268 SCIENCE AND METHOD. 



it is because they are very ancient, and we shall get an 

 inferior limit for their age. We shall likewise obtain a 

 superior limit, for this equilibrium is not ultimate and 

 cannot last for ever. Our spiral nebula; would be com- 

 parable to gases animated with permanent motions. 

 But gases in motion are viscous and their velocities 

 are finally expended. What corresponds in this case 

 to viscidity (and depends upon the chances of collision 

 of the molecules) is exceedingly slight, so that the 

 actual status may continue for a very long time, but 

 not for ever, so that our Milky Ways cannot be ever- 

 lasting nor become infinitely ancient. 



But this is not all. Consider our atmosphere. At 

 the surface an infinitely low temperature must prevail, 

 and the velocity of the molecules is in the neighbour- 

 hood of zero. But this applies only to the mean 

 velocity. In consequence of collisions, one of these 

 molecules may acquire (rarely, it is true) an enormous 

 velocity, and then it will leave the atmosphere, and 

 once it has left it, it will never return. Accordingly 

 our atmosphere is being exhausted exceedingly slowly. 

 By the same mechanism the Milky Way will also lose 

 a star from time to time, and this likewise limits its 

 duration. 



Well, it is certain that if we calculate the age of 

 the Milky Way by this method, we shall arrive at 

 enormous figures. But here a difficulty presents itself. 

 Certain physicists, basing their calculations on other 

 considerations, estimate that Suns can have but an ephe- 

 meral existence of about fifty millions of years, while 

 our minimum would be much greater than that. Must 

 we believe that the evolution of the Milky Way began 

 while matter was still dark? But how have all the 



