held by many astronomers, was controverted by others who maintained that the light, supposed to come from primitive fire-mist or nebulous matter, was in reality emitted by extensive sidereal groups, or vast universes too distant to show their individual stars. But, after some time, the round and the oval forms of many of these faint objects were looked upon as marks of a concentration around a centre, and the rare matter seemed to be emerging from its original chaotic state. It was thus that Kant, guided chiefly by the observations of Maupertuis, obtained a basis for his nebular hypothesis, which he published in 1755, and which, in essential features, differs little from that which has been held during the present century. Yet the subject excited little attention until many years afterward, when Sir William Herschel made his extensive and careful observations on planetary nebulæ, and pronounced them incipient solar systems, while he looked on irregular nebulosity as indicative of the presence of distant collections of stars.

As the doctrine founded on these observations was generalized by Laplace and supported by him with additional evidence, it obtained for a while much currency in astronomical circles; but it was seriously shaken in 1845, when many of the supposed embryonic systems of Herschel were resolved into stars by the powerful telescope of Lord Rosse. Yet, after a decline for a few years, the nebular hypothesis was revived on this side of the Atlantic by the announcement of Kirkwood's analogy; and some time afterward it obtained more decided support from the authority of Kirchhoff, as, on the discovery of spectrum analysis, it seemed to furnish a good explanation of solar phenomena. When Huggins obtained positive proof of the gaseous constitution of many of the irresolvable nebulæ, the tide of scientific opinion set more strongly in favor of the views of Herschel and Laplace; but it was soon checked when it was found that all the true nebulous objects had a uniformity of composition, and consisted entirely of hydrogen, nitrogen, and an unknown gas. It seems impossible that the vast diversity of material objects in future families of worlds could be afforded by the agency of three elements, one of which is noted for its reluctance to take part in chemical combinations. In addition to this difficulty, the periodical and the permanent changes, detected in certain nebulous objects by Hind and Holden, differ widely from a slow transition into a planetary system; and they are fatal to the idea that these cosmical clouds were in past ages impassive to physical influences and departed little from their primitive condition. But facts still more difficult of explanation, in regard to these celestial objects, have been made known by the recent observations on Nova Cygni ; and the apparent metamorphosis which was witnessed, of a temporary star into a nebula, was so little expected that theory seems much at fault; and it is evident that many of the views on this obscure department of astronomy must be either considerably modified or entirely abandoned.

According to the crude opinio's prevailing during the infancy of

modern science, matter and motion were all required for calling a world into existence; but it was soon found that, unless, in the beginning, the materials which formed the solar system moved with a certain order and regularity, they could never have risen from the chaotic to the cosmical condition. As all the planets move around the sun in the same direction, Laplace was led to believe that in remote times all must have been connected together; and such a primitive connection might be afforded if the sun and his attendants were originally a vast fire-mist, their matter being so much attenuated by heat that it extended far beyond its boundaries of the solar domain. He supposed that such an immense rarefied mass, on being set in motion by some cause which he does not specify, would ultimately be compelled by its own friction and by gravity to rotate with a uniform angular velocity in all its parts and around a common centre. In accordance with the principles of physical astronomy, he concluded that this rotation would become rapid as the immense solar nebula cooled and contracted, until at last the centrifugal force became great enough to overpower gravity and to throw off matter from the equator of the whirling mass. Laplace considered that, under the most probable circumstances, the nebulous matter thus thrown off, or abandoned by the shrinking spheroid, would all collect together to form a planet; but that, in some unusual cases, it would assume the expanded figure of a vast solar ring; and that, under certain conditions, it might break up into a number of asteroids. The singular group of bodies revolving between Mars and Jupiter is supposed to have come into existence in consequence of some rare accident, which made the great solar ring a prey to many centres of aggregation, instead of allowing it to coalesce around a single one. In all other cases, the cooling and contraction are said to have been successful in giving birth to a great planet, whenever the centrifugal force became sufficient to separate the equatorial portions of the rotating solar nebula. According to the views of Laplace, Neptune must be regarded as the first-born world of those already known; while Uranus is next in age, and the other planets were launched into being in a succession depending on their distances from the sun; so that Mercury is the youngest member of the solar family. It has been also concluded that from the condition of its birth each planet must have commenced its career as a rotating nebula; and that many of the larger ones, by subsequent cooling and contraction, were at certain periods enabled to throw off their equatorial matter, which in all but two instances was converted into a satellite. Of these minor worlds or moons, Saturn has succeeded in obtaining eight, in addition to the double ring, which in the eyes of Laplace appeared as two embryonic satellites, and which has been so often appealed to for proof of the world-making doctrine under consideration.

