_HISTORICAL SKETCH: THE DISSIPATION OF ENERGY._
182. In the last chapter we have endeavoured to exhibit the various
transmutations of energy, and, while doing so, to bring forward
evidence in favour of the theory of conservation, showing that it
enables us to couple together known laws, and also to discover new
ones--showing, in fine, that it bears about with it all the marks of a
true hypothesis.
It may now, perhaps, be instructive, to look back and endeavour to
trace the progress of this great conception, from its first beginning
among the ancients, up to its triumphant establishment by the labours
of Joule and his fellow-workers.
183. Mathematicians inform us that if matter consists of atoms or
small parts, which are actuated by forces depending only upon the
distances between these parts, and not upon the velocity, then it may
be demonstrated that the law of conservation of energy will hold good.
Thus we see that conceptions regarding atoms and their forces are
allied to conceptions regarding energy. A medium of some sort pervading
space seems also necessary to our theory. In fine, a universe composed
of atoms, with some sort of medium between them, is to be regarded as
the machine, and the laws of energy as the laws of working of this
machine. It may be that a theory of atoms of this sort, with a medium
between them, is not after all the simplest, but we are probably not
yet prepared for any more general hypothesis. Now, we have only to
look to our own solar system, in order to see on a large scale an
illustration of this conception, for there we have the various heavenly
bodies attracting one another, with forces depending only on the
distances between them, and independent of the velocities; and we have
likewise a medium of some sort, in virtue of which radiant energy is
conveyed from the sun to the earth. Perhaps we shall not greatly err
if we regard a molecule as representing on a small scale something
analogous to the solar system, while the various atoms which constitute
the molecule may be likened to the various bodies of the solar system.
The short historical sketch which we are about to give will embrace,
therefore, along with energy, the progress of thought and speculation
with respect to atoms and also with respect to a medium, inasmuch as
these subjects are intimately connected with the doctrines of energy.
_Heraclitus on Energy._
184. Heraclitus, who flourished at Ephesus, B.C. 500, declared that
fire was the great cause, and that all things were in a perpetual
flux. Such an expression will no doubt be regarded as very vague in
these days of precise physical statements; and yet it seems clear that
Heraclitus must have had a vivid conception of the innate restlessness
and energy of the universe, a conception allied in character to, and
only less precise than that of modern philosophers, who regard matter
as essentially dynamical.
_Democritus on Atoms._
185. Democritus, who was born 470 B.C., was the originator of the
doctrine of atoms, a doctrine which in the hands of John Dalton
has enabled the human mind to lay hold of the laws which regulate
chemical changes, as well as to picture to itself what is there taking
place. Perhaps there is no doctrine that has nowadays a more intimate
connection with the industries of life than this of atoms, and it
is probable that no intelligent director of chemical industry among
civilized nations fails to picture to his own mind, by means of this
doctrine, the inner nature of the changes which he sees with his eyes.
Now, it is a curious circumstance that Bacon should have lighted upon
this very doctrine of atoms, in order to point one of his philosophical
morals.
“Nor is it less an evil” (says he), “that in their philosophies and
contemplations men spend their labour in investigating and treating of
the first principles of things, and the extreme limits of nature, when
all that is useful and of avail in operation is to be found in what is
intermediate. Hence it happens that men continue to abstract Nature
till they arrive at potential and unformed matter; and again they
continue to divide Nature, until they have arrived at the atom; things
which, even if true, can be of little use in helping on the fortunes
of men.”
Surely we ought to learn a lesson from these remarks of the great
Father of experimental science, and be very cautious before we dismiss
any branch of knowledge or train of thought as essentially unprofitable.
_Aristotle on a Medium._
186. As regards the existence of a medium, it is remarked by Whewell
that the ancients also caught a glimpse of the idea of a medium, by
which the qualities of bodies, as colours and sounds are perceived, and
he quotes the following from Aristotle:--
“In a void there could be no difference of up and down; for, as in
nothing there are no differences, so there are none in a privation or
negation.”
Upon this the historian of science remarks, “It is easily seen that
such a mode of reasoning elevates the familiar forms of language, and
the intellectual connexions of terms, to a supremacy over facts.”
