110 JAMES CLEUK MAXWKLL 



molecule in its physical aspect was defined by 

 Maxwell in the following terms : 



** A molecule of a substance is a small body, such that if, on 

 the one hand, a number of similar molecules were assembled 

 together, they would form a mass of that substance ; while on 

 the other hand, if any portion of this molecule were removed, it 

 would no longer be able, along with an assemblage of other 

 molecules similarly treated, to make up a mass of the original 

 substance." 



We are to look upon a gas as an assemblage of 

 molecules Hying about in all directions. The path of 

 any molecule is a straight line, except during the 

 time when it is under the action of a neighbouring 

 molecule; this time is usually small compared with 

 that during which it is free. 



The simplest theory we could formulate would be 

 that the molecules behaved like elastic spheres, and 

 that the action between any two was a collision follow- 

 ing the laws which wo know apply to the collision of 

 elastic bodies. If the average distance between two 

 molecules be great compared with their dimensions, 

 the time during which any molecule is in collision 

 will be small compared with the interval between tho 

 collisions, and this is in accordance with the funda- 

 mental assumption just mentioned. It is not, 

 however, necessary to suppose an encounter between 

 two molecules to be a collision. One molecule may 

 act on another with a force, which depends on the 

 distance between them, of such a character that the 

 force is insensible except when the molecules are 

 extremely close together. 



It is not difficult to see how the pressure exerted 



