Popular Science Monthly/Volume 23/August 1883/On Radiation

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ON RADIATION.[1]
By JOHN TYNDALL, F. R. S.

SCIENTIFIC discoveries are not distributed uniformly in time. They appear rather in periodic groups. Thus, in the first two years of this century, among other gifts presented by men of science to the world, we have the voltaic pile; the principle of Interference, which is the basis of the undulatory theory of light; and the discovery by William Herschel of the dark rays of the sun.

Directly or indirectly, this latter discovery heralded a period of active research on the subject of radiation. Leslie's celebrated work, "On the Nature of Heat," was published in 1804, but he informs us, in the preface, that the leading facts which gave rise to the publication presented themselves in the spring of 1801. An interesting but not uncommon psychological experience is glanced at in this preface. The inconvenience of what we call ecstasy, or exaltation, is that it is usually attended by undesirable compensations. Its action resembles that of a tidal river, sometimes advancing and filling the shores of life, but afterward retreating and leaving unlovely banks behind. Leslie, when he began his work, describes himself as "transported at the prospect of a new world emerging to view." But further on the note changes, and before the preface ends he warns the reader that he may expect variety of tone, and perhaps defect of unity, in his disquisition. The execution of the work, he says, proceeded with extreme tardiness; and, as the charm of novelty w r ore off, he began to look upon his production with a coolness not usual in authors.

The ebb of the tide, however, was but transient; and to Leslie's ardor, industry, and experimental skill, we are indebted for a large body of knowledge in regard to the phenomena of radiation. In the prosecution of his researches he had to rely upon himself. He devised his own apparatus, and applied it in bis own way. To produce radiating surfaces, be employed metallic cubes, which to the present hour are known as Leslie's cubes. The different faces of these cubes be coated with different substances, and, filling the cubes with boiling water, be determined the emissive powers of the substances thus heated. These be found to differ greatly from each other. Thus, the radiation from a coating of lamp-black being called 100, that from the uncoated metallic surface of his cube was only 12. He pointed out the reciprocity existing between radiation and absorption, proving that those substances which emit heat copiously absorb it greedily. His thermoscopic instrument was the well-known differential-thermometer invented by himself. In experiment Leslie was very strong, but in theory he was not so strong. His notions as to the nature of the agent whose phenomena he investigated with so much ability are confused and incorrect. Indeed, he could hardly have formed any clear notion of the physical meaning of radiation before the undulatory theory of light, which was then on its trial, bad been established.

A figure still more remarkable than Leslie occupied the scientific stage at the same time namely, the vigorous, penetrating, and practical Benjamin Thompson, better known as Count Rumford, the originator of the Royal Institution. Rumford traversed a great portion of the ground occupied by Leslie, and obtained many of his results. As regards priority of publication, be was obviously discontented with the course which things had taken, and he endeavored to place both himself and Leslie in what he supposed to be their right relation to the subject of radiant heat. The two investigators were unknown to each other personally, and their differences hardly rose to scientific strife. There can hardly, I think, be a doubt that each of them worked independently of the other, and that, where their labors overlap, the honor of discovery belongs equally to both.

The results of Leslie and Rumford were obtained in the laboratory; but the walls of a laboratory do not constitute the boundary of its results. Nature's hand specimens are always fair samples, and, if the experiments of the laboratory be only true, they will be ratified throughout the universe. The results of Leslie and Rumford were in due time carried from the cabinet of the experimenter to the open sky, by Dr. Wells, a practicing London physician. And here let it be gratefully acknowledged that vast services to physics have been rendered by physicians. The penetration of Wells is signalized among other things by the fact recorded by the late Mr. Darwin, that, forty-five years before the publication of the "Origin of Species," the London doctor bad distinctly recognized the principle of Natural Selection, and that he was the first who recognized it. But Wells is 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 vapor 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 vapor 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 self-same 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 skepticism. He waited long in vain for a favorable 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.

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 circumstances, the electric current generated cold. He soldered together a 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 Identitats-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 subtile 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 vapors 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 vapors, 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 farther beyond the range of the microscope than the power of the microscope, at its maximum, lies beyond that of the unaided eye.

