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 tilings, 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 vortical 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.


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;[1] 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.


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 sonrce 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.[2]

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;[3] 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.

  1. This fact seems to have been known at a comparatively early period to Herschel and the elder Stephenson.
  2. This remark is due to Sir William Thomson.
  3. Mayer and Waterston seem first to have caught the rudiments of this idea.