1902.] on the Nebular Theory. 159 



Sun was swollen out into this great ball of glowing gas the materials 

 of the Earth were in a condition utterly different from their present 

 state. The Earth was then part of the great nebula itself from which 

 the Sun and all the solar system has been formed. 



Laplace bade us imagine a great primaeval nebula or fire-mist. 

 He reminded us how this must be incessantly radiating its heat and 

 gradually contracting. Laplace demonstrated that it was infinitely 

 probable such a nebula would have some movement of rotation ; he 

 pointed out the remarkable dynamical law by which the contracting 

 object would gradually accelerate its rotation, and he showed how 

 the inner parts would thus revolve more quickly than the outer 

 parts. Laplace bade us see how the denser parts of the nebula at 

 the centre gradually drew themselves together so as to form a sun. 

 He showed in like manner how the outer parts of the nebula gradually 

 cohered together to form the planets. By strictly dynamical reasoning 

 Laplace thus pointed out how from an extensive diffused nebula a 

 solar system with Sun and planets all complete, could be duly evolved 

 in the lapse of ages. Laplace bade us see how the subsidiary systems 

 of satellites appropriate to each planet came into being, he made it 

 plain that these satellites would revolve around their primaries, just 

 as the primaries revolve around the Sun, he bade us follow in ima^^i- 

 nation the progress of the whole system, from the widely difi*used 

 nebulosity — a mere stain of milky light in the sky on the one hand, 

 to an organised system of revolving worlds on the other. 



If Laplace lived now there are many objects in the heavens to 

 which he could point in vindication of his theory. The a^^e of 

 photography has dawned and the photographic plates have not only 

 illustrated in the most marvellous manner the Spiral Nebulse dis- 

 covered by Lord Rosse, but they have succeeded in disclosing many 

 other spiral nebulae. The photographs have even revealed on the 

 plate beautiful spiral nebulae invisible to any human eye, no matter 

 what may be the telescope to which it is applied. How strikingly 

 do the spiral nebulae elucidate Laplace's theory. We see in the Great 

 Spiral how the central part is condensed — doubtless in consequence 

 of the fact that the nebulous matter is drawing itself together. We see 

 how the future Sun may gradually become evolved, we see how 

 planets, also originally "without form and void," gradually come 

 into shape, drawing as they do so, their material from the same 

 primaeval fire-mist. We have in the photograph of the Great Spiral 

 a marvellous illustration of those principles of celestial evolution 

 which Laplace laid down for the formation of the solar system. I 

 try to imagine the astonishment and delight with which Kant or 

 Laplace would look on a photograph of the Great Spiral. If we 

 sought for the best picture of the great primaeval fire-mist which has 

 evolved into the solar system, I feel confident we could not obtain 

 anything so effective as a photograph of this wonderful object. 



I am permitted to illustrate this lecture by a series of photo- 

 graphs which have been most kindly sent to me by Professor Hale, 



