254 SCIENCE AND METHOD. 



word collision in its ordinary sense ; it is not necessary 

 that the two molecules should come into contact, but 

 only that they should come near enough to each other 

 for their mutual attraction to become perceptible. 

 The laws of the deviation they undergo are the same 

 as if there had been an actual collision. 



It seems at first that the orderless collisions of this 

 innumerable dust can only engender an inextricable 

 chaos before which the analyst must retire. But the 

 law of great numbers, that supreme law of chance, 

 comes to our assistance. In face of a semi-disorder 

 we should be forced to despair, but in extreme disorder 

 this statistical law re-establishes a kind of average or 

 mean order in which the mind can find itself again. 

 It is the study of this mean order that constitutes the 

 kinetic theory of gases ; it shows us that the velocities 

 of the molecules are equally distributed in all directions, 

 that the amount of these velocities varies for the dif- 

 ferent molecules, but that this very variation is subject 

 to a law called Maxwell's law. This law teaches us 

 how many molecules there are animated with such and 

 such a velocity. As soon as a gas departs from this 

 law, the mutual collisions of the molecules tend to 

 bring it back promptly, by modifying the amount 

 and direction of their velocities. Physicists have 

 attempted, and not without success, to explain in this 

 manner the experimental properties of gases — for 

 instance, Mariotte's (or Boyle's) law. 



Consider now the Milky Way. Here also we see 

 an innumerable dust, only the grains of this dust are 

 no longer atoms but stars ; these grains also move 

 with great velocities, they act at a distance one upon 

 another, but this action is so slight at great distances 



