There is no more interesting subject for scientific investigation
than the structure and operation, the anatomy and physiology of the
human body. That it is an amazingly complex and delicate mechanism,
performing a multitude of functions in a wonderfully perfect manner,
is, of course, an old story. That in the assimilation of its
nourishment and in the growth and repair of its tissue the body obeys
the laws of chemistry has long been understood. But that the body
obeys in everything the fundamental law of physics, namely, the law
of the conservation of energy, has not been so generally recognized.
For some years the writer was engaged in some investigations upon
this subject.[B] The development of the complex apparatus and unique
methods of the research required years of patient labor and study. One
of the features of the apparatus was an air-tight chamber, in which
a man, as the subject of the experiment, could be confined for any
desired period, eating, sleeping, working and living while under minute
observation. The experiments usually continued four or five days, but
were sometimes prolonged to eight or ten days, and the observations
were made and recorded day and night continuously for the entire period.
[B] The work was done at Wesleyan University, in collaboration
with Prof. W. O. Atwater, under the patronage of the
University and the U. S. Department of Agriculture.
The atmosphere within the chamber was maintained sufficiently pure
to make a prolonged sojourn within its walls entirely comfortable.
A current of fresh air, displacing as it entered an equal quantity
of air which contained the products of respiration, was maintained
continuously. The respired air was analyzed and measured, and the
products of respiration from lungs and skin accurately determined.
The ventilating air current was maintained by a pair of measuring
air pumps, driven by an electric motor. The air was dried, both
before entering and after leaving the chamber by freezing out its
moisture. This was done by passing it through a refrigerator where its
temperature was reduced far below the freezing point. The refrigerator
was operated by an ammonia machine, driven by an electric motor. The
quantity of air was automatically recorded by the pumps.
The chamber was so constructed and fitted with electrical and other
devices as to afford the means of measuring the quantity of heat which
the subject of the experiment gave off from his body. And in order to
keep the temperature of the room constant this heat was absorbed and
carried away by a stream of cold water, the latter flowing through a
series of copper pipes within the chamber, and coming out considerably
warmer than it entered. So delicate were the regulating devices that
the temperature could be maintained constant, hour after hour, to
within one or two hundredths of a degree. In some cases the man under
investigation worked regularly eight hours a day, the work done being
measured by apparatus designed for the purpose.
Food and drink were passed into the chamber three times a day through
an air-tight trap. Both were accurately weighed, their temperature
recorded and samples reserved for chemical analysis. Solid and liquid
excreta were likewise weighed and analyzed. The observations, analyses
and computations of a single experiment thus involved a vast amount of
labor and expense, which was only justified by the importance of the
question under investigation. In order to be able to understand just
what this question is, let us see what is meant by the conservation of
matter and energy in the physical world.
The impossibility of creating or destroying matter is very generally
recognized. Its forms or properties may be altered, chemical and
physical changes may be effected, it may, indeed, vanish from sight,
but its quantity remains unchanged. Thus ice may turn to water and
water to invisible steam, but the total quantity or mass of the
substance remains constant; and if by refrigeration the steam be
brought to the condition of ice again, there will be precisely the same
amount as before. These are physical changes and are easily effected.
We simply apply heat to melt the ice and then more heat to vaporize the
water. Conversely, withdrawing heat will condense the vapor to water,
when a further subtraction of heat will change the water into ice.
Again, wood disappears when burned and seems to be destroyed. And yet
we know that the weight of the resulting smoke and ashes is exactly
equal to that of the wood. The matter has been changed in form and
composition, but its mass cannot be altered. It is not so easy to bring
the smoke and ashes into combination again and so restore the matter to
its original form as in the case of ice and steam. But this is done by
nature. Ashes go to the soil, smoke into the atmosphere. The forces of
nature bring these elements together again in plant and tree, and so
it comes about that the materials resulting from the burning of wood
again become wood, and over and over again the cycle is repeated as
time rolls on. Many other examples might be cited to show what is meant
by the indestructibility of matter, or the conservation of matter; but
these will suffice to show that the one essential fact is that the
matter or stuff of a body cannot be destroyed.
