286

THE MINOR PLANETS.

possibility of the action of any explosive force of so tremendousa character. Our previous arguments (see Lecture IX., p. 219)against the existence of a thin crust covering a seething massof central fire in the case of the Earth , may also militate tosome extent against the theory of the explosion of a hypo-thetical planet.

We cannot afford space to discuss at any length the pos-sibility of such a supposed planet having been broken up byviolent collision with some other heavenly body. If such abody were of suitable size, and the collision sufficiently violent,it is quite conceivable that the heat generated might havealmost entirely converted both it and the planet into gas, andhave only left a comparatively small number of fragments in asolid form. But, if we assume the existence of such a hypo-thetical planet, it is very difficult to conceive where the otherbody should have come from, unless perchance it were a cometwith an unprecedentedly large and solid nucleus. Moreover,the same conclusions as before would follow, as to the sub-sequent passages of all the fragments, from time to time,through the point where the collision occurred, and through anopposite direction in longitude; and a similar difficulty wouldbe involved in the fact that this is not the case.

We therefore consider it to be a much more reasonablesupposition (as we shall more fully explain a little furtheron) that, at some long past epoch, when the matter of theSolar System may have been gradually aggregating from anebulous, or partly vaporous, condition to form the variousplanets which now circle round the Sun, a vast quantity ofmatter was in a long course of ages collected from theneighbouring regions of space into the huge bulk of Jupiter ,while a comparatively small amount was left between the orbitsof Jupiter and Mars , which only sufficed to generate a number•of much smaller bodies.

If so, such bodies may have been at first even more minutethan they are at present, and, by collision and mutual attraction,have gradually become fewer and larger. And such collisions,and the consequent fusing together of two or more, may stilloccasionally occur, although they must be exceedingly rare