Imágenes de páginas
PDF
EPUB

One striking example of the influence of period may be here recorded. Carbonic acid gas is one of the feeblest of absorbers of the radiant heat emitted by solid sources. It is, for example, extremely transparent to the rays emitted by the heated copper plate already referred to. There are, however, certain rays, comparatively few in number, emitted by the copper, to which the carbonic acid is impervious; and could we obtain a source of heat emitting such rays only, we should find carbonic acid more opaque than any other gas to the radiation from that source. Such a source is actually found in the flame of carbonic oxide, where hot carbonic acid constitutes the main radiating body. Of the rays emitted by our heated plate of copper, olefiant gas absorbs ten times the quantity absorbed by carbonic acid; of the rays emitted by a carbonic oxide flame, carbonic acid absorbs twice as much as olefiant gas. This wonderful change in the power of the former as an absorber is simply due to the fact that the periods of the hot and cold carbonic acid are identical, and the waves from the flame freely transfer their motion to the molecules which synchronize with them. Thus it is that the tenth of an atmosphere of carbonic acid, enclosed in a tube four feet long, absorbs 60 per cent. of the radiation from a carbonic oxide flame, while onethirtieth of an atmosphere absorbs 48 per cent. of the heat from the same origin. In fact the presence of the minutest quantity of carbonic acid may be detected by its action on the rays from the carbonic oxide flame. Carrying, for example, the dried human breath into a tube four feet long, the absorption there effected by the carbonic acid of the breath amounts to 50 per cent. of the entire radiation. Radiant heat may indeed be employed as a means of determining practically the amount of carbonic acid expired from the lungs. My assistant, Mr. Barrett, has, at my request, made this determination. The absorption produced by the breath, freed from its moisture, but retaining its carbonic acid, was first determined. Carbonic acid, artificially prepared, was then mixed with dry air in such proportions that the action of the mixture upon the rays of heat was the same as that of the dried breath. The percentage of the former being knowr, immediately gave that of the latter. The same breath, analyzed chemically by Dr. Frankland, and physically by Mr. Barrett, gave the following results:

Percentage of carbonic acid in the human breath.

[blocks in formation]

It is thus proved that in the quantity of ethereal motion which it is competent to take up, we have a practical measure of the carbonic acid of the breath, and hence of the combustion going on in the human lungs.

Still this question of period, though of the utmost importance, is not competent to account for the whole of the observed facts. The ether, as far as we know, accepts vibrations of all periods with the same readiness. To it the oscillations of an atom of oxygen are just as acceptable as those of a molecule of olefiant gas; that the vibrating oxygen then stands so far below the olefiant gas in radiant power must be referred not to period, but to some other peculiarity of the respective molecules. The atomic group which constitutes the molecule of olefiant gas produces many thousand times the disturbance caused by the oxygen because the group is able to lay a vastly more powerful hold upon the ether than the single atoms can. The cavities and indentations of a molecule composed of spherical atoms may be one cause of this augmented hold. Another, and possibly very potent one, may be, that the ether itself, condensed and entangled among the constituent atoms of a compound, virtually increases the magnitude of the group, and hence augments the disturbance. Whatever may be the fate of these attempts to visualize the physics of the process, it will still remain true, that to account for the phenomena of radiation and absorption we must

take into consideration the shape, size, and complexity of the molecules by which the ether is disturbed.

XVI. SUMMARY AND CONCLUSION.

Let us now cast a momentary glance over the ground that we have left behind. The general nature of light and heat was first briefly described: the compounding of matter from elementary atoms and the influence of the act of combination on radiation and absorption were considered and experimentally illustrated. Through the transparent elementary gases radiant heat was found to pass as through a vacuum, while many of the compound gases presented almost impassable obstacles to the calorific waves. This deportment of the simple gases directed our attention to other elementary bodies, the examination of which led to the discovery that the element iodine, dissolved in bisulphide of carbon, possesses the power of detaching, with extraordinary sharpness, the light of the spectrum from its heat, intercepting all luminous rays up to the extreme red, and permitting the calorific rays beyond the red to pass freely through it. This substance was then employed to filter the beams of the electric light, and to form foci of invisible rays so intense as to produce almost all the effects obtainable in an ordinary fire. Combustible bodies were burnt and refractory ones were raised to a white heat by the concentrated invisible rays. Thus, by exalting their refrangibility, the invisible rays of the electric light were rendered visible, and all the colors of the solar spectrum were extracted from utter darkness. The extreme richness of the electric light in invisible rays of low refrangibility was demonstrated, onetenth only of its radiation consisting of luminous rays. The deadness of the optic nerve to those invisible rays was proved, and experiments were then added to show that the bright and the dark rays of a body raised gradually to intense incandescence are strengthened together; that to reach intense white heat intense dark heat must be generated. A sun could not be formed or a meteorite rendered luminous on any other conditions. The light-giving rays constitute only a small fraction of the total radiation, their unspeakable importance to us being due to the fact that their periods are attuned to the special requirements of the

eye.