Although matter is protean and its transformations limitless, there
are certain changes which cannot be made. Iron cannot be turned into
silver, nor silver into gold, nor oxygen into nitrogen. There appear to
be indeed about seventy or eighty distinct kinds of matter, and so far
as we know one cannot be converted into another. They may be united in
countless combinations, but each is itself not only indestructible but
unchangeable. Why this is so is an interesting subject of speculation.
We do not positively know.
That energy is also something which cannot be created or destroyed is
not so generally recognized. Transformations of energy from one form
to another are constantly occurring before our very eyes; and yet we
seldom stop to think what the conservation of energy means in any given
case. Energy itself is often defined as that which has the capacity for
doing work, and work is done when force or resistance is overcome. A
hod carrier does work when, overcoming the force of gravity upon his
body and his hod of brick, he climbs to the top of a ladder; and the
work done is a measure of the energy expended. Energy stored up in his
body has been transferred to the brick in their elevated position, and
if they are allowed to fall to the ground their energy is turned into
heat, developed by their impact upon the ground. Again, work is done
by a windmill in pumping water up into an elevated reservoir, and the
so-called ‘potential’ energy which the water possesses in its elevated
position has all been transferred to the water from the wind which
drove the mill. If the water be allowed to flow down to the ground
again through a water motor the latter could drive machinery and so do
work; and the work it could do plus the heat produced by friction would
exactly equal the work done in pumping the water up to its elevated
position. Thus is the energy conserved, and not destroyed. More or less
of it is dissipated by friction, and lost, so far as useful effect may
go. But it all remains in existence, somewhere.
Again, coal is burned under the boiler of a steam engine. Heat is
produced, steam is generated, the engine does work. The coal possessed
a store of energy, potentially. That is, the coal had the capacity of
uniting with the oxygen of the air and setting free a store of energy.
This energy, potential or latent in the coal, becomes kinetic and
evident in the heat of the boiler and the work of the engine. Moreover,
the work done by the engine added to the heat given off by the boiler
and engine is exactly equal to the total store of energy possessed by
the coal. And if from a store of energy, either in the body of a man or
horse, or in a pile of wood or coal, a certain portion is expended in
doing work, the amount remaining is exactly the difference between that
expended and the original amount. In short, energy can be measured,
stored up and expended, just as truly as merchandise or money.
Thus the conservation of energy means that energy cannot be created
or destroyed; but it may be transferred from one body to another or
transformed from one form to another. Heat may be converted into work
and work into heat. The chemical energy of a zinc rod may be expended
to generate an electric current, and the latter passing through a coil
of wire or the filament of a lamp gives up its energy to produce heat
and light. The last form of this energy is equal in quantity to the
first.
Niagara represents a vast store of energy. Millions of tons of water
falling 160 feet could do a vast amount of mechanical work if properly
applied through water wheels. More than 50,000 horse power of useful
work is actually derived from Niagara’s waters, but this is only a
small fraction of the total. The energy is, however, given up in
falling, even though no useful work is done. In fact, the water is
slightly heated by the impact, and the amount of heat produced is
exactly equivalent to the mechanical energy lost by the water.
A cannon ball receives a large amount of kinetic energy from the
exploded powder as it leaves the muzzle of a great gun. If it be
suddenly stopped by a rigid target its mechanical or mass energy is at
once converted into heat; that is, into the vibratory motion of the
molecules. Ball and target are highly heated. Indeed, lead bullets are
often melted by the heat of impact. Meteors flying through space come
into our atmosphere and their speed is checked by its resistance. Part
or all of their kinetic energy is thus converted into heat. Both air
and meteor are heated; heated to so high a temperature that the meteor
becomes brilliantly luminous, and we call it a shooting star. The idea
of heat due to frictional resistance is common enough. The _exact
equivalence_ between the mechanical energy lost and the heat produced
is the thing to be especially noticed here.
