forces of heat, light, electricity or motion, we mean simply the
different modes of motion called by these names. And when we say that
they are correlated to each other, we mean simply that the mode of
motion called heat, light, electricity, is convertible into any of the
others, at pleasure. Correlation therefore implies convertibility, and
mutual dependence and relationship.
Having now defined the use of the term force, and shown that forces are
correlated which are convertible and mutually dependent, we go on to
study the evidences of such correlation among the motions of inorganic
nature usually called physical forces; and to ask what proof science can
furnish us that mechanical motion, heat, light, and electricity are thus
mutually convertible. As we have already hinted, the time was when these
forces were believed to be various kinds of imponderable matter, and
chemists and physicists talked of the union of iron with caloric as they
talked of its union with sulphur, regarding the caloric as much a
distinct and inconvertible entity as the iron and sulphur themselves.
Gradually, however, the idea of the indestructibility of matter extended
itself to force. And as it was believed that no material particle could
ever be lost, so, it was argued, no portion of the force existing in
nature can disappear. Hence arose the idea of the indestructibility of
force. But, of course, it was quite impossible to stop here. If force
cannot be lost, the question at once arises, what becomes of it when it
passes beyond our recognition? This question led to experiment, and out
of experiment came the great fact of force-correlation; a fact which
distinguished authority has pronounced the most important discovery of
the present century.[6] These experiments distinctly proved that when
any one of these forces disappeared, another took its place; that when
motion was arrested, for example, heat, light or electricity was
developed. In short, that these forces were so intimately related or
correlated—to use the word then proposed by Mr. Grove[7]—that when one
of them vanished, it did so only to reappear in terms of another. But
one step more was necessary to complete this magnificent theory. What
can produce motion but motion itself? Into what can motion be converted,
but motion? May not these forces, thus mutually convertible, be simply
different modes of motion of the molecules of matter, precisely as
mechanical motion is a motion of its mass? Thus was born the dynamic
theory of force, first brought out in any completeness by Mr. Grove, in
1842, in a lecture on the “Progress of Physical Science,” delivered at
the London Institution. In that lecture he said: “Light, heat,
electricity, magnetism, motion, are all convertible material affections.
Assuming either as the cause, one of the others will be the effect. Thus
heat may be said to produce electricity, electricity to produce heat;
magnetism to produce electricity, electricity magnetism; and so of the
rest.”[8]
A few simple experiments will help us to fix in our minds the great fact
of the convertibility of force. Starting with actual visible motion,
correlation requires that when it disappears as motion, it should
reappear as heat, light, or electricity. If the moving body be elastic
like this rubber ball, then its motion is not destroyed when it strikes,
but is only changed in direction. But if it be non-elastic, like this
ball of lead, then it does not rebound; its motion is converted into
heat. The motion of this sledge-hammer, for example, which if received
upon this anvil would be simply changed in direction, if allowed to fall
upon this bar of lead, is converted into heat; the evidence of which is
that a piece of phosphorus placed upon the lead is at once inflamed. So
too, if motion be arrested by the cushion of air in this cylinder, the
heat evolved fires the tinder carried in the plunger. But it is not
necessary that the arrest of motion should be sudden; it may be gradual,
as in the case of friction. If this cylinder containing water or alcohol
be caused to revolve rapidly between the two sides of this wooden
rubber, the heat due to the arrested motion will raise the temperature
of the liquid to the boiling point, and the cork will be expelled. But
motion may also be converted into electricity. Indeed electricity is
always the result of friction between heterogeneous particles.[9] When
this piece of hard rubber, for example, is rubbed with the fur of a cat,
it is at once electrified; and now if it be caused to communicate a
portion of its charge to this glass plate, to which at the same time we
add the mechanical motion of rotation, the strong sparks produced give
evidence of the conversion.
So, too, taking heat as the initial force, motion, light, electricity
may be produced. In every steam-engine the steam which leaves the
cylinder is cooler than that which entered it, and cooler by exactly the
amount of work done. The motion of the piston’s mass is precisely that
lost by the steam molecules which batter against it. The conversion of
heat into electricity, too, is also easily effected. When the junction
of two metals is heated, electricity is developed. If the two metals be
bismuth and antimony, as represented in this diagram, the currents flow
as indicated by the arrows; and by multiplying the number of pairs, the
effect may be proportionately increased. Such an arrangement, called a
thermo-electric battery, we have here; and by it the heat of a single
gas-burner may be made to move, when converted, this little electric
bell-engine. Moreover, heat and light have the very closest analogy;
exalt the rapidity with which the molecules move and light appears, the
difference being only one of intensity.