Yet, when examined with care and impartiality, the evidence derived from the condition of the Saturnian girdle will be found unfavorable, if

not fatal, to the views which it has been so frequently adduced to sustain. The superficial character of the examination which Laplace has given to this subject is betrayed by two statements which he makes in regard to it in two different parts of his writings. In setting forth the nebular hypothesis in his "Système du Monde," he asserts that the matter separated from a contracting nebula would take and maintain an annular figure, if there were a complete uniformity in its entire circuit and in its rate of cooling; but in the "Mécanique Céleste," in treating on Saturn's rings, he concludes that their preservation would be impossible without some decided irregularities in their structure. It is scarcely necessary to say that the annular appendage could not be of long duration if the conditions necessary for its existence or security in one age were fatal to it at another. On examining the alleged history of its birth, also, we feel at a loss for some cause of intermission in the work of detaching matter from the cooling nebula. It is difficult to imagine why, after the outer ring was completed, the separation of matter from Saturn, after a long continuance, should have ceased for a while; and why, after the completion of the inner ring, the centrifugal force again became weak, and that it has declined steadily until the present time, when, at the planet's equator, it is scarcely one-sixth of the force of gravity. Since the period when Saturn is supposed to have launched forth the zone of matter circulating nearest to him, his movements could be but little retarded by tidal action; and there seems to be no cause which could reduce his angular velocity of rotation during a contraction from the loss of primitive heat.

Kant, who regarded the rings as composed of aëriform matter separated from Saturn, was led to the natural inference that the time in which the planet turns once on his axis must be equal to that which the nearest annular zone requires to make a circuit around him. From such considerations, the eminent savant was induced to assign for the rotation of the planet the period of six hours, twenty-three minutes, and fiftythree seconds. But this theoretical or predicted length of Saturn's day is only about three-fifths of the actual value which was first revealed by the observations of Sir William Herschel, and lately determined with more precision by Prof. A. Hall. From the difficulties which the facts present in this case, Laplace endeavors to extricate his doctrine of planetary evolution by maintaining that it requires only that Saturn's day should be shorter than the period of revolution due to the inner ring, supposed to be one unbroken solid mass. But the basis on which this conclusion is founded has been exploded by modern researches, which show the impossibility of the existence of such vast solid annular structures; and Prof. Kirkwood, though long a supporter of the views of the great French astronomer, has lately pronounced the evidence obtained from the Saturnian system and from the inner moon of Mars as adverse to the nebular hypothesis.

