decompose a certain amount of some compound substance whose components have a great attraction for one another, so that by the reunion of these components a very high temperature might be produced; the heat of 212° would thus be converted into heat of a higher temperature which would be more available? or might not the radiant heat from the water be used to produce some kind of chemical decomposition, and thus the same result be obtained?

It is believed that any such result is, to say the least, unlikely; in fact, it has been remarked by Professor W. Thomson that an intimation to this effect seems to be given by nature, for we have no reason to think that either absorbed or radiant heat of low temperature is capable of producing powerful chemical changes of this nature.

There is no reason to think that this stratagem of bringing in chemical decomposition will increase the amount of available work to be obtained from one ton of hot water, and in our ignorance of the ultimate constitution of matter it would appear that the principle of the degradation and dissipation of energy, just as that of the conservation of energy, should be recognized as a principle having very strong claims to recognition, and as increasing these claims every day by the new facts which its employment as an instrument of research is constantly bringing to light.

At the same time it is but just to state that the principle of the conservation of energy is at present more fully established than that of the dissipation of energy.

386. Regarding, therefore, uniformly diffused heat as a form of energy utterly unavailable, and regarding mechanical energy as ultimately tending to assume this degraded form when it is reduced by friction, percussion, or otherwise, the question arises, Are there any influences at work tending thus fo degrade the motions of the universe? Now we know very well that all motions on the surface of our earth ulti

mately tend to be dissipated and converted into equally diffused heat; we know also that the heat of the sun and stars tends to assume this degraded form; but does the same reasong apply to celestial motions? This leads us to ask if there is a material medium pervading space; for if there is such a medium, however attenuated, something analogous to friction must necessarily take place, and the result would appear to be the conversion, at a rate more or less rapid, of the mechanical energy of the universe into the degraded form of diffused heat.

387. Medium pervading space. We have several reasons for assuming the existence of such a medium.

1. The various phenomena of light are best explained on the supposition that this agent consists of undulations in a medium pervading space.

Our proofs in favour of the undulatory theory of light are strengthened if we acknowledge the principle of the conservation of energy.

For the experiments of Mr. Bennet shewed all absence of momentum when the concentrated light of the sun was made to strike a piece of paper attached to the end of a straw delicately suspended, and acting as a lever which would change its position with the smallest momentum. Now let H denote the quantity of heat that struck the paper in unit of time. Then assuming for a moment that light consists of particles projected with great velocity from a heated body, that is to say, assuming the emission theory of light, and assuming also that in accordance with the principle of the conservation of energy the energy of the blow given to the paper by these particles. has been converted into heat, JH will denote this energy during an unit of time, but if m denote the mass and v thẻ velocity in feet per second of the light-particles, then (Art. mv2 314) the energy is denoted by

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Now H, or the heating effect, can be determined by observation, also v, or the velocity of light (in feet), is known; and hence the momentum, or mv, may be easily deduced; and it is thus found that this ought to have been quite perceptible in this experiment.

But as it was not perceptible we must conclude that light particles do not give a blow, and hence that the emission theory of light is not true; and if this theory be not true, we must have recourse to the undulatory or some similar theory which assumes the existence of a medium pervading space.

2. The continual shortening of the path of Encke's comet leads us to the same conclusion; that is to say, to a belief in the existence of a material medium pervading space.

If, therefore, there be a resisting medium pervading space, the visible motions of the universe must gradually be lessened in consequence; unless indeed we assume the existence of some unknown completely restorative process; but such a process is inconsistent both with the principle of the degradation of energy, and also with the shortening of the path of Encke's comet, whatever weight this may have.

There can, in fine, be little doubt that if we suppose the principle of the degradation of energy to hold throughout the universe it implies an element of decay in the present order of things, and the final transmutation of all available energy into uniformly diffused heat, unless we suppose the constitution of the universe to be such that this process of degradation will last an infinite time.

For a clear statement of this subject we may refer our readers to an article on 'Energy' that appeared lately in the North British Review, from which many of these remarks are taken.

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SOURCES OF ENERGY.

388. Let us now consider shortly the various supplies of energy of different kinds with which we are furnished, and also the ultimate sources of these supplies.

Of potential forms of energy we have-
1. The potential energy of fuel.
2. The potential energy of food.

3. The potential energy of a head of water.
4. The potential energy derived from the tides.
5. The potential energy of the chemical separation
implied in native sulphur, native iron, &c.

Then with regard to kinetic forms of energy, we have— 6. The kinetic energy of air in motion.

7. The kinetic energy of water in motion.

8. We may add to this catalogue the direct rays of the sun which are available for certain purposes; and also

9. The energy that may be derived from the unequal temperature of different parts of the earth. 389. Fuel and food. The potential energies of fuel and food are of essential importance to our existence.

By fuel we mean certain substances which are capable of combining with oxygen, and of supplying us as they so combine with a large amount of heat of high temperature.

Coal is the most important of such substances; and we can employ it either to warm ourselves and our habitations by means of the heat which it produces, or as an agent for generating mechanical effect in our various heat engines.

When we come to consider from what original source of energy the chemical separation of fuel is derived, we see that it is due to the sun's rays. These rays acting upon the leaves of plants produce those decompositions which form fuel. The energy of the sun's rays have in fact been transmuted into the potential energy of chemical separation.

This fact seems to have been recognized at a comparatively early period by Herschel and the elder Stephenson, and a curious tale is current about the latter, who, though well aware that it was the sun that drove his engines, could not give a very clear explanation of the subject; nevertheless the statement is undoubtedly true.

It is indeed true that the rays of the sun acting upon the leaves of plants in those remote ages when coal beds were being formed have laid up for man a stock of energy of inestimable value.

Food has the same origin as fuel, with the exception that vegetable food is being produced by the sun year by year, while the greater portion of our fuel, as we have just seen, has been produced ages ago. And here we may remark a very prominent distinction between vegetables and animals as regards energy. Vegetables serve to transmute the energy of the sun's rays into fuel and food. Animals, again, consume this food and transmute it partly into useful work but partly into the degraded form of diffused heat.

Joule, Carpenter, and Mayer seem to have been aware of the restrictions under which living beings are placed by the laws of energy, and to have seen that the power of an animal, as far as energy is concerned, is not creative but only directive.

A clear view of this subject was probably also held by Rumford, who remarked that if a certain amount of fodder were consumed by a horse he will do more work for it than would be done by an engine in which this fodder was burned as a source of heat. Finally, Helmholz has treated this question, as well as the whole subject of energy, in a very able manner. An animal is in fact an engine, and just as an engine must be fed with fuel, so an animal must be fed with food.

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