In the case of ordinary mechanical power it had been long known that the intervention of machinery did not create force, but only transformed it. If a weight of 1 lb., A, just balances a weight of 2 lb., B, by aid of a pulley, and by the addition of a minute fraction, such as a grain, raises it 1 foot, it will be invariably found that A has descended 2 feet. In other words, 1 lb. working through 2 feet does exactly the same work as 2 lbs. working through 1 foot. And whatever may be the intervening machinery the same thing holds good, and the work put in at one end comes out, neither more nor less, at the other, except for a minute loss due to friction and resistance of air. If a force equal to 1 lb. is made, by multiplying the intermediate machinery, to raise a ton a foot from the ground, exactly as much force must have been exerted as if the ton had been divided into 2,240 parts of 1 lb. each, and each part separately lifted.

But although energy cannot be created, at first sight it seems as if it might be destroyed, as when the ton falls to the ground and seems to have lost all its energy, whether of motion or of position. But here science steps in and shows us that it is not destroyed, but simply transformed into another sort of motion, which we call heat.

Some connection between mechanical work and heat had long been known, as in the familiar experiment of rubbing our hands together to warm them; and the practice known to most primitive races of obtaining fire by twirling a stick rapidly in a hole drilled in a block of wood; a practice described by the old Sanskrit word ‘pramantha,’ which means an instrument for obtaining fire by pressure or friction, and which, translated into Greek, has been immortalised by the legend of Prometheus. But it was reserved for recent years, and for an English philosopher, Dr. Joule, to give scientific precision and generality to this idea, by actually measuring the amount of heat produced by a given amount of work, and showing that they were in all cases convertible terms, so much heat for so much work, and so much work for so much heat. He did this by measuring accurately by a thermometer the heat added to a given amount of water by the work done by a set of paddles revolving in it, set in rapid motion by a known weight descending through a known space. The unit of work being taken as that sufficient to raise 1 kilogramme through 1 metre, and that of heat as that required to raise the temperature of one kilogramme of water by 1° Centigrade, the relation between them, as found by a vast number of careful experiments, is that of 424 to 1. That is, one unit of heat is equal to 424 units of work.

In this, and all cases requiring scientific precision, it is better to use the units of the metrical system than our clumsy English standards; but it may be sufficient for the ordinary reader to take the metre, which is about 39·37 inches, as practically a yard, and the kilogramme, which is 15,432 English grains, as practically equal to 2 lbs. This is sufficient to show the much greater energy of the invisible forces which act at minute distances, than that of gravity and other forces which do appreciable mechanical work, the energy of a weight falling from a height of more than 1,300 feet being only sufficient to heat its own weight by 1°.

This proof of the convertibility of work into heat gives much greater precision to our ideas respecting the real nature of heat and its kindred molecular and atomic energies. Heat is clearly not a material substance, for a body does not gain weight by becoming hotter. In the case of all ponderable matter down to the atoms, which are only of the size of cricket-balls compared to that of the earth, any combination which adds matter adds weight, and the weight of the product exactly equals the sum of the weights of the separate factors which have united to form it. Thus, if iron is burnt in oxygen gas, the product, oxide of iron or rust, weighs more than the original iron by just as much as the weight of the oxygen which has been consumed. But heat, light, and electricity add nothing to the weight of a body when they are added to it, and take nothing away when they are subtracted. The inference is unavoidable that heat, like light, is not ponderable matter, but an energy transmitted by waves of the imponderable medium known as ether. This is confirmed by finding that when a ray from the sun is analysed by passing through a refracting prism, one part of the spectrum shows light of various colours, while another gives heat. The hottest part of the spectrum lies in the red and beyond it, showing that the heat-waves are longer, and their oscillations slower, than those of light. Heat-waves also may be made to interfere, and to become polarised, in a manner analogous to the phenomena exhibited by those of light.

