Of the irregular nebulæ, Professor Proctor says, “It may well chance, as long since suggested by Professor Clark, of Cincinnati, and as more cautiously hinted by Dr. Huggins, that in the varieties of constitution observed in the irregular nebulæ, and the evidence such varieties afford of progressive changes, we may find not merely direct evidence of the development of suns and sun-systems from the great masses of nebulous matter, but even what would be a far more important and impressive result,—actual evidence of the development of so-called elements from substances really elementary, or, at any rate, one stage nearer the elementary condition than are our hydrogen, nitrogen, oxygen, carbon, and so forth. The peculiarity of the spectral indications of the presence of nitrogen and hydrogen in the nebula, that only one line of nitrogen and two or three lines of hydrogen are discernible, instead of a complete spectrum of either element as seen under any known conditions, seems suggestive of what may be called a more elemental condition of hydrogen and nitrogen.” Whether this be so, or whether these peculiarities are due to self-obscuration, or mutual reversal of the familiar lines due to the enormous disturbances of the nebular mass which must exist, it is certain that there is one terrestrial substance, at least, which acts invariably, in combination and chemical affinity, as a simple element in inorganic chemistry, but which is, in fact, compound,—to wit, the hypothetical radical ammonium, which is closely allied with the simple alkaline metals potassium and sodium, forming with them a single group; and yet, while the others have always remained as fixed, primitive elements, the hypothetical element ammonium alone is a composite substance consisting of hydrogen and nitrogen, two of the invariable gaseous constituents of all these nebulæ. In comets we find, vaguely expressed, an occasional strongly marked sodium line, and also the spectrum of carbon; in these gaseous nebulæ we find, as yet, no trace of carbon, and this element is so closely allied to hydrogen in its chemical affinities and reactions as to suggest that it may be the same element or some alloy of it, or in some allotropic form, as we find to be the case with other simple elements under special conditions. In organic chemistry—the chemistry of organic life—we find almost innumerable compound radicals which act as simple elements in combination, but which we can combine and separate into their constituents at will; to all intents and purposes, in their various reactions, they behave as elemental substances, and were it not that our analyses are able to resolve them, as the spectroscope resolves the nebulæ, we might well believe that here also we were dealing with simple primary elements. It is almost certain that great discoveries in this field of chemistry are not far distant, which will recall with wondering surprise the now universally exploded fallacies of the “Philosopher’s Stone” and the “Universal Solvent.” Indeed, we may find in the electrical energies of the planets and the self-repulsive force of the electrospheres of the earth and moon possible grounds for investigating anew some of the abandoned tenets of astrology, in the hope that the light of science may disclose some basis, at least, for what, at one time,—and for nearly all time, in fact,—was the universally accepted belief, not only of the ignorant, but of those the wisest and most learned of their day and generation. If the planets by their position can cloud the sun, nearly a million miles in diameter, with spots, or shed the brilliance of the aurora borealis over all our skies, may they not also cloud the embryonic intellect, or charge it with energies for a career of prosperity or of disaster? May not the unseen currents, or the electric storms around us, or the vast electrical phenomena of the sun as well affect the sprouting germs of the husbandman or some abnormally rapid development of an insect pest as the light, the warmth, the moisture, or the cold, which, to our coarser vision, are alone apparent? Fancy and fallacy revel luxuriantly where science fails, but truth existed long before science was systematized, and the supercilious condemnation of once generally accepted views without examination is merely pseudo-science, and scarcely a single grade higher in the scale than ignorant superstition itself. And every new advance in knowledge requires a new overhauling of abandoned material, just as a new advance in metallurgical knowledge enables us sometimes to work over again our once-rejected mining dumps with decided profit. Indeed, science itself is but a collection of observed facts reduced to system, and among the shrewd and practical miners there is a well-known saying, “The ore is where you find it,” which has frequently put scientific assertion to the blush.
Gaseous nebulæ (non-systemic in development).—Fig. 1, the Crab nebula; Fig. 2, Dumb-bell nebula (reduced from Nichol, after Lord Rosse); Fig. 3, nebula in Sobieski’s Crown; Fig. 4, Catherine-wheel nebula (from Flammarion).
In Fig. 1 gravity preponderates, and electrical repulsion drives the radiant matter upward and outward. This nebula resembles a comet in its phenomena; a large nebula in the neighborhood in rear of the Crab’s body would produce this effect.
Fig. 2 shows a bipolar form produced by repulsion acting against gravity; the two heads connected by a narrow strand, the lower head elongated by internal repulsion, and the horns curved upward by the attraction of gravity of the upper head. This figure suggests the division of a comet (like Biela’s) into two smaller comets.
In Fig. 3 gravity and electrical repulsion are nearly equal; the result is an elongated lineal nebula, warped into irregular curves by counter currents of space.
Fig. 4 is rotary, and the repulsive forces will probably entirely overcome gravity and result in the formation of an annular nebula with hollow center.
