74 NebulcB and Nebular Hypotheses. [Sess, 



that of a gas under electrical excitement in a vacuum tube. 

 In other words, the green, gaseous nebulae are not Ifiot ; they 

 owe their luminosity either to some kind of electrical stim- 

 ulus, or else, perhaps, to some kind of radio-activity. Kadium, 

 as you know, emits a curious luminescence, and it has been 

 found that helium is formed from radium. Since, then, 

 helium is one of the constituents of green nebulae, their light 

 may be due to a radio-active origin. 



But spiral nebulae are white, and their spectra are funda- 

 mentally different. Instead of a few bright lines we have in 

 their case a continuous spectrum, with a hint of dark lines 

 crossing it. The physical interpretation of such a spectrum 

 for such an object is extremely difficult, and is one of the 

 outstanding problems of astro-physics. But at all events this 

 spectrum proves that white nebulae are radically unlike green 

 nebulae ; and, indeed, the mere appearance of those of spiral 

 form almost certainly precludes the idea that they are a 

 natural development of the irregular class. As we shall find, 

 they have probably been formed quite otherwise. 



Two other types of nebulae demand a passing notice. One 

 is the dumb-bell type, the other the planetary. The dumb- 

 bell consists essentially of two nebulae joined by a narrow 

 neck, and the inference is that sooner or later the neck will 

 break and the nebulae will become separated. Each will then 

 contract and condense to form a star, so that in future ages 

 the dumb-bell will probably become what we call a binary 

 star, — two stars, that is to say, which are physically related 

 to one another by virtue of their common birth, and which 

 revolve round one another. Such coupled stars are very 

 common, — indeed the evidence goes to show that the great 

 majority of all the stars are double, although to the naked 

 eye they appear to be single. A star which, like our sun, 

 passes its life in single blessedness, is apparently the exception 

 and not the rule, — and it is worth noting as a consequence 

 that stars with families of planets must also be exceptional, 

 since no double star could conceivably generate a planetary 

 system like our own. This is a very serious argument against 

 the doctrine of the plurality of worlds. 



The other type of nebula to which I wish to refer is the 

 planetary, — so called because its members present the appear- 