Let us now take as a final example a locomotive engine. It takes on a
store of fuel and water and, directed by its engineer, sets out for
a day’s duty. The coal to be burned possesses a definite amount of
energy. Let us say every pound has one unit of energy, and suppose
5,000 pounds of coal are taken. What becomes of these 5,000 units of
energy, appearing as heat when the coal is burned?
1. A large amount of heat is required to keep the boiler and engine
hot, due to the loss of heat to the atmosphere. The engine cylinders,
as well as fire box and boiler, must be kept very hot; other parts of
the engine become more or less heated. All parts therefore continually
give off heat, and a large part of the heat produced by the burning
coal is thus expended.
2. A second portion is expended in doing work. If our locomotive
hauls a 500-ton train up a one-per cent. grade for 100 miles it would
be doing 2,640,000 foot-tons of work in addition to that required
to overcome the friction of the rails and the resistance of the
atmosphere. This would require nearly 500 units of energy which would
come from the heat of the coal. The work is done through the agency
of steam, but the energy of the steam comes from the burning coal. A
small amount of work is also done in pumping water from the tank on
the tender into the boiler and in pumping air into the reservoir for
the use of the air brakes. This may be called the internal work of the
engine. A second portion of the heat is therefore expended in internal
and external work.
3. The steam after expanding in the cylinders of the engine escapes
into the atmosphere. Although it has been cooled somewhat by expansion,
it is still hot, and carries a large amount of heat away with it.
Moreover, the smoke and hot air which pass out through the smokestack
carry away a large quantity of heat. Hot ashes likewise carry away
heat. Hence a third portion of heat is lost through smoke and steam and
ashes. And this is the largest portion of the total quantity of heat
generated by the burning coal.
When coal is burned, oxygen of the air unites chemically with the
carbon and hydrogen of the coal to form carbonic acid, or carbon
dioxid, as it is technically called, and water vapor. The incombustible
mineral matter of the coal remains as ashes. Hence smoke contains
carbonic acid gas and water vapor in addition to fine particles of
unburned coal carried away in the draft of air.
When the grade is steep a great deal of work must be done by the
locomotive, much steam is required, and the quantity of fuel burned is
large in proportion. When the road is level fuel burns less rapidly,
and when the train stops, still more slowly. At night the locomotive
rests, fires are banked and combustion is very slow. This process so
briefly and incompletely sketched, is more interesting as one examines
it closer, and a locomotive seems almost living when one considers
minutely its wonderful performance.
But interesting and instructive though the operation of the locomotive
may be, it is not for its own sake that I have mentioned it. It
is rather in order to point out a remarkable parallel between its
operation and that of a human body. A parallel, indeed, between the
operation of a complex inanimate engine of iron and steel, and a still
more complex living engine of flesh and bone and blood; both obeying
the law of the conservation of energy, as well as the other laws of
physics and chemistry.
Consider now a human body as a living engine. That man is more than
matter is, of course, conceded. But we here regard only the animal
body, guided by the brain as its engineer. The day begins, as with
the locomotive, by taking a store of fuel and water, namely, food
and drink. Food is not, however, burned in the body in a confined
receptacle, like coal in the fire box of an engine, but is digested,
assimilated and distributed through the body by means of the
circulating blood. And while some of it goes to repair bodily waste,
becoming tissue, other portions are oxidized or burned to produce
heat. Non-digestible parts of the food pass away from the body as
refuse, like ashes from the fire box of the engine. That the body
fat and muscular tissue are also burned, producing heat, is literally
true. A hibernating animal keeps his body warm all winter by burning
up his autumnal store of body fat. Even a well-fed body is constantly
wearing away, or burning away, and hence requires constant repair.
Thus we see two distinct functions for food, which should be carefully
distinguished.
