THE ORIGIN OF THE EARTH. 59 



eous body; but the vast dispersion and the enormous surface exposed 

 doubtless quickly reduced the more refractory portions to the liquid 

 and solid state, attended by some degree of aggregation into small 



Fig. 23. — Theoretical restoration of the parent nebula of the solar system. The 

 nuclei of the several planets may be indentified by their distances from the center. 

 The dimensions of the inner parts are made disproportionately large. 



accretions; hence the continuous spectrum which this class of nebulae 

 present. 



Nebular luminescence. — The problem of the luminescence of nebulae 

 is confessedly a puzzling one. There is little ground for assigning 

 general incandescence to matter so obviously scattered and tenuous, 

 and possessed of such enormous radiating surface. The assignment 

 of the light to collisions of meteorites, as has been done by Lockyer, 

 encounters both dynamic and spectroscopic difficulties. The recent 

 discoveries of the luminescent properties of radio-active matter, and 

 of its power to awaken luminescence in other matter, offers hope of 