Nevertheless, may it not be replied that our conceptions of matter are
deduced from the familiar experience, that certain portions of space
affect us in a certain manner; and, consequently, are we not entitled
to say there must be something where we experience the difference of
up or down? Is there, after all, a very great difference between this
argument and that of modern physicists in favour of a plenum, who tell
us that matter cannot act where it is not?
Aristotle seems also to have entertained the idea that light is not any
body, or the emanation of any body (for that, he says, would be a kind
of body), and that therefore light is an energy or act.
_The Ideas of the Ancients were not Prolific._
187. These quotations render it evident that the ancients had, in some
way, grasped the idea of the essential unrest and energy of things.
They had also the idea of small particles or atoms, and, finally, of a
medium of some sort. And yet these ideas were not prolific--they gave
rise to nothing new.
Now, while the historian of science is unquestionably right in his
criticism of the ancients, that their ideas were not distinct and
appropriate to the facts, yet we have seen that they were not wholly
ignorant of the most profound and deeply-seated principles of the
material universe. In the great hymn chanted by Nature, the fundamental
notes were early heard, but yet it required long centuries of patient
waiting for the practised ear of the skilled musician to appreciate
the mighty harmony aright. Or, perhaps, the attempts of the ancients
were as the sketches of a child who just contrives to exhibit, in a
rude way, the leading outlines of a building; while the conceptions
of the practised physicist are more allied to those of the architect,
or, at least, of one who has realized, to some extent, the architect’s
views.
188. The ancients possessed great genius and intellectual power, but
they were deficient in physical conceptions, and, in consequence,
their ideas were not prolific. It cannot indeed be said that we of the
present age are deficient in such conceptions; nevertheless, it may be
questioned whether there is not a tendency to rush into the opposite
extreme, and to work physical conceptions to an excess. Let us be
cautious that in avoiding Scylla, we do not rush into Charybdis. For
the universe has more than one point of view, and there are possibly
regions which will not yield their treasures to the most determined
physicists, armed only with kilogrammes and metres and standard clocks.
_Descartes, Newton, and Huyghens on a Medium._
189. In modern times Descartes, author of the vertical hypothesis,
necessarily presupposed the existence of a medium in inter-planetary
spaces, but on the other hand he was one of the originators of that
idea which regards light as a series of particles shot out from a
luminous body. Newton likewise conceived the existence of a medium,
although he became an advocate of the theory of emission. It is
to Huyghens that the credit belongs of having first conceived the
undulatory theory of light with sufficient distinctness to account for
double refraction. After him, Young, Fresnel, and their followers,
have greatly developed the theory, enabling it to account for the most
complicated and wonderful phenomena.
_Bacon on Heat._
190. With regard to the nature of heat, Bacon, whatever may be thought
of his arguments, seems clearly to have recognized it as a species
of motion. He says, “From these instances, viewed together and
individually, the nature of which heat is the limitation seems to be
motion;” and again he says, “But when we say of motion that it stands
in the place of a genus to heat, we mean to convey, not that _heat_
generates _motion_ or _motion heat_ (although even both may be true in
some cases), but that essential heat is motion and nothing else.”
Nevertheless it required nearly three centuries before the true theory
of heat was sufficiently rooted to develop into a productive hypothesis.
_Principle of Virtual Velocities._
191. In a previous chapter we have already detailed the labours in
respect of heat of Davy, Rumford, and Joule. Galileo and Newton, if
they, did not grasp the dynamical nature of heat, had yet a clear
conception of the functions of a machine. The former saw that what we
gain in power we lose in space; while the latter went further, and saw
that a machine, if left to itself, is strictly limited in the amount of
work which it can accomplish, although its energy may vary from that of
motion to that of position, and back again, according to the geometric
laws of the machine.
_Rise of true Conceptions regarding Work._
192. There can, we think, be no question that the great development
of industrial operations in the present age has indirectly furthered
our conceptions regarding work. Humanity invariably strives to escape
as much as possible from hard work. In the days of old those who had
the power got slaves to work for them; but even then the master had
to give some kind of equivalent for the work done. For at the very
lowest a slave is a machine, and must be fed, and is moreover apt to
prove a very troublesome machine if not properly dealt with. The great
improvements in the steam engine, introduced by Watt, have done as
much, perhaps, as the abolition of slavery to benefit the working man.
