THE STORY OF NINETEENTH-CENTURY SCIENCE 



specks of cosmic dust, as comets are composed of. Neb- 

 ulae are vast cometary clouds, with particles more or 

 less widely separated, giving off gases through meteoric 

 collisions, internal or external, and perhaps glowing also 

 with electrical or phosphorescent light. Gravity eventu- 

 ally brings the nebular particles into closer aggregations, 

 and increased collisions finally vaporize the entire mass, 

 forming planetary nebulas and gaseous stars. Contin- 

 ued condensation may make the stellar mass hotter and 

 more luminous for a time, but eventually leads to its 

 liquefaction, and ultimate consolidation the aforetime 

 nebula3 becoming in the end a dark or planetary star. 



The exact correlation Avhich Mr. Lockyer attempts to 

 point out between successive stages of meteoric con- 

 densation and the various types of observed stellar bod- 

 ies does not meet with unanimous acceptance. Mr. 

 Ranyard, for example, suggests that the visible nebulae 

 may not be nascent stars, but emanations from stars, 

 and that the true pre-stellar nebulae are invisible until 

 condensed to stellar proportions. But such details aside, 

 the broad general hypothesis that all the bodies of the 

 universe are, so to speak, of a single species that neb- 

 ulae (including comets), stars of all types, and planets, 

 are but varying stages in the life history of a single 

 race or type of cosmic organisms is accepted by the 

 dominant thought of our time as having the highest war- 

 rant of scientific probability. 



All this, clearly, is but an amplification of that nebu- 

 lar hypothesis which, long before the spectroscope gave 

 us warrant to accurately judge our sidereal neighbors, 

 had boldly imagined the development of stars out of 

 nebular and of planets out of stars. But Mr. Lockyer's 

 hypothesis does not stop with this. Having traced the 



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