If one stops taking food, it becomes difficult after a time for him to
move about and to keep warm. These results show that food has some
relation to the energy of the body, for motion and heat are forms of
energy. The relation of oxygen to the supply of energy has already been
discussed (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 (Chapter XV).
*Some Questions of Hygiene.*--The heat-producing capacity of the body
sustains a very important relation to the general health. A sudden chill
may result in a number of derangements and is supposed to be a
predisposing cause of _colds_. One's capacity for producing heat may be so
low that he is unable to respond to a sudden demand for heat, as in going
from a warm room into a cold one. As a consequence, the body is unable to
protect itself against unavoidable exposures.
_Impairment of the heat-producing capacity_ is brought about in many ways.
Several diseases do this directly, or indirectly, to quite an extent. In
health too great care in protecting the body from cold is the most potent
cause of its impairment. Staying in rooms heated above a temperature of
70 deg. F., wearing clothing unnecessarily heavy, and sleeping under an excess
of bed clothes, all diminish the power of the body to produce heat. They
accustom it to producing only a small amount, so that it does not receive
sufficient of what might be called _heat-producing exercise_. Lack of
physical exercise in the open air, as well as too much time spent in
poorly lighted and ventilated rooms, tends also to reduce one's ability to
produce heat. Moreover, since most of the heat of the body comes from the
union of oxygen and food materials at the cells, a lack of either of these
will interfere with the production of heat.
*Results of Exhaustion.*--Through overwork, or excesses in pleasurable
pursuits, one may make greater demands upon the energy of his body than it
can properly supply. The resulting condition, known as _exhaustion_, is
not only a matter of temporary inconvenience, but may through repetition
lead to a serious impairment of the health. It should be noted, in this
connection, that the energy of the body is spent in two general ways:
first, in carrying on the vital processes; and second, in the performance
of voluntary activities. Since, in all cases, there is a limit to one's
energy, it is easily possible to expend so much in the voluntary
activities that the amount left is not sufficient for the vital processes.
This leads to various disturbances and, among other things, renders the
body less able to supply itself with energy.
*The Problem of Increasing One's Energy.*--Since the energy supply is kept
up through the food and the oxygen, it might be inferred that the
introduction of these substances into the body in larger amounts would
increase the energy at one's disposal. This does not necessarily follow.
Oxidation at the cells is preceded by digestion, absorption, circulation,
and assimilation. It is followed and influenced by the removal of wastes
from the body. A careful study of the problem leads to the conclusion that
while the energy supply to the body does depend upon the introduction of
the proper amounts of food and oxygen, it also depends upon the efficiency
of the vital processes. The maximum amount of energy may, therefore, be
expected when the body is in a condition of perfect health. Hence, one
desiring to increase the amount of his energy must give attention to all
those conditions that improve the health.
*Effect of Stimulants on the Energy Supply.*--In the effort to get out of
the body as much as possible of work or of pleasure, various stimulants,
such as alcohol, tobacco, and strong tea and coffee, have been used.
Though these have the effect of giving a temporary feeling of strength and
of enabling the individual in some instances to accomplish results which
he could not otherwise have brought about, the general effect of their use
is to lessen, rather than to increase, the sum total of bodily power. The
student, for example, who drinks strong coffee in order to study late at
night is able to command less energy on the day following. While enabling
him to draw upon his reserve of nervous power for the time being, the
coffee deprives him of sleep and needed rest.
The danger of stimulants, so far as energy is concerned, is this: they
tend to exhaust the bodily reserve so that there is not sufficient left
for properly running the vital processes. Evidences of their weakening
effect are found in the feeling of discomfort and lassitude which result
when stimulants to which the body has become accustomed are withdrawn. Not
until one gets back his bodily reserve is he able to work normally and
effectively. Increase in bodily energy comes through health and not
through the use of stimulants.
*Summary.*--The body requires a continuous supply of energy. To obtain this
supply, materials possessing potential, or stored-up, energy are
introduced into it. The free oxygen of the air and the substances known as
foods, on account of the chemical relations which they sustain to each
other, contain potential energy and are utilized for supplying the body.
So long as the foods are not oxidized, the energy remains in the potential
form, but in the process of oxidation the potential energy is changed to
kinetic energy and made to do the work of the body.
*Exercises.*--1. In what different ways does the body use energy?
2. Show that a stone lying against the earth has no energy, while the same
stone above the earth has energy.
3. How does potential energy differ from kinetic energy?
4. What kind of energy is possessed by a bent bow? By a revolving wheel?
By a coiled spring? By the wind? By gunpowder?
5. How does decomposing water with electricity store energy?
6. Account for the energy possessed by the oxygen of the air and food
substances.
7. Trace the energy supply of the body back to the sun.
8. Why must both oxygen and food be introduced into the body in order to
supply it with energy?
9. How may overwork and overexercise diminish the energy supply of the
body?
10. How may one increase the amount of his energy?
PRACTICAL WORK
*Suggested Experiments.*--1. The change of kinetic into potential energy
may be shown by stretching a piece of rubber, by lifting a weight, and by
separating the armature from a magnet.
2. The change of potential into kinetic energy may be shown by letting
weights fall to the ground, by releasing the end of a piece of stretched
rubber, and by burning substances.
3. The change of one form of kinetic energy to another may be illustrated
by rubbing together two pieces of wood until they are heated, by ringing a
bell, and by causing motion in air or in water by heating them. If
suitable apparatus is at hand, the transformation of electrical energy
into heat, light, sound, and mechanical motion can easily be shown.
4. A weight connected by a cord with some small machine and made to run
it, will help the pupil to grasp the general principles in the storage of
energy through gravity. A vessel of water on a high support from which the
water is siphoned on to a small water wheel will serve the same purpose.
5. The storing of energy by chemical means may be illustrated by
decomposing potassium chlorate with heat or by decomposing water by means
of a current of electricity.
6. Study the transfer of energy from the body to surrounding objects, as
in moving substances and lifting weights.
Fill a half gallon jar two thirds full of water and carefully take the
temperature with a chemical thermometer. Hold the hand in the water for
four or five minutes and take the temperature again. Inference.