PART IV. COMETS IN VARIOUS STAGES OF MATURITY.

I. From a careful examination of those Comets whose motions are exactly known, on their successive returns to the perihelion, much valuable information of a different nature may be obtained. For if they happen to have undergone any change in their physical constitution, during the period of their absence, that change will probably be indicated by a corresponding variation in their appearance. Since the effect of the solar power is so great (whatever be the mode of its operation) in pushing away the nebulous matter of the Comet, into the form of a tail, it has been supposed that some of this nebulous matter may even be altogether detached from the attraction of the nucleus, so as to cause a gradual diminution in the Comet’s substance; and this effect, it is obvious, will be the more easily produced, if the gravitation of the nebulous particles to the nucleus be weakened by a rotatory motion of the Comet. Now, an attentive examination of those comets, whose approaches to the sun at the perihelion are near in respect of distance, and frequent in respect of time, may enable us to judge whether or not this supposition be well founded. But this is a point to which the attention of astronomers has been too recently directed, to be yet very satisfactorily fixed. Numerous data are requisite, which a constant and careful observation can alone supply, before any decisive result can be obtained. But certainly the observations of astronomers, as far as they have been made, with regard both to the diminished size of the nucleus of all comets after a perihelion passage, and the inferior brilliancy of Halley’s in particular, at its last appearance‍‍130, seems to confirm what other considerations abundantly suggest, that a partial abstraction of nebulous matter does take place at every approximation of a comet to the sun.

The question, therefore, very naturally occurs, whether a Comet, after a long succession of revolutions, will not be liable to become altogether annihilated by this dispersion of its nebulous matter? Herschell’s opinion respecting the constitution and formation of comets, here deserves our attention, as it satisfactorily resolves the difficulty which is now proposed. There is no individual perhaps in the annals of astronomy, who has contributed more to our knowledge of the heavens than Sir William Herschell, both by extending the limits of our vision into the most distant parts of the universe, and by investigating the laws which govern the more complicated phenomena of nature. But of all his contributions to the science, none are so important in themselves, or so well calculated to disclose to us the secret and marvellous operations going on in the workshop of Nature, as the discoveries which he has made concerning nebulæ. These nebulæ, it is supposed, are formed by the partial condensation of matter, probably the etherial medium itself diffused throughout the universe; and that their number must be prodigious, is sufficiently proved by the fact, that Herschell, by his own efforts alone, discovered 2000 of them. Some of the nebulæ are found to have so strong a resemblance to many comets, which, on account of their distance from the sun, can just be discerned from the earth, that they are not unfrequently confounded‍‍131; and it is only by a nearer approach, or by an intimate acquaintance with all the nebulæ in the same quarter of the heavens, that astronomers are able to distinguish them. Now, it is the opinion of Herschell, and his opinion is strongly supported by the authority of La Place‍‍132, that Comets are originally minute nebulæ, which, by the continual approximation of their particles, have at length acquired such a degree of density, as to be capable of being attracted by the sun, and of describing an orbit of their own. As the nebulous mass approaches the sun, one result, as we have seen, is the expansion of its parts, and their prolongation into what has been termed the Tail: But, another result, according to Herschell, and one no less important, is a gradual consolidation of the nebulous matter by the agency of the solar heat. “It is admitted on all hands,” says he, “that the act of shining denotes a decomposition, in which at least light is given out; but that many other elastic volatile substances escape at the same time, especially in so high a degree of rarefaction, is far from improbable. Since light then, certainly, and very likely other subtile fluids also, escape in great abundance during a considerable time before and after a comet’s nearest approach to the sun, I look,” says Herschell, “upon a perihelion passage in some degree as an act of consolidation‍‍133.”

