CHAPTER IV

.

THEORIES OF HEAT.

_The Dynamical Theory of Heat._

THAT the transmission of _radiant_ Heat takes place by means of the vibrations of a medium, as the transmission of Sound certainly does, and the transmission of Light most probably, is a theory which, as I have endeavored to explain, has strong arguments and analogies in its favor. But that Heat itself, in its essence and quantity, is Motion is a hypothesis of quite another kind. This hypothesis has been recently asserted and maintained with great ability. The doctrine thus asserted is, that Motion may be converted into Heat, and Heat into Motion; that Heat and Motion may produce each other, as we see in the rarefaction and condensation of air, in steam-engines, and the like: and that in all such cases the Motion produced and the Heat expended exactly measure each other. The foundation of this theory is conceived to have been laid by Mr. Joule of Manchester, in 1844: and it has since been prosecuted by him and by Professor Thomson of Glasgow, by experimental investigations of various kinds. It is difficult to make these experiments so as to be quite satisfactory; for it is {609} difficult to measure _all_ the heat gained or lost in any of the changes here contemplated. That friction, agitation of fluids, condensation of gases, conversion of gases into fluids and liquids into solids, produce heat, is undoubted: and that the quantity of such heat may be measured by the mechanical force which produces it, or which it produces, is a generalization which will very likely be found a fertile source of new propositions, and probably of important consequences.

As an example of the conclusions which Professor Thomson draws from this doctrine of the mutual conversion of motion and heat, I may mention his speculations concerning the cause which produces and sustains the heat of the sun.[16\B] He conceives that the support of the solar heat must be meteoric matter which is perpetually falling towards the globe of the sun, and has its motion converted into heat. He inclines to think that the meteors containing the stores of energy for future Sun-light must be principally within the earth's orbit; and that we actually see them there as the "Zodiacal Light," an illuminated shower, or rather tornado, of stones. The inner parts of this tornado are always getting caught in the Sun's atmosphere, and drawn to his mass by gravitation.

[Note 16\B: On the Mechanical Energies of the Solar System. _Edinb. Trans._ vol. XXI. part i. (1854), p. 67.]

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## BOOK XI.

ELECTRICITY.

GENERAL REMARKS.

ELECTRICITY in the form in which it was originally studied--Franklinic, frictional, or statical electricity--has been so completely identified with electricity in its more comprehensive form--Voltaic, chemical, or dynamical electricity--that any additions we might have to make to the history of the earlier form of the subject are included in the later science.

There are, however, several subjects which may still be regarded rather as branches of Electricity than of the Cognate Sciences. Such are, for instance, Atmospheric Electricity, with all that belongs to Thunderstorms and Lightning Conductors. The observation of Atmospheric Electricity has been prosecuted with great zeal at various meteorological observatories; and especially at the Observatory established by the British Association at Kew. The Aurora Borealis, again, is plainly an electrical phenomenon; but probably belonging rather to dynamical than to statical electricity. For it strongly affects the magnetic needle, and its position has reference to the direction of magnetism; but it has not been observed to affect the electroscope. The general features of this phenomenon have been described by M. de Humboldt, and more recently by M. de Bravais; and theories of the mode of its production have been propounded by MM. Biot, De la Rive, Kaemtz, and others.

Again, there are several fishes which have the power of giving an electrical shock:--the torpedo, the gymnotus, and the silurus. The agency of these creatures has been identified with electricity in the most general sense. The peculiar energy of the animal has been made to produce the effects which are produced by an electrical discharge or a voltaic current:--not only to destroy life in small animals, but to {611} deflect a magnet, to make a magnet, to decompose water, and to produce a spark.

_Dr. Faraday's Views of Statical Electric Induction._

According to the theories of electricity of Æpinus and Coulomb, which in this Book of our History are regarded as constituting a main part of the progress of this portion of science, the particles of the electric fluid or fluids exert forces, attractive and repulsive, upon each other in straight lines at a distance, in the same way in which, in the Newtonian theory of the universe, the

## particles of matter are conceived as exerting attractive forces upon

each other. An electrized body presented a conducting body of any form, determines a new arrangement of the electric fluids in the conductor, attracting the like fluid to its own side, and repelling the opposite fluid to the opposite side. This is Electrical _Induction_. And as, by the theory, the attraction is greater at the smaller distances, the distribution of the fluid upon the conductor in virtue of this Induction will not be symmetrical, but will be governed by laws which it will require a complex and difficult calculation to determine--as we have seen was the case in the investigations of Coulomb, Poisson, and others.

Instead of this action at a distance. Dr. Faraday has been led to conceive Electrical Induction to be the result of an action taking place between the electrized body and the conductor through lines of contiguous particles in the mass of the intermediate body, which he calls the _Dielectric_. And the irregularities of the distribution of the electricity in these cases of Induction, and indeed the existence of an action in points protected from direct action by the protuberant sides of the conductor, are the causes, I conceive, which lead him to the conclusion that Induction takes place in _curved lines_[17\B] of such contiguous particles.

[Note 17\B: _Researches_, 1165, &c.]

With reference to this, I may remark that, as I have said, the distribution of electricity on a conductor in the presence of an electrized body is so complex a mathematical problem that I do not conceive any merely popular way of regarding the result can entitle us to say, that the distribution which we find cannot be explained by the Coulombian theory, and must force us upon the assumption of an action in curved lines:--which is, indeed, itself a theory, and so vague a one {612} that it requires to be made much more precise before we can say what consequences it does or does not lead to. Professor W. Thomson has arrived at a mathematical proof that the effect of induction on the view of Coulomb and of Faraday must, under certain conditions, be necessarily and universally the same.

With regard to the influence of different _Dielectrics_ upon Induction, the inquiry appears to be of the highest importance; and may certainly necessitate some addition to the theory.

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## BOOK XII.

MAGNETISM.

_Recent Progress of Terrestrial Magnetism._

IN