principally known to us through his "Theory of Dew," which, prompted by the experiments of Leslie and Rumford, and worked out by the most refined and conclusive observations on the part of Wells himself, first revealed the cause of this beautiful phenomenon. Wells knew that through the body of our atmosphere invisible aqueous vapour is everywhere diffused. He proved that grasses and other bodies on which dew was deposited were powerful emitters of radiant heat; that when nothing existed in the air to stop their radiation, they became self-chilled; and that while thus chilled they condensed into dew the aqueous vapour of the air around them. I do not suppose that any theory of importance ever escaped the ordeal of assault on its first enunciation. The theory of Wells was thus assailed; but it has proved immovable, and will doubtless continue so to the end of time.

The interaction of scientific workers causes the growth of science to resemble that of an organism. From Faraday's tiny magnetoelectric spark, shown in this theatre half a century ago, has sprung the enormous practical development of electricity at the present time. Thomas Seebeck in 1822 discovered thermo-electricity, and eight years subsequently bars of bismuth and antimony were first soldered together by Nobili so as to form a thermo-electric pile. In the selfsame year Melloni perfected the instrument and proved its applicability to the investigation of radiant heat. The instrumental appliances of science have been well described as extensions of the senses of man. Thus the invention of the thermopile vastly augmented our powers over the phenomena of radiation. Melloni added immensely to our knowledge of the transmission of radiant heat through liquids and solids. His results appeared at first so novel and unexpected that they excited scepticism. He waited long in vain for a favourable Report from the Academicians of Paris; and finally, in despair of obtaining it, he published his results in the "Annales de Chimie." Here they came to the knowledge of Faraday, who, struck by their originality, brought them under the notice of the Royal Society, and obtained for Melloni the Rumford medal. The medal was accompanied by a sum of money from the Rumford fund; and this, at the time, was of the utmost importance to the young political exile, reduced as he was to penury in Paris. From that time until his death, Melloni was ranked as the foremost investigator in the domain of radiant heat.

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As regards the philosophy of the thermopile, and its relation to the great doctrine of the conservation of energy, now everywhere accepted, a step of singular significance was taken by Peltier in 1834. Up to that time it had been taken for granted that the action of an electric current upon a conductor through which it passed was always to generate heat. Peltier, however, proved that, under certain circum

stances, the electric current generated cold. He soldered together s bar of antimony and a bar of bismuth, end to end, thus forming of the two metals one continuous bar. Sending a current through this bar, he found that when it passed from antimony to bismuth across the junction, heat was always there developed, whereas when the direction of the current was from bismuth to antimony, there was a development of cold. By placing a drop of chilled water upon the junction of the two metals, Lenz subsequently congealed the water to ice by the passage of the current.

The source of power in the thermopile is here revealed, and a relation of the utmost importance is established between heat and electricity. Heat is shown to be the nutriment of the electric current. When one face of a thermopile is warmed, the current produced, which is always from bismuth to antimony, is simply heat consumed and transmuted into electricity.

Long before the death of Melloni, what the Germans call" Die Identitäts-Frage," that is to say, the question of the identity of light and radiant heat, agitated men's minds and spurred their inquiries. In the world of science men differ from each other in wisdom and penetration, and a new theoretic truth has always at first the minority on its side. But time, holding incessantly up to the gaze of inquirers the unalterable pattern of Nature, gradually stamps that pattern on the human mind. For twenty years Henry Brougham was able to quench the light of Thomas Young, and to retard, in like proportion, the diffusion of correct notions regarding the nature and propagation of radiant heat. But such opposing forces are, in the end, driven in, and the undulatory theory of light being once established, soon made room for the undulatory theory of radiant heat. It was shown by degrees that every purely physical effect manifested by light was equally manifested by the invisible form of radiation. Reflection, refraction, double refraction, polarization, magnetization, were all proved true of radiant heat, just as certainly as they had been proved true of light. It was at length clearly realized that radiant heat, like light, was propagated in waves through that wondrous luminiferous medium which fills all space, the only real difference between them being a difference in the length and frequency of the ethereal waves. Light, as a sensation, was seen to be produced by a particular kind of radiant heat, which possessed the power of exciting the retina.

And now we approach a deeper and more subtle portion of our subject. What, we have to ask, is the origin of the ether waves, some of which constitute light, and all of which constitute radiant heat? The answer to this question is that the waves have their origin in the vibrations of the ultimate particles of bodies. But we

must be more strict in our definition of ultimate particles. The ultimate particle of water, for example, is a molecule. If you go beyond this molecule and decompose it, the result is no longer water, but the discrete atoms of oxygen and hydrogen. The molecule of water consists of three such atoms held tightly together, but still capable of individual vibration. The question now arises: Is it the molecules vibrating as wholes, or the shivering atoms of the molecules, that are to be considered as the real sources of the ether waves? As long as we were confined to the experiments of Leslie, Rumford, and Melloni, it was difficult to answer this question. But when it was discovered that gases and vapours possessed-in some cases to an astonishing extent-the power both of absorbing and radiating heat, a new light was thrown upon the question.

You know that the theory of gases and vapours, now generally accepted, is that they consist of molecular or atomic projectiles darting to and fro, clashing and recoiling, endowed, in short, with a motion not of vibration but of translation. When two molecules clash, or when a single molecule strikes against its boundary, the first effect is to deform the molecule, by moving its atoms out of their places. But gifted as they are with enormous resiliency, the atoms immediately recover their positions, and continue to quiver in consequence of the shock. Held tightly by the force of affinity, they resemble a string stretched to almost infinite tension, and therefore capable of generating tremors of almost infinite rapidity. What we call the heat of a gas is made up of these two motions—the flight of the molecules through space, and the quivering of their constituent atoms. Thus does the eye of science pierce to what Newton called "the more secret and noble works of Nature," and make us at home amid the mysteries of a world lying in all probability vastly further beyond the range of the microscope than the power of the microscope, at its maximum, lies beyond that of the unaided eyc.

