Dr. J. Croll on the Origin of Nebulae. 7 



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 con- 

 dition 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 presently 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 

 pound of the mass, and a cubic foot of the sun at his present 

 density of 1*43 weighing 89 lbs., each cubic foot must have 

 possessed 8,900,000,000,000 foot-pounds. But when the mass 

 was expanded to occupy 274,000,000,000 times more space, 

 which it would do when it extended to the orbit of Neptune, 

 the heat possessed by each cubic foot would then amount to 

 only 32 foot-pounds. 



In point of fact, however, it would not even amount to that ; 

 for a quantity equal to upwards of 20 million years' heat would 

 necessarily be consumed in work against gravity in the expan- 

 sion of the mass ; all of which would, of course, be given back 

 in the form of heat as the mass contracted. During the ne- 

 bulous condition it would not exist as heat, so that only 

 19 foot-pounds out of the 32 foot-pounds generated by con- 

 cussion would then exist as heat. The density of the ne- 

 bula would be only - „ ^ that of hydrogen at ordinary 



•^ 16248160 J -iPi • i 



temperature and pressure. The 19 foot-pounds of heat m each 

 cubic foot would nevertheless be sufficient to maintain an ex- 

 cessive temperature ; for there would be in each cubic foot only 

 440 1 of a grain of matter. But although the temperature 

 would be excessive, the quantity both of light and heat in each 

 cubic foot would of necessity be small. The heat being only 

 ^ of a thermal unit, the light emitted would certainly be ex- 

 ceedingly feeble, resembling very much the electric light in a 

 vacuum-tube. 



Heat and Light of Nehuhe cannot result from Condensation. — 

 The fact that nebulas are not only self-luminous but indicate 

 the existence of hydrogen and nitrogen in an incandescent 

 condition proves that they must possess a considerable tempe- 



