f 3 2 IN STARRY REALMS. 



moon would be ranked as a large object of its class, while 

 even the greatest of them is perhaps not more than ten or 

 twelve times as great. Nor is the word cloud, as applied 

 to nebula, an appropriate one. What we mean by a 

 cloud is only a vast mass of watery vapour raised by 

 the sun from the sea, and poised aloft until such time as 

 it shall be again dispersed into invisible water, or until it 

 shall descend to the earth as rain. Such clouds are of 

 course within the limits of our atmosphere, and are rarely 

 more than a few miles above the earth's surface. The 

 light which renders clouds visible only comes from 

 reflected sunbeams, and consequently at night clouds be- 

 come invisible, though the astronomer is often only too 

 unpleasantly made acquainted with their presence by the 

 opacity with which they shut out the stars from his 

 view. 



Utterly different in all respects are the nebulae. They 

 are not masses of watery vapour. It may no doubt pos- 

 sibly be that water in some form is there, but it is not 

 water which we see. We are looking at some gaseous 

 material of a bluish hue. The light with which it glows 

 is no reflected sunlight. The nebula is indeed indebted 

 to no foreign source for that weird I had almost said 

 ghost-like radiance which it gives forth. The light 

 comes from the nebula itself. But how, it may well be 

 asked, should a purely gaseous substance be able to radiate 

 forth light ? It is easy for us to comprehend how stars 

 or suns or comparatively solid bodies can, in virtue of their 

 tremendous temperature, glow with heat like red-hot or 

 white-hot iron. It is true that flame is gas in an incan- 

 descent state, but in flame a vehement chemical union of 