There can be no doubt, therefore, that heat, like light, is an energy or mode of motion, transmitted by waves of an imponderable ether, and that it acts on the molecules and atoms of matter by the accumulated successive impulses of those waves on the molecules and atoms which are floating in it, or rather which are revolving in it, in definite groups and fixed orbits, like miniature solar systems or starry universes. We can now see how heat performs work, and why work can be transformed into it.

Heat performs work in two ways. First, it expands bodies—that is, it draws their molecules farther apart against the force of cohesion which binds them together or keeps them moving in definite orbits at definite distances. It is as if it increased the velocity, and therefore the centrifugal force of a system of planets, and so caused them to revolve in wider orbits. The expansion of mercury in a thermometer affords a familiar instance of this effect of heat and the readiest measure of its amount. Secondly, it increases the energy of the molecular motions, so that they dart about, collide, and vibrate with greater force. Thus, as heat increases, evaporation increases, for molecules on the surface are projected with so much force as to get beyond the sphere of the cohesive attraction which binds them to the system, and they dart off like comets into space. Finally, as heat increases, and more and more work is done, against the centripetal force of cohesion, most substances, and doubtless all if we could get heat enough, are converted from solids into fluids, and ultimately into gases, in which latter state the molecules have got altogether beyond the sphere of their mutual attraction, and tend to dart off indefinitely in the direction of their own proper centrifugal motions, unless confined, in which case they dart about, collide, rebound, and exercise pressure on the containing surface.

Conversely, if heat expands bodies, it is given out when they contract. Thus the enormous quantity of heat poured out for millions of years by the sun, is probably owing mainly to the mechanical force of contraction of the original cosmic matter condensing about the solar nucleus.

Again, when gases suddenly expand, their temperature falls, which is the principle by which artificial ice is procured, and frozen beef and mutton are brought from America and Australia, producing, such are the complicated relations of modern society, agricultural depression, fall of rents, and a serious aggravation of the Irish question.

As an example of the converse proposition of the transformation of heat into mechanical work, the steam-engine affords the aptest illustration. The original power came from the sun millions of years ago, and did work by enabling the leaves of plants to overcome the strong mutual affinity of carbon and oxygen in the carbonic dioxide in the air, and store up the carbon in the plant, where it remained since the coal era in the form of energy of position. By lighting the coal, or in other words separating its molecules more widely by heat, we enable them to exert once more their natural affinity for oxygen, and burn, that is recombine into carbonic dioxide. The heat thus produced turns water into steam, which passes through a cylinder, either into a condenser if the steam is at low pressure, or into the outer air if it has been superheated and brought to a higher pressure than that of the atmosphere. The difference of the pressure or elasticity of the steam in the boiler, and of the same steam when it is condensed or liberated, is available for doing work, and, being admitted and released alternately at the two ends of the cylinder, drives a piston up and down, which, by means of cranks and shafts, turns a wheel or does whatever work is required of it. In doing this, heat disappears, being converted into work, and the amount of heat would exactly equal that into which the work would be converted according to Joule’s law, if it could all be utilised without the loss necessarily incurred by friction, radiation, and the still more important absorption of latent heat required to convert water at boiling-point into vapour of the same temperature. This latter is not really an annihilation of the heat, but its conversion into work done in separating the molecules against the force of cohesion. The whole heat, therefore, is transformed into work, mainly molecular work in tearing molecules asunder, and the residue into mechanical work turning spindles and driving locomotives and steamboats.

