In the previous lesson we have seen that all movement of matter on the face of the earth involves a corresponding change of energy. We have also seen that while energy may be manifested in various forms, such as heat, chemical energy, potential energy, kinetic energy, etc., and may be changed from one of its various forms to another, none of it is ever lost, but that all of it tends to be dissipated into waste heat. Engines, as we have seen, whether animate or man-made, do not create energy, but merely utilize a supply of available energy for doing work. The available energy used by various engines usually occurs in two forms—mechanical energy as in the case of waterfalls or the wind, and chemical energy, as in the case of fuels and food.

Energy of Running Water. The end product of all of this energy is waste heat, but until now we have not inquired as to where it came from in the first place. Take the waterfall for example, which is continually dissipating energy. The water in the river was originally derived from rain, and this was in turn evaporated principally from the ocean. Now we have already seen that to evaporate water requires energy. At ordinary temperatures 585 gram calories of heat are required to evaporate 1 gram of water. Since ocean water does evaporate, this heat must be supplied, but where does it come from? Obviously, the only source of heat in the open ocean is the sunshine; the sun shines upon the ocean and other bodies of water, and its energy is used to produce evaporation. Another part of the sun’s energy heats the earth’s atmosphere, and, by causing it to expand, produces winds. In this manner the evaporated water is carried over the land. Then, upon cooling, the water vapor in the atmosphere condenses and falls as rain and snow, and this in turn produces rivers. Hence, the energy of a waterfall is originally derived from the energy of sunshine.

Energy of Plants and Animals. Where does the energy contained in food and fuels come from? We have already seen that when foods and fuels are combined chemically with oxygen the combustion produces chiefly carbon dioxide (CO2) and water vapor, while in the process heat is released. Since heat is not spontaneously created, a similar heat supply had to be provided when water vapor and carbon dioxide were originally united to produce the food and fuel products.

A large class of foods, such as grains, vegetables, etc., are derived directly from plants. A large number of fuels such as wood and coal are likewise plant products. Coal is simply the consolidated remains of forests which grew in past geological ages and have been preserved from decay by being buried under great thicknesses of rock. Hence, most of the energy contained in our food and fuel is derived directly from plants.

Some foods, and to a slight extent some fuels (whale oil, for example,) are derived not from plants, but from animals. In all cases, however, the energy contained in the animal tissues was derived from the animals’ diet of plants or other herbivorous animals. Thus, we see that all energy contained in animal tissue, and used to operate the animal bodies, is derived directly or indirectly from the chemical energy of plants.

The energy contained in petroleum has not yet been discussed. It has now been established beyond a doubt that petroleum has been derived from plants and animals of the geologic past which have been preserved from decay by burial under great thicknesses of rock. Hence, this energy is also derived from plants.

Chlorophyll. It remains to be seen where and how the plants get their energy. It is a matter of common observation on farms that a weed such as a cocklebur, if growing alone on an open piece of ground, will reach only a moderate height of about 3 feet and will spread laterally until its lateral diameter is also about 3 feet. If the cocklebur, however, is only one of a thick patch of cocklebur plants growing about 6 inches apart, then it will develop a long, slender stalk reaching a height of 5 or 6 feet, with almost no leaves except a small tuft directly on top. This same type of thing is true for all kinds of plants. Oak trees in an oak thicket have long slender trunks, whereas the same kind of oak trees when alone will form the familiar widely-branching tree.

When plants are placed in a house or cellar where little sunlight is available, the leaves usually lose the familiar green color and turn white or yellow, the plant loses its vigor of growth, and eventually dies. Grass on a shady lawn frequently dies out and has to be reset. Among plants the struggle for existence is, among other things, largely a struggle for sunshine. Raw materials from which plants are composed are chiefly carbon, hydrogen, and oxygen plus a small amount of nitrogen and mineral matter. Water is required by plants, and this water is derived from the moisture of the soil. The mineral matter, likewise, is the ordinary salts which are contained in solution by the water of the soil. The carbon is derived from the carbon dioxide which is contained in the air. The nitrogen is likewise derived from the air. We can represent this as follows:

6CO2 + 5H2O → C6H10O5 + 6O2

carbon dioxide    water    cellulose    oxygen

Cellulose plus lignin, a similar material, compose the woody material of plants. We have already seen that the chemical combination of wood with oxygen releases heat, as follows:

C6H10O5 + 6O2 → 6CO2 + 5H2O + heat

cellulose    oxygen    carbon dioxide    water

It will be noticed that the production of plant substance is chemically exactly the opposite from the burning of wood. Since energy is released when wood is burned, then an exact equal amount of energy must have been required when the wood was formed in the first place. Accordingly, the formation of wood may be represented:

6CO2 + 5H2O + energy → C6H10O5 + 6O2

carbon dioxide    water     cellulose     oxygen

Where does the energy come from? It has been found that, in this case, the energy is derived from the sunshine or other sources of light. This accounts for the fact that the plants seem to compete with each other for sunlight. The green substance in the leaves of plants is called chlorophyll. In the presence of chlorophyll solar energy is converted into chemical energy, as water and carbon dioxide combine to form plant substance.

