I. 



THE AIILKY WAY AND THE THEORY 



OF GASES. 



The considerations I wish to develop here have so 

 far attracted but h'ttle attention from astronomers. I 

 have merely to quote an ingenious idea of Lord 

 Kelvin's, which has opened to us a new field of re- 

 search, but still remains to be followed up. Neither 

 have I any original results to make known, and all 

 that I can do is to give an idea of the problems that 

 are presented, but that no one, up to this time, has 

 made it his business to solve. 



Every one knows how a great number of modern 

 physicists represent the constitution of gases. Gases 

 are composed of an innumerable multitude of mole- 

 cules which are animated with great velocities, and 

 cross and re-cross each other in all directions. These 

 molecules probably act at a distance one upon another, 

 but this action decreases very rapidly with the distance, 

 so that their trajectories remain apparently rectilineal, 

 and only cease to be so when two molecules happen 

 to pass sufficiently close to one another, in which case 

 their mutual attraction or repulsion causes them to 

 deviate to right or left. This is what is sometimes 

 called a collision, l)ut we must not understand this 