The intermediate machinery here, including the water in the boiler, is merely the means of applying the original energy in the particular way we desire. The essential thing is the transformation of a certain amount of heat into work by passing, in accordance with the laws of heat, from a hotter to a colder body. The last condition is indispensable, for the nature of heat is to seek an equilibrium by passing from hot to cold, and no work can be got out of it in the reverse way. On the contrary, work must be expended and turned into heat to restore the temperature which has run down. The case is analogous to that of water, which, if raised by evaporation or stored up in reservoirs at a level above the sea, can be made to turn a wheel while it is running down; but when it has all run down to the sea level, can do no more work, and can only be pumped up again to a higher level by the expenditure of fresh work. Owing to this tendency of heat we can see that, although matter and energy are to all appearance indestructible, the present constitution of the universe is not eternal. The animating energy of heat is always tending to obliterate differences of temperature, and bring all energy down to one uniform dead level of a common average, in which no further life, work, or motion are possible. Fortunately this consummation is far off, and for many tens or hundreds of millions of years the inhabitants of this tiny planet may feel fairly secure, and need not, like the late Dr. Cumming, of millenarian celebrity, introduce breaks in the leases of their houses to provide against the contingency of the world coming to an end at an early date.

Dismissing, then, to the remote future any speculations as to the failure of this essential element of active energy, let us rather consider the various protean forms in which it shows itself.

1. The energy of visible motion, which, as we have seen, may be transformed into an equivalent amount of energy of position.

2. Molecular energy, which causes the cohesive attraction, repulsion, and other proper motions of these minute and invisible particles of matter.

3. Energy of heat and light, which are transmitted by waves of the assumed imponderable medium called ether.

4. Energy of chemical action, by which the small ultimate particles of ponderable matter, called atoms, separate and combine into the various combinations of molecules constituting visible matter, in obedience to certain affinities, or inherent attractions and repulsions.

5. Electrical energy, which includes magnetism as a special instance.

All these forms of energy may exist, as in the case of visible energy, either as energies of motion or of position, and the actual constitution of the universe is due in a great measure to the alternation of these two energies. Thus all wave-motion, whether it be of the waves of the sea grinding down a rocky coast, of the air transmitting sound, or of ether transmitting light and heat, are instances of energies of motion and of position, conflicting with one another and alternately gaining the victory. So also a pound of gunpowder or dynamite has an immense energy of position, which, when its atoms are let loose from their mutual unstable connection by heat or percussion, manifests itself in an enormous energy of motion, which is more or less destructive according to the rapidity with which the atoms rush into new combinations.

Let us consider these different energies a little more in detail. The energy of visible motion is manifested principally by the law of gravity, under which all matter attracts other matter directly as the mass and inversely as the square of the distance. It is a universal and uniform law of matter, and can be traced without change or variation from the minutest atom up to the remotest double star. The energy of living force might, at first sight, be considered as another of the commonest causes of visible motion; but, when closely analysed, it will be found that what appears as such is only the result of molecular energy of position stored up in the living body by chemical changes during the slow combustion of food, and that nothing has been added by any hypothetical vital force. The conscious will seems to act in those cases simply as the signalman who shows a white flag may act on a train which has been standing on the line waiting for it. The energy which moves the train is due entirely to the difference of heat, which has been developed by the combustion of coal, between the steam in the boiler and the steam when allowed to escape into the air; and this energy came originally from the sun, whose rays enabled the leaves of growing plants to decompose carbonic dioxide and store up the carbon in the coal. Of this force of gravity causing visible motion we may say that it is comparatively a very weak force, which acts uniformly over all distances great or small.

Molecular energies, on the other hand, act with vastly greater force, but at very small distances, and appear sometimes as attractive and sometimes as repulsive forces. Thus solid bodies are held together by a force of cohesion which is very powerful, but acts only at very small distances, as we may see if we break a piece of glass and try to mend it by pressing the broken edges together. We cannot bring them near enough to bring the molecular attraction again into play and make the broken glass solid. But the same glass acts with repellent energy if another solid tries to penetrate it, so that we can walk on a glass floor without sinking into it. Heat also, by increasing the distance between the molecules, first weakens the cohesive force so that the solid becomes fluid, and finally overcomes it altogether, so that it passes into the state of gas in which the centripetal attraction of the molecules is extinguished, and they tend to recede further and further from each other under the centrifugal force of their own proper velocities. The great energy of molecular forces will be apparent from the fact that a bar of iron, in cooling 10° Centigrade, contracts with a force equal to a ton for each square inch of section, as exemplified in the tubular bridge across the Menai Straits, where space has to be allowed for the free contraction and expansion of the iron under changes of temperature.