II. This process of consolidation will evidently be the more powerful, the more that the Comet is subjected to the sun’s calorific action; a condition which depends upon two circumstances, one, the perihelion distance of the Comet, the other, the time in which it completes its revolution. It follows from this consideration, that we may be able even to estimate the degree of solidity which Comets have attained, simply by taking into account these two circumstances; and a reference to observation will at once shew whether or not the theory be correct. But, before attempting to apply this test, one remark must be made, which shews that the application of it may not in all cases be conclusive. If all Comets during their successive revolutions round the sun, were to remain totally exempt from the possibility of receiving any accession of foreign matter, tending to enlarge their bulk, then we might expect that the consideration of their perihelion distance and their period of revolution should always correspond with the amount of their solidity, or, in other words, the actual size of their nucleus. But, if we suppose with Herschell, La Place, and other eminent astronomers, that there exist multitudes of nebulæ throughout space in every different stage of maturity, from those whose formation has just commenced, to those whose condensation by the attraction of the particles has already so far advanced, as will soon render them capable of gravitating towards the sun, we must reckon it not impossible, that Comets, in the extensive range of their orbits, may occasionally meet with some of these nebulæ, and thus carry with them a new supply of unperihelioned matter in their next approach to the centre of the system. In this manner, the loss of substance to which, as we have above remarked, comets are exposed by volatilization, may possibly be restored; while, in process of time, they may acquire a magnitude and solidity considerably surpassing what could have arisen from the primitive quantity of their nebulous matter. Certainly we are not at liberty to suppose, that this fortuitous junction of a comet with nebulæ takes place frequently; but, in estimating the consolidation of different Comets, in order to find whether the result corresponds with what the frequency and nearness of their approach to the sun would lead us to expect, we ought to recollect that the test is not infallible, from the possibility of an accession of nebulous matter having occurred in the manner we have now described.

Herschel’s theory, with respect to the agency of the solar heat, in promoting the consolidation of comets, necessarily implies, that the envelope and tail gradually become less extensive, and that the nucleus, upon whose surface the nebulous matter consolidates, gradually increases in magnitude. In these respects, therefore, some difference ought to be indicated by the physical appearance of those comets whose perihelion distances and periods of revolution are not the same; a condition confirmed by the examination of several, that have been the most attentively observed. The second Comet of 1811 had a nucleus, which, according to the continental astronomers, amounted to 570 miles‍‍134; while its tail was 500,000 miles in length. The Comet of 1807 possessed a nucleus of less size, but a tail of greater brilliancy; the diameter of the one being only 538 miles, the length of the other 9,000,000. The first Comet of 1811, which, from its splendid appearance, has been termed the great Comet of 1811, was observed to have a smaller nucleus, but, on the other hand, its envelope and tail were far more extensive; the diameter of its nucleus was 428 miles, and its tail stretched out no less than 132,000,000 of miles. The first of these three Comets, then, according to Herschel’s theory, must have been subjected in a much greater degree to the consolidating influence of the sun’s heat than either of the other two, seeing that it had the largest nucleus, and the least quantity of nebulous matter: and the like result ought to be indicated with respect to all the three Comets, on a comparison of their respective periods and perihelion distances. The periodical revolution of the great Comet of 1811 is found to be 3383 years, and it approaches 1.55 nearer the sun at its perihelion, than the other Comet of 1811: the product of these two numbers is 5243. The periodical revolution of the Comet of 1807 is 1713 years, and its perihelion distance is 2.46 times less than that of the second Comet of 1811: the product of these two numbers is 4213. The periodical revolution of the second Comet of 1811, whose perihelion distance we have taken equal to 1 as the standard of comparison, is 875 years. These numbers, then, 5243, 4213, 875, representing inversely the result of the sun’s long continued action upon the nebulous matter of the three comets, correspond very nearly with the relative magnitudes of their nuclei, as indicated by observation; and hence the confirmation of Herschel’s theory is complete‍‍135. If this comparative view of Comets be verified by more extended observations, it will serve to give some insight into the origin and arrangement of these bodies, and inform us of the true place which they occupy in the planetary system. Nor will it be the least important result of the establishment of this theory, that it will enable astronomers to arrange comets according to the various stages of maturity at which, in the progress of consolidation, they have arrived. Observation has, in fact, already furnished us with an extensive scale of comets, which are distinguishable by means of this important criterion. Several have been seen which had no nucleus at all, presenting only a gradual thickening towards the middle parts, which were nearly translucent; while, on the other hand, there are many whose condensation has proceeded so far, by having been more subjected to the action of the solar heat, as to have a nucleus 100, 1000, or even 2000 miles in diameter. Those of the latter description approach, in all the circumstances of their physical character, to the nature of planetary bodies; and particularly, like them, are less exposed to those sudden changes from the violent action of the sun’s heat near their perihelion, which Comets of a smaller size and a looser texture are observed to undergo.