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-vapor, 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 vapor synchronize in their vibrations with the particular waves of ether which produce the sensation of yellow light. The vapor, 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-vapor 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 vapors are, as you know, not continuous, but formed of brilliant bands. Wishing, in 1861, to obtain the brilliant yellow band produced by incandescent sodium-vapor, 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 humoring 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-vapor, which spread outward and surrounded, as a cool envelope, the core of intensely heated vapor inside. By the cool vapor the rays from the hot were intercepted, but on lengthening the arc the outer vapor 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 vapor 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 skillful application of Stokes's 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 sand-paper, being covered with powdered uranium glass, a highly fluorescent body. Pushing the movable screen toward 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 vapor. 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 vapor 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 vapor 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 vapor, 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-vapor. Here is another flame of a rich blue color, 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-vapor 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 show how radiant heat may be made to play to the mind's eye the part of the microscope, in revealing to us something of the more secret and noble works of atomic Nature. Chemists are ever on the alert to notice analogies and resemblances in the atomic structures of different bodies. They long ago pointed out that a resemblance exists between that evil-smelling liquid, bisulphide of carbon, and carbonic acid. In the latter substance we have one atom of carbon united to two of oxygen, while in the former we have one atom of carbon united to two of sulphur. Attempts have been made to push the analogy still further by the discovery of a compound of carbon and sulphur which should be analogous to carbonic oxide, where the proportions, instead of one to two, are one to one, but hitherto, I believe, without success. Let us now see whether a little physical light can not reveal an analogy between carbonic acid and bisulphide of carbon more occult than any hitherto pointed out. For all ordinary sources of radiant heat the bisulphide, both in the liquid and vaporous form, is the most transparent, or diathermanous, of bodies. It transmits, for example, ninety per cent of the radiation from our hydrogen-flame, ten per cent only being absorbed. But when we make the carbonic-oxide flame our source of rays, the bisulphide shows itself to be a body of extreme opacity. The transmissive power falls from ninety to about twenty-five per cent, seventy-five per cent of the radiation being absorbed. To the radiation from the carbonic-oxide flame the bisulphide behaves like the carbonic acid. In other words, the group of atoms constituting the molecule of the bisulphide vibrate in the same periods as those of the atoms which constitute the molecule of the carbonic acid. And thus we have established a new, subtile, but most certain resemblance between these two substances. The time may come when chemists will make more use than they have hitherto done of radiant heat as an explorer of molecular condition.

The conception of these quivering atoms is a theoretic conception, but it is one which gives us a powerful grasp of the facts, and enables us to realize mentally the mechanism on which radiation and absorption depend. We will now turn to a more practical view of the subject. It is pretty well known that for a long series of years I conducted an amicable controversy with one of the most eminent experimenters of our time, as regards the action of the earth's atmosphere on solar and terrestrial radiation. My contention was that the great body of our atmosphere—its oxygen and nitrogen—had but little effect upon either the rays of the sun coming to us, or the rays of the earth darting away from us into space; but that mixed with the body of our air there was an attenuated and apparently trivial constituent which exercised a most momentous influence. That body, as many of you know, is aqueous vapor, the amount of which does not exceed one per cent of the whole atmosphere. Minute, however, as its quantity is, the life of our planet depends upon this vapor. Without it, in the first place, the clouds could drop no fatness. In this sense the necessity for its presence is obvious to all. But it acts in another sense as a preserver. Without it as a covering, the earth would soon be reduced to the frigidity of death. Observers were, and are, slow to take in this fact, which nevertheless is a fact, however improbable it may at first sight appear. The action of aqueous vapor upon radiant heat has been established by irrefragable experiments in the laboratory; and these experiments, though not unopposed, have been substantiated by some of the most accomplished meteorologists of our day.

I wished much to instruct myself a little by actual observation on this subject, under the open sky, and my first object was, to catch, if possible, states of the weather which would enable me to bring my views to a practical test. About a year ago, a little iron hut, embracing a single room, was erected for my benefit upon the wild moorland of Hind Head. From the plateau on which the hut stands there is a free outlook in all directions. Here, amid the heather, I had two stout poles fixed firmly in the ground eight feet asunder, and a stout cord stretched from one to the other. From the center of this cord a thermometer is suspended with its bulb four feet above the ground. On the ground is placed a pad of cotton-wool, and on this cotton-wool a second thermometer, the object of the arrangement being to determine the difference of temperature between the two thermometers, which are only four feet vertically apart.