Among the vapors of volatile liquids vast differences were also found to exist as regards their power of absorption. We followed, moreover, various molecules from a state of liquid to a state of gas, and found, in both states of aggregation, the power of the individual molecules equally asserted. The position of a vapor as an absorber of radiant heat was proved to be determined by that of the liquid from which it is derived. Reversing our conceptions, and regarding the molecules of gases and vapors not as the recipients, but as the originators of wave motion-not as absorbers, but as radiators--it was proved that the powers of absorption and radiation went hand in hand, the self-same chemical act which rendered a body competent to intercept the waves of ether rendering it competent in the same degree to generate them. Perfumes were next subjected to examination, and, notwithstanding their extraordinary tenuity, were found vastly superior, in point of absorptive power, to the body of the air in which they were diffused. We were led thus slowly up to the examination of the most widely diffused and most important of all vapors-the aqueous vapor of our atmosphere and we find in it a potent absorber of the purely calorific rays. The power of this substance to influence climate, and its general influence on the temperature of the earth, were then briefly dwelt upon. A cobweb spread above a blossom is sufficient to protect it from nightly chill; and thus the aqueous vapor of our air, attenuated as it is, checks the drain of terrestrial heat and saves the surface of our planet from the refrigeration which would assuredly accrue were no such substance interposed between it and the voids of We considered the influence of vibrating period and molecular form on absorption and radiation,

space.

and finally deduced from its action upon radiant heat the exact amount of carbonic acid expired by the human lungs.

Thus, in brief outline, have I placed before you some of the results of recent inquiries in the domain of radiation, and my aim throughout has been to raise in your minds distinct physical images of the various processes involved in our researches. It is thought by some that natural science has a tleadening influence on the imagination, and a doubt might fairly be raised as to the value of any study which would necessarily have this effect. But the experience of the last hour must, I think, have convinced you that the study of natural science goes hand in hand with the culture of the imagination. Throughout the greater part of this discourse we have been sustained by this faculty; we have been picturing atoms and molecules and vibrations and waves which eye has never seen nor ear heard, and which can only be discerned by the exercise of imagination. This, in fact, is the faculty which enables us to transcend the boundaries of sense, and connect the phenomena of our visible world with those of an invisible one. Without imagination we never could have risen to the conceptions which have occupied us here to-day; and in proportion to your power of exercising this faculty aright, and of associating definite mental images with the terms employed, will be the pleasure and the profit which you will derive from this lecture. The outward facts of nature are insufficient to satisfy the mind. We cannot be content with knowing that the light and heat of the sun illuminate and warm the world. We are led irresistibly to enquire what is light and what is heat; and this question leads us at once out of the region of sense into that of imagination. Thus pondering, and asking, and striving to supplement that which is felt and seen, but which is incomplete, by something unfelt and unseen which is necessary to its completeness, men of genius have in part discerned, not only the nature of light and heat, but also, through them, the general relationship of natural phenomena. The power of nature is the power of motion, of which all its phenomena are but special forms. It manifests itself in tangible and in intangible matter, being incessantly transferred from the one to the other, and incessantly transformed by the change. It is as real in the waves of the ether as in the waves of the sea, the latter being, in fact, nothing more than the heaped-up motion of the former, for it is the calorific waves emitted by the sun which heat our air, produce our winds, and hence agitate our ocean; and whether they break in foam upon the shore, or rub silently against the ocean's bed, or subside by the mutual friction of their own parts, the sea-waves finally resolve themselves into waves of ether, and thus regenerate the motion from which their temporary existence was derived. This connection is typical. Nature is not an aggregate of independent parts, but an organic whole. If you open a piano and sing into it a certain string will respond. Change the pitch of your voice; the first string ceases to vibrate, but another replies. Change again the pitch; the first two strings are silent, while another resounds. Now, in altering the pitch you simply change the form of the motion communicated by your vocal chords to the air, one string responding to one form and another to another. And thus is sentient man sung unto by nature, while the optic, the auditory, and other nerves of the human body are so many strings differently tuned and responsive to different forms of the universal power.

SYNTHETIC EXPERIMENTS RELATIVE TO METEORITES-APPROXIMATIONS TO

WHICH THESE EXPERIMENTS LEAD.*

BY M. DAUBRÉE, MEMBER OF THE INSTITUTE, INSPECTOR GENERAL OF MINES.

[Translated for the Smithsonian Institution.]

The study of meteorites touches on several fundamental questions of the physical history of the universe. Aside from the importance which these bodies present in a purely astronomical point of view, they are furthermore of interest to geology from their constitution itself, and this under a two-fold aspect. On the one hand, they are the only specimens of extra-terrestrial or cosmical bodies with which it is possible for us to have actual contact; or which can afford us any ideas respecting the constitution of the bodies scattered through the celestial spaces. On the other hand, the more thorough our study of them the better shall we recognize the bearing which they may have on sundry branches of knowledge, and particularly the history of our globe, as will be seen further Thus it is that meteorites constitute an essential as well as new chapter in geology; and notwithstanding the little attention hitherto accorded to their study by geologists, it cannot but be considered, on the grounds just stated, as meriting a place in the pages of the Annales des Mines.

on.

