394 NATURE AND MAN. 



act, — a gradual development, not the sudden springing of a com- 

 plete universe out of nothingness. And this is equally the case 

 whether the " six days," each with its evening and its morning, are 

 received in their literal sense, or are lengthened into indefinite 

 periods of time. 



Lucretius and other " atomic " philosophers attempted to give 

 a definite shape to this conception ; but it first found really 

 scientific expression in the "Nebular Hypothesis" of modern 

 astronomy, the combined doctrine of Laplace and the elder 

 Herschel. According to this, the original condition of the 

 universe was a difi"used " fire-mist " of unequal tenuity ; the 

 mutual attractions of whose particles would cause its denser 

 portions to gather round them the rarer matters of the intervening 

 spaces, would draw together the smaller collections thus formed 

 into larger clusters, and would thus "evolve" out of the uni- 

 versally but unequally diffused nebular matter a limited number 

 of separate substantial masses. At the same time, the inequality 

 in the movements of the different parts of the condensing fire-mist 

 would impart rotary motions to the clustering masses, just as 

 whirlpools are formed in water, or whirlwinds in air, by the action 

 of opposing currents ; and such rotation would lead to the 

 detachment of the outer parts of the clusters, which would then 

 draw together into planetary masses. These would retain their 

 rotary motion round their original centres, whilst acquiring, in the 

 act of concentration, a rotary motion around centres of their 

 own, and in their turn giving off" their outer portions to form 

 satellites. 



As regards the stellar universe, this hypothesis mainly rests on 

 the observations of the elder Herschel, which led him to the 

 conviction that beside the nebulae which the power of his tele- 

 scope enabled him to resolve into clusters of stars, there are some 

 which are still in the condition of patches of diffused faintly 

 luminous matter, in which the process of condensation has scarcely 

 begun ; others smaller but brighter, whose central parts look as if 

 they would soon form into stars ; others, again, in which stars had 

 actually begun to form ; and finally star-clusters, in which the 

 condensation is complete. Among the nearer stars, again, which 

 he considered to form part of our own particular cluster, he dis* 