III. From these observations, we shall be the better able to estimate the probability of a supposition, perhaps it may be said more speculative than useful, but nevertheless founded on philosophical principles, whether or not Comets be habitable bodies? It is very evident that such a supposition can never apply to the generality of comets; for, with regard to those whose consolidation is still only partial, the violent changes which take place in their constitution and structure, both at the perihelion and at the aphelion, are totally incompatible with all our ideas of either animal or vegetable existence. But with respect to those Comets, whose advanced state of maturity renders the sun’s influence incapable of materially affecting the surface of the nucleus, there seems to be no physical impossibility why many of them may not be the abode of living creatures, as well as the Earth and the other planets of the system.

Yet considering the extremes of distance from the sun, at which the Comets are placed in different parts of their eccentric orbits, it has been conceived, that the prodigious variations of heat and cold to which the inhabitants of a comet must be exposed, render the above supposition quite untenable. This, however, is an objection, which, though applicable to all comets, whatever be their state of consolidation, is truly more specious than substantial. Newton indeed calculated that the great Comet of 1680, which passed within 150,000 miles of the sun’s surface, must have been heated to a temperature 2000 times greater than red hot iron. But the simple fact, that the comet, even if its density had exceeded that of iron itself, was not instantly dissipated by the violence of such a combustion, indicates some error in the data on which this calculation is founded. Still though it should be allowed that the heat is not so great as Newton was inclined to estimate, it may be supposed that the variations of temperature to which a comet is subjected, are yet much too considerable for the existence and abode of beings, possessing constitutions at all analogous to those upon the Earth. But an application of the laws of chemical science to this subject, demonstrates that these extremes of heat and cold are by no means so excessive, as the mere alterations in the comet’s distance from the sun might perhaps lead us to imagine.

In the first place, it is well known, that, in the heating of bodies, when the compression to which they are subjected remains the same, there is a certain point, beyond which, whatever be the means employed, their temperature can never be elevated. Water, for instance, under the common atmospherical pressure, may be heated up as far as 212° of Fahrenheit; but all the heat which we employ in the endeavour to raise this temperature higher, is only dissipated in the ensuing evaporation. In like manner, the substance constituting a comet must have a certain point of its own, which, however near it may approach the sun, its mean temperature can never exceed. The tail of the comet may be expanded to a prodigious length, the nebulous envelope may become enlarged to an equal extent; even the materials on the surface of the nucleus, by volatilization, may pass into a gaseous or aërial form; but the planetary or solid body itself will experience no accession of heat beyond that point of maximum temperature, which its own nature and constitution determine.

In the second place, we may observe, that when, by any means, the density of bodies is made to change, by a process, whether of rarefaction, on the one hand, or of condensation, on the other, they are always found to undergo a corresponding diminution or increase of temperature‍‍136. When, therefore, in the approach of a comet to the sun, all the parts of its nebulous envelope and tail, which in the remoter regions of its course had been gathered close about the head, become expanded and attenuated, a very large proportion of the solar heat, which would otherwise have passed into the nucleus, and contributed to raise its temperature to a certain point, is carried off by the envelope and tail, in order to preserve an equilibrium among the several parts. Let us attempt to form some estimate of the actual loss of temperature thus sustained by the rarefaction. If we assume that the nebulous matter is elevated about 30 times its former height, the diminution of density, corresponding with the increase of volume, will amount to (30), or 27,000; and employing the formula given in the Supplement to the Encyclopædia Britannica, article ‘Climate,’ we have 45° × {27,000 - } or nearly 1,215,000 degrees of Fahrenheit, for the quantity of caloric abstracted. Now, Newton, judging from the proximity of the Comet of 1680 to the sun at its perihelion, shews that its temperature ought to be about 2000 times greater than the temperature of iron red hot, or about 9000 times greater than the heat of boiling water; the boiling point of water being 212° of Fahr., the sun communicated to this comet a supply of caloric amounting to 1,908,000°. But the loss, which, as we have just seen, must have been sustained by the rarefaction above supposed, amounted to two-thirds of this quantity; so that the actual influence of the sun, in raising the temperature of the comet, will undoubtedly be diminished in the same proportion. In a corresponding manner, when the comet retires towards its aphelion, where the heat of the sun becomes so much weakened on account of the distance, the condensation of the nebulous matter forming the tail and envelope serves not only to furnish the nucleus with continual supplies from the heat acquired at the perihelion, but even to render the warming influence of the solar rays much more efficacious than at a less remote part of the comet’s orbit.

