20 REPORT — 1891. 



Spencer showed that the observations of nebulae up to that time were 

 really in favour of an evolutional progress. 



In 1864 I brought the spectroscope to bear upon them ; the bright 

 lines which flashed upon the eye showed the source of the light of a 

 number of them to be glowing gas, and so restored these bodies to what 

 is probably their true place, as an early stage of sidereal life. 



At that early time our knowledge of stellar spectra was small. For this 

 reason partly, and probably also under the undue influence of theological 

 opinions then widely prevalent, I unwisely wrote in my original paper 

 in 1864, ' that in these objects we no longer have to do with a special 

 modification of our own type of sun, but find ourselves in presence of 

 objects possessing a distinct and peculiar plan of structure.' Two years 

 later, however, in a lecture before this Association, I took a truer position. 

 ' Our views of the universe,' I said, ' are undergoing important changes ; 

 let us wait for more facts with minds unfettered by any dogmatic theory, 

 and therefore free to receive the teaching, whatever it may be, of new 

 observations.' 



Let us turn aside for a moment from the nebulae in the sky to the 

 conclusions to which philosophers had been irresistibly led by a considera- 

 tion of the features of the solar system. We have before us in the 

 sun and planets obviously not a haphazard aggregation of bodies, but 

 a system resting upon a multitude of relations pointing to a common 

 physical cause. i?rom these considerations Kant and Laplace formulated 

 the nebular hypothesis, resting it on gravitation alone, for at that time 

 the science of the conservation of energy was practically unknown. These 

 philosophers showed how, on the supposition that the space now occupied 

 by the solar system was once fiUed by a vaporous mass, the formation 

 of the sun and planets could be reasonably accounted for. 



By a totally difi"erent method of reasoning, modern science traces 

 the solar system backward step by step to a similar state of things at 

 the beginning. According to Helmholtz the sun's heat is maintained 

 by the contraction of his mass, at the rate of about 220 feet a year. 

 Whether at the present time the sun is getting hotter or colder we do 

 not certainly know. We can reason back to the time when the sun was 

 sufliciently expanded to fill the whole space occupied by the solar system, 

 and was reduced to a great glowing nebula. Though man's life, the life 

 of the race perhaps, is too short to give us direct evidence of any distinct 

 stages of so august a process, still the probability is great that the 

 nebular hypothesis, especially in the more precise form given to it by 

 Roche, does represent broadly, notwithstanding some difficulties, the 

 succession of events through which the sun and planets have passed. 



The nebular hypothesis of Laplace requires a rotating mass of fluid 

 which at successive epochs became unstable from excess of motion, and 

 left behind rings, or more probably perhaps lumps, of matter from the 

 equatorial regions. 



