THE ASTRONOMICAL HISTORY OF WORLDS. 575 



tion as they aim to penetrate very far into space and time, the ordi- 

 nary means for avoiding error are wanting, and mathematical investiga- 

 tion 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 re- 

 specting 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 re- 

 semble 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 de- 

 duce from the large size of Jupiter and Saturn is equally weak and un- 

 satisfactory ; 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 gen- 

 erally 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 cen- 

 tre 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 hy- 

 pothesis 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- 



