MOLECULAR MOTIONS AND TEMPERATURE 161 



must be satisfied. First, there must be no subtraction 

 from their energy or no addition to it by the walls of 

 the box; and second, there must be no similar sub- 

 traction or addition on the part of the component parts 

 of the molecules. 



The first of these conditions requires that on the 

 average there shall be no interchange of molecular 

 energy, that is, heat, between the walls and the mole- 

 cules. In other words, they must be at the same 

 temperature. This does not mean that individual 

 molecules may not rebound with greater or less velocity, 

 but means that on the average the value of the kinetic 

 energy of a molecule, represented by wv 2 /2, must 

 remain constant. Individual molecules may lose energy 

 to the molecules of the walls, but in the succeeding 

 impacts other gas molecules must receive equivalent 

 additions of energy from the walls. This condition 

 requires, therefore, that on the average the kinetic 

 energy of the molecules constituting the walls, and 

 hence their velocities and momenta, must be such that 

 in their collisions with the molecules of the contained 

 gas there shall be no net interchanges of energy. 



If, however, the walls are not at the same temperature 

 such transfers of energy will occur until the partition 

 or division of the total energy possessed by the two 

 sets of molecules satisfies the above condition. It is 

 by such exchanges that two bodies which are at differ- 

 ent temperatures arrive at the same temperature, 

 when placed in contact, one cooling and the other 

 heating. This process, incidentally, is usually spoken 

 of as a transfer of heat by conduction. 



It is important to note that while we started with 