The hard work of the world has been put upon iron shoulders, that do
not smart; and, in consequence, we have had an immense extension of
industry, and a great amelioration in the position of the lower classes
of mankind. But if we have transferred our hard work to machines, it is
necessary to know how to question a machine--how to say to it, At what
rate can you labour? how much work can you turn out in a day? It is
necessary, in fact, to have the clearest possible idea of what work is.
Our readers will see from all this that men are not likely to err in
their method of measuring work. The principles of measurement have
been stamped as it were with a brand into the very heart and brain of
humanity. To the employer of machinery or of human labour, a false
method of measuring work simply means ruin; he is likely, therefore,
to take the greatest possible pains to arrive at accuracy in his
determination.
_Perpetual Motion._
193. Now, amid the crowd of workers smarting from the curse of labour,
there rises up every now and then an enthusiast, who seeks to escape
by means of an artifice from this insupportable tyranny of work.
Why not construct a machine that will go on giving you work without
limit without the necessity of being fed in any way. Nature must
have some weak point in her armour; there must surely be some way
of getting round her; she is only tyrannous on the surface, and in
order to stimulate our ingenuity, but will yield with pleasure to the
persistence of genius.
Now, what can the man of science say to such an enthusiast? He cannot
tell him that he is intimately acquainted with all the forces of
Nature, and can prove that perpetual motion is impossible; for, in
truth, he knows very little of these forces. But he does think that
he has entered into the spirit and design of Nature, and therefore he
denies at once the possibility of such a machine. But he denies it
intelligently, and works out this denial of his into a theory which
enables him to discover numerous and valuable relations between the
properties of matter--produces, in fact, the laws of energy and the
great principle of conservation.
_Theory of Conservation._
194. We have thus endeavoured to give a short sketch of the history of
energy, including its allied problems, up to the dawn of the strictly
scientific period. We have seen that the unfruitfulness of the earlier
views was due to a want of scientific clearness in the conceptions
entertained, and we have now to say a few words regarding the theory of
conservation.
Here also the way was pointed out by two philosophers, namely, Grove
in this country, and Mayer on the continent, who showed certain
relations between the various forms of energy; the name of Séguin
ought likewise to be mentioned. Nevertheless, to Joule belongs the
honour of establishing the theory on an incontrovertible basis: for,
indeed, this is preeminently a case where speculation has to be tested
by unimpeachable experimental evidence. Here the magnitude of the
principle is so vast, and its importance is so great, that it requires
the strong fire of genius, joined to the patient labours of the
scientific experimentalist, to forge the rough ore into a good weapon
that will cleave its way through all obstacles into the very citadel of
Nature, and into her most secret recesses.
Following closely upon the labours of Joule, we have those of William
and James Thomson, Helmholtz, Rankine, Clausius, Tait, Andrews,
Maxwell, who, along with many others, have advanced the subject; and
while Joule gave his chief attention to the laws which regulate the
transmutation of mechanical energy into heat, Thomson, Rankine, and
Clausius gave theirs to the converse problem, or that which relates to
the transmutation of heat into mechanical energy. Thomson, especially,
has pushed forward so resolutely from this point of view that he has
succeeded in grasping a principle scarcely inferior in importance to
that of the conservation of energy itself, and of this principle it
behoves us now to speak.
_Dissipation of Energy._
195. Joule, we have said, proved the law according to which work may
be changed into heat; and Thomson and others, that according to which
heat may be changed into work. Now, it occurred to Thomson that there
was a very important and significant difference between these two laws,
consisting in the fact that, while you can with the greatest ease
transform work into heat, you can by no method in your power transform
all the heat back again into work. In fact, the process is not a
reversible one; and the consequence is that the mechanical energy of
the universe is becoming every day more and more changed into heat.
It is easily seen that if the process were reversible, one form of a
perpetual motion would not be impossible. For, without attempting to
create energy by a machine, all that would be needed for a perpetual
motion would be the means of utilizing the vast stores of heat that
lie in all the substances around us, and converting them into work.
