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the limits of our firmament of stars. The tendency of some modern astronomers is to the opinion that all the nebulae are confined to distances no greater than the stars. Should this opinion become established, there are still some phenomena presented by the nebulae which seem to indicate central gravitation and rotary motion. While many are more or less irregular, others present a spherical (or at least a circular) form. Others are beautifully annular. Still others, as the nebula in Argo, present parabolic curves resembling the tails of cornets; and others, finally, like the nebulas in Canes Venatici and Virgo, are strikingly spiral. All these forms are irresistibly suggestive of central forces and axial rotations.

But the common opinion of astronomers seems to be in accord with that of the Herschels, that probably very many of the nebulae are really other firmaments of stars wholly external to ours, and removed to distances proportionate to the vast interstellar spaces of our own firmament. Our firmament, according to the system of gauging the star-depths employed by Sir William Herschel, is circumscribed by a definite boundary, and presents somewhat the form of a grindstone cleft around a portion of its periphery. It is true that the telescope reveals a multitude of stars not seen by the unaided eye; and, as the power increases, star after star rises in the far-off horizon of our view, till it would seem that the number is infinite and boundless. But our firmament has its outer periphery. When the most powerful instruments are steadily turned toward even the most populous portion of the sky, the vision threads its cold and devious way from rank to rank of glittering suns, till, finally, it stands upon the outer ramparts of the firmament and looks out upon immensity. What solemn emotions fill the soul when it succeeds in traveling beyond the stars, and gazes through the loop-holes of the firmament upon the blackness and emptiness beyond!

But what of the realms of space beyond the boundaries of our star-system? Across that dark and pathless interval the telescope has led our vision, and lo! upon the remotest confines of the universe hangs a faint film of light, like the feeble glow of a watchman's lamp upon the shore of a cold and dark and trackless sea. In other quarters of the heavens are other patches of light, the counter existences of this. These are the nebulae; and such, according to current views of stellar astronomy,* are their relative positions and distances.

Should such views be finally confirmed, we shall discover still stronger analogies with the phenomena of the solar system, and stretching over intervals of space too vast for even the imagination to span.

There is good ground for doubting, however, whether mere clouds of luminous vapor, which most of the irresolvable nebulae are supposed to be, would be visible to human eyes if really s0 far external to our firmament. Since some of the irresolvable nebulous matters have shown such a connection with stars as to demonstrate that they belong to our stellar system, it may be most reasonable to assume that all irresolvable nebulae are thus associated. Accordingly only resolvable nebulae would be regarded as external, while the irresolvable nebulae would be only specimens of formative matter in various stages of differentiation. This view is sustained by the existence of so called nebulous stars and planetary nebulae, in which the irresolvable nebulous matter presents itself condensed toward the center into a state of greater luminosity, which, in many examples, approaches or reaches the appearance of a veritable star or couple of stars.f

(5.) Inferences from the movements of light. Whether orbital and axial motions, and other evidences of the presence of gravity, be traced to the distances of the fixed stars and nebulaB or not, this proof of community of conditions certainly exists, that the flight of the luminous ray proclaims identical laws throughout the visible universe. Light is a phenomenon universally regarded as arising from inconceivably but measurably rapid vibrations of a subtile material fluid commonly known as ether.:]: Wherever light penetrates there is ether.

* Sir John Horschel: "Outlines of Astronomy," 4th ed., p. 537; "Troatiso on Astronomy," Am. ed., 1851, chap, xii; "Familiar Lectures on Science," p. 215; Nichol: "Architecture of the Heavens," letter i; Guillemin: "The Heavons," p. 366. On the contrary, soe Proctor: "Other Worlds than Ours," chap, xii; Rorison: "Replies to Essays and Reviews," pp. 270, 271; Whewell: "Plurality of Worlds," p. 142.

f For striking examples see the figures of Delaunay: "Op. Cit.," pp. 635, 636, 638.

% Aside from the necessity of some such medium for the propagation of light. and, as some of the latest speculations indicate, for the propagation of electricity also, the evidence for the existence of an ethereal fluid rests on slight disturbances To the remotest star—to the remotest nebulae—this tenuous fluid fills immensity; and throughout the height and depth, the length and breadth of the empire of matter, this omnipresent element is quick with the tremors generated by millions of suns. Light flies 186,000 miles in a second of time. The solar beam falls to the earth in eight and one third minutes, and reaches the orbit of Neptune in four hours. The light of the nearest star has occupied three and a half years in reaching us, and that of the remotest star which shows a parallax, seventy years. The light from the most distant star which shed a discernible light in the great telescope of Sir William Herschel had left its source eight thousand years before. Eminent authorities entertain the belief that light from some of the distant nebulas must have occupied 700,000 years in reaching the earth.*