While all scientific researches are exposed to uncertainty in propor

tion as they aim to penetrate very far into space and time, the ordinary means for avoiding error are wanting, and mathematical investigation is unavailable, in dealing with the supposed primitive fire-mist to which the birth of worlds has been ascribed. It would be hazardous to attempt to calculate or to trace the precise effects of gravity, motion, and friction, on matter more than 100,000,000 times more rarefied than the air we breathe, and diffused over a spheroidal space more than 6,000,000,000 miles in extent. It may seem easy to suppose all its parts rotating with regularity in the same direction around a common axis; but it would be very difficult to determine how many millions of years or centuries must elapse before such a regular rotation of the entire mass would be produced by an impulse at any locality. Inquiries respecting the arrangement of matter in the primitive solar nebula may seem to come within the scope of physical science; yet they have been hitherto unproductive of the evidence expected from them. Reasoning from the principles of hydrostatics, Kant regarded the great density of the planets near the sun and the rarity of Saturn as a proof of their nebulous origin; and he ventured to predict that, on future discoveries, the most remote members of the solar system would be found to resemble comets, in being composed of very light matter and deviating widely from circular paths in their revolutions. Yet time has shown the fallacy of his predictions, and of the proof on which he placed so much reliance. The evidence which late writers have endeavored to deduce from the large size of Jupiter and Saturn is equally weak and unsatisfactory; for the most distant planets are not the largest, and there is no definite law calling for an increased size of worlds in proportion as they are distant from the solar orb. According to the most generally received theory of its variability, the star Algol presents the case of a remote sun with a planet nearly as large as himself, yet confined to so small an orbit that the period of revolution is less than three days. But the defects and the utter inadequacy of the hypothesis are rendered most apparent when it is called on to furnish an account of the origin of binary systems, and to show the cause of the great eccentricities of the ellipses which pairs of distant suns describe around a common centre of gravity.

In modern times the doctrine of the nebular origin of worlds has been much modified by new speculations and inquiries; and it has been extended far beyond the state in which it was left by Herschel and Laplace. More than twenty years ago Helmholtz advanced the hypothesis that the sun's heat and light are produced by the contraction of his mass; and that, in concentrating from primitive nebulous diffusion and shrinking to its present dimensions, the solar orb has derived from the same cause the calorific energy which enlivened the ancient world. When the views of Mayer, who regarded falling meteors as the solar fuel, were exploded, chiefly through the discoveries by the spectroscope, the contraction theory of Helmholtz gained many votaries; and it be

came more attractive as it held out the hopes of giving a means of definitely measuring vast periods of time. It was calculated that the concentration from a widely-diffused nebula to its present size would produce as much heat as the sun would lose in 20,000,000 years, according to the present rate of radiation. This period was accordingly fixed as the age of the sun and the duration of solar light. Another step was soon taken in this direction by fixing a limit to the age of worlds. It was concluded, with much confidence, that less than 20,000,000 years have elapsed since the earth became a planet, and that previously it must have formed a part of the solar atmosphere. In Prof. Tait's "Recent Advances in Science," the estimate obtained in this way for the age of our globe is placed between 15,000,000 and 20,000,000 years; and geologists are given to understand that they must recognize the infallibility of mathematical authority and abstain from their usual extravagance in making exceedingly large drafts on the limited fund of time.

But the conditions on which this surrender of geological belief has been demanded are far more liberal than any which the eminent mathematician is legitimately authorized to offer. The estimate on which he relies has been made for an homogeneous nebula supposed to be equally dense at its borders and in its central regions, whereas there must be a preponderating density near the centre, according to the necessary inferences from the doctrines of Laplace. This early central condensation must be adopted to account for the great mass of the sun compared with that of the planets. In an able investigation on the subject, in this point of view, published in Silliman's Journal in 1864, Prof. Trowbridge concludes that, even in the earliest stage of planetary development, there must have been a very great concentration of matter around the central nucleus of our solar nebula. If we adopt the law which he deduces for the rapid increase of density toward the centre, it may be found that the amount of heat due to contraction since the supposed birth of our world would not be enough to compensate for the calorific waste which the sun sustains in 1,000,000 years.

To obtain information of the age of the earth's crust from the increase of subterranean temperature with the depth, according to the method devised by Fourier, is an object to which much labor has been devoted, and from which valuable fruits may be expected. On this principle, the time since the permanent solidification of the surface of our globe has been estimated by Sir William Thomson at about 100,000,000 years. But this estimate, which is obtained by taking 7,000° Fahr. as the highest limit of internal temperature, will appear too low when we consider the vast amount of heat arising from the primitive concentration of terrestrial materials, and the obstacles which central density or igneous fusion may present to its escape. Instead, however, of controverting the peculiar views of the eminent scientist on physical geology, I will only trace the consequences to which they lead. He maintains