in their passage through it. There is good reason to suppose that the product of chemical union assumes generally the liquid rather than the gaseous form, since it has frequently been noted that a faint continuous spectrum may be discerned along with that of the bright gaseous lines in many nebulæ, and which is not confined to any particular portion of them. We may now pretty clearly depict the condition of a nebula according to this hypothesis, when after the lapse of many ages a large number of comets and accompanying meteor-streams have been absorbed into its substance. It will consist of a greater or less residue of the original gaseous constituents, which, for reasons well known to spectroscopists, will still continue to yield most conspicuously the characteristic bright line spectrum, and dispersed throughout the mass an immense number of liquid nuclei, all tending towards the common centre of gravity in spiral orbits, the centre being occupied by a brilliant white-hot liquid mass. The temperature of such a nebula will be much higher than of one less developed-a fact which will tend to keep the density of the gaseous constituents at nearly the same level, in spite of the increased gravitational tendency to condensation near the centre. We may even conceive that this intense heat might be sufficient to render gaseous the liquid nuclei when a numerously bright-lined spectrum should make its appearance, but it would then be necessary to assume that the faintness of the spectrum would make it difficult to distinguish the difference between this and one truly continuous. We may, on the contrary, imagine the welldeveloped nebula to become a compact liquid mass, in this case also yielding a continuous spectrum. Nor does it seem unlikely that a considerable number of bodies in this latter state of existence may actually be discovered in the heavens. Many nebulæ, after having resisted the utmost efforts of astronomers, armed with the largest telescopes, to class them among the clusters of stars, have equally refused to be entered in the list of true nebulæ by yielding the discontinuous spectrum, and whose real condition remains, therefore, for the present an enigma. Of the two suppositions the second appears to me to be the most probable; yet if it be accepted, it will be necessary to reconstruct the nebular hypothesis, if it can still be maintained under the altered circumstances. There are, however, some facts which tend strongly to show that the first suggestion is the more correct one, or, in some cases at least, more closely approximates to the true condition of things. It is well known that the great nebula in Andromeda-one of those which defy alike the powers of the telescope and spectroscope-presents a very anomalous spectrum. Though apparently continuous, some portions of the red and orange are altogether wanting, and the more refrangible portion consists of a mottled band with uneven gradations of intensity. This is very much what would be expected if the spectrum were really one of numerous bright lines, nor is it a singular instance. Several other nebulæ have given rise to strong suspicions of a similar constitution; and no theory, so far as I am aware, has hitherto been advanced in explanation of these appearances. I need not point out that, if this supposition is correct, a state of things arises in which the nebular hypothesis in its later form will again apply, with the addition only that the gases of the nebulæ are in a much more complicated condition than was formerly supposed to be the case. It is to be remarked, that the number of comets which would be attracted to a nebula would increase in proportion as its mass increased with each succeeding capture, so that its development would proceed at an accelerated rate until a certain point was reached, when comparatively little of the gaseous elements remained. The comets would then describe their orbits around the newly-formed sun, and would leave only their smaller or outlying meteorites to swell its mass. But even these would greatly tend to compensate for the dissipation of heat by radiation, and would much retard the cooling process-a state of things that will continue to exist even in a perfectly formed system such as our own. I am thus led to regard the meteoric theory of the supply of solar heat as a part of the modified nebular hypothesis which I have suggested. If our sun is a nebulous star, surrounded by a far-extending atmosphere, in which are revolving a large number of meteoric bodies, visible possibly to us as the Zodiacal Light, the materials for keeping up a constant or nearly constant temperature for a considerable length of time are at hand, and every succeeding comet will add some to the number of those meteors which, unlike it, are unable to make their final escape, and pursue their orbits with scarcely diminished velocity. The smaller meteors in a stream, as presenting relatively a larger surface, will be more retarded than the larger, and will more rapidly fall upon the solar surface-so rapidly, indeed, that it does not seem unlikely that the universally held belief in the increased solar heat during years noted for large comets may have a sound foundation. Such a possibility is sufficient to invest the movements of comets with great practical interest, and the various circumstances producing the increased heat of such seasons must give rise to important investigations. Thus we should come to look upon the nearness of approach of a comet in perihelion as an essential element in such a discussion, and perhaps also the material composition of the comet itself. It has been suggested, with much show of reason, that the larger and nearer planets when in perihelion simultaneously have a considerable effect upon the solar surface by producing spots, &c., and these again, in determining the amount of heat radiated from the sun; may we not therefore expect that the arrival in perihelion of a comet from most remote regions, which, if of smaller mass, often approaches the sun much more nearly than they, may also produce a marked effect on the state of the solar atmosphere? Although the 11-year period of solar spot frequency is too well marked to admit of any question, there is already much evidence to prove that there exists many minor disturbances, secondary maxima and minima of solar activity, which remain to be explained, and which may possibly be due to the occasional and irregular approaches of comets. It is certain that the solution of no question can be of more service to the advancement of science than that of the real practical utility of comets in the economy of the universe. Speculation on this point has already been too long neglected, and unless it is carried on now as far and as correctly as the state of modern science allows, it is certain that it will mask some other results and hinder progress. Surely none can believe that these bodies are mere ignes-fatui, coming and going, without being of any service to us or to other systems; and although much mystery has always hung about them and still baffles our researches, perhaps the best way of attaining to the solution of it is by searching for some purpose that they may subserve. Without attempting altogether to set up the nebular hypothesis in as favourable a light as before, the above remarks and speculations may serve to indicate the position in which it stands at present, and the broader basis on which in future the question must be argued. A certain degree of unity of design seems to result from these theories, which is consonant with the order of nature. Every known body in the universe appears to have an important and appropriate function to perform in the development or maintenance of systems like our own-a function that is constant in all conceivable states of existence of those systems; and while my speculations do not contradict the various theories of others, they show a tendency to unite them into one consistent whole. Perhaps the most distinctive feature of these remarks is, that the effects I speak of must actually take place, whether or not they have the importance here attached to them. No one will deny that the nebula and the comet will constantly come into contact; and were we acquainted with all the materials so meeting, it would be a comparatively simple problem for the chemist to determine what compounds would result, and for the physicist to show in what state, gaseous or otherwise, they would afterwards remain. Both these questions require to be answered satisfactorily before it is possible to declare whether the celebrated hypothesis of Laplace is or is not the true key to the solution of the formation and history of the Solar System; and for these answers, at the present, it waits. |