The great principle of radiation, which affirms that all bodies absorb the same rays that they emit, is now a familiar one. When, for example, a beam of white light is sent through a yellow sodium flame, produced by a copious supply of sodium vapour, the yellow constituent of the white beam is stopped by the yellow flame, and if the beam be subsequently analyzed by a prism, a black band is found in the place of the intercepted yellow band of the spectrum. We have been led to our present theoretic knowledge of light by a close study of the phenomena of sound, which in the present instance will help us to a conception of the action of the sodium flame. The atoms of sodium vapour synchronize in their vibrations with the particular waves of ether which produce the sensation of yellow light. The vapour, therefore, can take up or absorb the motion of those

waves, as a stretched piano-string takes up or absorbs the pulses of a voice pitched to the note of the string. This action of sodium vapour may be shown by an experiment which startled and perplexed me on first making it, more than twenty years ago. The spectra of incandescent metallic vapours are, as you know, not continuous, but formed of brilliant bands. Wishing, in 1861, to obtain the brilliant yellow band produced by incandescent sodium vapour, I placed a bit of sodium in a carbon crucible, and volatilized it by a powerful voltaic current. A feeble spectrum overspread the screen, from which it was thought the sodium band would stand out with dominant brilliancy. To my surprise, at the very point where I expected this brilliant band to appear, a band of darkness took its place. By humouring the voltaic arc a little, the darkness. vanished, and the bright band which I had sought at the beginning was obtained. On reflection the cause was manifest.

The first ignition of the sodium was accompanied by the development of a large amount of sodium vapour, which spread outwards and surrounded, as a cool cool envelope, the core of intensely heated vapour inside. By the cool vapour the rays from the hot were intercepted, but on lengthening the arc the outer vapour in great part was dispersed, and the rays passed to the screen. This relation as to temperature was necessary to the production of the black band; for were the outside vapour as hot as the inside, it would, by its own radiation, make good the light absorbed.

An extremely beautiful experiment of this kind was lately made here by Professor Liveing, with rays which, under ordinary circumstances, are entirely invisible. Professor Dewar and Professor Liveing have been long working with conspicuous success at the ultra-violet spectrum. Using prisms and lenses of a certain kind, and a powerful dynamo-machine to volatilize our metals, like Professor Liveing, I cast a

spectrum upon the screen. Far beyond this terminal violet, waves

impinge upon the screen which have no sensible effect upon the organ of vision; they constitute what we call the ultra-violet spectrum. Professor Stokes has taught us how to render this invisible spectrum visible, and it is by a skilful application of Stokes' discovery that Liveing and Dewar bring the hidden spectrum out with wondrous strength and beauty.

A small second screen is at hand, which can be moved into the ultra-violet region. Felt by the fingers, the surface of this screen resembles sandpaper, being covered with powdered uranium glass, a highly fluorescent body. Pushing the movable screen towards the visible spectrum, at a distance of three or four feet beyond the violet, where only darkness existed before, light begins to appear. On pushing in the screen, the whole ultra-violet spectrum falls upon it, and is rendered visible from beginning to end.

The spectrum is not continuous, but composed for the most part of luminous bands derived from the white-hot crucible in which the metals are to be converted into vapour. I beg of you to direct your attention to one of these bands in particular. Here it is, of fair luminous intensity. My object now is to show you, with Professor Dewar's aid, the reversal, as it is called, of that band, which belongs to the vapour of magnesium, exactly as a moment ago you were shown the reversal of the sodium band. An assistant will throw a bit of magnesium into the crucible, and you are to observe what first takes place. The action is rapid, so that you will have to fix your eyes upon this particular strip of light. On throwing in the magnesium, the luminous band belonging to its vapour is cut away, and you have, for a second or so, a dark band in its place. I repeat the experiment three or four times in succession, with the same unfailing result. Here, as in the case of the sodium, the magnesium surrounded itself for a moment by a cool envelope of its .own vapour, which cut off the radiation from within, and thus produced the darkness.

And now let us pass on to an apparently different, but to a really similar result. Here is a feebly luminous flame, which you know to be that of hydrogen, the product of combustion being water vapour. Here is another flame of a rich blue colour, which the chemists present know to be the flame of carbonic oxide, the product of combustion being carbonic acid. Let the hydrogen flame radiate through a column of ordinary carbonic acid-the gas proves highly transparent to the radiation. Send the rays from the carbonic oxide flame through the same column of carbonic acid-the gas proves powerfully opaque. Why is this? Simply because the radiant, in the case of the carbonic oxide flame, is hot carbonic acid, the rays from which are quenched by the cold carbonic acid gas, exactly as the rays from the intensely heated sodium vapour were quenched a moment ago by the cooler envelope which surrounded it. Bear in mind the case is always one of synchronism. It is because the atoms of the cold acid vibrate with the same frequency as the atoms of the hot that the pulses sent forth from the latter are absorbed.

Newton, though probably not with our present precision, had formed a conception similar to that of molecules and their constituent atoms. The former he called corpuscles, which, as Sir John Herschel says, he regarded "as divisible groups of atoms of yet more delicate kind." The molecules he thought might be seen if microscopes could be caused to magnify three or four thousand times. But with regard to the atoms, he made the remark already alluded to:-" It seems impossible to see the more secret and nobler works of Nature within the corpuscles, by reason of their transparency."

I have now to ask your attention to an illustration intended to