Chemical energy, or the mutual attractions and repulsions of atoms, is even more powerful than that of molecules. It displays itself in their elective affinities, or what may be called the likes and dislikes, or loves and hatreds, of these ultimate particles. Perhaps the best illustration will be afforded by that ‘latest resource of civilisation,’ dynamite. This substance, or to give it its scientific name, nitro-glycerine, is composed of molecules each of which is a complex combination of nine atoms of oxygen, five of hydrogen, three of nitrogen, and three of carbon. Of these, oxygen and hydrogen have a strong affinity for one another, as is seen by their rushing together whenever they get the chance, and by their union forming the very stable compound, water. Oxygen and carbon have also a very strong affinity, and readily form the stable product carbonic dioxide gas. Nitrogen, on the other hand, is a very inert substance; its molecule consists of two atoms of itself which are bound together by a strong affinity, and can only be coaxed with difficulty into combinations with other elements, forming compounds which are, as it were, artificial structures, and very unstable. We see this in the air, which consists mainly of oxygen and nitrogen, but not in chemical combination, the oxygen being simply diluted by the nitrogen, as whisky is with water, with the same object of diluting the too powerful oxygen or too potent alcohol, and enabling the air-breather or whisky-drinker to take them into the system without burning up the tissues too rapidly. If nitrogen had more affinity for oxygen it would combine chemically with it, and we should live in an atmosphere of nitrous oxide, or laughing gas.

The molecule, therefore, of nitro-glycerine resembles a house of cards, so nicely balanced that it will just stand, but will fall to pieces at the slightest touch. When this is supplied by a slight percussion the molecule falls to pieces and is resolved into its constituent atoms, which rush together in accordance with their natural affinities, forming an immense volume of gas, partly of water in the form of steam where oxygen has combined with hydrogen, and partly of carbonic dioxide where it has combined with carbon, leaving the nitrogen atoms to pair off, and revert to their original form of two-atom molecules of nitrogen gas. It is as if ill-assorted couples, who had been united by matrimonial bonds tied by the manoeuvres of Belgravian mothers, found themselves suddenly freed by a decree of divorce a vinculo matrimonii, and rushed impetuously into each other’s arms, according to the laws of their respective affinities. So striking is the similitude that one of Goethe’s best-known novels, the ‘Wahlverwandschaften,’ takes its title from the human play of these chemical reactions. The enormous energy developed when these atomic forces are let loose and a vast volume of gas almost instantaneously created, is attested by the destructive force by which the hardest rocks are shattered to pieces and the strongest buildings overthrown.

These loves and hatreds, or, as they are termed, chemical affinities and repulsions of the atoms, are the principal means by which the material structure of the universe is built up from the original elements. The earth, or solid crust of the planet we inhabit, consists mainly of oxidised bases, and is due to the affinity of oxygen for silicon, calcium, aluminium, iron, and other primary elements of what are called metals. This affinity enables them to make stable compounds, which, under the existing conditions of temperature and otherwise, hold together and are not readily decomposed. Water in like manner, in all its forms of waves, seas, lakes, rivers, clouds, and invisible vapour, is due to the affinity between oxygen and hydrogen forming a stable compound. Salt again is owing to the affinity of chlorine for sodium, and so for nearly all the various products with which we are familiar, oxygen and nitrogen in the air we breathe being almost the only elements which exist in their primary and uncombined state in any considerable quantities, and form an essential part of the conditions which render our planet a habitable abode for man and other forms of life.

We shall see presently something more of the nature of these affinities, and the laws by which they act; but before entering on this branch of the subject we must consider the remaining form in which the one indestructible energy of the universe manifests itself, viz. that of electricity.