Again, if electricity be our starting point, we may accomplish its
conversion into the other forces. Heat results whenever its passage is
interrupted or resisted; a wire of the poorly conducting metal platinum
becoming even red-hot by the converted electricity. To produce light, of
course, we need only to intensify this action; the brightest artificial
light known, results from a direct conversion of electricity.
Enough has now been said to establish our point. What is to be
particularly observed of these pieces of apparatus is that they are
machines especially designed for the conversion of some one force into
another. And we expect of them only that conversion. We pass on to
consider for a moment the quantitative relations of this mutual
convertibility. We notice, in the first place, that in all cases save
one, the conversion is not perfect, a part of the force used not being
utilized, on the one hand, and on the other, other forces making their
appearance simultaneously. While, for example, the conversion of motion
into heat is quite complete, the inverse conversion is not at all so.
And on the other hand, when motion is converted into electricity, a part
of it appears as heat. This simultaneous production of many forces is
well illustrated by our little bell-engine, which converts the
electricity of the thermo-battery into magnetism, and this into motion,
a part of which expends itself as sound. For these reasons the question
“How much?” is one not easily answered in all cases. The best known of
these relations is that between motion and heat, which was first
established by Mr. Joule in 1849, after seven years of patient
investigation.[10] The apparatus which he used is shown in the diagram.
It consists of a cylindrical box of metal, through the cover of which
passes a shaft, carrying upon its lower end a set of paddles, immersed
in water within the box, and upon its upper portion a drum, on which are
wound two cords, which, passing in opposite directions, run over
pulleys, and are attached to known weights. The temperature of the water
within the box being carefully noted, the weights are then allowed to
fall a certain number of times, of course in their fall turning the
paddles against the friction of the liquid. At the close of the
experiment the water is found to be warmer than before. And by measuring
the amount of this rise in temperature, knowing the distance through
which the weights have fallen, it is easy to calculate the quantity of
heat which corresponds to a given amount of motion. In this way, and as
a mean of a large number of experiments, Mr. Joule found that the amount
of mass motion in a body weighing one pound, which had fallen from a
hight of 772 feet, was exactly equal to the molecular motion which must
be added to a pound of water, in order to heat it one degree Fahrenheit.
If we call the actual energy of a body weighing one pound which has
fallen one foot, a foot-pound, then we may speak of the mechanical
equivalent of heat as being 772 foot-pounds.
The significance and value of this numerical constant will appear more
clearly if we apply it to the solution of one or two simple problems.
During the recent war two immense iron guns were cast in Pittsburgh,
whose weight was nearly 112,000 pounds each, and which had a caliber of
20 inches.[11] Upon this diagram is a calculation of the effective blow
which the solid shot of such a gun, assuming its weight to be 1,000
pounds and its velocity 1,100 feet per second, would give; it is 902,797
tons![12] Now, if it were possible to convert the whole of this enormous
mechanical power into heat, to how much would it correspond? This
question may be answered by the aid of the mechanical equivalent of
heat; here is the calculation, from which we see that when 17 gallons of
ice-cold water are heated to the boiling point, as much energy is
communicated as is contained in the death-dealing missile at its highest
velocity.[13] Again, if we take the impact of a larger cannon-ball, our
earth, which is whirling through space with a velocity of 19 miles a
second, we find it to be 98,416,136,000,000,000,000,000,000,000,000
tons![14] Were this energy all converted into heat, it would equal that
produced by the combustion of 14 earths of solid coal.[15]
The conversion of heat into motion, however, as already stated, is not
as perfect. The best steam-engines economize only one-twentieth of the
heat of the fuel.[16] Hence if a steamship require 600 tons of coal to
carry her across the Atlantic, 570 tons will be expended in heating the
waters of the ocean, the heat of the remaining 30 tons only being
converted into work.
One other quantitative determination of force has also been made. Prof.
Julius Thomsen, of Copenhagen, has fixed experimentally the mechanical
equivalent of light.[17] He finds that the energy of the light of a
spermaceti candle burning 126½ grains per hour, is equal in mechanical
value to 13·1 foot-pounds per minute. The same conclusion has been
reached by Mr. Farmer, of Boston, from different data.[18]
If we pass from the actual physical energies or motions to consider for
a moment the potential energies or attractions, we find, also, an
intimate correlation. Since all energy not active in motion is potential
in attraction, it follows that in the attractions we have energy stored
up for subsequent use. The sun is thus storing up energy: every minute
it raises 2,000,000,000 tons of water to the mean hight of the clouds,
3½ miles; and the actual energy set free when this water falls is equal
to 2,757,000,000,000 horse-powers.[19] So when the oxygen and the zinc
of the ore are separated in the furnace, the actual energy of heat
becomes the potential energy of chemical attraction, which again becomes
actual in the form of electricity when the zinc is dissolved in an acid.
