nature to repeat with precision, owing to the necessity of waiting between each repetition for the thermometers to cool and become stationary.

But it should be observed that there is nothing in my results which contradicts the idea that simple heat may have in a very slight degree a power of transmissibility through glass: all I have assumed is, that it is sufficiently distinguishable in this respect from the heating power which accompanies the light, and which undergoes no diminution. Connected with these points, again, is the question, whether if simple heat can radiate through solid transparent media, it cannot also commence radiating IN them. It is commonly asserted that radiation can only take place, or commence, in elastic media. This, then, is an inquiry which will lead into a wide field of research, and may be found connected with the intimate nature of radiation. It will also be a question, whether, and how far, radiant heat passes through elastic media without heating them, and what support this gives to Leslie's theory of pulsations. The whole subject should be viewed in connexion with the admirable remarks of Sir J. Herschel in his Discourse on the Study of Natural Philosophy, p. 205.

The radiation of heat in vacuo is another point on which further inquiry is much wanted. The greater capacity of air for heat, as it is more rarefied, would occasion a more rapid abstraction from the hot body; and thus in an atmosphere of extreme rarity the cooling ought to be extremely rapid, and this must be accurately estimated in measuring the radiation. But it appears from the experiments of Gay-Lussac, (see Edinb. Phil. Journ. vi. 302,) that when air is reduced to the most extreme degree of rarefaction possible, a very considerable compression makes so little difference in its actual density, that the giving out of heat which ought to take place from diminishing its capacity is absolutely insensible.

But even in this case it is very questionable whether so complete an approach to a real vacuum is obtained as to warrant inferences respecting the radiation of heat in an actual vacuum.

In fact, we want a connected series of determinations to show the order and increase of conducting powers, as connected both with the radiation in and through different media, and the interception which they offer to its passage.

In solids it is presumed no radiation can commence; it is disputed whether it can continue even partially; but conduction goes on rapidly.

In liquids it has been disputed whether there can be radiation; and they are worse conductors than solids.

In elastic media radiation can commence and continue; but they are still worse conductors.

In vacuo it might be presumed by analogy that a yet more free radiation might take place; yet some experiments (as we have seen,) show the contrary; and here there is no conduction.

With regard to that portion of the heat which accompanies or belongs to light, the theory which I originally suggested, (merely as an hypothesis representing the facts,) viz. that it was simply the latent heat of light, developed of course when the light was absorbed, is connected with the hypothesis of the materiality of light; but it may be worth inquiry whether it does not apply even better to the elastic æther, in whose undulations light is now proved to consist.

Report on Thermo-electricity. By the Rev. JAMES CUMMING, F.R.S., Professor of Chemistry in the University of Cambridge.

IN communicating to the members of this Society an outline of the progress and present state of Thermo-electricity, I congratulate both them and myself on the allied branches of science having fallen into such able hands, that I should not be justified, even if it were my wish, to extend this Report beyond its immediate subject.

On one point more particularly I am happy,-since we are not so fortunate as to receive instruction from the discoverer himself, that Dr. Ritchie has undertaken to exhibit and explain to us the recent researches of Mr. Faraday. The continuous electrical currents, now made known to us by these experiments, seem so much more nearly connected with those in the thermoelectric circuit, than with those peculiar either to the common or galvanic electricity, that I should otherwise have thought it incumbent on me to make the notice of them a part of this Report. Divested, as the subject will thus be, of all extraneous matter, I shall therefore be enabled to say all that I think to be really necessary, and yet detain you but a short time from more important communications.

On a review of the labours of different experimentalists on Thermo-electricity, it soon became evident to me, that, to give anything like a luminous account of them, it would be necessary to make some attempt at a classification of their objects. This, I must confess, was no very easy matter; for in this, as in some other branches of experimental inquiry, I have found it difficult,

after reading a detail of an elaborate series of experiments, to discover what object was intended by them.

The first and most obvious inquiry seems to be into the circumstances which are necessary for thermo-electric excitation, or which modify its action. In this respect the original experiment of Seebeck left much room for further investigation. When he had found that a brass wire coiled round the ends of a bar of antimony exhibited magnetic action by the application of heat to one of the extremities of the bar, it was still doubtful whether this effect might not depend either on some peculiarity in one or the other of these metals, on their contact, or on the mode of their juncture.

The remarkable effects produced by helices in the hydroelectric circuit made it not improbable that much might depend on the wire being coiled round the bar. This was soon shown not to be the case, and that a circuit, however formed, provided it were composed of perfect conductors, was all that was necessary.