Electricity is the most subtle and the least understood of these forms. In its simplest form it appears as the result of friction between dissimilar substances. Thus if we rub a glass rod with a piece of silk, taking care that both are warm and dry, we find that the glass has acquired the property of attracting light bodies, such as little bits of paper, or balls of elder-pith. Other substances, such as sealing-wax and amber, have the same property. Pursuing our research further we find that this influence is not, like that of gravity, uniform and always acting in the same direction, but of two kinds, equal and opposite. If we touch the pith-ball by the excited glass rod, it will after contact be repelled; but if we bring the ball which has been excited by contact with the glass within the influence of a stick of sealing-wax which has been excited by rubbing it with warm dry flannel, the ball instead of being repelled is attracted.

Conversely, if the pith-ball has been first touched by excited sealing-wax, it will afterwards be repelled by excited sealing-wax and attracted by excited glass. It is clear, therefore, that there are two opposite electricities, and that bodies charged with similar electricities repel, and with unlike electricities attract, one another. For convenience, one of these electricities, that developed in glass, is called positive, and the other negative; and it has been clearly proved that one cannot exist without the other, and that whenever one electricity is produced, just as much is produced of an opposite description. If positive electricity is produced in glass by rubbing it with silk, just as much negative electricity is produced upon the silk.

Another primary fact is that some substances are able to carry away and diffuse or neutralise this peculiar influence called electricity, while others are unable to do so and retain it. The former are called conductors, the latter non-conductors. Thus, glass is an insulator or non-conductor, while metal is a conductor of electricity; and the reason why the substances rubbed together, as glass and silk, must be dry is that water, in all its forms, is a conductor which carries away the electricity as fast as it is produced.

These facts have given rise to a theory—which is after all not so much an explanation as a convenient mode of expressing the facts—of the existence of two opposite electric fluids, which, in the ordinary or unexcited body, are combined and neutralise one another, but are separated by friction, and flow in opposite directions, accumulating at opposite poles, or, it may be, one being accumulated at one pole, while the other is diffused through some conducting medium and lost sight of. The active electricity, be it positive or negative, thus accumulated at one pole, and retained there by the substance in contact with it being a non-conductor, disturbs by its influence the electrical equilibrium of any body brought near to it, separates its two fluids, and attracts the one opposite to itself. This attraction draws the light body towards it until contact ensues, when the electric fluid of the excited body flows into the smaller one, so that its opposite electricity is expelled, and it is in the same condition as its exciter, and therefore liable to be repelled by a similar exciter, or attracted by an opposite one which formerly repelled it.

It is evident, without going further, that there is a great analogy between electrical energy and those of heat and of chemical affinity. The same mechanical work—viz. friction—which generates heat, generates electricity. The chief difference seems to be that friction may be transformed into heat when the same substances are rubbed together, as in the case of obtaining fire by the friction of wood; but electricity can only be obtained by friction between dissimilar substances. Thus no electricity is obtained by rubbing glass upon glass, or silk upon silk, or upon glass covered with silk, though a slight difference of texture is sometimes sufficient to separate the electric fluids. Thus if two pieces of the same silk ribbon are rubbed together, lengthways, no electricity is produced, but if crossways, one is positively, and the other negatively, electrified. In this respect the analogy is evident to chemical affinity, which, in like manner, only acts between dissimilar bodies.

In order, however, to carry the proof of the identity of these forms of energy beyond the sphere of vague analogy, we must follow up electricity far beyond the simple manifestations of the glass rod and sealing-wax, and pursue it to its origin, in the transformations of chemical action and mechanical work, in the voltaic battery, the electric telegraph, the telephone, and the dynamo.