The work would no doubt, by means of friction and otherwise, be
ultimately reconverted into heat; but if the process be reversible, the
heat could again be converted into work, and so on for ever. But the
irreversibility of the process puts a stop to all this. In fact, I may
convince myself by rubbing a metal button on a piece of wood how easily
work can be converted into heat, while the mind completely fails to
suggest any method by which this heat can be reconverted into work.
Now, if this process goes on, and always in one direction, there can be
no doubt about the issue. The mechanical energy of the universe will
be more and more transformed into universally diffused heat, until the
universe will no longer be a fit abode for living beings.
The conclusion is a startling one, and, in order to bring it more
vividly before our readers, let us now proceed to acquaint ourselves
with the various forms of useful energy that are at present at our
disposal, and at the same time endeavour to trace the ultimate sources
of these supplies.
_Natural Energies and their Sources._
196. Of energy in repose we have the following varieties:--(1.) The
energy of fuel. (2.) That of food. (3.) That of a head of water. (4.)
That which may be derived from the tides. (5.) The energy of chemical
separation implied in native sulphur, native iron, &c.
Then, with regard to energy in action, we have mainly the following
varieties:--
(1.) The energy of air in motion. (2.) That of water in motion.
_Fuel._
197. Let us begin first with the energy implied in fuel. We can, of
course, burn fuel, or cause it to combine with the oxygen of the air;
and we are thereby provided with large quantities of heat of high
temperature, by means of which we may not only warm ourselves and cook
our food, but also drive our heat-engines, using it, in fact, as a
source of mechanical power.
Fuel is of two varieties--wood and coal. Now, if we consider the origin
of these we shall see that they are produced by the sun’s rays. Certain
of these rays, as we have already remarked (Art. 180), decompose
carbonic acid in the leaves of plants, setting free the oxygen, while
the carbon is used for the structure or wood of the plant. Now, the
energy of these rays is spent in this process, and, indeed, there
is not enough of such energy left to produce a good photographic
impression of the leaf of a plant, because it is all spent in making
wood.
We thus see that the energy implied in wood is derived from the sun’s
rays, and the same remark applies to coal. Indeed, the only difference
between wood and coal is one of age: wood being recently turned out
from Nature’s laboratory, while thousands of years have elapsed since
coal formed the leaves of living plants.
198. We are, therefore, perfectly justified in saying that the energy
of fuel is derived from the sun’s rays;[7] coal being the store which
Nature has laid up as a species of capital for us, while wood is our
precarious yearly income.
We are thus at present very much in the position of a young heir, who
has only recently come into his estate, and who, not content with the
income, is rapidly squandering his realized property. This subject has
been forcibly brought before us by Professor Jevons, who has remarked
that not only are we spending our capital, but we are spending the most
available and valuable part of it. For we are now using the surface
coal; but a time will come when this will be exhausted, and we shall be
compelled to go deep down for our supplies. Now, regarded as a source
of energy, such supplies, if far down, will be less effective, for we
have to deduct the amount of energy requisite in order to bring them to
the surface. The result is that we must contemplate a time, however far
distant, when our supplies of coal will be exhausted, and we shall be
compelled to resort to other sources of energy.
_Food._
199. The energy of food is analogous to that of fuel, and serves
similar purposes. For just as fuel may be used either for producing
heat or for doing work, so food has a twofold office to perform. In
the first place, by its gradual oxidation, it keeps up the temperature
of the body; and in the next place it is used as a source of energy,
on which to draw for the performance of work. Thus a man or a horse
that works a great deal requires to eat more food than if he does not
work at all. Thus, also, a prisoner condemned to hard labour requires
a better diet than one who does not work, and a soldier during the
fatigues of war finds it necessary to eat more than during a time of
peace.
Our food may be either of animal or vegetable origin--if it be the
latter, it is immediately derived, like fuel, from the energy of the
sun’s rays; but if it be the former, the only difference is that it has
passed through the body of an animal before coming to us: the animal
has eaten grass, and we have eaten the animal.
In fact, we make use of the animal not only as a variety of nutritious
food, but also to enable us indirectly to utilize those vegetable
products, such as grasses, which we could not make use of directly with
our present digestive organs.
_Head of Water._
200. The energy of a head of water, like that of fuel and food, is
brought about by the sun’s rays. For the sun vaporizes the water,
which, condensed again in upland districts, becomes available as a head
of water.
