1 90 7-1908.] Nebul(2 and Nebular Hypotheses. 73 



space and time. Masses that seemed almost amorphous when 

 imperfectly seen now show nuclei ; and curving lines of light, 

 which tell of movement and force, furrow them throughout. 

 ^Simple milky effusions prove to be far less common than had 

 been supposed, and complexity of constitution is already a 

 recognisable characteristic of most nebulae." 



But while many nebulae are thus irregular, and betray no 

 evidence of strict geometrical design, others — and by far the 

 largest number — exhibit the configuration of a more or less 

 perfect spiral. For a long time it was believed that these 

 two types^ — the irregular and the spiral — indicated different 

 stages in nebular evolution. On this view the irregular cloud- 

 like nebulae were literally " regions of lucid matter taking forms." 

 They were the least elaborated of sidereal objects, and had 

 just crossed the threshold of creation. How they had done 

 ;So no man ventured to say. It was supposed, however, that 

 under the unremitting influence of gravity these flimsy masses 

 would gradually contract. As Tennyson puts it, " the starry 

 tides " would " set towards the centre " — the material of 

 the nebula would slowly become more closely packed together 

 at the centre of gravity. Two new features would, it was 

 thought, result from this. First of all, the nebula would 

 grow liot. As it contracted its particles would occupy less 

 space ; they would therefore crowd together and jostle one 

 another more than formerly ; collisions between them would 

 occur more frequently, — and such collisions generate heat. 

 Secondly, the contraction would, it was thought, produce a 

 rotation of the mass, so that it might eventually be trans- 

 formed into a spiral. To a certain extent this hypothesis 

 undoubtedly affords a true picture of the probable develop- 

 ment of an irregular nebula — but the idea of the induced 

 rotation appears to be dynamically impossible, and for other 

 reasons the theory has been generally abandoned. 



Irregular nebulae are green in colour; spiral nebulae are 

 white. The spectroscope demonstrates that irregular nebulae 

 are gaseous, their principal constituents being helium, hydro- 

 gen, and a green gas which is unknown on earth, to which 

 the name " nebulium " has been given. Moreover, the spect- 

 rum is not like that of an incandescent gas — that is to say, of 

 a gas which is luminous because it is hot. It is rather like 



