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-
