HARDWICKE'S SCIENCE- G OS SIP. 



27 



outward and inward was a theoretical result of the 

 inevitable impacts between the parts of the rings, and 

 it is certain that considerable visible changes have 

 taken place in the rings since they were first observed 

 in 1659. As to the future of the rings, Professor 

 G. H. Darwin believes that the outward spreading 

 will, in time, carry many of the brickbats beyond 

 Roche's limit, where there will be no longer any 

 obstacle to aggregation into a celestial body, and a 

 ninth satellite will probably be formed. The 

 inward spreading will, in time, carry the meteorites 

 to the limits of Saturn's atmosphere, where, heated 

 by friction as they rush through the air, they will 

 be disintegrated and fall on to the planet as dust. After 

 a time, the length of which no estimate can be 

 formed, the rings will have vanished, leaving the 

 ninth satellite as their descendant. 



Saturn, which is only half as dense as Jupiter, is 

 by far the least dense planet, and this may account 

 for a ring having been formed so near to the planet 

 and so late in the planet's separate existence ; but 

 look at the subject how we may, Saturn's rings are 

 visible facts strongly favourable to the nebular 

 theory. 



Looking back for a moment, we have estab- 

 lished the prior existence of the solar system as a 

 nebulous mass. M. Plateau's experiment shows that 

 rings and planets may be formed from such a mass, 

 and in Saturn's system we see that a ring certainly 

 has been formed. 



The present physical condition of the various 

 planets affords further corroborative testimony in 

 favour of this theory. If all the planets have success- 

 ively originated from the same vaporous mass, they 

 must be composed in the main of similar chemical 

 elements, and this inference has thus far been 

 uniformly corroborated by spectroscopic observation 

 wherever there has been an opportunity of employing 

 it. Spectroscopic observation has, indeed, shown 

 much more than this, namely, that the'whole of the 

 matter of the universe is composed mainly of the 

 same physical matter as the solar system, so that 

 any deductions which can be made from the stellar 

 system may be safely used in discussing the solar 

 system. 



It is found that the density of the interior planets 

 of our solar system, compared with that of the more 

 remote, is about as five to one. The obvious con- 

 clusion is, that there is a preponderance of the 

 metallic elements in the interior planets and of 

 metalloids in the exterior. It is evident, as Mr. 

 Lockyer has shown, that when our solar system 

 existed in a nebulous condition, the metallic or 

 denser elements would occupy the interior portion 

 of the nebula, and the metalloids the exterior. 

 Taking a section of this nebula from its centre to its 

 circumference, the elements would, in the main, be 

 found arranged according to their densities — the 

 densest at the centre, and the least dense at the 



circumference. If we compare the planets with their 

 satellites, we find the same law holding true. The 

 satellites of Jupiter, for example, have a density of 

 only about one-fifth of that of the planet, or about 

 one twenty-fifth of that of our earth ; showing that, 

 when the planet was rotating as a nebulous mass, the 

 more dense elements were in the central parts, and 

 the less dense at the outer rim, where the satellites 

 were being formed. 



Again, if we take the case of our globe, we find, 

 as Mr. Lockyer remarks, the same distribution of 

 materials, proving that when the earth was in the 

 nebulous state, the metallic elements chiefly occupied 

 the central regions, and the metalloids those outer 

 parts which now constitute the earth's crust. 



All these facts show that the sifting and sorting of the 

 chemical elements, according to their densities, must 

 have taken place when our solar system was in the 

 condition of a nebula. But, further, it seems impos- 

 sible that this could have taken place had the 

 materials composing the nebula been in the solid 

 form, even supposing that they had taken the form 

 of clouds of stones. This is dead against the 

 meteoric theory of Mr. Proctor, which I shall refer 

 to later. 



In some of its stages, the nebula had a very much 

 higher temperature than that now possessed by the sun. 

 There must, during the sifting period, have been com- 

 plete chemical dissociation, so as to keep the metals 

 and the metalloids uncombined, and thus allow the 

 elements to arrange themselves according to their 

 densities. "The nebular hypothesis," remarks Mr. 

 Lockyer, " is almost worthless unless we assume very 

 high temperatures, because, unless you have heat 

 enough to get perfect dissociation, you will not have 

 that sorting out that always seems to follow the same 

 law." 



Before concluding, it will be shown why it is 

 believed^ very high temperature actually did exist 

 in the nebulous mass. In consequence of the 

 sorting and loss of heat referred to, the earth's ring 

 in starting was much denser and cooler than Saturn 

 or Jupiter : being much smaller it would cool more 

 quickly, so that while the earth has cooled into a 

 planet with a solid crust, Saturn and Jupiter are still, 

 to some extent, self-luminous. Herschel, Airy, and 

 others have observed what appear to be vast up- 

 heavals of portions of the surface of Saturn — the 

 so-called "square-shouldered " appearance of Saturn 

 — showing that there is not yet a solid crust to that 

 planet. Our moon starting from the earth with a 

 density which must have been nearly equal to the 

 density of the earth, but with a much smaller mass, 

 has cooled still more rapidly. The volcanic remains 

 on its surface evidence former heat. Professor 

 Frankland has shown that, assuming the solid 

 mass of the moon to contract in cooling at the 

 same rate as granite, its refrigeration, though only 

 180 F., would create cellular space equal to nearly 



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