There is, however, the difference that fuel and food are due to the
actinic power of the sun’s rays, while the evaporation and condensation
of water are caused rather by their heating effect.
_Tidal Energy._
201. The energy derived from the tides has, however, a different
origin. In Art. 133 we have endeavoured to show how the moon acts upon
the fluid portions of our globe, the result of this action being a very
gradual stoppage of the energy of rotation of the earth.
It is, therefore, to this motion of rotation that we must look as the
origin of any available energy derived from tidal mills.
_Native Sulphur, &c._
202. The last variety of available energy of position in our list is
that implied in native sulphur, native iron, &c. It has been remarked
by Professor Tait, to whom this method of reviewing our forces is due,
that this may be the primeval form of energy, and that the interior of
the earth may, as far as we know, be wholly composed of matter in its
uncombined form. As a source of available energy it is, however, of no
practical importance.
_Air and Water in Motion._
203. We proceed next to those varieties of available energy which
represent motion, the chief of which are air in motion and water in
motion. It is owing to the former that the mariner spreads his sail,
and carries his vessel from one part of the earth’s surface to another,
and it is likewise owing to the same influence that the windmill grinds
our corn. Again, water in motion is used perhaps even more frequently
than air in motion as a source of motive power.
Both these varieties of energy are due without doubt to the heating
effect of the sun’s rays. We may, therefore, affirm that with the
exception of the totally insignificant supply of native sulphur, &c.,
and the small number of tidal mills which may be in operation, all our
available energy is due to the sun.
_The Sun--a Source of High Temperature Heat._
204. Let us, therefore, now for a moment direct our attention to that
most wonderful source of energy, the Sun.
We have here a vast reservoir of high temperature heat; now, this
is a kind of superior energy which has always been in much request.
Numberless attempts have been made to construct a perpetual light,
just as similar attempts have been made to construct a perpetual
motion, with this difference, that a perpetual light was supposed to
result from magical powers, while a perpetual motion was attributed to
mechanical skill.
Sir Walter Scott alludes to this belief in his description of the grave
of Michael Scott, which is made to contain a perpetual light. Thus the
Monk who buried the wizard tells William of Deloraine--
“Lo, Warrior! now the Cross of Red
Points to the Grave of the mighty dead;
Within it burns a wondrous light,
To chase the spirits that love the night.
That lamp shall burn unquenchably
Until the eternal doom shall be.”
And again, when the tomb was opened, we read--
“I would you had been there to see
How the light broke forth so gloriously,
Stream’d upward to the chancel roof,
And through the galleries far aloof!
No earthly flame blazed e’er so bright.”
No earthly flame--there the poet was right--certainly not of this
earth, where light and all other forms of superior energy are
essentially evanescent.
_A Perpetual Light Impossible._
205. In truth, our readers will at once perceive that a perpetual light
is only another name for a perpetual motion, because we can always
derive visible energy out of high temperature heat--indeed, we do so
every day in our steam engines.
When, therefore, we burn coal, and cause it to combine with the
oxygen of the air, we derive from the process a large amount of high
temperature heat. But is it not possible, our readers may ask, to
take the carbonic acid which results from the combustion, and by
means of low temperature heat, of which we have always abundance
at our disposal, change it back again into carbon and oxygen? All
this would be possible if what may be termed the temperature of
disassociation--that is to say, the temperature at which carbonic acid
separates into its constituents--were a low temperature, and it would
also be possible if rays from a source of low temperature possessed
sufficient actinic power to decompose carbonic acid.
But neither of these is the case. Nature will not be caught in a
trap of this kind. As if for the very purpose of stopping all such
speculations, the temperatures of disassociation for such substances as
carbonic acid are very high, and the actinic rays capable of causing
their decomposition belong only to sources of exceedingly high
temperature, such as the sun.[8]
_Is the Sun an Exception?_
206. We may, therefore, take it for granted that a perpetual light,
like a perpetual motion, is an impossibility; and we have then to
inquire if the same argument applies to our sun, or if an exception
is to be made in his favour. Does the sun stand upon a footing of his
own, or is it merely a question of time with him, as with all other
instances of high temperature heat? Before attempting to answer this
question let us inquire into the probable origin of the sun’s heat.