Reasoning, again, from the analogy of the galvanic circuit, it might have been imagined that as three elements were necessary in the one, so two metallic elements with heat acting the part of the third might be required in the other; but it appeared from some of the earliest experiments, that metallic bars heated in contact with wires of the same metal gave considerable deviations with the galvanometer needle, and therefore that one metal alone sufficed for the development of thermo-electricity. The experiments of Dr. Trail in 1824 may be referred to this class; since, though made with slips of copper attached to the bar of antimony, yet, as the circuit was not completed through the copper, they properly exhibit the thermo-electricity of a single metal. One result, which is too important to be overlooked, is that the application of ice or heat to the centre of the bar produced opposite deviations in two needles placed between the centre and the extremities; whence he infers that "the direction of the compass needle may be considered as the resultant of two forces, the magnetism of composition of the earth, and its thermo-magnetism, which tends to place the needle east and west." How far this coincides with subsequent experiments I shall have occasion to point out to you hereafter.

But the most important researches on the thermo-electricity of a single metal were those made by Yelin in the same year; from which we learn, that all metallic bodies acquire magnetic properties when unequally heated, and that the series of their magnetic intensities when thus excited, is bismuth, antimony, zinc, silver, platina, copper, brass, gold, tin, lead :—that a metal acts differently according as the hot or cold part of it is placed

under the needle;-and that the magnetic action of metals unequally heated depends upon the form given them in casting; for which purpose masses of each metal, in the different forms of prisms, cylinders, and spheres both hollow and solid, were heated successively in different points, and examined by applying magnetic needles to their surfaces. From the different directions of the magnetism in cylinders of bismuth as they were cooled slowly or rapidly, he infers that there is some relation between the crystallization of metals and their magnetic properties. I may observe that I had previously shown that no difference either in the nature or quantity of the deviation could be detected in bismuth under similar circumstances, when forming a circuit with copper wires, &c.; the modification induced by slow or rapid crystallization is confined to the direction of the currents in the bar itself; and since fluid mercury is capable of becoming a thermo-electric element, crystallization is evidently not a primary agent in thermo-electric excitation or conduction, however it may modify its progress.

The latest experiments connected with this branch of the subject, are those of Mr. Sturgeon in 1831, on the thermoelectricity of homogeneous bodies, and the connexion between crystalline arrangement and thermo-electricity. The objects of his two papers appear to be to trace the directions of the magnetic currents in masses of metal, varying the form and the point of excitation; and so far they agree with those of Yelin. With these it appears that Mr. Sturgeon was not acquainted, as he says he is not aware that any experiments are yet before the public, illustrative of thermo-magnetic action in one solitary piece of metal. As a general result, it may be stated that, whether the mass of metal were in the form of a rectangular prism, a cylinder, or a cone, upon heating a point in the periphery of one extremity, the current proceeded longitudinally from the heated point on the same side of the axis, and returned on the other side, accompanied with transverse currents passing in opposite directions nearly at right angles to the longitudinal ones. With a large rectangular plate of zinc, when the heat was applied at one of the angles, the electric current was in the direction of the diagonal advancing, and returned along the edges. In this and all similar experiments, it seems that the direction of the electricity to or from the heated point, depends upon some peculiarity in each metal, which remains to be discovered; but that the course of the currents afterwards, with reference to the figure of the mass, depends solely upon the figures; and I think may be accounted for, by considering the whole as a congeries of wires, from which, ac

cording to their position with respect to each other and the exterior surface, the heat is conducted away with more or less rapidity; each portion, so far as the heat extends, acting both as a thermo-electric element and as a conductor of heat, but beyond that space acting simply as a conductor. The effects of crystalline structure in modifying thermo-electric action Mr. Sturgeon considers as arising from the lamina of each crystal being only in juxtaposition, and that therefore the heat passes more readily through the parts of each than from one to another. This hypothesis it is obvious is inapplicable to metals devoid of crystalline structure, as wires of copper or silver, and still more so to metals in a liquid state: but by conceiving each wire to be divided into an indefinite number of circular laminæ, we may suppose each of these to act as a layer of cold particles upon the lamina on one side and of hot upon those on the other, and the total effect of the whole to depend on their aggregate action; each bar or wire acting as an assemblage of an indefinite number of small plates, as the common magnet may be conceived to be composed of an indefinite number of atomic magnets. Still, admitting this mutual action of the metallic particles, the original induction of electricity by heat and its subsequent propagation remain to be explained. This Becquerel conceives may be accounted for on the hypothesis that, whenever a particle of a metal is heated, part of the neutral electric fluid which is attached to it is decomposed, the vitreous fluid being retained, and the resinous driven off and passing into the adjoining particles. In proportion as the heat extends by communication from particle to particle, similar effects take place in each of those that are acquiring heat, and the contrary in those that are losing it. Thus the first effect is only to produce an oscillatory movement of the electric fluid between the adjacent particles; but if the source of heat be permanent, the retrograde movements are prevented, and a continued current takes place. I can only observe as to this theory, that the hypothesis appears to assume the very point that was to be established. I am not aware of any experiments to prove such a decomposition of the electricities of an uninsulated particle of metal.

The next class of experiments to be mentioned are those which relate to the transmission and augmentation of thermoelectricity.

Reverting to the original experiment of Seebeck, the brass wire connecting the extremities of the bar of antimony might act simply as a conductor, or might modify at the same time that it transmitted the electricity, according as the susceptibility to this species of electric excitation was confined to antimony, or