What a conception is here for the mind to dwell upon! What proof of the age of the material universe, and its extent! Yet the same ether, like an ocean bathing continents on its opposite shores, pulsates through the systems of earth, sun, Arcturus, Polaris, and vanishing nebulae. "It is Light," says Sir John Herschel,f " and the free communication of it from

in the movements of certain comets of short period, especially Encke's. Mr. A. Hall (" Arner. Jour. ScL," [3,] ii, p. 404) has recently raised a doubt in reference to the correctness of the explanation of the retardation of Encke's comet Professor W. Stanley Jevons, also, in a late number of the London "Chemical News," attributes the retardation of the comet to electricity, and regards the hypothesis of a resisting medium as entirely imaginary. It is worthy of consideration, however, that the ethereal fluid, if it possess the properties of matter, must be increased in density and resisting power in the vicinity of great masses of matter, especially the sun; and that, hence, other things being equal, those comets having the shortest perihelion distances will experience the greatest effects. It is also worthy of remembrance that possibly the uniform motions of the planets about the sun may have imparted a vortical movement to the ether, which would accelerate or retard the motions of comets in accordance with their relation to the direction of the ethereal current. This would diminish its effect on all the comets of short period, since they all have direct motion. It must not be presumed, however, that this fluid is necessarily subject to the law of gravitation, and is possessed in every respect of the properties of ordinary matter. The nature of the ethereal medium, which the almost unanimous judgment of physicists holds to exist, is, at the present moment, the object of the profouudest researches and speculations. (Sir John Herschel: "Familiar Lectures on Scientific Subjects," lect. viii; Mac Vicar: "A Sketch of a Philosophy," parts ii, iii.)

* Guillemin: "The Heavens," p. 366.

f Sir John Herschel: "Familiar Lectures," p. 218.

the remotest regions of the universe, which alone can give, and does fully give ns, the assnrance of a uniform and all-pervading energy—a mechanism almost beyond conception complex, minute, and powerful, by which that influence, or rather that movement, is propagated. Our evidence of the existence of gravitation fails ns beyond the region of the double stars, or leaves us, at best, only a presumption, amounting to a moral conviction, in its favor. But the argument for a unity of design and action afforded by light stands unweakened by distance, and is co-extensive with the universe itself."

(6.) Revelations of the spectroscope. The culminating proof of identical conditions throughout the physical universe has been furnished by the spectroscope. This little instrument, of recent invention, takes the slender ray of light admitted through a narrow slit, and subjects it to a peculiar scrutiny —a searching examination, which extorts from it the secret of its origin, and of the body which sent it forth, and of the medium through which it has traveled. We can offer but a few words of explanation of this mysterious process, referring to the works of Schellen, Roscoe, Huggins, Lockyer, Brewster, Angstrom, and others, for fuller information.

Every one knows that solar light passed through a prism of glass undergoes decomposition into its seven primary colors, which may be projected on the opposite wall. Under proper adjustments this colored spectrum may be seen crossed by numerous dark lines. Light proceeding from other luminous sources presents other phenomena. 'The results of extended experiments upon artificial lights have established the three following principles:

1. The spectrum of an incandescent solid or liquid is continuous, that is, it presents no lines across it.

2. The spectrum of a glowing vapor or gas is crossed by numerous bright lines, and each different vapor gives a different set of bright lines.

3. The spectrum of an incandescent solid or liquid shining through a vapor (dark or incandescent) of lower temperature than the source of the light is crossed by numerous dark lines, and these dark lines occupy the same positions as the bright lines proper to the spectrum of the vapor.

From the third law it appears that vapors transmitting light from an incandescent solid or liquid, absorb exactly the same rays which they would themselves emit if incandescent. Light shining through the vapors of sodium presents a certain set of dark lines; but if the vapors of sodium are rendered incandescent, they produce a set of bright lines occupying exactly the places of the dark ones. This set of dark lines across any spectrum becomes, therefore, the evidence of the presence of sodium in the vapors through which the light passes.

Now, dark lines, as we said, cross the spectrum of the sun. This, first of all, proves that the light of the sun emanates from an incandescent solid or liquid, and passes through vapors of lower temperature in escaping to the earth. How sudden and unexpected a revelation of its constitution! A liquid nucleus with an envelope of glowing clouds*

But certain ones of these dark lines occupy exactly the positions of the bright lines of the spectrum of sodium. The vapor of sodium is therefore present in the gaseous envelope of the sun! Sodium is one of our most common substances. It is the basis of common salt consumed at every human meal. It gives the saltness to the waters of the universal ocean. This most familiar element enters largely into the constitution of the sun.

But another set of the dark lines of the solar spectrum corresponds to the bright lines of the spectrum of hydrogen. Now hydrogen is one of the two constituents of all the water which belongs to our planet. It is also a constituent of coal, of petroleum, and of all vegetable and animal substances. This familiar element abounds also in the sun!

Nor is this all. Physicists have studied these magical lines of the solar spectrum until they have detected the existence of sundry other substances in the constitution of the sun. We find there not only sodium and hydrogen, but iron, magnesium, barium, copper, zinc, calcium, chromium, nickel, and prob

* Father Soochi does not regard the liquid nucleus fully proven. The dark spectral lines would result if the photosphere of the sun were in such a state of condensation as to present an analogy to mist, and thus shine as a liquid; while the absorbent medium might be a non-luminous atmosphere external to the photosphere. Thus the central portion of the sun might be a non-luminous gas. (Secchi: "Op. Cit.," pp. 104-6.) Though this theory is adopted by M. Faye, ("Comtea rendus," 16 and 23 Jan., 1865; 27 July, 1868, torn, lxviii, 197,) we feel constrained to regard the doctrine of a molten nucleus the most plausible.

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