PHYSICAL SCIENCE. 25 



would seem the sole probable avenue to further knowledge ; 

 since any changes in the figure, condensation, luminousness, 

 or other aspects of these nebular systems must, upon every 

 analogy of the more proximate parts of the heavens, occupy 

 such immense periods of time as to place them beyond all 

 present reach. And we know too little of the duration of our 

 own species on the earth to venture on any assumption thus 

 remote in its fulfilment. 



These questions as to nebulous matter in space are deeply 

 interesting, retrospectively, as well as prospectively, in time. 

 Few subjects have so keenly exercised speculation of late as 

 the hypothesis, first sanctioned by Laplace, that our own solar 

 system, with its central sun, planets, moons, and comets, has 

 its origin in the concentration of the matter of a nebulous 

 sphere in successive zones ; each several planet being formed 

 by the condensation of vapour at these successive limits in the 

 plane of a common equator ; and the satellites being similarly 

 formed from the atmospheres of the planets. It does not 

 annul this theory to admit that there are great difficulties in 

 conceiving the cause of such aggregation of matter at certain 

 points, and of the permanent movements impressed on the 

 bodies thus formed. These difficulties, whatever they be, 

 have not prevented its eager appropriation by philosophers 

 who hold the doctrine of progressive developement according 

 to certain determinate laws, in the creation both of the 

 inorganic and organic world. They find a basis for the evo- 

 lution or transmutations they suppose, in this hypothesis of 

 the nebular origin of suns and planets ; and their argument 

 would be plausible were the hypothesis itself capable of being 

 verified. How far presumptive evidence may reach in 

 future towards such verification, we do not venture to say ; 

 but the sources of fresh knowledge are ever opening in this 

 as in other directions of research. The more careful study of 

 cometary phenomena ; of the numerous planetoids revolving 