In the first place, as already indicated, food repairs waste and builds
up the body. It makes blood, bone and muscular tissue. Herein we see a
departure from the parallel with the steam engine. A locomotive is a
machine which runs in a way determined by its builder. But it cannot
grow nor repair wear and tear. It requires a whole machine shop plus
skilful mechanics to do that. The body, on the other hand, not only
runs like a complex mechanism when supplied with energy, but also
builds itself up and repairs waste. We express this by saying that it
possesses vital force or life, but in just what vital force consists
is a matter of speculation and controversy. The raw material which is
employed in this work of repairing and building up is found in the
food. But not all food can be so utilized. Only those materials which
contain nitrogen, the so-called proteids, as lean meat, the casein of
milk and gluten of wheat, can be made use of in this most important
work of growth and repair.
In the second place, food is the fuel of the body and is just as truly
burned as is coal in a furnace. Moreover, the quantity of heat which
a piece of meat or a slice of bread yields when burned in the body is
just the same as if it had been burned in a stove. Complete combustion
yields a definite amount of heat wherever and whatever may be the place
and manner of burning. Any kind of food may serve as fuel for the body,
but those which consist mainly of sugar, starch and fat, which contain
no nitrogen and so cannot build up the body, are used chiefly as fuel.
These fuel foods form the bulk of our daily ration, comparatively
little being required for purposes of growth and repair.
We are hearing a good deal recently about alcohol as a food. When it is
remembered that alcohol contains no nitrogen it will be seen that it
cannot serve the first function of food, namely, the purpose of growth
and repair. It can, however, serve as fuel food, for when taken into
the body in small quantities it is assimilated and burned up, producing
the same amount of heat as if burned in a lamp. In sickness this may be
beneficial, at times when the body cannot assimilate other foods. But
the injurious effects of alcohol upon the digestive and nervous systems
are so important and far-reaching that its value as a fuel food sinks
into insignificance in comparison.
The process of combustion or burning in the fire box of our locomotive
consists, as has been said, in oxygen of the air uniting with the
carbon and hydrogen of the coal, forming carbonic acid and water, and
setting free a definite quantity of heat for every pound of fuel so
burned. So, in exactly the same way, oxygen, which has been taken up by
the blood from the air in the lungs, unites with carbon and hydrogen
in the tissues of the body and forms carbonic acid and water, yielding
precisely the same amount of heat as though the combustion had occurred
in a furnace. This idea of food, that it is literally fuel, is a very
suggestive one. And as fuels differ in the quantity of ash contained
and the amount of heat produced, so food materials differ in the
quantity of undigestible residue and in their heat-producing power.
Remembering the analogy of the steam engine, let us now inquire what
becomes of the energy supplied to the body in the fuel foods eaten,
and which is turned into heat by this process of combustion constantly
going on.
1. A large amount of heat is constantly being expended in keeping the
body warm. Like the locomotive, the body is warmer than the surrounding
air, and is constantly losing heat to the atmosphere. Unlike the
locomotive, however, the body has a nearly uniform temperature
throughout, namely, 98 degrees Fahr. The delicate regulation of
temperature which is automatically maintained in the animal body is one
of the wonders of physiology.
2. A second portion of energy is required to do the mechanical work of
the body. When a locomotive hauls a loaded train up grade, or steams
up grade alone, it is doing work in proportion to the total weight
and the height to which it is carried. So when a man walks up hill or
climbs a ladder he is lifting his body against the force of gravity,
and hence doing work. If his weight be 200 pounds he is doing twice
as much work as though he weighed only 100 pounds. If a man weighing
150 pounds climbs Bunker Hill Monument (220 feet), 33,000 foot-pounds
of work will then be done; and if he succeeds in making the ascent in
one minute, he would be doing work at the rate of one full horse power
for that minute. If he climbs a mountain two miles high in three hours
and twelve minutes he would be doing work in so lifting his body at
the rate of one quarter of a horse power. This is, of course, a faster
rate of work than an average man could maintain. In all the functions
of daily life the body is necessarily doing some mechanical work. Even
dressing and eating require a certain expenditure of energy, and in
ordinary business and manual labor the amount of mechanical work done
is considerable. Moreover, a large amount of work is done by the heart
in pumping the blood through the circulatory system, and by the chest
in respiration. This, then, the internal and external work done, as in
the locomotive, represents the second portion of energy derived from
the food eaten.
