THE PROBABLE AGE OF THE WORLD. 659 



awakened public interest in the debatable land which lies between 

 that which is certainly known to science and that which must always 

 defy inquiry. Before the appearance of that remarkable book, the 

 theory that the sun and its attendant planets were produced by the 

 condensation of a vast nebula was but little known to the unscientific 

 world. The idea was originally entertained by Sir William Herschel, 

 and affords one of the greatest proofs of his commanding genius. It 

 was afterward elaborated by Laplace ; but that great astronomer was 

 himself distrustful of it, and, while he expounded the mechanical laws 

 by which the proposed explanation could be supported, he was care- 

 ful to speak of it only as an hypothesis. As time goes on, it seems 

 probable that the saying of Arago will be accepted, and that the 

 views of Laplace will be universally acknowledged to be "those only 

 which, by their grandeur, their coherence, and their methematical 

 character, can be truly considered to form a physical cosmogony." 



But, though Laplace is thus credited by Arago with the origi- 

 nation of this grand conception, he was not its author. Sir William 

 Herschel gave the earliest sketch of the theory. His views w r ere ex- 

 pressed with so much precision, that one cannot help feeling a little 

 jealousy for the prior right of discovery of the English astronomer. 

 Herschel so plainly preceded Laplace, that it seems hard that Laplace 

 should have the credit of it. Herschel began to search after nebulae 

 in 1779, and soon formed a catalogue comprising an enormous num- 

 ber of them. By degrees it dawned upon his mind that the differ- 

 ences he observed in them were systematic, and at length occtirred 

 the magnificent intuition that the nebulae are stars in process of for- 

 mation. 



They lie in enormous numbers in every part of the heavens, and 

 apparently in every stage of progressive development. The slow 

 growth of worlds, extending over ages of time, cannot, of course, be 

 watched by any single observer. No more can a single tree among 

 the trees of a forest be so observed. But a forest contains specimens 

 of saplings, young trees, trees of vigorous growth, and trees in decay. 

 In like manner the heavens contain specimens of worlds in the mak- 

 ing, from the chaotic mass of vapory matter which forms the first 

 stage of cosmical existence to the perfect, self-luminous star. Her- 

 schel arranged them in classes showing this gradual development, 

 and he declares that each class is so nearly allied to the next, that 

 they do not differ so much as would the annual description of a hu- 

 man figure, if it were given from the birth of a child till he comes to 

 be a man in his prime. His catalogue arranges the objects he has act- 

 ually observed somewhat in the following fashion : first, patches of 

 extensive diffused nebulosity; "milky nebulosity," with condensa- 

 tion; round nebulae; nebulae with a nucleus; and soon till he reaches 

 stellar nebula?, nearly approaching the appearance of stars. 



The evidence grows irresistible as we read, that in these wonder- 