_Origin of the Sun’s Heat._
207. Now, some might be disposed to cut the Gordian knot of such an
inquiry by asserting that our luminary was at first created hot; yet
the scientific mind finds itself disinclined to repose upon such an
assertion. We pick up a round pebble from the beach, and at once
acknowledge there has been some physical cause for the shape into which
it has been worn. And so with regard to the heat of the sun, we must
ask ourselves if there be not some cause not wholly imaginary, but one
which we know, or at least suspect, to be perhaps still in operation,
which can account for the heat of the sun.
Now, here it is more easy to show what cannot account for the sun’s
heat than what can do so. We may, for instance, be perfectly certain
that it cannot have been caused by chemical action. The most probable
theory is that which was first worked out by Helmholtz and Thomson;[9]
and which attributes the heat of the sun to the primeval energy of
position possessed by its particles. In other words, it is supposed
that these particles originally existed at a great distance from each
other, and that, being endowed with the force of gravitation, they have
since gradually come together, while in this process heat has been
generated just as it would be if a stone were dropped from the top of a
cliff towards the earth.
208. Nor is this case wholly imaginary, but we have some reason
for thinking that it may still be in operation in the case of
certain nebulæ which, both in their constitution as revealed by the
spectroscope, and in their general appearance, impress the beholder
with the idea that they are not yet fully condensed into their ultimate
shape and size.
If we allow that by this means our luminary has obtained his wonderful
store of high-class energy, we have yet to inquire to what extent this
operation is going on at the present moment. Is it only a thing of the
past, or is it a thing also of the present? I think we may reply that
the sun cannot be condensing very fast, at least, within historical
times. For if the sun were sensibly larger than at present his total
eclipse by the moon would be impossible. Now, such eclipses have
taken place, at any rate, for several thousands of years. Doubtless a
small army of meteors may be falling into our luminary, which would
by this fall tend to augment his heat; yet the supply derived from
this source must surely be insignificant. But if the sun be not at
present condensing so fast as to derive any sufficient heat from this
process, and if his energy be very sparingly recruited from without,
it necessarily follows that he is in the position of a man whose
expenditure exceeds his income. He is living upon his capital, and is
destined to share the fate of all who act in a similar manner. We must,
therefore, contemplate a future period when he will be poorer in energy
than he is at present, and a period still further in the future when he
will altogether cease to shine.
_Probable Fate of the Universe._
209. If this be the fate of the high temperature energy of the
universe, let us think for a moment what will happen to its visible
energy. We have spoken already about a medium pervading space, the
office of which appears to be to degrade and ultimately extinguish
all differential motion, just as it tends to reduce and ultimately
equalize all difference of temperature. Thus the universe would
ultimately become an equally heated mass, utterly worthless as far as
the production of work is concerned, since such production depends upon
difference of temperature.
Although, therefore, in a strictly mechanical sense, there is a
conservation of energy, yet, as regards usefulness or fitness
for living beings, the energy of the universe is in process of
deterioration. Universally diffused heat forms what we may call the
great waste-heap of the universe, and this is growing larger year by
year. At present it does not sensibly obtrude itself, but who knows
that the time may not arrive when we shall be practically conscious of
its growing bigness?
210. It will be seen that in this chapter we have regarded the
universe, not as a collection of matter, but rather as an energetic
agent--in fact, as a lamp. Now, it has been well pointed out by
Thomson, that looked at in this light, the universe is a system that
had a beginning and must have an end; for a process of degradation
cannot be eternal. If we could view the universe as a candle not lit,
then it is perhaps conceivable to regard it as having been always in
existence; but if we regard it rather as a candle that has been lit,
we become absolutely certain that it cannot have been burning from
eternity, and that a time will come when it will cease to burn. We are
led to look to a beginning in which the particles of matter were in
a diffuse chaotic state, but endowed with the power of gravitation,
and we are led to look to an end in which the whole universe will be
one equally heated inert mass, and from which everything like life or
motion or beauty will have utterly gone away.
FOOTNOTES:
[7] This fact seems to have been known at a comparatively early period
to Herschel and the elder Stephenson.
[8] This remark is due to Sir William Thomson.
[9] Mayer and Waterston seem first to have caught the rudiments of this
idea.