The voltaic battery, in its simplest form, is a trough containing an acid liquid in which pairs of plates of different metals are immersed. It is evident that if the action of the acid on each metal were precisely the same, equal quantities of each would be dissolved in the acid, and the equilibrium of chemical energies would not be affected. But, the action being different, this equilibrium is disturbed, and if the sum of these disturbances for a number of separate pairs of plates can be accumulated, it will become considerable. This is done by connecting the plates of the same metal in each cell by a metallic wire covered by some non-conducting substance. There are, therefore, two wires, one to the right hand, the other to the left, the loose extremities of which are called the poles of the battery. If we test these poles as we did the glass rod and stick of sealing-wax, we find that one pole is charged with positive and the other with negative electricity. In other words, the chemical energy, whose equilibrium was disturbed by the unequal action of the acid on the plates of different metals, has been transformed into electrical energy manifesting itself, as it always does, under the condition of two equal and opposite polarities. If we connect these two poles with one another the two electricities rush together and unite, and there is established what is called an electrical current circulating round the battery. As the chemical action of the acid on the metals is not momentary but continuous, the acid taking up molecule after molecule of the metal, so also the current is continuous. When we call it a current, the term is used for the sake of convenience, for as the current, as we shall presently see, will flow along the wire or other conducting substance for immense distances, as across the Atlantic, with a velocity of many thousands of miles per second, we can, no more than in the case of light, figure it to ourselves as an actual transfer of material particles swept along as by a river running with this enormous velocity, but necessarily as a transmission of some form of motion travelling by waves or tremors through the all-pervading ether in which the atoms of the conducting wire are floating. Be this as it may, the effect of these electric currents is very varied and very energetic. It can produce intense heat, for if, instead of uniting the two poles, we connect them by a thin platinum wire, it will, in a few seconds, become heated to redness. If the connecting wire is thicker, heat will equally be generated but less intense, thus maintaining the analogy to the current which rushes with more impetuosity through a narrow than through a wide channel. If the poles are tipped with a solid substance like carbon, whose particles remain solid under great heat, when they are brought nearly together intense light is produced and the carbon slowly burns away. This produces what is called the arc light, which gives such a strong illuminating power and is coming into general use for lighting up large spaces.

Another transformation is back again into chemical energy, which is shown by the power of the electric current to decompose compound substances. If, for instance, the poles of a battery are plunged into a vessel containing water, the molecules of the water will be decomposed and bubbles of oxygen gas will rise from the positive, and of hydrogen from the negative, pole.

Another effect of electrical currents is that of attraction and repulsion on one another. If two parallel wires, free to move, carry currents flowing in the same direction as from positive to negative, or vice versÂ, they will attract one another; if in opposite directions, they will repel. Electrical currents also work by way of induction, that is, they disturb the electrical equilibrium of bodies brought within their influence and induce currents in them. Thus, if we have two circular coils of insulated wire placed near each other, one on the right hand, the other on the left, and connect the extremities of the right-hand coil with the poles of a battery, when the connection is first made and the current begins to flow, a momentary current in the opposite direction will pass through the left-hand coil. This will cease, and as long as the current continues to flow through the right-hand coil there will be no current through the other; but if we break the contact between the right-hand coil and the battery, there will be again a momentary current through the left-hand coil, but this time in the same direction as the other. The same effect will be produced if, instead of making and breaking contact in the right-hand coil, we keep the current constantly flowing through it, and make the right-hand coil alternately approach and recede from the other coil. In this case, when the right-hand coil approaches, it induces an opposite current in the left-hand one; and when it recedes, one in the same direction as that of the primary.

These phenomena of induction prepare us to understand the nature of magnets, and the magnetic effects produced by electrical currents. If an insulated wire is wrapped round a cylinder of soft or unmagnetic iron, and a current passed through the wire, the cylinder is converted into a magnet and becomes able to sustain weights. If the current ceases, the cylinder is no longer a magnet, and drops the weight. A magnet is therefore evidently a substance in which electric currents are circulating at right angles to its axis, and a permanent magnet is one in which such currents permanently circulate from the constitution of the body without being supplied from without. The earth is such a magnet, and also iron and other substances, under certain conditions.