Permit me at the outset to deal with the subject in a perfectly elementary way. In comparison with the cold of space, the earth must be regarded as a hot body, sending its rays, should nothing intercept them, across the atmosphere into space. The cotton-wool is chosen because it is a powerful, though not the most powerful, radiator. It pours its heat freely into the atmosphere, and by reason of its flocculence, which renders it a non-conductor, it is unable to derive from the earth heat which might atone for its loss. Imagine the cotton wool thus self-chilled. The air in immediate contact with it shares its chill, and the thermometer lying upon it partakes of the refrigeration. In calm weather the chilled air, because of its greater density, remains close to the earth's surface, and in this way we sometimes obtain upon that surface a temperature considerably lower than that of the air a few feet above it. The experiments of Wilson, Six, and Wells have made us familiar with this result. On the other hand, the earth's surface during the day receives from the sun more heat than it loses by its own radiation, so that, when the sun is active, the temperature of the surface exceeds that of the air.

These points will be best illustrated by describing the course of temperature for a day, beginning at sunrise and ending at 10.20 p. m. on the 4th of last March. The observations are recorded in the annexed table, at the head of which are named the place of observation, its elevation above the sea, and the state of the weather. The first column in the table contains the times at which the two thermometers were read. The column under "Air" gives the temperatures of the air, the column under "Wool" gives the temperatures of the wool, while the fourth column gives the differences between the two temperatures. It is seen at a glance that, from sunrise to 9.20 a. m., the cotton-wool is colder than the air; at 9.30 the temperatures are alike. This is the hour of "intersection," which is immediately followed by "inversion." Throughout the day, and up to 4 p. M., the wool is warmer than the air. At 4.5 p. m. the temperatures are again alike; while from that point downward the loss by terrestrial radiation is in excess of the gain derived from all other sources, the refrigeration reaching a maximum at 7.30 p. m., when the difference between the two thermometers amounted to 10º Fahr. When the observations are continued throughout the night, the greater cold of the surface is found to be maintained until sunrise, and for some hours beyond it. Had the air been perfectly still during the observations, the nocturnal chilling of the surface would have been in this case greater: for you can readily understand that even a light wind sweeping over the surface, and mixing the chilled with the warmer air, must seriously interfere with its refrigeration.

Hind Head, elevation 850 feet; sky cloudless; hoar-frost; wind light, from northeast. Course of temperature, March 4, 1883. PSM V23 D481 Earth temperature variations.jpgGlacial wind from northeast. Stars very bright.

Various circumstances may contribute to lessen, or even abolish, the difference between the two thermometers. Haze, fog, cloud, rain, snow, are all known to be influential. These are visible impediments to the outflow of beat from the earth; but we have now to deal with the powerful obstacle to that outflow to which reference has been already made, and which is entirely invisible. The pure vapor of water, for example, is a gas as invisible as the air itself. It is everywhere diffused through the air; but, unlike the oxygen and nitrogen of the atmosphere, it is not constant in quantity. We have now to examine whether meteorological observations do not clearly indicate its influence on terrestrial radiation:

January 16th.—Extremely serene; air almost a dead calm; shy without a cloud; light southwesterly air.

PSM V23 D482 Earth temperature variations.jpg With a view to this examination, I will choose a series of observations made during the afternoon and evening of a day of extraordinary calmness and serenity. The visible condition of the atmosphere at the time was that which has hitherto been considered most favorable to the outflow of terrestrial heat, and therefore best calculated to establish a large difference between the air and wool-thermometers. The 16th of last January was a day of this kind, when the observations recorded in the annexed table were made.

During these observations there was no visible impediment to terrestrial radiation. The sky was extremely pure; the moon was shining; Orion, the Pleiades, Charles's Wain, including the small companion star at the bend of the shaft, the North Star, and numbers of others, were clearly visible. After the last observations, my note-book contains the remark: "Atmosphere exquisitely clear; from zenith to horizon cloudless all around."