3. The warm air, carbonic acid gas and water vapor passing away from
the lungs in respiration carry with them a large amount of heat. This
corresponds to the loss of heat in the locomotive through the smoke
passing out the smokestack, and in both cases the loss is greater when
work is being done and less during inaction. The refuse products of the
body (as the ashes of the locomotive) also carry away heat. This is the
third portion of heat and is a large one.
Work is done in the locomotive by the expanding steam in the cylinders
of the engine. The steam is cooled as it expands. Hence heat disappears
when work is done; that is, is converted into mechanical energy, and a
steam engine is hence called a heat engine; an engine for converting
heat into work, according to the law of the conservation of energy.
As the pistons are pushed to and fro by the tremendous pressure of
the expanding steam, the reciprocating motion is communicated to
the great drivers of the engine by strong arms of steel. But how is
work done in the body? That is a question of prime importance and
of surpassing interest. When muscle contracts and force is exerted,
as when the body is lifted or an oar is pulled, muscular tissue (or
material stored in muscular tissue) is oxidized; that is, burned, and
heat is produced; yet not as much heat appears as would have appeared
on the combustion of the same amount of body material if no work had
been done. Apparently, then, heat has been converted into work. But we
cannot trace the process with the same clearness as in the cylinder of
a steam engine. Whether the potential energy of the body material is
directly converted into work, or whether combustion first produces heat
and a part of this heat is then converted into work, we do not know. In
other words, we do not know whether the animal body as a machine for
doing mechanical work is a heat engine or some other kind of engine.
This is a fundamental question, as well as a very difficult one, and
to a student of thermodynamics and physiology it prompts all sorts of
speculation.
When one tries to picture to himself how the potential energy of food
or body tissue can be directly converted into mechanical work, he is
apt to turn to the other alternative and imagine that in some way the
body is a heat engine. For we know that heat results from the oxidation
of tissue, and we also know how heat can be converted into mechanical
work. But we are at once confronted with a difficulty. One of the
fundamental laws of thermodynamics requires that when heat is converted
into work there shall be a difference of temperature between the source
of heat and the place to which the heated material employed passes
after doing the work. In other words, in a heat engine, whatever the
mechanism, there must be a fall of temperature, which is greater as the
relative amount of work, or efficiency, is greater. In the human body
the efficiency perhaps surpasses that of the best steam engines; hence
there should be a fall of temperature comparable with that between
the boiler and condenser of a steam engine. This may be 100 degrees
or more, and we do not know of any such difference of temperature in
the body. Indeed, we know, on the contrary, that the temperature of
the body is remarkably uniform, as already stated. It is possible,
however, that there are molecular differences of large amount. In other
words, if we could make an ultra-microscopic survey of temperature
in a muscle during contraction, there might be found places of high
temperature where combustion was occurring, and all the requirements of
a heat engine of molecular dimensions fulfilled. But this is a matter
of speculation. The process may yet be found to be electrical, or
something else quite different from that of a steam engine.
We thus find between the animal body and a locomotive engine a striking
parallel. In many particulars the chemical and physical processes going
on in the latter are found also in the former. In both, the fundamental
law of the conservation of energy is strictly observed. Nevertheless,
the animal body considered simply as a machine is far more complex in
its structure and operation than the engine, and far more of mystery
envelops its working. Much remains for the chemist and physicist and
physiologist to reveal, and no more fascinating field of research
exists.