This being established, it is easy to see why an electrical current deflects the magnetic needle. If such a needle is suspended freely near a wire parallel with it, on a current being passed through the wire it must attract if similar, or repel if dissimilar, the currents which are circulating at right angles to the axis of the needle, and thus tend to make the needle swing into a position at right angles with the wire so that its currents may be parallel to that of the needle. This is the reason why the needle in its ordinary condition points to the north and south, or rather to the magnetic poles of the earth, because its currents are influenced by the earth currents which circulate parallel to the magnetic equator. The deviation of the needle from this direction, caused by any other current, like that passed along the wire, will depend on the strength of the current, which may be measured by the amount of deflection of the needle. The direction in which the needle deflects, viz. whether the north pole swings to the right or to the left, will depend on the direction of the current through the wire. The direction of the circular currents which form a magnet is such that if you look towards the north pole of a freely suspended cylindrical magnet—i.e. if you stand on the north of it and look southwards—the positive current will ascend on your right hand, or on the west side, and descend on the east. It follows that unlike poles must necessarily attract, and like poles repel one another, for in the former case the circular currents which face each other are going in the same, and in the latter in opposite directions.

The reader is now in a position to understand the principle of the electric telegraph, that wonderful invention which has revolutionised human intercourse and, to a great extent, annihilated space and time. It originated in the discovery made by Oersted, a Danish savant, that the effect of an electric current was to make a magnet swing round, in the endeavour to place itself at right angles to it. The conducting power of insulated copper wire is such that it practically makes no difference whether one of the wires connected with the pole of a battery is two feet or 2,000 miles in length, and the earth, being a conducting medium, supplies an equal extension from the other pole, so that a closed electric circuit may be established across the Atlantic as easily as within the walls of a laboratory.

If, therefore, a magnetic needle is suspended at the American end, it will respond to every electrical current, and to any interruption, renewal, or reversal of that current established in England. The needle may thus be made to swing to the right or left, by forming or reversing a current through the wire; and it will return to its position whenever the current is interrupted, and repeat its movement whenever the current is renewed. In fact it may be made to move like the arm of the old-fashioned telegraph, or of a railway signal. It only remains to have a machine by which the operator can form and interrupt currents rapidly, and a code by which certain movements of the needle stand for certain letters of the alphabet, and you have the electric telegraph.

There are many ingenious applications of the machinery, but in principle they all resolve themselves into transformations of energy. Chemical energy is transformed into electric energy, and that again into mechanical work in moving the needle.

The telephone is another instance of similar transformations. Here spoken words create vibrations of the air, which cause corresponding vibrations in a thin plate or disc of metal at one end, which are conveyed by intermediate machinery to a similar disc at the other end, whose vibrations cause similar vibrations in the air, reproducing the spoken words at a distance which may be a great many miles from the speaker.

The great inventions of modern science which have so revolutionised society are all instances of the laws of the conservation of energy. Man makes the powers of nature available for his purposes by transforming them backwards and forwards, now into one, now into another form of energy, as required for the result he wishes to attain. He wants mechanical power to pump water or drive a locomotive or steamboat: he gets it from the steam-engine, by transforming the energy of heat in coal, which came ages ago from the energy of chemical action produced by the sun’s rays in the green leaves of growing plants. He wants to send messages in a few seconds across the Atlantic: he does it by transforming chemical energy into electricity in a voltaic battery, sending its vibrations along a conducting wire, and converting it at the far end into mechanical power, making a magnetic needle turn on its axis and give signals. If, instead of sending a message, he wants to hold a conversation at a distance, he invents the telephone, by which sound-vibrations of air are transformed into vibrations of a disc, then into electric currents, then into vibrations of a distant disc, and finally back again to spoken words. Or, if he wants light, he turns electricity into it by tipping the poles of his battery with carbon and bringing them close together.

