The Romance of the Heavens 39 



THE NEBULAR THEORY 

 2 



Nebulas are dim luminous cloud-like patches in the heavens, 

 more like wisps of smoke in some cases than anything else. Both 

 photography and the telescope show that they are very numer- 

 ous, hundreds of thousands being already known and the num- 

 ber being continually added to. They are not small. Most of 

 them are immensely large. Actual dimensions cannot be given, 

 because to estimate these we must first know definitely the dis- 

 tance of the nebulas from the earth. The distances 

 of some nebulae are known approximately, and we can there- 

 fore form some idea of size in these cases. The results are stag- 

 gering. The mere visible surface of some nebulae is so large that 

 the whole stretch of the solar system would be too small to form 

 a convenient unit for measuring it. A ray of light would require 

 to travel for years to cross from side to side of such a nebula. Its 

 immensity is inconceivable to the human mind. 



There appear to be two types of nebulas, and there is evi- 

 dence suggesting that the one type is only an earlier form of the 

 other ; but this again we do not know. 



The more primitive nebulas would seem to be composed of 

 gas in an extremely rarified form. It is difficult to convey an 

 adequate idea of the rarity of nebular gases. The residual gases 

 in a vacuum tube are dense by comparison. A cubic inch of air 

 at ordinary pressure would contain more matter than is contained 

 in millions of cubic inches of the gases of nebulas. The light of 

 even the faintest stars does not seem to be dimmed by passing 

 through a gaseous nebula, although we cannot be sure on this 

 point. The most remarkable physical fact about these gases is 

 that they are luminous. Whence they derive their luminosity 

 we do not know. It hardly seems possible to believe that ex- 

 tremely thin gases exposed to the terrific cold of space can be so 

 hot as to be luminous and can retain their heat and their lumin- 



