72 



KNOWLEDGE. 



February, 1913. 



the temperature of our luminary, or else have been 

 lost to the system for ever. 



We imagine that the Sun was rotating before the 

 initial catastrophe occurred, since this motion would 

 give a preponderance of direct revolution amongst 

 the ejected particles, although it is likely that that 

 of a large percentage may have been retrograde. This 

 diversity of motion would naturally cause frequent 

 collisions, and in this way other small centres of 

 gravity would gradually be formed. As these centres 

 increased in size, their attraction would deflect 

 neighbouring particles from their original course and 

 cause them to revolve round them. Further 

 collisions would ensue amongst the captured frag- 

 ments, involving a consequent loss of energy, 

 and they would thus be slowly drawn nearer 

 to, and finally upon, their various centres of attrac- 

 tion, while the dismembered remains of the original 

 sun, being in the centre of the nebula, would succeed, 

 owing to their superior mass, in gathering up the 

 greatest portion of the scattered contents. 



We may thus endeavour to picture to ourselves 

 our system slowly evolving, a vast aggregation of 

 particles of various sizes, extending far beyond the 

 present orbit of Neptune and becoming more and 

 more scattered as the distance from the common 

 centre increased. Occasionally we should witness 

 the collision of two fragments, and the consequent 

 generation of light and heat, and we should also 

 observe the increased frequency and violence of 

 these collisions in the vicinity of the Sun. Although 

 individually presenting every diversity of movement, 

 these particles would in the aggregate present a 

 certain orderly sequence, and a series of vortices 

 would be noticeable revolving round the Sun in the 

 same direction as its own rotation, the larger ones 

 lying in substantially the same plane : that of the 

 disturbing star's approach. The general appearance 

 of the system would closely resemble the spiral type 

 of nebulae which have been revealed to us in such 

 numbers by the telescope and the photographic 

 plate in recent years. It is a significant fact that 

 all these nebulae consist of two arms emanating 

 from a central nucleus at diametrically opposite 

 points. So far as we are aware the only force 

 capable of producing this symmetrical appearance 

 is that of disruptive tidal action, since, were the 

 coils thrown off owing to rapidity of rotation, jets 

 of matter would be ejected from every part of the 

 circumference, and if by explosive force from 

 indiscriminate points on the surface. 



This, then, is the modern idea of the primordial 

 nebula, a somewhat different one from the conception 

 of Laplace, insomuch as it consists of independent 

 particles revolving round a common centre of gravity 

 in elliptical orbits instead of a slowly rotating sphere 

 of glowing gas. It also has this advantage, in that 

 it presents a complete cycle of events, from star to 

 nebula and from nebula to star. This is analogous 

 in our conception of the infinite to the difference 

 between a circular and a straight line. There is less 

 difficulty in imagining the former as endless than 



the latter, although the origin of either may be 

 equally incomprehensible to our finite minds. 



It must not be imagined that the hypothesis here 

 outlined is entirely satisfactory, but as there appear 

 to be fewer drawbacks attached to its acceptance 

 than to that of most of the theories that have been 

 advanced from time to time, it is generally considered 

 probable that our evolution proceeded somewhat on 

 these lines. Accordingly, it would appear natural 

 that the planets and their satellites were both formed 

 by the capture and aggregation of smaller particles 

 in the same way. The lesser whorls would control 

 the particles in their immediate vicinity, but would 

 themselves be under the attraction of the larger 

 vortices, just as these would in their turn be con- 

 trolled by the Sun. 



It is, of course, mathematically impossible for one 

 body to capture another without the intervention of 

 a third force. This may be illustrated by the case 

 of a comet approaching our solar system. If the 

 former is a wanderer from interstellar space it is 

 obvious that although the solar attraction will 

 enormously increase its velocity as it draws nearer, 

 after the comet has passed perihelion, the Sun will 

 only be capable of controlling the same speed as it 

 was able to impart, and the visitor will leave the 

 confines of the solar system with its original 

 momentum unimpaired. If, however, the comet 

 passes near one of the planets, the attraction of the 

 latter may more than counterbalance the initial 

 velocity of the comet, and thus change its hyperbolic 

 orbit into an elliptical one. Another means whereby 

 a portion of the wanderer's energy could be dissipated 

 is by moving through a resisting medium. This is 

 a more satisfactory method of accounting for the 

 capture of a satellite than by means of a third 

 attractive force, since the latter would as often free 

 the satellite from the control of its primary, as assist 

 in making it captive. 



The capture theory, therefore, demands as a 

 necessary condition the existence, at all events in 

 the past, of a resisting medium. Professor See, to 

 whom the development of this hypothesis is due, 

 considers, and not without reason, that the presence 

 of this medium was very probable. For it is almost 

 certain that the vast quantity of gases which would 

 have been liberated with the more solid matter at the 

 birth of our solar system, would have existed in the 

 free state as a nebulous cloud for untold aeons 

 before combining with these solid elements, or 

 forming the atmospheres of the various worlds in 

 the course of evolution. This would render it 

 possible for a satellite entering the sphere of a 

 planet's influence to be eventually retained by the 

 latter, and would also have the effect of decreasing 

 the satellite's orbit. It would at the same time 

 cause an eccentric path to become more circular, 

 since a circle encloses a larger space than any 

 other curved line of equal length, and a body can 

 consequently revolve round a certain area with 

 less expenditure of energy if moving in a circular 

 orbit than if travelling in one of any other form. 



