PROBABLE ORIGIN OF NEBULJB. 305 



gaseous condition ; and it is well known that these 

 gases are exceedingly bad radiators. The oxyhydrogen 

 flame, though its temperature is only surpassed by 

 that of the voltaic arc, o-ives nevertheless a light so 

 feeble as scarcely to be visible in daylight. Now, 

 even supposing the enormous space occupied by a 

 nebula were due to excessive temperature, the light 

 emitted would yet not be intense were it derived from 

 nitrogen or hydrogen gas. The small luminosity of 

 nebulae, however, is due to a different cause. The 

 enormous space occupied by those nebulae is not so 

 much owing to the heat which they possess, as to the 

 fact that their materials were dispersed into space 

 before they had time to pass into the gaseous condition ; 

 so that, by the time this latter state was assumed, the 

 space occupied was far greater than was demanded 

 either by the temperature or the amount of heat 

 received. 



If we adopt the nebular hypothesis of the origin of 

 our solar system, we must assume that our sun's mass, 

 when in the condition of nebula, extended beyond the 

 orbit of the planet Neptune, and consequently filled 

 the entire space included within that orbit. Supposing 

 Neptune's orbit to have been its outer limit, which it 

 evidently was not, it would nevertheless have then 

 occupied 274,000,000,000 times the space that it does 

 at present. We shall assume, as before, that 50 million 

 years' heat was generated by the concussion. Of 

 course there might have been twice, or even ten times 

 that quantity; but it is of no importance what number 

 of years is in the meantime adopted. Enormous as 50 

 million of years' heat is, it yet gives, as we shall pre- 

 sently see, only 32 foot-pounds for each cubic foot. 

 The amount of heat due to concussion being equal, as 

 before stated, to 100,000,000,000 foot-pounds for each 



