Chapter VIII

). We are now to inquire more fully into the energy supply of the body, and to consider those conditions which make necessary the introduction of both food and oxygen for this purpose.

*Kinds of Bodily Energy.*--The healthy body has at any time a considerable amount of _potential_, or reserve, energy,--energy which it is not using at the time, but which it is able to use as its needs require. When put to use, this energy is converted into such forms of _kinetic_ energy(69) as are indicated by the different kinds of bodily power. These are as follows:

1. _Power of Motion._--The body can move itself from place to place and it can give motion to things about it.

2. _Heat Power._--The body keeps itself warm and is able to communicate warmth to its surroundings.

3. _Nervous Power._--Through the nervous system the body exercises the power of control over its different parts.

As motion, heat, and nervous power the body uses most of its energy.

*The Source of Bodily Energy.*--As already indicated, the energy of the body is supplied through the food and the oxygen. These contain energy in the potential form, which becomes kinetic (active) through their uniting with each other in the body. Somewhat as the power of the steam engine is derived from the combustion of fuel in the furnaces, the energy of the body is supplied through the oxidations at the cells. How the food and oxygen come to possess energy is seen by a study of the general methods by which energy is stored up and used.

[Fig. 81]

Fig. 81--*Simple device* for storing energy through gravity.

*Simple Methods of Storing Energy.*--Energy is stored by converting the kinetic into the potential form. Two of the simplest ways of doing this are the following:

1. _Storing of Energy through Gravity._--On account of the attraction between the earth and all bodies upon the earth, the mere lifting of a weight puts it in a position where gravity can cause it to move (Fig. 81). As a consequence _the raising of bodies above the earth's surface is a means of storing energy_--the energy remaining stored until the bodies fall. As they fall, the stored-up (potential) energy becomes kinetic and can be made to do work.

2. _Storing of Energy through Elasticity._--Energy is stored also by doing work in opposition to elasticity, as in bending a bow or in winding a clock spring. The bending, twisting, stretching, or compressing of elastic substances puts them in a condition of _strain_ which causes them to exert a pressure (called elastic force) that tends to restore them to their former condition. Energy stored by this means becomes active as the distorted or compressed substance returns to its former shape or volume.

These simple methods of storing energy will serve to illustrate the general principles upon which such storage depends:

1. To store energy, energy must be expended, or work done.

2. The work must be against some force, such as gravity or elasticity, which can undo the work, i.e., bring about an effect opposite to that of the work.

3. The stored energy becomes active (kinetic) as the force through which the energy was stored undoes the work, or puts the substance upon which the work was done into its former condition (gravity causing bodies to fall, etc.).

These principles are further illustrated by the

*Storing of Energy through Chemical Means.*--A good example of storing energy by chemical means is that of decomposing water with electricity. If a current of electricity is passed through acidulated water in a suitable apparatus (Fig. 82), the water separates into its component gases, oxygen and hydrogen. These gases now have power (energy) which they did not possess before they were separated. The hydrogen will burn in the oxygen, giving heat; and if the two gases are mixed in the right proportions and then ignited, they explode with violence. This energy was derived from the electricity. It was stored by _decomposing_ the water.

[Fig. 82]

Fig. 82--*Storing energy by chemical means.* Apparatus for decomposing water with electricity.

Energy is stored by chemical means by causing it to do work in opposition to the force of chemism, or chemical affinity. Instead of changing the form of bodies or moving them against gravity, it overcomes the force that causes atoms to unite and to hold together after they have united. Since in most cases the atoms on separating from any given combination unite at once to form other combinations, we may say that _energy is stored when strong chemical combinations are broken up and weak ones formed_. Energy stored by this means becomes active when the atoms of weak combinations unite to form combinations that are strong.(70)

*How Plants store the Sun's Energy.*--The earth's supply of energy comes from the sun. While much of this, after warming and lighting the earth's surface, is lost by radiation, a portion of it is stored up and retained. The sun's energy is stored both through the force of gravity(71) and by chemical means, the latter being the more important of the two methods. Plants supply the means for storing it chemically (Fig. 83). Attention has already been called to the fact (page 112) that growing plants are continually taking carbon dioxide into their leaves from the air. This they decompose, adding the carbon to compounds in their tissues and returning the oxygen to the air. It is found, however, that this process does not occur unless the plants are exposed to sunlight. The sunlight supplies the energy for overcoming the attraction between the atoms of oxygen and the atoms of carbon, while the plant itself serves as the instrument through which the sunlight acts. The energy for decomposing the carbon dioxide then comes from the sun, and through the decomposition of the carbon dioxide the sun's energy is stored--becomes potential. It remains stored until the carbon of the plant again unites with the oxygen of the air, as in combustion.

[Fig. 83]

Fig. 83--*Nature's device* for storing energy from the sun. See text.

*The Sun's Energy in Food and Oxygen.*--Food is derived directly or indirectly from plants and sustains the same relation to the oxygen of the air as do the plants themselves. (The elements in the food have an attraction for the oxygen, but are separated chemically from it.) On account of this relation they have potential energy--the energy derived through the plant from the sun. When a person eats the food and breathes the oxygen, this energy becomes the possession of the body. It is then converted into kinetic energy as the needs of the body require.

[Fig. 84]

Fig. 84--*Simple apparatus* for illustrating transformation of energy. Potential energy is converted into heat and heat into motion.

*From the Sun to the Cells.*--It thus appears that the body comes into possession of energy, and is able to use it, through a series of transferences and transformations that can be traced back to the sun.(72) Coming to the earth as kinetic energy, it is transformed into potential energy and stored in the compounds of plants and in the oxygen of the air. Through the food and the oxygen the potential energy is transferred to the cells of the body. Then by the uniting of the food and the oxygen at the cells (oxidation), the potential becomes kinetic energy and is used by the body in doing its work. The phrase "Child of the Sun" has sometimes been applied to man to express his dependence upon the sun for his supply of energy.

*Why Oxygen and Food are Both Necessary.*--The necessity for introducing both oxygen and food into the body for the purpose of supplying energy is now apparent. The energy which is used in the body is not the energy of food alone. Nor is it the energy of oxygen alone. It belongs to both. It is due to their attraction for each other and their condition of separation. It cannot, therefore, become kinetic except through their union. To introduce one of these substances into the body without the other, would neither introduce the energy nor set it free. They must both be introduced into the body and there caused to unite.

*Bodily Control of Energy.*--A fact of importance in the supply of energy to the body is that the rate of transformation (changing of potential to kinetic) is just sufficient for its needs. It is easily seen that too rapid or too slow a rate would prove injurious. The oxidations at the cells are, therefore, under such control that the quantity of kinetic energy supplied to the body as a whole, and to the different organs, is proportional to the work that is done. This is attained, in part at least, through the ability of the body to store up the food materials and hold them in reserve until they are to be oxidized (page 180).

*Animal Heat and Motion.*--Most of the body's energy is expended as heat in keeping warm. It is estimated that as much as five sixths of the whole amount is used in this way. The proportion, however, varies with different persons and is not constant in the same individual during different seasons of the year. This heat is used in keeping the body at that temperature which is best suited to carrying on the vital processes. All parts of the body, through oxidation, furnish heat. Active organs, however, such as the muscles, the brain, and the glands (especially the liver), furnish the larger share. The blood in its circulation serves as a _heat distributer_ for the body and keeps the temperature about the same in all its parts (page 33).

Next to the production of heat, in the consumption of the body's energy, is the production of motion. This topic will be considered in the study of the muscular system (