Solar Radiation. Almost all of the energy used by man, whether derived from wind or water power, from coal or oil, or from other animals or plants, is derived ultimately from the sunshine. Exceptions to this are energy derived from tides, or from volcanic heat from the earth’s interior. These exceptions are at present of little importance and will probably continue to be so in the future.

From the foregoing it is evident that most of the activity—most of the movements of matter—on the face of the earth are directly or indirectly the result of sunshine. The energy contained in the solar radiation as it impinges on the earth has been measured. It has been found that the solar radiation upon a square centimeter of surface taken at right angles to the sun’s rays will, if converted into heat, produce 1.94 gram calories of heat per minute of time. This relationship is strictly true only just outside the earth’s atmosphere; on the earth’s surface, the heat per minute is somewhat less than this due to the fact that some of the heat is absorbed by the earth’s atmosphere.

It may give one a better idea of the enormous quantity of energy contained in sunshine if he consider that the average sunshine per day on one square mile at Washington, D.C., would, if converted into mechanical work, equal 20 million horse-power hours. It is easy to see what an enormous amount of energy per day the total solar radiation on the entire earth must be.

Flow of Solar Energy. As energy is not destroyed, we must now determine how, with such an enormous amount of heat arriving daily from the sun the earth does not get continually hotter and hotter. We have geological evidence that the intensity of sunshine on the earth has been practically the same for many millions of years. We also know that the earth has had about the same temperature during that time. Therefore, the earth must be losing energy at about the same rate it is receiving it.

Let us trace the energy received from the sun. Of the total energy contained in the solar radiation which impinges upon the earth, approximately 37 percent is reflected back into space. Another part of the energy of the sunshine is directly absorbed by objects upon which it falls and is converted into heat; still another part produces the evaporation of water; another part is consumed in expanding the gases of the atmosphere and the ocean waters, producing winds and ocean currents. Finally, a part is converted by the chlorophyll of the plants into the chemical energy required by plant-eating animals and these latter finally become the food for carnivorous animals. As we have already shown, a part of the plant energy may be converted into mechanical work by means of man-made engines; in a similar manner the energy of waterfalls which ordinarily is dissipated as waste heat, may be made to drive machinery before finally being reduced to waste heat. The end product of all these processes is, however, low-temperature waste heat. Due to the fact that the earth is not getting hotter, the earth must be losing heat at the same rate it is receiving heat from the sunshine. This loss of heat is accomplished by means of long wavelength, invisible heat radiation which the earth radiates out into space. This type of thing is well illustrated in the case of a closed automobile, parked in the hot sunshine. The temperature in the car stays several degrees higher than the temperature outside the car, which is due to the fact that sunshine, which is short wave radiation, passes readily through the glass windows. When it strikes the cushions of the car it is absorbed and produces heat. These cushions then emit a long wave-length radiation, which can pass only with difficulty through the glass windows; consequently, the temperature of the interior of the car rises until enough heat can be radiated to allow the escaping energy to be equal to that coming in. Thereafter the temperature does not change. Clouds in the earth’s atmosphere act in a similar manner—they tend to block the escaping long wave-length radiation. That is the reason it rarely frosts on a cloudy night.


Solar radiation impinges upon the earth as a short wave-length radiation, and thereafter undergoes a series of energy changes, each one of which, in accordance with the Second Law of Thermodynamics, is at a lower scale of degradation than that preceding it. Finally, it is re-radiated back into space as spent long wave radiation. During, and as a consequence of this process, the wind blows, rivers flow, and plants and animals grow and propagate their kind, and most of the other events on the face of the earth take place.

Since the total flow of energy on the earth is practically a constant, it follows that there is not likely to be any cessation or diminution of this process for a long time to come. While the total flow of energy on the earth’s surface is essentially constant, the resulting picture, in terms of the configuration of the earth’s surface and of plant and animal life, is continuously changing. This change is itself unidirectional and irreversible; that is to say, it never repeats itself.


  • Physics of the Earth—III, Meteorology, Bull. of National Research Council No. 79 (1931).
  • The Data of Geochemistry, Clarke.
  • Elements of Physical Biology, Lotka (Chaps. 16-20).
  • Introduction to Geology, Branson and Tarr.
  • Photosynthesis, Spohr (Out of print).
  • Animal Life and Social Growth, Allee.