2l8 LIGHT SCIENCE FOR LEISURE HOURS. 



in the sun, is one possible origin. The mind of man, 

 unapt though it is to deal with time-intervals so 

 enormous as are required to transmute a giant orb from 

 the sun-like to the planetary condition, may yet accept 

 this interpretation, if no other present itself which is 

 not still more appalling. Only one other, as it seems 

 to me, remains, and this compels us to contemplate 

 time intervals compared with which those required to 

 change Uranus from sun to planet seem insignificant. 

 If, as we are taught by the nebular hypothesis of the 

 solar system, or, in fact, by any theory of its evolution 

 whatever, the planet Uranus was once in a vaporous 

 condition, extending as a mighty rotating disc far 

 beyond its present sphere, and probably far beyond the 

 path of its outermost satellite, we may conceive a 

 comet arriving from outer space to be captured by. the 

 resistance of the once vaporous planet, not by its mere 

 attractive force. But to what a result have we thus 

 been led ! If we accepted this view, rather than the 

 theory that Uranus had expelled the comet, we should 

 have first to carry our thoughts back almost to the 

 very beginning of our solar system, and then to 

 recognise, at that inconceivably distant epoch, comets 

 travelling from sun to sun, and some of them coming 

 from other suns towards ours, to be captured from 

 time to time by the resistance of the vaporous masses 

 out of which the planets of our system were one day 

 to be evolved. 



We do not know how the questions raised by such 

 thoughts should be answered, although, as has been 



