December 21, 1917] 



SCIENCE 



605 



known, it can be proved that this motion 

 would be so great that the force of gravity, 

 even of so great a mass, could not prevent 

 them from flying off into space and so be- 

 ing lost. 



Again, it can be calculated from the 

 facts at our disposal, where the rings 

 would be left by such a cooling nebula. It 

 is then found that the first ring, instead of 

 being in the position of the present orbit 

 of Neptune, would be inside the orbit of the 

 inmost planet, Mercury. Where fact and 

 theory do not agree, so miich the worse for 

 theory. 



These are typical of the niunerous and 

 insuperable objections to the acceptance of 

 the hypothesis. Within the last few years, 

 the belief has been gaining ground among 

 astronomers and geologists that this theory, 

 so long the accepted one, must in its turn 

 be discarded. 



The cogency of the difficulties which 

 have presented themselves whenever the 

 theorj' of Laplace has been critically stud- 

 ied cleared the way for the meteoritic 

 theory as presented by Lockyer and modi- 

 fied by Darwin. But here again the ob- 

 jections raised are so many and so reason- 

 able that it stands on no surer a founda- 

 tion than its predecessor. 



Of recent years, the Planetesimal 

 Theory of Chamberlin has been gaining 

 ever-increasing support. Like the authors 

 of preceding theories, he scanned the heav- 

 ens for facts which might have a bearing 

 upon the problem in hand. He saw, like 

 them, the brilliant masses of "star dust" 

 which we call nebula, but he saw also the 

 importance of the fact that there are two 

 distinct kinds of nebulae. One kind, 

 sometimes spherical, sometimes irregular 

 in shape, is composed of incandescent gas ; 

 the other, consisting of two tightly coiled 

 spiral arms, is evidently made up of solid 

 particles. In the latter only do we find in- 



dications of the important metallic elements 

 which occur in the earth. 



This suggested to him that the parent 

 nebula of our solar system was probably 

 one of the spiral type, and his first prob- 

 lem was to account for the origin of such a 

 nebula. An occurrence, famous in the 

 history of astronomy, has an important 

 bearing upon this. Nearly 350 years ago 

 (November, 1572), Tyeho Brahe, the fa- 

 mous Danish astronomer, was very much 

 astonished to observe a new star in the con- 

 stellation Cassiopeia. An hour before he 

 had scanned that part of the heavens and 

 saw nothing, and when he looked again 

 there it stood, a star of the first magnitude. 

 From night to night it gTcw in magnifi- 

 cence, surpassing in turn the fixed stars, 

 the planets, even Venus at her brightest, 

 until it could be seen at noonday. It had 

 now become the most glorious and brilliant 

 orb in the heavens, giving, it has been cal- 

 culated, 100,000 times as much light as our 

 sun. Then this strange luminary slowly 

 faded away, nightly becoming less bril- 

 liant, until, after the lapse of 17 months, 

 it sank into final darkness. 



How is this astonishing phenomenon to 

 be explained ? The general belief is that it 

 was probably due to the collision of two 

 great celestial bodies. Their energy of 

 motion was changed into molecular energy, 

 and the elements melted with fervent heat. 

 So hot indeed did they become, that a 

 great cloud of incandescent gas was the re- 

 sult, whose molecules were moving at such 

 rapid rates that they were whirled away 

 into space and so disappeared. Other stars 

 of this kind have frequently been observed 

 since then, but never has one so brilliant 

 been recorded. 



Now it may be that we have here a 

 typical example of the formation of a 

 gaseous nebula, though but a temporary 

 one. Had the impact been less violent it 

 might have been permanent. But what 



