430 THE WORLD MACHINE 



inch of water produces only about 1800 cubic inches of steam at 

 ordinary pressures. 



Water is practically incompressible, and we must therefore 

 conceive the molecules of which it is composed as in practical 

 contact. In a state of vapour, then, according to the kinetic 

 theory, the molecules are distant one from the other on the 

 average about 1800 times their own diameters. 



With the perfected air-pump it is possible to reduce the rare- 

 faction of a gas to less than a millionth of its density at ordinary 

 pressures. By means of liquid air this may be carried somewhat 

 farther. In a gas at this density we may conceive the molecules 

 then as eighteen million times and more their own diameters 

 apart. If the distance between alpha Centauri and our sun re- 

 presents anything like an average spacing of the stars, then, 

 pursuing our conception of the universe as a stellar gas with the 

 molecules as suns, its density would be something like air or 

 water vapour at less than one-millionth atmospheric pressure. 



If we conceived of a thousand dark suns for one blazing 

 star, this stellar gas would still be highly rarefied as compared 

 with ordinary air. It would still be exceedingly tenuous if we 

 conceived of a million dark bodies for every luminous sun. It 

 is almost impossible for balloonists to survive when the baro- 

 meter has dropped two-thirds. If, then, the imaginative Mr. 

 Wells were to write a story whose characters were vast beings 

 breathing this stellar air, he would have to picture them as able 

 to exist in a much more ethereal medium than those which live 

 and move about at the bottom of our sea of earthly atmosphere. 



But if the stars were spaced on the average of thirty million 

 times their own diameters, the chances of collisions between 

 suns would be almost infinitely small. It would probably not 

 begin to account for the number of new stars which blaze forth 

 in the sky. It is possible, of course, that a simple rush through 

 a relatively dense nebula might account for many of these ; 

 probably ^not all. The number of dark bodies must therefore be 

 considerable. Analogy with known suns like Sirius, Arcturus, 

 and Canopus would suggest many of them are of colossal size. 

 Even when they are cooled down to the crust-forming stage the 

 energy they contain is still prodigious. We conceive the interior 

 of the earth as a glowing gas practically rigid under the enormous 

 pressure which it sustains. We may believe this true of all dark 

 bodies, large and small. 



