and believe in Jesus. I think the love of Christ constrains me. I have not felt before that my work was very important; I have been expecting to work more earnestly by and by; but lately I have felt that Christ gives me something to do now for which he holds me responsible.”

“What have you tried to do for Christ?”

“I have been praying for some of my young friends, and especially for Ansel and Peter. And then I felt that I must talk with them as well as pray for them.”

“And can you, my young friends, be careless about your own salvation while Samuel is so anxious for you? Are you contented to live ‘having no hope and without God in the world’? Is your happiness here and hereafter more important to Samuel than to yourselves?”

“We are interested,” said Ansel. “We have been talking together about being Christians, but we don’t know what to do.”

“They said,” broke in Samuel, “that they wished I would ask you to preach a sermon and tell them what they must do to be saved. They wished to go on with these lessons, but they thought that perhaps you would be willing to preach a sermon just upon that subject.”

“You know that I often speak of that subject, and when persons have come to the inquiry-meeting I have told them what they must do. But I know that there must needs be ‘line upon line.’ If Ansel and Peter wish it, I will devote a sermon to the subject, and make it as plain as I can. Hardly anything gives me more pleasure than to explain the way of salvation when I know that my hearers are interested.”

“We do wish to have you preach upon that subject, and I am sure that you will have a great many interested hearers besides Ansel and myself.”“But, Samuel, did you not pray for Mr. Hume also, and talk with him?”

“I prayed for him, but I was afraid to speak with him. I have tried to pray for him a double portion because I could not speak with him.”

Tears gathered in Mr. Hume’s eyes; the thought came to him that his unbelief had raised a barrier between himself and both God and his people. This pious young man was afraid to come to him lest he should meet the scornful arguments and cold derision of a proud unbeliever. He felt humbled—he, a subtle, well-read unbeliever, and Samuel a pious lad yearning for the salvation of his soul, but daring only to pray in secret for him.

“Have not you, Mr. Hume, been treating Christ and the Holy Spirit as Samuel feared that you would treat him?”

“Perhaps so,” he answered. “I am sorry that Samuel did not come to me freely. I think he need not be afraid of me now. I also hope you will preach the sermon which Ansel and Peter wish to hear.”

Mr. Wilton assured them that he would do as they wished unless the Spirit clearly drew him to some other subject. “I always look,” he said, “to the Holy Spirit for direction in my preaching. ‘When he, the Spirit of truth, is come,’ said Jesus, ‘he will lead you into all truth.’ This was fulfilled pre-eminently, I suppose, in the inspired men who laid the foundation of the Church, but the Spirit still dwells in believers and leads those who love and follow Christ. The preacher of the gospel can do nothing without the power of the Spirit of God.”

And I, kind reader, will give you the outline of the sermon if the Spirit bids him preach it.