We see, then, that not only may any form of force or actual energy be
stored up as any form of attraction or potential energy, but that the
latter, from whatsoever source derived, may appear as heat, light,
electricity, or mechanical motion.
Having now established the fact of correlation for the physical forces,
we have next to inquire what are the evidences of the correlation of the
vital forces with them. But in the first place it must be remarked that
life is not a simple term like heat or electricity; it is a complex
term, and includes all those phenomena which a living body exhibits. In
this discussion, therefore, we shall use the term vital force to express
only the actual energy of the body, however manifested. As to the
attractions or the potential energy of the organism, nothing is more
fully settled in science than the fact that these are precisely the same
within the body as without it. Every particle of matter within the body
obeys implicitly the laws of the chemical and physical attractions. No
overpowering or supernatural agency comes in to complicate their action,
which is modified only by the action of the others. Vitality, therefore,
is the sum of the energies of a living body, both potential and actual.
Moreover, the important fact must be fully recognized that in living
beings we have to do with no new elementary forms of matter. Precisely
the same atoms which build up the inorganic fabric, compose the organic.
In the early days of chemistry, indeed, it was supposed that the
complicated molecules which life produced were beyond the reach of
simple chemical law. But as more and more complex molecules have been,
one after another, produced, chemistry has become re-assured, and now
doubts not her ability to produce them all. A few years hence, and she
will doubtless give us quinine and protagon, as she now gives us
coumarin and neurine, substances the synthesis of which was but
yesterday an impossibility.[20]
In studying the phenomena of living beings, it is important also to bear
in mind the different and at the same time the coördinate purposes
subserved by the two great kingdoms of nature. The food of the plant is
matter whose energy is all expended; it is a fallen weight. But the
plant-organism receives it, exposes it to the sun’s ray, and, in a way
yet mysterious to us, converts the actual energy of the sunlight into
potential energy within it. The fallen weight is thus raised, and energy
is stored up in substances which now are alone competent to become the
food of the animal. This food is not such because any new atoms have
been added to it; it is food because it contains within it potential
energy, which at any time may become actual as force. This food the
animal now appropriates; he brings it in contact with oxygen, and the
potential energy becomes actual; he cuts the string, the weight falls,
and what was just now only attraction, has become actual force; this
force he uses for his own purposes, and hands back the oxidized matter,
the fallen weight, to the plant to be again de-oxidized, to be again
raised. The plant then is to be regarded as a machine for converting
sunlight into potential energy; the animal, a machine for setting the
potential energy free as actual, and economizing it. The force which the
plant stores up is undeniably physical; must not the force which the
animal sets free by its conversion, be intimately correlated to it?
But approaching our question still more closely, let us, in illustration
of the vital forces of the animal economy, choose three forms of its
manifestation in which to seek for the evidences of correlation; these
shall be heat, evolved within the body; muscular energy or motion; and
lastly, nervous energy, or that form of force which, on the one hand,
stimulates a muscle to contract, and on the other, appears in forms
called mental.
The heat which is produced by the living body is obviously of the same
nature as heat from any other source; it is recognized by the same
tests, and may be applied for the same purposes. As to its origin, it is
evident that since potential energy exists in the food which enters the
body, and is there converted into force, a portion of it may become the
actual energy of heat. And since, too, the heat produced in the body is
precisely such as would be set free by the combustion of this food
outside of it, it is fair to assume that it thus originates. To this may
be added the chemical argument that while food capable of yielding heat
by combustion is taken into the body, its constituents are completely or
almost completely, oxidized before leaving it; and since oxidation
always evolves heat, the heat of the body must have its origin in the
oxidation of the food. Moreover, careful measurements have demonstrated
that the amount of heat given off by the body of a man weighing 180
pounds is about 2,500,000 units. Accurate calculations have shown, on
the other hand, that 288·4 grams of carbon and 12·56 grams of hydrogen
are available in the daily food for the production of heat. If burned
out of the body, these quantities of carbon and hydrogen would yield
2,765,134 heat units. Burned within it, as we have just seen, 2,500,000
units appear as heat; the rest in other forms of energy.[21] We
conceive, however, that no long argument is necessary to prove that
animal heat results from a conversion of energy within the body; or that
the vital force heat, is as truly correlated to the other forces as when
it has a purely physical origin.
