SEDEKEAL SYSTEMS. 77 



and shortening the time of revolution of Encke's and, 

 perhaps, also of Biela's comet. "We may conceive of this 

 impeding ethereal and cosmical matter that it is subject to 

 motion; that it gravitates, notwithstanding its extreme 

 tenuity, and is consequently condensed in the vicinity of the 

 great mass of the sun ; and even that it may be augmented in r 

 the course of myriads of years by emanations from the tails { 

 of comets. 



If we now leave the consideration of the attenuated va- 

 porous matter of the immeasurable regions of space (ovpavov 

 Xoproc) 33 , whether existing in a dispersed state, as a cosmical 

 ether without form or limits, or in the shape of nebulae, and 

 pass to those portions of the universe which are condensed 

 into solid spheres or spheroids, we approach a class of 

 phenomena exclusively designated as stars, or as the 

 sidereal universe. Here, too, we find different degrees 

 of solidity, or density, in the agglomerated matter. Our 

 own solar system presents all gradations of mean density 

 (or relation of volume to mass) . If we compare the planets, 

 from Mercury to Mars inclusive, with the Sun and with 

 Jupiter, and the two latter bodies with the still inferior 

 density of Saturn, we pass through a descending scale, in 

 which (taking terrestrial substances for illustration) the grada- 

 tions correspond respectively to the densities of antimony, of 

 honey, of water, and of deal wood. In comets (which, in point 

 of number of individual forms, constitute the largest portion 

 of our solar system), that which we call the head, or nucleus, 

 allows the light of stars to shine unimpaired through its 

 substance: perhaps in no case does the mass of a comet \ 

 equal the five-thousandth part of that of the earth, so 

 various do the processes of formation appear in the original, 



