Curiosities of Heat

by Lyman Beecher Tefft

“We ought at this point to look at those adjustments by which the earth receives just the amount of heat needed to maintain the requisite temperature. The importance of maintaining some certain average temperature cannot be over-estimated. Every animal and plant has its own habitat—that is, its natural dwelling-place or location—outside of which it perishes or maintains a stunted and precarious life. The habitat of animals and plants depends in a very great degree upon temperature. What a panorama would be seen if we could fly like a bird from the equator to the poles, and look down upon the ever-changing animal and vegetable life as we pass! How the luxuriant vegetation and flaunting colors of the tropics would shade off into the scantier vegetable life and more sober hues of the temperate zones, and these in turn die out and disappear in polar barrenness! We should see the lion and tiger give place to the bear and the wolf, the elephant and camel to the ox and horse, and these to the white bear and reindeer. This sublime panorama we see, in miniature, in ascending lofty mountains in the tropics. Around the base of the mountain flourish the rich and various productions of the torrid zone; a few thousand feet of elevation bring us among the productions of the temperate zones. The most valuable fruits and grains thrive. Then vegetation becomes scanty and stunted, and at last disappears. The top of Mt. Washington, 6234 feet high, in latitude forty-four degrees, is as bare of trees and plants and every form of vegetation as the north pole.

“The fitting temperature is almost as necessary to the animal tribes as to vegetable life. Animals which are native to the tropics do not thrive in colder countries, or if the difference of temperature be very great, they perish. A change from a cold to a warm region is equally disastrous. Man indeed transfers animals from their natural habitat by protecting them from the extremes of temperature, but this is, of course, no exception to the general principle of which I am speaking. A change of only a few degrees in the mean annual temperature would render this earth a hard place for even the human race to subsist. But the temperature of the earth depends upon many a wise adjustment—how many, we cannot tell. Will you tell us, Samuel, the first adjustment or arrangement upon which the temperature of the earth depends?”

“It must depend chiefly I think upon the intensity of the sun’s heat.”

“Whether or not that be the chief adjustment by which the right temperature is secured, it is at least a very important item. The intensity of the sun’s heat must, of course, be considered in connection with its distance from the earth. The distance of the sun is no less important than the power of his rays; indeed, in one sense, it is more important, for if the intensity of the sun’s heat were doubled, the temperature of the earth would be increased only twofold; whereas, if the earth were brought to one-half its present distance from the sun, the heat would be increased four times. Heat being one of the radiant forces, its intensity diminishes in proportion to the square of the distance through which it acts. If the earth were 190,000,000 of miles from the sun instead of 95,000,000, as it now is, the force of the sun’s rays would be diminished fourfold. The Creator has so fixed the distance of the earth and sun, and the power of the sun’s heat, as to give to this world a temperature suited to its various inhabitants.

“The temperature of the earth has also some dependence upon our atmosphere. Can you tell us, Ansel, how the temperature of the earth is affected by the atmosphere?”

“You have already told us that the atmosphere is diathermic, allowing the heat of the sun to fall upon the earth almost undiminished in force. If the air were so constituted as to intercept the sun’s rays, it is plain that the earth would receive less heat.”

“This adaptation of our atmosphere to transmit the sun’s rays,” said Mr. Wilton, “is more subtle than it appears at first sight. It is not merely a matter of depth and density, though those are important considerations, nor is it merely a question of the elements of which the atmosphere is composed. Simple gases are diathermic. The atmosphere is therefore made up of two simple gases, oxygen and nitrogen, not chemically combined, but mixed together. Compound gases intercept the passage of heat. Ammonia, composed of hydrogen and nitrogen chemically united, almost wholly stops it. Even ozone, which is nothing but oxygen in a changed or allotropic state, is not diathermic. The diathermic quality of the air depends, then, not only upon the fact that it is composed of simple elements mingled, but not chemically joined, but also upon the state, or condition, of those simple elements.

“Another point deserves attention. Oxygen is an element having a wide range of very strong and active affinities. It is ready to unite with every known substance, fluorine excepted. What if some other equally active element were mingled with oxygen to form the atmosphere? What if, in place of nitrogen, vapor of sulphur were substituted? What if hydrogen were put in the place of nitrogen? The two elements would combine in sudden combustion or explosion, and the atmosphere itself would perish. But nitrogen is a substance so sluggish and inert that it can be brought into union with oxygen only by indirect processes. Because the air is composed of one so inert element as nitrogen, the atmosphere is preserved, and, what is almost as important, it is kept, as it now is, composed of simple elements, and hence diathermic. If our atmosphere were a compound gas, the world would perish with cold.

“The temperature of the earth depends also upon certain qualities of the earth’s surface. I should be glad to have Mr. Hume explain this.”

“I suppose,” answered Mr. Hume, “that you refer to the qualities of the earth as an absorbent and conductor of heat. The earth must needs have the capacity of receiving and retaining the heat which falls upon it from the sun. If the earth’s surface were polished and brilliant, the heat of the sun would be reflected into space as from the surface of a mirror, and very small advantage would the earth receive from the solar heat. A dark soil absorbs heat more readily than a soil of lighter color, and a wet soil, on account of the high specific heat of water, requires more heat to raise its temperature than a dry soil. The mineral elements of the soil and its compactness or porosity also help to make up its capacity for receiving and retaining heat. The color and constitution of the soil sometimes go far toward making the climate of a region. The conducting qualities of the earth’s crust in its profoundest depths also must be taken into account. If the crust of the earth were composed of silver, or any other substance of like conducting quality, and the interior of the earth were molten rock, as it now is, the interior heat would be so rapidly conducted to the surface that everything upon the earth would be consumed.”