The belief that the muscular force exerted by an animal is created by
him is by no means confined to the very earliest ages of history.
Traces of it appear to the careful observer even now, although, as Dr.
Frankland says, science has proved that “an animal can no more
generate an amount of force capable of moving a grain of sand than a
stone can fall upward or a locomotive drive a train without fuel.”[22]
In studying the characters of muscular action we notice, first, that,
as in the case of heat, the force which it develops is in no wise
different from motion in inorganic nature. In the early part of the
lecture, motion produced by the contraction of muscle, was used to
show the conversion of mass-force into molecular force. No one in this
room believes, I presume, that the result would have been at all
different, had the motion been supplied by a steam-engine or a
water-wheel. Again, food, as we have seen, is of value for the
potential energy it contains, which may become actual in the body.
Liebig, in 1842, asserted that for the production of muscular force,
the food must first be converted into muscular tissue,[23] a view
until recently accepted by physiologists.[24] It has been conclusively
shown, however, within a few years, that muscular force cannot come
from the oxidation of its own substance, since the products of this
metamorphosis are not increased in amount by muscular exertion.[25]
Indeed, reasoning from the whole amount of such products excreted, the
oxidation of the amount of muscle which they represent would furnish
scarcely one-fifth of the mechanical force of the body. But while the
products of tissue-oxidation do not increase with the increase of
muscular exertion, the amount of carbonic gas exhaled by the lungs is
increased in the exact ratio of the work done.[26] No doubt can be
entertained, therefore, that the actual energy of the muscle is simply
the converted potential energy of the carbon of the food. A muscle,
therefore, like a steam-engine, is a machine for converting the
potential energy of carbon into motion. But unlike a steam-engine, the
muscle accomplishes this conversion directly, the energy not passing
through the intermediate stage of heat. For this reason, the muscle is
the most economical producer of mechanical force known. While no
machine whatever can transform all of the energy into motion—the most
economical steam-engines utilizing only one-twentieth of the heat—the
muscle is able to convert one-fifth of the energy of the food into
work.[27] The other four-fifths must, therefore, appear as heat.
Whenever a muscle contracts, then, four times as much energy appears
as heat as is converted into motion. Direct experiments by Heidenhain
have confirmed this, by showing that an important rise of temperature
attends muscular contraction;[28] a fact, however, apparent to any one
who has ever taken active exercise. The work done by the animal body
is of two sorts, internal and external. The former includes the action
of the heart, of the respiratory muscles, and of those assisting the
digestive process. The latter refers to the useful work the body may
perform. Careful estimates place the entire work of the body at about
800 foot-tons daily; of which 450 foot-tons is internal, 350 foot-tons
external work. And since the internal work ultimately appears as heat
within the body, the actual loss of heat by the production of motion
is the equivalent of the 350 foot-tons which represents external work.
This by a simple calculation will be found to be 250,000 heat units,
almost the precise amount by which the heat yielded by the food when
burned without the body, exceeds that actually evolved by the
organism. Moreover, while the total heat given off by the body is
2,500,000 units, the amount of energy evolved as work is equal to
about 600,000 heat units; hence the amount of work done by a muscle is
as above stated, one-fifth of the actual energy derivable from the
food. One point further. The law of correlation requires that the heat
set free when a muscle in contracting does work, shall be less than
when it effects nothing; this fact, too, has been experimentally
established by Heidenhain.[29] So, again, when muscular contraction
does not result in motion, as when one tries to raise a weight too
heavy for him, the energy which would have appeared as work, takes the
form of heat: a result deducible by the law of correlation from the
steam-engine.
The last of the so-called vital forces which we are to examine, is that
produced by the nerves and nervous centers. In the nerve which
stimulates a muscle to contract, this force is undeniably motion, since
it is propagated along this nerve from one extremity to the other. In
common language, too, this idea finds currency in the comparison of this
force to electricity; the gray or cellular matter being the battery, the
white or fibrous matter the conductors. That this force is not
electricity, however, Du Bois-Reymond has demonstrated by showing that
its velocity is only 97 feet in a second, a speed equaled by the
greyhound and the race-horse.[30] In his opinion, the propagation of a
nervous impulse is a sort of successive molecular polarization, like
magnetism. But that this agent is a force, as analogous to electricity
as is magnetism, is shown not only by the fact that the transmission of
electricity along a nerve will cause the contraction of the muscle to
which it leads, but also by the more important fact that the contraction
of a muscle is excited by diminishing its normal electrical current;[31]
a result which could take place only with a stimulus closely allied to
electricity. Nerve-force, therefore, must be a transmuted potential
energy.