MODERN VIEWS OF THE EARTH'S ORIGIN. 17 



estimate, extend far deeper than man can ever penetrate. Some other 

 facts bear upon the origin of the earth itself. 



Observations with the spectroscope prove that many of the fixed 

 stars are suns, more or less like the central luminary of our solar 

 system. Thus in the light from a star Aldebaran, Mr. Huggins found 

 the lines which indicate hydrogen, sodium, magnesium, calcium, 

 iron, bismuth, tellurium, antimony, and mercury. And many of the 

 fixed stars, like Sirius, give the light which is produced by incan- 

 descent hydrogen ; and there are very few of the fixed stars in which 

 hydrogen has not been detected. Thus the matter of the universe 

 appears to be as universal as the laws of force, by which its existence 

 is manifested and controlled. It is certain that each of these vast 

 burning worlds is slowly increasing in size by combining the gases of 

 its atmosphere with the superficial substances which burn ; and that 

 eventually they must all burn out when the atmosphere is exhausted, 

 or when the supply of combustible material comes to an end. And 

 this consideration leads up to what has been called the "nebular 

 hypothesis ; " because, if we suppose the sun, for instance, always to 

 have been about as large as it is now, it is difficult to conceive how 

 chemical combinations like those now occurring on its surface, could 

 have taken place throughout its entire mass from the centre outwards, 

 because the gases which support combustion are usually less dense than 

 the vapours which combustion would produce. And since these vapours 

 when condensed would so accumulate as to form a protecting envelope, 

 it has sometimes and naturally been supposed that the central mass of 

 the sun and of the planets may be less oxidised or earthy than the 

 part which is near the surface. The nebular hypothesis supposes that 

 before the stars existed, the materials of which they consist were diffused 

 in the heavens in a state of vapour. The nebulae in the firmament had 

 been observed to fill enormous areas in space, and to give a dull kind of 

 light which long ago suggested that they might be gaseous matter in 

 process of being condensed into worlds. And Sir William Herschel 

 speculated that if they condensed by their own gravity, they would 

 assume more or less spheroidal forms and be denser towards the centre. 

 Then assuming that local centres of condensation would come into 

 existence and gradually absorb the nebulous matter, the nebulae 

 would become resolved into clusters of stars. 1 There may be some 

 truth in this conception, though the nebulae are now known to be 

 dense masses of stars, and in no respect nebulous. And if the masses 

 of matter already condensed became drawn into contact with each 

 other by force of gravity, sufficient heat might be developed by the 

 concussion to melt or vaporise the whole mass. Such a heated mass 

 might then by rapid rotation throw off rings which would cool, break, 

 and condense into masses like planets, and if the condensation took 

 place with sufficient force, the planet might in its turn throw off a 

 ring which in due course would condense into a satellite. This is 



filiation, it is true ; but it is speculation supported by a number of 

 Miomical facts ; and it is mentioned now because it helps us to 

 1 HerscheFs " Astronomy." 

 VOL. I. B 



