8 MOLECULAR MOTION AND ITS ENERGY 3 



amount of work agree in showing a real cohesion in so far 

 as it was proved that particles of gas attract, and do not 

 repel, each other. Herein is a weighty support of our 

 /hypothesis, since in face of this fact the possibility of 

 explaining, as Newton attempted, the expansive tendency 

 of a gas by the repulsion of its particles disappears ; and 

 there remains as the only admissible hypothesis the oppo- 

 site view that in a motion of its molecules consists the 

 expansive tendency of a gas. 



4. Character of the Heat-motion in Gases 



It is now easy to detexxaine4he .way... in, .which we have 

 to represent to ourselves this molecular motion. We first 

 assume that the gas under consideration is removed from 

 the action of external forces such as gravity, for instance 

 and we then introduce the further assumption, which is 

 sufficiently, though not strictly, accurate, that there is no 

 cohesion in the gas worthy of account. 



If these two suppositions are realised the molecules of a 

 gaseous substance move freely without being subject to the 

 action of any force. Now, according to the law of inertia, 

 free motion without the action of force takes place with 

 unchanging speed in unchanging direction. Hence the 

 hypothesis which must form the groundwork of the theory 

 of gases is this : 



/ The heat-motion of the molecules of a gas consists in a 

 * uniform rectilinear translatory motion. 



We must add to this what is nearly obvious, namely, 

 that a molecule can proceed along its straight path only so 

 long as it meets with no obstruction. If it should strike a 

 wall or collide with another molecule, its motion must suffer 

 an abrupt change in direction by reason of the impenetra- 

 bility of matter. Two colliding molecules therefore rebound 

 from each other, and possibly just like two elastic balls. 



If, now, we take account of the action of external forces, 

 such as gravity, which is practically uniform, we have to 

 represent the paths of the molecules as no longer straight, 

 but in general curved, the path for a constant force being a 



