- 
8 COSMICAL ASPECTS OF GEOLOGY. [Boox I. — 
are usually estimated as in the following table, that of the earth 4 
being taken as the unit :-— 
Density of the Sun . - : : . . O25 
As Mercury : . ; «, ede 
4 Venus . . . . . . ae te 
* Earth . : 5 , : . » J00 
* Mars . < ‘ 3 : - « (ORG. 
es Jupiter . . . . ° ; : Oe Ze 
et Saturn . . . . : 4 « -ORTS 
is Uranus. - “ “ - . o OAR 
; Neptune ; ; . - - » OG 
It is to be observed, however, that “the densities here given are. 
mean densities, assuming that the apparent size of the planet or sun 
is the true size, 7.e., making no allowance for thousands of miles deep 
of cloudy atmosphere. Hence the numbers for Jupiter, Saturn, — 
and Uranus are certainly too small, that for the sun, much too 
small.” ‘Taking the figures as they stand, while they do not indi- 
cate a strict progression in the diminution of density, they state that 
the planets near the sun possess a density about twice as great as 
that of granite, but that those lying towards the outer limits of 
the system are composed of matter as light as cork. Again, in some 
cases, a similar relation has been observed between the densities of 
the satellites and their primaries. ‘The moon, for example, has a 
density little more than half that of the earth. The first satellite of 
Jupiter is less dense, though the other three are found to be more dense 
than the planet. Further, in the condition of the earth itself, a very 
light gaseous atmosphere forms the outer portion, beneath which lies 
a heavier layer of water, while within these two envelopes the materials 
forming the solid substance of the planet are so arranged that the 
outer layer or crust has only about half the density of the whole 
globe. Mr. Lockyer finds in the sun also evidence of the same 
tendency towards a stratified arrangement in accordance with relative 
densities, as will be immediately further alluded to. 
There seems therefore to be much probability in the hypothesis 
that, in the gradual condensation of the original nebula, each 
successive mass left behind represented the density of its parent 
shell, and consisted of progressively heavier matter. The remoter 
ape with their low density and vast absorbing atmospheres, may 
e supposed to consist of metalloids like the outer parts of the sun’s 
atmosphere, while the interior planets are no doubt mainly metallic. 
The rupture of each planetary ring would, it is conceived, raise the 
temperature of the resultant nebulous planet to such a height as to 
allow the vapours to rearrange themselves by degrees in successive 
layers, or rather shells, according to density. And when the planet 
gave off a satellite, that body might be expected to possess the 
composition and density of the outer layers of its primary.” 
1 Professor Tait, MS. note. 
2 Lockyer in Prestwich’s Inaugural Lecture, Oxford, 1875, and in Manchester Lectures, 
Why the Karil’s Chemistry 7s as it is. Readers interested in the historical development 
of geological opinion will find much suggestive matter bearing on the questions discussed 
above, in De la Beche’s “ Researches in Theoretical Geology,” 1834,—a work notably in 
advance of its time, 
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