THE NON-LIVING WORLD 175 



all our light, since the moon and planets reflect their light 

 on us, the light of the distant self-luminous stars being 

 quite insignificant in comparison with the direct and re- 

 flected solar light. The surface and atmosphere of the sun 

 form a region of intense energy and heat. Just as aqueous 

 vapours ascend from the surface of the earth and, after 

 condensation, fall back on it as showers of rain, so in 

 the sun metallic vapours are continually ascending, to 

 be condensed and fall down in showers of red-hot metal, 

 amidst flames of hydrogen,* thousands of miles high. 



Now, as we have seen,t light travels at the enormous 

 speed of 186,330 miles in a second, through whatever 

 intervenes between, and connects together, all the plane- 

 tary and stellar bodies. The hypothetical substance, 

 ether,:j: before noted as the medium of light, must, if 

 indispensable for luminous energy, be universally dif- 

 fused wherever light is transmitted. Wherever light 

 can travel, there must then be ether ; and light can 

 travel through every known interval, and through the 

 most perfect vacuum which we can make. Therefore, 

 neither can we make a real vacuum, nor can there be 

 any between us and the most distant visible star. The 

 distance from us of the nearest visible fixed star (Alpha 

 cenlauri) being about 272,000 times greater than that of 

 the sun which itself is about 92,700,000 miles away 

 the time which light must take in passing from it (the 

 nearest fixed star) to our eyes is three years. Even the 

 light of the sun takes eight minutes to come to us, and 

 that of the most distant known planet, Neptune, takes 

 five hours. The light of the most distant visible stars 

 probably takes centuries to reach us, so that in what we 



* See ante, p. 139. f See ante, p. 103. 



J See ante, p. 93. See ante, p. 102. 