The latest inventions of electrical science—the dynamo and the accumulator—afford remarkable instances of this convertibility of one primitive energy into different forms. In the instance just quoted of obtaining light from electricity by the voltaic battery, the cost has hitherto proved an obstacle to its adoption. The electrical energy is all obtained from the transformation of the heat produced in the cells by the chemical action on the metal used, which is commonly zinc. Now, the heat of combination of zinc with oxygen is only about one-sixth of that of coal, while the cost of zinc is about twenty times as great. Theoretically, therefore, energy got by burning zinc costs 120 times as much as that got by burning coal. Practically the difference is not nearly so great, for there is very little loss of energy in the battery by the process of conversion, while the best steam-engine cannot convert into work as much as twenty per cent, of the heat energy in the coal consumed. Still, after making every allowance, the cost of energy from zinc remains some twenty times as great as from coal, so that unless some process is found for obtaining back the zinc as a residual product, there is no prospect of this form of electricity being generally available for light or for mechanical power.

The dynamo is an instrument invented for the mechanical generation of electricity by taking advantage of the principle that electrical energy is produced by moving magnets near coils of wire, or coils of wire near magnets. A current is thus started by induction, and, once started, the mechanical power exerted in making the magnet or coils revolve is continually converted into electricity until the accumulated electrical energy becomes very powerful. The original energy comes of course from the coal burned in the steam-engine which makes the magnet or coils revolve.

The principle of the conservation of energy is well illustrated by the fact that as the dynamo generates an electric current if made to revolve, conversely it may be made to revolve itself if an electric current is sent through it from an exterior source. It is, therefore, available not only as a source of light in the former case, but as a direct source of mechanical power in the latter. It is on this principle that electric engines are constructed and electric railways are worked. Here also it is a question of cost and convenience, for you can only get electricity enough either to light a street or to drive an engine, by an original steam-engine or other motive power to work the dynamo, and a system of conducting wires to convey the electricity to the place where the light or power is wanted. Where the motive power is supplied by nature, as in the case of tidal or river currents or waterfalls, it is quite possible that power may be obtained in this way to compete with that obtained directly from the steam-engine; but there are as yet considerable practical difficulties to be overcome in the transmission of any large amount of energy for long distances.

To overcome some of these difficulties the accumulator has been invented, which affords yet another remarkable instance of the transformation of energy. It consists of two lead plates immersed in acidulated water. When a strong electrical current is sent through the water, it decomposes it, the oxygen going to one lead plate and the hydrogen to the other. The oxygen attacks the lead plate to which it goes, forming peroxide of lead; while the hydrogen reduces any oxide in the other plate, producing pure lead, and leaving a film of surplus hydrogen on the surface. The charging current is then reversed, so that the latter plate is now attacked and the former one reduced, when the current is again reversed. By continuing this process the surfaces of both lead plates become porous, so that they present a large surface, and can therefore hold a great deal of peroxide of lead. The charging current being now broken, the oxygen which has been forcibly separated from the liquid seeks to recombine with hydrogen; and if the two lead plates are joined by a wire, this effort of the oxygen generates an electrical current in the opposite direction to the original one, which is the current utilised. Electricity is thus stored up in a portable box, where it can be kept till wanted, when it is drawn out by connecting the plates, and as a large amount of energy has been accumulated the current which is produced lasts for a considerable time.

Unfortunately accumulators are bulky, heavy, and expensive, and nearly half the energy of the original charging current is lost in obtaining the reversed or working current. They are therefore not as yet adapted for general use, though perfectly capable of supplying either light or motive power, for both which purposes they have been successfully applied in special cases. The future both of electric power and electric lighting is now reduced entirely to a question of cost; and though it is hard to beat gas and the steam-engine, with cheap coal, and air and water for nothing, it is possible that by using natural sources of power to move dynamos, and by obtaining zinc back as a residual product in batteries, electricity may in certain cases carry the day.