A study of the beautiful mezzotint plates, from the drawings of the Earl of Rosse, contained in Professor Nichol’s splendid work, “The Architecture of the Heavens,” will clearly disclose the forms, as revealed by a powerful telescope, of many of these gaseous nebulæ. Of such nebulæ, Appleton’s Cyclopædia says, “nebulæ proper, or those which have not been definitely resolved, are found in nearly every quarter of the firmament, though abounding especially near those regions which have fewest stars. Scarcely any are found near the Milky Way, and the great mass of them lie in the two opposite spaces farthest removed from this circle. Their forms are very various, and often undergo strange and unexpected changes as the power of the telescope with which they are viewed is increased, so as not to be recognizable in some cases as the same objects.” An example of this is shown in Plate X. (Figs. 1 and 2) of Professor Nichol’s work, which gives a greatly enlarged view of those shown in Figs. 1 and 2 of Plate IX. (For Fig. 2 of Nichol’s Plate X., see illustration of nebula with double sun, in previous chapter.) Professor Nichol says, “In every instance examined, save one, the planetary nebulæ are nebulæ with hollow centers.” The inference which this writer makes, that such a planetary nebula consists of “a grand annular cluster of stars,” has been since disproved by the discoveries of the spectroscope, but the telescopic form remains true, and still awaits further interpretation. While the irresolvable nebulæ seem to seek some retired spot in space for their processes, like certain animals when incubating, this rule is not universal. Of this, Appleton’s Cyclopædia says, “The density of nebular distribution increased with the distance from the galactic zone for the irresolvable nebulæ, but diminished with that distance for the clusters …. There is not a gradual condensation of nebulæ towards two opposite regions, near the poles of the galactic zone, but the nebulæ are gathered into streams, nodules, and irregular aggregations such as we find in the grouping of stars …. Between stars and nebulæ their arrangement follows the law of contrast. There are two remarkable exceptions to this law,—the Magellanic Clouds. In these, where stars of all orders, from the ninth magnitude to irresolvable stellar aggregations, are as richly gathered as in the galactic zone, nebulæ of all orders are also gathered richly, even more so than anywhere else over the whole heavens.” In the same work, article “Nebula,” it is stated of the planetary nebulæ, “There are several which have perfectly the appearance of a ring, and are called annular nebulæ …. Some appear to be physically connected in pairs like double stars. Most of the small nebulæ have the general appearance of a bright central nucleus enveloped in a nebulous veil. This nucleus is sometimes concentrated as a star and sometimes diffused. The enveloping veil is sometimes circular and sometimes elliptical, with every degree of eccentricity between a circle and a straight line. There are some which, with a general disposition to symmetry of form, have great branching arms or filaments with more or less precision of outline. An example of this is Lord Rosse’s Crab nebula. Another remarkable object is the nebula in Andromeda, which is visible with the naked eye, and is the only one which was discovered before the invention of the telescope. Simon Marius (1612) describes its appearance as that of a candle shining through horn. Besides the above, which have comparatively regular forms, there are others more diffused and devoid of symmetry of shape. A remarkable example is the great nebula in Orion, discovered by Huygens in 1656 …. The great nebula in Argo is another example of this class.”
The number of nebulæ recognized in all the heavens is upward of five thousand, and new ones are being constantly discovered. Of these objects, Professor Nichol says, “The spiral figure is characteristic of an extensive class of galaxies. Majestic associations of orbs, arranged in this winding form, with branches issuing like a divergent geometric curve from a globular cluster.” These nebulæ, however, are not associations of orbs; they are gaseous nebulæ apparently in process of evolution. This author (Professor Nichol) presents views of such spiral nebulæ either foreshortened to the view, so as to form a long ellipse, or with the convolutions of the spiral apparently raised from the horizontal plane into a conical form, and showing the black streaks of space which lie between the convolutions, others seen in side view, others in front, and, in fact, presented to the eye in every position for observation. The author wrote before the days of the spectroscope, and that he should conceive these vast objects to be spirals made up of blazing suns like our Milky Way—vast galaxies, in fact—was an inevitable conclusion at that time; but we now know that these spiral nebulæ are gaseous, are apparently in process of manufacture, and we can see them in their different stages of evolution, and may perhaps learn something about the processes by which solar systems and galaxies of suns are formed. Of one of these strange but exceedingly instructive objects, Professor Ball, in his work “In the High Heavens,” says, “Fig. 3 represents one of the famous spiral nebulæ (that of Canes Venatici) discovered many years ago by the late Earl of Rosse. The object is invisible to the naked eye. It seems like a haze surrounding the stars, which the telescope discloses in considerable numbers, as shown in the picture. When viewed through an instrument of sufficient power, a marvellous spectacle is revealed. There are wisps and patches of glowing cloud-like material which shine not as our clouds do, by reflecting to us the sunlight. This celestial cloud is no doubt self-luminous; it is, in fact, composed of vapors so intensely heated that they glow with fervor. As I write, I have Lord Rosse’s elaborate drawing of this nebula before me, and on the margin of this stupendous object the nebula fades away so tenderly that it is almost impossible to say where the luminosity terminates. Probably this nebula will in some remote age condense down into more solid substances. It contains, no doubt, enough material to make many globes as big as our earth. Before, however, it settles down into dark bodies like the earth, it will have to pass through stages in which its condensing materials will form bright sun-like bodies. It seems as if this process of condensation might almost be witnessed at the present time in some parts of the great object. There are also some very striking nebulæ which are often spoken of as planetary. They are literally balls of bluish-colored gas or vapor, apparently more dense than that which forms the nebula now under consideration. Such globes are doubtless undergoing condensation, and may be regarded as incipient worlds.” Of these spiral nebulæ it is said, in Appleton’s Cyclopædia, “Many of them had been long known as nebulæ, but their characteristic spiral form had never been suspected. They have the appearance of a maelstrom of stellar matter, and are among the most interesting objects in the heavens.” Of their spectra it is said, “The bright-line spectrum is given by all the irregular nebulæ hitherto examined and by the planetary nebulæ.” That is to say, these nebulæ are gaseous in constitution, and have not yet reached the stage of solar condensation which marks the existence of individual suns.