A moment's attention bestowed on the column of differences in the foregoing table will repay us. Why should the difference at 6 p. m. be fully 5º less than at 5 p. m.; and again 5º less than at 8 and at 8.30 respectively? There was absolutely nothing in the aspect of the atmosphere to account for the approach of the two thermometers at six o'clock—nothing to account for their preceding and subsequent divergence from each other. Anomalies of this kind have been observed by the hundred, but they have never been accounted for, and they did not admit of explanation until it had been proved that the intrusion of a perfectly invisible vapor was competent to check the radiation, while its passing away reopened a doorway into space.

It is well to bear in mind that the difference between the two thermometers on the evening here referred to varied from 4º to 9º, the latter being the maximum.

Such observations might be multiplied, but, with a view to saving space, I will limit the record. On the evening of January 30th the atmosphere was very serene; there was no moon, but the firmament was powdered with stars. At 7.15 p. m. the difference between the two thermometers was 6º; while at 9.30 p. m. it was 4º, the wool-thermometer being in both cases the colder of the two. On February 3d, observations were made under similar conditions of weather, and with a similar result. At 7.15 p. m. the difference between the thermometers was 6º; while at 8.25 p. m. it was 4º. On both these evenings the sky was cloudless, the stars were bright, while the movement of the air was light, from the southwest.

In all these cases the air passing over the plateau of Hind Head had previously grazed the comparatively warm surface of the Atlantic Ocean, where it had charged itself with aqueous vapor to a degree corresponding to its temperature. Let us contrast its action with that of air coming to Hind Head from a quarter less competent to charge it with aqueous vapor. We were visited by such air on the 10th of last December, when the movement of the wind was light from the northeast, the temperature at the time being very low, and hence calculated to lessen the quantity of atmospheric vapor. Snow a foot deep covered the heather. At 8.5 a. m. the two thermometers were taken from the hut, having a common temperature of 35º. The one was rapidly suspended in the air, and the other laid upon the wool. I was not prepared for the result. A single minute's exposure sufficed to establish a difference of 5º between the thermometers; an exposure of five minutes produced a difference of 13; while after ten minutes' exposure the difference was found to be no less than 17º. Here follow some of the observations:

December 10th.—Deep snow; low temperature; sky clear; light northeasterly air.

TIME Air. Wool. Difference.
 a.m. Degrees. Degrees. Degrees.
8.10 29 16 13
8.15 29 12 17
8.20 27 12 15
8.30 26 11 15
8.40 26 10 16
8.45 27 11 16
8.50 29 11 18

During these observations, a dense bank of cloud on the opposite ridge of Blackdown virtually retarded the rising of the sun. It had, however, cleared the bank during the last two observations, and, touching the air-thermometer with its warmth, raised the temperature from 26º to 27º and 29º. The very large difference of 18º is in part to be ascribed to this raising of the temperature of the air-thermometer. I will limit myself to citing one other case of a similar kind. On the evening of the 31st of March, though the surface temperature was far below the dew-point, very little dew was deposited. The air was obviously a dry air. The sky was perfectly cloudless, while the barely perceptible movement of the air was from the northeast. At 10 p. m. the temperature of the air-thermometer was 37°, that of the wool-thermometer being 20°, a refrigeration of 17° being, therefore, observed on this occasion.

From the behavior of a smooth ball when urged in succession over short grass, over a gravel-walk, over a boarded floor, and over ice, it has been inferred that, were friction entirely withdrawn, we should have no retardation. In a similar way, when, under atmospheric conditions visibly the same, we observe that the refrigeration of the earth's surface at night markedly increases with the dryness of the atmosphere, we may infer what would occur if the invisible atmospheric vapor were entirely withdrawn. I am far from saying that the body of the atmosphere exerts no action whatever upon the waves of terrestrial heat; but only that its action is so small that, when due precautions are taken to have the air pure and dry, laboratory experiments fail to reveal any action. Without its vaporous screen, our solid earth would practically be in the presence of stellar space; and with that space, so long as a difference existed between them, the earth would continue to exchange temperatures. The final result of such a process may be surmised. If carried far enough, it would infallibly extinguish the life of our planet.—Contemporary Review.

 
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  1. A "Friday Evening Discourse," recently delivered in the Royal Institution.