In a recently published report on the progress of a part of geology, which may be called experimental geology, we had occasion to explain how far experiment had been made instrumental in solving the questions which relate to the origin of meteorites and the mode of their formation; this chapter it has been thought proper to reproduce here, with some developments, a portion of which had found a place in previous publications. The title sufficiently indicates the propriety of reducing the historical and descriptive part to a very succinct exposition.

CHAPTER I.

EXTRA-TERRESTRIAL ORIGIN OF METEORITES.—PHENOMENA WHICH ACCOMPANY THEIR FALL.

It is a long time since any doubt could be entertained that among the substances which fall from the atmosphere to the surface of the globe, there are some whose origin is incontestably foreign to the planet which we inhabit. Their descent makes itself known by a considerable production of light and sound which accompanies it, by the almost horizontal trajectory which they describe, and by the excessive velocity of the bolides which embody the substances in question.

Several recent falls, which have been studied with care, have enabled us to determine with more precision the circumstances which attend the arrival *Annales des Mines, &c, Paris, 1868.

+ Rapport sur les progrès de la géologie expérimentale. Imprimerie impériale, 1867.

of these masses on the earth. That these circumstances are constantly and identically reproduced, is extremely remarkable. The fall of meteorites is always accompanied by an incandescence sufficiently vivid to give to night an appearance of day, and to be perfectly perceptible even at noon-day. In consequence of this vivacity of their light, the arrival of meteorites in our atmosphere may be seen at very great distances; the fall at Orgueil (Tarn-et-Garonne,) of the 14th May 1864, was observed as far off as Gisors (Eure,) a distance of more than 500 kilometers,* (310 miles.)

The light in question is, moreover, of very transient duration. It is thought to be produced at the moment when the asteroid enters our atmosphere, and therefore at a great height, which, in the case of Orgueil, for example, has been computed at 65 kilometers, (40 miles.) It is owing to this incandescence that the trajectory of meteorites, which is in general but little inclined to the horizon, is susceptible of being observed. A trajectory of this nature was particularly verified for the bolide of Òrgueil just cited: proceeding from the west towards the east, this bolide was followed from Santander and other points of the coast of Spain to the place of its fall. The incandescence allows, moreover, of an appreciation of the velocity of the bolides, a velocity which has nothing analogous on the earth, and which can only be compared to that of the planets revolving in their orbits. This single circumstance would suffice to prove the cosmic origin of meteorites. The meteorite of Orgueil appeared to traverse about 20 kilometers (12 miles) per second; while in other cases velocities have been observed which could not be estimated at less than 30 kilometers, (18 miles.)

The appearance of the bolide is constantly accompanied by a trail of vapors, which are themselves not destitute of a certain effulgence. No instance of the fall of a meteorite has occurred without being preceded by an explosion and sometimes by several explosions. The noise of the explosion has been compared either to that of thunder or to that of cannon, according to the distance of the observers. It makes itself heard over a vast extent of country; sometimes at a distance of more than 100 kilometers in circuit, as in the fall at Orgeuil. If we reflect that it is produced in regions where the air, highly rarefied, lends itself very imperfectly to the propagation of sound, we shall readily be convinced that its intensity must be such as to surpass all else that is known to us. explosion a whizzing sound is heard, owing to the rapid passage of detached pieces in the air, which the Chinese compare to the noise made by the wings of wild geese, or to that of a cloth which is torn. It should be added that these phenomena have been observed not only in widely distant regions of the globe, but at all seasons, at all hours, and often when the sky is serene and cloudless and the air calm. Tempests, water-spouts, therefore, have no agency here.

After the

To obviate an objection which naturally presents itself to the mind, in relation to the velocity of these bodies, attention must be drawn to an essential distinction. The enormous velocity proper to the luminous body or bolide which is seen cleaving the atmosphere contrasts with that, incomparably more feeble, which the fragments possess at the moment of their arrival on the earth. The bolide moves like a body launched with a great initial velocity; on the contrary the fragments which reach us in the sequel of the explosion appear, in general, to possess only a velocity comparable to that which would correspond to their descent, moderated, besides, by the resistance of the air. It is to be added that, as the bolides move in all directions, their relative velocity, all else being equal, must necessarily vary according to the course of the trajectory with regard to the direction of the earth's rotation and motion in its orbit.

The stones of any one fall are more or less numerous, and are always burning hot on the surface at the moment of their arrival, without, however, having preserved their incandescence. At Orgueil stones fell upon 60 points, comprised

*A kilometre is 62 hundredths of a mile.

« AnteriorContinuar »