It appears, then, that the variations of heat and cold, to which Comets are exposed in the opposite points of their course, are by no means so great as to be incompatible with the supposition of their being fit abodes for animated beings: and if we recollect the facility with which our own bodies can adapt themselves to great and sudden extremes of temperature, as exemplified by various experiments, we may even conjecture these beings to possess a constitution not very dissimilar to that of the human species. Individuals, we know, have often allowed themselves to be confined for a considerable time in apartments heated to 260° and 280° of Fahr., without feeling much inconvenience; and though we cannot as easily ascertain the extent to which cold may be endured by the human frame, we know that it is frequently exposed, without any injurious effects, to an intensity far surpassing what is necessary for the congelation of mercury‍‍137. In order, then, to be capable of sustaining those variations of temperature to which a Comet may be subjected, it is not necessary that the constitution of its supposed inhabitants should be very different from the constitution of the beings belonging to the Earth. And when we recollect that these variations proceed in a gradual manner, not by the rapid transitions which we often experience on our own globe, the progress from one degree of temperature to another, as the Comet journeys onward in its course, may be little perceived by its inhabitants.

It is true that the atmosphere respired by these beings, while it is at one place a highly attenuated gas, is at another converted into a medium extremely dense; and therefore it may be difficult to conceive how animation can be supported in these opposite situations. But when Halley was able to breathe freely in a diving-bell, in which the compressed air was twelve times more dense than that on the tops of mountains,—and when the lungs, with all the other bodily organs, can so readily accommodate themselves to the most variable and trying circumstances, we do perceive how it is possible for respiration to be carried on, notwithstanding these changes in a comet’s atmosphere, which, though undoubtedly extensive, yet take place in a slow, and therefore harmless, manner. Another objection has been started to the existence of living beings on comets, on account of the alternations of light and darkness to which, in the opposite portions of their orbit, they are thought to be exposed. But I find it remarked by Bailly, that the Comet of 1680, supposing it at the aphelion to be 138 times more distant from the sun than the Earth, ought for this reason to receive five times as much light from the sun as we do from the full moon; and when we add to this the superior density of the comet’s atmosphere at this distant part of its orbit, it is capable of obtaining a still greater quantity of light by refraction‍‍138.

These explanations, then, if they be deemed correct, make it appear that the several changes which are produced upon the constitution of a Comet, in consequence of its varying distances from the sun, are not incompatible with our ideas of animated existence, and go so far as to render it not improbable, that the beings which inhabit Comets may even possess bodily frames resembling those of terrestrial beings. But why, it may be asked, are we so solicitous to establish this resemblance between ourselves and the inhabitants of a Comet, as if that were a condition which alone could render their existence possible? When we survey the wide field of animal organization which lies within the scope of our own experience, from Man, the proud lord of creation, to those tribes of zoophytes which we place lowest in the scale, do we not behold a continual succession of beings, as infinite in variety as in extent? If, then, upon the surface of our own little planet, we behold so diversified a picture of animal life, why should we deem it as either unnatural or unlikely, that Comets may be the residence of beings widely different from those which fall within the narrow sphere of human observation. What though these beings, from the peculiarities of their situation, be endowed with neither lungs, nor eyes, nor the feelings which afford the sensations of heat and cold, like unto our bodily organs? Does this want imply either any improbability as to their existence, or even any inferiority, compared with ourselves, in the scale of creation? Most certainly not: for, if we estimate the intelligence of beings by the knowledge which their place in the universe is fitted to impart, we are compelled to regard the Cometary inhabitants as of an order even superior to the creatures of the Earth. When, for example, they find themselves passing through the midst of the satellites, those small bodies, which we can scarcely discern with telescopes,—or when they are brought so close to the planet Saturn, that they can examine the wonderful phenomenon of his rings even with the naked eye,—or when at the perihelion passage, they are able to observe every thing on the surface of the Sun, that great luminary, the mysterious source of life, and light, and energy, to the system;—what spectacles of delightful contemplation must they enjoy, and what means of attaining an acquaintance with the works of Nature, infinitely greater than any which we shall ever command! Traversing, as they do, the whole extent of that system of which the Earth forms so insignificant a member, and directing their course far beyond its known limits into those regions of space, whose dark and unfathomable nature it will for ever baffle human penetration to explore, the beings who have their abode on Comets must be familiar with many important truths, of which we can obtain only a few casual glimpses, and witness such glorious and sublime displays of the manifold wonders of creation, as must afford to them the noblest conceptions of that Almighty Being, by whose wisdom they were constructed, and by whose power they are still sustained.